Modeling decohesion of a top-coat from a thermally-growing oxide in a thermal barrier coating

Abstract

Oxidation of the bond-coat and the shape-distortion which accompanies the growth of the oxide is a major mechanism which causes local interfacial decohesion between the ceramic top-coat and the growing oxide layer in a thermal barrier coating. The objective of this paper is to numerically study the growth of the oxide and the induced decohesion of the top-coat from a bond-coat which is not perfectly flat but has an initial sinusoidal-shaped surface undulation. Our numerical simulation study accounts for: (a) the diffusion of oxygen; (b) the oxidation of the bond-coat; (c) the anisotropic growth and the associated stresses and shape-distortion of the thermally-grown-oxide; (d) creep of the top-coat; and (e) the interfacial traction–separation response between the top-coat and the growing oxide.A parametric study is conducted by varying the amplitude of the initial sinusoidal undulation at the interface between the top-coat and the bond-coat. Our simulation results show that the decohesion of the top-coat depends strongly on the amplitude of the initial undulation — imperfections with a larger initial amplitude prove to be more detrimental.Our numerical study suggests that in order to improve the delamination-life of an air-plasma-sprayed thermal barrier coating it might be beneficial to produce bond-coat surfaces with shallow and evenly distributed dimples — rough enough to provide adherence of the top-coat to the bond-coat via a mechanical-keying effect, but not too rough so that it leads to excessive shape-distortion and premature decohesion. This may be accomplished by using shot-peening, so as to plastically-deform and reduce the roughness of the as-deposited bond-coat surface.