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Tribological properties of Ti3SiC2 coupled with different counterfaces
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Tribological properties of Ti3SiC2 coupled with different counterfaces Yuanyuan Zhua,b, Aiguo Zhoua,n, Yiqiu Jib, Jin Jiaa, Libo Wanga, Bin Wub, Qingfeng Zanb,nn a School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China Beijing Key Lab of Fine Ceramics, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
b
Received 27 December 2014; received in revised form 29 January 2015; accepted 29 January 2015
Abstract This paper reports the dry-sliding tribological properties of Ti3SiC2 at room temperature in air, coupled with different counterfaces, including Ti3SiC2, Al2O3, Si3N4, SiC, and GCr15-bearing-steel. Ti3SiC2 exhibited obviously different tribological properties with different sliding counterfaces. The lowest friction coefficient (0.43) and wear rate (2.09 10 4 mm3/Nm) were obtained in the Ti3SiC2/SiC friction pair. Increased friction coefficient (0.63) and wear rate (3.67 10 4 mm3/Nm) were observed if Ti3SiC2 slides against GCr15-bearing-steel. The highest friction coefficient (1.30) was observed in Ti3SiC2/Al2O3 friction pair and the highest wear rate (1.87 10 3 mm3/Nm) was observed in Ti3SiC2/Ti3SiC2 friction pair. Scanning electron microscopy and X-ray photoelectron spectroscopy showed two main wear mechanisms: mechanical wear and oxidation wear. Mechanical wear was the main mechanism for sliding against Ti3SiC2, Si3N4, or Al2O3. Grain removal was a significant tribological character of self-mated Ti3SiC2 friction pair. For Ti3SiC2/SiC friction pair, oxide wear played a more important role and more oxides were formed than those in other friction pairs. Oxide films protected the surface of Ti3SiC2/SiC friction pair from direct contact, and decreased wear rate and friction coefficient. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Friction; Ti3SiC2; Counterface; Wear
1. Introduction Titanium silicon carbide (Ti3SiC2) is a layered ternary compound that belongs to MAX phases; here, M is a transition metal, A is an A-group element, and X is carbon and/or nitrogen [1,2]. Ti3SiC2 combines the attributes of metals and ceramics, including low density, good thermal and electrical conductivity, good machinability, excellent oxidation resistance, and good mechanical properties at elevated temperatures [3–9]. All of these unique properties are attributed to the layered structure of Ti3SiC2, which is similar to that of graphite and MoS2 [10]. Thus, Ti3SiC2 can be used as a solid self-lubricating material with a low friction coefficient [11,12]. Myhra et al. [13] applied lateral force microscopy and reported that the friction coefficients of the basal planes n
Corresponding author. Tel.: þ86 391 3986936. Corresponding author. Tel.: þ 86 10 89796200. E-mail addresses: [email protected] (A. Zhou), [email protected] (Q. Zan). nn
of Ti3SiC2 range from 2 10 3 to 5 10 3 against a Si3N4 tip at loads of 0.15–0.19 N. Although Ti3SiC2 was supposed to have very low friction coefficient, in true engineering application, if coupled with other material, its tribological properties might not be very well under dry condition [12,14,15]. Zhang et al. [12] investigated the dry-sliding friction behaviors of Ti3SiC2 with an oscillating pin-on-flat tester and found that the friction coefficient of Ti3SiC2/Ti3SiC2 friction pair is fairly high (1.16–1.43), whereas that of the Ti3SiC2/diamond friction pair is as low as 0.06–0.1. Zhai et al. [16] found that a bulk Ti3SiC2 sliding against low carbon steel showed low friction coefficient of 0.27 and wear rate of 1.37 10 6 mm3/Nm for the sliding speed of 20 m/s and normal pressure of 0.8 MPa under dry condition. Therefore, for the true engineering application of this material, more work is still needed to understand the friction and wear behaviors. Many studies [14–21] showed that the tribological properties of Ti3SiC2 significantly vary under different test conditions, including temperature, sliding speed, applied load, atmosphere, and environment. Therefore, its tribological behavior is very sensitive to testing
http://dx.doi.org/10.1016/j.ceramint.2015.01.150 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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conditions. In our opinion, the influences of counterface materials are very important and need be further researched and fully understood. Another purpose of this paper is to find the possibility of applying this material in high-temperature gas-cooled reactors (HTRs). In this kind of reactors, due to high temperature and high radiation, only a few materials can meet the engineering requirement. Ti3SiC2 has excellent high-temperature properties and radiation-resistance properties, additionally, it has been proved to be a promising material applied in HTRs [22]. Ti3SiC2 should be considered as an ideal material for bearings in HTRs because of its excellent high-temperature and radiation-resistance properties as well as possible good tribological properties. The bearings in HTRs are used to reciprocally move graphite balls in low speed and short distance but long time. In order to verify this idea, we tested the tribological properties of Ti3SiC2 coupled with different materials in the simulated condition of bearings in reactors. In this paper, commercial Ti3SiC2 powders were used as raw materials and sintered by hot press in Ar atmosphere to prepare dense Ti3SiC2 bulks. The effects of five counterface materials (SiC, Si3N4, Al2O3, GCr15-bearing-steel, and Ti3SiC2) on the tribological behaviors of the bulk Ti3SiC2 were investigated. 