Decomposing of lubricating oil caused the blackening wear of porous polyimide—Or is it?

In actual applications, the pocket surface of the porous polyimide (PPI) cage often suffers blackening wear, which may lead to lubrication failure. However, the mechanism of blackening wear is still controversy. In this study, the impact of oxygen concentration on the blackening wear was investigated using a sealed double-contact friction tester by controlling the ventilation of argon gas. Then four types of oils, as well as steel balls and Si₃N₄ balls, were used to study the effect of oil decomposition on blackening wear. Finally, the effect of PPI decomposition was studied under dry friction condition for obtaining high friction temperature. The results indicate that iron oxides are the primary factor of blackening on the worn surface of PPI, while oil and PPI decomposition are the minor factor. At a concentration of 0.1% oxygen or when Si₃N₄ balls are used as rubbing-pair materials, there is only slight blackening wear on the PPI worn surface. Therefore, the influence of iron oxides on the blackening of PPI worn surfaces is crucial. Under low oxygen concentration (0.1%), higher unsaturation of oil may lead to slight blackening wear of PPI due to oil decomposition, but the impact is much lower. This study provides necessary insights into the mechanism of blackening wear of PPI cages.