Self-Reinforced Nitrogen-Rich Calcium–α-SiAlON Ceramics

Nitrogen-rich Ca–α-SiAlON ceramics with nominal compositions Ca _x Si_{12?2 x} Al_{2 x} N₁₆ and 0.2≤ x ≤2.6, extending along the Si₃N₄–1/2Ca₃N₂:3AlN tie line, were prepared from Si₃N₄, AlN, and CaH₂ precursors by hot pressing at 1800°C. The x values attained were determined by energy-dispersive X-ray (EDX) microanalysis and X-ray powder diffraction (XRPD) data using the Rietveld method. The results show that Ca–α-SiAlONs form continuously within the compositional range x =0 to at least x =1.82. Phase assemblages, lattice parameters, Vickers hardness, and fracture toughness were determined and correlated to the calcium content, x . Owing to a high sintering temperature and the use of CaH₂ as a precursor, grain growth was kinetically enhanced, resulting in self-reinforced microstructures with elongated grains. The obtained Ca–α-SiAlON ceramics demonstrate a combination of both high hardness ～21 GPa, and high fracture toughness ～5.5 MPa·m^1/2.