Wear debris compaction and friction film formation of polymer composites

Abstract

It is well established that the formation and stability of transfer films on the counterface play an important role in the friction and wear behavior of polymers or polymer composites sliding against a metal surface. However, there is a general lack of information in the literature on the formation of the friction film on the polymer composite itself; its formation mechanism and its importance in controlling friction and wear. This investigation was undertaken to study the nature of friction films on the polymer composite rubbing surface. Three friction couples in the configuration of automotive disc brakes were used for this study, which consisted of polymer composites as disc pad friction materials sliding against gray cast iron rotors. The present study shows that 1. (a) when stable friction films, commonly called “glaze”, are readily formed for a given friction couple, a stable friction level and low wear rates can be maintained at various temperatures, as long as the friction film is not destroyed; 2. (b) the ease of formation of a friction film and its stability are related to the ease of wear debris compaction and compacted pellet integrity, which in turn depends on the chemistry of the composite; 3. (c) the formation and stability of the friction film on the sliding surfaces depend upon the cohesive bond strength of the friction film itself as well as the adhesive bond strength between the film and the sliding surfaces. At the sliding interface between the composite friction material and the rotor, the friction film is continuously formed and sheared. When the shearing action overcomes the cohesive bond strength of the film, the film will split and adhere to both sliding surfaces. The role of both cohesive and adhesive bond strength in the formation of a friction film will be discussed. Two phenolic resins and a thermosetting polyimide resin were used in this study. The study revealed that the phenolic resins are superior to the polyimide resin in their ability to form a stable glaze at elevated temperatures, which is consistent with the fact that the disc pads made with phenolic resins have better friction and wear characteristics than those made with polyimide resin. In the case of the disc pads made with polyimide resin. massive material transfer to the rotor occurred at 430 °C.