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Superhard Nb–Si–N composite films synthesized by reactive magnetron sputtering
Applied Surface Science 252 (2006) 5215–5219 www.elsevier.com/locate/apsusc
Superhard Nb–Si–N composite films synthesized by reactive magnetron sputtering Yunshan Dong a, Yan Liu a, Jiawei Dai b, Geyang Li a,* a
State Key Lab of Metal Matrix Composites, Shanghai JiaoTong University, Shanghai 200030, PR China b Key Lab of the Ministry of Education for High Temperature Materials and Testing, Shanghai JiaoTong University, Shanghai 200030, PR China Received 7 May 2005; received in revised form 6 July 2005; accepted 2 August 2005 Available online 8 September 2005
Abstract By means of the reactive magnetron sputtering method, a series of Nb–Si–N composite films with different Si contents were deposited in an Ar, N2 and SiH4 mixture atmosphere. These films’ chemical composition, phase formation, microstructure and mechanical properties were characterized by the energy dispersive spectroscopy, X-ray diffraction, transmission electron microcopy, atomic force microscopy and nanoindentation. The experimental results showed that the silicon content in the Nb– Si–N composite films can be conveniently controlled by adjusting the SiH4 partial pressure in mixed gas. The hardness and elastic modulus of the Nb–Si–N films were remarkably increased with a small amount of silicon addition and reached their maximum values of 53 and 521 GPa, respectively, at 3.4 at.% Si. Such an obvious enhancement of mechanical properties is related to the increment of crystal defects in the Nb–Si–N films. With silicon content increasing in the films further, the mechanical properties decreased gradually to somewhat a bit lower than those of the NbN film. # 2005 Elsevier B.V. All rights reserved. Keywords: Nb–Si–N composite film; Reactive magnetron sputtering; Microstructure; Superhardness
1. Introduction Transition metal nitride (MeN) films have been extensively applied as hard coatings in cutting tools, molding dies and other industrial fields. With the development of dry and high-speed cutting technologies, there arises an increasing demand for coatings * Corresponding author. Tel.: +86 21 6292106; fax: +86 21 62932587. E-mail address: [email protected] (G. Li).
that exhibit higher hardness and better wear resistance. Since Veprek et al. [1] reported hardness as high as 80–105 GPa, even higher than that of the diamond film (70–90 GPa), in the Ti–Si–N nanocomposite films, many investigations have been focused on the Me–Si– N systems. Although no one has repeated such an amazing result so far, hardness of 40–60 GPa was obtained in some Me–Si–N nanocomposite films such as Ti–Si–N [2,3] V–Si–N [4] and W–Si–N [5]. As a result, it was profoundly recognized that the addition of silicon is an effective way for some MeN films to
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.007
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Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
get their mechanical properties enhanced. Veprek et al. believed that the refinement effect of the Si3N4 interfacial phase on the MeN crystals is the most important factor. Based on our previous work [6], the NbN film has superior mechanical properties than those of the TiN film. An enhancement on the high hardness of the NbN film can be expected by applying the same strengthening method mentioned above. In this work, a series of Nb–Si–N composite films with different Si contents were deposited and their composition, microstructure and mechanical properties were characterized.
2. Experiment Nb–Si–N composite films with a thickness of about 2 mm were all deposited in an ANELVA SPC-350 magnetron sputtering system without any deliberate bias or heating applied to the substrates. Pure Nb (99.99%) target with a diameter of 75 mm was sputtered by a radio frequency power. Mirror polished stainless steel substrates were ultrasonically cleaned in acetone and alcohol, and then mounted on the substrate holder in the vacuum chamber. After the base pressure reached 2 10 4 Pa, Ar (99.999%), N2 (99.999%) and SiH4 (99%) were introduced into the chamber through three separate gas manifolds. Among them, Ar and N2 partial pressures were both fixed at 3 10 1 Pa. With the SiH4 partial pressures changing from 0 to 6 10 2 Pa, a series of Nb–Si– N nanocomposite films with different Si contents were prepared. During deposition, the Nb target power was kept at 200 W. The chemical composition of the films was determined by energy dispersive spectroscopy (EDS) on an EDAX DX-4 energy dispersive analyzer. A Dmax-rC X-ray diffractometer (XRD) with Cu Ka radiation and a Philips CM200 FEG transmission electron microscope (TEM) were employed to characterize the microstructure of these samples. Mechanical properties were measured using a Fischerscope HV100 nanoindenter. Furthermore, a Nanoscope IIIa atomic force microscope (AFM) was used to observe the surface topography of the films and measure the diagonal length of the indentation in order to calibrate the hardness measurement results.
