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Dry sliding behaviors and friction surface characterization of Ti3Al0.8Si0.2Sn0.2C2 solid solution against S45C steel
Ceramics International 45 (2019) 2103–2110
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Dry sliding behaviors and friction surface characterization of Ti3Al0.8Si0.2Sn0.2C2 solid solution against S45C steel
T
⁎
Leping Caia, Zhenying Huanga,b, , Wenqiang Hua, Cong Leia, Hongxiang Zhaia, Yang Zhoua a b
Centre of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China Key Laboratory of Vehicle Advanced Manufacturing, Measuring and Control Technology (Beijing Jiaotong University), Ministry of Education, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ti3Al0.8Si0.2Sn0.2C2 Sliding friction Wear rate Tribofilm Friction surface analysis
Tribological performance of Ti3Al0.8Si0.2Sn0.2C2 solid solution sliding against S45C steel disk was investigated using a block-on-disk type tester at sliding speeds of 5–30 m/s under normal loads of 20–80 N. Ti3Al0.8Si0.2Sn0.2C2 exhibited a friction coefficient of 0.17–0.53 during the sliding process. It was shown that the wear rate decreased linearly as normal loads increasing from 20 to 80 N at 5 m/s, while the wear rates increased faintly in the range of 1.78–5.6 × 10−6 mm3/Nm with an increase in normal load when at other sliding speeds of 10 m/s, 20 m/s and 30 m/s. Both the friction coefficient and wear rate were found closely related to the tribooxidation induced tribofilm. Friction surface morphology and cross-section analyses revealed the tribofilm coverage played an effective performance, and the percentage of that was confirmed to increase with the sliding speed and normal load increasing. Phase composition and chemical states analyses of frictional surface showed that the tribofilm formed was a combination of TiO2, Al2O3, SnO2, SiO2, and Fe2O3 mixtures. Microstructure and physical state of the wear debris were also investigated for deeply understanding the contribution of the tribofilm.
1. Introduction In recent years, there are great demands for new materials with lower or moderate friction coefficient which can work in larger temperature range in industrial applications fields [1–4], such as structural parts like foil bearings, pantograph board and brake disk for high-speed train. Most solid lubricants like graphite and MoS2 are generally begin to oxidize at about 300 °C in air, result in a limited application area at relatively high temperatures [4]. Thus the appropriate wear-resistant material with suitable friction coefficient and low wear rate should be picked, and the self-adaptive friction surface formed at any time during the large-scaled friction process should be considered as well. Mn+1AXn (n = 1–3) phases are a class of ternary carbides and nitrides, where M is an early transition metal, A is the group IIIA or IVA element, X is either carbon or nitrogen, and MX slabs are separated by planar layers of A-elements [5]. This kind of materials has aroused people's interests recent years, they exhibit excellent properties, such as excellent capability in oxidation resistance [6,7], high damage tolerance [8], thermal shock resistance [9], and ductile-to-brittle transition [10], which are closely relevant to tribological performance. MAX phases hence are considered as promising candidates for tribological
applications. Typical MAX phases of Ti3SiC2 and Ti3AlC2 have been confirmed having outstanding tribological performance [11–16]. It is due to the unique nanolayered hexagonal structure of MAX phase, similar to that of graphite, the layered structure plays an important role in the friction process [12,13]. Besides, the important second point is due to the oxide film with self-lubricating effect. This self-generated tribofilm was composed mainly by various oxidation products contained disengagement A-site elements [14–18], it is primarily attributed to the coexisting multiple interlayer bonding force that the M-A bonds are weaker than the M-X bonds. Huang and Zhai et al. [15–18] studied the dry sliding tribological behaviors of Ti3SiC2 and Ti3AlC2 against low carbon steel disk using a block-on-disk tester. They found that both the friction coefficient and wear rate were dependent on the coverage of frictional films formed during sliding process. The frictional films were consisted of a mixture of O, Ti, Al or Si, and Fe on the friction surface. Ti3SiC2 was reported exhibiting a friction coefficient of 0.53–0.25 and a wear rate of 0.6–2.5 × 10−6 mm3/N m, and Ti3AlC2 exhibited a friction coefficient of 0.53–0.16 and a wear rate of 0.91–3.75 × 10−6 mm3/N m as the sliding speed increased from 5 m/s to 60 m/s, respectively, both showing excellent friction performance [15–18]. Souchet et al. [19]
⁎ Corresponding author at: Centre of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (Z. Huang).
https://doi.org/10.1016/j.ceramint.2018.10.115 Received 2 August 2018; Received in revised form 14 October 2018; Accepted 14 October 2018 Available online 15 October 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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have investigated the friction properties of Ti3SiC2/steel and Ti3SiC2/ Si3N4 tribo-pair, an instantaneous heat-induced film contained alumina grinding dust was reported forming on the friction surface. The compacted wear debris was acted as a thin film, leading to quite low wear rate and friction coefficient of Ti3SiC2. Take advantages of the excellent mechanical properties and frictional performance, MAX phases were common applied as frictional enhancement phases [20–22] or as tribological protective coatings [23–25]. Ti3SiC2 powder can be used as lubrication phases to enhance the wear resistance of NiAl intermetallic compound for the proposed applications such as turbine blade tips [21]. Ti3SiC2, Ti2AlN, and Cr2AlC thin films were applied against a counterpart bearing steel [24]. The selected MAX phases exhibit excellent tribological properties obviously, which can be applied as the most promising candidate for tribological system, even adapted in harsher conditions, e.g., working at elevated temperatures, or in corrosion environment [26–28]. It can be estimated that adjusting the phase composition and oxide proportion of the dominated friction film will lead to different physical form, which continue to affect the tribological properties of MAX phases, by providing that preparing MAX phase solid solutions through adding various A elements. In reality, Huang et al. [29] have investigated that the friction coefficient was continuously adjusted in a range of 0.1–0.4 according to the different content of Sn by forming Ti3Al(Sn)C2 solid solution, and still maintained at low wear rate. At same dry sliding conditions, the tribological oxide film of Ti3AlC2 was in a fused state during sliding process, while that of Ti3Al(Sn)C2 solid solution existed in a soft state, resulting in larger friction coefficient. Accordingly, the friction coefficient of Ti3Al(Sn)C2 could be tailored by adjusting the content of Sn dopant in the solid solution to enlarge its application range. Solid solution treatment was also verified as an effective way to optimize the mechanical performance of MAX phase [30]. Some recent work focused on oxidation behaviors of MAX phase solid solution were studied as well [31,32]. The oxidation rate of Ti2Al1xSnxC solid solution was reported could be accelerated by substituting Al by Sn in Ti2Al1-xSnxC at lower temperature [32]. It is conceivable that MAX phase solid solution is more likely to generate tribological oxide film during the friction process. The preceding discussions offered potential guidance for the development of MAX phases in friction field by adding varieties of A-site elements consciously. As yet, there are few studies focus on the tribological properties of the various A-site elements (three or more) MAX phase solid solutions. A new typed MAX phase solid solution of Ti3Al0.8Si0.2Sn0.2C2 has been synthesized in our previous work [33], Ti3Al0.8Si0.2Sn0.2C2 were observed having a characteristic layered structure and exhibited an obvious solid solution effect. The present work tried to investigate the tribological performance of Ti3Al0.8Si0.2Sn0.2C2 MAX phase solid solution. Friction coefficient and wear resistance were studied, and special attention was paid to the chemical composition and physical state on the friction surface to further understand how frictional performances were affected by sliding contact surfaces. The morphology and composition of wear debris were analyzed as well.
Fig. 1. Schematic representation of the friction tester.
