Selective Chemical Stripping of Thin Film Coatings Using Hydrogen
Peroxide and Potassium Oxalate
Samuel Bastien
Department of Chemical Engineering
McGill University, Montréal
September 2011
A thesis submitted to McGill University in partial fulfillment of the requirements of the degree
of Master’s of Applied Sciences
© Samuel Bastien, 2011
ACKNOWLEDGMENTS
I would like to extend my thanks to the following people and organizations for their support and advice
throughout the course of this project:



Dr. Dimitrios Berk (Research Supervisor)
The students, faculty and support staff of the Department of Chemical Engineering of McGill
University, especially Rabib Chaudhury, Benjamin Ross and the other members of my laboratory
group.
The Eugenie Ulmer Lamothe Fund
i
TABLE OF CONTENT
ABSTRACT...................................................................................................................................................... 1
CHAPTER 1 - Introcution ............................................................................................................................... 2
1.1: Properties of TiAlN Coatings .............................................................................................................. 2
1.2: Methods and Mechanisms for the Removal of the Coating and Substrate ...................................... 3
1.2.1: Activation of Hydrogen Peroxide by Transition Metal Ion .......................................................... 5
1.2.2: Activation of Hydrogen Peroxide by Organic Compounds .......................................................... 5
1.2.3: Stability of Hydrogen Peroxide and Organic Acids ...................................................................... 8
1.3: Literature Review ............................................................................................................................... 8
1.4: Objectives ........................................................................................................................................ 10
CHAPTER 2 – Materials, Methods and Set-up ............................................................................................ 11
2.1: Samples ............................................................................................................................................ 11
2.2: Reactor set-up.................................................................................................................................. 11
2.3: Experimental procedure .................................................................................................................. 13
2.4: Measurements ................................................................................................................................. 13
CHAPTER 3 - Results .................................................................................................................................... 15
3.1: Preliminary Experiments .................................................................................................................. 15
3.1.1: Establishment of a Measurement Technique............................................................................ 15
3.1.2: SEM analysis of samples ........................................................................................................... 17
3.2: Effect of Experimental Conditions ................................................................................................... 19
3.2.1: Selected Experimental Conditions ............................................................................................. 19
3.2.2: Qualitative Observations and Description of a Stripping Experiment ...................................... 21
3.2.3: Control Experiments .................................................................................................................. 24
3.3: Results .............................................................................................................................................. 25
3.3.1: Room Temperature and 50oC Experiments ............................................................................... 25
3.3.2: 75oC Experiments ...................................................................................................................... 29
3.3.3: Coating with a Different Composition....................................................................................... 31
CHAPTER 4 – Discussion of Results ............................................................................................................. 33
4.1: Evaluation of the Removal Rate of the Coating and Substrate ....................................................... 33
4.1.1: Calculated Stripping Rates at Room Temperature and 50oC .................................................... 34
ii
4.1.2: Calculated Stripping Rates at 75oC ........................................................................................... 38
4.1.3: Calculated Stripping Rate for Coatings B and C ........................................................................ 41
4.1.4: Analysis of the Calculated Stripping Rates ................................................................................ 43
4.2: Selectivity of the Stripping Technique ............................................................................................. 44
4.2.1: Selectivity at Room Temperature .............................................................................................. 45
4.2.2: Selectivity at 75oC...................................................................................................................... 47
4.2.3: Selectivity for Coating B and C .................................................................................................. 49
4.3: Evaluation of the Environmental Friendliness of the Proposed Technique .................................... 50
CHAPTER 5 – Conclusions and Recommendation ....................................................................................... 52
REFERENCES ................................................................................................................................................ 54
APPENDIX 1 - METHOD FOR THE MEASUREMENT OF THE CONCENTRATION OF HYDROGEN PEROXIDE .... I
APPENDIX 2 - SAMPLE CALCULATIONS ........................................................................................................ III
APPENDIX 3 - RAW DATA WITH CALCULATED ERROR ................................................................................. IV
LIST OF TABLES
Table 1.1: Half reaction cell potential for many common oxidizing agents (Kotz et al., 2009) .................... 4
Table 1.2: pKa values of common organic acids ........................................................................................... 7
Table 1.3: Selected Equilibrium Constant of Oxalic Acid in Aqueous Solution (Gokel, 2004) ...................... 7
Table 3.1: ICP-OES Measurements Results for One Experiment ................................................................ 16
LIST OF FIGURES
Figure 1.1: Peracid Formation Mechanism (Strukul, 1992) .......................................................................... 6
Figure 2.1: Side View of the Used Impeller ................................................................................................. 12
Figure 2.2: Sample Holder........................................................................................................................... 12
Figure 2.3: Side View of Sample Holder with Sample Inserted................................................................... 12
Figure 3.1: Profilometry Analysis of a Coated Sample ................................................................................ 16
Figure 3.2: SEM picture of a Coated Sample, 5000x Magnification Rate ................................................... 18
Figure 3.3: SEM Picture of an Uncoated Sample, 5000x Magnification Rate ............................................. 18
Figure 3.4: Comparison of coated sample before (right) and after (left) stripping .................................... 24
Figure 3.5: The Effect of the Reactant Composition on the Observed Mass Loss of Uncoated Samples at
Room Temperature, 6 Hours Reaction Time .............................................................................................. 27
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Figure 3.6: The Effect of the Reactant Composition of the Observed Mass Loss of Coated Samples at
Room Temperature, 6 Hours Reaction Time .............................................................................................. 28
Figure 3.7: The Effect of the Reaction Time on the Observed Mass Loss at 50oC, 4.4 mol/L H2O2 and 0.150
mol/L K2C2O4 ............................................................................................................................................... 28
Figure 3.8: The Effect of the Reactant Composition on the Observed Mass Loss of Coated and Uncoated
Samples at 50oC, 0.150 mol/L and 2 Hours Reaction Time......................................................................... 29
Figure 3.9: The Effect of the Etchant Composition on the Observed Mass Loss of Uncoated Samples at
75oC, 20 Minutes Reaction Time ................................................................................................................. 30
Figure 3.10: The Effect of the Etchant Composition on the Observed Mass Loss of Coated Samples at
75oC, 20 Minutes Reaction Time ................................................................................................................. 31
Figure 3.11: The Effect of the Coating Composition on the Observed Mass Loss at 75oC, 20 Minutes
Reaction Time ............................................................................................................................................. 32
Figure 4.1: The Effect of Hydrogen Peroxide Concentration on the Calculated Stripping Rate of Uncoated
Samples at Room Temperature .................................................................................................................. 35
Figure 4.2: The Effect of Potassium Oxalate Concentration on the Calculated Stripping Rate of Uncoated
Samples at Room Temperature .................................................................................................................. 36
Figure 4.3: The Effect of Hydrogen Peroxide Concentration on the Calculated Stripping Rate of Coated
Samples at Room Temperature .................................................................................................................. 36
Figure 4.4: The Effect of Potassium Oxalate Concentration on the Calculated Stripping Rate of Coated
Samples at Room Temperature .................................................................................................................. 37
Figure 4.5: The Effect of Total Reaction Time on the Calculated Stripping Rate at 50oC, 4.4 mol/L H2O2
and 0.150 mol/L K2C2O4 .............................................................................................................................. 37
Figure 4.6: The Effect of Hydrogen Peroxide Concentration on the Calculated Stripping Rate of Coated
and Uncoated Samples at 50oC, 0.150 mol/L K2C2O4 .................................................................................. 38
Figure 4.7: The Effect of Hydrogen Peroxide Concentration on the Calculated Stripping Rate of Uncoated
Samples at 75oC .......................................................................................................................................... 39
Figure 4.8: The Effect of Potassium Oxalate Concentration on the Calculated Stripping Rate of Uncoated
Samples at 75oC .......................................................................................................................................... 40
Figure 4.9: The Effect of Hydrogen Peroxide Concentration on the Calculated Stripping Rate of Coated
Samples at 75oC .......................................................................................................................................... 40
Figure 4.10: The Effect of Potassium Oxalate Concentration on the Calculated Stripping Rate of Coated
Samples at 75oC .......................................................................................................................................... 41
Figure 4.11: The Effect of the Reactant Composition on the Stripping Rate of Coatings B and C at 75oC . 42
Figure 4.12: Comparison of the Stripping Rate of the Different Coatings for Similar Experimental
Conditions ................................................................................................................................................... 42
Figure 4.13: Comparison of the Stripping Rate of Uncoated Samples at Different Experimental
Temperatures (0.150 mol/L Potassium Oxalate) ........................................................................................ 43
Figure 4.14: Comparison of the Stripping Rate of Coated Samples at Different Experimental
Temperatures (0.150 mol/L Potassium Oxalate) ........................................................................................ 44
Figure 4.15: The Effect of Hydrogen Peroxide Concentration on the Calculated Selectivity for Room
Temperature Experiments .......................................................................................................................... 46
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Figure 4.16: The Effect of Potassium Oxalate Concentration on the Calculated Selectivity for Room
Temperature Experiments .......................................................................................................................... 46
Figure 4.17: The Effect of Hydrogen Peroxide Concentration on the Calculated Selectivity for 50oC
Experiments ................................................................................................................................................ 47
Figure 4.18: The Effect of Hydrogen Peroxide Concentration on the Calculated Selectivity for 75oC
Experiments ................................................................................................................................................ 48
Figure 4.19: The Effect of Potassium Oxalate Concentration on the Calculated Selectivity for 75oC
Experiments ................................................................................................................................................ 49
Figure 4.20: Selectivity as a Function of Reactant Molar Ratio for Experiments at 75oC ........................... 49
Figure 4.21: The Effect of Reactant Concentration on the Calculated Selectivity for Coatings B and C at
75oC ............................................................................................................................................................. 50
v
ABSTRACT
Titanium Aluminum Nitride (TiAlN) is an important industrial coating that improves hardness
and corrosion resistance. The objective of the present work is to develop chemical methods which
selectively remove TiAlN coatings deposited on Titanium substrates. The selected stripping solution
consists of hydrogen peroxide (H2O2) and potassium oxalate (K2C2O4). Coupons of Ti-6-4 coated with a
notional 10 micron thick coating of TiAlN were exposed to a stripping solution at varying temperature
and compositions. Overall, it was found that increasing the temperature of reaction or the
concentration of reactants led to an increase in stripping rate of the coating and substrate. The
selectivity also increased with an increase in temperature or potassium oxalate concentration, but
decreased with an increase in hydrogen peroxide concentration. The highest stripping rate that was
obtained for the coating was of 39 µm/hr at a temperature of 75oC, a concentration of hydrogen
peroxide of 5.9 mol/L and a potassium oxalate concentration of 0.226 mol/L. At the same conditions,
uncoated samples were found to be stripped at a rate of 6.6 µm/hr. The best selectivity that was
obtained was of 6.8, at a potassium oxalate concentration of 0.226 mol/L, a hydrogen peroxide
concentration of 4.4 mol/L and a 75oC temperature. It was also found that the ratio of Ti:Al in the
coating had a major effect on its chemical resistance to H2O2 and K2C2O4 mixtures.
