Fretting wear of polysiloxane-polyimide copolymer coatings as a function of varying humidity

Abstract

The friction and wear of coatings of polysiloxane-polyimide copolymers 17 μm thick were measured in reciprocating sliding contact with a chromium steel sphere. The compositions studied included 0%, 5% and 12% polysiloxane. The humidity levels were less than 10%, 40% and greater than 90%. The rate of wear was indicated by the time required for the sphere to wear through the film and contact the metallic (steel and aluminum) substrate. The wear rate of the unmodified polyimide at the highest humidity was 28 times greater than that at the lowest humidity. The wear rates for the 5% and 12% polysiloxane compositions were less sensitive to the different humidity levels than that of the unmodified polyimide. The wear rate increased as the percentage of polysiloxane increased at a given humidity level. The wear process consisted of the following sequence of events: formation of ridges in the contact area, movement of material to the edge of the area, detachment of ridges to form ribbons, breakage of ribbons to form wear particles and wear particle agglomeration as a result of entrapment between the sphere and the coating surface. The presence of moisture appeared to have an antiplasticizing effect as indicated by cracks between the ridges.