Evaluation of Thermal Shock Fracture of Functionally Gradient Materials

This paper presents some results of laser heating thermal shock tests performed on plasma-sprayed thermal barrier coatings. The coatings tested were single-layer ZrO₂ (stabilized with 83% Y₂O₃), and Functionally Gradient Materials (FGMs) which consisted of multi-layer with varying mixture ratios of ZrO₂ and Ni-based superalloy (NiCrAlY). FGMs are recently proposed coating materials which are aimed at mitigating thermally induced stresses in the coating. A CO₂ laser was used for the thermal shock tests, where a laser beam with a preset spot size, duration and intensity was irradiated onto the coating surfaces. Concurrently with the laser experiments acoustic-emission (AE) monitoring was made to defect the onset of the thermal shock fracture. In order to evaluate the stress state of coatings induced by the laser heating and to discuss the fracture process, numerical computations of transient thermal stresses was made using the finite element method. Based on the results, critical laser power density at the onset of coating failure was defined in order to characterize the thermal shock resistance of the coatings. It was shown that the critical power density of the FGMs is significantly higher than that of the single-layer coating, indicating the effectiveness of the FGMs to reduce the thermal expansion mismatch stress.