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Microstructure and mechanical properties of vanadium carbide coatings synthesized by reactive magnetron sputtering
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 611–614
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructure and mechanical properties of vanadium carbide coatings synthesized by reactive magnetron sputtering Xiaoyan Wu, Guangze Li, Yanghui Chen, Geyang Li * State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 26 June 2008 Accepted 19 September 2008
Keywords: Vanadium carbide Hard coatings Microstructure Mechanical properties Magnetron sputtering
a b s t r a c t Vanadium carbide coatings with different carbon contents were synthesized in Ar–C2H2 mixture by reactive magnetron sputtering. Energy-dispersive X-ray analysis, X-ray diffraction analysis, scanning electron microscopy, atomic force microscopy and nanoindentation were employed to investigate the effect of C2H2 partial pressure on the composition, phase, microstructure and mechanical properties of the coatings. The results show that vanadium carbide coatings can be synthesized at a low partial pressure of C2H2. The c-VC coating with columnar crystal is obtained when the proportion of C2H2 partial pressure is only 3% in the mixture. It reaches the peak hardness of 35.5 GPa and elastic modulus of 358 GPa, respectively. The presence of excessive carbon with increased C2H2 partial pressure in the h-VC coating leads to the significant decrease of hardness and elastic modulus. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The transition metal nitrides and carbides with high hardness play important parts in surface engineering [1,2]. The successful application of titanium nitride coating started the ‘‘golden revolution” in the field of cutting tools in the 1970s, which powerfully promoted the development of the automation and mass production in the manufacturing industry. Other nitride coatings, like chromium nitride [3], zirconium nitride [4], and titanium aluminum nitride [5] with distinguished properties came forth. The emergences of these materials provided a wider range of selection to meet different machining requirements. Compared with nitrides, carbides have even higher hardness. However, the development of these promising materials is limited due to their relative complicated structures and synthesis process. Up to now, only a few carbide coatings like titanium carbide and titanium carbon nitride [2] have been investigated and used in cutting tools. As one of the hardest transition metal carbides, vanadium carbide coating [6,7] shows some other distinguished properties in cutting tool applications. For example, V2O5, as a solid lubricant, forming on the coating surface in the machining process improves the wear resistance of cutters [8,9]. Till now, scant researches concentrate on the vanadium carbide coatings. It was reported by Ferro et al. [6] that the electron beam deposited cubic VC coating from the carbide target reaches a high hardness of 25 GPa. In the research of Aouni et al. [10], a series of vanadium carbide coatings with different carbon contents were prepared by reactive dc diode magnetron
* Corresponding author. Tel.: +86 21 54742261; fax: +86 21 54742268. E-mail address: [email protected] (G. Li). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.014
sputtering with the vanadium target in the Ar–CH4 mixture. It was found that with the variation of partial pressure of CH4 (3% 15.7% of the mixture), monophase or multiphase coatings like vanadium solid solution, V2C, VC and VC + C were obtained due to different carbon contents. Unfortunately, no results about corresponding mechanical properties were reported in their paper. In this paper, a series of vanadium carbide coatings were synthesized by reactive magnetron sputtering with different C2H2 partial pressures and a systematical study was carried on to investigate the effects of different C2H2 partial pressures on the composition, phase, microstructure and mechanical properties of the coatings. 2. Experimental details A series of vanadium carbide coatings with different carbon contents were synthesized by reactive magnetron sputtering on ANELVA SPC-350 multi-target magnetron sputtering system. The 3 inch diameter vanadium target (99.99% in purity) was controlled by the radio frequency cathode. Mirror polished stainless steel substrates were ultrasonically cleaned in acetone and alcohol and then mounted on the substrate holder. The distance is 5 cm between target surface and the substrate in the chamber. When the background vacuum reached 2 10 3 Pa, Ar (99.999% in purity) and C2H2 (99.9% in purity) were separately introduced into the chamber. The total pressure of the mixture was kept at 3.2 10 1 Pa while C2H2 partial pressure (PC2 H2 ) varied from 5 10 3 to 25 10 3 Pa. During the deposition, the target power was kept at 200 W and the deposition time was 45 min for each sample. No heating and deliberate bias voltage was applied to the substrates.
