Plasma hot machining for new engineering materials

Abstract A hot-machining technique using plasma jet heating has been employed to improve the machinability of glasses and engineering ceramics such as Pyrex, mullite, alumina, zirconia and silicon nitride, and a sintered high speed steel (HSS). Plasma heating is aimed not only at reducing the hardness of ceramics but also at changing the chip formation from discontinuous to continuous. The turning forces markedly decreased as the workpiece was heated to temperatures above 1050 °C in the case of silicon nitride. An increased heating temperature changed the segmented chips produced in ordinary cutting into continuous ones, accompanied by a better finished surface with fewer defects. Tool wear was also decreased by means of hot machining, e.g. by a factor of 8 in turning silicon nitride with a diamond tool. However, the application to alumina and zirconia did not show significant improvements in machinability. The application to HSS showed general benefits: prevention of chatter, decrease in tool wear by a factor of 2, etc. However, it was critical to select an appropriate heating temperature because of oxidation and segregation of the machined surface. Finally, a new machining technology, which makes full use of surface cracks induced in the workpiece during rapid heating and cooling, is proposed.