Lubrication failure mechanism of rolling-sliding steel ball against oil-impregnated porous polyimide and bearing steel in double friction pair

Oil-impregnated Porous Polyimide (O-PPI) cages have been widely applied in aerospace bearings because of their exceptional performance and self-lubricating characteristics, where the surface blackening failure of the O-PPI cage during the operation process significantly shortens the service lifetime. However, the blackening failure mechanisms are still not fully understood due to the complex multiple contacts between steel balls, the cage, and the inner/outer rings in the bearing system. This study aims to deeply detect the surface blackening failure mechanism of PPI materials with a relevant improvement measuring method. A double-contact rolling-sliding ball holder was developed to simulate the contact and the motion in aerospace bearings with PPI cages, where the blackening phenomenon on the O-PPI surface was replicated as the bearing ball rubbed against the PPI and steel surfaces. Comparative experiments and subsequent characterization results revealed that the iron debris generated at the ball-steel sliding interface was infiltrated into the PPI surface pores along the rolling motion of the steel ball, and then the splitting reaction of PAO oil at the steel-PPI interface catalyzed by the iron (Fe) during friction resulted in the formation of blackening products and further lubrication failure. Based on these findings, effective countermeasures are proposed to prevent similar failures in the future. This analysis is expected to enhance the operational reliability and enlarge the service lifetime of the space-bearing systems.