Wear and friction of PEEK composites, dry or lubricated

The present study investigates the wear and friction properties of Poly Ether Ether Ketone (PEEK) sliding against stainless steel. The materials are neat PEEK, PEEK with 10% Polytetrafluorethylene (PTFE), PEEK with 30% carbon fibers (CF) and PEEK with 30% glass fibers (GF). Adding fibers to PEEK (CF or GF) increased the wear by a factor of four. This may seem surprising but it is in accordance with the Ratner-Lancaster correlation which postulates that the wear rate is inversely proportional to the product of the strain and strength at break. Adding fibers may increase the strength but it decreases the strain at break even more. Water lubrication increases the wear by a factor of ten, except in the case of CF where the wear is halved. To investigate the influence of the polarity of the lubricant, the very polar water has been replaced by the quasi-nonpolar n-heptane. This decreases the wear significantly. The n-heptane lubricated PEEK with CF shows the lowest wear rate in the present study. In general the addition of a liquid lubricant decreases the friction whereas it is the opposite when fibers are added. But friction seems to be decoupled from the wear response in the present study.