Table 1 Composition of Nb–Si–N films No.
1#
2#
3#
4#
5#
PSiH4 (10 2Pa) Si (at.%) Nb (at.%) N (at.%)
0 0 43.4 56.6
0.5 2.7 40.6 56.7
1.0 3.4 40.3 56.3
2.0 4.6 39.0 56.4
3.0 6.3 38.1 55.6
3. Results and discussion 3.1. Composition and microstructure of films The composition of Nb–Si–N films listed in Table 1 shows that all films are slightly rich in nitrogen. With the increase of Si content, Nb content in the films decreases correspondingly while the N content keeps almost unchanged. Fig. 1 displays the XRD patterns of Nb–Si–N composite films with different silicon contents. The NbN film presents a cubic structure with sharp (1 1 1) and (2 2 0) diffraction peaks, which means that the film grows well along these preferred orientations. With a small amount of silicon addition, a notable shift towards lower diffraction side can be observed for both the (1 1 1)c-NbN and (2 2 0)c-NbN diffraction peaks, indicating an obvious increase of interplanar distance in the films. Meanwhile, a weak diffraction peak emerges at about 618 overlapping with the (2 2 0)c-NbN diffraction peak. In combination with
Fig. 1. XRD patterns of Nb–Si–N composite films.
Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
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Fig. 2. HRTEM images (a–c) and SAED pattern of the Nb–Si–N film (3.4 at.% Si).
electron diffraction analysis shown later on, this peak can be assigned as (1 1 0) diffraction of the hexagonalstructured NbN (h-NbN). With Si content increasing further, all these diffraction peaks gradually broaden and weaken, which reveals that the whole film exists in a nanocrystalline or partially amorphous structure. The HRTEM images and selected area electron diffraction (SAED) pattern of the Nb–Si–N film with 3.4 at.% Si are shown in Fig. 2. Fig. 2a reveals that the Nb–Si–N film is composed of two phases with grain size ranging from 30 to 100 nm. No amorphous Si3N4 interfacial phase, as reported by Veprek in Ti–Si–N nanocomposite films, is observed among these NbN grains. When the field of view is focused on the individual NbN grain, a fine and dense strip-like structure is observed, as shown in Fig. 2b. By magnifying this image to a larger scale (Fig. 2c), an obvious distortion of crystal lattice and large amounts of dislocations are clearly exhibited in such a strip structure. The SAED pattern (Fig. 2d) indicates that the Nb–Si–N film is composed of c-NbN and h-NbN mixture crystalline phases, coincided with XRD results. The broadening of diffraction rings is due to the lack of crystal perfection in films.
selecting a proper penetration load. To avoid the influence of substrate distortion on the measurement results, a two-step penetration method [7] was used. The first step penetration of this method employs a load large enough, e.g. 100 mN as used in this paper, to get the curve of loading hardness, HU (Universal hardness [8]) vs. penetration load. As shown in Fig. 3, each curve exhibits a platform with high hardness value where the penetration load is in the rage of 4– 16 mN because the film’s hardness is higher than that of the stainless steel substrate. When penetrating with a load selected from this range, the deformation under the indenter tip is limited within the film and does not
3.2. Mechanical properties of films For hard films, the measurement of mechanical properties is very difficult due to the uncertainty in
Fig. 3. HU variation of Nb–Si–N films vs. penetration load.
