Environmental Engineering Reference
In-Depth Information
inertness of the coating is put on the priority list along with improved mechanical
properties of the alloy substrate, one may also consider the use of nonoxide ce-
ramic coatings, keeping in mind that components of the coating material will
diffuse into the substrate. Due to reactivity of such coating constituents with the
substrate metallic elements during long-term application at high temperatures, a
sequence of intermetallic phases may be formed. Such a situation is illustrated
in Fig. 6.44b. Embrittlement of the substrate alloy by solid-state reactions with the
coating material is often a possibility during a long exposure period and should be
avoided since the composition and microstructure of the substrate alloy is always
optimized to achieve the desired mechanical properties. The unique technical
solution to the problem is the combination of a high-strength alloy with a highly
alloyed coating of the preferentially oxidizable alloy constituents having the capa-
bility to form self-healing layers as illustrated in Fig 6.44c. Since it is recognized
that the addition of elements like Cr, Al, and Si in sufficient quantity to the
substrate alloys seriously affects the mechanical properties of the alloys, these
elements are often used in limited quantity in the alloy manufacturing process,
which provides limited resistance to the substrate alloys as shown in Fig. 6.44d.
On the other hand, if mechanical properties of the alloy become the prime con-
cern, and to eliminate the chance of embrittlement caused by aluminum and sili-
con, alloys having the best mechanical properties could be produced without the
addition of these elements, thus allowing totally uncontrolled degradation of the
alloys under service conditions as illustrated in Fig. 6.44e.
Therefore, the main goal of researchers and developers during recent decades
has been to achieve a coating-alloy combination of maximum or full chemical
resistance on the exposed surfaces of the metallic material with unaltered mechan-
ical properties of the substrate during the expected service period of the compo-
nent.
6.8.1 Requirements of Coating-Substrate System
In oxidizing environments at high temperatures, a coating in general owes its
oxidation resistance to the formation of a protective oxide layer. Therefore, in
selecting coating materials, it is important that the coating-substrate system meet
the following requirements [2,67,68]:
1.
The coating should be chemically and thermally stable (forming an integral
coating-metal/alloy system) during service life of the component.
2.
It should have properties compatible with those of the metallic substrate.
3.
The rate of interdiffusion of the elements in the integral system (i.e., between
coating and substrate alloy) must be slow during the desired service life.
4.
The protective layer and the metallic substrate should have matching thermal
expansion coefficients to avoid cracking and exfoliation of the coating during
thermal cycling.
