Fretting wear is a surface degradation process caused by oscillatory motion and contact slipping. During gross slip, high local stresses, and plastic deformation in the surface and subsurface can lead to the creation of a nanosized grained structure called Tribologically Transformed Structure (TTS). The mechanisms of formation of TTS under fretting are not fully understood and the properties of the TTS layer needs characterization, especially since this layer will be the one on which fretting takes place. The microstructure and mechanical behaviour of TTS in an alpha-beta Ti-6Al-4V alloy has been studied.
Cross-sections of wear scars were observed after polishing and chemical etching. TTS appears in the contact along two other structures: a Third Body Layer (TBL) made of compacted debris and a General Deformed Layer (GDL) which is the plastic zone under the TTS. All structures have the same chemical compositions as the initial bulk material except for the nitrided TBL. TTS is characterized by a very high hardness compared to the bulk. The mechanical behavior of TTS was further studied thanks to in-situ notch beam test. Very low tenacity was found confirming that TTS is a brittle layer.
TTS nanostructure was then observed with a Transmission Electron Microscope (TEM). It shows extreme grain refinement and is composed of two alternated zones. The first zone I is composed of α grains with size of 20 to 50 nm with texturation. Zone II comprises nanosized equiaxed grains whose sizes range from 5 to 20 nm without textures. Further analyses of texturation in Zone I have been conducted with an ASTAR device which produces TEM orientation imaging. Preferential orientation of the normal to the basal plane at around 45° to the fretting loading is observed.