Oxygen Impurities in Ti-Si-N and Related Systems are Hindering the Phase
Segregation, Formation of Stable Nanostructure and Degrading the Cutting
Performance of Tools Coated with the Nanocomposites
S. Veprek,1 M.G.J. Veprek-Heijman,1 M. Jílek,2 M. Píška,3 X. Zeng,4 A. Bergmaier 5 and Q. F. Fang 6
1
Technical University Munich, Germany, 2SHM Ltd. Sumperk, Czech Republic, 3Brno Technical University, Czech
Republic, 4Singapore Institute of Manufacturing Technology, Singapore, 5Universität der Bundeswehr Munich,
Germany, 6Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, P.R. China
Abstract: We have shown earlier, that an oxygen impurity content of >0.4 at.%
(4000 ppm) strongly degrades the hardness of the nc-TiN/a-Si3N4
nanocomposites. Here we show that such impurities also hinder the phase
segregation and formation of stable and strong nanostructure consisting of 3-4 nm
size TiN nanocrystals "glued" together by about 1 monolayer thick interfacial
Si3N4-like layer, thus apparently stabilizing the solid solution at high temperature
of >900°C, as reported by other researchers. At the impurity content of only few
hundred ppm, the segregation is completed, and a stable nanostructure formed at
temperature of ≤550°C. By decreasing the impurity content from 2000-3000 ppm
down to about ≤1000 ppm in an industrial PVD coating equipment, the life time
of cutting tools has been increased by a factor of ≥2. The results to be presented
will underline the need for the improvement of the purity of the nanocomposite
coatings applied in industry as well as used for fundamental studies in academia.
Keywords: Superhard Nanocomposites; Impurities; Oxygen; Thermal Stability; TiSiN
1. Introduction
Plasma-induced chemical vapor deposition (CVD)
and physical vapor deposition (PVD) are important
techniques for the deposition of large variety of
protective and functional coatings on machine parts,
tools for machining, forming, stamping, injection
moulding, and the like. This paper deals with the
issue of the reproducibility of high hardness of ≥ 50
GPa in superhard nanocomposites nc-TmN/a-Si3N4
consisting of 3-4 nm small transition nitride (TmN)
nanocrystals "glued" together by about 1 monolayer
(1 ML) thin interfacial layer of silicon nitride, which
were summarized in several reviews [1] and recent
papers [2].
Impurities are often found in plasmas due to
desorption from the walls which are bombarded by
chemically active species, such as radicals and
atoms, and by energetic ions of which Ar+ is
particularly efficient because of its high mass and
resulting high yield for physical sputtering. Oxygen
and moisture are the most common impurities which
occur at a concentration of 0.5 to several at.% [3],
unless care has been taken to avoid them. It has been
shown that for example a hydrogen glow discharge
plasma can be reproducibly operated with oxygen
impurity of less than 3 ppm (0.000003 at.%) when
the necessary precautions have been taken [4].
Unfortunately, the majority of plasmas used for the
deposition of protective coatings contain impurities
up to several at.% (for some examples see [5] and
references therein). Figure 1, which we reproduce
here for the convenience of the reader, shows that ≥
0.4 at.% of oxygen degrade the hardness of the ncTiN/a-Si3N4 nanocomposites because it substitutes
for nitrogen in the Si3N4-like interfacial layer [7]
thus weakening the Ti-N bonds adjacent to it. In
addition to a low hardness, such coatings are brittle,
and have also lower oxidation resistance. Highquality nanocomposites sustain ≥ 15% elastic,
reversible deformation, i.e. they have high resistance
against brittle fracture [8]. Here we shall show that
oxygen impurities hinder the formation of stable
nanostructure thus apparently stabilizing the Ti-Si-N
solid solution. An improvement of the nanostructure
by reducing the impurities and increasing the
deposition temperature significantly increases the
cutting performance of coated tools.
