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The effect of intrinsic parameters on the critical load as measured with the scratch test method
Surface and Coatings Technology 137 Ž2001. 146᎐151
The effect of intrinsic parameters on the critical load as measured with the scratch test method N.X. Randall a,U , G. Favaro a , C.H. Frankel b a
CSEM Instruments, Jaquet-Droz 1, CH-2007 Neuchatel, ˆ Switzerland b Pennsyl¨ ania State Uni¨ ersity, Uni¨ ersity Park, PA 16802, USA
Received 13 March 2000; received in revised form 29 September 2000; accepted 29 September 2000
Abstract In the scratch test method, scratches are generated on the coated sample using a diamond indenter Žusually a Rockwell C profile. which is drawn across the surface under either constant or progressively increasing load. The sample is displaced at constant speed and at a certain load damage occurs along the scratch path. This value of critical load, Lc , can be used to accurately characterise the adhesive strength of the coating᎐substrate system. With modern scratch testing instruments, the critical load can be determined by acoustic emission, optical microscopy, variation in penetration depth or variation in the tangential frictional force between tip and sample. However, it is difficult to express the adherence of a coating᎐substrate system in a quantitative way because the critical load depends on several parameters related to the testing conditions. Experience in the scratch testing field has shown that various different radii of diamond indenter are often required in order to adequately characterise the adhesion of modern thin films and coatings. The constant reduction in thickness of coatings, as well as the increase in development of softer Žpolymeric. coatings has meant that a whole range of indenter geometries are needed to cover the scope of elastic and plastic properties measured by scratch testing. This paper considers the effect of intrinsic parameters such as scratching speed, loading rate and diamond indenter radius on the measured critical load for scratch tests performed on various different coating᎐substrate combinations ŽTiN, W, DLC, Al and Au.. Results are presented which suggest definite correlations between critical load and indenter radius, and which should aid users in making the right choice of indenter for a given coating᎐substrate system. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: ŽB. Scratch test; ŽX. Critical load
1. Introduction In conventional scratch testing, scratches are generated on the sample using a diamond stylus Žusually a Rockwell C profile of radius 200 m with the Revetest w1x., which is drawn across the sample under either constant or progressively increasing load. Elastic andror plastic deformation occurs at specific points
U
Corresponding author. Tel.: q41-32-7205-448; fax: q41-32-7205730. E-mail address: [email protected] ŽN.X. Randall..
along the scratch path, such critical points being observed by optical microscopy or by variations in acoustic emission and frictional force w2᎐5x. Not all the observed failure events in scratch testing are related to detachment at the coating᎐substrate interface and only certain, therefore, can be truly used as a measure of adhesion. Other types of failure, such as cohesive damage within the coating or substrate, may be equally important in evaluating the in-service behaviour of a coated component in a specific application. In many cases, the scratch test has now become accepted as a versatile tool for assessing the mechanical integrity of a surface, whether bulk or coated, and
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 0 9 7 - 5
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
has found application in many different fields of materials research w6᎐9x. The driving forces for the failure of the coating᎐substrate system in the scratch test are a combination of elastic᎐plastic indentation stresses, frictional stresses and the residual internal stress present in the coating. The normal load at which failure occurs is called the critical load, Lc . When a progressive load scratch is performed, a number of consecutive coating failure events may be observed as the load is increased, the final event corresponding usually to total delamination. The critical load depends on coating adhesion, but also on several other parameters; some are directly related to the test itself Žintrinsic parameters. whereas others are related to the coating᎐substrate combination Žextrinsic parameters.. Experience in the scratch testing field has shown that various different radii of diamond indenter are often required in order to adequately characterise the adhesion of modern thin films and coatings. The constant reduction in thickness of coatings, as well as the increase in development of softer Žpolymeric. coatings has meant that a whole range of indenter geometries are needed to cover the scope of elastic and plastic properties measured by scratch testing. Previous work w10x has shown that the average pressure, Pm , applied by a spherical indenter tip can be approximated to the substrate hardness, H, by the equation: Pm f H s
2 Lc b2
Ž1.
