Processing effects on porosity-property correlations in plasma sprayed yttria-stabilized zirconia coatings

For plasma sprayed thermal barrier coatings (TBCs), control of thermal conductivity is critical since low thermal conductivity depends not only on the intrinsic property of the yttria-stabilized zirconia (YSZ) TBC, but also on the morphology of pores and cracks introduced during spray process. They are closely linked to process methodology as well as to chemistry, structure and morphology of the ceramic feed materials. This paper addresses the influence of feedstock characteristics on particle state in the plasma and the resultant coating properties. In addition, substrate temperature, angle-of-impact and thermal cycling effects on porosity (quantity and morphology) and its resultant influence on thermal conductivity and elastic modulus of plasma sprayed YSZ TBCs. 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 and modulus. Furthermore, higher substrate temperatures and low particle velocity lead to lower porosity and improved inter-splat contact and, thus, enhanced coating properties. Sintering during thermal cycling reduces porosity and increases thermal conductivity and modulus.