[XXX] nc-TiN/a-Si3N4 P CVD: Cl < 0.5 at.%
Cl = 0.5 - 0.8 at.%
Cl = 1.0 - 3.5 at.%
nc-TiN/a-Si3N4/a- & nc-TiSi2 P CVD: Cl < 0.5 at.%
Cl > 0.5 at.%
nc-TiN/a-Si3N4/a-TiSi2Li Shizhi 2004, Cl 0.7 - 1.0 at.%
nc-TiN/a-Si3N4 RMSputt - Centr. Cathode
nc-TiN/a-Si3N4 RMSputt - Planar Cathode + SiH 4+ H2
nc-TiN/a-Si3N4 RMSputt of Ti & Si in N 2 - Planar Cathode & Outgas
nc-TiN/a-Si3N4 RMSputt of Ti & Si in N 2 - Planar Cathode
+ "Ti-Si-N" Vaz et al. RMSputt
110
Plastic Hardness [GPa]
100
90
80
70
6
4
2
60
50
40
30
+
[H]=0.9 at.%
0
[H]=10 at.%
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
1/Coverage [Nanocrystal/Oxygen Atoms]
120
2.0
Oxygen Content [at%]
The formation of the stable nanostructure occurs by
self-organization upon spinodal decomposition and
phase segregation which is thermodynamically
driven by a sufficient nitrogen activity (its partial
pressure in the plasma) and kinetically controlled by
diffusion [11]. However, as it will be shown in this
paper, oxygen impurities of more than few 100 ppm
hinder the diffusion due to the formation of very
strong Si-O bonds thus forming immobile clusters.
The completion of the phase segregation and
formation of stable nanostructure is usually followed
by the measurement of the hardness and crystallite
size. However, this technique has relatively low
sensitivity. For this reason we used the technique of
internal friction measurement described in Ref. [12],
to which we refer for further details.
3. Results
3.1. Oxygen hinders the decomposition of solid
solution and formation of stable nanostructure
Fig. 1: Effect of oxygen on the maximum achievable hardness in ncTiN/a-Si3N4 nanocomposites deposited by plasma CVD and by reactive
magnetron sputtering in 5 different apparatuses from 3 different
countries. The dotted line corresponds to the reciprocal coverage of the
nanocrystals with oxygen atoms (Reprinted with permission from Ref.
[6], Copyright 2005, American Vacuum Society).
2.5
Coating # 24/260702-1
-3
Internal Friction [10 ]
2.0
2. Experimental
nc-TiN/a-Si3N4 nanocomposite coatings with the
lowest oxygen impurity concentration of few 100
ppm were deposited by plasma induced CVD,
whereas those deposited by reactive sputtering had
impurity content of 2000-2500 ppm (see reviews [1]
and references therein). nc-(Ti1-xAlx)N/a-Si3N4
nanocomposites were deposited by means of vacuum
arc evaporation in industrial coating equipment π80
which is available from company PLATIT [9], and
in an industrial coating unit "ORM" that is used in
everyday job coating at company SHM [10]. We
refer to the homepages of the companies for further
details. In the standard industrial operation, the
impurities are in the range of 1500-2000 ppm, but
with more care and pre-cleaning they are
reproducibly reduced to 1000 ppm [5] and even to
700 ppm [1c].
as deposited
1.5
annealed at 450 °C
1.0
annealed 650 °C
0.5
annealed 750 °C
0.0
0
100
200
300
400
500
600
0
Temperature [ C]
Fig. 2: Example of internal friction measurement and vanishing of the
friction peak when, upon annealing in nitrogen at the temperature
indicated the phase segregation is completed.
Figure 2 shows an example of the internal friction
measurement in nc-TiN/a-Si3N4 coatings deposited
by reactive magnetron sputtering. The peak seen in
as deposited coatings corresponds to energy losses
of the vibrating reed due to thermally activated
jumps of atoms or clusters between different,
unstable sites within the grain boundaries. Upon
annealing in nitrogen, the diffusion controlled phase
segregation continues resulting in a decrease of the
friction peak and its vanishing when the segregation
and formation of stable nanostructure is completed.
Figure 3 shows an example of the vanishing of the
friction peak and increase of the hardness and
Young's modulus upon annealing of an industrial nc(Ti1-xAlx)N/a-Si3N4 coating in nitrogen. Because the
impurity concentration in this coating was less than
that in the coatings deposited by sputtering (Fig. 2)
the friction peak vanishes and the hardness stabilizes
already after annealing at 600 °C.
Fig. 3: Example of the dependence of the intensity of the internal
friction signal ("Peak height"), of Young's modulus and hardness on the
annealing temperature of nc-(Ti1-xAlx)N/a-Si3N4 coatings deposited in an
industrial coating unit of the SHM company.
Annealing Temperature (°C)
1600
a-TiSiN with high Si ?
