The Research On Fabricate Process And Microstructure Analysis Of Boron Based Ultra High Temperature Ceramics

Ultra high temperature ceramics (UHTCs) is becoming more and more important in national defence and astronavigation region. In-situ reactive synthesis provides a new method to fabricate UHTCs as another way. In this thesis, a series of ZrB2-SiC composites have been fabricated by in-situ reactive synthesis successfully, and the volume content of SiC is 10, 15, 20, 25 respectively. The designed compositions, reaction thermodynamics, reaction kinetics, preparation process and microstructure of ZrB2-SiC CMCs have been investigated. The relations between the content of SiC and phase compositions, microstructures and mechanical properties have been investigated.The thermodynamics has been calculated with the Zr-B4C-Si as raw materials, the process of reactive synthesis has been investigated, and the results showed that ZrB2, SiC, ZrC would be the products finally. The raw materials will reactive fully with sustaining for 60 mins above 1200℃, and phases in every specimen consistent with the outcomes of thermodynamics calculation in the final. The ZrB2 is close-packed hexagonal with cell parameter a=3.170 , c=3.530 , ZrC is face-centered cubic with cell parameter a=4.669A ,β-SiC is face-centered cubic with cell parameter a=4.358 . However, theβ-SiC will change toα-SiC in ZrBAoAo oAo2-20vol%SiC sample when sintered at 1900℃.The density increased as the amount of SiC increased. The SiC grains distribute between ZrB2 grains, preventing the grain boundary moving ahead and reducing the size of grains remarkable. It's the finest grains in the sample ZrBB2-20vol%SiC, the ZrB2 grain is columnar with raial size 0.4~1μm and axial size 1.5~3μm, the SiC grain is equiaxed with the size less than 2μm, the ZrC grain is equiaxed with the size 1.5~2μm. The ZrB2 grains change from columnar to equiaxed partly when SiC volume content reaches 25vol%, and the ZrB2 grains aggregate obviously.When the volume content of SiC is 20vol%, bending strength and fracture toughness reached the maximum values, 645.8MPa and 5.66 MPa·m1/2 respectively, it's due to compact structure, fine columnar grains and limited ZrC...