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Deposition and characterization of Ti(C,N,O) coatings by unbalanced magnetron sputtering
Surface and Coatings Technology 163 – 164 (2003) 233–237
Deposition and characterization of Ti(C,N,O) coatings by unbalanced magnetron sputtering J.H. Hsieha,*, W. Wub, C. Lia, C.H. Yub, B.H. Tanc a School of MPE, Nanyang Technological University, Singapore 639798, Singapore Department of Materials Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC c Gintic Institute of Manufacturing Technology, Singapore 638075, Singapore
b
Abstract Several TiCNO coatings were deposited using an unbalanced magnetron sputtering system. The coating properties as a function of oxygenynitrogen flow ratio were studied using glow discharge optical spectrometer, X-ray diffraction, scratch testing and nanoindentation measurement. The tribological properties of the coatings were then investigated using a ball-on-disk setup with alumina balls. The results show that coating properties and performance are greatly affected by the flow rate of oxygen. With oxygen flow rate at 4 sccm during deposition, the TiCNO coating shows the lowest wear rate among all. Further increase in oxygen flow rate caused a decrease of hardness, adhesion and wear resistance, while together with the increase of friction coefficient. Also found is that the color of these coatings changes as a function of oxygen flow rate, indicating that these coatings can be used as decorating thin films with wide variety of choices. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetron sputtering; Hard coating; X-ray diffraction; Wear
1. Introduction Titanium carbonitride (TiCxN1yx or Ti(C,N)) coatings are, in general, used to improve tool life by combining the properties of tough TiN and hard TiC. The advantages of these coatings over other coating materials, such as TiN or TiAlN, stem from its superior friction behavior in contact with steel, high hardness and residual stress w1,2x. Because of their low friction, the coatings are especially durable at low cutting speeds w1x. The combined effect of low friction behavior and high residual stress helps to prevent cutting-edge deformation for high speed steels and reduces the risk of cuttingedge chipping on carbide. In addition, the coating’s high hardness provides excellent resistance to wear w2x. To further improve the properties of Ti(C,N) coating, alloying the coating with certain amount of oxygen is hinted by other researchers as a plausible way w3,4x. The oxygen-containing Ti(C,N) coating (or Ti(C,N,O)) is expected to exhibit good resistance to friction wear and corrosion due to the small atomic size of oxygen *Corresponding author. Tel.: q65-790-4330; fax: q65-791-1859. E-mail address: [email protected] (J.H. Hsieh).
and the inertness of oxide, which in turn creates high hardness and compressive stress w5,6x. However, only few publications in the literature ever discussed the deposition, properties and effectiveness of Ti(C,N,O) coatings. In this paper, Ti(C,N,O) coatings were prepared by unbalanced magnetron sputtering with the variation of oxygenynitrogen flow ratio. The effects of oxygen content on some critical properties of coatings, such as hardness, adhesion, wear and friction, were studied and discussed. 2. Experimental 2.1. Coating deposition Ti(C,N) and Ti(C,N,O) coatings with various composition were deposited in a closed field unbalanced reactive D.C. magnetron sputtering system. The details about the system can be found elsewhere w7x. Polished and cleaned M2 steel disks with a hardness of 63HRC and dimensions of 50 mm in diameter and 6 mm in thickness were used as the substrates for coating. The base pressure of the chamber is 6.65=10y4 Pa. Before deposition, the substrates were sputter-cleaned using
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 9 4 - 2
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
234
Table 1 Gas flow rate and some properties of Ti(C,N,O) coatings
1 2 3 4 5 6
N2 flow rate (sccm)
O2 flow rate (sccm)
Hardness (GPa)
Adhesion (load, N)
C:N:O
Color
22 20 18 16 14 12
0 2 4 6 8 10
26 30 32 30 29 24
54 60 65 45 40 38
1:2.0:0 1:3.8:0.2 1:3.5:0.4 1:5.0:0.4 1:3.5:0.5 1:3.5:0.65
Pink brown Pink purple Red brown Yellow brown Red brown Gray brown
argon plasma. During deposition, the bias voltage and argon working pressure were y60 V and 0.44 Pa (3.3 mTorr), respectively. The deposition rate of the coatings was approximately 0.4–0.45 nmys. In order to maintain a good adhesion for these coatings, TiyTiN was used as an intermediate layer. The Ti (C,N,O) coatings were then deposited on top of the TiyTiN layer by sputtering four Ti targets simultaneously. The total flow rate of oxygen and nitrogen was kept at 22 sccm during deposition. However, various oxygen and nitrogen flow rates were used to obtain different C, N and O ratios in the deposition. The flow of C4H10 into the chamber was controlled by optical emission spectroscopy, which is set at 50%. After deposition, glow discharge optical spectroscopy was used to examine the chemical composition of the coatings. The average thickness of these coatings was approximately 3 mm.
