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Study of adhesion of TiN grown on a polymer substrate
Surface & Coatings Technology 201 (2007) 6742 – 6744 www.elsevier.com/locate/surfcoat
Study of adhesion of TiN grown on a polymer substrate C. Chaiwong ⁎, D.R. McKenzie, M.M.M. Bilek Applied and Plasma Physics, School of Physics, The University of Sydney, NSW 2006, Australia Available online 3 November 2006
Abstract TiN films were deposited on polycarbonate substrates by cathodic vacuum arc using the plasma immersion ion implantation and deposition (PIII&D) method. The biaxial intrinsic stress in the film deposited using PIII&D with 3 kV applied bias was 0.3 GPa — much lower than that found in films deposited without the application of high-voltage pulsed bias. It was found that the dominant mechanism for generating stress in the TiN film was thermal stress arising from the large difference between the thermal expansion coefficient of TiN and that of the polymer. Tensile testing was used to ascertain film adhesion and a model was used to estimate the adhesion between the film and the substrate. It was found that PIII&D strongly reduced the stress in the TiN film and increased the adhesion to the polycarbonate. The ultimate shear strength of adhesion is of the same order of magnitude as that of TiN on stainless steel. © 2006 Elsevier B.V. All rights reserved. PACS: 52.77.Dq; 85.40.sZ; 63.37.Hk; 62.20Mk; 65.40.De Keywords: Cathodic vacuum arc; PIII&D; Adhesion; Thermal expansion mismatch
1. Introduction Titanium nitride has a wide range of applications due to a unique combination of properties such as high hardness, good chemical inertness, and excellent wear resistance. TiN can be deposited onto steels, such as punching dies and high speed tools for increasing their performance [1–3]. It may also be a suitable protective layer for the surface of polymeric materials. Recently, research has been performed on applying titanium nitride onto thermoplastic substrates [4–8]. The films were deposited by magnetron sputtering and the morphologies of the films on different polymer substrates e.g. PA, PC, PEEK and PBT were analysed. The deposition parameters, activation of the polymer surface prior to deposition, and the use of a metallic intermediate layer were found to play important roles on the microstructure of the coatings and the film–substrate adhesion. Adhesion to the polymer surface for the service life of the product is central to the success of the protective coating and methods to assess the adhesion under typical service conditions are needed. Recently, it has been shown that filtered cathodic vacuum arc deposition using the plasma immersion ion implantation and deposition (PIII&D) method substantially reduces intrinsic ⁎ Corresponding author. Tel.: +61 2 93515962; fax: +61 2 9351 7726. E-mail address: [email protected] (C. Chaiwong). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.046
stress in the deposited materials [9]. The stress in TiN films can be reduced by the impact of energetic ions [10]. In this paper, we investigate the adhesion of TiN films grown on polycarbonate using a combination of filtered cathodic vacuum arc and PIII&D. The adhesion strength between the film and the substrate was evaluated. 2. Experimental procedure The substrate material was 1-mm thick polycarbonate (Lexan®) sheet. The sheet was cut into a tensile sample with a parallel gauge section of length 12 mm and width 3 mm and used in the as-received state. Filtered cathodic vacuum arc with titanium cathode was employed for deposition of the film. The cathodic arc system [11] consists of a cathode chamber connected to the substrate chamber by a 140-cm long filter duct. The titanium plasma was ignited by a mechanical striker making contact with the cathode. The wall of the duct is surrounded by a series of co-axial coils which when supplied with a current act as a curved magnetic solenoid to guide the plasma stream. The plasma density and hence the deposition rate in the substrate chamber can be altered by changing the current in the magnetic filter coils. An arc current of 60 A was used. The deposition rate was kept low (2 nm/min) to minimize substrate heating. This low deposition rate was achieved by
