A numerical investigation of the effect of thermal aging, processing and humidity on initiation and delayed cracking in plasma-sprayed coatings

Abstract

The effect of aging by the relaxation of the initial thermal stresses related to the processing on the initiation and propagation of the inter-splat and the intra-spat cracks in plasma sprayed zirconia is analyzed using finite element model and a description of failure with cohesive surfaces. A multi-scale approach is adopted in which the inter-splat and intra-splat crack growth is described with a rate-temperature and humidity dependent cohesive zone model that mechanically represents the reaction-rupture mechanism underlying stress and environmentally assisted sub-critical failure. It is found that the relaxation of the initial thermal stresses generates a significant initial damage at the inter-splat scale by the nucleation of inter-splat cracks and a minor initial damage at the intra-splat scale. The results show that the rate of inter-splat cracks increases with the relative humidity and especially with the temperature at which the relaxation occurs. The effect of the initial damage generated by the thermal aging on the resistance of the polycrystal of plasma sprayed zirconia against intra-splat slow crack growth under static fatigue loading is investigated. The results show that the initial damage at the intra-splat scale does not affect its resistance against intra-splat slow crack growth. However, the initial damage at the inter-splat scale leads to an increase in the slow cracking rate for a loading level KI and a reduction in the threshold load K0 below which no slow crack growth occurs as the individual splat is embedded in a damaged equivalent continuum representing the overall splat structure. The aim of this work is to provide a reliable predictions and insight in long lasting applications of plasma sprayed ceramic materials.