- Email: [email protected]
CrN superlattice
Surface and Coatings Technology 177 – 178 (2004) 204–208
Multilayered coatings with alternate pure Ti and TiNyCrN superlattice Q. Yang*, D.Y. Seo, L.R. Zhao Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario, Canada K0A 1R6
Abstract Multilayered coatings containing alternating layers of pure Ti and TiNyCrN superlattice were deposited with controlled layer thickness and superlattice bilayer period using pulsed DC magnetron sputtering technique. The preferred orientation of the TiNy CrN superlattice constituent was either (1 1 1), (2 0 0) or random depending on the sub-structure between the Ti and superlattice layers. Nano-indentation testing showed that mechanical properties of the coatings varied with the bilayer period, crystallographic orientation and volume fraction of superlattice layers in the coatings. For the same bilayer period and superlattice volume fraction, coatings with (2 0 0)-oriented superlattice layers were harder than those with randomly and (1 1 1)-oriented superlattices. Scratch testing revealed severe interfacial failure between coatingysubstrate and superlatticeyTi layers. Due to the brittle nature of the superlattice layers and weak interfacial strength, these multilayered coatings exhibited poor performance in erosion testing involving high-speed particle impingements, which caused extensive cracking and localized interfacial detachments between the coating and substrate and between the superlattice and pure Ti layers. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Magnetron supttering; Erosion; Multilayer; Coating
1. Introduction Multilayered coatings demonstrate enhanced engineering performance over single-layered counterparts, and thus could have an advantage to combine attractive properties of constituent materials. Multilayered coatings containing both hard and tough layers, such as TiNyTi w1x and W–NyW w2x, have been fabricated for the potential erosion resistant application. It was shown that the multilayered W–CyW coating improved erosion resistance of Ti alloy substrate by at least two orders of magnitude w3x. In a similar ‘multiscalar design’ approach, multilayered coatings consisting of alternate layers of hard superlattice structure (strengthening phase) and soft toughening phase layers were deposited on Si wafers w4–6x. In multiscalar coating systems, the monolithic tough layer is much thicker than the individual layers within a superlattice stack. By tailoring the scales of the hard and tough phases, the coatings, while exhibiting high hardness, are expected to have enhanced fracture toughness as shown in the multilayered MoyW system w6x, and therefore might be a good candidate for *Corresponding author. Tel.: q1-613-9934335; fax: q1-6139906870. E-mail address: [email protected] (Q. Yang).
the erosion protections. However, little research has been carried out so far to explore the potential of multiscalar multilayered coatings for this application. In this study, multiscalar multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers, as schematically shown in Fig. 1, were deposited on Ti– 6Al–4V substrate using magnetron sputtering technique. The coatings were characterized by X-ray diffraction (XRD), nano-indentation and scratch testing. Erosion testing involving high-speed hard particle impingements was also performed to evaluate erosion-protection performance and erosion-damage mechanism of the coatings. 2. Experimental Flat Ti–6Al–4V disc specimens were mechanically polished down by finally using 1-mm diamond paste, followed by ultrasonic cleaning before deposition. A TEER 650 closed-field unbalanced magnetron sputtering deposition system with pulsed DC plasma generators was used to deposit multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers (Fig. 1) through periodically turning on and off the supply of reactive gas N2. The base pressure was ;10y6 Torr and working
0257-8972/04/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.033
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
205
Fig. 1. Schematic diagram of the multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers.
pressure during deposition stage was 1–2 mTorr. The thickness of the individual Ti and TiNyCrN superlattice layers was controlled by the N2 on-and-off time. The total thickness of the coatings was ;5 mm. Crystallographic structures of the coatings were characterized by high-angle XRD. The bilayer period was measured by using small-angle XRD method. Mechanical properties of some multilayered coatings were measured using a CSM nano-hardness tester with a maximum load of 50 mN. The erosion testing was performed by following the ASTM standard G76-02. The erosive media are angular alumina (Al2O3) powders with an average size of 50 mm. The particle-N2 gas stream, which provides a particle flying speed of 84 mys, was directed towards the coated specimens at 908 impingement angle.