2.2. Coating characterization A nano-indenter (Nanotest 550) was used to measure the hardness of the coatings. For analysis, the Oliverand-Pharr approach was used. Scanning electron microscopy (SEM) (Cambridge 360S) was performed on the fractured cross-sections of the coatings in order to examine the coatings’ morphology. The texture was studied using X-ray diffraction (XRD) (Philips X’pert) with Cu Ka radiation. For adhesion evaluation, a scratch tester (Teer ST-2200) was used. During scratch testing, the load was increased from 10 to 110 N with a loading rate of 50 Nymin. The critical load for adhesion failure was identified when a sudden change of the first order derivative in the friction vs. load curve was observed. Examination of the scratches using optical microscope
Fig. 1. XRD patterns of steel substrate, TiCN coating, and TiCNO coating (O2s10 sccm).
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
235
Fig. 2. Effects of O2yN2 flow ratios on I(1 1 1)yI(2 0 0) and NyC ratios in the coatings.
revealed that this sudden change always indicated the beginning of cohesive failure (transverse cracking). 2.3. Wear test Wear tests were conducted using a pin-on-disk wear tester (ASTM G99) under a normal load of 5 N at 30 cmys (18 mymin). These tests were run against alumina balls in an atmospheric environment (RH 50%, RT) without lubrication. These alumina balls were well polished with 10 mm in diameter. The amount of wear was determined by measuring appropriate linear dimensions of the wear tracks before and after the test using Talysurf surface profilometer. Linear measurement was then converted to wear volume. Wear coefficient was computed from wear volume loss divided by sliding distance (1000 m) and normal load (5 N). 3. Results and discussion 3.1. Coating properties Table 1 lists the deposition conditions and some coatings’ properties including hardness, adhesion, color and the ratios of C:N:O. From this table, it can be observed that all properties are affected by the variation of O2 yN2 flow ratios. Especially, when the oxygen flow rate was at 4 sccm (nitrogen flow rates18 sccm), the hardness and adhesion of Ti(C,N,O) coatings reached their maximum. Further increase in oxygen flow rate would decrease the values of both. From the chemical composition, it is worthy of noting that addition of oxygen enhanced the incorporation of nitrogen into the
coatings. This is probably due to oxygen etching of the deposited carbon atoms. The maximum content of nitrogen can be obtained when the oxygen flow rate was at 6 sccm. Moreover, due to different chemical compositions, the color of these coatings varied as a function O2 yN2 flow ratios, indicating that these coatings have a potential to be used as decorating thin films with wide variety of choices. 3.2. Structural analysis From the XRD study, it is found that all the Ti(C,N,O) coatings have similar diffraction patterns to TiCN coatings with strong texture of (1 1 1) or (2 0 0), as shown in Fig. 1. Besides (1 1 1) and (2 0 0) peaks, relatively weak (2 2 0) and (2 2 2) peaks are also observed. The effect of O2 yN2 flow ratios on texture is presented in Fig. 2. It can be seen that the addition of oxygen to TiCN coating tends to increase the intensity of (1 1 1). The (1 1 1) plane is known to be the most closely packed and normally exhibits the lowest surface energy w8x. When the oxygen flow rate was at 6 sccm (OyNs 0.375), the I(1 1 1)yI(2 0 0) ratio reached its maximum. By checking the ratios of CyN in the coatings, it can be seen that the (1 1 1) texture is more dominant for higher nitrogen concentration. This result is in good agreement with previous study on TiCN w8x. 3.3. Morphology Fig. 3 shows the cross-sectional SEM micrographs of the Ti(C,N,O) coatings with the variation of O2 yN2 flow ratios. It is observed that the morphologies change
236
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
Fig. 3. Cross-sectional SEM micrographs of the Ti(C,N,O) deposited under various O2 yN2 flow ratios: (a) O2yN2 s0; (b) O2yN2s2y20; (c) O2yN2s4y18; (d) O2yN2s8y14.
