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Examination of mechanical properties and failure mechanisms of TiN and Ti−TiN multilayer coatings
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ELSEVIER
Surface and Coatings Technology 76-77 (1995) 297-302
Examination of mechanical properties and failure mechanisms of TiN and Ti-TiN multilayer coatings KJ. Ma, A. Bloyce, T. Bell School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
Abstract A number of studies have been carried out to establish mechanical properties of single and multilayer hard coatings. However, the mechanisms of deformation, cracking and delamination of coatings under ploughing and shear stress are not fully understood. A fractured cross-sectional specimen preparation technique through hardness indentation and scratch tests on hard coatings has been used in conjunction with high resolution SEM to observe deformation and fracture behaviour occurring as a result of these tests. TiN and Ti-TiN multilayer coatings were deposited on M2 high speed steel and silicon substrates using an unbalanced magnetron sputtering system. Hardness measurements and scratch tests were performed to monitor the mechanical properties. X-ray diffraction was used for phase identification. Coatings comprising fine columnar TiN behaved like closely congregated strong fibres: they were found to accommodate a large amount of ploughing and shear stress through densification and shear deformation. On increasing the load above a certain value, rupture of heavily deformed TiN initiated at defect locations and the cracks propagated and coalesced into macrocracks. When the applied load was increased to near the critical load, close packed columns separated from each other and detached from the substrate, resulting in total failure. For Ti-'TiN multilayers, hardness and critical load are related to the different monolayer thickness of the Ti and TiN. The Ti layers dissipate most of the energy by means of shear deformation during the scratch test. At higher scratch loads, cracks occurred at Ti-TiN interfaces or at multilayersubstrate interfaces depending on the relative interface strengths. The influences of substrate hardness on the indentation crack pattern and scratch failure mechanism are also briefly covered in this paper. Keywords: Multilayer; Failure mechanism; Mechanical properties; Indentation tests; Scratch tests
1. Introduction A few publications have indicated that multilayer coatings may be superior to single layer coatings for some applications [1-4], but a systematic study concerning their failure mechanisms is lacking. A number of studies have, however, been carried out in order to establish mechanical properties of single and multilayer coatings. Microhardness indentation tests have been commonly used to measure mechanical properties of hard coatings, either deriving mathematical models for the contributions to measured hardness from the different components of the coating-substrate system [5,6], or producing elastic or elastic-plastic contact mechanics simulations of various coating-substrate systems [7,8]. Page and colleagues [9,10] recently noted the substrate effects on the response of the crack patterns of the hard coatings and proposed a model to explore nest crack phenomena. In order to measure coating adhesion, scratch critical load, L c , is widely used to identify the scratch test failure mode, and analyses of scratch tests using optical micro0257-8972/95/$09.50 © 1995 Elsevier Science SA All rights reserved SSDI 0257-8972(95)02585-5
scopy, scanning electron microscopy, acoustic emission and friction force measurement are commonplace [10-16]. Despite progressive improvements in both indentation and scratch tests, some basic questions have not been fully answered concerning the resultant deformation and fracture behaviour. Multilayer coatings have been developed and applied on cemented carbide cutting tools for many years. Most of these coatings consist of three or more monolayers, each approximately 1-31lm thick, such as TiCTiCN-TiN, TiC-TiCN-AI 2 0 3 or other similar combinations [1,15]. Alternated multilayers including Ti-TiN, Ti-BN, TiC-TiB 2 , TiC-AI 2 0 3 , TiN-TiB, TiN-TiCNDLC have also been studied [16-20]. The alternating multilayer design is based on the introduction of more interfaces which allow cracks to be deflected, thereby dissipating energy and hence improving toughness. More recently, some articles have suggested that multilayer coatings with superlattice structures have potential for tribological applications owing to their high hardness and strength [21-22], but with an associated reduction in the toughness and scratch adhesion.
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Elucidation of the failure mechanisms of multilayer coatings is dependent on fine scale microstructure analysis of the coating and substrate. This feature has never been examined due to the resolution limitation of the conventional SEM.
2. Experimental techniques
3. Results and discussion
3.1. Microstructure and phase and analysis (a) TiN single layer coatings X-ray diffraction showed a (111) preferred growth orientation: (200) and (220) orientations also appear. The phase structure and preferred orientation are similar for the three layer thicknesses.