layer (-100 nm) is deposited as a transition layer (Type B). As a result, the Type B coatings show no preferred orientations both for the Ti and superlattice layers. Fig. 3 presents the cross-sectional fracture surface of a Type B coating showing the multilayered structure. However, if several alternate layers of Cr and Ti with individual
3. Results and discussion The superlattice stacks consisting of tens of alternating layers of TiN and CrN, are considered as one constituent while pure Ti layers are considered as another constituent in this paper. Multilayered coatings of 6– 12 constituent layers were deposited with various superlattice bilayer periods and volume fractions, and three different interface sub-structures between pure Ti and TiNyCrN superlattice layers. XRD experiments revealed that the preferred orientation of the superlattice layers varies with the interface sub-structure. Fig. 2 presents the schematics of the sub-structures and corresponding XRD spectra. The Type A coatings, which have no sub-layers between Ti and superlattice layers, grew (0 0 0 2)-oriented Ti layers and (1 1 1)-oriented TiNyCrN superlattice layers. (0 0 0 2) and (1 1 1) are the closed packed planes for the hcp-structured Ti and B1-structured (face centred cubic) superlattice, respectively. The small lattice misfit (-2%) between these two closed packed planes, facilitated the formation of (0 0 0 2)-oriented Ti layers and (1 1 1)-oriented TiNy CrN superlattice layers. This crystallographic relation cannot be retained if a body centred-cubic structured Cr
Fig. 2. The schematics showing three different interface structures between pure Ti layer and TiNyCrN superlattice layer (a) and corresponding XRD spectra (b).
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
206
Fig. 3. The cross-sectional fracture surface morphology of a Type B coating.
layer thickness less than 15 nm are deposited as a transition zone (Type C), the superlattice layers exhibit a (2 0 0) preferred orientation. Furthermore, testing results show that the change in superlattice volume percentage does not affect the crystallographic orientation of the superlattice layers. The mechanical properties of multilayered coatings with different superlattice volume fractions and preferred orientations have been measured. The effective Young’s modulus E* and the hardness H of the coatings were determined from the load–displacement curves using Oliver–Pharr method w7x. The mechanical properties measured from the specimens with superlattice bilayer period of ;10 nm, are listed in Table 1. For pure TiNy CrN superlattice coatings, (2 0 0)-oriented coatings yield higher hardness and effective Young’s modulus than the (1 1 1)-oriented counterpart. It is not out of expectation that the similar effect will apply to the multilayered coatings having the same superlattice bilayer period L and volume fraction Vsup. In Table 1, for the coatings with Ls;10 nm and Vsups50%, the hardness decreases from 15.1 to 14.0, to 11.6 GPa, and the effective Young’s modulus decreases from 216 to 211, to 176 MPa for the superlattice constituents with (2 0 0), random and (1 1 1) preferred orientation, respectively. For coatings with the same superlattice preferred orientation, the hardness decreases with decrease in Vsup.
The superlattice coatings with three different superlattice orientations, i.e. (1 1 1), random and (2 0 0), all exhibited adhesion strengths lower than 15 N. The multilayered coating with (1 1 1)-oriented superlattice layers experienced localized spallation adjacent to the track. However, the coating with (2 0 0)-oriented TiNy CrN layers suffered more severe spallations. The spallation areas usually have step-like edges indicating weak interfacial adhesion as shown in Fig. 4. It has been reported that the large hardness difference between the coating and substrate can lead to low adhesion strength w8x. For multilayered Tiysuperlattice coatings, the large hardness difference between the hard superlattice and the soft pure Ti layers may cause weakening in the interfacial strength, which is detrimental to erosion resistance of the coatings. In this study, the multilayered coatings were subjected to erosion tests at 908 impingement angle by high-speed
Table 1 The mechanical properties of coatings Vsup (%)
Orientation
H (GPa)
E* (GPa)
100 100 62 50 ;50 ;50 38
(2 0 0) (1 1 1) Random (2 0 0) (1 1 1) Random (2 0 0)
40.1 28.7 17.8 15.1 11.6 14.0 12.7
398 347 242 216 176 211 173
Fig. 4. The morphology of a spallation area showing the interfacial detachments between coatingysubstrate and Tiysuperlattice layers for a multilayered coating.
