Microstructure-property relations of hot-pressed silicon carbide-aluminum nitride compositions at room and elevated temperatures

A series of SiC-AlN compositions of 0, 10, 25, 50, 75, 90, and 100 mol% AlN were hot pressed at 2100 degrees C for a 1 h soak ata pressure of 35 MPa under vacuum. 2H-wurtzite SiC-AlN solid-solution structures were formed for compositions with 25-100 mol% AlN. The associated lattice parameters for these solid solutions followed Vegard's law, The microstructures varied with composition; the number of needlelike grains decreased for compositions up to 25 mol% AlN and the amount of equiaxed grains increased for compositions with 25-100 mol% AlN. Densities for all the specimens were >99% of the theoretical density. Coefficients of thermal expansion varied from 4.80 x 10(-6)/degrees C to 6.25 x 10(-6)/degrees C in the 20 degrees-1400 degrees C range. Young's moduli varied from 451 GPa to 320 GPa at room temperature (RT) and retained 98%, 96%, and 94% of their RT values at 500 degrees, 1000 degrees, and 1250 degrees C, respectively. These three properties correlated linearly with composition, RT microhardness varied from 21.6 GPa to 11.2 GPa and correlated linearly with composition within the solid-solution range. Flexural strengths increased from 487 MPa to 604 MPa from 0 mol% AlN to 25 mol% AIN and then decreased to 284 MPa for 100 mol% AlN, At 1250 degrees C, flexural strengths decreased from 90% to 65% of the RT values. Fracture toughness increased from 3.6 MPa.m(1/2) to 4.2 MPa.ml(1/2) from 0 mol% AlN to 10 mol% AlN and then decreased to 2.5 MPa.m(1/2) for 100 mol% AlN.