Page 432 - Handbook of Materials Failure Analysis

430 CHAPTER 16 Degradation of protective PVD coatings

if the mismatch of coating/substrate hardness and modulus increases, the positive
influence of PVD coating deposition decreases leading to a drop of fatigue life below
that of uncoated steel.
The improvement of fatigue strength was explained by the process of crack ini-
tiation and its development. According to Hotta et al. [89], a hard PVD coating
causes suppression of the crack initiation in the substrates by a decrease of the defor-
mation of the substrate surface. Similarly, Kim et al. [58] have observed that under
testing at lower stress level, crack initiation is retarded by a hard coating layer on the
substrate surface, which acts as a barrier to slip deformation of the plain surface.
Depending on the stress amplitude, different processes in fatigue fracture occurs.
When low amplitude stress is applied, fatigue fracture process occurred as a conse-
quence of the propagation of a single crack [87,103,106,113]. This crack might be
initiated in a substrate [87,100,103,113] or at a coating/surface interface [106]. The
nucleation of the fatigue crack is initiated in a substrate if it is connected with non-
metallic inclusions located inside the steel substrate [100,113]. Fracture nucleates at
the inclusion due to the stress concentration induced by the metallurgical disconti-
nuity, which propagates both toward the surface of the specimen and the inner por-
tion of the steel, causing the typical fish eye morphology on the fracture surface.
However, if substrate is made from steel characterized by high level of cleanness,
than cracks nucleate at the coating [100]. Thus, to improve the fatigue limit by
the deposition of PVD coating, a good adhesion is mandatory. A hard well-bonded
PVD coating is effective in preventing the crack initiation in the substrates.
In the case of the high amplitude of the stresses, fatigue fracture develops by a co-
propagation of multiple cracks initiated at the coating surface, especially at the
defects that are spots of the stress concentration. PVD coatings are ceramic materials
that possess high strength and low toughness. As a result, PVD coatings are too brittle
and stiff to accommodate large elastic strain imposed by substrate deflections under
load, so that multi-cracks propagate quickly inside the coating and merge on a frac-
ture plane of specimen leading to a decrease of fatigue life of the substrate
[58,88,100,101]. The other place of crack initiation is the coating surface at local
micro-defects, such as micro-pits and voids [91,92,94–96,104,105,112]. Each
micro-defect acts as a center of stress concentration that favors crack initiation.
Cassar et al. [105] pointed that the main reason of the decrease of fatigue strength
is delamination caused by the substantially different elastic moduli of the substrate
and coating. Thus, the main reason of lowering the fatigue strength was the mismatch
of coating/substrate deformation causing coating delamination and creation of voids
at coating-substrate interface, and micro-defects of the coating.

7 DEGRADATION CAUSED BY DYNAMIC LOADING (EROSION)
Erosion of materials (cavitation erosion, liquid impact erosion, and solid particle ero-
sion) is caused by repeated action of dynamic loads on small area of exposed mate-
rial. This loading may last only few microseconds or even nanoseconds with a high