2.1. Materials Hardened and annealed M2 tool steel approximately 25 x 20 mm in area and 2 mm thickness, and Si(100) wafer were used as substrates. The M2 substrates were ground and polished with 6 urn and 1 urn diamond pastes and then cut with a diamond saw to produce several deep slits 0.5 mm in width and 1.9 mm in depth on the back face. 2.2. Deposition process The TiN and Ti-TiN multilayers were coated using a r.f'-reactive sputtering system (Teer 450/4 unbalanced magnetron sputtering system). Sputter durations (the time span for deposition of each component of a multilayer coating) of 1200, 280, 180, 90, 45, 20, 10, 5, 2 s were used. A shield could be used in order to minimise the nitrogen uptake by the coating during the Ti deposition stages. Deposition with and without the shield was carried out to compare its influence on microstructure and mechanical properties. The total thickness of the multilayer films was in the range of 1.8-2.5 urn with TiN as the last layer. Single layer TiN was coated with three different thickness (1.8, 2.6, 4.5 urn) to study the influence of coating thickness on the L e .
(b) Ti-TiN multilayer coatings For Ti-TiN multilayer systems, XRD showed that structure and relative intensity varied with the sputter duration (or layer thickness) and deposition condition (with or without the shield). In the cases of Ti-TiN multilayers coated without the shield, the Ti and TiN were the principal phases for a sputter duration of 1200 s. Small peaks corresponding to Ti1N were also noted. For shorter sputter durations, the TiN and Ti1N became the dominant phases and the relative strength of the (XTi peaks decreased. This result can be attributed to an interfacial chemical reaction during deposition producing a Ti1N transition layer. For sputter durations of less than 45 s, peaks corresponding to (X- Ti phase were not present in the XRD patterns. For the multilayers coated using the shield, the intermediate reaction phase, Ti1N, was present to a lesser degree than in the coatings produced without the shield. Ti and TiN remained the main phases for sputter duration of 45 s +45 s and weak peaks from Ti1N were also measured. When the sputter duration of each layer was less than 20 s, TiN and Ti1N become the predominant phases in the multilayer. 3.2. Microhardness
2.3. Mechanical properties testing
Hardness measurements were made using Vickers microhardness testing over the load range 10-500 g. L; for each coating was determined using a Teer ST-200 scratch tester with loading rate dL/dt = 100 N min- 1 and the table speed dx/d; = 10 mm min -1. All the indentation tests and constant load scratch tests were carried out along the back side of the slit in order to examine the microstructure using fracture cross-sections. 2.4. Analysis A high resolution Hitachi S-4000 field emission gun (FEG)-SEM was necessary to examine microstructure and failure mechanisms of TiN and Ti-TiN coatings, and a Philips 6300 SEM in the backscattered electron mode was used to observe the crack patterns because of the higher degree of contrast. Phase structure and orientation were determined with an MAC-3 kW X-ray diffractometer (XRD).
(a) TiN single layer coatings The measured microhardness values of the TiN coatings was '" 3500 HVO.Ol and there was no significant difference in the measured hardness values for the different substrate conditions, and this value therefore reflected the hardness of the coating alone.
(b) Ti-TiN multilayer coatings In the Ti-TiN multilayer system, the indentation microhardness strongly depends on the sputter duration and interface reaction conditions. For the multilayers coated without the shield, the hardness is higher than for those coated with the shield. This result can be attributed to TizN formation at the Ti-TiN interface, and the shield prevented or at least led to a thinner reaction zone formation at the interface. The amount of interface product, Ti1N, increased with a reduction in the sputter duration, which again favoured an increase in hardness.
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3.3. Indentation crack patterns and mechanisms (a) TiN single layer coatings Fig. 1 shows the indentation crack patterns observed for the TiN coating on hardened and annealed M2 substrates. Only radial cracks were observed for the hardened M2 and the Si substrate, which were initiated at a load of 100 g for hardened M2 and 10 g for Si substrate, and the length of which increased with increase of the applied load. However, the annealed M2 substrate samples exhibited ring or nest cracks, which were observed in almost all the specimens. It has been reported that nest cracks occur because the coating does not conform to the deformation produced in the substrate [9J, which results in coating cracking, rather than adhesive failure between the coating and the substrate. Indentation with an applied load of 25 g caused nest cracks which continued to develop with increase of the indentation size, whilst exhibiting almost the same inter-
crack spacing for the different applied load tests. The inter-crack spacing became smaller and the critical load for the nest crack initiation decreased with decreasing substrate hardness. Fig. 2(a) shows the microstructural responses during
(a)
..
t/lrn
... I
IOJ.l111
Fig. 1. Indentation crack pattern occurring in (a) hardened M2, 929 HV, 2.6 urn TiN and (b) annealed M2, 285 HV, 2.6 urn TiN, applied load of 300 g.
