On the effects of particle size and preform porosity on the mechanical properties of reaction-bonded boron carbide infiltrated with Al-Si alloy at 950 °C

The infiltration of boron carbide preforms with Al alloys at relatively low temperature prevents the formation of the undesired Al4C3 phase. In the present study the effect of boron carbide powder particle size on the mechanical properties and phase composition of composites infiltrated with Al-20%Si alloys at 950 degrees C was investigated. According to XRD analysis, the infiltrated composites contain Al8C7B4, AlB2 and AlB12, as well as non-reacted Al and Si that originated from solidification of Al-Si alloy. The presence of small amounts of SiC was noted in specimens fabricated from fine boron carbide powder. No evidence for the formation of non-desired aluminum carbide phase was obtained. Infiltration of ceramic preforms with virtually the same green density generated composites with an elastic modulus and bending strength that continuously decreased from 270 GPa and 405 MPa to 195 GPa and 345 MPa for powder with 90% particles close to 3 mu m and powder with 90% particles close to 180 mu m, respectively. These results ambiguously confirm that boron carbide particle size strongly affects mechanical properties of reaction-bonded composites infiltrated with Al-Si alloy at 950 degrees C and reflect the amount of newly formed ceramic phases appearing during infiltration and the presence of defects at the metal-ceramic interface.