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Mechanical properties of deposited carbon thin films on sapphire substrates using atomic force microscopy (AFM)
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10159–10162 www.elsevier.com/locate/ceramint
Short communication
Mechanical properties of deposited carbon thin films on sapphire substrates using atomic force microscopy (AFM) Suat Patn, M. Zafer Balbağ, Şadan Korkmaz Physics Department, Eskisehir Osmangazi University, 26480, Turkey Received 10 December 2013; received in revised form 13 February 2014; accepted 13 February 2014 Available online 22 February 2014
Abstract In this study, the mechanical properties of nano-layered carbon thin films deposited on Al2O3 single crystal substrates with varying orientations in A (1120), M (1010), R (1102) and C (0001) planes were investigated using depth-sensing nanoindentation techniques. All of the nano-layered films were deposited by a thermionic vacuum arc (TVA). In this experiment, a high purity amorphous carbon rod was used. Single crystal Al2O3 plates were used as the substrate material. All of the substrates were a commercial product that is readily available. For the surface topography, the roughness and depth-sensing nano-hardness of the deposited films were analyzed using an Ambios Q-scope atomic force microscope. An F20 thin film measurement system was used for the determination of thickness and reflection properties of the deposited thin films. Indention depths were determined as 5 nm, 10 nm and 15 nm. In these indentation depths, hardness values were calculated in the range of 8–25 GPa. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Deposition; Nanomaterials; Sapphire; Thin films
1. Introduction The nanomechanical behaviors of the surfaces of layered materials are of great interest at the nanometer scale and are studied using nanoindentations with atomic force microscopy [1,2]. Nanoindentation is a hardness measurement method applied to thin films and bulk materials of small volumes [3]. Generally, nanoindentation analysis is applied to depths at the nanometer scale [2–6]. Atomic force microscopy is a powerful technique for determining the nanomechanical properties of nano-structured materials [5,7]. The depth scale for nanoindentation has been selected to be up to 40–50 nm [1–7]. This region is elastic and elastoplastic. The indentation depth should only be approximately 10–20% of the total film thickness to avoid substrate effects [5–7]. Typically, the applied loads to the indenter were on the order of nNs [5–7]. The hardness (H) and Young's modulus (E) of the coated carbon layers are calculated from the load-unload curve of the n
Corresponding author. Tel./fax: þ 90 222 2393750x2822. E-mail address: [email protected] (S. Pat).
http://dx.doi.org/10.1016/j.ceramint.2014.02.041 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
nanoindentation experiment [1–11]. Oliver and Pharr's method is the best technique for determining the mechanical properties at the nanoscale for nano-layered and monolayer thin films. In addition, this technique may determine the depth profile of mechanical properties. This technique is also called the depth-sensing nanoindentation technique [12]. For hardness calculations, the coated layer hardness is calculated using H¼
Pmax ; A
ð1Þ
where Pmax is the maximum loading, and A is the projected area of the indentation impression of the pushup force used on the indenter in the AFM. This effect determines the elastic properties of the layer. The elastic properties are determined using pffiffiffi π S pffiffiffi ; ð2Þ Er ¼ 2 A where S is the slope of the unloading curve of the nanoindentation, and Er is the reduced modulus.
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Nanoindentation tests, hardness, and Young's modulus of the deposited thin films were determined using Oliver and Pharr's method. E can be obtained from the following equation: 1 1 v2 1 v2i þ ¼ Er E Ei
ð3Þ
whereE i and vi are Young's modulus and Poisson coefficient of the indenter used in the AFM, respectively. E and v are Young's modulus and Poisson coefficient of the tested coated layer, respectively. In this study, the mechanical properties of nano-layered thin films deposited on Al2O3 single crystal substrates with varying orientations in A (1120), M (1010), R (1102) and C (0001) planes were investigated using depth-sensing nanoindentation techniques. All of the nano-layered films were deposited by a thermionic vacuum arc (TVA). In this experiment, a high purity amorphous carbon rod was used. Single crystal Al2O3
plates were used as the substrate material. All of the substrates were a commercial product that is readily available (purchased from MTI cooperation). For the surface topography, the roughness and depth-sensing nano-hardness of deposited films were analyzed using an Ambios Q-scope AFM (atomic force microscope). An F20 thin film measurement system was used for the determination of the thickness and reflection properties of the deposited thin films. 2. Sample preparation TVA is an anodic plasma vacuum arc. TVAs have a number of significant advantages as compared to deposited thin films, such as their pure thin film deposition, compactness, low roughness, nano-structures, homogeneity, good adhesive properties, fast processing times of a few minutes, low working budgets, and high deposition rate, among others [11,13–17].
