Research On Microstructure Characterization And Mechanical Properties Of Ultra High Temperature Ceramic Subjected To Thermo/ Mechanical/Oxygenic Environment

As non-ablative thermal protection materials, ultra-high temperature ceramic(UHTC) materials have excellent mechanical and physical properties at high temperature. In the family of ultra-high temperature ceramic materials, ZrB2 has minimum theoretical density, it is widely used in the aerospace field. In the service process, the material often suffer from extremely harsh environment filled by thermal/mechanical/oxygen. It is vital to research the mechanical properties of the material in this extreme environment and the evolution of the mechanical properties for improving the reliability.Firstly, the uniaxial tension test methods of UHTC materials during room/high temperature are built according to ASTM standards and national standards. The methods are proved to be reasonable using the finite element method and the uniaxial tension tests at room and high temperature in vacuum, and the intrinsic strength is got simultaneously.Secondly, The specimens are pre-oxidized at difficult temperatures and time, then the changes of tensile strength influenced by temperature and time are got compared with the intrinsic strength of the material at room temperature, the strength reaches the maximum level at 1200 ℃, 30 min while at 1000 ℃, 30 min, the strength reaches the minimum level. Mechanism of the strength affected by oxidation are found using scanning electron microscope(SEM) and energy dispersive spectrometer(EDS), it suggests that the strength is improved when the effect on strength due to surface micro-defects healed by oxidation product is greater than the new defects due to oxidation; on the contrary, the strength decreased.Finally, the thermal/ mechanical/oxygen uniaxial tension test methods are built by the design of high temperature clamp, the induction coil and the temperature measurement, the key environment control factors are got using orthogonal experimental design, it suggests that the temperature is the most important factor and the tension stress is the less. Compared with the strength at high temperature in vacuum, the oxidation effects on high-temperature strength are studied. It shows that the strength reaches the maximum level at 800 ℃-10min-60 MPa and the minimum level at 1000 ℃-30min-0MPa. According to SEM and EDS, the mechanism of the oxidation effect on strength at high-temperature in vacuum is similar with the situation at room temperature; What’s more, the tensile stress has a certain effect on the oxidation behavior, mainly because of the tensile stress increases the oxygen diffusion channel size, thus accelerates the reaction rate.