Environmental Engineering Reference
In-Depth Information
Figure 5.28
Cracking and spallation caused by compressive oxide stresses. Route I
(a) shear cracks develop; (b) wedge crack grows between shear cracks; (c) local athermal
stress relaxation; (d) thermal stress relaxation. Route II: (e, f) localized decohesion can lead
to buckling; (f) buckles may spread laterally and coalesce; (g, h) tensile cracks develop in
regions of tensile stress and lead to spallation [Ref. 51].
(route I), initial failure of the oxide occurs by compressive shear cracking. Subse-
quent cooling of the sample results in differential contraction strains that drive
wedges of the adjacent oxide layer under the segment bounded by the shear cracks
and thus produce gradual decohesion at the interface. Alternatively, if interfacial
adhesion is poor, e.g., due to coalescence of voids or segregation of elements
such as sulfur, then compressive straining will initiate wrinkling and buckling
of the oxide (route II) over these areas of weaker adhesion. It may also happen
for individual wrinkles or buckles to spread laterally by propagation of crack
along the interface. Spallation results when through-thickness cracks form prefer-
entially in regions of high tensile stress in the oxide. The above-mentioned two
different routes to spallation are denoted as ''wedging'' and ''buckling,'' respec-
tively [51]. The two modes of failure have been well demonstrated in the oxida-
tion of 20Cr-25Ni austenitic steel where chromia is the protective oxide layer.
The buckling configuration arises on the steel containing no silicon and the wedg-
ing configuration (characterized by an inclined oxide failure plane) on a steel
containing 0.6% Si. Presence of this small amount of silicon in the second steel