Fig. 1. AFM images of nanolayered carbon thin films deposited on different Al2O3 single crystal oriented substrates in A (1120), M (1010), R (1102) and C (0001) planes.
S. Pat et al. / Ceramics International 40 (2014) 10159–10162
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Fig. 2. (a) Hardness and (b) Young’s modulus of nano-layered carbon thin films deposited on different Al2O3 single crystal oriented substrates in A (1120), M (1010), R (1102) and C (0001) planes.
A TVA is also an anodic plasma generator that works in high vacuum conditions (10 5–10 6 Torr) for pure material plasmas. A TVA with a double Wehnelt was designed to deposit the thin films. All of the thin films were deposited on single crystal (sapphire) substrates with varying orientations using a thermionic vacuum arc in a high vacuum. The Al2O3 single crystal substrates were purchased from MTI Corporation. 3. Results and discussions The thicknesses of nano-layered thin films deposited on sapphire substrates were measured to be 40–60 nm using a Filmetrics F20 device. All of the AFM images were obtained using a 10 mm 10 mm scale. These coated surface images are shown in Fig. 1. The ranged scale in Fig. 1 is 2 mm. All of the substrates are deposited simultaneously. As shown in Fig. 1, the substrates exhibit varying surface morphologies. The average roughnesses of nano-layered films were determined using 40 lines at a 10 mm 10 mm scale using AFM. The average roughness values of the deposited thin films were determined to be approximately 2–5 nm. Indention depths were determined as 5 nm, 10 nm and 15 nm. Using Oliver and Pharr's method, the calculated hardness and Young's modulus of the nano-layers are shown in Fig. 2 (a) and (b). As the indentation depth increases, the hardness and Young's modulus of the coated nano-layered thin films decreases, as observed in Fig. 2. The hardness of the sapphire is independent of the crystals’ orientations [18]. This hardness value is approximately 20 GPa [18]. In addition, Young's modulus for A (1120), M (1010), R (1102) and C (0001) planes is 345 GPa [19]. Fig. 2 shows the hardness depth profile of the nano-layered films. The calculated hardness values of the nano-layered thin films deposited on different Al2O3 single crystal substrates in A (1120), M (1010), R (1102) and C (0001) planes are approximately 20–25 GPa. These values are in good agreement with those found in the literature [20]. According to the literature, coated, nano-layered thin films have high mechanical nano-hardness.
4. Conclusion In addition, the mechanical properties of the nano carbon layered thin films deposited on different Al2O3 single crystal substrates were analyzed using a depth-sensing nanoindentation technique. Indention depths were determined as 5 nm, 10 nm and 15 nm. In these indentation depths, hardness values were calculated in the range of 8–25 GPa. When the nanoindentation depth increased, the nano-hardness and Young's modulus of the samples decreased. The reasons for this decrease are complex, but dislocations and the surface energy distribution play important roles. All of the carbon coated samples showed elastoplastic properties. The surface properties of the coated samples varied according to the AFM images. The mechanical properties of the samples were almost identical. Based on these results, we conclude that the crystal orientation of the substrates does not have a significant influence on the mechanical properties of the deposited layers. However, the formation of the surface changed because of the surface energy distributions. The depths analyzed during this small operation were investigated successfully. Acknowledgments The authors thank Eskisehir Osmangazi University Scientific Research Projects Commission (ESOGU-BAP) for the support through Grant 2001219026. References [1] J. Fraxedas, S. Garcia-Manyes, P. Gorostiza, F. Sanz, Nanoindentation: toward the sensing of atomic interactions, Proc. Natl. Acad. Sci. USA 99 (2002) 5228–5232. [2] J. Caro, P. Gorostiza, F. Sanz, J. Fraxedas, Nanomechanical properties of surfaces of molecular organic thin films, Synth. Met. 121 (2001) 1417–1418. [3] A.A. Elmustafa, D.S. Stone, Stacking fault energy and dynamic recovery: do they impact the indentation size effect?, Mater Sci. Eng. A 358 (2003) 1–8.
