Synthesis of Reaction Bonded Silicon Nitride from High Purity Si Powders
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SYNTHESIS OF REACTION BONDED SILICON NITRIDE FROM HIGH PURITY Si POWDERS JOHN S. HAGGERTY Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT Significantly improved reaction kinetics and properties have been realized for reaction bonded Si3N4 (RBSN) by using Si powders having unusual physical and chemical characteristics combined with clean, oxygen-free processing conditions. For instance, complete nitridation can be achieved 0in < 1 h at 11500C or < 10 m at 0 1250 C without pre-nucleation, or < 1 h at 1100 C with pre-nucleation. These compare with normal schedules of -100 h at 1470-1500 0C. Resulting strengths (up to 870 MPa) and oxidation resistances of the RBSN essentially equal those of fully dense Si3 N4 even though RBSN includes 25% open porosity. The results show that every aspect of the process, ranging from powder synthesis to < ppm levels of oxygen in the nitriding atmosphere, have demonstrable effects on the reaction kinetics and the resulting properties of the parts. With careful processing, reaction formed ceramics can compete advantageously with fully dense counterparts. INTRODUCTION AND RESEARCH OBJECTIVES Reaction bonded Si 3N4 (RBSN) is the most developed of the reaction formed ceramic processes. Others, including vapor-phase methods such as chemical vapor infiltration (CVI), a variety of liquid-phase methods such as reaction bonded SiC (RBSC), various polymer derived ceramics, and the LanxideTm process are also receiving increased attention. The relatively low processing temperatures, the absence of densification aids that degrade high-temperature mechanical properties, and the absence of shrinkage during fabrication make these processes particularly attractive for the fabrication of ceramic matrix composites. Realizing the potential advantages of these processes requires precise microstructural evolution through careful control of synthesis process variables. RBSN is a relatively new material. It was first seriously investigated in the early 1950's in Great Britain by several wonderfully inventive researchers [1 -4]. This early research demonstrated the potential advantages of using this reaction forming process by making large and complex parts as monoliths, and by using RBSN as a cement to join other Si 3 N4 elements. The general status of RBSN as a structural ceramic material prior to the initiation of our synthesis research is summarized in Table I. Although the details are complex, RBSN was precluded from demanding applications because problems associated with intrinsic residual porosity (unreliable and low values of strength and oxidation resistance, etc.) overwhelmed important advantages (net-shape, purity, superior high temperature properties, etc.). RBSN is made by heating a porous Si preform, typically a green Si powder compact, in a nitriding gas, which is usually N2 , a N2/H2 mixture, or NH3 , until it is completely reacted to Si 3N4 . The 22% volumetric increase caused by the nitridation reaction occurs within the void space of the compact [4], so a Si body that is 65
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