Studies on SiBN(C)-ceramics: Oxidation- and Crystallization Behavior Lead the Way to Applications
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H.-P. BALDUS AND G. PASSING Bayer AG, ZF-MFA, D-51368 Leverkusen, Germany
ABSTRACT
The thermal stability of amorphous borosilicon nitride (Si 3B3N7 ) and borosilicon carbonitride (SiBN 3C) between 1000*C and 2000"C both in air and under inert conditions is reported. Both materials are derived from polymerization and subsequent pyrolysis of a "single source" precursor. On heating in vacuum or nitrogen SiBN 3C remains amorphous up to 190("C whereas Si3 B3N7 crystallizes at about 1800"C under these conditions. At about 2000"C the SiBN(C)-materials decompose into SiC, BN, BC and N2. Oxidation studies performed by TEM- and SEM-investigations of oxidized borosilicon carbonitride grains reveal that an interlayer consisting of B, N, and only little 0 is formed between the oxide scale on the surface and the inner bulk material. The interlayer does not disappear at temperatures above 1450"C in contrast to the Si 2N 2O-interlayer observed on oxidized silicon nitride. The oxidation kinetics of the new ceramics are established in the range from 1000"C to 1600"C indicating a very high oxidation resistance. Possible applications as matrix materials as well as materials for fibers and coatings are discussed.
1. INTRODUCTION Silicon nitride and silicon carbide are materials which are suitable for turbo superchargers, turbines for jet engines and as protective walls of rocket nozzles and combustion chambers by virtue of their strength and their corrosion resistance. However, thermal shock resistance, oxidation resistance and high temperature resistance of sintered bodies made of silicon nitride and silicon carbide are still insufficient under high thermal stresses and/or highly corrosive atmospheres. On the other hand, it has been shown that the thermal shock resistance of composite materials made by mixing and hot pressing of Si 3N4- and BN-powders is considerably improved compared to ceramic bodies made of pure silicon nitride [1, 2]. Moreover, composite materials consisting of Si 3N4 and TiN show improved hardness and bending strength in comparison to pure silicon nitride [3]. Synthesis of these composites is usually performed by co-milling of the binary compounds; this however, leads to an inhomogeneous elemental distribution and impurities in the material. Furthermore, this method is not suitable for making fibers or coatings. However, it has recently been shown that pyrolysis of precaramic polysilazanes leads to composites of Si 3N 4 and SiC exhibiting a good homogeneity [4]. These observations led to investigations on new preceramic polymers including elements other than silicon as a means of making novel multinary ceramics or nanocomposites with improved mechanical properties[5-8]. This objective has been accomplished by mixing polysilazanes with heteroelement compounds and successive pyrolysis of the mixture. However, on a atomic scale resulting materials do not exhibit a really homogenous elemental distribution due to the use of large polymeric building blocks as educts which affects physical properties of the final cer
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