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S. Pat et al. / Ceramics International 40 (2014) 10159–10162
[4] S.G. Corcoran, R.J. Colton, E.T. Lilleodden, W.W. Gerberich, Anomalous plastic deformation at surfaces: nanoindentation of gold single crystals, Phys. Rev. B 55 (1997) 16057–16060. [5] S. Garcia-Manyes, A.G. Guell, P. Gorostiza, F. Sanz, Nanomechanics of silicon surfaces with atomic force microscopy: an insight to the first stages of plastic deformation, J. Chem. Phys. (2005) 123. [6] S. Hara, T. Kumagai, S. Izumi, S. Sakai, Multiscale analysis on the onset of nanoindentation-induced delamination: effect of high-modulus Ru overlayer, Acta Mater. 57 (2009) 4209–4216. [7] D. Tranchida, Z. Kiflie, S. Acierno, S. Piccarolo, Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: viscoelastic characterization of a model material, Meas. Sci. Technol. (2009) 20. [8] A. Rar, G.M. Pharr, W.C. Oliver, E. Karapetian, S.V. Kalinin, Piezoelectric nanoindentation, J. Mater. Res. 21 (2006) 552–556. [9] A. Bolshakov, W.C. Oliver, G.M. Pharr, An explanation for the shape of nanoindentation unloading curves based on finite element simulation, Thin Films: Stresses and Mechanical Properties V, 356, , 1995, p. 675–680. [10] G.M. Pharr, W.C. Oliver, Nanoindentation of silver-relations between hardness and dislocation-structure, J. Mater. Res. 4 (1989) 94–101. [11] S. Pat, S. Temel, N. Ekem, S. Korkmaz, M. Ozkan, M.Z. Balbag, Diamond-like carbon coated on polyethylene terephthalate by Thermionic Vacuum Arc, J. Plast. Film Sheeting 27 (2011) 127–137. [12] Z.Q. Fang, M. Hu, W. Zhang, X.R. Zhang, H.B. Yang, Mechanical properties of porous silicon by depth-sensing nanoindentation techniques, Thin Solid Films 517 (2009) 2930–2935.
[13] S. Pat, N. Ekem, T. Akan, O. Kusmuş, S. Demirkol, R. Vladoiu, C. P. Lungu, G. Musa, Study on thermionic vacuum arc – a novel and advanced technology for surface coating, J. Optoelectron. Adv. Mater. 7 (2005) 2495–2499. [14] N. Ekem, S. Pat, G. Musa, M.Z. Balbağ, I. Cenik, R. Vladoiu, Carbon thin film deposition by thermionic vacuum arc (TVA), J. Optoelectron. Adv. Mater. 10 (2008) 672–674. [15] G. Musa, I. Mustata, M. Blideran, V. Ciupina, R. Vladoiu, G. Prodan, E. Vasile, H. Ehrich, Thermionic vacuum arc (TVA) – new technique for high purity carbon thin film deposition, Acta Phys. Slovaka 5 (2005) 417–421. [16] G. Musa, I. Mustata, V. Ciupina, R. Vladoiu, G. Prodan, C.P. Lungu, H. Ehrich, Thermionic vacuum arc (TVA) – carbon thin film deposition, J. Optoelectron. Adv. Mater. 7 (2005) 2485–2487. [17] G. Musa, I. Mustata, M. Blideran, V. Ciupina, R. Vladoiu, G. Prodan, E. Vasile, Nanostructured carbon thin film deposition using Thermionic vacuum arc (TVA) technology, J. Optoelectron. Adv. Mater. 5 (2003) 667–673. [18] A.B. Sinani, N.K. Dynkin, P.V. Konevskiy, L.A. Lytvynov, Dependence of sapphire hardness on loading type and orientation, Funct. Mater. 16 (2009) 456–459. [19] Technical data sheet of sapphire wafers and substrates 〈http://www. valleydesign.com/sappprop.htm〉, 2012. Access date: 26.09.2012. [20] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2001) 129–281.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10159–10162 www.elsevier.com/locate/ceramint
Short communication
Mechanical properties of deposited carbon thin films on sapphire substrates using atomic force microscopy (AFM) Suat Patn, M. Zafer Balbağ, Şadan Korkmaz Physics Department, Eskisehir Osmangazi University, 26480, Turkey Received 10 December 2013; received in revised form 13 February 2014; accepted 13 February 2014 Available online 22 February 2014
Abstract In this study, the mechanical properties of nano-layered carbon thin films deposited on Al2O3 single crystal substrates with varying orientations in A (1120), M (1010), R (1102) and C (0001) planes were investigated using depth-sensing nanoindentation techniques. All of the nano-layered films were deposited by a thermionic vacuum arc (TVA). In this experiment, a high purity amorphous carbon rod was used. Single crystal Al2O3 plates were used as the substrate material. All of the substrates were a commercial product that is readily available. For the surface topography, the roughness and depth-sensing nano-hardness of the deposited films were analyzed using an Ambios Q-scope atomic force microscope. An F20 thin film measurement system was used for the determination of thickness and reflection properties of the deposited thin films. Indention depths were determined as 5 nm, 10 nm and 15 nm. In these indentation depths, hardness values were calculated in the range of 8–25 GPa. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Deposition; Nanomaterials; Sapphire; Thin films
