Surface optimization of CVD grown silicon carbide interlayer on graphite for plasma sprayed yttria topcoat

Abstract

Chemical Vapor Deposition (CVD) technique is employed to deposit uniform, smooth, and dense SiC interlayer over High-Density Graphite (HDG) substrate, as an interlayer for subsequent deposition of yttria (Y2O3) topcoat by Atmospheric Plasma Spray (APS) process. In any thermal spray process, since the bonding is purely mechanical interlocking, the surface roughening of smooth CVD grown SiC surface becomes essential. Different surface preparation techniques like alumina grit blasting, plasma etching, laser ablation, and chemical etching were attempted to create rough anchoring patterns on SiC surface. Chemical etching of SiC interlayer by using molten eutectic NaOH/KOH is found to be effective in introducing the desire anchoring sites for Y2O3 splats during plasma spraying. The microstructural features and surface roughness of CVD grown SiC and after subsequent surface roughening treatments were compared using SEM/EDS and profilometer, respectively. The performance and its durability evaluation were carried out by thermal cycling studies on Y2O3 coated samples with and without SiC interlayer at 1723 and 1823 K that shows considerable improvement in the life of plasma sprayed Y2O3 coating with SiC interlayer.