YTTERBIUM-STABILIZED ALPHA-SIALON CERAMICS

Dense, Yb-doped alpha-sialon ceramics of the composition YbxSi12-4.5xA l4.5xO1.5xN16-1.5x, with 0.2 less than or equal to X less than or equa l to 1.2, together with some Yb-based alpha- and alpha-beta-sialon cer amics containing an excess of glassy phase, have been prepared by hot- pressing at 1800 degrees C. These materials were subsequently heat-tre ated in the temperature range 1150-1750 degrees C for different period s. It was found that Yb-doped alpha-sialon forms within a wide composi tion region, with x ranging from 0.3 to more than 1.0, and that a-sial on is stable over a large temperature interval and during heat treatme nt times of up to 30 days. The alpha-sialon phase coexists with a liqu id phase at T greater than or equal to 1550 degrees C and with crystal line phases such as the Yb garnet and/or J phase at T less than or equ al to 1450 degrees C. Further heat treatment at 1450 degrees C reveale d that initially (during the first 24 h) the alpha-sialon phase reacts with the residual liquid grain-boundary phase, yielding a material wh ich contains an Yb-stabilized garnet phase, besides the alpha-sialon p hase, but no glassy phase. This phase assembly is stable upon further heat treatment (30 days). Samples containing Yb-doped alpha-sialon in conjunction with an amorphous intergranular phase show very high hardn ess (H-v10 = 22 Gpa) and fracture toughness in the range of 4.2 MPa m( 1/2) at room temperature, and mechanical data in the same range were o btained for samples exposed to prolonged heat treatment at 1450 degree s C. Elongated cr-sialon grains were found in samples with high x valu es, but their presence did not affect the mechanical properties signif icantly.