Page 431 - Handbook of Materials Failure Analysis

6 Fatigue Strength of PVD Coating 429

20
TiN (Kim et al. [58])
18 TiAIN (Kim et al. [58])
TiCN (Kim et al. [58])
16
Increase of fatigue endurance (%) 12 8
CrN (Baragetti et al. [100])
14

10

4 6

2
0
0 500 1000 1500 2000 2500 3000 3500
Coating hardness (HV)
FIGURE 16.11
Correlation between coating hardness and improvement of fatigue endurance based on Refs.
[58,100].

With increasing coating hardness, the fatigue life of coated specimens increased as
well (Figure 16.11)[58,87,100,102]. However, according to Kim et al. [58], with
increasing coating hardness up to 2800 HV the fatigue strength increased, but after
exceeding this hardness a decrease in fatigue endurance was noticed. This shows that
there exists a certain coating hardness, above which fatigue strength decreases. This
hardness limit is connected with the fact that very hard coatings are often also very
stiff and can fracture in a brittle mode, especially if the substrate is much softer than
the coating. Experimental observations confirmed that if very hard coating is depos-
ited on soft substrate, coating cannot sustain the substrate elastic strain and cracks in
multiple points rapidly arise and grow causing a decrease of fatigue limit
[100,105,112]. Thus, the substrate hardness, the mismatch of coating/substrate hard-
ness are more important for fatigue life of coated specimens than the coating hard-
ness alone [99,100,104,105,110]. In the experiments, in which aluminum alloys
[100,112] or Ti-6Al-4V titanium alloy [99,105,106] were used as substrates, depo-
sition of hard PVD coatings always caused a drop of fatigue endurance.
Hotta et al. [89] attributed the improvement of number cycles to failure of coated
steels in rotating bending tests to an increase of the substrate hardness together with
the simultaneous reduction of tensile residual stress in the substrate. The existence of
hard, crack-free, and well-bonded coatings deposited on the hard substrates is
expected to be effective in preventing the crack initiation and development. Thus,