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The properties of (Ti,Al)N coatings deposited by inductively coupled plasma assisted d.c. magnetron sputtering
Surface and Coatings Technology 142᎐144 Ž2001. 999᎐1004
The properties of ž Ti,Al/ N coatings deposited by inductively coupled plasma assisted d.c. magnetron sputtering H.S. Park a , D.H. Jung a , H.D. Naa , J.H. Joo b, J.J. Lee a,U a
b
School of Materials Science and Engineering, Seoul National Uni¨ ersity, Shillim-dong, Kwanak-gu, 151-742, Seoul, South Korea Department of Materials Science and Engineering, Kunsan National Uni¨ ersity, Mt68 Miryong-dong, 573-701, Kunsan, South Korea
Abstract ŽTi,Al.N coatings were deposited on M2 high speed steel substrates by inductively coupled plasma ŽICP. assisted d.c. magnetron sputtering, and the structure and mechanical properties such as hardness, Young’s modulus, wear resistance and adhesion strength were investigated. A TiAl alloy target ŽTirAl ᎏ 50:50 at.%. was sputtered in an Ar and N2 atmosphere with 400 W for d.c. magnetron power as well as 400 W for ICP power at 80 mtorr of working pressure. Both the hardness and adhesion strength of the coating were found to increase with increasing substrate bias voltage. The hardness value was higher than 6500 HK 0.01 at the bias voltage higher than ᎐50 V. The Young’s modulus of the coating had a maximum value of approximately 450 GPa at y50 V. The wear properties of coatings also improved with the application of the substrate bias voltage. It was found that the grain size decreased Ž- 100 nm., and the columnar structure, which was observed in the absence of bias, disappeared when a bias voltage was applied. The high hardness and good wear property was attributed to the microstructure change from a columnar structure with facet shaped grains to a denser one with small and round shaped grains. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma-sputtering; ŽTi,Al.N; Knoop hardness; Wear test
1. Introduction Recently, research has been focused on improving the ion density and ion flux in magnetron sputtering. The ion density can be increased by using high density plasma, e.g. inductively coupled plasma ŽICP. w1᎐6x or electron cyclotron resonance ŽECR. plasma w7x. By ICP-assisted magnetron sputtering ŽICP-sputtering., high density plasma is created at the region between the target and the substrate by adding inductively coupled r.f. plasma to a conventional magnetron sputtering. In its early stages, ICP-sputtering was applied for the deposition of metals such as Ti, Al, AlCu and Cu in the semiconductor process. Later, hard and protective coatings such as TiN, CN, Cr-B coatings were also produced by ICP-sputtering w8᎐12x. Previously, the auU
Corresponding author. Tel.: q82-2-880-5511; fax: q82-2-8864156. E-mail address: [email protected] ŽJ.J. Lee..
thors reported that the hardness of TiN could be considerably increased Žover 6500 HK 0.01 . by ICPsputtering, owing to the dense structure and high compressive residual stress in the coating w8x. However, few studies have been reported on the mechanical properties and microstructure of ternary compounds produced by ICP-sputtering. In this study, ŽTi,Al.N coatings were deposited on M2 high speed steel substrates by ICP-sputtering at various substrate bias voltages. The structure, morphology and mechanical properties such as hardness, Young’s modulus, adhesion strength and wear resistance of the coatings were investigated.
2. Experimental ŽTi,Al.N coatings were deposited on M2 high speed steel substrates by ICP assisted d.c. magnetron sputtering. Fig. 1 shows a schematic diagram of the sputtering system used in this work. The ICP was generated in the
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 2 1 7 - 8
1000
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
for an equivalent distance of 350 m at room temperature. The morphology of the films was observed using both scanning electron microscopy ŽSEM. and transmission electron microscopy ŽTEM.. The surface roughness of the coatings was investigated by atomic force microscopy ŽAFM, PSI Co...
