High-Temperature Tribological Performance of Ti3SiC2/TiAl Self-Lubricating Composite Against Si3N4...

High-Temperature Tribological Performance of Ti3SiC2/TiAl Self-Lubricating Composite Against Si3N4 in Air

Zengshi Xu, Xiaoliang Shi, Qiaoxin Zhang, Wenzheng Zhai, Jie Yao, Long Chen, Qingshuai Zhu, and Yecheng Xiao

(Submitted December 20, 2013; in revised form March 18, 2014; published online April 4, 2014)

More durable, low-friction bearing materials are needed for turbine components and other high-temper-ature bearing applications. The current study presents the sliding friction and wear behavior of a Ti3SiC2/TiAl composite (TTC) against a Si3N4 counterface at temperatures ranging from 25 to 800 �C in air. Theload was 10N, and the sliding speed was 0.2 m/s for all tests. The friction coefficient of both the TiAl and theTTC composite increased with rising temperature up to 400 �C, but beyond that, the friction coefficient ofthe TTC decreased, making it more lubricative than TiAl at the higher temperatures. It is proposed that athigh-temperatures, Ti3SiC2-formed oxides that were incorporated into a well-consolidated tribo-filmdrastically reducing the friction coefficients and wear rates of the TTC material.

Keywords electron microscopy, metal matrix composites,mechanical testing, powder metallurgy, tribology

1. Introduction

Thanks to their remarkable properties such as low density,high specific strength, elastic modulus retention, high dimen-sional, and good environmental stabilities, TiAl-based inter-metallics have been widely selected as ideal high-temperaturestructural and engine materials (Ref 1-5). In recent years, TiAlturbocharger turbine wheel has been used for commercial cars(Ref 1), and other TiAl products like low-pressure turbineblade, corner-beam, transition-duct beam, etc., have alsoreached the engineering technology level (Ref 4). However,their applications are significantly hindered because of the poorductility at ambient temperature and low creep resistance atelevated temperature (Ref 6).

Because sliding contact occurs in many potential applicationsof TiAl-based intermetallics, which is associated with frictionand wear, it is vital to study their tribological behavior undersliding conditions. A number of investigations on slidingtribological performance of TiAl-based intermetallic materialshave been done at room temperature (Ref 7-11). Li et al. (Ref 8)reported the sliding wear of TiAl intermetallics against steel andceramics of Al2O3, Si3N4, and WC/Co at room temperature.Cheng et al. (Ref 11) investigated the tribological behavior of aTi-46Al-2Cr-2Nb alloy under liquid paraffine lubrication againstAISI 52100 steel ball in ambient environment and at varyingloads and sliding speeds. Nevertheless, until now, only severalstudies have been done on the tribological behavior of TiAlintermetallic materials at elevated temperatures. The fretting

wear behavior of a Ti-48Al-2Cr-2Nb alloywas investigated in airfrom room temperature to 600 �C (Ref 12). Tribological behaviorof a Ti-46Al-2Cr-2Nb alloy was investigated against a Si3N4

ceramic ball at a constant speed of 0.188 ms�1 and an appliedload of 10 N from 20 to 900 �C (Ref 13). Consequently, it isnecessary to further investigate the elevated temperature tribo-logical behavior of TiAl-based intermetallic materials.

It is well known that the layered carbide-like ternarycompound Ti3SiC2 has ignited a worldwide attention due to itsremarkable properties. It displays not only metal properties,including fine thermal conductance, better conductivity, highelastic and shear modulus, easy machining property, andplasticity at high temperature, but also ceramic properties, i.e.,high yield strength, highmelting point, and high thermal stability(Ref 14-18). A number of studies on its tribological behaviorhave also been reported, which indicate that Ti3SiC2 has goodtribological properties (Ref 19-21). Hence, Ti3SiC2 may be anideal candidate as a structural ceramic for high temperature and anew solid lubricant material at high temperatures. To the author�sknowledge, reports on Ti3SiC2 as a solid high-temperaturelubricant in composites are very rare, especially in TiAl matrixcomposite. Thus, it is valuable to explore the high-temperaturelubrication mechanism of Ti3SiC2 in TiAl matrix composite.