Titanium Aluminum Nitride (TiAlN) est un revêtement industriel important puisqu’il améliore la
dureté et la résistance à la corrosion. L’objectif de ce travail est de développer des techniques chimiques
qui enlèvent de façon sélective des revêtements de TiAlN déposés sur des substrats de Titane. La
solution chimique sélectionnée consiste de peroxyde d’hydrogène (H2O2) et d’oxalate de potassium
(K2C2O4). Des échantillons de Ti-6-4 couverts d’une couche de TiAlN d’une épaisseur notionnelle de 10
micromètres ont été exposés à plusieurs solutions chimiques avec des températures et concentrations
variées. De façon générale, nous avons trouvé que si on augmentait la température de la réaction ou la
concentration des réactants, cela faisait augmenter les vitesses de dégradation du revêtement et du
substrat. La sélectivité augmentait aussi avec une hausse de la température ou de la concentration
d’oxalate de potassium, mais diminuait avec une hausse de la concentration de peroxyde d’hydrogène.
La plus haute vitesse de dissolution du revêtement qui a été obtenue était de 39 µm/hr à une
température de 75oC, une concentration de peroxyde d’hydrogène de 5.9 mol/L et une concentration
d’oxalate de potassium de 0.226 mol/L. À des conditions similaires, le substrat se dissolvait à une vitesse
de 6.6 µm/hr. La meilleure sélectivité obtenue était de 6.8, à une concentration d’oxalate de potassium
de 0.226 mol/L, une concentration de peroxyde d’hydrogène de 4.4 mol/L et une température de 75oC.
Nous avons aussi trouvé que le ratio de Ti:Al dans le revêtement a un impact majeur sur sa résistance
chimique aux solutions de H2O2 et de K2C2O4.
1
CHAPTER 1
INTRODUCTION
The application of TiAlN ceramic coatings increases the hardness and corrosion resistance of
mechanical parts. Very often, they will degrade to a point where it needs to be replaced. However, in
order to deposit a new high quality coating, it is important to remove the original coating and perform
necessary reparations on the component before applying the new one.
This coating is already in use on many alloys. The main hurdle that currently prevents the use of
this coating on titanium alloys (such as Ti-6-4) is that many of the known chemicals that will strip this
coating will also attack the substrate. This thesis focuses on the development of a chemical stripping
(etching) method to remove such TiAlN coatings from titanium alloy substrates.
1.1: Properties of TiAlN Coatings
TiAlN coatings are a relatively new class of coatings that have begun to replace TiN coatings,
notably on machining tools. Compared to TiN coatings, TiAlN coatings show a higher hardness, a superior wear resistance, a lower thermal expansion, a lower thermal conductivity and enhanced corrosion
resistance. TiAlN coatings also produce oxides of titanium (TiO2) and alumina (Al2O3) on their surfaces
(Chen et al., 2009). Properties of TiAlN coatings vary depending on the ratio of titanium to aluminum
found in the coating. Hence, these coatings are often represented with the chemical formula Ti1-xAlxN.
Kim and Kim (1997) have reported that such a coating with a value of x = 0.12 did not show any oxidation in air up to 700oC, but started to oxidize at 800oC. Kim et al. (2005) reported that TiAlN coatings had
a hardness coefficient ranging from 30 to 40 GPa, and an elastic modulus ranging from 350 to 425 GPa,
depending on the value of x. Similar results were also reported by Santana et al. (2004), who also reported that annealing the coating at 1000oC for 30 minutes will result in a reduction in hardness if x ≤ 0.6,
2
but will result in an increase in hardness for x > 0.6. They also reported the same tendency for the elastic
modulus.
Some work has been done on the chemical stability of TiAlN coatings, although most of the work
is related to high temperature environment (Kim and Kim, 1997; Donohue et al., 1999; Cunha et al.,
1999), salty environment (Ding et al., 2008; Yoo et al., 2008; Souto and Alanyali, 2000) or in a hydrofluoric gas atmosphere (Choi and Park, 2000). These investigations were performed in order to evaluate
the corrosion behaviour of TiAlN in a simulated industrial conditions where TiAlN coatings are used, such
as on machine tools.
1.2: Methods and Mechanisms for the Removal of the Coating and Substrate
Metals and metallic compounds can be oxidized by the following basic oxidizing reaction
(Callister, 2005):
Although oxidation of most metals can be enhanced by the use of electrochemical means, the
TiAlN coating used in this project is a ceramic compound and is thus very resistant to oxidizing reactions.
Oxidation of such a compound usually involves only a simple chemical dissolution process (Callister,
2005) which necessitates finding an oxidizing agent that will be a strong oxidizer towards TiAlN. Table
1.1 shows the half reactions of some typical oxidizing agents, as well as their standard reduction potential Eo for aqueous solutions at 25oC (Kotz et al., 2009). A high Eo is indicative of a strong oxidizing agent
whereas a low (negative) Eo is indicative of a strong reducing agent.
3
Table 1.1: Half reaction cell potential for many common oxidizing agents (Kotz et al., 2009)
Reduction Half-Reaction
Eo (V)
+2.87
+1.77
+1.685
+1.51
+1.50
+1.36
+1.33
+1.229
+1.08
+0.96
+0.89
+0.855
+0.799
+0.789
+0.771
+0.535
+0.40
+0.337
+0.15
0.00
Most of the strong oxidizing agents in Table 1.1 are based either on dissolved heavy metals or
on halogen compounds. Hydrogen peroxide (H2O2) is an interesting choice as a starting oxidizing agent
because of its relatively high standard reduction potential, and also because it degrades into harmless
H2O and O2 when it reacts. Hydrogen peroxide is also an interesting compound to use for the dissolution
of metals or metallic ceramics because the metallic ions it oxidizes have a high solubility in hydrogen
peroxide; an acid solution of hydrogen peroxide will have about the same solubility as aqua regia in this
regard (a mixture of 25% nitric acid and 75% hydrochloric acid) (Schumb, 1955).
Although H2O2 is a strong oxidant, it can sometimes react quite slowly with some compounds. In
those cases, it may be required to activate the peroxide bond to make it more reactive towards the
4
selected species. Unless otherwise indicated the following discussion on the activation of hydrogen peroxide is based on Strukul (1992). Activation, in this case, means the formation of an intermediate peroxide compound which will be more reactive towards the desired species. There are two common
methods for the activation of hydrogen peroxide: activation by a transition-metal ion, and activation by
an organic compound.
1.2.1: Activation of Hydrogen Peroxide by Transition Metal Ion
Activation of hydrogen peroxide by dissolved heavy metals ions occurs by a reaction similar to
the Fenton reaction which produces highly reactive hydroxyl radicals. When combining hydrogen peroxide and Titanium ions, for example, the following reactions occur (Ardon, 1965):
Additionally, hydrogen peroxide is able to react with metal ions such as Ti(IV), V(V), Mo(VI),
W(VI) and others while still keeping its peroxo bond intact. This has the net effect of increasing the
peroxygen’s electrophilic or nucleophilic character with respect to the original molecule.
Activation of hydrogen peroxide by transition metal ions is not the main method by which hydrogen peroxide will be activated in this thesis; however, some of it occurs since coatings of TiAlN and
its Ti substrate will be dissolved into the reacting medium and those metal ions will activate hydrogen
peroxide as a side reaction.
1.2.2: Activation of Hydrogen Peroxide by Organic Compounds
Activation of hydrogen peroxide can be done by combining it with organic acids, acyl halides or
acid anhydrides (Swern, 1949), according to the following general reaction mechanism in an acidic
medium (Edwards, 1962):
5
where
Of these methods, the most effective and frequently used involves the generation of a peracid
(also called a peroxyacid) through the reaction mechanism in Figure 1.1 whereby the products in square
brackets are intermediate compounds that cannot be isolated:
Figure 1.1: Peracid Formation Mechanism (Strukul, 1992)
When using low aliphatic carboxylic acids such as formic or acetic acid, this reaction is reversible. The formation of a peracid also requires the presence of a strong acid catalyst to ensure a sufficiently rapid reaction rate. There are a few relatively strong organic acids, such as trifluoroacetic acid
and formic acid, which are able to catalyze the formation of peracid without the need of a strong acid in
supplement. Table 1.2 shows the acidic strength (pKa) of some of the most powerful and common organic acids (Gokel, 2004). A low pKa is indicative of a strong acid, whereas a high pKa is indicative of a poor
acid. Some organic acids have many pKa values because they are composed of many acid groups; pKa1 is
the acidic strength of the acid when all H+ ions are attached to the molecule, pKa2 is the strength of the
acid when one H+ ion has been released, and so forth.
6
Table 1.2: pKa values of common organic acids
Compound
pKa1
pKa2
pKa3
Uric Acid
5.40
5.53
Acetic Acid
4.756
Benzoic Acid
4.204
L-(+)-Lactic Acid
3.858
Formic Acid
3.751
Citric Acid
3.128 4.761 6.396
Oxalic Acid
1.271 4.272
Trifluoroacetic Acid
0.50
When preparing a peracid, selection of the proper acid to use is very important because it will
be the major determining factor in the strength of the acid. Temperature will also affect the strength of
the acid, but to a much lesser extent, as can be seen in Table 1.3 for the case of oxalic acid.
Table 1.3: Selected Equilibrium Constant of Oxalic Acid in Aqueous Solution (Gokel, 2004)
Temperature (oC) 0
5
10
15
20
25
30
35
40
50
pKa2
4.210 4.216 4.227 4.240 4.254 4.272 4.295 4.318 4.349 4.409
Generally, when activating hydrogen peroxide by transferring it from HOOH to ZOOH, it has
been found that the most efficient way to go about doing so is by making a significant change in the
alkalinity of the leaving group (X in Figure 1.1), while having Z be a strong electron-withdrawing group.
Generally the most important factor in determining the reactivity of the final activated peroxide compound ZOOH will be the low basicity of the leaving group XO-. If an organic acid was used as a precursor
for the formation of a peracid, the leaving group would then be OH-. This makes the use of the basic
conjugate of an acid more interesting because the leaving group is less basic.
When using a peracid as an oxidizing agent, it is not necessary to isolate it from the carboxylic
acid and the hydrogen peroxide from which it was made. One can simply immerse the substance to be
oxidized into a mixture of organic acid and hydrogen peroxide. The peracid will be continuously formed
through the mechanism discussed earlier, and it will be simultaneously consumed as it oxidizes the desi-
7
red substance. This in situ technique is most often used when the carboxylic acid used is aliphatic like
those of Table 1.2 (Swern, 1949).
1.2.3: Stability of Hydrogen Peroxide and Organic Acids
Pure hydrogen peroxide will not degrade to any significant level, except when it is at high temperature in the vapour phase (Schumb, 1955). With the basic decomposition reaction of hydrogen peroxide of
, the equilibrium is shifted heavily towards the products; however, no de-
composition of pure hydrogen peroxide is observed because of the high activation energy of this reaction (Ardon, 1965). Any decomposition of hydrogen peroxide that is observed is quite exothermic
(-98.3 kJ/mol) and is due to the presence of impurities such as metal ions, metal oxides or hydroxides
(Strukul, 1992). An example of the decomposition of hydrogen peroxide in the presence of dissolved
titanium ions was given in Section 1.2.1.