observed having a plate-like shape and layered structure, the average grain size was estimated to be ~ 10 µm in length and ~ 3 µm in width, the volume density was determined to be 4.68 g/cm3 and Vickers hardness was 5.4 ± 0.3 GPa [33]. Ti3Al0.8Si0.2Sn0.2C2 samples were cut into blocks of 10 mm × 10 mm × 12 mm. Dry sliding friction and wear behaviors of Ti3Al0.8Si0.2Sn0.2C2 were evaluated using a homemade block-on-disk type high-speed tester, the schematic representation of the friction tester was shown in Fig. 1. A S45C steel (after quenched and tempered) disk of Φ300 mm × 10 mm was selected as the friction counterpart, the frictional contact area was 10 mm × 10 mm. Quenched and tempered S45C steel is widely used as structural equipment like wheel gear, wheel axle, wheel hub and plunger due to excellent integrated mechanical properties. Therefore, tribological behaviors of Ti3Al0.8Si0.2Sn0.2C2 sliding against S45C steel were investigated in order to provide guidance for Ti3Al0.8Si0.2Sn0.2C2 applying as matching parts of structural equipment in this study. Vickers hardness of the S45C steel was determined with a TH700 hardness tester, ten indentation tests were carried out at 98 N with a dwell time of 15 s. Hardness of the S45C steel disk was tested to be 3.2 ± 0.1 GPa. Friction tests were carried out with sliding speeds in a range of 5–30 m/s, and the applied normal load was 20–80 N for each sliding speed. The sliding distance of a continuous process was of 12,000 m. Friction coefficients were automatically measured and recorded using the computer system of the friction tester, the wear quantity of the samples was measured by weighing method, which measured the mass loss of the samples per sliding distance and per normal load. Pre-abrasion was made at the first stage to eliminate any possible influence on the friction surface, and the tests for every given conditions were repeated three times to obtain the average value. The phase composition and microstructure of fractured surface were identified by XRD analysis and a Scanning Electron Microscope (SEM, ZEISS EVO 18, German) equipped with an Energy Dispersive X-ray Spectrometer (EDXS, Bruker Nano XFlash detector 5010). EDS compositions of the friction surface were presented in at% (atomic percent). The chemical states of elements on the worn surface were examined by (ESCALAB 250Xi) X-ray Photoelectron Spectroscopy (XPS). The surface topography of the tribological surface was observed by Atomic Force Microscopy (AFM) using a Dimension Fastscan™ atomic force microscope. The wear debris was collected and analyzed by a Thermogravimetric and Differential Scanning Calorimetric (TG&DSC: SDT Q600) to identify the fusing temperature of the frictional film in the temperature range room temperature ~ 1300 °C with a heating rate of 5 °C/min in Ar atmosphere. The Gibbs free energies of the related
2. Experimental procedures Polycrystalline bulk Ti3Al0.8Si0.2Sn0.2C2 samples were prepared by two-time hot-pressing. The processing details have been reported elsewhere [33], in brief, dense Ti3Al0.8Si0.2Sn0.2C2 bulks were two-time hot-pressed at 1450 °C for 30 min with a pressure of 30 MPa in Ar using synthesized Ti3Al0.8Si0.2Sn0.2C2 powders (Ti3Al0.8Si0.2Sn0.2C2 powders were pressurelessly sintered from Ti, Al, Si, Sn and TiC powders, detailed process was reported in Ref. [33]) as raw materials. Phase identification of the bulk samples was carried out by X-Ray Diffraction (XRD) analysis with Cu Kα radiation (X′ PERT-PRO MPDT, Netherlands). The bulk samples consisted of dominant Ti3Al0.8Si0.2Sn0.2C2 phase from XRD analysis, no impurity phases like TiC, Al3Ti, Ti6Sn5 or Ti5Si3 were detected [33]. Most of the Ti3Al0.8Si0.2Sn0.2C2 grains were 2104
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from 2.76 × 10−6 mm3/N m to 4.34 × 10−6 mm3/N m. When at higher sliding speed of 30 m/s, the wear rate was 3.91 × 10−6 mm3/ Nm corresponding to 20 N, and that was 5.6 × 10−6 mm3/N m corresponding to 80 N. The results showed that wear rates were greatly affected accordingly to sliding speeds. When at sliding speeds of 10 m/s, 20 m/s and 30 m/s, the speed-dependent changed trend was reversed to that of friction coefficient in Fig. 2(a). The friction coefficient and wear rate showed a similar changing phenomenon in Ti3Al0.8Sn0.4C2 [34]. The friction coefficient of Ti3Al0.8Sn0.4C2 bulk showed a variation tendency from 0.48 to 0.3 and a low wear rate from 0.4 × 10−6 mm3/N m to 4.3 × 10−6 mm3/N m within the sliding speed changing from 5 m/s to 10 m/s against low carbon steel. Predictably, the wear rate of Ti3Al0.8Si0.2Sn0.2C2 could be governed by comprehensive effects of sliding speed and load, relating to the state of the friction surface. 3.2. Microstructure analysis of friction surfaces The friction surfaces of Ti3Al0.8Si0.2Sn0.2C2 were observed by SEM (see Fig. 3) to further understand the variation trend of friction coefficient and wear rate. Friction surfaces were observed different in images with the normal load changing. The percentage of frictional film coverage increased with normal load increasing, as shown in Fig. 3(a)(b), the tribological film distributed sporadically in case of 5 m/s and 20 N while it gradually became continuously when the normal load increased to 80 N. This tribological film was also strongly speed-dependent, the friction surfaces were almost completely covered by continuous and smoother tribological film (see Fig. 3(b)–(e)) as the sliding speeds increased from 5 m/s to 30 m/s. Regional map scanning was applied to analyze the composition of tribological films. Table 1 lists the main chemical constituents of the different regions as denoted by yellow dashed frames in Fig. 3. It was observed that all of the worn surface contained O elements under any friction condition, indicating the tribo-oxidation reacted during the friction process. O contents in regions of zone D-F were observed higher than those in the other regions, it suggested that sufficiently continuous and compact tribofilms were gradually covered on the friction surface, the main mechanisms were conceived to be oxidation wear and twobody abrasion wear. Friction films did positive effects on the friction behaviors, which was conducive to preventing the friction surface from adhering onto the counterpart and was liable for the decreased friction coefficient during the sliding effectively, as demonstrated in previous studies [15–18,34,35]. Thus it was reasonable to conclude that Ti3Al0.8Si0.2Sn0.2C2 demonstrated lower friction coefficient when at higher sliding speeds of 20 m/s and 30 m/s. Fe element was transferred from S45C steel counterpart, which was attributed to the adhesive wear behavior of tribological pairs during sliding process. It was no doubt that material tends to transfer from soft to the hard one, the terms soft and hard herein were defined by Vickers hardness (HV) of materials. The HV value of Ti3Al0.8Si0.2Sn0.2C2 is about 5.4 ± 0.3 GPa [33], higher than that of S45C steel, 3.2 ± 0.1 GPa. Then the transference from S45C steel (a softer material) to Ti3Al0.8Si0.2Sn0.2C2 (a harder material) might perform. In the light of EDS analysis, small amounts of Fe were detected on the selected area at 30 m/s, revealing that the transfer of Fe element from S45C steel counterpart was reduced, precisely should be ascribed to the smooth and almost full coverage of tribo-oxidation film, which apparently prevented the scraping and adhesive wear behaviors between the contact surfaces. Zhai and Huang et al. [15–18] have investigated the interaction between sliding wear and oxidation of Ti3SiC2 and Ti3AlC2. Results revealed that the tribo-oxidation was a procession of formation, constant consumption and re-composition. That's to say, if the tribofilms happened to wear off, new ones could be formed to take their place instantaneously. Actually, it is conceivable that the oxidation occurring on the Ti3Al0.8Si0.2Sn0.2C2 surface is proportional to the friction work μLV (L is the normal load, V is the sliding speed, and μ is the friction coefficient [36]), the rate of generating oxides is an increasing function
Fig. 2. (a) Coefficient of friction and (b) wear rate of Ti3Al0.8Si0.2Sn0.2C2 sliding against S45C steel disk at different sliding speeds as a function of normal load.
possible formed products were calculated by the HSC Chemistry 6.0 software. 3. Results and discussions 3.1. Friction and wear behaviors Fig. 2(a) shows typical curves of kinetic friction coefficients of Ti3Al0.8Si0.2Sn0.2C2 sliding against S45C steel under normal loads of 20–80 N and sliding speeds of 5–30 m/s. It was obvious that the friction coefficients were speed- and load-dependent. For the low sliding speed of 5 m/s, the friction coefficient increased from 0.46 to 0.53 with increasing the normal load from 20 to 40 N, then it gradually reduced from 0.46 to 0.43 with increasing the normal load from 50 to 80 N, almost kept stable. For the sliding speed of 10 m/s, the friction coefficients retained at around 0.35 as the load increased. A diverse change was found at higher sliding speed of 20 m/s, the friction coefficient increased from 0.26 to 0.35 with increasing the normal load from 20 to 80 N, showing a monotonous increase trend within the increase of normal load. Similar load-dependent change was exhibited at 30 m/s, the corresponding friction coefficient was 0.18 under the normal load of 20 N, and that was 0.3 when the normal load increased to 80 N. Sliding speed seemed as an assignable factor clearly, the friction coefficient was notably reduced with the increasing sliding speed, and normal load also effected the friction coefficient, which varied according to the corresponding sliding speed. It is suggested that the friction behavior could be governed by both of the sliding speed and normal load. Wear rates of Ti3Al0.8Si0.2Sn0.2C2 are shown in Fig. 2(b) as functions of normal load and sliding speed. For the lower sliding speed of 5 m/s, the wear rate was 14 × 10−6 mm3/Nm corresponding to 20 N, and that was 4.1 × 10−6 mm3/N m corresponding to 80 N, it showed a significant decrease as normal load increased. However, for the sliding speed of 10 m/s, the wear rate increased mildly from 1.78 × 10−6 mm3/N m to 2.84 × 10−6 mm3/N m with the increase in normal load from 20 to 80 N. For that of 20 m/s, the wear rate increased slightly 2105
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Fig. 3. SEM micrographs of Ti3Al0.8Si0.2Sn0.2C2 friction surfaces under the different sliding speeds and the normal loads: (a) 5 m/s and 20 N; (b) 5 m/s and 80 N; (c) 10 m/s and 80 N; (d) 20 m/s and 80 N; and (e) 30 m/s and 80 N.
