STRUCTURAL-CHANGES IN PLASMA-SPRAYED ZRO2 COATINGS AFTER HOT ISOSTATIC PRESSING

Plasma-sprayed 8 wt.% Y2O3 partially stabilized ZrO2 coatings were sub jected to hot isostatic pressing (HIP) treatment. Changes in microstru cture, composition and fracture mode were examined from the as-sprayed and hipped coatings by X-ray diffraction, scanning electron microscop y and electron probe microanalysis. A dense and crack-less ZrO2 coatin g can be achieved by applying HIP treatment. During HIP treatment, the non-transformable and high yttria content tetragonal phase experience s a certain degree of recrystallization, forming fine equiaxed grains. Because of sintering effects and subsequent inter-growth between lame llae and between splats, a metallurgical bonding is realized between t hem, resulting in a fracture mode of the hipped coating as mainly tran s-lamella or trans-splat. After HIP treatment, a thin layer of Al2O3 d evelops at the interface between the ZrO2 coating and the metallic inn er layer of Ni-5 wt.% Al alloy (bond coat). Because of obvious inter-d iffusion between elements during HIP treatment, a wide transition laye r is formed in the bond coat-substrate interface. The Kirkendall pores , formed in the bond coat neighboring the bond coat-transition layer i nterface and within the transition layer, as a result of two chemical species diffusing in opposite direction at different rates, can be par tially closed up or reduced in size by applying HIP treatment.