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AlN superlattice coatings using closed-field unbalanced magnetron sputtering process
Surface and Coatings Technology 171 (2003) 91–95
Synthesis of CrNyAlN superlattice coatings using closed-field unbalanced magnetron sputtering process Gwang S. Kima,*, Sang Y. Leea, Jun H. Hahnb, Sang Y. Leec a Department of Materials Engineering, HanKuk Aviation University, KoYang, KyungKi-Do 412-791, South Korea Chemical Metrology and Materials Evaluation Division, Korea Research Institute of Standard and Science, Taejon 305-600, South Korea c Metal Working and Plasticity Group, KIMM, Changwon, KyungNam 641-010, South Korea
b
Abstract Synthesis of CrNyAlN superlattice coatings with various composition (CryAl at.%) and superlattice period (l) using closedfield unbalanced magnetron sputtering method was studied in this work. The coatings were characterized in terms of crystal phase, chemical composition, microstructure and mechanical properties by transmission electron microscopy (TEM), X-ray diffractometry (XRD), glow discharge optical emission spectroscopy and nano-indentation test. Results from TEM and XRD analysis showed that the crystal structure of AlN layer in the CrNyAlN superlattice coatings has a metastable cubic lattice structure rather than an equilibrium hexagonal structure (wurtzite-type), matching coherently with the CrN layer, which has a NaCl type lattice structure. The maximum hardness and plastic deformation resistance (H 3 yE 2 ) of CrNyAlN superlattice film, when the atomic concentration ratio of Cr to Al is 0.98 and the superlattice period (l) is 4.8 nm, are approximately 37 and 0.48 GPa, respectively. These values are 1.6 and 2.5 times higher than those of the CrN single layer coating (23.5 and 0.17 GPa), respectively. These enhancement effects in superlattice films could be attributed to the resistance to dislocation glide across interface between the CrN and AlN layers. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: CrNyAlN superlattice coatings; Unbalanced magnetron sputtering; TEM; Nano-indentation
1. Introduction In the early industrial stage, the single layer hard coatings such as TiN, CrN, AlN and DLC coatings played an important role. These coatings were applied to molds, punch, cutting tools and other machine parts to improve their wear, oxidation and corrosion resistance w1–3x. However, in recent years an industrial interest in hard coatings has been focused on further improvement of their chemical and mechanical properties. Two ways to improve the properties of single layer hard coatings seem to be of interest. One way is the creation of multiphase hard coatings like TiAlN, TiCrN and TiCN. The other way is the deposition of more complex multilayer coatings such as CrNyNbN w4x, TiNyCrN w5x and WCy TiAlN w6x. The latter is effective in improving chemical and mechanical properties, including hardness and fracture toughness, as compared to homogeneous single *Corresponding author. E-mail address: [email protected] (G.S. Kim).
layer coatings. Furthermore, the hardness of superlattice coatings which are composed of two different alternating nanometer-scale multilayers with a superlattice period (l), i.e. a bilayer thickness of two kinds of materials, ranging from 4 to 9 nm w7x, increases greater than that of the materials of the individual components of bilayer. This hardness enhancement is a very complex phenomenon but several models w8–11x, which explain superlattice strengthening, have already been developed and the most typical model is based on restricted dislocation movement within and between layers in the superlattice coating. Chu and Barnett et al. w12x explained that the difference in the dislocation line energy between two layers, which is proportional to the difference in the elastic shear modulus of the two materials, provides a barrier to the motion of dislocation across the boundary. The energy needed to move dislocation across boundary is dependent on the thickness of layers or superlattice period. A narrow or sharp boundary requires more energy than a diffuse boundary. Therefore, it predicts a peak in hardness when there is a difference in shear
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(03)00244-5
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G.S. Kim et al. / Surface and Coatings Technology 171 (2003) 91–95
Fig. 1. The typical elemental depth profile of CrNyAlN superlattice film deposited at the 1 kW of Cr target power.