In the present work, the friction and wear properties ofTi3SiC2/TiAl composite (TTC) produced by in situ techniqueusing spark plasma sintering (SPS) against Si3N4 ceramic ballfrom room temperature (RT, 25 �C) to 800 �C are studied. Forthe purpose of comparison, the tribological properties of TiAl-based alloy (TA) are also investigated under the sameconditions. Furthermore, the effect of Ti3SiC2 on friction andwear properties of TTC at elevated temperatures has beenanalyzed and discussed in detail.

2. Experimental Details

Commercially available Ti (20 lm in average size, 99.9% inpurity), Al (20 lm in average size, 99.9% in purity), B (25 lmin average size, 99.9% in purity), Nb (10 lm in average size,

Zengshi Xu, Xiaoliang Shi, Qiaoxin Zhang, Wenzheng Zhai,Jie Yao, Long Chen, Qingshuai Zhu, and Yecheng Xiao, School ofMechanical and Electronic Engineering, Wuhan University ofTechnology, 122 Luoshi Road, Wuhan 430070, China. Contacte-mails: [email protected] and [email protected].

JMEPEG (2014) 23:2255–2264 �ASM InternationalDOI: 10.1007/s11665-014-0969-9 1059-9495/$19.00

Journal of Materials Engineering and Performance Volume 23(6) June 2014—2255

99.9% in purity), Cr (10 lm in average size, 99.9% in purity),and Ti3SiC2 (5 lm in average size, 99.9% in purity, produced byvacuum solid state reaction in our laboratory) powders wereused as the starting materials. The composite powders of TiAl-based alloy matrix consisted of Ti, Al, Nb, Cr, and B powderswith molar ratio of 48:47:2:2:1. Referring to our previousexperiment data, the optimum additive amount of solid lubricantTi3SiC2 was in the rage of 10-15 wt.%. Hence, the additiveamount of Ti3SiC2 was 13 wt.% in the work. These raw materialpowders were mixed together by high energy ball-milling invacuum for 12 h. Balls and vials were made of hard alloy, andthe charge ratio (ball to powder mass ratio) employed was 10: 1.After being mixed and dried, the mixtures were set into acylindrical graphite mold with an inner diameter of 20 mm tosinter by SPS using a D.R.Sinter� SPS3.20 (Sumitomo Coal &Mining, now SPS Syntex Inc.) apparatus at 1100 �C under apressure of 45 MPa for 10 min in pure Ar atmosphereprotection. The as-prepared specimen surfaces were ground toremove the layers on the surfaces and polished mechanicallywith successive grades of emery papers down to 1200 grit,5 lm up to a mirror finish, to make the following tests.

2.1 Mechanical and Tribological Measurements

The microhardness of each as-received specimen was mea-sured, according to the ASTM standard E92-82 (Ref 22), using aHVS-1000 Vicker�s hardness instrument. The density of eachas-prepared specimen was determined using Archimedes� methodaccording to the ASTM Standard B962-08 (Ref 23). Thetribological tests were carried out on a HT-1000 ball-on-diskhigh-temperature tribometer (made in Zhong Ke Kai HuaCorporation, China) according to the ASTM Standard G99-95(Ref 24). The as-prepared samples disk was sliding against acommercially available Si3N4 ball with 6 mm in diameter [1676HV1, surface roughness (Ra) of 0.01 lm]. The reason why wechose a Si3N4 ball as a counterpart was attributed to the elevatedtemperature oxidation resistance of Si3N4 ceramic. According toour previous study and experience in the past, it was better for us toexplore the tribological properties of Ti3SiC2/TiAl compositeunder the load of about 10 N and the speed of about 0.2 m/s. Wehave studied the tribological behavior of TiAl matrix self-lubricating composites containing silver from 25 to 800 �C under10 N load and 0.234 m/s speed (Ref 25). Herein, the wear testparameters were 10 N load, 0.2 m/s speed, test temperaturesRT-800 �C, radius ofwear track 2 mm, and 40 min testing time forthe different temperatures. All the testswere conducted at a relativehumidity of 55-75%. The friction coefficient was automaticallymeasured and recorded in real time by the computer system of thefriction tester. Wear rate was calculated by the following formula:

W ¼ V

PS; ðEq 1Þ

where V was the wear volume in mm3, P was the appliedload in N, and S was the total sliding distance in mm. Theprofiles of the worn scar cross section were measured using asurface profilometer to determine the wear volume. Five testswere conducted for each set of testing conditions, and theaverage data were used as the evaluating data.

