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An experimental method for coating–substrate interface investigation
Materials Characterization 55 (2005) 173 – 178
An experimental method for coating–substrate interface investigation Bojan Podgornik a,*, Olle Wa¨nstrand b a
Centre for Tribology and Technical Diagnostics, University of Ljubljana, Bogisiceva 8, SI-1000 Ljubljana, Slovenia b Balzers AG, Arstaa¨ngsva¨gen 31 E, S-117 43 Stockholm, Sweden Received 21 January 2005; received in revised form 12 March 2005; accepted 15 March 2005
Abstract Investigations of coated surfaces indicate that in many cases the coating–substrate interface is the weakest part of the coated component, with the coating-to-substrate adhesion being used to evaluate the strength of the coating–substrate interface. While modeling of the coated surface depends on coating and substrate material properties, which are not easy to determine, standard experimental methods do not allow a direct study of the interface. The aim of the present paper is to describe a supplementary method for coating–substrate interface investigation, which can provide further information on the interface properties. D 2005 Elsevier Inc. All rights reserved. Keywords: Coatings; Interface; Adhesion; Scratch test
1. Introduction There has been increased interest in the use of tribological coatings on mechanical components in engines and transmissions, on tools in the manufacturing industry, on disc drives in the computer industry, on precision instruments, and on artificial human joints [1]. New coating deposition techniques developed over the last two decades offer a wide variety of
* Corresponding author. Tel.: +38 6 1 4771 463; fax: +38 6 1 4771 469. E-mail address: [email protected] (B. Podgornik). 1044-5803/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.04.011
possibilities to tailor surfaces with many different materials and structures. In particular, chemical vapour deposition (CVD) and physical vapour deposition (PVD) techniques have made it possible to deposit thin coatings in the micrometre range even at room temperature [2]. A tribological contact with two loaded surfaces in relative motion is a very complex system, which becomes even more complex when coatings are introduced on the surface [1,2]. Certainly, it is not easy to understand nor simulate or predict tribological behaviour and performance of coated surfaces. However, different investigations have indicated [3– 5] that in many cases the coating–substrate interface
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B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
represents the weakest part of the coated component, with the coating-to-substrate adhesion being used to evaluate the strength of the coating–substrate interface. Different models [6,7] and experimental methods [8,9] are used nowadays to evaluate the strength of the coating–substrate interface, with Scratch-testing [9,10] and Rockwell C indentation [11] being the most widely used. However, while modeling depends on coating and substrate material properties, which are not easy to determine [12], experimental methods do not allow a direct microscopic study of the interface. The aim of the present paper is to present a supplementary method for coating–substrate interface investigation, which can provide further information on the interface properties.
ng ishi Pol ction dire
Fig. 2. Polishing of the scratched sample.
of the analyzed area relative to the scratch channel end, it is possible to correlate SEM pictures, findings and features to the load, as shown in Fig. 3. Another way of preparing the sample, which would not require any polishing, would be to cut the sample in the middle of the scratch using an Focus Ion beam equipment.
3. Practical use of the method 2. Method description The proposed method for coating–substrate interface investigation is based on a standard scratch test, where an up to 10 mm long scratch is made on a coated sample under a linearly increasing load, using a diamond tip. In a second step the scratched sample is cut at the border of the scratch parallel to the scratch, as shown in Fig. 1. The sample, which has been cut off, is then mounted in a holder with a micrometre screw, as used for TEM sample preparation, and carefully polished until the middle of the scratch channel is reached (Fig. 2). After polishing, the sample can be nital etched and analyzed by high resolution scanning electron microscope (SEM). By knowing loading rate and position
In order to demonstrate the applicability of the above described experimental method, it was applied on a number of PVD coated steel samples. 3.1. Sample preparation Samples selected for this experiment were made of AISI 4140 steel, quenched and tempered (650 8C) to a mean hardness of 300 HV0.5 and ground to a R a value of ~ 0.4 Am. Four groups of samples, denoted A–D, were prepared in terms of thermochemical treatment (Table 1) [3], and coated by monolayer TiN and multilayer TiAlN coating, respectively. Details of the coating deposition process are given in Table 2 and Ref. [3].
10 mm
0 10 20 30 40 50 60 70 80 90 100 Load [N] Fig. 1. Cutting of the scratched sample.
Fig. 3. Corresponding loads for the loading rate of 10 N/mm.