modulus between two layer materials and sharp interface between layers. These prominent mechanical properties of superlattice coatings compelled us to consider a CrNyAlN superlattice to improve oxidation and mechanical properties of CrN single layer coating further. Therefore, the CrNy AlN superlattice coatings with various compositions (CryAl at.%) and superlattice period (l) were synthesized using closed-field unbalanced magnetron sputtering method. The relationships between their superlattice period (l), microstructure, hardness and elastic modulus were investigated. 2. Experimental details The CrNyAlN superlattice coatings were deposited on silicon wafer of (1 0 0) orientation by means of closed-field unbalanced magnetron sputtering which is an exceptionally versatile technique for the deposition of high-quality, well-adhered films. Before deposition of films, the substrate of Si wafer was cleaned by the conventional cleaning process. The base pressure of sputtering chamber (diameter 250 mm, Ls300 mm) was pumped down to less than 3=10y5 Torr. In the deposition conditions of CrNyAlN superlattice coatings, the power density of Al target was maintained at 63.7 Wycm2 (pulsed DC: frequency 19 kHz and duty cycle 65%), while the power of Cr target was varied between DC 0.5 and 1.0 kW to produce coatings with different composition ratios (CryAl at.%) and superlattice period (l). Other deposition conditions such as the distance of target-to-substrate, substrate bias voltage and substrate rotation speed were fixed to 60 mm, y100 V (pulsed DC with frequency 10 kHz and duty cycle 50%) and 3 rpm, respectively. During the deposition process, Ar pressure was initially set at
2.4=10y3 Torr and the reactive gas of N2 was subsequently added to obtain desired gas composition, maintaining a total working pressure constant at 3.3=10y3 Torr. The structure of coatings deposited under the above conditions comprises an initial Cr adhesion layer, a CrN based layer and then a CrNyAlN superlattice layer. The phase and texture of CrNyAlN superlattice coatings were characterized by X-ray diffractometry (XRD: SEIFERT 3000PTS) using Cu Ka X-ray. The microstructure of coating was observed using cross-sectional transmission electron microscopy (TEM) and crystal structure was analyzed in detail using transmission electron diffraction (TED) pattern. The TEM studies were performed in a JEM-3011 instrument operated at 300 kV. The chemical compositions of coatings were determined from the elemental depth profiles measured by glow discharge optical emission spectroscopy (LECO GDS 850A). The intrinsic hardness (H) and elastic modulus (E) of coatings, where the substrate effect is not included, were measured using a nano-indentation instrument (Nano-indenter II developed by MTS Instrument Co.) with a Berkovich diamond tip by fixing the indentation depth to 300 nm which is less than 10% of the total thickness of film. From measured values of H and E, the ratios H 3 yE 2 of each film, which is proportional to the resistance of the coating to plastic deformation, were calculated. 3. Results and discussion 3.1. Chemical compositions and crystal structures of films Fig. 1 illustrates the chemical compositions depth profile of CrNyAlN superlattice coating deposited at a Cr target power of 1 kW. It is shown that the carbon and oxygen elements are rarely found throughout the depth of coating, so it is clear that the coatings were deposited on substrate without carbon and oxygen contamination. The increase in chromium content observed adjacent to the coatingysubstrate interface results from the Cr adhesion layer and CrN based layer deposited in order to improve adhesion property prior to deposition of the superlattice layer (CrNyAlN). The average relative atomic concentration ratios of Cr to Al and that of N to (CrqAl) in CrNyAlN superlattice layer are listed in Table 1 as a function of Cr target power. It appeared that the relative atomic concentration ratio of Cr to Al increased from approximately 0.97 to 2.14 as the Cr Table 1 Relative ratios of CryAl and Ny(CrqAl) atomic percent in the CrNyAlN superlattice coatings as a function of Cr target power Cr target power (kW) Relative ratio of CryAl (at.%) Relative ratio of Ny(AlqCr) (at.%)
0.5 0.97 1.14
0.6 1.38 1.12
0.8 1.89 1.09
1 2.14 1.13
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target power was raised from 0.5 to 1 kW and that of N to (CrqAl) in all the coatings remains at approximately 1.1 (from 1.09 to 1.14). The results of X-ray diffraction analyses on CrNy AlN superlattice coatings deposited with various atomic concentration ratios of Cr to Al (at.%) are shown in Fig. 2. Normally, the AlN single layer film has an equilibrium hexagonal structure (wurtzite-type) w13x, however in Fig. 2, the peaks of this wurtzite-structured AlN were not shown but the B1 NaCl type superlattice peak of (1 1 1) which is preferred orientation as well as that of (2 0 0) and (3 1 1) was observed. This analysis indicates that the AlN layer in CrNyAlN superlattice films has a metastable cubic structure. 3.2. Microstructure of CrNyAlN superlattice film The bright-field cross-sectional TEM micrograph of CrNyAlN superlattice film with CryAl atomic ratio of 0.95 is shown in Fig. 3a. The film is dense columnar structure and CrN and AlN layers alternating in growth direction are shown as bright and dark layers, respectively. The AlN layers appear to be slightly lighter than the CrN layers because of the lower scattering factor of Al compared to Cr. HR-TEM (Fig. 3b) analysis revealed that superlattice period (l) of CrNyAlN superlattice film with CryAl atomic ratio of 0.95 is approximately 4.9 nm and the layers are well-defined and slightly nonplanar. The lattice fringes having a spacing (d111) of ˚ (CrN: d111s2.3902 A ˚ reported in the JCPDS 2.4626 A card) are continuous through the CrN and AlN layers. This result shows that the crystal structure of AlN in
Fig. 3. The cross-sectional TEM images of CrNyAlN superlattice film deposited at the 0.5 kW of Cr target power: (a) XTEM image; (b) HR-TEM image.
