Grinding energy and mechanisms for ceramics
A technological basis for efficient ceramic machining requires a fundamental understanding of the prevailing grinding mechanisms. Most past research on grinding mechanisms for ceramics has followed either the "indentation fracture mechanics" approach or the "machining" approach. The indentation fracture mechanics approach likens abrasive workpiece interactions to idealized small-scale indentations. The machining approach typically involves measurement of cutting forces together with microscopic observations of grinding debris and surfaces produced. Both approaches provide important insights into the grinding mechanisms for ceramic materials. However, up to now, no physical model has been presented which can quantitatively account for the energy associated with grinding of ceramics.
The present research was undertaken to investigate grinding mechanisms for ceramics and to account for the energy expended. SEM observations of grinding debris for various ceramics and a glass over a wide range of conditions indicate material removal mainly by brittle fracture associated with lateral cracking and crushing. However, the ground surfaces reveal extensive ductile flow with characteristic scratches along the grinding direction and smearing. Ductile flow typically extends to a depth of 1-5 $\mu$m below the ground surface. For silicon nitride, etching with hydrofluoric acid removed the smeared layer, which would indicate that it consists of a glassy phase probably formed by oxidation at elevated grinding temperatures.
Although material removal appears to occur mainly by brittle fracture, most of the grinding energy is apparently associated with ductile flow. An order of magnitude analysis indicates that the energy expended by brittle fracture constitutes a negligible portion of the total grinding energy. An upper bound plowing analysis is presented which can account the specific energy in terms of the geometry of the plowed groove. A new model has been developed which relates the grinding power to the rate of plowed surface area generated by the diamond cutting points on the wheel surface interacting with the workpiece. Over a wide range of grinding conditions, the power increases approximately proportionally with the rate of surface area generated, which suggests a nearly constant energy per unit area of plowed surface. Values obtained for energy per area for plowing are much bigger than the corresponding fracture surface energies, which further indicates that most of the grinding energy is associated with ductile flow.
0794: Materials science