3. Results and discussion
Fig. 1. Schematic diagram of the ICP assisted sputtering apparatus.
region between the sputtering target and the substrate plane by applying 13.56 MHz r.f power through a tuning network to a two turn coil ŽRFI coil. of a water-cooled copper tube. The ICP power was 400 W. It has been found in a previous work w8x that the hardness of TiN coatings increased with the increase in the ICP power, but remained relatively unchanged above 300 W. The d.c. magnetron power was also fixed at 400 W on a 2-inch target cathode. A TiAl alloy target Žpurity s 99.9%, TirAl ᎏ 50:50 at.%. was sputtered in an Ar and N2 atmosphere. The d.c. substrate bias voltage was varied between 0, y50 and ᎐100 V. The base pressure of the system was - 1 = 10y6 torr and the substrate temperature was 300⬚C. The total pressure of mixed gas ŽAr and N2 . was fixed at 80 mtorr. Prior to deposition, the substrates were pretreated by the ICP process in the same deposition chamber in order to improve the adhesion strength of the coating. The pretreatment was carried out with a 400-W r.f. power for ICP and with a substrate bias voltage of only ᎐100 V during 20 min in an Ar atmosphere at 20 mtorr. The thickness of the ŽTi,Al.N coatings was 2.0" 0.2 m in most cases. For structural analysis of the coatings, X-ray diffraction ŽXRD. and transmission electron microscopy ŽTEM. were used. A Knoop hardness indenter with a load of 10 g and an MTS nanoindenter II determined the coating hardness and Young’s modulus, respectively. For the nano-indentation test, a continuous stiffness measurement ŽCSM. method with a Berkovich indenter was adopted. The adhesion property of the coatings was investigated using a CSEM scratch tester ŽREVESTER. as well as a Rockwell C indenter. The scratch tester conditions were d Lrdt s 10 mmrmin and d Nrdt s 100 Nrmin with a 200-m diamond stylus. For evaluating the wear resistance, the ball-on-disk method was applied. A S440C steel ball ŽHv 550. with a diameter of 6 mm was slid across the surface of specimens at a speed of 0.14 mrs and with 5 N normal force
Fig. 2 shows change in substrate current as a function of the substrate bias voltage. The substrate current increased with increasing bias voltage. The substrate current usually showed negative values when only d.c. magnetron plasma occurred. However, it became positive with assistance of inductively coupled plasma occurred by applying r.f. power onto RFI coils. This suggests that a large amount of positive ions such as Tiq, Alq and inert gas ions are created and impinged on the substrate under ICP conditions. Fig. 3 shows the Knoop hardness of ŽTi,Al.N coatings at various bias voltages. In the absence of bias, the hardness was approximately 3500 HK 0.01 which is similar to other values reported in the literature w13᎐18x. However, the hardness increased considerably up to 6500 HK 0.01 at a bias of ᎐50 V and changed little when the bias was further increased to ᎐100 V. The Young’s modulus of ŽTi,Al.N coatings, which was determined from the initial stiffness of load vs. indenter displacement curve w19x, had a maximum value of approximately 450 GPa at ᎐50 V ŽFig. 4.. The Young’s modulus was 350 GPa for the coating produced in the absence of bias, and was almost the same as that reported by Roos et al. w20x. The Young’s modulus was slightly lower than the maximum value when the bias voltage was ᎐100 V. The decrease of the Young’s modulus with the indentation depth is caused by the substrate effect, as the high speed steel substrate is much softer than the coating.
Fig. 2. The changes in the substrate current of the ŽTi,Al.N coatings with the substrate bias voltage.
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
1001
Fig. 3. Micro-hardness change of the ŽTi,Al.N coatings with the substrate bias voltage.
The critical load values determined from the scratch test were between 20 and 25 N for all samples. However, from the Rockwell C indentation test ŽFig. 5., the coating produced at ᎐100 V had the best adhesion property. No visible cracks were observed around the indented mark on the coating produced at ᎐100 V, which contrasts with those at 0 and ᎐50 V. Results from the wear test are presented in Table 1. The coating produced at ᎐100 V showed an excellent wear resistance owing to the high hardness and adhesion strength.
Fig. 5. Rockwell C indentation marks at 150 kgf wŽa. 0 V; Žb. y50 V; Žc. y100 Vx.
Fig. 4. Young’s modulus of the ŽTi,Al.N coatings measured by the nano-indentation test.