Fig. 2. The XRD patterns of CrNyAlN superlattice films with various relative atomic concentration ratios of xsCryAl: (a) xs0.97; (b) xs1.38; (c) xs1.89; (d) xs2.14.
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and no diffractions corresponding to wurtzite-type AlN was observed. These results were also consistent with the results from XRD analysis. 3.3. Mechanical properties of films
Fig. 4. The hardness and elastic modulus of CrN film, AlN film and CrNyAlN superlattice films dependent on Cr target power.
the CrNyAlN superlattice grown on CrN base layer has a metastable cubic lattice structure (c-AlN) instead of equilibrium hexagonal structure (wurtzite-type: w-AlN) because of growth template effect w14x and the interface of each layer in superlattice was matched coherently. Furthermore, the TED pattern of this film (as shown in the insert of Fig. 3a) is a spotty ring pattern and matches a typical diffraction pattern of NaCl crystal structure
Fig. 4 shows the hardness and elastic modulus of CrNyAlN superlattice coatings dependent on Cr target power. The values of hardness were measured by nanoindenter using a continuous stiffness method (CSM), which is applying a vibration of 45 Hz during the continuous loading–unloading indentation. Therefore the CSM method enables us to measure the continuous hardness of film from initial indentation depth to final indentation depth w9x. The hardness and elastic modulus of each coating determined at the indentation depth of 100 nm were listed in Table 2. The hardness of CrNy AlN superlattice coatings were measured to be in a range from approximately 31 to 37 GPa and the maximum hardness of approximately 37 GPa was found at a Cr target power of 0.5 kW, which corresponds to a Cry Al atomic percentage ratio of 0.97 and a superlattice period (l) of 0.49 nm. Thus, at Cr target power of 0.5 kW, CrN and AlN components of the multilayer have an approximately equal thickness with a composition ratio of 1:1. Previous work has shown that a maximum hardness would be expected when the individual components of the superlattice have an equal thickness w15x. The high hardness of coating is not necessarily to the prime requirement for a wear resistance. Elastic modulus of coating can be an equally important factor w16x. According to Johnson analysis w17x, the yield pressure Py in a rigid-ball on elasticyplastic plate contact can be determined by the equation. Pys0.78r2ŽH3yE2.
(1)
where r is the contacting sphere radius: thus the ratio of H 3 yE 2 (plastic deformation resistance) should be a strong indicator of a coating’s resistance to plastic deformation in these types of loaded contact. Therefore, the likelihood of plastic deformation is reduced in materials with high hardness and low modulus w13x. In general, a low modulus is also desirable as it allows the given load to be distributed over a wider area. In the
Table 2 Mechanical constants of CrN film, AlN film and CrNyAlN superlattice films deposited at different values of Cr target power A
AlN
CrN
CrNyAlN (Crs0.5 kW)
CrNyAlN (Crs0.6 kW)
CrNyAlN (Crs0.8 kW)
CrNyAlN (Crs1 kW)
CryAl (at.%) l (nm) H (GPa) E (GPa) H 3yE 2 (GPa)
– – 23.6 258.18 0.197
– – 26.1 319.33 0.174
0.97 4.9 37.3 328.26 0.481
1.38 6.3 35.2 341.46 0.374
1.89 9.7 32.0 342.78 0.278
2.14 12.5 31.6 350.56 0.256
l, superlattice periods; H, hardness; E, elastic modulus and H 3 yE 2, plastic deformation resistance.
