Wear mechanisms and in-service surface modifications of a Stellite 6B Co–Cr alloy

In the present paper, the in-service wear mechanisms and cross-sectional microstructural evolution of a Stellite 6B cobalt-based alloy subjected to sliding contact conditions were analysed by focused ion beam (FIB), transmission electron microscopy (TEM) and nanoindentation techniques. Samples under investigation consisted of a lip-seal adopted in a tunnel boring machine (TBM). In the working conditions, the lip-seal is in sliding contact with a rubber seal in a pressurised oil environment. In this specific case, an unexpected low wear resistance was observed during service, and a long machine downtime was necessary to remove the component. Results of FIB-SEM/EDS characterisation showed deep grooving due to sand (SiO2) particles embedded in the rubber seal inside the contact area, while three-body abrasive wear is observed immediately outside it. Microstructural evaluation of Stellite 6 B Co-based alloy of the cross-section also showed the presence of a nano-crystalline hardened layer with diffuse presence of stacking faults. The presence of a tribo-film with a complex structure was also clearly observed. It is concluded that wear resistance of such components could be significantly enhanced either by a proper control of abrasive contaminant particles in the lubricant oil or by introducing surface microstructural modifications of the component (e.g. a nanostructured thick coating).