B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
175
Table 1 Details of substrate treatment processes and resulting surface hardness [3] Treatment Plasma nitriding + 5 Am gV Plasma nitriding Hardening Tempering
A B C D
Atmosphere
Temperature [8C]
Time [h]
Case depth [mm]
Hardness [HV0.1]
75%H2–25%N2 99.4%H2–0.6%N2 oil oil
540 540 870/250 870/650
28 17 2/1 2/1
0.55 0.3 through through
935 F 40 705 F 25 615 F 20 300 F 10
Table 2 Deposition parameters and resulting coating properties [3] Coating
Process
Temperature [8C]
TiN TiAlN
Sputtering Sputtering
450 450
Bias [V] 200 200
Interlayer [Am]
Hardness [GPa]
Young’s modulus [GPa]
Ti 0.1 Ti 0.1
28.5 F 1.8 31.4 F 2.8
386 F 26 407 F 37
3.2. Experimental procedure
mond stylus and a loading rate of 10 N/mm was used to form grooves on the above described samples. Standard evaluation of the results included recording of the acoustic emission signal, caused
A commercial scratch tester, the CSEM Revetest, equipped with a 200 Am radius Rockwell-C dia-
(b)
(d)
(c)
2500 A B C D
(e)
2000
1500
1000
AE [x10 db]
(a)
500
0
10
20
30
40
50
60
70
80
90
0 100
FN [N]
Fig. 4. (a) Acoustic emission signals during scratch testing of TiN-coated AISI 4140 steel and corresponding coating failures: (b) tensile cracking, (c) coating debris removal at the rim of the scratch, (d) complete removal of the coating and (e) coating debris removal at the bottom of the scratch-hardened substrate.
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B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
purposes standard Rockwell-C adhesion tests were also carried out [11]. 3.3. Results
Fig. 5. Critical loads of the first failure (L c1) and total removal (L c2) of the coating.
by crack formation, and a post-test optical microscope determination of critical loads L c1 (first adhesive or cohesive failure) and L c2 (complete delamination of the coating) [9,10]. For comparison
Fig. 4 shows an acoustic emission signal for a TiN coating deposited on different thermochemically treated steel substrates (Fig. 4a) and corresponding coating failure modes (Fig. 4b–e). In the case of the plasma-nitrided substrate (treatment A and B) tensile cracking of the coating (Fig. 4b) propagating from the bottom of the scratch towards the scratch-channel rims was observed at a critical load of approximately 35 N. An increased load led to coating debris removal, observed at the rim of the scratch (Fig. 4c), and finally to complete removal of the coating, as shown in Fig. 4d. A similar coating failure mechanism with coating debris removal was also observed at the bottom of the scratch channel (Fig. 4e) for the hardened substrate
Fig. 6. Subsurface coating failures of TiAlN coating deposited on hardened steel; (a) cracking, (b) delamination and (c) flaking.
B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
177
Fig. 7. Scratch-channel cross-sections of TiAlN coating on (a) hardened-steel substrate at a load of 35 N and (b) 20 N, and (c) plasma-nitrided steel substrate without and (d) with compound layer at a load of 40 N.
(treatment C and D). Hardening, however, was found to decrease critical loads compared to plasma nitriding, as shown in Fig. 5. For both TiN and TiAlN coatings the same coating failure mechanisms with comparable critical loads were observed (Fig. 5). The rockwell-C adhesion test confirmed improved coating-to-substrate adhesion obtained by plasma nitriding of the substrate. The coating deposited on a hardened substrate peeled off circularly around an indentation mark, while only cracks spreading out radially from the indentation mark were observed for the coating deposited on the plasma-nitrided substrate with or without a compound layer. Regarding Rockwell-C indentation results, the coating-to-substrate adhesion can be defined as sufficient for all substrate treatments used. However, for the plasmanitrided substrate (treatment A and B) coating adhe-
Fig. 8. Crack deflection at the porous zone of the compound layer.
