| In this thesis, Zr B2-based ultra high temperature ceramics was chosen as the research object. For the material of Intrinsic brittleness and internal defects affecting its performance in extreme conditions.ultrasonic non-destructive testing and Nano-CT implements were used to achieve accurate characterization and quantification of internal defects.Firstly, By ultrasonic nondestructive testing method to study the influences of different parameters on the detection results, and the conclusions was drawn that The higher the frequency of transducer, the better the detection resolution. Using 230 MHz high frequency transducer to test the different components of ultra high temperature ceramic, and analyzing the advantages and disadvantages of ultrasonic testing Combined with the microstructure. The surface roughness after polishing materials was tested, it is foud that the addition of graphite layer reduces the hardness of the material,however, it makes the material more polished and has the lowest surface roughness.Next, the laser was adopted to process the micron level of defects on the surface of voidless Zr O2 and non dense Zr B2 ceramic,based on Zr O2 fitting function of defect depth and intensity, it is drawn that Critical defect size affecting Zr B2 ceramic strength.Secondly, according to Nano-CT method researching relative density distribution in different position of body, diameter distribution, volume distribution and orientation of hole. After test, it is verified that the hole causes the internal defects during the sintering process of ultra high temperature ceramics. Based on this, the Zr B2 ceramics was studied which density is about 93%, and it is found the density difference is very big in different body. Besides, according to the study using the VG Studio MAX software to set a direction then calculate the average density of each area, it is proved that the porosity in the middle part is higher than the edge of body parts.Finally, Si C particles and graphite layer was studied about their effect on internal holes of Zr B2 ceramic. It is found that internal hole number is logarithmic relationship decreasing with increase of the hole diameter. Then a cube was selected in which side length was 300 microns, finding the internal diameter of less than 5 microns is accounted for the vast majority of the number of holes. Adopting hole X, Y, and Z shaft hole boundary box size ratio to research oriented of holes along the hot pressing direction. It is drawn that the hole in the middle of the body orientation is more obvious.Using the same methods to analyze the feature information of the internal hole in Zr B2-Si C ceramics, it is verified that the addition of Si C particles promotes the sintering,inhibits grain growth, improves the relative density. At the same time,it reduces the diameter and volume of holes, markedly higher orientation of holes. All is for theaverage size of Si C particle is bout 500 nm, it can fills holes when its lattice diffusion is in the final-stage sintering, reduce the diameter of holes and the hole, bring more pressure on unit surface area of hole. The addition of graphite layer improves the material fracture toughness. Compared with the Zr B2-Si C ceramic, graphite layer increases diameter and volume of holes as well as the orientation of holes. Because of the different coefficient of thermal expansion between graphite and Zr B2, around the graphite layer, there are exist a certain amount of holes and high orientation,so holes orientation of Zr B2-Si C-G is improved. |
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