In-situ Fabrication Of B₄C CNTs Composite Ceramics And Their Toughening Mechanism

Boron carbide ?B₄C? has many excellent properties, such as high hardness, high modulus,low density as well as good wear resistance, anti-oxidation, anti-corrosion and good neutron absorption properties, which make it possess wide application prospect in the fields of cutting tools, protective armor materials, nuclear shielding materials and corrosion and wear resistance materials and so on. However, the application of boron carbide has been greatly limited by its low fracture toughness and high sintering temperature resulted by the strong covalent bonds of itself.In this study, in-situ reaction was adopted to lower the sintering temperature of B₄C and carbon nano tubes ?CNTs? was used to enhance the fracture toughness of B₄C. High purity boron?B?, graphite ?Gr? and CNTs were mixed with the help of planetary ball milling machine and CNTs toughed boron carbide ?CNTS/B₄C? composite ceramics were prepared by spark plasma sintering ?SPS?. Phase composition, microstructure and mechanical properties of composite were characterized by the mearusement of X-ray diffraction ?XRD?, scanning electronic microscopy?SEM?, laser Raman spectroscopy ?LRS? and so on. The densification process of B₄C and the effect of sintering temperature, holding time, powder size and the volume percent of CNTs on the densification process and mechanical properties were studied. At the same time, the toughning mechanism of CNTs reinforced B₄C ?4B+C? composite ceramics was also studied.Firstly, boron and graphite powder with mole ratio of 4:1 were milled by dry milling and wet milling when ball to powder ratio was 10:1. Homogeneous 4B+C powder mixture with media particle size of 280 nm was obtained after 9 h planetary milling at the presence of ethanol. It turned out that diameters of 4B+C by dry milling and wet milling both decreased with the increment of milling time. However, attributed to the prevention of ethanol to the aggragation of particles, wet milling could get more fine powder compared with dry milling. Besides, CNTs could be homogenously dispersed in mixed powders by ultrasonic wave vibration and mechanical stirring.Secondly, accordinging to DSC of 9h milled 4B+C powder mixture and XRD of sintered B₄C_4B+C ceramics at different temperatre, the starting reaction temperature of B and C from 4B+C powder mixture was 1160 ? and the production of B₄C_4B+C occured at 1200 ?. The production of B₄C ?4B+C? gradually increase when increased temperature and the reaction completed at 1700 ?. And SPS sintering process of B₄C ?4B+C? could be composed of three stages according to temperature and displacement curves of B₄C ?4B+C?. The shrinkage below 700 0C in first stage mainly originated from the mechanical compression of powders. Although B₄C_4B+Ccould be found in 1200 ? sintered sample, the low temperature was difficult to get high relative density, therefore little displacement was generated in second stage from 700 ? to 1550 ?. In third stage,begun at 1550 ?, samples shrinked greatly induced by the synthesis of B₄C ?4B+C?and the densification of B₄C (4B+C. With the increase of the sintering temperature, the relative density of B₄C ?4B+C? gradually increased and fully densification was achieved at 1800 ?. The longer holding time the bigger relative density when the time is less than 30 minutes, and the decreased relative density was obtained in 60 minutes holding time. So, the following sintering parameters could be proper in this paper: 100 ?/min, 1800 ?, 10 minutes and 30 MPa. Besides,the densification process of 10vol%CNTs/B₄C was similar to that of B₄C ?4B+C?. The relative density and hardness of composite decreased with the increase of CNTs addition.At last, the mechanical properties of sintered samples were studied. It found that the relative density and hardness of B₄C ?4B+c?, 10vol%CNTs/B₄C and B₄C ?B₄C? both added with the increment of the sintering temperature and B₄C ?4B+C? with smaller grain size than B₄CB₄C. Fully dense B₄C ?4B+C? reached maximum of hardness 39.6 GPa at 1800 ?. Less than 30 minutes, the relative density and hardness of B₄C ?4B+C? increased with the augment of holding time and then decreased when the holding time was 60 min. The addition of CNTs reduced the relative density and hardness of B₄C ?4B+C?. At the same time, the bending strength and fracture toughness of composite firstly increased and then decreased with the change of volume percent of CNTs. The bending strength and fracture toughness of composite reached peak value when the volume percent of CNTs was 7%. Crack bridging and deflection, fiber pull-out and breakage were observed in materials which would be beneficial to the augment of mechanical properties.