Research On Improving The Mechanical And Wear Properties Of Silicon Nitride Ceramics Based On Interface Stress Control

Silicon nitride ceramics have excellent material properties such as low density,high strength,high-temperature resistance,wear resistance,and corrosion resistance,and have broad application prospects in the field of key basic components of high-end mechanical equipment.However,key basic components such as silicon nitride ceramic bearings and nozzles are prone to failure and scrapping during use due to brittle fracture,severe wear,and other reasons,seriously affecting their service life.Therefore,improving the mechanical properties and wear resistance of silicon nitride ceramics is of great significance for promoting their application in key basic components.The introduction of toughening phases has been proven to be one of the most effective methods for improving the fracture toughness of materials.However,when improving the fracture toughness of silicon nitride ceramics through toughening,due to the significant differences in material properties such as thermal expansion coefficient and sintering activity between the toughened phase and silicon nitride ceramics,the material interface will debond due to thermal mismatch stress,resulting in defects such as microcracks and pores,reducing the strength and wear performance of the material.Therefore,regulating the distribution of interface thermal mismatch stress and suppressing the generation of interface defects is crucial for simultaneously improving the fracture toughness,material strength,and wear resistance of silicon nitride ceramics.This study aims to improve the mechanical and wear properties of silicon nitride ceramics by changing the type of toughening phase and regulating the thermal mismatch stress at the interface.The research content and results are as follows:（1）The mechanism of intergranular glass phase formation and the thermal stress distribution law of silicon nitride grain interface during the sintering process of silicon nitride ceramics were studied.By adjusting the Y₂O₃ content from 1%to 9%,four types of Y₂Si₂O₇,YSiO₂N,Y4Si₂O₇N₂,and Y₂Si₃O₃N₄ intergranular glass phases were generated inside the material,targeting the sintering of silicon nitride ceramics with different ratios of Al₂O₃ and Y₂O₃ as sintering aids.Meanwhile,the thermal mismatch stress at the interface between these four intergranular phases and silicon nitride grains was calculated using the bond valence model and fracture theory.The calculation results show that the interfacial thermal mismatch stress caused by the Y₂Si₃O₃N₄ intergranular glass phase is 22-35%higher than that caused by the other three types of intergranular glass phases.The fracture toughness of the silicon nitride sample containing this structure is increased by 1.6%-25%compared to the silicon nitride sample containing the other three intergranular glass phases,and the bending strength is reduced by 1.2%-10.8%.In addition,the larger interface thermal mismatch stress intensifies the grain shedding during the wear process,reducing the wear performance of silicon nitride ceramics.（2）The influence of interface thermal mismatch stress using SiC particles as toughening phase on material strength and wear performance was studied.A core-shell structure SiC Glass phase was formed in situ inside silicon nitride ceramic materials through pre sintering process.It was found that when the material was lowered from sintering temperature to room temperature,there was a significant residual compressive stress distributed inside the Glass phase shell,which suppressed the debonding of the SiC Glass phase interface and the generation of interface defects,thereby improving the strength and wear performance of the material.At the same time,there is a high thermal mismatch tensile stress distributed at the interface between SiC Glass phase and silicon nitride matrix in the core-shell structure,which promotes crack propagation along the Glass phase Si₃N₄ interface and improves the material’s fracture toughness.（3）We studied the properties of silicon nitride ceramics prepared by introducing a core-shell structure W-glass phase into the material,with low hardness metal W particles as toughening and lubricating phases.After introducing the core-shell structure W-glass phase into silicon nitride ceramics,it was found that the bending strength of the material increased from985MPa to 1168 MPa,and the fracture toughness increased from 6.5 MPa·m^1/2 to 9.0 MPa·m^1/2.The W-glass phase of the core-shell structure slows down the mechanical fracture of the worn sub surface during the wear process and weakens the cracking and peeling of the oxide film on the worn surface.Compared with the silicon nitride samples without and without the addition of W particles,the friction coefficient decreased by 43.8%and 32.1%respectively,and the wear rate decreased by 61%and 24%,respectively.Further proof has been provided that introducing a core-shell structure into silicon nitride ceramics can significantly improve the mechanical and wear properties of the material.（4）The study compared the influence of SiC,W,Mo,Cr particles and their corresponding core-shell structures on the mechanical and wear properties of silicon nitride ceramic materials.It was found that the use of introducing a core-shell structure to suppress interface debonding is not suitable for metal particles with a high coefficient of thermal expansion.In the core-shell structure,when the thermal expansion coefficients of the materials between the core and shell differ too much,the interface will also experience debonding.When toughening silicon nitride ceramic materials with metal particles,the bending strength of the material is not only affected by interface defects,but also related to the interface bonding state between silicon nitride grains and metal particles.Especially when the metal particles are subjected to plastic deformation caused by the compression of silicon nitride grains,it can hinder the reduction of the material’s bending strength.After comparison,the comprehensive mechanical properties of silicon nitride ceramics with Cr Glass phase core-shell structure were the best.Compared with the samples without toughening and lubricating phases,the fracture toughness and bending strength of the material increased by 24%and 27%respectively,and the wear rate decreased by 73%.This thesis combines fracture theory and experimental analysis to explore the mechanism of interface thermal mismatch stress generation.Meanwhile,considering the influence of interface thermal mismatch stress on material defects such as microcracks and pores,a method for controlling interface thermal mismatch stress to suppress material interface defects was designed.This provides new ideas for improving the mechanical and wear properties of silicon nitride ceramics,and greatly helps to promote the application of key basic components of silicon nitride ceramics.