2. Experimental procedures Polycrystalline Ti3SiC2 bulk ceramic was prepared by hotpressing. Commercial Ti3SiC2 powders (200 mesh, 99.0% purity, Beijing Hutongwangyi Trade Co., Ltd., China) were pre-compressed under 10 MPa in a BN-sprayed graphite die and then hot-pressed at 1400 1C under 30 MPa for 90 min in flowing Ar gas. The heating rate was 10 1C/min. The bulk density of hot-pressed specimens was measured using Archimedes' method. As-prepared specimen surfaces were ground and polished with diamond paste to obtain an average surface roughness of 0.1 μm prior to mechanical test. The samples were cut into rectangular bars with dimensions of 3 mm 4 mm 30 mm for flexural strength testing. Threepoint flexural strength was test at a crosshead speed of 0.5 mm/ min. The Vickers hardness of the samples was measured using a micro-hardness tester (AMH32, LECO Co., USA) operated with a load of 500 g and a dwell time of 15 s. This test was conducted 10 times, and the average value was reported. The phase composition of Ti3SiC2 samples was determined by an X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu Kα radiation at 40 kV, 40 mA, and scanning rate of 0.021/s. The fractured surface of the samples was observed with a field-emission scanning electron microscope (FESEM, LEO-1530, Germany). Friction and wear tests were carried out on an oscillating friction and wear tester (SRV-IV, Optimol, Germany) with ring-on-disc configurations for Ti3SiC2/Ti3SiC2 pair and ballon-disc contact configurations for other pairs. The disc (lower specimen) was made of Ti3SiC2 with the dimension of ∅24 mm 7.9 mm and surface roughness of Ra ¼ 0.1 mm. The ring (upper specimen) for Ti3SiC2/Ti3SiC2 pair was made of Ti3SiC2 with an internal diameter of 17 mm and an external
diameter of 20 mm. For other pairs, commercial SiC, Si3N4, Al2O3, and GCr15-bearing-steel balls with a diameter of 10 mm (upper specimen) were selected as counterface materials. The upper specimens were driven to reciprocate on a stationary disc at a frequency of 10 Hz, stroke of 1 mm, load of 34 N, duration of 30 min, and temperature of 27 1C in air. These experimental parameters were selected according to the practical working situation of the bearings in HTRs. Frequency and stoke were chosen to get a friction speed that was 4–5 times of the bearings speed in HTRs. The load of 34 N was chosen based on the highest stress ( 1.5 GPa) applied on the bearings. Friction coefficients were automatically measured and recorded using the computer system of the friction tester. The mass loss of the Ti3SiC2 discs was measured using an electronic balance (accuracy ¼ 10 5 g) after every friction process. Tests for every given condition were repeated three times, and the average value was used to evaluate data. The morphologies of the worn surfaces were analyzed by FESEM. Depth profiles across wear tracks were measured with a Form Talysurf instrument (PGI 1230, Taylor Hobson, UK). X-ray photoelectron spectroscopy (XPS) analysis was performed using a PHI Quantera SXM instrument with Al Kα radiation (energy ¼ 1486.6 eV). 3. Results Fig. 1 shows the XRD pattern of as-prepared Ti3SiC2 samples. According to PDF card 40-1132, all diffraction peaks correspond to Ti3SiC2 phase. Fig. 2 presents an SEM micrograph of the fractured surface of Ti3SiC2. This sample is almost fully dense and has typical layered structure of Ti3SiC2. The density, Vickers hardness, and flexural strength of the samples are listed in Table 1. The relative density is about 98% of the theoretical density (4.53 g/cm3). Fig. 3 depicts the curves of friction coefficient versus sliding time for Ti3SiC2 coupled with itself, SiC, Si3N4, Al2O3, and steel under 34 N for 30 min with a frequency of 10 Hz and a stroke of 1 mm. The friction coefficients begin at relatively low values, abruptly increase to high values, and then rapidly decline to steady values with increasing sliding time. The abrupt increase in friction coefficient at the initial stage of testing can
Fig. 1. XRD pattern of as-prepared Ti3SiC2 sample.
Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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Fig. 2. SEM micrograph of the fractured surface of Ti3SiC2 sample.
