Properties and performance of polysiloxane-derived ceramic matrix in heat resistant composites reinforced with R-glass or fine ceramic fibres

Eight commercially available polysiloxane resins from 3 producers in the Czech Republic, Poland, and the U.S.A. were scrutinised by considering their weight loss after pyrolysis in inert atmosphere to 1200 degrees C For polymethylsiloxane resins the loss did not exceed 14% while for polymethylphenylsiloxane ones it amounted to 20, even 30 %. Selected resins were used as matrix precursors in laboratory manufacture of ceramic matrix composites according to a polymer pyrolysis route. Rovings of continuous R-glass (Vetrotex Saint Gobain), or Nicalon NL202 (Nippon Carbon) and Nextel 720 (3M) fine ceramic fibres were used as unidirectional reinforcement of composites pyrolysed up to 750 degrees C or 1000 degrees C, respectively. Initial transformation stages from a polymer to a ceramic matrix were monitored by measuring the dynamic Young's and shear moduli, which yielded a pronounced minimum of the shear modulus for composites pyrolysed to 400-600 degrees C manifesting a radical polymer reconstruction in this temperature region. The temperature dependence of the static Young's modulus is almost flat up to 600 degrees C or 1000 degrees C for composites reinforced with R-glass or ceramic fibres, respectively; at higher temperatures, however, a rapid fall of modulus takes place. Microscopic observation of the composites subjected to 2 h oxidation in air at 1200 degrees C revealed that the polymethylphenylsiloxane-based matrix is more prone to damaging than its polymethylsiloxane counterpart. The observed splitting after oxidation of initially continuous matrix to segments is accompanied by a decrease of the shear modulus.