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sion is classified as HF1 and for the hardened and tempered ones (treatment C and D) as HF3 [11]. In order to investigate the origin of the coating failure, cross-sections of scratched samples were analysed with a field-emission-gun scanning electron microscope (FEG-SEM) LEO 1550. Fig. 6 shows that coating failure in the shape of cracks (Fig. 6a), delamination (Fig. 6b) or flaking (Fig. 6c) normally starts at the interface, as observed for both coatings (TiN and TiAlN). Comparison of different substrate pre-treatments also showed that regardless of the substrate pre-treatment used, coating failure starts at the interface [3,13]. Therefore, for all coating–substrate pre-treatment combinations, the interface was found to be the weakest part of the composite. Plasma nitriding, however, was found to improve the substrate load-carrying capacity with reduced plastic deformation of the substrate, as shown in Fig. 7. In the case of a hardened or tempered substrate (treatments C and D) considerable plastic deformation, with coating particles embedded into the substrate material, was observed at loads below 40 N (Fig. 7a), while the coated plasma-nitrided substrate (treatment B) displayed reduced plastic deformation of the substrate without coating spallation (Fig. 7c). Large plastic deformation of the hardened substrate also caused cracking of the coating along the scratch, as shown in Fig. 7b. On the other hand, superior load-carrying capacity without any plastic deformation even at loads over 50 N was obtained in the case of the plasma-nitrided substrate with an intermediate compound layer (treatment A), as shown in Fig. 7d. Although the compound layer displayed some porosity at the surface it gives very good coating-to-substrate adhesion (Figs. 4 and 5). In addition, it seems that in a similar way to the interfaces in multilayer coatings it can increase the cracking resistance of the coating [5]. The porous zone of the compound layer prevents the cracks that start at the surface from propagating directly into the substrate and vice versa (Fig. 8).
4. Conclusions ! The rockwell-C adhesion test gives us information about coating–substrate interface strength or coating-to-substrate adhesion, however, it is only an
indentation method, which provides qualitative information, limited to 6 stages (HF1–HF6). ! The scratch-test gives qualitative and quantitative information about coating–substrate interface strength under relative motion. However, it does not give direct insight into the process taking place at the interface, i.e. crack initiation and propagation. ! The preparation of scratch-channel cross-sections and their analysis represents an additional tool and possibilities in the process of coating–substrate interface investigation.
References [1] Holmberg K, Laukkanen A, Ronkainen H. Tribological optimisation of coated components by stress modelling and surface fracture assessment. Proc. COST 532 Conf.: Triboscience and Tribotechnology. Ghent, Belgium, October 18–19. [2] Holmberg K, Matthews A. Coatings tribology: properties, technologies and applications in surface engineering. NY7 Elsevier; 1994. [3] Podgornik B. Influence of substrate pre-treatment on the tribological properties of hard coatings during sliding, PhD thesis, University of Ljubljana, Ljubljana, Slovenia; 2000. [4] Mishina H. Microscopic real observation of failure, wear and deformation on coating/subsurface. Tribol Int 1999; 32:137 – 44. [5] Wiklund U, Hedenqvist P, Hogmark S. Multilayer cracking resistance in bending. Surf Coat Technol 1997;97:773 – 8. [6] Sainsot Ph, Leroy JM, Villechaise B. Effect of surface coatings in a rough normally loaded contact. In: Owson D, et al, editors. Mechanics of coatings. Amsterdam7 Elsevier; 1990. p. 151 – 6. [7] Diao DF, Kato K, Hayashi K. The local yield map of hard coatings under sliding contact. In: Dowson D, et al, editors. Thin films in tribology. Amsterdam7 Elsevier; 1993. p. 419 – 27. [8] Bhushan B, Gupta BK. Handbook of tribology; materials, coatings and surface treatments. NY7 McGraw-Hill; 1991. [9] Heinke W, Leyland A, Matthews A, Berg G, Friedrich C, Broszeit E. Evaluation of PVD nitride coatings, using impact, Scratch and Rockwell-C adhesion test. Thin Solid Films 1995;270:431 – 8. [10] Bull SJ. Failure modes in scratch adhesion testing. Surf Coat Technol 1991;50:25 – 32. [11] Jehn H, Reiners G, Siegel N. DIN Fachbericht 39, Characterisierung du¨nner Schichten. Berlin7 Beuth Verlag; 1993. [12] Holmberg K, Laukkanen A, Ronkainen H, Wallin K, Varjus S. A model for stresses, crack generation and fracture toughness calculation in scratched TiN-coated steel surfaces. Wear 2003;254:278 – 91. [13] Wiklund U, Gunnars J, Hogmark S. Influence of residual stresses on fracture and delamination of Thin Hard Coatings. Wear 1999;232:262 – 9.