G.S. Kim et al. / Surface and Coatings Technology 171 (2003) 91–95
case of CrNyAlN superlattice coating synthesized at the Cr target power of 0.5 kW, when individual components of the superlattice have an equal thickness, the elastic modulus (E) was measured to be approximately 328 GPa (minimum value) and the ratio of H 3 yE 2 (plastic deformation resistance) was calculated to be 0.481 GPa, which is approximately 2.5 times higher than that (0.174 and 0.197 GPa, respectively), of the CrN and AlN single layer coatings. Therefore, in the case of CrNyAlN superlattice films, it is certain that the plastic deformation due to the wear is reduced larger than that of other films when CrN and AlN layers have an approximately equal thickness, and the effect of superlattice hardening may be attributed to the resistance to dislocation glide across interface between CrN layer and AlN layer and each interface also functions like a grain boundary in a Hall–Petch related mechanism such that dislocations pile-up at the interfaces and strain hardens layer. Furthermore, each interface serves as crack tip deflectors which strengthens the coatings. 4. Conclusions The main results obtained in this work could be summarized as follows: (1) The CrNyAlN superlattice coatings with a superlattice period in the range of ls4.9–12.5 nm have been produced by closed-field unbalanced magnetron sputtering process. In all cases, the CrNyAlN superlattice coatings exhibit FCC structure. The crystal orientations were (1 1 1), (2 0 0) and (3 1 1) orientations. (2) The crystal structure of AlN layer in the CrNy AlN superlattice coatings grown on CrN based layer has a metastable cubic lattice structure instead of equilibrium hexagonal structure (wurtzite-type) because of growth template effect and the coherent interface matching between CrN and AlN. (3) The hardness of CrNyAlN superlattice coatings were measured to be in the range from approximately
95
31 to 37 GPa. The maximum hardness was found to be 37 GPa at a Cr target power of 0.5 kW, which corresponds to a CryAl atomic percentage ratio of 0.97 and a superlattice period (l) of 0.49 nm. (4) The H 3 yE 2 ratios (plastic deformation resistance) of CrNyAlN superlattice coatings were calculated to be in the range from approximately 0.256 to 0.481. The superlattice film has a maximum value of H 3 yE 2 when the individual components of the superlattice have an equal thickness. References w1x S.Y. Lee, J.W. Chung, K.B. Kim, J.G. Han, S.S. Kim, Surf. Coat. Technol. 86y87 (1996) 325. w2x S.Y. Lee, J.W. Chung, H.J. Park, J.G. Han, J.H. Lee, J. Kor. Inst. Met. Mater. 35 (1997) 1734. w3x G.S. Kim, S.Y. Lee, E.Y. Lee, J. Kor. Inst. Met. Mater. 37 (1999) 578. w4x D.C. Cameron, R. Aimo, Z.H. Wang, K.A. Pischow, Surf. Coat. Technol. 142–144 (2001) 567–572. w5x Q. Yang, C. He, L.R. Zhao, J.-P. Immarigeon, Scripta Mater. 46 (2002) 293–297. w6x J.S. Yoon, H.Y. Hee, J.G. Han, S.H. Yang, J. Musil, Surf. Coat. Technol. 142–144 (2001) 596–602. w7x J. Musil, Surf. Coat. Technol. 125 (2000) 322–330. w8x X. Chu, S.A. Barnett, J. Appl. Phys. 77 (1995) 4403. w9x T.F. Page, G.M. Pharr, J.C. Hay, et al., Mater. Res. Soc. Symp. Proc. (1998) 522. w10x X. Chu, S.A. Barnet, M.S. Wong, W.D. Sproul, Surf. Coat. Technol. 57 (1993) 13. w11x W.D. Sproul, Surf. Coat. Technol. 86–87 (1996) 170. w12x X.T. Zeng, S. Mridha, U. Chai, J. Mater. Pro. Technol. 89–90 (1999) 528–531. w13x T.Y. Tsui, G.M. Pharr, W.C. Oliver, et al., Mater. Res. Soc. Symp. Proc. 383 (1995) 447. w14x A. Inspektor, C.E. Brauer, Surf. Coat. Technol. 68–69 (1994) 359. w15x D.B. Lewis, I. Wadsworth, V. Valvoda, Surf. Coat. Technol. 116–119 (1999) 285. w16x A. Leyland, A. Matthews, Wear 246 (2000) 1–11. w17x K.L. Johnson, Contact Mechanics, Cambridge University Press, London, UK, 1985, ISBN: 0-521-34796-3.
