- Email: [email protected]
Formulated self-lubricating carbon coatings on carbide ceramics
Wear 271 (2011) 1974–1979
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Formulated self-lubricating carbon coatings on carbide ceramics J. Sui a,b , J. Lu a,∗ a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 19 November 2010 Accepted 21 November 2010
Keywords: Chlorination Carbide-derived carbon Self-lubricating Microstructure
a b s t r a c t SiC and Ti3 SiC2 are very important materials for many tribological applications. However, they are not lubricious under dry sliding. Chlorination was conducted at 1000 ◦ C for 5 h in a quartz tube to fabricate carbide-derived carbon (CDC) coatings on SiC and Ti3 SiC2 blocks. Unlubricated sliding friction and wear tests were conducted on a UMT-2MT tribometer with a ball-on-block configuration. Compared to bare SiC and Ti3 SiC2 blocks, SiC and Ti3 SiC2 blocks with the CDC coatings were self-lubricating in sliding against a Si3 N4 ball at room temperature. The wear of the Si3 N4 ball was too low to measure in sliding against the CDC coatings. In addition, the CDC coating on a SiC block took advantages over the CDC coating on a Ti3 SiC2 block in terms of friction coefficient, wear resistance and load-carrying capacity. The microstructure of the two CDC coatings was characterized by three aspects, i.e. pore size, grain size and shape, and composition (carbon species). The two CDC coatings are highly porous materials but of different grain size, grain shape and composition. The CDC coating on SiC was composed of nanocrystalline and disorder carbon of equiaxed grains while that on Ti3 SiC2 was composed of plate-like microcrystalline graphite. The microstructural effect on the tribological behavior of the CDC coatings on SiC and Ti3 SiC2 blocks was investigated and discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The importance of binary carbides (e.g. SiC, WC, and TiC) has already been well recognized. Ternary carbides (e.g. Ti3 SiC2 ) are also important materials but is less known. In recent years, Ti3 SiC2 has received considerable attention because of the unusual combination of metallic and ceramic properties [1]. On the one hand, it is relatively soft, easily machinable, electrically and thermally conductive, and behaves plastically at high temperature. On the other hand, it is oxidation resistant, has high melting point, and, most importantly, it maintains its strength at high temperature. Today, binary carbides and ternary carbides are also very important materials for many tribological applications [1,2]. However, these carbides do not feel self-lubricated under dry sliding [3,4]. Some researches revealed Ti3 SiC2 possessed excellent tribological properties due to the formation of tribo-induced oxides on the contact surface of Ti3 SiC2 [5]. It should be noted that the conditions allowing the formation of lubricious oxides on frictional surface are limited to either high sliding speed or high temperature. Therefore, it is essential to fabricate self-lubricating surfaces on carbides. Carbon-based coatings, e.g. amorphous carbon and diamond like carbon (DLC) coating, have been used to improve the tribological
∗ Corresponding author. Tel.: +86 931 4968198; fax: +86 931 8277088. E-mail address: [email protected] (J. Lu). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.11.015
performance in various environments for sliding parts, including advanced ceramics [6]. An alternative approach to improve the tribological behavior of carbide ceramic is carbide-derived carbon (CDC) process [7–9]. The CDC coating can be obtained by transforming carbide to carbon layer in chlorine or a chlorine-containing atmosphere at elevated temperature and is thought to have several advantages over DLC coatings, e.g. high growth rate, good coating adherence, unlimited thickness, and low residual stress [9,10]. Binary and ternary carbides can be chlorinated at elevated temperature to fabricate a CDC coating on the surface [11,12]. In the CDC process, because CCl4 is thermodynamically unstable at high temperature (e.g. 1000 ◦ C), the role of Cl2 gas is to selectively etch Si or metal in the carbides and produce gaseous chlorides of Si and metals. In addition, since the carbide lattice is used as a template and metal is extracted layer by layer, atomic-level control can be achieved in the CDC process [10]. As binary carbides (e.g. SiC) and ternary carbides (e.g. Ti3 SiC2 ) have different crystal structures and microstructures, they can serve as different templates for the CDC coatings of different microstructures (arrangement of carbon atoms, grain shape, etc.) and thereby different properties (e.g. tribological property). The first topic of this paper is to prepare the CDC coatings on carbides, and investigate the friction and wear of the CDC coatings. From the viewpoint of materials science, both binary and ternary carbides should be included and hence typical binary and ternary carbides, i.e. SiC and Ti3 SiC2 , were selected. Another significance
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
of this material selection is that SiC and Ti3 SiC2 are two different templates for the CDC coatings. Comparison on the tribological behavior of the two CDC coatings is another topic of this paper. A ball-on-block (disk) configuration is widely accepted to evaluate the tribological performance of a carbon-based coating/film. Using this tribo-system in this paper will allow a comparison of experimental data with published data. The balls used in the published paper are made of steel, Al2 O3 and Si3 N4 . For a DLC coating (hardness from tens to several tens GPa), a hard counterpart material is preferred. It was found that the steel ball suffered from chemical attack of adsorbed Cl2 in the CDC coating and resulted in high friction and wear [13]. The inert materials (e.g. Al2 O3 and Si3 N4 ) are highly preferred to eliminate tribo-corrosion. On the other hand, the availability of the material should also be considered. In this paper, Si3 N4 balls are selected.
