Accurate Characterization And Method Study On The Mechanical Properties Of Zirconium Boride Based Ultra-high Temperature Ceramics

As ultra-high temperature ceramics(UHTCs)with high temperature resistance,eroded resistance,mechanical and chemical stability under high temperature,they have become one of the most prospective materials for the application in spaceflight,ultrasonic aircraft and propulsion system of rocket.However,their high flaw sensitivity,low fracture toughness and poor thermal shock resistance have always limited the engineering applications.Accurate characterization and evaluation of mechanical properties of UHTCs are important prerequisites for the material optimization design,material selection and engineering application.In this paper,studies were carried out from the following three aspects:flaw-strength relationship,accurate measurement of fracture toughness and reasonable characterization and evaluation of thermal shock resistance of leading edges for ZrB2,ZrB2-SiC and ZrB2-SiC-G ceramics,respectively.These results could lay the foundation for the accurate characterization of the mechanical properties of UHTCs and the perfection of test methods.Micron-sized flaws(as small as 5 μm)with different sizes,shapes,angles and positions were successfully introduced in UHTCs specimens by adjusting the femtosecond laser processing parameters.The three-dimensional shape of flaws can be accurately measured through coherent correlation interferometer.Quantitative results indicate that in double logarithmic coordinates,the strengths first remain unchanged at small flaw sizes,then decrease linearly with the increase of flaw length after a critical value.The linearly decreasing slope is dependent on the material properties,but is irrelevant with the flaw sharpness.Under mixed-mode loading condition,the effect of flaw on the strength gradually decreased with the decrease of flaw angle,and the strength value can be well predicted by the maximum strain energy release rate criterion.Furthermore,we also successfully introduced the two collinear flaws for the first time,and the results showed that when the flaw spacing is very small,the inner tips of the two flaws will start to propagate first and the flaws tend to coalescence before fracture happened owing to the strong interaction between the flaws.In order to accurately predict the material strength,based on the Emmerich’s through hole-strength model,we established a flaw-strength model of UHTCs by adding boundary conditions and experimental correction coefficients.This model has been verified that can accurately predict the strength of UHTCs with different grain sizes,which provides us a theoretical basis for the design of high-strength ceramics.Sharp notches with the tip radius less than 1 μm can be controllably introduced in almost any ceramics by adjusting the nanosecond laser processing parameters and the notching process takes only a few seconds.This method provide us an effective way to measure the fracture toughness of UHTCs accurately.The effect of notch morphology on fracture toughness measured values was systematically studied.Results show that the measured fracture toughness is closely related to the notch tip radius,notch depth and opening angle.The fracture toughness value almost remains constant when the notch tip radius(p)is less than the critical notch tip radius,and then increased gradually with increasing notch tip radius,namely,the fracture toughness value is proportional to pk,where k depending on the material itself.For ZrB2,ZrB2-SiC and ZrB2-SiC-G ceramics,fracture toughness can be accurately obtained when the ratio of notch depth and specimen height is between 0.15 and 0.6.Too deep or too shallow notches will leading to the overestimation or underestimation of fracture toughness.When the opening angle is less than 60°,the fracture toughness value remains stable and the result is accurate.When the opening angle exceeds 60°,the fracture toughness value will increase rapidly with the increase of the opening angle.It is very difficult to reasonably characterize and evaluate the thermal shock resistance of UHTCs leading edge by the traditional water quenching method.We innovatively proposed a novel water spraying method,where the assessment environment can be accurately controlled and the test results agreed well with that of wind tunnel test.Combined with the quantitative temperature measurement technology,the effects of material composition,sampling direction,leading edge size,shape,thermal shock time,thermal shock initiation temperature and cooling rate on the thermal shock resistance of UHTCs leading edges were systematically studied.The introduction of graphite flakes could significantly improve the thermal shock resistance of leading edges.The critical failure temperature increases from 350℃for ZrB2-SiC to 650℃ for ZrB2-SiC-G,and no split occurs during thermal shock.However,the thermal shock resistance of the ZrB2-SiC-G leading edge is very sensitive to the sampling direction,which could be mainly attributed to the directional arrangement of graphite flakes and the anisotropy of thermal conductivity.When the sampling direction is constant,the thermal shock resistance of leading edge is most affected by the ratio of width to length,followed by the wedge angle and is also affected by both the R value and the thickness.The larger the ratio of width to length and the smaller the wedge angle,the worse the thermal shock resistance of leading edge.The crack initiation time is dependent on the severity of assessment environment.The harsher the environment,the shorter the cracking time.The crack propagation during thermal shock could be considered as a quasistatic process(crack speed was lower than 1 cm/s)that needed to be driven by continuous cooling.When the cooling rate is constant,the decrease of initial temperature would lead to the increase of thermal stress,accompanied by a more serious damage of material,which was mainly due to the nonlinear relationship between thermal expansion coefficient and temperature.