Fig. 6 shows results from XRD. The preferred orientation changes from Ž111. to Ž220. with increasing bias voltage. But TiN Ž111. or Ž200. orientations have been the preferred orientations mainly reported in the liter-
Table 1 Results from the wear test on ŽTi,Al.N coatings Substrate bias voltage
0V
y50 V
y100 V
Wear rate of coating Žmm3 rNm. Wear rate of steel ball Žmm3 rNm.
3.84= 10y5
2.057= 10y5
0.839= 10y5
4.53= 10y6
3.21= 10y6
3.469= 10y6
1002
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
Fig. 6. XRD patterns of the ŽTi,Al.N coatings with the substrate bias voltage.
ature w13᎐18x. It is not easy to find out the cause of the preferred orientation change. However, we could assume that high ion density created by the inductively coupled plasma and high energy ion bombardment generated by applied bias voltage are the main causes for the change of the preferred orientation. Molarius et al. investigated the orientation of TiN, ŽTi,Al.N coatings as a function of substrate current density and bias voltage in the triode ion plating method w21x. The preferred orientation of TiN films was changed from TiN Ž200. to TiN Ž220. with increasing substrate current density Žwith increasing ion bombardment of the substrate . at a fixed substrate bias voltage ᎐100 V. Under the same substrate current density, the increase of substrate bias voltage changed the preferred orientation from none at Vs s ᎐100 V to TiN Ž220. at Vs ) ᎐150 V.
Fig. 7. Surface and cross-sectional micrographs of the ŽTi,Al.N coatings wŽa.,Žb. 0 V; Žc.,Žd. y50 V; Že.,Žf. y100 Vx.
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004 Table 2 Surface roughness of the ŽTi,Al.N coatings measured by AFM Substrate bias voltage
0V
y50 V
y100 V
RŽrms.
˚ 86.9 A
˚ 39.4 A
˚ 64.4 A
RŽaverage.
˚ 67.9 A
˚ 31.4 A
˚ 53.1 A
In ICP-sputtering, deposition and re-sputtering processes would progress more intensely as a result of the energetic ions. This phenomenon can be seen in Fig. 7. Without the bias voltage, the coatings were grown with a columnar structure with trigonal-shaped facets at the surface, which is an indirect indication of the Ž111. preferred orientation, as shown by XRD. When a bias voltage was applied, however, the colum-
1003
nar structure disappeared and changed to a denser one with many pits at the surface. These pits were assumed to be formed by energetic ion bombardment with the bias voltage. The coating roughness is shown in Table 2. The specimen formed at ᎐50 V showed the best surface uniformity among all samples, while the coatings either without the bias voltage or with ᎐100 V had a rather high surface roughness. It appears that the high roughness observed at 0 V was due to the facet shaped morphology, while, at ᎐100 V, the high bombardment effect increased the surface roughness. Fig. 8 shows TEM results of the coatings. Many trigonal shaped facets could be found with smaller grains in the non-biased ŽTi,Al.N coating ŽFig. 8a,c.. The size of a trigonal facet was between 50 and 200 nm. At ᎐100 V, the trigonal shaped facets disappeared and only the
Fig. 8. Plane-view TEM micrographs and SADP of the ŽTi,Al.N coatings wŽa.,Žc.,Že. without bias; Žb.,Žd.,Žf. y100 Vx.
1004
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
smaller grains remained ŽFig. 8b,d.. Munz et al. ¨ observed that the TiAlN film microstructure changed with substrate bias voltage in d.c. magnetron sputtering deposition w22x. Under low substrate bias voltage at Vs - 80 V, the film had a columnar structure exhibiting a high intercolumn, as well as intragrain, porosity. Increasing Vs above 80 V, however, resulted in average grain size decreased from 105 nm at Vs s 0 V to approximately 35 nm. There was also a corresponding decrease in the void density at high substrate biases, the columnar structure was interrupted and the dislocation density increased. As the result of structure change, the micro-hardness of the films increased with increasing substrate bias. Knotek et al. also reported that the ŽTi,Al x .N coatings grow in a dense and compact structure when a substrate bias voltage is applied w17x. Hence, it is suggested that the increase in substrate bias voltage resulted in the structural change from a columnar structure with facet shaped grains to a denser one with smaller grains. Therefore, the increase of film microhardness as a function of substrate bias can be understood in terms of the change in film microstructure with ion irradiation during deposition.
