A mechanics-based approach to cyclic oxidation
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I.
INTRODUCTION
HIGH-TEMPERATURE components often rely on a protective coating for enhanced oxidation resistance. Several types of thermal coatings, which include NiCrAlY, CoCrAlY, and nickel aluminide, have been developed over the years for high-temperature structural alloys such as Nibase superalloys. Many of these protective coatings derive their excellent oxidation resistance from the formation of an adherent, continuous oxide layer, e.g., Al2O3, which prevents the metallic elements in the coating and substrate from further reaction with oxygen at excessive rates.[1] The protective oxide layer, however, is often quite brittle and can fail by spallation during thermal cycling.[2–10] Spallation reduces the thickness of the protective oxide layer and can increase the rate of oxidation of the oxide-forming element, e.g., Al, in the coating.[2,3,4] The effectiveness of a protective oxide, e.g., Al2O3, is substantially reduced when the continuous oxide layer becomes discontinuous due to depletion of the oxide-forming metallic element from the coating.[1,11,12] Under this circumstance, breakaway oxidation occurs and causes a drastic reduction in coating life.[13,14,15] The interplays between oxidation and spallation must therefore be taken into account when considering the cyclic oxidation resistance and usable life of a thermal coating. The spallation of oxide scales is generally considered to involve two processes:[5–10] (1) the formation of oxide cracks and (2) the separation of spalls from the scale either by decohesion at the oxide/coating interface or by fracture on planes within the oxide itself. The manner by which these processes proceed varies with the stress state.[5–10] In tension, the formation of microcracks that are aligned normal to the tensile stress axis is relatively easy, but the decohesion of the spalls from the substrate or coating is difficult because of the absence of large shear stresses on the interface. In compression, oxide spallation can occur by either a wedging or a buckling process depending on the
interface strength.[9] The wedging process, which is prevalent in systems with a weak oxide and a strong interface, proceeds with the formation of shear cracks in the oxide under a compressive stress and the subsequent detachment of the spalls from the interface. The role of shear cracks in oxide spallation has been reviewed recently by Evans.[9] Several authors[9,10,16] have presented analyses of oxide spallation based on the presence of shear cracks in the oxide scale. In contrast, the buckling process, which is prevalent in systems with a strong oxide and a weak interface, commences with the formation of interface cracks that lead to buckling of the oxide layer under compression. Buckling, in turn, induces tensile stresses and microcracks in the detached layer and causes the subsequent fracture and spallation of the oxide. The mechanics of oxide spallation have been analyzed by a number of investigators.[5–10] Oxide spallation by wedging has been treated by Evans and Lobb[16] and by Sch
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