Effect of Porosity on the Friction Properties of Porous Polyimide Impregnated with Poly-α-Olefin in Different Lubrication Regimes

Porous polyimide materials (PPIs) having different porosity with a constant pore size were fabricated to further understand the effect of porosity on the friction properties in the boundary lubrication, mixed lubrication and elastohydrodynamic lubrication (EHL) regime, respectively. The tribological behaviors of non-porous polyimide (PI0) immersed in poly-α-olefin oil bath and PPIs impregnated with poly-α-olefin were firstly compared, and the effects of load and porosity on the friction performances were investigated. Results show that as the speed increases, the PPIs impregnated with oil exhibit the same friction coefficient (COF) change trend as PI0 immersed in poly-α-olefin oil, which is consistent with the stribeck curve, but the COFs in whole velocity range are higher, and the EHL arrived at a higher speed. The EHL critical speed of PPIs impregnated with oil, as well as the COFs during whole lubricating regime are closely related to the load and the porosity of the material. It mainly depends on two opposite effects of porosity on the friction properties. On one hand, PPIs with higher porosity will release more oil from pores under compression to form a lubricating film, on the other hand, higher porosity will lead to higher contact pressure under the same load, and provide larger contact areas as a result of the increase in the elastic collapse area, which will aggravate the solid–solid direct rubbing and increase the COFs.