On the Crack Propagation in Hard Coatings: Implications in Fatigue Resistance

The fatigue life of a sprayed component can be reduced if a crack is initiated in the coating, propagates towards the interface, and advances into the substrate. This is predominantly the case of hard coatings on soft substrates. The intersplat interfaces in the coating can act as crack initiation sites, and as the coatings are inherently built particle by particle, there are many possible points where cracks can start. Studies in fatigue resistance of sprayed components have revealed that in many cases the cracks grow from the coating into the substrate. However, this is not always the rule. In the present study, the crack propagation is studied by means of wedge indentation (static approach), and fractography analysis in samples after fatigue tests. Samples were prepared with WC-Co on steel substrates in different residual stress states as well as with graded structures (top layer 100% WC-Co, graded to 30%WC-Co and 70%NiAl). Hard-chrome plated coatings are also tested for comparison with the sprayed coatings. The results have shown that compressive residual stress in the coating benefits the fatigue life as it delays the crack propagation. In the case of graded structures, the purely metallic regions stop or blunt the cracks. The possible effects of residual stresses, thicknesses, and adhesion/cohesion strength are discussed to limit crack growth from the coating into the substrate. Processing strategies are proposed to produce coatings that do not reduce the fatigue life of the component.