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X. Wu et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 611–614
Table 1 Composition of vanadium carbide coatings at different C2H2 partial pressures. Sample P C2 H2 (10 C (at.%) V (at.%)
3
Pa)
1
2
3
4
5
5 27.07 72.93
10 48.76 51.24
15 54.94 45.06
20 66.42 33.58
25 81.04 18.96
Mechanical properties including Vicker’s hardness (HV) and elastic modulus (E) of all coatings were measured on the Fischerscope H100 VP nanoindentor. In order to obtain reliable mechanical properties of the coatings, a maximum load of 10 mN was used for measurement based on a two-step penetration method [11,12]. The hardness and elastic modulus value for each sample was an average of at least 20 measurements. 3. Results and discussion 3.1. Composition and microstructure
Fig. 1. XRD patterns of vanadium carbide coatings.
The phase constituting vanadium carbide coatings was determined by X-ray diffraction (XRD) on the Dmax-2550/PC diffractor with Cu Ka excitation radiation. The growth structure and the chemical composition were characterized by the FEI SIRION 200 field emission scanning electron microscope (SEM) and the attached OXFORD INCA quantitative energy dispersive X-ray analysis (EDX), respectively. The surface morphology of the coatings was observed using the Nanoscope IIIa atomic force microscope (AFM).
The composition of vanadium carbide coatings acquired at different C2H2 partial pressures analyzed by EDX is listed in Table 1. The results show the partial pressure of C2H2 is very low when C2H2 serves as the reactive gas in the synthesis of vanadium carbide coatings by reactive magnetron sputtering. Moreover, the carbon content in the coating is sensitive to the C2H2 partial pressure. The carbon content of coatings increases from about 27 at.% to over 80 at.% with a slight increase of P C2 H2 from 5 10 3 to 25 10 3 Pa. XRD patterns of vanadium carbide coatings with different carbon contents are shown in Fig. 1. In sample 1 with the low carbon content of nearly 30 at.%, no diffraction peak corresponding to vanadium is found while two peaks around 2h = 41° and 79° correspond to those of V2C. V2C exist in either orthorhombic a-V2C or hexagonal b-V2C and some of their peaks overlap at 2h = 41° and 79°. Considering the influence of texture in the coating, the information from the scanty XRD peaks is not sufficient to identify the exact structure of V2C. A set of sharp peaks are found in the XRD pattern of sample 2 (50 at.% C). These peaks correspond to those of NaCl-type VC (cVC). Yet VC might transfer to the ordered cubic V8C7 or hexagonal V6C5 at a relative low temperature [13,14], the characteristic peaks of which might overlap those of c-VC. Taking into account of fast cooling process in the vapor deposition, the ordering might be prohibited in this non equilibrium process. This coating is called c-VC in general in this paper. When the carbon content reaches 55 at.%, the (1 0 1) plane peak of hexagonal VC (h-VC) appears in the XRD pattern of sample 3. This coating consists of both c-VC and h-VC.
Fig. 2. Cross-sectional fracture micrographs of vanadium carbide coatings by SEM (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4.
X. Wu et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 611–614
Once the carbon content is more than 66 at.%, the main crystalline phase in both samples 4 and 5 transforms into h-VC. The gradually broadened peaks indicate the poor crystallization. It is related to the excessive carbon in the coatings, which probably exists in amorphous state. Fig. 2 shows cross-sectional fracture micrographs of vanadium carbide coatings. Columnar crystals are found in the V2C coating (Fig. 2a). With the increased carbon content, more distinguished columnar crystals appear in both the fracture of the c-VC coating (Fig. 2b) and that of the c-VC and h-VC coexisting coating (Fig. 2c). When carbon content is more than 66 at.%, an amorphous fracture, the substitution of the columnar one, is shown (Fig. 2d). This growth structure transformation of the coatings is related to the presence of amorphous carbon. Moreover, the SEM graphs suggest that with the increase of the carbon content, the thicknesses of the different coatings show a slight decrease. It indicates that the deposition rate of coatings rarely depends on the proportion of C2H2 in the mixture, although it significantly influences the growth structures. The surface morphology and the corresponding roughness (Rq) obtained from AFM are displayed in Fig. 3. In Fig. 3a, the compact cellular structure with a roughness of 3.42 nm is shown in the low carbon content coating. With the increase of carbon content, the enhanced roughness in either sample 2 (Fig. 3b) or sample 3 (Fig. 3c) is 6.10 and 6.81 nm, respectively, which is contributed to the crystalline perfection in the coatings. However, once the amorphous carbon appears due to the excessive carbon, the coating shows a smooth growth morphology with a roughness only of 2.91 nm (Fig. 3d).