corresponding normal load and assumed a linear descending tendency. The sliding roughness of friction surfaces after 5 m/s, 80 N and 30 m/s, 80 N were determined with an AFM (Fig. 4). The average surface roughness (Rq) were 102 nm and 53.6 nm for sliding speeds of 5 m/s and 30 m/s, respectively. Friction surface under 30 m/s was found to be extremely smooth, which was consistent with the observing in SEM micrographs in Fig. 3(e). White dotted areas were taken from the regional continuous tribofilm on both friction surfaces to compare with the roughness throughout the region to determine the roughness of local continuous frictional film. Surface roughness of the white dotted area were 13.7 nm in Fig. 4(a) and 18.2 nm in Fig. 4(c). The higher entire surface roughness after sliding speed of 5 m/s indicated that the tribological film covered unevenly, only sporadic impacted tribofilms were consisted on the worn surface. Inversely, the tribological film covered uniformly and continuously on the friction surface at 30 m/s under 80 N. Combining with the foregoing SEM microstructure, tribofilm formed at 30 m/s under 80 N was found to be extremely smooth. It was conceivable that the impacted continuous frictional oxide film formed on the worn surface was responsible for the relative low friction coefficient and moderate wear rate. Smoother friction surface could lead to a better wear resistance. Besides, when at relative low sliding speed (5 m/s or 10 m/s), local continuous tribofilm slightly played an effective role to reduce the abrasion and adhesive wear. Thus for Ti3Al0.8Si0.2Sn0.2C2, although the friction coefficients were higher when at 5 m/s and 10 m/s than that at 20 m/s and 30 m/s, the corresponding friction coefficients were still keep at lower level compared with other ceramic material worked with no produced tribo-oxide film under
Table 1 Main chemical constituents of the regions noted by yellow dashed frame in Fig. 3. Friction condition
Region
Main chemical constituents
5 m/s, 20 N
Zone Zone Zone Zone Zone Zone Zone Zone Zone Zone
Ti25.6Al7.1Si2.1Sn1.9O33.4Fe2.1 Ti19.1Al4.9Si1.5Sn1.5O45.7Fe6.9 Ti21.5Al5.6Si1.7Sn1.6O44.4Fe6.8 Ti9.7Al1.4Si0.6Sn0.7O55.4Fe22.3 Ti13.5Al2.8Si1.1Sn1.0O53.8Fe18.2 Ti20.3Al5.4Si1.5Sn1.4O55.2Fe7.5 Ti13.7Al3.4Si1.1Sn0.9O52.4Fe17.0 Ti13.5Al3.3Si1.1Sn0.9O52.5Fe16.9 Ti19.7Al5.4Si2.0Sn0.9O54.8Fe2.7 Ti17.6Al4.8Si2.2Sn1.1O51.5Fe3.5
5 m/s, 80 N 10 m/s, 80 N 20 m/s, 80 N 30 m/s, 80 N
A B C D E F G H I J
of the μ, L or V. In general, the oxide film can be formed on the friction surface only if the generating rate was larger than the consuming rate, while a larger generating rate of oxides could result into a larger wear rate. Since there would be more oxides generated at higher sliding speed due to the higher friction-induced temperature, in terms of the collected wear rate in this work, wear rates of 30 m/s, 20 m/s and 10 m/s increased by degrees. When at sliding speed of 5 m/s, the friction surface was ploughed and scratched heavily for lacking of the oxide film at lower normal load, consequently the severe friction surface induced higher wear rate. When the normal load increased gradually, the state of tribofilm was changed to be extensive and smooth, hence, the wear rate curve at 5 m/s showing in Fig. 3(b) was relied heavily on the 2106
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Fig. 4. AFM analysis in scanning mode on 10 µm × 10 µm Ti3Al0.8Si0.2Sn0.2C2 surface after sliding at 5 m/s, 80 N for 3 × 12,000 m of (a) isometric view; (b) top view; and at 30 m/s, 80 N for 3 × 12,000 m of (c) isometric view; (d) top view.
was difficult to obtain XRD pattern of the tribological film because Xray could readily penetrate the tribological film and reached into the sub-surface. Hence, XPS analysis was applied here (see Fig. 6) in order to further identify the elements chemical states of tribological film on the worn surface. In Fig. 6, the characteristic XPS peaks of MAX phase disappeared after sliding, indicating that Ti, Al, Si and Sn existed in other compounds on the worn surfaces. The XPS peaks at 458.9 eV and 464.6 eV were corresponding to Ti2p3/2 and Ti2p1/2 in TiO2 [37,38], the peak at 75.02 eV was assigned to Al2O3 [21,37,38], the peak at 102.05 eV was in agreement with the XPS shifts of SiO2 [21,38], and the peaks at 487.02 eV and 495.75 eV ware associated with SnO2, as has been reported [39]. The peaks corresponded to Fe2p in Fe2O3 were attached as well. No XPS spectra of Ti2p, Al2p, Si2p or Sn2p was assigned to Ti3Al0.8Si0.2Sn0.2C2 compound, the tribological film formed on the worn surface was a total combination of wear debris contained various oxides. The friction and wear behaviors were verified to be governed by the oxide films, rather the Ti3Al0.8Si0.2Sn0.2C2 surface. Fig. 7 shows standard Gibbs free energies (ΔG) of formation for SnO2, Fe2O3, SiO2, TiO2, and Al2O3 calculated by the HSC6.0 Chemistry software to verify the possibility of oxidation products formation during friction process. The sequence of the phase formation was Al2O3﹥TiO2﹥ SiO2﹥Fe2O3﹥SnO2, Al2O3 was relatively more easily produced. Combining with the former EDS analysis of zone I and zone J in Fig. 3(e) and XPS analysis, TiO2 and Al2O3 were confirmed as predominant constituents of the tribofilm. It was established that TinO2n-1 and Al2O3 phases contributed to good wear resistance [15–18,35,37,38], in Refs. [15–18,23–29], SiO2, Al2O3, SnO2 and Fe2O3 played positive role in wear resistance as well. That's to say, the tribofilms containing various oxides including mixtures of Ti-Al-Si-Sn-Fe-O contributed to the lubricity and wear resistance. However, the tribofilm was too thin to identify the exact distribution of each oxide, maybe it will be deeper
vacuum or argon atmosphere [35,37] and most engineering materials. The tribofilm was further analyzed by cross-sectioning. Fig. 5 shows the cross-section SEM micrographs of the friction surface after three continuous sliding processes (3 × 12,000 m) performed at different sliding speeds. Tribofilm formed at sliding speed of 5 m/s was not shown here for comparison because the uneven surface was too thin to measure the thickness, but have an analogous appearance. It can be clearly observed that continuous and adherent films were formed on the friction surfaces. The tribofilm thickness at 10 m/s was estimated to be about 0.6 µm, whereas those at 20 and 30 m/s were estimated to be 1.46 and 1.44 µm, indicating that tribofilms generated more easily at higher sliding speed. The friction induced heat accelerated the oxidation of contact surfaces on account of the increasing sliding speeds. Moreover, continuous oxide films were maintained on friction surface when the generating rate was equal to consuming rate, then the film thickness was basically steady, playing lubricate and anti-attrition effects. EDS line scanning analysis was performed to identify the compositions of the substrate and films. Obviously, Ti, Al, Si, Sn and C elements were distributed uniformly in the whole substrate, suggesting that Asite elements of Al, Si and Sn were almost homogeneous despite of the tribo-oxidation process. Contents of Ti, Al, Si and Sn were obviously lower in tribological film than those in substrate while O and Fe contents increased. It can be estimated that the friction film was mainly composed of an oxide mixture of various elements. When at sliding speeds of 20 m/s and 30 m/s, the increase of O content was more pronounced. Higher sliding speed correlative with friction induced heat accelerated the tribo-oxidation process. As described above, all the tribofilms were mainly composed of mixture oxides of Ti, Al, Si and Sn elements, and Fe element was detected as well, which was transferred from the counterpart. In reality, it 2107
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Fig. 5. Cross-section of the friction surface at the sliding conditions of (a) 10 m/s, 80 N; (b) 20 m/s, 80 N; (c) 30 m/s, 80 N. (d), (e) and (f) are the corresponding EDS line scanning analysis of 10 m/s, 20 m/s and 30 m/s along the yellow lines.
Fig. 6. XPS spectra of Ti, Al, Si, Sn and Fe elements on the friction surface of Ti3Al0.8Si0.2Sn0.2C2 after sliding at 30 m/s and 80 N.
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Fig. 7. Standard Gibbs free energies of formation for SnO2, Fe2O3, SiO2, TiO2, and Al2O3.