where b is the half-width of the scratch. Because b is proportional to the indenter tip radius, R, Eq. Ž1. shows that the critical load must be proportional to R 2 . Steinmann et al. w2x showed experimentally that the critical load variation is not proportional to R 2 , but varies from R 0.85 to R1.10 , depending on the value of d Lrd x Žthe ratio between the loading rate, d Lrdt, and the scratching speed, d xrdt .. It is interesting to note that if the ratio d Lrd x remains constant then the critical load is independent of the loading rate and the scratching speed.
2. Experimental 2.1. Influence of tip size on critical load The samples used for this study consisted of TiN and W deposited on steel substrates, and diamond-like carbon ŽDLC., Al and Au deposited on silicon substrates. Full details for all samples are summarised in Table 1. A combination of materials was chosen which covered a range of properties, e.g. elasticrplastic deformation,
147
Table 1 Summary of tested coating᎐substrate combinations a Coating
Deposition method
Thickness Žm.
Substrate
TiN W DLC Al Au
CVD CVD CVD Sputtering Sputtering
1.50 1.20 0.56 0.50 0.50
440C steel 440C steel Silicon wafer Silicon wafer Silicon wafer
a
Pure Si was chosen as the substrate for the thinner coatings in order to have a very low surface roughness Žnegligible influence on the critical loads..
hardness, coating᎐substrate interfacial strength, etc., and whose thicknesses were adapted in order to use the widest range of contact conditions and diamond indenter radii. The instrument used was a CSEM Micro Scratch Tester ŽMST. which has a load range up to 30 N. Spherical diamond indenters were used with radii of 20, 50, 100 and 500 m. Five scratches were performed with each indenter on each sample and average values of critical load were calculated. For comparison of critical load as a function of indenter radius, the scratch length was maintained at 5 mm and the loading rate was kept constant for all measurements at 30 N miny1 . Either two or three critical loads were determined for each sample type by optical microscopy inspection of the damaged area after scratching. In some cases, the acoustic emission signal was also used to confirm the measured values. 2.2. Influence of scratching speed and loading rate on critical load The samples used for this study consisted of a commercial paint coating on an aluminium substrate that had been anodised in sulfuric acid to provide better interfacial bonding. This coating᎐substrate system was chosen because it has a much lower critical load than hard coatings Ž- 10 N with a standard diamond indenter of radius 20 m. but exhibits brittle fracture making it easy to determine the critical load from optical microscopy and from fluctuations in the frictional signal. The influence of the scratching speed was investigated by varying over the range 2᎐10 mm miny1 , whilst maintaining the scratch length at 4 mm. Critical load variations were measured over the entire range for different loading rates Ž10᎐30 N miny1 .. The influence of the loading rate was investigated by varying over the range 2᎐10 N miny1 for three different values of scratching speed Ž1᎐5 mm miny1 .. Three scratches were made for each critical load point and an average value computed.
148
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
Fig. 1. Optical micrographs of critical failure points along a progressive load scratch performed on a Tungsten sample. The first failure, Lc1 Ža., corresponds to initial cracking, the second failure, Lc2 Žb., to extensive cracking and final failure, Lc3 Žc. to partial delamination of the coating from the steel substrate. Scratch direction is from left to right.