Table 1 Mechanical properties of as-prepared Ti3SiC2 ceramic. Sample
Density (g/cm3) Vickers hardness (GPa) Flexural strength (MPa)
Ti3SiC2 4.44
4.54
495
Fig. 3. Friction coefficient curves of Ti3SiC2 sliding against itself, SiC, Si3N4, Al2O3, and GCr15-bearing-steel under 34 N for 30 min with a frequency of 10 Hz and a stroke of 1 mm.
be attributed to the removal of surface contaminants by rubbing [12,23,24]. From Fig. 3, it can be concluded that the friction properties significantly depend on counterface materials. Ti3SiC2/SiC pair exhibits the lowest friction coefficient (0.43) after the breaking-in period (250 s), with a very small fluctuation in the stable period. In the stable period, the small fluctuation in friction coefficients can be ascribed to the plastic flow of stressed surface and decreases of stiffness during the drysliding test [24,25]. The friction pairs of Ti3SiC2/Ti3SiC2, Ti3SiC2/Si3N4, and Ti3SiC2/Al2O3 exhibit high friction coefficients of 1.08, 1.17, and 1.30, respectively. Relatively large fluctuations are observed during the sliding process of these pairs. The friction coefficient of the Ti3SiC2/steel pair is approximately 0.63. Depend on counterface materials, the five
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friction coefficients vary in a range of 0.8. It indicates that the friction coefficient is very sensitive to the nature of the counterface material under similar testing conditions. Fig. 4 presents the wear rates of Ti3SiC2 sliding against different counterfaces. Ti3SiC2 exhibits the highest wear rate of 1.87 10 3 mm3/Nm if it slides against itself. The wear rate of Ti3SiC2 decreases to 1.41 10 3 and 1.02 10 3 mm3/Nm if it slides against Al2O3 and Si3N4, respectively. The carbide exhibits lower wear rates of 2.09 10 4 and 3.67 10 4 mm3/Nm if it slides against SiC and steel, respectively. These results can be directly and clearly observed from the wear track profiles of Ti3SiC2 sliding against different counterfaces (Fig. 5). The SEM morphologies of the worn surfaces of Ti3SiC2 sliding against different counterfaces are shown in Fig. 6. Many potholes can be observed in Fig. 6a. It indicates that some Ti3SiC2 grains are removed from the worn surfaces of Ti3SiC2 sliding against itself. For Ti3SiC2 slides against Al2O3, a small number of potholes can be found on the worn surface, as shown in Fig. 6b. And the wear scar is relatively wide and deep (Fig. 5). These potholes in Fig. 6a and b consequentially increase the sliding resistance and thus produce a high friction coefficient. Similar results have been reported in previous
Fig. 4. Wear rates of Ti3SiC2 sliding against different counterfaces.
Fig. 5. Wear track profiles of Ti3SiC2 sliding against different counterfaces.
Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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studies [24]. The worn surface of Ti3SiC2 sliding against Si3N4 is relatively smooth, and a few potholes may be detected (Fig. 6c). On the worn surface of Ti3SiC2 sliding against SiC, some oxide particles are present (Fig. 6d). And the wear scar is relatively narrow and shallow (Fig. 5). Discontinuous patches and cracks can be detected on the worn surface of Ti3SiC2 sliding against steel (Fig. 6e). Elemental chemical states on the worn surfaces of Ti3SiC2 sliding against different counterfaces were analyzed through XPS, and the results are shown in Fig. 7. It is found that Ti and Si elements on the worn surfaces are partially oxidized. Fig. 7a shows the spectrum of Ti2p. At this figure, the peaks at 464.8 and 458.9 eV are assigned to TiO2 and the peak at 454.8 eV is attributed to Ti3SiC2. For worn surface against SiC, the peaks assigned to TiO2 are stronger than the peaks of other worn
surfaces. It indicates that the relative amount of TiO2 on the Ti3SiC2 worn surface against SiC is higher than that on other worn surfaces. Fig. 7b shows the spectrum of Si2p. The peak at 102.3 eV is assigned to SiOx, and the peak at 99.1 eV is attributed to Ti3SiC2. The peaks assigned to Ti3SiC2 on worn surfaces against steel and Al2O3 are relatively weaker than the peaks on other worn surfaces (Fig. 7a and b). Hence, these two worn surfaces have fewer Ti3SiC2. Fig. 7c shows the spectrum of Al2p on the worn surface of Ti3SiC2 sliding against Al2O3. The peak at 74.3 eV is assigned to Al2O3. Fig. 7d shows the spectrum of Fe2p on the worn surface of Ti3SiC2 sliding against steel. The peaks at 724.4 and 711.0 eV are assigned to Fe2O3. These results indicate that counterface materials are transferred to the worn surfaces of Ti3SiC2 and partially oxidized.
Fig. 6. SEM morphologies of the worn surface of Ti3SiC2 slid against (a) itself, (b) Al2O3, (c) Si3N4, (d) SiC, and (e) GCr15 bearing steel. Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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Fig. 7. XPS spectra of (a) Ti2p, (b) Si2p, (c) Al2p, and (d) Fe2p on the worn surfaces of Ti3SiC2 sliding against different counterfaces.