Fig. 9. TG&DSC analysis curves of the wear debris.
characterized by other advanced methods. 3.3. Analysis of the wear debris SEM images of wear debris from Ti3Al0.8Si0.2Sn0.2C2 after sliding is shown in Fig. 8. Wear debris existed in the forms of fine pulverized scrap when at the sliding speed of 5 m/s. Easy-cutting and impacting between the counterparts were serious, which were ascribed to the lack of continuous tribofilm and the three-body abrasion wear. At medium sliding speed of 10 m/s and 20 m/s, the wear debris was plate shaped and uniform-sized. When the sliding speed increased to 30 m/s, large tracts of wear debris were formed, and some of them could be peeled off as pulverized scrap along with friction. The more generated oxides caused more consumed oxides, resulting into higher wear rate, which was in consistent with the foregoing discussion in variation of the wear rate. To validate the fusibility of the tribofilm, the fusion point of the wear debris was identified by TG&DSC analysis, showing in Fig. 9. There was an endothermic peak in an approximate range of 900–1300 °C, indicating the oxide film would be fused when the temperature of contact area reaches or exceeds 900 °C. This behavior was quite different from that exhibited by the oxide film on the Ti3Al0.8Sn0.4C2 friction surface [34], where no significant endothermic peak was indicated in the range of 100–1200 °C, implying the oxide film
Fig. 10. XRD analysis of the wear debris.
was not fused within the temperature range that the friction surfaced reached. Different characteristics of the tribo-induced oxides might make a difference. In order to make clear the endothermic process, XRD analysis (showing in Fig. 10) of the wear debris was carried out. It can be seen that the wear debris contained various oxygenated compound, basically corresponding to EDS analysis on the friction surface in Fig. 3(e), which in turn, could verify the tribofilm was generated by the impacted wear debris. In present study, glassy phases SiO2 were
Fig. 8. SEM micrographs of wear debris generated at the sliding conditions of (a) 5 m/s, 80 N; (b) 10 m/s, 80 N; (c) 20 m/s, 80 N; (d) 30 m/s, 80 N. 2109
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evidently detected in oxide scale. It was determined that SiO2 could formed as viscous or liquid glass phases at high temperature and easily diffused into everywhere [40]. Besides, the aluminosilicate glass phases were formed as well from XRD result, they were mainly composed of Al2O3 and SiO2 and might be caused by the dissolution of Al2O3 into SiO2. These glassy phases have a lower softening point under a level of 1000 °C, which would lead to an endothermic process. The glassy phases have fluidity and can fill pores and other defects of the friction surface during the friction process, thus the oxide films were exactly in a liquid stage. Clearly, such a liquid oxide film must have a significant lubricating effect on the friction surface and consequently playing an important role in the friction and wear resistance.
ceramics, Am. Ceram. Soc. Bull. 92 (2013) 20–27. [9] Y.W. Bao, X.H. Wang, H.B. Zhang, Y.C. Zhou, Thermal shock behavior of Ti3AlC2 between 200 °C and 1300 °C, J. Eur. Ceram. Soc. 25 (14) (2005) 3367–3374. [10] Z.M. Sun, Progress in research and development on MAX phases: a family of layered ternary compounds, Inter. Mater. Rev. 56 (2011) 144–166. [11] S. Gupta, M.W. Barsoum, On the tribology of the MAX phases and their composites during dry sliding: a review, Wear 271 (2011) 1878–1894. [12] S. Myhra, J.W.B. Summers, E.H. Kisi, Ti3SiC2–a layered ceramic exhibiting ultralow friction, Mater. Lett. 39 (1999) 6–11. [13] T. El-Raghy, P. Blau, M.W. Barsoum, Effect of grain size on friction and wear behavior of Ti3SiC2, Wear 238 (2000) 125–130. [14] Y. Zhang, G.P. Ding, Y.C. Zhou, B.C. Cai, Ti3SiC2–a self-lubricating ceramic, Mater. Lett. 55 (2002) 285–289. [15] H.X. Zhai, Z.Y. Huang, M.X. Ai, Y. Zhou, Z.L. Zhang, S.B. Li, Tribophysical properties of polycrystalline bulk Ti3AlC2, J. Am. Ceram. Soc. 88 (11) (2005) 3270–3274. [16] H.X. Zhai, Z.Y. Huang, Y. Zhou, Z.L. Zhang, Y.F. Wang, M.X. Ai, Oxidation layer in sliding friction surface of high-purity Ti3SiC2, J. Mater. Sci. 39 (21) (2004) 6635–6637. [17] Z.Y. Huang, H.X. Zhai, M.L. Guan, X. Liu, M.X. Ai, Oxide-film-dependent tribological behaviors of Ti3SiC2, Wear 262 (2007) 1079–1085. [18] Z.Y. Huang, H.X. Zhai, W. Zhou, X. Liu, M.X. Ai, Tribological behaviors and mechanisms of Ti3AlC2, Tribol. Lett.. 27 (2007) 129–135. [19] A. Souchet, J. Fontaine, M. Belin, T.L. Mogne, J.L. Loubet, M.W. Barsoum, Tribological duality of Ti3SiC2, Tribol. Lett. 18 (2005) 3245–3248. [20] S. Gupta, D. Filimonov, T. Palanisamy, T. El-Raghy, M.W. Barsoum, Ta2AlC and Cr2AlC Ag-based composites-New solid lubricant materials for use over a wide temperature range against Ni-based superalloys and alumina, Wear 262 (2007) 1479–1489. [21] X.L. Shi, M. Wang, W.Z. Zhai, Z.S. Xu, Q.X. Zhang, Y. Chen, Influence of Ti3SiC2 content on tribological properties of NiAl matrix self-lubricating composites, Mater. Des. 45 (2013) 179–189. [22] S. Wang, S.Y. Zhu, J. Cheng, Z.H. Qiao, J. Yang, W.M. Liu, Microstructural, mechanical and tribological properties of Al matrix composites reinforced with Cu coated Ti3AlC2, J. Alloy. Compd. 690 (2017) 612–620. [23] M. Rester, J. Neidhardt, P. Eklund, J. Emmerlich, H. Ljungcrantz, L. Hultman, C. Mitterer, Annealing studies of nanocomposite Ti-Si-C thin films with respect to phase stability and tribological performance, Mater. Sci. Eng. A 429 (2006) 90–95. [24] M. Hopfeld, R. Grieseler, A. Vogel, H. Romanus, P. Schaaf, Tribological behavior of selected Mn+1AXn phase thin films on silicon substrates, Surf. Coat. Technol. 257 (2014) 286–294. [25] J. Emmerlich, G. Gassner, P. Eklund, H. Hogberg, L. Hultman, Micro and macroscale tribological behavior of epitaxial Ti3SiC2 thin films, Wear 264 (2008) 914–919. [26] S. Gupta, D. Filimonov, T. Palanisamy, M.W. Barsoum, Tribological behavior of select MAX phases against Al2O3 at elevated temperatures, Wear 265 (2008) 560–565. [27] S.F. Ren, J.H. Meng, J.B. Wang, J.J. Lu, S.R. Yang, Tribocorrosion behavior of Ti3SiC2 in the dilute and concentrated sulfuric acid solutions, Wear 269 (2010) 50–59. [28] S. Wang, J. Cheng, S.Y. Zhu, Z.H. Qiao, J. Yang, Low friction properties of Ti3AlC2/ SiC tribo-pair in sea water environment, Tribol. Int. 103 (2016) 228–235. [29] Z.Y. Huang, H. Xu, H.X. Zhai, Y.Z. Wang, Y. Zhou, Strengthening and tribological surface self-adaptability of Ti3AlC2 by incorporation of Sn to form Ti3Al(Sn)C2 solid solutions, Ceram. Int. 41 (3) (2014) 3701–3709. [30] Y.C. Zhou, J.X. Chen, J.Y. Wang, Strengthening of Ti3AlC2 by incorporation of Si to form Ti3Al1-xSixC2 solid solutions, Acta Mater. 54 (2006) 1317–1322. [31] H.B. Zhang, Y.C. Zhou, Y.W. Bao, M.S. Li, Improving the oxidation resistance of Ti3SiC2 by forming a Ti3Si0.9Al0.1C2 solid solution, Acta Mater. 52 (2004) 3631–3637. [32] G.P. Bei, B.J. Pedimonte, T. Fey, P. Greil, Oxidation Behavior of MAX Phase Ti2Al(1x)SnxC Solid Solution, J. Am. Ceram. Soc. 96 (5) (2013) 1359–1362. [33] L.P. Cai, Z.Y. Huang, W.Q. Hu, C. Lei, S.S. Wo, X.K. Li, H.X. Zhai, Y. Zhou, Fabrication and microstructure of a new ternary solid solution of Ti3Al0.8Si0.2Sn0.2C2 with high solid solution strengthening effect, Ceram. Int. 44 (2018) 9593–9600. [34] H. Xu, Z.Y. Huang, H.X. Zhai, M.Q. Li, X.H. Liu, Y. Zhou, Fabrication, mechanical properties, and tribological behaviors of Ti3Al0.8Sn0.4C2 solid solution by two-time hot-pressing method, Int. J. Appl. Ceram. Technol. 12 (4) (2015) 783–789. [35] S. Wang, J.Q. Ma, S.Y. Zhu, J. Cheng, Z.H. Qiao, J. Yang, W.M. Liu, High temperature tribological properties of Ti3AlC2 ceramic against SiC under different atmospheres, Mater. Des. 67 (2015) 188–196. [36] J.R. Barber, Thermoelastic instabilities in the sliding of comforming solids, Proc. R. Soc. A 312 (1969) 381–394. [37] J.Q. Ma, J.Y. Hao, L.C. Fu, Z.H. Qiao, J. Yang, W.M. Liu, Q.L. Bi, Intrinsic selflubricity of layered Ti3AlC2 under certain vacuum environment, Wear 297 (2013) 824–828. [38] Y.Y. Zhu, A.G. Zhou, Y.Q. Ji, J. Jia, L.B. Wang, B. Wu, Q.F. Zan, Tribological properties of Ti3SiC2 coupled with different counterfaces, Ceram. Int. 41 (2015) 6950–6955. [39] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, in: J. Chastain, R.C. King (Eds), MN: Physical Electronics Inc., Eden Prairie, 1992, pp. 126–127. [40] S.B. Li, G.M. Song, Y. Zhou, A dense and fine-grained SiC/Ti3Si(Al)C2 composite and its high-temperature oxidation behavior, J. Eur. Ceram. Soc. 32 (2012) 3435–3444.
