Plasma spray deposition of tri-layer environmental barrier coatings

An air plasma spray process has been used to deposit tri-layer environmental bather coatings consisting of a silicon bond coat, a mullite inter-diffusion bather, and a Yb2SiO5 top coat on SiC substrates. Solidified droplets in as-deposited Yb2SiO5 and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb2SiO5 + Yb2O3 two-phase top coat and 2:1 mullite (2Al(2)O(3)*SiO2) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 degrees C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb2O3 in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al2O3. Mud-cracks were found in the Yb2SiO5 layer and in precipitated Al2O3 due to the thermal expansion mismatch between these coating phases and the substrate. (C) 2014 Elsevier Ltd. All rights reserved.