4. Discussion From aforementioned results, the tribological behaviors of Ti3SiC2 are strongly affected by counterface materials. According to previous reports [3,26], Ti3SiC2 can be oxidized to form dense adhesive and layered scales consisting of SiO2 and TiO2 on the surface at high temperatures in air. And, during the dry-sliding friction process in air, temperature is increased due to heat generated by friction. Thus Ti3SiC2 on the worn surface is oxidized. The oxide film formed on the worn surface consists of TiO2 and SiOx as shown by the XPS results in Fig. 7a and b. The oxide film can decrease friction coefficient and wear rate [27,28]. If Ti3SiC2 slides against itself, the TiO2 and SiO2 produced by friction are less than those of the other friction pairs (Fig. 7a and b). As a result, fewer oxide films are formed in the selfmated Ti3SiC2 couple, which results in a high friction coefficient and wear rate. And due to weak interfacial strength, Ti3SiC2 grains are easily removed to form rough worn surface. If coupling with Al2O3, the resulted wear scar is relatively wide and deep as shown in Fig. 5, and extensive wear debris is observed on the worn surfaces. Thus, the wear rate is high for the pair of Ti3SiC2 against Al2O3. The XPS results (Fig. 7) show the presence of mixed oxides of TiO2, SiOx, and Al2O3
on the worn surface of Ti3SiC2. Thus Al elements are transferred from the Al2O3 ball to the worn surface of Ti3SiC2. For Ti3SiC2/steel pair, discontinuous patches and cracks are present on the Ti3SiC2 worn surface (Fig. 6e) and the oxide film is comprised of TiO2, SiOx, and Fe2O3 (Fig. 7). The Fe2O3 detected on the worn surface of Ti3SiC2 through XPS is attributed to Fe elements that are transferred from steel ball and oxidized in air. The formation of microcracks on the surface layer may be due to shear stress under the ball during friction. Hence, moderate friction coefficients and wear rates are obtained in the Ti3SiC2/steel ball couple. Ti3SiC2/SiC friction pair demonstrates the lowest friction coefficient and wear rate after the breaking-in period. Before friction test, Ti3SiC2 grains in the bulk sample are randomly oriented. During friction test, however, grains with basal plane perpendicular to the sliding direction are easily fractured and removed due to the high hardness of SiC. After the breaking-in period, these grains are consumed and the surface layer of Ti3SiC2 sample has preferred orientation that most basal planes are parallel to the sliding direction, which have very low friction coefficient [11,13]. Hence, the friction coefficient decreases to as low as 0.43 on average. In addition, as shown in Fig. 7a and b, the peaks of TiO2 and SiOx are stronger than
Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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those of the other friction pairs. Therefore, the oxide content on the surface layer of worn surface against SiC is higher than that of other friction pairs. This also can be ascribed to the high hardness of SiC. Oxide films can be easily formed and preferably adhere onto the surface. These films protect the surface from direct contact between couples, thereby reducing friction coefficient and wear losses of Ti3SiC2. Overall, Ti3SiC2 exhibits the lowest wear rate and friction coefficient if it slides against SiC. 5. Conclusions Ti3SiC2 exhibits different tribological properties if it slides against different counterface materials. The friction pair of Ti3SiC2/SiC demonstrates the lowest friction coefficient of approximately 0.43, and Ti3SiC2/steel shows a moderate friction coefficient of 0.63. The Ti3SiC2/Ti3SiC2, Ti3SiC2/Si3N4, and Ti3SiC2/Al2O3 friction pairs present high friction coefficients of 1.08, 1.17, and 1.30, respectively. Ti3SiC2/SiC pair exhibits the lowest wear rate of 2.09 10 4 mm3/Nm. High wear rates are obtained if Ti3SiC2 slides against itself, Al2O3, and Si3N4. Mechanical wear is the main mechanism if Ti3SiC2 slides against Ti3SiC2, Si3N4, and Al2O3. Grain removal is a significant tribological character of self-mated Ti3SiC2. For Ti3SiC2/SiC friction pair, oxide wear played a more important role and more oxides were formed than those in other friction pairs. Oxide films protected the surfaces of the couples from direct contact, and decreased the wear rate and friction coefficient of Ti3SiC2. Acknowledgments This work was supported by National Natural Science Foundation of China (51205111, 51305226, 51472075), Plan for Scientific Innovation Talent of Henan Province (134100510008), Program for Innovative Research Team of Henan Polytechnic University (T2013-4), State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (KF201313). References [1] M.W. Barsoum, The M(N þ 1)AX(N) phases: a new class of solids; thermodynamically stable nanolaminates, Prog. Solid State Chem. 28 (2000) 201–281. [2] M.W. Barsoum, T. El-Raghy, The MAX phases: unique new carbide and nitride materials - ternary ceramics turn out to be surprisingly soft and machinable, yet also heat-tolerant, strong and lightweight, Am. Sci. 89 (2001) 334–343. [3] M.W. Barsoum, T. ElRaghy, L.U.J.T. Ogbuji, Oxidation of Ti3SiC2 in air, J. Electrochem. Soc. 144 (1997) 2508–2516. [4] T. ElRaghy, A. Zavaliangos, M.W. Barsoum, S.R. Kalidindi, Damage mechanisms around hardness indentations in Ti3SiC2, J. Am. Ceram. Soc. 80 (1997) 513–516. [5] I.M. Low, S.K. Lee, B.R. Lawn, M.W. Barsoum, Contact damage accumulation in Ti3SiC2, J. Am. Ceram. Soc. 81 (1998) 225–228.
