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Effect of TiC coated MWCNT content on friction and wear behavior of MWCNT–Ti3SiC2 composites
Materials Research Bulletin 48 (2013) 315–323
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Effect of TiC coated MWCNT content on friction and wear behavior of MWCNT–Ti3SiC2 composites Xiaoliang Shi a,b,*, Zhiwei Zhu a, Mang Wang a, Wenzheng Zhai b, Zengshi Xu a, Jie Yao a, Siyuan Song a, Qiaoxin Zhang a,b a b
School of Mechanical and Electronic Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 August 2012 Received in revised form 6 October 2012 Accepted 17 October 2012 Available online 26 October 2012
Ti3SiC2 doped with various contents of TiC coated multi-walled carbon nanotubes (MWCNTs) were fabricated by spark plasma sintering. The effect of TiC coated MWCNT content on friction and wear properties of MWCNT–Ti3SiC2 composites was investigated in this paper. The results showed that MWCNT–Ti3SiC2 composites doped with 3 wt.% TiC coated MWCNTs had the best mechanical and tribological properties, and the relative density and Vicker’s microhardness were 98.7% and HV1 806.2 respectively. When the counterpart was GCr15 steel ball, the friction coefficient and wear rate were 0.40 and 1.80 10 4 mm3 N 1 m 1 at the load of 10 N and sliding speed of 0.234 m s 1 for 20 min at room temperature. Whereas, with Si3N4 ceramic ball counterpart, the friction coefficient and wear rate were 0.52 and 4.87 10 4 mm3 N 1 m 1 respectively. The excellent tribological performance could be attributed to the synergetic effect of the enhanced TiC coated MWCNTs and Ti3SiC2 matrix, which provided the composites with good deformation resistance and high load bear capacity, as well as the excellent balance between strength and lubricity. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Chemical synthesis C. Electron microscopy D. Mechanical properties
1. Introduction Carbon nanotubes (CNTs) have been received a great deal of attention due to their superior mechanical and physical properties. In particular, multi-walled carbon nanotubes (MWCNTs) have been expected to be used in industrial applications as their price has steadily decreased [1]. CNTs have been considered as useful and attractive additives to organic materials [2,3], bulky metal [4], metallic coating [5], bulky ceramic [6] and ceramic coating [7] in order to improve the wear resistance and lower friction of mechanical components. According to the theoretical consideration, a friction coefficient between the walls of MWCNTs should be extremely low. That is, MWCNTs have a significant selflubricant property by nano-ball bearing effects [8]. Gupta and Barsoum [9] reported that the layered hexagonal MAX phases were thermodynamically stable nanolaminates displaying unusual and sometimes unique properties. Their layered nature suggested they might have excellent promise as solid lubricant materials. Recently, first generation MAX phase based composites shafts were successfully tested against Nibased superalloy at 50,000 rpm from room temperature till
* Corresponding author. Tel.: +86 27 87651793; fax: +86 27 87651793. E-mail address: [email protected] (X. Shi). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.10.028
550 8C during thermal cycling in a foil bearing rig. This study further demonstrates the potential of MAX phases and their composites in different tribological applications. Ti3SiC2 is a representative among MAX ternary compounds (Mn + 1AXn where, M: early transition metal, A: group IIIA or IVA element, X: C and/or N, n = 1–3), which combines merits of both metals and ceramics, such as low density, high Young’s modulus, good thermal and electrical conductivity, excellent thermal shock resistance, high temperature strength, damage tolerance, and easy machinability [10–12]. Furthermore, Ti3SiC2 performs low friction coefficient, coefficient of thermal expansion, and good self-lubricating property. Therefore it is a promising candidate for new high temperature structural materials, electrode materials in molten metal, electric brush materials and self-lubricating materials. Unfortunately, its low hardness (Vickers hardness of 4 GPa), creep strength and wear resistance restrict the potential application of Ti3SiC2. El-Raghy et al. [13] reported that wear rates of fine (5 mm) and coarse (100 mm) grained Ti3SiC2 sliding against a 440C stainless steel pin in pin-on-disk type tester were as large as 4.25 10 3 mm3 N 1 m 1 and 1.34 10 3 mm3 N 1 m 1 respectively. It shows that Ti3SiC2 is not very wear resistant because of its relatively low hardness, shear strength and ratio of hardness to elastic modulus. Gupta et al. [14] tested the layered Mn + 1ACn ternary carbides – MAX phases – Ta2AlC, Ti2AlC, Cr2AlC and Ti3SiC2 under dry sliding conditions against alumina at 550 8C. At
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0.4, the friction coefficient of Ti3SiC2 was the lowest measured, but the wear rate, at 2 10 4 mm3 N 1 m 1, was high. Moreover, Gupta et al. [15] researched the tribological behavior, at 25 and 550 8C of some layered (MAX phase) ternary carbides including Ti3SiC2, tested against Ni-based superalloys, SA (Inconel-718 and Inconel-600). The results showed that at room temperature, the wear rates were relatively high (10 4 mm3 N 1 m 1) and no correlation was found between the wear rates and the friction coefficients. Thus, researchers attempted to improve its poor wear resistance by adding some enhanced phases (e.g. Al2O3, TiC, SiC and TiB2) to Ti3SiC2 matrix [16–19]. Hu et al. [16] studied out that the wear rates of the Al2O3-reinforced Ti3SiC2 composites decreased with increasing Al2O3 content. The enhanced wear resistance is mainly attributed to the hard Al2O3 particles nail the surrounding soft matrix and decentrale the shear stresses under the sliding ball to reduce the wear losses. Sun et al. [17] studied the wear and friction properties of Ti3SiC2/7 wt.% TiC material against AISI 52100 steel pair. The results showed the friction coefficient was not sensitive to the load, and the average wear rate and friction coefficient were 9.9 10 5 mm3 N 1 m 1 and 0.4–0.5 respectively. Wan et al. [18] investigated sliding friction and wear behaviors of Ti3Si(Al)C2 and SiC-reinforced Ti3Si(Al)C2 composites against AISI 52100 bearing steel ball using a ball-on-flat, reciprocating tribometer at room temperature. The results showed that compared to monolithic Ti3Si(Al)C2, these SiC particle-reinforced composites exhibited improved wear resistance. The wear coefficients decreased significantly with the increase in SiC content, which meant that the wear resistance could be greatly enhanced by adding SiC particles into Ti3Si(Al)C2. Ti3Si(Al)C2/ 30 vol.% SiC composite showed the highest wear resistance. Yang et al. [19] studied that the non-lubricated, ball-on-flat sliding friction and wear properties of in situ (TiB2 + TiC)/Ti3SiC2 composites against bearing steel ball. The results showed that both friction coefficient and wear rate initially increased and then dramatically decreased with TiB2 content increasing from 0 vol.% to 20 vol.%. (TiB2 + TiC)/Ti3SiC2 composites with 15 vol.% and 20 vol.% TiB2 showed excellent friction and wear resistance, whose friction coefficient and wear rate were much lower than those of TiC/Ti3SiC2 composite. And it also is a meaningful attempt to improve the tribological properties of Ti3SiC2 by combining Ti3SiC2 with MWCNTs. To the best of our knowledge, meager information is available as regards the development of Ti3SiC2 matrix self-lubricating composites reinforced by MWCNTs. In order to obtain a clean matrix/ reinforcement interface and improve the bonding strength between MWCNTs and Ti3SiC2 matrix, as well as realize the stability of MWCNTs during the fabricated process of the composites, TiC coating on the surface of MWCNTs prepared by vacuum slow vapor deposition process was used as coupling agents, which could lead to better improvement in mechanical properties and tribological properties of MWCNT–Ti3SiC2 composites. Compared with the traditional powder metallurgy techniques, the main advantages of spark plasma sintering (SPS) are the fast heating speed and cooling speed, short sintering time, fine grain size of the prepared materials, controlled organizational structure, energy saving and environmental protection and so on [20–22]. In this study, MWCNT–Ti3SiC2 composites with different contents of TiC coated MWCNTs were prepared by SPS. The effects of TiC coated MWCNTs on the dry sliding friction and wear behavior of MWCNT–Ti3SiC2 composites at the load of 10 N and sliding speed of 0.234 m s 1 for 20 min at the room temperature were investigated through the determination of friction coefficients and wear rates and the analysis of the compositions of wear debris, the worn surfaces of both MWCNT–Ti3SiC2 composites and friction ball pairs.
