Fabrication And Analysis Of Coated ZrB₂-ZrO₂ Ultra High Temperature Ceramic

Since the discovery of modem high-temperature technology, materials are required with excellent high temperature properties to adapt to the harsh operating environment. Zirconium diboride (ZrB₂) is widely used in high-temperature ceramics, composite materials, refractory material, electrode materials and nuclear materials, due to its high melting point and high hardness, good conductivity and heat conductivity and many other advantages. But its brittleness, low bending strength, high temperature oxidation and other shortcomings restrict the scope of application. Therefore, people employ a variety of toughening means to develop different types of ZrB₂ composite. In this paper, coated ZrB_2-ZrO₂ composite powder is prepared by non-homogeneous nucleation measure. The relation of synthesis conditions and composite structure of coated powder is researched to determine the optimal synthesis conditions; the coated ZrB_2-ZrO₂ composite powder is sintered by spark plasma sintering technique to prepare the ZrB_2-ZrO₂ composite materials. The physical properties, thermal shock resistance, microstructure and high temperature oxidation of the ZrB_2-ZrO₂ composite materials are researched.In the preparation of the coated ZrB_2-ZrO₂ composite powder, we study the influence of the pH value, the zirconia addition, the consistency of ZrB₂ scattered solution, the consistency of ZrOCl₂ solution, titration speed to the microscopic structure of coated powder. The results showed: when experimental conditions were as follows: in pH=5, titrate the ZrOCl₂ solution (0.04mol/L) to ZrB₂ scattered solution (0.03g/ml); and then in pH=9 titrate YCl₃ solution (1.5ml/min) (which are 1.5ml/min), the powder coating samples can achieve good coating effects.The microstructure and phase composition of coated ZrB_2-ZrO₂ composite powder are tested by SEM, TEM, XRD method. Test results show that: the main component of coated powder is ZrB₂ and monoclinic ZrO₂. Nano-zirconia grain bonds on to the surface of ZrB₂ powder.The coated ZrB₂-ZrO₂ composite powder is sintered by spark plasma sintering technique to prepare the ZrB_2-ZrO₂ composite materials. We study the influence of the zirconia addition, sintering temperature, sintering time to the density and microstructure of the ZrB_2-ZrO₂ composite materials. The results showed that the density of composite materials can achieve to 97.5% in the condition that 10vol% ZrO₂ addition, sintering at 1950℃, holding 10 min, sintering pressure 30MPa.The microstructure and phase composition of ZrB_2-ZrO₂ composite material are tested by SEM, TEM, XRD, EPMA method. The main components of composite materials are ZrB₂ and tetragonal ZrO₂. The ZrO₂ crystal particles are very small, when sintering completed, the crystal particles are not raised significantly, which are filling in the boundaries of zirconium diboride grain and the triangle region.In addition, the high-temperature oxidation behavior of pure ZrB₂ ceramics and ZrB₂-ZrO₂ composite material are researched by constant temperature oxidization and cycle oxidization method. The results show: the oxidation resistance of ZrB_2-ZrO₂ composite material is better than that of pure ZrB₂ ceramics, especially in the second stage in which the oxide layer formed.