Organic acids can degrade when they are mixed with hydrogen peroxide. It was found that when
oxalic acid is mixed with hydrogen peroxide, acid conditions will promote its conversion to CO2, whereas
basic conditions will retard this conversion (Schumb, 1955).
1.3: Literature Review
Because it is a relatively novel material, there is only a limited amount of information available
on the chemical stripping of TiAlN coatings in the literature.
Bonnachi et al. (2003) reported that TiAlN films can be etched with a mixture of hydrogen peroxide and potassium oxalate in alkaline conditions (pH 14) at a rate ranging from 168nm/h to 294 nm/h,
but there is no mention of the reacting conditions in which these experiments were performed (temperature and concentration of reactants). They have also reported that a mixture of potassium permanganate and concentrated sulphuric acid will transform a layer of TiAlN into a black crust that can then be
8
easily removed by mechanical polishing. An XPS depth profile analysis of this crust shows that aluminum
oxides segregate at the top and seem to be soluble, and that underneath this aluminum oxide there is a
titanium dioxide layer, and that underneath still remains some TiAlN. This treatment was found to be
comparable with high temperature corrosion.
Koo et al. (2001) also report that etch pit tests can be performed on TiAlN films using a 1:2:6
molar solution of NH4OH:H2O2:H2O. An etch pit test measures the density of dislocations on a material;
an etching solution is applied to the material, and a higher etching rate is observed near those dislocations, resulting in pits. It is important to note, though, that etch pits will only appear in their experiments
when the samples had been annealed at a temperature of at least 600oC for 1 hour, and that the size of
the etch pits increased with annealing temperature. There is no mention of the rate at which this stripping will occur in this article.
Some studies were performed on different methods for the chemical stripping of TiN and AlN
coatings. Kim et al. (1999) report that a 1:1 solution of NH4OH:H2O2 creates etching pits on the surface
of TiN coatings. Sandhu et al. (1993) report that the same mixture is able to etch TiN at a rate ranging
from 1 to 10 Å/min depending on the operating pressure at which they were fabricated. Both of these
papers state that the etch rate will vary depending on the method with which the coatings were produced. Witvrouw et al. (2000) reports that a 14.2 mol/L HF solution is able to degrade TiN at a rate of 0.4 ±
0.2 nm/min, that a mixture of buffered HF with glycerol etches TiN at a rate of 0.06 ± 0.05 nm/min, and
that HF vapour etches TiN at a rate of 0.06 ± 0.02 nm/min. TiN coatings were also found to be able to be
degraded by a 40:2:1 mixture of de-ionized water, hydrogen peroxide and ammonium hydroxide
(Triyoso et al., 2006).
Pearton et al. (1993) report that AlN films can be slowly etched in buffered HF solutions,
although that this may be due to the dissolution of aluminum oxides that form on the surface. Mileham
et al. (1995) reports that H3PO4 at 65 to 85oC can etch AlN coatings at a rate of 500 Å/s, and that solu-
9
tions based on KOH could etch AlN at a rate up to 104 Å/min depending on the temperature and the
quality of the coating itself. Zhuang and Edgar (2005) further report that sodium hydroxide at 75oC
etches AlN at a rate of 50 nm/min.
1.4: Objectives
The objective of the research project reported in the present thesis is to develop technologies
for the stripping of TiAlN coatings. Because of the strong mechanical properties of these coatings
different alternatives based on wet chemical stripping, dry plasma etching and laser etching are to be
developed. This thesis investigates the removal of TiAlN by wet chemical stripping method.
The method developed is to be optimized for:

Speed of removal of the coating

Selectivity over the substrate

Environmental friendliness
Based on the information presented in the theoretical background and literature review, the
TiAlN coating will be stripped using a mixture of hydrogen peroxide (H2O2) and potassium oxalate
(K2C2O4). This mixture should produce a peracid with a high strength, due to the fact that the oxalate
structure itself is very electron withdrawing (as per its low pKa). The basic conjugate of oxalic acid was
chosen since its leaving group will have a lower basicity than its acid form, which should lead to a
stronger peracid.
10
CHAPTER 2
MATERIALS, METHODS AND SET-UP
2.1: Samples
Experiments were performed on coupons of Ti-6-4 (a Titanium alloy with 6% Aluminum and 4%
Vanadium), some of which were coated with Titanium Aluminum Nitride (TiAlN) by our industrial partner. Both coated and uncoated coupons had a dimension of 2.5 cm × 5 cm × 1.1 mm, with a total surface
area of approximately 25 cm2. The coupons were coated on both side, each side had a notional coating
thickness of 10 µm. The coating covered most, but not all, of the surface area of the coupon: it covered
2.5 cm × 4.1 cm of both of the main faces of the coupon. The coating surface area was then approximately 20.5 cm2.
Three different TiAlN coatings designated as A, B and C which varied on their ration of Ti:Al were
used throughout the course of this project. Most of the experiments that were performed were done
using coating A, unless explicitly mentioned.
2.2: Reactor set-up
A polytetrafluoroethylene (Teflon) beaker having a volume of one litre was used as the reaction
vessel. The reactor had an inner diameter of 92mm, a height of 155mm and a wall thickness of 5mm.
Teflon was chosen as the material of the reactor because of its chemical inertness and resistance to corrosion (Plastomertech, 2009). A beaker cover, which was a slightly concave piece of Teflon, was also
placed on the top of the beaker to reduce liquid loss by evaporation and prevent potential spills from
occurring. This beaker was deposited in a bath with an operating temperature ranging from ambient to
275oC. Mixing was induced in the reactor from the top, using a glass stir rod with a Teflon impeller
11
(Figure 2.1) and a variable speed stirrer with digital control. The temperature of the reaction mixture
and the hot water bath fluid were monitored using alcohol thermometers.
Figure 2.1: Side View of the Used Impeller
Samples were inserted in the reactor using a sample holder, which is a cuboid of Teflon of dimensions 15.1 cm × 2.7 cm × 1.4 cm (Figure 2.2). A slit was made at the bottom of the sample holder,
having a thickness slightly bigger than that of the samples themselves, so that they could be inserted
with ease while minimizing free space for movement of liquid. The slit had a depth of 0.9 cm so that
when a coated sample was inserted into the slit, only the uncoated portion of the sample was covered
inside the sample holder (Figure 2.3). The sample was further fastened into the sample holder using a
PTFE screw.
Figure 2.2: Sample Holder
Figure 2.3: Side View of Sample Holder with Sample Inserted
12
2.3: Experimental procedure
The reaction mixture was prepared by mixing the appropriate amount of 50 weight % hydrogen
peroxide (Fisher Scientific, reagent grade) with the appropriate amount of pure potassium oxalate crystals (Fisher Scientific, reagent grade) and by adding the appropriate amount of water in order to obtain
the desired reactant concentration. 730mL of the reaction mixture was placed in the reactor, and the
reactant temperature was increased to the desired value by the hot water bath while at the same time
mixing was initiated in the reactor at the desired speed. When the reaction mixture reached the desired
temperature, the sample was inserted into the sample holder and then fully immersed into the reactant
for the planned amount of time. When the reaction time had elapsed, the sample was removed from
the reactor and immediately rinsed with water to remove any reactants that may linger on its surface.
2.4: Measurements
The results shown in this thesis were obtained using the weight difference of the samples before
and after stripping. The thickness of the sample that was removed can also be estimated by assuming
that the mass loss that was observed occurred uniformly on the exposed surface of the sample. The
difference in thickness of the coating or the substrate could not be measured accurately because of its
small magnitude.
The samples to be stripped were first cleaned by scrubbing with a mixture of water and laboratory detergent, then rinsed with de-ionized water and then with acetone. They were then dried for 1
hour in an oven at 105oC, and then placed in a dessicator at room temperature for 1 hour to dry and
cool down. They were then weighed twice using an analytical balance with a ± 0.1 mg precision. This
drying and weighing procedure was repeated a second time. If all of the weight measurements taken
agreed closely, the samples were individually stored in plastic bags until needed. Otherwise, this procedure was repeated as many times as necessary until the last four weight measurements were similar.
13
The mass of the sample was taken as the average of the last 4 weight measurements that were taken.
This procedure was performed twice for each sample, once before it was stripped and a second time
after it was stripped.
A technique to measure the concentration of hydrogen peroxide of the reactant was also developed for this project. A detailed step-by-step explanation of this procedure can be found in Appendix 1.
Using this procedure, the concentration of hydrogen peroxide of the reactant was measured before and
after the stripping reaction.
A Dektak3ST Surface Profile Measuring System (commonly known as a Profilometer) from Veeco
Instruments Inc. was used at the beginning of this project. This apparatus was used according to the
standard operating procedure of the apparatus without any need for pre-treatment of the sample.
A TraceScan Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) system
coupled with a mini-flow nebulizer and a baffled cyclonic spray chamber was also used at the beginning
of this project to measure dissolved concentrations of Vanadium, Aluminum and Titanium in the reactant, using wavelengths of 310.2, 396.1 and 336.1 nm, respectively. Samples of the reactant were taken
before and after stripping, and were first digested using nitric acid (to solubilise all ions) at 95oC for 1
hour, and then diluted to a known volume before being analyzed by the ICP-OES.
14
CHAPTER 3
RESULTS
3.1: Preliminary Experiments
3.1.1: Establishment of a Measurement Technique
Three techniques were investigated in order to quantify the extent at which a sample was stripped: profilometry, Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES), and weight
difference.

Profilometry: This technique works by masking a part of a sample when it is being etched, effectively protecting it from the stripping solution. The profilometer will then be able to measure
the difference in height between those two sections of the samples after etching. This reading
gives the total thickness of the sample that was removed by the stripping process.

ICP-OES: The ICP-OES is an instrument which enables the precise measurement of the concentration of dissolved metal ions in an aqueous solution. By measuring the concentration of those
ions in the stripping solution both before and after chemical stripping, one can then calculate
the total mass of material which has been transferred from the sample to the reacting mixture.

Weight difference: This technique is a more direct method to obtain the same information given
by the ICP-OES one. By measuring the mass of a sample both before and after chemical stripping, one can calculate the mass of the sample which has been removed during chemical stripping.
Experiments were performed to investigate which of these techniques would be most suitable
for this project. Profilometry readings were taken from a few samples that were not subject to any stripping. Figure 3.1 shows one of these readings. This reading is very unstable, with detected changes in the
local height of a sample of about 3 µm. Considering that the coating itself has a notional thickness of
15
10 µm, this is a level of error that is much too high, which makes this technique unsuitable for the needs
of this project.
Figure 3.1: Profilometry Analysis of a Coated Sample
Both the ICP-OES and the weight difference techniques were used in the same experiments for
comparison. The concentrations of vanadium, aluminum and titanium in the stripping solution as measured by ICP-OES for one of these experiments are shown in Figure 3.1. Vanadium was measured because it is present in the substrate. This particular experiment was performed at room temperature with
concentrations of 0.150 mol/L of potassium oxalate and 5.91 mol/L of hydrogen peroxide for a total of 6
hours.