C. Chaiwong et al. / Surface & Coatings Technology 201 (2007) 6742–6744
defocusing the magnetic filter coils to reduce the density of the plasma transported to the substrate chamber. Prior to TiN deposition, a thin layer of Ti was deposited onto the surface of the substrate in order to enhance the adhesion of the TiN film. Nitrogen reactive gas was introduced into the system for TiN deposition. The substrate holder was mounted on a highvoltage, insulating feed-through and connected to the pulser unit (RUP 3-3A, GBS-Elektronik GmbH, Germany). The highvoltage negative pulses were set to 3 kV. The pulse width and frequency setting in this work were 10 μs and 1.2 kHz, respectively. The films were deposited at room temperature without cooling or heating system. During deposition, a Luxtron® fluoroptic temperature probe (Luxtron Corporation, CA) was applied to measure the temperature on the back side of the samples. The temperature was recorded until the deposition was completed. Film thickness was measured after deposition with a Tencor™ surface profilometer. The stress of the film was deduced by measuring the radius of curvature of a 20 mm × 20 mm × 0.25 mm sheet before and after deposition and by applying Stoney's equation [12]. After deposition, tensile test was used to evaluate the adhesion between the film and the substrate. A small tensile device positioned under an optical microscope was used. The displacement and load during straining at a crosshead speed of 3 μm/s were measured and images of the surface were captured every 2 s. Post-analysis of the sample was done by a Phillips scanning electron microscopy (XL30).
6743
Fig. 2. The stress–strain curve of the dog-bone shape sample during the tensile test. There are 2 significant regions to be considered: the strain at which the cracks developed (2%), and the strain at which the cracks saturate (4%).
stress is dominant. The thermal stress of the film can be calculated from: r¼
Ef ðaf −as ÞðTd −Tr Þ; 1−mf
ð1Þ
Fig. 1 presents an SEM micrograph of the TiN film on polycarbonate. The film thickness is 80 nm. There is no evidence of cracks. The temperature of the sample ranged from 23 °C at the beginning to 45 °C at the end of the deposition. The compressive stress of the film is 0.3 GPa. The stress is remarkably small compared to 2.3 GPa of the film deposited without PIII&D. Such high level of stress causes spontaneous delamination of the film. The residual stress in the films is a result of two mechanisms: the mismatch in the thermal expansion coefficient between the coating and the substrate and the intrinsic stress [9]. In our case, due to a large difference between the thermal expansion coefficient of polycarbonate and that of titanium nitride (αs = 7αf), the thermal
where Ef is Young's modulus of the film, νf is Poisson's ratio of the film, αf and αs are the thermal expansion coefficient of the film and the substrate, respectively, Td is the deposition temperature, and Tr is the room temperature. The parameters for TiN and polycarbonate are: E f = 350 GPa, ν f = 0.25, α f = 9.4 × 10 − 6 K − 1 , αs = 70 × 10− 6 K− 1. For the deposition temperature in our experiment, the resultant thermal stress is 0.6 GPa. This is in the same order of magnitude as the measured stress for films deposited using the PIII&D process during deposition. Fig. 2 shows the typical stress–strain curve of the test. The sample was strained until a plastic deformation neck developed. The strain at which the film started to crack is about 2% and the crack saturation occurred at about 4% strain. The SEM micrograph of the sample after the test is shown in Fig. 3. The parallel cracks observed due to the tensile loading are typical of brittle coatings and oriented perpendicular to the tensile axis. The crack spacing is of the order of a few microns. The film is
Fig. 1. SEM micrograph of the 80-nm thick titanium nitride film on the polycarbonate substrate.
Fig. 3. SEM micrograph of the dog-bone shape sample after the tensile test. The tensile axis is in vertical direction.
