Spalling failure of α-alumina films grown by oxidation. II. Decohesion nucleation and growth

Buckling and spalling of thermally-grown alumina films have been studied on Fe–Cr–Al heat-resisting alloy using microscopy and photostimulated piezospectroscopy. Buckles nucleate and propagate with time after cooling to room temperature at a constant residual stress in the film and, apparently, in the absence of sufficiently large pre-existing flaws at the oxide–metal interface. The size of the slowly growing buckles is typically far below the critical size deduced in regular buckling mechanics. The growth rate of the buckles depends on the residual stress and, especially, on the thickness of the oxide. An abrupt increase of the buckle size followed by cracking at the buckle perimeter produces circular spalls. The relationship between the spall size, residual stress and oxide thickness is discussed.