Formation And Performance Of B₄c/al Anti-damage Composite

Boron carbide skeletons with tri-dimension co-continuous opening pores were sintered by pressureless sintering methods at 1800℃, then was infiltrated by Al in vacuum environment or in Argon atmosphere. A composite with co-continuous phases of ceramics and metal were successfully fabricated, which was almost fully condensed to the density higher than 96% with mass density lower than 2.6g/cm³.Compared with the specimens infiltrated with Al in Argon atmosphere under 1 bar, the quality of the others prepared by infiltration under vacuum condition turns out to be better. By adjusting the sizes distribution of boron carbide particles, the content of each phase of the final products could be under controlled, through which the mechanical properties of those composites could be regulated.By choosing proper organic additive, thin sheet of B₄C porous ceramics with large area could be obtained with the method of rolling forming process. PVB has been proved by experiment be an effectual binder as well as PEG2000 as dispersant and plasticizer. With these two kinds of organic additives, the flexibility and the surface quality of green body of the sheets are guaranteed. One principle to pick binders for boron carbide rolling forming is to avoid the polycondensation between the -OH from boric acid and that from the polymer, which causes cross-linked among polymer chains.The flexural strength of B₄C/Al composite strongly depends on the strength of boron carbide skeleton, which is determined by sintering quality. The sintering quality has little influence on elastic modulus of those composites. The values of the elastic modulus of those composites are related to the content of material phases. Experiments show that by the preparation technique, the values of fracture toughness of the composites go on higher level while bending strength is higher; which could not gain rational explanation from the classical Griffith Theory. Since the composite has a macro-homogeneous, micro-inhomogeneous structure, some large deviation would be brought about from the traditional hardness measurement. Accurate hardness measurement requires low indenting load, and indenting area being large enough so that we can make sure the statistically representative of the point tested.The fracture behavior of B₄C/Al composite is controlled by crack propagation. Crack propagation will experience four stages: rapid crack propagating in brittle ceramic phase; pinning appears when crack tips contact metal toughening phase; crack deflecting at phase interface or propagating backward along the extended surface; toughening phase bridging frame, materials fail but retain the residual strength of the toughening phase. At the last three phases, the resistant of crack propagation comes from toughening phase.