Patent ID: 7157394

Claim:
A silicon nitride cutting tool material consisting essentially of a dense crystalline and whisker beta silicon nitride matrix with an interstitial amorphous glass phase that consists of a mixture of silica (SiO 2 ), yttria (Y 2 O 3 ), magnesia (MgO) and zirconia (ZrO 2 ) that exhibits enhanced machining performance when turning gray cast iron parts, wherein (a) the total silica (SiO 2 ) in the material is in an amount greater than 1.8 w/o (weight percent) but less than 3.0 w/o (weight percent); (b) the weight ratio of yttria (Y 2 O 3 ) to magnesia (MgO) in the material is between 2.0 to 1.0 and 1.0 to 2.0; (c) the weight ratio of zirconia (ZrO 2 ) to magnesia (MgO) in the material is between 0.89 to 1.0 and 1.55 to 1.0; (d) having a manufacturing sintering temperature, or post sintering thermal treatment temperature of between 1775° C. and 1800° C.; (e) the amounts of yttria (Y 2 O 3 ), magnesia (MgO) and zirconia (ZrO 2 ) in the material are finitely controlled so that a mixed glass made up of stoichiometric Y 2 Si 2 O 7 , MgSiO 3 and ZrSiO 4 glasses is achieved.