Fracture Toughness of Amorphous Precursor Derived Si-C-N Ceramics
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Fracture Toughness of Amorphous Precursor Derived Si-C-N Ceramics A. Bauer, A. Zimmermann, F. Aldinger Max-Planck-Institut für Metallforschung and Institut für Nichtmetallische Anorganische Materialien, Universität Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, 70569 Stuttgart, Germany
Stuttgart,
ABSTRACT Fully amorphous ceramics in the system silicon-carbon-nitrogen were produced with the polymer precursor route using the commercially available polysilazane CerasetTM. Besides their high temperature thermal stability, these ceramics show excellent high temperature creep resistance. Not many investigations have been dedicated to the fracture mechanics of these materials. This paper provides data on toughness measurements utilizing bulk and indentation techniques. The double cantilever beam method (DCB) was used to study crack propagation. To determine the intrinsic toughness, the crack opening displacements (COD) of indentation cracks were determined.
INTRODUCTION Thermolysis of polymer precursors leads to amorphous ceramics, which are considered to be candidates for high-temperature applications [1,2]. Bulk specimens can be produced by compaction of polymer powders [3]. Besides their high temperature stability [4], an outstanding high temperature creep resistance was found for these bulk materials [5-7]. This behavior can be attributed to processing without sintering aids with low melting point which are required for the sintering of non-oxide ceramics, e.g. silicon nitride and silicon carbide. For structural applications, the fracture toughness needs to be considered in conjunction with creep and thermal shock behavior. Since very few data on the fracture toughness of precursor derived materials are available, the current work wants to close this gap. The amorphous precursor-derived ceramics under investigation exhibit predominantly covalent bonds, and brittle fracture is expected as found for oxide glasses [8].
PROCESSING AND EXPERIMENTAL SETUP The polysilazane CerasetTM [9] was used as a pre-ceramic polymer. The liquid was handled in an argon-filled glove-box to avoid oxygen contamination. To receive a non-meltable solid, the polymer was thermally cross-linked for 2 h at a temperature of 380 °C. The hard glassy body was pulverized with a tungsten carbide ball mill and sieved with a mesh size of 128 µm. The polymer powders were densified in graphite dies under uniaxial compressive stress of 48 MPa for 30 min at temperatures of 290 °C. The green body was then thermolyzed under argon atmosphere at 1050 °C for four hours, using a heating ramp of 25 K/h. One side of the
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bulk specimen was polished with a diamond particle suspension of 1 µm. Only one batch has been used for the toughness measurements to avoid the influence of the processing parameters. Crack propagation of monolithic precursor-derived ceramics has been studied using the double cantilever beam method. The dimensions of the bulk specimen were chosen to be 2x3x25 mm3. The specimen were notched on one side. A crack is initiated at th
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