Study On Gel-Casting Processing Of ZrB₂-SiC Composites

ZrB2-SiC composites are materials of particular interest because of the excellent combination of high melting point, high hardness, high electrical thermal conductivity. These properties make it an attractive candidate for high temperature applications. Nevertheless, the preparation technology of ZrB2-SiC composite is immature by now. In order to prepare the composite with complex shap, high density and little defects, we use sol-gel method in this experiment. The sol-gel method is a new near net-shape forming technology invented in the 1990s in the USA. Polymerization chemical method was used flexibly in the forming of ceramic powder. In this way,we can prepared organic processed ceramic bodies with high density and excellent uniformity. We get the finished product after demoulding, drying and debinding.ZrB2-SiC composites were prepared by sol-gel process. The effects of PH, dispersant, the solid volume content and the AM content on the viscosity of the ZrB2-SiC slurry were investigated. The result show that the solid content of the slurry is 45vol% and the viscosity is 608 MPa.s when the content of dispersant is 0.92wt%, PH is 10.3, the content of AM is 3.2wt%.ZrB2-SiC composites were prepared by hot-pressing. The results of mechanical properties off the samples with different SiC content, SiC particle size, holding time and sintering pressure were analyzed and compared after testing. This shows that the condition of the optimum mechanical properties is:sintering temperature is 1750℃; SiC content is 20vol%; holding time is 30mins; sintering pressure is 30MPa, and the mechanical properties at those condition are the best. The optimum mechanical properties of ZrB2-SiC composites are:HRA is 85; Fracture toughness is 7.8MP.am1/2; Bending strength is 669.5MPa.Residual stress, crack deflection, crack bridging and grain refinement mechanisms play an important role in the strengthening and toughening of ZrB2-SiC composites. Grain refinement not only improves strengthening and toughening of composites, but increases the grain boundaries, and contributes for confining the crack propagation.The phase composing and microstructure of ZrB2-SiC composites were investigated by XRD, SEM and EDS. The results show that there is no new phase after sintering, and the primary phases are ZrB2 and SiC. The SiC particles are distributed uniformly throughout the ZrB2 matrix. The oxidation resistance of ZrB2-SiC composites with different temperatures were studied at different oxidation temperatures. The result shows that the oxidation resistance of the sample is optimum when the sintering temperature is 1750℃.