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SiO2 nanomultilayers synthesized by reactive sputtering
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Materials Letters 62 (2008) 1621 – 1623 www.elsevier.com/locate/matlet
Microstructure and mechanical properties of VN/SiO2 nanomultilayers synthesized by reactive sputtering Jianling Yue a,b , Wenji Zhao a , Geyang Li a,⁎, Wei Gao b a
State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200030, China b School of Engineering, The University of Auckland, 20 Symonds Street, Auckland, New Zealand Received 10 June 2007; accepted 19 September 2007 Available online 26 September 2007
Abstract VN/SiO2 nanomultilayers with various individual SiO2 and VN layer thickness were prepared by reactive sputtering process. The crystallization condition of the SiO2 layers and how they influence the characteristics of the multilayers were studied. The results reveal that under the template effect of the crystalline VN layer, as-deposited amorphous SiO2 crystallizes at very low thickness (b ∼ 1 nm), and then grows epitaxially with VN layer in the VN/SiO2 nanomultilayers. This leads to a remarkable increase in hardness. If the SiO2 layers' thickness is further increased, they gradually transform into amorphous structure and block the epitaxial growth of the multilayers, resulting in a quick decline in hardness. On the other hand, the hardness of the multilayers is not sensitive to the VN layer thickness. © 2007 Elsevier B.V. All rights reserved. Keywords: Coatings; Epitaxial growth; Hardness
1. Introduction During high speed and dry cutting, the temperature at the tip of a cutting tool can reach up to 1000 °C. Such cutting tools require a high performance coating. Most of the nitride hard coatings, such as TiN and TiAlN, will oxidize and lose their protective function in such high temperature [1]. Therefore, it is crucial to improve the hardness and high temperature oxidation resistance of the tool coatings to withstand the rigorous conditions. Although oxides exhibit good oxidation resistance, they have seldom been used individually for cutting tool coatings due to their low hardness. In the earlier practice, Al2O3 layers with thickness of about 0.5 μm were inserted into the nitride coatings to improve the oxidation resistance [2]. However, the results were not satisfactory due to their low hardness. It was found in 1987 that multilayers deposited alternately by two kinds of materials at nanometers thickness exhibit superhardness effect, an anomalous increase of hardness [3]. Based on this concept, Sproul [4] suggested combining two oxides to ⁎ Corresponding author. Tel.: +86 21 62932106; fax: +86 21 62932785. E-mail address: [email protected] (G. Li). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.040
synthesize nanomultilayers which may exhibit both high temperature oxidation resistance and sufficient hardness. Efforts following this idea were made in the Al2O3/ZrO2 [4] and Y2O3/ ZrO2 [5] nanomultilayers. Unfortunately, there is no significant increase in hardness which can be due to the following two reasons. Firstly, there is only a small difference in the shear modulus between the two oxides. Secondly, the multilayers form an amorphous structure. Recently, a strategy to synthesize a new kind of superhard coatings, nitrides/oxides nanomultilayers, was brought forward. Such coatings are presumed to exhibit both high temperature oxidation resistance and sufficient hardness. TiN/SiO2 [6] and TiN/Al2O3 [7] nanomultilayers have been successfully synthesized using this strategy. Studies revealed that when the thickness of the oxide layer is below about 1 nm, normally amorphous SiO2 or Al2O3 can crystallize under the “template effect” of the crystalline TiN layers. The crystallized oxide layers grow epitaxially with nitride layers, accompanied with a remarkable hardness increase. In spite of their great scientific value, the nitrides/oxides nanomultilayers had little practicality because they were deposited by directly sputtering compound targets (TiN, Al2O3, SiO2) that are very low in deposition rate.
