Fabrication,Microstructures And Mechanical Properties Of ZrB₂-based Ultra High Temperature Ceramics

ZrB₂ based ceramics are famous ultra high temperature materials with extremely high melting point?>3000 °C?,excellent thermal conductivity and stability,high oxidation and corrosion resistances,high capturing ability of neutrons,and so on.However,because of the covalent bond structure,low self-diffusivity and oxide impurities?like ZrO2 andB₂O3?in the composites,the sintering of ZrB₂ powder requires high temperature and external high pressure,and for these reasons,the application of ZrB₂ based ceramics was limited.The purpose of this dissertation is to prepare dense ZrB₂ ceramics,by adding some high temperature resistant materials such as SiC,TaSi₂ and ZrSi₂,using hot pressing sintering or spark plasma sintering technology,and to explore the methods and processes for the preparation of ZrB₂ based composite ceramics.By analyzing the microstructure,mechanical properties and high temperature mechanical properties,the following results could be obtained.?1?ZrB₂-SiC-ZrSi₂ composites containing different fractions?0.5-5 vol.%?of ZrSi₂ were fabricated by spark plasma sintering at 1600 °C under a uniaxial pressure of 30 MPa.When the adding amount of ZrSi₂ is small,the material density is poorer;only when the adding amount was more than 5 vol.%,higher density of ZrB₂-SiC-TaSi₂ composites could be obtained,and the relative density reached 92.1%.As the amount of added TaSi₂ increased,the flexural strength of composite was slowly increased in the range of 230-293 MPa;the hardness of materials was gradually enhanced in the range of 381-873 HV5.?2?By using hot pressing sintering,nearly full dense ZrB₂-SiC-TaSi₂ composites were prepared when the adding amount of TaSi₂ was over 1 vol.%,and the maximum relative density was 98.3% when the addition amount was 3 vol.%.As the amount of the added TaSi₂ increased,the flexural strength of composite was gradually decreased in the range of 108-154 MPa;while the hardness first increased and then decreased,reaching the maximum hardness of about 1703 HV5 when the adding amount was for 3 vol.%.?3?ZrB₂-SiC-ZrSi₂ composites containing different fractions?0.5-5 vol.%?of ZrSi₂ were fabricated by spark plasma sintering.With the increase of the adding amount of ZrSi₂,the relative density of the materials increased first and then decreased,with its relative density reaching the maximum of about 85.6% when 3 vol.% of ZrSi₂ was added.The appearance of agglomerates began to reduce the relative density,and at the same time,the mechanical properties decreased;The flexural strength initially increased and then decreased with the addition of ZrSi₂,reaching a maximum of about 330 MPa when 3 vol.% of ZrSi₂ was added.The hardness of the composites would decrease after the addition of ZrSi₂ in the range of 960-1200 HV5.?4?ZrB₂-SiC-TaSi₂ composites were prepared by hot pressing sintering,and the mechanical properties was higher,compared with its SPSed counterparts.The densification of the ZrB₂-SiC-ZrSi₂ composites could be enhanced along with the increase of the added fraction of ZrSi₂,with its relative density reaching the maximum of about 90.8% when 3 vol.% of ZrSi₂ was added.The flexural strength initially increased and then decreased with the addition of ZrSi₂,reaching the maximum of about 503 MPa when 3 vol.% of ZrSi₂ was added;and the hardness of the composites was stable in the range of 985-1198 HV5.The formation of a thin and dense glass layer on the sample surface would enhance the high temperature oxidation resistance during short annealing time.The high temperature thermal shock resistance of the ZrB₂-SiC-ZrSi₂ composites would remain stable when the thermal shock temperature difference was 300 °C,and fell down sharply when it increased up to 500 °C.