Surface engineering to improve the durability and lubricity of Ti–6Al–4V alloy

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 TiB2 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.