3. Results and discussion The optical micrographs in Fig. 1 show the three distinct critical points measured on the Tungsten sam-
ple. Such points, corresponding to initial cracking, extensive cracking and final failure were highly reproducible and remained the same even when the radius of the diamond indenter was varied. However, for all tested samples, the point of initial cracking seemed more marked for small radii Ži.e. 20 m. than for large radii Ži.e. 500 m.. This can be explained by the fact that larger radii spread the load over a greater area of the coating and therefore produce lower pressure zones around the advancing indenter tip. It should be noted that the critical failure points were less distinct Žby optical microscopy. for the softer and less brittle coatings tested, namely Al and Au. Some typical micrographs of critical points measured on the TiN sample are shown in Fig. 3. The evolution of critical load as a function of indenter radius is shown graphically for each tested material in Figs. 2 and 3. As a general rule, the measured critical load is seen to increase quite dramatically as the indenter radius is increased. In some cases Že.g. W., the critical load increases by an order of magnitude between a 20-m and a 200-m indenter radius. This clearly demonstrates the importance of indenter choice when making comparative measurements. Numerical simulations w11x have shown that a smaller radius produces less interfacial shearing of the coating material due to less confinement of the material between the indenter and the substrate. Other studies w2x suggest that the indenter radius should be as large as possible with respect to the coating thickness in order to assure sufficient deformation of the substrate. However, the elastic recovery is higher when large radius indenters are used because of the hydrostatic nature of deformation. From contact mechanics w12,13x can be derived relationships between indenter radius and the distance of the stress maximum from the surface; for adhesion testing of coatings this has important implications because such a point of maximum stress should be positioned at the coating᎐substrate interface to encourage delamination. The choice of indenter radius is also subject to the load application range of the instrument used; large radius indenters require more load because the contact area is greater over the surface of the coating material and also because the confinement produces a greater pressure in the coating. The variation of the critical load as a function of scratching speed is displayed in Fig. 4a for three different loading rate values Žcovering the range 10᎐30 N miny1 .. The critical load in each case is seen to decrease as the scratching speed is increased. Additionally, the overall critical load values increase with increasing loading rate. The variation of the critical load as a function of loading rate is displayed in Fig. 4b for three different scratching speed values Žcovering
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
149
Fig. 2. Variation in the critical load as a function of different indenter radii for each critical failure point measured on Ža. TiN, Žb. W, Žc. DLC, Žd. Al and Že. Au.
the range 1᎐5 mm miny1 .. For the lower scratching speed values, the critical load is seen to increase significantly as the loading rate is increased. However, for the 5 mm miny1 scratching speed, the critical load remains virtually constant over the loading rate range measured. Note that in both sets of measurements shown in Fig. 4, the scratch length was maintained at 4 mm so this factor can be excluded in the subsequent discussion as a potential influence on the results. These results correlate well with previous work carried out on TiC and TiN coatings using a CSEM Revetest instrument Žload range up to 200 N. w2x which also showed an increase in critical load as the loading rate increases or the table speed decreases. However, other studies on plasma-sprayed zirconia coatings w14x have shown that the loading rate and scratching speed have no prominent effect on the critical load Žthese results were also measured on a higher
load Revetest instrument .. It is clear that any change in the loading rate or the scratching speed results in a variation in the load applied per unit scratch length. By decreasing the value of d Lrd x, it has been suggested w2x that the probability of the indenter encountering defective adhesion in the coating increases because the same load range is applied over a longer scratch length. The probability of the indenter meeting a point having poor adhesion depends on the frequency of the occurrence of such points along the scratch length. Therefore, for the coating to debond at a weakly bonded area, the indenter should reach such a point and the applied load at that point should be greater than or equal to that required to overcome the bond strength of the coating᎐substrate interface. The instantaneous load measured at this point will thus correspond to the measured critical load. It can be summarised that the dependence of critical
150
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
caused the scratch length to change and so comparative results would not have been produced.
4. Conclusions This study has investigated the effect of intrinsic parameters Žnamely scratching speed, loading rate and diamond indenter radius. on the critical load, as measured using a CSEM Micro Scratch Tester Žload range - 30 N., and follows on from previous studies made using a CSEM Revetest Žload range - 200 N.. The following conclusions may by drawn: 1. The measured critical load increases quite dramatically as the indenter radius is increased. In some cases Že.g. W., the critical load increases by an order of magnitude between a 20- and a 200-m indenter radius. For comparative measurements, the choice of indenter geometry is obviously of fundamental importance.
