Machinability and brittleness of glass-ceramics

The relationship between the machinability and the brittleness of glass-ceramic materials is investigated. A brittleness index (B), given by the ratio of the hardness to the fracture toughness, is proposed as a parameter for estimating the machinability. This approach is confirmed by considering experimental data from the literature on turning operations of mica-containing glass-ceramics. It is shown that machinability parameters, such as the slope of the log-log plot of the specific cutting energy versus the cutting rate, or the specific cutting energy al low cutting rates, are in good agreement with the brittleness indices for seven different glass-ceramics. In order for a glass-ceramic to be machinable, it was found that the brittleness index of the material should be lower than B approximate to 4.3 mu m(-1/2). (C) 1997 Elsevier Science S.A.