1975
SiC (1.53 m). To make the surface roughness of the two samples comparable, the CDC coating on Ti3 SiC2 was carefully ground to Ra = 1.46 m before tribological tests. After grinding, the thickness of the CDC coating on Ti3 SiC2 was around 210 m. 2.3. Characterization Micro-Raman measurement was performed on Raman Scope System 2000 Spectrometer (Renishaw, UK) equipped with an Ar laser with a wavelength of 514.5 nm and an optical microscope for focusing the incident laser beam. Scanning electron microscopy (SEM, JEOL JSM-5600LV) was employed to investigate the microstructure of the CDC coatings and morphologies of the worn surfaces and wear debris. 3. Results and discussion
2. Experimental details
3.1. Friction and wear of the two CDC coatings under unlubricated condition
2.1. Materials and preparation ␣-SiC block fabricated by pressureless sintering was commercially available from Shanghai Institute of Ceramics, Chinese Academy of Sciences and was a single-phase ceramic. The density, mean grain size and porosity of SiC were 3.1 g/cm3 , 2.3 m and 6%, respectively. A Ti3 SiC2 disk was prepared by using an in situ hot pressing/solid–liquid reaction process described elsewhere [14]. Ti3 SiC2 was composed of Ti3 SiC2 and 3 wt.% TiC as the impurity. The density, mean grain size and porosity of Ti3 SiC2 were 4.49 g/cm3 , 6 m and 1%, respectively. Before chlorination, the SiC and Ti3 SiC2 samples were sectioned into 20 mm × 10 mm × 2 mm cubes, followed by grinding (diamond wheel) and polishing (diamond paste) to a surface roughness Ra of 0.02 m and 0.06 m, respectively. The apparatus and principle for synthesis of the CDC coating were described elsewhere [3]. The chlorination of SiC and Ti3 SiC2 samples was performed in the 5% Cl2 + Ar ambience at 1000 ◦ C for 5 h. Reactions involved in the chlorination are: SiC(s) + 2Cl2 (g) = SiCl4 (g) + C(s)
(1)
Ti3 SiC2 (s) + 8Cl2 (g) = 3TiCl4 (g) + SiCl4 (g) + 2C(s)
(2)
After chlorination, post-treatment of samples at 1000 ◦ C for 60 min with Ar gas at a flow rate of 50 sccm was conducted to remove the residual Cl2 adsorbed in the CDC sample. Finally, all samples were ultrasonically cleaned in an alcohol bath to remove small particles on the surface. The thickness of the two as received CDC coatings determined by cross-sectioned sample on scanning electron microscopy (SEM) was 100 m on SiC and 230 m on Ti3 SiC2 . 2.2. Friction and wear tests Unlubricated sliding friction and wear tests were performed on an UMT-2MT tribo-meter with a ball-on-block configuration at room temperature. This configuration provides a stable contact between the ball and block. In addition, it is easy to make observation (e.g. plastic flow, etc.) on the worn surface of the blocks. The relative humidity was 40–50%. A Si3 N4 ball made oscillating movement (with a 2.5 mm amplitude) on top of bare carbides (SiC and Ti3 SiC2 ) or CDC coatings. The Si3 N4 balls had a diameter of 3 mm and a surface roughness Ra of 0.02 m. The applied load varied from 5 to 30 N, and the sliding velocity was in the range of 2–20 Hz (corresponding to 0.02–0.2 m/s in linear speed). The sliding duration was 60 min for all tests unless otherwise stated. The friction coefficients were automatically recorded by the tribo-meter. The average surface roughness Ra of as received CDC coating on Ti3 SiC2 was ca. 5 m, which was higher than that of the CDC coating on
As seen in Table 1, the average friction coefficient of a bare SiC disk sliding against a Si3 N4 ball was around 0.40 at room temperature. And the average friction coefficient of a bare Ti3 SiC2 block sliding against a Si3 N4 ball was as high as 0.82. The wear rates of the bare SiC block and Si3 N4 ball were on the order of magnitude of 10−5 mm3 N−1 m−1 and this can be classified as moderate wear. However, severe wear occurred to the bare Ti3 SiC2 block based on two figures. Firstly, the wear was so high that a wear track with a depth of several millimeters was found after running for only 2 min. And secondly, the estimated wear rates of the bare Ti3 SiC2 block and Si3 N4 ball were on the order of magnitude of 10−2 mm3 N−1 m−1 . The two CDC coatings, however, were lubricious in sliding against a Si3 N4 ball, see Table 1. Friction coefficients of the CDC coating on Ti3 SiC2 against a Si3 N4 ball were between 0.20 and 0.30. The CDC coating on SiC showed better friction-reducing ability than that of the CDC coating on Ti3 SiC2 because the average friction coefficient was lower than 0.10. In addition, the wear rates of the two CDC coatings were much lower than that of the corresponding substrates. Most importantly, the wears of Si3 N4 balls in sliding against the two CDC coatings were too low to measure, see Table 1. This is very important improvement for an engineering application. The two CDC coatings were lubricious over a wide range of load and speed, see Fig. 1. Friction coefficients of the CDC coating on SiC sliding against Si3 N4 were as low as 0.08 at loads from 5 N to 30 N, see Fig. 1a. For the CDC coating on Ti3 SiC2 sliding against Si3 N4 , increasing load slightly increased friction coefficient. No data were given at load higher than 20 N for the CDC coating on Ti3 SiC2 . The reason was that the wear rates of the CDC coating on Ti3 SiC2 were very high and friction coefficients were high and unstable. For the two CDC coatings, increasing sliding speed can reduce friction coefficient, see Fig. 1b. Typical frictional traces of the two CDC coatings were plotted to show the fluctuation of friction coefficient with sliding duration, see Fig. 1c. The frictional trace of the CDC coating on SiC was smooth. In summary, self-lubricating surfaces on SiC and Ti3 SiC2 can be prepared by chlorination at 1000 ◦ C. 3.2. Comparison on the tribological behaviors of the two CDC coatings Although the two CDC coatings were lubricious, the tribological behaviors of the two CDC coatings were different. Comparison on the tribological behaviors of CDC coatings on Ti3 SiC2 and SiC was made on three aspects, i.e. friction coefficient, wear rate and loadbearing capacity.
