Synergistic Oxidational Effect of Hydroxyl and Sulfate Radicals Induced an Efficient Removal Mechanism in Photochemical Mechanical Polishing of Yttrium Aluminum Garnet

Monocrystalline yttrium aluminum garnet (YAG) has a higher hardness and stable chemical properties. Therefore, conventional chemical mechanical polishing of monocrystalline YAG has a low-efficiency problem. This paper proposed advanced oxidation processes based on hydroxyl radical (•OH, HR) and sulfate radical anion (SO4•–, SRA) to assist the photochemical mechanical polishing (PCMP) of monocrystalline YAG. A series of experiments were carried out to investigate the effects of HR and SRA on the material removal rate (MRR), roughness (Sa), and surface morphology of monocrystalline YAG. Meanwhile, the chemical reaction mechanism between HR/SRA and YAG was revealed based on XPS and radical substitution reactions. The results demonstrated that HR and SRA synergistically assisted PCMP of monocrystalline YAG showed optimal results with MRR = 208.11 nm/min and Sa = 0.17 nm. In particular, compared to the conditions with no UV irradiation, the MRR could be increased by nearly 3 times and Sa could be reduced by about 56.41%. In addition, under the synergistic oxidational effect of HR and SRA, hydroxides (Al(OH)3, Y(OH)3), sulfates (Al2(SO4)3, Y2(SO4)3), and oxygen (O2) could be produced on the surface of monocrystalline YAG. The above findings led to the development of a polishing approach to attain a high MRR and surface quality on laser crystals.