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Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
3.4 at.% Si and then declines to 361 GPa at 6.3 at.% Si. What illustrated in Fig. 5 is the surface topography of the Nb–Si–N film containing 3.4 at.% Si and indentation scar on it. The surface is compact and grows uniformly in a cellular-like structure. In this figure, the diagonal length of indentation scar is scaled as 710 nm, from which the hardness is calculated out as 55 GPa, close to the value of 53 GPa that was measured by the nanoindenter.
Fig. 4. Hardness and elastic modulus of Nb–Si–N films as a function of Si content.
spread to the substrate. Therefore, the value obtained by using such a load represents the film’s intrinsic hardness. Considering a uniform load should be applied to all samples, 15 mN was selected as a substrate free load for the second step penetration. After calculating the measurement results of the second step penetration according to the Oliver and Pharr’s formula [9], the Vickers hardness HV and elastic modulus of films as a function of Si content are obtained. As shown in Fig. 4, the hardness of NbN film is 37 GPa. This hardness is radically increased with a small amount of Si addition and reaches a maximum value of 53 GPa at 3.4 at.% Si. Thereafter, with Si content increasing further, the hardness decreases gradually. It falls to 32.5 GPa when Si content is 6.3 at.%, even a bit lower than that of NbN film. The elastic modulus has the same variation tendency as the hardness. It reaches its maximum value of 521 GPa at
Fig. 5. Surface topography of the Nb–Si–N film (3.4 at.% Si) and indentation shape.
4. Discussion Currently, the superhardness in Me–Si–N composite films is mostly interpreted using the nanocomposites microstructure model and the corresponding strengthening mechanism originally proposed by Veprek [10] for the nc-MeN/a-Si3N4 coating systems. This model believes that the amorphous phase Si3N4 does not resolve in MeN grains, while it wets on their surface and prevents them from growing up. Then a microstructure with MeN nanocrystals being separated and wrapped by the a-Si3N4 interface forms consequently, which is written as nc-MeN/a-Si3N4. The grain size of the MeN nanocrystals is less than 10 nm and the thickness of the Si3N4 interfacial phase is thinner than 1 nm. In such a structure, dislocations can not proliferate or move inside these MeN nanocrystals; and a strong bonding force between MeN and Si3N4 is able to prohibit the occurrence of the anti Hall-Petch effect, the decline of mechanical properties induced by grain-boundary sliding in most nanomaterials. As a result, a remarkable hardness enhancement was achieved in Me–Si–N nanocomposite films. But based on our studies here, NbN grains are not markedly refined and the important amorphous Si3N4 interfacial phase is not found among the grain boundaries in films with superhardness. Therefore it is obvious that the enhancement of Nb–Si–N films’ mechanical properties can not be explained by the above-mentioned nanocomposite model proposed by Veprek. As discussed above, the XRD and HRTEM analysis reveal that remarkable lattice distortion and large amounts of dislocations developed in the films with a small amount of silicon addition. The solution of
Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
silicon atoms into the NbN lattice may result in the formation of such crystal defects. Nose et al. [11] found a similar phenomenon in the Zr–Si–N system that a solution of silicon atoms into the ZrN lattice aggravates the crystal distortion and then strengthens the films. Analogously, these crystal defects of NbN grains prohibit the dislocation movement during the deformation of Nb–Si–N films, and thus enhance their mechanical properties. But with a further increase of Si content, the film will transform to an amorphous structure due to the existence of excessive crystal defects in NbN grains. Correspondingly, hardness and elastic modulus gradually decline. 5. Conclusion (1) Nb–Si–N composite films can be synthesized in an Ar, N2 and SiH4 mixture atmosphere by reactive sputtering. The Si content in the films can be conveniently controlled by adjusting the SiH4 partial pressure in the mixed gas. (2) At a low Si content, although NbN grains in the films are not refined, remarkable lattice distortion and large amounts of crystal defects occur, contributing to the strengthening of films. The hardness and elastic modulus exhibit their maximum values of 53 and 521 GPa, respectively, at 3.4 at.% Si. With silicon content increasing further, the film exists in a nanocrystalline or
5219
amorphous structure, accompanied with an obvious decline of mechanical properties. Acknowledgement Gratefully acknowledge the financial support offered by the Science and Technology Foundation of Shanghai, Grant no. 037252052 to this paper.