4. Conclusion ŽTi,Al.N coatings were deposited on M2 high speed steel substrates using inductively coupled plasma assisted magnetron sputtering. By applying a substrate bias voltage over ᎐50 V, the hardness and adhesion strength of the coating increased considerably. The hardness of the coatings deposited with a bias voltage of ᎐50 V as well as ᎐100 V increased up to 6500 HK 0.01. Without a bias voltage, the coating had a columnar structure with trigonal shaped facets at the surface, while, at ᎐50 and ᎐100 V, the columnar structure with the facets disappeared, and changed to a dense one. The preferred orientation of the coating changed from Ž111. to Ž220. at the presence of the bias voltage. The coating produced at ᎐50 V showed a smooth surface with the roughness of approximately 30 ˚ Ž R average .. However, the surface roughness was high A due to the trigonal shaped facets at 0 V, and the energetic ion bombardment at ᎐100 V.
Acknowledgements
This work has been supported by EESRI ŽGrant No. 99-019., which is funded by KEPCO.
References w1x Y.-W. Kim, J. Moser, I. Petrov, J.E. Greene, S.M. Rossnagel, J. Vac. Sci. Technol. A12 Ž6. Ž1994. 3169. w2x W. Wang, J. Foster, A.E. Wendt, J.H. Booske, T. Onuoha, P.W. Sandstrom, Appl. Phys. Lett. 71 Ž12. Ž1997. 1622 Ž22 September.. w3x J.M. Schneider, W.D. Sproul, A. Matthews, J. Vac. Sci. Technol. A15 Ž3. Ž1997. 1084. w4x J.M. Schneider, W.D. Sproul, A. Matthews, Surf. Coat. Technol. 94r95 Ž1997. 179. w5x J. Hopwood, F. Qian, J. Appl. Phys. 78 Ž1995. 758. w6x S.M. Rossnagel, J. Hopwood, J. Vac. Sci. Technol. B12 Ž1994. 449. w7x W.M. Holber, J.S. Logan, J.J. Grabarz et al., J. Vac. Sci. Technol. A11 Ž1993. 2903. w8x J.W. Lim, H.S. Park, T.H. Park, J.J. Lee, J. Vac. Sci. Technol. A18 Ž2. Ž2000. 524. w9x Y. Kusano, Z.H. Barber, J.E. Evetts, I.M. Hutchings, Surf. Coat. Technol. 124 Ž2000. 104. w10x M. Zhou, M. Nose, Y. Makino, K. Nogi, Thin Solid Films 343r344 Ž1999. 234. w11x M. Zhou, Y. Makino, M. Nose, K. Nogi, Thin Solid Films 339 Ž1999. 203. w12x C.Y. Hsu, F.C.N. Hong, Diamond Relat. Mater. 8 Ž1999. 1315. w13x C.T. Huang, J.G. Duh, Surf. Coat. Technol. 71 Ž1995. 259. w14x B.Y. Shew, J.L. Huang, Surf. Coat. Technol. 71 Ž1995. 30. w15x I. Ikeda, H. Satoh, Thin Solid Films 195 Ž1991. 99. w16x B.F. Coll, R. Fontana, P. Sathrum, Mat. Sci. Eng. A140 Ž1991. 816. w17x O. Knotek, M. Atzor, F. Jungblit, Surf. Coat. Technol. 39r40 Ž1989. 445. w18x F. Adibi, I. Petrov, J.E. Greene, J. Vac. Sci. Technol. A11 Ž1. Ž1993. 136. w19x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 Ž6. Ž1992. 1564 ŽJune.. w20x J.R. Roos, J.P. Celis, E. Vancoille et al., Thin Solid Films 193r194 Ž1990. 547. w21x J.M. Molarius, A.S. Korhonen, E. Harju, R. Lappalainen, Surf. Coat. Technol. 33 Ž1987. 117. w22x G. Hakansson, J.-E. Sundgren, D. Mcintyre, J.E. Greene, W.-D. Munz, ¨ Thin Solid Films 153 Ž1987. 55.
