Environmental Engineering Reference
In-Depth Information
3.
Ceramic coatings
4.
Noble metals, which under operational conditions do not form compounds
with the aggressive constituents of the environment.
During service of the coating-alloy system at high temperatures, interdiffusion
occurs between the alloy and the coating constituents. This leads to significant
changes in the composition and microstructure of both alloy and the coating mate-
rial in the vicinity of the coating-alloy interface. Such interdiffusion in many
instances leads to an increase in adherence of the coating to the substrate and to
an improvement in its mechanical properties. Accordingly, coated alloys are often
subjected to a special thermal treatment known as ''diffusion annealing.'' During
this process, a partial intermixing of the alloy constituents and the coating mate-
rial takes place, owing to which a gradual compositional change occurs. This is
particularly important in the cases of alloys used under thermal cycling conditions
because it accommodates differences in thermal expansion of the coating and the
substrate, thus ensuring improved stability. On the other hand, a rapid diffusion
process may lead to reduction or complete exhaustion of the element providing
protection during the short service period of the alloy. At the same time, mechani-
cal properties of an alloy may also get adversely altered due to a change in its
composition. A particularly disadvantageous situation may arise due to formation
of chemical compounds having a negative effect on the mechanical properties
of the coating-alloy system.
A large number of methods are available for developing coatings on alloy
substrates. These include electroplating, hot dipping in molten metals or fused
salts, spraying (oxyfuel and plasma techniques), slurry spraying, cladding, enam-
eling, vapor deposition or chemical transport reactions (pack cementation, fluid-
ized-bed technique, pyrolitic deposition), vacuum evaporation, etc. Depending
on the technique adopted for the production of protective coatings with enriched
content of the stable oxide-forming elements, they are broadly classified into
two groups: diffusion coatings and overlay coatings. Diffusion coatings are
formed through diffusional interactions between the coating material and the sub-
strate alloy. On the other hand, overlay coatings do not involve a direct alloying
reaction with the substrate, though often a diffusion step is included to improve
the bonding between the coating and the substrate. Some methods for producing
the two types of coatings are discussed below.
Diffusion Coatings
Pack Chromizing.
The pack chromizing process by pack cementation [69] as
developed during World War II is still often used to increase the service life of
stationary gas turbine blades and is considered as a representative of the diffusion
technique. Coatings so developed are considered to be among the most effective
