Study On Microstructure And Properties Of Si₃N₄/5083Al Composites Prepared By SPS Technology

In the aerospace,precision instruments and other fields,there is often a problem of device size mismatch due to a abrupt change in ambient temperature,which has a great impact on the service life of the device.The key reason for this problem is the mismatch of thermal expansion coefficients.Although the commonly used Si C/Al or B4C/Al composite materials can meet the thermal expansion coefficient requirements,the introduction of high volume fraction reinforcement will inevitably lead to a decrease in the plasticity of the material.Therefore,in this paper,Si₃N₄ with lower thermal expansion coefficient is used as the reinforcement and 5083 Al as the matrix to prepare Si₃N₄/5083 Al composite by SPS technology.While ensuring that the thermal expansion coefficient of the composite matches the bearing mesh,the plasticity of the material can be improved by reducing the volume fraction of the reinforcement.This paper first takes the 25% Si₃N₄/5083 Al composite material as an example to study the preparation process of the composite material.The research results of the ball milling process show that: compared with high-energy planetary ball milling,the use of low-speed horizontal ball milling can make Si₃N₄ and 5083 Al uniformly mixed,and at the same time will not have a large impact on the original material's morphology,and finally determined the optimal ball milling process parameters are: ball milling time is 48 h,ball milling speed is 80 rpm.The results of the research on the SPS process show that: under the process parameters of sintering temperature of 575? and pulse condition of ton:toff=2:1 and sintering pressure of 40 MPa,the composite material is dense,without obvious defects,and the density is as high as 98.3%.Its comprehensive mechanical properties are excellent,with tensile strength reaching 482 MPa and bending strength reaching 825 MPa.Subsequently,15%,25% and 35%vol.Si₃N₄/5083 Al composites were prepared using the optimal process parameters,and their microstructures were systematically studied.The experimental results show that with the increase of the volume fraction of Si₃N₄,obvious agglomeration of Si₃N₄ particles appears in the composite,resulting in a slight decrease in density.There is an interfacial reaction in the composite material.The thickness of the interfacial reaction layer is about 10 nm.The interfacial reaction products are Al N and Mg3Al2N4.Combined with the characterization of the microstructure of the composite material,a brief analysis of the SPS sintering mechanism of the composite material: with the increase of the sintering pressure and sintering temperature,the Al matrix closely contacted with the Si₃N₄ particles melts to form a reaction layer containing Si₃N₄ particles-interface-liquid phase Al layer,and the particle rearrangement under the driving of the liquid phase Al forms Si₃N₄ clusters,and the sintering of the composite material is realized after cooling.Finally,the mechanical properties and thermophysical properties of composites with different volume fractions were studied.The results of the study on the mechanical properties show that: as the volume fraction of Si₃N₄ increases,the hardness of the composite gradually increases.When the volume fraction of Si₃N₄ is 35%,the hardness reaches 172.3N/mm2;while the tensile strength and bending strength show the first trend of the decline after the increase reaches the maximum when the volume fraction of Si₃N₄ is 25%.The results of thermophysical properties research show that as the volume fraction of Si₃N₄ increases,the thermal expansion coefficient and thermal conductivity of the composite gradually decrease,and then the thermal expansion coefficient of the 35% Si₃N₄/5083 Al composite is the lowest,reaching 12.236×10^-6/?,which satisfies matching requirements for thermal expansion coefficient of bearing steel(12.236×10^-6/?).