Hard sialon ceramics reinforced with SiC nanoparticles

SiC nanoparticles were added to a range of rare-earth doped Ln-sialon (Ln=Nd, Dy, Yb) ceramics to prepare a series of increased hardness sialon/SiCp composites. The composites, of overall composition Ln0.33Si9.3Al2.7O1.7N14.3 and containing 10 and 20 wt.% of SiC particles, were sintered by hot-pressing at 1800°C and then heat-treated at 1450°C to modify the microstructure and properties by α⇔β sialon transformation. Enhanced hardness was found in proportion to the content of SiC particles and with increasing α:β sialon phase ratio in the matrix. The kind of grain boundary phases which occurred also affected the results. In Nd-densified samples with or without SiC particles, the α-sialon phase was unstable under the heat-treatment conditions used and transformed substantially to β-sialon in elongated grain morphology, with Nd-melilite phase as the dominant grain boundary crystalline phase. These materials were tough with a highest indentation fracture toughness K1C of 7.0 MPa m1/2. In contrast, Dy- and Yb-densified samples with or without SiC addition showed good stability for the α-sialon phase under the same conditions, with only a small amount of β-sialon phase produced and Dy- or Yb-garnet as the main grain boundary phase. These composites were hard and brittle with a maximum Vickers hardness HV10 of 20 GPa.