Fig. 4. Effects of O2yN2 flow ratios during deposition on wear and friction coefficients.
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
from columnar structure (O2 flow rates0 sccm), as shown in Fig. 3a to dense zone T structure (O2 flow rates4 sccm), as shown in Fig. 3c. Further increase of oxygen changes the morphology back to columnar structure as shown in Fig. 3d. The change of the structure seems to be matched with the hardness value. The coating with dense zone T structure in Fig. 3c exhibits the highest hardness at 32 GPa.
237
would increase the incorporation of nitrogen into the coatings, which in turn enhance the (1 1 1) texture. This is probably caused by the etching effect of oxygen on the deposited carbon atoms. With oxygen flow rate at 4 sccm during deposition, the TiCNO coating had the lowest wear rate among all. Further increase in oxygen flow rate caused the decrease of the hardness, adhesion and wear resistance, together with the increase of friction coefficient.
3.4. Tribological properties Acknowledgments The tribological behaviors of the coatings were studied comparatively at a sliding speed of 30 cmys (18 my min) and under a normal load of 5 N sliding against alumina pin in atmospheric environment. Fig. 4 shows the results of the tribological tests. The coating deposited with oxygen flow rate at 4 sccm (OyNs4y18) shows the lowest wear coefficient. This is probably related to its high hardness, dense structure and low friction coefficient. Overall, as shown in Fig. 4, the coatings’ wear coefficient is closely related to the friction coefficient, which is one of the characteristics of TiCN coatings. Higher flow rate of oxygen ()6 sccm) during deposition would create coatings with columnar structure, and higher nitrogen content. This would in turn result in higher friction coefficient and therefore higher wear rate. 4. Conclusions The properties and performance of Ti(C,N,O) coatings prepared in this study are greatly affected by the flow ratios of oxygen to nitrogen. Addition of oxygen
We would like to express sincere gratitude to Dr Zeng Xianting and Ms Liu Yuchan of Gintic Institute of Manufacturing Technology for their help during this study. References w1x G. Baravian, G. Sultan, E. Damond, H. Detour, Surf. Coat. Technol. 76y77 (1995) 687. w2x O. Knotek, F. Loffler, G. Kramer, Surf. Coat. Technol. 61 (1993) 320. w3x V.D. Kal’ner, A.K. Verner, Metal Sci. Heat Treat. 36 (1994) 196. w4x T. Wierzchon, J.R. Sobiecki, D. Krupa, Surf. Coat. Technol. 59 (1993) 217. w5x Y. Shi, H. Peng, Y. Xie, G. Xie, C. Zhao, Surf. Coat. Technol. 132 (1998) 26. w6x A. Stanishevsky, R. Lappalainen, Surf. Coat. Technol. 123 (2000) 101. w7x D.P. Monaghan, D.G. Teer, K.C. Laing, R.D. Arnell, Surf. Coat. Technol. 59 (1993) 21. w8x J.M. Schneider, A. Voevodin, C. Rebholz, A. Matthews, Surf. Coat. Technol. 74–75 (1995) 312.
