Effect of Gas Environment on Friction Behavior and Tribofilm Formation of PEEK/Carbon Fiber Composite

This study investigated the friction and wear behaviors of polyetheretherketone composites containing carbon fibers (PEEK-CF) sliding against stainless steel in terms of their dependencies on gas environments. Sliding tests with a block-on-ring apparatus were conducted in humid/dry air and dry nitrogen at normal temperature and nitrogen at cryogenic temperature of 113K. Friction and wear significantly decreased when sliding occurred in low-humidity, low-oxygen, and low-temperature environments. The friction coefficient, initially 0.25 in ambient humid air, decreased to below 0.10 in the dry nitrogen environment and 0.03 in nitrogen at 113K. Microscopic observations, surface roughness measurements, and surface composition analysis of worn surfaces were conducted after the sliding tests. After sliding in humid air, the surface roughness of the PEEK-CF block was increased from its initial value by adhesions of large wear particles. However, the surface roughness of the block decreased due to the formations of tribofilms that were micrometers thick and composed of fine wear particles of polymer and carbon fibers after sliding in dry nitrogen. X-ray photoelectron spectroscopy analysis and the Raman spectroscopy analysis indicated increased carbon content with graphite structures on the worn surface after sliding in dry nitrogen. Consequently, it was considered that PEEK-CF changed its wear process in the dry nitrogen environment and formed a smooth surface with carbon particles containing graphite, which is thought to be advantageous in reducing friction.