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Tribological properties of Ti3SiC2 coupled with different counterfaces Yuanyuan Zhua,b, Aiguo Zhoua,n, Yiqiu Jib, Jin Jiaa, Libo Wanga, Bin Wub, Qingfeng Zanb,nn a School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China Beijing Key Lab of Fine Ceramics, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
b
Received 27 December 2014; received in revised form 29 January 2015; accepted 29 January 2015
Abstract This paper reports the dry-sliding tribological properties of Ti3SiC2 at room temperature in air, coupled with different counterfaces, including Ti3SiC2, Al2O3, Si3N4, SiC, and GCr15-bearing-steel. Ti3SiC2 exhibited obviously different tribological properties with different sliding counterfaces. The lowest friction coefficient (0.43) and wear rate (2.09 10 4 mm3/Nm) were obtained in the Ti3SiC2/SiC friction pair. Increased friction coefficient (0.63) and wear rate (3.67 10 4 mm3/Nm) were observed if Ti3SiC2 slides against GCr15-bearing-steel. The highest friction coefficient (1.30) was observed in Ti3SiC2/Al2O3 friction pair and the highest wear rate (1.87 10 3 mm3/Nm) was observed in Ti3SiC2/Ti3SiC2 friction pair. Scanning electron microscopy and X-ray photoelectron spectroscopy showed two main wear mechanisms: mechanical wear and oxidation wear. Mechanical wear was the main mechanism for sliding against Ti3SiC2, Si3N4, or Al2O3. Grain removal was a significant tribological character of self-mated Ti3SiC2 friction pair. For Ti3SiC2/SiC friction pair, oxide wear played a more important role and more oxides were formed than those in other friction pairs. Oxide films protected the surface of Ti3SiC2/SiC friction pair from direct contact, and decreased wear rate and friction coefficient. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Friction; Ti3SiC2; Counterface; Wear
1. Introduction Titanium silicon carbide (Ti3SiC2) is a layered ternary compound that belongs to MAX phases; here, M is a transition metal, A is an A-group element, and X is carbon and/or nitrogen [1,2]. Ti3SiC2 combines the attributes of metals and ceramics, including low density, good thermal and electrical conductivity, good machinability, excellent oxidation resistance, and good mechanical properties at elevated temperatures [3–9]. All of these unique properties are attributed to the layered structure of Ti3SiC2, which is similar to that of graphite and MoS2 [10]. Thus, Ti3SiC2 can be used as a solid self-lubricating material with a low friction coefficient [11,12]. Myhra et al. [13] applied lateral force microscopy and reported that the friction coefficients of the basal planes n
Corresponding author. Tel.: þ86 391 3986936. Corresponding author. Tel.: þ 86 10 89796200. E-mail addresses: [email protected] (A. Zhou), [email protected] (Q. Zan). nn
of Ti3SiC2 range from 2 10 3 to 5 10 3 against a Si3N4 tip at loads of 0.15–0.19 N. Although Ti3SiC2 was supposed to have very low friction coefficient, in true engineering application, if coupled with other material, its tribological properties might not be very well under dry condition [12,14,15]. Zhang et al. [12] investigated the dry-sliding friction behaviors of Ti3SiC2 with an oscillating pin-on-flat tester and found that the friction coefficient of Ti3SiC2/Ti3SiC2 friction pair is fairly high (1.16–1.43), whereas that of the Ti3SiC2/diamond friction pair is as low as 0.06–0.1. Zhai et al. [16] found that a bulk Ti3SiC2 sliding against low carbon steel showed low friction coefficient of 0.27 and wear rate of 1.37 10 6 mm3/Nm for the sliding speed of 20 m/s and normal pressure of 0.8 MPa under dry condition. Therefore, for the true engineering application of this material, more work is still needed to understand the friction and wear behaviors. Many studies [14–21] showed that the tribological properties of Ti3SiC2 significantly vary under different test conditions, including temperature, sliding speed, applied load, atmosphere, and environment. Therefore, its tribological behavior is very sensitive to testing
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Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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conditions. In our opinion, the influences of counterface materials are very important and need be further researched and fully understood. Another purpose of this paper is to find the possibility of applying this material in high-temperature gas-cooled reactors (HTRs). In this kind of reactors, due to high temperature and high radiation, only a few materials can meet the engineering requirement. Ti3SiC2 has excellent high-temperature properties and radiation-resistance properties, additionally, it has been proved to be a promising material applied in HTRs [22]. Ti3SiC2 should be considered as an ideal material for bearings in HTRs because of its excellent high-temperature and radiation-resistance properties as well as possible good tribological properties. The bearings in HTRs are used to reciprocally move graphite balls in low speed and short distance but long time. In order to verify this idea, we tested the tribological properties of Ti3SiC2 coupled with different materials in the simulated condition of bearings in reactors. In this paper, commercial Ti3SiC2 powders were used as raw materials and sintered by hot press in Ar atmosphere to prepare dense Ti3SiC2 bulks. The effects of five counterface materials (SiC, Si3N4, Al2O3, GCr15-bearing-steel, and Ti3SiC2) on the tribological behaviors of the bulk Ti3SiC2 were investigated. 