Shear Testing for Characterizing the Adhesive and Cohesive Coating Strength without
the Need of Adhesives
St. Siegmann1, M. Dvorak1, H. Grützner2, K. Nassenstein3, A. Walter4
1
EMPA – Materials Science and Technology, Thun, Switzerland,
Luckenbach, Germany; 4Walter+Bai, Löhningen, Switzerland
2
IFAM, Bremen, Germany;
3
GTVmbH,
The characterization of the adhesive and cohesive strength of thermally sprayed coatings is often evaluated
according to given standardized testing procedures. These tests require the preparation of normally large coupons
which have to be fixed together using an appropriate adhesive. Additionally they need time for preparation (e.g.
annealing/curing of the adhesive) and require test equipments normally not available at job shops for coating
development. One of the largest limitations of these tests is the applicability only for non-porous coatings, and in
some cases the limited strength of the adhesive.
Within a European CRAFT research project on “standards, measurements and testing”, a new shear test method
was developed to characterize the mode and value of failure of thermally sprayed layers in a more reliable and less
limited manner. This new shear test does not need any adhesive and yields more intrinsic information on coating
quality than conventional tensile tests.
1
Introduction
2
The quality of thermally sprayed coatings is
characterized by different means. First, the
microstructure plays an important role in coating
mechanical properties [1, 2] and secondly the surface
preparation for coating adhesion [3-9].
One method commonly used to characterize the bond
strength of thermally sprayed coatings is the pull-off
test standardized e.g. in EN 582 [10], ISO 14916 [11],
or ASTM C 633 [12], ASTM F 1147 [13], etc. The
limitation of all these tests is the need to fix a counter
body to the coating to be measured using an
appropriate adhesive.
Depending on coating chemistry and microstructure,
the bond strength values can be much higher than
that of the adhesive or the adhesive may even
penetrate the coating to form a compound. This effect
can significantly influence the values of the bond
strength of the original coating, as round-robin tests
have recently shown.
One of the aims of this investigation was to overcome
these limitations and suggesting a test more realistic
to loading conditions of coatings in practice, like the
resistance against shear loads.
So far different shear test arrangements have been
proposed and described in the literature, e.g. [14-26].
This new shear test method requires only little effort in
producing the samples as well as preparing and
carrying out the test, but nevertheless yields
reproducible and interpretable results. The results can
be categorized in three different kind of coating
failures explained later in paragraph “5.1 Fracture
modes”. This testing seems to be best suited to fulfill
the required demands and to simulate loading
conditions in practice.
2.1
Experimental set up
Shear test device
The shear test device consists of a simple table size
equipment capable to deliver a maximum force of 20
kN. To our estimation, this should be enough to shear
even well bonded VPS-, HVOF-coatings, or coatings
made out of self fluxing alloys.
The requirements for sample preparation should not
be highly demanding and any post treatment, which
could affect the specimens like cutting or adhesively
bonding a counterpart, was avoided.
A hard metal plate (as commonly used for milling
tools) is utilized to introduce the force in parallel and
close to the coating-substrate interface. The force is
increased until coating failure occurs. Loaddisplacement
measurements
yield
important
information about coating cracking, delamination,
rupture and other adhesion or cohesion properties.
An exact fixation of the sample in the sample holder
and precise alignment of the specimen is the only
requirement. The shear distance can be adjusted by a
micrometer screw at the back side of the sample.
Shear distance is preferably set between 50 and 100
µm above the interface.
The following Figure 1 shows a schematic drawing of
the test arrangement developed within this
investigation.
Siegmann, S., M. Dvorak, H. Grützner, K. Nassenstein and A. Walter: Shear testing for characterizing the adhesive and cohesive coating strength without the need
of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5
test load
N6
N6
load cell
data
processing
10 5
guided punch
5
displacement
gauge
5
30 +0/-0.05
shear plate
coating
sample
holder
Figure 3: Sketch and dimension of the new shear test
sample.
specimen
Figure 1: Principle of the new shear test device.
Figure 2 shows as an overview the first prototype built
during the project. The equipment consists of the
measurement unit and read-out, as well as an optional
PC for data recording and evaluation. The close up in
Figure 2 shows a magnification of the sample fixation
and shear plate at starting position.