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
207
Fig. 5. The typical surface morphologies at different erosion track locations of the coating with (1 1 1)-oriented superlattice (Ls;10 nm) and superlattice volume fraction of 50%.
erosive Al2O3 particles. The erosion track of multilayered coatings did not reveal the layered structure during testing. Instead, localized spallations were observed as a result of the particle impingements (Fig. 5a). The erosion crater created by a single particle impact (Fig. 5b) exhibited plastic deformation and crack formation, especially at the crater edge. Due to the large difference in hardness between the soft Ti and hard superlattice constituents, the hard layers responded to the impact by formation of cracks because of their inability to accommodate the plastic deformation produced in the soft layer. The repeated impingements of particles on the same location caused coating fragmentation (Fig. 5c). Further particle bombardments gradually enlarged the fragmentation spots and eventually led to localize coating spallations as shown in Fig. 5d. The exposed substrate surface, together with the step-like areas near the edges, indicates that the accumulated particle impingements were responsible for the localized detachments at the coatingysubstrate interface and Tiysuper-
lattice layer interface. Erosive damage in cracking and delamination modes has also been reported for TiNyTi mulitlayered coatings w9x. The edges of the spalled areas are the weak points, which could easily expand under impingements. Ultimately, the spallation areas will connect with each other to form a large area of substrate exposure. The multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers exhibited a poor erosion performance because of the weak interfacial strength. The erosion resistance of the coatings appear to be governed by the adhesion between coatingysubstrate, as well as the cohesion between hard and soft layers, as failures often occur at layer interfaces as shown in this study and in a TiNyTi multilayered system w10x. This problem could be solved by introducing compositional graded toughening layers instead of pure metal layers, to improve mechanical compatibility between the hardening and toughening constituents. More work is under-
208
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
way to modify the microstructure of multilayered coatings with enhanced erosion performance. 4. Summary Multilayered coatings containing alternate layers of pure Ti and TiNyCrN superlattice with controlled preferred orientation, bilayer period and volume fraction, have been successfully deposited by magnetron sputtering. The preferred orientation of the TiNyCrN superlattice layers can be controlled by controlling the sub-structure between the Ti and superlattice layers. The weak interfacial strength between coatingysubstrate and superlatticeyTi is negatively affected by the large hardness difference between the superlattice and Ti layers. The weak interfacial strength of the multilayered coatings is primarily responsible for the lack of satisfactory erosion protection performance, and more work is underway to address the problem. Acknowledgments The authors wish to acknowledge the financial support from the Institute for Aerospace Research, National
Research Council of Canada, under the IAR New Initiative Fund Program. They also thank R. McKellar for his technical support in this project. References w1x A. Leyland, A. Matthews, Surf. Coat. Technol. 70 (1994) 19. w2x Y. Gachon, P. Ienny, A. Rorner, G. Farges, M.C. SainteCatherine, A.B. Vannes, Surf. Coat. Technol. 113 (1999) 140. w3x E. Quesnel, Y. Pauleau, P. Monge-Cadet, M. Burn, Surf. Coat. Technol. 62 (1993) 474. w4x M. Vill, D.P. Adams, S.M. Yalisove, J.C. Bilello, Acta Metall. Mater. 43 (1995) 427. w5x D.P. Adams, M. Will, J. Tao, J.C. Bilello, S.M. Yalisove, J. Appl. Phys. 74 (1993) 1015. w6x M. Vill, D.P. Adams, S.M. Yalisove, J.C. Bilello, Mat. Res. Soc. Symp. Proc. 308 (1993) 759. w7x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. w8x E. Harry, A. Rouzaud, P. Juliet, Y. Pauleau, Thin Solid Films 342 (1999) 207. w9x A. Leyland, A. Matthews, Surf. Coat. Technol. 70 (1994) 19. w10x M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 90 (1997) 217.
