Effect of humidity on the tribological behavior of carbon-carbon composites

The present work is a study of the effect of humidity on the tribological behavior of carbon-carbon composites, including two-dimensional PAN/pitch (designated TM), PAN/CVI (designated E), and pitch/resin/CVI (designated A) formulae. Results indicate that there exist close relationships among friction coefficient, wear rate, and worn surface morphology. Whenever a morphological transition has occurred, transitions in friction and wear also occur. Three different debris morphologies are identified. The smooth type I and type III debris films can lubricate, but not the powdery type II debris. The relative humidity level has a strong effect on the tribological behavior of all three composites. Low humidity and high sliding speed generally accelerate the occurrence of type I-to-type II transitions in friction, wear, and debris morphology, whereas high humidity and low speed enhance the formation of type III debris. At any humidity level, the type I morphology is always accompanied by low friction and wear. After the type I-to-type II transition, higher friction coefficient and wear rate are observed in the low humidity than in the high humidity. Under the present conditions, composite E appears to be the most sensitive to the humidity effect, while composite A appears the least sensitive.