Microstructure Regulation And Second Phase Toughening Of Silicon Nitride Ceramics

In the continuous hot-dip galvanizing production line,friction pair materials such as bushings and bushings used in conjunction with sinking rollers and stabilizing rollers are required to withstand multiple effects of repeated erosion of molten zinc and aluminum liquid,large stresses,and sliding friction with bearings.The damage of these materials is very serious,the service life is not high,and improvement is urgently needed.Si₃N₄ ceramics have become the material of choice for this component with excellent mechanical properties and stability.However,the brittleness of Si₃N₄ ceramic material is still its short board.The brittle fracture that may occur during use will cause the entire production line to stop running.The shutdown replacement and commissioning will affect the quality and production efficiency of the hot-dip galvanized product of the entire unit.In addition,with increasing wear and tear,the friction between the bearing pairs increases rapidly,which may cause the steel plate to fail to drive the rollers to rotate,causing scratches on the hot-dip galvanized steel plate and posing hidden risks to product quality.Therefore,improving the fracture toughness and friction and wear resistance of Si₃N₄ ceramic materials,while maintaining its high mechanical strength and resistance to metal melt corrosion,is of great significance for improving the product quality and production efficiency of hot-dip galvanizing production lines.According to the practical application requirements of hot dip galvanizing production line,this paper improves the mechanical properties and wear resistance of Si₃N₄ ceramics by introducing the second phase and adjusting the grain size distribution.First,a new additive phase-metal tungsten particles?W?was introduced into the Si₃N₄ceramic material,the possible chemical reaction between W and Si₃N₄ matrix was studied,and the technical means to suppress the reaction were explored;through high performance W/Si₃N₄ ceramic material preparation technology and other studies have investigated the influence of W introduction on the densification,mechanical properties and friction and wear resistance of Si₃N₄ matrix.Secondly,the effect of metal tungsten fiber?W_f?as a toughening on the fracture behavior of Si₃N₄ ceramics was explored.Then,by introducing iron silicide?Fe Si_x?into the Si₃N₄ ceramic matrix,the microstructure of the Si₃N₄ ceramic material is adjusted,which in turn affects the mechanical properties of the material.Finally,the W-Fe₅Si₃ core-shell structure was designed and prepared,and the mechanical properties of Si₃N₄ ceramic materials were further optimized through finite element analysis combined with experimental verification and other studies.The main findings are as follows.Thermodynamic analysis shows that W and Si₃N₄ cannot coexist at high temperature,and a certain N₂ gas pressure can effectively suppress the chemical reaction between the two.The experiment confirmed the above analysis,and successfully prepared W/Si₃N₄ ceramic material under the conditions of 100 bar N₂ and1750 ^oC.When the amount of W introduced is?5 vol%,a dense W/Si₃N₄ ceramic material can be prepared.When the W content is?10 vol%,the relative density of W/Si₃N₄ ceramics decreases with increasing W content.When the amount of W introduced is 3 vol%,the fracture toughness of W/Si₃N₄ ceramics reaches 10.8±0.5MPa m^1/2,which is 24%higher than that of the sample without W introduced.This is due to the formation of weaker between W and Si₃N₄ Interfacial bonding,the latter is conducive to crack deflection and Si₃N₄ grain extraction,so that fracture toughness is improved.In the friction and wear process,the W element in W/Si₃N₄ ceramics is oxidized and enters the abrasive debris in the form of oxides,which promotes the adhesion of these abrasive debris and forms a smoother and complete adhesion layer on the surface of the grinding ball,thereby improving Si₃N₄ ceramic friction and wear resistance.When W/Si₃N₄ ceramic is matched with GCr15 bearing steel ball,the introduction of 5vol%W can reduce the wear rate from 3.39×10^-6 mm³ N^-1 m^-1 to 1.83×10^-6 mm³ N^-1m^-1,a decrease of 46%.When W/Si₃N₄ ceramic is matched with commercial Si₃N₄ ball,the introduction of 5 vol%W can reduce the wear rate from 4.70×10^-6 mm³ N^-1 m^-1 to0.38×10^-6 mm³ N^-1 m^-1,reduced by more than an order of magnitude.When the amount of W_f introduced is?2 vol%,a dense W_f/Si₃N₄ ceramic material can be prepared.In the process of material preparation,W_f is easily oxidized and finally transformed into Si O₂.The W_f introduced into the Si₃N₄ ceramic matrix is prone to grain coarsening during the sintering process,and the orientation structure of the grains is also destroyed.The introduction of W_f failed to achieve the expected effect of improving the fracture toughness of Si₃N₄ ceramic materials,which is related to the rough interface between the two is not conducive to the extraction of Wf and the rapid degradation of W_f performance.The introduction of Fe Si₂ can play the role of forming?-Si₃N₄ seeds in situ and adjusting the grain size distribution.The introduction of 0.2 wt%Fe Si₂ increased the bending strength of Si₃N₄ ceramics from 1016±46 MPa to 1086±48 MPa,the fracture toughness increased from 7.1±0.1 to 9.8±0.5 MPa·m^1/2,and the fracture toughness increased by 38%.The introduced Fe Si₂ finally exists in the form of the second phase of Fe₅Si₃.The high thermal expansion coefficient of Fe₅Si₃ particles causes large radial residual tensile stress and tangential residual compressive stress around the interface.By modifying the thermal stress model,it is found that when the second phase particles are close to the surface of the material,the radial residual stress decreases sharply,while the tangential residual stress increases sharply,forming a large residual compressive stress on the Fe₅Si₃/Si₃N₄ ceramic surface,resulting in material strength.Rise.The introduction of 0.5 wt%Fe Si₂ increased the bending strength of Si₃N₄ ceramics from1016±46 MPa to 1242±26 MPa,and the bending strength increased by 22%.The interface of large-size Fe₅Si₃ particles generates more elastic strain energy,which is easy to cause the interface to peel off and cause the decrease of the bending strength of Si₃N₄ ceramics.By introducing Fe Si₂/WSi₂ composite powder,W-Fe₅Si₃ core-shell structure was formed in Si₃N₄ ceramics.The W core with low thermal expansion coefficient can suppress the radial shrinkage of the Fe₅Si₃ shell.The introduction of W-Fe₅Si₃ core-shell structure can effectively improve the size and distribution of residual thermal stress around the interface and reduce the occurrence of interface peeling.With the increase in the amount of Fe Si₂/WSi₂ composite powder introduced??1wt%?,the bending strength and fracture toughness of the material showed a continuous increase trend.