Ultra-high temperature ceramics (UHTCs) are mainly composed of refractory metal borides, carbides and nitride compounds with high melting point, which are key thermal structural materials such as nose cone, wing leading edge, scramjet engine leading edge strutches and combustion chamber of ultra-high sound speed aerospace aircraft. Antioxidation and toughening are the difficulties in UHTCs research and the bottleneck problems that limit their application. The introduction of SiC second phase can improve the oxidation resistance of UHTCs, reduce the sintering temperature and inhibit the high-temperature grain growth, which is the focus of recent research. Polymer precursor ceramics (PDC) is prepared by polymer molding and pyrolysis, and the composition (chemical composition, phase composition) and microstructure (nano/micron structure) of the ceramics are controlled by the atomic scale design of the polymer precursor to obtain the desired properties. In this paper, for the first time, through the design and synthesis of anaerobic single source polymer precursor, ZrC-ZrB2-SiC nanocomposite ceramics were successfully prepared by PDC method, which provides a new idea for the preparation of this kind of UHTCs multiphase ceramics.
In general, the self-diffusion coefficient of refractory metal borides and carbides is very low, and the powder sintering method requires a high temperature of nearly 2000℃ to make them compact. At this high temperature, the ceramic grains grow rapidly. The grain size of the multiphase UHTCs prepared by this method is in the sub-micron-micron level, and it is impossible to prepare nano-ceramics with one phase less than 100 nm. PDC method is an effective way to prepare nano-UHTCs, but there are few reports on its polymer precursors. Generally, oxygen-containing polymer precursors are used to form refractory metal oxides by pyrolysis at low temperatures, and then are prepared by high temperature carbothermal reduction reaction. Carbothermal reduction produces a large amount of CO and CO2, resulting in very low ceramic yield and forming defects such as pores in the material. In addition, when used for substrate and surface modification such as CF /C and CF /SiC, the oxides in precursor cracking products can react with carbon fibers to cause damage. Therefore, anaerobic polymer precursors are the focus and development direction of multiphase UHTCs precursors. In this paper, multiphase ZrC-ZrB2-SiC nano-ceramics were prepared by heat treatment at 1400℃ using anaerobic polymer precursor to avoid high temperature carbothermal reduction reaction. Preceramic polymer system is studied in the process of transition from organic and inorganic structure evolution, examines the ceramic from the amorphous crystalline chemical composition, phase composition in the process of evolution, evaluated the high-temperature properties of nanocomposite ceramics, the results show that the PDC prepared ZrC - ZrB2 - SiC nanometer ceramics after 1800 ℃ heat treatment, remains below 100 nanoscale microstructure and low thermal weightlessness, confirmed that this kind of materials, excellent resistance to high temperature performance.