Ductile-regime grinding of brittle materials

Traditionally, the grinding of hard materials has resulted in accurate contours but a poor quality, fractured surface. Polishing, on the other hand, provides a good surface finish at the expense of contour accuracy. Through recent advances in precision actuation and control, a new form of machining has emerged in which a stiff, accurate grinding machine is controlled in real-time to provide extremely small material removal rates, similar to those encountered in polishing. Under these conditions, the material removal mechanism changes from brittle fracture to plastic deformation, resulting in a much improved surface finish. Research in the Precision Engineering Center at NCSU has characterized the mechanical conditions and material properties that influence the brittle-to-ductile transition. The micro-plasticity of otherwise brittle materials has been modeled in terms of the mechanical properties of the material surface, and quantitative techniques have been developed for the measurement of surface ductility. An experimental apparatus has been designed and built to plunge grind hard materials. Using this device, transitions from brittle to ductile material removal have been demonstrated for a broad range of materials, including single crystals, amorphous glasses, and advanced ceramics. The dependence of grinding ductility on the infeed rate has been established, and a series of exploratory experiments have further delineated the mechanical and material parameters that control the ductility of the material removal regime.