An experimental method for coating–substrate interface investigation Bojan Podgornik a,*, Olle Wa¨nstrand b a
Centre for Tribology and Technical Diagnostics, University of Ljubljana, Bogisiceva 8, SI-1000 Ljubljana, Slovenia b Balzers AG, Arstaa¨ngsva¨gen 31 E, S-117 43 Stockholm, Sweden Received 21 January 2005; received in revised form 12 March 2005; accepted 15 March 2005
Abstract Investigations of coated surfaces indicate that in many cases the coating–substrate interface is the weakest part of the coated component, with the coating-to-substrate adhesion being used to evaluate the strength of the coating–substrate interface. While modeling of the coated surface depends on coating and substrate material properties, which are not easy to determine, standard experimental methods do not allow a direct study of the interface. The aim of the present paper is to describe a supplementary method for coating–substrate interface investigation, which can provide further information on the interface properties. D 2005 Elsevier Inc. All rights reserved. Keywords: Coatings; Interface; Adhesion; Scratch test
1. Introduction There has been increased interest in the use of tribological coatings on mechanical components in engines and transmissions, on tools in the manufacturing industry, on disc drives in the computer industry, on precision instruments, and on artificial human joints [1]. New coating deposition techniques developed over the last two decades offer a wide variety of
* Corresponding author. Tel.: +38 6 1 4771 463; fax: +38 6 1 4771 469. E-mail address: [email protected] (B. Podgornik). 1044-5803/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.04.011
possibilities to tailor surfaces with many different materials and structures. In particular, chemical vapour deposition (CVD) and physical vapour deposition (PVD) techniques have made it possible to deposit thin coatings in the micrometre range even at room temperature [2]. A tribological contact with two loaded surfaces in relative motion is a very complex system, which becomes even more complex when coatings are introduced on the surface [1,2]. Certainly, it is not easy to understand nor simulate or predict tribological behaviour and performance of coated surfaces. However, different investigations have indicated [3– 5] that in many cases the coating–substrate interface
174
B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
represents the weakest part of the coated component, with the coating-to-substrate adhesion being used to evaluate the strength of the coating–substrate interface. Different models [6,7] and experimental methods [8,9] are used nowadays to evaluate the strength of the coating–substrate interface, with Scratch-testing [9,10] and Rockwell C indentation [11] being the most widely used. However, while modeling depends on coating and substrate material properties, which are not easy to determine [12], experimental methods do not allow a direct microscopic study of the interface. The aim of the present paper is to present a supplementary method for coating–substrate interface investigation, which can provide further information on the interface properties.
ng ishi Pol ction dire
Fig. 2. Polishing of the scratched sample.
of the analyzed area relative to the scratch channel end, it is possible to correlate SEM pictures, findings and features to the load, as shown in Fig. 3. Another way of preparing the sample, which would not require any polishing, would be to cut the sample in the middle of the scratch using an Focus Ion beam equipment.
3. Practical use of the method 2. Method description The proposed method for coating–substrate interface investigation is based on a standard scratch test, where an up to 10 mm long scratch is made on a coated sample under a linearly increasing load, using a diamond tip. In a second step the scratched sample is cut at the border of the scratch parallel to the scratch, as shown in Fig. 1. The sample, which has been cut off, is then mounted in a holder with a micrometre screw, as used for TEM sample preparation, and carefully polished until the middle of the scratch channel is reached (Fig. 2). After polishing, the sample can be nital etched and analyzed by high resolution scanning electron microscope (SEM). By knowing loading rate and position
In order to demonstrate the applicability of the above described experimental method, it was applied on a number of PVD coated steel samples. 3.1. Sample preparation Samples selected for this experiment were made of AISI 4140 steel, quenched and tempered (650 8C) to a mean hardness of 300 HV0.5 and ground to a R a value of ~ 0.4 Am. Four groups of samples, denoted A–D, were prepared in terms of thermochemical treatment (Table 1) [3], and coated by monolayer TiN and multilayer TiAlN coating, respectively. Details of the coating deposition process are given in Table 2 and Ref. [3].
10 mm
0 10 20 30 40 50 60 70 80 90 100 Load [N] Fig. 1. Cutting of the scratched sample.
Fig. 3. Corresponding loads for the loading rate of 10 N/mm.
