Page 433 - Handbook of Materials Failure Analysis

7 Degradation Caused by Dynamic Loading (Erosion) 431

local-impact pressure of GPa. Material resistance to deformation and fracture under
dynamic loading is lower in general than that under quasi-static ones. It has been
found that bulk materials, that have good fatigue strength, often also have good ero-
sion resistance. This relationship was found to be also true in the case of PVD coat-
ings [33,44–46,50,56,75,114–118].
According to Gerdes et al. [114], TiN coating deposited on Ti-6Al-4V improved
an incubation period approximately three times and a cumulative volume loss
decreased 30 times in water droplet erosion. TiN PVD coatings, regardless of
the deposition techniques (cathodic arc, electron beam, unbalanced magnetron sput-
tering), were the most resistant against solid particle erosion [56]. In Mann et al.
[119] was shown that TiAlN PVD coating was very effective in preventing degra-
dation against slurry erosion. However, investigations of cavitation erosion resis-
tance showed that in case of very intensive erosion deposition of PVD coating
caused mainly an increase of incubation period [120]. It should be added that all coat-
ing defects such as micro-droplets and pinholes act as nucleation centers for wear
leading to reduced incubation time [45,121]. Many micro-droplets are removed dur-
ing erosion process leaving small pinholes on the coating surface. All surface defects
are the spots of stress concentration that favor spots of cracks initiation. However, a
large number of tiny micro-pinholes does not affect a crack initiation and an increase
of coating degradation [122].
Similarly to bulk materials, erosion resistance of PVD coatings depends on many
factors, such as mechanical properties of coating and substrate (hardness, Young’s
modulus, tensile strength, and fatigue strength), thermal properties of coating and
substrate, microstructure (grain sizes, phases, number and size of defects) and a num-
ber of layers in PVD coating, adhesion of a coating to substrate, coating thickness
and also on surface roughness. A huge number of factors influencing the resistance
of PVD coating to erosion make it impossible to determine the impact of individual
factors on the overall behavior of the coating and substrate under the dynamic load-
ing in micro-regions. However, some attempts were made in the literature.
Jianxin et al. [50] have shown that the resistance to solid particle erosion
increases with increasing coating hardness and adhesion (critical load). On the other
hand, investigations shown in Iwai et al. [117], Krella and Czyz ˙niewski [46], and
Krella [44] cannot confirm such relation. As in the case of fatigue, there is a coating
hardness, above which erosion resistance decreases [44]. In Krella [44] was per-
formed an attempt to determine an effect of the properties of a coating and a substrate
on the resistance of the entire coating-substrate system against cavitation erosion, but
only arc-evaporated coatings were investigated.
As it was mentioned earlier, coating ability to resist erosion degradation depends
on the substrate hardness. In case of quite soft substrate, for example, austenitic steel,
degradation of PVD coatings under dynamic load in cavitation erosion begins from
the coating-substrate undulation. This undulation was caused by plastic deformation
of the substrate and shear or/and sliding of columnar grains of coating. Undulation of
substrate and cyclic strain of coating caused by short lasting loading and elastic
behavior of a coating (especially a monolayer coating) may lead to the coating