1976
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
Table 1 Friction and wear data obtained at 5 N and 0.02 m/s. Tribo-couple
Friction coefficient
Bare SiC against a Si3 N4 ball CDC at SiC against a Si3 N4 ball Bare Ti3 SiC2 against a Si3 N4 ball CDC at Ti3 SiC2 against a Si3 N4 ball
0.40 0.08 0.82 0.21
± ± ± ±
0.12 0.01 0.21 0.03
Wear ratea (mm3 N−1 m−1 ) Block
Si3 N4 ball
10−5 Too low to measure 10−2 10−5
10−5 Too low to measure 10−2 Too low to measure
a Because the duration of test is different from one tribo-couple to another, the accurate data are not provided. The duration of bare Ti3 SiC2 against Si3 N4 was much less than 60 min.
As seen in Table 1 and Fig. 1, the friction coefficients of the CDC coating on Ti3 SiC2 were higher than that of the CDC coating on SiC. Meanwhile, a stronger scatter of the friction coefficients in case of the CDC coating on Ti3 SiC2 was observed, see Fig. 1c. It will be discussed in Section 3.3. The wear resistance of the CDC coating on SiC was excellent because the wear of the coating was too mild to measure. A good example is shown in Fig. 2c, in which the original craters (can be seen in Fig. 2a) can still be seen on the wear track. As a contrast, the wear track on the CDC coating on Ti3 SiC2 (Fig. 2d) was deep and wide (indicating an obvious wear) and had quite different morphology from that of the unworn surface (Fig. 2b). The load-bearing capacity of the CDC coating can be defined as the load which an abrupt increment of wear rate is detected. As load increased, the wear rates of the CDC coating on Ti3 SiC2 increased considerably. The wear of the CDC coating on Ti3 SiC2 at 20 N were
too severe to perform a 5-min test. Therefore, the load-bearing capacity of the CDC coating on Ti3 SiC2 was poor. The load-bearing capacity of the CDC coating on SiC, however, was much higher than that of the CDC coating on Ti3 SiC2 . 3.3. Microstructural effect 3.3.1. Microstructure of the two CDC coatings As seen in Section 3.2, the tribological behaviors of the two CDC coatings were quite different. It is very important to figure out the role of microstructure of the two CDC coatings. Generally, the three elements to describe the microstructure of the two CDC coatings were pore, grain (size and shape) and composition (carbon species). The morphologies of the pores of the two CDC coatings were basically different, see Fig. 3. The pores for the CDC coating on SiC were crater-like with diameters of both several
Fig. 1. Friction coefficient as a function of (a) contact load (sliding speed is fixed at 0.05 m/s) and (b) sliding speed (contact load is fixed at 5 N) of the two CDC coatings in sliding against a Si3 N4 ball in air. (c) Frictional traces of the two CDC coatings on SiC (load: 20 N, speed: 0.05 m/s) and Ti3 SiC2 (load: 5 N, speed: 0.05 m/s) in sliding against a Si3 N4 ball in air.
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
1977
Fig. 2. SEM micrographs of the CDC coating on SiC (a) the unworn surface and (c) the worn surfaces (the wear track was darker than the unworn area), the CDC coating on Ti3 SiC2 (b) the as ground surface and (d) the worn surfaces.
tens of micrometers and sub-micrometers (Fig. 3a). Namely, rough surface with porous structure at micro- and nanometer scales was the main characteristic of the CDC coating on SiC. However, cracks with sizes of both micro- and nano-scales, the latter was considered originating from cleavage of lamellar Ti3 SiC2 , were found in the CDC coating on Ti3 SiC2 . The grains of the CDC coating on SiC were obviously equiaxed. Sheets with a thickness of nanometers are seen in Fig. 3b. It is easy to understand from the crystal structure and grain of the two templates (SiC and Ti3 SiC2 ), because the rearrangement during the CDC process depends on the template. In a CDC process, after removal of non-carbon atoms (Si for SiC and Ti, Si for Ti3 SiC2 ), carbon from different carbides differs naturally in network structure [11,15]. Moreover, because of the different volumes of carbon atomics in carbide unit cells, the pore volumes of CDC from different carbides differ greatly from each other. The theoretical porosities of CDC derived from Ti3 SiC2 and -SiC are 75.5% and 57.2%, respectively [12]. This might be one of the factors resulted in different surface structure of carbon from Ti3 SiC2 and SiC. Raman spectrum of polycrystalline graphite showed a double-resonance effect: D-band at ∼1350 cm−1 and G-band at ∼1580 cm−1 . In many papers, Raman was always employed to investigate the effect of chlorination temperature on composition and microstructure of CDC. The same conclusion [12,16] that the graphitization of CDC increased with increasing chlorination temperature could be educed. The Raman analysis of CDC from Ti3 SiC2 and SiC showed two peaks corresponding to D and G bands (Fig. 4). As for CDC from SiC, the D-band was centered at around 1335 cm−1 , while G-band was at around 1593 cm−1 . The distance between positions of D-band and G-band was larger than that of microcrystalline graphite. In detail, the
D-band was downshifted, while G-band was upshifted. Moreover, the intensity of D-band was found to be stronger than that of G-bands, suggesting a nanocrystalline and disorder structure of CDC from SiC. Most differently, the intensity of G-band was much higher than that of D-band in spectrum of CDC from Ti3 SiC2 , showing high graphitization. The positions of D-band and G-band in Raman spectrum of Ti3 SiC2 were very close to that of microcrystalline graphite. The distance between positions of D-band and G-band was reduced compared with that of CDC from SiC. 3.3.2. Role of microstructure As seen in Table 1, the wear of Si3 N4 ball was too low to measure. In this connection, wear debris were mainly from the CDC coatings. Wear debris from the CDC coating on SiC was hardly collected. In addition, very mild material flow on the worn surface of the CDC coating on SiC was observed (Fig. 3c). As a contrast, plastic flow on the worn surface of the CDC coating on Ti3 SiC2 was evident (Fig. 3d) and in this way a large amount of fragments was generated and removed from the tribo-interface as wear debris, see Fig. 5. The wear debris in Fig. 5 and particles of the CDC coating on Ti3 SiC2 had similar shape. The nanocrystalline and disorder structure of CDC on SiC seemed to be more resistant to plastic flow than the microcrystalline graphite on Ti3 SiC2 . The weak bonding among CDC particles on Ti3 SiC2 was responsible for the weak resistance to plastic flow. Large amount of wear debris and plastic flow at the tribo-interface between the CDC coating on Ti3 SiC2 and Si3 N4 led to a unstable friction trace, see Fig. 1c. The role of pores on the friction and wear of the CDC coating was still unknown. It seems a tough job to uncover the role of pores
1978
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
Fig. 5. SEM micrograph of wear debris collected from the CDC coating on Ti3 SiC2 in sliding against Si3 N4 .
