Performance of Ceramic Cutting Tool Materials in Dependence on their Specific Properties.

Pure oxide (Al2O3) ceramic was considered for machining operations in Germany by Osenberg as early as 1930. The properties of these materials have been improved since that time by optimization of chemical composition and processing techniques in order to meet the stringent mechanical requirements of metal cutting. Mowadays, ceramic tools cover a wide field of application as turning or face milling of steel and cast iron. The following ceramic tool materials are of importance in manufacturing engineering today: aluminium oxide with additives of titanium carbide and titanium nitride, hot-pressed silicon nitride. SiC-whisker reinforced aluminium oxide and Si-Al-O-N. Because ceramics are hard and brittle, fracture resulting from structural defects is more likely than in metallic materials. The complex crystalline structure with high lattice energies prevents healing processes such as flowing or creeping (1). Accordingly, stress maxima at crack tips are not reduced by plastic flow, so the speed of crack propagation increases. The reliability of cutting process can be enhanced if critical application conditions and critical loadings on the tool are recognized and avoided. Development work led to increasing fracture toughness of ceramic tools. In the class of Al2O3-ceramics this could be achieved by addition of ZrO2. The zierconia phase helps to retard crack propagation by transformation from a metastable state to a stable state when a crack is initiated. That phase transformation is associated with volume change, causing compressive stresses at the crack tip and preventing propagation (2).  Introduction of the metallic phases TiN and TiC provides adequate edge strength and high resistance of thermal shock. The addition of SiC-whiskers in the brittle aluminium matrix results in higher strength an improvement in the fracture toughness. Silicon nitride based tools were developed in the late 1970s. These tool materials possess higher fracture toughness, that make it less prone to catastophic failure in a cutting process. The high thermal shock resistance of Si3N4 is a result of their good thermal conductivity and low coefficient of thermal expansion (Fig. 1).