2. Experimental details 2.1. Materials 2.1.1. TiC coating on MWCNTs Raw MWCNTs (diameter 40–60 nm, length 40–1000 nm) were firstly acidified by ultrasonicating in the mixture solution of 65 wt.% nitric acid and 98 wt.% sulfuric acid (H2SO4:HNO3 = 3:1, volume ratio) and refluxed at 100 8C for 2 h. Then the resultant solid was washed with deionized water till pH = 6. The solid was then collected, and dried in vacuum at 80 8C. The TiC coating on MWCNTs was very simple and convenient to produce. Commercial Ti and TiH2 powders (99.9% pure, Longxi Technology Development Co., Ltd., Shanghai, China) were provided as the titanium source. Firstly, a proper amount of pure Ti and TiH2 powders were dispersed and handled in the mixture of hydrofluoric acid and acetone solution (2:8 at weight ratio) using ultrasonic vibration for 30 min. After dispersing and handling, the product was filtered and dried at 90 8C in a vacuum oven. The pretreated Ti and TiH2 mixture powders were set on the bottom of an alumina crucible, and pretreated MWCNTs were placed upon the pretreated powders via a carbon felt. This apparatus [23] was covered with an alumina lid to keep the Ti source gas pressure inside the crucible and heated at 800 8C continuously with a heating rate of 5 8C min 1, and then kept at 800 8C for 1 h in a vacuum of about 10 3 Pa. After the coating treatment, the TiC coated MWCNTs were obtained. 2.1.2. Sintering of MWCNT–Ti3SiC2 composites MWCNT–Ti3SiC2 composites with different contents of TiC coated MWCNTs were prepared by SPS. The weight fractions of TiC coated MWCNTs in the composites were fixed at 0 wt.%, 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.% and 5 wt.%, respectively. MWCNT–Ti3SiC2 composites without TiC coated MWCNTs were denoted as MTA. While MTB, MTC, MTD, MTE and MTF represented MWCNT–Ti3SiC2 composites with 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.% and 5 wt.% TiC coated MWCNTs, respectively. The fabrication process of Ti3SiC2 powder was described in detail elsewhere [24], whose average particle size was about 5 mm. The starting powders were mixed by high energy ball-milling in vacuum. Balls and vials were made of hard alloy, and the charge ratio (ball to powder mass ratio) employed was 10:1. The milling time and speed are 10 h and 200 rpm respectively. After being mixed and dried, the mixtures were then sintered by SPS at 1200 8C under a pressure of 30 MPa for 10 min in pure Ar atmosphere protection. And the heating rate was 100 8C min 1. The cylindrical graphite molds with an inner diameter of 20 mm were used. The as-prepared specimen surfaces were ground to remove the layer on the surface and polished mechanically with emery papers down to 1200 grad, and then with 0.05 mm wet polishing diamond pastes. 2.2. Vicker’s microhardness and density The Vicker’s microhardness of each as-received specimen was measured using a HVS-1000 Vicker’s hardness instrument with a load of 1 kg and a dwell time of 8 s. Five tests were conducted and the mean value was given. The density of as-prepared specimens was determined by Archimedes’ principle. 2.3. Tribological test The tribological tests were conducted on a HT-1000 ball-ondisk high temperature tribometer. The disks, which were the asprepared materials, were cleaned with acetone and then dried in hot air before test. The counterpart ball was the commercial GCr15 steel ball (composition: 1.01 C, 1.50 Cr, 0.30 Mn, 0.25 Si wt.%, as USA 52100) with a diameter of 5 mm (HV0.2 690, Ra 0.2–0.3 mm).
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Moreover, the counterpart ball of the commercial Si3N4 ceramic ball with a diameter of 5 mm (about HV 15 GPa) was also used for MWCNT–Ti3SiC2 composites with optimum amount of TiC coated MWCNTs. The test temperature was room temperature. The sliding speed was 0.234 m s 1, and the applied load was 10 N. The friction radius was 2 mm, and the test time was 20 min. The friction coefficients were automatically measured and recorded in real time by the computer system of the friction tester. The wear quantity of the MWCNT–Ti3SiC2 composites was measured by the weighting method, which measured the mass loss of the samples for every friction process. The tests for every given conditions were repeated three times to obtain reliable data, and the average value was used as the evaluating data. 2.4. Analysis The surfaces of the as-prepared specimens and wear debris were examined by XRD with CuKa radiation at 30 kV and 40 mA at a scanning speed of 0.01 s 1 for the identification of the phase constitution. The morphologies and compositions of worn surfaces of MWCNT–Ti3SiC2 composites, GCr15 steel ball and Si3N4 ceramic ball friction pairs against the composites with optimum amount of TiC coated MWCNTs, as well as wear debris were analyzed by a SIRION 200 field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS). 3. Results and discussion 3.1. Microstructure analysis Fig. 1 exhibited the FESEM morphologies of fractured surface of MTD. As shown in Fig. 1(a), the composites had a dense and homogeneous microstructure. The average grain size of Ti3SiC2 particles was about 8 mm. Although the aggregation phenomenon of TiC coated MWCNTs still existed (see Fig. 1(b)), the entangled bundles of TiC coated MWCNTs were homogenously dispersed in the Ti3SiC2 matrix. Moreover, as shown in Fig. 1(a), there was a clean interface of Ti3SiC2 matrix/TiC coated MWCNTs existing, which would lead to better and improved mechanical and tribological properties of the composites. 3.2. Vicker’s microhardness and density Fig. 2 shows variation of relative density and Vicker’s microhardness HV1 of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs. As shown in Fig. 2, MTD exhibited the highest Vicker’s microhardness of HV1 806, which was much higher than HV1 470 of MTA. However, except the HV1 571.8 of MTE, the Vicker’s hardness of MTB, MTC or MTF was lower than that of MTA. Furthermore, the relative density of MTD was
Fig. 2. Variation of relative density and Vicker’s microhardness HV1 of MWCNT– Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs.
98.7%, which was the highest one among all the MWCNT–Ti3SiC2 composites. The relative density of MTA was 97.1%. It showed that the addition of 3 wt.% TiC coated MWCNTs could improve the strength of the Ti3SiC2, which could provide the MWCNT–Ti3SiC2 composites with good deformation resistance and high load bear capacity. However, the aggregation phenomenon of MWCNTs became more serious when the amounts of TiC coated MWCNTs in MTE or MTF were surplus. Moreover, the content of the TiC coated MWCNTs in MTB or MTC was lower, and the uniform dispersion of MWCNTs could not be well realized. Thus, the strength of the composites would be deteriorated. 3.3. Compositions of MWCNT–Ti3SiC2 composites Fig. 3 was the XRD patterns of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs. It could be found that the diffraction peaks primarily belonged to the Ti3SiC2 and TiC phases. Additionally, the (0 0 2) crystal plane of MWCNTs also appeared. The diffraction peak intensities of TiC phase obviously became stronger with the increase in doped amounts of TiC coated MWCNTs, which displayed four crystal planes of (1 1 1), (2 0 0), (2 2 0) and (3 1 1). In addition, the Ti3SiC2 displayed fifteen crystal planes of (0 0 4), (0 0 6), (1 0 1), (1 0 3), (1 0 4), (0 0 8), (1 0 5), (1 0 8), (1 0 9), (1 1 0), (1 0 2), (2 0 4), (1 1 8), (2 0 5) and (2 0 6). The TiC phase could be the enhanced phase to improve the wear resistance of MWCNT–Ti3SiC2 composites. Although the diffraction peak intensities of TiC in Fig. 3(e) (MTE) and Fig. 3(f) (MTF) were stronger than that in Fig. 3(d) (MTD), the Vicker’s microhardness of MTD was the
Fig. 1. FESEM micrographs of fractured surface of MTD.
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Fig. 3. XRD patterns of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs: 0 wt.% (a), 1 wt.% (b), 2 wt.% (c), 3 wt.% (d), 4 wt.% (e) and 5 wt.% (f).
highest one (see Fig. 2). Thus, MTD could have the best wear resistance.
Fig. 5. Variation of friction coefficients of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair.
Fig. 4 shows the typical measuring curves of the dynamic friction coefficients of MWCNT–Ti3SiC2 composites against GCr15 steel ball friction pair at a constant sliding speed of 0.234 m s 1 and load of 10 N at room temperature. It was obvious that the curve of the friction coefficient of MTD was relatively smooth (see Fig. 4(d)), which was about 0.4. However, the fluctuation ranges of friction coefficients of MTA (see Fig. 4(a)), MTB (see Fig. 4(b)), MTC (see Fig. 4(c)), MTE (see Fig. 4(e)) and MTF were larger. And the friction coefficients were in the range of 0.3–0.8. Among all the samples, MTF had the lowest COF of 0.3–0.45. Fig. 5 shows variation of the friction coefficient of MWCNT– Ti3SiC2 composites with varying TiC coated MWCNTs weight fraction. As shown in Fig. 5, the friction coefficient of MTA against GCr15 steel ball friction pair was about 0.62. And the friction coefficients of MTB, MTC, MTD, MTE and MTF were 0.43, 0.42, 0.40, 0.54 and 0.35, respectively.