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J. Yue et al. / Materials Letters 62 (2008) 1621–1623
In present work, VN/SiO2 nanomultilayers have been synthesized by reactive sputtering in order to make the strategy of nitrides/oxides nanomultilayers more industrially practical. 2. Experimental procedure VN/SiO2 nanomultilayers and VN, SiO2 monolithic films were prepared by an ANELVA SPC-350 magnetron sputtering system. The targets of V (99.99%) and SiO2 (99.9%) with each diameter of 75 mm were controlled by two RF cathodes respectively. Mirror polished stainless steel substrates were cleaned in acetone before being placed on the rotatable substrate holder. Before the deposition, the system was evacuated to a base pressure of 4 × 10− 4 Pa. Then the system was filled with sputtering gas Ar (99.999%) and reactive gas N2 (99.999%), with pressure of 2.4 × 10− 1 Pa and 8 × 10− 2 Pa respectively. No deliberate bias or heating was applied to the substrate during the deposition. By controlling the time the substrate stops in front of the targets and the target power, nanomultilayers with different SiO2 and VN layer thickness (lSiO2 and lVN, respectively) were obtained. The overall thickness for each specimen was about 2 μm. In order to find out whether the SiO2 in the multilayers reacts with N2 during deposition, a SiO2 monolithic film was prepared in the same conditions as the VN/SiO2 multilayers. EDXA DX-4 energy dispersive spectroscope (EDS), Philips CM200 FEG high-resolution transmission electron microscope (HRTEM) and Rigaku Dmax-rC X-ray diffractometer (XRD)
Fig. 1. Low-magnified (a) and high-magnified (b) cross-sectional HRTEM images of the VN (3.6 nm)/SiO2 (0.9 nm) nanomultilayers.
Fig. 2. Low-angle XRD patterns of the VN/SiO2 multilayers with various lSiO2 (lVN = 3.6 nm).
using Cu Kα (40 kV 150 mA) radiation were used to characterize the composition and microstructure of these films. The hardness of the films was measured by using Fisherscopoe H100VP nanoindenter. 3. Results and discussion EDS results show that SiO2 monolithic film is nitrogen-free, indicating that the reactive sputtering process in Ar-N2 mixture atmosphere can be used to prepare the VN/SiO2 nanomultilayers. Fig. 1 shows the cross-sectional HRTEM images of the VN (3.6 nm)/SiO2 (0.9 nm) sample. The dark and light layers correspond to the VN and SiO2 layers respectively. Fig. 1(a) clearly shows that the multilayer has a well-defined layer structure with planar modulation layers. Fig. 1(b) is the highly magnified image of the same sample. It shows that the lattice fringes continuously penetrate several modulation layers and their interfaces. These results indicate that the SiO2 layers are entirely crystallized, and grow epitaxially with the VN layers. The low angle XRD patterns of multilayers with different SiO2 layer thickness are shown in Fig. 2. The low angle diffraction peaks are clearly observed for all the samples, indicating an obvious layer structure. According to the low angle patterns, the modulation period, Λ, can be figured out in terms of the modified Bragg Formula [8].
Fig. 3. XRD patterns of the substrate, the VN monolithic film, and the multilayers with various lSiO2 (lVN = 3.6 nm).
J. Yue et al. / Materials Letters 62 (2008) 1621–1623
Considering lVN is kept at 3.6 nm for all the samples, their lSiO2 can be worked out, as captioned in Fig. 2. The XRD patterns indicate the amorphous character of the SiO2 monolithic film deposited in this work (not shown here). Fig. 3 shows the XRD patterns of the VN/SiO2 multilayers with various lSiO2, along with those of the substrate and monolithic VN film. The monolithic VN film presents the preferred orientation (111) after taking away the diffraction peak of the substrate that overlaps VN (200). For VN/SiO2 multilayers, the intensity of the VN (200) increases rapidly with lSiO2 varying from 0.45 to 0.9 nm, while VN (111) restrained almost the same intensity. When lSiO2 is 0.9 nm, the intensity of the VN (200) reaches the maximum value, which is much higher than that of the monolithic VN film. This is the result of the crystallization of SiO2 layers and their epitaxial growth with the VN, as indicated by the HRTEM. With the further increase of lSiO2, the intensity of the VN (200) decreased gradually. This suggests that the SiO2 layers transform into amorphous structure. The HRTEM and XRD results clearly reveal that the SiO2 layers below a critical thickness (∼1 nm) crystallize and grow epitaxially with the VN layers, which can be explained by the minimization of the interfacial energy. It is hard to confirm the crystal structure of the SiO2 layers as their thickness is as