Fig. 2. Fracture SEM images through indentations on (a) annealed M2 + TiN, (b) hardened M2 + TiN, (c) silicon + TiN.
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indentation tests for the TiN coated on soft M2 substrates. The columnar TiN behaved like perfect elastic media without plastic deformation revealed in the coatings, which transferred the indentation load very rapidly into the substrate and resulted in substrate deformation. Because the coatings do not accommodate the deformation produced in the substrate, sliding and splitting separation between the adjacent columns occurred around the indentation edges, which resulted in the nest cracks. In the case of TiN coated on hardened M2 substrates, instead of splitting, densification and shear deformation of the columns was observed in the TiN coatings near the surface, as shown in Fig. 2(b). For the TiN films coated on Si substrate, significant densification and shear deformation occurred in the coatings for indentation loads larger than 10 g (Fig. 2(c)).
(a)
Iprn -"""---
(b) Ti-TiN multilayer coatings For the Ti-TiN multilayers, indentation crack patterns were closely related to the substrate hardness, sputter duration and the use of the shield during the deposition process. A greater thickness of Ti favours greater energy dissipation, since more shear deformation occurs, avoiding tensile nest cracking (Fig. 3(a)). If the interface effect becomes dominant, reducing the amount of rL- Ti present, the degree of possible shear deformation is reduced and hence tensile nest cracks appear around the indentation edge (Fig. 3(b)). The inter-crack spacing becomes smaller and the critical load for the nest crack initiation decreases with increasing hardness of the Ti-TiN multilayer coatings. 3.4. Failure mechanisms ofcoatings during scratch tests (a) TiN single layer coatings Fine columnar TiN coatings were found to accommodate ploughing and shear stress through densification and shear deformation (Fig.4(a)). No adhesive failure occurred even at a load of 55 N. On increasing the load above a certain value, rupture of heavily deformed TiN columns is preferentially initiated at defect locations such as column growth steps or surface defects (Fig.4(b)), which coalesces into macro cracks resulting in microflakes. When the applied load was further increased to near the critical load, columnar TiN separated and detached from the substrate, resulting in total failure as shown in Fig.4(c). Microflaking occurred most frequently at the edge of scratches where maximum tensile stress developed due to tangential traction and the plastic pile up of the substrate material [23] (Fig. 5(a)). Plastic flow of columnar TiN occurs in a direction across the scratch and cracks are then nucleated at defect locations in this region and propagate due to the tangential traction, resulting in microflakes (Fig.5(b)). The annealed substrates allow a stress situation which produces microflaking and total failure at lower applied loads in the
Fig. 3. Fracture SEM images through indentations on Ti-TiN multilayer coatings for (a) sputter duration 1200 s, (b) sputter duration 280 s, without shield.
scratch test. The microstructural response of TiN to scratch testing can be attributed to its columnar structure causing it to behave like a group of strong fibres. (b) Ti-TiN multilayer coatings Thicker Ti components in the multilayer allow substantial shear deformation during scratch testing (Fig.6(a)). It is clear that maximum shear stress was developed at the surface due to the friction effects, and a gradient of the shear deformation may be observed in the Ti and TiN layers. However, when the applied stress is above a certain value, ductile failure within the Ti layer occurs. For relatively thin Ti layers, the interface effect dominates and less shear can occur; cracking is preferentially initiated at the interface between the substrate and the multilayer rather than at the stronger interface between Ti-Ti2N-TiN or TiN-Ti2N layers, i.e. a typical brittle type scratch failure results (Fig. 6(b)). An intermediate thickness of Ti favoured both higher energy absorption in shear deformation and prevention
K.J Ma et al.lSurface and Coatings Technology 76-77 ( /995) 297-]02
(a)
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Ipm
Fig. 5. SEM images of scratch tracks on hardened M2 + TiN showing microflaking (a) on the surface, (b) fracture cross-section, partly in the scratch and partly adjacent to the scratch.
