Tribofilm Formation of Zinc Dialkyldithiophosphate as Oil Additives Impregnated in Porous Polyimide under Different Contacts.

To improve the tribological properties of porous polyimide (PPI), ZDDP-mixed PAO4 was impregnated in PPI (denoted as ZPPI), and the tribological properties of ZPPI under single- and double-contacts were investigated. In the single-contact of ZPPI-steel, a rough and thick tribofilm was formed on the steel ball, which could protect the steel surface but resulted in large fluctuations in the friction coefficient. In the double-contact of ZPPI-steel-steel, ZDDP formed a uniform and thinner tribofilm on steel surfaces, leading to a lower friction. ZDDP could inhibit the formation of iron oxides significantly in the double-contact, while the antioxidant effect of ZDDP in the single-contact of ZPPI-steel was not obvious. ZnS and ZnO generated from ZDDP were adsorbed in the ZPPI pores, which aggravated the blackening of the ZPPI worn surface.