Phase Evolution and Microstructural Behavior in Plasma-sprayed YPO 4 Coating upon Thermal Cycling

Atmospheric plasma spray (APS) coating of Yttrium Phosphate (YPO 4 ) may offer excellent environmental durability, outstanding chemical inertness, high melting point, and matching thermal expansion coefficients over a range of substrate materials of interest. The study investigates for the first time the high temperature thermal cycling behavior of APS coating of YPO 4 , developed on graphite discs without any bond coat. Generation of commonly solid P 2 O 5 in gas phase during development of coating and thermal cycling enabling it to acquire some unique properties are few of the interesting aspects observed. Identifying formation of new phases, investigating microstructural changes inside the coating, their origin and consequences are the primary interests. Heating the APS coated graphite disk up to 1250, 1300, 1350, and 1400 °C at a rate of 12 °C/min, holding it at the temperatures for 2 h followed by natural cooling to ambient temperature are the steps followed in the thermal cycling. To understand suitability for nuclear application, the highest temperature in a thermal cycle is chosen above the melting point of Uranium. Induced structural and phase changes upon thermal cycling are investigated using x-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and energy-dispersive x-ray (EDAX) analysis. Raman spectroscopy and Fourier Transform Infrared (FTIR) spectroscopy are used to further confirm the structural and chemical nature of the coating developed. Excellent chemical compatibility of the YPO 4 coating with graphite has been demonstrated at temperatures up to 1400 °C. Conversion of YPO 4 to Y 2 O 3 and P 2 O 5 upon thermal cycling, densification through pore closures, and possible improvements in thermal barrier and strain tolerance capacity due to formation of voids are some of the interesting results obtained.