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Development of Si-O-C based ceramic matrix composites produced via pyrolysis of a polysiloxane
The traditional ceramic processing techniques of CMCs such as hot pressing are high-temperature, high-cost processes, and unsuitable for manufacturing complex and near-net shapes. Fabrication of ceramic matrix composites (CMCs) from pyrolytic conversion of preceramic polymers has gained considerable attention in recent years due to their unique combination of low temperature processing, applicability of versatile plastic shaping technologies and microstructural control capabilities.In the present work, phenyl (PPS) and methyl (PMS) containing polysiloxanes were pyrolyzed at elevated temperatures (900-1500oC) without filler addition under argon atmosphere to investigate the thermal conversions and phase formations in thepolymer matrix. X-Ray diffraction (XRD) and Infrared spectroscopy (FTIR) techniques were used for this purpose. It was found that pyrolysis of the polymers under inert atmosphere up to 1300oC lead to amorphous silicon oxycarbide (SiOxCy) ceramics.Conversions at higher temperatures caused the transformation into the crystalline b-SiC phases. % Weight changes of the samples without filler addition were also followed by measuring the masses before and after pyrolyzation of the samples.CMC monoliths were fabricated with the addition of 60-80 wt% active and inert fillers by hot pressing under 15 MPa pressure and pyrolysis at elevated temperatures between 900-1500 C under inert argon and reactive nitrogen atmosphere. Effects of the filler type and ratio, pyrolysis temperature and atmosphere on the phase formations were investigated by using XRD, SEM-EDX and TGA techniques. The results showed that with the incorporation of active Ti fillers, formation of TiC, TiSi, and TiO within the amorphous matrix occurred due to the reactions between the Ti and the polymer decomposition products. However, no new phase development was observed in the case of inert SiC particulate addition. SEM-EDX analysis was also performed to monitor the new phase formations. Mass loss and densification values of the CMCs were measured to investigate the effect of active filler controlled polymer pyrolysis process (AFCOP).Weight changes were considerably affected in the case of addition of the fillers into the ceramic structure due to the reduction of the polymer ratio in the composite systems and the reactions between polymer and filler particles. As an example weight loss of 27 % was measured for PPS samples without filler addition, while 17 % weight reduction was measured for PMS without filler addition after pyrolysis at 1500oC. On the other hand, it was found that for the composite systems the weight loss values were reduced to 2 %.Mechanical property characterization of samples with and without filler addition was done by Vickers Indentation tests. It was found that ceramics that is the product of the pyrolysis of the polymer without filler addition exhibited the maximum hardness values (8.88 GPa for neat PPS, 10.67 GPa for neat PMS) at 1100 C, which is the optimum temperature for crack free samples with the least amount of porosity. Also, the composite system exhibited the hardness values up to 14 GPa.