Fractography of alumina fibre reinforced ex-polysiloxane matrix composites

摘要

The influence of repeated impregnation - pyrolysis (applied once or twice) and heat treatment temperature (HTT = 800 or 1000°C) on selected mechanical properties and fractographic features of unidirectional ceramic matrix composites with alumina-mullite fibre (Nextel 720) reinforcement and matrix derived from polymethylphenylsiloxane resin Lukosil 901 was pursued. Application of the 2nd impregnation and increasing the HTT led to lowering the open porosity from ≈11-12 to 7-9 % but the influence of impregnation was higher than that of HTT. Load - deflection characteristics of all specimens were brittle. Bending strength values (257 - 277 MPa) were rather insensitive to the processing parameters but their scatter decreased significantly after the 2nd impregnation - pyrolysis. The repeated impregnation was more effective in a slight increase of the Young's modulus than the increase of HTT. The shear modulus behaved similarly but its differences were by far more pronounced. The fracture surface roughness in macroscale was higher in once and twice impregnated specimens pyrolysed at 800°C (1-3 mm) than in those treated at 1000°C (≈0.6 mm). Toughening mechanisms effective in fracture surface formation like bundle pull-out or individual fibre pull-out and their mutual proportion are discussed.