Page 426 - Handbook of Materials Failure Analysis

424 CHAPTER 16 Degradation of protective PVD coatings

45
Cheng et al. [67]
40 Duck et al. [68]
Krella [77]
35
Zhao et al. [74]
30
Hardness (GPa) 25

20

15

10
5

0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Modulation period (nm)
FIGURE 16.7
Relation between hardness and modulation period.

Zhao et al. [72] has obtained that in a multi-layer coating are generated lower
maximum radial stress, shear stress and equivalent stress than in a monolayer coating
under the same load. This shows that plasticity of multi-layer coating is higher than
that of the monolayer. On the other hand, in tribological test (ball-on-disk tests),
Ti/TiN multi-layer coatings had worse wear resistance than a TiN monolayer coating
[67]. According to Cheng et al. [67], with increasing thickness of Ti layer, plastic
deformation of this metallic layer was easier and increases the mismatch of plastic
deformation between Ti and TiN layers. Thus, if Ti layer is quite thick, a hard and
brittle top TiN layer is removed at lower stress than in case when Ti layer is much
thinner than TiN layer.

5 DEFORMATION OF MULTI-LAYER COATINGS

As it was mentioned earlier, in deformation process of multi-layer coatings, soft
layers play a significant role. They act as buffer layers that favor energy dissipation
by shear deformation, the drop of intrinsic stress and the increase of strain
[61,65,69,82]. Investigations of TiAlN/CrN multi-layer coatings showed that with
increasing periodic thickness from 7 to 18 nm, increases resistance to fracture and
ability to deformation with a substrate [64].