Fig. 6 shows variation of wear rates of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair. Although the relative density of MTA was 97.1% (see Fig. 2), the wear rate of MTA was only 1.76 10 3 mm3 N 1 m 1. The results observed by El-Raghy et al. [13] and Gupta et al. [14,15] also showed that the wear rates of Ti3SiC2 at room temperature were relatively high. The wear rates of MTB and MTC were 1.10 10 2 mm3 N 1 m 1 and 3.89 10 2 mm3 N 1 m 1 respectively, which were much higher than that of MTA. The wear resistance of Ti3SiC2 was not enhanced by the doped 1 wt.% or 2 wt.% TiC coated MWCNTs, and the wear resistance was lowed. The doped amounts of the 1 wt.% or 2 wt.% TiC coated MWCNTs were lower, and the uniform dispersion of MWCNTs could not be well realized. Thus, the mechanical and tribological properties of the composites were deteriorated. As shown in Fig. 2, the relative densities of MTB and MTC were only 92.0% and 87.6% respectively, which were much lower than 97.1% of MTA. When the doped TiC MWCNTs was 3 wt.%, MTD had the lowest wear rate of 1.80 10 4 mm3 N 1 m 1. The wear rates of MWCNT–Ti3SiC2 composites increased with the
Fig. 4. Curves of friction coefficients of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs with sliding time at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair: 0 wt.% (a), 1 wt.% (b), 2 wt.% (c), 3 wt.% (d), 4 wt.% (e) and 5 wt.% (f).
Fig. 6. Variation of wear rates of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair.
3.4. Friction and wear behavior
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Fig. 7. FESEM morphologies and EDS patterns of worn surfaces of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair: 0 wt.% (a and b), 1 wt.% (c and d), 2 wt.% (e and f), 3 wt.% (g and h), 4 wt.% (i and j) and 5 wt.% (k and l).
increase in doped amounts of TiC coated MWCNTs. When the doped contents of TiC coated MWCNTs were 4 wt.% and 5 wt.%, the wear rates of the composites were 5.85 10 4 mm3 N 1 m 1 and 6.22 10 4 mm3 N 1 m 1. The aggregation phenomenon of MWCNTs could become more serious when the amounts of TiC coated MWCNTs in the composites were 4 wt.% and 5 wt.%. Moreover, the uniform dispersion of MWCNTs also was not realized. The relative densities of MTE and MTF were 93.4% and 94.7% respectively, which were lower than 98.7% of MTD. With the 3 wt.% TiC coated MWCNTs, the MWCNTs could be homogeneously dispersed in the MWCNT–Ti3SiC2 composites. Meanwhile, there was a clean interface of Ti3SiC2 matrix/TiC coated MWCNTs (see Fig. 1(c)). Some MWCNTs were well bonded to the Ti3SiC2 matrix in ‘‘bridging’’ manner. It would effectively enhance the mechanical properties such as hardness and toughness for the well-known reinforcing effect of MWCNTs. Meanwhile, the well bonded MWCNTs could have a load-bearing action, which might increase wear resistance of the friction surface. Consequently, as shown in Fig. 2, MTD had the higher Vicker’s microhardness of HV1 806 and relative density of 98.7 wt.%. MWCNTs have a significant selflubricant property by nano-ball bearing effects [8]. Thus, MTD against GCr15 steel ball pair had the excellent tribological properties among the samples, whose friction coefficient and wear rate were about 0.40 and 1.80 10 4 mm3 N 1 m 1 respectively. 3.5. Morphologies, compositions of wear debris and the worn surfaces of MWCNT–Ti3SiC2 composites and counterpart balls Fig. 7 shows FESEM morphologies and EDS patterns of worn surfaces of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair. As shown in Fig. 7(a), the coarse worn surface of MTA was covered with some deformed wear debris. A small amount of the
tribo-layers called ‘glazes’ [25] were formed. Fig. 7(b) was a result of EDS analysis for the B area shown in Fig. 7(a). The EDS pattern exhibited the presence of C, O, Si, Ti, Fe and a small amount of Al. The Al came from the Al addition that was used to accelerate the Ti3SiC2 formation. The contents of O and Fe were 23.35 at.% and 14.75 at.% respectively. It showed that the transfer of metal from GCr15 steel ball to the worn surface of MTA was serious. And the high O content indicated that the worn surface should be consisted of an amount of oxides. The slight traces in the smooth worn surface in Fig. 7(c) suggested that the wear mechanism of MTB was microcutting. A lot of the wear particle debris were only trapped on the worn surface. Fig. 7(d) was a result of EDS analysis for the D area shown in Fig. 7(c). The EDS pattern exhibited that the content of Fe was only 0.65 at.%. It showed that the transfer of material from GCr15 steel ball to the monolithic Ti3SiC2 surface was mild. Fig. 7(e) clearly shows that the wear debris was only trapped flow on the worn surface of MTC. Fig. 7(f) was a result of EDS analysis for the F area shown in Fig. 7(e). The EDS pattern exhibited the absence of Fe on the worn surface. According to Fig. 7(c–f), the hard asperities of GCr15 steel ball counterface just ploughed or cut soft MTB or MTC, and the wear rates should be high. There were a few trapped wear debris and discontinuous glazes existing on the smooth worn surface in Fig. 7(g), which suggested that the wear mechanism of MTD was microcutting and delamination. Fig. 7(h) was a result of EDS analysis for the H area shown in Fig. 7(g). The contents of O and Fe were 17.78 at.% and 2.25 at.% respectively. The material transfer of GCr15 steel ball to MTD was mild. The glazes should be consisted of an amount of oxides. There a few discontinuous glazes existing on the coarse worn surface in Fig. 7(i) suggested that the wear mechanism of MTE was microcutting and delamination. Fig. 7(j) was a result of EDS analysis for the J area shown in Fig. 7(i). The C content was 17.20 at.%. And the contents of O and Fe were 49.78 at.% and 28.96 at.% respectively. The higher Fe content indicated that the
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Fig. 8. FESEM morphologies and EDS patterns of wear debris of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair: 0 wt.% (a and b), 3 wt.% (c and d), 4 wt.% (e and f) and 5 wt.% (g and h).
material transfer from GCr15 steel pair was serious. The worn surface should contain oxides of Fe, Mn and Cr. The C content was 17.20 at.%, which was lower. However, the C did not come from the Ti3SiC2 matrix but the doped TiC coated MWNCTs because the contents of Si and Ti were only 1.30 at.% and 2.18 at.% respectively. The coarse worn surface and serious material transfer between friction pairs showed that the friction coefficient could be high. As shown in Fig. 7(k), the worn surface of MTF was nearly covered with a continued glaze film. Thus, the friction coefficient should be
low. Meanwhile, microfracture and delamination of the glaze also existed. Fig. 7(l) was a result of EDS analysis for the L area shown in Fig. 7(k). The EDS result showed that the glaze was consisted of oxides, matrix wear debris and C. Fig. 8 shows FESEM morphologies and EDS patterns of wear debris of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair. As shown in Fig. 8(a), most of wear debris obtained from MTA against GCr15 steel ball pair under
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Fig. 9. XRD patterns of wear debris of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs against GCr15 steel friction pair at the sliding speed of 0.234 m s 1 and load of 10 N: 0 wt.% (a), 3 wt.% (b).
the dry sliding friction and wear were fine equiaxed and few of them were lamellar. It indicated that the wear debris was mainly a product of having been directly generated from MTA. Fig. 8(b) was a result of EDS analysis for the Fig. 8(a). As shown in Fig. 8(b), the EDS pattern exhibited the presence of C, O, Al, Si, Ti and Fe. The contents of O and Fe were 24.09 at.% and 9.42 at.%, respectively. It showed that the wear debris was composed of oxides and the matrix wear debris. As shown in Fig. 8(c), most of wear debris obtained from MTD against GCr15 steel ball pair under the dry sliding friction and wear were lamellar and few of them were fine equiaxed. It indicated that the wear debris was mainly a product of the glazes rather than having been directly generated from MTD. Fig. 8(d) was a result of EDS analysis for the Fig. 8(c). The contents of C, O, Al, Si, Ti and Fe were 32.38 at.%, 32.18 at.%, 1.41 at.%, 6.68 at.%, 16.48 at.% and 10.86 at.%, respectively. It showed that the wear debris was composed of oxides, Ti3SiC2 and Sarkar et al. [26] investigated the unlubricated friction and wear properties of
Ti3SiC2 against steel. The results showed that under the selected fretting conditions, Ti3SiC2/steel tribocouple exhibited a transition in friction as well as wear behavior with coefficient of friction varying between 0.5 and 0.6 and wear rate in the order of 10 5 mm3 N 1 m 1. Raman analysis revealed that the fretting wear was accompanied by the triboxidation with the formation of TiO2, SiO2, and Fe2O3. Gupta et al. [14] point out that tribofilms, which were mainly comprised of X-ray amorphous oxides of the M and A elements and, in some cases, unoxidized grains of the corresponding MAX phases, were formed on the contact surfaces. Gupta et al. [15] indicated that the tribofilms resulted in low (<10 6mm3 N 1 m 1) wear rates and friction coefficients <0.5. The results in this research were similar to that observed by Sarkar et al. [26] and Gupta et al. [14,15]. The glazes actually had not only a significant antifriction effect, but also a protective action for the friction surfaces. Thus, the friction coefficient of MTD was about 0.40, and the wear rate was 1.80 10 4 mm3 N 1 m 1. As shown
Fig. 10. FESEM morphologies and EDS of worn surface of GCr15 steel (a and b) and Si3N4 ceramic ball (c and d) friction pair against MTD at the sliding speed of 0.234 m s load of 10 N.
