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Tribological behavior of Ti–Si–N coating layers prepared by a hybrid system of arc ion plating and sputtering techniques
Surface and Coatings Technology 179 (2004) 83–88
Tribological behavior of Ti–Si–N coating layers prepared by a hybrid system of arc ion plating and sputtering techniques Oc-Nam Parka, Jong Hyun Parka, Seog-Young Yoona, Mi-Hye Leeb, Kwang Ho Kima,* a
School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea b Technical Appraisal Center, Korea Technology Credit Guarantee Fund, Busan 600-777, South Korea Received 28 February 2003; accepted in revised form 2 May 2003
Abstract Ti–Si–N coating layers were deposited onto WC–Co substrates by a hybrid system of arc ion plating and sputtering. The coating layers were prepared with different Si contents to investigate the effect of Si content on their tribological behaviors. For this study, the dry sliding wear experiments were conducted on Ti–Si–N-coated WC–Co discs at three different sliding speeds, 0.1, 0.3, 0.5 mys, against steel (SUJ2) and alumina balls using a conventional ball-on-disc sliding wear apparatus. In the case of steel ball, the average friction coefficient slightly decreased with increasing the sliding speed regardless of Si content, due to adhesive wear behavior between coating layer and steel ball. At constant sliding speed, the average friction coefficient decreased with increase of Si content. This behavior was attributed to the formation of self-lubricating tribo-layers such as SiO2 or Si(OH)2. Conversely, in the case of the alumina ball, the average friction coefficient increased with increasing sliding speed regardless of Si content, indicating that the abrasive wear behavior was more dominant when the coating layers slid against an alumina ball. From our experimental results, it was found that the tribological behavior of Ti–Si–N coating layers was affected by factors such as Si content, sliding speed, and type of counterpart materials. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Hybrid coating system; Ti–Si–N; Friction coefficient; Wear
1. Introduction For wear-protective hard coatings, the ternary systems of Ti–X–N hard coatings, where XsAl, Si, Cr, C, etc., have been explored and are attracting major interests w1–3x because of their high hardness and good oxidation resistance compared to previous TiN coatings which has been widely applied to tools and dies. One of those ternary coating systems, Ti–Si–N, is expected to be a good candidate for hard coatings because it shows very high hardness, i.e. superhardness (040 GPa) caused by the characteristic microstructure, described as a nanocomposite consisting of nanocrystalline TiN crystallites embedded in amorphous silicon nitride matrix w4–7x. Therefore, several groups have focused on synthesis and characterization of Ti–Si–N coating layers by plasma*Corresponding author. Tel.: q82-51-510-2391; fax: q82-51-5103660. E-mail address: [email protected] (K.H. Kim).
enhanced chemical vapor deposition techniques w6–8x and by sputtering techniques w9–11x. The arc ion plating (AIP) technique has strong merits of good adhesion and high deposition rate because the process is characterized by a very high ionization and high current density compared to other processes w12– 14x. Although the AIP technique is recognized as an excellent process for hard coatings, it was rarely adopted for Ti–Si–N coatings. In our previous paper w15x, Ti– Si–N coating layer has been successfully synthesized by a hybrid coating system of AIP and sputtering techniques, where TiN was deposited by AIP technique while Si was incorporated by sputtering. In this work, the tribological behavior of Ti–Si–N coating layers prepared by the hybrid coating system was examined against the steel and alumina balls using a conventional ball-on-disc sliding wear apparatus. The tribological behaviors were described using not only the average friction coefficient, but also worn surface mor-
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00769-2
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Table 1 Typical deposition conditions for Ti–Si–N coatings by hybrid coating system Process
Variable
Value
Ar ion bombardment
Temperature Bias voltage Pressure Time
300 8C y800 V 8.0=10y1 Pa 15 min
Coating
Base pressure Working pressure Working gas ratio Arc material Sputter material Arc current Sputter current Substrate temperature Substrate bias voltage Rotational velocity of substrate
6.7=10y4 Pa 8.0=10y2 Pa N2:Ars3:1 Ti (99.99%) Si (99.99%) 60 A 0–2.0 A 300 8C y100 V 25 rpm
2. Experimental
electron probe micro-analysis (EPMA, Shimadzu, EPMA1600). The hardness of the coating layer was determined using a micro-zone Vickers hardness testing system (Akashi, MZT) under a load of 30 mN.
