Formation of YOF on Y2O3 through surface modification with NH4F and its plasma-resistance behavior

Yttrium oxyfluoride (YOF) is known for exhibiting superior plasma-resistance properties compared to Y2O3, particularly in a fluorocarbon plasma environment. The formation of the YOF layer directly on the surface of Y2O3 via surface modification may be considered as a viable and cost-effective method. In this study, we employed ammonium fluoride (NH4F) salt for the surface modification of Y2O3. By using a 40 wt% NH4F aqueous salt solution, the YOF layer with a thickness of about 8.6 mu m was efficaciously formed in-situ on the surface of Y2O3 based on the fluorination mechanism. The composition and microstructure of the modified Y2O3 surface was thoroughly characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD analysis revealed the formation of distinct intermediate ammonium yttrium fluoride phases depending upon the heat treatment temperature (150 - 500 degrees C) through the fluorination of Y2O3 with NH4F. A stable YOF phase was formed ultimately at 300 degrees C. Plasma etching tests were performed using a mixture of CF4/Ar/O2 gases for 1 h. The microstructures of specimens are observed by SEM before and after plasma treatments. Plasma exposure tests demonstrated that specimens heat-treated at 500 degrees C for 2 h exhibit excellent plasma-resistance behavior with minimal surface damage, which is attributed to the presence of stable and dense YOF phase.