Table 3.1: ICP-OES Measurements Results for One Experiment
Element Concentration before
Standard deviation of
Concentration after Standard deviation of
stripping (mg/L)
measurement (mg/L)
stripping (mg/L)
measurement (mg/L)
V
0.0000
0.0173
0.2243
0.0838
Al
0.1335
0.0360
8.955
0.198
Ti
0.0168
0.0030
12.590
0.0393
16
Considering that the total reaction mixture volume was of 730 mL, the reactant before stripping
contained about 0.11 mg of dissolved metal ions whereas the reactant after stripping contained about
15.89 mg of dissolved metal ions. Thus, during the course of the stripping experiment, the sample released 15.78 mg of metal ions into the stripping solution according to the ICP-OES technique. The weight of
this same sample was also measure before and after chemical stripping. It was 8.9487 g and 8.9322 g,
respectively. According to the weight difference technique, then, the sample lost 16.5 mg during stripping.
Both techniques gave results that were fairly consistent, with the weight difference technique
usually having a slightly higher weight loss than the ICP-OES. This was true for all samples which were
tested with both techniques. Finally, it was decided to use only the weight difference technique for all
experiments because it was simpler, faster and cheaper to use than the ICP-OES technique, and also
because it gave a more direct measurement of the mass loss of a sample than the ICP-OES. All results
reported in this thesis are reported from the weight difference technique.
3.1.2: SEM analysis of samples
Some samples were investigated using a scanning electron microscope (SEM) prior to being
stripped. Figure 3.2, shows a coated sample at a 5000 magnification rate. Its surface is very uneven and
contains a lot of spherical particles due to the way the coating was applied. Figure 3.3 shows an uncoated sample at a 5000 magnification rate, which has a much smoother surface, because it was polished.
The vertical lines which can be observed on its surface are due to the polishing process.
17
Figure 3.2: SEM picture of a Coated Sample, 5000x Magnification Rate
Figure 3.3: SEM Picture of an Uncoated Sample, 5000x Magnification Rate
18
3.2: Effect of Experimental Conditions
3.2.1: Selected Experimental Conditions
The experiments were designed to evaluate the effect of the reaction temperature, the concentration of hydrogen peroxide and the concentration of potassium oxalate on the stripping rate of both
coated and uncoated samples. In order to facilitate the comparison of the results of the coated and uncoated samples, all of the experiments for both of these types of samples were performed at the same
experimental conditions. Additional considerations were also given to the reaction time, the composition of the coating and the reproducibility of the experiments, as discussed below.
I.
Reaction Temperature
The temperatures used for the reaction were room temperature (22 ± 1oC), 50oC and 75oC. A
greater emphasis was placed on the higher temperature experiments (75oC) because, as will be shown
later, their results proved to be better in terms of stripping rate and selectivity.
II.
Concentration of Reactants
All of the experiments were performed with varying concentrations of hydrogen peroxide and
potassium oxalate. The use of concentrations of potassium oxalate higher than 0.226 mol/L proved to
be difficult, as it was difficult to make all of the K2C2O4 crystals dissolve suggesting that its solubility limit
was approached. All experiments were done with an excess of hydrogen peroxide with respect to potassium oxalate.
As was mentioned before, the TiAlN coatings were removed using a mixture of hydrogen peroxide and potassium oxalate which once in solution formed the active peracids (peroxalic acid). There
are no available data on the equilibrium constants for the production of peroxalic acid using hydrogen
peroxide and potassium oxalate. However, it was possible to calculate the maximum concentration of
19
peracid structures in solution as simply twice the concentration of potassium oxalate which was added
to the reaction mixture, since each oxalate molecule has the potential to form two peracid bonds.
III.
Reaction Time
Since the purpose of these experiments was to obtain data that can eventually lead to the calcu-
lation of the rate at which stripping of the coating or substrate occurs, it was very important to use an
appropriate total reaction time. The total reaction time had to be high enough so that the measured
change in the mass loss is large compared to the precision of the analytical method that was used. On
the other hand, the reaction time cannot be so large that the entirety of the coating is removed during
the course of the reaction since in that case there would be no way of knowing at what point during the
reaction the coating is completely removed. It was then necessary to perform several preliminary tests
to evaluate an acceptable reaction time that would be between both of these limits; they were found to
be 6 hours for room temperature experiments, 1 to 4 hours for 50oC and 20 minutes for 75oC experiments. For each of these experimental times, it was found that some coating was left covering the entire
surface of the coated samples when they were removed from the reactor.
An additional set of experiments was performed at 50oC by varying the reaction time to evaluate
its effect on the observed mass loss of both coated and uncoated samples.
IV.
Composition of the Coating
A set of experiments was performed to evaluate the effect of the composition of the coating
using samples A, B, and C on its stripping rate. Unless otherwise specified, the coating that was used was
coating A that was described in Section 2.1.
20
V.
Reproducibility of Experiments
Most of the experimental conditions tested in this thesis were performed in triplicates or dupli-
cates. To make the graphs that will be shown in this thesis easier to read and understand, only the average of the replicates of each experimental condition will be shown. Appendix 3 gives the variability and
repeatability of each experimental condition, in tabular form, for all of the data obtained for all experimental conditions along with their calculated average, standard deviation and residual standard deviations.
3.2.2: Qualitative Observations and Description of a Stripping Experiment
I.
Stripping Solution Colour
When a fresh hydrogen peroxide and potassium oxalate mixture is prepared, it is transparent
and colourless. After either a coated or uncoated sample has been stripped by the solution for a set
period of time, the colour of the reaction mixture would often change to a yellow tinge. This colour is
most likely due to the presence of dissolved titanium ions in solution which creates a complex with
hydrogen peroxide (Schumb, 1955) called pertitanic acid according to the following reaction (Eisenberg,
1943):
During the course of this project, it was actually possible to get a qualitative idea of the magnitude of stripping by simply observing the colour of the reacting solution just after stripping. Furthermore,
if the colour of the solution remained colourless after an experiment, it always meant that the sample
on which a stripping experiment was performed did not experience any significant loss in mass.
21
II.
Temperature Increase
For all experiments which were performed at room temperature and 50oC, no changes in tem-
perature were ever observed. For the experiments performed at 75oC, a small increase in temperature
ranging from 1 to 2oC was usually observed over the course of the 20 minutes of the experiment. This
increase in temperature can be due to two possible reactions which occur simultaneously: either from
the stripping reaction itself which may be exothermic or either due to the breakdown of hydrogen peroxide molecules accelerated by the presence of metal ions in solution, as explained in Section 1.2.3.
Because of the comparatively small extent of the stripping reaction the breakdown of hydrogen peroxide is believed to be the main reason for the temperature increase.
III.
Concentration of Hydrogen Peroxide
The concentration of hydrogen peroxide of the reactant was also measured before and after
most experiments according to the procedure described in Appendix 1. Using this procedure on samples
of hydrogen peroxide of known concentration, it was found that the method can be used to measure
H2O2 concentrations within a precision of about ± 0.15 mol/L. This procedure does not measure specifically the concentration of hydrogen peroxide, but rather the concentration of any peroxide bonds,
which includes any peracids which have been produced due to the presence of potassium oxalate. In general, it was found that for most experiments, the concentration of peroxide bonds after the reaction
was equal to the concentration before the reaction, when taking into account the experimental error of
the method. This was to be expected: a calculation (shown in Appendix 2) which compares approximately the total amount of moles of the sample that was stripped to the total amount of moles of hydrogen peroxide present in the solution shows that the latter is larger by a factor of at least 600. Thus, the
total amount of hydrogen peroxide that was consumed during the stripping reaction itself was negligible
compared to the error in the measurement technique.
22
Two experiments were performed on coated samples with an initial temperature of 75oC, both
with a concentration of potassium oxalate of 0.150 mol/L and of hydrogen peroxide of 4.4 mol/L, but
were left to run for a much longer time than usual (1 hour instead of 20 minutes). Since the reactor was
not equipped with a controller, in both cases a large increase in the temperature of the reactant was
observed, with a total increase of 15oC in one case and 25oC in the other (the solution was boiling). For
both of these experiments, the hydrogen peroxide concentration decreased noticeably from 4.4 mol/L
to 2.4 mol/L in the first case, and from 4.4 mol/L to 0.88 mol/L in the second. Considering that the rate
of mass loss of the samples that was observed in both of these cases is relatively similar to the rates
calculated for other experiments at 75oC with the same initial concentrations, the increase in temperature that was observed in these experiments and also in all experiments performed at 75oC was then
due to the breakdown of hydrogen peroxide rather than the stripping of the coating or the substrate.
Due to the large change in temperature observed with the experiments, the results from those two
experiments are not included in the main body of the results section below.
IV.
Visual change in the coating
When a coated sample was exposed to either a 50 or 75oC mixture of hydrogen peroxide and
potassium oxalate, it changed colour from grey to black, as shown in Figure 3.4. This darkened coating
had a lower adherence to the substrate since it was possible to remove part of the stripped coating
simply by rubbing gently on it. All of the weight loss results that are presented in this thesis take into
account this phenomenon since all samples have been vigorously scrubbed and cleaned prior to weighing in order to remove any coating that could be removed this way. This phenomenon was not observed
for uncoated samples or for coated samples that were exposed to a room temperature mixture of
hydrogen peroxide and potassium oxalate. It is possible that there is a change in the chemistry of the
attack of a peracid on the coating when changing the temperature from room temperature to 50oC.
23
Figure 3.4: Comparison of coated sample before (right) and after (left) stripping
V.
Changes in pH of the Reacting Medium
The pH of the reacting mixture was also measured before and after experiments. In general,
there was an increase in pH ranging from 0.1 to 0.5 units during the reaction although no correlation
could be made on the magnitude of the pH change and the observed mass loss or the experimental conditions. There was a trend, however, regarding the initial concentration of the reactants and the initial
pH: the higher the hydrogen peroxide concentration, the lower the initial pH (since H2O2 is a weak acid
(Ardon, 1965)), and the higher the potassium oxalate concentration, the higher the initial pH (since
potassium oxalate is the basic conjugate of an organic acid). Depending on the concentrations of the
reactants the initial pH of the reactant was between 4.5 and 6.5.
3.2.3: Control Experiments
Several control experiments were also performed throughout this project in order to test the individual effect of either hydrogen peroxide or potassium oxalate on the samples. When both uncoated
and coated samples were exposed to a 4.4 mol/L solution of hydrogen peroxide at room temperature
without any potassium oxalate barely any mass loss was observed. In addition, when coated and uncoa-
24
ted samples were exposed to a 0.150 mol/L solution of potassium oxalate without any hydrogen peroxide at room temperature, there was no mass loss. In order to test if hydrogen peroxide was more reactive at higher temperatures, both coated and uncoated samples were exposed to a solution of 4.4 mol/L
hydrogen peroxide without any oxalate at 75oC. Once again an insignificant mass loss was observed.
