Effect Of Particle Size Of Starting Materials On Densification And Mechanical Of ZrB₂-SiC Multiphase Ceramics

Ultra-high temperature ceramics have been widely applied in national defence and astronavigation region because of their physical and chemical stability at high temperature and under reactive atmosphere.Among them,ZrB₂-SiC system attracts more attention due to its excellent comprehensive properties,such as good mechanical properties and oxidation resistance.Like other non-oxides ceramics,high temperature (more than 1900℃) hot pressing is necessary to be used to densify ZrB2-SiC ceramics due to the strong B-B covalent bond and low self-diffusion coefficient of B atoms.In order to decrease the sintering temperature of ZrB₂-SiC ceramics,additives are usually used.Previous researches mainly focused on the effect of the additives on densification and microstructures of ZrB₂-SiC.As we know,the addition of additives would cause the degradation of mechanical properties and oxidation resistance at elevated temperature.In addition,the properties of ceramics could be changed by using raw materials with different particle sizes.Therefore,it is necessary to study the effect of particle size on densification and properties of ZrB₂-SiC ceramics.In the present work,ZrB₂-SiC multi-phase ceramics with a fixed volume ratio of ZrB₂:SiC being 80:20 was prepared,in which raw materials of ZrB₂ and SiC with different particle sizes were used.The effect of particle size of raw material and sintering temperature on the densification,development of microstructure and mechanical properties of ZrB₂-SiC ceramics was investigated.The results indicated that the densification of ZrB₂-SiC ceramics was affected by particle size of both ZrB₂ matrix and SiC and the sintering temperature.When coarse powder of ZrB₂(average particle size 14μm) was used,the relative densities of ZrB₂-SiC were increased from 88%to 100%with the particle size of SiC decreasing from 5 to 0.45μm and sintering temperature increasing from 1800 to 2000℃,while the relative densities were not affected by the particle size of SiC and the sintering temperature when fine raw powder ZrB₂(1.4μm) was used as the relative densities changing between 98%and 99%.Thus,it is possible to get fully densified materials hot-pressed at 1900℃by suitable raw powder match. In this work,we also investigated the effect of particle size of ZrB₂ and SiC raw materials and hot-pressing temperature on the microstructures and mechamical properties The results of microstructures and mechanical properties of ZrB₂-SiC ceramics sintered at 1900℃and 2000℃indicated that the fracture toughness of ZrB₂-SiC composites decreased with increasing the particle size of SiC when coarse powder ZrB₂ was used,while the fracture toughness of ZrB₂-SiC was not changed much with the particle size of SiC when fine powder ZrB₂ was used.It is noticed that the fracture toughness of ZrB₂-SiC was increased significantly from about 5 MPa·m^1/2 to 6.5 MPa·m^1/2 when hot pressing temperature increased from 1900℃to 2000℃. The flexural strength of the dense ZrB₂-SiC sample sintered at 2000℃is 912MPa with fine grain sizes of both ZrB₂ and SiC.