Multilayered coatings with alternate pure Ti and TiNyCrN superlattice Q. Yang*, D.Y. Seo, L.R. Zhao Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario, Canada K0A 1R6
Abstract Multilayered coatings containing alternating layers of pure Ti and TiNyCrN superlattice were deposited with controlled layer thickness and superlattice bilayer period using pulsed DC magnetron sputtering technique. The preferred orientation of the TiNy CrN superlattice constituent was either (1 1 1), (2 0 0) or random depending on the sub-structure between the Ti and superlattice layers. Nano-indentation testing showed that mechanical properties of the coatings varied with the bilayer period, crystallographic orientation and volume fraction of superlattice layers in the coatings. For the same bilayer period and superlattice volume fraction, coatings with (2 0 0)-oriented superlattice layers were harder than those with randomly and (1 1 1)-oriented superlattices. Scratch testing revealed severe interfacial failure between coatingysubstrate and superlatticeyTi layers. Due to the brittle nature of the superlattice layers and weak interfacial strength, these multilayered coatings exhibited poor performance in erosion testing involving high-speed particle impingements, which caused extensive cracking and localized interfacial detachments between the coating and substrate and between the superlattice and pure Ti layers. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Magnetron supttering; Erosion; Multilayer; Coating
1. Introduction Multilayered coatings demonstrate enhanced engineering performance over single-layered counterparts, and thus could have an advantage to combine attractive properties of constituent materials. Multilayered coatings containing both hard and tough layers, such as TiNyTi w1x and W–NyW w2x, have been fabricated for the potential erosion resistant application. It was shown that the multilayered W–CyW coating improved erosion resistance of Ti alloy substrate by at least two orders of magnitude w3x. In a similar ‘multiscalar design’ approach, multilayered coatings consisting of alternate layers of hard superlattice structure (strengthening phase) and soft toughening phase layers were deposited on Si wafers w4–6x. In multiscalar coating systems, the monolithic tough layer is much thicker than the individual layers within a superlattice stack. By tailoring the scales of the hard and tough phases, the coatings, while exhibiting high hardness, are expected to have enhanced fracture toughness as shown in the multilayered MoyW system w6x, and therefore might be a good candidate for *Corresponding author. Tel.: q1-613-9934335; fax: q1-6139906870. E-mail address: [email protected] (Q. Yang).
the erosion protections. However, little research has been carried out so far to explore the potential of multiscalar multilayered coatings for this application. In this study, multiscalar multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers, as schematically shown in Fig. 1, were deposited on Ti– 6Al–4V substrate using magnetron sputtering technique. The coatings were characterized by X-ray diffraction (XRD), nano-indentation and scratch testing. Erosion testing involving high-speed hard particle impingements was also performed to evaluate erosion-protection performance and erosion-damage mechanism of the coatings. 2. Experimental Flat Ti–6Al–4V disc specimens were mechanically polished down by finally using 1-mm diamond paste, followed by ultrasonic cleaning before deposition. A TEER 650 closed-field unbalanced magnetron sputtering deposition system with pulsed DC plasma generators was used to deposit multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers (Fig. 1) through periodically turning on and off the supply of reactive gas N2. The base pressure was ;10y6 Torr and working
0257-8972/04/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.033
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
205
Fig. 1. Schematic diagram of the multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers.
pressure during deposition stage was 1–2 mTorr. The thickness of the individual Ti and TiNyCrN superlattice layers was controlled by the N2 on-and-off time. The total thickness of the coatings was ;5 mm. Crystallographic structures of the coatings were characterized by high-angle XRD. The bilayer period was measured by using small-angle XRD method. Mechanical properties of some multilayered coatings were measured using a CSM nano-hardness tester with a maximum load of 50 mN. The erosion testing was performed by following the ASTM standard G76-02. The erosive media are angular alumina (Al2O3) powders with an average size of 50 mm. The particle-N2 gas stream, which provides a particle flying speed of 84 mys, was directed towards the coated specimens at 908 impingement angle.