2.1. Deposition
2.2. Dry sliding wear test
phology and wear debris composition with the variation of the Si content and sliding speed.
Ti–Si–N coating layers were deposited onto WC–Co substrates by the hybrid coating system, where AIP method was combined with a magnetron sputtering technique. The substrates (with a disc shape, 20 mm in diameter) were ground and polished for them to have an average surface roughness (Ra) of approximately 0.3 mm. The substrates were thoroughly cleaned in an ultrasonic bath cleaner using acetone. An arc cathode gun for Ti source and a DC sputter gun for Si source were installed on each side of chamber wall (chamber size: 600=600=500 mm3). A rotational substrate holder was located on a straight line between two sources. Each distance from the arc and sputter sources to substrates holder was 350 and 250 mm, respectively. Ar gas (99.999%) was introduced into sputtering target holder to increase the sputtering rate, and N2 gas (99.999%) was injected near substrate holder. Prior to deposition, the substrates were further cleaned by Ar ion bombardment at a bias voltage of y800 V for 15 min. A typical deposition conditions for Ti–Si–N coatings by our hybrid coating system are summarized in Table 1. The roughness of substrate and coating layer were measured with a stylus (a-STEP) instrument. The crystal structure of coating layer was characterized by Xray diffraction (Rigaku, DyMax-2400 diffractometer) using Cu Ka radiation (generator settings of 25 kV and 10 mA). The cross-sectional morphology and thickness of coating layer were observed by scanning electron microscopy (SEM, Hitachi S-4200). Compositional analyses of the Ti–Si–N films were carried out with
Friction and wear behavior were evaluated in sliding tests using a conventional ball-on-disc wear apparatus. The apparatus comprised a rotating shaft arrangement on which a specimen holder made of stainless steel was attached. A vertical loading arrangement, attached to a ball, lowered onto the rotating specimens so as to produce a circular wear track. All tests were conducted at an ambient temperature (f25 8C) and a laboratory environmental relative humidity (25–30% RH) under a normal load of 1 N with steel and alumina balls (diameter, 6 mm) as counterpart materials. The steel and alumina ball had average hardnesses of 700 Hv0.2 and 2100 Hk, respectively. In order to investigate the tribological behavior of the coating layer with different Si contents, the dry sliding wear experiments were carried out at three different sliding speeds, 0.1, 0.3 and 0.5 mys, and total sliding distance was approximately 500 m. Optical microscopy and SEM were employed to observe the morphology of wear track after each sliding experiment. The width and depth of wear track were estimated with SEM and stylus. Energy dispersive spectroscopy (EDS) was used to reveal the compositions of wear debris formed with counterpart materials during wear experiment. 3. Results and discussion 3.1. Coating layers Fig. 1 represents the X-ray diffraction patterns for the Ti–Si–N coating layers with different Si contents. The diffraction patterns showed a typical one of polycrystal-
O.-N. Park et al. / Surface and Coatings Technology 179 (2004) 83–88
Fig. 1. X-ray diffraction patterns of Ti–Si–N coating layers with various Si contents.
line TiN with multiple orientations of (1 1 1), (2 0 0) and (2 2 0) at a Si content of 5 at.%. Such multiple orientations of TiN crystallites with Si incorporation have been reported in other deposition systems w10,16,17x. At the Si content of 7 at.%, the coating layer had most randomly oriented microstructure of (1 1 1), (2 0 0), (2 2 0) and (3 1 1). However, as the Si content increased above 7 at.%, such a random orientation of the microstructure trended to disappear, and the peaks became broad. These peak broadening phenomena, in general, result from both diminution of grain size and residual stress induced in the crystal lattice. However, the XRD peak broadening in Ti–Si–N coating layers (as shown in Fig. 1) was reported to be due to size reduction of TiN crystallites with increasing of Si content w10x. On the other hand, any XRD peak corresponding to crystalline Si3N4 was not found, indicating that Si would exist as an amorphous Si3N4 phase. The thickness of the obtained coating layer was approximately 2.5 mm regardless of Si content. Fig. 2 shows the microhardness of Ti–Si–N coating layer as a function of Si content. During indentation, the ratio of maximum penetration depth (h) to thickness of film (t) was 0.08, which would avoid the influence of substrate effect w18x. The hardness largely increased, and reached a maximum value of 4500 kgymm2 at the Si content of 7 at.%, and dropped with further increase of Si content. This high hardness would be explained by taking into consideration the nanocomposite structure described by Veprek w6x.
