Figure 8. SN curves of the Nano, Micro, NM and Substrate. The open dots represent specimens that did not fracture during the test.
Some studies which tested thicker coatings denoted better fatigue performance of the coatings compared to the substrate [7 - 9, 12, 23]. However, our study aimed to comparatively evaluate the selected coatings. Therefore, the study allowed to observe that the coatings performed equally in fatigue, regardless of the powder’s granulometry. Fatigue performance can be enhanced by some means which provide substantial compressive residual stresses of the coated material, that hinder crack initiation [1]. The residual stresses induced on the surface by the HVOF deposition process are usually positive, but in some cases the stresses can be compressive, especially in metal-ceramic composites, as is our case.
Figure 9 gives representative images of all the coating specimens fractured during the tests. Striations evidence that the fatigue nucleates at the coating surface.