Page 429 - Handbook of Materials Failure Analysis

6 Fatigue Strength of PVD Coating 427

6 FATIGUE STRENGTH OF PVD COATING
Due to high importance of the fatigue damage of many elements in operation, the
study of resistance and degradation mechanism of thin, hard-coated steels has been
carried out since the 1980s of last century. Performed studies do not show unambig-
uous effect of PVD coatings deposition on fatigue resistance, however, some corre-
lations have been found.
Fatigue investigations of systems of hard PVD coating-substrate have shown that
kind of fatigue tests and parameters, at which these fatigue tests are performed, coat-
ing properties (e.g., thickness, hardness, elastic modulus, and grain size), and also
substrate properties have an influence on the obtained fatigue results.
Early studies of an effect of deposition of PVD coatings on fatigue resistance
showed an increase of fatigue strength under the tests of R¼ l, as compared with
that of uncoated specimen [12,87–98] and an decrease of fatigue strength under the
tests of R¼0 or 0.1 [88,93,99], except for the region of low stress amplitude
[88,93,100–102]. However, some exceptions from this rule have been observed.
The positive effect of PVD of coatings on the fatigue life under the tests of
R¼0.l or R¼0 was obtained in Baragetti et al. [100], Kim et al. [58], Lecis et al.
[103], Sundaram [104], and Zhou et al. [102], while a decrease of fatigue strength
of TiN and TiN/Ti multi-layer coatings under the tests of R¼ l was obtained in
Cassar et al. [105] and Kolkman [106]. It should be noted that in Kolkman [106],
the thickness of tested coating was over 40 μm, while in Baragetti et al. [100],
Ferreira et al. [88], Hotta et al. [89], Kim et al. [58], Shiozawa et al. [93], Su
et al. [94–96], Trapezon and Lyashenko [97], Xin et al. [98], and Zhou et al.
[102], where the positive effect was obtained, coating thickness was in range of
1-15 μm. This confirms that in addition to the test parameters, the thickness of coat-
ing has also a great influence on fatigue resistance.
However, investigations of an effect of thickness of PVD coating on fatigue
strength do not give unambiguous result. According to Murakami et al. [107] and
Kim et al. [58], the fatigue life increased with increasing film thickness in the range
of 2-6 μm. On the other hand, investigations performed by Su et al. [94] and
Sundaram [104] showed the decrease of fatigue life of TiN-coated steel with increas-
ing film thickness in the range of 3-7 μm. Investigations shown in Su et al. [94] and
Sundaram [104] are in complete opposition to that shown in Murakami et al. [107].
Trapezon and Lyashenko’s investigations [97] showed that the influence of coating
thickness on fatigue resistance is dependent on deposition parameters and structure
of coating (the grain size and number of discontinuities of the deposited coating). An
increase of thickness of TiN coatings in the range of 1-15 μm deposited at a temper-
ature T¼550 °C and pressure P¼4.0 10 1 Pa caused an increase of fatigue resis-
tance, but in case of coatings deposited at a lower temperature (500 and 450 °C) an
increase of coating thickness had an opposite effect, despite the deposition pressure
1 1
(3.5 10 Pa and 5.0 10 Pa, respectively) [97]. The relations between a per-
centage increase of fatigue endurance and coating thickness based on Trapezon
and Lyashenko’s investigations is shown at Figure 16.10.