Study On Preparation And Properties Of Si₃N₄ Cutting Tool Based On Phase And Microstructure Tailoring

Silicon nitrideceramics are feature with high hardness,strength,toughness and wear resistance and good thermal stability,and are widely used in the fields of high-speed cutting tools and high-precision bearing balls etc.The phases of Si₃N₄ ceramics are mainly?-Si₃N₄ and?-Si₃N₄.Among them,?-Si₃N₄ grains are usually equiaxed,demonstrating high hardness but low fracture toughness,while?-Si₃N₄ grains are equiaxed or long rod-shaped,possessing higher fracture toughness and lower hardness.Therefore,the ratio of?-/?-Si₃N₄phase and microstructure of the grains significantly affect the mechanical properties of Si₃N₄ceramics,which in turn may affect the cutting performance.In order to reveal the relations between the phase and microstructure and the cutting performance of Si₃N₄ ceramics,this work symmetrically investigated the influences of the tailoring of phase composition and microstructure on the mechanical properties and the cutting performance of Si₃N₄ ceramics,by means of sintering process,sintering aids and the secondary strengthening phase,aiming to provide the theory and guidance for the optimization of the phase and microstructure oftools.The main contents and conclusions are as follows:Firstly,Si₃N₄ cutting tools were obtained by gas-pressure sintering at 1930?C with the introduction of TiO₂ additive,which demonstrated the typical?-Si₃N₄ phase and the typical bimodal microstructure?equiaxed grains and elongated grains?.The results showed that?-Si₃N₄ phase was the dominating product,and equiaxed grains and elongated grains coexisted within the coarse-grained bimodal microstructure,after gas-pressure sintering at high temperature.After introducing the secondary phase of TiO₂,Ti N formed via in-situ reaction and refined the microstructure,alongside the increasing hardness?from?16.8 GPa to?17.2GPa?and the lower fracture toughness(?6.4 MPa?m^1/2 to?5.8 MPa?m^1/2).It was noteworthy that the cutting length increased from 1760 m to 2130 m when continuously cutting of cast iron,indicating the fine-grained microstructure and high hardness contributed to improve the cutting performance of Si₃N₄ ceramic tools.Then,with the purpose of refinement of the microstructure of Si₃N₄ ceramics and enhancement of the hardness,Si₃N₄ ceramic tools with the co-existing phases of?-/?-Si₃N₄and equiaxed fine-grained microstructure were prepared by spark plasma sintering and adjusting the dwelling time.It was found that the fraction of?-Si₃N₄ phase increased from 4.9wt.%to 49.7 wt.%,when the SPS holding time was shortened from 10 min to 0 min at 1800?C.Consequently,the microstructure changes from a bimodal structure with coexistence of equiaxed grains and large elongated grains to the ultrafine equiaxed structure.Vickers hardness increased from?17.5 GPa to?20.1 GPa,and the fracture toughness decreased from?5.3 MPa?m^1/2to?3.9 MPa?m^1/2.The remarkable increasing of cutting length was observed from 1200 m to 2400 m,but the tip collapsed because of the low fracture toughness.Therefore,in order to further improve the cutting performance of Si₃N₄ ceramic tools,the fracture toughness was expected to improve while maintaining high hardness.Finally,in order to obtain Si₃N₄ ceramics with high hardness and high toughness,a low-temperature hot-press sintering process?1500?C?combined with the secondary phases of Zr B2 and Ti B2 additive was developed to prepare the cutting ceramics with?-Si₃N₄ as the major phase and fine-grained bimodal microstructure.The results demonstrated that the introduction of Zr B2 and Ti B2 in 2.5 vol.%could promote the phase transformation from?-to?-Si₃N₄ and the yields of elongated?-Si₃N₄ grains,resulting in the fine-grained bimodal microstructure.Si₃N₄ ceramic with TiB₂ aid showed high hardness??21.1 GPa?and high toughness(?4.7 MPa?m^1/2),whose cutting length increased from 1780 m to 2480 m and no tip collapsing was observable.Therefore,by adjusting the phase composition and microstructure,a high?-Si₃N₄ ratio content and a fine-grained bimodal structure could simultaneously increase hardness and toughness,and improve its cutting performance of the Si₃N₄ ceramic.