Fracture evaluation of thermal barrier coating system by thermal load testing

Abstract

The fracture process in thermal barrier coatings (TBCs) was investigated by conducting furnace heating tests. The uniform heating tests were conducted using TBC specimens with a top coat (YSZ) and bond coat (NiCrAlY), and the fracture process was examined by monitoring AE events. Two types of TBCs with different roughness of the bond coat surface were tested to examine the effect of the interface property on the fracture process. SEM observation in conjunction with the AE activity revealed that a number of distributed microcracks were observed to initiate along the vicinity of the top coat/bond coat interface only at the cooling stage of heating test. The coalescence of those discontinous microcracks resulted in the delamination and eventual spallation of the top coat due to buckling deformation when the TBC was further cooled down. The AE results were used to obtain the temperature condition at which the significant delamination took place in the TBCs and to determine the temperature drop ΔT_i from the maximum heating temperature T_<max>. It was shown that ΔT_i was approximately constant irrespective of T_<max>, suggesting the importance of the thermal shrinkage after T_<max> for the delamination growth.