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.