It was found that the surface film acts as a protective layer that ‘seals’ crack initiations due to the applied alternating stress. Once the surface
layer is damaged, the ‘seal’ loses its function. In other words, depending on the cyclic load level, the coating’s inability to
deform plastically (i.e. resistance to brittle cracks) can affect the whole structure’s resistance to fatigue crack initiation. Such
thin coatings, however, were found not to largely influence the bulk property of the coating–substrate system because of
their extremely small area compared to the dimensions of the substrate [13