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Scratch adhesion test of reactively sputtered TiN coatings on a soft substrate
Thin Solid Films, 136 (1986) 57-67
57
METALLURGICAL AND PROTECTIVE COATINGS
S C R A T C H A D H E S I O N TEST O F R E A C T I V E L Y S P U T T E R E D TiN COATINGS ON A SOFT SUBSTRATE J. H. JE, E. GYARMATI AND A. NAOUMIDIS Institut ffir Reaktorwerkstoffe, Kernforschungsanlage Jfilich G.m.b.H., D-5170 ( F.R.G. ) (Received May 28, 1985; revised September 20, 1985; accepted October 18, 1985)
Reactively sputtered TiN coatings on the relatively soft substrate, Crofer 1700 (Vickers' hardness about 176 HV), were subjected to a scratch test for adhesion. The critical load of the coating/substrate combination is defined as the load at the onset of coating failure revealed as cracking or loss of the coating that causes an increase in acoustic emission. For example, at the critical load of a thicker coating (approximately 1.5 ~tm), not only cracking but also loss of the coating were observed with an increase in acoustic emission. However, at the critical loads of thinner coatings (1.2 Ixm or less), coating loss was not observed in spite of an increase in the emission but coating cracking did occur. A failure model is suggested for the given coating plus soft substrate system. Using this model, the maximum observed in a typical acoustic emission curve is explained.
1. INTRODUCTION
M a n y methods have been put forward to test for adhesion and these have been reviewed 1'2. A major experimental difficulty encountered with adhesion testing of a thin coating is that of making a mechanical linkage to the coating so that the adherence to the substrate can be measured. The scratch test circumvents this linkage problem and has often been used to study adhesion 3 6 The scratch test is one simple method currently available for testing thin, hard and well-adhering coatings. With this method a diamond stylus is drawn under selected loads across the coating surface and acoustic signals induced in the stylus are recorded. Under a sufficient load the coating fails, giving rise to an enhanced acoustic emission. The mechanism is believed to be that the stylus produces a shearing force at the coatingsubstrate interface around the rim of the indentation so that under the critical load the coating becomes detached. The load under which detachment of the coating occurs is taken as a measure of the adhesion. A major problem associated with the scratch test is that the relationship between the measured critical load and the actual strength of adhesion (the energy required to propagate a crack along the substratecoating interface) is not yet clear 7. Various definitions of the critical load have been proposed 7 ~o. Recently Perry 1~ introduced the concept of the onset of coating loss 0040-6090/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
58
J.H.
JE, E. G Y A R M A T I , A. N A O U M I D I S
as the critical load. He believes this to be a measure of the effective load-bearing capacity of a coating/substrate combination. The objectives of this work were (a) to establish the failure model of the reactively sputtered TiN coatings on a relatively soft substrate, Crofer 1700 (Vickers' hardness about 176 HV), (b) to define the critical load for the given coating/substrate system and (c) to analyse the acoustic emission signal caused by detachment of the coating. 2.
E X P E R I M E N T A L DETAILS
The substrates studied in this work were Crofer 1700 discs (hardness about 176 HV) 2 m m thick and 25 m m in diameter with the composition given in Table I, and one 1.4301 stainless steel disc (hardness about 210 HV) for comparison of the microscopic picture of the scratched channel with the acoustic signals. The substrates were given a final polish using 0.2/am alumina and were cleaned in ultrasonic baths of acetone and ethyl alcohol. TABLE 1 THE CHEMICAL COMPOSITION OF CROFER 1700
Element Content ( w t . ~ )
Cr 17.5 19.0
C ~<0.14
Mn ~<0.50
S ~<0.03
Si ~< 1.50
P ~<0.03
Fe Balance
TiN was reactively sputtered onto the unheated substrate in mixed A r - N 2 discharges using a Hereaus Z 4 0 0 universal magnetron sputtering system. The experimental conditions are described in Table II. The coatings were deposited over an area of diameter 2 cm on the discs. The thicknesses were measured by a ball grinding method 12. This method had a range of error _+0.2/am for the TiN coating on Crofer 1700. Film thicknesses thus measured ranged from 0.7 to 1.5/am. The profile of height around the end of a channel was measured with a Dektak II instrument (Sloan Technology Corporation, U.S.A.). The X-ray diffraction patterns were recorded. The TiN coatings showed the preferred (111) orientation perpendicular to the substrate surface. Estimation of the T A B L E II THE EXPERIMENTAL CONDITIONS OF T i N COATING BY REACTIVE SPUTTERING
Sample
Uac (V)
1n
(A)
Un (V)
PAr ( m b a r )
PN2 ( m b a r )
Thickness
Substrate
(lam) 1 2 3 4 5
750 700 700 400 500
Ua¢, c a t h o d e voltage. 1a, c u r r e n t . UB, bias voltage.