3
1
2
An alignment from the backside can be done, if the
lengths of the samples are machined precisely
enough (30 +0/-0.05mm). The hole at the backside
can be used for fixation as safety catch during
rotational spraying. Grit blasting in addition reduces
the length of the sample in such a way, that a collision
of the shear plate with the substrate can be avoided. If
the required precision of the length can not be
assured, the measurement of the shear distance has
to be done from coating surface by subtracting its
thickness. The material of the test piece can be the
same like afterwards in production.
The test specimen can be put together to a bundle of
5 to 10 pieces for spraying (Figure 4). Copper or
Aluminum foils in between the samples help to
separate the specimen after coating.
Figure 2: Shear testing machine with computer for
data acquisition and read out (Walter+Bai, CH); Close
up (top right) of the sample fixation (1), sticking out
coating (2), and shear plate (3) viewed from the side.
During this study, most samples were tested with a
displacement rate of the shear plate of 30 µm/second
until rupture occurred. The test turned out to be not
very sensitive against this parameter.
2.2
Figure 4: Sample fixation for rotational coating of
bundle of shear test specimen.
Shear Test Samples
A first task was the definition of the sample geometry.
The samples should be cheap in mass production and
as small as possible providing enough area for
statistical relevance and reproducibility. Different
geometries of samples with 30 mm length, 10 mm
width, and various heights of 3 mm, 5 mm, and 10 mm
have been tested. After numerous experiments a
height of 5 mm turned out to be adequate (Figure 3).
3
Assessment of stress distribution during
shear loading
Even for tensile test loading we cannot expect plain
stress conditions at the interface between coating and
substrate. However stress distribution within the
sample by shear loading is deviating remarkably from
that by tensile load. The stress tensor generated
during shear test can be assessed by a FEM
Siegmann, S., M. Dvorak, H. Grützner, K. Nassenstein and A. Walter: Shear testing for characterizing the adhesive and cohesive coating strength without the need
of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5
1000
Coating
Figure 5: Stress distribution in loading direction (3).
Strong compressive stress is generated directly under
the edge of the shear plate (dark line), whereas at the
interface between coating and substrate strong tensile
stress is efficacious (grey line between two dark
lines). However tension decay rather fast i.e. in the
centre of the interface stress is already reduced by
more than one magnitude.
In Figure 6 the maximal stress at the interface is
plotted as function of shear plate displacement Dz
(coating deformation) for coating thicknesses from
200 to 1000 µm. Straight lines arise due to the
assumption that only elastic strain becomes effective.
The black lines indicate “shear stress” if shear force is
related to the coated area of the sample. Obviously
this is not meaningful because this stress does not
characterize loading capacity of the coating.
100
s max
s max / area
10
1
0
10
15
20
Dz [µm]
Figure 6: Upper curve: maximum stress, lower curve:
stress related to coated area as function of coating
thickness.
4
Substrate
200
300
400
600
800
1000
10000
s [MPa]
calculation. The calculation offers valuable clues
about the effect of different test parameters like
specimen height and shear distance. It shows that
tensile, compression, and shear forces are effective
within coating and substrate in all directions. For the
simplified model it is assumed that coating and parent
material are homogeneous, undergo only elastic strain
and bonding is not disturbed by flaws. Based on
former investigations Young’s modulus of the sprayed
coating is taken much lower than that of the base
material. Shear distance is set to 100 µm from the
interface and different coating thicknesses between
200 and 1000 µm have been evaluated.
The principal stress in direction 3 parallel to the
interface assuming coating compression by the shear
plate of 20 µm is shown in Figure 5.
5
Coating materials and techniques
Different types of thermally sprayed coatings have
been evaluated using the standardized bond strength
test (EN 582) and are compared to the results of the
new shear test.
The spraying techniques covered a wide range of
coating systems commonly used on the market like
atmospheric plasma (APS), vacuum plasma (VPS),
high velocity oxygen fuel (liquid and gas HVOF),
powder flame (FS), wire flame (WFS), wire arc (WAS),
and even cold gas spraying (CGS).
The results from more than 14 different coating
materials (6 metals, 3 hard metals and 5 ceramics)
have been compared.
The surface preparation and spraying was done at the
different project partners according to their standard
parameters. Surface roughness of the grit blasted
samples was measured by UBM laser profilometer on
non-coated samples. The sprayed coatings were
finally characterized by metallographic cross-sections.
Coating hardness was measured with a conventional
micro-hardness tester and universal hardness testing
machine type Zwick ZHU 2,5. The thicknesses of the
different coatings ranged between 200 to 400 µm.
