Research On The Microstructure And Fracture Mechanism Of Oxide Structural Ceramics At Cryogenic Temperatures

With the progress of cryogenic technology and the materials widely used in cryogenic field,more and more scientists have started to focus on the application of materials used in cryogenic temperatures,which have excellent room and high temperature properties.The cryogenic fields often accompanied with corrosion,abrasion,strong electric field and magnetic field.The advanced structural ceramics have an irreplaceable application in cryogenic field because of their high strength,wear resistance,low thermal conductivity and electrical insulating.However,there are few research studying the cryogenic properties and their mechanism of advanced structural ceramics.In this paper,the influence of glass phase content,fracture mode and martensitic phase transformation on cryogenic properties for some typical oxide ceramics have been studied.The effects of different Y₂O₃ content on the fracture toughness of ZrO₂ ceramics have been studied at different temperatures.The change of monoclinic ZrO₂ phase at room and cryogenic temperatures has been measured by in-situ Raman.The influence of Y₂O₃ content on the toughening mechanism of ZrO₂ has been researched.Meanwhile,the reason why fracture toughness of 4 mol%Y₂O₃ stabilized ZrO₂ ceramic deteriorated at cryogenic temperature has been analyzed.The cryogenic properties of Al₂O₃ have been systematically studied.The study of translucent Al₂O₃ shows that the mechanical properties including flexural strength,fracture toughness and compressive strength were increased at 77 K.At the same time,Al₂O₃ containing a series of different glass phase content have been prepared.Their room and cryogenic mechanical properties have been studied,as well as the effect of glass phase content on their cryogenic properties.While the crack propagation path of 99%Al₂O₃ has been analyzed by statistical method at 500 K,298 K and 77 K,in order to study the change of fracture mode at cryogenic temperatures.The statistical analysis of indentation crack propagation paths of 99%Al₂O₃ illustrated that the proportion of transgranular fracture increased when temperature decreased from 500 K to 77 K.ZTA ceramics have been prepared at different sintering temperature by Spark Plasma Sintering?SPS?and Hot-pressing Sintering?HP?.The mechanical properties of ZTA have been investigated at different temperatures.It has been indicated that the flexural strength and fracture toughness of ZTA improved at cryogenic temperature.Subsequently,the reason why mechanical properties increased at cryogenic temperature has been studied.The toughening mechanism of ZTA at cryogenic temperature has been studied as well.In-situ Raman analysis revealed that the t-ZrO₂ phase in ZTA samples could have stress-induced martensitic transformation toughening effect.The toughening effect became stronger at 77 K,resulting in the increase of cryogenic mechanical properties.Mechanical properties of machinable mica glass-ceramics has been tested at room and cryogenic temperatures.Compared with 298 K,the flexural strength increased by 28%at 77 K,while the increasement of fracture toughness was 24%.Compressive strength of machinable mica glass-ceramics has also increased,while the higher value was obtained at 195 K.SEM analysis showed that more mica chips were pulled out at 77 K,it may increase the mechanical properties.Meanwhile,the reason for the improvement of cryogenic mechanical properties of machinable mica glass-ceramics has been investigated.The cryogenic properties analysis of these typical oxide ceramics has indicated that their mechanical properties were not deteriorated at cryogenic temperatures,some of them even increased significantly.Taking their good thermal stability at cryogenic temperatures into account,these oxide ceramics are considering as ideal candidates for cryogenic fields.