Synthesis of CrNyAlN superlattice coatings using closed-field unbalanced magnetron sputtering process Gwang S. Kima,*, Sang Y. Leea, Jun H. Hahnb, Sang Y. Leec a Department of Materials Engineering, HanKuk Aviation University, KoYang, KyungKi-Do 412-791, South Korea Chemical Metrology and Materials Evaluation Division, Korea Research Institute of Standard and Science, Taejon 305-600, South Korea c Metal Working and Plasticity Group, KIMM, Changwon, KyungNam 641-010, South Korea
b
Abstract Synthesis of CrNyAlN superlattice coatings with various composition (CryAl at.%) and superlattice period (l) using closedfield unbalanced magnetron sputtering method was studied in this work. The coatings were characterized in terms of crystal phase, chemical composition, microstructure and mechanical properties by transmission electron microscopy (TEM), X-ray diffractometry (XRD), glow discharge optical emission spectroscopy and nano-indentation test. Results from TEM and XRD analysis showed that the crystal structure of AlN layer in the CrNyAlN superlattice coatings has a metastable cubic lattice structure rather than an equilibrium hexagonal structure (wurtzite-type), matching coherently with the CrN layer, which has a NaCl type lattice structure. The maximum hardness and plastic deformation resistance (H 3 yE 2 ) of CrNyAlN superlattice film, when the atomic concentration ratio of Cr to Al is 0.98 and the superlattice period (l) is 4.8 nm, are approximately 37 and 0.48 GPa, respectively. These values are 1.6 and 2.5 times higher than those of the CrN single layer coating (23.5 and 0.17 GPa), respectively. These enhancement effects in superlattice films could be attributed to the resistance to dislocation glide across interface between the CrN and AlN layers. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: CrNyAlN superlattice coatings; Unbalanced magnetron sputtering; TEM; Nano-indentation
1. Introduction In the early industrial stage, the single layer hard coatings such as TiN, CrN, AlN and DLC coatings played an important role. These coatings were applied to molds, punch, cutting tools and other machine parts to improve their wear, oxidation and corrosion resistance w1–3x. However, in recent years an industrial interest in hard coatings has been focused on further improvement of their chemical and mechanical properties. Two ways to improve the properties of single layer hard coatings seem to be of interest. One way is the creation of multiphase hard coatings like TiAlN, TiCrN and TiCN. The other way is the deposition of more complex multilayer coatings such as CrNyNbN w4x, TiNyCrN w5x and WCy TiAlN w6x. The latter is effective in improving chemical and mechanical properties, including hardness and fracture toughness, as compared to homogeneous single *Corresponding author. E-mail address: [email protected] (G.S. Kim).
layer coatings. Furthermore, the hardness of superlattice coatings which are composed of two different alternating nanometer-scale multilayers with a superlattice period (l), i.e. a bilayer thickness of two kinds of materials, ranging from 4 to 9 nm w7x, increases greater than that of the materials of the individual components of bilayer. This hardness enhancement is a very complex phenomenon but several models w8–11x, which explain superlattice strengthening, have already been developed and the most typical model is based on restricted dislocation movement within and between layers in the superlattice coating. Chu and Barnett et al. w12x explained that the difference in the dislocation line energy between two layers, which is proportional to the difference in the elastic shear modulus of the two materials, provides a barrier to the motion of dislocation across the boundary. The energy needed to move dislocation across boundary is dependent on the thickness of layers or superlattice period. A narrow or sharp boundary requires more energy than a diffuse boundary. Therefore, it predicts a peak in hardness when there is a difference in shear
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(03)00244-5
92
G.S. Kim et al. / Surface and Coatings Technology 171 (2003) 91–95
Fig. 1. The typical elemental depth profile of CrNyAlN superlattice film deposited at the 1 kW of Cr target power.
