Nitrogen-rich Ca-a-SiAlON ceramics with nominal compositions Ca x Si 12À2x Al 2x N 16 and 0.2rxr2.6, extending along the Si 3 N 4 -1/2Ca 3 N 2 :3AlN tie line, were prepared from Si 3 N 4 , AlN, and CaH 2 precursors by hot pressing at 18001C. 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-a-SiAlONs form continuously within the compositional range x 5 0 to at least x 5 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 CaH 2 as a precursor, grain growth was kinetically enhanced, resulting in self-reinforced microstructures with elongated grains. The obtained Ca-a-SiAlON ceramics demonstrate a combination of both high hardness B21 GPa, and high fracture toughness B5.5 MPa . m 1/2 .