Environmental Engineering Reference
In-Depth Information
5.
A protective coating should exhibit some mechanical ''elasticity'' under op-
erating conditions to accommodate creep and plastic deformation.
6.
A coating material should resist damages from impact, erosion, and abrasion
depending on the specific applications of the metallic components,
7.
It should exhibit a spontaneous ''self-healing'' property for self-repair in
case failure occurs due to cracking or spallation of the layer. So the coating
should act as a reservoir for the highly oxidizable metallic constituent/con-
stituents for early development of a protective scale,
8.
It should be relatively easy to apply the coating on substrates, and the defects
that may occur during handling of the component should be reparable without
accompanying adverse effects on the sound neighboring areas.
Consequently, the development of a truly satisfactory coating that meets all
of the above requirements is a difficult task. Accordingly, compromises are often
made, depending on the specific application of the coated material in a particular
environment. Moreover, because of coating-environment and coating-substrate
reactions, the structures of the actual protective coating systems are complex.
Multilayered coating systems are often found to be the most successful; in prac-
tice, even single-layer coatings often become multilayered during service due to
coating-substrate and coating-environment reactions (e.g., silicide coatings on
refractory metals such as Mo, W, etc.). So, in the selection of protective systems
for components of high-temperature utility, three main factors deserve consider-
ation: the service or application conditions of the component, the structural alloy,
and the system of protection itself. The design of the component sets the service
stresses, temperature, thermal cyclings, and so forth. The alloy properties are
governed by composition, microstructure, and the processing steps, which control
the high-temperature stability. Finally, the selection of a suitable protective sys-
tem is decided by its resistance to environmental effects. The possible interactions
of these three constituents as illustrated in Fig. 6.45 serve as a guide in the design,
formulation, and evolution of the best possible coating-alloy system that guaran-
tees the expected service life of the component [68].
Stability and Compatibility of Oxides
Since the high-temperature oxidation resistance of metallic materials is contin-
gent on the development and maintenance of a protective oxide layer, it is perti-
nent to have a first-hand knowledge of the stability, diffusion characteristics,
compatibility with the substrate, evaporation, and so forth, of such oxides to serve
as a basis for the assessment of potential coating materials. The protective layer
should consist of an oxide or mixture of oxides with the maximal stability. On
the basis of a thermodynamic property like free energy of formation, the oxides
of primary interest include BeO, MgO, CaO, Al
2
O
3
,Y
2
O
3
,La
2
O
3
, SiO
2
, TiO
2
,
