Etude des Transformations Tribologiques Superficielles de TA6V formées par fretting : mécanismes de création, propriétés micromécaniques et modélisation de l’usure

Fretting is a surface degradation phenomenon occurring under low-amplitude alternating sliding. During fretting loading, superficial layers undergo microstructural transformations, appearing white after chemical etching, and become brittle. These layers are named Tribologically Transformed Structures (TTS) and contribute to debris formation. This research aims to understand the genesis of TTS in a plane-plane contact of Ti-Al-4V alloy subjected to fretting loading and to characterize their mechanical behavior.First, TTS formation was evaluated under various contact conditions, particularly by modifying the contact pressure. The kinetics of TTS formation are studied by analyzing the localization and morphology of TTS for different fretting cycle numbers. EDX chemical analyses, optical observations, and SEM of cross-sectional cuts of fretting scars after chemical etching are conducted. The results show that TTS initially appear locally as islands before forming a single, enlarged zone at the center of the contact with a thickness smaller than 100 microns. However, pressures inferior or equal to 200 MPa do not allow TTS to form, suggesting the establishment of a pressure threshold for their appearance in the contact.The second part of the study focuses on the characterization of the microstructure and crystallographic texture of TTS. Since TTS are nanograin materials, TEM analyses are needed to describe their structure. The phases are identified through electron diffraction patterns, while EDS and EELS chemical analyses are executed. The Astar method is employed to establish the local texture of TTS zones at a nanoscopic resolution. It emerges that TTS consist of two alternating layers of nanograins. One layer comprises larger alpha phase grains (20 to 50 nm) with a distinct crystallographic texture, while the other consists of smaller grains (also of the alpha phase) with a few nanometers in diameter and lacking an untextured. The presence of nitrogen is also detected in this layer. A mechanism of TTS formation by continuous dynamic recrystallization coupled with localization of plastic deformation into bands is introduced to explain the observed heterogeneous microstructure.The destruction of TTS is also a critical issue. However, the very low thickness of TTS (<100 microns) renders the determination of their mechanical characteristics challenging. Flexural tests of notched micro-cantilevers, machined and loaded in a SEM-FIB, were used to identify the fracture toughness of TTS and revealed their significant brittleness. Their heterogeneous microstructure also impacts the crack propagation path.Finally, a novel numerical approach has been implemented to estimate cumulative levels of plastic deformation within the contacts. This simulation is conducted in two stages. First, wear is simulated using a multiphysical model that takes into account surface oxidation phenomena. Then, an elasto-plastic calculation is performed on the worn surface to estimate the cumulative plastic deformation during fretting. These simulations confirm the appearance of TTS in the form of islands via a plastic process, highlighting the utility of simulations in explaining TTS formation.