Browsing by Author "Kumar Nelamane Vijayakumar, Ravi"

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The influence of crystallization on the high temperature deformation behavior of precursor derived Si-B-C-N ceramics
(2005) Kumar Nelamane Vijayakumar, Ravi; Aldinger, Fritz (Prof. Dr.)
Si-B-C-N ceramics obtained from two different polymer precursors, boron modified polyvinylsilazanes with chemical composition (B[C2H4-Si(H)NH]3)n (denoted as MW-33) and (B[C2H4Si(CH3)NH]3)n (denoted as T2-1) respectively, were investigated. Compression creep experiments on MW-33 derived material were performed in constant load and load change experiments. It could be seen that the creep rate decreases continuously with time and the dependence of the strain rate, , for the amorphous and nano-crystalline ceramics is similar, including the absence of any asymptotic behavior till 300 h of testing time. For the crystalline material the deformation rates are about one order of magnitude smaller than for the amorphous counterpart, which holds for the whole testing time. The deformation rates decreases by about 2 ½ orders of magnitude reaching a value of 10^-9 1/s after 300 h at the end of the load experiment. Newtonian viscosities of amorphous and nano-crystalline Si-B-C-N ceramics derived from load change experiments were found to be very similar with values of 10^15 to 10^16 Pa•s at 1400 °C. It was observed that the decreased strain rates in the crystallized materials as compared to the as-thermolyzed materials was mainly due to the densification as a result of annealing and structural changes in the matrix phase rather than due to the dispersion of the crystallites. Compression creep experiments on T2-1 derived ceramics exhibit at the beginning of the creep experiments the same creep rates and similar time and temperature dependences of the strain rates. Strain rates in the order of as low as 10^-9 1/s was observed at 1350 °C after a duration of 300 h of creep testing time. However, at higher temperatures the strain rates becomes constant and even increases after longer durations. This behavior was attributed to a strong influence of oxidation. The high creep resistance of the original non-oxide microstructure prevails for a while albeit oxidation starts from beginning of the test. At later stages, the creep resistance is compromised due to the softening of the load carrying bridges by oxidation. When the oxidation effects dominate, the creep curves tend to have a positive slope with increasing creep rates over time, as observed in the creep experiments carried out for this material which is reflected most significantly at 1500 °C. The oxidation behavior of these materials is dependent on the processing conditions, in particular, the size of the polymer particles used to compact the bulk body. If the ceramic can be processed in such a way that the number of open pore channels and the diameter of the open pores are minimized, the oxidation of these materials can be enormously reduced, and henceforth the creep resistance of the materials increased.