Effect of treatment time on the oil lubrication behavior of thermally oxidized CP-Ti sliding against AISI 52100 Steel

Outstanding corrosion resistance and low density are advantages of commercially pure titanium (CP-Ti). But poor wear resistance becomes the limitation of its use in different fields. Oil lubrication is one of the direct methods employed to reduce friction and wear and to improve material performance. However, titanium is a metal that is difficult to be lubricated by conventional oil lubricants. Simple surface treatments such as thermal oxidation (TO),  normally requires a long period of time for oxygen diffusion. To promote sustainability by reducing energy consumption, short time surface treatments are more preferable.  The aim of this research work is to discover the effectiveness of short time TO treatments in improving the oil lubrication behavior of CP-Ti under a high contact load. CP-Ti samples were TO treated at 625 ℃, times ranging from 0.5 h to 24 h. Friction tests were carried out under oil lubricated conditions, under 60N. The counterbody material was AISI 52100 steel ball. The results demonstrate that under such boundary lubrication conditions, all the TO treated samples can be effectively lubricated with low coefficient of friction (COF) below 0.09 and low wear rates in the order of 10^-5mm³m^-1N^-1. This work discovered that very short time treatment (0.5 h) is sufficient to change the surface characteristics of CP-Ti, effective oil lubrication can be achieved. This is attributed to the thin rutile oxide (TiO₂) layer on the surface and the oxygen diffusion zone at the subsurface. EDS and Raman spectroscopy analysis revealed the existence of zinc, phosphorus and sulfur compounds on the steel counterface, indicating the formation of a tribofilm on the counterface. The formation of such a tribofilm  can provide excellent wear protection under high loads. The short TO treatment time indicates less energy consumption in manufacturing and thus is more preferable in practical applications.