Processing and characterization of ceramics and ceramic composites synthesized by pyrolysis of siloxane-based precursors

The broad objective of this research was to develop a continuous fiber ceramic matrix composite using the polymer pyrolysis approach. Composite systems based on Allied Signal's Blackglas{dollar}rmsp{lcub}TM{rcub}{dollar} matrices were chosen for this study. The research study comprised of three sub-topics. In the first, some of the basic aspects of polymer pyrolysis process by itself was studied. These studies were directed towards understanding of some of the basic issues relating to processing of silicon oxycarbide ceramics. In particular, the role of polymer chemistry and polymer architecture, on the structure and composition of the final ceramic was determined. Results show that the carbon and the "free carbon" content in the ceramic is dependent on the nature of the functional groups present in the starting precursor. Precursors with combinations of vinyl, hydro, methyl and phenyl groups were evaluated. The most significant outcome of the study showed that with an increase in vinyl content in the precursors, the carbon content and structural integrity in the ceramic is increased at the cost of a decrease in reticulated carbon. Ceramic yields are maximized, when number of vinyl and hydro groups in the starting precursor were equal.; The second area of research deals with processing of Blackglas matrix composites with Nextel 440 fibers. Here, the cure and the pyrolysis behavior of Blackglas resin and composite was studied. DSC studies on curing of Blackglas resin indicate that with an increase of catalyst content from 0.1% to 1.0%, the onset and peak temperature of cure are decreased, coupled with an increase in the enthalpy of cure indicating a greater extent of crosslinking. However, the pyrolysis yield is relatively insensitive to cure conditions. Cure pressure and pyrolysis atmosphere are variables in the processing of Blackglas matrix composite. Variations in the cure pressure had no discernible effect on the chemistry of the pyrolyzate. A higher cure pressure however resulted in the top and bottom ply damage. Pyrolysis in argon environment resulted in the incorporation of {dollar}sim{dollar}12 wt.% C in the composite while pyrolysis in ammonia resulted in 3.9 wt.% N and 1.5 wt.% C in the composite. One common feature was that all of the composites showed a high degree of matrix cracks in the as processed conditions.; In the third part of this research, the mechanical behavior of Blackglas/Nextel 440 and Blackglas Nextel 312 composites were studied as a function of processing conditions and heat treatment. The processing variables studied were, autoclave cure pressure, pyrolysis environment and pyrolysis temperatures. The flexural strength decreases when processed at higher autoclave pressures, pyrolyzed at higher temperatures and pyrolyzed in ammonia. However, the strain to failure did not follow the same trends, and increased for samples pyrolyzed at lower temperatures and in ammonia. Exposure of the samples to high temperatures in air resulted in loss of flexure strength, modulus and strain to failure. The micromechanical damage characteristics also changed with processing conditions and heat treatment. Samples pyrolyzed in ammonia showed greater fiber pullout and damage tolerance with increased strain to failure. With heat treatment, the samples failed in a brittle manner with almost no damage tolerance through crack deflections or pullout.