WEAR MECHANISMS AND IN-SERVICE SURFACE MODIFICATIONS OF A STELLITE 6B CO-CR ALLOY

Abstract: 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 are analysed. The analysis is carried out using focused ion beam (FIB), transmission electron microscopy (TEM), 3D confocal profilometer 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 those conditions, an unexpected low wear resistance was observed during service. As a consequence, a long machine downtime was necessary to replace the damaged component. Results of FIB-SEM/EDS characterisation showed severe grooving due to the presence of sand (SiO2) particles embedded in the rubber seal in the contact area. In addition, three body abrasive wear is observed immediately outside the contact area. 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 a proper protective coating (e.g. a thermal sprayed thick coating)