3. Results and discussion
6744
C. Chaiwong et al. / Surface & Coatings Technology 201 (2007) 6742–6744
considered to be well-adhered to the substrate since no dramatic buckling or spallation is evident. The interfacial shear strength parallel to the surface can be evaluated by a tensile test, such as the one we have used, by pulling the sample along the plane of the film. When a load is applied to the substrate, shear stresses are developed in the vicinity of the interface, resulting from the difference in Young's modulus between the substrate and the film. The strain at which cracks begin to appear gives a measure of the tensile fracture strength of the film. On further straining, the density of cracks increases but beyond a certain strain limit, the crack density tends to stabilize. The saturation spacing between the cracks was analysed to yield the ultimate shear strength of the interface in the case of hard coatings deposited on metal substrates [13–15]. In this work, we will apply this method to a system in which the substrate is a polymer which also deforms plastically while the coating deforms elastically as in the metal substrate case. According to the model [12], the ultimate shear strength τ for decohesion is expressed as s¼
kd rF ; k
ð2Þ
where δ is the film thickness, λ is the maximum crack spacing, σF is the film fracture stress. The strain at which the crack begins to appear corresponds to the fracture strain εF of the film. In our experiment, it is determined to be 2%. The fracture stress is given by rF ¼ eF Ef ;
ð3Þ
which is equal to 7 GPa in this experiment, since Ef = 350 GPa for TiN. We measured the maximum crack spacing to be 3 μm. Therefore, we calculate the ultimate shear strength of the interface to be 0.6 GPa using Eq. (2). The residual stress in the film is not taken into account in this model. Since a compressive residual stress is in the film after deposition, the fracture strength used in Eq. (2) will be equal to the apparent fracture stress obtained from Eq. (3), plus the compressive residual stress i.e. σF + σr. Since the compressive residual stress in the film is 0.3 GPa, thus, the corrected fracture stress is 6.7 GPa. From the point of view of the thin film, the applied tensile stress is first offset by the compressive residual stress in the film before a tensile stress is experienced by the film. It is, however, the resultant tensile stress which would eventually cause cracking to occur. The ultimate shear strength of adhesion is of the same order of magnitude as that of TiN on stainless steel [15–17]. We believe that the excellent adhesion of TiN on polycarbonate observed is due to the interface mixing and stress relaxation provided by employing PIII&D during film deposition. To assess the degree of interface mixing likely to occur in this system we carried out theoretical calculations of the implantation range of ions using a Monte-Carlo simulation program, SRIM 2003 (the Stopping and Range of Ions in Matter). The average range of 3-keV titanium ions in polycarbonate was calculated to be 94 A. SRIM predicted that 3-keV titanium and nitrogen ions implanted into 100-A titanium layer will have an average range of 42 A and 53 A, with 1.4 and 38% transmission, respectively. A calculation of implantation into the composite structure predicted
that titanium and nitrogen ions implanted through the 100-A titanium layer into the polycarbonate will have an average range of 43 A and 96 A in the titanium on polycarbonate structure, respectively. It can be seen that the process achieves implantation into the substrate causing interface mixing at both the Ti/TiN and Ti/polycarbonate interfaces as well as a modification of the material deposited onto the substrate during growth. Adhesion may also be enhanced by the presence of the Ti interlayer. 