Mechanisms of strain rate effect on the material removal and surface formation of single-crystal silicon in robotic polishing

In robotic polishing, the strain rate effect induced by machining speed considerably affects material removal mechanism and machined surface quality. This study conducted gradient speed polishing experiments and molecular dynamics (MD) simulation (two-grain model) to investigate the surface morphology, material removal behavior, internal stress, temperature and subsurface damage during single-crystal silicon robotic polishing process. The results indicate that increasing the polishing speed can significantly enhance the material removal rate, however, excessive speed may lead to surface quality degradation. As omega(q) increases from 50 to 200 rpm, material removal depth (MRD) increased from 2.777 mu m to 5.151 mu m, approximately 1.8 times, and material removal rate (MRR) rose from 0.276 mm(3) to 0.419 mm(3), about 1.5 times. Notably, the roughness Sa has also increased sharply from 6.149 nm to 18.782 nm, surging more than 3 times. Besides, the thicknesses of the amorphous layer decreased from similar to 63 nm to similar to 36 nm, demonstrating a 45 % reduction. The MD results are consistent with the experimental observations. The results show that as the machining speed of grains rises, the number of the piled-up atoms and side flow ratio increase significantly, indicating a higher machining efficiency. Simultaneously, fewer atoms exhibit large displacement, and the deformation in the subsurface can be more confined to the superficial layer of the workpiece, indicating a reduction in subsurface damage (SSD). Additionally, fewer over-coordinated atoms are remained upon the processed surface, reflecting improved crystalline quality of the finished surface. The internal stress also decreases significantly with the rise in workpiece temperature, which is attributed to the thermal softening effect occurring under high strain rates.