Abstract: A method is disclosed comprising placing in a hot pressing environment a controlled mixture of a first powder of Si.sub.3 N.sub.4 containing a controlled amount of SiO.sub.2 (with or without a limited amount of milling media) with a second powder consisting essentially of Y.sub.2 O.sub.3, hot pressing the controlled mixture, and cooling the resultant unitary object.A novel silicon nitride comprising a unitary object is disclosed which can be shaped to form a novel cutting tool useful for machining metal such as cast iron. The object has selected crystalline phases formed beta Si.sub.3 N.sub.4 and beta prime Si.sub.3 N.sub.4 and has intergranular crystalline secondary phases consisting essentially of YSiO.sub.2 N, Y.sub.10 Si.sub.17 N.sub.4 O.sub.23 and Y.sub.2 Si.sub.2 O.sub.7 with the YSiO.sub.2 N phase representing at least a 25% volume fraction of the secondary phases and comprising essentially all of the yittria-silicon oxide.

Abstract: A method is disclosed comprising placing in a hot pressing environment a controlled mixture of a first powder of Si.sub.3 N.sub.4 containing a controlled amount of SiO.sub.2 (with or without a limited amount of milling media) with a second powder consisting essentially of Y.sub.2 O.sub.3, hot pressing the controlled mixture, and cooling the resultant unitary object.A novel silicon nitride comprising a unitary object is disclosed which can be shaped to form a novel cutting tool useful for machining metal such as cast iron. The object has selected crystalline phases formed beta Si.sub.3 N.sub.4 and beta prime Si.sub.3 N.sub.4 and as intergranular crystalline secondary phases consisting essentially of YSiO.sub.2 N, y.sub.10 Si.sub.17 N.sub.4 O.sub.23 and Y.sub.2 Si.sub.2 O.sub.7 with the YSiO.sub.2 N phase representing at least a 25% volume fraction of the secondary phases and comprising essentially all of the yittria-silicon oxide.

Abstract: A method of machining cast iron by using a ceramic cutting tool of specific composition at specified removal rates which results in increased tool life.

Abstract: A process for machine cutting of cast iron with a shaped tool which is a ceramic having as constituent ingredients about 4 to about 12% by weight Y.sub.2 O.sub.3 and a balance consisting essentially of Si.sub.3 N.sub.4 and which has a density of at least 3.25 gm./cm.sup.3.

Abstract: A method is disclosed for removing stock from a metal part comprised of steel, malleable cast iron or nodular cast iron. A moving relationship is established between the part and a cutting tool formed of a substantially full dense hot pressed ceramic material consisting essentially of a generally uniform mixture of 50-70% by weight Al.sub.2 O.sub.3 and the remainder substantially Si.sub.3 N.sub.4. The tool is advanced into the surface of the part at a predetermined depth of cut and feed rate which results in the removal of stock from the part during periods of relative movement. A portion of the Al.sub.2 O.sub.3 may be replaced by up to 15% stabilized ZrO.sub.2 or HfO.sub.2 to further improve strength at higher temperatures.

Abstract: A process for continuous or interrupted machine cutting of solid cast iron by milling, turning or boring with a shaped tool, wherein said shaped tool is a ceramic consisting essentially of Si.sub.3 N.sub.4 and 4-12% by weight Y.sub.2 O.sub.3, produced by hot pressing at a pressure and a temperature of 1700.degree.-1750.degree. C. to achieve substantially full density.