Fig. 3. Optical micrographs of critical failure points along a progressive load scratch performed on a TiN sample. The first failure, Lc1 Ža., corresponds to first chipping and the second failure, Lc2 Žb. to extensive chipping along the sides of the scratch path. Scratch direction is from left to right.
load on loading rate and scratching speed is highly influenced by the particular coating᎐substrate material pair measured. It is interesting to note that the samples measured in this study consisted of a polymeric paint coating on an aluminium substrate. The variation of the critical load as a function of loading rate Žincreasing at low speeds but constant at higher speeds. can be attributed to the rate dependence of the scratch test for particular types of material. The plastic deformation of the aluminium substrate could also be a contributory factor but is not thought to exert significant influence over the loading rate range considered. Additionally, such paint coatings are brittle and therefore relieve the strain during scratching by cracking and delamination; at the 5 mm miny1 scratching speed, the strain hardening becomes negligible which accounts for the critical load remaining virtually constant with varying loading rate. It was not possible to investigate higher values of scratching speed as these would have
Fig. 4. Variation of the critical load as a function of Ža. scratching speed and Žb. loading rate for measurements made with a spherical diamond indenter of radius 20 m on a commercial paint coating.
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
2. The measured critical load is found to decrease as the scratching speed is increased, but is found to increase significantly as the loading rate is increased. However, the scratch test is observed to be rate-dependent and variations from the observed trends are possible for certain coating᎐substrate combinations. 3. The effect of intrinsic parameters on the critical load are seen to follow the same correlation for the Micro Scratch Tester than for the Revetest instrument. 4. When quoting practical values of adhesion, it is imperative that all the parameters which can influence the results be mentioned if true comparisons are to be made. Future work is envisaged to investigate the effects of extrinsic parameters on the critical load, such as substrate hardness, coating thickness, surface roughness, tangential force and acoustic emission. Such studies should help to pave the way for better standardisation of scratch testing methods and allow different instrument users to compare results more accurately. Additional measurement parameters, such as the penetration depth during and after scratch testing, will also aid
151
interpretation of results and provide a better understanding of failure modes encountered. References w1x C. Julia-Schmutz, H.E. Hintermann, Surf. Coat. Technol. 48 Ž1991. 1. w2x P.A. Steinmann, Y. Tardy, H.E. Hintermann, Thin Solid Films 154 Ž1987. 333. w3x J. Valli, J. Vac. Sci. Technol. A 4 Ž1986. 3007. w4x P.A. Steinmann, P. Laeng, H.E. Hintermann, Mater. Tech. 13 Ž1985. 85. w5x P.J. Burnett, D.S. Rickerby, Thin Solid Films 157 Ž1988. 233. w6x R. Consiglio, N.X. Randall, B. Bellaton, J. vonStebut, Thin Solid Films 332 Ž1998. 151. w7x A.J. Perry, Thin Solid Films 107 Ž1983. 167. w8x J. Valli, V. Makela, A. Matthews, Surf. Eng. 2 Ž1986. 49. w9x K. Adamsons, G. Blackman, B. Gregorovich, L. Lin, R. Matheson, Prog. Org. Coat. 34 Ž1998. 64. w10x J. Ahn, K.L. Mittal, R.H. MacQueen, Adhesion measurement of thin films, thick films and bulk coatings, ASTM 640 Ž1978. 134. w11x R. Jayachandran, M.C. Boyce, A.S. Argon, J. Adhes. Sci. Tech. 7 Ž8. Ž1993. 189. w12x G.M. Hamilton, Proc. Instn. Mech. Eng. 197C Ž1983. 53. w13x G.M. Hamilton, L.E. Goodman, J. Appl. Mech. 33 Ž1966. 371. w14x D.K. Das, M.P. Srivastava, S.V. Joshi, R. Sivakumar, Surf. Coat. Technol. 46 Ž1991. 331.
