Size Effect of Yttria Stabilized Zirconia Addition on Fracture Toughness and Thermal Conductivity of Plasma Sprayed Aluminum Oxide Composite Coatings

Aluminum oxide (Al2O3) is a conventional structural ceramic, but its applications are limited due to higher brittleness and low fracture toughness (similar to 3.2 MPa center dot m(1/2)). Yttria stabilized zirconia (YSZ, fracture toughness 10 MPa center dot m(1/2)) is an ideal reinforcement for Al2O3 matrix to improve its fracture toughness. Al2O3-8 mol.% YSZ composite coatings with various length scale (nanometer- to micrometer-sized) and pretreated (spray dried) YSZ particles were processed using plasma spraying technique. Adherent and uniform coatings with similar to 85% densification were deposited having varying YSZ distribution. X-Ray diffraction analysis confirmed the formation of gamma-Al2O3 due to the rapid solidification of Al2O3 splats. Micro hardness of the deposits showed decrease in hardness (up to 12%) with respect to that of pure Al2O3. Fracture toughness shows similar to 28% enhancement with YSZ addition. Improved fracture toughness is ascribed to the distribution of YSZ particles and transformation toughening of YSZ splats present in the coatings. Typical thermal conductivity was estimated to be similar to 4 W/m center dot K for plasma sprayed coatings. Results display the superiority of spray dried YSZ particles over nano and micro size YSZ addition on fracture toughness, which can make them potential thermal barrier coatings (TBCs).