These control experiments suggest that a mixture of hydrogen peroxide and potassium oxalate
is required for the removal of either the coating or the substrate to any significant degree. This corroborates the peracid formation mechanism outlined in section 1.2.2. Although hydrogen peroxide by itself
possesses an oxidizing potential and is thus able to attack either the coating or the substrate, it does so
at a negligible rate, thus it can be concluded that the oxidation by hydrogen peroxide alone in the remaining experiments is negligible when compared to the oxidation done by the peracid.
3.3: Results
In this section, the results will only be presented, followed by a brief discussion of their reproducibility. Chapter 4 will discuss in greater detail the conclusions that can be drawn from these results.
3.3.1: Room Temperature and 50oC Experiments
All of the room temperature experiments (22 ± 1 oC) were performed in triplicates for each experimental condition for a total of 6 hours and with a mixing speed of 60 RPM. Since the reaction rate
that was measured was quite slow, there is little possibility that these experiments were performed in a
mass transfer limited regime. Figure 3.5 and Figure 3.6 show the measured loss in mass of uncoated and
coated samples respectively when exposed to a mixture of hydrogen peroxide and potassium oxalate at
room temperature.
Figure 3.7 shows the results of the experiments performed at 50oC with a hydrogen peroxide
concentration of 4.4 mol/L, a potassium oxalate concentration of 0.150 mol/L and with a varying total
25
reaction time. These reactant concentrations were used for this experimental set because they proved
to have the highest calculated selectivity for the room temperature experiments. These experiments
were performed twice for a reaction time of 2 and 3 hours, and once for a reaction time of 1 and 4
hours. The mixing rate was increased to 300 RPM in order to ensure that the reaction was not mass
transfer limited.
Figure 3.7 shows that the extent of the reaction (i.e. the amount removed) increases linearly
with respect to time, for both coated and uncoated samples, which means that the rate of the reaction
is constant and that the reaction conditions (concentrations of reactants and reaction temperature) do
not change with time, as was discussed in Section 3.2.2.
Figure 3.8 shows the result of the experiments performed at 50oC and a 0.150 mol/L concentration of potassium oxalate and with a varying concentration of hydrogen peroxide. Each experimental
condition was performed either once or in duplicate. In all cases, the total reaction time was of 2 hours,
with a mixing rate of 300 RPM. Due to time constraints, only three different concentrations of hydrogen
peroxide were tested at 50oC. Instead, a greater number of experiments to study the effect of concentration on the rate of reaction were performed at the higher temperature of 75oC because, as will be
discussed later, experiments at 75oC proved to have results that were more promising.
The room temperature experiments show a fairly high degree of reproducibility, except for the
experiments in absence of potassium oxalate. The residual standard deviation (% RSD) for each experimental condition calculated from the data in Figure 3.5 and Figure 3.6 is shown respectively in Table
A3.1 and A3.2 in Appendix 3. The uncoated samples have a % RSD from 0.7 to 6.8 (with an average of
3.6 % for all experimental conditions), which is an excellent level of reproducibility. The coated samples
have a % RSD which ranges from 1.8 to 22.3 (with an average of 13.0 for all experimental conditions),
which is an acceptable level of reproducibility. This higher % RSD for the coated sample can be explained
by looking at the SEM pictures of the samples shown in Figure 3.2 and Figure 3.3. The coated samples’
26
bulbous shape at the microscopic level will vary from location to location on the sample, and also from
one sample to another. This will then create a difference in the actual surface area between each sample, which will lead to a difference in the results observed. The uncoated samples, on the other hand,
having all been polished in the same manner, are more likely to have a similar actual surface area from
one sample to the next, explaining their more similar results.
Figure 3.5: The Effect of the Reactant Composition on the Observed Mass Loss of
Uncoated Samples at Room Temperature, 6 Hours Reaction Time
0.6
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
0.301 mol/L K2C2O4
Mass Loss (mg/cm2)
0.5
0.4
0.3
0.2
0.1
0.0
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
27
Figure 3.6: The Effect of the Reactant Composition of the Observed Mass Loss of
Coated Samples at Room Temperature, 6 Hours Reaction Time
0.8
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
0.301 mol/L K2C2O4
0.7
0.6
Mass Loss (mg/cm2)
0.5
0.4
0.3
0.2
0.1
0.0
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
Figure 3.7: The Effect of the Reaction Time on the Observed Mass Loss at 50oC,
4.4 mol/L H2O2 and 0.150 mol/L K2C2O4
12
Uncoated
Coated
Mass Loss (mg/cm2)
10
8
6
4
2
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Reaction time (hr)
28
Figure 3.8: The Effect of the Reactant Composition on the Observed Mass Loss of
Coated and Uncoated Samples at 50oC, 0.150 mol/L and 2 Hours Reaction Time
6
Uncoated
Coated
Mass Loss (mg/cm2)
5
4
3
2
1
0
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
3.3.2: 75oC Experiments
Experiments at 75oC were performed in triplicates when the hydrogen peroxide concentration
was of 2.9, 4.4 and 5.9 mol/L, while they were only performed once at other hydrogen peroxide concentrations. All of these experiments were performed for a total reaction time of 20 minutes, with a mixing
speed of 300 RPM. The use of such a short reaction time was justified in Section 3.2.1.
Table A3.7 and A3.8 in Appendix 3 gives the residual standard deviation of all experimental conditions found in Figure 3.9 and Figure 3.10 respectively. For the 75oC experiments the coated samples
show a fair degree of reproducibility, with an average % RSD of 11.5 across all experimental conditions.
The uncoated samples, on the other hand, show a poorer degree of reproducibility, with an average
% RSD of 15.6 across all experimental conditions. When comparing the reproducibility of these
experiments to the room temperature ones, the coated samples have the same average % RSD, but the
29
uncoated samples have a much higher average % RSD at 75oC than at room temperature (15.5 % vs
3.6 %). This is probably due to the oxide layer present on the surface of the uncoated samples, which
will vary in thickness from one sample to the next (Effah et al., 1995). Since the total loss in mass for the
75oC experiments is high compared to the room temperature experiments, it is quite possible that the
entirety of this oxide layer has been removed, as well as some of the underlying non-oxidized portion.
The difference in the thickness of this oxide layer would then lead to the difference in the total amount
of sample that has been removed for similar experimental conditions at 75oC.
Figure 3.9: The Effect of the Etchant Composition on the Observed Mass Loss of
Uncoated Samples at 75oC, 20 Minutes Reaction Time
1.2
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
Mass Loss (mg/cm2)
1.0
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
7
8
9
10
H2O2 Concentration (mol/L)
30
Figure 3.10: The Effect of the Etchant Composition on the Observed Mass Loss of
Coated Samples at 75oC, 20 Minutes Reaction Time
7
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
6
Mass Loss (mg/cm2)
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
H2O2 Concentration (mol/L)
3.3.3: Coating with a Different Composition
The following set of experiments was performed using samples with coating B and C (see section
2.1) which both have a different Ti:Al ratio compared to coating A. They were performed at 75oC with a
mixing rate of 300 RPM for a total reaction time of 20 minutes. These conditions and the concentrations
that were tested were selected because they were found to have demonstrated some of the most optimal conditions for coating A. Each experimental condition was performed at least in duplicates.
Table A3.9 and A3.10 in Appendix 3 gives the residual standard deviation of all the experiments
in Figure 3.11. Overall, these experiments show a fair level of reproducibility, with an average % RSD of
10.9 across all experimental condition, which is similar to that of the samples with coating A at 75oC
(Figure 3.10) and at room temperature (Figure 3.6). On average, coated samples have roughly the same
degree of reproducibility regardless of the stripping conditions.
31
Figure 3.11: The Effect of the Coating Composition on the Observed Mass Loss at
75oC, 20 Minutes Reaction Time
0.9
0.150 mol/L K2C2O4, coating B
0.226 mol/L K2C2O4, coating B
0.150 mol/L K2C2O4, coating C
0.226 mol/L K2C2O4, coating C
0.8
Mass Loss (mg/cm2)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
H2O2 Concentration (mol/L)
32
CHAPTER 4
DISCUSSION OF RESULTS
As stated in the objective section of this thesis, the chemical stripping procedure developed in
this project to remove the coating has to be fast, selective, and environmentally friendly. It is then necessary to evaluate the results presented in the results section according to these three criteria in order
to find an optimum experimental condition.
4.1: Evaluation of the Removal Rate of the Coating and Substrate
The graphs which have been presented in Chapter 3 show the raw data which have been collected expressed as a mass loss per unit surface area. These data can be expressed as stripping rates (or
coating/substrate removal rates) by the following calculation:
where:
S = Stripping Rate (µm/hr)
ΔM = Observed Mass Loss (mg/cm2)
ρ = Density of the Coating or the Substrate (g/cm3)
t = Experiment Duration (hr)
Section 4.1.1 to 4.1.3 will present the data that have been presented in Figure 3.5 to Figure 3.11
and which have been converted into the calculated stripping rate of the coating or the substrate, using a
notional density of 4.64 g/cm3 for the coating and 4.42 g/cm3 for the substrate (Brandes, 1998). The calculated stripping rate for room temperature and 75oC experiments is presented twice for ease of presentation: once as a function of hydrogen peroxide concentration, and once as a function of potassium
oxalate concentration.
33
4.1.1: Calculated Stripping Rates at Room Temperature and 50oC
Figure 4.1 to Figure 4.4 present the calculated stripping rates for the room temperature
experiments. The maximum average stripping rate that was obtained for coated samples at room
temperature is about 0.27 µm/hr when the potassium oxalate concentration is 0.150 mol/L and the
hydrogen peroxide concentration is 5.9 mol/L. At such a rate, removing a coating with a notional thickness of 10 µm would take about 37 hours, which is extremely long and far beyond a practical level. The
uncoated samples have an associated stripping rate that is slightly slower than that of the coated sample for the same reactant concentration. At the same conditions, an uncoated sample is stripped at a
rate of about 0.12 µm/hr.
The general trend from Figure 4.1 to Figure 4.4 is that increasing either the hydrogen peroxide
or the potassium oxalate concentration will lead to an increase in the stripping rate both for coated and
uncoated samples. The stripping rate is more sensitive to the potassium oxalate concentration than the
hydrogen peroxide concentration. This is mainly due to the fact that those experiments are done at a
high excess of hydrogen peroxide with respect to potassium oxalate, which implies that a small increase
in the potassium oxalate concentration will have a larger effect towards the formation of peracids than
an increase in the hydrogen peroxide concentration.
The stripping rates of coated samples that were obtained at room temperature (around 100 to
250 nm/hr) seem to be in the same general range as the range obtained by Bonnachi et al. (2003) of 168
to 294 nm/hr. The comparison cannot be taken further, however, since the authors of this article mention neither the concentrations of hydrogen peroxide and potassium oxalate nor the reaction temperature; however they do mention that these experiments were performed at pH 14. Since hydrogen peroxide is decomposed rapidly in basic solution by OH- catalysis (Ardon, 1965), leading to a release of a
large quantity of heat, the results obtained by Bonnachi et al. were probably due to a reaction that
34
occurred at a relatively high temperature and low H2O2 concentration because of the decomposition of
hydrogen peroxide.