modulus between two layer materials and sharp interface between layers. These prominent mechanical properties of superlattice coatings compelled us to consider a CrNyAlN superlattice to improve oxidation and mechanical properties of CrN single layer coating further. Therefore, the CrNy AlN superlattice coatings with various compositions (CryAl at.%) and superlattice period (l) were synthesized using closed-field unbalanced magnetron sputtering method. The relationships between their superlattice period (l), microstructure, hardness and elastic modulus were investigated. 2. Experimental details The CrNyAlN superlattice coatings were deposited on silicon wafer of (1 0 0) orientation by means of closed-field unbalanced magnetron sputtering which is an exceptionally versatile technique for the deposition of high-quality, well-adhered films. Before deposition of films, the substrate of Si wafer was cleaned by the conventional cleaning process. The base pressure of sputtering chamber (diameter 250 mm, Ls300 mm) was pumped down to less than 3=10y5 Torr. In the deposition conditions of CrNyAlN superlattice coatings, the power density of Al target was maintained at 63.7 Wycm2 (pulsed DC: frequency 19 kHz and duty cycle 65%), while the power of Cr target was varied between DC 0.5 and 1.0 kW to produce coatings with different composition ratios (CryAl at.%) and superlattice period (l). Other deposition conditions such as the distance of target-to-substrate, substrate bias voltage and substrate rotation speed were fixed to 60 mm, y100 V (pulsed DC with frequency 10 kHz and duty cycle 50%) and 3 rpm, respectively. During the deposition process, Ar pressure was initially set at
2.4=10y3 Torr and the reactive gas of N2 was subsequently added to obtain desired gas composition, maintaining a total working pressure constant at 3.3=10y3 Torr. The structure of coatings deposited under the above conditions comprises an initial Cr adhesion layer, a CrN based layer and then a CrNyAlN superlattice layer. The phase and texture of CrNyAlN superlattice coatings were characterized by X-ray diffractometry (XRD: SEIFERT 3000PTS) using Cu Ka X-ray. The microstructure of coating was observed using cross-sectional transmission electron microscopy (TEM) and crystal structure was analyzed in detail using transmission electron diffraction (TED) pattern. The TEM studies were performed in a JEM-3011 instrument operated at 300 kV. The chemical compositions of coatings were determined from the elemental depth profiles measured by glow discharge optical emission spectroscopy (LECO GDS 850A). The intrinsic hardness (H) and elastic modulus (E) of coatings, where the substrate effect is not included, were measured using a nano-indentation instrument (Nano-indenter II developed by MTS Instrument Co.) with a Berkovich diamond tip by fixing the indentation depth to 300 nm which is less than 10% of the total thickness of film. From measured values of H and E, the ratios H 3 yE 2 of each film, which is proportional to the resistance of the coating to plastic deformation, were calculated. 3. Results and discussion 3.1. Chemical compositions and crystal structures of films Fig. 1 illustrates the chemical compositions depth profile of CrNyAlN superlattice coating deposited at a Cr target power of 1 kW. It is shown that the carbon and oxygen elements are rarely found throughout the depth of coating, so it is clear that the coatings were deposited on substrate without carbon and oxygen contamination. The increase in chromium content observed adjacent to the coatingysubstrate interface results from the Cr adhesion layer and CrN based layer deposited in order to improve adhesion property prior to deposition of the superlattice layer (CrNyAlN). The average relative atomic concentration ratios of Cr to Al and that of N to (CrqAl) in CrNyAlN superlattice layer are listed in Table 1 as a function of Cr target power. It appeared that the relative atomic concentration ratio of Cr to Al increased from approximately 0.97 to 2.14 as the Cr Table 1 Relative ratios of CryAl and Ny(CrqAl) atomic percent in the CrNyAlN superlattice coatings as a function of Cr target power Cr target power (kW) Relative ratio of CryAl (at.%) Relative ratio of Ny(AlqCr) (at.%)
0.5 0.97 1.14
0.6 1.38 1.12
0.8 1.89 1.09
1 2.14 1.13
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target power was raised from 0.5 to 1 kW and that of N to (CrqAl) in all the coatings remains at approximately 1.1 (from 1.09 to 1.14). The results of X-ray diffraction analyses on CrNy AlN superlattice coatings deposited with various atomic concentration ratios of Cr to Al (at.%) are shown in Fig. 2. Normally, the AlN single layer film has an equilibrium hexagonal structure (wurtzite-type) w13x, however in Fig. 2, the peaks of this wurtzite-structured AlN were not shown but the B1 NaCl type superlattice peak of (1 1 1) which is preferred orientation as well as that of (2 0 0) and (3 1 1) was observed. This analysis indicates that the AlN layer in CrNyAlN superlattice films has a metastable cubic structure. 3.2. Microstructure of CrNyAlN superlattice film The bright-field cross-sectional TEM micrograph of CrNyAlN superlattice film with CryAl atomic ratio of 0.95 is shown in Fig. 3a. The film is dense columnar structure and CrN and AlN layers alternating in growth direction are shown as bright and dark layers, respectively. The AlN layers appear to be slightly lighter than the CrN layers because of the lower scattering factor of Al compared to Cr. HR-TEM (Fig. 3b) analysis revealed that superlattice period (l) of CrNyAlN superlattice film with CryAl atomic ratio of 0.95 is approximately 4.9 nm and the layers are well-defined and slightly nonplanar. The lattice fringes having a spacing (d111) of ˚ (CrN: d111s2.3902 A ˚ reported in the JCPDS 2.4626 A card) are continuous through the CrN and AlN layers. This result shows that the crystal structure of AlN in
Fig. 3. The cross-sectional TEM images of CrNyAlN superlattice film deposited at the 0.5 kW of Cr target power: (a) XTEM image; (b) HR-TEM image.
