Influence of Porosity on Wear Resistance of Sintered Titanium under Boundary Lubrication

The influence of the porosity of commercially pure (CP) sintered titanium on its tribotechnical characteristics in a tribo-pair with bronze under boundary lubrication conditions was investigated. The investigations were carried out on samples of sintered titanium with a porosity of 3 and 9% fabricated by powder metallurgy (pressing and vacuum sintering). CP titanium (non-porous) fabricated by the traditional technology of casting and hot deformation was used for comparison. Tribotechnical characteristics were investigated using an SMT-2 friction machine with automatic registration of changes of the friction coefficient and temperature near the friction zone under specific load of 1 MPa. The duration and friction rate were 3000 s and 0.6 m/s, respectively. The friction investigations were carried out according to the “disk–block” scheme. I‑40 industrial oil was used. It was determined that the presence of pores negatively affects the tribotechnical characteristics of titanium. For tribo-pairs with titanium porosity of 3 and 9% at the steady-state stage of wear, the friction coefficient increases by approximately 1.7 and 2.2 times, respectively, compared to the tribo-pair with nonporous titanium. At the same time, an increase of the temperature near the friction zone by 20 and 40% occurs due to the low thermal conductivity and high chemical activity of titanium and processes of diffusion of micro-welding (adhesion) of surfaces were accelerated. The wear intensity of the tribo-pair increases by 21 and 66% with increasing porosity. According to the gravimetric analysis of the samples after tribological tests, it was established that the wear intensity of the counterbody (bronze) is approximately 1.5–2 times higher than the wear intensity of titanium, regardless of its porosity. According to the metallographic analysis of titanium worn surfaces, it was found that, regardless of the porosity, for all tribo-pairs during friction, adhesion and fatigue wear mechanisms were realized. The decrease in the wear resistance of porous titanium compared to nonporous titanium occurs due to the initiation of cracks by surface pores, which provokes delamination of surface layers and intensifies fatigue wear during friction.