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 u nder vacuum. 2H-wurtzite SiC-AlN solid-solution structures were formed for compositions with 25-100 mol% AlN. The associated lattice parameters for th ese 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 the oretical 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 ra nge. 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 deg rees, and 1250 degrees C, respectively. These three properties correlated l inearly 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 to ughness 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.