Figure 4.5 and Figure 4.6 presents the calculated stripping rates for the 50oC experiments. The
similar stripping rates that were calculated in Figure 4.5 for various reaction times further confirm the
conclusion that experiments with different total reaction times can be compared by calculating their
rates. The measured stripping rates at 50oC were significantly higher than those at room temperature, in
the range of 5 to 6 µm/hr for a coated sample. The same coating with a 10 µm notional thickness that
was discussed for the room temperature experiments can now completely be removed in 1.5 to 2 hours
rather than 37, a significant improvement. The uncoated samples, on the other hand, are stripped at
rates ranging from 0.9 to 1.9 µm/hr. The results in Figure 4.6 tend to show that the stripping rate of
both coated and uncoated samples is relatively independent of the hydrogen peroxide concentration for
the conditions that were tested.
Figure 4.1: The Effect of Hydrogen Peroxide Concentration on the Calculated
Stripping Rate of Uncoated Samples at Room Temperature
0.20
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
0.301 mol/L K2C2O4
0.18
Stripping Rate (µm/hr)
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
35
Figure 4.2: The Effect of Potassium Oxalate Concentration on the Calculated
Stripping Rate of Uncoated Samples at Room Temperature
0.20
2.9 mol/L H2O2
4.4 mol/L H2O2
5.9 mol/L H2O2
0.18
Stripping Rate (µm/hr)
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Potassium Oxalate Concentration (mol/L)
Figure 4.3: The Effect of Hydrogen Peroxide Concentration on the Calculated
Stripping Rate of Coated Samples at Room Temperature
0.30
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
0.301 mol/L K2C2O4
0.25
Stripping Rate (µm/hr)
0.20
0.15
0.10
0.05
0.00
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
36
Figure 4.4: The Effect of Potassium Oxalate Concentration on the Calculated
Stripping Rate of Coated Samples at Room Temperature
0.30
2.9 mol/L H2O2
4.4 mol/L H2O2
5.9 mol/L H2O2
Stripping Rate (µm/hr)
0.25
0.20
0.15
0.10
0.05
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Potassium Oxalate Concentration (mol/L)
Figure 4.5: The Effect of Total Reaction Time on the Calculated Stripping Rate at
50oC, 4.4 mol/L H2O2 and 0.150 mol/L K2C2O4
7
Uncoated
Coated
Stripping Rate (µm/hr)
6
5
4
3
2
1
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Reaction time (hr)
37
Figure 4.6: The Effect of Hydrogen Peroxide Concentration on the Calculated
Stripping Rate of Coated and Uncoated Samples at 50oC, 0.150 mol/L K2C2O4
7
Uncoated
Coated
Stripping Rate (µm/hr)
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
4.1.2: Calculated Stripping Rates at 75oC
Figure 4.7 and Figure 4.8 present the calculated stripping rates at 75oC for the uncoated samples
whereas Figure 4.9 and Figure 4.10 presents the rates for the coated samples. The calculated stripping
rates are quite high when compared to those obtained at room temperature and 50oC, with a maximum
of about 39 µm/hr for coated samples when the concentration of potassium oxalate is of 0.226 mol/L
and hydrogen peroxide of 5.9 mol/L. At this rate, a coating with a notional thickness of 10 µm is completely removed in about 15 minutes which is quite fast and is reasonable for an industrial application.
At this same condition, the uncoated samples are stripped at a rate of 6.6 µm/hr; during those same 15
minutes which would be required to completely remove the coating, the uncoated portions of a sample
would be stripped of about 1.6 µm.
38
The same general trend which was observed for the room temperature experiments was also
observed for these experiments: the higher the concentration of either hydrogen peroxide or potassium
oxalate, the higher the stripping rate. Because of the poorer reproducibility associated with the uncoated samples, this trend is more difficult to observe. As can be seen in Figure 4.9, however, increasing the
concentration of hydrogen peroxide beyond 5.9 mol/L did not lead to any increase in the stripping rate
since hydrogen peroxide was in excess. This suggests there is a maximum to the amount of formation of
peracids that can be formed at a given potassium oxalate concentration. This can be seen when increasing the potassium oxalate concentration at a constant hydrogen peroxide concentration leads to an increase in the stripping rate. Since potassium oxalate is the limiting reactant in these experiments, there
were no observed maximum effects with regards to potassium oxalate concentration (Figure 4.10).
Figure 4.7: The Effect of Hydrogen Peroxide Concentration on the Calculated
Stripping Rate of Uncoated Samples at 75oC
9
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
8
Stripping Rate (µm/hr)
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
H2O2 Concentration (mol/L)
39
Figure 4.8: The Effect of Potassium Oxalate Concentration on the Calculated
Stripping Rate of Uncoated Samples at 75oC
9
2.9 mol/L H2O2
4.4 mol/L H2O2
5.9 mol/L H2O2
7.3 mol/L H2O2
8.8 mol/L H2O2
8
Stripping Rate (µm/hr)
7
6
5
4
3
2
1
0
0.00
0.05
0.10
0.15
0.20
0.25
Potassium Oxalate Concentration (mol/L)
Figure 4.9: The Effect of Hydrogen Peroxide Concentration on the Calculated
Stripping Rate of Coated Samples at 75oC
45
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
40
Striping Rate (µm/hr)
35
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10
H2O2 Concentration (mol/L)
40
Figure 4.10: The Effect of Potassium Oxalate Concentration on the Calculated
Stripping Rate of Coated Samples at 75oC
45
2.9 mol/L H2O2
4.4 mol/L H2O2
5.9 mol/L H2O2
7.3 mol/L H2O2
8.8 mol/L H2O2
40
Stripping Rate (µm/hr)
35
30
25
20
15
10
5
0
0.00
0.05
0.10
0.15
0.20
0.25
Potassium Oxalate Concentration (mol/L)
4.1.3: Calculated Stripping Rate for Coatings B and C
Figure 4.11 shows the calculated stripping rates associated with coating B and C at different
potassium oxalate and hydrogen peroxide concentrations. It shows that the stripping rates that were
calculated for coatings B and C are quite low considering that the experiments were performed at 75oC.
The maximum stripping rate that was calculated for coating B was of around 5 µm/hr whereas for coating C it was around 1.4 µm/hr, which implies that a coating with a notional thickness of 10 µm can be
removed in around 2 hours and 7 hours, respectively. The different concentrations that were tested
with coating B and C suggest that changing the concentration of hydrogen peroxide of the stripping
solution will not have a significant effect on the final result whereas increasing the concentration of
potassium oxalate will lead to a slight increase in the stripping rate.
Figure 4.12 compares the results of coating A, B and C at similar conditions. The results clearly
demonstrate that the composition of the TiAlN coating is very important in determining the stripping
41
rate of a sample. It shows that for all cases the stripping rate of coating A is much greater than the
stripping rate of coating B, which in turns is much greater that the stripping rate of coating C.
Figure 4.11: The Effect of the Reactant Composition on the Stripping Rate of
Coatings B and C at 75oC
6
0.150 mol/L K2C2O4, coating B
0.226 mol/L K2C2O4, coating B
0.150 mol/L K2C2O4, coating C
0.226 mol/L K2C2O4, coating C
4
3
2
1
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
H2O2 Concentration (mol/L)
Figure 4.12: Comparison of the Stripping Rate of the Different Coatings for
Similar Experimental Conditions
35
Stripping rate (µm/hr)
Stripping Rate (µm/hr)
5
30
25
20
15
Coating A
10
Coating B
5
Coating C
0
2.95 mol/L H2O2 4.42 mol/L H2O2 2.95 mol/L H2O2 4.42 mol/L H2O2
0.150 mol/L
K2C2O4
0.150 mol/L
K2C2O4
0.226 mol/L
K2C2O4
0.226 mol/L
K2C2O4
42
4.1.4: Analysis of the Calculated Stripping Rates
The data that have been presented show that the temperature of reaction, the concentration of
both reactants and the composition of the coating are all important in determining the stripping rate of
the coating and the substrate. As can be seen in Figure 4.13 and Figure 4.14, temperature is the most
important of these conditions. For similar reactant concentrations, an increase in temperature from
room temperature to 50oC will lead to a 20 to 35 fold increase in the stripping rate of the coating and a
14 to 16 fold increase in the stripping rate of the substrate. Increasing the temperature from 50oC to
75oC leads to a more modest increase in the stripping rate, in the range of a 3 to 6 fold increase for both
the coating and the substrate.
The second most important factor which affects the total stripping rate of the coating was its
chemical composition, as was shown with Figure 4.12. Since the rates calculated with coatings B and C
are quite low, peracid based mixtures are probably not the best etchant that can be used on them. Additional work would probably need to be done with other types of mixture to find a suitable one for these
coatings.
Figure 4.13: Comparison of the Stripping Rate of Uncoated Samples at Different
Experimental Temperatures (0.150 mol/L Potassium Oxalate)
6
Stripping rate (µm/hr)
5
4
Room Temperature
3
50 C
75 C
2
1
0
2.95 mol/L H2O2 4.42 mol/L H2O2 5.91 mol/L H2O2
43
Figure 4.14: Comparison of the Stripping Rate of Coated Samples at Different
Experimental Temperatures (0.150 mol/L Potassium Oxalate)
Stripping rate (µm/hr)
30
25
20
Room Temperature
15
50 C
10
75 C
5
0
2.95 mol/L
H2O2
4.42 mol/L
H2O2
5.91 mol/L
H2O2
In general, increasing the concentration of either of the reactant was found to lead to an increase in the stripping rate of the coating and the substrate. With the concentrations that were used, it was
found that the concentration of potassium oxalate was more important than the concentration of hydrogen peroxide in determining the stripping rate of the coating and the substrate. This is due to the
fact that hydrogen peroxide was always in excess relative to potassium oxalate in the experiments that
were performed. If the experiments had been performed with potassium oxalate in excess, the reverse
would probably have been true.
4.2: Selectivity of the Stripping Technique
As was stated in the objective of this project, a high selectivity is desired. It can be calculated
from the graphs shown in the results section for a given experimental condition according to the
following formula:
where
S = Selectivity
44
ΔMc = Average measured mass loss for coated sample (mg/cm2)
ΔMu = Average measured mass loss for uncoated sample (mg/cm2)
Selectivity was calculated from the results in Figure 3.5 to Figure 3.11, and is shown below in
Figure 4.15 to Figure 4.20.The calculated selectivity for room temperature and 75oC experiments is presented twice for ease of understanding: once as a function of hydrogen peroxide concentration, and
once as a function of potassium oxalate concentration.
4.2.1: Selectivity at Room Temperature
The selectivity for the room temperature experiments (Figure 4.15 and Figure 4.16) is relatively
low, ranging from 1.2 to 2.7 depending on the experimental condition. There seems to be an optimum
in the selectivity at a potassium oxalate concentration of 0.150 mol/L at this temperature (Figure 4.15).
Although the calculated selectivity for the 0 mol/L potassium oxalate condition is high, it was due to the
division of two very small numbers with large standard deviation, and it is thus not very reliable.