0.13 0.10 0.10 0.18 0.12
250 200 200 130 330
5 x 10 2 5x10 -2 5 x 10 - 2 1.2 x 10 - 2 5 x [0 2
3 x 10 3 3x10 3 3 x 10 3 8 x 10 - 4 5 X lO-3
1.5 1.2 0.8 0.7 4.0
Crofer Crofer Crofer Crofer Stainless steel
A D H E S I O N TEST OF
TiN
C O A T I N G S O N SOFT S U B S T R A T E
59
crystallite size (approximately 10-20 nm) according to the broadening of X-ray lines leads to the assumption that the substrate temperature is relatively low in this magnetron sputtering system. Adhesion was studied using a scratch testing unit 7 designed by the Laboratoire Suisse de Recherches Horlog6res, Neuch~tel, Switzerland. The unit has a diamond stylus in the form of a Rockwell C 120 ° cone with a spherical tip of radius 200 ~m 13 In addition, the scratch equipment was fitted with an acoustic signal detector in the form of an accelerometer mounted just above the diamond stylus. It registered the signal emitted in the range centred around 200 kHz. Samples mounted on a slide dish were scratched with a standard scratch speed of 10 m m min 1 for a length of 3 mm. The spacing was 1 m m to prevent mutual interference effects. The voltage output signal was recorded in order to determine whether a correlation could be established between the signal, the scratch characteristics and the loss of coating adhesion. The uncoated Crofer 1700 showed almost no acoustic emission below the stylus load of 44.1 N. Detailed morphologies of the scratch channels were viewed with an optical microscope. 3. RESULTS Optical micrographs of typical failure modes of the TiN coatings are illustrated in Figs. 1 and 3. The acoustic emission curves showing the m a x i m u m and minimum signals are plotted as a function of the stylus load in Figs. 2 and 4. For the specimen coated with 1.5 lam TiN (sample 1) under low stylus loads, the coatings were smoothed out with the coating conforming to the contours of the substrate (Fig. l(a)). As the stylus load was increased, the coating started to crack perpendicularly to the steel surface at the edge of the channel. This crack coincides with the well-known ring crack due to tensile stress near the edge of the contact x4. These loads are still below the critical load, i.e. the coating was not detached at this stage and the acoustic signal did not increase at this stage (Fig. 2, region a). In Fig. l(c) cracks are seen to begin to form perpendicularly to the direction of the moving stylus. Also, microcracks appear behind these cohesive cracks. A slight detachment of the coating is observed behind the crack. The onset of coating loss was accompanied by a sudden increase in the acoustic emission (Fig. 2, region b). As the stylus load was further increased, the number of the cracks and the microcracks increased (Fig. l(d)) while.the acoustic emission signal steadily increased (Fig. 2, region b). The very high normal load can either cause the detached coating particles to be completely pressed back into the substrate or cause a partial removal, thus leaving a smooth surface within the channel, as seen in Fig. l(e) and in Fig. 2, region c. In the case of the 0.7 ~tm TiN coating (sample 4), the onset of coating cracking (Fig. 3(a)) was also accompanied by a sudden increase in the acoustic signal (Fig. 4). Some microcracks also appear behind the cohesive cracks. In contrast with the results obtained with the first coating of thickness 1.5 tam (sample 1), no removal of the coating was found behind the cohesive crack. Cracking perpendicular to the
60
(a)
J . H . JE, E. GYARMATI, A. NAOUMIDIS
(b)
(c)
(d)
(e) Fig. 1. Optical micrographs of channels produced during scribing from right to left under various stylus loads in Crofer 1700 coated with TiN about 1.5 gm thick (sample 1): (a) 1.96 N; (b) 4.90 N; (c) 7.84N; (d) 8.82 N; (e) 13.7 N. d i r e c t i o n of the m o v i n g stylus o c c u r r e d a h e a d of the m o v i n g stylus as s h o w n in Fig. 3(c). F i g u r e 5 illustrates the h e i g h t profile m a d e at the e n d of the c h a n n e l seen in Fig.
ADHESION TEST OF TiN COATINGS ON SOFT SUBSTRATE
> 300 E
,,--ct-~.Iq
b~l
~
c
61
*'1
2~o
o 180
120
60
10
2'0
3b stylus load N
Fig. 2. Accelerometer signal maxima and minima taken from the curves recorded within the scratch channel length between 0.5 and 2.5 m m in the case of sample 1.
(a)
(b)
(c) (d) Fig. 3. Optical micrographs of channels produced during scribing from right to left under various stylus loads in Crofer 1700 coated with TiN about 0.7/am thick (sample 4): (a) 1.96 N; (b) 6.86 N; (c) 8.82 N; (d) 19.6 N.
62
J . H . JE, E. GYARMATI, A. NAOUMIDIS
72iiii stylus {oad N Fig. 4. Accelerometer signal maxima and minima taken from the curves recorded within the scratch channel length between 0.5 and 2.5 mm in the case of sample 4.
height prn B 6 4 2 0 -2
~ 2'0
~ gO
1;0
"~1111111111111IT ~hickne' 1L0
1;0
2½0
2;0
3;0 length 0m Fig. 5. Height profile around the end of the channel produced in a TiN coating 0.7 ~m thick on Crofer 1700.