2.2 Microstructure Examination

XRD studies with CuKa radiation at 30 kV and 40 mA at ascanning speed of 0.01�s�1 were carried out on the as-prepared

composites to analyze the phase compositions. The surfacesand worn surfaces of TTC were characterized using electronprobe microanalyzer (EPMA, JAX-8230) and energy dispersivespectroscopy (EDS, Inca X-Act). The phases of worn surface ofTTC at 600 �C were analyzed by a VG Multilab 2000 x-rayphotoelectron spectroscope (XPS). The fractured surfaces ofTTC were analyzed by field emission scanning electronmicroscope (FESEM, FEI-SIRION). The morphologies andelement distribution of cross sections of wear scar were alsoobserved and analyzed by EPMA and EDS.

3. Results and Discussion

3.1 Microstructure and Mechanical Properties

XRD results of the as-prepared samples are given in Fig. 1.It can be found that the phases of the as-prepared TA are mainlycomposed of TiAl, as well as a small amount of TiC and Ti3Alphases, as is evident from Fig. 1 (TA). The newly formed phaseTiC is attributed to a high-temperature complex reactionbetween TiAl and graphite atmosphere caused by the graphitemolds during the fabrication process (Ref 25). After addingTi3SiC2 lubricant, the peaks of free Ti3SiC2 phase appear in theTTC as shown in Fig. 1 (TTC).

The microstructure characteristic and elemental distributionof TTC are illustrated in Fig. 2. According to the EDS analysis,the gray area is the continuous bulk TiAl phase, and the lightgray is the Ti3SiC2 phase, while the dark area is the TiC phase.Meanwhile, it can be observed that the Ti3SiC2 phase has anapproximately uniform distribution on the surface of TTC. Thepresence of Ti3SiC2 phase can also be seen in the XRD patternof TTC as shown in Fig. 1 (TTC).

Figure 3 shows the microstructure of fractured surface ofTTC characterized by FESEM, indicating that TTC has a denseand homogeneous microstructure. Hence, TTC should haveexcellent mechanical properties. Herein, the measured relativedensity of TTC is up to 99.5%; the measured Vicker�s hardness

Fig. 1 XRD patterns of the composites: TA (TiAl) and TTC(Ti3SiC2/TiAl)

2256—Volume 23(6) June 2014 Journal of Materials Engineering and Performance

of 690.3 HV1 for the TTC is about 27.78% higher than that ofTA (540.1 HV1).

3.2 Tribological Behaviors

Figure 4(a) and (b) exhibits the friction coefficient and wearrate of TA and TTC as a function of temperature. It is clear thatthe variation trends of both friction coefficient and wear ratewith temperature for TA and TTC are the same. The frictioncoefficient and wear rate firstly increase and reach to the highestpoint at 400 �C, then decrease to the lowest value withincreasing test temperature up to 800 �C. Furthermore, it can beobserved that the friction coefficients and wear rates of TTC arecomparable to those of TA at 25-400 �C, while the gaps infriction coefficients and wear rates between TTC and TAbeyond 400 �C significantly appear. The reason for these is thatTi3SiC2 being a newly known high-temperature lubricant(Ref 14-19) plays a dominant role in friction-reduction andanti-wear properties at high temperatures of 600 and 800 �C.As shown in Fig. 4(a), the friction coefficients of TA and TTCat the test temperatures from 25 to 400 �C are in the range of0.44-0.52. The friction coefficients of TTC are observed toremarkably decrease from 0.46 to 0.34 at 800 �C after addingthe Ti3SiC2 lubricant. Meanwhile, as is seen obviously fromFig. 4(b), the wear rates for TA and TTC at the same testtemperatures from 25-400 �C are comparable. For the hightemperatures of 600 and 800 �C, the wear rates of TTC fallfrom 3.42 to 1.21910�4 mm3/N/m and from 2.65 to

0.859 10�4 mm3/N/m by the addition of Ti3SiC2 lubricant,respectively.

