Processing And Characterization Of Polysiloxane Derived Ceramic Matrix Composites

The preceramic polymer pyrolysis route to process ceramics and ceramic matrix composites has been being actively developed since 1976 because it offers many potential advantages such as low processing temperature, controllable ceramic compositions through molecular design, and near-net-shape technologies. However, the most often used polycarbosilane and polysilazane precursors are expensive and not commercially available, which prevent the preceramic polymer pyrolysis route from going into wide application.In this dissertation, hydrogen-containing polysiloxane (HPSO), which is not only very cheap ($3 per kg) but also commercially available, was selected as precursor for silicon oxycarbide ceramics in order to lower manufacturing cost of ceramics from preceramic polymers and make the preceramic polymer pyrolysis route go into wide application. Divinylbenzene (DVB) and vinyl-containing polysiloxane (VPSO), which are also commercially available and have low prices of $2 per kg and $20 per kg, were selected as cross-linking reagents for HPSO, respectively. The curing and pyrolysis behavior of DVB/HPSO and VPSO/HPSO and the processing, microstructures, and properties of DVB/HPSO precursor derived ceramic matrix composites were investigated by means of some techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), thermoanalysis, and element analysis.It is found that chloroplatinic acid (HaPtCl₆), which was dissolved in anhydrous ethanol, is an effective catalyst for the curing of DVB/HPSO and VPSO/HPSO. Catalyst content, temperature and mass ratios of DVB/HPSO and VPSO/HPSO have significant influence on curing degree and ceramic yield. The DVB/HPSO and VPSO/HPSO with a mass ratio of 0.5:1 and 4:1, respectively, have the highest ceramic yield (76.0% and 74.6%, respectively). The cured DVB/HPSO and VPSO/HPSO are both colorless transparent hard solids. The results above indicate that DVB/HPSO and VPSO/HPSO are desirable precursors for silicon oxycarbide ceramics.The pyrolysates and pyrolysis mechanism of DVB/HPSO and VPSO/HPSO were characterized. The pyrolysates of DVB/HPSO and VPSO/HPSO consist of Si, O, C in which O atoms all bond to Si, someC atoms bond to Si and the rest C exist as free carbon. Pyrolysis temperature plays a key role in determining compositions, structure, and crystalline of the pyrolysates of DVB/HPSO and VPSO/HPSO. The structure and composition of pyrolysate of DVB/HPSO are different from those of pyrolysate of VPSO/HPSO. The difference results from the different structures of cured DVB/HPSO and VPSO/HPSO. The pyrolysis of DVB/HPSO and VPSO/HPSO can both be divided into two stages. The apparent activation energies of the first stage of DVB/HPSO and VPSO/HPSO are 208.38kJ/mol and 234.4 IkJ/mol, respectively. The pyrolysis mechanism functions of this stage of DVB/HPSO and VPSO/HPSO are Avrami-Erofeev and Zhuralev-Lesokin-TemPelman equation, respectively. The rates of pyrolysis reactions in this stage are controlled by random nucleation and three-dimensional diffusion, respectively. The apparent activation energies of the second stage of DVB/HPSO and VPSO/HPSO are 339.89kJ/mol and 484.76kJ/mol, respectively. The pyrolysis mechanism functions of the second stage of DVB/HPSO and VPSO/HPSO are both parabola principle, and the rates of pyrolysis reactions in this stage are both controlled by one-dimensional diffusion. The results of curing and pyrolysis of DVB/HPSO have not been reported ever before.The processing, microstructures, and properties of DVB/HPSO derived Si-O-C ceramic matrix composites were investigated. It is found that SiC particle can not only reduce volume shrinkage of DVB/HPSO during pyrolysis but also reinforce monolithic Si-O-C ceramics derived from DVB/HPSO. The flexural strength of SiCp/Si-O-C with a SiC particle content of 55wt% is 201.32MPa that is four times of that of monolithic Si-O-C ceramics. However, this SiCp/Si-O-C exhibits poor oxidaton and thermal shock resistance in static air a...