Effects of surface pore size on the tribological properties of oil-impregnated porous polyimide material

The friction properties of oil-impregnated porous polyimide (PPI) material are closely related to surface pore size which is subject to changes during processing. Though the pores inside PPI, designed according to requirements of applications, could not be changed conveniently, surface pores are easy to be modified. To study the effects of surface pores, pores on PPI surface was enlarged by laser scanning and shrunk by spreading a thin layer of finer polyimide particles on PPI surface during molding. Oil absorption of PPI materials was tested with a U-tube test rig and a contact angle goniometer. The friction tests were carried out on a ball-on-disc tribometer. The results shows that though larger surface pores may increase oil absorption-extrusion, the friction coefficient gets higher. The friction coefficient can be decreased by shrinking surface pores. Larger surface pores make oil extrude easily but lead to low capacity of oil film, vice versa. Therefore, a balance exists between oil absorption-extrusion and the capacity of oil film by fabricating proper surface pore size. In this paper, the oil-impregnated friction coefficient is reduced by 18.8 % through spreading finer powder filtered by 400 mesh on PPI material surface.