4. Conclusions We have investigated the deposition of TiN films on polycarbonate substrates. The film was compressively stressed due to a mismatch of thermal expansion coefficients. The stress in the film deposited with PIII&D was found to be remarkably reduced compared to that of the film deposited without PIII&D. The adhesion of the film to the substrate was studied by means of a tensile test. Adhesion strength comparable to that found for TiN films on steel substrates was achieved. The good adhesion is attributed to the reduction of stress and interface mixing induced by using PIII&D during deposition. Acknowledgements We wish to thank B. Latella from the Australian Nuclear Science and Technology Organization (ANSTO) for the discussion about tensile test. C. Chaiwong would like to thank the Thai Government and AINSE Postgraduate Research Awards for financial support. References [1] P. Hedenqvist, M. Olsson, P. Wallen, A. Kassman, S. Hogmark, S. Jacobson, Surf. Coat. Technol. 41 (1990) 243. [2] O. Knotek, A. Barimani, B. Bosserhoff, F. Loffler, Thin Solid Films 193/ 194 (1990) 557. [3] J.R. Roos, J.P. Celis, E. Vancoile, H. Veltrop, S. Bolens, F. Jungblut, J. Eberbrinf, H. Holmberg, Surf. Coat. Technol. 193/194 (1990) 547. [4] E. Lugscheider, S. Barwulf, M. Riester, H. Hilgers, Surf. Coat. Technol. 116–119 (1999) 1172. [5] E. Lugscheider, K. Bobzin, M. Maes, A. Kramer, Thin Solid Films 459 (2004) 313. [6] M. Riester, S. Barwulf, E. Lugscheider, H. Hilgers, Surf. Coat. Technol. 116–119 (1999) 1001. [7] M. Ahern, Surf. Coat. Technol. 43/44 (1990) 279. [8] I. Grimberg, B. Bouaifi, U. Draugelates, K. Soifer, B.Z. Weiss, Surf. Coat. Technol. 68/69 (1994) 166. [9] C. Chaiwong, D.R. McKenzie, M.M.M. Bilek, Surf. Coat. Technol. (in press). [10] S.H.N. Lim, D.G. McCulloch, M.M.M. Bilek, D.R. McKenzie, Surf. Coat. Technol. 174–175 (2003) 76. [11] R.N. Tarrant, N. Fujisawa, M.V. Swain, N.L. James, D.R. McKenzie, J.C. Woodard, Surf. Coat. Technol. 156 (2002) 143. [12] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation, and Surface Evolution, Cambridge University Press, New York, 2003. [13] D.C. Agrawal, R. Raj, Acta Metall. 37 (1989) 1265. [14] B.F. Chen, J. Hwang, I.F. Chen, G.P. Yu, J.-H. Huang, Surf. Coat. Technol. 126 (2000) 91. [15] B.F. Chen, J. Hwang, G.P. Yu, J.H. Huang, Thin Solid Films 352 (1999) 173. [16] M.H. Shiao, F.S. Shieu, Thin Solid Films 358 (2000) 159. [17] W.J. Chou, G.P. Yu, J.H. Huang, Surf. Coat. Technol. 149 (2002) 7.
Study of adhesion of TiN grown on a polymer substrate C. Chaiwong ⁎, D.R. McKenzie, M.M.M. Bilek Applied and Plasma Physics, School of Physics, The University of Sydney, NSW 2006, Australia Available online 3 November 2006
Abstract TiN films were deposited on polycarbonate substrates by cathodic vacuum arc using the plasma immersion ion implantation and deposition (PIII&D) method. The biaxial intrinsic stress in the film deposited using PIII&D with 3 kV applied bias was 0.3 GPa — much lower than that found in films deposited without the application of high-voltage pulsed bias. It was found that the dominant mechanism for generating stress in the TiN film was thermal stress arising from the large difference between the thermal expansion coefficient of TiN and that of the polymer. Tensile testing was used to ascertain film adhesion and a model was used to estimate the adhesion between the film and the substrate. It was found that PIII&D strongly reduced the stress in the TiN film and increased the adhesion to the polycarbonate. The ultimate shear strength of adhesion is of the same order of magnitude as that of TiN on stainless steel. © 2006 Elsevier B.V. All rights reserved. PACS: 52.77.Dq; 85.40.sZ; 63.37.Hk; 62.20Mk; 65.40.De Keywords: Cathodic vacuum arc; PIII&D; Adhesion; Thermal expansion mismatch