Fig. 4. Fracture SEM images through scratch tracks on hardened M2 + TiN for (a) 55 N load, (b) 60 N load, (c) 68 N load.
of ductile shear failure within the Ti layer, and hence produced a maximum value for Lc for a multilayer coating.
4. Conclusions Indentation crack patterns, radial or nest cracks, are related to the substrate and coating structure. For the
TiN-Si and TiN-hardened M2 coating systems, columnar TiN accommodates the indentation load and the opposing force from the interface by densification and shear deformation, which dissipate the energy and avoid or delay the occurrence of tensile nest cracks. Radial cracking is possible and occurs due to the stress concentration. For the TiN-annealed M2 system, the substrate absorbs most of the energy from the elastic TiN by plastic deformation which causes little opposing force to the coatings. Columnar TiN accommodates spontaneous plastic deformation of the substrate by sliding and splitting between adjacent columns resulting in cracks around indenter edges, i.e. nest cracks. For Ti-TiN multilayer systems, indentation hardness and ductile (no crack) or brittle (tensile nest cracks) failure phenomena during indentation test depend on the amount of ductile material present in the coating. Thin layers of Ti play an important role in dissipation of energy by shear deformation and thus avoid cracking.
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(a)
_1.....-_
Optimisation of Ti layer thickness plays an important role in energy dissipation by shear deformation, which delays critical shear and tensile stress developed at substrate-coating interface resulting in a higher L e . Decreasing the thickness of alternate layers of Ti-'TiN or Ti-'TizN-TiN causes the interface to dominate mechanical properties and L e , i.e. the apparent hardness increases but L e decreases. The technique used in this study, still in its early stages and applied to a relatively simple system (Ti-'TiN), provide a new viewpoint from which to study the deformation and failure behaviour occurring in thin films, and may lead to improvements in the design of higher performance, fourth-generation hard coatings. Detailed discussions of the phenomena shown in this paper will be reported elsewhere.
Acknowledgements The authors are grateful to Teer Coatings Ltd. for the provision of coating and scratch test facilities and to Dr. D.G. Teer for fruitful discussion.
References
Fig. 6. Fracture SEM images through scratch tracks on hardened M2 + Ti-TiN multilayer coating for (a) sputter duration 1200 s, (b) sputter duration 180 s, without shield.
Nest cracking is initiated from the surface where maximum tensile stress was developed and then propagated into the coatings. The inter-crack spacing becomes smaller and the critical load for the nest crack initiation decreases with increasing coating hardness or decreasing substrate hardness. Scratch tests on coatings comprising fine columnar TiN produced a response analogous to closely congregated strong fibres which were found to accommodate a large amount of ploughing and shear stress through densification and shear deformation. On increasing the load above a certain value, rupture of heavily deformed TiN initiated at defect locations and coalesced into macrocracks, causing microflaking along the edge of the scratch track or inside the track. When the applied load was increased to near the L e , close-packed columns separated from each other and detached from the substrate, resulting in total failure. Soft substrates allow higher shear stress and tensile stress to be developed at the substrate-eoating interface at a lower applied load in the scratch test, hence giving rise to a lower L e •
[1] W. Schintlmeister, W. Wallgram and J. Kanz, Thin Solid Films, 107 (1983) 117. [2] V.V. Lyubimov, A.A. Voevodin, YS. Timofeev and IK Arkhipov, Surf. Coat. Technol., 52 (1992) 145. [3] D.P. Monaghan, D.G. Teer, P.A. Logan, 1. Efeoglu and RD. Arnell, Surf. Coat. Techno/., 60 (1993) 525. [4] R. Fella, H. Holleck and H. Schulz, Surf. Coat. Technol., 36 (1988) 257. [5] B. Jonsson and S. Hogrnark, Thin Solid Films, 114 (1984) 257. [6] PJ. Burnett and D.S. Rickerby, Thin Solid Films, 148 (1987) 41. [7J A.K. Bhattacharya and W.D. Nix, Int. J. Solids Structures, 24 (1988) 1287. [8] Y. Sun, A. Bloyce and T. Bell, Thin Solid Films, in press. [9] r.c Knight, T.F. Page and 1.M. Hutchings, Surf Eng., 5 (1989) 213. [10] M.R McGurk, H.W. Chandler, P.e. Twigg and T.F. Page, Surf Coat. Technol .. 68/69 (1994) 576. [11] P.A. Stainmann, Y Tardy and H.E. Hintermann, Thin Solid Films, 154 (1987) 333. [12] S.l. Bull, Surf. Coat. Technol., 50 (1991) 25. [13] l. Valli, J. Vac. Sci. Technol., A4 (1986) 3007. [14] A.J. Perry, Surf. Eng., 2 (1986) 183. [15] W. Schintleister, O. Pacher, T. Krall, W. Wallgram and T. Raine, Powder Metall., 13 (1981) 26. [16] R. Hubler, A. Schroer, W. Ensinger, G.K. Wolf, W.H. Schreiner and LJ.R Baumvol, Surf Coat. Technol., 60 (1993) 561. [17] R. Fella and H. Holleck, Mater. Sci. Eng., A140 (1991) 676. [18] H. Holleck and H. Schulz, Thin Solid Films, 153 (1987) 11 [19] T. Friesen, J. Haupt, W. Gissler and A. Barna, Surf Coat. Technol., 48 (1991) 169. [20] D.P. Monaghan, K.e. Laing, P.A. Loagan, P. Teer and D.G. Teer, Mater. World, June (1993) 347. [21] X. Chu, M.S. Wong, W.D. Sproul and SA Barnett, Surf Coat. Technol., 61 (1993) 251. [22] U. Helmersson, S. Todorova, SA Barnett, l.E. Sundgren, r.c Markert and l.E. Greene, J. Appl. Phys., 62 (1987) 481. [23] R.D. Arnell, Surf Coat. Technol., 43/44 (1990) 674.
