Tribochemical polishing of silicon nitride.

摘要

Tribochemical polishing (TCP) is a new polishing technology in which the material removal occurs by a chemical reaction that is stimulated by friction. It does not rely on abrasive particles. Like chemomechanical polishing, it produces defect-free, ultra smooth surfaces and stress-free material with a polishing rate similar to that of dry polishing. Silicon nitride is one of the most widely used ceramics for high performance ball bearings and for passivation or insulating interlayers in microelectronics. TCP of silicon nitride was explored as a function of load, speed, chemical solutions and their concentrations. Over 30 chemicals such as acids, bases, salts, inorganic and organic oxidizers were studied. Distilled water shows high polishing rates but some solid wear debris are produced by surface fracture. Two effective polishing fluids have been identified. Hydrogen peroxide in some concentrations produces high polishing rates and pure tribochemical behavior. Chromic acid in all concentrations produces the best results. Optical, electron and atomic force microscopy reveal that the tribochemically polished surfaces possess ultra smooth and defect-free surfaces: The surface roughness is Ra {dollar}le{dollar} 5 A on a 50 {dollar}mu{dollar}m cut-off length and Ra {dollar}leq{dollar} 20 A at 150 {dollar}mu{dollar}m. The surface residual stress of tribochemically polished surfaces was measured by X-ray diffraction and found to be compressive at 50 MPa (7.3 Ksi). The fracture strength was measured by biaxial stress method: No degradation of the fracture strength was found. The chemical reaction was examined: The presence of ammonia and silicic acid as reaction products was verified by chemical titration and micro-FTIR. The tribochemically polished surface was found free of contamination by RBS, SIMS and XPS. Angle resolved XPS shows that the surface consists of a monolayer of adventitious carbon, 2-5 A of SiO{dollar}sb2{dollar}, 10-15 A of SiO{dollar}sb{lcub}rm x{rcub}{dollar}N{dollar}sb{lcub}rm y{rcub}{dollar} and the Si{dollar}sb3{dollar}N{dollar}sb4{dollar} substrate. This layer results from storage in air. In the polishing of large area samples, the polishing rate is about 3-4 nm/min. Silicon nitride, stainless steel and cast iron are effective tool materials, therefore, this technique is easily adaptable to standard polishing equipments.