On fretting of XC38 steel: Surface treatments effect

In this work, the authors report on the fretting wear behaviour of polished and treated XC38 specimens. For fretting experiments, due to microdisplacements at the interface between two contacting surfaces, two types of damage can be observed: crack initiation and debris formation. Crack damage has been found in XC38 when it was brought into contact with 100Cr6 under fretting. This is the case in a train's axis–axle assembly when subjected to vibration. To enhance the durability of this system under fretting, three treatments of the XC38 surface were investigated: shot peening, Dalic coating and Mo coating. Shot peening, which is already well known for improving fatigue resistance of steels, is shown to have a beneficial effect on the crack initiation and propagation under fretting wear loading, as cracks observed on specimens after cylinder on flat fretting tests are shorter in shot peened specimens than in polished ones. It is also demonstrated that the crack nucleation threshold was pushed back towards the highest loadings. These results were explained by relaxation phenomenon of residual compressive stress during fretting cycles. For the hard coatings on the XC38 (Dalic and Mo), crack nucleation resistance was improved compared to the polished steel. However, the crack propagation rate has shown no stable evolution as a function of fretting loading conditions (normal force, displacement amplitude). This instability is linked to the coating properties such as homogeneities, thickness, etc.