MECHANICAL-PROPERTIES OF OXIDE SCALES

At high temperatures most alloys rely oh the protective effect of oxide scales formed by the reaction between oxygen from the environment and components of the alloy. The protective effect of these scales may, however, be impaired if stresses lead to cracking or spalling. Therefore, the mechanical properties of the scales play a vital part in protection, in particular under service conditions of the materials where the presence of stresses cannot be excluded The paper provides a survey of the existing models that describe the mechanical properties of scales (the emphasis being on mechanical scale failure), the measuring techniques and the data. It will be demonstrated that, in the case of tensile conditions, the situation is relatively well understood and that a fair amount of data exists. With regard to compressive conditions scale failure consists of a combination of different failure steps, which can each be described by a quantitative model, but there is still clearly a need for experimental verification While the measuring techniques for the properties under tensile stresses are fairly well developed, those for the behavior under compressive stresses, in particular those characterizing scale adhesion, are still under discussion and relatively few experimentally determined data are available in this case. Nevertheless, existing knowledge can be put to good use in assessing scale behavior under the effect of stresses, based on the models and data given in the paper.