Experimental Study and Numerical Simulation of Y2O3 Coatings Deposited by Plasma Spraying at Different Torch Powers

Thermodynamically stable Y2O3 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 Y2O3 plasma spray coatings is found to be uncertain due to its characteristic microstructure composed of various intrinsic process-dependent defects. In this study, Y2O3 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 Y2O3 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 Y2O3 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 ? under an inert argon atmosphere, followed by microstructure, phase and mechanical property analysis to compare the durability of the Y2O3 coating sprayed at different plasma spraying powers. The results indicated that the Y2O3 coating deposited at 30 kW with optimum superheat similar to 375 degrees 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 degrees. The linear increase in indentation modulus, hardness and fracture toughness with thermal cycles is attributed to the densification in the Y2O3 coating due to sintering. With optimized Y2O3 deposition parameters, the highest durability of the coating is demonstrated, and the failure mechanism is elaborated.