4. Conclusions The intrinsic dry sliding tribological performance of a new typed Ti3Al0.8Si0.2Sn0.2C2 solid solution was investigated in this paper. Friction coefficient and wear rate were measured and the results showed that Ti3Al0.8Si0.2Sn0.2C2 exhibited a good friction stability as well as quite low friction coefficient and wear rate. Actually the low friction coefficient and wear rate were strongly speed-dependent and faintly normal load-dependent in the sliding speed range of 5–30 m/s under the applied normal load of 20–80 N. The friction coefficient was significantly reduced while the wear rate increased with an increase of sliding speed. Both of friction coefficient and wear rate showed a monotonous increase trend within the increase in normal load, except that the wear rate was significant dependent on normal load at lower sliding speed of 5 m/s. Comparatively continuous self-generated tribofilm with a certain thickness was confirmed playing an effective role in the excellent tribological performance through characterizing the friction surfaces. The tribofilm was consisted of a mixture oxides of Ti-AlSi-Sn-Fe-O attached to the friction surface. It was precisely account of the smoother and continuous tribofilm formed by higher sliding speed induced higher friction heat that the friction coefficient showed a decreasing trend with the increase in sliding speed. Due to the tribo-oxidation was a procession of formation, constant consumption and recomposition, frictional wear debris could be peeled off as pulverized scrap along with friction. Large tracts of wear debris was found generated at higher sliding speed, and then resulted into higher wear rate. Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities under Grant no. 2018YJS146, by National Natural Science Foundation of China (NSFC) under Grant nos. 51572017 and 51871011, and by Beijing Government Funds for the Constructive Project of Central Universities. The financial supports by them are greatly appreciated. References [1] C. Muratore, A.A. Voevodin, Chameleon coatings: adaptive surfaces to reduce friction and wear in extreme environments, Annu. Rev. Mater. Res. 39 (2009) 297–324. [2] S.M. Aouadi, B. Luster, P. Kohli, C. Muratore, A.A. Voevodin, Progress in the development of adaptive nitride-based coatings for high temperature tribological applications, Surf. Coat. Technol. 204 (2009) 962–968. [3] W. Wang, Application of a high temperature self-lubricating composite coating on steam turbine components, Surf. Coat. Technol. 177 (2004) 12–17. [4] H. Heshmat, P. Hryniewicz, J.F. Walton II, J.P. Willis, S. Jahanmir, C. DellaCorte, Low-friction wear-resistant coatings for high-temperature foil bearings, Tribol. Int. 38 (2005) 1059–1075. [5] M.W. Barsoum, The Mn+1AXn phases: a new class of solids: thermodynamically stable nanolaminates, Prog. Solid. State Chem. 28 (2000) 201–281. [6] M.W. Barsoum, N. Tzenov, A. Procopio, T. El-Raghy, M. Ali, Oxidation of Tin+1AlXn (n = 1-3 and X = C, N): II experimental results, J. Electrochem. Soc. 148 (8) (2001) B234–B239. [7] X.H. Wang, Y.C. Zhou, High-temperature oxidation behavior of Ti2AlC in Air, Oxid. Met. 59 (2003) 303–320. [8] M. Radovic, M.W. Barsoum, MAX phases: bridging the gap between metals and
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Dry sliding behaviors and friction surface characterization of Ti3Al0.8Si0.2Sn0.2C2 solid solution against S45C steel
T
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Leping Caia, Zhenying Huanga,b, , Wenqiang Hua, Cong Leia, Hongxiang Zhaia, Yang Zhoua a b
Centre of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China Key Laboratory of Vehicle Advanced Manufacturing, Measuring and Control Technology (Beijing Jiaotong University), Ministry of Education, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ti3Al0.8Si0.2Sn0.2C2 Sliding friction Wear rate Tribofilm Friction surface analysis
Tribological performance of Ti3Al0.8Si0.2Sn0.2C2 solid solution sliding against S45C steel disk was investigated using a block-on-disk type tester at sliding speeds of 5–30 m/s under normal loads of 20–80 N. Ti3Al0.8Si0.2Sn0.2C2 exhibited a friction coefficient of 0.17–0.53 during the sliding process. It was shown that the wear rate decreased linearly as normal loads increasing from 20 to 80 N at 5 m/s, while the wear rates increased faintly in the range of 1.78–5.6 × 10−6 mm3/Nm with an increase in normal load when at other sliding speeds of 10 m/s, 20 m/s and 30 m/s. Both the friction coefficient and wear rate were found closely related to the tribooxidation induced tribofilm. Friction surface morphology and cross-section analyses revealed the tribofilm coverage played an effective performance, and the percentage of that was confirmed to increase with the sliding speed and normal load increasing. Phase composition and chemical states analyses of frictional surface showed that the tribofilm formed was a combination of TiO2, Al2O3, SnO2, SiO2, and Fe2O3 mixtures. Microstructure and physical state of the wear debris were also investigated for deeply understanding the contribution of the tribofilm.
1. Introduction In recent years, there are great demands for new materials with lower or moderate friction coefficient which can work in larger temperature range in industrial applications fields [1–4], such as structural parts like foil bearings, pantograph board and brake disk for high-speed train. Most solid lubricants like graphite and MoS2 are generally begin to oxidize at about 300 °C in air, result in a limited application area at relatively high temperatures [4]. Thus the appropriate wear-resistant material with suitable friction coefficient and low wear rate should be picked, and the self-adaptive friction surface formed at any time during the large-scaled friction process should be considered as well. Mn+1AXn (n = 1–3) phases are a class of ternary carbides and nitrides, where M is an early transition metal, A is the group IIIA or IVA element, X is either carbon or nitrogen, and MX slabs are separated by planar layers of A-elements [5]. This kind of materials has aroused people's interests recent years, they exhibit excellent properties, such as excellent capability in oxidation resistance [6,7], high damage tolerance [8], thermal shock resistance [9], and ductile-to-brittle transition [10], which are closely relevant to tribological performance. MAX phases hence are considered as promising candidates for tribological
applications. Typical MAX phases of Ti3SiC2 and Ti3AlC2 have been confirmed having outstanding tribological performance [11–16]. It is due to the unique nanolayered hexagonal structure of MAX phase, similar to that of graphite, the layered structure plays an important role in the friction process [12,13]. Besides, the important second point is due to the oxide film with self-lubricating effect. This self-generated tribofilm was composed mainly by various oxidation products contained disengagement A-site elements [14–18], it is primarily attributed to the coexisting multiple interlayer bonding force that the M-A bonds are weaker than the M-X bonds. Huang and Zhai et al. [15–18] studied the dry sliding tribological behaviors of Ti3SiC2 and Ti3AlC2 against low carbon steel disk using a block-on-disk tester. They found that both the friction coefficient and wear rate were dependent on the coverage of frictional films formed during sliding process. The frictional films were consisted of a mixture of O, Ti, Al or Si, and Fe on the friction surface. Ti3SiC2 was reported exhibiting a friction coefficient of 0.53–0.25 and a wear rate of 0.6–2.5 × 10−6 mm3/N m, and Ti3AlC2 exhibited a friction coefficient of 0.53–0.16 and a wear rate of 0.91–3.75 × 10−6 mm3/N m as the sliding speed increased from 5 m/s to 60 m/s, respectively, both showing excellent friction performance [15–18]. Souchet et al. [19]
⁎ Corresponding author at: Centre of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (Z. Huang).
