Tests for the corrosion of β-Si6-z Al z OzN8-z ( z =0, 1, 2, 3) (β-Si3 N4 and β-SiAlON) ceramics were carried out at 1300°C for 3-24 h in NaCl vapor of various concentrations (1.67 × 10-2, 3.33 × 10-2, 5.0 × 10-2 g l-1), which was carried by flowing Ar gas. The densified Si3N4 and SiAlON ceramics were fabricated by HIPing under N2 of 150 MPa at 1700°C. The corroded surface was observed by optical and scanning electron microscopy (OM and SEM). The phases produced during corrosion were identified by X-ray diffraction. The thickness of the corroded scale was determined by cross sectional SEM observation. The Si3N4 ceramics were hardly corroded by NaCl vapor, while the z =1 and 2 SiAlONs were slightly corroded with the formation of bubbles on the surface; the z=3 ceramics were severely corroded with the formation of Al2O3 needle crystals and fine mullite crystals, depending on the NaCl vapor concentration. Quantitative X-ray microanalysis showed that 2 at.% Na is contained in all the scales of the SiAlONs. The severe corrosion of the z=3 SiAlON was explained on the basis of the kinetic results for the thickness of the scale.
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