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A SiAlON ceramic body made from a starting powder mixture that includes silicon nitride powder and one or more powders that provide aluminum, oxygen, nitrogen, and two selected rare earth elements to the SiAlON ceramic body wherein the selected rare earth elements are selected from at least two grou

A SiAlON ceramic body made from a starting powder mixture that includes silicon nitride powder and one or more powders that provide aluminum, oxygen, nitrogen, and two selected rare earth elements to the SiAlON ceramic body wherein the selected rare earth elements are selected from at least two groups of the following three groups of rare earth elements wherein Group I comprises La, Ce, Pr, Nd, Pm, Sm and Eu, and Group II comprises Gd, Tb, Dy and Ho, and Group III comprises Er, Tm, Yb and Lu. The SiAlON ceramic body includes a two phase composite that includes an alpha prime SiAlON phase and a beta prime SiAlON phase wherein the alpha prime SiAlON phase contains one or more of the selected rare earth elements excluding La and Ce. The silicon nitride powder makes up at least about 70 weight percent of the starting powder mixture wherein the beta-silicon nitride content of the silicon nitride powder has a lower limit equal to zero weight percent and an upper limit equal to about 1.6 weight percent of the silicon nitride powder.

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What is claimed is: 1. A SiAlON ceramic body made from a starting powder mixture that includes silicon nitride powder and one or more powders that provide aluminum, oxygen, nitrogen, ytterbium and lanthanum, and the SiAlON ceramic body comprising: a two phase composite comprising an alpha prime SiA

