Lubrication of ceramics by tribopolymerization: A designed experiment to determine effects of monomer structure, load, and speed on wear

An experimental study of ceramic lubrication by tribopolymerization at high lends and high speeds, using a pin-on-disk (fixed ball-on-flat) machine wi th alumina-on-alumina, is presented. In order to extend the range of applie d loads and sliding velocities beyond those used in previous studies, a thr ee-factor two-level designed experiment was carried out to determine the ef fects of monomer structure, load, and speed on wear. Five monomers of widel y varying chemical structure were used at one percent concentration in a hy drocarbon carrier fluid, hexadecane. They consisted of (a) one condensation -type monomer, a partial glycol ester of a longchain dimer acid, and (b) fo ur vinyl-type addition monomers. Two levels of load (40 and 160 N) and spee d (0.25 and 1.0 m/s) - each varying by a factor of four were used; thus the range of frictional heat generation was 16 to 1. The results of this study were rather surprising and changed our thinking o n the mechanism(s) by which monomers can act to reduce ceramic wear. For ex ample, at low speeds - regardless of load - the monomers used were very eff ective in reducing wear; with reductions ranging from 44 to 98 percent depe nding on the monomer and load. However, at high speeds, the monomers were g enerally ineffective; in some cases, increases in wear were observed. This was unexpected. Possible explanations for this behavior - including surface temperature eff ects and tribochemical reactions - are discussed. Results presented on Four ier Transform Infrared (FTIR) Spectroscopy of worn ceramic surfaces and wea r debris show that the film-formation from the monomer solutions is complex , involving a combination of aluminum soap formation as well as evidence of oligomer/polymer formation in some cases, notably diallyl phthalate.