layer (-100 nm) is deposited as a transition layer (Type B). As a result, the Type B coatings show no preferred orientations both for the Ti and superlattice layers. Fig. 3 presents the cross-sectional fracture surface of a Type B coating showing the multilayered structure. However, if several alternate layers of Cr and Ti with individual
3. Results and discussion The superlattice stacks consisting of tens of alternating layers of TiN and CrN, are considered as one constituent while pure Ti layers are considered as another constituent in this paper. Multilayered coatings of 6– 12 constituent layers were deposited with various superlattice bilayer periods and volume fractions, and three different interface sub-structures between pure Ti and TiNyCrN superlattice layers. XRD experiments revealed that the preferred orientation of the superlattice layers varies with the interface sub-structure. Fig. 2 presents the schematics of the sub-structures and corresponding XRD spectra. The Type A coatings, which have no sub-layers between Ti and superlattice layers, grew (0 0 0 2)-oriented Ti layers and (1 1 1)-oriented TiNyCrN superlattice layers. (0 0 0 2) and (1 1 1) are the closed packed planes for the hcp-structured Ti and B1-structured (face centred cubic) superlattice, respectively. The small lattice misfit (-2%) between these two closed packed planes, facilitated the formation of (0 0 0 2)-oriented Ti layers and (1 1 1)-oriented TiNy CrN superlattice layers. This crystallographic relation cannot be retained if a body centred-cubic structured Cr
Fig. 2. The schematics showing three different interface structures between pure Ti layer and TiNyCrN superlattice layer (a) and corresponding XRD spectra (b).
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
206
Fig. 3. The cross-sectional fracture surface morphology of a Type B coating.
layer thickness less than 15 nm are deposited as a transition zone (Type C), the superlattice layers exhibit a (2 0 0) preferred orientation. Furthermore, testing results show that the change in superlattice volume percentage does not affect the crystallographic orientation of the superlattice layers. The mechanical properties of multilayered coatings with different superlattice volume fractions and preferred orientations have been measured. The effective Young’s modulus E* and the hardness H of the coatings were determined from the load–displacement curves using Oliver–Pharr method w7x. The mechanical properties measured from the specimens with superlattice bilayer period of ;10 nm, are listed in Table 1. For pure TiNy CrN superlattice coatings, (2 0 0)-oriented coatings yield higher hardness and effective Young’s modulus than the (1 1 1)-oriented counterpart. It is not out of expectation that the similar effect will apply to the multilayered coatings having the same superlattice bilayer period L and volume fraction Vsup. In Table 1, for the coatings with Ls;10 nm and Vsups50%, the hardness decreases from 15.1 to 14.0, to 11.6 GPa, and the effective Young’s modulus decreases from 216 to 211, to 176 MPa for the superlattice constituents with (2 0 0), random and (1 1 1) preferred orientation, respectively. For coatings with the same superlattice preferred orientation, the hardness decreases with decrease in Vsup.
The superlattice coatings with three different superlattice orientations, i.e. (1 1 1), random and (2 0 0), all exhibited adhesion strengths lower than 15 N. The multilayered coating with (1 1 1)-oriented superlattice layers experienced localized spallation adjacent to the track. However, the coating with (2 0 0)-oriented TiNy CrN layers suffered more severe spallations. The spallation areas usually have step-like edges indicating weak interfacial adhesion as shown in Fig. 4. It has been reported that the large hardness difference between the coating and substrate can lead to low adhesion strength w8x. For multilayered Tiysuperlattice coatings, the large hardness difference between the hard superlattice and the soft pure Ti layers may cause weakening in the interfacial strength, which is detrimental to erosion resistance of the coatings. In this study, the multilayered coatings were subjected to erosion tests at 908 impingement angle by high-speed
Table 1 The mechanical properties of coatings Vsup (%)
Orientation
H (GPa)
E* (GPa)
100 100 62 50 ;50 ;50 38
(2 0 0) (1 1 1) Random (2 0 0) (1 1 1) Random (2 0 0)
40.1 28.7 17.8 15.1 11.6 14.0 12.7
398 347 242 216 176 211 173
Fig. 4. The morphology of a spallation area showing the interfacial detachments between coatingysubstrate and Tiysuperlattice layers for a multilayered coating.
