Cryogenic sliding induced subsurface deformation and tribological behavior of pure titanium

Abstract

The cryogenic sliding induced surface deformation and tribological behavior of TA2 pure titanium were investigated in the present work. Friction and wear tests under dry sliding condition at room and cryogenic temperatures were conducted separately. An external cryogenic device was designed to produce a steady cryogenic environment through liquid nitrogen during the sliding process. Microstructure of subsurface under beneath the worn surface were detected by Optical Microscope (OM), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The results showed that sliding at room temperature induced a severe plastic deformation (up to 50 μm in depth), while the deformation layer was converted to a work-hardening layer (10 μm) and a gradient microstructure was formed under cryogenic sliding condition. Sliding at cryogenic temperature induced the formation of dislocation walls and α phase sub-grains, which hindered the dislocation motion during sliding process and was responsible for the decrease of plastic deformation. Furthermore, the refinement of α phase grains in this hardening layer could be attributed to the strain accumulation and severe local deformation at cryogenic temperature. With the effects of subsurface deformation, wear rate of TA2 samples was decreased by up to 66% under cryogenic sliding condition. The wear mechanism followed the general characteristics of oxidation and adhesive wear under dry sliding wear condition at room temperature. Under cryogenic sliding, a successive tribological behavior consisting of strain concentration, cracks generation and propagation, and fatigue fracture can be induced by the formation of work-hardening layer in the subsurface of the sample.