Morphology of SiAlON–Al2O3– MgO Ceramic Products Synthesized in Arc-discharge Plasma

This work addresses the influence of magnesium oxide on the structure, phase composition, and morphology of ceramic composites of the SiAlON–Al 2 O 3 –MgO system, which are formed under the non-equilibrium conditions of the arc-discharge plasma synthesis. Purpose: The aim of this work is to determine the effect of magnesium oxide concentration on the morphology, interfacial boundary characteristics, elemental distribution, and density of the ceramic product of the SiAlON–Al 2 O 3 –MgO system synthesized via a direct exposure to the arc-discharge plasma. Methodology : Scanning electron microscopy, local energy-dispersive X-ray spectroscopy, and elemental distribution mapping are employed to investigate the morphology and chemical heterogeneity of ceramic products of the SiAlON–Al 2 O 3 –MgO system obtained by the arc-discharge nitrogen plasma flow on powder compacts. Research findings: Three precursor mixture compositions are investigated, i.e., a conventionally oxygen-free Si 3 N 4 –AlN (70/30 wt.%) mixture and two compositions with a fixed (5 wt.%) Al 2 O 3 content containing 5 and 15 wt.% of MgO, respectively. The exposure time is varied in the range of 30 to 50 s at 100 A current. A zonal structure with pronounced interdiffusion of Si and Al (2.97 and 15.71 wt.% Al at adjacent analysis points) appears in the oxygen-free composition. However, the low (~1.2 wt.%) oxygen content and nitrogen deficiency indicate a partial dissociation of nitrides and incomplete development of the β-SiAlON stoichiometry. The introduction of 5 wt.% MgO and 5 wt.% Al 2 O 3 leads to the formation of a cellular framework with the growth of whiskers in the pore space and core-shell structures. Increasing the MgO content to 15 wt.% levels out the morphological interface while preserving the chemical zoning, which is registered only by EDS maps. It is demonstrated that the MgO content determines the material resistance to decompression (de-densification), i.e., with the exposure time growth from 30 to 50 s, the relative change in density ρ1/ρ0 is 0.98→0.84 for the oxygen-free composition and 1.01→0.99 for the composition with 15 wt.% MgO.