Evaluation of fatigue resistance of a gradient CrNx coating applied to turbine blades

In order to protect metallic materials from solid particle erosion, the gradient CrNx ceramic coatings with thickness ranging from 32 to 40 μm were deposited on the 10%Cr heat-resistant steel using a hybrid deposition system. The fatigue tests were conducted under three-point bending at a stress ratio of 0.1. The S-N curves of the substrate and coated CrNx specimens were obtained. SEM observations on the fracture surfaces of the fatigue specimens were carried out. The results have indicated that the gradient CrNx coating increased the fatigue life of the substrate at low stresses and increased the fatigue limit by 9.1% due to the strengthening effect of coating. Whereas at high stresses, it decreased the fatigue life of the substrate and widened the scatter bands of fatigue life compared to the substrate because of the relatively high notch sensitivity (droplets on the coating). The fracture analysis for the coated specimens has shown that the fatigue cracks nucleated on the surface and propagated towards the substrate.