4. Conclusions The CDC coatings on SiC and Ti3 SiC2 were prepared by chlorination at 1000 ◦ C. The unlubricated friction and wear of the two CDC coatings in sliding against Si3 N4 were investigated on an UMT-2MT tribometer with a ball-on-block configuration at room temperature. Comparison on the tribological properties of the two CDC coatings was made. The microstructural effect of the CDC coating was discussed. The following conclusions can be drawn:
Fig. 3. SEM micrographs of the two CDC coatings on (a) SiC and (b) Ti3 SiC2 .
by experimental investigation because it is impossible to fabricate two kinds of CDC coatings on SiC and Ti3 SiC2 (one is porous and the other is low porous or free of porous). This is the limitation of this study.
(1) CDC coatings on SiC and Ti3 SiC2 were self-lubricating in sliding against a Si3 N4 ball. The wear of the Si3 N4 ball is not measurable. (2) The CDC coating on a SiC block took advantages over the CDC coating on a Ti3 SiC2 block in terms of friction coefficient, wear resistance and load-carrying capacity. The typical friction coefficient of the CDC coating on a SiC block was as low as 0.08, which was only ca. 40% of the friction coefficient of the CDC coating on a Ti3 SiC2 block. (3) The CDC coating on SiC had a nanocrystalline and disorder structure of carbon and excellent tribological behavior. The CDC coating on Ti3 SiC2 was microcrystalline graphite and subject to plastic flow at the tribo-interface and yielded high wear. Acknowledgements The authors were acknowledged the finical support from National Science Foundation of China (NSFC, 50675216). References
Fig. 4. Raman spectra of the two CDC coatings on SiC and Ti3 SiC2 .
[1] H.B. Zhang, Y.W. Bao, Y.C. Zhou, Current status in layered ternary carbide Ti3 SiC2 , a review, J. Mater. Sci. Technol. 25 (2009) 1–37. [2] Y. Gogotsi, R.A. Andrievski, Materials Science of Carbides Nitrides and Borides, Kluwer Academic Publishers, Dordrecht, 1999. [3] D. Sarkar, B. Basu, S.J. Cho, M.C. Chu, S.S. Hwang, S.W. Park, Tribological properties of Ti3 SiC2 , J. Am. Ceram. Soc. 88 (2005) 3245–3248. [4] S.F. Ren, J.H. Meng, J.J. Lu, S.R. Yang, Tribological behavior of Ti3 SiC2 sliding against Ni-based alloys at elevated temperatures, Tribol. Lett. 31 (2008) 129–137. [5] Z.Y. Huang, H.X. Zhai, M.L. Guan, X. Liu, M.X. Ai, Y. Zhou, Oxide-film-dependent tribological behaviors of Ti3 SiC2 , Wear 262 (2007) 1079–1085. [6] A. Erdemir, O. Eryilmaz, Superlubricity in diamond carbon films, in: A. Erdemir, J.-M. Martin (Eds.), Superlubricity, 2007, pp. 253–271. [7] Y.G. Gogotsi, I.-D. Jeon, M.J. McNallan, Carbon coatings on silicon carbide by reaction with chlorine-containing gases, J. Mater. Chem. 7 (1997) 1841–1848. [8] B. Carroll, Y. Gogotsi, A. Kovalchenko, A. Erdemir, M.J. McNallan, Effect of humidity on the tribological properties of carbide-derived carbon (CDC) films on silicon carbide, Tribol. Lett. 15 (2003) 51–55.
J. Sui, J. Lu / Wear 271 (2011) 1974–1979 [9] H.-J. Choi, H.-T. Bae, J.-K. Lee, B.-C. Na, M.J. McNallan, D.-S. Lim, Sliding wear of silicon carbide modified by etching with chlorine at various temperatures, Wear 266 (2009) 214–219. [10] A. Nikitin, Y. Gogotsi, Nanostructured carbide-derived carbon, Encyclopaedia Nanosci. Nanotechnol. 7 (2004) 553–573. [11] G.N. Yushin, E.N. Hoffman, A. Nikitin, H.H. Ye, M.W. Barsoum, Y. Gogotsi, Synthesis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide, Carbon 43 (2005) 2075–2082. [12] Y. Gogotsi, A. Nikitin, H. Ye, W. Zhou, J. Fischer, B. Yi, et al., Nanoporous carbide-derived carbon with tunable pore size, Nat. Mater. 2 (2003) 591– 594.
