Figure 9. (a) Image that represents all the coating specimens fractured during the tests; (b) SEM image of the interface between coating (C), top of figure, and substrate (S) showing striations in both sides. Sample Nano.
Secondary cracks (Figure 10) on the side of the specimen which undergoes positive fatigue stress agree with this finding. Critical stresses for fatigue nucleation were more easily approached at the coatings than the substrate. This may be because the elastic modulus of the layer is larger than the substrate, making it less prone to elastic straining the layer without crack formation. Therefore, several nucleation sites emerged early on the layer’s surface, before that the fatigue cycles were enough to nucleate the fatigue in the substrate. The reduction of the cycles for fatigue nucleation in the coatings leads, finally, to their lower fatigue performance as compared to the substrate. When the crack hits the substrate, however, it progresses in only one plane. The reduction of the cycles for fatigue nucleation in the coatings leads, finally, to their lower fatigue performance as compared to the substrate. It also shows that the reduction in fatigue resistance is associated with easier nucleation of fatigue in the layers than in the substrate.