1
and
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Fig. 11. Friction coefficients (a) and wear rates (b) of MTD against GCr15 steel and Si3N4 ceramic balls at the sliding speed of 0.234 m s
in Fig. 8(e), most of wear debris obtained from MTE against GCr15 steel ball pair under the dry sliding friction and wear were equiaxed and few of them were fine lamellar. It indicated that the wear debris was mainly a product of having been directly generated from MTE. Fig. 8(f) was a result of EDS analysis for the Fig. 8(e). The contents of C, O, Si, Ti and Fe were 21.78 at.%, 37.01 at.%, 0.96 at.%, 1.69 at.% and 38.55 at.%, respectively. It showed that the wear debris was mainly composed of C and oxides of Fe. As shown in Fig. 8(g), most of wear debris obtained from MTF against GCr15 steel ball pair under the dry sliding friction and wear were lamellar and few of them were fine equiaxed. It indicated that the wear debris was mainly a product of the glazes rather than having been directly generated from MTF. Fig. 8(h) was a result of EDS analysis for the Fig. 8(g). The contents of C, O, Si, Ti and Fe were 24.71 at.%, 26.45 at.%, 1.97 at.%, 4.49 at.% and 42.38 at.%, respectively. It showed that the wear debris was also mainly composed of C and oxides of Fe. The glazes actually had not only a significant antifriction effect, but also a protective action for the friction surfaces. And the friction coefficient of MTF was only about 0.35. However, the Vicker’s microhardness of the
1
and load of 10 N.
composites was only HV1 422.5. Thus, the wear rate was 6.22 10 4 mm3 N 1 m 1, which was higher than that of MTD. Fig. 9 shows the XRD patterns of wear debris of MTA and MTD against GCr15 steel friction pair at the sliding speed of 0.234 m s 1 and load of 10 N. As shown in Fig. 9(a), there were TiO2, Fe2O3, Al2O3 and Ti3SiC2 phases existing in the wear debris of MTA. As shown in Fig. 9(b), there were C, TiO2, Fe2O3, SiO2, Al2O3 and Ti3SiC2 phases existing in the wear debris of MTD. The C phase came from the doped TiC coated MWCNTs. Fig. 10 shows FESEM morphologies and EDS of worn surface of GCr15 steel (a and b) ball and Si3N4 ceramic ball (c and d) friction pairs against MTD at the sliding speed of 0.234 m s 1 and load of 10 N. As shown in Fig. 10(a), the worn surface of GCr15 steel ball counterpart showed distinct parallel grooves throughout the worn surface. These grooves were formed due to the ploughing tendency of asperities of MTD, whose Vicker’s microhardness (HV1 806) was much higher than HV0.2 690 of GCr15 steel ball counterpart. As shown in Fig. 10(b), the contents of C, O, Al, Si, Ti and Fe were 51.22 at.%, 16.68 at.%, 1.14 at.%, 16.10 at.%, 12.55 at.% and 2.31 at.%, respectively. The content of Fe was much lower than that of
Fig. 12. FESEM morphologies and EDS of worn surfaces of MTD against Si3N4 ceramic (a–c) and GCr15 steel (d) balls at the sliding speed of 0.234 m s
1
and load of 10 N.
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original GCr15 steel. EDS analysis on worn surface of GCr15 steel ball indicated that GCr15 steel was coated by the mixture of C, Ti3SiC2 and oxides, which was beneficial to the low friction coefficient and wear rate. As shown in Fig. 10(c), there were a few discontinuous glazes existing on the coarse worn surface of Si3N4 ceramic ball. As shown in Fig. 10(d), the contents of C, O, Al, Si, Ti and N were 38.80 at.%, 23.79 at.%, 1.78 at.%, 13.93 at.%, 20.46 at.% and 1.23 at.%, respectively. The content of N was much lower than that of original Si3N4 ceramic ball. It indicated that Si3N4 ceramic ball was coated by the mixture of C, Ti3SiC2 and oxides, which was beneficial to the low friction coefficient and wear rate. However, distinct microfracture and delamination of the glazes existed on the worn surface (see Fig. 10(c)), and it could cause the high friction coefficient. Thus, as shown in Fig. 11(a and b), the friction coefficient of MTD against Si3N4 ceramic ball friction pair was 0.52, which was higher than 0.40 of MTD against GCr15 steel ball friction pair. Meanwhile, the Vicker’s microhardness of Si3N4 ceramic ball counterpart (HV 15 GPa) was much higher than HV1 806 of MTD. The hard asperities of Si3N4 ceramic ball counterface easily abraded soft MTD, thus the wear rate of MTD against Si3N4 ceramic ball friction pair was 4.87 10 4 mm3 N 1 m 1, which was higher than 1.80 10 4 mm3 N 1 m 1 of MTD against GCr15 steel ball friction pair (see Fig. 11(b)). Fig. 12 shows FESEM morphologies and EDS of worn surfaces of MTD against Si3N4 ceramic (a–c) and GCr15 steel (d) balls at the sliding speed of 0.234 m s 1 and load of 10 N. As shown in Fig. 12(a and b), the delamination of the glazes was obviously found, which revealed the existence of mechanically mixed layer (MML). The formation of MML indicated mild wear [27]. The MML was composed of finer equiaxed particles. As shown in Fig. 12(c), the EDS analysis showed that the glazes were composed of oxides, C and Ti3SiC2. As shown Fig. 12(d), the delamination of MML also existed. Moreover, the delamination phenomenon was more serious than that of MTD against GCr15 steel ball. And the remained amounts of trapped fine equiaxed particles on the worn surface were less than that on MTD against GCr15 steel ball. During the sliding friction and wear process, the hard asperities of Si3N4 ceramic ball counterface easily removed the soft MML which was subjected to tangential shears during sliding, thus the wear rate of MTD against Si3N4 ceramic ball friction pair was 4.87 10 4 mm3 N 1 m 1. 4. Conclusions (1) MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs were fabricated by SPS at 1200 8C under a pressure of 30 MPa for 10 min in pure Ar atmosphere protection. MTD had the highest Vicker’s microhardness and relative density, which were HV1 806.2 and 98.7%. Meanwhile, MTD also owned excellent tribological properties. With the sliding speed of 0.234 m s 1 and load of 10 N at room temperature, the friction coefficient and wear rate of MTD against GCr15 steel ball counterpart were 0.40 and 1.80 10 4 mm3 N 1 m 1 respectively. And the friction coefficient and wear rate of MTD against GCr15 steel ball counterpart were 0.52 and 4.87 10 4 mm3 N 1 m 1 respectively.