3(c). The position of the crack ahead of the end of the channel in Fig. 3(c) is indicated with an arrow in Fig. 5. This crack lies about 50 lam in front of the peak in the deformed material. This picture shows that the crack does not form by bending. Partial loss of the coating was found at a stylus load of 19.6 N (Fig. 3(d)). The failure mode for samples 2 and 3 is very similar to that of sample 4. 4. DISCUSSION
In adhesion scratch testing, the mode of the coating failure appears to vary according to the nature of both the substrate and the coating, the coating thickness and also the method by which the coating is produced. The failure mode observed for the reactively sputtered TiN coatings on Crofer 1700 is very different from that reported for chemically vapour-deposited TiN coatings 11 and ion-plated TiN coatings 15. It could of course be due either to differences in coating thickness or to differences in the substrate materials. Acoustic emissions are stress waves emitted by sudden localized changes in stress form, e.g. crack formation ~6. Figure 6 shows an optical micrograph of the channel and the associated acoustic signal which is obtained with a TiN-coated
ADHESION TEST OF
TiN COATINGS ON SOFT SUBSTRATE
63
intensity of l acoustic emission
Start time
(channel length) Fig. 6. Comparison of the optical micrograph of the channel produced during scribing lrom right to leJt under a siylus load of 6.86 N in stainless steel coated with TiN about 4.0 gm thick with the corresponding acoustic signals.
stainless steel disc under a stylus load of 6.86 N. The number and the positions of the cracks coincide with the acoustic signal peaks. This indicates that each acoustic signal peak is due to an individual crack formation. In Fig. 3(c) the crack forms ahead of the stylus. The crack formation must have been followed by an increase in the acoustic signals, i.e. by the formation of one peak in the signals. Hamilton and G o o d m a n x4 have calculated the stress field created by a circular sliding contact and have shown that the point of m a x i m u m stress in the surface occurs at the front rim of
64
J . H . JE, E. GYARMATI, A. NAOUMIDIS
the stylus. Outside the contact area, all stresses decay to zero at least as rapidly as the square of the distance from the centre of the stylus 14. From the observed formation of a cohesive crack ahead of the stylus, it is concluded that the coating failure occurs in the front rim of the stylus. Laugier x7 suggested a simple energy balance criterion by which stored elastic energy just ahead of the stylus is released to form new surfaces, i.e. surfaces formed on detachment of the coating when the critical normal load is reached. Of course the initiating mechanism may also be thought of as a critical interfacial shear stress at the front rim of the stylus. Once the coating is detached in the front rim of the stylus, the adhesive crack can easily propagate forwards releasing the stored energy just ahead of the stylus. Thus the initiation of an adhesive crack and its propagation can give rise to the increase in the acoustic signals. The coating may not be removed at this stage. As the stylus moves forward, the raised portion of the coating ahead of the stylus may be pushed back into contact with the substrate 17. Figure 7 illustrates the failure model suggested for the TiN coatings on the soft substrate, Crofer 1700, during the scratch test. At first, elastic energy is stored just ahead of the stylus (Fig. 7(a)). The stored energy causes de-adhesion of the coating from the substrate at the front rim of the stylus (Fig. 7(b)). The de-adhesion propagates forwards (Fig. 7(c)). When the propagation of the de-adhesion is stopped, a cohesive crack which is perpendicular to the direction of the moving stylus is generated at the boundary between the detached and the undetached surfaces (Fig. 7(d)). Finally when the stylus moves over the detached surface,
II II I
i I I I I
I .,. .........
~..~hesion
chon~'ne"|i
"~stylL~
edge
(a)
(b)
(c)
I I I
I I cohesivecrackingof ]
coating
I
"--"i~~.""." .,1"""
(d)
""-micro-
cracking
(e)
Fig. 7. Failure model of the reactively sputtered TiN coatings on Crofer 1700 by the scratch test: (a) the elastic energy is stored just ahead of the stylus; (b) the stored energy causes de-adhesion between the coating and the substrate; (c) de-adhesion propagates ahead of the stylus; (d) stopping of de-adhesion by cohesive cracking of the coating; (e) the movement of the stylus on the detached surface causes microcracking.
ADHESION TEST OF
TiN
COATINGS ON SOFT SUBSTRATE
65
microcracking occurs as a result of the brittleness of the TiN coating (Fig. 7(e)). As the stylus moves further, this process is repeated. Figures 1 and 3 are photomicrographs illustrating a typical pattern formed by these cohesive cracks and trailing microcracks. The critical load as the load at the onset of coating loss as defined by Perry 11 is believed not to be applicable for thin TiN coatings on the soft substrate Crofer 1700. Of course, in the case of the 1.5 I~m TiN coating, it is clear that the critical load is the load at the onset of the coating loss which, in turn, causes an increase in the acoustic emission. However, in the case of the thinner coatings (1.2 ktm or less), coating loss was not observed even though cracking and the enhanced acoustic emission did begin to occur. In such a thin hard coating on a soft substrate, the detached coating can be pressed back into the substrate. By the above failure model, therefore, the critical load is here defined as the load required for the onset of the coating failure revealed as cracking or loss of the coating which, in turn, causes an increase in the acoustic emission. This type of failure can occur preferentially at the weakest points of the interface and is illustrated in Figs. l(c) and 3(a). Critical loads based on the above definition are plotted as a function of the coating thickness in Fig. 8. In spite of the diverse coating conditions, the critical load increases almost linearly with the coating thickness over the entire range of the coating thicknesses investigated. This linear dependence seems to hold whether the coatings are formed under the same (samples 2 and 3) or under different (samples 1 and 4 of Table II) sputtering conditions. In a scratch test, the stress required to detach the coating by shear failure at the interface is transmitted through the coating 17. Thicker layers may well require a greater surface stress to achieve the same shear stress at or near the interface, thus resulting in higher critical loads in thicker layers. Therefore, it is not certain that the apparent increase in critical load with coating thickness represents a real increase in adhesion. A linear extrapolation of the critical loads according to Fig. 8 is not to be presumed for coating thicknesses outside the range of experimentation, namely 0.7-1.5 t~m. With coating thicknesses below 0.7 Ixm, for example, it is quite possible that adhesion would increase, instead of decrease according to a linear extrapolation, because of a decrease in stresses in the coating-substrate interface. An explanation for the existence of a m a x i m u m in the diagram of the intensity of the acoustic signal v e r s u s the stylus load has not yet been given. That is here explained as follows. For example, in the case of TiN on Crofer 1700 (sample 3), the critico[ rood N 8 6 J 4 2 0
4016
018 1'0 112 ,14 ,'6 tooting thickness l~m
Fig. 8. Critical load as a function of coating thickness of the reactively sputtered TiN coating on Crofer 1700.