The effect of intrinsic parameters on the critical load as measured with the scratch test method N.X. Randall a,U , G. Favaro a , C.H. Frankel b a
CSEM Instruments, Jaquet-Droz 1, CH-2007 Neuchatel, ˆ Switzerland b Pennsyl¨ ania State Uni¨ ersity, Uni¨ ersity Park, PA 16802, USA
Received 13 March 2000; received in revised form 29 September 2000; accepted 29 September 2000
Abstract In the scratch test method, scratches are generated on the coated sample using a diamond indenter Žusually a Rockwell C profile. which is drawn across the surface under either constant or progressively increasing load. The sample is displaced at constant speed and at a certain load damage occurs along the scratch path. This value of critical load, Lc , can be used to accurately characterise the adhesive strength of the coating᎐substrate system. With modern scratch testing instruments, the critical load can be determined by acoustic emission, optical microscopy, variation in penetration depth or variation in the tangential frictional force between tip and sample. However, it is difficult to express the adherence of a coating᎐substrate system in a quantitative way because the critical load depends on several parameters related to the testing conditions. Experience in the scratch testing field has shown that various different radii of diamond indenter are often required in order to adequately characterise the adhesion of modern thin films and coatings. The constant reduction in thickness of coatings, as well as the increase in development of softer Žpolymeric. coatings has meant that a whole range of indenter geometries are needed to cover the scope of elastic and plastic properties measured by scratch testing. This paper considers the effect of intrinsic parameters such as scratching speed, loading rate and diamond indenter radius on the measured critical load for scratch tests performed on various different coating᎐substrate combinations ŽTiN, W, DLC, Al and Au.. Results are presented which suggest definite correlations between critical load and indenter radius, and which should aid users in making the right choice of indenter for a given coating᎐substrate system. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: ŽB. Scratch test; ŽX. Critical load
1. Introduction In conventional scratch testing, scratches are generated on the sample using a diamond stylus Žusually a Rockwell C profile of radius 200 m with the Revetest w1x., which is drawn across the sample under either constant or progressively increasing load. Elastic andror plastic deformation occurs at specific points
U
Corresponding author. Tel.: q41-32-7205-448; fax: q41-32-7205730. E-mail address: [email protected] ŽN.X. Randall..
along the scratch path, such critical points being observed by optical microscopy or by variations in acoustic emission and frictional force w2᎐5x. Not all the observed failure events in scratch testing are related to detachment at the coating᎐substrate interface and only certain, therefore, can be truly used as a measure of adhesion. Other types of failure, such as cohesive damage within the coating or substrate, may be equally important in evaluating the in-service behaviour of a coated component in a specific application. In many cases, the scratch test has now become accepted as a versatile tool for assessing the mechanical integrity of a surface, whether bulk or coated, and
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 0 9 7 - 5
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
has found application in many different fields of materials research w6᎐9x. The driving forces for the failure of the coating᎐substrate system in the scratch test are a combination of elastic᎐plastic indentation stresses, frictional stresses and the residual internal stress present in the coating. The normal load at which failure occurs is called the critical load, Lc . When a progressive load scratch is performed, a number of consecutive coating failure events may be observed as the load is increased, the final event corresponding usually to total delamination. The critical load depends on coating adhesion, but also on several other parameters; some are directly related to the test itself Žintrinsic parameters. whereas others are related to the coating᎐substrate combination Žextrinsic parameters.. Experience in the scratch testing field has shown that various different radii of diamond indenter are often required in order to adequately characterise the adhesion of modern thin films and coatings. The constant reduction in thickness of coatings, as well as the increase in development of softer Žpolymeric. coatings has meant that a whole range of indenter geometries are needed to cover the scope of elastic and plastic properties measured by scratch testing. Previous work w10x has shown that the average pressure, Pm , applied by a spherical indenter tip can be approximated to the substrate hardness, H, by the equation: Pm f H s
2 Lc b2
Ž1.
where b is the half-width of the scratch. Because b is proportional to the indenter tip radius, R, Eq. Ž1. shows that the critical load must be proportional to R 2 . Steinmann et al. w2x showed experimentally that the critical load variation is not proportional to R 2 , but varies from R 0.85 to R1.10 , depending on the value of d Lrd x Žthe ratio between the loading rate, d Lrdt, and the scratching speed, d xrdt .. It is interesting to note that if the ratio d Lrd x remains constant then the critical load is independent of the loading rate and the scratching speed.