https://doi.org/10.1016/j.ceramint.2018.10.115 Received 2 August 2018; Received in revised form 14 October 2018; Accepted 14 October 2018 Available online 15 October 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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have investigated the friction properties of Ti3SiC2/steel and Ti3SiC2/ Si3N4 tribo-pair, an instantaneous heat-induced film contained alumina grinding dust was reported forming on the friction surface. The compacted wear debris was acted as a thin film, leading to quite low wear rate and friction coefficient of Ti3SiC2. Take advantages of the excellent mechanical properties and frictional performance, MAX phases were common applied as frictional enhancement phases [20–22] or as tribological protective coatings [23–25]. Ti3SiC2 powder can be used as lubrication phases to enhance the wear resistance of NiAl intermetallic compound for the proposed applications such as turbine blade tips [21]. Ti3SiC2, Ti2AlN, and Cr2AlC thin films were applied against a counterpart bearing steel [24]. The selected MAX phases exhibit excellent tribological properties obviously, which can be applied as the most promising candidate for tribological system, even adapted in harsher conditions, e.g., working at elevated temperatures, or in corrosion environment [26–28]. It can be estimated that adjusting the phase composition and oxide proportion of the dominated friction film will lead to different physical form, which continue to affect the tribological properties of MAX phases, by providing that preparing MAX phase solid solutions through adding various A elements. In reality, Huang et al. [29] have investigated that the friction coefficient was continuously adjusted in a range of 0.1–0.4 according to the different content of Sn by forming Ti3Al(Sn)C2 solid solution, and still maintained at low wear rate. At same dry sliding conditions, the tribological oxide film of Ti3AlC2 was in a fused state during sliding process, while that of Ti3Al(Sn)C2 solid solution existed in a soft state, resulting in larger friction coefficient. Accordingly, the friction coefficient of Ti3Al(Sn)C2 could be tailored by adjusting the content of Sn dopant in the solid solution to enlarge its application range. Solid solution treatment was also verified as an effective way to optimize the mechanical performance of MAX phase [30]. Some recent work focused on oxidation behaviors of MAX phase solid solution were studied as well [31,32]. The oxidation rate of Ti2Al1xSnxC solid solution was reported could be accelerated by substituting Al by Sn in Ti2Al1-xSnxC at lower temperature [32]. It is conceivable that MAX phase solid solution is more likely to generate tribological oxide film during the friction process. The preceding discussions offered potential guidance for the development of MAX phases in friction field by adding varieties of A-site elements consciously. As yet, there are few studies focus on the tribological properties of the various A-site elements (three or more) MAX phase solid solutions. A new typed MAX phase solid solution of Ti3Al0.8Si0.2Sn0.2C2 has been synthesized in our previous work [33], Ti3Al0.8Si0.2Sn0.2C2 were observed having a characteristic layered structure and exhibited an obvious solid solution effect. The present work tried to investigate the tribological performance of Ti3Al0.8Si0.2Sn0.2C2 MAX phase solid solution. Friction coefficient and wear resistance were studied, and special attention was paid to the chemical composition and physical state on the friction surface to further understand how frictional performances were affected by sliding contact surfaces. The morphology and composition of wear debris were analyzed as well.
Fig. 1. Schematic representation of the friction tester.
observed having a plate-like shape and layered structure, the average grain size was estimated to be ~ 10 µm in length and ~ 3 µm in width, the volume density was determined to be 4.68 g/cm3 and Vickers hardness was 5.4 ± 0.3 GPa [33]. Ti3Al0.8Si0.2Sn0.2C2 samples were cut into blocks of 10 mm × 10 mm × 12 mm. Dry sliding friction and wear behaviors of Ti3Al0.8Si0.2Sn0.2C2 were evaluated using a homemade block-on-disk type high-speed tester, the schematic representation of the friction tester was shown in Fig. 1. A S45C steel (after quenched and tempered) disk of Φ300 mm × 10 mm was selected as the friction counterpart, the frictional contact area was 10 mm × 10 mm. Quenched and tempered S45C steel is widely used as structural equipment like wheel gear, wheel axle, wheel hub and plunger due to excellent integrated mechanical properties. Therefore, tribological behaviors of Ti3Al0.8Si0.2Sn0.2C2 sliding against S45C steel were investigated in order to provide guidance for Ti3Al0.8Si0.2Sn0.2C2 applying as matching parts of structural equipment in this study. Vickers hardness of the S45C steel was determined with a TH700 hardness tester, ten indentation tests were carried out at 98 N with a dwell time of 15 s. Hardness of the S45C steel disk was tested to be 3.2 ± 0.1 GPa. Friction tests were carried out with sliding speeds in a range of 5–30 m/s, and the applied normal load was 20–80 N for each sliding speed. The sliding distance of a continuous process was of 12,000 m. Friction coefficients were automatically measured and recorded using the computer system of the friction tester, the wear quantity of the samples was measured by weighing method, which measured the mass loss of the samples per sliding distance and per normal load. Pre-abrasion was made at the first stage to eliminate any possible influence on the friction surface, and the tests for every given conditions were repeated three times to obtain the average value. The phase composition and microstructure of fractured surface were identified by XRD analysis and a Scanning Electron Microscope (SEM, ZEISS EVO 18, German) equipped with an Energy Dispersive X-ray Spectrometer (EDXS, Bruker Nano XFlash detector 5010). EDS compositions of the friction surface were presented in at% (atomic percent). The chemical states of elements on the worn surface were examined by (ESCALAB 250Xi) X-ray Photoelectron Spectroscopy (XPS). The surface topography of the tribological surface was observed by Atomic Force Microscopy (AFM) using a Dimension Fastscan™ atomic force microscope. The wear debris was collected and analyzed by a Thermogravimetric and Differential Scanning Calorimetric (TG&DSC: SDT Q600) to identify the fusing temperature of the frictional film in the temperature range room temperature ~ 1300 °C with a heating rate of 5 °C/min in Ar atmosphere. The Gibbs free energies of the related
2. Experimental procedures Polycrystalline bulk Ti3Al0.8Si0.2Sn0.2C2 samples were prepared by two-time hot-pressing. The processing details have been reported elsewhere [33], in brief, dense Ti3Al0.8Si0.2Sn0.2C2 bulks were two-time hot-pressed at 1450 °C for 30 min with a pressure of 30 MPa in Ar using synthesized Ti3Al0.8Si0.2Sn0.2C2 powders (Ti3Al0.8Si0.2Sn0.2C2 powders were pressurelessly sintered from Ti, Al, Si, Sn and TiC powders, detailed process was reported in Ref. [33]) as raw materials. Phase identification of the bulk samples was carried out by X-Ray Diffraction (XRD) analysis with Cu Kα radiation (X′ PERT-PRO MPDT, Netherlands). The bulk samples consisted of dominant Ti3Al0.8Si0.2Sn0.2C2 phase from XRD analysis, no impurity phases like TiC, Al3Ti, Ti6Sn5 or Ti5Si3 were detected [33]. Most of the Ti3Al0.8Si0.2Sn0.2C2 grains were 2104
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from 2.76 × 10−6 mm3/N m to 4.34 × 10−6 mm3/N m. When at higher sliding speed of 30 m/s, the wear rate was 3.91 × 10−6 mm3/ Nm corresponding to 20 N, and that was 5.6 × 10−6 mm3/N m corresponding to 80 N. The results showed that wear rates were greatly affected accordingly to sliding speeds. When at sliding speeds of 10 m/s, 20 m/s and 30 m/s, the speed-dependent changed trend was reversed to that of friction coefficient in Fig. 2(a). The friction coefficient and wear rate showed a similar changing phenomenon in Ti3Al0.8Sn0.4C2 [34]. The friction coefficient of Ti3Al0.8Sn0.4C2 bulk showed a variation tendency from 0.48 to 0.3 and a low wear rate from 0.4 × 10−6 mm3/N m to 4.3 × 10−6 mm3/N m within the sliding speed changing from 5 m/s to 10 m/s against low carbon steel. Predictably, the wear rate of Ti3Al0.8Si0.2Sn0.2C2 could be governed by comprehensive effects of sliding speed and load, relating to the state of the friction surface. 3.2. Microstructure analysis of friction surfaces The friction surfaces of Ti3Al0.8Si0.2Sn0.2C2 were observed by SEM (see Fig. 3) to further understand the variation trend of friction coefficient and wear rate. Friction surfaces were observed different in images with the normal load changing. The percentage of frictional film coverage increased with normal load increasing, as shown in Fig. 3(a)(b), the tribological film distributed sporadically in case of 5 m/s and 20 N while it gradually became continuously when the normal load increased to 80 N. This tribological film was also strongly speed-dependent, the friction surfaces were almost completely covered by continuous and smoother tribological film (see Fig. 3(b)–(e)) as the sliding speeds increased from 5 m/s to 30 m/s. Regional map scanning was applied to analyze the composition of tribological films. Table 1 lists the main chemical constituents of the different regions as denoted by yellow dashed frames in Fig. 3. It was observed that all of the worn surface contained O elements under any friction condition, indicating the tribo-oxidation reacted during the friction process. O contents in regions of zone D-F were observed higher than those in the other regions, it suggested that sufficiently continuous and compact tribofilms were gradually covered on the friction surface, the main mechanisms were conceived to be oxidation wear and twobody abrasion wear. Friction films did positive effects on the friction behaviors, which was conducive to preventing the friction surface from adhering onto the counterpart and was liable for the decreased friction coefficient during the sliding effectively, as demonstrated in previous studies [15–18,34,35]. Thus it was reasonable to conclude that Ti3Al0.8Si0.2Sn0.2C2 demonstrated lower friction coefficient when at higher sliding speeds of 20 m/s and 30 m/s. Fe element was transferred from S45C steel counterpart, which was attributed to the adhesive wear behavior of tribological pairs during sliding process. It was no doubt that material tends to transfer from soft to the hard one, the terms soft and hard herein were defined by Vickers hardness (HV) of materials. The HV value of Ti3Al0.8Si0.2Sn0.2C2 is about 5.4 ± 0.3 GPa [33], higher than that of S45C steel, 3.2 ± 0.1 GPa. Then the transference from S45C steel (a softer material) to Ti3Al0.8Si0.2Sn0.2C2 (a harder material) might perform. In the light of EDS analysis, small amounts of Fe were detected on the selected area at 30 m/s, revealing that the transfer of Fe element from S45C steel counterpart was reduced, precisely should be ascribed to the smooth and almost full coverage of tribo-oxidation film, which apparently prevented the scraping and adhesive wear behaviors between the contact surfaces. Zhai and Huang et al. [15–18] have investigated the interaction between sliding wear and oxidation of Ti3SiC2 and Ti3AlC2. Results revealed that the tribo-oxidation was a procession of formation, constant consumption and re-composition. That's to say, if the tribofilms happened to wear off, new ones could be formed to take their place instantaneously. Actually, it is conceivable that the oxidation occurring on the Ti3Al0.8Si0.2Sn0.2C2 surface is proportional to the friction work μLV (L is the normal load, V is the sliding speed, and μ is the friction coefficient [36]), the rate of generating oxides is an increasing function
Fig. 2. (a) Coefficient of friction and (b) wear rate of Ti3Al0.8Si0.2Sn0.2C2 sliding against S45C steel disk at different sliding speeds as a function of normal load.