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Fig. 2. Microhardness values of Ti–Si–N coating layers as a function of Si content.
less of Si content. This behavior would be related with the adhesive wear behavior between the steel and Ti– Si–N coating layer because the coating layers were harder than steel w19x. At constant sliding speed conditions, the friction coefficient decreased with increase of Si content. The friction coefficient was not dependent on the hardness of coating layer, considering that the hardness varied with the Si content as shown in Fig. 2.
3.2. Friction coefficient Fig. 3 represents the average friction coefficient of the Ti–Si–N coating layers with different Si contents as a function of sliding speed against steel (Fig. 3a) and alumina (Fig. 3b) balls. In the case of a steel ball, the average friction coefficient of Ti–Si–N coating layer slightly decreased with increase of sliding speed regard-
Fig. 3. Average friction coefficient of Ti–Si–N coating layers as a function of sliding speed against (a) steel and (b) alumina counterpart materials.
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explained by the possible tribochemical reactions that Si3N4 reacted with H2O to produce SiO2 or Si(OH)2 tribo-layers, which probably played an role as selflubricating layer. The formation of tribo-layers like SiO2 or Si(OH)2 would be more activated with increasing Si content. EDS analyses for the wear debris revealed the presence of Ti, Si, Fe and O species (Fig. 5c and f). These results reflect that the mass transfer from the steel ball happened during the sliding wear test regardless of Si content, and it was suggested that the debris and wear track were oxidized during the sliding wear process with aid of the humidity w19,20x.
Fig. 4. Wear track depth and width after dry sliding wear test for Ti– Si–N coating layer against the steel ball at three different sliding speeds, 0.1, 0.3, 0.5 mys with various Si contents. (Total sliding distance, 500 m.)
This result would be explained by a possible tribochemical reaction, often taking place in Si3N4 ceramics, e.g. Si3N4 reacts with ambient H2O to produce SiO2 or Si(OH)2 tribo-layers w19,20x. On the other hand, in the case of alumina ball, the average friction coefficient of Ti–Si–N coating layer slightly increased with increasing sliding speed, and the friction coefficient increased with increase of Si content. These results were opposite behaviors compared with the case of steel ball, and indicate that the abrasive wear behavior is more dominant when the coatings slide against an alumina ball.
3.3.2. Coating layers against alumina ball Fig. 6 shows the wear track depth and width of Ti– Si–N coating layers after sliding wear test against alumina ball at various sliding speeds and Si contents, and the total sliding distance was 500 m. In contrast to the case of steel ball, the wear depth and width were dependent on the sliding speed. As the sliding speed increased, the width increased, and the depth decreased. This behavior would be related with the abrasive wear behavior between coating layers and alumina ball. At low Si content (5 and 7 at.%), the wear track width did
3.3. Wear behavior 3.3.1. Coating layers against steel Fig. 4 shows the wear track depth and width of Ti– Si–N coating layers with various Si content after sliding wear test against steel ball at three different sliding speeds of 0.1, 0.3, 0.5 mys, and total sliding distance was approximately 500 m. Overall, the width of wear track decreased with increasing the sliding speed regardless of Si content. This would be due to the decrease of running time as the sliding speed increased. On the other hand, the depth of wear track was almost undetectable regardless of experimental conditions. This behavior indicates that steel ball material was smeared onto coating layers rather than digging into them, and would be related with the adhesive wear behavior between coating layers and steel ball because the coating layers were harder than steel w21x. Fig. 5 shows a comparison of wear behavior for Ti– Si–N coating layers having two different Si contents when they slid against steel ball at a sliding speed of 0.1 mys. At high Si content of 13 at.%, the worn surface of wear track was fairly smooth (Fig. 5d and e), while the debris was smeared onto the wear track at low Si content of 5 at.% (Fig. 5a and b). This result could be
Fig. 5. Morphology and EDS analysis for wear tracks after dry sliding wear test against steel ball. (a) Optical micrograph of wear track, (b) SEM micrograph for selected area (rectangular area in (a)), (c) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 5 at.%; and (d) Optical micrograph of wear track, (e) SEM micrograph for selected area (rectangular area in (d)), (f) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 13 at.%.