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(2) The entangled bundles of TiC coated MWCNTs were homogenously dispersed in the Ti3SiC2 matrix of MTD. And there was a clean interface of Ti3SiC2 matrix/TiC coated MWCNTs existing, which was beneficial to improved mechanical and tribological properties. (3) The mechanically mixed layer was formed on the worn surface of MTD during the sliding friction and wear process. And microcutting and delamination were dominant wear mechanisms of MTD against GCr15 steel ball or Si3N4 ceramic ball counterpart. The wear debris was composed of C, TiO2, Fe2O3, SiO2, Al2O3 and Ti3SiC2. (4) The smooth worn surface of GCr15 steel ball against MTD was coated by the mixture of C, Ti3SiC2 and oxides. Moreover, there were a few discontinuous glazes existing on the coarse worn surface of Si3N4 ceramic ball against MTD. The worn surface of Si3N4 ceramic ball was also coated by the mixture of C, Ti3SiC2 and oxides. The antifriction films were beneficial to the low friction coefficient and wear rate. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2010-II-020); the National Natural Science Foundation of China (51275370); the Project for Science and Technology Plan of Wuhan City; the Program for New Century Excellent Talents in University of Ministry of Education of China; the Nature Science Foundation of Hubei Province; and the Academic Leader Program of Wuhan City (201150530146). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effect of TiC coated MWCNT content on friction and wear behavior of MWCNT–Ti3SiC2 composites Xiaoliang Shi a,b,*, Zhiwei Zhu a, Mang Wang a, Wenzheng Zhai b, Zengshi Xu a, Jie Yao a, Siyuan Song a, Qiaoxin Zhang a,b a b
School of Mechanical and Electronic Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 August 2012 Received in revised form 6 October 2012 Accepted 17 October 2012 Available online 26 October 2012
Ti3SiC2 doped with various contents of TiC coated multi-walled carbon nanotubes (MWCNTs) were fabricated by spark plasma sintering. The effect of TiC coated MWCNT content on friction and wear properties of MWCNT–Ti3SiC2 composites was investigated in this paper. The results showed that MWCNT–Ti3SiC2 composites doped with 3 wt.% TiC coated MWCNTs had the best mechanical and tribological properties, and the relative density and Vicker’s microhardness were 98.7% and HV1 806.2 respectively. When the counterpart was GCr15 steel ball, the friction coefficient and wear rate were 0.40 and 1.80 10 4 mm3 N 1 m 1 at the load of 10 N and sliding speed of 0.234 m s 1 for 20 min at room temperature. Whereas, with Si3N4 ceramic ball counterpart, the friction coefficient and wear rate were 0.52 and 4.87 10 4 mm3 N 1 m 1 respectively. The excellent tribological performance could be attributed to the synergetic effect of the enhanced TiC coated MWCNTs and Ti3SiC2 matrix, which provided the composites with good deformation resistance and high load bear capacity, as well as the excellent balance between strength and lubricity. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Chemical synthesis C. Electron microscopy D. Mechanical properties
1. Introduction Carbon nanotubes (CNTs) have been received a great deal of attention due to their superior mechanical and physical properties. In particular, multi-walled carbon nanotubes (MWCNTs) have been expected to be used in industrial applications as their price has steadily decreased [1]. CNTs have been considered as useful and attractive additives to organic materials [2,3], bulky metal [4], metallic coating [5], bulky ceramic [6] and ceramic coating [7] in order to improve the wear resistance and lower friction of mechanical components. According to the theoretical consideration, a friction coefficient between the walls of MWCNTs should be extremely low. That is, MWCNTs have a significant selflubricant property by nano-ball bearing effects [8]. Gupta and Barsoum [9] reported that the layered hexagonal MAX phases were thermodynamically stable nanolaminates displaying unusual and sometimes unique properties. Their layered nature suggested they might have excellent promise as solid lubricant materials. Recently, first generation MAX phase based composites shafts were successfully tested against Nibased superalloy at 50,000 rpm from room temperature till
* Corresponding author. Tel.: +86 27 87651793; fax: +86 27 87651793. E-mail address: [email protected] (X. Shi). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.10.028
550 8C during thermal cycling in a foil bearing rig. This study further demonstrates the potential of MAX phases and their composites in different tribological applications. Ti3SiC2 is a representative among MAX ternary compounds (Mn + 1AXn where, M: early transition metal, A: group IIIA or IVA element, X: C and/or N, n = 1–3), which combines merits of both metals and ceramics, such as low density, high Young’s modulus, good thermal and electrical conductivity, excellent thermal shock resistance, high temperature strength, damage tolerance, and easy machinability [10–12]. Furthermore, Ti3SiC2 performs low friction coefficient, coefficient of thermal expansion, and good self-lubricating property. Therefore it is a promising candidate for new high temperature structural materials, electrode materials in molten metal, electric brush materials and self-lubricating materials. Unfortunately, its low hardness (Vickers hardness of 4 GPa), creep strength and wear resistance restrict the potential application of Ti3SiC2. El-Raghy et al. [13] reported that wear rates of fine (5 mm) and coarse (100 mm) grained Ti3SiC2 sliding against a 440C stainless steel pin in pin-on-disk type tester were as large as 4.25 10 3 mm3 N 1 m 1 and 1.34 10 3 mm3 N 1 m 1 respectively. It shows that Ti3SiC2 is not very wear resistant because of its relatively low hardness, shear strength and ratio of hardness to elastic modulus. Gupta et al. [14] tested the layered Mn + 1ACn ternary carbides – MAX phases – Ta2AlC, Ti2AlC, Cr2AlC and Ti3SiC2 under dry sliding conditions against alumina at 550 8C. At
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0.4, the friction coefficient of Ti3SiC2 was the lowest measured, but the wear rate, at 2 10 4 mm3 N 1 m 1, was high. Moreover, Gupta et al. [15] researched the tribological behavior, at 25 and 550 8C of some layered (MAX phase) ternary carbides including Ti3SiC2, tested against Ni-based superalloys, SA (Inconel-718 and Inconel-600). The results showed that at room temperature, the wear rates were relatively high (10 4 mm3 N 1 m 1) and no correlation was found between the wear rates and the friction coefficients. Thus, researchers attempted to improve its poor wear resistance by adding some enhanced phases (e.g. Al2O3, TiC, SiC and TiB2) to Ti3SiC2 matrix [16–19]. Hu et al. [16] studied out that the wear rates of the Al2O3-reinforced Ti3SiC2 composites decreased with increasing Al2O3 content. The enhanced wear resistance is mainly attributed to the hard Al2O3 particles nail the surrounding soft matrix and decentrale the shear stresses under the sliding ball to reduce the wear losses. Sun et al. [17] studied the wear and friction properties of Ti3SiC2/7 wt.% TiC material against AISI 52100 steel pair. The results showed the friction coefficient was not sensitive to the load, and the average wear rate and friction coefficient were 9.9 10 5 mm3 N 1 m 1 and 0.4–0.5 respectively. Wan et al. [18] investigated sliding friction and wear behaviors of Ti3Si(Al)C2 and SiC-reinforced Ti3Si(Al)C2 composites against AISI 52100 bearing steel ball using a ball-on-flat, reciprocating tribometer at room temperature. The results showed that compared to monolithic Ti3Si(Al)C2, these SiC particle-reinforced composites exhibited improved wear resistance. The wear coefficients decreased significantly with the increase in SiC content, which meant that the wear resistance could be greatly enhanced by adding SiC particles into Ti3Si(Al)C2. Ti3Si(Al)C2/ 30 vol.% SiC composite showed the highest wear resistance. Yang et al. [19] studied that the non-lubricated, ball-on-flat sliding friction and wear properties of in situ (TiB2 + TiC)/Ti3SiC2 composites against bearing steel ball. The results showed that both friction coefficient and wear rate initially increased and then dramatically decreased with TiB2 content increasing from 0 vol.% to 20 vol.%. (TiB2 + TiC)/Ti3SiC2 composites with 15 vol.% and 20 vol.% TiB2 showed excellent friction and wear resistance, whose friction coefficient and wear rate were much lower than those of TiC/Ti3SiC2 composite. And it also is a meaningful attempt to improve the tribological properties of Ti3SiC2 by combining Ti3SiC2 with MWCNTs. To the best of our knowledge, meager information is available as regards the development of Ti3SiC2 matrix self-lubricating composites reinforced by MWCNTs. In order to obtain a clean matrix/ reinforcement interface and improve the bonding strength between MWCNTs and Ti3SiC2 matrix, as well as realize the stability of MWCNTs during the fabricated process of the composites, TiC coating on the surface of MWCNTs prepared by vacuum slow vapor deposition process was used as coupling agents, which could lead to better improvement in mechanical properties and tribological properties of MWCNT–Ti3SiC2 composites. Compared with the traditional powder metallurgy techniques, the main advantages of spark plasma sintering (SPS) are the fast heating speed and cooling speed, short sintering time, fine grain size of the prepared materials, controlled organizational structure, energy saving and environmental protection and so on [20–22]. In this study, MWCNT–Ti3SiC2 composites with different contents of TiC coated MWCNTs were prepared by SPS. The effects of TiC coated MWCNTs on the dry sliding friction and wear behavior of MWCNT–Ti3SiC2 composites at the load of 10 N and sliding speed of 0.234 m s 1 for 20 min at the room temperature were investigated through the determination of friction coefficients and wear rates and the analysis of the compositions of wear debris, the worn surfaces of both MWCNT–Ti3SiC2 composites and friction ball pairs.
