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A SiAlON material that includes a two phase composite of an alpha prime SiAlON phase and a beta prime SiAlON phase. The alpha prime phase contains ytterbium. The alpha prime SiAlON phase being present in an amount between about 25 weight percent and about 85 weight percent of the two phase composite

A SiAlON material that includes a two phase composite of an alpha prime SiAlON phase and a beta prime SiAlON phase. The alpha prime phase contains ytterbium. The alpha prime SiAlON phase being present in an amount between about 25 weight percent and about 85 weight percent of the two phase composite. The SiAlON material further includes an intergranular phase.

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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, and a rare earth to the SiAlON ceramic body, the SiAlON ceramic body comprising:a two phase composite comprising an alpha prime SiAlON ph

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, and a rare earth to the SiAlON ceramic body, 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 the rare earth therein; the alpha prime SiAlON phase being present in an amount greater than or equal to about 20 weight percent of the two phase composite; the starting silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture, the silicon nitride powder in the starting powder mixture contains beta-silicon nitride powder wherein the beta-silicon nitride powder comprises less than or equal to about 1.6 weight percent of the starting silicon nitride powder; and the SiAlON ceramic body having a Vickers hardness (18.5 Kg load) equal to or greater than about 16.5 GPa, and a fracture toughness (KIC) equal to or greater than about 5.5 MPam1/2. 2. The SiAlON ceramic body of claim 1 wherein at least sonic of a silicon component of the alpha prime SiAlON phase originates form the silicon nitride powder in the starting powder mixture, and at least some of the silicon component of the beta prime SiAlON phase originates from the silicon nitride powder in the starting powder mixture.3. The SiAlON ceramic body of claim 1 having a Vickers hardness (18.5 Kg load) equal to at least about 17.5 GPa and a fracture toughness as measured by the Evans & Charles method equal to at least about 6.0 MPa·m1/2.4. The SiAlON ceramic body of claim 1 having a Vickers hardness (18.5 Kg load) equal to at least about 18.5 GPa and a fracture toughness as measured by the Evans & Charles method equal to at least about 7.0 MPa·m1/2.5. The SiAlON ceramic body of claim 1 having a fracture toughness as measured by the Evans & Charles method equal to at least about 8.0 MPa·m1/2.6. The SiAlON ceramic body of claim 1 wherein the beta prime SiAlON phase being of the formula Si6-zAlz OzN8-z wherein z is greater than 0.3 and less than 1.5.7. The SiAlON ceramic body of claim 1 wherein z is greater than 0.7 and less than 1.5.8. The SiAlON ceramic body of claim 1 wherein z is greater than 0.3 and less than 0.6.9. The SiAlON ceramic body of claim 1 wherein the alpha prime SiAlON phase being present in an amount between about 60 weight percent and about 80 weight percent of the two phase composite.10. The SiAlON ceramic body of claim 1 wherein the alpha prime SiAlON phase being present in an amount between about 45 weight percent and about 85 weight percent of the two phase composite.11. 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 a rare earth to the SiAlON ceramic body, 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 the rare earth therein; the alpha prime SiAlON phase being present in an amount greater than or equal to about 20 weight percent of the two phase composite; the starting silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture; the SiAlON ceramic body having a Vickers hardness (18.5 Kg load) equal to or greater than about 16.5 GPa, and a fracture toughness (KIC) equal to or greater than about 5.5 MPam1/2; and the silicon nitride starting powder having an amount of beta silicon nitride equal to about zero. 12. The SiAlON ceramic body of claim 1 wherein the SiAlON ceramic body further includes an integranular phase comprising one or more of a glassy phase and an integranular crystalline phase.13. The SiAlON ceramic body of claim 1 wherein the rare earth is selected from the group consisting of ytterbium, erbium, thulium, scandium and lutetium.14. The SiAlON ceramic body of claim 1 wherein the rare earth comprises ytterbium.15. A ceramic body made from a starting powder mixture that includes silicon nitride powder, aluminum nitride powder, alumina powder, and ytterbium oxide powder, the ceramic body comprising:a two phase composite comprising alpha prime SiAlON phase and beta prime SiAlON phase, and the alpha prime SiAlON phase containing ytterbium therein; the alpha prime SiAlON phase being present in an amount between about 25 weight percent and about 50 weight percent of the two phase composite; the sintered ceramic body having a fracture toughness (KIC) of greater than or equal to about 6.0 MPam1/2; the sintered ceramic body having a Vickers hardness (18.5 kg load) of greater than or equal to about 17.0 GPa, and the starting silicon nitride powder has less than or equal to about 1.6 weight percent beta-silicon nitride. 