References [1] S. Veprek, A. Niederfoher, K. Moto, T. Bolom, H.D. Mannling, P. Nesladek, Surf. Coat. Technol. 133 (2000) 152. [2] X.Z. Ding, X.T. Zeng, Y.C. Liu, Q. Yang, L.R. Zhao, J. Vac. Sci. Technol. A 22 (2004) 2351. [3] E. Ribeiro, L. Rebouta, S. Carvalho, F. Vaz, G.G. Fuentes, R. Rodriguez, M. Zazpe, E. Alves, P. Goudeau, J.P. Riviere, Surf. Coat. Technol. 188 (2004) 351. [4] S. Veprek, Surf. Coat. Technol. 1 (1997) 15. [5] T. Fu, Z.F. Zhou, K.Y. Li, Y.G. Shen, Mater. Lett. 59 (2005) 618. [6] Z.H. Han, X.P. Hu, J.W. Tian, G.Y. Li, M.Y. Gu, Surf. Coat. Technol. 179 (2004) 188. [7] J.W. Tian, Z.H. Han, Q.X. Lai, X.J. Yu, G.Y. Li, Chin. J. Mech. Eng. 39 (2003) 71. [8] Germany Normal. DIN 50359-1: 1997-10. Universal ha¨rtepru¨fung [s]. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [10] S. Veprek, H. Veprek, G.J. Maritza, P. Karvankova, J. Prochazka, Thin Solid Films 476 (2005) 1. [11] M. Nose, W.A. Chiou, M. Zhou, T. Mae, M. Meshii, J. Vac. Sci. Technol. A 20 (2002) 823.
Superhard Nb–Si–N composite films synthesized by reactive magnetron sputtering Yunshan Dong a, Yan Liu a, Jiawei Dai b, Geyang Li a,* a
State Key Lab of Metal Matrix Composites, Shanghai JiaoTong University, Shanghai 200030, PR China b Key Lab of the Ministry of Education for High Temperature Materials and Testing, Shanghai JiaoTong University, Shanghai 200030, PR China Received 7 May 2005; received in revised form 6 July 2005; accepted 2 August 2005 Available online 8 September 2005
Abstract By means of the reactive magnetron sputtering method, a series of Nb–Si–N composite films with different Si contents were deposited in an Ar, N2 and SiH4 mixture atmosphere. These films’ chemical composition, phase formation, microstructure and mechanical properties were characterized by the energy dispersive spectroscopy, X-ray diffraction, transmission electron microcopy, atomic force microscopy and nanoindentation. The experimental results showed that the silicon content in the Nb– Si–N composite films can be conveniently controlled by adjusting the SiH4 partial pressure in mixed gas. The hardness and elastic modulus of the Nb–Si–N films were remarkably increased with a small amount of silicon addition and reached their maximum values of 53 and 521 GPa, respectively, at 3.4 at.% Si. Such an obvious enhancement of mechanical properties is related to the increment of crystal defects in the Nb–Si–N films. With silicon content increasing in the films further, the mechanical properties decreased gradually to somewhat a bit lower than those of the NbN film. # 2005 Elsevier B.V. All rights reserved. Keywords: Nb–Si–N composite film; Reactive magnetron sputtering; Microstructure; Superhardness
1. Introduction Transition metal nitride (MeN) films have been extensively applied as hard coatings in cutting tools, molding dies and other industrial fields. With the development of dry and high-speed cutting technologies, there arises an increasing demand for coatings * Corresponding author. Tel.: +86 21 6292106; fax: +86 21 62932587. E-mail address: [email protected] (G. Li).