2. Experimental details 2.1. Materials 2.1.1. TiC coating on MWCNTs Raw MWCNTs (diameter 40–60 nm, length 40–1000 nm) were firstly acidified by ultrasonicating in the mixture solution of 65 wt.% nitric acid and 98 wt.% sulfuric acid (H2SO4:HNO3 = 3:1, volume ratio) and refluxed at 100 8C for 2 h. Then the resultant solid was washed with deionized water till pH = 6. The solid was then collected, and dried in vacuum at 80 8C. The TiC coating on MWCNTs was very simple and convenient to produce. Commercial Ti and TiH2 powders (99.9% pure, Longxi Technology Development Co., Ltd., Shanghai, China) were provided as the titanium source. Firstly, a proper amount of pure Ti and TiH2 powders were dispersed and handled in the mixture of hydrofluoric acid and acetone solution (2:8 at weight ratio) using ultrasonic vibration for 30 min. After dispersing and handling, the product was filtered and dried at 90 8C in a vacuum oven. The pretreated Ti and TiH2 mixture powders were set on the bottom of an alumina crucible, and pretreated MWCNTs were placed upon the pretreated powders via a carbon felt. This apparatus [23] was covered with an alumina lid to keep the Ti source gas pressure inside the crucible and heated at 800 8C continuously with a heating rate of 5 8C min 1, and then kept at 800 8C for 1 h in a vacuum of about 10 3 Pa. After the coating treatment, the TiC coated MWCNTs were obtained. 2.1.2. Sintering of MWCNT–Ti3SiC2 composites MWCNT–Ti3SiC2 composites with different contents of TiC coated MWCNTs were prepared by SPS. The weight fractions of TiC coated MWCNTs in the composites were fixed at 0 wt.%, 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.% and 5 wt.%, respectively. MWCNT–Ti3SiC2 composites without TiC coated MWCNTs were denoted as MTA. While MTB, MTC, MTD, MTE and MTF represented MWCNT–Ti3SiC2 composites with 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.% and 5 wt.% TiC coated MWCNTs, respectively. The fabrication process of Ti3SiC2 powder was described in detail elsewhere [24], whose average particle size was about 5 mm. The starting powders were mixed by high energy ball-milling in vacuum. Balls and vials were made of hard alloy, and the charge ratio (ball to powder mass ratio) employed was 10:1. The milling time and speed are 10 h and 200 rpm respectively. After being mixed and dried, the mixtures were then sintered by SPS at 1200 8C under a pressure of 30 MPa for 10 min in pure Ar atmosphere protection. And the heating rate was 100 8C min 1. The cylindrical graphite molds with an inner diameter of 20 mm were used. The as-prepared specimen surfaces were ground to remove the layer on the surface and polished mechanically with emery papers down to 1200 grad, and then with 0.05 mm wet polishing diamond pastes. 2.2. Vicker’s microhardness and density The Vicker’s microhardness of each as-received specimen was measured using a HVS-1000 Vicker’s hardness instrument with a load of 1 kg and a dwell time of 8 s. Five tests were conducted and the mean value was given. The density of as-prepared specimens was determined by Archimedes’ principle. 2.3. Tribological test The tribological tests were conducted on a HT-1000 ball-ondisk high temperature tribometer. The disks, which were the asprepared materials, were cleaned with acetone and then dried in hot air before test. The counterpart ball was the commercial GCr15 steel ball (composition: 1.01 C, 1.50 Cr, 0.30 Mn, 0.25 Si wt.%, as USA 52100) with a diameter of 5 mm (HV0.2 690, Ra 0.2–0.3 mm).
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Moreover, the counterpart ball of the commercial Si3N4 ceramic ball with a diameter of 5 mm (about HV 15 GPa) was also used for MWCNT–Ti3SiC2 composites with optimum amount of TiC coated MWCNTs. The test temperature was room temperature. The sliding speed was 0.234 m s 1, and the applied load was 10 N. The friction radius was 2 mm, and the test time was 20 min. The friction coefficients were automatically measured and recorded in real time by the computer system of the friction tester. The wear quantity of the MWCNT–Ti3SiC2 composites was measured by the weighting method, which measured the mass loss of the samples for every friction process. The tests for every given conditions were repeated three times to obtain reliable data, and the average value was used as the evaluating data. 2.4. Analysis The surfaces of the as-prepared specimens and wear debris were examined by XRD with CuKa radiation at 30 kV and 40 mA at a scanning speed of 0.01 s 1 for the identification of the phase constitution. The morphologies and compositions of worn surfaces of MWCNT–Ti3SiC2 composites, GCr15 steel ball and Si3N4 ceramic ball friction pairs against the composites with optimum amount of TiC coated MWCNTs, as well as wear debris were analyzed by a SIRION 200 field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS). 3. Results and discussion 3.1. Microstructure analysis Fig. 1 exhibited the FESEM morphologies of fractured surface of MTD. As shown in Fig. 1(a), the composites had a dense and homogeneous microstructure. The average grain size of Ti3SiC2 particles was about 8 mm. Although the aggregation phenomenon of TiC coated MWCNTs still existed (see Fig. 1(b)), the entangled bundles of TiC coated MWCNTs were homogenously dispersed in the Ti3SiC2 matrix. Moreover, as shown in Fig. 1(a), there was a clean interface of Ti3SiC2 matrix/TiC coated MWCNTs existing, which would lead to better and improved mechanical and tribological properties of the composites. 3.2. Vicker’s microhardness and density Fig. 2 shows variation of relative density and Vicker’s microhardness HV1 of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs. As shown in Fig. 2, MTD exhibited the highest Vicker’s microhardness of HV1 806, which was much higher than HV1 470 of MTA. However, except the HV1 571.8 of MTE, the Vicker’s hardness of MTB, MTC or MTF was lower than that of MTA. Furthermore, the relative density of MTD was
Fig. 2. Variation of relative density and Vicker’s microhardness HV1 of MWCNT– Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs.
98.7%, which was the highest one among all the MWCNT–Ti3SiC2 composites. The relative density of MTA was 97.1%. It showed that the addition of 3 wt.% TiC coated MWCNTs could improve the strength of the Ti3SiC2, which could provide the MWCNT–Ti3SiC2 composites with good deformation resistance and high load bear capacity. However, the aggregation phenomenon of MWCNTs became more serious when the amounts of TiC coated MWCNTs in MTE or MTF were surplus. Moreover, the content of the TiC coated MWCNTs in MTB or MTC was lower, and the uniform dispersion of MWCNTs could not be well realized. Thus, the strength of the composites would be deteriorated. 3.3. Compositions of MWCNT–Ti3SiC2 composites Fig. 3 was the XRD patterns of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs. It could be found that the diffraction peaks primarily belonged to the Ti3SiC2 and TiC phases. Additionally, the (0 0 2) crystal plane of MWCNTs also appeared. The diffraction peak intensities of TiC phase obviously became stronger with the increase in doped amounts of TiC coated MWCNTs, which displayed four crystal planes of (1 1 1), (2 0 0), (2 2 0) and (3 1 1). In addition, the Ti3SiC2 displayed fifteen crystal planes of (0 0 4), (0 0 6), (1 0 1), (1 0 3), (1 0 4), (0 0 8), (1 0 5), (1 0 8), (1 0 9), (1 1 0), (1 0 2), (2 0 4), (1 1 8), (2 0 5) and (2 0 6). The TiC phase could be the enhanced phase to improve the wear resistance of MWCNT–Ti3SiC2 composites. Although the diffraction peak intensities of TiC in Fig. 3(e) (MTE) and Fig. 3(f) (MTF) were stronger than that in Fig. 3(d) (MTD), the Vicker’s microhardness of MTD was the
Fig. 1. FESEM micrographs of fractured surface of MTD.
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Fig. 3. XRD patterns of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs: 0 wt.% (a), 1 wt.% (b), 2 wt.% (c), 3 wt.% (d), 4 wt.% (e) and 5 wt.% (f).
highest one (see Fig. 2). Thus, MTD could have the best wear resistance.
Fig. 5. Variation of friction coefficients of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair.
Fig. 4 shows the typical measuring curves of the dynamic friction coefficients of MWCNT–Ti3SiC2 composites against GCr15 steel ball friction pair at a constant sliding speed of 0.234 m s 1 and load of 10 N at room temperature. It was obvious that the curve of the friction coefficient of MTD was relatively smooth (see Fig. 4(d)), which was about 0.4. However, the fluctuation ranges of friction coefficients of MTA (see Fig. 4(a)), MTB (see Fig. 4(b)), MTC (see Fig. 4(c)), MTE (see Fig. 4(e)) and MTF were larger. And the friction coefficients were in the range of 0.3–0.8. Among all the samples, MTF had the lowest COF of 0.3–0.45. Fig. 5 shows variation of the friction coefficient of MWCNT– Ti3SiC2 composites with varying TiC coated MWCNTs weight fraction. As shown in Fig. 5, the friction coefficient of MTA against GCr15 steel ball friction pair was about 0.62. And the friction coefficients of MTB, MTC, MTD, MTE and MTF were 0.43, 0.42, 0.40, 0.54 and 0.35, respectively.
