Analysis of the application of ZrN coatings for the mitigation of the development of fretting wear processes at the surfaces of push fit joint elements

The purpose of this article is to assess the influence of a ZrN coating on the development of fretting wear in a model of a wheel/axle push fit joint in a rail vehicle. The examinations of the fretting wear process in rail vehicle wheel sets were conducted on two groups of models consisting of a shaft and sleeve connected by means of a push-fit joint, in compliance with the rules for the representation of the real condition. Wear tests of a joint loaded with a vertical force were conducted on a fatigue testing machine permitting oscillatory tangential displacement, which is responsible for the development of fretting wear. Uncoated shafts were the first model, and shafts coated with a ZrN coating were the second one. The tolerance was 0.02 mm, the model loading force was 550 N and the number of fatigue cycles was 7 × 10⁶. The destruction of models was evaluated in, for example, the computer image analysis process with the use of the Image Pro Plus 7.0 software. Macroscopic observations showed wear traces for both model groups. In the first group, wear traces were observed on either side of the axle seat, and those traces had the form of a 2–3 mm wide ring comprising the entire shaft circumference. The random location of the traces of wear was observed in the second model. The microscopic observations of the destruction areas demonstrated mainly the presence of material build-ups, the local surface abrasion and micropits. The analysis of the chemical composition of material build-ups showed the presence of iron and oxygen, a fact which confirms the shearing of the microirregularities of the sleeve top layer. Plastically deformed material build-ups become oxidised, hence the brown colour of the worn-out areas can be observed at the sample surface during macroscopic observations. The significant mitigation of fretting wear on samples with ZrN coatings has been demonstrated. This shows the need for fretting wear tests to be performed on a real object to confirm the scale of improvement of the life of forced-in joints, thus contributing to the higher reliability of rail vehicle wheel sets.