Page 422 - Handbook of Materials Failure Analysis

420 CHAPTER 16 Degradation of protective PVD coatings

tended to exhibit small chipped or spalled regions, while on substrates whose surface
is almost free of microcracks, adhesive failure is evidenced as a complete detachment
of the coating at relatively high loads, usually initiated at through-thickness cracks
(cohesive failure) [62].
The initial deformation of the coating, at very low indentation depth, was found to
be elastic or elastic-plastic [10]. At higher indentation depths, when a substrate starts
to flow plastically, a PVD coating is likely to follow the deformed substrate. Based
on experimental observations, Bhowmick et al. [10] have proposed a model of PVD
coating deformation (Figure 16.4a). According to this model, deformation of
columnar-grained PVD coating occurs via shearing and sliding at intercolumnar
boundary under the action of a shear stress. This model was later confirmed by
Jayaram et al. [32] in testing thin TiN coatings (Figure 16.4b). When the plastic

A B

δ
Film Shear at
inter-
columnar
D boundary
C
Substrate
Plastic zone

(a)

δ 1 μm

(b)
FIGURE 16.4
Model of deformation of columnar-grained coating [10] (a) and cross-section of an
indented TiN columnar coating [32] (b), permission from Elsevier.