Gas-Tight Coatings Produced by Very Low Pressure Plasma Spraying

A certain level of porosity is always presented in thermal spray ceramic coatings. LPPS or some specific APS processes can allow to reduce it but it is difficult to obtain coatings with a low thickness (less than 100 m) which are fully gas-tight because of cracks or interconnected pores. This gas-tight property is for example very suitable for the ceramic electrolyte of solid oxide fuel cells (SOFC). Some ZrO₂-Y₂O₃ coatings were obtained by the VLPPS process, a LPPS system operating at a pressure of about 100 Pa. The specific structure of these coatings is a mixture of condensed vapors and splats. The results are very satisfying because ZrO₂-Y₂O₃ coatings with a thickness of about 70 micrometers are tight under a hydrogen pressure of 2x105 Pa. This paper presents some ZrO₂-Y₂O₃ coatings obtained by different processes (APS, LPPS and VLPPS) and their properties.