By increasing the temperature from room temperature to 50oC, the selectivity increases from a
range of 1.2 to 2.7 to a range of 3 to 6 (Figure 4.17). The small amount of experiments that were performed at 50oC tends to suggest that increasing the hydrogen peroxide will lead to a decrease in the selectivity.
45
Figure 4.15: The Effect of Hydrogen Peroxide Concentration on the Calculated
Selectivity for Room Temperature Experiments
3.0
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
0.301 mol/L K2C2O4
2.5
Selectivity
2.0
1.5
1.0
0.5
0.0
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
Figure 4.16: The Effect of Potassium Oxalate Concentration on the Calculated
Selectivity for Room Temperature Experiments
3.0
2.9 mol/L H2O2
4.4 mol/L H2O2
5.9 mol/L H2O2
2.5
Selectivity
2.0
1.5
1.0
0.5
0.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Potassium Oxalate Concentration (w%)
46
Figure 4.17: The Effect of Hydrogen Peroxide Concentration on the Calculated
Selectivity for 50oC Experiments
7
6
Selectivity
5
4
3
0.150 mol/L K2C2O4
2
1
0
0
1
2
3
4
5
6
7
H2O2 Concentration (mol/L)
4.2.2: Selectivity at 75oC
The selectivity at 75oC ranges from around 3 to 7 depending on the concentration of the reactants that was used. Increasing the concentration of potassium oxalate clearly leads to an increase in the
selectivity, as can be seen in Figure 4.18. Increasing the concentration of hydrogen peroxide seems to
lead to a slight decrease in the selectivity, as shown in Figure 4.19. This is especially true when the hydrogen peroxide concentration is increased to 7.3 and 8.8 mol/L. As was discussed in section 4.1.3,
these concentrations are also those that were found to be a maximum in the stripping rate of the coated samples. It is possible then that changing the hydrogen peroxide concentration will have no real
effect on the selectivity up to the point where it is so much in excess that the selectivity will start
decreasing.
More generally, the lower the molar ratio of hydrogen peroxide to potassium oxalate that was
used, the higher the selectivity will be, as can be seen in Figure 4.20. Since all of the experiments were
47
done with a high ratio of hydrogen peroxide to potassium oxalate (the lowest being 13), it might be possible that there is a maximum in the selectivity at a H2O2 to potassium oxalate ratio closer to unity. The
molar ratio of hydrogen peroxide and potassium oxalate is probably not the only factor affecting selectivity because the fit that is observed in Figure 4.18 can only be considered to be decent at best. The individual concentration of each reactant is probably also important in determining the selectivity of the
process.
Figure 4.18: The Effect of Hydrogen Peroxide Concentration on the Calculated
Selectivity for 75oC Experiments
8
7
Selectivity
6
5
0 mol/L K2C2O4
0.038 mol/L K2C2O4
0.075 mol/L K2C2O4
0.150 mol/L K2C2O4
0.226 mol/L K2C2O4
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
H2O2 Concentration (mol/L)
48
Figure 4.19: The Effect of Potassium Oxalate Concentration on the Calculated
Selectivity for 75oC Experiments
8
7
5
4
2.9 mol/L H2O2
4.4 mol/L H2O2
5.9 mol/L H2O2
7.3 mol/L H2O2
8.8 mol/L H2O2
3
2
1
0
0.00
0.05
0.10
0.15
0.20
0.25
Potassium Oxalate Concentration (mol/L)
Figure 4.20: Selectivity as a Function of Reactant Molar Ratio for Experiments at
75oC
8
7
6
Selectivity
Selectivity
6
5
y = 21.475x-0.409
R² = 0.8064
4
3
2
1
0
0
20
40
60
80
100
120
140
160
180
H2O2:Oxalate Molar Ratio
4.2.3: Selectivity for Coating B and C
The selectivity for coating B and C was really low, in the range of 1 to 1.5 for coating B and 0.2 to
0.4 for coating C. This extremely low selectivity for both these coatings was predictable considering that
49
the measured stripping rate for both of these coatings was much lower than for coating A. This makes
the use of chemical stripping by hydrogen peroxide and potassium oxalate for the removal of coating B
and C a poor choice since this method would most probably damage the substrate to a level that is
unacceptable.
Of all the experiments that were performed, the best selectivity that was obtained was of 6.8
for experiments using coating A, at a temperature of 75oC and a hydrogen peroxide concentration of 4.4
mol/L and a potassium oxalate concentration of 0.226 mol/L.
Figure 4.21: The Effect of Reactant Concentration on the Calculated Selectivity
for Coatings B and C at 75oC
1.8
1.6
1.4
Selectivity
1.2
1.0
0.150 mol/L K2C2O4, coating B
0.8
0.226 mol/L K2C2O4, coating B
0.6
0.150 mol/L K2C2O4, coating C
0.226 mol/L K2C2O4, coating C
0.4
0.2
0.0
2
2.5
3
3.5
4
4.5
5
H2O2 Concentration (mol/L)
4.3: Evaluation of the Environmental Friendliness of the Proposed Technique
Since the late 1980s, mixtures of hydrogen peroxide and acetic acid (which yield peracetic acid)
have been used as a method for chemical disinfection of industrial wastewater. The main advantage of
using such a mixture as a chemical disinfectant is the fact that it is not affected by pH, it does not yield
persistent residuals and byproducts, it has a short contact time and it has a high effectiveness as a bac50
tericide and virucide. Measurements have been made to identify the disinfection byproducts of peracetic acid, and they were found to be acetic acid, oxygen, methane, carbon dioxide and water, which are
all relatively harmless in the concentrations in which they are used (Metcalf and Eddy, 2003).
From the information about peracids which was outlined in the theoretical section, the information in the above paragraph will also apply to mixtures of hydrogen peroxide and potassium oxalate.
This implies that, when only considering the environmental impact of a stripping solution, mixtures of
peracids based on low aliphatic organic acids such as hydrogen peroxide and potassium oxalate is the
best choice within the experimental space. This is because it will not have any negative environmental
impact when it is disposed because of its lack of harmful byproducts, and also because it will reduce the
total load of the wastewater treatment plant that will handle it since it will disinfect more toxic wastewater produced by other industries. This is one of the significant reasons why hydrogen peroxide based
mixtures were selected as the method to be investigated for the removal of the coating.
This is not to say, however, that the method proposed in this thesis will have absolutely no environmental impact. Since the purpose of the stripping process itself is to remove the TiAlN coating, it is
inevitable that a relatively large quantity of Ti and Al ions will be dissolved into the stripping solution.
This dissolution of metal ions into solution would occur regardless of the choice in stripping solution,
which still makes the use of hydrogen peroxide and an organic acid the best choice for this project.
There are already some existing techniques for the recovering of metal ions in wastewater which are
very effective (Metcalf and Eddy, 2003).
51
CHAPTER 5
CONCLUSIONS AND RECOMMENDATION
The results that have been obtained so far demonstrate that using mixtures of hydrogen peroxide (H2O2) and potassium oxalate (K2C2O4) for the stripping of TiAlN coatings is feasible for an industrial
application. This stripping solution is especially interesting for this purpose because of its very low environmental impact.
It was found that increasing the temperature of the stripping reaction led to both an increase in
the stripping rate of the coating and an increase in the selectivity of the coating over its substrate
(Titanium). It was also found that increasing the concentration of either hydrogen peroxide or potassium
oxalate led to an increase in the stripping rate of both the coating and the substrate. Since all of the
experiments were performed in high excess of hydrogen peroxide, the stripping rate was found to be
much more sensible to a change in potassium oxalate concentration than to a change in hydrogen peroxide concentration. The selectivity was found to increase with a higher potassium oxalate concentration and to decrease with a higher hydrogen peroxide concentration. The highest stripping rate that was
obtained for the coating was of 39 µm/hr at a temperature of 75oC, a concentration of hydrogen peroxide of 5.9 mol/L and a potassium oxalate concentration of 0.226 mol/L. At these conditions, a coating
with a notional thickness of 10 µm can be completely dissolved in around 15 minutes. At the same conditions, uncoated samples (the substrate) were found to be stripped at a rate of 6.6 µm/hr. The best
selectivity that was obtained was of 6.8, at a potassium oxalate concentration of 0.226 mol/L and a
hydrogen peroxide concentration of 4.4 mol/L. The results that have been obtained so far suggest that
decreasing the molar ratio of hydrogen peroxide to potassium oxalate in the stripping solution should
lead to a further increase in the selectivity of the reaction. Further experiments should be performed in
this regard by decreasing the concentration of hydrogen peroxide in the reactant in order to obtain a
52
reaction that should have a higher selectivity. Additional experiments should also be performed using
other precursors than potassium oxalate for the formation of peracids. As was discussed in Chapter 1,
the kinetics of formation of a peracid and its subsequent reactivity will be altered depending on the
organic acid that is used. Using other peracids could then lead to an improved selectivity and/or
stripping rate.
It was also found that the composition of the coating is very important in determining its stripping rate and the selectivity. A new chemical formula that is not based on hydrogen peroxide should be
investigated for the stripping of coatings B and C.
53
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Deposited HfO2 Resulting from Annealing with a TiN Capping Layer. Applied Physics Letter 89, 132903
(2006).
A. Witvrouw, B. du Bois, P. de Moor, A. Verbist, C. Van Hoof, H. Bender, Kris Baert. A Comparison
Between Wet HF Etching and Vapor HF Etching for Sacrificial Oxide Removal. Proc. SPIE, Vol. 4174, 130
(2000).
Y.H. Yoo, D.P. Lee, J.G. Kim, S.K. Kim, P.V. Vinh. Corrosion behaviour of TiN, TiAlN, TiAlSiN thin films
deposited on tool steel in the 3.5 wt.% NaCl solution. Thin Solid Films 516 (2008) 3544-3548.
D. Zhuang and J.H. Edgar. Wet etching of GaN, AlN, and SiC: a review. Materials Science and Engineering
R 48 (2005) 1-46.
55
APPENDIX
APPENDIX 1
METHOD FOR THE MEASUREMENT OF THE CONCENTRATION OF
HYDROGEN PEROXIDE
Principle
Hydrogen Peroxide (H2O2) reacts with excess Potassium Iodide (KI) in the presence of an
ammonium molybdate catalyst ((NH4)6Mo7O24∙4H2O) to produce triiodide ions (I3-). These ions can then
be subsequently titrated with a standard thiosulfate solution (S2O32-).
Reagents




Potassium Iodide, powder form.
Acid mixture. Dissolve 0.18 g of ammonium molybdate in 750 mL of deionised water. While
stirring, slowly add 300 mL of concentrated sulfuric acid. Wear safety goggles and gloves when
handling the concentrated sulfuric acid.
Sodium thiosulfate solution (certified)
Starch solution (optional). Weigh 1 g of soluble starch to a 150 mL beaker. While stirring,
gradually add about 5 mL of water until a paste is formed. Add the paste to 100 mL of boiling
water. Cool and add 5 g of potassium iodide. Stir until dissolution is complete and transfer to a
plastic bottle.
Required materials



Some Erlenmeyer flasks.
