Temperature And Strain Measurement And Mechanical Properties Testing Of ZrB₂-Based Ceramics At Ultrahigh Temperature

Ultra-high temperature ceramics(UHTCs)are a family of materials that include borides,carbides and nitrides of the group IVB and VB transition metals.Their melting temperatures usually exceed 3000?.Among UHTCs,Zr B2 shows unique combination of chemical stability,high electrical and thermal conductivities,and resistance to erosion/corrosion,which makes it suitable for the cases of extreme chemical and thermal environments.The testing of the thermal-mechanical properties of UHTCs is important for their applications,but there is still a lack of the corresponding testing devices.A home-made testing device that can meet the need has been developed,but two key problems remain unsolved,which are related to the measurements of temperature and strain,respectively.In this thesis,aiming at these two problems,we preliminarily explored the temperature and strain measurement methods at ultrahigh temperature and developed the corresponding devices.The developed temperature measurement method integrates an industrial camera and a 2-color pyrometer and can achieve the reconstruction of the temperature distribution over the interested part of surface of the testing specimen.In this method,the relationship between temperature and gray level should at first be established,the parameters in this relationship can be identified with the temperature and grayscale data obtained in an experiment.Using the relationship,the whole temperature distribution can be reconstructed from the grayscale image captured by camera.The application of conventional strain measurement methods can hardly be used in the cases of ultrahigh temperature.A non-contact video-extensometer method is suggested for the measurement of the axial strain of the specimen tested at ultrahigh temperature.In this method,some special gauge points are designed on the specimen tested,and the locations of these gauge points are tracked in real-time using an industrial camera and processed with a correlation algorithm.The change of the gauge length between two gauge points is simultaneously determined,with which the axial strain can be obtained.Thermal shock and thermal fatigue experiments were conducted to investigate the effects of tensile prestress on the residual flexural strength and thermal fatigue life of Zr B2-SiC-graphite specimens.For repeated thermal shock under different levels of tensile prestress,the residual flexural strengths of the specimens subjected to 10 and 30 cycles of thermal shock between room temperature and 2000? are measured.It was shown that for the specimen subjected to 10 cycles of thermal shock,the residual flexural strength does not show a distinct change under different levels of tensile prestress.But the residual flexural strength reduces distinctly in the cases of 30 cycles of thermal shock.In the thermal fatigue tests,specimens under different levels of tensile prestress are subjected to thermal cycles between 25? and 2000? until fracture.The results show that thermal fatigue life decreases remarkably with the increase of level of the applied tensile prestress.The SEM metallographs show different fracture morphologies under different levels of tensile prestress,implying different fracture modes.Thermal fatigue of specimens under different levels of tensile prestress subjected to thermal cycles between 25? and 2200? is also tested.Although the thermal fatigue life shows the same tendency,the maximum thermal fatigue life is sharply reduced,which can be presumably attributed to the decrease of the bear loading capability of the material at ultrahigh temperatures.