What is claimed is: 1. A SiAlON ceramic body made from a starting powder mixture that includes silicon nitride powder and one or more powders that provide aluminum, oxygen, nitrogen, ytterbium and lanthanum, and the SiAlON ceramic body comprising: a two phase composite comprising an alpha prime SiAlON phase and a beta prime SiAlON phase, and the alpha prime SiAlON phase contains ytterbium wherein there is no detectable amount of lanthanum in the alpha prime SiAlON phase; and the silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture. 2. The SiAlON ceramic body of claim 1 wherein the beta-silicon nitride content of the silicon nitride powder has a lower limit equal to zero weight percent and an upper limit equal to about 1.6 weight percent of the silicon nitride powder. 3. The SiAlON ceramic body of claim 1 wherein the silicon nitride powder in the starting powder mixture contains about zero weight percent beta silicon nitride. 4. A SiAlON ceramic body made from a starting powder mixture that includes silicon nitride powder and one or more powders that provide aluminum, oxygen, nitrogen, at least one of La and Ce and at least one selected rare earth element to the SiAlON ceramic body wherein the selected rare earth element is selected from at least one group of the following two groups of rare earth elements wherein Group II comprises Gd, Tb, Dy and Ho, and Group III comprises Er, Tm, Yb and Lu, and the SiAlON ceramic body comprising: a two phase composite comprising an alpha prime SiAlON phase and a beta prime SiAlON phase, and the alpha prime SiAlON phase containing one or more of the selected rare earth elements wherein there being no detectable amount of either La or Ce in the alpha prime SiAlON phase; and the silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture. 5. The SiAlON ceramic body of claim 4 wherein the Vickers hardness (18.5 kg load) is greater than or equal to about 17 GPa, and the fracture toughness (KIC Evans & Charles) is greater than or equal to about 6.8 MPa쨌m1/2. 6. The SiAlON ceramic body of claim 4 wherein the Vickers hardness (18.5 kg load) is greater than or equal to about 18.5 GPa, and the fracture toughness (KIC Evans & Charles) is greater than or equal to about 6.8 MPa쨌m1/2. 7. The SiAlON ceramic body of claim 4 wherein the alpha prime SiAlON phase is presenting an amount greater than or equal to about 35 weight percent of the two phase composite, Vickers hardness (18.5 kg load) is greater than or equal to about 18 GPa, and the fracture toughness (KIC Evans & Charles) is greater than or equal to about 6.8 MPa쨌m1/2. 8. The SiAlON ceramic body of claim 4 wherein the alpha prime SiAlON phase is presenting an amount greater than or equal to about 50 weight percent of the two phase composite. 9. The SiAlON ceramic body of claim 4 wherein the silicon nitride starting powder comprise at least about 80 weight percent of the starting powder mixture, the alpha prime SiAlON phase being present in an amount greater than or equal to about 25 weight percent of the two phase composite, the Vickers hardness (18.5 kg load) being greater than or equal to about 17 GPa, and the fracture toughness (KIC Evans & Charles) being greater than or equal to about 6.8 MPa쨌m1/2. 10. The SiAlON ceramic body of claim 4 wherein the silicon nitride starting powder comprise at least about 85 weight percent of the starting powder mixture, the alpha prime SiAlON phase being present in an amount greater than or equal to about 35 weight percent of the two phase composite, the Vickers hardness (18.5 kg load) being greater than or equal to about 18 GPa, and the fracture toughness (KIC Evans & Charles) being greater than or equal to about 6.8 MPa쨌m1/2. 11. The SiAlON ceramic body of claim 4 wherein the silicon nitride starting powder contains about zero weight percent beta silicon nitride. 12. The SiAlON ceramic body of claim 4 wherein the alpha prime SiAlON phase being present in an amount greater than or equal to about 25 weight percent of the two phase composite the Vickers hardness (18.5 kg load) being greater than or equal to about 16.5 GPa, and the fracture toughness (KIC Evans & Charles) is greater than or equal to about 5.4 MPa쨌m2. 13. The SiAlON ceramic body of claim 4 wherein the Vickers hardness (18.5 kg load) being greater than or equal to about 16.0 GPa, and the fracture toughness (KIC Evans & Charles) is greater than or equal to about 4.4 MPa쨌m 1/2. 14. The SiAlON ceramic body of claim 4 further including a coating scheme on the SiAlON ceramic body comprising one or more coating layers. 15. The SiAlON ceramic body of claim 14 wherein the coating scheme includes a coating layer of alumina applied by chemical vapor deposition. 16. The SiAlON ceramic body of claim 14 wherein the coating scheme includes a coating layer of titanium aluminum nitride applied by physical vapor deposition. 17. The SiAlON ceramic body of claim 4 wherein the ceramic body having a composition falling within the area bounded by the trapezoid defined by points A, B, C, and D in FIG. 7. 18. The SiAlON ceramic body of claim 4 wherein the body comprises a cutting insert having a rake surface and a flank surface, and a cutting edge at the juncture of the rake surface and the flank surface. 19. The SiAlON ceramic body of claim 4 wherein the starting powder mixture further provides a liquid phase-forming element, and the liquid phase-forming element is selected from the group comprising one or more of Ba, Mg, Sr and Sc. 20. The SiAlON ceramic body of claim 19 wherein the ceramic body having a composition falling within the area bounded by the trapezoid defined by points A, B, C and D in FIG. 7. 21. The SiAlON ceramic body of claim 4 wherein the beta-silicon nitride content of the silicon nitride powder has a lower limit equal to zero weight percent and an upper limit equal to about 1.6 weight percent of the silicon nitride powder. 22. A SiAlON ceramic body made from a starting powder mixture that includes silicon nitride powder and one or more powders that provide aluminum, oxygen, nitrogen, and at least one of La and Ce and at least one selected rare earth element to the SiAlON ceramic body wherein the selected rare earth element is selected from at least one group of the following two groups of rare earth elements wherein Group II comprises Gd, Tb, Dy and Ho, and Group III comprises Er, Tm, Yb and Lu, and the SiAlON ceramic body comprising:: a two phase composite comprising an alpha prime SiAlON phase and a beta prime SiAlON phase, and the alpha prime SiAlON phase containing one or more of the selected rare earth wherein there being no detectable amount of either La or Ce in the alpha prime SiAlON phase; the ceramic body having a composition falling within the space bounded by the trapezoid defined by points A, B, C and D in FIG. 7; and the silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture. 23. The SiAlON ceramic body of claim 22 wherein the beta-silicon nitride content of the silicon nitride powder has a lower limit equal to zero weight percent and an upper limit equal to about 1.6 weight percent of the silicon nitride powder. 24. The SiAlON ceramic body of claim 22 wherein the silicon nitride powder in the starting powder mixture contains about zero weight percent beta silicon nitride. 25. A SiAlON ceramic body made from a starting powder mixture that includes silicon nitride powder and one or more powders that provide aluminum, oxygen, nitrogen, ytterbium and cerium, and the SiAlON ceramic body comprising: a two phase composite comprising an alpha prime SiAlON phase and a beta prime SiAlON phase, and the alpha prime SiAlON phase contains ytterbium wherein there is no detectable amount of cerium in the alpha prime SiAlON phase; and the silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture. 26. The SiAlON ceramic body of claim 25 wherein the beta-silicon nitride content of the silicon nitride powder has a lower limit equal to zero weight percent and an upper limit equal to about 1.6 weight percent of the silicon nitride powder. 27. The SiAlON ceramic body of claim 25 wherein the silicon nitride powder in the starting powder mixture contains about zero weight percent beta silicon nitride.