Experimental Characterization And Modeling Of Oxidation Properties Of ZrB₂- Based Ultrahigh-Temperature Ceramics At High Temperatures

Ultrahigh-temperature ceramics are a family of materials including borides, carbides, and nitrides of early transition metals, of which the melting temperatures are generally over 3000 oC. Zirconium diboride, a key member of these materials, is specifically considered as one of the most promising candidates suitable for a future generation of thermal protection system used in aerospace field due to its unique combination of extremely high melting temperature, relatively low density, high thermal and electrical conductivities, high strength and modulus, and good physical and chemical stabilities. However, the relatively poor oxidation resistance at high temperature may limit its applications. In this thesis, studies on the high?temperature oxidation properties were conducted, and the main contributions include three aspects: developing a testing device, experimental characterization, and modeling.Taking advantage of the ideal electrical conductivities of Zr B2 based ultrahigh- temperature ceramics and in order to study their oxidation properties, a high-temperature testing device was designed to be capable of applying multi-physical loadings, such as electrical, thermal, mechanical, and magnetic loading. The testing system has many merits, such as extremely high heating temperature, ultrahigh heating rate, low energy consuming, and high testing efficiency, etc. It can measure the real-time temperature field and capture the surface morphology of the testing specimen simultaneously. Additionally, the effect of the applied stress on oxidation behaviour can be studied using this apparatus by applying external tensile loading, bending loading or the both.High-temperature oxidation properties of Zr B2 based ceramics with Si C additive were investigated experimentally using the developed testing device. The ultrahigh heating rate of nearly 3900oC/s was obtained during a rapid heating process. A further theoretical analysis showed that the inputted electrical current, temperature, material properties, and dimensions of the specimen govern the heating rate. The oxidation behavior from the initial heating to the failure of the specimen can be divided into three different stages. In each stage, the longitudinal distribution of the temperature in the specimen was measured, and the corresponding morphology of the surface of the specimen was observed synchronously, in which the impacts of oxidation on temperature measurement can be characterized. And an explanation of the failure mechanism of the specimen was suggested.Finally, a thermo-chemo-mechanical model was proposed to describe quantitatively the oxidation properties of normally pure Zr B2 by using the micromechanics and thermodynamics methods. Taking the structural interaction of the oxide scale and substrate into consideration, the model can describe the relation between the stress state in the oxide and diffusion property of oxygen through the oxide. The prediction results, for example, the evolution of oxide thickness, are in excellent agreement with the experimental data. The effects of some influencing factors were discussed and a reasonable value of lateral growth parameter was recommended.