Effects of thin layer on friction and wear of cast iron under severe sliding conditions

Abstract

The wear tests using a pin-on-disk type test rig were carried out in order to clarify the dry friction and wear characteristics of cast iron under severe sliding conditions (high sliding speed and high contact pressure), and the frictional force, the wear rate and the temperature rise of cast iron pin were measured. Moreover, the crystalline structure of cast iron pin after sliding test was observed by microscope. Then, the relations among them were investigated. For a while from starting when the temperature of sliding surface of cast iron pin is not so high, the wear of cast iron pin was small (the mild wear), but its wear rate remarkably increases when the temperature rise becomes high enough to soften the cast iron pin near the sliding surface (the severe wear or so-called thermal wear). In the condition of thermal wear, the coefficient of friction is independent of the contact pressure but decreases with the increment of sliding speed. On the other hand, the wear rate of cast iron pin is little influenced with the sliding speed but proportionally increases with the increment of contact pressure. From the microscopic observations of the crystalline structure of cast iron pin after sliding test, it was seen that the crystallines near the sliding surface became soft because of the severe temperature rise and flew to the sliding direction. And the thickness of the layer in which the crystallines of cast iron pin flew increased with the increment of sliding speed but was not so influenced with the contact pressure. The way how the thickness of this thin layer changes with the sliding speed and the contact pressure is deeply related with the friction and wear characteristics.