Deposition and characterization of Ti(C,N,O) coatings by unbalanced magnetron sputtering J.H. Hsieha,*, W. Wub, C. Lia, C.H. Yub, B.H. Tanc a School of MPE, Nanyang Technological University, Singapore 639798, Singapore Department of Materials Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC c Gintic Institute of Manufacturing Technology, Singapore 638075, Singapore
b
Abstract Several TiCNO coatings were deposited using an unbalanced magnetron sputtering system. The coating properties as a function of oxygenynitrogen flow ratio were studied using glow discharge optical spectrometer, X-ray diffraction, scratch testing and nanoindentation measurement. The tribological properties of the coatings were then investigated using a ball-on-disk setup with alumina balls. The results show that coating properties and performance are greatly affected by the flow rate of oxygen. With oxygen flow rate at 4 sccm during deposition, the TiCNO coating shows the lowest wear rate among all. Further increase in oxygen flow rate caused a decrease of hardness, adhesion and wear resistance, while together with the increase of friction coefficient. Also found is that the color of these coatings changes as a function of oxygen flow rate, indicating that these coatings can be used as decorating thin films with wide variety of choices. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetron sputtering; Hard coating; X-ray diffraction; Wear
1. Introduction Titanium carbonitride (TiCxN1yx or Ti(C,N)) coatings are, in general, used to improve tool life by combining the properties of tough TiN and hard TiC. The advantages of these coatings over other coating materials, such as TiN or TiAlN, stem from its superior friction behavior in contact with steel, high hardness and residual stress w1,2x. Because of their low friction, the coatings are especially durable at low cutting speeds w1x. The combined effect of low friction behavior and high residual stress helps to prevent cutting-edge deformation for high speed steels and reduces the risk of cuttingedge chipping on carbide. In addition, the coating’s high hardness provides excellent resistance to wear w2x. To further improve the properties of Ti(C,N) coating, alloying the coating with certain amount of oxygen is hinted by other researchers as a plausible way w3,4x. The oxygen-containing Ti(C,N) coating (or Ti(C,N,O)) is expected to exhibit good resistance to friction wear and corrosion due to the small atomic size of oxygen *Corresponding author. Tel.: q65-790-4330; fax: q65-791-1859. E-mail address: [email protected] (J.H. Hsieh).
and the inertness of oxide, which in turn creates high hardness and compressive stress w5,6x. However, only few publications in the literature ever discussed the deposition, properties and effectiveness of Ti(C,N,O) coatings. In this paper, Ti(C,N,O) coatings were prepared by unbalanced magnetron sputtering with the variation of oxygenynitrogen flow ratio. The effects of oxygen content on some critical properties of coatings, such as hardness, adhesion, wear and friction, were studied and discussed. 2. Experimental 2.1. Coating deposition Ti(C,N) and Ti(C,N,O) coatings with various composition were deposited in a closed field unbalanced reactive D.C. magnetron sputtering system. The details about the system can be found elsewhere w7x. Polished and cleaned M2 steel disks with a hardness of 63HRC and dimensions of 50 mm in diameter and 6 mm in thickness were used as the substrates for coating. The base pressure of the chamber is 6.65=10y4 Pa. Before deposition, the substrates were sputter-cleaned using
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 9 4 - 2
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
234
Table 1 Gas flow rate and some properties of Ti(C,N,O) coatings
1 2 3 4 5 6
N2 flow rate (sccm)
O2 flow rate (sccm)
Hardness (GPa)
Adhesion (load, N)
C:N:O
Color
22 20 18 16 14 12
0 2 4 6 8 10
26 30 32 30 29 24
54 60 65 45 40 38
1:2.0:0 1:3.8:0.2 1:3.5:0.4 1:5.0:0.4 1:3.5:0.5 1:3.5:0.65
Pink brown Pink purple Red brown Yellow brown Red brown Gray brown
argon plasma. During deposition, the bias voltage and argon working pressure were y60 V and 0.44 Pa (3.3 mTorr), respectively. The deposition rate of the coatings was approximately 0.4–0.45 nmys. In order to maintain a good adhesion for these coatings, TiyTiN was used as an intermediate layer. The Ti (C,N,O) coatings were then deposited on top of the TiyTiN layer by sputtering four Ti targets simultaneously. The total flow rate of oxygen and nitrogen was kept at 22 sccm during deposition. However, various oxygen and nitrogen flow rates were used to obtain different C, N and O ratios in the deposition. The flow of C4H10 into the chamber was controlled by optical emission spectroscopy, which is set at 50%. After deposition, glow discharge optical spectroscopy was used to examine the chemical composition of the coatings. The average thickness of these coatings was approximately 3 mm.
