Fig.6 Morphology of fretting region; (a) P=50 MPa, (b) P=150 MPa, (c) P=250 MPa, DP, σe =400MPa;(d) P=150MPa, SP, σe= 400MPa.
Under DP, when the contact stress is 50 MPa, which is a relatively low value, the amount of deformation caused by the fretting of the sample and the fretting pad is limited, the relative displacement is large, and the fretting process is nearly in the slip regime, as shown in Fig.6 (a), although there are a large number of scratch marks and micro abrasive debris, the scratch marks are shallow, and the deformation and damage in the fretting region are not very severe; With the increase of contact stress, the adhesion between the specimen and the fretting pad is too close to reduce the relative displacement. Compared with low contact stress, the fretting region has deeper scratch marks, and the surface of the fretting region is severely worn. When the contact stress is 150 MPa, as shown in Fig.6 (b), a partial stick area has appeared in the center of the fretting region and there is an obvious furrow phenomenon in the slip area; When the contact stress is further increased to 250 MPa, the fretting region is then basically in an stick state, as shown in Fig.6 (c), there are no furrows due to sliding. Due to lower contact stress, the relative displacement in Fig.6 (b) is larger compared to Fig.6 (c). During the fretting fatigue process, part of the material in the adhesion area tears due to the large relative displacement. As a result, the surface material of the fretting region is torn in Fig.6(b), and a large number of pits are generated. By compared with Fig.6 (c), Fig.6 (b) is more severely damaged due to the severe tearing of the surface material and the coupling effect with the fretting wear.
Under SP, as the contact stress increases, the surface wear of the fretting zone also becomes more serious. Compared with the morphology of the fretting zone of the DP, due to the larger amount of strain under the same conditions under SP, there is a relatively large amount of relative displacement. Therefore, under the same contact stress, the adhesion of SP is lower, and surface tearing is weaker than that of the DP. When the contact stress is 150 MPa, stick and slip regions can also be observed on the surface of the fretting zone. There are more abrasive debris in the slip region and a small amount of surface material tear in the stick region.