613
3.2. Mechanical properties Fig. 4 shows the variation of hardness and elastic modulus of coatings with carbon content. The hardness and elastic modulus of the V2C coating with 27.07 at.% C are 31.4 and 262 GPa, respectively. For the c-VC coating with 48.76 at.% C, the peak hardness and elastic modulus of are achieved, 35.5 and 358 GPa, respectively. A slight decreased hardness and elastic modulus are measured in the c-VC and h-VC coexisting coating. With the further increase of carbon content, the hardness and elastic modulus drop significantly due to the amorphous carbon in the coatings.
Fig. 4. The dependence of hardness and elastic modulus of coatings on the carbon content.
Fig. 3. AFM micrographs of vanadium carbide coatings (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4.
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X. Wu et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 611–614
4. Conclusions Vanadium carbide coatings can be synthesized by reactive magnetron sputtering method in Ar–C2H2 mixture. The composition, phase, microstructure and the corresponding mechanical properties of the coatings show the sensitivity to the partial pressure of C2H2. Thus, the c-VC coating with favorable mechanical properties is acquired at a low C2H2 proportion of about 3.1%. It reaches the peak hardness of 35.5 GPa and elastic modulus of 358 GPa. With the further increase of carbon content, the hardness and elastic modulus decrease significantly because the amorphous carbon appears in the coating. Acknowledgement This study is supported by National Natural Science Foundation of China (No. U0774001). References [1] Sundgren JE, Hentzell TG. A review of the present state of art in hard coatings grown from the vapor phase. J Vac Sci Technol A: Vac Surf Films 1986;4(5):2259–79. [2] Robinson GM, Jackson MJ. A review of micro and nanomachining from a materials perspective. J Mater Process Technol 2005;167(2–3):316–37.
[3] Berg G, Friedrich C, Broszeit E, Berger C. Development of chromium nitride coatings substituting titanium nitride. Surf Coat Technol 1996;86–87(Part 1): 184–91. [4] Liu F, Meng YD, Ren ZX, Shu XS. Microstructure, hardness and corrosion resistance of ZrN films prepared by inductively coupled plasma enhanced RF magnetron sputtering. Plasma Sci Technol 2008;10(2):170–5. [5] Derflinger VH, Schutze A, Ante M. Mechanical and structural properties of various alloyed TiAlN-based hard coatings. Surf Coat Technol 2006;200(16– 17):4693–700. [6] Ferro D, Rau JV, Generosi A, Albertini RA, Latini A, Barinov SM. Electron beam deposited VC and NbC thin films on titanium: hardness and energy-dispersive X-ray diffraction study. Surf Coat Technol 2008;202(10):2162–8. [7] Chicco B, Borbidge WE, Summerville E. Experimental study of vanadium carbide and carbonitride coatings. Mater Sci Eng A 1999;266(1–2):62–72. [8] Woydt M, Skopp A, Dorfel I, Witke K. Wear engineering oxides/anti-wear oxides. Wear 1998;218(1):84–95. [9] Chen JG, Kirn CM, Fruhberger B, DeVries BD, Touvelle MS. A NEXAFS determination of the oxidation state of vanadium carbide on V(110): observation of charge transfer from vanadium to carbon. Surf Sci 1994;321(1–2):145–55. [10] Aouni A, Weisbecker P, Loi TH, Grosse EB. Search for new materials in sputtered V1-xCx films. Thin Solid Films 2004;469–470:315–21. [11] Tian JW, Han ZH, Lai QX, Yu XJ, Li GY, Gu MY. Two-step penetration: a reliable method for the measurement of mechanical properties of hard coatings. Surf Coat Technol 2004;176(3):267–71. [12] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7(6):20. [13] Okamoto H. Carbon–vanadium. J Phase Equilibr 1991;12(6):699. [14] Lipatnikov VN, Gusev AI, Ettmayer P, Lengauer W. Phase transformations in non-stoichiometric vanadium carbide. J Phys: Condens Matter 1999;11:22.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructure and mechanical properties of vanadium carbide coatings synthesized by reactive magnetron sputtering Xiaoyan Wu, Guangze Li, Yanghui Chen, Geyang Li * State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 26 June 2008 Accepted 19 September 2008
Keywords: Vanadium carbide Hard coatings Microstructure Mechanical properties Magnetron sputtering
