Synthesis of Mg-stabilized nitrogen-rich alpha-sialons along Si₃N₄–0.5Mg₃N₂:3AlN line via field-assisted sintering

The present study reports for the first time the synthesis and evaluation of magnesium (Mg)-doped nitrogen-rich (N-rich) sialon ceramics exploring the possible existence of Mg-stabilized single-phase alpha-sialon (on the Mg-alpha-sialon plane). Mg-stabilized N-rich sialons, with the general formula of Mg_xSi_12−2xAl_2xN₁₆ having x value in the range of 0.2–2.2 for the composition laying along the Si₃N₄–0.5Mg₃N₂:3AlN line, were synthesized at 1500°C, using nano-size starting powder precursors and field-assisted (or spark plasma) sintering technique. Consolidated sialon ceramic samples were characterized for their microstructure, phase stability regime, and physical and mechanical properties. Although a relatively low synthesis temperature was adopted, well densified sialon samples were achieved; however, the densification of the samples became difficult with a higher x value (containing high Mg₃N₂/AlN content). Contrary to what was expected to take place, a single-phase Mg-doped sialon was not obtained near the N-rich line (on Mg-alpha-sialon plane). Such distinctive behavior in Mg-doped sialons was supposed to be due to the formation of a highly stable Mg-containing aluminum nitride polytype phase, which consumed most of the high-temperature transient liquid phase. Mg-doped N-rich sialon sample having the maximum amount of alpha phase depicted a remarkable hardness (HV₁₀) of 21.4 GPa and a fracture toughness of 3.5 MPa m^1/2.