GROWTH AND ADHESION OF OXIDE SCALES ON AL2O3-FORMING ALLOYS AND COATINGS

The main parameters affecting the modes and mechanisms of alpha -Al//2O//3 scale development and adhesion on high temperature alloys and coatings are reviewed and discussed, with emphasis on the more recent contributions. Such scales are reasonably protective but are susceptible to cracking or spalling, especially during thermal cycling. In general, this can be attributed to the low strength of the scale-alloy bonds arising from impurity segregations, the nature of the scale growth process, the development of oxide growth stresses and the resulting overall scale configuration which may involve convoluted scales. Superimposition of differential contraction stresses can cause considerable scale cracking and spalling on cooling. Some of the more important mechanisms accounting for the beneficial effects of reactive elements or oxide dispersions on scale adhesion-cohesion are discussed.