Thermal Oxidation of Sintered Beta-Sialon (z=3) Ceramics in Atmospheres with Water Vapor

Thermal oxidation of β-sialon (Si6-zAlzOzN8-z, z = 3) ceramic was investigated at 1300 to 1600 ̊C in dry (Ar/O2 = 90/10 kPa), wet O2 (Ar/O2/H2O = 80/10/10 kPa) and wet (Ar/H2O = 90/10 kPa) atmospheres. Oxidation was followed from weight gains using thermogravimetry and the oxidized sample was characterized by scanning electron microscope, electron probe microanalysis and X-ray powder diffraction. Oxidation kinetics in the three atmospheres were represented by a parabolic rate model except for at 1600 ̊C. The wet and wet O2 oxidation rates were greater than the dry oxidation, the degree decreasing with rising temperature from 1300 to 1500 ̊C, while wet and wet O2 oxidation at 1600 ̊C was even faster again than the dry oxidation. It was found that the scale formed by the dry, wet and wet O2 oxidation consisted of mullite and cristobalite at 1300 1500 ̊C and of mullite and SiO2 rich aluminosilicate glass at 1600 ̊C. Introduction β-sialon is expected to be useful for engineering ceramics such as cutting tools, refractory materials and metal-forming tools, because of their high hardness, low thermal expansion, high strength, and high chemical stability. [1, 2] In high temperature conditions, oxidation becomes important for their practical uses. Water vapor has been reported to accelerate oxidation of Si-based ceramics, such as SiC, Si3N4. [3, 4] Oxidation of sintered β-sialon ceramics in dry atmosphere has been reported in detail by Perrson et al. [5] However, there is a very few reports of oxidation in atmosphere containing water vapor. [6, 7] The present work describes the effect of water vapor on oxidation of sintered β-sialon (z = 3) ceramics at 1300 to 1600 ̊C from viewpoints of the kinetics and microstructural observation of oxidized samples. Experimental Sintered β-sialon was fabricated by hot pressing of β-sialon (z = 3) powder without additive at 1850 ̊C and 24 MPa for 2 h in N2 atmosphere. The relative density of the sintered body was about 95%. The β-sialon powder was commercially purchased (Ube Industry); Purity of the powder was not reported by the supplier. X-ray diffraction pattern (XRD) of the β-sialon powder and sintered body showed the major peaks of β-sialon with a trace of 15R sialon. The polished sample of about 15 × 5 × 1 mm was oxidized in a flowing mixture of Ar/O2 (90/10 kPa, dry), Ar/H2O (90/10 kPa, wet) and Ar/O2/H2O (80/10/10 kPa, wet O2) at a temperature range of 1300 1600 ̊C for up to 100 h. The sample was hanged with an alumina wire in a fused quartz tube (at 1300 ̊ 1500 ̊C) or in an alumina tube (at 1600 ̊C). The weight change during oxidation was monitored by using an electronic microbalance (CAHN 2000). After the TG experiments, the oxidized sample was characterized by XRD, scanning electron microscope with energy dispersion analysis unit Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 893-896 doi:10.4028/www.scientific.net/KEM.264-268.893 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:14:02) (SEM-EDX), and electron probe micro analyzer with wave dispersion analysis unit (EPMA). Results and Discussion Oxidation kinetics. Figure 1 shows log-log plots of weight gains due to oxidation of β-sialon ceramics. Since large weight gains were observed in the wet atmosphere without oxygen, water vapor oxidizes β-sialon as an oxidant. Except for an early stage oxidation (< 1 20 h), slopes of the plots at 1300 1500 ̊C were about 0.5 and thus the oxidation is represented by a parabolic rate law (Eq. 1), applicable to the diffusion limiting rates, dW = kp t 0.5 (1) where dW is weight gain, kp parabolic rate constant and t time. The slopes of plots at 1600 ̊C, however, were 0.6 0.8 for the three oxidation, suggesting that the oxidation is not represented by diffusion limiting rate model. The kp values for the dry, wet and wet O2 oxidation at 1300 – 1500 ̊C are summarized in Table 1. The kp values for the wet oxidation were 15, 2.7 and 1.4 times greater than for the dry oxidation at 1300, 1400 and 1500 ̊C, respectively, while the wet O2 and wet oxidation proceeds at very similar rates. No great difference of the kp between the wet and wet O2 oxidation suggests that oxygen in the wet O2 atmosphere does not greatly contribute to oxidation of β-sialon. At 1600 ̊C, the wet and wet O2 oxidation for 50 h gave similar weight gains (6.2 and 6.3 mg cm, respectively), becoming five times greater than that (1.2 mg cm) for the dry oxidation. These results show that water vapor accelerates the oxidation of β-sialon, particularly at 1300 and 1600 ̊C. The Arrhenius plots of the kp (Fig. 2) show that the apparent activation energy (Ea) for the dry, wet and wet O2 oxidation was 860, 600, 600 kJ mol, respectively, with the same value for the wet O2 and wet oxidation (Table 1). These Ea values were large, compared with the reported values (300 400 kJmol at 1000 1400 ̊C) for the dry oxidation of β-sialon. [5] (a) 1600oC