Investigation Of Properties And Microstructure Tailoring Of Boride-Based UHTCs

Zr_B2 with unique properties including high melting point,high hardness,excellent thermal shock resistance,oxidation resistance and ablation resistance,are considered as promising candidates for hypersonic applications.In order to meet the requirements in complex environments,the properties improvement of ultra-high temperature ceramics is still the main issue of current research.At present,there are some deficiencies in the research of high temperature properties and related mechanism,which will hinder the application of Zr_B2 in this field.In this study,Zr_B2 ceramics with different content of tungsten were sintered by hot pressing at 1500oC-2100oC.The self-made Zr_B2 powders were used as raw material and tungsten as additive,and the densification,microstructure and high temperature mechanical properties were investigated.At 1500oC,the diffusion from boron to tungsten formed W₂B.At 1700oC,W₂B was converted to WB.The grains with coreshell structure were found in Zr_B2-5vol%W ceramics sintered at 2000°C,the core was Zr_B2 matrix,and the shell was?Zr,W?B2-x solid solution.The shell inhibited the grain growth and densification of Zr_B2 ceramics at the temperature below 2000oC.The thickness of the shell increased and the core-shell structure of some grains disappeared when the sintering temperature up to 2100?.The densification of Zr_B2 ceramics was promoted by W-Zr-B liquid phase.WB solubilized the oxygen atoms from Zr_B2,purified the grain boundary and enhanced the grain boundary's strength.Flexural strength of ceramics with 3-10vol% W were much higher than that of Z0 at 25-1000oC.The solid solution?Zr,W?B2-x with saturated boron vacancies was transformed into?Zr,W?B2-x and?Zr,W?B under the action of tensile stress at 1600oC,which destroyed the surface state and reduced the flexural strength.The slurry with solid content of 20vol% was prepared by using self-made Zr_B2 powder as raw material and WC as additive.1wt% PEI was added as dispersant.The green body was produced by slip casting under strong magnetic field with intensity of 12 T.Zr_B2-5vol%WC ceramics were sintered by hot pressing at 2100°C,and its orientation,microstructure and high temperature mechanical properties were studied.Zr_B2-5vol%WC ceramics obtained by strong magnetic field alignment had textured structure,and its Lorentz orientation factor was 0.95.The alignment removed the largescale pores produced by anisotropic growth and improved the density of Zr_B2.Texturing was beneficial to eliminate the residual thermal stress and improved the flexural strength of Zr_B2 ceramics at low temperature??1000oC?.The plate-like grains formed an interlocked structure via hot pressing combined with strong magnetic field alignment,which improved the flexural strength at 1600?.