that exhibit higher hardness and better wear resistance. Since Veprek et al. [1] reported hardness as high as 80–105 GPa, even higher than that of the diamond film (70–90 GPa), in the Ti–Si–N nanocomposite films, many investigations have been focused on the Me–Si– N systems. Although no one has repeated such an amazing result so far, hardness of 40–60 GPa was obtained in some Me–Si–N nanocomposite films such as Ti–Si–N [2,3] V–Si–N [4] and W–Si–N [5]. As a result, it was profoundly recognized that the addition of silicon is an effective way for some MeN films to
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.007
5216
Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
get their mechanical properties enhanced. Veprek et al. believed that the refinement effect of the Si3N4 interfacial phase on the MeN crystals is the most important factor. Based on our previous work [6], the NbN film has superior mechanical properties than those of the TiN film. An enhancement on the high hardness of the NbN film can be expected by applying the same strengthening method mentioned above. In this work, a series of Nb–Si–N composite films with different Si contents were deposited and their composition, microstructure and mechanical properties were characterized.
2. Experiment Nb–Si–N composite films with a thickness of about 2 mm were all deposited in an ANELVA SPC-350 magnetron sputtering system without any deliberate bias or heating applied to the substrates. Pure Nb (99.99%) target with a diameter of 75 mm was sputtered by a radio frequency power. Mirror polished stainless steel substrates were ultrasonically cleaned in acetone and alcohol, and then mounted on the substrate holder in the vacuum chamber. After the base pressure reached 2 10 4 Pa, Ar (99.999%), N2 (99.999%) and SiH4 (99%) were introduced into the chamber through three separate gas manifolds. Among them, Ar and N2 partial pressures were both fixed at 3 10 1 Pa. With the SiH4 partial pressures changing from 0 to 6 10 2 Pa, a series of Nb–Si– N nanocomposite films with different Si contents were prepared. During deposition, the Nb target power was kept at 200 W. The chemical composition of the films was determined by energy dispersive spectroscopy (EDS) on an EDAX DX-4 energy dispersive analyzer. A Dmax-rC X-ray diffractometer (XRD) with Cu Ka radiation and a Philips CM200 FEG transmission electron microscope (TEM) were employed to characterize the microstructure of these samples. Mechanical properties were measured using a Fischerscope HV100 nanoindenter. Furthermore, a Nanoscope IIIa atomic force microscope (AFM) was used to observe the surface topography of the films and measure the diagonal length of the indentation in order to calibrate the hardness measurement results.
Table 1 Composition of Nb–Si–N films No.
1#
2#
3#
4#
5#
PSiH4 (10 2Pa) Si (at.%) Nb (at.%) N (at.%)
0 0 43.4 56.6
0.5 2.7 40.6 56.7
1.0 3.4 40.3 56.3
2.0 4.6 39.0 56.4
3.0 6.3 38.1 55.6
3. Results and discussion 3.1. Composition and microstructure of films The composition of Nb–Si–N films listed in Table 1 shows that all films are slightly rich in nitrogen. With the increase of Si content, Nb content in the films decreases correspondingly while the N content keeps almost unchanged. Fig. 1 displays the XRD patterns of Nb–Si–N composite films with different silicon contents. The NbN film presents a cubic structure with sharp (1 1 1) and (2 2 0) diffraction peaks, which means that the film grows well along these preferred orientations. With a small amount of silicon addition, a notable shift towards lower diffraction side can be observed for both the (1 1 1)c-NbN and (2 2 0)c-NbN diffraction peaks, indicating an obvious increase of interplanar distance in the films. Meanwhile, a weak diffraction peak emerges at about 618 overlapping with the (2 2 0)c-NbN diffraction peak. In combination with
Fig. 1. XRD patterns of Nb–Si–N composite films.
Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
5217
Fig. 2. HRTEM images (a–c) and SAED pattern of the Nb–Si–N film (3.4 at.% Si).
electron diffraction analysis shown later on, this peak can be assigned as (1 1 0) diffraction of the hexagonalstructured NbN (h-NbN). With Si content increasing further, all these diffraction peaks gradually broaden and weaken, which reveals that the whole film exists in a nanocrystalline or partially amorphous structure. The HRTEM images and selected area electron diffraction (SAED) pattern of the Nb–Si–N film with 3.4 at.% Si are shown in Fig. 2. Fig. 2a reveals that the Nb–Si–N film is composed of two phases with grain size ranging from 30 to 100 nm. No amorphous Si3N4 interfacial phase, as reported by Veprek in Ti–Si–N nanocomposite films, is observed among these NbN grains. When the field of view is focused on the individual NbN grain, a fine and dense strip-like structure is observed, as shown in Fig. 2b. By magnifying this image to a larger scale (Fig. 2c), an obvious distortion of crystal lattice and large amounts of dislocations are clearly exhibited in such a strip structure. The SAED pattern (Fig. 2d) indicates that the Nb–Si–N film is composed of c-NbN and h-NbN mixture crystalline phases, coincided with XRD results. The broadening of diffraction rings is due to the lack of crystal perfection in films.
selecting a proper penetration load. To avoid the influence of substrate distortion on the measurement results, a two-step penetration method [7] was used. The first step penetration of this method employs a load large enough, e.g. 100 mN as used in this paper, to get the curve of loading hardness, HU (Universal hardness [8]) vs. penetration load. As shown in Fig. 3, each curve exhibits a platform with high hardness value where the penetration load is in the rage of 4– 16 mN because the film’s hardness is higher than that of the stainless steel substrate. When penetrating with a load selected from this range, the deformation under the indenter tip is limited within the film and does not
3.2. Mechanical properties of films For hard films, the measurement of mechanical properties is very difficult due to the uncertainty in
Fig. 3. HU variation of Nb–Si–N films vs. penetration load.
5218
Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
3.4 at.% Si and then declines to 361 GPa at 6.3 at.% Si. What illustrated in Fig. 5 is the surface topography of the Nb–Si–N film containing 3.4 at.% Si and indentation scar on it. The surface is compact and grows uniformly in a cellular-like structure. In this figure, the diagonal length of indentation scar is scaled as 710 nm, from which the hardness is calculated out as 55 GPa, close to the value of 53 GPa that was measured by the nanoindenter.
Fig. 4. Hardness and elastic modulus of Nb–Si–N films as a function of Si content.
spread to the substrate. Therefore, the value obtained by using such a load represents the film’s intrinsic hardness. Considering a uniform load should be applied to all samples, 15 mN was selected as a substrate free load for the second step penetration. After calculating the measurement results of the second step penetration according to the Oliver and Pharr’s formula [9], the Vickers hardness HV and elastic modulus of films as a function of Si content are obtained. As shown in Fig. 4, the hardness of NbN film is 37 GPa. This hardness is radically increased with a small amount of Si addition and reaches a maximum value of 53 GPa at 3.4 at.% Si. Thereafter, with Si content increasing further, the hardness decreases gradually. It falls to 32.5 GPa when Si content is 6.3 at.%, even a bit lower than that of NbN film. The elastic modulus has the same variation tendency as the hardness. It reaches its maximum value of 521 GPa at
Fig. 5. Surface topography of the Nb–Si–N film (3.4 at.% Si) and indentation shape.
