DIRECT OBSERVATION OF PRECERAMIC AND ORGANIC BINDER DECOMPOSITION IN 2-D MODEL MICROSTRUCTURES

Preceramic and organic binder decomposition processes were studied dur ing thermolysis to determine how the physico-chemical properties of th e binder affected the microstructural development of the ceramic compo nent. Specifically, the behavior of two organic polymers, poly(methyl methacrylate) (PMMA) and a cross-linked poly(methyl methacrylate) (x-P MMA), and two preceramic polymers, polycarbosilane (PCS) and vinylic p olysilane (VPS) was observed as a function of temperature. Binder-fill ed two-dimensional (2-D) model microstructures were fabricated to simu late ceramic green bodies whose pores were completely filled with bind er. Examination of these 2-D samples by hot-stage optical microscopy e nabled direct observations of pore development and changes in polymer morphology during binder thermolysis. These observations revealed that the mass transport processes involved during thermolysis, as well as the developing microstructural features, depend on the properties of t he binder system during thermal decomposition. The organic polymers we re investigated because of their chemical similarity and markedly diff erent physical behavior upon heating. It was shown that thermoplastic polymers (e.g., PMMA) are influenced by capillary forces during thermo lysis, while thermosetting polymers (e.g., x-PMMA) do not flow within these porous microstructures. Both of the preceramic polymers displaye d a range of physical behavior over the temperatures studied. The deco mposition chemistry and weight loss at a given temperature combined wi th the associated physical behavior had a dramatic effect on the final distribution of the pyrolyzed product (amorphous silicon carbide and glassy carbon) formed during thermolysis. The pyrolysis product formed from PCS was observed to segregate to the smaller pore channels in th e 2-D microstructures, while the pyrolysis product formed from VPS was observed to be homogeneously distributed in these model microstructur es. This work offers guidelines to improve the microstructural homogen eity of ceramic-ceramic composites derived from particulate-preceramic polymer green bodies.