Effect of Al₂O₃addition on the characteristics of 8YSZ thermal barrier coating and its performance evaluation in engine test

Utilization of thermal barrier coatings (TBCs) on the compression ignition engine piston enables them to run at higher temperatures and, as a consequence, achieve higher thermal efficiency. TBC deposited on the piston by virtue of low thermal conductivity reduces the heat loss, thereby mitigating the fuel requirement and improving the life of the piston. Effects of the addition of 20 % Al₂O₃ (w/w) into 8 % Yttria stabilized Zirconia (8YSZ) as a top coat and NiCrAlY bond coat on different characteristics have been investigated. Pure 8YSZ was also studied as a reference. All the coatings were deposited by the atmospheric plasma spray coating technique. A bond coat of thickness 150 μm was first deposited on grit blasted aluminum alloy sample (Al4032) of size 20 × 20 × 5 mm³for improved adhesion, followed by deposition of top coat with a thickness of 350 μm. Detailed characterization tests utilizing scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry, roughness test, micro-hardness test and scratch test have been done to analyze various properties of the TBCs. Adhesion and cohesive strength of 20 %Al₂O₃-8YSZ was found to be higher than those of pure 8YSZ coating. Following the characterization of these coatings, a brief experimental study was also performed in an engine test setup to evaluate the addition of 20 %Al₂O₃ into 8YSZ as top coat. 8YSZ coated piston resulted in relatively higher NO_x emission at lower engine loads, while 20 %Al₂O₃-8YSZ coating exhibited its superior thermal stability characteristics while testing at higher engine load (3 kW).