ELSEVIER
Surface and Coatings Technology 76-77 (1995) 297-302
Examination of mechanical properties and failure mechanisms of TiN and Ti-TiN multilayer coatings KJ. Ma, A. Bloyce, T. Bell School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
Abstract A number of studies have been carried out to establish mechanical properties of single and multilayer hard coatings. However, the mechanisms of deformation, cracking and delamination of coatings under ploughing and shear stress are not fully understood. A fractured cross-sectional specimen preparation technique through hardness indentation and scratch tests on hard coatings has been used in conjunction with high resolution SEM to observe deformation and fracture behaviour occurring as a result of these tests. TiN and Ti-TiN multilayer coatings were deposited on M2 high speed steel and silicon substrates using an unbalanced magnetron sputtering system. Hardness measurements and scratch tests were performed to monitor the mechanical properties. X-ray diffraction was used for phase identification. Coatings comprising fine columnar TiN behaved like closely congregated strong fibres: they were found to accommodate a large amount of ploughing and shear stress through densification and shear deformation. On increasing the load above a certain value, rupture of heavily deformed TiN initiated at defect locations and the cracks propagated and coalesced into macrocracks. When the applied load was increased to near the critical load, close packed columns separated from each other and detached from the substrate, resulting in total failure. For Ti-'TiN multilayers, hardness and critical load are related to the different monolayer thickness of the Ti and TiN. The Ti layers dissipate most of the energy by means of shear deformation during the scratch test. At higher scratch loads, cracks occurred at Ti-TiN interfaces or at multilayersubstrate interfaces depending on the relative interface strengths. The influences of substrate hardness on the indentation crack pattern and scratch failure mechanism are also briefly covered in this paper. Keywords: Multilayer; Failure mechanism; Mechanical properties; Indentation tests; Scratch tests
1. Introduction A few publications have indicated that multilayer coatings may be superior to single layer coatings for some applications [1-4], but a systematic study concerning their failure mechanisms is lacking. A number of studies have, however, been carried out in order to establish mechanical properties of single and multilayer coatings. Microhardness indentation tests have been commonly used to measure mechanical properties of hard coatings, either deriving mathematical models for the contributions to measured hardness from the different components of the coating-substrate system [5,6], or producing elastic or elastic-plastic contact mechanics simulations of various coating-substrate systems [7,8]. Page and colleagues [9,10] recently noted the substrate effects on the response of the crack patterns of the hard coatings and proposed a model to explore nest crack phenomena. In order to measure coating adhesion, scratch critical load, L c , is widely used to identify the scratch test failure mode, and analyses of scratch tests using optical micro0257-8972/95/$09.50 © 1995 Elsevier Science SA All rights reserved SSDI 0257-8972(95)02585-5
scopy, scanning electron microscopy, acoustic emission and friction force measurement are commonplace [10-16]. Despite progressive improvements in both indentation and scratch tests, some basic questions have not been fully answered concerning the resultant deformation and fracture behaviour. Multilayer coatings have been developed and applied on cemented carbide cutting tools for many years. Most of these coatings consist of three or more monolayers, each approximately 1-31lm thick, such as TiCTiCN-TiN, TiC-TiCN-AI 2 0 3 or other similar combinations [1,15]. Alternated multilayers including Ti-TiN, Ti-BN, TiC-TiB 2 , TiC-AI 2 0 3 , TiN-TiB, TiN-TiCNDLC have also been studied [16-20]. The alternating multilayer design is based on the introduction of more interfaces which allow cracks to be deflected, thereby dissipating energy and hence improving toughness. More recently, some articles have suggested that multilayer coatings with superlattice structures have potential for tribological applications owing to their high hardness and strength [21-22], but with an associated reduction in the toughness and scratch adhesion.
