Damage Process of Thermal Barrier Coating Subjected to Thermal Cycling Under High Heat Flux

Abstract

In order to develop an evaluation method for resistance to thermal cycling of thermal barrier coatings (TBCs) for gas turbines, thermal cycle tests were performed by means of a high heat-flux heating apparatus, where the surface ofTBC specimens was heated cyclically by plasma gas flow and the other surface was cooled by water. This experiment nearly simulated a thermal condition in gas turbines. The experimental observation clarifiedthe damage processes in TBC systems under high heat flux. At first, cracks were initiated in the direction ofthe coating thickness (vertical cracks). When they reached at the vicinity of the boundary between a top coat and a bond coat, the cracks growing along the boundary (transverse cracks) were initiated. The transverse cracks coalesced each other, to lead the top coat to delamination. After the thermal cycle test, the porosity ofthe top coat decreased due to sintering. The tensile stress increased due to the top coat sintering, as well as the thermal fatigue due to thermal cycling, promotes the initiation and growth of vertical cracks. In the thermal cycle tests on a more porous TBC with lower content of impurities such as SiO 2 , the growth rate of vertical cracks was smaller and the coalescence of transverse cracks was less frequent than the denser TBC including more impurities. This results from the decrease in Young's modulus and thermal conductivity due to the porosity increase, the decrease in the impurity content, and the difference of sprayed particle shape.