Towards Understanding and Restraining the Mechanical Relaxation Effect in Polishing Silicon Carbide with a Detachable Bonnet Tool

Bonnet polishing has shown promising shape adaptability and processing quality in the surface finishing of glass-ceramic optical components with complex surfaces. This study proposed a new robotic bonnet polishing device with a detachable polishing cloth to realize relatively high polishing accuracy on sintered silicon carbide (S-SiC) with low cost and pollution. During polishing, the contact state between the secondary-thermoformed polishing cloth and workpiece was found to follow a complex force curve, including a short-term force drop caused by quasi-stress relaxation and a long-term force rise caused by creep. Therefore, a predictive model was established to modify the removal efficiency error induced by force fluctuation and to estimate the remaining service time. On this basis, a new approach was proposed to accelerate the initial-stage creep and restrain the force drop during polishing via temperature control. The results obtained in tool influence function and form correction experiments proved that the temperature threshold to suppress the stress relaxation effect of the polishing pad is around 45 degrees C. Meanwhile, the pre-treatment time of the pad can be compressed by more than half at this temperature to maintain the high determinacy and efficiency of the polishing process. After polishing, almost no subsurface damage was detected near the surface of the S-SiC block. However, due to the impact of original micropores and the scratches caused by the abrasive aggregation effect, the roughness is challenging to continue decreasing after Ra reaches 3 similar to 4 nm.