Synthesis And Spark Plasma Sintering Process Of Zr B₂-SiC Ccomposite Powders

During the preparation of Zr B2-Si C ultra-high temperature ceramic composites, the Zr B2 and Si C powders need to be wet milled to obtain composite powders in which milling media will be introduced in the milling process and can not guarantee the uniform distribution of Si C and Zr B2 powders. The long sintering time and significant growth of grain in the preparation of Zr B2-Si C composites by traditional hot-pressing sintering.To solve these problems, in this paper, Zr B2-Si C composite powders with high purity, homogeneous distribution of each phase, uniform and samll particle size were prepeared by sol-gel and carbothermal reduction method. The in-situ nano graphite sheets found in the Zr B2-Si C composites changed the fracture mode of the composites, a new method of in-situ preparing Zr B2-Si C-G composites was found.The precursors of synthesized Zr B2 powders were zirconium n-propanol(Zr(OPr)4),boric acid(B(OH)3) and sorbitol(C6H14O6) and the molar ratio of the precursors was 1:3:1.7. The precursors of synthesized Zr B2-Si C composite powders were zirconium n-propanol(Zr(OPr)4),boric acid(B(OH)3), sorbitol(C6H14O6) and TEOS(Si(OC2H5)4) and the molar ratio of the precursors was 1:3:0.37:2.1. The particle size of synthesized Zr B2 powder was about 1~2 μm and the size of Si C particles was in the sub-micron level. Through the heat-differential thermal analysis(TG-DSC) and infrared spectroscopy(FTIR), made clear of the transformation process and chemical bonding composition of the network structure in the precursor and confirmed the started temperature of synthesizing Zr B2-Si C composite powders was about 1350 oC and the ceramic yield of Zr B2-Si C precursors was about 20 wt.%. The X-ray diffraction analysis of Zr B2 and Zr B2-Si C ceramic powders indicated that no impurity phase was existed. The X-ray photoelectron spectroscopy analysis showed the presence of oxygen on Zr B2 surface. The EDS spectra of Zr B2 demonstrated the existed primarily element of Zr B2 powder were Zr and B. The EDS spectra of Zr B2-Si C composite powders reaveled the main elements were Zr, B, Si and C.The Zr B2-Si C composites were prepared by spark plasma sintering at 1850 oC and 1950 oC, respectively. The densification process of Zr B2-Si C composites sintered at different temperatures has been studied respectively. The densification process can be divided into three stages, the first stage: particles rearrangement process; the second stage: rapid densification process; the third stage: grains grown up, pores and voids further narrowed and closed. The grain sizes of sintered Zr B2 and Si C were about 5 μm and 2 μm respectively. The Si C grains uniformly distributed between Zr B2 grains so as to solve the difficulty of uniform distribution between the phases of the composite and the significant growth of grains. The nano lamellar substance was found among Zr B2 grains. The energy dispersive spectroscopy demonstrated the lamellar substance had high carbon content and the Raman spectra indicated that it had been partially graphitized.The peak of graphite was also found in the X-ray diffraction which indicated the nano lamellar substance was graphite sheet indeed. The in-situ nano graphite sheets changed the fracture mode of Zr B2-Si C composite. The average three-point bending strength of the Zr B2-Si C composites sintered at 1850 oC and 1950 oC were 282 and 318 MPa respectively, while the average fracture toughness were 2.94 and 3.09 MPa?m1/2 respectively.