Thermal stability of air plasma spray and solution precursor plasma spray thermal barrier coatings

Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional air plasma spray (APS) and solution precursor plasma spray (SPPS) processes. Both TBCs were isothermally heat treated from 1200 degrees to 1500 degrees C for 100 h. Changes in the phase content, microstructure, and hardness were investigated. The nontransformable tetragonal (t') phase is the predominant phase in both the as-sprayed APS and SPPS TBCs. APS and SPPS coatings exhibit similar thermal stability behavior such as densification rate, hardness increase, and grain coarsening rate. Both the as-received and heat-treated APS and SPPS TBCs show a bimodal pore size distribution with nano- and micro-size pores. After 1400 degrees C/100 h heat treatment, equiaxed grains replace the columnar structure in APS TBCs and the splat structure disappears. Vertical cracks remain after the 1500 degrees C/100 h exposure in SPPS TBCs. The monoclinic phase appears in APS TBCs after a 1400 degrees C/100 h exposure and in SPPS coatings after a 1500 degrees C/100 h exposure.