Limits to adherence of oxide scales

Abstract

Strains can cause oxide scales to fracture, often leading to loss of protectiveness and creation of spalling debris. Fracture mechanics is used to identify a number of criteria under which uniform scales may be expected to fail as a result of rapidly applied strains. The failure mode most frequently found occurs when the strain ɛ builds up in the scale until the strain energy density per unit area exceeds the fracture surface energy γ of the oxide. This produces spalling when ɛ>(2·8 γ/hE)1/2, where h is the scale thickness and E is the Young's modulus of the oxide. Two further regimes are identified in thin scales. First, as the external strain is applied to the oxide via the metal substrate, it is obvious that no further stress can be applied to the oxide if the substrate has itself been stressed beyond yield. This condition gives rise to a second regime, that of extended oxide adherence on substrates under external tensile strains in which the oxide cracks and forms a series of islands, but remains attached to the deformed metal. This regime accounts for the reduced spalling observed when oxidised cylinders of annealed 9% Cr steels were subjected to torsional straining. A third regime arises when the oxide thickness is less than its comminution limit, when the flaw size necessary for brittle fracture exceeds the oxide thickness and the oxide then yields in a ductile manner without cracking. This condition accounts for the retention of passivity by the thin films found on metals at low temperatures. The results are presented as a series of failure mode maps of failure strain versus oxide thickness for various oxide systems such as Fe3O4, Cr2O3, Al2O3, SiO2 and NiO. The spalling observed is found to lie within the predicted regions, illustrating both the validity of the model and that the strength of the oxide metal interface usually approaches that of the bulk oxide. The necessary materials data for the maps (surface energy, Young's modulus, hardness) are found from a critical survey of oxide properties. The boundary for brittle spalling is found to lie at a similar position for each oxide, because the γ/E ratio of the oxides tends to be fairly constant.MST /1129