Research On The Mechanical Properties And Sintering Behavior Of WC-doped Silicon Nitride Ceramics Consolidated By Spark Plasma Sintering/Pressure Sintering

Silicon nitride（Si₃N₄） with small specific gravity, low thermal expansion coefficient, good thermal shock resistance, good self-lubrication is an ideal structural ceramic.Since the discovery of silicon nitride in 1857, the structure,properties and sintering technologys of silicon nitride have been studied thoroughly.Both the preparation processes,sintering aids and various toughening mechanisms of silicon nitride ceramics are quite mature.The existing research direction is very difficult to have a big breakthrough.As a representative material of carbide alloy,WC has a excellent characters such as high hardness,excellent wear resistance and corrosion resistance.Different from the previous work,a novel approch was adopted to fabricate the silicon nitride by doping WC via mechanical ball milling.The sintering behavior and properties of WC-doped silicon nitride ceramics were studied by spark plasma sintering and air pressure sintering.The fully densified and good mechanical properties of β-phase silicon nitride ceramics were prepared by spark plasma sintering at lower temperatures.The results show that,as the increase of the sintering temperature,the density, flexural strength,fracture toughness of samples increased.The flexural strength,fracture toughness of the samples sintered at 1500 oC were 289.2MPa,3.45 MPa·m1/2 and150.9GPa,respectively.The Phase composition and microstructure analysis shows that the β-phase content of silicon nitride ceramics could reach 99%,The elongated columnar silicon nitride grains were uniform and well developed.Tight connection between the grains and less grain boundaries pores.The green body was prepared by dry pressing Process under 12 MPa for 90 s.Fully dense β-phase silicon nitride ceramics were obtained under 8MPa sintering pressure at1780℃.The highest value of flexural strength,fracture toughness of the samples were967.2MPa、8.9M?Pam1/2,respectively.The silicon nitride grains are well developed and distributed evenly, and the inner pores are almost completely isolated.Further increase the sintering temperature,abnormal grain growth will occur resulting in the increase of weight loss, decreas of density. The fracture behavior of silicon nitride ceramics is gradually changed from the transcrystalline rupture to the intergranular fracture, and the fracture toughness of the material is improved.The density,microstructure and mechanical properties improved by WC sintering aid was elaborated in detail according to the test results of XRD,XRF and SEM. Part of WC solided solution to the matrix lattice during the sintering process.In this experiment, with the increase of the content of WC, the amount of solid solution increases gradually.The introduction of WC is helpful to the densification process ofsilicon nitride ceramics and the transformation of the α�'β phase.Micron size of WC spherical particles are dispersed in the matrix of silicon nitride ceramics, resulting in a series of mechanisms, such as dispersion strengthening mechanism, particle pinning and so on.It is found that the flexural strength of silicon nitride ceramic is significantly improved.The highest shtength value of 1091.2MPa was obtained,which is improved by 48.4% compared to pure silicon nitride ceramic fabricated at the same condition.