Ceramics-Silikáty 54, (4) 345 - 351 (2010)

Unidirectional fibre reinforced composites were prepared, with polysiloxane matrix converted pyrolytically to silicon oxycarbide to a varying degree depending on pyrolysis conditions. As reinforcement, continuous E-glass or basalt fibres were used in order to compare their eventually different effect in the composites. The samples were made from wet-wound prepregs and first cured at 180-250°C and thereafter pyrolysed at 420-750°C. Flexural strength, Young’s and shear moduli of the final composites were measured. The curing of the siloxane resin at lower temperatures was investigated by thermogravimetry. After pyrolysis, the reinforcing fibres were characterized by light microscopy in order to assess their eventual deformation and damaging at elevated temperatures. The composites pyrolysed at the highest temperatures differ very strongly in flexural strength but not in moduli. The low strength of the glass-reinforced composites is due to their brittle failure and is obviously caused by strong fibre-matrix bonding, while the high strength of the basalt reinforced ones results from a weak bonding which makes fibre pull-out possible. The tendency to fibre pull-out was evaluated by comparing loading characteristics of the samples. A probable explanation of the different fibre bonding tendency is suggested, based on reactivity comparison of iron-containing and iron-free silicates with silica.