| Boron carbide is a kind of important special ceramics, which has excellent properties, such as high hardness, low density, high melting point, and high capability for neutron absorption, as well as good resistance to chemical corrosion. However, the shortage of boron carbide material is its extreme susceptibility to brittle fracture, which limit its application. In order to improve its fracture toughness, CeB6/B4C ceramic material were prepared by in-situ synthesis and hot pressed sintering method. The mechanical properties and microstructure of pure B4C ceramic material, hot pressed sintering CeB6/B4C ceramic material and in-situ synthesis CeB6/B4C ceramic material were studied respectively, and the thermodynamics and kinetics of CeO2-B4C-C system were researched, the interface microstructure, and in-situ reaction hot pressed sintering mechanism, as well as toughening and reinforcing mechanisms of in-situ synthesis CeB6/B4C ceramics were also investigated. The studied results indicated that:In the case of 2040℃×20MPa×40min, the density of pure B4C ceramic material is 2.419g/cm3, the Vickers-microhardness 31.34 GPa, the bending strength 268.76 MPa, the fracture toughness 3.14 MPa·m1/2. With the temperature increasing, its porosity decreases first, then increases. The effect of sintering time on the porosity of pure B4C ceramic material isn't obvious. However, its crystal grain grows with sintering time, and especially grows more fast when sintering time is much longer at high temperature.In the case of 1950℃×20MPa×40min, the relative density of hot pressed sintering CeB6/B4C ceramic material is 96.95%, the Vickers-microhardness 39.97 GPa, the bending strength 313.81 MPa, the fracture toughness 4.55 MPa-m1/2, which enhance 0.96%, 27.53%,16.76% and 44.90% respectively compared to that of pure B4C ceramic material. The sintering temperature of boron carbide ceramics is decreased by adding second-phase CeB6. The porosity of hot pressed sintering CeB6/B4C ceramic material decreases and there is no crystal grain growth.In the case of 1950℃×20MPa×40min, the relative density of in-situ synthesis ceramic material is 97.92%, the Vickers-microhardness 42.01 GPa, the bending strength 346.75 MPa, the fracture toughness 5.95 MPa·m1/2 which improve 1.01%, 5.10%,10.50% and 30.77% respectively compared to that of hot pressed sintering CeB6/B4C ceramic material, and improve 2.01%,34.04%,29.02% and 89.49% respectively compared to that of pure B4C ceramic material. A amorphous transition layer of 5nm-width CeB6, synthesized in-situ by CeO2 and B4C, make the interface combine very well, and intensify the combination strength of interfaces. The in-situ synthesized CeB6 greatly impact grain boundary mobility, and inhibits grain growth. Therefore, in-situ synthesized CeB6 disperse among the structure, and play good role at grain refinement. The main toughening and reinforcing mechanisms of in-situ synthesis CeB6/B4C ceramic material were:fine grain toughening and reinforcing, the crack deflection caused by the residual stress resulting from the difference in thermal expansion coefficient between CeB6 and B4C, the second-phase CeB6 pullout and intergranular crack.The chemical reaction among CeO2, B4C and C under high temperature belongs to CeO2-B4C-C multicomponent chemical reaction system. The TG-DTA and XRD experimental results illustrates the process of CeO2, B4C and C generating CeB6 undergo five steps, firstly, at 650～910℃, CeO2, B4C and C react into CeBO3, CeBC and B, and the apparent activation energy (E1) is 271.5kJ·mol-1, the series (n1) is 0.24; secondly, at 1270～1309℃, CeO2 and B4C react into CeBO3, B and CeB4, at the same time, CeBO3, CeBC and B4C react into CeB4, and the apparent activation energy (E2) is 241.9kJ·mol-1, the series(n2) is 0.16; thirdly, at 1317～1370℃, CeBO3, B4C and C react into CeB4 and B, and the apparent activation energy (E3) is 715.1kJ·mol, the series(n3) is 0.41; fourthly, in 1371～1420℃, CeO2, B4C and C react into CeB6 and CO; fifthly, at the temperature of 1460℃,CeB4 and B further react into CeB6, and the apparent activation energy (E4) is 327.1 kJ·mol-1, the series(m4) is 0.13. With the prolonging time, the content of CeB6 increases but the content of CeB4 and B decrease, the content of CeB6 is 97.8% at 2 h, and most CeB4 changes into CeB6. It is feasible that in-situ synthesis CeB6/B4C ceramic material by the reaction of CeO2, B4C and C above 1460℃.
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