| ZrB2-SiC, as a member of ultrahigh-temperature ceramics(UHTCs), has been used as a candidate for the extreme chemical and extreme thermal environments associated with hypersonic aerospace vehicles because of its combination of excellent properties, such as high melting point, high hardness, high electrical and thermal conductivities, resistance to corrosion and oxidation. The traditional route for ZrB2-based ceramics is dry processing route, which gives way to the gelcasting (one of wet forming methods) for high-quality complex-shaped ceramic parts, because it has limitations on the control of particle-particle interactions, the homogeneity of particle packing and especially the complex geometry of final parts. The production of near-net shape components by gelcasting would not only reduce the time and cost, but also decrease the risks of cracks and defects during the electrical discharge machining. Hence, gelcasting is a promising technology for more reliable ceramics. In this dissertation, preparation of ZrB2-SiC by optimized gelcasting was investigated.The influence of monomer, the ratio of monomers, initiator and catalyst on the gelation was tested from the pure gel perspective. It has been concluded that less monomer content (4wt.%) is the best choice, and the gel has coordinative uniformity and strength when the ratio of monomers is10/1. Addition of catalyst is not suitable for complete network structure. The optimal content of initiator is a range from0.3wt.%to0.5wt.%, and the appropriate temperature for solidification is70℃The Zeta potential of ZrB2and SiC as well as the rheological property of ZrB2-SiC were studied, and the particles could co-disperse in aqueous media using0.1wt.%PAA as dispersant at the pH value of10. The optimal ball milling time is18h with milling speed at250rpm. The ZrB2-SiC slurry with a high solid loading (53vol.%) and low viscosity (～0.2Pa S) was obtained.The gelcasting process was systematically investigated, including degassing, drying and binder burnout. The optimal time for solidification at70℃is30～45min. The ceramic was dried at ambient temperature for20h firstly,and then the drying temperature was increased after shrinkage of green body was ended. The heating rate for binder burnout was30℃/h (200～450℃). The influence of monomer and initiator on the properties of green body was also studied. The green strength and relative density was increased with the increase of monomer, and the green body with more than4wt.%monomer content could be machined.The pressureless sintering of ZrB2-SiC was investigated with a varity of sintering aids. The ZrB2-SiC ceramic with97.2%relative density was achieved at2000℃for2h with B4C and carbon (in the form of phenolic resin) as sintering aids. The flexural strength and fracture toughness of sintered ceramic was402.3MPa and4.68MPa·m1/2The complex-shaped sintered ceramic was obtained at last.The dispersity of nano ZrB2and nano SiC powder and the rheological property of ZrB2-SiCnano suspension was characterized. About40vol.%suspension was achieved by optimal content (0.1wt.%) dispersion PAA-NH4was added. The ZrB2-SiCnano ceramic with94%relative density was obtained at2000℃by versatile gelcasting and pressureless sintering, which showed flexural strength of448MPa and fracture toughness of4.72MPa·m1/2. |
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