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KJ. Ma et al.ISurface and Coatings Technology 76-77 ( 1995) 297-302
Elucidation of the failure mechanisms of multilayer coatings is dependent on fine scale microstructure analysis of the coating and substrate. This feature has never been examined due to the resolution limitation of the conventional SEM.
2. Experimental techniques
3. Results and discussion
3.1. Microstructure and phase and analysis (a) TiN single layer coatings X-ray diffraction showed a (111) preferred growth orientation: (200) and (220) orientations also appear. The phase structure and preferred orientation are similar for the three layer thicknesses.
2.1. Materials Hardened and annealed M2 tool steel approximately 25 x 20 mm in area and 2 mm thickness, and Si(100) wafer were used as substrates. The M2 substrates were ground and polished with 6 urn and 1 urn diamond pastes and then cut with a diamond saw to produce several deep slits 0.5 mm in width and 1.9 mm in depth on the back face. 2.2. Deposition process The TiN and Ti-TiN multilayers were coated using a r.f'-reactive sputtering system (Teer 450/4 unbalanced magnetron sputtering system). Sputter durations (the time span for deposition of each component of a multilayer coating) of 1200, 280, 180, 90, 45, 20, 10, 5, 2 s were used. A shield could be used in order to minimise the nitrogen uptake by the coating during the Ti deposition stages. Deposition with and without the shield was carried out to compare its influence on microstructure and mechanical properties. The total thickness of the multilayer films was in the range of 1.8-2.5 urn with TiN as the last layer. Single layer TiN was coated with three different thickness (1.8, 2.6, 4.5 urn) to study the influence of coating thickness on the L e .
(b) Ti-TiN multilayer coatings For Ti-TiN multilayer systems, XRD showed that structure and relative intensity varied with the sputter duration (or layer thickness) and deposition condition (with or without the shield). In the cases of Ti-TiN multilayers coated without the shield, the Ti and TiN were the principal phases for a sputter duration of 1200 s. Small peaks corresponding to Ti1N were also noted. For shorter sputter durations, the TiN and Ti1N became the dominant phases and the relative strength of the (XTi peaks decreased. This result can be attributed to an interfacial chemical reaction during deposition producing a Ti1N transition layer. For sputter durations of less than 45 s, peaks corresponding to (X- Ti phase were not present in the XRD patterns. For the multilayers coated using the shield, the intermediate reaction phase, Ti1N, was present to a lesser degree than in the coatings produced without the shield. Ti and TiN remained the main phases for sputter duration of 45 s +45 s and weak peaks from Ti1N were also measured. When the sputter duration of each layer was less than 20 s, TiN and Ti1N become the predominant phases in the multilayer. 3.2. Microhardness
2.3. Mechanical properties testing
Hardness measurements were made using Vickers microhardness testing over the load range 10-500 g. L; for each coating was determined using a Teer ST-200 scratch tester with loading rate dL/dt = 100 N min- 1 and the table speed dx/d; = 10 mm min -1. All the indentation tests and constant load scratch tests were carried out along the back side of the slit in order to examine the microstructure using fracture cross-sections. 2.4. Analysis A high resolution Hitachi S-4000 field emission gun (FEG)-SEM was necessary to examine microstructure and failure mechanisms of TiN and Ti-TiN coatings, and a Philips 6300 SEM in the backscattered electron mode was used to observe the crack patterns because of the higher degree of contrast. Phase structure and orientation were determined with an MAC-3 kW X-ray diffractometer (XRD).
