The Study of Thermal Properties and Micro Structures of YSZ

Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve a better efficiency by allowing increased turbine inlet temperature and decreasing the amount of cooling air used. Plasma spraying is one of the most reliable methods to produce TBCs, which are generally comprised of a top coating of ceramic and a bond-coat of metal. Usually, the top coating is Yttria-Stabilized-Zirconia (YSZ), providing the thermal barrier effect. The bond-coat is typically a layer of M-Cr-Al-Y (where “M” stands for “metal”), employed to improve the attachment between the ceramic top-coat and the substrate. Due to the extreme temperature gradient presented in the plasma jet and the wide particle size distribution, during the coating process, injected ceramic powders may experience a significantly different heating process. Different heating history, coupled with the substrate preheating temperature, may affect the thermal properties of the YSZ layers. In this paper, four sets of mol 8% YSZ disks are fabricated under controlled temperatures of 1100°C, 1200°C, 1400°C and 1600°C. Subsequently the thermal properties and the microstructures of these YSZ disks are studied. The results indicate a strong microstructure change at a temperature slightly below 1400°C. For a high sintering temperature, a dense YSZ layer can be formed, which is good for gas tight operation; At low sintering temperature, say 1200°C, a porous YSZ layer is formed, which has the advantage of low thermal conductivity. For gas turbine TBC applications, a robust low thermal conductivity YSZ layer is desirable, while for Solid Oxide Fuel Cells, a gas-tight YSZ film must be formed. This study offers a general guideline on how to prepare YSZ layers, mainly by controlling the heating process, to form microstructures with desired properties.