| Ultra-high temperature ceramics(UHTCs) is the ceramic materials which is using in a temperature of upper 2000 oC, such as Zr B2, Hf B2, Si C, Zr C, Zr B2/Si C/C et al. The good properties like high melting point, high thermal conductivity and excellent mechanical properties make UHTCs particularly attractive and potential candidates for use as sharp wing leading edges(WLEs) and nose tips for thermal protection of hypersonic vehicles during re-entry conditions. This play a great important role in our aerospace career and security of country.However, the sintering of Zr B2 requires very high temperature and high pressure owing to the high melting point, strong covalent bonding, low self-diffusion coefficients and difficulties in diffusion. Furthermore, the oxide like B2O3 which are exist on the surface of the Zr B2 materials will prevent the densification of the materials. There is two ways can be useful to densification of the Zr B2 ceramic. One is to make an external field to promote the materials densification; another is to apply some sintering additives. In this paper, three new sintering methods have been used to densify the Zr B2 ceramics. Proper sintering additives and in-situ reaction were used in the densification system. Vickers’ hardness, bulk density, flexural strength and fracture toughness were tested. The microstructure and phase fraction of sintered specimens were analyzed with scanning electron microscopy(SEM) and X-ray diffraction(XRD). The anti-oxidation and thermal shock resistance of these samples making through different sintering methods have been studied.Zr B2 and Si C mixed powders were sintered by high pressure and high temperature sintering. The flexural strength of the sintered composites was increased at first and decreased at later with the increasing amount of Si C. Little Si C phase could repatch the pores and cracks beyond the Zr B2 powders and form silicate glasses at high temperature which could seal the surface flaws to further improve the flexural strength. However, while the amount of Si C is too much, the flexural strength would decreased as the aggregation of Si C.Zr B2-Si C composites were fabricated by high pressure and high temperature sintering. In situ reaction of Zr B2, B4 C and C were being used into the material system. With the increase of the temperature and pressure, vickers’ hardness, flexural strength and other mechanical properties of the sintered samples were improved. The maximal flexural strength of the sintered samples is 320 MPa. Residual stress of the inner-materials may influent a lot to the strength and some of them can be wipe off by annealing. The flexural strength of the sintered samples could increased to 520 MPa after annealing.Zr B2, Zr Si2, B4 C and C mixed powders were sintered by spark plasma sintering. With the in-situ reaction of Zr Si2, B4 C and C, densified Zr B2-Si C composites were fabricated at a low temperature of 1600 oC. The sintered samples have a good flexural strength, which could up to 1114 MPa. Anti-oxidation and thermal shock resistance is well. Mass gain beyond the oxidation is not large and the critical thermal shock resistance temperature is 600 oC.A novel vibration-assisted hot-pressing(VAHP) technique has been used to fabricate Zr B2-Si C composites. Adding Al, B4 C and C powders as sintering additives to Zr B2-Si C matrix with vibration sintering. Full dense Zr B2-Si C-Al3BC3 composites had been fabricated with a temperature of 1600 oC and oscillatory pressure of 50 t using vibration sintering. Flexural strength was 600 MPa, which is bigger than nomal samples under constant pressure. And this material exhibited good anti-oxidation and thermal shock resistance. The mass gain beyond the oxidation is not large and the critical thermal shock resistance temperature is 756 oC. Compared to constant pressure, Sintering under oscillatory pressure exhibited many differences. It may generate much severe plastic deformation of ceramics, help remove the isolated pores, lead to high density and more uniform microstructure, lead to a lower grain- boundary energy, and finally got us a high strength sample. |
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