A burette OR an automatic titrator. The automatic titrator will be used in manual mode in this
method.
An accurate scale, which can weigh with a precision of ± 0.1 mg.
Procedure



Take 5 mL of the acid solution and add it to an Erlenmeyer flask (250 or 500 mL).
Weigh at least 0.2 g of potassium iodide, and add it to the Erlenmeyer flask by dissolving it in
deionised water (about 20 mL).
Precisely weigh 0.1 mL of the peroxide-containing solution to be analyzed.
I





Add the weighed sample to the Erlenmeyer flask. Thoroughly wash the container where the
sample was weighed to make sure that all of the peroxide has been transferred to the
Erlenmeyer flask. The solution should now be anywhere from an orange to a very thick red
colour, depending on the concentration of the peroxide solution that was added.
Using either the burette or the automatic titrator, gradually add the sodium thiosulfate solution
to the Erlenmeyer flask, until the solution inside the flask is of a light straw colour. During step 5
to 7, make sure mixing is always occurring inside the Erlenmeyer flask.
Optional step: Add a few drops of the starch solution, which will make the solution change to a
blue/purple colour, making it easier to detect.
Continue to slowly add sodium thiosulfate to the flask, until the solution inside is colourless and
transparent. Wait a few seconds, there might be a slight colour that reappears. If it does
happen, simply add in some more sodium thiosulfate to make it become colourless again.
Note down the total volume of sodium thiosulfate that was added to the flask.
Calculation
The hydrogen peroxide concentration of the measured sample can be calculated as:
Where:
V: Volume of sodium thiosulfate added to the flask.
N: Normality of the sodium thiosulfate.
W: Mass of the sample that was added.
II
APPENDIX 2
SAMPLE CALCULATIONS
2.1: Estimation of the moles of stripped samples with respect to the moles of available hydrogen
peroxide
Taking as an example an experiment performed at 75oC with a coated sample, 2.95 mol/L of hydrogen
peroxide and 0.226 mol/L of potassium oxalate, it was measured that one coated samples was stripped
by 4.5 mg/cm2. Since the surface area were stripping occurred was 20.5 cm2, and assuming that the
coating has a molecular weight close to that of aluminum (26.98 g/mol), the amount of coating that was
removed is estimated as being 3.41 × 10-3 mol.
Since the volume of the reaction mixture was 730 mL, the total amount of hydrogen peroxide present
was then 2.15 mol.
For this specific reaction, the total amount of moles of hydrogen peroxide that was present was 600
times greater than the total amount of the coating that was stripped. Considering that the reaction
concentration that was used here was the one where the highest total mass loss was measured for a low
hydrogen peroxide concentration, all other reaction conditions should have a ratio that is even higher
than this one.
III
APPENDIX 3
RAW DATA WITH CALCULATED ERROR
Table A3.1: Measured data for each uncoated experimental conditions from Fig 3.5
H2O2
K2C2O4
Measured Average
Standard
%RSD
Concentration Concentration mass loss mass loss deviation
(mol/L)
(mol/L)
(mg/cm2) (mg/cm2) (mg/cm2)
2.9
0.075
0.138
0.132
0.006
4.63
0.126
0.132
2.9
0.150
0.161
0.155
0.007
4.51
0.159
0.145
0.156
4.4
0
0.002
0.014
0.012
81.47
0.015
0.026
4.4
0.038
0.284
0.288
0.005
1.76
0.294
0.287
4.4
0.075
0.268
0.277
0.019
6.76
0.265
0.299
4.4
0.150
0.202
0.201
0.006
3.12
0.194
0.206
4.4
0.226
0.445
0.475
0.027
5.58
0.485
0.495
4.4
0.301
0.487
0.497
0.009
1.78
0.502
0.501
5.9
0.075
0.255
0.253
0.002
0.74
0.251
0.252
5.9
0.150
0.304
0.310
0.011
3.41
0.322
0.304
IV
Table A3.2: Measured data for each coated experimental conditions from Fig 3.6
H2O2
K2C2O4
Measured Average Standard
%RSD
Concentration Concentration mass loss mass loss deviation
(mol/L)
(mol/L)
(mg/cm2) (mg/cm2) (mg/cm2)
2.9
0.075
0.204
0.275
0.061
22.35
0.301
0.318
2.9
0.150
0.430
0.424
0.019
4.41
0.402
0.438
0.004
0.031
0.023
75.28
4.4
0
0.045
0.044
0.351
0.410
0.086
21.00
4.4
0.038
0.370
0.509
0.545
0.443
0.115
25.98
4.4
0.075
0.467
0.318
0.382
0.457
0.067
14.58
4.4
0.150
0.509
0.480
0.576
0.553
0.056
10.13
4.4
0.226
0.594
0.489
0.626
0.657
0.088
13.45
4.4
0.301
0.756
0.588
0.538
0.540
0.018
3.29
5.9
0.075
0.559
0.523
0.766
0.755
0.014
1.86
5.9
0.150
0.739
0.760
Table A3.3: Measured data for each uncoated experimental condition from Fig 3.7
Reaction
Measured Mass Average Mass Standard Deviation
% RSD
time (hr)
Loss (mg/cm2)
Loss (mg/cm2)
(mg/cm2)
1
0.37
0.37
n/a
n/a
2
1.14
0.93
0.30
31.8
0.72
3
1.92
1.49
0.61
41.4
1.05
4
1.40
1.40
n/a
n/a
V
Table A3.4: Measured data for each coated experimental condition from Fig 3.7
Reaction Measured Mass Average Mass Standard Deviation
% RSD
time (hr)
Loss (mg/cm2)
Loss (mg/cm2)
(mg/cm2)
1
1.90
1.90
n/a
n/a
2
5.29
5.65
0.51
9.0
6.01
3
7.33
8.07
1.04
12.9
8.80
4
10.5
10.5
n/a
n/a
Table A3.5: Measured data for each uncoated experimental condition from Fig 3.8
H2O2 Concentration Measured Mass Average Mass Standard Deviation % RSD
(mol/L)
Loss (mg/cm2)
Loss (mg/cm2)
(mg/cm2)
2.95
0.79
0.79
n/a
n/a
4.42
1.14
0.93
0.30
31.8
0.72
5.91
1.69
1.69
n/a
n/a
Table A3.6: Measured data for each coated experimental condition from Fig 3.8
H2O2 Concentration Measured Mass Average Mass Standard Deviation % RSD
(mol/L)
Loss (mg/cm2)
Loss (mg/cm2) (mg/cm2)
2.95
4.95
4.95
n/a
n/a
4.42
5.29
5.65
0.51
9.0
6.01
5.91
5.03
5.04
n/a
n/a
VI
Table A3.7: Measured data for each uncoated experimental condition from Fig 3.9
H2O2
K2C2O4
Measured Average
Standard
% RSD
Concentration Concentration Mass Loss Mass Loss deviation
(mol/L)
(mol/L)
(mg/cm2) (mg/cm2) (mg/cm2)
2.95
0.038
0.28
0.28
0.007
2.4
0.27
0.29
2.95
0.075
0.41
0.37
0.04
9.9
0.34
0.36
2.95
0.150
0.34
0.41
0.08
18.1
0.41
0.49
2.95
0.226
0.77
0.69
0.17
24.4
0.49
0.80
4.42
0
0.01
n/a
n/a
n/a
4.42
0.038
0.28
0.36
0.06
17.7
0.40
0.39
4.42
0.075
0.58
0.48
0.12
24.0
0.36
0.50
4.42
0.150
0.45
0.63
0.17
26.6
0.77
0.66
4.42
0.226
0.53
0.76
0.20
25.9
0.83
0.90
5.91
0.038
0.52
0.50
0.02
3.7
0.49
0.49
5.91
0.075
0.70
0.64
0.06
8.9
0.60
0.62
5.91
0.150
0.82
0.75
0.08
10.1
0.76
0.67
5.91
0.226
1.17
1.00
0.15
14.9
0.92
0.90
7.33
0.075
0.74
n/a
n/a
n/a
7.33
0.150
1.14
n/a
n/a
n/a
7.33
0.226
1.04
n/a
n/a
n/a
8.82
0.075
0.87
n/a
n/a
n/a
8.82
0.150
1.10
n/a
n/a
n/a
8.82
0.226
0.95
n/a
n/a
n/a
I
Table A3.8: Measured data for each coated experimental condition from Fig 3.10
H2O2
K2C2O4
Measured Average
Standard
% RSD
Concentration Concentration Mass Loss Mass Loss deviation
(mol/L)
(mol/L)
(mg/cm2) (mg/cm2) (mg/cm2)
2.95
0.038
0.86
1.06
0.29
27.1
0.92
1.39
2.95
0.075
1.25
1.79
0.56
31.4
1.76
2.38
2.95
0.150
2.75
2.61
0.51
19.6
3.04
2.05
2.95
0.226
3.71
4.09
0.41
10.1
4.02
4.52
4.42
0
0
n/a
n/a
n/a
4.42
0.038
1.05
1.09
0.06
5.2
1.07
1.15
4.42
0.075
1.98
2.05
0.12
5.7
1.99
2.18
4.42
0.150
4.07
3.93
0.14
3.6
3.93
3.79
4.42
0.226
4.54
4.99
0.39
7.9
5.29
5.12
5.91
0.038
1.39
1.34
0.08
5.7
1.25
1.38
5.91
0.075
2.23
2.41
0.18
7.3
2.59
2.42
5.91
0.150
4.30
4.33
0.31
7.1
4.65
4.04
5.91
0.226
5.68
6.04
0.45
7.4
5.89
6.54
7.33
0.075
2.80
n/a
n/a
n/a
7.33
0.150
4.08
n/a
n/a
n/a
7.33
0.226
5.48
n/a
n/a
n/a
8.82
0.075
2.49
n/a
n/a
n/a
8.82
0.150
4.17
n/a
n/a
n/a
8.82
0.226
5.19
n/a
n/a
n/a
II
Table A3.9: Measured data for each coated experimental condition with coating B from Fig 3.11
H2O2
K2C2O4
Measured
Average
Standard
% RSD
Concentration
Concentration
Mass Loss
Mass Loss deviation
(mol/L)
(mol/L)
(mg/cm2)
(mg/cm2) (mg/cm2)
2.95
0.150
0.73
0.66
0.08
12.5
0.69
0.57
2.95
0.226
0.93
0.83
0.14
16.3
0.74
4.42
0.150
0.50
0.55
0.07
12.0
0.59
4.42
0.226
0.73
0.75
0.04
5.0
0.78
Table A3.10: Measured data for each coated experimental condition with coating C from Fig 3.11
H2O2
K2C2O4
Measured
Average
Standard
% RSD
Concentration
Concentration
Mass Loss
Mass Loss deviation
(mol/L)
(mol/L)
(mg/cm2)
(mg/cm2) (mg/cm2)
2.95
0.150
0.17
0.16
0.01
7.6
0.15
2.95
0.226
0.14
0.17
0.04
22.6
0.20
4.42
0.150
0.15
0.16
0.01
9.0
0.17
4.42
0.226
0.23
0.22
0.01
2.3
0.22
III