Synthesis and phase evolution of Mg–Si–C–N ceramics prepared by pyrolysis of magnesium-filled poly(ureamethylvinyl)silazane precursor

Abstract

Mg–Si–C–N ceramics were synthesized by reactive pyrolysis of poly(ureamethylvinyl)silazane (PUMVS) polymeric precursor filled with Mg metal powder. Pyrolysis of crosslinked polymeric composites with Mg loadings between 10 and 50 wt.% was carried out in argon atmosphere at temperatures up to 1600 °C. The metal–precursor reactions, ceramization and phase evolution were studied using thermogravimetry, infrared spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. Predominantly X-ray amorphous Mg–Si–C–N materials, containing up to 15 mol% Mg, were obtained at 1000 °C, with a homogeneous distribution of magnesium at a length scale above 100 nm. The incorporation of Mg in the materials proceeds in two steps: (i) conversion of the Mg metal to magnesium compounds and (ii) solid state diffusion, assisted by vapour phase transport, of Mg into the amorphous Si–C–N–H bulk. Mg retention in the materials at temperatures above 900 °C is realized due to the formation of Si–Mg–N bonds. Primary crystallization of SiC is initiated by magnesium induced partitioning of nitrogen in the Mg–Si–C–N materials. Stability of the amorphous Mg–Si–C–N phase and crystallization of MgSiN₂ at higher pyrolysis temperatures is governed by the free carbon content in the materials and the nitrogen partial pressure in the thermolysis atmosphere. The results indicate the feasibility of synthesizing monolithic MgSiN₂-based ceramics through precursor pyrolysis.