a b s t r a c t Vanadium carbide coatings with different carbon contents were synthesized in Ar–C2H2 mixture by reactive magnetron sputtering. Energy-dispersive X-ray analysis, X-ray diffraction analysis, scanning electron microscopy, atomic force microscopy and nanoindentation were employed to investigate the effect of C2H2 partial pressure on the composition, phase, microstructure and mechanical properties of the coatings. The results show that vanadium carbide coatings can be synthesized at a low partial pressure of C2H2. The c-VC coating with columnar crystal is obtained when the proportion of C2H2 partial pressure is only 3% in the mixture. It reaches the peak hardness of 35.5 GPa and elastic modulus of 358 GPa, respectively. The presence of excessive carbon with increased C2H2 partial pressure in the h-VC coating leads to the significant decrease of hardness and elastic modulus. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The transition metal nitrides and carbides with high hardness play important parts in surface engineering [1,2]. The successful application of titanium nitride coating started the ‘‘golden revolution” in the field of cutting tools in the 1970s, which powerfully promoted the development of the automation and mass production in the manufacturing industry. Other nitride coatings, like chromium nitride [3], zirconium nitride [4], and titanium aluminum nitride [5] with distinguished properties came forth. The emergences of these materials provided a wider range of selection to meet different machining requirements. Compared with nitrides, carbides have even higher hardness. However, the development of these promising materials is limited due to their relative complicated structures and synthesis process. Up to now, only a few carbide coatings like titanium carbide and titanium carbon nitride [2] have been investigated and used in cutting tools. As one of the hardest transition metal carbides, vanadium carbide coating [6,7] shows some other distinguished properties in cutting tool applications. For example, V2O5, as a solid lubricant, forming on the coating surface in the machining process improves the wear resistance of cutters [8,9]. Till now, scant researches concentrate on the vanadium carbide coatings. It was reported by Ferro et al. [6] that the electron beam deposited cubic VC coating from the carbide target reaches a high hardness of 25 GPa. In the research of Aouni et al. [10], a series of vanadium carbide coatings with different carbon contents were prepared by reactive dc diode magnetron
* Corresponding author. Tel.: +86 21 54742261; fax: +86 21 54742268. E-mail address: [email protected] (G. Li). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.014
sputtering with the vanadium target in the Ar–CH4 mixture. It was found that with the variation of partial pressure of CH4 (3% 15.7% of the mixture), monophase or multiphase coatings like vanadium solid solution, V2C, VC and VC + C were obtained due to different carbon contents. Unfortunately, no results about corresponding mechanical properties were reported in their paper. In this paper, a series of vanadium carbide coatings were synthesized by reactive magnetron sputtering with different C2H2 partial pressures and a systematical study was carried on to investigate the effects of different C2H2 partial pressures on the composition, phase, microstructure and mechanical properties of the coatings. 2. Experimental details A series of vanadium carbide coatings with different carbon contents were synthesized by reactive magnetron sputtering on ANELVA SPC-350 multi-target magnetron sputtering system. The 3 inch diameter vanadium target (99.99% in purity) was controlled by the radio frequency cathode. Mirror polished stainless steel substrates were ultrasonically cleaned in acetone and alcohol and then mounted on the substrate holder. The distance is 5 cm between target surface and the substrate in the chamber. When the background vacuum reached 2 10 3 Pa, Ar (99.999% in purity) and C2H2 (99.9% in purity) were separately introduced into the chamber. The total pressure of the mixture was kept at 3.2 10 1 Pa while C2H2 partial pressure (PC2 H2 ) varied from 5 10 3 to 25 10 3 Pa. During the deposition, the target power was kept at 200 W and the deposition time was 45 min for each sample. No heating and deliberate bias voltage was applied to the substrates.
