Pergamon Acfa marer. Vol. 46, No. 7, pp. 2401-2405, 1998 0 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S1359-6454(97)00397-Z 1359-6454/98 $19.00 + 0.00 zyxwvutsrqp SILICON NITRIDE: FROM SINTERING TO JOINING MANI GOPAL13, L. C. DE JONGHE’” and G. THOMAS’” ‘Department of Materials Science and Mineral Engineering, University of California at Berkeley, Berkeley, CA 94720, U.S.A. and Center for Advanced Materials, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. Abstract-In earlier work, silicon nitride (SisNd) powders were sintered with a mixture of rare-earth oxide (REaOs) and silica (SiOs). When the chemistry and sintering schedule is carefully controlled, the intergra- nular phase is a crystalline REaSia07 phase, which imparts improved mechanical and oxidation properties to the S&N+ Based on the premise that a material that is successful in joining individual grains will also be suitable for joining bulk pieces, the process has been extended to join bulk SisN4. This method of joining causes the elimination of the joint, as the mechanism of joining is the same as that by which Si3N4 powder sinters. By analogy with the results for sintered compacts, it is predicted that the rare-earth oxide joined parts may also exhibit improved high temperature performance. 0 1998 zyxwvutsrqponmlkjihgfedcbaZYXWVUTS Acta M etallurgica Inc. 1. SILICON NITRIDE: SINTERING The microstructure of silicon nitride (Si3N4) cer- amics consists of equiaxed a-SisN4 and elongated fl- Si3N4 grains surrounded by an intergranular phase [Fig. l(a)]. The intergranular phase is formed from the sintering aids that are used to densify the ceramics [l]. The mechanical properties of Si3N4 ceramics are controlled by the nature of this inter- granular phase [2]. When Si3N4 powders are sin- tered with MgO, Y203, Al2O3 and SiOZ, the intergranular phase is a glass. With increasing tem- perature, the viscosity of the glass decreases permit- ting the Si3N4 grains to slide past each other. This causes a rapid decrease in strength above 1200°C. This degradation in strength can be minimized by forming a crystalline intergranular phase, which would prevent the sliding of grains. Sintering with RE20s and SiOZ largely eliminates the glassy phase due to crystallization [Figs l(b)-(d)] to form the RE&O, phase (determined by energy dispersive spectroscopy, electron and X-ray diffraction) [3,4]. The SisN4 therefore retains a higher fraction of its room temperature strength at high temperatures [5]. Sintering aids such as MgO, Y203, A&O3 and SiOz yield essentially glassy reaction products and induce poor oxidation resistance. While all S&N4 grains are covered with a layer of oxide that should give the ceramic good oxidation resistance, at high temperatures the silica reacts with the sintering additives to form a flux. It has been proposed [6] that this flux provides enhanced oxygen transport to the S&N4 surface resulting in an increase in the oxidation rates. A phase that can provide good oxi- dation resistance is one in equilibrium with SiOZ. Such a phase is found in the RE-Si-N-O system, and has the stoichiometry RE&O,. Figure 2 is a simplified isothermal section of the REzOs-Si02- SisN4 phase diagram, and illustrates the compatibil- ities and the basis of our grain boundary design. Experimental measurements in bulk S&N4 sintered with REzOs and SiOa show greatly improved oxi- dation resistance [7,8]. Hence, having a crystalline RE#.i207 as the inter- granular phase has a two fold advantage - the strength does not rapidly decrease at high tempera- tures and the oxidation rates are decreased. This behavior is independent of the specific rare-earth oxide used, be it YZOs, SmzOs, Gd203, DyzOs, ErzOs or Yb203 [5,8]. Detailed analyses of strength and creep [5], oxidation [8] and fatigue [9] have been discussed in earlier papers. 2. SILICON NITRIDE: JOINING This idea of sintering SisN4 with RE203 and SiOz is extended to the joining of bulk Si3N4 with the sintering aid as the interlayer. The rationale behind this approach is that if these sintering aids produce good bonding between individual powders or grains, then they should also be suitable for bond- ing bulk material. This concept of joining S&N4 cer- amics using sintering aids has been used by previous workers [lo-131. However, the sintering aids they used includes A1203 which promotes the formation of glassy phases [14]. The strength of these joints are therefore expected to rapidly degrade above 12OO”C, as has been observed in bulk SisN4 [2]. For this reason, A1203 has not been used in our joint material even though it facilitates sintering at lower temperatures. To ensure wetting and joining of the ceramics, a liquid phase must be formed in the interlayer. In the YbZO,-SiOz system, the melting point of Yb2Si207 is 1850°C [15], which is the temperature above which Si3N4 powders are typically sintered. For the joining process, the temperature is lowered by utilizing the deep eutectic present near the disili- 240 1