possible formed products were calculated by the HSC Chemistry 6.0 software. 3. Results and discussions 3.1. Friction and wear behaviors Fig. 2(a) shows typical curves of kinetic friction coefficients of Ti3Al0.8Si0.2Sn0.2C2 sliding against S45C steel under normal loads of 20–80 N and sliding speeds of 5–30 m/s. It was obvious that the friction coefficients were speed- and load-dependent. For the low sliding speed of 5 m/s, the friction coefficient increased from 0.46 to 0.53 with increasing the normal load from 20 to 40 N, then it gradually reduced from 0.46 to 0.43 with increasing the normal load from 50 to 80 N, almost kept stable. For the sliding speed of 10 m/s, the friction coefficients retained at around 0.35 as the load increased. A diverse change was found at higher sliding speed of 20 m/s, the friction coefficient increased from 0.26 to 0.35 with increasing the normal load from 20 to 80 N, showing a monotonous increase trend within the increase of normal load. Similar load-dependent change was exhibited at 30 m/s, the corresponding friction coefficient was 0.18 under the normal load of 20 N, and that was 0.3 when the normal load increased to 80 N. Sliding speed seemed as an assignable factor clearly, the friction coefficient was notably reduced with the increasing sliding speed, and normal load also effected the friction coefficient, which varied according to the corresponding sliding speed. It is suggested that the friction behavior could be governed by both of the sliding speed and normal load. Wear rates of Ti3Al0.8Si0.2Sn0.2C2 are shown in Fig. 2(b) as functions of normal load and sliding speed. For the lower sliding speed of 5 m/s, the wear rate was 14 × 10−6 mm3/Nm corresponding to 20 N, and that was 4.1 × 10−6 mm3/N m corresponding to 80 N, it showed a significant decrease as normal load increased. However, for the sliding speed of 10 m/s, the wear rate increased mildly from 1.78 × 10−6 mm3/N m to 2.84 × 10−6 mm3/N m with the increase in normal load from 20 to 80 N. For that of 20 m/s, the wear rate increased slightly 2105
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Fig. 3. SEM micrographs of Ti3Al0.8Si0.2Sn0.2C2 friction surfaces under the different sliding speeds and the normal loads: (a) 5 m/s and 20 N; (b) 5 m/s and 80 N; (c) 10 m/s and 80 N; (d) 20 m/s and 80 N; and (e) 30 m/s and 80 N.
corresponding normal load and assumed a linear descending tendency. The sliding roughness of friction surfaces after 5 m/s, 80 N and 30 m/s, 80 N were determined with an AFM (Fig. 4). The average surface roughness (Rq) were 102 nm and 53.6 nm for sliding speeds of 5 m/s and 30 m/s, respectively. Friction surface under 30 m/s was found to be extremely smooth, which was consistent with the observing in SEM micrographs in Fig. 3(e). White dotted areas were taken from the regional continuous tribofilm on both friction surfaces to compare with the roughness throughout the region to determine the roughness of local continuous frictional film. Surface roughness of the white dotted area were 13.7 nm in Fig. 4(a) and 18.2 nm in Fig. 4(c). The higher entire surface roughness after sliding speed of 5 m/s indicated that the tribological film covered unevenly, only sporadic impacted tribofilms were consisted on the worn surface. Inversely, the tribological film covered uniformly and continuously on the friction surface at 30 m/s under 80 N. Combining with the foregoing SEM microstructure, tribofilm formed at 30 m/s under 80 N was found to be extremely smooth. It was conceivable that the impacted continuous frictional oxide film formed on the worn surface was responsible for the relative low friction coefficient and moderate wear rate. Smoother friction surface could lead to a better wear resistance. Besides, when at relative low sliding speed (5 m/s or 10 m/s), local continuous tribofilm slightly played an effective role to reduce the abrasion and adhesive wear. Thus for Ti3Al0.8Si0.2Sn0.2C2, although the friction coefficients were higher when at 5 m/s and 10 m/s than that at 20 m/s and 30 m/s, the corresponding friction coefficients were still keep at lower level compared with other ceramic material worked with no produced tribo-oxide film under
Table 1 Main chemical constituents of the regions noted by yellow dashed frame in Fig. 3. Friction condition
Region
Main chemical constituents
5 m/s, 20 N
Zone Zone Zone Zone Zone Zone Zone Zone Zone Zone
Ti25.6Al7.1Si2.1Sn1.9O33.4Fe2.1 Ti19.1Al4.9Si1.5Sn1.5O45.7Fe6.9 Ti21.5Al5.6Si1.7Sn1.6O44.4Fe6.8 Ti9.7Al1.4Si0.6Sn0.7O55.4Fe22.3 Ti13.5Al2.8Si1.1Sn1.0O53.8Fe18.2 Ti20.3Al5.4Si1.5Sn1.4O55.2Fe7.5 Ti13.7Al3.4Si1.1Sn0.9O52.4Fe17.0 Ti13.5Al3.3Si1.1Sn0.9O52.5Fe16.9 Ti19.7Al5.4Si2.0Sn0.9O54.8Fe2.7 Ti17.6Al4.8Si2.2Sn1.1O51.5Fe3.5
5 m/s, 80 N 10 m/s, 80 N 20 m/s, 80 N 30 m/s, 80 N
A B C D E F G H I J
of the μ, L or V. In general, the oxide film can be formed on the friction surface only if the generating rate was larger than the consuming rate, while a larger generating rate of oxides could result into a larger wear rate. Since there would be more oxides generated at higher sliding speed due to the higher friction-induced temperature, in terms of the collected wear rate in this work, wear rates of 30 m/s, 20 m/s and 10 m/s increased by degrees. When at sliding speed of 5 m/s, the friction surface was ploughed and scratched heavily for lacking of the oxide film at lower normal load, consequently the severe friction surface induced higher wear rate. When the normal load increased gradually, the state of tribofilm was changed to be extensive and smooth, hence, the wear rate curve at 5 m/s showing in Fig. 3(b) was relied heavily on the 2106
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Fig. 4. AFM analysis in scanning mode on 10 µm × 10 µm Ti3Al0.8Si0.2Sn0.2C2 surface after sliding at 5 m/s, 80 N for 3 × 12,000 m of (a) isometric view; (b) top view; and at 30 m/s, 80 N for 3 × 12,000 m of (c) isometric view; (d) top view.