4. Discussion Currently, the superhardness in Me–Si–N composite films is mostly interpreted using the nanocomposites microstructure model and the corresponding strengthening mechanism originally proposed by Veprek [10] for the nc-MeN/a-Si3N4 coating systems. This model believes that the amorphous phase Si3N4 does not resolve in MeN grains, while it wets on their surface and prevents them from growing up. Then a microstructure with MeN nanocrystals being separated and wrapped by the a-Si3N4 interface forms consequently, which is written as nc-MeN/a-Si3N4. The grain size of the MeN nanocrystals is less than 10 nm and the thickness of the Si3N4 interfacial phase is thinner than 1 nm. In such a structure, dislocations can not proliferate or move inside these MeN nanocrystals; and a strong bonding force between MeN and Si3N4 is able to prohibit the occurrence of the anti Hall-Petch effect, the decline of mechanical properties induced by grain-boundary sliding in most nanomaterials. As a result, a remarkable hardness enhancement was achieved in Me–Si–N nanocomposite films. But based on our studies here, NbN grains are not markedly refined and the important amorphous Si3N4 interfacial phase is not found among the grain boundaries in films with superhardness. Therefore it is obvious that the enhancement of Nb–Si–N films’ mechanical properties can not be explained by the above-mentioned nanocomposite model proposed by Veprek. As discussed above, the XRD and HRTEM analysis reveal that remarkable lattice distortion and large amounts of dislocations developed in the films with a small amount of silicon addition. The solution of
Y. Dong et al. / Applied Surface Science 252 (2006) 5215–5219
silicon atoms into the NbN lattice may result in the formation of such crystal defects. Nose et al. [11] found a similar phenomenon in the Zr–Si–N system that a solution of silicon atoms into the ZrN lattice aggravates the crystal distortion and then strengthens the films. Analogously, these crystal defects of NbN grains prohibit the dislocation movement during the deformation of Nb–Si–N films, and thus enhance their mechanical properties. But with a further increase of Si content, the film will transform to an amorphous structure due to the existence of excessive crystal defects in NbN grains. Correspondingly, hardness and elastic modulus gradually decline. 5. Conclusion (1) Nb–Si–N composite films can be synthesized in an Ar, N2 and SiH4 mixture atmosphere by reactive sputtering. The Si content in the films can be conveniently controlled by adjusting the SiH4 partial pressure in the mixed gas. (2) At a low Si content, although NbN grains in the films are not refined, remarkable lattice distortion and large amounts of crystal defects occur, contributing to the strengthening of films. The hardness and elastic modulus exhibit their maximum values of 53 and 521 GPa, respectively, at 3.4 at.% Si. With silicon content increasing further, the film exists in a nanocrystalline or
5219
amorphous structure, accompanied with an obvious decline of mechanical properties. Acknowledgement Gratefully acknowledge the financial support offered by the Science and Technology Foundation of Shanghai, Grant no. 037252052 to this paper.
References [1] S. Veprek, A. Niederfoher, K. Moto, T. Bolom, H.D. Mannling, P. Nesladek, Surf. Coat. Technol. 133 (2000) 152. [2] X.Z. Ding, X.T. Zeng, Y.C. Liu, Q. Yang, L.R. Zhao, J. Vac. Sci. Technol. A 22 (2004) 2351. [3] E. Ribeiro, L. Rebouta, S. Carvalho, F. Vaz, G.G. Fuentes, R. Rodriguez, M. Zazpe, E. Alves, P. Goudeau, J.P. Riviere, Surf. Coat. Technol. 188 (2004) 351. [4] S. Veprek, Surf. Coat. Technol. 1 (1997) 15. [5] T. Fu, Z.F. Zhou, K.Y. Li, Y.G. Shen, Mater. Lett. 59 (2005) 618. [6] Z.H. Han, X.P. Hu, J.W. Tian, G.Y. Li, M.Y. Gu, Surf. Coat. Technol. 179 (2004) 188. [7] J.W. Tian, Z.H. Han, Q.X. Lai, X.J. Yu, G.Y. Li, Chin. J. Mech. Eng. 39 (2003) 71. [8] Germany Normal. DIN 50359-1: 1997-10. Universal ha¨rtepru¨fung [s]. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [10] S. Veprek, H. Veprek, G.J. Maritza, P. Karvankova, J. Prochazka, Thin Solid Films 476 (2005) 1. [11] M. Nose, W.A. Chiou, M. Zhou, T. Mae, M. Meshii, J. Vac. Sci. Technol. A 20 (2002) 823.