1979
[13] F. Gao, J.J. Lu, W.M. Liu, Tribological behavior of carbon-derived carbon coating on SiC polycrystal against SAE52100 steel in moderately humid air, Tribol. Lett. 27 (2007) 339–345. [14] J. Sui, Y.J. Zhang, S.F. Ren, M. Rinke, J.J. Lu, J.H. Jia, Carbon coating with combined super-hydrophobic and self-lubricating properties on titanium silicon carbide, Carbon 47 (2009) 629–634. [15] R. Dash, J. Chmiola, G. Yushin, Y. Gogotsi, G. Laudisio, J. Singer, J.S. Kucheyev, Titanium carbide derived nanoporous carbon for energy-related applications, Carbon 44 (2006) 2489–2497. [16] Z.G. Cambaz, G.N. Yushin, Y. Gogotsi, Formation of carbide-derived carbon on -silicon carbide whiskers, J. Am. Ceram. Soc. 89 (2006) 509–514.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Formulated self-lubricating carbon coatings on carbide ceramics J. Sui a,b , J. Lu a,∗ a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 19 November 2010 Accepted 21 November 2010
Keywords: Chlorination Carbide-derived carbon Self-lubricating Microstructure
a b s t r a c t SiC and Ti3 SiC2 are very important materials for many tribological applications. However, they are not lubricious under dry sliding. Chlorination was conducted at 1000 ◦ C for 5 h in a quartz tube to fabricate carbide-derived carbon (CDC) coatings on SiC and Ti3 SiC2 blocks. Unlubricated sliding friction and wear tests were conducted on a UMT-2MT tribometer with a ball-on-block configuration. Compared to bare SiC and Ti3 SiC2 blocks, SiC and Ti3 SiC2 blocks with the CDC coatings were self-lubricating in sliding against a Si3 N4 ball at room temperature. The wear of the Si3 N4 ball was too low to measure in sliding against the CDC coatings. In addition, the CDC coating on a SiC block took advantages over the CDC coating on a Ti3 SiC2 block in terms of friction coefficient, wear resistance and load-carrying capacity. The microstructure of the two CDC coatings was characterized by three aspects, i.e. pore size, grain size and shape, and composition (carbon species). The two CDC coatings are highly porous materials but of different grain size, grain shape and composition. The CDC coating on SiC was composed of nanocrystalline and disorder carbon of equiaxed grains while that on Ti3 SiC2 was composed of plate-like microcrystalline graphite. The microstructural effect on the tribological behavior of the CDC coatings on SiC and Ti3 SiC2 blocks was investigated and discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The importance of binary carbides (e.g. SiC, WC, and TiC) has already been well recognized. Ternary carbides (e.g. Ti3 SiC2 ) are also important materials but is less known. In recent years, Ti3 SiC2 has received considerable attention because of the unusual combination of metallic and ceramic properties [1]. On the one hand, it is relatively soft, easily machinable, electrically and thermally conductive, and behaves plastically at high temperature. On the other hand, it is oxidation resistant, has high melting point, and, most importantly, it maintains its strength at high temperature. Today, binary carbides and ternary carbides are also very important materials for many tribological applications [1,2]. However, these carbides do not feel self-lubricated under dry sliding [3,4]. Some researches revealed Ti3 SiC2 possessed excellent tribological properties due to the formation of tribo-induced oxides on the contact surface of Ti3 SiC2 [5]. It should be noted that the conditions allowing the formation of lubricious oxides on frictional surface are limited to either high sliding speed or high temperature. Therefore, it is essential to fabricate self-lubricating surfaces on carbides. Carbon-based coatings, e.g. amorphous carbon and diamond like carbon (DLC) coating, have been used to improve the tribological
∗ Corresponding author. Tel.: +86 931 4968198; fax: +86 931 8277088. E-mail address: [email protected] (J. Lu). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.11.015
performance in various environments for sliding parts, including advanced ceramics [6]. An alternative approach to improve the tribological behavior of carbide ceramic is carbide-derived carbon (CDC) process [7–9]. The CDC coating can be obtained by transforming carbide to carbon layer in chlorine or a chlorine-containing atmosphere at elevated temperature and is thought to have several advantages over DLC coatings, e.g. high growth rate, good coating adherence, unlimited thickness, and low residual stress [9,10]. Binary and ternary carbides can be chlorinated at elevated temperature to fabricate a CDC coating on the surface [11,12]. In the CDC process, because CCl4 is thermodynamically unstable at high temperature (e.g. 1000 ◦ C), the role of Cl2 gas is to selectively etch Si or metal in the carbides and produce gaseous chlorides of Si and metals. In addition, since the carbide lattice is used as a template and metal is extracted layer by layer, atomic-level control can be achieved in the CDC process [10]. As binary carbides (e.g. SiC) and ternary carbides (e.g. Ti3 SiC2 ) have different crystal structures and microstructures, they can serve as different templates for the CDC coatings of different microstructures (arrangement of carbon atoms, grain shape, etc.) and thereby different properties (e.g. tribological property). The first topic of this paper is to prepare the CDC coatings on carbides, and investigate the friction and wear of the CDC coatings. From the viewpoint of materials science, both binary and ternary carbides should be included and hence typical binary and ternary carbides, i.e. SiC and Ti3 SiC2 , were selected. Another significance
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
of this material selection is that SiC and Ti3 SiC2 are two different templates for the CDC coatings. Comparison on the tribological behavior of the two CDC coatings is another topic of this paper. A ball-on-block (disk) configuration is widely accepted to evaluate the tribological performance of a carbon-based coating/film. Using this tribo-system in this paper will allow a comparison of experimental data with published data. The balls used in the published paper are made of steel, Al2 O3 and Si3 N4 . For a DLC coating (hardness from tens to several tens GPa), a hard counterpart material is preferred. It was found that the steel ball suffered from chemical attack of adsorbed Cl2 in the CDC coating and resulted in high friction and wear [13]. The inert materials (e.g. Al2 O3 and Si3 N4 ) are highly preferred to eliminate tribo-corrosion. On the other hand, the availability of the material should also be considered. In this paper, Si3 N4 balls are selected.
