Pore Size Control and Tribological Properties of Oil-Impregnated Porous Polyimide With Similar Porosity

Abstract

Pore size is critical to the oil-storage performance and tribological properties of porous polyimide (PPI). To study the effect of pore size, four PPIs with different pore sizes and similar porosities were fabricated. The mechanical, oil-absorption, and tribological properties of the PPIs were investigated. According to the results, PPIs with larger pores demonstrate faster oil absorption and a higher oil-filling rate. However, larger pores contribute to a rougher surface and lower oil retention. These dual effects result in PPIs with larger pores displaying a low friction coefficient at low speeds but a high friction coefficient at high speeds. The effect of pore sizes on elastohydrodynamic (EHD) film and Stribeck curves were discussed from seepage effect and viscoelasticity of PPI, respectively. The permeability of the four PPIs was calculated based on mercury intrusion test results. Larger pores lead to higher permeability of PPI at lower pressure, which makes it easier to reduce thickness of EHD film, causing high friction and wear. The damping effect of PPIs, rather than a viscous flow, results in a slight increase in friction coefficient at high speed under micro-oil lubrication. Considering the tribological and oil-absorption properties, the preferred particle size of polyimide molding powder is 25–48 μm.