1. Introduction The nanomechanical behaviors of the surfaces of layered materials are of great interest at the nanometer scale and are studied using nanoindentations with atomic force microscopy [1,2]. Nanoindentation is a hardness measurement method applied to thin films and bulk materials of small volumes [3]. Generally, nanoindentation analysis is applied to depths at the nanometer scale [2–6]. Atomic force microscopy is a powerful technique for determining the nanomechanical properties of nano-structured materials [5,7]. The depth scale for nanoindentation has been selected to be up to 40–50 nm [1–7]. This region is elastic and elastoplastic. The indentation depth should only be approximately 10–20% of the total film thickness to avoid substrate effects [5–7]. Typically, the applied loads to the indenter were on the order of nNs [5–7]. The hardness (H) and Young's modulus (E) of the coated carbon layers are calculated from the load-unload curve of the n
Corresponding author. Tel./fax: þ 90 222 2393750x2822. E-mail address: [email protected] (S. Pat).
http://dx.doi.org/10.1016/j.ceramint.2014.02.041 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
nanoindentation experiment [1–11]. Oliver and Pharr's method is the best technique for determining the mechanical properties at the nanoscale for nano-layered and monolayer thin films. In addition, this technique may determine the depth profile of mechanical properties. This technique is also called the depth-sensing nanoindentation technique [12]. For hardness calculations, the coated layer hardness is calculated using H¼
Pmax ; A
ð1Þ
where Pmax is the maximum loading, and A is the projected area of the indentation impression of the pushup force used on the indenter in the AFM. This effect determines the elastic properties of the layer. The elastic properties are determined using pffiffiffi π S pffiffiffi ; ð2Þ Er ¼ 2 A where S is the slope of the unloading curve of the nanoindentation, and Er is the reduced modulus.
10160
S. Pat et al. / Ceramics International 40 (2014) 10159–10162
Nanoindentation tests, hardness, and Young's modulus of the deposited thin films were determined using Oliver and Pharr's method. E can be obtained from the following equation: 1 1 v2 1 v2i þ ¼ Er E Ei
ð3Þ
whereE i and vi are Young's modulus and Poisson coefficient of the indenter used in the AFM, respectively. E and v are Young's modulus and Poisson coefficient of the tested coated layer, respectively. In this study, the mechanical properties of nano-layered thin films deposited on Al2O3 single crystal substrates with varying orientations in A (1120), M (1010), R (1102) and C (0001) planes were investigated using depth-sensing nanoindentation techniques. All of the nano-layered films were deposited by a thermionic vacuum arc (TVA). In this experiment, a high purity amorphous carbon rod was used. Single crystal Al2O3
plates were used as the substrate material. All of the substrates were a commercial product that is readily available (purchased from MTI cooperation). For the surface topography, the roughness and depth-sensing nano-hardness of deposited films were analyzed using an Ambios Q-scope AFM (atomic force microscope). An F20 thin film measurement system was used for the determination of the thickness and reflection properties of the deposited thin films. 2. Sample preparation TVA is an anodic plasma vacuum arc. TVAs have a number of significant advantages as compared to deposited thin films, such as their pure thin film deposition, compactness, low roughness, nano-structures, homogeneity, good adhesive properties, fast processing times of a few minutes, low working budgets, and high deposition rate, among others [11,13–17].
