Effect of temperature on the wear of unfilled and filled liquid crystal polymers

Abstract

The friction and wear properties of unfilled and filled liquid crystal polymers (LCPs) (polyester derived from terephthalic acid, P-hydroxybenzoic acid and P,P-biphenol) were examined in the longitudinal (L), transverse (T) and normal (N) directions of the polymer molecules. When rubbed against an abrasive paper, the wear rate of the unfilled LCP depended on the sliding direction. The maximum wear rates were observed in the T direction, the minimum wear rates in the N direction and medium wear rates in the L direction. The wear rates steeply increased above 100 °C. In the LCPs filled with graphite only, or graphite in combination with polytetrafluoroethylene (PTFE), the orientation was minimized but there was no minimizing effect on the wear rates. When rubbed against a chromium-plated disk at room temperature, the wear rates of the unfilled LCP were of the order of 10 −7 mm 3 N −1 m −1. The wear rates steeply increased above room temperature and reached a figure of the order of 10 −4 mm 3 N −1 m −1 at 100 °C. The filled LCPs in the N direction showed very low wear rates of the order of 10 −8 mm 3 N −1 m −1 at room temperature. However, the wear rates in the L and T directions were of the order of 10 −7 mm 3 N −1 m −1 at room temperature. The wear rates for the graphite-filled LCP steeply increased beyond room temperature and maximum wear rates of the order of 10 −5 mm 3 N −1 m −1 occurred at 120 °C in the temperature range examined. At 160 °C, the wear rates were much lower than those at 120 °C. The wear rates for the LCP filled with graphite in combination with PTFE also steeply increased above room temperature. However, the wear rates at temperatures from 60 to 160 °C were lower than those of the graphite-filled LCP. The friction coefficients for the filled LCPs were relatively high at room temperature but tended to decrease above 60 °C. The filled graphite and PTFE had obvious reducing effects on the wear rates at elevated temperatures.