66
J. H. JE, E. GYARMATI, A. NAOUMIDIS
number of cohesive cracks increases as the load increases. This causes an increase in the intensity of the acoustic emission. This increase in intensity with stylus load (Fig. 4) is accompanied by an increase in the density of cohesive cracks, both reaching a maximum simultaneously, at 6.86 N in the example illustrated in Fig. 3(b). As the load is further increased, the elastic energy stored ahead of the stylus can increase slightly so that the length of the propagation of the adhesive crack increases, i.e. the frequency of formation of the cohesive cracks decreases with the load (Fig. 3(c)). The intensity of the associated acoustic emission thereby decreases with the load (Fig. 4). Loss and detachment of a coating do not always occur simultaneously. It is well known that the detached coatings can be pressed back into the substrate by the movement of the loaded stylus, especially in the case of brittle coatings 8. In Fig. 3, no loss of coating was detected below a stylus load of 11.8 N which is even greater than the critical load. In contrast, loss of coating was detected in Fig. l(c) even though it is only a local removal around the cohesive cracks. The microcracks in Fig. l(c) are seen to remain behind the cohesive crack. Of course, the coatings are removed at sufficiently high stylus loads, but at the critical load, which is the essential criterion for mechanical failure of the coated material 11, the thinner coatings tend not to be removed in spite of the de-adhesion of the coating. 5. SUMMARIZING REMARKS
A failure model for scratch testing of reactively sputtered TiN coatings on the soft substrate Crofer 1700 is proposed. According to this model, elastic energy is stored just ahead of the scratching stylus causing de-adhesion of the coating from the substrate at the front rim of the stylus and the de-adhesion propagates forward. When the propagation of de-adhesion is stopped, a cohesive crack is generated at the boundary between the detached and the undetached surface. As the stylus moves on the detached surface, microcracking occurs because of the brittleness of the TiN coating. The critical load for the TiN coating on Crofer 1700 corresponds to the load for the onset of coating failure revealed as cracking or loss of coating which leads to the observed increase in acoustic emission. A m a x i m u m in the acoustic emission is observed as a function of the stylus load. According to the failure model described, this is associated with a corresponding rise and maximization of the cohesive crack density in its variation with the load. ACKNOWLEDGMENT
The authors are grateful to Dr. R. Benz for many useful discussions and for critical reading of the manuscript. REFERENCES 1 2 3
C. Weaver, Chem. bTd. ( N Y ) , 1 (1965) 370. B.N. Chapman, J. Vac. Sci. Technol., 11 (1974) 106. O.S. Heavens, J. Phys. Radium, l l (1950) 355.
ADHESION TEST OF
4 5 6 7 8 9 10 11 12 13 14 15 16 17
TiN COATINGS ON SOFT SUBSTRATE
67
P. Benjamin and C. Weaver, Proc. R. Soc. London, Ser. A, 254 (1960) 163. P. Benjamin and C. Weaver, Proc. R. Soc. London, Set, A, 274 (1963) 267. C. Weaver, J. Vac. Sci. Technol., 12(1975) 18. A.J. Perry, Thin Solid Films, 107 (1983) 167. D.W. Butler, C. T. H. Stoddart and P. R. Stuart, J. Phys. D., 3 (1970) 877. J.E. Greene, J. Woodhouse and M. Pestes, Rev. Sei. lnstrum., 45 (1974) 747. J. Oroshnik and N. K. Croll, Adhesion measurements of thin films, A S T M Spec. Tech. Publ. 640, 1978, p. 158. A.J. Perry, ThinSolidFilms, 78(1981)77. K. Herffand E. Roeder, Pract. Metallogr., 5 (1968) 615. J. Hamersky, Thin Solid Films, 3 (1969) 263. G.M. Hamilton and L. E. Goodman, J. Appl. Mech., 33 (1966) 371. A.J. Perry, ThinSolidFilms, 81(1981)357. H . N . G . Wadley and R. Mehrabian, Mater. Sci. Eng., 65 (1984) 245. M.T. Laugier, Thin Solid Films, 117 (1984) 243.
