α-SiAlON–SiC composites obtained by gas-pressure sintering and hot-pressing

The objective of this work was the obtaining “in situ” of α-SiAlON–SiC composite, using an alternative rare-earth oxide mixture, RE2O3, as sintering additive, by two different sintering processes. As sintering additive, 20 vol.% of AlN–RE2O3 in a molar ratio of 90:10 was mixed to the α-Si3N4 powder. In the Si3N4–AlN–RE2O3 powder mixture, 0, 10, 15 and 20 wt.% of SiC were added. The powder batches were milled, dried and compacted by cold isostatic pressing. Two different sintering processes were used: gas-pressure sintering at 1950 °C for 1 h under 1.5 MPa of N2 atmosphere, or uniaxial hot-pressing at 1750 °C, for 30 min under pressure of 20 MPa. The sintered samples were characterized by X-ray diffraction, scanning electron microscopy and mechanical properties. XRD patterns indicate only α-SiAlON (α′) and β-SiC as crystalline phases. It was observed that the SiC addition did not influence the α-SiAlON formation, although the growth of elongated α′-grains is substantially decreased. The hot-pressed composites presented better mechanical properties, exhibiting fracture toughness of 5 MPa m1/2 and hardness around 21.5 GPa.