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
207
Fig. 5. The typical surface morphologies at different erosion track locations of the coating with (1 1 1)-oriented superlattice (Ls;10 nm) and superlattice volume fraction of 50%.
erosive Al2O3 particles. The erosion track of multilayered coatings did not reveal the layered structure during testing. Instead, localized spallations were observed as a result of the particle impingements (Fig. 5a). The erosion crater created by a single particle impact (Fig. 5b) exhibited plastic deformation and crack formation, especially at the crater edge. Due to the large difference in hardness between the soft Ti and hard superlattice constituents, the hard layers responded to the impact by formation of cracks because of their inability to accommodate the plastic deformation produced in the soft layer. The repeated impingements of particles on the same location caused coating fragmentation (Fig. 5c). Further particle bombardments gradually enlarged the fragmentation spots and eventually led to localize coating spallations as shown in Fig. 5d. The exposed substrate surface, together with the step-like areas near the edges, indicates that the accumulated particle impingements were responsible for the localized detachments at the coatingysubstrate interface and Tiysuper-
lattice layer interface. Erosive damage in cracking and delamination modes has also been reported for TiNyTi mulitlayered coatings w9x. The edges of the spalled areas are the weak points, which could easily expand under impingements. Ultimately, the spallation areas will connect with each other to form a large area of substrate exposure. The multilayered coatings with alternate pure Ti and TiNyCrN superlattice layers exhibited a poor erosion performance because of the weak interfacial strength. The erosion resistance of the coatings appear to be governed by the adhesion between coatingysubstrate, as well as the cohesion between hard and soft layers, as failures often occur at layer interfaces as shown in this study and in a TiNyTi multilayered system w10x. This problem could be solved by introducing compositional graded toughening layers instead of pure metal layers, to improve mechanical compatibility between the hardening and toughening constituents. More work is under-
208
Q. Yang et al. / Surface and Coatings Technology 177 – 178 (2004) 204–208
way to modify the microstructure of multilayered coatings with enhanced erosion performance. 4. Summary Multilayered coatings containing alternate layers of pure Ti and TiNyCrN superlattice with controlled preferred orientation, bilayer period and volume fraction, have been successfully deposited by magnetron sputtering. The preferred orientation of the TiNyCrN superlattice layers can be controlled by controlling the sub-structure between the Ti and superlattice layers. The weak interfacial strength between coatingysubstrate and superlatticeyTi is negatively affected by the large hardness difference between the superlattice and Ti layers. The weak interfacial strength of the multilayered coatings is primarily responsible for the lack of satisfactory erosion protection performance, and more work is underway to address the problem. Acknowledgments The authors wish to acknowledge the financial support from the Institute for Aerospace Research, National
Research Council of Canada, under the IAR New Initiative Fund Program. They also thank R. McKellar for his technical support in this project. References w1x A. Leyland, A. Matthews, Surf. Coat. Technol. 70 (1994) 19. w2x Y. Gachon, P. Ienny, A. Rorner, G. Farges, M.C. SainteCatherine, A.B. Vannes, Surf. Coat. Technol. 113 (1999) 140. w3x E. Quesnel, Y. Pauleau, P. Monge-Cadet, M. Burn, Surf. Coat. Technol. 62 (1993) 474. w4x M. Vill, D.P. Adams, S.M. Yalisove, J.C. Bilello, Acta Metall. Mater. 43 (1995) 427. w5x D.P. Adams, M. Will, J. Tao, J.C. Bilello, S.M. Yalisove, J. Appl. Phys. 74 (1993) 1015. w6x M. Vill, D.P. Adams, S.M. Yalisove, J.C. Bilello, Mat. Res. Soc. Symp. Proc. 308 (1993) 759. w7x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. w8x E. Harry, A. Rouzaud, P. Juliet, Y. Pauleau, Thin Solid Films 342 (1999) 207. w9x A. Leyland, A. Matthews, Surf. Coat. Technol. 70 (1994) 19. w10x M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 90 (1997) 217.