The properties of ž Ti,Al/ N coatings deposited by inductively coupled plasma assisted d.c. magnetron sputtering H.S. Park a , D.H. Jung a , H.D. Naa , J.H. Joo b, J.J. Lee a,U a
b
School of Materials Science and Engineering, Seoul National Uni¨ ersity, Shillim-dong, Kwanak-gu, 151-742, Seoul, South Korea Department of Materials Science and Engineering, Kunsan National Uni¨ ersity, Mt68 Miryong-dong, 573-701, Kunsan, South Korea
Abstract ŽTi,Al.N coatings were deposited on M2 high speed steel substrates by inductively coupled plasma ŽICP. assisted d.c. magnetron sputtering, and the structure and mechanical properties such as hardness, Young’s modulus, wear resistance and adhesion strength were investigated. A TiAl alloy target ŽTirAl ᎏ 50:50 at.%. was sputtered in an Ar and N2 atmosphere with 400 W for d.c. magnetron power as well as 400 W for ICP power at 80 mtorr of working pressure. Both the hardness and adhesion strength of the coating were found to increase with increasing substrate bias voltage. The hardness value was higher than 6500 HK 0.01 at the bias voltage higher than ᎐50 V. The Young’s modulus of the coating had a maximum value of approximately 450 GPa at y50 V. The wear properties of coatings also improved with the application of the substrate bias voltage. It was found that the grain size decreased Ž- 100 nm., and the columnar structure, which was observed in the absence of bias, disappeared when a bias voltage was applied. The high hardness and good wear property was attributed to the microstructure change from a columnar structure with facet shaped grains to a denser one with small and round shaped grains. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma-sputtering; ŽTi,Al.N; Knoop hardness; Wear test
1. Introduction Recently, research has been focused on improving the ion density and ion flux in magnetron sputtering. The ion density can be increased by using high density plasma, e.g. inductively coupled plasma ŽICP. w1᎐6x or electron cyclotron resonance ŽECR. plasma w7x. By ICP-assisted magnetron sputtering ŽICP-sputtering., high density plasma is created at the region between the target and the substrate by adding inductively coupled r.f. plasma to a conventional magnetron sputtering. In its early stages, ICP-sputtering was applied for the deposition of metals such as Ti, Al, AlCu and Cu in the semiconductor process. Later, hard and protective coatings such as TiN, CN, Cr-B coatings were also produced by ICP-sputtering w8᎐12x. Previously, the auU
Corresponding author. Tel.: q82-2-880-5511; fax: q82-2-8864156. E-mail address: [email protected] ŽJ.J. Lee..
thors reported that the hardness of TiN could be considerably increased Žover 6500 HK 0.01 . by ICPsputtering, owing to the dense structure and high compressive residual stress in the coating w8x. However, few studies have been reported on the mechanical properties and microstructure of ternary compounds produced by ICP-sputtering. In this study, ŽTi,Al.N coatings were deposited on M2 high speed steel substrates by ICP-sputtering at various substrate bias voltages. The structure, morphology and mechanical properties such as hardness, Young’s modulus, adhesion strength and wear resistance of the coatings were investigated.
2. Experimental ŽTi,Al.N coatings were deposited on M2 high speed steel substrates by ICP assisted d.c. magnetron sputtering. Fig. 1 shows a schematic diagram of the sputtering system used in this work. The ICP was generated in the
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 2 1 7 - 8
1000
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
for an equivalent distance of 350 m at room temperature. The morphology of the films was observed using both scanning electron microscopy ŽSEM. and transmission electron microscopy ŽTEM.. The surface roughness of the coatings was investigated by atomic force microscopy ŽAFM, PSI Co...