2.2. Coating characterization A nano-indenter (Nanotest 550) was used to measure the hardness of the coatings. For analysis, the Oliverand-Pharr approach was used. Scanning electron microscopy (SEM) (Cambridge 360S) was performed on the fractured cross-sections of the coatings in order to examine the coatings’ morphology. The texture was studied using X-ray diffraction (XRD) (Philips X’pert) with Cu Ka radiation. For adhesion evaluation, a scratch tester (Teer ST-2200) was used. During scratch testing, the load was increased from 10 to 110 N with a loading rate of 50 Nymin. The critical load for adhesion failure was identified when a sudden change of the first order derivative in the friction vs. load curve was observed. Examination of the scratches using optical microscope
Fig. 1. XRD patterns of steel substrate, TiCN coating, and TiCNO coating (O2s10 sccm).
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
235
Fig. 2. Effects of O2yN2 flow ratios on I(1 1 1)yI(2 0 0) and NyC ratios in the coatings.
revealed that this sudden change always indicated the beginning of cohesive failure (transverse cracking). 2.3. Wear test Wear tests were conducted using a pin-on-disk wear tester (ASTM G99) under a normal load of 5 N at 30 cmys (18 mymin). These tests were run against alumina balls in an atmospheric environment (RH 50%, RT) without lubrication. These alumina balls were well polished with 10 mm in diameter. The amount of wear was determined by measuring appropriate linear dimensions of the wear tracks before and after the test using Talysurf surface profilometer. Linear measurement was then converted to wear volume. Wear coefficient was computed from wear volume loss divided by sliding distance (1000 m) and normal load (5 N). 3. Results and discussion 3.1. Coating properties Table 1 lists the deposition conditions and some coatings’ properties including hardness, adhesion, color and the ratios of C:N:O. From this table, it can be observed that all properties are affected by the variation of O2 yN2 flow ratios. Especially, when the oxygen flow rate was at 4 sccm (nitrogen flow rates18 sccm), the hardness and adhesion of Ti(C,N,O) coatings reached their maximum. Further increase in oxygen flow rate would decrease the values of both. From the chemical composition, it is worthy of noting that addition of oxygen enhanced the incorporation of nitrogen into the
coatings. This is probably due to oxygen etching of the deposited carbon atoms. The maximum content of nitrogen can be obtained when the oxygen flow rate was at 6 sccm. Moreover, due to different chemical compositions, the color of these coatings varied as a function O2 yN2 flow ratios, indicating that these coatings have a potential to be used as decorating thin films with wide variety of choices. 3.2. Structural analysis From the XRD study, it is found that all the Ti(C,N,O) coatings have similar diffraction patterns to TiCN coatings with strong texture of (1 1 1) or (2 0 0), as shown in Fig. 1. Besides (1 1 1) and (2 0 0) peaks, relatively weak (2 2 0) and (2 2 2) peaks are also observed. The effect of O2 yN2 flow ratios on texture is presented in Fig. 2. It can be seen that the addition of oxygen to TiCN coating tends to increase the intensity of (1 1 1). The (1 1 1) plane is known to be the most closely packed and normally exhibits the lowest surface energy w8x. When the oxygen flow rate was at 6 sccm (OyNs 0.375), the I(1 1 1)yI(2 0 0) ratio reached its maximum. By checking the ratios of CyN in the coatings, it can be seen that the (1 1 1) texture is more dominant for higher nitrogen concentration. This result is in good agreement with previous study on TiCN w8x. 3.3. Morphology Fig. 3 shows the cross-sectional SEM micrographs of the Ti(C,N,O) coatings with the variation of O2 yN2 flow ratios. It is observed that the morphologies change
236
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
Fig. 3. Cross-sectional SEM micrographs of the Ti(C,N,O) deposited under various O2 yN2 flow ratios: (a) O2yN2 s0; (b) O2yN2s2y20; (c) O2yN2s4y18; (d) O2yN2s8y14.