Fig. 6 shows variation of wear rates of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair. Although the relative density of MTA was 97.1% (see Fig. 2), the wear rate of MTA was only 1.76 10 3 mm3 N 1 m 1. The results observed by El-Raghy et al. [13] and Gupta et al. [14,15] also showed that the wear rates of Ti3SiC2 at room temperature were relatively high. The wear rates of MTB and MTC were 1.10 10 2 mm3 N 1 m 1 and 3.89 10 2 mm3 N 1 m 1 respectively, which were much higher than that of MTA. The wear resistance of Ti3SiC2 was not enhanced by the doped 1 wt.% or 2 wt.% TiC coated MWCNTs, and the wear resistance was lowed. The doped amounts of the 1 wt.% or 2 wt.% TiC coated MWCNTs were lower, and the uniform dispersion of MWCNTs could not be well realized. Thus, the mechanical and tribological properties of the composites were deteriorated. As shown in Fig. 2, the relative densities of MTB and MTC were only 92.0% and 87.6% respectively, which were much lower than 97.1% of MTA. When the doped TiC MWCNTs was 3 wt.%, MTD had the lowest wear rate of 1.80 10 4 mm3 N 1 m 1. The wear rates of MWCNT–Ti3SiC2 composites increased with the
Fig. 4. Curves of friction coefficients of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs with sliding time at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair: 0 wt.% (a), 1 wt.% (b), 2 wt.% (c), 3 wt.% (d), 4 wt.% (e) and 5 wt.% (f).
Fig. 6. Variation of wear rates of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair.
3.4. Friction and wear behavior
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Fig. 7. FESEM morphologies and EDS patterns of worn surfaces of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair: 0 wt.% (a and b), 1 wt.% (c and d), 2 wt.% (e and f), 3 wt.% (g and h), 4 wt.% (i and j) and 5 wt.% (k and l).
increase in doped amounts of TiC coated MWCNTs. When the doped contents of TiC coated MWCNTs were 4 wt.% and 5 wt.%, the wear rates of the composites were 5.85 10 4 mm3 N 1 m 1 and 6.22 10 4 mm3 N 1 m 1. The aggregation phenomenon of MWCNTs could become more serious when the amounts of TiC coated MWCNTs in the composites were 4 wt.% and 5 wt.%. Moreover, the uniform dispersion of MWCNTs also was not realized. The relative densities of MTE and MTF were 93.4% and 94.7% respectively, which were lower than 98.7% of MTD. With the 3 wt.% TiC coated MWCNTs, the MWCNTs could be homogeneously dispersed in the MWCNT–Ti3SiC2 composites. Meanwhile, there was a clean interface of Ti3SiC2 matrix/TiC coated MWCNTs (see Fig. 1(c)). Some MWCNTs were well bonded to the Ti3SiC2 matrix in ‘‘bridging’’ manner. It would effectively enhance the mechanical properties such as hardness and toughness for the well-known reinforcing effect of MWCNTs. Meanwhile, the well bonded MWCNTs could have a load-bearing action, which might increase wear resistance of the friction surface. Consequently, as shown in Fig. 2, MTD had the higher Vicker’s microhardness of HV1 806 and relative density of 98.7 wt.%. MWCNTs have a significant selflubricant property by nano-ball bearing effects [8]. Thus, MTD against GCr15 steel ball pair had the excellent tribological properties among the samples, whose friction coefficient and wear rate were about 0.40 and 1.80 10 4 mm3 N 1 m 1 respectively. 3.5. Morphologies, compositions of wear debris and the worn surfaces of MWCNT–Ti3SiC2 composites and counterpart balls Fig. 7 shows FESEM morphologies and EDS patterns of worn surfaces of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair. As shown in Fig. 7(a), the coarse worn surface of MTA was covered with some deformed wear debris. A small amount of the
tribo-layers called ‘glazes’ [25] were formed. Fig. 7(b) was a result of EDS analysis for the B area shown in Fig. 7(a). The EDS pattern exhibited the presence of C, O, Si, Ti, Fe and a small amount of Al. The Al came from the Al addition that was used to accelerate the Ti3SiC2 formation. The contents of O and Fe were 23.35 at.% and 14.75 at.% respectively. It showed that the transfer of metal from GCr15 steel ball to the worn surface of MTA was serious. And the high O content indicated that the worn surface should be consisted of an amount of oxides. The slight traces in the smooth worn surface in Fig. 7(c) suggested that the wear mechanism of MTB was microcutting. A lot of the wear particle debris were only trapped on the worn surface. Fig. 7(d) was a result of EDS analysis for the D area shown in Fig. 7(c). The EDS pattern exhibited that the content of Fe was only 0.65 at.%. It showed that the transfer of material from GCr15 steel ball to the monolithic Ti3SiC2 surface was mild. Fig. 7(e) clearly shows that the wear debris was only trapped flow on the worn surface of MTC. Fig. 7(f) was a result of EDS analysis for the F area shown in Fig. 7(e). The EDS pattern exhibited the absence of Fe on the worn surface. According to Fig. 7(c–f), the hard asperities of GCr15 steel ball counterface just ploughed or cut soft MTB or MTC, and the wear rates should be high. There were a few trapped wear debris and discontinuous glazes existing on the smooth worn surface in Fig. 7(g), which suggested that the wear mechanism of MTD was microcutting and delamination. Fig. 7(h) was a result of EDS analysis for the H area shown in Fig. 7(g). The contents of O and Fe were 17.78 at.% and 2.25 at.% respectively. The material transfer of GCr15 steel ball to MTD was mild. The glazes should be consisted of an amount of oxides. There a few discontinuous glazes existing on the coarse worn surface in Fig. 7(i) suggested that the wear mechanism of MTE was microcutting and delamination. Fig. 7(j) was a result of EDS analysis for the J area shown in Fig. 7(i). The C content was 17.20 at.%. And the contents of O and Fe were 49.78 at.% and 28.96 at.% respectively. The higher Fe content indicated that the
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Fig. 8. FESEM morphologies and EDS patterns of wear debris of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair: 0 wt.% (a and b), 3 wt.% (c and d), 4 wt.% (e and f) and 5 wt.% (g and h).
material transfer from GCr15 steel pair was serious. The worn surface should contain oxides of Fe, Mn and Cr. The C content was 17.20 at.%, which was lower. However, the C did not come from the Ti3SiC2 matrix but the doped TiC coated MWNCTs because the contents of Si and Ti were only 1.30 at.% and 2.18 at.% respectively. The coarse worn surface and serious material transfer between friction pairs showed that the friction coefficient could be high. As shown in Fig. 7(k), the worn surface of MTF was nearly covered with a continued glaze film. Thus, the friction coefficient should be
low. Meanwhile, microfracture and delamination of the glaze also existed. Fig. 7(l) was a result of EDS analysis for the L area shown in Fig. 7(k). The EDS result showed that the glaze was consisted of oxides, matrix wear debris and C. Fig. 8 shows FESEM morphologies and EDS patterns of wear debris of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs at the sliding speed of 0.234 m s 1 and load of 10 N with GCr15 steel friction pair. As shown in Fig. 8(a), most of wear debris obtained from MTA against GCr15 steel ball pair under
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Fig. 9. XRD patterns of wear debris of MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs against GCr15 steel friction pair at the sliding speed of 0.234 m s 1 and load of 10 N: 0 wt.% (a), 3 wt.% (b).
the dry sliding friction and wear were fine equiaxed and few of them were lamellar. It indicated that the wear debris was mainly a product of having been directly generated from MTA. Fig. 8(b) was a result of EDS analysis for the Fig. 8(a). As shown in Fig. 8(b), the EDS pattern exhibited the presence of C, O, Al, Si, Ti and Fe. The contents of O and Fe were 24.09 at.% and 9.42 at.%, respectively. It showed that the wear debris was composed of oxides and the matrix wear debris. As shown in Fig. 8(c), most of wear debris obtained from MTD against GCr15 steel ball pair under the dry sliding friction and wear were lamellar and few of them were fine equiaxed. It indicated that the wear debris was mainly a product of the glazes rather than having been directly generated from MTD. Fig. 8(d) was a result of EDS analysis for the Fig. 8(c). The contents of C, O, Al, Si, Ti and Fe were 32.38 at.%, 32.18 at.%, 1.41 at.%, 6.68 at.%, 16.48 at.% and 10.86 at.%, respectively. It showed that the wear debris was composed of oxides, Ti3SiC2 and Sarkar et al. [26] investigated the unlubricated friction and wear properties of
Ti3SiC2 against steel. The results showed that under the selected fretting conditions, Ti3SiC2/steel tribocouple exhibited a transition in friction as well as wear behavior with coefficient of friction varying between 0.5 and 0.6 and wear rate in the order of 10 5 mm3 N 1 m 1. Raman analysis revealed that the fretting wear was accompanied by the triboxidation with the formation of TiO2, SiO2, and Fe2O3. Gupta et al. [14] point out that tribofilms, which were mainly comprised of X-ray amorphous oxides of the M and A elements and, in some cases, unoxidized grains of the corresponding MAX phases, were formed on the contact surfaces. Gupta et al. [15] indicated that the tribofilms resulted in low (<10 6mm3 N 1 m 1) wear rates and friction coefficients <0.5. The results in this research were similar to that observed by Sarkar et al. [26] and Gupta et al. [14,15]. The glazes actually had not only a significant antifriction effect, but also a protective action for the friction surfaces. Thus, the friction coefficient of MTD was about 0.40, and the wear rate was 1.80 10 4 mm3 N 1 m 1. As shown
Fig. 10. FESEM morphologies and EDS of worn surface of GCr15 steel (a and b) and Si3N4 ceramic ball (c and d) friction pair against MTD at the sliding speed of 0.234 m s load of 10 N.
