Thermal barrier coatings typically fail on cooling after prolonged thermal cycling or isothermal exposure. The mechanics of spalling requires that first a critical sized portion of the coating separates from the underlying material, then buckles and finally spalls away. The critical size for buckling depends on the thickness of the coating but is several millimeters for typical zirconia coatings 150 mu m thick. As-deposited coatings do not have interface separations but they form on thermal cycling as described in this work based on observations of coating cross-sections combined with the stress redistribution in the thermally grown oxide imaged using a piezospectroscopic luminescence method. Analysis of the images reveals that small, isolated regions of damage initially form and then grow, linking up and coalescing to form percolating structures across the coating until the buckling condition is attained, the buckle extends and failure occurs by spallation. The piezospectroscopic imaging of the stresses in the thermally grown oxide formed by oxidation beneath thermal barrier coatings provides a form of "stress tomography" enabling the subcritical separations to be monitored.