O.-N. Park et al. / Surface and Coatings Technology 179 (2004) 83–88
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Fig. 6. Wear track depth and width after dry sliding wear test for Ti– Si–N coating layer against the alumina ball at three different sliding speeds, 0.1, 0.3, 0.5 mys with various Si contents. (Total sliding distance, 500 m.)
not change much with sliding speed compared to the case of high Si content (10 and 13 at.%). On the contrary, the depth was changed largely with sliding speed at low Si content compared to the case of high Si content. These behaviors could be due to the formation of tribo-layers like SiO2 or Si(OH)2 with increase of the Si content. Fig. 7 shows the examples of surface morphology and composition analyses for the wear tracks after the sliding wear test against alumina ball at sliding speed of 0.1 mys. The wear debris was not smeared on the wear track and was accumulated at the boundary of wear track (Fig. 7b and e). EDS analyses (Fig. 7c and f) for the wear debris revealed the presence of Ti, Si, Al and O species, and the relative content of Al, O decreased with increase of Si content. This result suggested that the tribo-layers of SiO2 or Si(OH)2 were easily formed with increase of Si content. In addition, the wear debris disappeared from wear track after cleaning with acetone. This indicates that the white debris was not chemically but physically adhered on the coating surface. From the results of Figs. 6 and 7, the abrasive wear behavior is more dominant when the coating layers slide against alumina ball. 4. Conclusions Ti–Si–N coating layers were deposited onto WC–Co substrates by a hybrid system of AIP and sputtering techniques. The coating layer showed the fairly high hardness of 4500 kgymm2 at the Si content of 7 at.%. When the dry sliding wear test was carried out against steel ball, the average friction coefficient slightly decreased with increasing the sliding speed regardless of Si content. It was due to adhesive wear behavior between coating layer and steel ball. At constant sliding speed, the average friction coefficient decreased with
Fig. 7. Morphology and EDS analysis for wear tracks after dry sliding wear test against alumina ball. (a) Optical micrograph of wear track, (b) SEM micrograph for selected area wrectangular area in (a)x, (c) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 5 at.%; and (d) Optical micrograph of wear track, (e) SEM micrograph for selected area wrectangular area in (d)x, (f) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 13 at.%.
increase of Si content. This behavior was attributed to the formation of self-lubricating tribo-layers such as SiO2 or Si(OH)2. On the contrary, in the case of alumina ball, the average friction coefficient increased with increasing the sliding speed regardless of Si content, indicating that the abrasive wear behavior was more dominant when the coating layers slid against alumina ball. It was found from our experimental results that the tribological behavior of Ti–Si–N coating layer was affected by factors such as Si content, sliding speed and kinds of counterpart materials. Acknowledgments This work was performed through National Research Laboratory (NRL) program supported by Ministry of Science and Technology of Korea (MOST). References w1x M.S. Wong, Y.C. Lee, Surf. Coat. Technol. 120–121 (1999) 194. w2x D.B. Lee, M.H. Kim, Y.C. Lee, S.C. Kwon, Surf. Coat. Technol. 141 (2001) 232.
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w3x J.D. Bressan, R. Hesse, E.M. Silva Jr., Wear 250 (2001) 561. w4x X. Sun, J.S. Reid, E. Kolawa, M.-A. Nicolet, J. Appl. Phys. 81 (2) (1997) 656. w5x L. Rebouta, C.J. Tavares, R. Aimo, et al., Surf. Coat. Technol. 133–134 (2000) 234. w6x S. Veprek, Surf. Coat. Technol. 97 (1997) 15. w7x S. Veprek, S. Reiprich, L. Shizhi, Appl. Phys. Lett. 66 (20) (1995) 2640. w8x K.H. Kim, B.H. Park, Chem. Vapor Depos. 5 (1999) 275. w9x M. Diserens, J. Patscheider, F. Levy, ´ Surf. Coat. Technol. 120– 121 (1999) 158. w10x M. Diserens, J. Patscheider, F. Levy, ´ Surf. Coat. Technol. 108– 109 (1998) 241. w11x L. Le Brizoual, A. Granier, F. Clenet, P. Briaud, G. Lemperiere, G. Turban, Surf. Coat. Technol. 116–119 (1999) 922. w12x M. Sakaki, T. Sakakibara, IEEE Trans. Plasma Sci. 22 (1994) 1049.