2. Experimental 2.1. Influence of tip size on critical load The samples used for this study consisted of TiN and W deposited on steel substrates, and diamond-like carbon ŽDLC., Al and Au deposited on silicon substrates. Full details for all samples are summarised in Table 1. A combination of materials was chosen which covered a range of properties, e.g. elasticrplastic deformation,
147
Table 1 Summary of tested coating᎐substrate combinations a Coating
Deposition method
Thickness Žm.
Substrate
TiN W DLC Al Au
CVD CVD CVD Sputtering Sputtering
1.50 1.20 0.56 0.50 0.50
440C steel 440C steel Silicon wafer Silicon wafer Silicon wafer
a
Pure Si was chosen as the substrate for the thinner coatings in order to have a very low surface roughness Žnegligible influence on the critical loads..
hardness, coating᎐substrate interfacial strength, etc., and whose thicknesses were adapted in order to use the widest range of contact conditions and diamond indenter radii. The instrument used was a CSEM Micro Scratch Tester ŽMST. which has a load range up to 30 N. Spherical diamond indenters were used with radii of 20, 50, 100 and 500 m. Five scratches were performed with each indenter on each sample and average values of critical load were calculated. For comparison of critical load as a function of indenter radius, the scratch length was maintained at 5 mm and the loading rate was kept constant for all measurements at 30 N miny1 . Either two or three critical loads were determined for each sample type by optical microscopy inspection of the damaged area after scratching. In some cases, the acoustic emission signal was also used to confirm the measured values. 2.2. Influence of scratching speed and loading rate on critical load The samples used for this study consisted of a commercial paint coating on an aluminium substrate that had been anodised in sulfuric acid to provide better interfacial bonding. This coating᎐substrate system was chosen because it has a much lower critical load than hard coatings Ž- 10 N with a standard diamond indenter of radius 20 m. but exhibits brittle fracture making it easy to determine the critical load from optical microscopy and from fluctuations in the frictional signal. The influence of the scratching speed was investigated by varying over the range 2᎐10 mm miny1 , whilst maintaining the scratch length at 4 mm. Critical load variations were measured over the entire range for different loading rates Ž10᎐30 N miny1 .. The influence of the loading rate was investigated by varying over the range 2᎐10 N miny1 for three different values of scratching speed Ž1᎐5 mm miny1 .. Three scratches were made for each critical load point and an average value computed.
148
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
Fig. 1. Optical micrographs of critical failure points along a progressive load scratch performed on a Tungsten sample. The first failure, Lc1 Ža., corresponds to initial cracking, the second failure, Lc2 Žb., to extensive cracking and final failure, Lc3 Žc. to partial delamination of the coating from the steel substrate. Scratch direction is from left to right.
