Porosity-Thermal Conductivity Relationships in Plasma Sprayed Zirconia Coatings

Thermal barrier coatings (TBCs) are used on heat engine parts to impart protection to components against failure under excessive heat loads, to increase inlet temperatures with consequent improvements in efficiency, and to reduce requirements for cooling. Control of thermal conductivity is addressed since low thermal conductivity depends not only on the nature of the yttria stabilized zirconia (YSZ) layer, but also on the morphology of pores and cracks, which are closely linked to process parameters. This paper will present the influence of feedstock characteristics (particle size distribution and powder morphology) and thermal cycling on porosity content and thermal conductivity of zirconia coatings. The results show increased porosity with particle size, due to an increase in the degree of particle fragmentation and unmelted particles, leading to lower thermal conductivity. Coatings sprayed with powders of different powder morphology yielded changes in porosity and interlamellar contact, thus, influenced thermal conductivity. Sintering effects during thermal cycling resulted in reduced porosity and increased thermal conductivity.