w13x H. Musil, J. Vyskocil, S. Kadlec, in: M.H. Fancombe, J.L. Vossen (Eds.), Mechanical and Dielectric Properties, Academis Press, London, 1993, p. 79. w14x P.C. Johnson, in: M.H. Francombe, J.L. Vossen (Eds.), Contemporary, Preparative Techniques, Academic Press, London, 1989, p. 129. w15x K.H. Kim, S.-R. Choi, S.-Y. Yoon, Surf. Coat. Technol. 161 (2–3) (2002) 243. w16x S.H. Kim, J.W. Jang, S.S. Kang, K.H. Kim, J. Mater. Proc. Technol. 130–131 (2002) 283. w17x L. Shizhi, S. Yulong, P. Hongrui, Plasma Chem. Plasma Process. 12 (3) (1992) 287. w18x G.M. Pharr, W.C. Oliver, MRS Bull. 17 (7) (1992) 28. w19x S. Wilson, A.T. Alpas, Wear 245 (1996) 223. w20x J. Takadoum, H. Houmid-Bennani, D. Mairey, J. Eur. Ceram. Soc. 18 (1998) 553. w21x K.N. Andersen, E.J. Bienk, K.O. Schweitz, et al., Surf. Coat. Technol. 123 (2000) 219.
Tribological behavior of Ti–Si–N coating layers prepared by a hybrid system of arc ion plating and sputtering techniques Oc-Nam Parka, Jong Hyun Parka, Seog-Young Yoona, Mi-Hye Leeb, Kwang Ho Kima,* a
School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea b Technical Appraisal Center, Korea Technology Credit Guarantee Fund, Busan 600-777, South Korea Received 28 February 2003; accepted in revised form 2 May 2003
Abstract Ti–Si–N coating layers were deposited onto WC–Co substrates by a hybrid system of arc ion plating and sputtering. The coating layers were prepared with different Si contents to investigate the effect of Si content on their tribological behaviors. For this study, the dry sliding wear experiments were conducted on Ti–Si–N-coated WC–Co discs at three different sliding speeds, 0.1, 0.3, 0.5 mys, against steel (SUJ2) and alumina balls using a conventional ball-on-disc sliding wear apparatus. In the case of steel ball, the average friction coefficient slightly decreased with increasing the sliding speed regardless of Si content, due to adhesive wear behavior between coating layer and steel ball. At constant sliding speed, the average friction coefficient decreased with increase of Si content. This behavior was attributed to the formation of self-lubricating tribo-layers such as SiO2 or Si(OH)2. Conversely, in the case of the alumina ball, the average friction coefficient increased with increasing sliding speed regardless of Si content, indicating that the abrasive wear behavior was more dominant when the coating layers slid against an alumina ball. From our experimental results, it was found that the tribological behavior of Ti–Si–N coating layers was affected by factors such as Si content, sliding speed, and type of counterpart materials. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Hybrid coating system; Ti–Si–N; Friction coefficient; Wear
1. Introduction For wear-protective hard coatings, the ternary systems of Ti–X–N hard coatings, where XsAl, Si, Cr, C, etc., have been explored and are attracting major interests w1–3x because of their high hardness and good oxidation resistance compared to previous TiN coatings which has been widely applied to tools and dies. One of those ternary coating systems, Ti–Si–N, is expected to be a good candidate for hard coatings because it shows very high hardness, i.e. superhardness (040 GPa) caused by the characteristic microstructure, described as a nanocomposite consisting of nanocrystalline TiN crystallites embedded in amorphous silicon nitride matrix w4–7x. Therefore, several groups have focused on synthesis and characterization of Ti–Si–N coating layers by plasma*Corresponding author. Tel.: q82-51-510-2391; fax: q82-51-5103660. E-mail address: [email protected] (K.H. Kim).