thin as 3–6 atomic planes. Nevertheless, the SiO2 layers are more likely to form a pseudocrystal structure, i.e., the same structure as VN, because no XRD reflection of SiO2 is observed. Such kinds of pseudocrystal growth have been reported in other systems [9–11]. Since SiO2 is more likely to grow into amorphous structure under sputtering conditions, the epitaxial growth of the multilayers may no longer be energetically favorable with the further increase of lSiO2. SiO2 layers gradually transform into amorphous structure when its thickness exceeds 1 nm. Consequently, the newly arriving VN particles have to renucleate on the amorphous SiO2 layers. The epitaxial growth of the multilayers is therefore blocked. The hardness test results show that nanoindentation hardness (HV) of the multilayers (lVN = 3.6 nm) increases quickly with the increase of lSiO2, and reaches a maximum hardness of 34 GPa when lSiO2 is 0.9 nm, which is much higher than the hardness of VN and SiO2 monolithic film (about 24 GPa and 9 GPa, respectively). Further increasing lSiO2 to 1.7 nm, the hardness of multilayers slightly decreases to 25 GPa, but is still higher than the hardness of VN monolithic film. Several mechanisms have been proposed to explain the superhardness effect of nanomultilayers, which include dislocation blocking by layer interfaces [12], strain effects at layer interfaces [13] and Hall– Petch strengthening [14]. Some common requirements in these theories can be summarized as follows. Firstly, the shear moduli difference between the two modulation layers needs to be as large as possible. Secondly, the thickness of the modulation layers must be small so that the dislocations cannot be generated or slipped within the individual layers. Lastly, the two modulation layers have to form a sharp coherent interface. To meet these requirements, the dislocations in the multilayers have to overcome the additional resistance force when slipping across the interfaces. Thus, considerable hardness increase can be obtained. In this study, when lSiO2 is below about 1 nm, VN/SiO2 nanomultilayers meet all the above requirements and show enhanced
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hardness. However, when SiO2 layers transform into amorphous structure, the coherent interfaces are damaged, resulting in the decrease of the multilayer hardness. In order to analyze the effects of the VN layer on the mechanical properties of the multilayers, another series of samples with different lVN (3.6–37 nm) and the same lSiO2 (0.9 nm) were prepared. The results reveal that the hardness of the multilayers is not sensitive to the thickness of VN layers. Even when lVN is increased to 10 nm, the hardness of the multilayers still retains higher than 30 GPa. It means that the production efficiency of this kind of nitride/oxide multilayer can be remarkably improved by increasing the deposition rate of nitride layer. The reactive sputtering process used successfully in this work can significantly increase the deposition rate of VN layers and this makes the VN/SiO2 nanomultilayers more hopeful for industrial applications.
4. Conclusions With the template effect of the VN layers, the SiO2 in the multilayers crystallizes when SiO2 layer thickness is below about 1 nm. The two modulation layers grow epitaxially. Correspondingly, the hardness of the multilayers increases remarkably to 34 GPa. When the SiO2 layer thickness is larger than 1 nm, the SiO2 grows into amorphous structure and blocks the epitaxial growth of the multilayers. Therefore, the hardness of the multilayer decreases obviously. Because the deposition rate of the nitride layers is greatly increased, the reactively synthesized VN/SiO2 nanomultilayers and their preparation method are likely to be applied in some industrial field. Acknowledgement The work is financially supported by National Natural Chinese Foundation of China, under grant no. 50571062. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
S. Paldey, et al., Mater. Sci. Eng. A. 342 (2003) 58. A.A. Layyous, et al., Surf. Coat. Technol. 56 (1992) 89. U. Helmersson, et al., J. Appl. Phys. 62 (1987) 481. W.D. Sproul, Science. 273 (1996) 889. W.D. Sproul, Surf. Coat. Technol. 86–87 (1996) 170. L. Wei, et al., Appl. Phys. Lett. 86 (2005) 021919. L. Wei, et al., J. Appl. Phys. 98 (2005) 074302. C. Kim, et al., Thin Solid Films 240 (1994) 52. D.W. Wu, et al., Acta Phys. Sin. 48 (1999) 904. J.J. Lao, et al., Acta Phys. Sin. 53 (2004) 1961. L. Wei, et al., Acta Phys. Sin. 54 (2005) 4846. J.S. Koehler, Phys. Rev. B. 2 (1970) 547. M. Kato, et al., Acta Metall. 28 (1980) 285. P.M. Anderson, et al., Nanostruct. Mater. 5 (1995) 349.
