Effects of critical plasma spraying parameters on microstructure and mechanical properties of LaPO4-8YSZ thick composite coatings

LaPO4-8YSZ composite coatings with a thickness of similar to 1 mm have been developed for increasing thermal protection of combustion chamber components in gas turbines and diesel engines. A serious problem concerning the thick coatings is the high residual stress, which results in poor adhesion and low thermal shock resistance. In this study, thick LaPO4-8YSZ composite coatings in different compositions were produced by atmospheric plasma spraying at four different critical plasma spraying parameters (CPSP; 0.83, 0.94, 1.06 and 1.17 kW/Lpm). The effects of CPSP on microstructure, residual stress and mechanical properties of LaPO4-8YSZ thick composite coatings were investigated. The results shows that the porosity of the coatings is decreased with the increase of CPSP from 0.83 to 1.17. The decreasing porosity leads to the increase of Young's modulus and microhardness of the coatings, while the fracture toughness of the coatings shows a decreasing tendency. The residual stress in the composite coatings increases with increasing CPSP. Correspondingly, the thermal shock resistance of the coatings decreases with the increase of CPSP. The 5 wt% LaPO4-8YSZ composite coating has the lowest residual stress at CPSP 0.83 and it exhibits the longest thermal cycle life at 1000 degrees C (1355 +/- 134 cycles).