Adhesion Failure
Surface degradation events characterized by the detachment and loss of a protective oxide layer expose the underlying metal to accelerated oxidation or corrosion. Occurrence of oxide scale spallation typically results from the accumulation of elastic energy within the scale during thermal cycling. The failure occurs when the stored energy exceeds the fracture energy of the interface or the scale itself, leading to a loss of the barrier.
Thermal Mismatch
Differences in the thermal expansion coefficients of the metal and its oxide generate substantial stress during cooling from high temperatures. The metal usually contracts more than the oxide, placing the scale under compression. If the compressive stress becomes too high, the scale may buckle and then snap off the surface.
Mass Loss
Gravimetric analysis tracks the weight changes of a sample to quantify the extent of the damage. A sudden drop in weight indicates a major loss of the protective layer. Repetitive spallation and regrowth cycles lead to the depletion of the alloying elements that form the protective scale.
Surface Protection
Modification of the alloy chemistry can improve the bond strength at the interface. Small additions of reactive elements like yttrium or hafnium change the scale growth mechanism.
