Effects of preheating temperature and cooling rate on two-step residual stress in thermal barrier coatings considering real roughness and porosity effect

In this research, a finite element model was developed in order to simulate the two-step residual stress distribution of a thermal barrier coating system, considered to be used in diesel engine cylinder head, with a real roughness and real porosity. Two steps including the bond coat and the top coat deposition processes were taken into account. The real geometry of coating layers, including the roughness and the porosity, was also considered based on a scanning electron microscopy image. Then, effects of the convective heat transfer coefficient and initial substrate and substrate/bond coat preheating temperatures on the residual stress were studied. Obtained results illustrate that the residual stress, regarding the two steps of coating processes, are different, particularly in the substrate. Numerical results indicate that while the convective heat transfer coefficient in BC deposition had no striking effect, it was an influential factor in the residual stress distribution when TC was deposited. Moreover, preheating temperature for depositions of the bond coat layer and the top coat layer has a pivotal role in stress distribution of all layers. In addition, consideration of the porosity in the top coat layer causes remarkable decrease in axial stress distribution in substrate and much more local stress concentrations in TC layer compared to those of BC layer due to larger and more irregular pores.