TABLE OF CONTENTS Superalloys used in high-temperature industrial processes are generally developed with optimized structural properties such as tensile, creep, and fatigue strength while maintaining microstructural stability over a wide temperature range. The attractive mechanical properties are achieved to some extent at the expense of environmental resistance. The environmental protection, therefore, has to be provided by compatible thin metallic coatings (Sivakumar and Mordike, 1989), which are not designed to carry load, but have constituents and microstruc-tures to provide good oxidation and corrosion protection.
At the heart of the successful environmental performance of metallic coatings lies the formation of thin oxide scales on the coating surface, that limit access of oxygen and corroding salts. For these scales to provide extended protection, a number of requirements have to be satisfied (Nicholls, 2000; Nicholls and Hancock, 1987). Many of the solutions to the requirements described below play multiple roles in protecting the coated article.
Oxidation/Corrosion Resistance
Thermodynamically stable, protective surface scale of uniform thickness
Slow growth rate of protective surface scale
Adherent surface scale
High concentration of scale former
Stability
No undesired phase changes within the coating
Low diffusion rate across interface at use temperature
Adequate compositional stability across interface
Minimized brittle phase formation
Adhesion
Good adherence of coating to substrate
Matched coating/substrate properties to reduce thermal stress
Minimized growth stresses (process parameters related)
Optimized surface condition (rough or smooth)
Structural Properties
Can withstand service-related creep, fatigue, and impact loading of surface without failure of function