57
METALLURGICAL AND PROTECTIVE COATINGS
S C R A T C H A D H E S I O N TEST O F R E A C T I V E L Y S P U T T E R E D TiN COATINGS ON A SOFT SUBSTRATE J. H. JE, E. GYARMATI AND A. NAOUMIDIS Institut ffir Reaktorwerkstoffe, Kernforschungsanlage Jfilich G.m.b.H., D-5170 ( F.R.G. ) (Received May 28, 1985; revised September 20, 1985; accepted October 18, 1985)
Reactively sputtered TiN coatings on the relatively soft substrate, Crofer 1700 (Vickers' hardness about 176 HV), were subjected to a scratch test for adhesion. The critical load of the coating/substrate combination is defined as the load at the onset of coating failure revealed as cracking or loss of the coating that causes an increase in acoustic emission. For example, at the critical load of a thicker coating (approximately 1.5 ~tm), not only cracking but also loss of the coating were observed with an increase in acoustic emission. However, at the critical loads of thinner coatings (1.2 Ixm or less), coating loss was not observed in spite of an increase in the emission but coating cracking did occur. A failure model is suggested for the given coating plus soft substrate system. Using this model, the maximum observed in a typical acoustic emission curve is explained.
1. INTRODUCTION
M a n y methods have been put forward to test for adhesion and these have been reviewed 1'2. A major experimental difficulty encountered with adhesion testing of a thin coating is that of making a mechanical linkage to the coating so that the adherence to the substrate can be measured. The scratch test circumvents this linkage problem and has often been used to study adhesion 3 6 The scratch test is one simple method currently available for testing thin, hard and well-adhering coatings. With this method a diamond stylus is drawn under selected loads across the coating surface and acoustic signals induced in the stylus are recorded. Under a sufficient load the coating fails, giving rise to an enhanced acoustic emission. The mechanism is believed to be that the stylus produces a shearing force at the coatingsubstrate interface around the rim of the indentation so that under the critical load the coating becomes detached. The load under which detachment of the coating occurs is taken as a measure of the adhesion. A major problem associated with the scratch test is that the relationship between the measured critical load and the actual strength of adhesion (the energy required to propagate a crack along the substratecoating interface) is not yet clear 7. Various definitions of the critical load have been proposed 7 ~o. Recently Perry 1~ introduced the concept of the onset of coating loss 0040-6090/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
58
J.H.
JE, E. G Y A R M A T I , A. N A O U M I D I S
as the critical load. He believes this to be a measure of the effective load-bearing capacity of a coating/substrate combination. The objectives of this work were (a) to establish the failure model of the reactively sputtered TiN coatings on a relatively soft substrate, Crofer 1700 (Vickers' hardness about 176 HV), (b) to define the critical load for the given coating/substrate system and (c) to analyse the acoustic emission signal caused by detachment of the coating. 2.
E X P E R I M E N T A L DETAILS
The substrates studied in this work were Crofer 1700 discs (hardness about 176 HV) 2 m m thick and 25 m m in diameter with the composition given in Table I, and one 1.4301 stainless steel disc (hardness about 210 HV) for comparison of the microscopic picture of the scratched channel with the acoustic signals. The substrates were given a final polish using 0.2/am alumina and were cleaned in ultrasonic baths of acetone and ethyl alcohol. TABLE 1 THE CHEMICAL COMPOSITION OF CROFER 1700
Element Content ( w t . ~ )
Cr 17.5 19.0
C ~<0.14
Mn ~<0.50
S ~<0.03
Si ~< 1.50
P ~<0.03
Fe Balance
TiN was reactively sputtered onto the unheated substrate in mixed A r - N 2 discharges using a Hereaus Z 4 0 0 universal magnetron sputtering system. The experimental conditions are described in Table II. The coatings were deposited over an area of diameter 2 cm on the discs. The thicknesses were measured by a ball grinding method 12. This method had a range of error _+0.2/am for the TiN coating on Crofer 1700. Film thicknesses thus measured ranged from 0.7 to 1.5/am. The profile of height around the end of a channel was measured with a Dektak II instrument (Sloan Technology Corporation, U.S.A.). The X-ray diffraction patterns were recorded. The TiN coatings showed the preferred (111) orientation perpendicular to the substrate surface. Estimation of the T A B L E II THE EXPERIMENTAL CONDITIONS OF T i N COATING BY REACTIVE SPUTTERING
Sample
Uac (V)
1n
(A)
Un (V)
PAr ( m b a r )
PN2 ( m b a r )
Thickness
Substrate
(lam) 1 2 3 4 5
750 700 700 400 500
Ua¢, c a t h o d e voltage. 1a, c u r r e n t . UB, bias voltage.