In addition and for comparison some bulk materials
like aluminum, brass (CuZn37) and cold working steel
bars were tested as well. The bars were
metallographically investigated to observe any grain
orientation or surface hardening due to preliminary
fabrication processes. The idea was to compare the
results gained by the new shear test with universal
hardness and strength results from conventional
material characterization.
Siegmann, S., M. Dvorak, H. Grützner, K. Nassenstein and A. Walter: Shear testing for characterizing the adhesive and cohesive coating strength without the need
of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5
Table 1: Spray methods and coating materials used
including bond strength [MPa] and maximum shear
force [N] values.
Coating
Material
Spray
Methode
Bond
strength
[MPa]
Metal
Ti
316L
Cu
NiCr 80/20
NiCr 80/20
Al/Zn
13%Cr-steel
VPS
APS
CGS
AS
FS
WAS
WFS
Ceramic
TiO2
ZrO2-Y2O3
Al2O3/TiO2
87/13
Al2O3
Cr2O3
HV-G
APS
FS
APS
APS
APS
WFS
WFS
Cermet
WCCo88/12
WCCo88/12
74WC20Cr3C26Ni
Cr3C2NiCr75/25
Mo
Mo
Mo
Spec.
fused
Galv.
materials
NiCrBSi
Hard-Cr
HV-G
HV-K
HV-K
HV-K
Max. Shear-Force
[N]
90 ± 5
57 ± 1
14 ± 4
84 ± 2
80 ± 2
53 ± 4
66 ± 6
1439 ± 80
1512 ± 91
970 ± 88
2283 ± 90
916 ± 94
633 ± 27
1582
F(a/c)
a
a
a
c
a
c
a
58 ± 10
78 ± 8
88 ± 4
1020 ± 263
475 ± 28
444 ± 53
c/a
a
a
69 ± 5
67 ± 5
776 ± 63
1110 ± 135
a
a
54 ± 2
48 ± 2
54 ± 3
2343 ± 66
3119 ± 753
1662 ± 82
c
92 ± 4
848 ± 95
c
54 ± 4
54 ± 8
51 ± 8
848 ± 95
884 ± 123
1635 ± 365
c
c/a
c
-
4225 ± 330
7429
c
a
c
the interface between coating and substrate. Only
some coating residues in depressions remaining from
grit blasting become visible on the substrate surface.
The shear distance does not exert a remarkable effect
on the result as long as the coating is loaded not only
close to the surface. Even if the fracture starts with
some distance it will run into the interface. Influence of
sample height on the measured value decreases with
increasing height.
Examples for this fracture mode are APS sprayed
coatings from Al2O3/TiO2 97/3, 316L or copper
sprayed by cold gas deposition (Figure 7).
Figure 7: Examples of mode I failure (adhesion <
cohesion) for steel 316L (left) and Cu (right) on
substrate material of 1.4301.
Mode II:
The crack path depends upon shear distance. If the
shearing is done closely from interface, the coating
will detach along the interface, whereas with larger
shear distance fracture occurs within the coating
(adhesion ≅ cohesion). Especially with sample heights
more than 5 mm coating cracks and detaches only
partially. Examples for this type of fracture mode are
TiO2 and Mo. Mode II is distinguishable from mode III
only when corresponding series of samples are tested
with different shear distances (Figure 8).
HV-G=HVOF-gas, HV-K=HVOF-kerosene
F = fracture mode: a = adhesive, c = cohesive.
5
5.1
Results
Fracture modes
Following the different fracture modes observed
during the project, a rough classification in three main
categories determined by the relationship between
cohesion of coating and adhesion between coating
and substrate is proposed:
Mode I adherence < coherence
Mode II adherence ≅ coherence
Mode III adherence > coherence
Mode I:
The coating detaches completely at maximum shear
load. Independent of sample height fracture occurs in
Figure 8: Example of mode II failure (adhesion ≅
cohesion) for Cr2O3/ TiO2 coatings.
The scattering of shear strength values in mode II is
larger, since deviations in shear distance adjustment
enter into test results. This comes true even more for
mode III fracture. This type of fracture should
therefore be divided into two sub-modes.
