Wear characteristics of an Al-Si alloy impregnated graphite composite containing 56 vol% graphite and its matrices in contact with bearing steel were investigated under insufficient lubrication. Pin-on-disk type wear tests in air were conducted at various levels of relative humidity (RH) under dry sliding and drip feed lubrication (0.005 to 0.02 cm3) before each test by using base oil for engines. Changes in friction and wear were continuously monitored during the tests. Four types of wear mode for the composite were obtained: steady wear with a low rate (Type 1), high wear after a lubricating oil shortage (Type 2), no wear after pin lifting (Type 3) and steady wear with a high rate (Type 4). Type 1 appears over the whole range of RH (5 to 80%). Type 2 is found at low RH levels of 5 and 10%. Type 3 occurs at a frequency of 30 percents between 10 and 35% RH. Type 4 is seen over the whole RH range under dry sliding. The appearance of each wear mode at a specified RH depends on wear behavior of the matrices associated with RH under drip feed lubrication.
