Partial-gross slip fretting transition of martensitic stainless steels

Based on critical reviews of the well-known dependence of fretting process upon the sliding amplitude, experiments were performed to verify this dependence. One of the main critical points is that the experiments which led to this result were performed controlling the imposed displacement amplitude instead of the real sliding amplitude. Therefore, the difference between the real displacement amplitude and the imposed amplitude due to compliances of the test rigs components was not considered. Fretting tests were performed using a high precision test rig. One of the main peculiarities of this test rig is that there is no difference between the imposed sliding amplitude and the real amplitude. The fretting process parameters of experiments were room temperature, two normal load (contact pressure 15, 25 MPa), four strokes (10, 15, 20, 50 μm), two martensitic stainless steels (X20Cr13, M152) and different durations from 15 to 160 M-cycles. Friction coefficient was computed using the hysteresis loops measured during the wear test. The worn surfaces were measured using an optical instrument based on focus variation. Wear volumes were accurately computed with a procedure that takes in to account the roughness of the surfaces. Results show that the friction coefficient is independent of slip amplitude, normal load and steel type if the hysteresis loops shape is parallelogrammatic and the contact surfaces are effectively conformal. When these conditions are not observed, the friction coefficient is dependent on normal load, even in contrast with its increase. Wear volume shows linear evolution in gross slip regime while it is non-linear in partial slip. Wear rate is independent of slip amplitude and normal load in partial slip regime at low ratio amplitude contact length. In contrast, wear rate depends on slip amplitude and exhibits a sharp increase near the transition partial-gross slip.