Mode IIIa:
Whereas hard coatings failing in this mode splinter in
small particles, soft and often porous coatings
crumble and loaded layer of the coating is scraped off
Siegmann, S., M. Dvorak, H. Grützner, K. Nassenstein and A. Walter: Shear testing for characterizing the adhesive and cohesive coating strength without the need
of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5
(adhesion > cohesion). Examples are Ti, Zn/Al, and
WC-Co 88/12 (Figure 9 right).
5
Aluminium (bulk)
CuZn37 (bulk)
Cold working steel (bulk)
Mode IIIb:
In this mode fracture path runs along the coating
parallel to the interface and the coating part under the
shear plate detaches prompt like in mode I.
Fmax [kN]
4
3
2
1
0
0
100
200
300
400
500
600
Shear distance [µm]
Figure 11: Maximum forces [kN] as a function of
distance from the surface into the bulk of aluminum,
CuZn37 and cold working steel (soft annealed).
Figure 9: (Left) examples of failure mode IIIb
(adhesion > cohesion) for NiCr 80/20 coatings and
(right) side view of Ti coating in failure mode IIIa.
The different modes can easily be distinguished when
using the type of fracture and the typical loaddisplacement curves, known from mechanical tensile
tests, as shown in Figure 10.
As can be seen from Figure 11 the maximum shear
force necessary to detach a “slice” of bulk material is
linearly increasing with increasing distance from
surface (i.e. thickness). After dividing the maximum
shear force values by the representative areas
involved, the result agrees qualitatively well with the
known material rupture strength.
Shear Strength [N/mm2]
0
16
0
14
0
12
0
10
80
60
40
0
20
Fmax for mode I, II, IIIb
Hard chrome (Galvanic)
cold w orking steel (Bulk)
Load
[kN]
NiCrBSi (fused) (FS)
316 L (VPS)
Fmax for mode IIIa
WCCo 88/12 (HVOF-K)
Ti (VPS)
WCCo 88/12 (HVOF-G)
ductile behavior
(no peak, but deviation
form linear curve)
NiCr 80 20 (WAS)
CuZn37 (Bulk)
Cr3C2-NiCr 75/25 (HVOF-K)
Mo (WFS)
Displacement [a.u.]
316L (WFS)
316 L (APS)
Aluminum (Bulk)
Figure 10: Typical load-displacement curves showing
different kinds of coating failures (as discussed in the
text).
Cr2O3 (APS)
TiO2 (HVOF-G)
Cu (CG)
NiCr 80/20 (FS)
Mo (APS)
For comparing the observed shear strength of the
coatings with known bulk materials, different materials
like aluminum, brass (CuZn37) and cold working steel
(1.2210; soft annealed) were prepared and measured
by shear testing, tensile strength measurement, and
different hardness tests (see next paragraph).
The results of the shear tests are shown in Figure 11.
Al2O3 (APS)
Al/Zn (WAS)
UB 100 (Adhesive)
ZrO2-Y2O3 92/8 (APS)
Al2O3 TiO2 87/13 (FS)
Figure 12: Results of shear strength [N/mm2]
measurements for differently sprayed coatings
including fused NiCrBSi, galvanic coatings, adhesive
(UB100), and selected bulk materials (mild steel,
brass (CuZn37), and Aluminum).
Siegmann, S., M. Dvorak, H. Grützner, K. Nassenstein and A. Walter: Shear testing for characterizing the adhesive and cohesive coating strength without the need
of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5
5.2
Hardness
400
ceramic
y = 0.1109x
R2 = 0.9978
350
metal
300
Shearforce/ln(HU) [a.u.]
The shear strength values calculated from maximum
shear force divided by the sample square area
(50mm2) delivers a shear strength value, which seems
to be always lower than the expected tensile adhesive
strength. This is in good correlation to the
expectations and theoretical predictions.
y = 0.1295x
2
R = 0.9637
hardmetal
250
y = 0.1073x
R2 = 0.9924
200
150
100
50
Different kind of hardness measurements, like Vickers
hardness (HV0.2), Martens hardness (HM) [27], and
Universal Hardness (HU) were performed on all
samples including the bulk materials.
The data were compared with the shear test results
and correlations were found. The following Figure 13
shows such a tendency of maximum shear forces in
relation to the measured coating universal hardness
(HU). The different material classes like metals, hard
metals and ceramics can easily be distinguished and
each fitted best using a logarithmic function.
