The adhesion of alumina films to metallic alloys and coatings

The adherence of protective oxide scales to alloy substrates is governed by the stored elastic energy in the scale which drives delamination and the f racture resistance of the alloy oxide interface. Clearly any modifications to the alloy or the exposure environment which decreases the former or incr eases the latter will improve the durability of a given system. The stored elastic energy is determined by the stress level in the scale an d the scale thickness. The stress state in the scale is determined by stres ses which arise during the oxide formation (Growth Stresses). stresses prod uced during temperature changes as the re suit of thermal expansion mismatc h between the oxide and the alloy (Thermal Stresses), and any stress relaxa tion which occurs as the result of creep of the scale or alloy. The fracture energy of the interface is a function of the composition at th e interface, the microstructure in the interfacial region, and the composit ion of the exposure environment. This paper focuses on the results of studies of a variety of alloys and coa tings. all of which form continuous alumina scales, in which it has been at tempted to evaluate the effects of various alloy and exposure parameters on the stress state in the scale, the microstructure of the alloy/oxide inter face, and the fracture resistance of the interface. The alloy parameters in clude alloy type, sulfur content, and reactive element content. The exposur e parameters include oxidation temperature, temperature profile during expo sure, and water vapor and sulfur contents of the atmosphere.