2. Experimental procedures Polycrystalline Ti3SiC2 bulk ceramic was prepared by hotpressing. Commercial Ti3SiC2 powders (200 mesh, 99.0% purity, Beijing Hutongwangyi Trade Co., Ltd., China) were pre-compressed under 10 MPa in a BN-sprayed graphite die and then hot-pressed at 1400 1C under 30 MPa for 90 min in flowing Ar gas. The heating rate was 10 1C/min. The bulk density of hot-pressed specimens was measured using Archimedes' method. As-prepared specimen surfaces were ground and polished with diamond paste to obtain an average surface roughness of 0.1 μm prior to mechanical test. The samples were cut into rectangular bars with dimensions of 3 mm 4 mm 30 mm for flexural strength testing. Threepoint flexural strength was test at a crosshead speed of 0.5 mm/ min. The Vickers hardness of the samples was measured using a micro-hardness tester (AMH32, LECO Co., USA) operated with a load of 500 g and a dwell time of 15 s. This test was conducted 10 times, and the average value was reported. The phase composition of Ti3SiC2 samples was determined by an X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu Kα radiation at 40 kV, 40 mA, and scanning rate of 0.021/s. The fractured surface of the samples was observed with a field-emission scanning electron microscope (FESEM, LEO-1530, Germany). Friction and wear tests were carried out on an oscillating friction and wear tester (SRV-IV, Optimol, Germany) with ring-on-disc configurations for Ti3SiC2/Ti3SiC2 pair and ballon-disc contact configurations for other pairs. The disc (lower specimen) was made of Ti3SiC2 with the dimension of ∅24 mm 7.9 mm and surface roughness of Ra ¼ 0.1 mm. The ring (upper specimen) for Ti3SiC2/Ti3SiC2 pair was made of Ti3SiC2 with an internal diameter of 17 mm and an external
diameter of 20 mm. For other pairs, commercial SiC, Si3N4, Al2O3, and GCr15-bearing-steel balls with a diameter of 10 mm (upper specimen) were selected as counterface materials. The upper specimens were driven to reciprocate on a stationary disc at a frequency of 10 Hz, stroke of 1 mm, load of 34 N, duration of 30 min, and temperature of 27 1C in air. These experimental parameters were selected according to the practical working situation of the bearings in HTRs. Frequency and stoke were chosen to get a friction speed that was 4–5 times of the bearings speed in HTRs. The load of 34 N was chosen based on the highest stress ( 1.5 GPa) applied on the bearings. Friction coefficients were automatically measured and recorded using the computer system of the friction tester. The mass loss of the Ti3SiC2 discs was measured using an electronic balance (accuracy ¼ 10 5 g) after every friction process. Tests for every given condition were repeated three times, and the average value was used to evaluate data. The morphologies of the worn surfaces were analyzed by FESEM. Depth profiles across wear tracks were measured with a Form Talysurf instrument (PGI 1230, Taylor Hobson, UK). X-ray photoelectron spectroscopy (XPS) analysis was performed using a PHI Quantera SXM instrument with Al Kα radiation (energy ¼ 1486.6 eV). 3. Results Fig. 1 shows the XRD pattern of as-prepared Ti3SiC2 samples. According to PDF card 40-1132, all diffraction peaks correspond to Ti3SiC2 phase. Fig. 2 presents an SEM micrograph of the fractured surface of Ti3SiC2. This sample is almost fully dense and has typical layered structure of Ti3SiC2. The density, Vickers hardness, and flexural strength of the samples are listed in Table 1. The relative density is about 98% of the theoretical density (4.53 g/cm3). Fig. 3 depicts the curves of friction coefficient versus sliding time for Ti3SiC2 coupled with itself, SiC, Si3N4, Al2O3, and steel under 34 N for 30 min with a frequency of 10 Hz and a stroke of 1 mm. The friction coefficients begin at relatively low values, abruptly increase to high values, and then rapidly decline to steady values with increasing sliding time. The abrupt increase in friction coefficient at the initial stage of testing can
Fig. 1. XRD pattern of as-prepared Ti3SiC2 sample.
Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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Fig. 2. SEM micrograph of the fractured surface of Ti3SiC2 sample.
