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 part ially stabilized ZrO2 ceramic top coat were subjected to hot isostatic pressing (HIP). Transmission electron microscopy (TEM) with qualitati ve microanalysis was utilized to characterize the microstructural chan ges occurring during HIP treatment and to clarify the nature of the ce ramic-metal interface [ZrO2-(Ni-Al)] both in the as-sprayed and hipped states. During HIP treatment, the as-sprayed ZrO2 experienced a certa in degree of hot plastic deformation and recrystallization, forming de nse and crackless fine-equiaxed tetragonal grains. However, due to non uniformities of the as-sprayed structure, the degree of recrystallizat ion of the original columnar grains changed from location to location, resulting in a recrystallized microstructure ranging from coarse equi axed grains 1-2 mu m in size to co-existing grains of equiaxed and col umnar shapes, including still intact columnar grains. Through TEM obse rvations 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 uneve nly thick and even discontinuous in some locations. The amorphous inte rfacial layer is composed largely of the Ni-Al bond coat materials tog ether with some ZrO2 ceramic material. After HIP treatment, the amorph ous ZrO2-(Ni-Al) interfacial layer is replaced by a continuous, dense and polycrystalline Al2O3 layer which can act as a diffusion barrier f or elements both from the ZrO2 coating to the bond coat and to the sub strate, and also in the opposite direction.