0.13 0.10 0.10 0.18 0.12
250 200 200 130 330
5 x 10 2 5x10 -2 5 x 10 - 2 1.2 x 10 - 2 5 x [0 2
3 x 10 3 3x10 3 3 x 10 3 8 x 10 - 4 5 X lO-3
1.5 1.2 0.8 0.7 4.0
Crofer Crofer Crofer Crofer Stainless steel
A D H E S I O N TEST OF
TiN
C O A T I N G S O N SOFT S U B S T R A T E
59
crystallite size (approximately 10-20 nm) according to the broadening of X-ray lines leads to the assumption that the substrate temperature is relatively low in this magnetron sputtering system. Adhesion was studied using a scratch testing unit 7 designed by the Laboratoire Suisse de Recherches Horlog6res, Neuch~tel, Switzerland. The unit has a diamond stylus in the form of a Rockwell C 120 ° cone with a spherical tip of radius 200 ~m 13 In addition, the scratch equipment was fitted with an acoustic signal detector in the form of an accelerometer mounted just above the diamond stylus. It registered the signal emitted in the range centred around 200 kHz. Samples mounted on a slide dish were scratched with a standard scratch speed of 10 m m min 1 for a length of 3 mm. The spacing was 1 m m to prevent mutual interference effects. The voltage output signal was recorded in order to determine whether a correlation could be established between the signal, the scratch characteristics and the loss of coating adhesion. The uncoated Crofer 1700 showed almost no acoustic emission below the stylus load of 44.1 N. Detailed morphologies of the scratch channels were viewed with an optical microscope. 3. RESULTS Optical micrographs of typical failure modes of the TiN coatings are illustrated in Figs. 1 and 3. The acoustic emission curves showing the m a x i m u m and minimum signals are plotted as a function of the stylus load in Figs. 2 and 4. For the specimen coated with 1.5 lam TiN (sample 1) under low stylus loads, the coatings were smoothed out with the coating conforming to the contours of the substrate (Fig. l(a)). As the stylus load was increased, the coating started to crack perpendicularly to the steel surface at the edge of the channel. This crack coincides with the well-known ring crack due to tensile stress near the edge of the contact x4. These loads are still below the critical load, i.e. the coating was not detached at this stage and the acoustic signal did not increase at this stage (Fig. 2, region a). In Fig. l(c) cracks are seen to begin to form perpendicularly to the direction of the moving stylus. Also, microcracks appear behind these cohesive cracks. A slight detachment of the coating is observed behind the crack. The onset of coating loss was accompanied by a sudden increase in the acoustic emission (Fig. 2, region b). As the stylus load was further increased, the number of the cracks and the microcracks increased (Fig. l(d)) while.the acoustic emission signal steadily increased (Fig. 2, region b). The very high normal load can either cause the detached coating particles to be completely pressed back into the substrate or cause a partial removal, thus leaving a smooth surface within the channel, as seen in Fig. l(e) and in Fig. 2, region c. In the case of the 0.7 ~tm TiN coating (sample 4), the onset of coating cracking (Fig. 3(a)) was also accompanied by a sudden increase in the acoustic signal (Fig. 4). Some microcracks also appear behind the cohesive cracks. In contrast with the results obtained with the first coating of thickness 1.5 tam (sample 1), no removal of the coating was found behind the cohesive crack. Cracking perpendicular to the
60
(a)
J . H . JE, E. GYARMATI, A. NAOUMIDIS
(b)
(c)
(d)
(e) Fig. 1. Optical micrographs of channels produced during scribing from right to left under various stylus loads in Crofer 1700 coated with TiN about 1.5 gm thick (sample 1): (a) 1.96 N; (b) 4.90 N; (c) 7.84N; (d) 8.82 N; (e) 13.7 N. d i r e c t i o n of the m o v i n g stylus o c c u r r e d a h e a d of the m o v i n g stylus as s h o w n in Fig. 3(c). F i g u r e 5 illustrates the h e i g h t profile m a d e at the e n d of the c h a n n e l seen in Fig.
ADHESION TEST OF TiN COATINGS ON SOFT SUBSTRATE
> 300 E
,,--ct-~.Iq
b~l
~
c
61
*'1
2~o
o 180
120
60
10
2'0
3b stylus load N
Fig. 2. Accelerometer signal maxima and minima taken from the curves recorded within the scratch channel length between 0.5 and 2.5 m m in the case of sample 1.
(a)
(b)
(c) (d) Fig. 3. Optical micrographs of channels produced during scribing from right to left under various stylus loads in Crofer 1700 coated with TiN about 0.7/am thick (sample 4): (a) 1.96 N; (b) 6.86 N; (c) 8.82 N; (d) 19.6 N.
62
J . H . JE, E. GYARMATI, A. NAOUMIDIS
72iiii stylus {oad N Fig. 4. Accelerometer signal maxima and minima taken from the curves recorded within the scratch channel length between 0.5 and 2.5 mm in the case of sample 4.
height prn B 6 4 2 0 -2
~ 2'0
~ gO
1;0
"~1111111111111IT ~hickne' 1L0
1;0
2½0
2;0
3;0 length 0m Fig. 5. Height profile around the end of the channel produced in a TiN coating 0.7 ~m thick on Crofer 1700.
