An investigation into the effect of Triode Plasma Oxidation (TPO) on the tribological properties of Ti6Al4V

Abstract

Improving the tribological properties of titanium alloys has been the subject of extensive research for many

years. A number of thermochemical processes have been developed for that purpose. In this study, surface

hardening of Ti6Al4V is achieved by Triode Plasma Oxidation (TPO) which differs from conventional diode

plasma treatments through the use of a third electrode; a negatively biased tungsten filament to enhance

the ionisation levels in the plasma. The resultant surface generally consists of a top oxide layer with an oxy- gen diffusion zone lying immediately underneath it. The effects of process parameters such as substrate tem- perature, current density and oxygen partial pressure have been investigated. Surface hardness

measurements at various indentation loads were carried out to assess the changes in hardness with depth

across the diffusion layer. The hardness profiles obtained confirmed the gradual decrease in hardness with

treatment depth and provided an indication of the thickness of the hardened layer produced. Ball-on-plate

reciprocating sliding wear data and glancing angle XRD analyses of the oxidised samples are also presented.

The results indicate that a harder and deeper case is achieved at both high substrate temperature and high

oxygen partial pressure. Furthermore, XRD data show that the substrate temperature strongly affects the

structure of the oxide layer produced. All TPO-treated samples exhibit significantly better wear performance

compared to the untreated material.