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

In practical applications, the pocket surface of a porous polyimide (PPI) cage often experiences blackening wear, which may lead to lubrication failure. However, the mechanism of blackening wear is still controversial. In this study, the impact of oxygen concentration on blackening wear was investigated using a sealed double-contact friction tester by controlling the ventilation with 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 conditions to obtain high friction temperatures. The results indicate that iron oxides are the primary factors of blackening on the worn surfaces of PPIs, whereas oil and PPI decompositions are minor factors. At an oxygen concentration of 0.1% or when Si₃N₄ balls are used as rubbing pair materials, there is only slight blackening wear on the worn surface of the PPI. Therefore, the influence of iron oxides on the blackening of worn PPI surfaces is crucial. At low oxygen concentration (0.1%), a relatively high degree of unsaturation of the oil may lead to slight blackening wear of the PPI due to oil decomposition, but its impact is minimal. This study provides necessary insights into the mechanism of blackening wear in PPI cages.