3(c). The position of the crack ahead of the end of the channel in Fig. 3(c) is indicated with an arrow in Fig. 5. This crack lies about 50 lam in front of the peak in the deformed material. This picture shows that the crack does not form by bending. Partial loss of the coating was found at a stylus load of 19.6 N (Fig. 3(d)). The failure mode for samples 2 and 3 is very similar to that of sample 4. 4. DISCUSSION
In adhesion scratch testing, the mode of the coating failure appears to vary according to the nature of both the substrate and the coating, the coating thickness and also the method by which the coating is produced. The failure mode observed for the reactively sputtered TiN coatings on Crofer 1700 is very different from that reported for chemically vapour-deposited TiN coatings 11 and ion-plated TiN coatings 15. It could of course be due either to differences in coating thickness or to differences in the substrate materials. Acoustic emissions are stress waves emitted by sudden localized changes in stress form, e.g. crack formation ~6. Figure 6 shows an optical micrograph of the channel and the associated acoustic signal which is obtained with a TiN-coated
ADHESION TEST OF
TiN COATINGS ON SOFT SUBSTRATE
63
intensity of l acoustic emission
Start time
(channel length) Fig. 6. Comparison of the optical micrograph of the channel produced during scribing lrom right to leJt under a siylus load of 6.86 N in stainless steel coated with TiN about 4.0 gm thick with the corresponding acoustic signals.
stainless steel disc under a stylus load of 6.86 N. The number and the positions of the cracks coincide with the acoustic signal peaks. This indicates that each acoustic signal peak is due to an individual crack formation. In Fig. 3(c) the crack forms ahead of the stylus. The crack formation must have been followed by an increase in the acoustic signals, i.e. by the formation of one peak in the signals. Hamilton and G o o d m a n x4 have calculated the stress field created by a circular sliding contact and have shown that the point of m a x i m u m stress in the surface occurs at the front rim of
64
J . H . JE, E. GYARMATI, A. NAOUMIDIS
the stylus. Outside the contact area, all stresses decay to zero at least as rapidly as the square of the distance from the centre of the stylus 14. From the observed formation of a cohesive crack ahead of the stylus, it is concluded that the coating failure occurs in the front rim of the stylus. Laugier x7 suggested a simple energy balance criterion by which stored elastic energy just ahead of the stylus is released to form new surfaces, i.e. surfaces formed on detachment of the coating when the critical normal load is reached. Of course the initiating mechanism may also be thought of as a critical interfacial shear stress at the front rim of the stylus. Once the coating is detached in the front rim of the stylus, the adhesive crack can easily propagate forwards releasing the stored energy just ahead of the stylus. Thus the initiation of an adhesive crack and its propagation can give rise to the increase in the acoustic signals. The coating may not be removed at this stage. As the stylus moves forward, the raised portion of the coating ahead of the stylus may be pushed back into contact with the substrate 17. Figure 7 illustrates the failure model suggested for the TiN coatings on the soft substrate, Crofer 1700, during the scratch test. At first, elastic energy is stored just ahead of the stylus (Fig. 7(a)). The stored energy causes de-adhesion of the coating from the substrate at the front rim of the stylus (Fig. 7(b)). The de-adhesion propagates forwards (Fig. 7(c)). When the propagation of the de-adhesion is stopped, a cohesive crack which is perpendicular to the direction of the moving stylus is generated at the boundary between the detached and the undetached surfaces (Fig. 7(d)). Finally when the stylus moves over the detached surface,
II II I
i I I I I
I .,. .........
~..~hesion
chon~'ne"|i
"~stylL~
edge
(a)
(b)
(c)
I I I
I I cohesivecrackingof ]
coating
I
"--"i~~.""." .,1"""
(d)
""-micro-
cracking
(e)
Fig. 7. Failure model of the reactively sputtered TiN coatings on Crofer 1700 by the scratch test: (a) the elastic energy is stored just ahead of the stylus; (b) the stored energy causes de-adhesion between the coating and the substrate; (c) de-adhesion propagates ahead of the stylus; (d) stopping of de-adhesion by cohesive cracking of the coating; (e) the movement of the stylus on the detached surface causes microcracking.
ADHESION TEST OF
TiN
COATINGS ON SOFT SUBSTRATE
65
microcracking occurs as a result of the brittleness of the TiN coating (Fig. 7(e)). As the stylus moves further, this process is repeated. Figures 1 and 3 are photomicrographs illustrating a typical pattern formed by these cohesive cracks and trailing microcracks. The critical load as the load at the onset of coating loss as defined by Perry 11 is believed not to be applicable for thin TiN coatings on the soft substrate Crofer 1700. Of course, in the case of the 1.5 I~m TiN coating, it is clear that the critical load is the load at the onset of the coating loss which, in turn, causes an increase in the acoustic emission. However, in the case of the thinner coatings (1.2 ktm or less), coating loss was not observed even though cracking and the enhanced acoustic emission did begin to occur. In such a thin hard coating on a soft substrate, the detached coating can be pressed back into the substrate. By the above failure model, therefore, the critical load is here defined as the load required for the onset of the coating failure revealed as cracking or loss of the coating which, in turn, causes an increase in the acoustic emission. This type of failure can occur preferentially at the weakest points of the interface and is illustrated in Figs. l(c) and 3(a). Critical loads based on the above definition are plotted as a function of the coating thickness in Fig. 8. In spite of the diverse coating conditions, the critical load increases almost linearly with the coating thickness over the entire range of the coating thicknesses investigated. This linear dependence seems to hold whether the coatings are formed under the same (samples 2 and 3) or under different (samples 1 and 4 of Table II) sputtering conditions. In a scratch test, the stress required to detach the coating by shear failure at the interface is transmitted through the coating 17. Thicker layers may well require a greater surface stress to achieve the same shear stress at or near the interface, thus resulting in higher critical loads in thicker layers. Therefore, it is not certain that the apparent increase in critical load with coating thickness represents a real increase in adhesion. A linear extrapolation of the critical loads according to Fig. 8 is not to be presumed for coating thicknesses outside the range of experimentation, namely 0.7-1.5 t~m. With coating thicknesses below 0.7 Ixm, for example, it is quite possible that adhesion would increase, instead of decrease according to a linear extrapolation, because of a decrease in stresses in the coating-substrate interface. An explanation for the existence of a m a x i m u m in the diagram of the intensity of the acoustic signal v e r s u s the stylus load has not yet been given. That is here explained as follows. For example, in the case of TiN on Crofer 1700 (sample 3), the critico[ rood N 8 6 J 4 2 0
4016
018 1'0 112 ,14 ,'6 tooting thickness l~m
Fig. 8. Critical load as a function of coating thickness of the reactively sputtered TiN coating on Crofer 1700.
