Gas jet assisted vapor deposition of yttria stabilized zirconia

Abstract

A gas jet assisted electron beam evaporation process for synthesizing yttria stabilized zirconia (YSZ) coatings has recently been reported. The process uses a rarefied inert gas jet to entrain and transport vapor to a substrate. The gas jet enables the lateral spreading of the flux to be controlled and large fractions of the vapor to be deposited on samples of relatively small size. When the gas pressure is high, coatings grown at 1050°C and below have a columnar structure and a high pore fraction. The total pore volume fraction, the morphology of the inter- and intracolumn pores and the coating texture are all observed to be a strong function of the gas pressure in the chamber with increasing chamber pressure leading to larger intercolumnar pore spacings, wider pores, a higher total pore volume fraction, and a reduction in the coating texture. A direct simulation Monte Carlo simulation approach has been used to investigate vapor transport for the various gas pressures explored in this study. The simulation indicates that as the gas pressure increases, binary scattering events between the vapor and background gas broaden the vapor molecule incidence angle distribution. This intensifies flux shadowing and results in the incorporation of voids in the coating. Increasing the gas pressure also results in a rapid increase in the vapor phase nucleation of YSZ clusters. This observation coincides with a transition from a [200] textured columnar morphology at moderate pressures to a nanogranular structure with no texture and a very high nanoscopic pore volume fraction at high pressures.