Tribofilm Formation of Zinc Dialkyldithiophosphate as Oil Additives Impregnated in Porous Polyimide-Steel Soft Contact

As a commonly used lubricant additive, ZDDP can form antiwear tribofilms on metal surfaces, reduce the wear of rolling elements and grooves, and thus holds potential for application in bearing lubrication. However, in the actual operation of bearings using oil-impregnated porous polyimide (iPPI) as the bearing cage, there are both steel-steel contacts and iPPI-steel contacts. The characteristic of the ZDDP tribofilms under iPPI-steel soft contact remains unclear. Herein, ZDDP was added as a lubricant additive to poly-alpha-olefin oil (PAO4), and its frictional behavior under soft contact (iPPI-steel) and hard contact (steel-steel) at different temperatures was investigated. Results demonstrated that raising temperature promotes ZDDP tribochemical activity and increases the tribofilm-affected area on the steel surface. The formed tribofilm protects the steel surface and reduces wear. FIB-SEM characterization demonstrated that compared with hard contact, the ZDDP tribofilms formed on the steel ball surface under soft contact exhibit stronger continuity and greater thickness, thereby providing more effective protection for the steel ball surface. This study provides necessary insights for the application of ZDDP in bearings with iPPI as the bearing cage.