Analysis of Tribological Properties of Expanded Graphite—Alloy Steel Pair Under High Loads in Dry and Humid Conditions

Expanded graphite significantly improves the tribological properties of materials in friction pairs, but there is a lack of research in the literature on its cooperation with metals and the effect of water on friction and wear mechanisms. It is particularly important to understand the phenomenon of graphite layer formation on the steel surface and its effect on tribological properties. The aim of this study is to evaluate the tribological properties of an expanded graphite–alloy steel combination operating under selected loads in both dry and humid conditions. The tests were carried out on a block-on-ring tribological tester (where the blocks were made of expanded graphite and the rings were made of AISI 4130 steel) at a rotational speed of 150 rpm, with loads of 200 and 650 N. The frictional behavior was analyzed on the basis of the measured values of the friction torque and the coefficient of friction (COF) calculated from it (and the applied load). In dry conditions, the friction torque was stable, while in humid conditions it showed cyclical changes. An increase in load from 200 to 650 N caused an increase in the average friction torque by 235% in dry conditions and by 209% in humid conditions. The presence of water reduced the friction pair temperature by 12% at 200 N and by 18% at 650 N; however, it simultaneously increased graphite consumption–by 1179% at 200 N and by 100% at 650 N. The amount of graphite deposited on the steel surface depended on the load–in humid conditions, it increased by 114% at 200 N, while it decreased by 250% at 650 N. The conducted research expanded the understanding of the influence of operating conditions on the tribological properties of the expanded graphite–alloy steel pair. It also provided new data on the friction and wear mechanisms of this material combination in humid conditions, which may have significant engineering applications.