(a) TiN single layer coatings The measured microhardness values of the TiN coatings was '" 3500 HVO.Ol and there was no significant difference in the measured hardness values for the different substrate conditions, and this value therefore reflected the hardness of the coating alone.
(b) Ti-TiN multilayer coatings In the Ti-TiN multilayer system, the indentation microhardness strongly depends on the sputter duration and interface reaction conditions. For the multilayers coated without the shield, the hardness is higher than for those coated with the shield. This result can be attributed to TizN formation at the Ti-TiN interface, and the shield prevented or at least led to a thinner reaction zone formation at the interface. The amount of interface product, Ti1N, increased with a reduction in the sputter duration, which again favoured an increase in hardness.
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3.3. Indentation crack patterns and mechanisms (a) TiN single layer coatings Fig. 1 shows the indentation crack patterns observed for the TiN coating on hardened and annealed M2 substrates. Only radial cracks were observed for the hardened M2 and the Si substrate, which were initiated at a load of 100 g for hardened M2 and 10 g for Si substrate, and the length of which increased with increase of the applied load. However, the annealed M2 substrate samples exhibited ring or nest cracks, which were observed in almost all the specimens. It has been reported that nest cracks occur because the coating does not conform to the deformation produced in the substrate [9J, which results in coating cracking, rather than adhesive failure between the coating and the substrate. Indentation with an applied load of 25 g caused nest cracks which continued to develop with increase of the indentation size, whilst exhibiting almost the same inter-
crack spacing for the different applied load tests. The inter-crack spacing became smaller and the critical load for the nest crack initiation decreased with decreasing substrate hardness. Fig. 2(a) shows the microstructural responses during
(a)
..
t/lrn
... I
IOJ.l111
Fig. 1. Indentation crack pattern occurring in (a) hardened M2, 929 HV, 2.6 urn TiN and (b) annealed M2, 285 HV, 2.6 urn TiN, applied load of 300 g.
Fig. 2. Fracture SEM images through indentations on (a) annealed M2 + TiN, (b) hardened M2 + TiN, (c) silicon + TiN.
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indentation tests for the TiN coated on soft M2 substrates. The columnar TiN behaved like perfect elastic media without plastic deformation revealed in the coatings, which transferred the indentation load very rapidly into the substrate and resulted in substrate deformation. Because the coatings do not accommodate the deformation produced in the substrate, sliding and splitting separation between the adjacent columns occurred around the indentation edges, which resulted in the nest cracks. In the case of TiN coated on hardened M2 substrates, instead of splitting, densification and shear deformation of the columns was observed in the TiN coatings near the surface, as shown in Fig. 2(b). For the TiN films coated on Si substrate, significant densification and shear deformation occurred in the coatings for indentation loads larger than 10 g (Fig. 2(c)).
(a)
Iprn -"""---
(b) Ti-TiN multilayer coatings For the Ti-TiN multilayers, indentation crack patterns were closely related to the substrate hardness, sputter duration and the use of the shield during the deposition process. A greater thickness of Ti favours greater energy dissipation, since more shear deformation occurs, avoiding tensile nest cracking (Fig. 3(a)). If the interface effect becomes dominant, reducing the amount of rL- Ti present, the degree of possible shear deformation is reduced and hence tensile nest cracks appear around the indentation edge (Fig. 3(b)). The inter-crack spacing becomes smaller and the critical load for the nest crack initiation decreases with increasing hardness of the Ti-TiN multilayer coatings. 3.4. Failure mechanisms ofcoatings during scratch tests (a) TiN single layer coatings Fine columnar TiN coatings were found to accommodate ploughing and shear stress through densification and shear deformation (Fig.4(a)). No adhesive failure occurred even at a load of 55 N. On increasing the load above a certain value, rupture of heavily deformed TiN columns is preferentially initiated at defect locations such as column growth steps or surface defects (Fig.4(b)), which coalesces into macro cracks resulting in microflakes. When the applied load was further increased to near the critical load, columnar TiN separated and detached from the substrate, resulting in total failure as shown in Fig.4(c). Microflaking occurred most frequently at the edge of scratches where maximum tensile stress developed due to tangential traction and the plastic pile up of the substrate material [23] (Fig. 5(a)). Plastic flow of columnar TiN occurs in a direction across the scratch and cracks are then nucleated at defect locations in this region and propagate due to the tangential traction, resulting in microflakes (Fig.5(b)). The annealed substrates allow a stress situation which produces microflaking and total failure at lower applied loads in the
Fig. 3. Fracture SEM images through indentations on Ti-TiN multilayer coatings for (a) sputter duration 1200 s, (b) sputter duration 280 s, without shield.
