Influence of iron dilution on plastic deformation mechanisms in cobalt-based alloys: Consequence of phase transformations on tribological behavior

The chemical composition and microstructure of metallic parts are parameters that affect plastic deformation accommodation mechanisms under friction stresses. The content of alloying elements impacts the stacking fault energy and the plastic deformation, and thus affects the tribological behavior. Depending on this content and the levels of mechanical stresses, plastic deformation of some alloys obtained under non-equilibrium conditions can occur. It is caused by different mechanisms, such as perfect slip and/or partial dislocation slip, as well as by phase transformation. The purpose of this study is to determine the influence of the plastic deformation accommodation mechanisms on tribological behavior by studying the effect of the iron content in cobalt-based alloys. For manufacturing purposes, cobalt-based coatings are produced on steel substrate using a additive manufacturing process (SLM). With this process, the microstructures of the cobalt-based coatings are essentially metastable FCC phase at room temperature with different contents of diluted iron. Tribological tests were carried out with a ball-to-disc contact. Iron contents were estimated by EDS-SEM. Analytical techniques such as XRD and EBSD were used to identify the microstructural changes observed in TTS due to tribological loading. The friction coefficient is linked to the evolution of the plastic deformation mechanisms activated to accommodate the contact. In particular, in addition to work-hardening phenomena, phase transformations are possible, namely a metastable FCC phase gives an HCP phase and a metastable FCC phase gives an α′-BCC phase under tribological loading. Both types of transformations can occur, individually or simultaneously, depending on the iron content in the coating.