Route from U. R. Evans’ original approach to today's understanding of scale failure

Abstract

A first quantitative description of the mechanical failure of surface scales was given by U. R. Evens in 1937. This approach can still form the basis for today's understanding, but the important role of physical defects is hidden in the energy term for the adhesional work to be overcome for scale failure. In the present paper, an extended approach is described that is based primarily on defect growth kinetics during high temperature exposure and relates the critical strain to scale failure to physical defect size via a ‘fingerprint’ parameter η. If η has been determined once for an oxide/metal system by metallography and mechanical four-point bending tests, critical oxide failure strains can be assessed simply by metallographic determination of defect sizes without additional mechanical testing. This seems to be of particular interest for the assessment of scale strain tolerance on service exposed components, e.g. in power stations. So far, the extended concept has been confirmed for Ni and Ti oxide scales; further work is currently in progress.