Materials Letters 62 (2008) 1621 – 1623 www.elsevier.com/locate/matlet
Microstructure and mechanical properties of VN/SiO2 nanomultilayers synthesized by reactive sputtering Jianling Yue a,b , Wenji Zhao a , Geyang Li a,⁎, Wei Gao b a
State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200030, China b School of Engineering, The University of Auckland, 20 Symonds Street, Auckland, New Zealand Received 10 June 2007; accepted 19 September 2007 Available online 26 September 2007
Abstract VN/SiO2 nanomultilayers with various individual SiO2 and VN layer thickness were prepared by reactive sputtering process. The crystallization condition of the SiO2 layers and how they influence the characteristics of the multilayers were studied. The results reveal that under the template effect of the crystalline VN layer, as-deposited amorphous SiO2 crystallizes at very low thickness (b ∼ 1 nm), and then grows epitaxially with VN layer in the VN/SiO2 nanomultilayers. This leads to a remarkable increase in hardness. If the SiO2 layers' thickness is further increased, they gradually transform into amorphous structure and block the epitaxial growth of the multilayers, resulting in a quick decline in hardness. On the other hand, the hardness of the multilayers is not sensitive to the VN layer thickness. © 2007 Elsevier B.V. All rights reserved. Keywords: Coatings; Epitaxial growth; Hardness
1. Introduction During high speed and dry cutting, the temperature at the tip of a cutting tool can reach up to 1000 °C. Such cutting tools require a high performance coating. Most of the nitride hard coatings, such as TiN and TiAlN, will oxidize and lose their protective function in such high temperature [1]. Therefore, it is crucial to improve the hardness and high temperature oxidation resistance of the tool coatings to withstand the rigorous conditions. Although oxides exhibit good oxidation resistance, they have seldom been used individually for cutting tool coatings due to their low hardness. In the earlier practice, Al2O3 layers with thickness of about 0.5 μm were inserted into the nitride coatings to improve the oxidation resistance [2]. However, the results were not satisfactory due to their low hardness. It was found in 1987 that multilayers deposited alternately by two kinds of materials at nanometers thickness exhibit superhardness effect, an anomalous increase of hardness [3]. Based on this concept, Sproul [4] suggested combining two oxides to ⁎ Corresponding author. Tel.: +86 21 62932106; fax: +86 21 62932785. E-mail address: [email protected] (G. Li). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.040
synthesize nanomultilayers which may exhibit both high temperature oxidation resistance and sufficient hardness. Efforts following this idea were made in the Al2O3/ZrO2 [4] and Y2O3/ ZrO2 [5] nanomultilayers. Unfortunately, there is no significant increase in hardness which can be due to the following two reasons. Firstly, there is only a small difference in the shear modulus between the two oxides. Secondly, the multilayers form an amorphous structure. Recently, a strategy to synthesize a new kind of superhard coatings, nitrides/oxides nanomultilayers, was brought forward. Such coatings are presumed to exhibit both high temperature oxidation resistance and sufficient hardness. TiN/SiO2 [6] and TiN/Al2O3 [7] nanomultilayers have been successfully synthesized using this strategy. Studies revealed that when the thickness of the oxide layer is below about 1 nm, normally amorphous SiO2 or Al2O3 can crystallize under the “template effect” of the crystalline TiN layers. The crystallized oxide layers grow epitaxially with nitride layers, accompanied with a remarkable hardness increase. In spite of their great scientific value, the nitrides/oxides nanomultilayers had little practicality because they were deposited by directly sputtering compound targets (TiN, Al2O3, SiO2) that are very low in deposition rate.