3. Results and discussion
Fig. 1. Schematic diagram of the ICP assisted sputtering apparatus.
region between the sputtering target and the substrate plane by applying 13.56 MHz r.f power through a tuning network to a two turn coil ŽRFI coil. of a water-cooled copper tube. The ICP power was 400 W. It has been found in a previous work w8x that the hardness of TiN coatings increased with the increase in the ICP power, but remained relatively unchanged above 300 W. The d.c. magnetron power was also fixed at 400 W on a 2-inch target cathode. A TiAl alloy target Žpurity s 99.9%, TirAl ᎏ 50:50 at.%. was sputtered in an Ar and N2 atmosphere. The d.c. substrate bias voltage was varied between 0, y50 and ᎐100 V. The base pressure of the system was - 1 = 10y6 torr and the substrate temperature was 300⬚C. The total pressure of mixed gas ŽAr and N2 . was fixed at 80 mtorr. Prior to deposition, the substrates were pretreated by the ICP process in the same deposition chamber in order to improve the adhesion strength of the coating. The pretreatment was carried out with a 400-W r.f. power for ICP and with a substrate bias voltage of only ᎐100 V during 20 min in an Ar atmosphere at 20 mtorr. The thickness of the ŽTi,Al.N coatings was 2.0" 0.2 m in most cases. For structural analysis of the coatings, X-ray diffraction ŽXRD. and transmission electron microscopy ŽTEM. were used. A Knoop hardness indenter with a load of 10 g and an MTS nanoindenter II determined the coating hardness and Young’s modulus, respectively. For the nano-indentation test, a continuous stiffness measurement ŽCSM. method with a Berkovich indenter was adopted. The adhesion property of the coatings was investigated using a CSEM scratch tester ŽREVESTER. as well as a Rockwell C indenter. The scratch tester conditions were d Lrdt s 10 mmrmin and d Nrdt s 100 Nrmin with a 200-m diamond stylus. For evaluating the wear resistance, the ball-on-disk method was applied. A S440C steel ball ŽHv 550. with a diameter of 6 mm was slid across the surface of specimens at a speed of 0.14 mrs and with 5 N normal force
Fig. 2 shows change in substrate current as a function of the substrate bias voltage. The substrate current increased with increasing bias voltage. The substrate current usually showed negative values when only d.c. magnetron plasma occurred. However, it became positive with assistance of inductively coupled plasma occurred by applying r.f. power onto RFI coils. This suggests that a large amount of positive ions such as Tiq, Alq and inert gas ions are created and impinged on the substrate under ICP conditions. Fig. 3 shows the Knoop hardness of ŽTi,Al.N coatings at various bias voltages. In the absence of bias, the hardness was approximately 3500 HK 0.01 which is similar to other values reported in the literature w13᎐18x. However, the hardness increased considerably up to 6500 HK 0.01 at a bias of ᎐50 V and changed little when the bias was further increased to ᎐100 V. The Young’s modulus of ŽTi,Al.N coatings, which was determined from the initial stiffness of load vs. indenter displacement curve w19x, had a maximum value of approximately 450 GPa at ᎐50 V ŽFig. 4.. The Young’s modulus was 350 GPa for the coating produced in the absence of bias, and was almost the same as that reported by Roos et al. w20x. The Young’s modulus was slightly lower than the maximum value when the bias voltage was ᎐100 V. The decrease of the Young’s modulus with the indentation depth is caused by the substrate effect, as the high speed steel substrate is much softer than the coating.
Fig. 2. The changes in the substrate current of the ŽTi,Al.N coatings with the substrate bias voltage.
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
1001
Fig. 3. Micro-hardness change of the ŽTi,Al.N coatings with the substrate bias voltage.
The critical load values determined from the scratch test were between 20 and 25 N for all samples. However, from the Rockwell C indentation test ŽFig. 5., the coating produced at ᎐100 V had the best adhesion property. No visible cracks were observed around the indented mark on the coating produced at ᎐100 V, which contrasts with those at 0 and ᎐50 V. Results from the wear test are presented in Table 1. The coating produced at ᎐100 V showed an excellent wear resistance owing to the high hardness and adhesion strength.
Fig. 5. Rockwell C indentation marks at 150 kgf wŽa. 0 V; Žb. y50 V; Žc. y100 Vx.
Fig. 4. Young’s modulus of the ŽTi,Al.N coatings measured by the nano-indentation test.