Fig. 1. AFM images of nanolayered carbon thin films deposited on different Al2O3 single crystal oriented substrates in A (1120), M (1010), R (1102) and C (0001) planes.
S. Pat et al. / Ceramics International 40 (2014) 10159–10162
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Fig. 2. (a) Hardness and (b) Young’s modulus of nano-layered carbon thin films deposited on different Al2O3 single crystal oriented substrates in A (1120), M (1010), R (1102) and C (0001) planes.
A TVA is also an anodic plasma generator that works in high vacuum conditions (10 5–10 6 Torr) for pure material plasmas. A TVA with a double Wehnelt was designed to deposit the thin films. All of the thin films were deposited on single crystal (sapphire) substrates with varying orientations using a thermionic vacuum arc in a high vacuum. The Al2O3 single crystal substrates were purchased from MTI Corporation. 3. Results and discussions The thicknesses of nano-layered thin films deposited on sapphire substrates were measured to be 40–60 nm using a Filmetrics F20 device. All of the AFM images were obtained using a 10 mm 10 mm scale. These coated surface images are shown in Fig. 1. The ranged scale in Fig. 1 is 2 mm. All of the substrates are deposited simultaneously. As shown in Fig. 1, the substrates exhibit varying surface morphologies. The average roughnesses of nano-layered films were determined using 40 lines at a 10 mm 10 mm scale using AFM. The average roughness values of the deposited thin films were determined to be approximately 2–5 nm. Indention depths were determined as 5 nm, 10 nm and 15 nm. Using Oliver and Pharr's method, the calculated hardness and Young's modulus of the nano-layers are shown in Fig. 2 (a) and (b). As the indentation depth increases, the hardness and Young's modulus of the coated nano-layered thin films decreases, as observed in Fig. 2. The hardness of the sapphire is independent of the crystals’ orientations [18]. This hardness value is approximately 20 GPa [18]. In addition, Young's modulus for A (1120), M (1010), R (1102) and C (0001) planes is 345 GPa [19]. Fig. 2 shows the hardness depth profile of the nano-layered films. The calculated hardness values of the nano-layered thin films deposited on different Al2O3 single crystal substrates in A (1120), M (1010), R (1102) and C (0001) planes are approximately 20–25 GPa. These values are in good agreement with those found in the literature [20]. According to the literature, coated, nano-layered thin films have high mechanical nano-hardness.
4. Conclusion In addition, the mechanical properties of the nano carbon layered thin films deposited on different Al2O3 single crystal substrates were analyzed using a depth-sensing nanoindentation technique. Indention depths were determined as 5 nm, 10 nm and 15 nm. In these indentation depths, hardness values were calculated in the range of 8–25 GPa. When the nanoindentation depth increased, the nano-hardness and Young's modulus of the samples decreased. The reasons for this decrease are complex, but dislocations and the surface energy distribution play important roles. All of the carbon coated samples showed elastoplastic properties. The surface properties of the coated samples varied according to the AFM images. The mechanical properties of the samples were almost identical. Based on these results, we conclude that the crystal orientation of the substrates does not have a significant influence on the mechanical properties of the deposited layers. However, the formation of the surface changed because of the surface energy distributions. The depths analyzed during this small operation were investigated successfully. Acknowledgments The authors thank Eskisehir Osmangazi University Scientific Research Projects Commission (ESOGU-BAP) for the support through Grant 2001219026. References [1] J. Fraxedas, S. Garcia-Manyes, P. Gorostiza, F. Sanz, Nanoindentation: toward the sensing of atomic interactions, Proc. Natl. Acad. Sci. USA 99 (2002) 5228–5232. [2] J. Caro, P. Gorostiza, F. Sanz, J. Fraxedas, Nanomechanical properties of surfaces of molecular organic thin films, Synth. Met. 121 (2001) 1417–1418. [3] A.A. Elmustafa, D.S. Stone, Stacking fault energy and dynamic recovery: do they impact the indentation size effect?, Mater Sci. Eng. A 358 (2003) 1–8.