1975
SiC (1.53 m). To make the surface roughness of the two samples comparable, the CDC coating on Ti3 SiC2 was carefully ground to Ra = 1.46 m before tribological tests. After grinding, the thickness of the CDC coating on Ti3 SiC2 was around 210 m. 2.3. Characterization Micro-Raman measurement was performed on Raman Scope System 2000 Spectrometer (Renishaw, UK) equipped with an Ar laser with a wavelength of 514.5 nm and an optical microscope for focusing the incident laser beam. Scanning electron microscopy (SEM, JEOL JSM-5600LV) was employed to investigate the microstructure of the CDC coatings and morphologies of the worn surfaces and wear debris. 3. Results and discussion
2. Experimental details
3.1. Friction and wear of the two CDC coatings under unlubricated condition
2.1. Materials and preparation ␣-SiC block fabricated by pressureless sintering was commercially available from Shanghai Institute of Ceramics, Chinese Academy of Sciences and was a single-phase ceramic. The density, mean grain size and porosity of SiC were 3.1 g/cm3 , 2.3 m and 6%, respectively. A Ti3 SiC2 disk was prepared by using an in situ hot pressing/solid–liquid reaction process described elsewhere [14]. Ti3 SiC2 was composed of Ti3 SiC2 and 3 wt.% TiC as the impurity. The density, mean grain size and porosity of Ti3 SiC2 were 4.49 g/cm3 , 6 m and 1%, respectively. Before chlorination, the SiC and Ti3 SiC2 samples were sectioned into 20 mm × 10 mm × 2 mm cubes, followed by grinding (diamond wheel) and polishing (diamond paste) to a surface roughness Ra of 0.02 m and 0.06 m, respectively. The apparatus and principle for synthesis of the CDC coating were described elsewhere [3]. The chlorination of SiC and Ti3 SiC2 samples was performed in the 5% Cl2 + Ar ambience at 1000 ◦ C for 5 h. Reactions involved in the chlorination are: SiC(s) + 2Cl2 (g) = SiCl4 (g) + C(s)
(1)
Ti3 SiC2 (s) + 8Cl2 (g) = 3TiCl4 (g) + SiCl4 (g) + 2C(s)
(2)
After chlorination, post-treatment of samples at 1000 ◦ C for 60 min with Ar gas at a flow rate of 50 sccm was conducted to remove the residual Cl2 adsorbed in the CDC sample. Finally, all samples were ultrasonically cleaned in an alcohol bath to remove small particles on the surface. The thickness of the two as received CDC coatings determined by cross-sectioned sample on scanning electron microscopy (SEM) was 100 m on SiC and 230 m on Ti3 SiC2 . 2.2. Friction and wear tests Unlubricated sliding friction and wear tests were performed on an UMT-2MT tribo-meter with a ball-on-block configuration at room temperature. This configuration provides a stable contact between the ball and block. In addition, it is easy to make observation (e.g. plastic flow, etc.) on the worn surface of the blocks. The relative humidity was 40–50%. A Si3 N4 ball made oscillating movement (with a 2.5 mm amplitude) on top of bare carbides (SiC and Ti3 SiC2 ) or CDC coatings. The Si3 N4 balls had a diameter of 3 mm and a surface roughness Ra of 0.02 m. The applied load varied from 5 to 30 N, and the sliding velocity was in the range of 2–20 Hz (corresponding to 0.02–0.2 m/s in linear speed). The sliding duration was 60 min for all tests unless otherwise stated. The friction coefficients were automatically recorded by the tribo-meter. The average surface roughness Ra of as received CDC coating on Ti3 SiC2 was ca. 5 m, which was higher than that of the CDC coating on
As seen in Table 1, the average friction coefficient of a bare SiC disk sliding against a Si3 N4 ball was around 0.40 at room temperature. And the average friction coefficient of a bare Ti3 SiC2 block sliding against a Si3 N4 ball was as high as 0.82. The wear rates of the bare SiC block and Si3 N4 ball were on the order of magnitude of 10−5 mm3 N−1 m−1 and this can be classified as moderate wear. However, severe wear occurred to the bare Ti3 SiC2 block based on two figures. Firstly, the wear was so high that a wear track with a depth of several millimeters was found after running for only 2 min. And secondly, the estimated wear rates of the bare Ti3 SiC2 block and Si3 N4 ball were on the order of magnitude of 10−2 mm3 N−1 m−1 . The two CDC coatings, however, were lubricious in sliding against a Si3 N4 ball, see Table 1. Friction coefficients of the CDC coating on Ti3 SiC2 against a Si3 N4 ball were between 0.20 and 0.30. The CDC coating on SiC showed better friction-reducing ability than that of the CDC coating on Ti3 SiC2 because the average friction coefficient was lower than 0.10. In addition, the wear rates of the two CDC coatings were much lower than that of the corresponding substrates. Most importantly, the wears of Si3 N4 balls in sliding against the two CDC coatings were too low to measure, see Table 1. This is very important improvement for an engineering application. The two CDC coatings were lubricious over a wide range of load and speed, see Fig. 1. Friction coefficients of the CDC coating on SiC sliding against Si3 N4 were as low as 0.08 at loads from 5 N to 30 N, see Fig. 1a. For the CDC coating on Ti3 SiC2 sliding against Si3 N4 , increasing load slightly increased friction coefficient. No data were given at load higher than 20 N for the CDC coating on Ti3 SiC2 . The reason was that the wear rates of the CDC coating on Ti3 SiC2 were very high and friction coefficients were high and unstable. For the two CDC coatings, increasing sliding speed can reduce friction coefficient, see Fig. 1b. Typical frictional traces of the two CDC coatings were plotted to show the fluctuation of friction coefficient with sliding duration, see Fig. 1c. The frictional trace of the CDC coating on SiC was smooth. In summary, self-lubricating surfaces on SiC and Ti3 SiC2 can be prepared by chlorination at 1000 ◦ C. 3.2. Comparison on the tribological behaviors of the two CDC coatings Although the two CDC coatings were lubricious, the tribological behaviors of the two CDC coatings were different. Comparison on the tribological behaviors of CDC coatings on Ti3 SiC2 and SiC was made on three aspects, i.e. friction coefficient, wear rate and loadbearing capacity.
1976
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
Table 1 Friction and wear data obtained at 5 N and 0.02 m/s. Tribo-couple
Friction coefficient
Bare SiC against a Si3 N4 ball CDC at SiC against a Si3 N4 ball Bare Ti3 SiC2 against a Si3 N4 ball CDC at Ti3 SiC2 against a Si3 N4 ball
0.40 0.08 0.82 0.21
± ± ± ±
0.12 0.01 0.21 0.03
Wear ratea (mm3 N−1 m−1 ) Block
Si3 N4 ball
10−5 Too low to measure 10−2 10−5
10−5 Too low to measure 10−2 Too low to measure
a Because the duration of test is different from one tribo-couple to another, the accurate data are not provided. The duration of bare Ti3 SiC2 against Si3 N4 was much less than 60 min.
