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ANALYSIS OF FRICTION AND WEAR OFTITANIUM ALLOYS

Aadarsh Mishra1*

*Corresponding Author: Aadarsh Mishra,aadarshm9@gmail.com

Due to the lower-cost processing of titanium, its application in the engines as light weight materialhas renewed its interest in the tribological behavior. A pin on disk sliding friction test was conductedon the titanium alloy (Ti-6Al-4V). Alloy disks were slid against the bearing ball composed ofstainless steels at the speeds of 0.2 and 0.8 m/s. When the sliding speed is higher the coefficientof friction and wear rate are lower. For steel the wear rate is the least. Microstructural studyconfirms that Ti alloys have the tendency to transfer material to their counter face and there arepossible tribological reactions. Degradation of mechanical properties of contact areas of reactionproducts takes place.

Keywords: Titanium, Material transfer and tribological reaction

INTRODUCTIONTitanium exhibits superior properties as atribomaterial in comparison with aluminum andmagnesium which are light weight alloys.Titanium alloys have an excellent resistance forheat and corrosion and they are much harderthan Magnesium and aluminum alloys. Ascompared to aluminum and magnesium theyhave high affinity for oxygen which leads toformation of an adherent surface oxide namelytitanium oxide which acts as a solid lubricant.Titanium alloys also exhibit good metallurgical,heat treatment and mechanical properties dueto which it finds a lot of application in aerospace

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Mechanical Engineering, Manipal Institute of Technology, Manipal University, Manipal, Karnataka 576104, India.

industry. Tribological concerns for Ti inaerospace componentshave focused mainly ontheir fretting behavior, leading to researchonsurface treatments like ion implantation andsolidfilm lubrication (Kustas and Misra, 1992;Waterhouse and Iwabuchi, 1985). Needs in thechemical process industry motivated a 1991study of the galling and sliding wear behaviorofcommercial-purity Ti and alloy Ti-6Al-4V(Budinski, 1991). In the Budinski (1991)experiment the best wear and friction resultsfor Ti alloys were obtained for anodized counter-surfaces coated with MoS2 solid-film or withpolytetrafluoroethylene (PTFE), butthe abrasion

Research Paper

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Int. J. Mech. Eng. & Rob. Res. 2014 Aadarsh Mishra, 2014

resistance was poor. Many more studies wereconducted on the sliding wear mechanisms ofTi alloy. Molinari et al. (1997) highlighted themechanisms responsible for the wearresistance under different load and slidingspeed conditions in self-mated Ti-6Al-4V disk-on-disk sliding tests (Molinari et al., 1997), andDong and Bell (1999) reported unexpectedlyhigh wear rates for alumina sliding against Ti-6Al-4V (pin-on-disk tests). Recentdevelopments in Ti processing forecast theavailability of lower-cost Ti and that hasprompted further interestin exploring thetribological behavior of Ti alloys as bearingmaterials (EHKT Technologies, 2002).

EXPERIMENTAL PROCEDURETitanium alloy Ti-6Al-4V was tested in this study.Friction and wear tests were conducted byusing pin-on-disk apparatus. Ball sliderdiameter was taken as 7 mm while thestainless steel was taken as a metallicmaterials. The disk was made up of titaniumalloy of diameter 50 mm and thickness of 8.5mm. Polishing of disk surface is done by usingpolishing papers while the wear tracks of anaverage diameter of 25 mm are used. Thesliding speed was varied from 0.2 to 0.8 m/s.A 5 N normal load is applied keeping slidingdistance as 350 mm. The friction force wasmonitored by using computerized forcemeasurement system while the wear volumesare determined by using weight changemeasuring system. The wear volumesnormalized by the applied load and the slidingdistance of pin was defined as the wear factor.

The tests were conducted in the ambientair conditions with an average temperature of24 ºC and an average humidity of 55-60%.

RESULTS AND DISCUSSIONWith stainless steel the coefficient of frictionfluctuates between 0.4-0.5. Due to simultaneousbreakage and formation of transfer layer thefluctuation in coefficient of friction is large. Theother reason can include localized fracture intransfer layer. Titanium alloy commonly transfersto thecounterface when rubbing against othermetals (Budinski, 1991; Molinariet al., 1997; andDong and Bell, 1999). The surface morphologyand surface analysis by using microstructureswas done. Wear scar on the stainless steel wasthe basic identification for the formation oftransfer layer. Lower coefficient of friction wasobserved at 0.8 m/s while the coefficient of frictionwas comparatively higher at 0.2 m/s.

