Page 434 - Handbook of Materials Failure Analysis

432 CHAPTER 16 Degradation of protective PVD coatings

delamination. The coating delaminated under cyclic dynamic loading easily
undergoes shear fracture through the entire coating thickness that ends with coating
removal. Another effect of coating undulation is an initiation and increase of tensile
stresses in a coating at the bottom and top of undulation—“hills.” These stresses lead
to crack initiation and its progress. However, delamination contributes to the stress
release at the bottom of hills and undulations. Delaminated coating fractures easily
under impact. In Figure 16.12 is shown fracture of TiN coating caused by cavitation.
This indicates that coating fractures in delaminated parts are in I and II mode
(by tensile and by shearing), and at the tops of “hills,” they are mostly in I mode
(by tensile).
In the case of hard substrate, the undulation is slight or sometimes does not occur
[54]. However, cyclic loading causes cyclic strain of the coating and substrate and
generates complex stress state in the coating. After a number of cycles, stress state in
weak points of coating (places with some defects) may exceed the coating strength
and may lead to coating fracture. Thus, cracks are caused mostly by shearing. This
shows that the erosion behavior of the coating is strongly influenced by the substrates
hardness. With increasing substrate hardness, fracture mechanism is changing from
I mode to II mode of cracking.
Investigations of an improvement of erosion resistance by applying multi-layer
coating instead of monolayer do not give unambiguous results. According to
Bromark et al. [75] and Gachon et al. [123], erosion or wear rate decreased if
multi-layer coatings were used instead of monolayer ones, while according to Iwai
et al. [117] applying of multi-layer coatings may give an opposite result. The
decrease of the solid particle erosion resistance of multi-layer coatings was explained
by a very small scale of the wear fragments and a very low thickness of soft layers
that played the role of the defects that facilitate a removal of coating fragments.

Fracture at delaminated
part of the coating

Fracture at the top
of undulation

Magn 20 μm
1000x 22TiN
FIGURE 16.12
Fracture of a TiN coating deposited on austenitic steel caused by cavitation.
Krella and Czyzniewski [54], permission from Elsevier.