1. Introduction Titanium nitride has a wide range of applications due to a unique combination of properties such as high hardness, good chemical inertness, and excellent wear resistance. TiN can be deposited onto steels, such as punching dies and high speed tools for increasing their performance [1–3]. It may also be a suitable protective layer for the surface of polymeric materials. Recently, research has been performed on applying titanium nitride onto thermoplastic substrates [4–8]. The films were deposited by magnetron sputtering and the morphologies of the films on different polymer substrates e.g. PA, PC, PEEK and PBT were analysed. The deposition parameters, activation of the polymer surface prior to deposition, and the use of a metallic intermediate layer were found to play important roles on the microstructure of the coatings and the film–substrate adhesion. Adhesion to the polymer surface for the service life of the product is central to the success of the protective coating and methods to assess the adhesion under typical service conditions are needed. Recently, it has been shown that filtered cathodic vacuum arc deposition using the plasma immersion ion implantation and deposition (PIII&D) method substantially reduces intrinsic ⁎ Corresponding author. Tel.: +61 2 93515962; fax: +61 2 9351 7726. E-mail address: [email protected] (C. Chaiwong). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.046
stress in the deposited materials [9]. The stress in TiN films can be reduced by the impact of energetic ions [10]. In this paper, we investigate the adhesion of TiN films grown on polycarbonate using a combination of filtered cathodic vacuum arc and PIII&D. The adhesion strength between the film and the substrate was evaluated. 2. Experimental procedure The substrate material was 1-mm thick polycarbonate (Lexan®) sheet. The sheet was cut into a tensile sample with a parallel gauge section of length 12 mm and width 3 mm and used in the as-received state. Filtered cathodic vacuum arc with titanium cathode was employed for deposition of the film. The cathodic arc system [11] consists of a cathode chamber connected to the substrate chamber by a 140-cm long filter duct. The titanium plasma was ignited by a mechanical striker making contact with the cathode. The wall of the duct is surrounded by a series of co-axial coils which when supplied with a current act as a curved magnetic solenoid to guide the plasma stream. The plasma density and hence the deposition rate in the substrate chamber can be altered by changing the current in the magnetic filter coils. An arc current of 60 A was used. The deposition rate was kept low (2 nm/min) to minimize substrate heating. This low deposition rate was achieved by
C. Chaiwong et al. / Surface & Coatings Technology 201 (2007) 6742–6744
defocusing the magnetic filter coils to reduce the density of the plasma transported to the substrate chamber. Prior to TiN deposition, a thin layer of Ti was deposited onto the surface of the substrate in order to enhance the adhesion of the TiN film. Nitrogen reactive gas was introduced into the system for TiN deposition. The substrate holder was mounted on a highvoltage, insulating feed-through and connected to the pulser unit (RUP 3-3A, GBS-Elektronik GmbH, Germany). The highvoltage negative pulses were set to 3 kV. The pulse width and frequency setting in this work were 10 μs and 1.2 kHz, respectively. The films were deposited at room temperature without cooling or heating system. During deposition, a Luxtron® fluoroptic temperature probe (Luxtron Corporation, CA) was applied to measure the temperature on the back side of the samples. The temperature was recorded until the deposition was completed. Film thickness was measured after deposition with a Tencor™ surface profilometer. The stress of the film was deduced by measuring the radius of curvature of a 20 mm × 20 mm × 0.25 mm sheet before and after deposition and by applying Stoney's equation [12]. After deposition, tensile test was used to evaluate the adhesion between the film and the substrate. A small tensile device positioned under an optical microscope was used. The displacement and load during straining at a crosshead speed of 3 μm/s were measured and images of the surface were captured every 2 s. Post-analysis of the sample was done by a Phillips scanning electron microscopy (XL30).
6743
Fig. 2. The stress–strain curve of the dog-bone shape sample during the tensile test. There are 2 significant regions to be considered: the strain at which the cracks developed (2%), and the strain at which the cracks saturate (4%).
