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A composite article and cutting tool are prepared by densification to form a body consisting essentially of particles of hard refractory material uniformly distributed in a matrix consisting essentially of a first phase and a second phase, said first phase consisting essentially of crystalline silic

A composite article and cutting tool are prepared by densification to form a body consisting essentially of particles of hard refractory material uniformly distributed in a matrix consisting essentially of a first phase and a second phase, said first phase consisting essentially of crystalline silicon nitride and said second phase being an intergranular refractory phase comprising silicon nitride and a suitable densification aid selected from the group consisting of yttrium oxide, zirconium oxide, hafnium oxide and the lanthanade rare earth oxides and mixture thereof.

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A method of machining cast iron comprising turning a cast iron work piece on a lathe at an effective speed of up to 1000 surface feet per minute; moving a densified composite ceramic cutting tool consisting essentially of particles of hard refractory material uniformly distributed in a two phase mat

A method of machining cast iron comprising turning a cast iron work piece on a lathe at an effective speed of up to 1000 surface feet per minute; moving a densified composite ceramic cutting tool consisting essentially of particles of hard refractory material uniformly distributed in a two phase matrix, said two phase matrix consisting essentially of a first phase and a second intergranular phase, said particles of hard refractory material being present in an amount from about 1 to about 60 volume percent of said densified composite ceramic cutting tool, said particles of hard refractory material selected from the group consisting of the carbides, nitrides, and mixtures thereof of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, and combinations thereof, said particles having an average size of less than about 20 microns, said first phase consisting essentially of crystalline silicon nitride, said second intergranular phase consisting essentially of silicon nitride and a densification aid, said densification aid selected from the group consisting of yttrium oxide, hafnium oxide, the lanthanide rare earth oxides, and mixtures thereof, wherein said densification aid comprises from about 1 to about 25 weight percent of said two phase matrix, said two phase matrix containing less than 3 weight percent aluminum oxide, said densified composite ceramic article having a high density, a high abrasion resistance, a high hardness, a high fracture strength and a resistance to oxidation at temperatures greater than 1200°C. across the face of said cast iron workpiece at a rate of up to 0.020 inches per revolution; and cutting said cast iron workpiece with said densified composite ceramic cutting tool to effect a depth of cut of up to 0.050 inches per pass.