From the above analyses, it can be concluded that TTCwhen compared with TA only exhibits excellent tribologicalproperties at the elevated temperatures of 600 and 800 �C. Itindicates that Ti3SiC2 can actually act as an ideal hightemperature solid lubricant in TTC but does not well work at25-400 �C. We will emphatically analyze and discuss thelubrication mechanism of Ti3SiC2, which governs the tribolog-ical behaviors of TTC at elevated temperatures.

3.3 Worn Surface Analysis

Figure 5 presents the morphologies of worn surfaces of TAand TTC tested at different temperatures from 25 to 400 �C. Itcan be obviously observed that the morphologies of wornsurfaces of TA and TTC at the same testing temperatures from25 to 400 �C (Fig. 5a-f) slightly differ and show grooves, weardebris, and delamination, which support the comparablefriction coefficients and wear rates for TA and TTC at thetesting temperatures from 25 to 400 �C (see Fig. 4a and b). It isalso noted that the Ra of TA and TTC increases gradually withthe increase in test temperature, as listed in Table 1. Therougher surface corresponds with the increase of frictioncoefficients with the increase in test temperature from 25 to400 �C, as presented in Fig. 4(a). Similar trends have also beenobserved by other researchers (Ref 25, 26).

Fig. 2 Microstructure and elemental distribution of TTC


Figure 6 shows the EPMA morphologies of worn surfacesof TA and TTC obtained at 600 �C. It is clear to see fromFigure 6 that the morphology of worn surface of TTC isdifferent from that of TA. Some wider but shallow groovescharacterizing the abrasive nature of wear can be observed onthe worn surface of TA that is covered with local compactedtribo-films, as evident from Fig. 6(a). The elemental composi-tions of region marked A on the worn surface (Ra = 1.943 lm)of TA shown in Fig. 6(a) have been listed in Table 2. The EDSanalysis results indicate that the compacted tribo-films mainlyconsist of Al-Ti-Oxides. However, as is evident from Fig. 6(b),it shows a smooth surface (Ra = 0.733 lm) covered withcompacted tribo-films, and no such grooves can be seen on theworn surface of TTC. Moreover, the extent of covered tribo-films in Fig. 6(b) is larger in comparison to that observed inFig. 6(a). Besides, the smooth surface is covered with loosewear debris at a small location that appears bright. Theelemental compositions of different regions marked A and B inFig. 6(b) have been listed in Table 2. The EDS analysis resultsindicate that the compacted tribo-film is an oxide-rich tribo-filmmainly consisting of Al-Ti-Si-Oxides, while the wear debrismainly consist of Al-Ti-Si-Oxides. It is well known that it wasquite difficult to directly obtain a XRD pattern of the tribo-filmon the worn surface, because the x-ray can readily penetrate thetribo-film and reach into the surface of the matrix. Hence, inorder to further identify the information of phase compositionof the tribo-film on the worn surface, the XPS analysis is used

herein. From the XPS results (see Fig. 7), it can be found thatthe peaks assigned to TiO2, SiO2, and Al2O3 are detected. Thepresence of Si-Oxide implies that Ti3SiC2 has been decom-posed and oxidized during the friction process (Ref 26). It isclear from the XPS results that the tribo-films formed on theworn surface of TTC mainly consist of Al-Ti-Si-Oxides, whichreduces TTC-Si3N4 contact and provides the low shear strengthjunctions at the interface, thus reducing the plowing componentof friction force which in turn reduces the wear rate and frictioncoefficient (Ref 27, 28). This should be responsible for thelower friction coefficient and less wear rate of TTC at 600 �C,as seen in Fig. 4(a) and (b).

The typical EPMAmorphologies of worn surfaces of TA andTTC at 800 �C are presented in Fig. 8, indicating that themorphology of the worn surface of TTC is distinct from that ofTA. TA exhibits plastic smearing and local adhesive tribo-filmson the rough worn surface (Ra = 1.571 lm), as seen in Fig. 8(a).EDS analysis indicates that the adhesive tribo-film marked byrectangle in Fig. 8(a)mainly consists of Ti-Al-Oxides.Whereas asmooth surface (See Fig. 8b, Ra = 0.376 lm) covered by asmooth and well-compacted tribo-film mainly consists of Al-Ti-Si-Oxides as suggested by the EDS analysis. In order to furtherexplore the tribo-film on the worn surface of TTC, the subsurface

Fig. 3 Microstructure of fractured surface of TTC

Fig. 4 Friction coefficient (a) and wear rate (b) of TA and TTCwith temperature


Fig. 5 EPMA morphologies of worn surfaces: (a) TA at 25 �C, (b) TTC at 25 �C, (c) TA at 200 �C, (d) TTC at 200 �C, (e) TA at 400 �C,(f) TTC at 400 �C

Table 1 The average roughness (lm) of the worn surfaces of TA and TTC tested at different temperaturesfrom 25 to 800 �C

Sample

Surface roughness, lm

25 �C 200 �C 400 �C 600 �C 800 �C

TA 0.653 1.274 1.968 1.943 1.571TTC 0.498 1.013 1.647 0.733 0.376


Fig. 6 EPMA morphologies of worn surfaces of (a) TA and (b) TTC at 600 �C

Table 2 EDS analysis of element (at.%) in regions marked in Fig. 6

Region

Element, at.%

O Al Si Ti Nb Cr C

A 45.71 30.73 ÆÆÆ 21.13 0.81 1.62 ÆÆÆB 38.85 28.22 14.12 15.39 1.07 2.35 ÆÆÆC 32.97 20.17 18.06 18.19 0.75 0.77 9.09

Fig. 7 XPS spectra of elements on the worn surface of TTC at 600 �C: (a) Ti, (b) Si and (c) Al


analysis is carried out on the worn surface of TTC by cross-sectioning it vertically to the sliding direction, and the location ofthe cross-sectional position is shown in Fig. 8(b). Figure 9exhibits the microstructure and elemental distribution of crosssection of wear scar of TTC after tests at 800 �C. It is clear toidentify the significant stratification morphologies remarked asA, B, and C. The chemical compositions of the different regionsaremeasured by the EDS elementalmappings of Ti, Al, Si, C, andO, as shown in Fig. 9. It can be observed that the amounts of Cdistributing in the A and B regions are higher than those in the Cregion, implying that part of Ti3SiC2 lubricant in the TTCovercomes the potential barrier and is squeezed to the slidingsurface under the frictional heat and the cyclic stress during thefriction process. The presence of O element indicates thatoxidation reactions occur and form oxide debris, which togetherform a well-consolidated tribo-film as A region in Fig. 9. Inaddition, it can be seen that the chemical compositions of Ti, Al,Si, C, and O unevenly distribute in different regions, asdetermined in Fig. 9. From these analyses, the followingconclusions can be drawn: (I) the A region is the oxide-richtribo-filmmainly consisting of Al-Ti-Si-Oxides; (II) the B regionis the mechanically mixed layer (MML), which mainly consistsof Ti3SiC2 and some oxide debris; (III) the C region is thesubstrate. It is well known that a good tribo-film on the frictionsurface results in the excellent tribological behaviors, while apoor tribo-film on the friction surface results in the badtribological behaviors (Ref 28). Therefore, TTC exhibits thelowest friction coefficient and least wear rate at 800 �C, asdetermined in Fig. 4(a) and (b).

According to the above analyses, the mechanism of Ti3SiC2

governing the elevated temperature tribological behaviors ofTTC during the friction process can be proposed. Figure 10(a-d) provides a schematic representation to illustrate the wholefriction and wear process. As produced observed in Fig. 10(a),Ti3SiC2 is well proportionately embedded within the TTC (seeFig. 2, 3). At the onset of the high-temperature friction process,on the ambient temperature and the applied contact pressure,part of Ti3SiC2 in the TTC overcomes the potential barrier andis squeezed to the sliding surface, as is seen in Fig. 10(b). Then,in the process of subsequent friction, on the elevated temper-ature of sliding contact interface, part of Ti3SiC2 on the slidingsurface (see Fig. 10b) has been decomposed and oxidized to