stress is dominant. The thermal stress of the film can be calculated from: r¼
Ef ðaf −as ÞðTd −Tr Þ; 1−mf
ð1Þ
Fig. 1 presents an SEM micrograph of the TiN film on polycarbonate. The film thickness is 80 nm. There is no evidence of cracks. The temperature of the sample ranged from 23 °C at the beginning to 45 °C at the end of the deposition. The compressive stress of the film is 0.3 GPa. The stress is remarkably small compared to 2.3 GPa of the film deposited without PIII&D. Such high level of stress causes spontaneous delamination of the film. The residual stress in the films is a result of two mechanisms: the mismatch in the thermal expansion coefficient between the coating and the substrate and the intrinsic stress [9]. In our case, due to a large difference between the thermal expansion coefficient of polycarbonate and that of titanium nitride (αs = 7αf), the thermal
where Ef is Young's modulus of the film, νf is Poisson's ratio of the film, αf and αs are the thermal expansion coefficient of the film and the substrate, respectively, Td is the deposition temperature, and Tr is the room temperature. The parameters for TiN and polycarbonate are: E f = 350 GPa, ν f = 0.25, α f = 9.4 × 10 − 6 K − 1 , αs = 70 × 10− 6 K− 1. For the deposition temperature in our experiment, the resultant thermal stress is 0.6 GPa. This is in the same order of magnitude as the measured stress for films deposited using the PIII&D process during deposition. Fig. 2 shows the typical stress–strain curve of the test. The sample was strained until a plastic deformation neck developed. The strain at which the film started to crack is about 2% and the crack saturation occurred at about 4% strain. The SEM micrograph of the sample after the test is shown in Fig. 3. The parallel cracks observed due to the tensile loading are typical of brittle coatings and oriented perpendicular to the tensile axis. The crack spacing is of the order of a few microns. The film is
Fig. 1. SEM micrograph of the 80-nm thick titanium nitride film on the polycarbonate substrate.
Fig. 3. SEM micrograph of the dog-bone shape sample after the tensile test. The tensile axis is in vertical direction.
3. Results and discussion
6744
C. Chaiwong et al. / Surface & Coatings Technology 201 (2007) 6742–6744
considered to be well-adhered to the substrate since no dramatic buckling or spallation is evident. The interfacial shear strength parallel to the surface can be evaluated by a tensile test, such as the one we have used, by pulling the sample along the plane of the film. When a load is applied to the substrate, shear stresses are developed in the vicinity of the interface, resulting from the difference in Young's modulus between the substrate and the film. The strain at which cracks begin to appear gives a measure of the tensile fracture strength of the film. On further straining, the density of cracks increases but beyond a certain strain limit, the crack density tends to stabilize. The saturation spacing between the cracks was analysed to yield the ultimate shear strength of the interface in the case of hard coatings deposited on metal substrates [13–15]. In this work, we will apply this method to a system in which the substrate is a polymer which also deforms plastically while the coating deforms elastically as in the metal substrate case. According to the model [12], the ultimate shear strength τ for decohesion is expressed as s¼
kd rF ; k
ð2Þ
where δ is the film thickness, λ is the maximum crack spacing, σF is the film fracture stress. The strain at which the crack begins to appear corresponds to the fracture strain εF of the film. In our experiment, it is determined to be 2%. The fracture stress is given by rF ¼ eF Ef ;
ð3Þ