Table 1 Mechanical properties of as-prepared Ti3SiC2 ceramic. Sample
Density (g/cm3) Vickers hardness (GPa) Flexural strength (MPa)
Ti3SiC2 4.44
4.54
495
Fig. 3. Friction coefficient curves of Ti3SiC2 sliding against itself, SiC, Si3N4, Al2O3, and GCr15-bearing-steel under 34 N for 30 min with a frequency of 10 Hz and a stroke of 1 mm.
be attributed to the removal of surface contaminants by rubbing [12,23,24]. From Fig. 3, it can be concluded that the friction properties significantly depend on counterface materials. Ti3SiC2/SiC pair exhibits the lowest friction coefficient (0.43) after the breaking-in period (250 s), with a very small fluctuation in the stable period. In the stable period, the small fluctuation in friction coefficients can be ascribed to the plastic flow of stressed surface and decreases of stiffness during the drysliding test [24,25]. The friction pairs of Ti3SiC2/Ti3SiC2, Ti3SiC2/Si3N4, and Ti3SiC2/Al2O3 exhibit high friction coefficients of 1.08, 1.17, and 1.30, respectively. Relatively large fluctuations are observed during the sliding process of these pairs. The friction coefficient of the Ti3SiC2/steel pair is approximately 0.63. Depend on counterface materials, the five
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friction coefficients vary in a range of 0.8. It indicates that the friction coefficient is very sensitive to the nature of the counterface material under similar testing conditions. Fig. 4 presents the wear rates of Ti3SiC2 sliding against different counterfaces. Ti3SiC2 exhibits the highest wear rate of 1.87 10 3 mm3/Nm if it slides against itself. The wear rate of Ti3SiC2 decreases to 1.41 10 3 and 1.02 10 3 mm3/Nm if it slides against Al2O3 and Si3N4, respectively. The carbide exhibits lower wear rates of 2.09 10 4 and 3.67 10 4 mm3/Nm if it slides against SiC and steel, respectively. These results can be directly and clearly observed from the wear track profiles of Ti3SiC2 sliding against different counterfaces (Fig. 5). The SEM morphologies of the worn surfaces of Ti3SiC2 sliding against different counterfaces are shown in Fig. 6. Many potholes can be observed in Fig. 6a. It indicates that some Ti3SiC2 grains are removed from the worn surfaces of Ti3SiC2 sliding against itself. For Ti3SiC2 slides against Al2O3, a small number of potholes can be found on the worn surface, as shown in Fig. 6b. And the wear scar is relatively wide and deep (Fig. 5). These potholes in Fig. 6a and b consequentially increase the sliding resistance and thus produce a high friction coefficient. Similar results have been reported in previous
Fig. 4. Wear rates of Ti3SiC2 sliding against different counterfaces.
Fig. 5. Wear track profiles of Ti3SiC2 sliding against different counterfaces.
Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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studies [24]. The worn surface of Ti3SiC2 sliding against Si3N4 is relatively smooth, and a few potholes may be detected (Fig. 6c). On the worn surface of Ti3SiC2 sliding against SiC, some oxide particles are present (Fig. 6d). And the wear scar is relatively narrow and shallow (Fig. 5). Discontinuous patches and cracks can be detected on the worn surface of Ti3SiC2 sliding against steel (Fig. 6e). Elemental chemical states on the worn surfaces of Ti3SiC2 sliding against different counterfaces were analyzed through XPS, and the results are shown in Fig. 7. It is found that Ti and Si elements on the worn surfaces are partially oxidized. Fig. 7a shows the spectrum of Ti2p. At this figure, the peaks at 464.8 and 458.9 eV are assigned to TiO2 and the peak at 454.8 eV is attributed to Ti3SiC2. For worn surface against SiC, the peaks assigned to TiO2 are stronger than the peaks of other worn
surfaces. It indicates that the relative amount of TiO2 on the Ti3SiC2 worn surface against SiC is higher than that on other worn surfaces. Fig. 7b shows the spectrum of Si2p. The peak at 102.3 eV is assigned to SiOx, and the peak at 99.1 eV is attributed to Ti3SiC2. The peaks assigned to Ti3SiC2 on worn surfaces against steel and Al2O3 are relatively weaker than the peaks on other worn surfaces (Fig. 7a and b). Hence, these two worn surfaces have fewer Ti3SiC2. Fig. 7c shows the spectrum of Al2p on the worn surface of Ti3SiC2 sliding against Al2O3. The peak at 74.3 eV is assigned to Al2O3. Fig. 7d shows the spectrum of Fe2p on the worn surface of Ti3SiC2 sliding against steel. The peaks at 724.4 and 711.0 eV are assigned to Fe2O3. These results indicate that counterface materials are transferred to the worn surfaces of Ti3SiC2 and partially oxidized.
Fig. 6. SEM morphologies of the worn surface of Ti3SiC2 slid against (a) itself, (b) Al2O3, (c) Si3N4, (d) SiC, and (e) GCr15 bearing steel. Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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Fig. 7. XPS spectra of (a) Ti2p, (b) Si2p, (c) Al2p, and (d) Fe2p on the worn surfaces of Ti3SiC2 sliding against different counterfaces.
