TEM characterization of plasma-sprayed thermal barrier coatings and ceramic-metal interfaces after hot isostatic pressing

Plasma-sprayed thermal barrier coatings (TBCs) consisting of both a Ni-5%wt. Al bond coat (Ni-Al) and a thermally insulating 8%wt. Y2O3 partially stabilized ZrO2 ceramic top coat were subjected to hot isostatic pressing (HIP). Transmission electron microscopy (TEM) with qualitative microanalysis was utilized to characterize the microstructural changes occurring during HIP treatment and to clarify the nature of the ceramic-metal interface [ZrO2-(Ni-Al)] both in the as-sprayed and hipped states. During HIP treatment, the as-sprayed ZrO2 experienced a certain degree of hot plastic deformation and recrystallization, forming dense and crackless fine-equiaxed tetragonal grains. However, due to nonuniformities of the as-sprayed structure, the degree of recrystallization of the original columnar grains changed from location to location, resulting in a recrystallized microstructure ranging from coarse equiaxed grains 1-2 mu m in size to co-existing grains of equiaxed and columnar shapes, including still intact columnar grains. Through TEM observations of a plasma-sprayed ZrO2-(Ni-Al) interface more than 20 mu m long, it was found that the interface is amorphous in nature and unevenly thick and even discontinuous in some locations. The amorphous interfacial layer is composed largely of the Ni-Al bond coat materials together with some ZrO2 ceramic material. After HIP treatment, the amorphous ZrO2-(Ni-Al) interfacial layer is replaced by a continuous, dense and polycrystalline Al2O3 layer which can act as a diffusion barrier for elements both from the ZrO2 coating to the bond coat and to the substrate, and also in the opposite direction.