enhanced chemical vapor deposition techniques w6–8x and by sputtering techniques w9–11x. The arc ion plating (AIP) technique has strong merits of good adhesion and high deposition rate because the process is characterized by a very high ionization and high current density compared to other processes w12– 14x. Although the AIP technique is recognized as an excellent process for hard coatings, it was rarely adopted for Ti–Si–N coatings. In our previous paper w15x, Ti– Si–N coating layer has been successfully synthesized by a hybrid coating system of AIP and sputtering techniques, where TiN was deposited by AIP technique while Si was incorporated by sputtering. In this work, the tribological behavior of Ti–Si–N coating layers prepared by the hybrid coating system was examined against the steel and alumina balls using a conventional ball-on-disc sliding wear apparatus. The tribological behaviors were described using not only the average friction coefficient, but also worn surface mor-
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00769-2
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Table 1 Typical deposition conditions for Ti–Si–N coatings by hybrid coating system Process
Variable
Value
Ar ion bombardment
Temperature Bias voltage Pressure Time
300 8C y800 V 8.0=10y1 Pa 15 min
Coating
Base pressure Working pressure Working gas ratio Arc material Sputter material Arc current Sputter current Substrate temperature Substrate bias voltage Rotational velocity of substrate
6.7=10y4 Pa 8.0=10y2 Pa N2:Ars3:1 Ti (99.99%) Si (99.99%) 60 A 0–2.0 A 300 8C y100 V 25 rpm
2. Experimental
electron probe micro-analysis (EPMA, Shimadzu, EPMA1600). The hardness of the coating layer was determined using a micro-zone Vickers hardness testing system (Akashi, MZT) under a load of 30 mN.
2.1. Deposition
2.2. Dry sliding wear test
phology and wear debris composition with the variation of the Si content and sliding speed.
Ti–Si–N coating layers were deposited onto WC–Co substrates by the hybrid coating system, where AIP method was combined with a magnetron sputtering technique. The substrates (with a disc shape, 20 mm in diameter) were ground and polished for them to have an average surface roughness (Ra) of approximately 0.3 mm. The substrates were thoroughly cleaned in an ultrasonic bath cleaner using acetone. An arc cathode gun for Ti source and a DC sputter gun for Si source were installed on each side of chamber wall (chamber size: 600=600=500 mm3). A rotational substrate holder was located on a straight line between two sources. Each distance from the arc and sputter sources to substrates holder was 350 and 250 mm, respectively. Ar gas (99.999%) was introduced into sputtering target holder to increase the sputtering rate, and N2 gas (99.999%) was injected near substrate holder. Prior to deposition, the substrates were further cleaned by Ar ion bombardment at a bias voltage of y800 V for 15 min. A typical deposition conditions for Ti–Si–N coatings by our hybrid coating system are summarized in Table 1. The roughness of substrate and coating layer were measured with a stylus (a-STEP) instrument. The crystal structure of coating layer was characterized by Xray diffraction (Rigaku, DyMax-2400 diffractometer) using Cu Ka radiation (generator settings of 25 kV and 10 mA). The cross-sectional morphology and thickness of coating layer were observed by scanning electron microscopy (SEM, Hitachi S-4200). Compositional analyses of the Ti–Si–N films were carried out with
Friction and wear behavior were evaluated in sliding tests using a conventional ball-on-disc wear apparatus. The apparatus comprised a rotating shaft arrangement on which a specimen holder made of stainless steel was attached. A vertical loading arrangement, attached to a ball, lowered onto the rotating specimens so as to produce a circular wear track. All tests were conducted at an ambient temperature (f25 8C) and a laboratory environmental relative humidity (25–30% RH) under a normal load of 1 N with steel and alumina balls (diameter, 6 mm) as counterpart materials. The steel and alumina ball had average hardnesses of 700 Hv0.2 and 2100 Hk, respectively. In order to investigate the tribological behavior of the coating layer with different Si contents, the dry sliding wear experiments were carried out at three different sliding speeds, 0.1, 0.3 and 0.5 mys, and total sliding distance was approximately 500 m. Optical microscopy and SEM were employed to observe the morphology of wear track after each sliding experiment. The width and depth of wear track were estimated with SEM and stylus. Energy dispersive spectroscopy (EDS) was used to reveal the compositions of wear debris formed with counterpart materials during wear experiment. 3. Results and discussion 3.1. Coating layers Fig. 1 represents the X-ray diffraction patterns for the Ti–Si–N coating layers with different Si contents. The diffraction patterns showed a typical one of polycrystal-
O.-N. Park et al. / Surface and Coatings Technology 179 (2004) 83–88
Fig. 1. X-ray diffraction patterns of Ti–Si–N coating layers with various Si contents.
