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.

Oil-impregnated porous polyimide (iPPI) materials are usually used as retainer for bearings. In these bearings, balls and rings, balls and retainers are two different kinds of contact. In this paper, the friction and wear properties of iPPI were investigated using steel (disc)–steel (ball)–iPPI (pin) double-contact friction test rig for simulating the actual contact in bearings. The results show that compared with that of iPPI–steel single contact, the friction coefficient of iPPI–steel in double contacts is lower and decreases with the amount of additional oil. The surface of iPPI in single contact suffers more wear compared with that in double contacts. Different from single contact, the worn surfaces of iPPI in double contacts are blackened. The Raman spectra of worn surfaces of balls and discs indicate that α-Fe₂O₃ and Fe₃O₄ were formed during rubbing of the double contacts. Many nanoscale iron oxide particles are found on the worn surfaces of iPPI in double contacts; on the contrary, few particles could be found on the surface in single contact. In double-contact friction, the nanoscale wear debris penetrates inside the iPPI material through the process of extruding and recycling of oil, which is the mechanism of the blackening of the iPPI worn surfaces. The studies show that the double-contact friction method is a new and effective method to study the friction in bearings, especially for those with polymer retainer.

This study fabricated textures on the stator surface of a traveling wave ultrasonic motor (USM) using laser and investigated the tribological behavior of a polytetrafluoroethylene (PTFE) composite friction material and stator. Initially, the effect of textures with different densities was tested. As the results suggested, the generation of large transfer films of PTFE composite was prevented by laser surface texturing, and adhesive wear reduced notably despite the insignificant decrease in load capacity and efficiency. Next, the 100-h test was performed to further study the effects of texture. Worn surface and wear debris were observed to discuss wear mechanisms. After 100 h, the form of wear debris changed into particles. The wear mechanisms of friction material sliding against the textured stator were small size fatigue and slight abrasive wear. The wear height of friction material decreased from 3.8 μm to 1.1 μm. This research provides a method to reduce the wear of friction materials used in travelling wave USMs.