Oxidation Resistance of Nd-Si-Al-O-N Glasses and Glass-Ceramics

Glasses containing up to 20% (e.o.) nitrogen were prepared in Nd-Si-Al-O-N system. Compositions, which crystallise in single and multiphase materials, were heat-treated under various conditions, and oxynitride glasses were found to be self-nucleating. The crystalline phases formed depend on both the composition of the parent glass and the heat-treatment conditions. X-ray investigations show that U-phase (Nd₃Si₃Al₃O₁₂N₂) is stable at 1150°C for many crystallisation durations. The microstructural changes were studied by scanning electron microscopy (SEM) and image analysis. The reactivities in air of different glasses and glass-ceramics were compared in the temperature range of 975-1250°C. Sialon glasses oxidised rapidly in air at a temperature above the glass transition temperature by forming a porous oxide scale, due to the evolution of gaseous species at the internal interface. The nature and morphology of oxidised samples were characterised by optical microscopy, X-ray diffraction (XRD), and SEM. The nature and the concentration of the crystalline phases present in the glass-ceramics were found to control the oxidation behaviour of the resulting materials.