line TiN with multiple orientations of (1 1 1), (2 0 0) and (2 2 0) at a Si content of 5 at.%. Such multiple orientations of TiN crystallites with Si incorporation have been reported in other deposition systems w10,16,17x. At the Si content of 7 at.%, the coating layer had most randomly oriented microstructure of (1 1 1), (2 0 0), (2 2 0) and (3 1 1). However, as the Si content increased above 7 at.%, such a random orientation of the microstructure trended to disappear, and the peaks became broad. These peak broadening phenomena, in general, result from both diminution of grain size and residual stress induced in the crystal lattice. However, the XRD peak broadening in Ti–Si–N coating layers (as shown in Fig. 1) was reported to be due to size reduction of TiN crystallites with increasing of Si content w10x. On the other hand, any XRD peak corresponding to crystalline Si3N4 was not found, indicating that Si would exist as an amorphous Si3N4 phase. The thickness of the obtained coating layer was approximately 2.5 mm regardless of Si content. Fig. 2 shows the microhardness of Ti–Si–N coating layer as a function of Si content. During indentation, the ratio of maximum penetration depth (h) to thickness of film (t) was 0.08, which would avoid the influence of substrate effect w18x. The hardness largely increased, and reached a maximum value of 4500 kgymm2 at the Si content of 7 at.%, and dropped with further increase of Si content. This high hardness would be explained by taking into consideration the nanocomposite structure described by Veprek w6x.
85
Fig. 2. Microhardness values of Ti–Si–N coating layers as a function of Si content.
less of Si content. This behavior would be related with the adhesive wear behavior between the steel and Ti– Si–N coating layer because the coating layers were harder than steel w19x. At constant sliding speed conditions, the friction coefficient decreased with increase of Si content. The friction coefficient was not dependent on the hardness of coating layer, considering that the hardness varied with the Si content as shown in Fig. 2.
3.2. Friction coefficient Fig. 3 represents the average friction coefficient of the Ti–Si–N coating layers with different Si contents as a function of sliding speed against steel (Fig. 3a) and alumina (Fig. 3b) balls. In the case of a steel ball, the average friction coefficient of Ti–Si–N coating layer slightly decreased with increase of sliding speed regard-
Fig. 3. Average friction coefficient of Ti–Si–N coating layers as a function of sliding speed against (a) steel and (b) alumina counterpart materials.
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explained by the possible tribochemical reactions that Si3N4 reacted with H2O to produce SiO2 or Si(OH)2 tribo-layers, which probably played an role as selflubricating layer. The formation of tribo-layers like SiO2 or Si(OH)2 would be more activated with increasing Si content. EDS analyses for the wear debris revealed the presence of Ti, Si, Fe and O species (Fig. 5c and f). These results reflect that the mass transfer from the steel ball happened during the sliding wear test regardless of Si content, and it was suggested that the debris and wear track were oxidized during the sliding wear process with aid of the humidity w19,20x.
Fig. 4. Wear track depth and width after dry sliding wear test for Ti– Si–N coating layer against the steel ball at three different sliding speeds, 0.1, 0.3, 0.5 mys with various Si contents. (Total sliding distance, 500 m.)
This result would be explained by a possible tribochemical reaction, often taking place in Si3N4 ceramics, e.g. Si3N4 reacts with ambient H2O to produce SiO2 or Si(OH)2 tribo-layers w19,20x. On the other hand, in the case of alumina ball, the average friction coefficient of Ti–Si–N coating layer slightly increased with increasing sliding speed, and the friction coefficient increased with increase of Si content. These results were opposite behaviors compared with the case of steel ball, and indicate that the abrasive wear behavior is more dominant when the coatings slide against an alumina ball.