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X. Wu et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 611–614
Table 1 Composition of vanadium carbide coatings at different C2H2 partial pressures. Sample P C2 H2 (10 C (at.%) V (at.%)
3
Pa)
1
2
3
4
5
5 27.07 72.93
10 48.76 51.24
15 54.94 45.06
20 66.42 33.58
25 81.04 18.96
Mechanical properties including Vicker’s hardness (HV) and elastic modulus (E) of all coatings were measured on the Fischerscope H100 VP nanoindentor. In order to obtain reliable mechanical properties of the coatings, a maximum load of 10 mN was used for measurement based on a two-step penetration method [11,12]. The hardness and elastic modulus value for each sample was an average of at least 20 measurements. 3. Results and discussion 3.1. Composition and microstructure
Fig. 1. XRD patterns of vanadium carbide coatings.
The phase constituting vanadium carbide coatings was determined by X-ray diffraction (XRD) on the Dmax-2550/PC diffractor with Cu Ka excitation radiation. The growth structure and the chemical composition were characterized by the FEI SIRION 200 field emission scanning electron microscope (SEM) and the attached OXFORD INCA quantitative energy dispersive X-ray analysis (EDX), respectively. The surface morphology of the coatings was observed using the Nanoscope IIIa atomic force microscope (AFM).
The composition of vanadium carbide coatings acquired at different C2H2 partial pressures analyzed by EDX is listed in Table 1. The results show the partial pressure of C2H2 is very low when C2H2 serves as the reactive gas in the synthesis of vanadium carbide coatings by reactive magnetron sputtering. Moreover, the carbon content in the coating is sensitive to the C2H2 partial pressure. The carbon content of coatings increases from about 27 at.% to over 80 at.% with a slight increase of P C2 H2 from 5 10 3 to 25 10 3 Pa. XRD patterns of vanadium carbide coatings with different carbon contents are shown in Fig. 1. In sample 1 with the low carbon content of nearly 30 at.%, no diffraction peak corresponding to vanadium is found while two peaks around 2h = 41° and 79° correspond to those of V2C. V2C exist in either orthorhombic a-V2C or hexagonal b-V2C and some of their peaks overlap at 2h = 41° and 79°. Considering the influence of texture in the coating, the information from the scanty XRD peaks is not sufficient to identify the exact structure of V2C. A set of sharp peaks are found in the XRD pattern of sample 2 (50 at.% C). These peaks correspond to those of NaCl-type VC (cVC). Yet VC might transfer to the ordered cubic V8C7 or hexagonal V6C5 at a relative low temperature [13,14], the characteristic peaks of which might overlap those of c-VC. Taking into account of fast cooling process in the vapor deposition, the ordering might be prohibited in this non equilibrium process. This coating is called c-VC in general in this paper. When the carbon content reaches 55 at.%, the (1 0 1) plane peak of hexagonal VC (h-VC) appears in the XRD pattern of sample 3. This coating consists of both c-VC and h-VC.
Fig. 2. Cross-sectional fracture micrographs of vanadium carbide coatings by SEM (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4.
X. Wu et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 611–614
Once the carbon content is more than 66 at.%, the main crystalline phase in both samples 4 and 5 transforms into h-VC. The gradually broadened peaks indicate the poor crystallization. It is related to the excessive carbon in the coatings, which probably exists in amorphous state. Fig. 2 shows cross-sectional fracture micrographs of vanadium carbide coatings. Columnar crystals are found in the V2C coating (Fig. 2a). With the increased carbon content, more distinguished columnar crystals appear in both the fracture of the c-VC coating (Fig. 2b) and that of the c-VC and h-VC coexisting coating (Fig. 2c). When carbon content is more than 66 at.%, an amorphous fracture, the substitution of the columnar one, is shown (Fig. 2d). This growth structure transformation of the coatings is related to the presence of amorphous carbon. Moreover, the SEM graphs suggest that with the increase of the carbon content, the thicknesses of the different coatings show a slight decrease. It indicates that the deposition rate of coatings rarely depends on the proportion of C2H2 in the mixture, although it significantly influences the growth structures. The surface morphology and the corresponding roughness (Rq) obtained from AFM are displayed in Fig. 3. In Fig. 3a, the compact cellular structure with a roughness of 3.42 nm is shown in the low carbon content coating. With the increase of carbon content, the enhanced roughness in either sample 2 (Fig. 3b) or sample 3 (Fig. 3c) is 6.10 and 6.81 nm, respectively, which is contributed to the crystalline perfection in the coatings. However, once the amorphous carbon appears due to the excessive carbon, the coating shows a smooth growth morphology with a roughness only of 2.91 nm (Fig. 3d).