Fig. 2. The XRD patterns of CrNyAlN superlattice films with various relative atomic concentration ratios of xsCryAl: (a) xs0.97; (b) xs1.38; (c) xs1.89; (d) xs2.14.
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and no diffractions corresponding to wurtzite-type AlN was observed. These results were also consistent with the results from XRD analysis. 3.3. Mechanical properties of films
Fig. 4. The hardness and elastic modulus of CrN film, AlN film and CrNyAlN superlattice films dependent on Cr target power.
the CrNyAlN superlattice grown on CrN base layer has a metastable cubic lattice structure (c-AlN) instead of equilibrium hexagonal structure (wurtzite-type: w-AlN) because of growth template effect w14x and the interface of each layer in superlattice was matched coherently. Furthermore, the TED pattern of this film (as shown in the insert of Fig. 3a) is a spotty ring pattern and matches a typical diffraction pattern of NaCl crystal structure
Fig. 4 shows the hardness and elastic modulus of CrNyAlN superlattice coatings dependent on Cr target power. The values of hardness were measured by nanoindenter using a continuous stiffness method (CSM), which is applying a vibration of 45 Hz during the continuous loading–unloading indentation. Therefore the CSM method enables us to measure the continuous hardness of film from initial indentation depth to final indentation depth w9x. The hardness and elastic modulus of each coating determined at the indentation depth of 100 nm were listed in Table 2. The hardness of CrNy AlN superlattice coatings were measured to be in a range from approximately 31 to 37 GPa and the maximum hardness of approximately 37 GPa was found at a Cr target power of 0.5 kW, which corresponds to a Cry Al atomic percentage ratio of 0.97 and a superlattice period (l) of 0.49 nm. Thus, at Cr target power of 0.5 kW, CrN and AlN components of the multilayer have an approximately equal thickness with a composition ratio of 1:1. Previous work has shown that a maximum hardness would be expected when the individual components of the superlattice have an equal thickness w15x. The high hardness of coating is not necessarily to the prime requirement for a wear resistance. Elastic modulus of coating can be an equally important factor w16x. According to Johnson analysis w17x, the yield pressure Py in a rigid-ball on elasticyplastic plate contact can be determined by the equation. Pys0.78r2ŽH3yE2.
(1)
where r is the contacting sphere radius: thus the ratio of H 3 yE 2 (plastic deformation resistance) should be a strong indicator of a coating’s resistance to plastic deformation in these types of loaded contact. Therefore, the likelihood of plastic deformation is reduced in materials with high hardness and low modulus w13x. In general, a low modulus is also desirable as it allows the given load to be distributed over a wider area. In the
Table 2 Mechanical constants of CrN film, AlN film and CrNyAlN superlattice films deposited at different values of Cr target power A
AlN
CrN
CrNyAlN (Crs0.5 kW)
CrNyAlN (Crs0.6 kW)
CrNyAlN (Crs0.8 kW)
CrNyAlN (Crs1 kW)
CryAl (at.%) l (nm) H (GPa) E (GPa) H 3yE 2 (GPa)
– – 23.6 258.18 0.197
– – 26.1 319.33 0.174
0.97 4.9 37.3 328.26 0.481
1.38 6.3 35.2 341.46 0.374
1.89 9.7 32.0 342.78 0.278
2.14 12.5 31.6 350.56 0.256
l, superlattice periods; H, hardness; E, elastic modulus and H 3 yE 2, plastic deformation resistance.