Musil et al. 2007
1400
1200
1000
800
Int. Friction (Sputtering)
Int. Friction (Vacuum Arc)
Int. Friction (Plasma CVD)
400
0
5000
10000
15000
From the results in Fig. 4 we conclude that oxygen
impurities strongly hinder the diffusion which is
needed for the decomposition of the solid solution
and segregation of the stoichiometric TiN and Si3N4
which are, as it is well documented in the literature,
immiscible [15]. In a stoichiometric Ti-Si-N system
with oxygen impurity of only few 100 ppm, the
decomposition of the solid solution is completed
already at a temperature of ≤ 550°C. Thus, the
apparently high stability of the Ti-Si-N solid
solutions reported by Flink et al. [13] and by Musil
et al. [14] is due to oxygen impurities in the
coatings. One can understand this detrimental effect
because the binding energy of Eb(Si-O) =452 kJ/mol
is much higher than any other in the system: Eb(SiN)=322 kJ/mol, Eb(Si-Si)=340 kJ/mol (see [16]
Table 9.5); also the binding energy of Ti-O and Ti-N
is smaller than that of Si-O (see [17] p. F 222 ff). Let
us also emphasize that minor oxygen impurities of
about 1 wt. % stabilize the α-Si3N4 phase [18], and
cause the amorphization of nc-Si [19].
3.2. Improving the cutting performance of tools
Recrystallization (Vacuum Arc)
Flink et al. 2009
600
observed in the coatings deposited at 550 °C) and by
sputtering, and in industrial nc-(Ti1-xAlx)N/a-Si3N4
coatings deposited by vacuum arc technology. In
addition, we show also the results of Flink et al. (TiSi-N coatings deposited by vacuum arc technique
[13]), and of Musil et al. (Ti-Si-N coatings with high
Si-content deposited by sputtering [14]). These
researchers did not measure internal friction but
reported the temperature at which their coatings recrystallized.
20000
Oxygen Impurity Content (at. ppm)
Fig. 4: Dependence of the annealing temperature at which the phase
segregation is completed and internal friction peak vanished on oxygen
impurity (see text), and the temperature at which the Ti-Si-N solid
solutions recrystallized (Flink et al. 2009 [13]) and the coatings of Musil
et al. with high Si content oxidized and recrystallized [14].
Figure 4 summarizes the results of internal friction
measurements on several nc-TiN/a-Si3N4 coatings
deposited by plasma CVD (no friction peak
Low oxygen impurities are not easy to achieve in
large industrial coating equipment because the walls
are coated with thick deposits which become
contaminated whenever the coating chamber is
opened for exchange of the tools. Moreover, it is not
easy to operate at high temperature the carousel that
holds several 10 kg of tools subjected to 3-fold
rotation and has to work reliably for long time.
Therefore the majority of industrial coating units use
deposition temperature of less than about 450°C.
Figure 5 shows the significant improvement of the
cutting performance of indexable inserts by about a
factor of 2.2 in dry turning of tough steel DIN C45,
when the oxygen impurities were decreased to about
1000 ppm and the deposition temperature increased
to about 535°C. In the case of expensive tools one
utilizes the increase of the life time whereas in the
case of cheaper tools one takes the advantage of
faster machining which increases the productivity.
Too Life Time (min)
50
40
30
20
Improved
deposition 1
4 Inserts
10
Improved
deposition 2
4 Inserts
Standard
0
Fig. 5: Comparison of the lifetime of indexable inserts made of
cemented
carbide
and
coated
with
nc-(Ti1-xAlx)N/a-Si3N4
nanocomposites using the standard LARC coating technology [9] and
the improved one (see text). Dry turning of steel DIN C45, speed of 130
m/min, feed rate 0.18 mm/revolution and axial depth of cut 1.5 mm. The
deposition of the coatings has been done at the Singapore Institute of
Manufacturing Technology (SIMTech) using the industrial coating
equipment π80 from the company PLATIT [9], the cutting tests at the
Brno Technical University (CZ).
4. Conclusions
Impurities, in particularly oxygen, degrade the
mechanical properties of the nanocomposite coatings
by decreasing their hardness to ≤ 35 GPa making
them brittle, decreasing their oxidation resistance,
and increasing the temperature needed for the
formation of fully segregated stable nanostructure.
The oxygen impurity of 700-1000 ppm which are
presently routinely achieved in the industrial coating
units ORM, π80 and π300 of the SHM company
should be still decreased in order to achieve the
formation of stable nanostructure at deposition
temperature of < 500°C, to make the process fully
compatible with tools made of high speed steel.
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