Formation mechanism of grinding wheel oxide layer in the ultrasonic-assisted ELID grinding of silicon carbide ceramics

As a substrate material for lightweight optical mirrors, silicon carbide (SiC) ceramics are extensively employed in deep space exploration applications. Nevertheless, conventional grinding/milling and polishing processes are prone to induce subsurface damage while exhibiting inherently low processing efficiency. Therefore, this study proposes the combination of ultrasonic vibration and online electrolytic sharpening, firstly employing ultrasonic-assisted ELID (Electrolytic In-process Dressing) (U-ELID) grinding to address these machining challenges associated with SiC ceramics. Theoretical analysis and multi-physics field simulations reveal that the formation mechanism of the grinding wheel oxide layer under high-frequency ultrasonic vibration is the critical factor governing the surface integrity of the machined workpiece. Subsequently, through the integration of multi-physics field simulation, pre-sharpening experiments, and grinding trials, this research establishes a thickness growth model for the oxide layer formed during U-ELID pre-sharpening. It further investigates the formation characteristics of the oxide layer during the pre-sharpening stage in U-ELID grinding of SiC ceramics, revealing the underlying mechanisms and inherent laws governing the influence of ultrasonic vibration on the uniformity and compactness of the oxide layer. The results demonstrate that the synergistic effects of ultrasonic high-frequency mechanical action, ultrasonic cavitation, and ultrasonic acoustic streaming act upon the oxide layer. Compared to conventional ELID grinding, the oxide layer generated during U-ELID grinding exhibits significantly higher compactness and superior grinding performance. Specifically, under the same power supply voltage, the porosity of the oxide layer formed during U-ELID pre-sharpening is reduced by 51%, with narrower cracks and fewer pits induced by abrasive grain detachment. As the grinding depth decreases from 5 μm to 2 μm, the arithmetic mean surface roughness (Sa) of the ELID ground surface decreases by 35%, whereas that of the U-ELID ground surface decreases by 44.2%. Compared with ELID grinding, the reduction rate of Sa for the U-ELID ground surface is increased by 9.2%. Additionally, the surface scratches are finer and more uniform. This study, for the first time, unveils the mechanism by which the introduction of ultrasonic vibration influences oxide layer formation on grinding wheels during U-ELID grinding. The identified synergistic mechanism offers a novel pathway for achieving ductile-mode grinding of SiC ceramics, holding significant promise for advancing the machining precision of optical components to the nanoscale and supporting the demands of national major engineering projects such as deep space exploration.