Methods of measuring adhesion for thermally grown oxide scales

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Abstract
High temperature alloys and coatings rely on the formation of adherent scales to protect against further oxidation, but scale spallation is often problematic. Despite the technical importance of the problem, ‘practical adhesion’, which refers to the separation of the oxide from the metal, has mainly been treated qualitatively in the past. Various techniques now exist such that the subject can be assessed in quantitative or semiquantitative terms. Some of the techniques are described in this paper, and their weaknesses and strengths are discussed. The experimental methods addressed here include tensile pulling, microindentation, scratch tests, residual stress induced delamination, laser-induced or shock wave induced spallation, double-cantilever beam bending and several four-point beam bending approaches. To date, there is not a universal, easy test for oxide adhesion measurement that can provide reproducible information on interfacial fracture energy for a variety of oxide/metal systems. Much experimentation is still needed to increase confidence in many of the existing tests, and the fundamental mechanics for some present techniques also require further development.

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