Research On The Fracture Mechanismand Crack Propagation Of Non-Oxide Ceramics At Cryogenic Temperatures

With the progress of technology,cryogenic technology has been widely used in the field of aeronautics and astronautics,thermonuclear fusion,and superconducting.Structural ceramics with high hardness,wear resistance,and electrical insulation can be essential complementary with the metal and polymer materials for some cryogenic applications,and obtain unique advantages.However,there are few studies on the fracture mechanism and properties of the non-oxide ceramics at low temperatures.In this work,the relationship between fracture behavior,crack propagation,crack bridging,pores and mechanical properties of several non-oxide ceramics at cryogenic temperatures were investigated,and the results revealed the mechanisms and reasons of property changes at low temperatures.As the temperature decreased from 293 K to 77 K,the fracture toughness,bending strength and Weibull modulus of aluminum nitride ceramics were found to increase at the same time.The main reason for these changes are the increase of fracture surface energyat low temperatures.On the other hand,by comparing the different fracture modes between silicon nitride ceramics and aluminum nitride ceramics at low temperatures,we found that the properties,volume fraction and distribution of grain boundary phase have an effect on the fracture mode of ceramics at low temperatures.The steady-state fracture toughness of silicon nitride ceramics increases by 25.4% from 5.19MPa·m1/2 to 6.51MPa·m1/2as the temperature is reduced from 293 K to 77 K.With the help of in situ Raman Spectroscopy,the bridging stress of a certain bridge structure was determination.The maximum stress increased from 0.7GPa at 293 K to 1.0GPa at 77 K,which is one of the main reasons for the enhanced toughness of silicon nitride ceramics at cryogenic temperatures.4323 crack deflections and 320 bridging structures in 60 cracks thatgenerated at different temperatures were investigated for statistical analysis.Although the residual stress caused by low temperature can affect the proportion of intergranular fracture,the angle distribution of intergranular crack deflection and the probability of deflection turning into bridging are determined by material microstructures,without influence of temperature changes.In the system of Si3N4-Y2O3-Al2O3,the change of sintering additive mass fraction and the ratio of Y2O3/Al2O3 will lead to variations of grain boundary phase volume,thermal expansion coefficient and microstructure of silicon nitride ceramics.Those changes all have an influence on the low temperature properties of materials,but the microstructure change is the dominant factor.At low temperatures,the elastic modulus of silicon carbide grain increase at low temperatures,and the enhanced extension resistance of pore-induced cracks lead to enhanced fracture strength of the material.On the other hand,the effective sizes of pore-induced cracks in pressureless sintered silicon carbide ceramics will decrease and lead to increasing Weibull modulus.