(b) Open void defects
The coated foreign particles are loosely bound to the coating
and, during deposition, a part of them can spontaneously flake
off due to high compressive stress in the coating above critical
thickness. Part of them flakes off during the cooling of the
vacuum chamber due to thermal stress. This leads to the
formation of through-voids (Fig. 2g). Actually voids can be
interpreted as ‘‘inverse’’ flakes. If the flake is detached during
deposition, the crater surface is covered by a thinner layer of
hard coating with rough and porous features (Fig. 2h).