3. Results and discussion The optical micrographs in Fig. 1 show the three distinct critical points measured on the Tungsten sam-
ple. Such points, corresponding to initial cracking, extensive cracking and final failure were highly reproducible and remained the same even when the radius of the diamond indenter was varied. However, for all tested samples, the point of initial cracking seemed more marked for small radii Ži.e. 20 m. than for large radii Ži.e. 500 m.. This can be explained by the fact that larger radii spread the load over a greater area of the coating and therefore produce lower pressure zones around the advancing indenter tip. It should be noted that the critical failure points were less distinct Žby optical microscopy. for the softer and less brittle coatings tested, namely Al and Au. Some typical micrographs of critical points measured on the TiN sample are shown in Fig. 3. The evolution of critical load as a function of indenter radius is shown graphically for each tested material in Figs. 2 and 3. As a general rule, the measured critical load is seen to increase quite dramatically as the indenter radius is increased. In some cases Že.g. W., the critical load increases by an order of magnitude between a 20-m and a 200-m indenter radius. This clearly demonstrates the importance of indenter choice when making comparative measurements. Numerical simulations w11x have shown that a smaller radius produces less interfacial shearing of the coating material due to less confinement of the material between the indenter and the substrate. Other studies w2x suggest that the indenter radius should be as large as possible with respect to the coating thickness in order to assure sufficient deformation of the substrate. However, the elastic recovery is higher when large radius indenters are used because of the hydrostatic nature of deformation. From contact mechanics w12,13x can be derived relationships between indenter radius and the distance of the stress maximum from the surface; for adhesion testing of coatings this has important implications because such a point of maximum stress should be positioned at the coating᎐substrate interface to encourage delamination. The choice of indenter radius is also subject to the load application range of the instrument used; large radius indenters require more load because the contact area is greater over the surface of the coating material and also because the confinement produces a greater pressure in the coating. The variation of the critical load as a function of scratching speed is displayed in Fig. 4a for three different loading rate values Žcovering the range 10᎐30 N miny1 .. The critical load in each case is seen to decrease as the scratching speed is increased. Additionally, the overall critical load values increase with increasing loading rate. The variation of the critical load as a function of loading rate is displayed in Fig. 4b for three different scratching speed values Žcovering
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
149
Fig. 2. Variation in the critical load as a function of different indenter radii for each critical failure point measured on Ža. TiN, Žb. W, Žc. DLC, Žd. Al and Že. Au.
the range 1᎐5 mm miny1 .. For the lower scratching speed values, the critical load is seen to increase significantly as the loading rate is increased. However, for the 5 mm miny1 scratching speed, the critical load remains virtually constant over the loading rate range measured. Note that in both sets of measurements shown in Fig. 4, the scratch length was maintained at 4 mm so this factor can be excluded in the subsequent discussion as a potential influence on the results. These results correlate well with previous work carried out on TiC and TiN coatings using a CSEM Revetest instrument Žload range up to 200 N. w2x which also showed an increase in critical load as the loading rate increases or the table speed decreases. However, other studies on plasma-sprayed zirconia coatings w14x have shown that the loading rate and scratching speed have no prominent effect on the critical load Žthese results were also measured on a higher
load Revetest instrument .. It is clear that any change in the loading rate or the scratching speed results in a variation in the load applied per unit scratch length. By decreasing the value of d Lrd x, it has been suggested w2x that the probability of the indenter encountering defective adhesion in the coating increases because the same load range is applied over a longer scratch length. The probability of the indenter meeting a point having poor adhesion depends on the frequency of the occurrence of such points along the scratch length. Therefore, for the coating to debond at a weakly bonded area, the indenter should reach such a point and the applied load at that point should be greater than or equal to that required to overcome the bond strength of the coating᎐substrate interface. The instantaneous load measured at this point will thus correspond to the measured critical load. It can be summarised that the dependence of critical
150
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
caused the scratch length to change and so comparative results would not have been produced.
4. Conclusions This study has investigated the effect of intrinsic parameters Žnamely scratching speed, loading rate and diamond indenter radius. on the critical load, as measured using a CSEM Micro Scratch Tester Žload range - 30 N., and follows on from previous studies made using a CSEM Revetest Žload range - 200 N.. The following conclusions may by drawn: 1. The measured critical load increases quite dramatically as the indenter radius is increased. In some cases Že.g. W., the critical load increases by an order of magnitude between a 20- and a 200-m indenter radius. For comparative measurements, the choice of indenter geometry is obviously of fundamental importance.
