Corrosion of HIPed β-Si_6-zAl_zO_z N_8-z (z =0, 1, 2, 3) ceramics by NaCl vapor

Tests for the corrosion of β-Si_6-z Al _z O_zN_8-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.