form loose Ti-Si-Oxides (Ref 26), as is shown in Fig. 10(c). Atthe same time, TiAl substrate on the worn surface has also beenoxidized to form loose Ti-Al-Oxides during the sliding frictionprocess. This is in a good agreement with the aforementionedEPMA, EDS, and XPS studies shown in Fig. 6, 7, and 8. Underthe frictional heat and cyclic stress in the process of subsequentfriction, these oxides become soft and spread gradually, finallytogether with the formation of a well-consolidated tribo-filmmainly consisting of Al-Ti-Si-Oxides and MML mainlycontaining Ti3SiC2 and some oxide debris on the above regionof substrate as seen in Fig. 10(d), as is confirmed from Fig. 9.The frictional heat generated during the friction process can risethe temperature of sliding contact interface, which not onlyassists formation of oxides, but also assists soft of oxides tosmear easily on the worn surface. Consequently, it is necessaryto add a calculation of the estimated frictional heating. Becausethe worn surface is covered by a smooth and well-compactedtribo-film during the friction process at elevated temperatures,resulting in stable and the lowest friction coefficient and leastwear rate. Hence, we can approximately assume that the Si3N4

ball moves along a smooth and horizontal plane during thefriction process. Herein, the calculation of the estimatedfrictional heating can be theoretically calculated according tothe equation of Q = FS, where F is the friction force in N, and Sis the total sliding distance in mm. The friction force F istheoretically calculated using the equation F = lP, where l isthe average friction coefficient, and P is the applied load in N.Based on analysis and calculation, the calculation of theestimated frictional heating is in the range of 31.69-35.42 J. Itis well known that such a tribo-film plays a dominant role inreducing the friction coefficients and wear rates during thesliding friction (Ref 27, 28), which is responsible for the lowerfriction coefficients and less wear rates of TTC at elevatedtemperatures of 600 and 800 �C (see Fig. 4a and b).

4. Conclusions

Tribological properties of TA and TTC, fabricated by in situtechnique using SPS at 1100 �C for 10 min under 45 MPa inpure Ar atmosphere protection, are performed using a ball-on-disk

Fig. 8 EPMA morphologies of worn surfaces of (a) TA and (b) TTC at 800 �C


high-temperature tribometer against Si3N4 counterface at theconditions of 10 N-0.2 m/s in a range of testing temperaturesfrom RT to 800 �C. Both the friction coefficient and wear ratefor TA and TTC firstly increase and reach to the highest pointat 400 �C, then decrease to the lowest value with increasingtest temperature up to 800 �C. Furthermore, the frictioncoefficients and wear rates of TTC are comparable to those ofTA at 25-400 �C, while the gaps in friction coefficients andwear rates between TTC and TA beyond 400 �C significantlyappear. It suggests that Ti3SiC2 can actually act as an idealelevated temperature solid lubricant in TTC but does not well

work at temperatures from 25 to 400 �C. A brief represen-tation for the lubrication mechanism of Ti3SiC2 governingthe excellent high-temperature tribological properties of TTCis as follows. Firstly, Ti3SiC2 lubricant is oxidized to formTi-Si-Oxides under the frictional heat. Under the cyclicstress and frictional heat in the process of subsequent friction,these oxides become soft and spread gradually. Finally, awell-consolidated rich-oxide tribo-film forms on the wornsurface of TTC, which drastically reduces the frictioncoefficients and wear rates of TTC at high temperatures of600 and 800 �C.

Fig. 9 Microstructure and elemental distribution of cross section of wear scar of TTC obtained at 800 �C


Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (51275370); the Nature Science Foundation ofHubei Province (2012FFB05104); the Fundamental Research Fundsfor the Central Universities (2013-ZY-049); the Project for Scienceand Technology Plan of Wuhan City (2013010501010139); theAcademic Leader Program ofWuhan City (201150530146); and theProject for Teaching and Research project of Wuhan University ofTechnology (2012016). The authors also wish to gratefully thank theMaterial Research and Testing Center of Wuhan University ofTechnology for their assistance.

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High-Temperature Tribological Performance of Ti3SiC2/TiAl Self-Lubricating Composite Against Si3N4...

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