| Boron carbide ceramics have widely applied in the mechanical, chemical, metallurgic and military fields, owing to the properties of super high hardness, wearing resistance, corroding resistance and so on. However, compared with other ceramics such as SiC, Si3N4 and TiC, its bending strength and fracture toughness are lower. Moreover, the oxidation of boron carbide in high temperature has greatly restricted its use in air. These factors limit its application seriously.In this research, first of all, the different hot-pressing sintering temperatures and different sizes of B4C powders effect of pure B4C ceramics microstructure and mechanical properties are investigated.Then based on the studies at home and abroad on the second phase toughened B4C matrix composites, in this paper, reactive hot-pressing(RHP) is used to prepare ZrB2/B4C ceramic material by adding ZrO2 and C into B4C matrix. In-situ synthetic mechanism is studied as well as the influence of second-phase's content (by adjusting addition of ZrO2 and C) to ZrB2/B4C ceramic material microstructure and mechanical properties. Phase, grain size, fracture mechanism of ZrB2/B4C ceramic materials are investigated by X-ray diffraction, scanning electron microscope(SEM). In addition, the oxidation resistance of pure B4C ceramic material and ZrB2/B4C ceramic material is studied in this paper too. The oxidation thermodynamics and kinetic are also referred to.The results of the mechanical properties test about pure B4C ceramic materials show that relative density, bending strength and fracture toughness increased first but decreased finally and Vicker-hardness increased as the sintering temperature increasing. The better technics is that hot-pressing sintering temperature is 1950℃and the grain size of B4C powders is 1.5μm.The results of the mechanical properties test about ZrB2/B4C ceramic materials show that the better content of ZrB2 is 20wt%. The values of its relative density, Vicker-hardness, bending strength and fracture toughness are 99.3%,36.12GPa,533.33MPa and 6.95MPa-m1/2 respectively. Compare to pure B4C ceramic materials, above-mentioned mechanical properties increased 0.61%,17.23%,15.23% and 35.48% respectively. The results of the microstructure, fracture surface and mechanical properties analysis show that the mechanisms of the ZrB2/B4C ceramic material's toughness promotion are the crack deflection and branch caused by the residual stress resulting from the unmatched thermal expansion coefficient between ZrB2 and B4C and grain refinement because of second phase ZrB2. The fracture pattern of ZrB2/B4C ceramic material is the mixture of intergranular and transgranular fractures.The oxidation performance of pure B4C ceramic material and ZrB2/B4C ceramic material are investigated at different tempertures. It is shown that the oxidation about ZrB2/B4C ceramic material is selective oxidation. B4C is oxidized firstly and its oxidation is very greatly. The oxidation of ZrB2 only at 850℃is increased and at this temperature the oxidation film is compactness. After the oxidized activation energy computation, the pure B4C ceramic material and ZrB2/B4C ceramic material oxidized activation energy are 98.11kJ/mol and 147kJ/mol respectively. By comparison, ZrB2/B4C ceramic material have higher oxidation resistance than pure B4C ceramic material. The oxidation mechanism of these two ceramic materials as follows:at earliest oxidation phase, chemical reaction at the material surface controls oxidation process. The speed of oxidation is very fast. As oxidation times increasing, the speed of oxidation is decreased. This moment, the diffusion reaction of O2 pass oxidation film controls oxidation process. |
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