Effect of temperature on the tribological and dynamic mechanical properties of liquid crystalline polymer

The effects of temperature on the tribological and dynamic mechanical properties (DMP) of aromatic liquid-crystal poly(ester-co-amide) were investigated. Specimens for the DMA test and friction and wear test were prepared by compression molding at elevated temperature. The dynamic mechanical properties of the resulting compression-molded liquid crystal polymer (LCP) specimens were evaluated on a Perkin-Elmer DMA instrument, in a three-point bending mode, at a heating rate of 10 degrees/min and frequency of 1 Hz from room temperature to 250 degrees C. The friction and wear behavior of the compression-molded LCP specimens sliding against SA52100 steel in a ball-on-disk configuration was evaluated on a high-temperature friction and wear tester from 25 to 300 degrees C. The worn surface morphologies of the compression-molded LCP specimens were observed on a scanning electron microscope. It was found that the tribological and dynamic mechanical properties of the LCP strongly depended on the test temperature. Namely, the friction coefficients decreased significantly and the wear rate increased considerably with increasing temperature. This was attributed to the enhanced softening and plastic deformation of the LCP at elevated temperature. The LCP experienced severe plastic deformation, flowing and local melting owing to the destruction of the stiff backbones of the polymeric molecules at excessively high temperature, thus the largest wear rate of the tested LCP was generated at 300 T. Such a trend in the friction and wear behavior of the compression-molded LCP with temperature conformed well to the corresponding worn surface morphologies. Namely, the LCP worn surface was characterized by mild plastic deformation and micro-cracking as the LCP disk slid against the steel ball at 50 degrees C, whilst at 100 degrees C, the plastic deformation and adhesion became severe and the micro-cracking disappeared owing to the plastic flow. The plastic deformation and adhesion became increasingly more severe with further increase of the test temperature up to 150, 200, and 250 degrees C, owing to the increased plastic flow and local softening and melting of the LCP. Moreover, the storage modulus of the compression-molded LCP greatly decreased with increasing temperature, as did the loss modulus, while the complex modulus and tan delta increased with increasing temperature, which agreed well with the corresponding trend of the tribological behavior with temperature. It was confirmed that the dynamic mechanical properties of the polymer were an important factor of the high temperature tribological. behavior. (c) 2004 Elsevier Ltd. All rights reserved.