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J. Yue et al. / Materials Letters 62 (2008) 1621–1623
In present work, VN/SiO2 nanomultilayers have been synthesized by reactive sputtering in order to make the strategy of nitrides/oxides nanomultilayers more industrially practical. 2. Experimental procedure VN/SiO2 nanomultilayers and VN, SiO2 monolithic films were prepared by an ANELVA SPC-350 magnetron sputtering system. The targets of V (99.99%) and SiO2 (99.9%) with each diameter of 75 mm were controlled by two RF cathodes respectively. Mirror polished stainless steel substrates were cleaned in acetone before being placed on the rotatable substrate holder. Before the deposition, the system was evacuated to a base pressure of 4 × 10− 4 Pa. Then the system was filled with sputtering gas Ar (99.999%) and reactive gas N2 (99.999%), with pressure of 2.4 × 10− 1 Pa and 8 × 10− 2 Pa respectively. No deliberate bias or heating was applied to the substrate during the deposition. By controlling the time the substrate stops in front of the targets and the target power, nanomultilayers with different SiO2 and VN layer thickness (lSiO2 and lVN, respectively) were obtained. The overall thickness for each specimen was about 2 μm. In order to find out whether the SiO2 in the multilayers reacts with N2 during deposition, a SiO2 monolithic film was prepared in the same conditions as the VN/SiO2 multilayers. EDXA DX-4 energy dispersive spectroscope (EDS), Philips CM200 FEG high-resolution transmission electron microscope (HRTEM) and Rigaku Dmax-rC X-ray diffractometer (XRD)
Fig. 1. Low-magnified (a) and high-magnified (b) cross-sectional HRTEM images of the VN (3.6 nm)/SiO2 (0.9 nm) nanomultilayers.
Fig. 2. Low-angle XRD patterns of the VN/SiO2 multilayers with various lSiO2 (lVN = 3.6 nm).
using Cu Kα (40 kV 150 mA) radiation were used to characterize the composition and microstructure of these films. The hardness of the films was measured by using Fisherscopoe H100VP nanoindenter. 3. Results and discussion EDS results show that SiO2 monolithic film is nitrogen-free, indicating that the reactive sputtering process in Ar-N2 mixture atmosphere can be used to prepare the VN/SiO2 nanomultilayers. Fig. 1 shows the cross-sectional HRTEM images of the VN (3.6 nm)/SiO2 (0.9 nm) sample. The dark and light layers correspond to the VN and SiO2 layers respectively. Fig. 1(a) clearly shows that the multilayer has a well-defined layer structure with planar modulation layers. Fig. 1(b) is the highly magnified image of the same sample. It shows that the lattice fringes continuously penetrate several modulation layers and their interfaces. These results indicate that the SiO2 layers are entirely crystallized, and grow epitaxially with the VN layers. The low angle XRD patterns of multilayers with different SiO2 layer thickness are shown in Fig. 2. The low angle diffraction peaks are clearly observed for all the samples, indicating an obvious layer structure. According to the low angle patterns, the modulation period, Λ, can be figured out in terms of the modified Bragg Formula [8].
Fig. 3. XRD patterns of the substrate, the VN monolithic film, and the multilayers with various lSiO2 (lVN = 3.6 nm).
J. Yue et al. / Materials Letters 62 (2008) 1621–1623
Considering lVN is kept at 3.6 nm for all the samples, their lSiO2 can be worked out, as captioned in Fig. 2. The XRD patterns indicate the amorphous character of the SiO2 monolithic film deposited in this work (not shown here). Fig. 3 shows the XRD patterns of the VN/SiO2 multilayers with various lSiO2, along with those of the substrate and monolithic VN film. The monolithic VN film presents the preferred orientation (111) after taking away the diffraction peak of the substrate that overlaps VN (200). For VN/SiO2 multilayers, the intensity of the VN (200) increases rapidly with lSiO2 varying from 0.45 to 0.9 nm, while VN (111) restrained almost the same intensity. When lSiO2 is 0.9 nm, the intensity of the VN (200) reaches the maximum value, which is much higher than that of the monolithic VN film. This is the result of the crystallization of SiO2 layers and their epitaxial growth with the VN, as indicated by the HRTEM. With the further increase of lSiO2, the intensity of the VN (200) decreased gradually. This suggests that the SiO2 layers transform into amorphous structure. The HRTEM and XRD results clearly reveal that the SiO2 layers below a critical thickness (∼1 nm) crystallize and grow epitaxially with the VN layers, which can be explained by the minimization of the interfacial energy. It is hard to confirm the crystal structure of the SiO2 layers as