High temperature wear mechanisms in thermally oxidized titanium alloys for engine valve applications

Titanium alloys are promising lightweight materials to replace the conventional use of stainless steels in engine valve applications, but their high temperature wear resistance should be improved. In this study, a thermal oxidation (TO) treatment (at 600 degrees C for 60 h) was utilized on Ti-6Al-4V to generate a surface rutile, TiO2, layer with a hardened oxygen diffusion zone underneath. Sliding wear tests were performed on untreated Ti-6Al-4V and TO-Ti-6Al-4V disks against cast iron pins at 350 degrees C and 550 degrees C, typical temperatures at which intake and exhaust engine valves operate. In addition, high temperature impact tests were conducted to study the deformation and fracture mechanisms of surface layers produced on TO-Ti-6Al-4V. The results showed that compared with untreated Ti-6Al-4V, TO-Ti-6Al-4V exhibited lower sliding wear losses at both test temperatures. The lowest wear loss was observed at 550 degrees C due to the preservation of both TiO2 layer and oxygen diffusion zone, and the propensity of impact crack growth was also reduced. The oxygen diffusion zone reduced the extent of subsurface plastic deformation and contributed to improved wear resistance.