MICROSTRUCTURE DEVELOPMENT OF OXYCARBIDE COMPOSITES DURING ACTIVE-FILLER-CONTROLLED POLYMER PYROLYSIS

The microstructure development of a ceramic composite material fabrica ted by active-filler-controlled polymer pyrolysis (AFCOP) was investig ated. During heating of a polysiloxane precursor mixed with titanium p owder in argon atmosphere up to 1400-degrees-C, thermally induced deco mposition of the polymer phase is combined with simultaneous carburiza tion of the transition metal filler. Precipitation of nanocrystalline titanium carbide at the filler particle surface starts above 400-degre es-C, and larger, faceted carbide particles have grown above 800-degre es-C. A skeleton of turbostratic carbon is formed above 800-degrees-C in the polymer-derived silicon oxycarbide matrix from which b-silicon carbide and cristobalite crystallize above 1000-degrees-C. A pronounce d reduction in linear shrinkage involved in polymer-ceramic conversion is observed. The shrinkage reduction ranges from more than 25% for th e filler-free precursor to less than 10% in the presence of 30 vol% of the titanium filler. Thus, active-filler-controlled pyrolysis offers the possibility of controlling shrinkage and porosity formation during polymer-ceramic conversion in order to fabricate bulk components from organometallic polymer precursor systems.