B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
175
Table 1 Details of substrate treatment processes and resulting surface hardness [3] Treatment Plasma nitriding + 5 Am gV Plasma nitriding Hardening Tempering
A B C D
Atmosphere
Temperature [8C]
Time [h]
Case depth [mm]
Hardness [HV0.1]
75%H2–25%N2 99.4%H2–0.6%N2 oil oil
540 540 870/250 870/650
28 17 2/1 2/1
0.55 0.3 through through
935 F 40 705 F 25 615 F 20 300 F 10
Table 2 Deposition parameters and resulting coating properties [3] Coating
Process
Temperature [8C]
TiN TiAlN
Sputtering Sputtering
450 450
Bias [V] 200 200
Interlayer [Am]
Hardness [GPa]
Young’s modulus [GPa]
Ti 0.1 Ti 0.1
28.5 F 1.8 31.4 F 2.8
386 F 26 407 F 37
3.2. Experimental procedure
mond stylus and a loading rate of 10 N/mm was used to form grooves on the above described samples. Standard evaluation of the results included recording of the acoustic emission signal, caused
A commercial scratch tester, the CSEM Revetest, equipped with a 200 Am radius Rockwell-C dia-
(b)
(d)
(c)
2500 A B C D
(e)
2000
1500
1000
AE [x10 db]
(a)
500
0
10
20
30
40
50
60
70
80
90
0 100
FN [N]
Fig. 4. (a) Acoustic emission signals during scratch testing of TiN-coated AISI 4140 steel and corresponding coating failures: (b) tensile cracking, (c) coating debris removal at the rim of the scratch, (d) complete removal of the coating and (e) coating debris removal at the bottom of the scratch-hardened substrate.
176
B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
purposes standard Rockwell-C adhesion tests were also carried out [11]. 3.3. Results
Fig. 5. Critical loads of the first failure (L c1) and total removal (L c2) of the coating.
by crack formation, and a post-test optical microscope determination of critical loads L c1 (first adhesive or cohesive failure) and L c2 (complete delamination of the coating) [9,10]. For comparison
Fig. 4 shows an acoustic emission signal for a TiN coating deposited on different thermochemically treated steel substrates (Fig. 4a) and corresponding coating failure modes (Fig. 4b–e). In the case of the plasma-nitrided substrate (treatment A and B) tensile cracking of the coating (Fig. 4b) propagating from the bottom of the scratch towards the scratch-channel rims was observed at a critical load of approximately 35 N. An increased load led to coating debris removal, observed at the rim of the scratch (Fig. 4c), and finally to complete removal of the coating, as shown in Fig. 4d. A similar coating failure mechanism with coating debris removal was also observed at the bottom of the scratch channel (Fig. 4e) for the hardened substrate
Fig. 6. Subsurface coating failures of TiAlN coating deposited on hardened steel; (a) cracking, (b) delamination and (c) flaking.
B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
177
Fig. 7. Scratch-channel cross-sections of TiAlN coating on (a) hardened-steel substrate at a load of 35 N and (b) 20 N, and (c) plasma-nitrided steel substrate without and (d) with compound layer at a load of 40 N.
(treatment C and D). Hardening, however, was found to decrease critical loads compared to plasma nitriding, as shown in Fig. 5. For both TiN and TiAlN coatings the same coating failure mechanisms with comparable critical loads were observed (Fig. 5). The rockwell-C adhesion test confirmed improved coating-to-substrate adhesion obtained by plasma nitriding of the substrate. The coating deposited on a hardened substrate peeled off circularly around an indentation mark, while only cracks spreading out radially from the indentation mark were observed for the coating deposited on the plasma-nitrided substrate with or without a compound layer. Regarding Rockwell-C indentation results, the coating-to-substrate adhesion can be defined as sufficient for all substrate treatments used. However, for the plasmanitrided substrate (treatment A and B) coating adhe-
Fig. 8. Crack deflection at the porous zone of the compound layer.