10162
S. Pat et al. / Ceramics International 40 (2014) 10159–10162
[4] S.G. Corcoran, R.J. Colton, E.T. Lilleodden, W.W. Gerberich, Anomalous plastic deformation at surfaces: nanoindentation of gold single crystals, Phys. Rev. B 55 (1997) 16057–16060. [5] S. Garcia-Manyes, A.G. Guell, P. Gorostiza, F. Sanz, Nanomechanics of silicon surfaces with atomic force microscopy: an insight to the first stages of plastic deformation, J. Chem. Phys. (2005) 123. [6] S. Hara, T. Kumagai, S. Izumi, S. Sakai, Multiscale analysis on the onset of nanoindentation-induced delamination: effect of high-modulus Ru overlayer, Acta Mater. 57 (2009) 4209–4216. [7] D. Tranchida, Z. Kiflie, S. Acierno, S. Piccarolo, Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: viscoelastic characterization of a model material, Meas. Sci. Technol. (2009) 20. [8] A. Rar, G.M. Pharr, W.C. Oliver, E. Karapetian, S.V. Kalinin, Piezoelectric nanoindentation, J. Mater. Res. 21 (2006) 552–556. [9] A. Bolshakov, W.C. Oliver, G.M. Pharr, An explanation for the shape of nanoindentation unloading curves based on finite element simulation, Thin Films: Stresses and Mechanical Properties V, 356, , 1995, p. 675–680. [10] G.M. Pharr, W.C. Oliver, Nanoindentation of silver-relations between hardness and dislocation-structure, J. Mater. Res. 4 (1989) 94–101. [11] S. Pat, S. Temel, N. Ekem, S. Korkmaz, M. Ozkan, M.Z. Balbag, Diamond-like carbon coated on polyethylene terephthalate by Thermionic Vacuum Arc, J. Plast. Film Sheeting 27 (2011) 127–137. [12] Z.Q. Fang, M. Hu, W. Zhang, X.R. Zhang, H.B. Yang, Mechanical properties of porous silicon by depth-sensing nanoindentation techniques, Thin Solid Films 517 (2009) 2930–2935.
[13] S. Pat, N. Ekem, T. Akan, O. Kusmuş, S. Demirkol, R. Vladoiu, C. P. Lungu, G. Musa, Study on thermionic vacuum arc – a novel and advanced technology for surface coating, J. Optoelectron. Adv. Mater. 7 (2005) 2495–2499. [14] N. Ekem, S. Pat, G. Musa, M.Z. Balbağ, I. Cenik, R. Vladoiu, Carbon thin film deposition by thermionic vacuum arc (TVA), J. Optoelectron. Adv. Mater. 10 (2008) 672–674. [15] G. Musa, I. Mustata, M. Blideran, V. Ciupina, R. Vladoiu, G. Prodan, E. Vasile, H. Ehrich, Thermionic vacuum arc (TVA) – new technique for high purity carbon thin film deposition, Acta Phys. Slovaka 5 (2005) 417–421. [16] G. Musa, I. Mustata, V. Ciupina, R. Vladoiu, G. Prodan, C.P. Lungu, H. Ehrich, Thermionic vacuum arc (TVA) – carbon thin film deposition, J. Optoelectron. Adv. Mater. 7 (2005) 2485–2487. [17] G. Musa, I. Mustata, M. Blideran, V. Ciupina, R. Vladoiu, G. Prodan, E. Vasile, Nanostructured carbon thin film deposition using Thermionic vacuum arc (TVA) technology, J. Optoelectron. Adv. Mater. 5 (2003) 667–673. [18] A.B. Sinani, N.K. Dynkin, P.V. Konevskiy, L.A. Lytvynov, Dependence of sapphire hardness on loading type and orientation, Funct. Mater. 16 (2009) 456–459. [19] Technical data sheet of sapphire wafers and substrates 〈http://www. valleydesign.com/sappprop.htm〉, 2012. Access date: 26.09.2012. [20] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2001) 129–281.