Microstructure and properties of various fluorine-containing SiAlON ceramics synthesized by HIPing

An attempt was made to prepare various F-doped β-, O-, X-, and α-SiAlONs from a mixture of Si3N4, SiO2, Al2O3, AlN, or Y2O3 using AlF3 or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z, x, and m/n values determined for β-, O-, and α-SiAlONs by X-ray diffraction. When AlF3 was used, a single phase ceramic (O-SiAlON) was produced from a mixture of α-Si3N4 and SiO2 at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of α-Si3N4, AlN, and Y2O3 with AlF3 produced β-/Y-α-SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of α-Si3N4 and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from α-Si3N4 and SiO2 at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y-α-SiAlON mixture showed the growth of O- and β-SiAlON and Y-α-SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses.