SiAlON-Boron Nitride Porous Composites: In Situ Synthesis, Microstructure and Properties

Sialon-boron nitride porous composites were prepared based on the in situ n itridation of AlB2 to form AlN and BN in Si 3N4 and Al2O3 in this work. The fabricated composites by pressureless sintering at 1800 °C for 2 h under N2 atmosphere of 8 atm show closed type pores and demonstrate relatively high bending strength by comparison with tha of conventionally processed composites. In addition, due to the large volume expansion of 226% during the nit ridation process of AlB2, Sialon-BN composites with low-shrinkage in the reaction synthesizi ng process are obtained. Introduction Comparing to the sintering processes of mechanically mixed c omponent phases, ceramic composites fabricated from various reaction processes demonstrate s ome unique features such as special microstructures, high chemical and microstructural st ability at high temperature, and even cheap starting materials [1]. Many reactions were designed in our previous work to produce various ceramic composites, for example, Al 2O3-MoSi2 from controlled oxidized MoSi 2 plus Al [2], Mo(Al, Si)2-SiC from MoSi 2-Al-C [3], TiB2-TiN from BN-Ti [4], AlN-TiB 2 from BN-Ti-Al [5], and SiC-TiB2 (or ZrB2) from B4C-Si-Ti (or Zr) [6-8]. BN ceramics, which shows very low elast ic modulus among ceramic materials, has been widely used as the sec ond phase to reduce the elastic modulus and improve the thermal shock resistance of monolithic ceramic materials, such as in Si3N4-BN and Sialon-BN [9-11], SiC-BN [12], AlN-BN[13] and Alon-BN[14] compos ite . Unfortunately, the strength of BN-containing composites generally decreases with increasing the BN content compared to the monolithic ceramics. One of the main re sons is that the densification behavior of BN composite becomes bad with the addition of BN due to the poor s interability of BN. The other is that because BN is weak, BN agglomerates or large BN particles/platelets that are difficult to avoid their existence in starting powders may act a s fr cture flaws. This means that BN dispersoids with fine particle size and homogeneous distribution are the k y factors to obtain high strength composites. In situ formation of fine and homogeneously distribut ed BN in matrix through reactions is an attractive way to solve this problem. There are some reports on in situ reaction synthesis of BN composites. For example, composites of SiO 2-BN, Al2O3-BN and mullite-BN from the in situ reactions of Si 3N4 + B2O3, AlN + B2O3 or Si3N4 + AlN + B2O3, respectively [15]. The authors formerly proposed some novel reactions to in situ synt he ize various nonoxide-boron nitride (Nobn) composites. SiC-BN composites were either pr epared from the reaction between Si 3N4, B4C and C or Si 3N4, B2O3 (or H3BO3) and C [1,16,17]. We found that the in Key Engineering Materials Online: 2003-04-15 ISSN: 1662-9795, Vol. 237, pp 123-128 doi:10.4028/www.scientific.net/KEM.237.123 © 2003 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-13/03/20,21:55:25) situ formed very fine BN flakes were homogeneously distributed in the SiC matrix and sintering aids that would form liquid phase at sintering temperatures promoted th reaction, the crystallization and grain growth of BN flakes [18]. These composi tes demonstrate very high strength even at high BN content [19]. Si 3N4-SiC-BN ternary system composites can also be produced from the above reaction [20]. Very recently, we reported the r esults on the preparation of AlN-BN, Alon-BN and Si 3N4-BN composites based on the nitridation of AlB 2 and SiB6 [21-23]. In this article, the preliminary results on in situ Sialon-BN porous composites w ill be reported. Experimental Procedure The starting powders were Si 3N4 (E-10 grade, Ube Industries, Japan), AlB 2 (particle size <10 μm, purity >99%, High Purity Chemicals Laboratory, Saitama, Japan), SiB 6 (particle size <5 μm, purity >99%, High Purity Chemicals Laboratory, Saitama, Japan) and A1 2O3 (particle size 0.2 μm, Daimei Chemical Co., Japan). 1 wt% Y 2O3 (Shin-etsu Chemical Co., Japan) was used as the sintering aid. For investigating the nitridation process of AlB 2, firstly, thermogravimetry-differential thermal analysis (TG-DTA) was conducted (TG-DTA2020 type, Mac Science Co., Tokyo, Ja pan) from room temperature to 1700 °C at a heating rate of 10 °C /min in flowing high purity nitrogen. Sequentially, AlB2 powder compacts (about 0.5 g) shaped by uniaxially pressing under 60 MPa were nitrided at 600°C to 1900°C with a temperature rising rate of 10 °C /min under 1 or 8 atm nitrogen pressure atmospheres for 1 or 5 h. Then the weight changes were measured to det ermine the nitridation reaction ratio and X-ray diffraction (XRD) using CuK α radiation was conducted to determine the phase compositions of the obtained nitrided powder compacts after ground t o powders. According to the result on the nitridation process, processing parameters for t he preparation of Sialon-BN composites were determined as 1600 °C × 5 h + 1800°C × 2 h under nitrogen atmosphere of 8 atm. The raw powders calculated stoichiometrically according to the i n situ reactions were mixed by ball milling in ethanol using Si 3N4 balls. After reaction sintering at the conditions mentioned above, the obt ained rectangular specimens were ground into bars of 3 × 4 × 20 mm. Water displacement method was used to determine the density. The three-point bending strength was measured on bars with a span of 16 mm and a crosshead speed of 0.5 mm/min. The strength data were an average of t hr e measurements. The pulse-echo method was used to measure the Young’s modulus and Poisson’s r atio [16]. The fracture surfaces were observed by scanning electron microscopy (SEM). Results and Discussion The nitridation reaction of AlB 2 is as follow: AlB2 + 1.5N2 → AlN + 2BN (1) The calculated volume contents of AlN and BN in the obtained product accor ding to this reaction are 36.51% and 63.49%, respectively, and the weight contents are 45.23% and 54.77%, res pectively. On the other hand, this nitridation reaction shows a large volume expans ion of 226%. This volume expansion should compensate part of the sintering shrinkage during the i n situ reaction sintering process and result in a lower total shrinkage. The total in situ reaction for the synthesis of Sialon-BN composites is as f ollow: Si3N4 + Al2O3 + AlB2 + N2 → Si6-zAl zOzN8-z + BN (2) If we set z = 2, then the in situ reaction becomes as: 4Si3N4 + 2Al2O3 + 2AlB2 + 3N2 → 3Si4Al 2O2N6 + 4BN (3) This composite is designated as 14Z2. The calculated volume contents of Sialon and BN in the 124 SiAlONs