Effect Of Transition Metal Borides On Phase Composition,Microstructure And Mechanical Properties Of Silicon Nitride-based Ceramics

Silicon nitride（Si₃N₄）ceramics are important structural ceramics due to excellent mechanical properties,corrosion resistance and wear resistance,which are widely used in cutting tools and bearings.Recent researches show that high hardness-high toughness Si₃N₄-based ceramics with bimodal microstructure can be prepared by hot-pressing sintering at low temperature（1500℃）with a small amount of（2.5 vol%）transition metal borides（ZrB₂,HfB₂）.In order to further improve the mechanical properties of Si₃N₄-based ceramics,in this thesis,the phase composition,relative density,microstructure and mechanical properties of Si₃N₄-based ceramics prepared at low temperature were studied by means of reaction-bonded sintering,spark plasma sintering and introduction of high entropy borides with high hardness,combined with transition metal borides（ZrB₂ and HfB₂）.The specific research contents and results are as follows:Firstly,Si₃N₄-ZrB₂ ceramics with different contents of ZrB₂（0,2.5,5,10 vol%）were prepared by nitridation of silicon powder and hot pressing sintering at 1500℃.The effects of ZrB₂ content on phase composition,relative density,microstructure and mechanical properties of Si₃N₄-based ceramics were studied.The results show that ZrB₂ significantly promotes the phase transformation of Si₃N₄,and the content ofβ-Si₃N₄ increases from 40%to more than 90%.The hardness of Si₃N₄-based ceramics decreased from 20.0±0.3 GPa to13.9±0.9 GPa due to grain coarsening,highβ-Si₃N₄ content and decrease of relative density.The addition of 2.5 vol%ZrB₂ resulted in a significant bimodal microstructure,and the fracture toughness increased from 3.9±0.3 MPa·m^1/2 to 5.0±0.2 MPa·m^1/2.With the further increase of ZrB₂ content,the microstructure is transformed into coarse-grained equiaxialβ-Si₃N₄ grains,leading to a significant decrease in fracture toughness.The results show that self-toughened Si₃N₄-based ceramic can be prepared with 2.5 vol%ZrB₂ at low temperature by nitridation of Si powder and hot pressing sintering,but the hardness of ceramics is low（16.4±0.5 GPa）.Then,HfB₂ with different contents（0,1,2.5,5 vol%）was introduced to Si₃N₄-based ceramics at 1500℃and 1600℃by spark plasma sintering.The effects of HfB₂ content on phase composition,relative density,microstructure and mechanical properties of Si₃N₄-based ceramics were studied.The results show that HfB₂ does not significantly promote the phase transformation of Si₃N₄ at 1500℃,therefore the main phase isα-Si₃N₄ and the microstructure is basically equiaxed fine grain structure.The hardness of Si₃N₄ slightly decreases from 21.4±0.2 GPa to 20.0±0.5 GPa,and the fracture toughness gradually increased from 3.3±0.1 MPa·m^1/2 to 4.4±0.4 MPa·m^1/2.At 1600℃,the addition of HfB₂ can significantly promote the phase transformation of Si₃N₄,the content ofβ-Si₃N₄ increases from 38%to more than 70%,and the microstructure shows a significant bimodal structure with fine equiaxed grains and long rod-like grains.The reaction between HfB₂ and Si₃N₄ is more serious.As a result,the relative density and hardness of ceramic was markedly decreased,while the fracture toughness significantly increased.The results show that the Si₃N₄-based ceramic with 1 vol%HfB₂ prepared by SPS at 1600℃have excellent comprehensive mechanical properties,with hardness of 18.6±0.3 GPa and fracture toughness of 7.7±0.3 MPa·m^1/2.Then,Si₃N₄-based ceramics with different contents of high entropy borides(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂（0,1,2.5,5 vol%）were prepared by hot pressing sintering at different temperatures（1500℃,1600℃）.The effects of(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ content on phase composition,relative density,microstructure and mechanical properties of Si₃N₄-based ceramics were studied.At 1500℃,(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ promotes the phase transformation of Si₃N₄.The main phase of ceramics is stillα-Si₃N₄,and the microstructure is a bimodal structure consisting of a few rod-like grains and equiaxed fine grains.Most of(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ reacts with Si₃N₄,leading to the decrease of relative density and hardness（from 99.0%and 22.3±0.5 GPa to 96.0%and 17.9±0.2 GPa,respectively）and the increase of fracture toughness(from 3.4±0.1 MPa·m^1/2 to 4.9±0.2MPa·m^1/2).At 1600℃,(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ can obviously promote the phase transformation of Si₃N₄ with the increase ofβ-Si₃N₄ content from 39.5%to more than 70%,which makes the microstructure of the ceramics show a significant bimodal structure.However,the more serious reaction between(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ and Si₃N₄ results in a further decrease of the relative density and hardness,but fracture toughness increases significantly.The results show that the Si₃N₄-based ceramic prepared by hot pressing at1600℃with 1 vol%(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ have good comprehensive mechanical properties(hardness of 19.1±0.3 GPa,fracture toughness of 6.2±0.3 MPa·m^1/2).Since most of(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ will be consumed by reaction,the relative density of the materials will decrease,which leads to the decrease of the hardness of Si₃N₄-based ceramics.Finally,using spark plasma sintering,the effects of(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ content on the phase composition,relative density,microstructure and mechanical properties of Si₃N₄-based ceramics at 1500℃and 1600℃were investigated.At 1500℃,the effect of(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ on the phase transformation of Si₃N₄ is not obvious.The main phase of the ceramics is stillα-Si₃N₄,and the microstructure is equiaxed fine grain structure with a few rod-like grains.The serious reaction of(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ leads to the decrease of the relative density and hardness of the ceramics,but the fracture toughness is not improved significantly.The hardness and fracture toughness of the sample containing 5vol%(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B2 are 17.6±0.2 GPa and 3.8±0.2 MPa·m1/2,respectively.At1600℃,Si₃N₄-based ceramics with(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ have high ratio ofβ-Si₃N₄（more than 50%）and bimodal microstructure,but the increased reaction results in a further decrease in relative density and hardness,but a increase in fracture toughness.At 1600℃,Si₃N₄-based ceramics with 1 vol%(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ have excellent comprehensive mechanical properties,with hardness of 20.2±0.2 GPa and fracture toughness of 6.9±0.2MPa·m^1/2.Compared with hot pressing sintering,SPS cannot avoid the reaction between(Hf_0.2Zr_0.2Ta_0.2Cr_0.2Ti_0.2)B₂ and Si₃N₄.However,Si₃N₄ prepared by SPS has higher hardness and fracture toughness.