EB-PVD helium-tight zirconia ceramic coating on porous ceramic substrate

Abstract

The dense helium impermeable zirconia coating deposited on highly porous NiO-ZrO2 substrate with EB-PVD were studied with transmission and scanning electron microscopy. The plausible condensation mechanisms of zirconia from its vapour phase onto zirconia and nickel oxide phases of the substrate were established. The condensation of zirconia was identified as a two-stage process with two mechanisms of growth, planar and cellular, similar to solidification from the liquid phase. ZrO2 condensation on ZrO2 and NiO phases occurs with different routes. During the planar growth on the ZrO2, the ZrO2 film nucleates and grows up to ~0.3-0.5 mm with no orientation preference as continuous 'ayer by layer' defective film. The planar ZrO2 film on the NiO of ~0.5-0.7 mm thickness is discontinuous and porous; it consists of oriented structurally perfect 'islands'. The differences found are probably the result of differences in the melting temperature of ZrO2 and NiO, and Zr and Ni affinities to oxygen as well as their different behavior under high vacuum. The deposition affected zone surrounding the ZrO2 nucleation site is established.