Fatigue-like failure of thermally oxidised titanium in reciprocating pin-on-plate wear tests

Abstract

Thermal oxidation (TO) is a simple method for increasing the tribological performance of titanium alloys. In previous studies however delamination of the oxide layer has been observed in reciprocating sliding tests. This study seeks to clarify the effects of oxide thickness on interfacial failure in reciprocating pin-on-plate tests and to contrast this behaviour with that shown in conventional (scratch and Rockwell C (Rc) indentation) adhesion tests. Commercial purity titanium samples were TO treated in air at 700 °C for 16, 25 and 36 h, resulting in oxide thicknesses of 3.3, 4.3 and 4.8 μm respectively. These samples were subjected to reciprocating pin-on-plate tests conducted with a stroke of 7.5 mm at 1 Hz with a 7.94 mm diameter silicon nitride ball in a mineral base oil. A critical load that decreased with increasing oxide thickness was found above which progressive failure occurred at the oxide-metal interface. This failure started as small discrete blisters that grew along the wear track with further passes until the free-standing oxide ruptured. A characteristic increase in friction coefficient was observed concurrently with the oxide rupture. Failure during the scratch tests progressed from oxide delamination to oxide rupture with increasing scratch load. The scratch load at which oxide delamination was first observed initially increased then decreased with increasing oxide thickness, in contrast with the reciprocating sliding results. Delamination and oxide rupture were also observed around the Rc indentations and the radius of the interfacial failure zone increased with increasing oxide thickness.