16. The ceramic body of claim 15 wherein the content of the starting alumina powder being greater than the content of the starting aluminum nitride powder.17. The ceramic body of claim 15 wherein the starting silicon nitride powder has an amount of beta silicon nitride equal to about zero.18. The ceramic body of claim 15 having a fracture toughness as measured by the Evans & Charles method equal to at least about 7.0 MPa·m1/2.19. The ceramic body of claim 15 having a fracture toughness as measured by the Evans & Charles method equal to at least about 8.0 MPa·m1/2.20. The ceramic body of claim 15 wherein the beta prime SiAlON phase being of the formula Si6-zAlzOzN8-zwherein z is greater than 0.3 and less than 1.5.21. The ceramic body of claim 20 wherein z is greater than 0.7 and less than 1.5.22. The ceramic body of claim 20 wherein z is greater than 0.3 and less than 0.6.23. A ceramic body made from a starting powder mixture that includes silicon nitride powder, ytterbium oxide powder, and at least one or more powders together containing aluminum, oxygen and nitrogen, the ceramic body comprising:a two phase composite comprising alpha prime SiAlON phase and beta prime SiAlON phase, and the alpha prime phase containing ytterbium therein; the alpha prime SiAlON phase being present in an amount greater than or equal to about 20 weight percent of the two phase composite; at least some of a silicon component of the alpha prime SiAlON phase originates from the silicon nitride powder in the starting powder mixture, and at least some of the silicon component of the beta prime SiAlON phase originates from the silicon nitride powder in the starting powder mixture; the starting silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture, the silicon nitride powder in the starting powder mixture contains beta-silicon nitride powder wherein the beta-silicon nitride powder comprises less than or equal to about 1.6 weight percent of the starting silicon nitride powder; and the SIALON ceramic body having a Vickers hardness (18.5 Kg load) equal to or greater than about 16.5 Gpa, and a fracture toughness (KIC) equal to or greater than about 5.5 Mpam1/2. 24. A ceramic body made from a starting powder mixture that includes silicon nitride powder, ytterbium oxide powder, and at least one or more powders together containing aluminum, oxygen and nitrogen, the ceramic body comprising:a two phase composite comprising alpha prime SiAlON phase and beta prime SiAlON phase, and the alpha prime phase containing ytterbium therein; the alpha prime SiAlON phase being present in an amount greater than or equal to about 20 weight percent of the two phase composite; at least some of a silicon component of the alpha prime SiAlON phase originates form the silicon nitride powder in the starting powder mixture, and at least some of the silicon component of the beta prime SiAlON phase originates from the silicon nitride powder in the starting powder mixture; the starting silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture; the silicon nitride starting powder having an amount of beta silicon nitride equal to about zero; and the SiAlON ceramic body having a Vickers hardness (18.5 Kg load) equal to or greater than about 16.5 Gpa, and a fracture toughness (KIC) equal to or greater than about 5.5 Mpam1/2. 25. The ceramic body of claim 23 wherein the SiAlON ceramic body further includes an intergranular phase comprising one or more of a glassy phase and an intergranular crystalline phase.26. The SiAlON ceramic body of claim 1 further including a coating scheme on the SiAlON ceramic body comprising one or more coating layers.27. The SiAlON ceramic body of claim 26 wherein the coating scheme includes a coating layer of alumina applied by chemical vapor deposition.28. The SiAlON ceramic body of claim 27 wherein the alumina coating layer is applied on the SiAlON ceramic body.29. The SiAlON ceramic body of claim 26 wherein the coating scheme includes a coating layer of titanium aluminum nitride applied by physical vapor deposition.30. The SiAlON ceramic body of claim 11 wherein the rare earth is selected from the group comprising ytterbium, erbium, thulium, scandium and lutetium.31. 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 or more rare earths selected from the group comprising ytterbium, erbium, thulium, scandium and lutetium to the SiAlON ceramic body, the SiAlON ceramic body comprising:a two phase composite consisting of an alpha prime SiAlON phase and a beta prime SiAlON phase, and the alpha prime SiAlON phase containing the rare earth therein; the alpha prime SiAlON phase being present in an amount greater than or equal to about 20 weight percent of the two phase composite; the starting silicon nitride powder comprises at least about 70 weight percent of the starting powder mixture, the silicon nitride powder in the starting powder mixture contains beta-silicon nitride powder wherein the beta-silicon nitride powder comprises less than or equal to about 1.6 weight percent of the starting silicon nitride powder; and the SiAlON ceramic body having a Vickers hardness (18.5 Kg load) equal to or greater than about 16.5 GPa, and a fracture toughness (KIC) equal to or greater than about 3.5 MPam1/2. 32. The SiAlON ceramic body of claim 31 wherein the silicon nitride starting powder has an amount of beta silicon nitride equal to about zero.33. The SiAlON ceramic body of claim 32 wherein the rare earths comprise two or more of the rare earths selected from the group consisting of ytterbium, erbium, thulium, scandium and lutetium.34. The ceramic body of claim 31 further including a coating scheme on the ceramic body comprising one or more coating layers.35. The ceramic body of claim 34 wherein the coating scheme includes a coating layer of alumina applied by chemical vapor deposition.36. The ceramic body of claim 35 wherein the alumina coating layer is applied on the ceramic body.37. The ceramic body of claim 34 wherein the coating scheme includes a coating layer of titanium aluminum nitride applied by chemical vapor deposition.