Ceramic material and method of manufacture

Claim 1

A metalcutting insert made by the method of producing a ceramic product, comprising the steps of forming a powder mixture consisting essentially of a first component consisting of compounds containing the elements of silicon, aluminum, oxygen and nitrogen in proportions such that the ratio of the total number of silicon and aluminum atoms to the total number of oxygen and nitrogen atoms lies in the range 0.735 to 0.77 and such that said compounds react together with the second component during the subsequent sintering process to produce a double phase ceramic material wherein a first phase obeys the general formula: Si 6-z Al z O z N 8-z where z is between 0.38 and 1.5; wherein a second phase obeys the general formula (Si, Al) 12 M x (O, N) 16 ; and wherein up to 10 percent by weight of a second component is in the form of an oxide of at least one of the further elements, yttrium, scandium, cerium, lanthanum and the metals of the lanthanide series; and wherein sintering said mixture in a non-reactive environment comprises placing said mixture in a graphite pot and covering with a mixture of boron nitride and silicon nitride powders while surrounding said graphite pot and said silicon nitride and boron nitride powders with a nitrogen gas, with or without the application of pressure, at a temperature between 1600 degrees Centigrade and 2000 degrees Centigrade and for a time, decreasing with increasing temperature, of at least ten minutes to at least five hours so as to produce a ceramic material containing said double phase ceramic material together with a second phase containing said at least one further element, which is a glassy phase.