613
3.2. Mechanical properties Fig. 4 shows the variation of hardness and elastic modulus of coatings with carbon content. The hardness and elastic modulus of the V2C coating with 27.07 at.% C are 31.4 and 262 GPa, respectively. For the c-VC coating with 48.76 at.% C, the peak hardness and elastic modulus of are achieved, 35.5 and 358 GPa, respectively. A slight decreased hardness and elastic modulus are measured in the c-VC and h-VC coexisting coating. With the further increase of carbon content, the hardness and elastic modulus drop significantly due to the amorphous carbon in the coatings.
Fig. 4. The dependence of hardness and elastic modulus of coatings on the carbon content.
Fig. 3. AFM micrographs of vanadium carbide coatings (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4.
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X. Wu et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 611–614
4. Conclusions Vanadium carbide coatings can be synthesized by reactive magnetron sputtering method in Ar–C2H2 mixture. The composition, phase, microstructure and the corresponding mechanical properties of the coatings show the sensitivity to the partial pressure of C2H2. Thus, the c-VC coating with favorable mechanical properties is acquired at a low C2H2 proportion of about 3.1%. It reaches the peak hardness of 35.5 GPa and elastic modulus of 358 GPa. With the further increase of carbon content, the hardness and elastic modulus decrease significantly because the amorphous carbon appears in the coating. Acknowledgement This study is supported by National Natural Science Foundation of China (No. U0774001). References [1] Sundgren JE, Hentzell TG. A review of the present state of art in hard coatings grown from the vapor phase. J Vac Sci Technol A: Vac Surf Films 1986;4(5):2259–79. [2] Robinson GM, Jackson MJ. A review of micro and nanomachining from a materials perspective. J Mater Process Technol 2005;167(2–3):316–37.
[3] Berg G, Friedrich C, Broszeit E, Berger C. Development of chromium nitride coatings substituting titanium nitride. Surf Coat Technol 1996;86–87(Part 1): 184–91. [4] Liu F, Meng YD, Ren ZX, Shu XS. Microstructure, hardness and corrosion resistance of ZrN films prepared by inductively coupled plasma enhanced RF magnetron sputtering. Plasma Sci Technol 2008;10(2):170–5. [5] Derflinger VH, Schutze A, Ante M. Mechanical and structural properties of various alloyed TiAlN-based hard coatings. Surf Coat Technol 2006;200(16– 17):4693–700. [6] Ferro D, Rau JV, Generosi A, Albertini RA, Latini A, Barinov SM. Electron beam deposited VC and NbC thin films on titanium: hardness and energy-dispersive X-ray diffraction study. Surf Coat Technol 2008;202(10):2162–8. [7] Chicco B, Borbidge WE, Summerville E. Experimental study of vanadium carbide and carbonitride coatings. Mater Sci Eng A 1999;266(1–2):62–72. [8] Woydt M, Skopp A, Dorfel I, Witke K. Wear engineering oxides/anti-wear oxides. Wear 1998;218(1):84–95. [9] Chen JG, Kirn CM, Fruhberger B, DeVries BD, Touvelle MS. A NEXAFS determination of the oxidation state of vanadium carbide on V(110): observation of charge transfer from vanadium to carbon. Surf Sci 1994;321(1–2):145–55. [10] Aouni A, Weisbecker P, Loi TH, Grosse EB. Search for new materials in sputtered V1-xCx films. Thin Solid Films 2004;469–470:315–21. [11] Tian JW, Han ZH, Lai QX, Yu XJ, Li GY, Gu MY. Two-step penetration: a reliable method for the measurement of mechanical properties of hard coatings. Surf Coat Technol 2004;176(3):267–71. [12] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7(6):20. [13] Okamoto H. Carbon–vanadium. J Phase Equilibr 1991;12(6):699. [14] Lipatnikov VN, Gusev AI, Ettmayer P, Lengauer W. Phase transformations in non-stoichiometric vanadium carbide. J Phys: Condens Matter 1999;11:22.