As seen in Table 1 and Fig. 1, the friction coefficients of the CDC coating on Ti3 SiC2 were higher than that of the CDC coating on SiC. Meanwhile, a stronger scatter of the friction coefficients in case of the CDC coating on Ti3 SiC2 was observed, see Fig. 1c. It will be discussed in Section 3.3. The wear resistance of the CDC coating on SiC was excellent because the wear of the coating was too mild to measure. A good example is shown in Fig. 2c, in which the original craters (can be seen in Fig. 2a) can still be seen on the wear track. As a contrast, the wear track on the CDC coating on Ti3 SiC2 (Fig. 2d) was deep and wide (indicating an obvious wear) and had quite different morphology from that of the unworn surface (Fig. 2b). The load-bearing capacity of the CDC coating can be defined as the load which an abrupt increment of wear rate is detected. As load increased, the wear rates of the CDC coating on Ti3 SiC2 increased considerably. The wear of the CDC coating on Ti3 SiC2 at 20 N were
too severe to perform a 5-min test. Therefore, the load-bearing capacity of the CDC coating on Ti3 SiC2 was poor. The load-bearing capacity of the CDC coating on SiC, however, was much higher than that of the CDC coating on Ti3 SiC2 . 3.3. Microstructural effect 3.3.1. Microstructure of the two CDC coatings As seen in Section 3.2, the tribological behaviors of the two CDC coatings were quite different. It is very important to figure out the role of microstructure of the two CDC coatings. Generally, the three elements to describe the microstructure of the two CDC coatings were pore, grain (size and shape) and composition (carbon species). The morphologies of the pores of the two CDC coatings were basically different, see Fig. 3. The pores for the CDC coating on SiC were crater-like with diameters of both several
Fig. 1. Friction coefficient as a function of (a) contact load (sliding speed is fixed at 0.05 m/s) and (b) sliding speed (contact load is fixed at 5 N) of the two CDC coatings in sliding against a Si3 N4 ball in air. (c) Frictional traces of the two CDC coatings on SiC (load: 20 N, speed: 0.05 m/s) and Ti3 SiC2 (load: 5 N, speed: 0.05 m/s) in sliding against a Si3 N4 ball in air.
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
1977
Fig. 2. SEM micrographs of the CDC coating on SiC (a) the unworn surface and (c) the worn surfaces (the wear track was darker than the unworn area), the CDC coating on Ti3 SiC2 (b) the as ground surface and (d) the worn surfaces.
tens of micrometers and sub-micrometers (Fig. 3a). Namely, rough surface with porous structure at micro- and nanometer scales was the main characteristic of the CDC coating on SiC. However, cracks with sizes of both micro- and nano-scales, the latter was considered originating from cleavage of lamellar Ti3 SiC2 , were found in the CDC coating on Ti3 SiC2 . The grains of the CDC coating on SiC were obviously equiaxed. Sheets with a thickness of nanometers are seen in Fig. 3b. It is easy to understand from the crystal structure and grain of the two templates (SiC and Ti3 SiC2 ), because the rearrangement during the CDC process depends on the template. In a CDC process, after removal of non-carbon atoms (Si for SiC and Ti, Si for Ti3 SiC2 ), carbon from different carbides differs naturally in network structure [11,15]. Moreover, because of the different volumes of carbon atomics in carbide unit cells, the pore volumes of CDC from different carbides differ greatly from each other. The theoretical porosities of CDC derived from Ti3 SiC2 and -SiC are 75.5% and 57.2%, respectively [12]. This might be one of the factors resulted in different surface structure of carbon from Ti3 SiC2 and SiC. Raman spectrum of polycrystalline graphite showed a double-resonance effect: D-band at ∼1350 cm−1 and G-band at ∼1580 cm−1 . In many papers, Raman was always employed to investigate the effect of chlorination temperature on composition and microstructure of CDC. The same conclusion [12,16] that the graphitization of CDC increased with increasing chlorination temperature could be educed. The Raman analysis of CDC from Ti3 SiC2 and SiC showed two peaks corresponding to D and G bands (Fig. 4). As for CDC from SiC, the D-band was centered at around 1335 cm−1 , while G-band was at around 1593 cm−1 . The distance between positions of D-band and G-band was larger than that of microcrystalline graphite. In detail, the
D-band was downshifted, while G-band was upshifted. Moreover, the intensity of D-band was found to be stronger than that of G-bands, suggesting a nanocrystalline and disorder structure of CDC from SiC. Most differently, the intensity of G-band was much higher than that of D-band in spectrum of CDC from Ti3 SiC2 , showing high graphitization. The positions of D-band and G-band in Raman spectrum of Ti3 SiC2 were very close to that of microcrystalline graphite. The distance between positions of D-band and G-band was reduced compared with that of CDC from SiC. 3.3.2. Role of microstructure As seen in Table 1, the wear of Si3 N4 ball was too low to measure. In this connection, wear debris were mainly from the CDC coatings. Wear debris from the CDC coating on SiC was hardly collected. In addition, very mild material flow on the worn surface of the CDC coating on SiC was observed (Fig. 3c). As a contrast, plastic flow on the worn surface of the CDC coating on Ti3 SiC2 was evident (Fig. 3d) and in this way a large amount of fragments was generated and removed from the tribo-interface as wear debris, see Fig. 5. The wear debris in Fig. 5 and particles of the CDC coating on Ti3 SiC2 had similar shape. The nanocrystalline and disorder structure of CDC on SiC seemed to be more resistant to plastic flow than the microcrystalline graphite on Ti3 SiC2 . The weak bonding among CDC particles on Ti3 SiC2 was responsible for the weak resistance to plastic flow. Large amount of wear debris and plastic flow at the tribo-interface between the CDC coating on Ti3 SiC2 and Si3 N4 led to a unstable friction trace, see Fig. 1c. The role of pores on the friction and wear of the CDC coating was still unknown. It seems a tough job to uncover the role of pores
1978
J. Sui, J. Lu / Wear 271 (2011) 1974–1979
Fig. 5. SEM micrograph of wear debris collected from the CDC coating on Ti3 SiC2 in sliding against Si3 N4 .
