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

Currently, the rapid development of cryogenic techniques makes them required fora wide range of applications such as aeronautics&astronautics, superconducting fields,nuclear fusion and so on. Structural ceramics are promising cryogenic structuralmaterials which are prior to some kinds of metals and polymer materials in somespecific applications. However, more systematical and intensive researches on theproperties of structural ceramics at cryogenic temperatures are quite required becauseonly a small quantity of researches have been reported on such issues.By considering the relationship between cryogenic temperatures and typicaltoughening mechanisms of structural ceramics, the correlation between cryogenicproperties and several important issues including transformation, fracture mechanismsand residual stress have been investigated.A more pronounced R-curve behavior of3Y-TZP unanticipated by theconventional interpretation was found at cryogenic temperatures, which wasaccompanied by the concomitant increment of the fracture strength, toughness andWeibull modulus at cryogenic temperatures. This is due to the enhanced transformationtoughening effects at cryogenic temperatures. In addition, the transformation zoneparameters, discerned from accurate measurements with Raman microprobespectroscopy, were used to evaluate the shielding stress intensity factors. Bycomparison with the obtained experimental parameters from R-curve measurement andanalysis from the viewpoint of fractography at room and cryogenic temperatures, thefactors which may affect the cryogenic fracture toughness of3Y-TZP have beendiscussed.The critical grain sizes of2Y-TZP at ambient and cryogenic temperatures havebeen determined. The grain size dependence of fracture toughness revealed that themaximum toughness value corresponds to the critical grain size at each temperature. Onthe basis of nucleation and thermodynamic theories, an established linear relationshipbetween inverse critical grain size and temperature has been explained and a basicunderstanding of grain size dependence of toughness at room temperature has been alsogained. It is believed that this relationship can provide an effective way to optimize the fracture toughness of2Y-TZP ceramics at cryogenic temperatures.Fracture toughness values of Si₃N₄and RBSiC ceramics were found to beenhanced at cryogenic temperatures, respectively. On application of higher residualstress at77K, a larger number of Si₃N₄grains become involved in the crack deflectionprocess, leading to a larger percentage of intergranular fracture and thus enhancedfracture toughness of Si₃N₄ceramics. Moreover, the enhanced fracture toughness ofRBSiC ceramics at77K could be explained by the stronger resistance to crackpropagation resulting from higher residual stress at77K. Flexural strengths of Al₂O₃ceramics with no obvious toughening mechanisms were also found to be similar at bothcryogenic and ambient temperatures. However, fracture toughness of99%Al₂O₃ceramics tended to increase with decreasing temperatures.As discussed above, the mechanical properties of structural ceramics can be similaror even higher compared to those at ambient temperatures. Therefore, such favorableperformances coupled with low thermal conductivity and good thermal stability makestructural ceramics candidate materials for thermal insulation and supportingapplications in cryogenic engineering.