G.S. Kim et al. / Surface and Coatings Technology 171 (2003) 91–95
case of CrNyAlN superlattice coating synthesized at the Cr target power of 0.5 kW, when individual components of the superlattice have an equal thickness, the elastic modulus (E) was measured to be approximately 328 GPa (minimum value) and the ratio of H 3 yE 2 (plastic deformation resistance) was calculated to be 0.481 GPa, which is approximately 2.5 times higher than that (0.174 and 0.197 GPa, respectively), of the CrN and AlN single layer coatings. Therefore, in the case of CrNyAlN superlattice films, it is certain that the plastic deformation due to the wear is reduced larger than that of other films when CrN and AlN layers have an approximately equal thickness, and the effect of superlattice hardening may be attributed to the resistance to dislocation glide across interface between CrN layer and AlN layer and each interface also functions like a grain boundary in a Hall–Petch related mechanism such that dislocations pile-up at the interfaces and strain hardens layer. Furthermore, each interface serves as crack tip deflectors which strengthens the coatings. 4. Conclusions The main results obtained in this work could be summarized as follows: (1) The CrNyAlN superlattice coatings with a superlattice period in the range of ls4.9–12.5 nm have been produced by closed-field unbalanced magnetron sputtering process. In all cases, the CrNyAlN superlattice coatings exhibit FCC structure. The crystal orientations were (1 1 1), (2 0 0) and (3 1 1) orientations. (2) The crystal structure of AlN layer in the CrNy AlN superlattice coatings grown on CrN based layer has a metastable cubic lattice structure instead of equilibrium hexagonal structure (wurtzite-type) because of growth template effect and the coherent interface matching between CrN and AlN. (3) The hardness of CrNyAlN superlattice coatings were measured to be in the range from approximately
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31 to 37 GPa. The maximum hardness was found to be 37 GPa at a Cr target power of 0.5 kW, which corresponds to a CryAl atomic percentage ratio of 0.97 and a superlattice period (l) of 0.49 nm. (4) The H 3 yE 2 ratios (plastic deformation resistance) of CrNyAlN superlattice coatings were calculated to be in the range from approximately 0.256 to 0.481. The superlattice film has a maximum value of H 3 yE 2 when the individual components of the superlattice have an equal thickness. References w1x S.Y. Lee, J.W. Chung, K.B. Kim, J.G. Han, S.S. Kim, Surf. Coat. Technol. 86y87 (1996) 325. w2x S.Y. Lee, J.W. Chung, H.J. Park, J.G. Han, J.H. Lee, J. Kor. Inst. Met. Mater. 35 (1997) 1734. w3x G.S. Kim, S.Y. Lee, E.Y. Lee, J. Kor. Inst. Met. Mater. 37 (1999) 578. w4x D.C. Cameron, R. Aimo, Z.H. Wang, K.A. Pischow, Surf. Coat. Technol. 142–144 (2001) 567–572. w5x Q. Yang, C. He, L.R. Zhao, J.-P. Immarigeon, Scripta Mater. 46 (2002) 293–297. w6x J.S. Yoon, H.Y. Hee, J.G. Han, S.H. Yang, J. Musil, Surf. Coat. Technol. 142–144 (2001) 596–602. w7x J. Musil, Surf. Coat. Technol. 125 (2000) 322–330. w8x X. Chu, S.A. Barnett, J. Appl. Phys. 77 (1995) 4403. w9x T.F. Page, G.M. Pharr, J.C. Hay, et al., Mater. Res. Soc. Symp. Proc. (1998) 522. w10x X. Chu, S.A. Barnet, M.S. Wong, W.D. Sproul, Surf. Coat. Technol. 57 (1993) 13. w11x W.D. Sproul, Surf. Coat. Technol. 86–87 (1996) 170. w12x X.T. Zeng, S. Mridha, U. Chai, J. Mater. Pro. Technol. 89–90 (1999) 528–531. w13x T.Y. Tsui, G.M. Pharr, W.C. Oliver, et al., Mater. Res. Soc. Symp. Proc. 383 (1995) 447. w14x A. Inspektor, C.E. Brauer, Surf. Coat. Technol. 68–69 (1994) 359. w15x D.B. Lewis, I. Wadsworth, V. Valvoda, Surf. Coat. Technol. 116–119 (1999) 285. w16x A. Leyland, A. Matthews, Wear 246 (2000) 1–11. w17x K.L. Johnson, Contact Mechanics, Cambridge University Press, London, UK, 1985, ISBN: 0-521-34796-3.