The effect of surface roughness orientation on PEEK (polyetheretherketone) transfer film volume in multi-directional and linear sliding

Abstract

Polymer transfer films are thought to reduce friction and wear during sliding. In such cases, a continuous, uniform transfer film is thought to yield better wear performance. However, several polymers, including the thermoplastic polyetheretherketone (PEEK), do not always display this behavior. Recent works analyzing transfer film quality of PEEK resulted in no clear correlation to wear. Currently, the mechanisms for PEEK transfer film development are unknown, but there is evidence suggesting roughness orientation relative to sliding and frictional heating play key roles. In this work, the development of PEEK transfer film is explored in relation to multi-directional versus linear sliding, roughness orientation and temperature rise. Three distinct wear paths were chosen for wear tests. The transfer film of the square wear paths was analyzed using white light profilometry and imaging software to obtain the volume and area coverage by the film. The temperature rise during sliding of the bulk polymer pin was recorded with infrared camera radiometry for linear reciprocating tests. Scratch tests and chemical etching were conducted on the polymer pin surface to evaluate any directional bias or crystallinity orientation induced by sliding. It was found that wear debris and polymer chain orientation play no noticeable role in PEEK's transfer film formation. The transfer film gradient increased with frictional heating, and transfer film color changed under certain conditions. This color changed also correlated to reduced wear. This study also confirms that transfer film development is strongly dependent on roughness orientation, and its effects are examined.