Abstract
► Oil-lubricated friction and wear tests of both commercial and experimental coatings and surface treatments for titanium alloy Ti–6Al–4V were conducted to determine which held promise for applications in diesel engines. ► Eighteen materials and surface treatment combinations were tested against Type AISI 52100 bearing steel under reciprocating sliding and in engine-conditioned formulated diesel oil. ► Eight of eighteen candidates passed the friction criteria (friction coefficient less than 0.15) and wear criteria (stable sliding with minimal vibration). ► The eight treatments that passed our criteria included a nitriding treatment, a titanium nitride coating, an oxygen diffusion treatment, a diamond-like coating, and a commercial chromium-nitride coating. The fuel efficiency of ground vehicles, like heavy trucks, can be improved by reducing engine weight. While primarily known for its use in aerospace structures, titanium alloy Ti–6Al–4V has the potential to replace heavier steel in certain friction and wear-critical diesel engine components like connecting rods, intake valves, movable turbocharger vanes, and pistons. While Ti–6Al–4V exhibits excellent corrosion resistance, good fatigue strength, and acceptable fracture toughness, it has poor sliding characteristics. Titanium alloys have a propensity to fail by galling, and often exhibit high and unstable friction coefficients. In the current work, selected surface engineering techniques were compared to determine which best enhance the tribological performance of Ti–6Al–4V alloy and another alloy, 60Ni–40Ti. Candidate treatments included diffusion treatments, hard coatings (TiN and CrN), a soft coating (Cu–Ni–In), titanium-matrix TiB 2 in situ-formed composite, and shot peening. Diffusion treatments included oxygen diffusion, nitriding, and carburizing. In addition to studying the effects of individual surface engineering approaches, some were combined in an attempt to maximize their effects, but at the same time retain the mechanical properties of the titanium alloy achieved by proper heat treatment. Both dry and lubricated friction and wear tests were conducted using ASTM G133 (linearly reciprocating ball-on-flat). The ball specimens were AISI 52100 bearing steel. Lubricated tests were performed in engine-conditioned diesel engine oil. Test coupons were characterized using microindentation, stylus and optical interferometry, and metallographic examination. Surface engineering methods significantly improved the wear performance of Ti–6Al–4V alloy, but their relative rankings varied significantly between oil-lubricated and non-lubricated conditions.
Published Version
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