66
J. H. JE, E. GYARMATI, A. NAOUMIDIS
number of cohesive cracks increases as the load increases. This causes an increase in the intensity of the acoustic emission. This increase in intensity with stylus load (Fig. 4) is accompanied by an increase in the density of cohesive cracks, both reaching a maximum simultaneously, at 6.86 N in the example illustrated in Fig. 3(b). As the load is further increased, the elastic energy stored ahead of the stylus can increase slightly so that the length of the propagation of the adhesive crack increases, i.e. the frequency of formation of the cohesive cracks decreases with the load (Fig. 3(c)). The intensity of the associated acoustic emission thereby decreases with the load (Fig. 4). Loss and detachment of a coating do not always occur simultaneously. It is well known that the detached coatings can be pressed back into the substrate by the movement of the loaded stylus, especially in the case of brittle coatings 8. In Fig. 3, no loss of coating was detected below a stylus load of 11.8 N which is even greater than the critical load. In contrast, loss of coating was detected in Fig. l(c) even though it is only a local removal around the cohesive cracks. The microcracks in Fig. l(c) are seen to remain behind the cohesive crack. Of course, the coatings are removed at sufficiently high stylus loads, but at the critical load, which is the essential criterion for mechanical failure of the coated material 11, the thinner coatings tend not to be removed in spite of the de-adhesion of the coating. 5. SUMMARIZING REMARKS
A failure model for scratch testing of reactively sputtered TiN coatings on the soft substrate Crofer 1700 is proposed. According to this model, elastic energy is stored just ahead of the scratching stylus causing de-adhesion of the coating from the substrate at the front rim of the stylus and the de-adhesion propagates forward. When the propagation of de-adhesion is stopped, a cohesive crack is generated at the boundary between the detached and the undetached surface. As the stylus moves on the detached surface, microcracking occurs because of the brittleness of the TiN coating. The critical load for the TiN coating on Crofer 1700 corresponds to the load for the onset of coating failure revealed as cracking or loss of coating which leads to the observed increase in acoustic emission. A m a x i m u m in the acoustic emission is observed as a function of the stylus load. According to the failure model described, this is associated with a corresponding rise and maximization of the cohesive crack density in its variation with the load. ACKNOWLEDGMENT
The authors are grateful to Dr. R. Benz for many useful discussions and for critical reading of the manuscript. REFERENCES 1 2 3
C. Weaver, Chem. bTd. ( N Y ) , 1 (1965) 370. B.N. Chapman, J. Vac. Sci. Technol., 11 (1974) 106. O.S. Heavens, J. Phys. Radium, l l (1950) 355.
ADHESION TEST OF
4 5 6 7 8 9 10 11 12 13 14 15 16 17
TiN COATINGS ON SOFT SUBSTRATE
67
P. Benjamin and C. Weaver, Proc. R. Soc. London, Ser. A, 254 (1960) 163. P. Benjamin and C. Weaver, Proc. R. Soc. London, Set, A, 274 (1963) 267. C. Weaver, J. Vac. Sci. Technol., 12(1975) 18. A.J. Perry, Thin Solid Films, 107 (1983) 167. D.W. Butler, C. T. H. Stoddart and P. R. Stuart, J. Phys. D., 3 (1970) 877. J.E. Greene, J. Woodhouse and M. Pestes, Rev. Sei. lnstrum., 45 (1974) 747. J. Oroshnik and N. K. Croll, Adhesion measurements of thin films, A S T M Spec. Tech. Publ. 640, 1978, p. 158. A.J. Perry, ThinSolidFilms, 78(1981)77. K. Herffand E. Roeder, Pract. Metallogr., 5 (1968) 615. J. Hamersky, Thin Solid Films, 3 (1969) 263. G.M. Hamilton and L. E. Goodman, J. Appl. Mech., 33 (1966) 371. A.J. Perry, ThinSolidFilms, 81(1981)357. H . N . G . Wadley and R. Mehrabian, Mater. Sci. Eng., 65 (1984) 245. M.T. Laugier, Thin Solid Films, 117 (1984) 243.