Fig. 3. Optical micrographs of critical failure points along a progressive load scratch performed on a TiN sample. The first failure, Lc1 Ža., corresponds to first chipping and the second failure, Lc2 Žb. to extensive chipping along the sides of the scratch path. Scratch direction is from left to right.
load on loading rate and scratching speed is highly influenced by the particular coating᎐substrate material pair measured. It is interesting to note that the samples measured in this study consisted of a polymeric paint coating on an aluminium substrate. The variation of the critical load as a function of loading rate Žincreasing at low speeds but constant at higher speeds. can be attributed to the rate dependence of the scratch test for particular types of material. The plastic deformation of the aluminium substrate could also be a contributory factor but is not thought to exert significant influence over the loading rate range considered. Additionally, such paint coatings are brittle and therefore relieve the strain during scratching by cracking and delamination; at the 5 mm miny1 scratching speed, the strain hardening becomes negligible which accounts for the critical load remaining virtually constant with varying loading rate. It was not possible to investigate higher values of scratching speed as these would have
Fig. 4. Variation of the critical load as a function of Ža. scratching speed and Žb. loading rate for measurements made with a spherical diamond indenter of radius 20 m on a commercial paint coating.
N.X. Randall et al. r Surface and Coatings Technology 137 (2001) 146᎐151
2. The measured critical load is found to decrease as the scratching speed is increased, but is found to increase significantly as the loading rate is increased. However, the scratch test is observed to be rate-dependent and variations from the observed trends are possible for certain coating᎐substrate combinations. 3. The effect of intrinsic parameters on the critical load are seen to follow the same correlation for the Micro Scratch Tester than for the Revetest instrument. 4. When quoting practical values of adhesion, it is imperative that all the parameters which can influence the results be mentioned if true comparisons are to be made. Future work is envisaged to investigate the effects of extrinsic parameters on the critical load, such as substrate hardness, coating thickness, surface roughness, tangential force and acoustic emission. Such studies should help to pave the way for better standardisation of scratch testing methods and allow different instrument users to compare results more accurately. Additional measurement parameters, such as the penetration depth during and after scratch testing, will also aid
151
interpretation of results and provide a better understanding of failure modes encountered. References w1x C. Julia-Schmutz, H.E. Hintermann, Surf. Coat. Technol. 48 Ž1991. 1. w2x P.A. Steinmann, Y. Tardy, H.E. Hintermann, Thin Solid Films 154 Ž1987. 333. w3x J. Valli, J. Vac. Sci. Technol. A 4 Ž1986. 3007. w4x P.A. Steinmann, P. Laeng, H.E. Hintermann, Mater. Tech. 13 Ž1985. 85. w5x P.J. Burnett, D.S. Rickerby, Thin Solid Films 157 Ž1988. 233. w6x R. Consiglio, N.X. Randall, B. Bellaton, J. vonStebut, Thin Solid Films 332 Ž1998. 151. w7x A.J. Perry, Thin Solid Films 107 Ž1983. 167. w8x J. Valli, V. Makela, A. Matthews, Surf. Eng. 2 Ž1986. 49. w9x K. Adamsons, G. Blackman, B. Gregorovich, L. Lin, R. Matheson, Prog. Org. Coat. 34 Ž1998. 64. w10x J. Ahn, K.L. Mittal, R.H. MacQueen, Adhesion measurement of thin films, thick films and bulk coatings, ASTM 640 Ž1978. 134. w11x R. Jayachandran, M.C. Boyce, A.S. Argon, J. Adhes. Sci. Tech. 7 Ž8. Ž1993. 189. w12x G.M. Hamilton, Proc. Instn. Mech. Eng. 197C Ž1983. 53. w13x G.M. Hamilton, L.E. Goodman, J. Appl. Mech. 33 Ž1966. 371. w14x D.K. Das, M.P. Srivastava, S.V. Joshi, R. Sivakumar, Surf. Coat. Technol. 46 Ž1991. 331.