Page 423 - Handbook of Materials Failure Analysis

4 Properties of Multi-Layer Coatings 421

Columnar shear crack Inclined shear crack
Film thickness
1.6 3.5 4.9 5.9
0
AI
2
Substrate hardness 4 6
MS

SS
8
HSS
10
FIGURE 16.5
A plot showing the change in failure modes with film thickness and substrate hardness.
Jayaram et al. [32], permission from Elsevier.

deformation in the substrate rises to a level that the coating is not able to deform
elastic-plastically and absorb the deformation energy, then it breaks causing the
relaxation of strain energy. According to Figure 16.4a, columnar-grained coatings
should dominate columnar shear cracks. However, investigation performed by
Jayaram et al. [32] showed that besides columnar shear cracks occur inclined cracks
and the kind of cracks generated in columnar TiN coatings depends on the coating
thickness and substrate hardness (Figure 16.5). Columnar shear cracks arise mostly
in thin films deposited on hard substrates, while in thick coatings deposited on soft
substrates arise inclined shear cracks.
The substrate hardness has an influence on stress at which plastic flow of sub-
strate begins, while with increasing coating thickness increases the number of defects
in coating structure. In the case of a soft substrate, the deformation of a coating
develops due to sliding and spitting of the adjacent columns, and circumferential
cracks arise at the edge of a spherical indentation [1,47,61,63]. In the case of a hard
substrate, the deformation of a coating develops due to shear at intercolumnar bound-
ary and radial cracks are formed, but circumferential cracks hardly arise [10,61]. In
case of dense noncolumnar coatings, brittle circumferential cracks arise despite the
hardness of a substrate [61].

4 PROPERTIES OF MULTI-LAYER COATINGS
Multi-layer coatings consist mostly of several repetition of two layers—one harder
and one softer. They may be made of ceramic and metallic layers, or two different
ceramic layers. Properties of layer material influence properties of multi-layer coat-
ing. Schematic correlation between properties of layers and properties of multi-layer
coating is presented in Figure 16.6.