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.