which is equal to 7 GPa in this experiment, since Ef = 350 GPa for TiN. We measured the maximum crack spacing to be 3 μm. Therefore, we calculate the ultimate shear strength of the interface to be 0.6 GPa using Eq. (2). The residual stress in the film is not taken into account in this model. Since a compressive residual stress is in the film after deposition, the fracture strength used in Eq. (2) will be equal to the apparent fracture stress obtained from Eq. (3), plus the compressive residual stress i.e. σF + σr. Since the compressive residual stress in the film is 0.3 GPa, thus, the corrected fracture stress is 6.7 GPa. From the point of view of the thin film, the applied tensile stress is first offset by the compressive residual stress in the film before a tensile stress is experienced by the film. It is, however, the resultant tensile stress which would eventually cause cracking to occur. The ultimate shear strength of adhesion is of the same order of magnitude as that of TiN on stainless steel [15–17]. We believe that the excellent adhesion of TiN on polycarbonate observed is due to the interface mixing and stress relaxation provided by employing PIII&D during film deposition. To assess the degree of interface mixing likely to occur in this system we carried out theoretical calculations of the implantation range of ions using a Monte-Carlo simulation program, SRIM 2003 (the Stopping and Range of Ions in Matter). The average range of 3-keV titanium ions in polycarbonate was calculated to be 94 A. SRIM predicted that 3-keV titanium and nitrogen ions implanted into 100-A titanium layer will have an average range of 42 A and 53 A, with 1.4 and 38% transmission, respectively. A calculation of implantation into the composite structure predicted
that titanium and nitrogen ions implanted through the 100-A titanium layer into the polycarbonate will have an average range of 43 A and 96 A in the titanium on polycarbonate structure, respectively. It can be seen that the process achieves implantation into the substrate causing interface mixing at both the Ti/TiN and Ti/polycarbonate interfaces as well as a modification of the material deposited onto the substrate during growth. Adhesion may also be enhanced by the presence of the Ti interlayer. 4. Conclusions We have investigated the deposition of TiN films on polycarbonate substrates. The film was compressively stressed due to a mismatch of thermal expansion coefficients. The stress in the film deposited with PIII&D was found to be remarkably reduced compared to that of the film deposited without PIII&D. The adhesion of the film to the substrate was studied by means of a tensile test. Adhesion strength comparable to that found for TiN films on steel substrates was achieved. The good adhesion is attributed to the reduction of stress and interface mixing induced by using PIII&D during deposition. Acknowledgements We wish to thank B. Latella from the Australian Nuclear Science and Technology Organization (ANSTO) for the discussion about tensile test. C. Chaiwong would like to thank the Thai Government and AINSE Postgraduate Research Awards for financial support. References [1] P. Hedenqvist, M. Olsson, P. Wallen, A. Kassman, S. Hogmark, S. Jacobson, Surf. Coat. Technol. 41 (1990) 243. [2] O. Knotek, A. Barimani, B. Bosserhoff, F. Loffler, Thin Solid Films 193/ 194 (1990) 557. [3] J.R. Roos, J.P. Celis, E. Vancoile, H. Veltrop, S. Bolens, F. Jungblut, J. Eberbrinf, H. Holmberg, Surf. Coat. Technol. 193/194 (1990) 547. [4] E. Lugscheider, S. Barwulf, M. Riester, H. Hilgers, Surf. Coat. Technol. 116–119 (1999) 1172. [5] E. Lugscheider, K. Bobzin, M. Maes, A. Kramer, Thin Solid Films 459 (2004) 313. [6] M. Riester, S. Barwulf, E. Lugscheider, H. Hilgers, Surf. Coat. Technol. 116–119 (1999) 1001. [7] M. Ahern, Surf. Coat. Technol. 43/44 (1990) 279. [8] I. Grimberg, B. Bouaifi, U. Draugelates, K. Soifer, B.Z. Weiss, Surf. Coat. Technol. 68/69 (1994) 166. [9] C. Chaiwong, D.R. McKenzie, M.M.M. Bilek, Surf. Coat. Technol. (in press). [10] S.H.N. Lim, D.G. McCulloch, M.M.M. Bilek, D.R. McKenzie, Surf. Coat. Technol. 174–175 (2003) 76. [11] R.N. Tarrant, N. Fujisawa, M.V. Swain, N.L. James, D.R. McKenzie, J.C. Woodard, Surf. Coat. Technol. 156 (2002) 143. [12] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation, and Surface Evolution, Cambridge University Press, New York, 2003. [13] D.C. Agrawal, R. Raj, Acta Metall. 37 (1989) 1265. [14] B.F. Chen, J. Hwang, I.F. Chen, G.P. Yu, J.-H. Huang, Surf. Coat. Technol. 126 (2000) 91. [15] B.F. Chen, J. Hwang, G.P. Yu, J.H. Huang, Thin Solid Films 352 (1999) 173. [16] M.H. Shiao, F.S. Shieu, Thin Solid Films 358 (2000) 159. [17] W.J. Chou, G.P. Yu, J.H. Huang, Surf. Coat. Technol. 149 (2002) 7.