Measurement And Characterization On Residual Stress Of ZrC-SiC_w Ultrahigh Temperature Composite Ceramics

Researching of the excellent performance of ultra high temperatureceramics(UHTCs) has a great influence on the key technical problems in the fieldsof aviation and aerospace. Residual stress of the composites will appear accordingto the difference of the thermal expansion coefficient in the process of hot-pressingsintering. Harmful residual stress can damage the composites while the beneficialresidual stress can enhance the strength and the toughness of the composites. Thestudy and measurement of the residual stress which have been the key points anddifficulties of the research both in China and abroad because of the high brittle, arevery important on the development of the materials.By means of a lot of exploratory experiments, this paper optimized theparameters of the X-ray stress analysis method according to the compositionstructure of the ZrC-SiCwultrahigh temperature composite ceramics. Theparameters includes the wavelength, the diffraction Angle, the diffraction crystaldiffraction line, the ? Angle, the elastic changshu, the spot diameter and theexposure time. The paper also analyzes three kinds of method used for setting theXRD peak to find the best one. X-ray stress analysis method and the Eshelbyequivalent inclusion theory were used to acquire the surface residual stress of theZrC-SiCwcomposites. Analysis the results of the residual stress contacting themechanical properties and the microstructure of the ceramics to explore theinfluence law of the content and distribution of the whisker to the surface residualstress of ZrC-SiCwcomposites and to find the relationship between the surfaceresidual stress and the properties of the composites such as flexural strength,fracture toughness and the antioxidant ablation performance.Via a lot of exploratory experiments, we can find that the Cauchy is theoptimal solution to set the XRD peak rather than the other two common method:Parabola and Gaussian for the ZrC-SiCwcomposites according that Cauchy peakfinding method has a best match to the real diffraction peak.Through the contrast of the residual stress, the mechanical properties andablation properties of the ZrC-SiCwcomposites, we can discover that the surfaceresidual compressive stress can enhance the strength and the toughness of thecomposites. The strengthening and toughening mechanisms are as follows: first ofall, the surface residual compressive stress can prevent crack propagation. Secondly,a part of load can be offset by the compressive stress. Finally, the base material ZrCis pulled by the SiC whisker along the length direction of the whisker while crushed by SiC along the radial direction for that the linear expansion coefficient of SiC islower than that of ZrC matrix. the crack extending along the length direction willlead to the transgranular fracture, crack deflection and whisker bridging. Theresidual compressive stress will force the crack closing if the crack extends alongthe radial direction of SiC. The surface residual compressive stress can enhance thestrength and the toughness and damage the ablation/oxidation resistance propertiesof the composites.Via the deeply analysis of the surface residual compressive stress and the SiCwhisker content in the material composition, The phenomenon can be find that thesurface residual compressive stress grows with the increasing of SiC whiskercontent in the material composition. Excessive residual stress will be harm to theproperties of composites. Uneven dispersing of SiCwcauses the uneven distributionof the residual stress in the composites.