14000
y = 6643.1Ln(x) - 40617
R2 = 0.925
Universal hardness (HU)
12000
y = 3424.7Ln(x) - 15301
R2 = 0.7121
10000
ceramic
hardmetal
metal
8000
6000
4000
y = 1748.4Ln(x) - 10824
2
R = 0.8243
2000
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Shear force [N]
Figure 13: Results of the shear test on metallic,
ceramic, and hard metal coatings sprayed with 7
different types of thermal spray systems (wire arc,
wire and powder flame, atmospheric plasma, vacuum
plasma, HVOF (gas and liquids), cold gas). The
maximum shear force measured is depicted against
the universal hardness (HU).
Using the logarithm of the universal hardness of each
material as a weighing factor, a clear linear tendency
can be seen for all metals, hard metals and ceramic
coatings (Figure 14).
The correlation coefficients of all material classes
tested were entirely above R2 = 0.96. The slope of the
linear tendency for all classes is close to 0.11 times
the shear force, except for the metals, which are close
to 0.13. The correlation coefficient of the metals was
the lowest (R2 = 0.96) and all the other ones above
0.99. The measured correlation between shear load
and the logarithm of universal hardness indicates the
similar resistance against penetration of an indenter,
but with different geometries.
0
0
500
1000
1500
2000
2500
3000
3500
4000
Shearforce [N]
Figure 14: Shear force as a function of shear force
divided by the logarithm of the universal hardness
(HU) showing linear behavior for all material classes
measured.
6
Conclusions
Compared to the conventional tensile adhesion test
the proposed new shear test delivers more
information regarding the coating microstructure and
resistance to applied mechanical loads as e.g.
compression, shearing, rolling, or abrasive wear like in
real applications.
A further advantage of the newly developed shear test
is the possibility to take load displacement plots as
well. The integration of the load displacement curve
delivers information on energy absorption capacity of
the layers or coating-layer interface.
Reliability and also good transferability of
measurement results could be shown [23]. The
scattering of measured shear load data’s was in
general less than 10%.
These promising results recommend the shear test as
alternative to determine the load resistance of
thermally sprayed coatings. It seems that this type of
test fulfills the requirements to become a standard.
7
Acknowledgement
The authors would like to acknowledge the European
Commission and the Swiss Federal Office for
Education and Sciences (BBW) for funding the project
“Shear Test for Thermally Sprayed Coatings” N°:
CRAF-CR-1999-70303 within the CRAFT-module and
all the other project partners involved: Buser
Oberflächentechnik AG, CH (S. Isch); Metallisation
Limited, GB (T. Lester); OBZ Dresel & Grasme
GmbH, DE (D. Grasme); Euroflamm Italiana srl; Erling
Jensen Aps (H. Møller), The authors are indebted to
“Linde AG” (P. Heinrich) outside of the consortium for
their steady interest in the development and for
samples production.
Siegmann, S., M. Dvorak, H. Grützner, K. Nassenstein and A. Walter: Shear testing for characterizing the adhesive and cohesive coating strength without the need
of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5
8
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of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5
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International Journal of Adhesion and Adhesives,
22 (2002), 2, p. 119-127
[23] Grützner, H.; Dvorak, M.; Siegmann, S.;
Nassenstein, K.: A New Shear Test for
Characterization of Coating Adhesive or
Cohesive Failure, ITSC 2004 Thermal Spray
Solutions - Advances in Technology and
Applications, Osaka, Japan, DVS, (2004), p. 364368
[24] DIN 50161, Testing of Thermally Sprayed
Metallic Coats; Determination of the Adhesive
Shear Strength in Shearing Test (withdrawn)
[25] ASTM D 1002, Standard Test Method for
Apparent Shear Strength of Single-Lap-Joint
Adhesively Bonded Metal Specimens by Tension
Loading (Metal-to-Metal)
[26] ASTM F 1044, Standard Test Method for Shear
Testing of Calcium Phosphate Coatings and
Metallic Coatings
[27] Wilde, H.-R. ; Wehrstedt, A.: Martens Hardness
HM - an international accepted designation for
Hardness under Test Force,
Materialwissenschaft und Werkstofftechnik, 31
(2000), 10, p. 937-940
Siegmann, S., M. Dvorak, H. Grützner, K. Nassenstein and A. Walter: Shear testing for characterizing the adhesive and cohesive coating strength without the need
of adhesives, Proceedings of ITSC 2005 Thermal Spray connects: Explore its surfacing potential! (2005), p. 823-829, ISBN 3-87155-793-5