Fig. 4. Effects of O2yN2 flow ratios during deposition on wear and friction coefficients.
J.H. Hsieh et al. / Surface and Coatings Technology 163 – 164 (2003) 233–237
from columnar structure (O2 flow rates0 sccm), as shown in Fig. 3a to dense zone T structure (O2 flow rates4 sccm), as shown in Fig. 3c. Further increase of oxygen changes the morphology back to columnar structure as shown in Fig. 3d. The change of the structure seems to be matched with the hardness value. The coating with dense zone T structure in Fig. 3c exhibits the highest hardness at 32 GPa.
237
would increase the incorporation of nitrogen into the coatings, which in turn enhance the (1 1 1) texture. This is probably caused by the etching effect of oxygen on the deposited carbon atoms. With oxygen flow rate at 4 sccm during deposition, the TiCNO coating had the lowest wear rate among all. Further increase in oxygen flow rate caused the decrease of the hardness, adhesion and wear resistance, together with the increase of friction coefficient.
3.4. Tribological properties Acknowledgments The tribological behaviors of the coatings were studied comparatively at a sliding speed of 30 cmys (18 my min) and under a normal load of 5 N sliding against alumina pin in atmospheric environment. Fig. 4 shows the results of the tribological tests. The coating deposited with oxygen flow rate at 4 sccm (OyNs4y18) shows the lowest wear coefficient. This is probably related to its high hardness, dense structure and low friction coefficient. Overall, as shown in Fig. 4, the coatings’ wear coefficient is closely related to the friction coefficient, which is one of the characteristics of TiCN coatings. Higher flow rate of oxygen ()6 sccm) during deposition would create coatings with columnar structure, and higher nitrogen content. This would in turn result in higher friction coefficient and therefore higher wear rate. 4. Conclusions The properties and performance of Ti(C,N,O) coatings prepared in this study are greatly affected by the flow ratios of oxygen to nitrogen. Addition of oxygen
We would like to express sincere gratitude to Dr Zeng Xianting and Ms Liu Yuchan of Gintic Institute of Manufacturing Technology for their help during this study. References w1x G. Baravian, G. Sultan, E. Damond, H. Detour, Surf. Coat. Technol. 76y77 (1995) 687. w2x O. Knotek, F. Loffler, G. Kramer, Surf. Coat. Technol. 61 (1993) 320. w3x V.D. Kal’ner, A.K. Verner, Metal Sci. Heat Treat. 36 (1994) 196. w4x T. Wierzchon, J.R. Sobiecki, D. Krupa, Surf. Coat. Technol. 59 (1993) 217. w5x Y. Shi, H. Peng, Y. Xie, G. Xie, C. Zhao, Surf. Coat. Technol. 132 (1998) 26. w6x A. Stanishevsky, R. Lappalainen, Surf. Coat. Technol. 123 (2000) 101. w7x D.P. Monaghan, D.G. Teer, K.C. Laing, R.D. Arnell, Surf. Coat. Technol. 59 (1993) 21. w8x J.M. Schneider, A. Voevodin, C. Rebholz, A. Matthews, Surf. Coat. Technol. 74–75 (1995) 312.