In this study, a novel finishing method, entitled clustered magnetorheological finish (CMRF), was proposed to improve surface finish of the silicon nitride ( ${\mathrm{Si}}_3{\mathrm{N}}_4$ ) balls with ultra fine precision. The effects of different polishing parameters including rotation speeds, eccentricities and the machining gaps on surface finish of ${\mathrm{Si}}_3{\mathrm{N}}_4$ balls were investigated by analyzing the roughness, sphericity and the micro morphology of the machined surface. The experimental results showed that the polishing parameters significantly influenced the surface finish. The best surface finish was obtained by using the polishing parameters: the machining gap of 0.8 mm, the eccentricity of 10 mm and the rotation ratio of 3/4. To further investigate the influence of the polishing parameters on the surface finish, an analytical model was also developed to analyze the kinematics of the ceramic ball during CMRF process. The resulting surface finish, as a function of different polishing parameters employed, was evaluated by analyzing the visualized finishing trace and the distribution of the contact points. The simulative results showed that the distribution and trace of the contact points changed with different polishing parameters, which was in accordance with the results of experiments.

Si₃N₄ ceramic is generally recognized as being difficult to machine due to its hardness and brittleness. It is necessary to control the normal load applied and the machined depth of the abrasive particles in order to eliminate surface/subsurface damage and defects during the grinding or polishing. In this study, scratch experiments were conducted on the polished surface of Si₃N₄ specimens to investigate the brittle–ductile transformation and the evolution of material removal mechanisms. In addition, the cracking behaviour of Si₃N₄ ceramic was characterized by indentation tests. The Vickers indentation produced cracks that exhibited good developmental integrity and geometric symmetry. The results indicate that the scratch track can be divided into three stages: the ductile regime, the brittle–ductile coexisting stage, and the brittle fracture regime. The critical loads and the corresponding penetration depths of cracking occurrence in Si₃N₄ were recorded. The material removal of Si₃N₄ ceramic was primary attributed to ductile regime removal when the load was less than 9.8 N. Microcrack initiation on the subsurface was observed when the penetration depth of the scratch tip reached 8 μm or the depth of the indentation tip reached 3.2 μm. Microcracks expanded rapidly as the load was further increased, resulting in a brittle fracture of the Si₃N₄ ceramic.