Experimental Study and Numerical Simulation of Y 2 O 3 Coatings Deposited by Plasma Spraying at Different Torch Powers

Thermodynamically stable Y 2 O 3 coatings produced by plasma spraying act as an excellent chemical barrier for molten uranium interaction with graphite crucibles used in uranium melting pyrochemical reprocessing applications. The establishment of the structure–property relationship, performance and life assessment of Y 2 O 3 plasma spray coatings is found to be uncertain due to its characteristic microstructure composed of various intrinsic process-dependent defects. In this study, Y 2 O 3 coating is deposited on high-density graphite at three different plasma spray powers, i.e., 25, 30 and 35 kW, with in situ in-flight particle temperature and velocity measurements using a spray diagnostic system. The Y 2 O 3 splat formation is studied in detail using a computational fluid dynamics tool. To study the shape and propagation characteristics of the splat, the numerical simulations were performed at three different plasma spray powers. The numerically obtained spread factor and shape characteristics of the Y 2 O 3 splats are correlated with the in-house experimental observations, which highlight the role of velocity and temperature of the in-flight particle at impact on the bonding characteristics. A thermal cycling study was performed at 1550 °C under an inert argon atmosphere, followed by microstructure, phase and mechanical property analysis to compare the durability of the Y 2 O 3 coating sprayed at different plasma spraying powers. The results indicated that the Y 2 O 3 coating deposited at 30 kW with optimum superheat ~ 375 °C for molten particles resulted in the formation of dense pancake splats with minimal shrinkage cracks and residual stresses in accordance with the simulation analysis. Corresponding lamellar structures with 12-15% porosity have contributed to the maximum durability, i.e., 10 cycles in the thermal fatigue test at 1550 °C. The linear increase in indentation modulus, hardness and fracture toughness with thermal cycles is attributed to the densification in the Y 2 O 3 coating due to sintering. With optimized Y 2 O 3 deposition parameters, the highest durability of the coating is demonstrated, and the failure mechanism is elaborated.