178
B. Podgornik, O. Wa¨nstrand / Materials Characterization 55 (2005) 173–178
sion is classified as HF1 and for the hardened and tempered ones (treatment C and D) as HF3 [11]. In order to investigate the origin of the coating failure, cross-sections of scratched samples were analysed with a field-emission-gun scanning electron microscope (FEG-SEM) LEO 1550. Fig. 6 shows that coating failure in the shape of cracks (Fig. 6a), delamination (Fig. 6b) or flaking (Fig. 6c) normally starts at the interface, as observed for both coatings (TiN and TiAlN). Comparison of different substrate pre-treatments also showed that regardless of the substrate pre-treatment used, coating failure starts at the interface [3,13]. Therefore, for all coating–substrate pre-treatment combinations, the interface was found to be the weakest part of the composite. Plasma nitriding, however, was found to improve the substrate load-carrying capacity with reduced plastic deformation of the substrate, as shown in Fig. 7. In the case of a hardened or tempered substrate (treatments C and D) considerable plastic deformation, with coating particles embedded into the substrate material, was observed at loads below 40 N (Fig. 7a), while the coated plasma-nitrided substrate (treatment B) displayed reduced plastic deformation of the substrate without coating spallation (Fig. 7c). Large plastic deformation of the hardened substrate also caused cracking of the coating along the scratch, as shown in Fig. 7b. On the other hand, superior load-carrying capacity without any plastic deformation even at loads over 50 N was obtained in the case of the plasma-nitrided substrate with an intermediate compound layer (treatment A), as shown in Fig. 7d. Although the compound layer displayed some porosity at the surface it gives very good coating-to-substrate adhesion (Figs. 4 and 5). In addition, it seems that in a similar way to the interfaces in multilayer coatings it can increase the cracking resistance of the coating [5]. The porous zone of the compound layer prevents the cracks that start at the surface from propagating directly into the substrate and vice versa (Fig. 8).
4. Conclusions ! The rockwell-C adhesion test gives us information about coating–substrate interface strength or coating-to-substrate adhesion, however, it is only an
indentation method, which provides qualitative information, limited to 6 stages (HF1–HF6). ! The scratch-test gives qualitative and quantitative information about coating–substrate interface strength under relative motion. However, it does not give direct insight into the process taking place at the interface, i.e. crack initiation and propagation. ! The preparation of scratch-channel cross-sections and their analysis represents an additional tool and possibilities in the process of coating–substrate interface investigation.
References [1] Holmberg K, Laukkanen A, Ronkainen H. Tribological optimisation of coated components by stress modelling and surface fracture assessment. Proc. COST 532 Conf.: Triboscience and Tribotechnology. Ghent, Belgium, October 18–19. [2] Holmberg K, Matthews A. Coatings tribology: properties, technologies and applications in surface engineering. NY7 Elsevier; 1994. [3] Podgornik B. Influence of substrate pre-treatment on the tribological properties of hard coatings during sliding, PhD thesis, University of Ljubljana, Ljubljana, Slovenia; 2000. [4] Mishina H. Microscopic real observation of failure, wear and deformation on coating/subsurface. Tribol Int 1999; 32:137 – 44. [5] Wiklund U, Hedenqvist P, Hogmark S. Multilayer cracking resistance in bending. Surf Coat Technol 1997;97:773 – 8. [6] Sainsot Ph, Leroy JM, Villechaise B. Effect of surface coatings in a rough normally loaded contact. In: Owson D, et al, editors. Mechanics of coatings. Amsterdam7 Elsevier; 1990. p. 151 – 6. [7] Diao DF, Kato K, Hayashi K. The local yield map of hard coatings under sliding contact. In: Dowson D, et al, editors. Thin films in tribology. Amsterdam7 Elsevier; 1993. p. 419 – 27. [8] Bhushan B, Gupta BK. Handbook of tribology; materials, coatings and surface treatments. NY7 McGraw-Hill; 1991. [9] Heinke W, Leyland A, Matthews A, Berg G, Friedrich C, Broszeit E. Evaluation of PVD nitride coatings, using impact, Scratch and Rockwell-C adhesion test. Thin Solid Films 1995;270:431 – 8. [10] Bull SJ. Failure modes in scratch adhesion testing. Surf Coat Technol 1991;50:25 – 32. [11] Jehn H, Reiners G, Siegel N. DIN Fachbericht 39, Characterisierung du¨nner Schichten. Berlin7 Beuth Verlag; 1993. [12] Holmberg K, Laukkanen A, Ronkainen H, Wallin K, Varjus S. A model for stresses, crack generation and fracture toughness calculation in scratched TiN-coated steel surfaces. Wear 2003;254:278 – 91. [13] Wiklund U, Gunnars J, Hogmark S. Influence of residual stresses on fracture and delamination of Thin Hard Coatings. Wear 1999;232:262 – 9.