4. Discussion From aforementioned results, the tribological behaviors of Ti3SiC2 are strongly affected by counterface materials. According to previous reports [3,26], Ti3SiC2 can be oxidized to form dense adhesive and layered scales consisting of SiO2 and TiO2 on the surface at high temperatures in air. And, during the dry-sliding friction process in air, temperature is increased due to heat generated by friction. Thus Ti3SiC2 on the worn surface is oxidized. The oxide film formed on the worn surface consists of TiO2 and SiOx as shown by the XPS results in Fig. 7a and b. The oxide film can decrease friction coefficient and wear rate [27,28]. If Ti3SiC2 slides against itself, the TiO2 and SiO2 produced by friction are less than those of the other friction pairs (Fig. 7a and b). As a result, fewer oxide films are formed in the selfmated Ti3SiC2 couple, which results in a high friction coefficient and wear rate. And due to weak interfacial strength, Ti3SiC2 grains are easily removed to form rough worn surface. If coupling with Al2O3, the resulted wear scar is relatively wide and deep as shown in Fig. 5, and extensive wear debris is observed on the worn surfaces. Thus, the wear rate is high for the pair of Ti3SiC2 against Al2O3. The XPS results (Fig. 7) show the presence of mixed oxides of TiO2, SiOx, and Al2O3
on the worn surface of Ti3SiC2. Thus Al elements are transferred from the Al2O3 ball to the worn surface of Ti3SiC2. For Ti3SiC2/steel pair, discontinuous patches and cracks are present on the Ti3SiC2 worn surface (Fig. 6e) and the oxide film is comprised of TiO2, SiOx, and Fe2O3 (Fig. 7). The Fe2O3 detected on the worn surface of Ti3SiC2 through XPS is attributed to Fe elements that are transferred from steel ball and oxidized in air. The formation of microcracks on the surface layer may be due to shear stress under the ball during friction. Hence, moderate friction coefficients and wear rates are obtained in the Ti3SiC2/steel ball couple. Ti3SiC2/SiC friction pair demonstrates the lowest friction coefficient and wear rate after the breaking-in period. Before friction test, Ti3SiC2 grains in the bulk sample are randomly oriented. During friction test, however, grains with basal plane perpendicular to the sliding direction are easily fractured and removed due to the high hardness of SiC. After the breaking-in period, these grains are consumed and the surface layer of Ti3SiC2 sample has preferred orientation that most basal planes are parallel to the sliding direction, which have very low friction coefficient [11,13]. Hence, the friction coefficient decreases to as low as 0.43 on average. In addition, as shown in Fig. 7a and b, the peaks of TiO2 and SiOx are stronger than
Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150
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those of the other friction pairs. Therefore, the oxide content on the surface layer of worn surface against SiC is higher than that of other friction pairs. This also can be ascribed to the high hardness of SiC. Oxide films can be easily formed and preferably adhere onto the surface. These films protect the surface from direct contact between couples, thereby reducing friction coefficient and wear losses of Ti3SiC2. Overall, Ti3SiC2 exhibits the lowest wear rate and friction coefficient if it slides against SiC. 5. Conclusions Ti3SiC2 exhibits different tribological properties if it slides against different counterface materials. The friction pair of Ti3SiC2/SiC demonstrates the lowest friction coefficient of approximately 0.43, and Ti3SiC2/steel shows a moderate friction coefficient of 0.63. The Ti3SiC2/Ti3SiC2, Ti3SiC2/Si3N4, and Ti3SiC2/Al2O3 friction pairs present high friction coefficients of 1.08, 1.17, and 1.30, respectively. Ti3SiC2/SiC pair exhibits the lowest wear rate of 2.09 10 4 mm3/Nm. High wear rates are obtained if Ti3SiC2 slides against itself, Al2O3, and Si3N4. Mechanical wear is the main mechanism if Ti3SiC2 slides against Ti3SiC2, Si3N4, and Al2O3. Grain removal is a significant tribological character of self-mated Ti3SiC2. For Ti3SiC2/SiC friction pair, oxide wear played a more important role and more oxides were formed than those in other friction pairs. Oxide films protected the surfaces of the couples from direct contact, and decreased the wear rate and friction coefficient of Ti3SiC2. Acknowledgments This work was supported by National Natural Science Foundation of China (51205111, 51305226, 51472075), Plan for Scientific Innovation Talent of Henan Province (134100510008), Program for Innovative Research Team of Henan Polytechnic University (T2013-4), State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (KF201313). References [1] M.W. Barsoum, The M(N þ 1)AX(N) phases: a new class of solids; thermodynamically stable nanolaminates, Prog. Solid State Chem. 28 (2000) 201–281. [2] M.W. Barsoum, T. El-Raghy, The MAX phases: unique new carbide and nitride materials - ternary ceramics turn out to be surprisingly soft and machinable, yet also heat-tolerant, strong and lightweight, Am. Sci. 89 (2001) 334–343. [3] M.W. Barsoum, T. ElRaghy, L.U.J.T. Ogbuji, Oxidation of Ti3SiC2 in air, J. Electrochem. Soc. 144 (1997) 2508–2516. [4] T. ElRaghy, A. Zavaliangos, M.W. Barsoum, S.R. Kalidindi, Damage mechanisms around hardness indentations in Ti3SiC2, J. Am. Ceram. Soc. 80 (1997) 513–516. [5] I.M. Low, S.K. Lee, B.R. Lawn, M.W. Barsoum, Contact damage accumulation in Ti3SiC2, J. Am. Ceram. Soc. 81 (1998) 225–228.
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Please cite this article as: Y. Zhu, et al., Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.01.150