was difficult to obtain XRD pattern of the tribological film because Xray could readily penetrate the tribological film and reached into the sub-surface. Hence, XPS analysis was applied here (see Fig. 6) in order to further identify the elements chemical states of tribological film on the worn surface. In Fig. 6, the characteristic XPS peaks of MAX phase disappeared after sliding, indicating that Ti, Al, Si and Sn existed in other compounds on the worn surfaces. The XPS peaks at 458.9 eV and 464.6 eV were corresponding to Ti2p3/2 and Ti2p1/2 in TiO2 [37,38], the peak at 75.02 eV was assigned to Al2O3 [21,37,38], the peak at 102.05 eV was in agreement with the XPS shifts of SiO2 [21,38], and the peaks at 487.02 eV and 495.75 eV ware associated with SnO2, as has been reported [39]. The peaks corresponded to Fe2p in Fe2O3 were attached as well. No XPS spectra of Ti2p, Al2p, Si2p or Sn2p was assigned to Ti3Al0.8Si0.2Sn0.2C2 compound, the tribological film formed on the worn surface was a total combination of wear debris contained various oxides. The friction and wear behaviors were verified to be governed by the oxide films, rather the Ti3Al0.8Si0.2Sn0.2C2 surface. Fig. 7 shows standard Gibbs free energies (ΔG) of formation for SnO2, Fe2O3, SiO2, TiO2, and Al2O3 calculated by the HSC6.0 Chemistry software to verify the possibility of oxidation products formation during friction process. The sequence of the phase formation was Al2O3﹥TiO2﹥ SiO2﹥Fe2O3﹥SnO2, Al2O3 was relatively more easily produced. Combining with the former EDS analysis of zone I and zone J in Fig. 3(e) and XPS analysis, TiO2 and Al2O3 were confirmed as predominant constituents of the tribofilm. It was established that TinO2n-1 and Al2O3 phases contributed to good wear resistance [15–18,35,37,38], in Refs. [15–18,23–29], SiO2, Al2O3, SnO2 and Fe2O3 played positive role in wear resistance as well. That's to say, the tribofilms containing various oxides including mixtures of Ti-Al-Si-Sn-Fe-O contributed to the lubricity and wear resistance. However, the tribofilm was too thin to identify the exact distribution of each oxide, maybe it will be deeper
vacuum or argon atmosphere [35,37] and most engineering materials. The tribofilm was further analyzed by cross-sectioning. Fig. 5 shows the cross-section SEM micrographs of the friction surface after three continuous sliding processes (3 × 12,000 m) performed at different sliding speeds. Tribofilm formed at sliding speed of 5 m/s was not shown here for comparison because the uneven surface was too thin to measure the thickness, but have an analogous appearance. It can be clearly observed that continuous and adherent films were formed on the friction surfaces. The tribofilm thickness at 10 m/s was estimated to be about 0.6 µm, whereas those at 20 and 30 m/s were estimated to be 1.46 and 1.44 µm, indicating that tribofilms generated more easily at higher sliding speed. The friction induced heat accelerated the oxidation of contact surfaces on account of the increasing sliding speeds. Moreover, continuous oxide films were maintained on friction surface when the generating rate was equal to consuming rate, then the film thickness was basically steady, playing lubricate and anti-attrition effects. EDS line scanning analysis was performed to identify the compositions of the substrate and films. Obviously, Ti, Al, Si, Sn and C elements were distributed uniformly in the whole substrate, suggesting that Asite elements of Al, Si and Sn were almost homogeneous despite of the tribo-oxidation process. Contents of Ti, Al, Si and Sn were obviously lower in tribological film than those in substrate while O and Fe contents increased. It can be estimated that the friction film was mainly composed of an oxide mixture of various elements. When at sliding speeds of 20 m/s and 30 m/s, the increase of O content was more pronounced. Higher sliding speed correlative with friction induced heat accelerated the tribo-oxidation process. As described above, all the tribofilms were mainly composed of mixture oxides of Ti, Al, Si and Sn elements, and Fe element was detected as well, which was transferred from the counterpart. In reality, it 2107
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Fig. 5. Cross-section of the friction surface at the sliding conditions of (a) 10 m/s, 80 N; (b) 20 m/s, 80 N; (c) 30 m/s, 80 N. (d), (e) and (f) are the corresponding EDS line scanning analysis of 10 m/s, 20 m/s and 30 m/s along the yellow lines.
Fig. 6. XPS spectra of Ti, Al, Si, Sn and Fe elements on the friction surface of Ti3Al0.8Si0.2Sn0.2C2 after sliding at 30 m/s and 80 N.
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Fig. 7. Standard Gibbs free energies of formation for SnO2, Fe2O3, SiO2, TiO2, and Al2O3.
Fig. 9. TG&DSC analysis curves of the wear debris.
characterized by other advanced methods. 3.3. Analysis of the wear debris SEM images of wear debris from Ti3Al0.8Si0.2Sn0.2C2 after sliding is shown in Fig. 8. Wear debris existed in the forms of fine pulverized scrap when at the sliding speed of 5 m/s. Easy-cutting and impacting between the counterparts were serious, which were ascribed to the lack of continuous tribofilm and the three-body abrasion wear. At medium sliding speed of 10 m/s and 20 m/s, the wear debris was plate shaped and uniform-sized. When the sliding speed increased to 30 m/s, large tracts of wear debris were formed, and some of them could be peeled off as pulverized scrap along with friction. The more generated oxides caused more consumed oxides, resulting into higher wear rate, which was in consistent with the foregoing discussion in variation of the wear rate. To validate the fusibility of the tribofilm, the fusion point of the wear debris was identified by TG&DSC analysis, showing in Fig. 9. There was an endothermic peak in an approximate range of 900–1300 °C, indicating the oxide film would be fused when the temperature of contact area reaches or exceeds 900 °C. This behavior was quite different from that exhibited by the oxide film on the Ti3Al0.8Sn0.4C2 friction surface [34], where no significant endothermic peak was indicated in the range of 100–1200 °C, implying the oxide film
Fig. 10. XRD analysis of the wear debris.
was not fused within the temperature range that the friction surfaced reached. Different characteristics of the tribo-induced oxides might make a difference. In order to make clear the endothermic process, XRD analysis (showing in Fig. 10) of the wear debris was carried out. It can be seen that the wear debris contained various oxygenated compound, basically corresponding to EDS analysis on the friction surface in Fig. 3(e), which in turn, could verify the tribofilm was generated by the impacted wear debris. In present study, glassy phases SiO2 were
Fig. 8. SEM micrographs of wear debris generated at the sliding conditions of (a) 5 m/s, 80 N; (b) 10 m/s, 80 N; (c) 20 m/s, 80 N; (d) 30 m/s, 80 N. 2109
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evidently detected in oxide scale. It was determined that SiO2 could formed as viscous or liquid glass phases at high temperature and easily diffused into everywhere [40]. Besides, the aluminosilicate glass phases were formed as well from XRD result, they were mainly composed of Al2O3 and SiO2 and might be caused by the dissolution of Al2O3 into SiO2. These glassy phases have a lower softening point under a level of 1000 °C, which would lead to an endothermic process. The glassy phases have fluidity and can fill pores and other defects of the friction surface during the friction process, thus the oxide films were exactly in a liquid stage. Clearly, such a liquid oxide film must have a significant lubricating effect on the friction surface and consequently playing an important role in the friction and wear resistance.
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4. Conclusions The intrinsic dry sliding tribological performance of a new typed Ti3Al0.8Si0.2Sn0.2C2 solid solution was investigated in this paper. Friction coefficient and wear rate were measured and the results showed that Ti3Al0.8Si0.2Sn0.2C2 exhibited a good friction stability as well as quite low friction coefficient and wear rate. Actually the low friction coefficient and wear rate were strongly speed-dependent and faintly normal load-dependent in the sliding speed range of 5–30 m/s under the applied normal load of 20–80 N. The friction coefficient was significantly reduced while the wear rate increased with an increase of sliding speed. Both of friction coefficient and wear rate showed a monotonous increase trend within the increase in normal load, except that the wear rate was significant dependent on normal load at lower sliding speed of 5 m/s. Comparatively continuous self-generated tribofilm with a certain thickness was confirmed playing an effective role in the excellent tribological performance through characterizing the friction surfaces. The tribofilm was consisted of a mixture oxides of Ti-AlSi-Sn-Fe-O attached to the friction surface. It was precisely account of the smoother and continuous tribofilm formed by higher sliding speed induced higher friction heat that the friction coefficient showed a decreasing trend with the increase in sliding speed. Due to the tribo-oxidation was a procession of formation, constant consumption and recomposition, frictional wear debris could be peeled off as pulverized scrap along with friction. Large tracts of wear debris was found generated at higher sliding speed, and then resulted into higher wear rate. Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities under Grant no. 2018YJS146, by National Natural Science Foundation of China (NSFC) under Grant nos. 51572017 and 51871011, and by Beijing Government Funds for the Constructive Project of Central Universities. The financial supports by them are greatly appreciated. References [1] C. Muratore, A.A. Voevodin, Chameleon coatings: adaptive surfaces to reduce friction and wear in extreme environments, Annu. Rev. Mater. Res. 39 (2009) 297–324. [2] S.M. Aouadi, B. Luster, P. Kohli, C. Muratore, A.A. Voevodin, Progress in the development of adaptive nitride-based coatings for high temperature tribological applications, Surf. Coat. Technol. 204 (2009) 962–968. [3] W. Wang, Application of a high temperature self-lubricating composite coating on steam turbine components, Surf. Coat. Technol. 177 (2004) 12–17. [4] H. Heshmat, P. Hryniewicz, J.F. Walton II, J.P. Willis, S. Jahanmir, C. DellaCorte, Low-friction wear-resistant coatings for high-temperature foil bearings, Tribol. Int. 38 (2005) 1059–1075. [5] M.W. Barsoum, The Mn+1AXn phases: a new class of solids: thermodynamically stable nanolaminates, Prog. Solid. State Chem. 28 (2000) 201–281. [6] M.W. Barsoum, N. Tzenov, A. Procopio, T. El-Raghy, M. Ali, Oxidation of Tin+1AlXn (n = 1-3 and X = C, N): II experimental results, J. Electrochem. Soc. 148 (8) (2001) B234–B239. [7] X.H. Wang, Y.C. Zhou, High-temperature oxidation behavior of Ti2AlC in Air, Oxid. Met. 59 (2003) 303–320. [8] M. Radovic, M.W. Barsoum, MAX phases: bridging the gap between metals and
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