We investigated the tribological behavior of polyacetal polyoxymethylenecomposite filled with rice bran ceramics particles under water lubrication, compared to those of polyoxymethylene composites filled with glass beads and glass fibers. Furthermore, the local contact pressure between a particle and the paired ball was calculated based on a simple contact model. The polyoxymethylene/rice bran ceramics composite showed low wear ( ws < 10−8 mm2/N) and low friction ( μ < 0.10) under water lubrication, irrespective of the normal load and sliding velocity. On a Stribeck curve, the lubrication condition of the polyoxymethylene/rice bran ceramics composite appeared to be near hydrodynamic lubrication. The specific wear rates of the polyoxymethylene/rice bran ceramics composite were the lowest of the composites, regardless of the bearing characteristic number. A smooth worn surface was observed for both the polyoxymethylene/rice bran ceramics composite and the paired ball. The dimensionless roughness parameters of the polyoxymethylene/rice bran ceramics composite were smaller than 1.0, irrespective of the bearing characteristic number. The friction coefficients of the polyoxymethylene composites decreased exponentially with decreasing dimensionless roughness parameter; the low friction of the polyoxymethylene/rice bran ceramics composite resulted from the small roughness parameters. The rice bran ceramics particles indicated a small contact pressure per particle, which was nearly half the Vickers hardness of the steel ball. As a result, the rice bran ceramics particles rarely damaged the steel ball with increasing surface roughness. Therefore, the low friction of the polyoxymethylene composite filled with the rice bran ceramics particles was attributable to the decrease in the roughness, e.g., polishing without the formation of a transfer film and the filler detachment.
