Preparation And Mechanical Properties Of Second Phase Doped Silicon Carbide Composite Ceramics By SPS

In recent years,silicon carbide（SiC）ceramics have become an important class of high-temperature structural ceramics because of their good mechanical properties of high stiffness,good resistance to high temperatures,chemical stability and unique electrical properties.However,SiC is a highly covalently bonded compound with a low self-diffusion coefficient.These properties lead to poor sinterability,high sintering temperatures,and difficulty in forming dense structures even at high temperatures and pressures.At the same time,the low fracture toughness of silicon carbide ceramics and their tendency to be brittle at room temperature have led to difficulties in processing and molding,limiting the engineering applications of these materials.In order to reduce the sintering temperature of silicon carbide and improve the mechanical properties,in this paper,SiC ceramics,SiC-B₄C multiphase ceramics and SiC-GNPs multiphase ceramics were prepared by Spark Plasma Sintering using silicon carbide powder as the main raw material and boron carbide nanoparticles and graphene nanosheets（GNPs）as additional phases to study the effects of different sintering temperatures and pressures on the microstructure and mechanical properties of SiC ceramics,and the effects of different raw material compositions on the microstructure and mechanical properties of the two composite ceramics were investigated.The SiC ceramics were firstly prepared and the effects of different sintering process parameters on their microstructure and mechanical properties were investigated,and their relative densities,mechanical properties and microstructures were analyzed.The observation of the changes of microstructure and mechanical properties showed that increasing both the sintering temperature and pressure effectively promoted the sintering of the samples and improved their properties.The best overall properties were obtained at a sintering temperature of 1850°C and a sintering pressure of 30MPa.The relative density of SiC ceramics was 96.8%,the Vickers hardness was 27.58GPa,the flexural strength was 490MPa,and the fracture toughness was 4.79MPa·m^1/2.Next,SiC-B₄C multiphase ceramics were prepared by SPS sintering using nano-B₄C as an additional phase.The results showed that the addition of nano-B₄C could effectively promote the sintering of the samples and improve the mechanical properties of the materials.The best properties were obtained at a sintering temperature of 1850°C,a sintering pressure of 30MPa,and a nano-B₄C addition of 15wt%.At this time,the relative density of SiC-B₄C composite ceramics was 98.1%,the Vickers hardness was 29.93GPa,the flexural strength was 499MPa,and the fracture toughness was 4.88MPa·m^1/2.Finally,SiC-GNPs multiphase ceramics were prepared with graphene nanosheets as an additional phase.The results show that the addition of graphene nanosheets decreases the sample densities.However,compared with SiC ceramics and SiC-B₄C multiphase ceramics,some mechanical properties of SiC-GNPs multiphase ceramics are not reduced.Even at lower densities,the flexural streng and fracture toughness of SiC-GNPs multiphase ceramics were comparable to both.At 1850°C,30MPa,and 3%GNPs addition,the SiC-GNPs multiphase ceramics achieved a relative density of 94.4%,a Vickers hardness of 24.81GPa,a flexural strength of 498MPa,and a fracture toughness of 4.93MPa·m^1/2.