In-situ Preparation Of B₄C-W₂B₅ Composite Ceramics And Investigations Of Their Trengthening And Toughening Mechanism

The key problems of restricting the development of B₄C ceramics are the high densification sintering temperature ?>2200 ?? and the mismatch of fracture toughness and hardness. In this paper, the heat release via in-situ reaction of boron?B?-graphite?Gr?-WC system was used to reduce the densification sintering temperature, and the fracture toughness of the B₄C matrix was reinforced by the trip-shaped W₂B₅ via in-situ reaction between WC-B. Graphite, boron and WC powders were ball milled by high energy planet ball milling ?HEPBM? and the powder mixture was consolidated by spark plasma sintering ?SPS?, as a result, W₂B₅ reinforced B₄C composite ceramics at a relative lower temperature and a fast consolidation speed with a higher relative density were successfully prepared. The sintering parameters were shown as following: sintering pressure was set as 30 MPa, the three-step sintering temperature was 1100 ? -1550 ?- 1800 ?and the duration time were set as 5-5-6 min. The influences of sintering, temperature, content of in-situ formed W₂B₅ and Gr in the composites on microstructure,room temperature mechanical properties were studied. Densification behavior and strengthening and toughening mechanisms were also investigated.The results showed that in-situ reaction between B-Gr-WC can promote the densification process of B₄C composite ceramics, the relative density of composite ceramics can reach 100%and the hardness can attain to 36.4 GPa sintered at 1700 ?. Compatibly, B₄C ceramics using B₄C powder as raw powder sintered at the same temperature showed the density of only 98% and the hardness of 35 GPa. When the sintering temperature was increased from 1600 ? to 1800 ?, the density and hardness of B₄C-W₂B₅ composite ceramics were increased gradually and which showed a higher value than B₄C ceramics prepared by B₄C raw powder. The result indicated that the in-situ reaction of WC, B and Gr is an exothermic reaction, which activated the sintering activity of the powder mixture, promoted the densification behavior of the composite ceramic,thus decreased the densification temperature.The effect of the different content of W₂B₅ on the properties of B₄C was the object of this study, in the process the hardness and fracture toughness of B₄C can achieve balance with different WC content as the sintering aid. The microstructure analysis shown that in-situ formed strip-shaped W₂B₅ particles were dispersed homogenously in B₄C matrix. With the increasing content of WC, the content of the white-gray strip-shape W₂B₅ particles showed increasing tendency and the grain size was refined by pinning-effect of W₂B₅ which evenly distributed in B₄C matrix. Especially, W₂B₅ particle pull-out phenomenon was found in the fracture surface,the crack deflection and bridging of W₂B₅ grains was discovered in the propagation of cracks.The experimental results showed that when W₂B₅ : B₄C>1: 5 ?molar ratio?, the density of composite ceramics can reach 100%, the hardness and fracture toughness had a maximum value of 37.9 GPa,7.8 MPa-m1/2 when W₂B₅ : B₄C =1 : 4, the fracture toughness increased by 58%compared with B₄C.with the increase of W₂B₅, the hardness declined but the fracture toughness increases, when W₂B₅ : B₄C = 1 : 2, the hardness is 28.8 GPa, fracture toughness can reach 10.9 MPa·m1/2. Crack bridging, deflection, and pull out of W₂B₅ particles were the main toughening mechanisms of composites.In order to research the effect of the Gr on the properties of the as-fabricated composite ceramics, the powder mixture of B-Gr-WC with various amounts of Gr powder were blended and consolidated by SPS. TEM observation indicated that the length of W₂B₅ particles was about 1?m and the width approximately 0.3 ?m, the length of graphite was about 200?280 nm,the width was about 70?85 nm,there were no pores and microcracks were present among W₂B₅ and Gr and B₄C particles. The results of HRTEM showed that the interface among the W₂B₅, B₄C and the residual Gr were very clean and intimate boundary, there were no any reaction phase and amorphous layer can be detected in the interfacial zone. The firm interface bonding can bear larger load when the composite ceramic subjected to extremal load, which can be greatly beneficial to the mechanical properties of composite ceramics. The experimental results showed that appropriate content of 5 vol% Gr showed positive effect on the mechanical properties which achieved an optimal counter-balance of fracture toughness and hardness, the relative density was 99.9%, the Vickers hardness can reach to 30.2 GPa, and the fracture toughness was 11.9 MPa·m1/2 when the sintering temperature was set at 1700 ?. But the relative density, hardness and fracture toughness were declined. The cracks come across the soft nano-sized graphite, it can be rapidly released the stress of structure at crack-tip, which can be effectively slowing the propagation of the crack, thus, enhanced the fracture toughness, which were main toughening mechanisms.