3.3.2. Coating layers against alumina ball Fig. 6 shows the wear track depth and width of Ti– Si–N coating layers after sliding wear test against alumina ball at various sliding speeds and Si contents, and the total sliding distance was 500 m. In contrast to the case of steel ball, the wear depth and width were dependent on the sliding speed. As the sliding speed increased, the width increased, and the depth decreased. This behavior would be related with the abrasive wear behavior between coating layers and alumina ball. At low Si content (5 and 7 at.%), the wear track width did
3.3. Wear behavior 3.3.1. Coating layers against steel Fig. 4 shows the wear track depth and width of Ti– Si–N coating layers with various Si content after sliding wear test against steel ball at three different sliding speeds of 0.1, 0.3, 0.5 mys, and total sliding distance was approximately 500 m. Overall, the width of wear track decreased with increasing the sliding speed regardless of Si content. This would be due to the decrease of running time as the sliding speed increased. On the other hand, the depth of wear track was almost undetectable regardless of experimental conditions. This behavior indicates that steel ball material was smeared onto coating layers rather than digging into them, and would be related with the adhesive wear behavior between coating layers and steel ball because the coating layers were harder than steel w21x. Fig. 5 shows a comparison of wear behavior for Ti– Si–N coating layers having two different Si contents when they slid against steel ball at a sliding speed of 0.1 mys. At high Si content of 13 at.%, the worn surface of wear track was fairly smooth (Fig. 5d and e), while the debris was smeared onto the wear track at low Si content of 5 at.% (Fig. 5a and b). This result could be
Fig. 5. Morphology and EDS analysis for wear tracks after dry sliding wear test against steel ball. (a) Optical micrograph of wear track, (b) SEM micrograph for selected area (rectangular area in (a)), (c) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 5 at.%; and (d) Optical micrograph of wear track, (e) SEM micrograph for selected area (rectangular area in (d)), (f) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 13 at.%.
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Fig. 6. Wear track depth and width after dry sliding wear test for Ti– Si–N coating layer against the alumina ball at three different sliding speeds, 0.1, 0.3, 0.5 mys with various Si contents. (Total sliding distance, 500 m.)
not change much with sliding speed compared to the case of high Si content (10 and 13 at.%). On the contrary, the depth was changed largely with sliding speed at low Si content compared to the case of high Si content. These behaviors could be due to the formation of tribo-layers like SiO2 or Si(OH)2 with increase of the Si content. Fig. 7 shows the examples of surface morphology and composition analyses for the wear tracks after the sliding wear test against alumina ball at sliding speed of 0.1 mys. The wear debris was not smeared on the wear track and was accumulated at the boundary of wear track (Fig. 7b and e). EDS analyses (Fig. 7c and f) for the wear debris revealed the presence of Ti, Si, Al and O species, and the relative content of Al, O decreased with increase of Si content. This result suggested that the tribo-layers of SiO2 or Si(OH)2 were easily formed with increase of Si content. In addition, the wear debris disappeared from wear track after cleaning with acetone. This indicates that the white debris was not chemically but physically adhered on the coating surface. From the results of Figs. 6 and 7, the abrasive wear behavior is more dominant when the coating layers slide against alumina ball. 4. Conclusions Ti–Si–N coating layers were deposited onto WC–Co substrates by a hybrid system of AIP and sputtering techniques. The coating layer showed the fairly high hardness of 4500 kgymm2 at the Si content of 7 at.%. When the dry sliding wear test was carried out against steel ball, the average friction coefficient slightly decreased with increasing the sliding speed regardless of Si content. It was due to adhesive wear behavior between coating layer and steel ball. At constant sliding speed, the average friction coefficient decreased with
Fig. 7. Morphology and EDS analysis for wear tracks after dry sliding wear test against alumina ball. (a) Optical micrograph of wear track, (b) SEM micrograph for selected area wrectangular area in (a)x, (c) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 5 at.%; and (d) Optical micrograph of wear track, (e) SEM micrograph for selected area wrectangular area in (d)x, (f) EDS analysis of wear debris for Ti–Si–N coating layer having Si content of 13 at.%.
increase of Si content. This behavior was attributed to the formation of self-lubricating tribo-layers such as SiO2 or Si(OH)2. On the contrary, in the case of alumina ball, the average friction coefficient increased with increasing the sliding speed regardless of Si content, indicating that the abrasive wear behavior was more dominant when the coating layers slid against alumina ball. It was found from our experimental results that the tribological behavior of Ti–Si–N coating layer was affected by factors such as Si content, sliding speed and kinds of counterpart materials. Acknowledgments This work was performed through National Research Laboratory (NRL) program supported by Ministry of Science and Technology of Korea (MOST). References w1x M.S. Wong, Y.C. Lee, Surf. Coat. Technol. 120–121 (1999) 194. w2x D.B. Lee, M.H. Kim, Y.C. Lee, S.C. Kwon, Surf. Coat. Technol. 141 (2001) 232.
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