Fig. 6 shows results from XRD. The preferred orientation changes from Ž111. to Ž220. with increasing bias voltage. But TiN Ž111. or Ž200. orientations have been the preferred orientations mainly reported in the liter-
Table 1 Results from the wear test on ŽTi,Al.N coatings Substrate bias voltage
0V
y50 V
y100 V
Wear rate of coating Žmm3 rNm. Wear rate of steel ball Žmm3 rNm.
3.84= 10y5
2.057= 10y5
0.839= 10y5
4.53= 10y6
3.21= 10y6
3.469= 10y6
1002
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
Fig. 6. XRD patterns of the ŽTi,Al.N coatings with the substrate bias voltage.
ature w13᎐18x. It is not easy to find out the cause of the preferred orientation change. However, we could assume that high ion density created by the inductively coupled plasma and high energy ion bombardment generated by applied bias voltage are the main causes for the change of the preferred orientation. Molarius et al. investigated the orientation of TiN, ŽTi,Al.N coatings as a function of substrate current density and bias voltage in the triode ion plating method w21x. The preferred orientation of TiN films was changed from TiN Ž200. to TiN Ž220. with increasing substrate current density Žwith increasing ion bombardment of the substrate . at a fixed substrate bias voltage ᎐100 V. Under the same substrate current density, the increase of substrate bias voltage changed the preferred orientation from none at Vs s ᎐100 V to TiN Ž220. at Vs ) ᎐150 V.
Fig. 7. Surface and cross-sectional micrographs of the ŽTi,Al.N coatings wŽa.,Žb. 0 V; Žc.,Žd. y50 V; Že.,Žf. y100 Vx.
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004 Table 2 Surface roughness of the ŽTi,Al.N coatings measured by AFM Substrate bias voltage
0V
y50 V
y100 V
RŽrms.
˚ 86.9 A
˚ 39.4 A
˚ 64.4 A
RŽaverage.
˚ 67.9 A
˚ 31.4 A
˚ 53.1 A
In ICP-sputtering, deposition and re-sputtering processes would progress more intensely as a result of the energetic ions. This phenomenon can be seen in Fig. 7. Without the bias voltage, the coatings were grown with a columnar structure with trigonal-shaped facets at the surface, which is an indirect indication of the Ž111. preferred orientation, as shown by XRD. When a bias voltage was applied, however, the colum-
1003
nar structure disappeared and changed to a denser one with many pits at the surface. These pits were assumed to be formed by energetic ion bombardment with the bias voltage. The coating roughness is shown in Table 2. The specimen formed at ᎐50 V showed the best surface uniformity among all samples, while the coatings either without the bias voltage or with ᎐100 V had a rather high surface roughness. It appears that the high roughness observed at 0 V was due to the facet shaped morphology, while, at ᎐100 V, the high bombardment effect increased the surface roughness. Fig. 8 shows TEM results of the coatings. Many trigonal shaped facets could be found with smaller grains in the non-biased ŽTi,Al.N coating ŽFig. 8a,c.. The size of a trigonal facet was between 50 and 200 nm. At ᎐100 V, the trigonal shaped facets disappeared and only the
Fig. 8. Plane-view TEM micrographs and SADP of the ŽTi,Al.N coatings wŽa.,Žc.,Že. without bias; Žb.,Žd.,Žf. y100 Vx.
1004
H.S. Park et al. r Surface and Coatings Technology 142᎐144 (2001) 999᎐1004
smaller grains remained ŽFig. 8b,d.. Munz et al. ¨ observed that the TiAlN film microstructure changed with substrate bias voltage in d.c. magnetron sputtering deposition w22x. Under low substrate bias voltage at Vs - 80 V, the film had a columnar structure exhibiting a high intercolumn, as well as intragrain, porosity. Increasing Vs above 80 V, however, resulted in average grain size decreased from 105 nm at Vs s 0 V to approximately 35 nm. There was also a corresponding decrease in the void density at high substrate biases, the columnar structure was interrupted and the dislocation density increased. As the result of structure change, the micro-hardness of the films increased with increasing substrate bias. Knotek et al. also reported that the ŽTi,Al x .N coatings grow in a dense and compact structure when a substrate bias voltage is applied w17x. Hence, it is suggested that the increase in substrate bias voltage resulted in the structural change from a columnar structure with facet shaped grains to a denser one with smaller grains. Therefore, the increase of film microhardness as a function of substrate bias can be understood in terms of the change in film microstructure with ion irradiation during deposition.