4. Conclusions The CDC coatings on SiC and Ti3 SiC2 were prepared by chlorination at 1000 ◦ C. The unlubricated friction and wear of the two CDC coatings in sliding against Si3 N4 were investigated on an UMT-2MT tribometer with a ball-on-block configuration at room temperature. Comparison on the tribological properties of the two CDC coatings was made. The microstructural effect of the CDC coating was discussed. The following conclusions can be drawn:
Fig. 3. SEM micrographs of the two CDC coatings on (a) SiC and (b) Ti3 SiC2 .
by experimental investigation because it is impossible to fabricate two kinds of CDC coatings on SiC and Ti3 SiC2 (one is porous and the other is low porous or free of porous). This is the limitation of this study.
(1) CDC coatings on SiC and Ti3 SiC2 were self-lubricating in sliding against a Si3 N4 ball. The wear of the Si3 N4 ball is not measurable. (2) The CDC coating on a SiC block took advantages over the CDC coating on a Ti3 SiC2 block in terms of friction coefficient, wear resistance and load-carrying capacity. The typical friction coefficient of the CDC coating on a SiC block was as low as 0.08, which was only ca. 40% of the friction coefficient of the CDC coating on a Ti3 SiC2 block. (3) The CDC coating on SiC had a nanocrystalline and disorder structure of carbon and excellent tribological behavior. The CDC coating on Ti3 SiC2 was microcrystalline graphite and subject to plastic flow at the tribo-interface and yielded high wear. Acknowledgements The authors were acknowledged the finical support from National Science Foundation of China (NSFC, 50675216). References
Fig. 4. Raman spectra of the two CDC coatings on SiC and Ti3 SiC2 .
[1] H.B. Zhang, Y.W. Bao, Y.C. Zhou, Current status in layered ternary carbide Ti3 SiC2 , a review, J. Mater. Sci. Technol. 25 (2009) 1–37. [2] Y. Gogotsi, R.A. Andrievski, Materials Science of Carbides Nitrides and Borides, Kluwer Academic Publishers, Dordrecht, 1999. [3] D. Sarkar, B. Basu, S.J. Cho, M.C. Chu, S.S. Hwang, S.W. Park, Tribological properties of Ti3 SiC2 , J. Am. Ceram. Soc. 88 (2005) 3245–3248. [4] S.F. Ren, J.H. Meng, J.J. Lu, S.R. Yang, Tribological behavior of Ti3 SiC2 sliding against Ni-based alloys at elevated temperatures, Tribol. Lett. 31 (2008) 129–137. [5] Z.Y. Huang, H.X. Zhai, M.L. Guan, X. Liu, M.X. Ai, Y. Zhou, Oxide-film-dependent tribological behaviors of Ti3 SiC2 , Wear 262 (2007) 1079–1085. [6] A. Erdemir, O. Eryilmaz, Superlubricity in diamond carbon films, in: A. Erdemir, J.-M. Martin (Eds.), Superlubricity, 2007, pp. 253–271. [7] Y.G. Gogotsi, I.-D. Jeon, M.J. McNallan, Carbon coatings on silicon carbide by reaction with chlorine-containing gases, J. Mater. Chem. 7 (1997) 1841–1848. [8] B. Carroll, Y. Gogotsi, A. Kovalchenko, A. Erdemir, M.J. McNallan, Effect of humidity on the tribological properties of carbide-derived carbon (CDC) films on silicon carbide, Tribol. Lett. 15 (2003) 51–55.
J. Sui, J. Lu / Wear 271 (2011) 1974–1979 [9] H.-J. Choi, H.-T. Bae, J.-K. Lee, B.-C. Na, M.J. McNallan, D.-S. Lim, Sliding wear of silicon carbide modified by etching with chlorine at various temperatures, Wear 266 (2009) 214–219. [10] A. Nikitin, Y. Gogotsi, Nanostructured carbide-derived carbon, Encyclopaedia Nanosci. Nanotechnol. 7 (2004) 553–573. [11] G.N. Yushin, E.N. Hoffman, A. Nikitin, H.H. Ye, M.W. Barsoum, Y. Gogotsi, Synthesis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide, Carbon 43 (2005) 2075–2082. [12] Y. Gogotsi, A. Nikitin, H. Ye, W. Zhou, J. Fischer, B. Yi, et al., Nanoporous carbide-derived carbon with tunable pore size, Nat. Mater. 2 (2003) 591– 594.
1979
[13] F. Gao, J.J. Lu, W.M. Liu, Tribological behavior of carbon-derived carbon coating on SiC polycrystal against SAE52100 steel in moderately humid air, Tribol. Lett. 27 (2007) 339–345. [14] J. Sui, Y.J. Zhang, S.F. Ren, M. Rinke, J.J. Lu, J.H. Jia, Carbon coating with combined super-hydrophobic and self-lubricating properties on titanium silicon carbide, Carbon 47 (2009) 629–634. [15] R. Dash, J. Chmiola, G. Yushin, Y. Gogotsi, G. Laudisio, J. Singer, J.S. Kucheyev, Titanium carbide derived nanoporous carbon for energy-related applications, Carbon 44 (2006) 2489–2497. [16] Z.G. Cambaz, G.N. Yushin, Y. Gogotsi, Formation of carbide-derived carbon on -silicon carbide whiskers, J. Am. Ceram. Soc. 89 (2006) 509–514.