their thickness is as thin as 3–6 atomic planes. Nevertheless, the SiO2 layers are more likely to form a pseudocrystal structure, i.e., the same structure as VN, because no XRD reflection of SiO2 is observed. Such kinds of pseudocrystal growth have been reported in other systems [9–11]. Since SiO2 is more likely to grow into amorphous structure under sputtering conditions, the epitaxial growth of the multilayers may no longer be energetically favorable with the further increase of lSiO2. SiO2 layers gradually transform into amorphous structure when its thickness exceeds 1 nm. Consequently, the newly arriving VN particles have to renucleate on the amorphous SiO2 layers. The epitaxial growth of the multilayers is therefore blocked. The hardness test results show that nanoindentation hardness (HV) of the multilayers (lVN = 3.6 nm) increases quickly with the increase of lSiO2, and reaches a maximum hardness of 34 GPa when lSiO2 is 0.9 nm, which is much higher than the hardness of VN and SiO2 monolithic film (about 24 GPa and 9 GPa, respectively). Further increasing lSiO2 to 1.7 nm, the hardness of multilayers slightly decreases to 25 GPa, but is still higher than the hardness of VN monolithic film. Several mechanisms have been proposed to explain the superhardness effect of nanomultilayers, which include dislocation blocking by layer interfaces [12], strain effects at layer interfaces [13] and Hall– Petch strengthening [14]. Some common requirements in these theories can be summarized as follows. Firstly, the shear moduli difference between the two modulation layers needs to be as large as possible. Secondly, the thickness of the modulation layers must be small so that the dislocations cannot be generated or slipped within the individual layers. Lastly, the two modulation layers have to form a sharp coherent interface. To meet these requirements, the dislocations in the multilayers have to overcome the additional resistance force when slipping across the interfaces. Thus, considerable hardness increase can be obtained. In this study, when lSiO2 is below about 1 nm, VN/SiO2 nanomultilayers meet all the above requirements and show enhanced
1623
hardness. However, when SiO2 layers transform into amorphous structure, the coherent interfaces are damaged, resulting in the decrease of the multilayer hardness. In order to analyze the effects of the VN layer on the mechanical properties of the multilayers, another series of samples with different lVN (3.6–37 nm) and the same lSiO2 (0.9 nm) were prepared. The results reveal that the hardness of the multilayers is not sensitive to the thickness of VN layers. Even when lVN is increased to 10 nm, the hardness of the multilayers still retains higher than 30 GPa. It means that the production efficiency of this kind of nitride/oxide multilayer can be remarkably improved by increasing the deposition rate of nitride layer. The reactive sputtering process used successfully in this work can significantly increase the deposition rate of VN layers and this makes the VN/SiO2 nanomultilayers more hopeful for industrial applications.
4. Conclusions With the template effect of the VN layers, the SiO2 in the multilayers crystallizes when SiO2 layer thickness is below about 1 nm. The two modulation layers grow epitaxially. Correspondingly, the hardness of the multilayers increases remarkably to 34 GPa. When the SiO2 layer thickness is larger than 1 nm, the SiO2 grows into amorphous structure and blocks the epitaxial growth of the multilayers. Therefore, the hardness of the multilayer decreases obviously. Because the deposition rate of the nitride layers is greatly increased, the reactively synthesized VN/SiO2 nanomultilayers and their preparation method are likely to be applied in some industrial field. Acknowledgement The work is financially supported by National Natural Chinese Foundation of China, under grant no. 50571062. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
S. Paldey, et al., Mater. Sci. Eng. A. 342 (2003) 58. A.A. Layyous, et al., Surf. Coat. Technol. 56 (1992) 89. U. Helmersson, et al., J. Appl. Phys. 62 (1987) 481. W.D. Sproul, Science. 273 (1996) 889. W.D. Sproul, Surf. Coat. Technol. 86–87 (1996) 170. L. Wei, et al., Appl. Phys. Lett. 86 (2005) 021919. L. Wei, et al., J. Appl. Phys. 98 (2005) 074302. C. Kim, et al., Thin Solid Films 240 (1994) 52. D.W. Wu, et al., Acta Phys. Sin. 48 (1999) 904. J.J. Lao, et al., Acta Phys. Sin. 53 (2004) 1961. L. Wei, et al., Acta Phys. Sin. 54 (2005) 4846. J.S. Koehler, Phys. Rev. B. 2 (1970) 547. M. Kato, et al., Acta Metall. 28 (1980) 285. P.M. Anderson, et al., Nanostruct. Mater. 5 (1995) 349.