The contact area between titanium alloy andstainless steel was observed to exhibit highertemperature which causes reduction in shearstress and lower the value of coefficient offriction. Wear rates observed are higher at bothspeeds of 0.2 and 0.8 m/s. Similardependency of sliding speed is confirmed by(Dong and Bell, 1999). High wear rates occurin case of Titanium disks when sliding against

Figure 1: Microstructure of Ti-6Al-4V

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Int. J. Mech. Eng. & Rob. Res. 2014 Aadarsh Mishra, 2014

stainless steels was done. Wear rates are ofthe order 10-3 mm3/Nm. A relatively more wearis generated by harder sliders when disks ofTi were used. Dong and Bell also reported ahigher wear rate of an aluminaball than that ofa steel ball when sliding against a Ti64 disk(Dong and Bell, 1999). The microstructuralanalysis shows that the wear on the metal canbe either abrasive wear or adhesive wear.Since the wear debris particles are larger andflatter at the sliding speed of 0.2 m/s in thewear process. Therefore this type of wear wastermed as an Abrasive wear. On the other handthe wear debris particles at the sliding speedof 0.8 m/s were smaller with large patches oftransferred material confirming that the typeof wear is adhesive wear. The wear resistanceof stainless steel was observed to ne inverselyproportional to its hardness number. Theabrasive wear of ceramics has been proposedto be a functionof both hardness and fracturetoughness (Evans and Marshall, 1980). Wearresistance has a significant effect on thefracture toughness of the material. Fischer(1993) has demonstrated that, in the case ofyttria stabilizedzirconia ceramics, the wearresistance increases with the fourth power offracture toughness.

To evaluate the sliding wear the pin on diskexperiment is used but if the disk is not normalto the axis of rotation it may cause vibrationsto occur. It depends on the distance betweenthe point of contact of pin and disk and thecenter of rotation. Catastrophic failures mayoccur due to plastic shearing in the materials.The catastrophic failures include cracking andcrushing of the contact surfaces. The maindisadvantage involves the loss of material andfragmentation of debris. Rice (1979) and Rice

et al. (1979) have studied the wear rate andmechanism of compound impact (impact andsliding)on metals and superalloys. Thematerial with lower fracturetoughness had ahigher wear rate and gave strong evidenceforsubsurface damage. Mechanically deformedsurfaces usuallyhave different chemicalreactivity than purely thermallystressed solids(Heinicke, 1984). The wear process of thematerial is affected significantly by Tribo-chemical reactions. Titanium oxides have thelower value of Gibbs free energy of formationthan the value of Gibbs free energy of Titaniumnitride. The main reason can be that Titaniumoxides are amorphous in nature due to severeplastic deformation. Titanium aluminides werealso suspected by Dong and Bell (1999)basedon the XRD analysis of the wear debrisproduced by aluminasliding against Ti-6Al-4V.

CONCLUSIONThe titanium alloy Ti-6Al-4V was observed toexhibit excellent tribological properties whilesling against stainless steel in the pin-on-diskapparatus. Fluctuations in the value ofcoefficient of friction occur because ofsimultaneous formation and breakage oftransfer layer. The transfer layers areperiodically formed and fractures occur inthem. When the sliding speed is lower thenthe coefficient of friction was observed to behigh with lager fluctuation and higher wear rate.Other main reasons for the high wear rateinclude fracture toughness and tribologicalproperties.

ACKNOWLEDGMENTI would also like to dedicate this research workto my father late R S Mishra and mother K LMishra.

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REFERENCES1. Budinski K G (1991), “Tribological

Properties of Titanium Alloys”, Wear,Vol. 151, pp. 203-217.

2. Dong H and Bell T (1999), “TribologicalBehavior of Alumina Sliding AgainstTi6Al4V in Unlubricated Contact”, Wear,Vols. 225-229, pp. 874-884.

3. EHKT Technologies (2002),“Opportunities for Low Cost Titanium inReduced Fuel Consumption, ImprovedEmissions, and Enhanced DurabilityHeavy-Duty Vehicles”.

4. Evans A G and Marshall D B (1980),Fundamentals of Friction and Wear ofMaterials, p. 439, ASM.

5. Fischer T E (1993), “Friction and Wear ofCeramics”, Scripta Metall. Mater. ,Vol. 24, pp. 833-838.

6. Heinicke G (1984), Tribochemistry, CarlHanser Verlag Munchen Wien, Berlin.

7. Kustas F M and Misra M S (1992),Friction and Wear of Titanium Alloys, in

P J Blau (Ed.), ASM Handbook, Vol. 18,pp. 778-784, Friction, Lubrication, andWear Technology, ASM International.

8. Molinari A, Straffelini T B and Bacci T(1997), “Dry Sliding Wear Mechanisms ofthe Ti6Al4V Alloy”, Wear, Vol. 208,pp. 105-112.

9. Rice S L (1979), “The Role ofMicrostructure in the Impact Wear of TwoAluminum Alloys”, ASME Proc. WearMater., pp. 27-34.

10. Rice S L, Nowotny H and Wayne S F(1979), “Characteristics of MetallicSubsurface Zones in Sliding and ImpactWear”, ASME Proc. Wear Mater .,pp. 47-52.

11. Waterhouse R B and Iwabuchi A (1985),“The Effect of Ion Implantation on theFretting Wear of Four Titanium Alloys atTemperatures up to 600 °C”, inProceedings of the InternationalConference on Wear of Materials,pp. 471-484, ASME, New York.