Sulfuric acid modified magnetorheological finishing of polycrystalline magnesium aluminate spinel

Polycrystalline magnesium aluminate spinel has excellent optical, mechanical, and chemical properties; making it an ideal material for critical optical parts under extreme conditions, and it has broad application prospects in aerospace. The high hardness and polycrystalline nature of the material bring significant challenges to high-precision spinel processing, especially in magnetorheological finishing (MRF), where low processing efficiency, polishing ripples and grain effect appear during polishing, leading to degradation of processing accuracy, surface quality and optical performance. To achieve high-efficiency and high-quality MRF of spinel, a new method of surface modification of spinel with sulfuric acid followed by MRF is proposed. The effect of sulfuric acid modification on the removal efficiency and surface roughness of spinel MRF was studied. Furthermore, the material properties of the spinel surface before and after modification were analyzed by nanoindentation, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Additionally, the mechanism of sulfuric acid etching assisted MRF was investigated. The sulfuric acid modification softens the spinel surface and removes the surface damage layer, and the modified layer can be used as a sacrificial layer to suppress the grain effect. The results showed that the sulfuric acid modification improved the MRF efficiency by 2.46 times, and decreased the peak-valley values of the grain effect and the average residual stress by 69.6% and 54.5%, respectively.