1
and
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Fig. 11. Friction coefficients (a) and wear rates (b) of MTD against GCr15 steel and Si3N4 ceramic balls at the sliding speed of 0.234 m s
in Fig. 8(e), most of wear debris obtained from MTE against GCr15 steel ball pair under the dry sliding friction and wear were equiaxed and few of them were fine lamellar. It indicated that the wear debris was mainly a product of having been directly generated from MTE. Fig. 8(f) was a result of EDS analysis for the Fig. 8(e). The contents of C, O, Si, Ti and Fe were 21.78 at.%, 37.01 at.%, 0.96 at.%, 1.69 at.% and 38.55 at.%, respectively. It showed that the wear debris was mainly composed of C and oxides of Fe. As shown in Fig. 8(g), most of wear debris obtained from MTF against GCr15 steel ball pair under the dry sliding friction and wear were lamellar and few of them were fine equiaxed. It indicated that the wear debris was mainly a product of the glazes rather than having been directly generated from MTF. Fig. 8(h) was a result of EDS analysis for the Fig. 8(g). The contents of C, O, Si, Ti and Fe were 24.71 at.%, 26.45 at.%, 1.97 at.%, 4.49 at.% and 42.38 at.%, respectively. It showed that the wear debris was also mainly composed of C and oxides of Fe. The glazes actually had not only a significant antifriction effect, but also a protective action for the friction surfaces. And the friction coefficient of MTF was only about 0.35. However, the Vicker’s microhardness of the
1
and load of 10 N.
composites was only HV1 422.5. Thus, the wear rate was 6.22 10 4 mm3 N 1 m 1, which was higher than that of MTD. Fig. 9 shows the XRD patterns of wear debris of MTA and MTD against GCr15 steel friction pair at the sliding speed of 0.234 m s 1 and load of 10 N. As shown in Fig. 9(a), there were TiO2, Fe2O3, Al2O3 and Ti3SiC2 phases existing in the wear debris of MTA. As shown in Fig. 9(b), there were C, TiO2, Fe2O3, SiO2, Al2O3 and Ti3SiC2 phases existing in the wear debris of MTD. The C phase came from the doped TiC coated MWCNTs. Fig. 10 shows FESEM morphologies and EDS of worn surface of GCr15 steel (a and b) ball and Si3N4 ceramic ball (c and d) friction pairs against MTD at the sliding speed of 0.234 m s 1 and load of 10 N. As shown in Fig. 10(a), the worn surface of GCr15 steel ball counterpart showed distinct parallel grooves throughout the worn surface. These grooves were formed due to the ploughing tendency of asperities of MTD, whose Vicker’s microhardness (HV1 806) was much higher than HV0.2 690 of GCr15 steel ball counterpart. As shown in Fig. 10(b), the contents of C, O, Al, Si, Ti and Fe were 51.22 at.%, 16.68 at.%, 1.14 at.%, 16.10 at.%, 12.55 at.% and 2.31 at.%, respectively. The content of Fe was much lower than that of
Fig. 12. FESEM morphologies and EDS of worn surfaces of MTD against Si3N4 ceramic (a–c) and GCr15 steel (d) balls at the sliding speed of 0.234 m s
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and load of 10 N.
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original GCr15 steel. EDS analysis on worn surface of GCr15 steel ball indicated that GCr15 steel was coated by the mixture of C, Ti3SiC2 and oxides, which was beneficial to the low friction coefficient and wear rate. As shown in Fig. 10(c), there were a few discontinuous glazes existing on the coarse worn surface of Si3N4 ceramic ball. As shown in Fig. 10(d), the contents of C, O, Al, Si, Ti and N were 38.80 at.%, 23.79 at.%, 1.78 at.%, 13.93 at.%, 20.46 at.% and 1.23 at.%, respectively. The content of N was much lower than that of original Si3N4 ceramic ball. It indicated that Si3N4 ceramic ball was coated by the mixture of C, Ti3SiC2 and oxides, which was beneficial to the low friction coefficient and wear rate. However, distinct microfracture and delamination of the glazes existed on the worn surface (see Fig. 10(c)), and it could cause the high friction coefficient. Thus, as shown in Fig. 11(a and b), the friction coefficient of MTD against Si3N4 ceramic ball friction pair was 0.52, which was higher than 0.40 of MTD against GCr15 steel ball friction pair. Meanwhile, the Vicker’s microhardness of Si3N4 ceramic ball counterpart (HV 15 GPa) was much higher than HV1 806 of MTD. The hard asperities of Si3N4 ceramic ball counterface easily abraded soft MTD, thus the wear rate of MTD against Si3N4 ceramic ball friction pair was 4.87 10 4 mm3 N 1 m 1, which was higher than 1.80 10 4 mm3 N 1 m 1 of MTD against GCr15 steel ball friction pair (see Fig. 11(b)). Fig. 12 shows FESEM morphologies and EDS of worn surfaces of MTD against Si3N4 ceramic (a–c) and GCr15 steel (d) balls at the sliding speed of 0.234 m s 1 and load of 10 N. As shown in Fig. 12(a and b), the delamination of the glazes was obviously found, which revealed the existence of mechanically mixed layer (MML). The formation of MML indicated mild wear [27]. The MML was composed of finer equiaxed particles. As shown in Fig. 12(c), the EDS analysis showed that the glazes were composed of oxides, C and Ti3SiC2. As shown Fig. 12(d), the delamination of MML also existed. Moreover, the delamination phenomenon was more serious than that of MTD against GCr15 steel ball. And the remained amounts of trapped fine equiaxed particles on the worn surface were less than that on MTD against GCr15 steel ball. During the sliding friction and wear process, the hard asperities of Si3N4 ceramic ball counterface easily removed the soft MML which was subjected to tangential shears during sliding, thus the wear rate of MTD against Si3N4 ceramic ball friction pair was 4.87 10 4 mm3 N 1 m 1. 4. Conclusions (1) MWCNT–Ti3SiC2 composites with varying weight fraction of TiC coated MWCNTs were fabricated by SPS at 1200 8C under a pressure of 30 MPa for 10 min in pure Ar atmosphere protection. MTD had the highest Vicker’s microhardness and relative density, which were HV1 806.2 and 98.7%. Meanwhile, MTD also owned excellent tribological properties. With the sliding speed of 0.234 m s 1 and load of 10 N at room temperature, the friction coefficient and wear rate of MTD against GCr15 steel ball counterpart were 0.40 and 1.80 10 4 mm3 N 1 m 1 respectively. And the friction coefficient and wear rate of MTD against GCr15 steel ball counterpart were 0.52 and 4.87 10 4 mm3 N 1 m 1 respectively.
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(2) The entangled bundles of TiC coated MWCNTs were homogenously dispersed in the Ti3SiC2 matrix of MTD. And there was a clean interface of Ti3SiC2 matrix/TiC coated MWCNTs existing, which was beneficial to improved mechanical and tribological properties. (3) The mechanically mixed layer was formed on the worn surface of MTD during the sliding friction and wear process. And microcutting and delamination were dominant wear mechanisms of MTD against GCr15 steel ball or Si3N4 ceramic ball counterpart. The wear debris was composed of C, TiO2, Fe2O3, SiO2, Al2O3 and Ti3SiC2. (4) The smooth worn surface of GCr15 steel ball against MTD was coated by the mixture of C, Ti3SiC2 and oxides. Moreover, there were a few discontinuous glazes existing on the coarse worn surface of Si3N4 ceramic ball against MTD. The worn surface of Si3N4 ceramic ball was also coated by the mixture of C, Ti3SiC2 and oxides. The antifriction films were beneficial to the low friction coefficient and wear rate. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2010-II-020); the National Natural Science Foundation of China (51275370); the Project for Science and Technology Plan of Wuhan City; the Program for New Century Excellent Talents in University of Ministry of Education of China; the Nature Science Foundation of Hubei Province; and the Academic Leader Program of Wuhan City (201150530146). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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