4. Conclusion ŽTi,Al.N coatings were deposited on M2 high speed steel substrates using inductively coupled plasma assisted magnetron sputtering. By applying a substrate bias voltage over ᎐50 V, the hardness and adhesion strength of the coating increased considerably. The hardness of the coatings deposited with a bias voltage of ᎐50 V as well as ᎐100 V increased up to 6500 HK 0.01. Without a bias voltage, the coating had a columnar structure with trigonal shaped facets at the surface, while, at ᎐50 and ᎐100 V, the columnar structure with the facets disappeared, and changed to a dense one. The preferred orientation of the coating changed from Ž111. to Ž220. at the presence of the bias voltage. The coating produced at ᎐50 V showed a smooth surface with the roughness of approximately 30 ˚ Ž R average .. However, the surface roughness was high A due to the trigonal shaped facets at 0 V, and the energetic ion bombardment at ᎐100 V.
Acknowledgements
This work has been supported by EESRI ŽGrant No. 99-019., which is funded by KEPCO.
References w1x Y.-W. Kim, J. Moser, I. Petrov, J.E. Greene, S.M. Rossnagel, J. Vac. Sci. Technol. A12 Ž6. Ž1994. 3169. w2x W. Wang, J. Foster, A.E. Wendt, J.H. Booske, T. Onuoha, P.W. Sandstrom, Appl. Phys. Lett. 71 Ž12. Ž1997. 1622 Ž22 September.. w3x J.M. Schneider, W.D. Sproul, A. Matthews, J. Vac. Sci. Technol. A15 Ž3. Ž1997. 1084. w4x J.M. Schneider, W.D. Sproul, A. Matthews, Surf. Coat. Technol. 94r95 Ž1997. 179. w5x J. Hopwood, F. Qian, J. Appl. Phys. 78 Ž1995. 758. w6x S.M. Rossnagel, J. Hopwood, J. Vac. Sci. Technol. B12 Ž1994. 449. w7x W.M. Holber, J.S. Logan, J.J. Grabarz et al., J. Vac. Sci. Technol. A11 Ž1993. 2903. w8x J.W. Lim, H.S. Park, T.H. Park, J.J. Lee, J. Vac. Sci. Technol. A18 Ž2. Ž2000. 524. w9x Y. Kusano, Z.H. Barber, J.E. Evetts, I.M. Hutchings, Surf. Coat. Technol. 124 Ž2000. 104. w10x M. Zhou, M. Nose, Y. Makino, K. Nogi, Thin Solid Films 343r344 Ž1999. 234. w11x M. Zhou, Y. Makino, M. Nose, K. Nogi, Thin Solid Films 339 Ž1999. 203. w12x C.Y. Hsu, F.C.N. Hong, Diamond Relat. Mater. 8 Ž1999. 1315. w13x C.T. Huang, J.G. Duh, Surf. Coat. Technol. 71 Ž1995. 259. w14x B.Y. Shew, J.L. Huang, Surf. Coat. Technol. 71 Ž1995. 30. w15x I. Ikeda, H. Satoh, Thin Solid Films 195 Ž1991. 99. w16x B.F. Coll, R. Fontana, P. Sathrum, Mat. Sci. Eng. A140 Ž1991. 816. w17x O. Knotek, M. Atzor, F. Jungblit, Surf. Coat. Technol. 39r40 Ž1989. 445. w18x F. Adibi, I. Petrov, J.E. Greene, J. Vac. Sci. Technol. A11 Ž1. Ž1993. 136. w19x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 Ž6. Ž1992. 1564 ŽJune.. w20x J.R. Roos, J.P. Celis, E. Vancoille et al., Thin Solid Films 193r194 Ž1990. 547. w21x J.M. Molarius, A.S. Korhonen, E. Harju, R. Lappalainen, Surf. Coat. Technol. 33 Ž1987. 117. w22x G. Hakansson, J.-E. Sundgren, D. Mcintyre, J.E. Greene, W.-D. Munz, ¨ Thin Solid Films 153 Ž1987. 55.