scratch test. The microstructural response of TiN to scratch testing can be attributed to its columnar structure causing it to behave like a group of strong fibres. (b) Ti-TiN multilayer coatings Thicker Ti components in the multilayer allow substantial shear deformation during scratch testing (Fig.6(a)). It is clear that maximum shear stress was developed at the surface due to the friction effects, and a gradient of the shear deformation may be observed in the Ti and TiN layers. However, when the applied stress is above a certain value, ductile failure within the Ti layer occurs. For relatively thin Ti layers, the interface effect dominates and less shear can occur; cracking is preferentially initiated at the interface between the substrate and the multilayer rather than at the stronger interface between Ti-Ti2N-TiN or TiN-Ti2N layers, i.e. a typical brittle type scratch failure results (Fig. 6(b)). An intermediate thickness of Ti favoured both higher energy absorption in shear deformation and prevention
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Fig. 5. SEM images of scratch tracks on hardened M2 + TiN showing microflaking (a) on the surface, (b) fracture cross-section, partly in the scratch and partly adjacent to the scratch.
Fig. 4. Fracture SEM images through scratch tracks on hardened M2 + TiN for (a) 55 N load, (b) 60 N load, (c) 68 N load.
of ductile shear failure within the Ti layer, and hence produced a maximum value for Lc for a multilayer coating.
4. Conclusions Indentation crack patterns, radial or nest cracks, are related to the substrate and coating structure. For the
TiN-Si and TiN-hardened M2 coating systems, columnar TiN accommodates the indentation load and the opposing force from the interface by densification and shear deformation, which dissipate the energy and avoid or delay the occurrence of tensile nest cracks. Radial cracking is possible and occurs due to the stress concentration. For the TiN-annealed M2 system, the substrate absorbs most of the energy from the elastic TiN by plastic deformation which causes little opposing force to the coatings. Columnar TiN accommodates spontaneous plastic deformation of the substrate by sliding and splitting between adjacent columns resulting in cracks around indenter edges, i.e. nest cracks. For Ti-TiN multilayer systems, indentation hardness and ductile (no crack) or brittle (tensile nest cracks) failure phenomena during indentation test depend on the amount of ductile material present in the coating. Thin layers of Ti play an important role in dissipation of energy by shear deformation and thus avoid cracking.
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Optimisation of Ti layer thickness plays an important role in energy dissipation by shear deformation, which delays critical shear and tensile stress developed at substrate-coating interface resulting in a higher L e . Decreasing the thickness of alternate layers of Ti-'TiN or Ti-'TizN-TiN causes the interface to dominate mechanical properties and L e , i.e. the apparent hardness increases but L e decreases. The technique used in this study, still in its early stages and applied to a relatively simple system (Ti-'TiN), provide a new viewpoint from which to study the deformation and failure behaviour occurring in thin films, and may lead to improvements in the design of higher performance, fourth-generation hard coatings. Detailed discussions of the phenomena shown in this paper will be reported elsewhere.
Acknowledgements The authors are grateful to Teer Coatings Ltd. for the provision of coating and scratch test facilities and to Dr. D.G. Teer for fruitful discussion.
References
Fig. 6. Fracture SEM images through scratch tracks on hardened M2 + Ti-TiN multilayer coating for (a) sputter duration 1200 s, (b) sputter duration 180 s, without shield.
Nest cracking is initiated from the surface where maximum tensile stress was developed and then propagated into the coatings. The inter-crack spacing becomes smaller and the critical load for the nest crack initiation decreases with increasing coating hardness or decreasing substrate hardness. Scratch tests on coatings comprising fine columnar TiN produced a response analogous to closely congregated strong fibres which were found to accommodate a large amount of ploughing and shear stress through densification and shear deformation. On increasing the load above a certain value, rupture of heavily deformed TiN initiated at defect locations and coalesced into macrocracks, causing microflaking along the edge of the scratch track or inside the track. When the applied load was increased to near the L e , close-packed columns separated from each other and detached from the substrate, resulting in total failure. Soft substrates allow higher shear stress and tensile stress to be developed at the substrate-eoating interface at a lower applied load in the scratch test, hence giving rise to a lower L e •
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