Microstructure,Mechanical Properties And Thermal Conductivity Of The Short Carbon Fiber Reinforced Magnesium Matrix Composites

Magnesium matrix composites have broad application prospects in aerospace,automotive,electronic instruments and other industries because they not only have the properties of low density,high specific strength and thermal conductivity,but also have some excellent characteristics of reinforcements.Carbon fiber is a very ideal reinforcement for magnesium matrix composites on account of its excellent performance such as low density,thermal conductivity,corrosion resistance and low thermal expansion.Thermal conductivity is an important physical performance of materials,and the heat dissipation of materials directly affects the reliability and service life of the device.Presently,many literatures focus on the research about the microstructure,mechanical properties and corrosion resistance of magnesium alloys.However,few literatures focus on the thermal conductivity of magnesium alloys.To solve this problem,the microstructure,mechanical properties and thermal conductivity of short carbon fiber reinforced magnesium matrix composites were studied in this paper.In this paper,the carbon fiber was chopped by ball milling method and then the nickel plating was carried out on the surface of carbon fibers.The coated short carbon fibers and magnesium powers were mixed by ultrasonic dispersion and mechanical agitation in the mixed solution of polyethylene glycol and ethanol.Finally the composites were prepared with powder metallurgy technology.Optical microscopy,scanning electron microscopy and XRD diffraction were used to observe and analyze the microstructure and the phase composition of the materials,and the mechanical properties and thermal properties also measured.The following conclusions were obtained:After adjustment of the plating parameters of carbon fiber,in the bath temperature of 65℃,pH value of 8,in the environment of ultrasonic plating for 5 minutes,a uniform,continuous and compact electroless nickel plating layer forms on the surface of carbon fibers.Through theoretical calculation,the thickness of the coating is about 0.36μm.The coating formed on carbon fiber surface is Ni-P alloy containing Ni3P.Through experiment,when the pressure is 30MPa,the composite material has good microstructure and performance.Magnesium matrix composites with different reinforcements were prepared under this condition.Most of the carbon fibers in the composites distribute at the interface.Compared to pure magnesium,the hardness and compressive strength of the specimens increase obviously.When the content of short carbon fibers is 1.0vol.%,its hardness reaches 65.3HV,higher than that of pure magnesium by 86.6%.Compressive strength and yield strength are 169.0MPa and 99.6MPa,higher than those of pure magnesium by 42.0%and 31.3%,respectively.The nickel coating for the short carbon fibers improves the interfacial bonding between matrix and reinforcement,and improves the properties of the composites.During the hot pressing sintering process,the matrix reacts with the Ni element in the coating to generate Mg2Ni,which solves the problem of poor wettability between the matrix and reinforcement,and improves the interfacial bonding property.By comparing the theoretical calculation with the actual experimental data,it is found that the enhancement mechanism of magnesium matrix composite reinforced by coated short carbon fibers can be mainly attributed the load transfer effect.The reinforcements reduce the thermal conductivity of the materials.When the content of short carbon fibers is 0.8vol.%,compared to pure magnesium,thermal conductivity decreases by 3.4%.With the increase of carbon fiber content,the thermal conductivity drops.The thermal conductivity of the composite with the uncoated short carbon fibers is worse than that of the composite with the coated short carbon fibers.The thermal conductivity of the composite with the coated short carbon fibers is 120.6 W/m·K.The distribution of reinforcements also affects the thermal conductivity of composites.In the vertical load direction,the thermal conductivity is 120.6 W/m·K,higher than in the parallel load direction by 4.2%.When the content of coated short carbon fibers is 1.0vol.%,the composite possesses a good combination of mechanical properties and thermal conductivity.