Creep Behaviour of a Crystallized Si-B-C-N Ceramic Obtained from the Polymeric Precursor T2-1(B[C 2 H 4 Si(CH 3 )NH] 3 )
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Creep behaviour of a crystallized Si-B-C-N ceramic obtained from the polymeric precursor T2-1(B[C2H4Si(CH3)NH]3)n Ravi Kumar N .V., André Zimmermann 1, Fritz Aldinger Max-Planck-Institut für Metallforschung and Institut für Nichtmetallische Anorganische Materialien, Universität Stuttgart, Pulvermetallurgisches Laboratorium, Heisenbergstr. 3, D70569, Stuttgart, Germany. 1
Present Address: Robert Bosch GmbH, Corporate Research and Development
ABSTRACT: The boron modified poly(vinyl)silazane polymer precursor with the chemical composition (B[C2H4Si(CH3)NH]3)n was milled and sieved. The polymer particles from different size fractions were compacted using a graphite die in a uni-axial warm pressing machine at a pressure of 48 MPa and in the temperature range 250°C to 330°C. The green bodies were pyrolysed in an argon atmosphere at a temperature of 1300°C when the organic polymer converts into an inorganic amorphous Si-B-C-N ceramic. These amorphous ceramics were annealed under various conditions of temperature, nitrogen overpressure and holding time in order to crystallize them and to produce nano-crystalline microstructures. Compression creep experiments were carried out in atmospheric ambience at loads varying from 5 – 100 MPa and in the temperature range 1350°C – 1500°C to investigate the high temperature deformation behaviour of the crystalline material. The interest is to understand the mechanisms of deformation in these nanocrystalline Si-B-C-N ceramics at elevated temperatures and to compare the results with that of amorphous ceramics. The investigation also includes the determination of viscosity of the material at high temperatures. INTRODUCTION: Amorphous Si-B-C-N ceramics have been investigated to a reasonable extent to understand the high temperature deformation behaviour by carrying out extensive compression creep experiments. The studies showed that these materials yield highly creep resistant class of materials. The relatively high creep resistance comes from the fact that these materials lack the presence of any oxidic grain boundary phase, resulting from low melting point sintering additives as in the case of conventionally sintered materials. The deformation in amorphous SiB-C-N ceramics at high temperatures was analysed using the free volume model which is generally used to describe the deformation behaviour in amorphous metallic glasses [1]. But these amorphous Si-B-C-N ceramics are metastable and devitrify at high temperatures of the order of 1800°C. Also, traces of nanocrystallites have been observed by TEM studies at even lower temperatures during long term creep tests. This motivates us to crystallize these quarternary amorphous systems prior to creep studies and investigate the influence of the crystallinity on the high temperature deformation behaviour.
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