Study On Properties Of Surface Modified Carbon Fiber Reinforced Magnesium Matrix Composites

With its unique advantages,such as light weight,high specific strength,high stiffness,low thermal expansion,good dimensional stability,high temperature resistance,good thermal conductivity and other properties,continuous carbon fiber reinforced magnesium matrix composite（C_f/Mg）has become a research hotspot of a new generation of lightweight,high-performance structural materials,and has a broad application prospect in the aerospace,military defense,and civil and other fields.However,due to the inertness of the carbon fiber surface,the wettability of magnesium alloy and carbon fiber is poor,which makes it difficult to prepare C_f/Mg composites.In addition,Al element in magnesium alloy will cause serious interfacial reaction with carbon fiber,which will damage carbon fiber and seriously restrict the improvement of mechanical properties of composites.These problems limit the wide application of C_f/Mg composites.In this paper,based on the interface problem of composite,and for the purpose of preparing high-performance C_f/Mg composite,the modified carbon fiber reinforced magnesium matrix composites were prepared by liquid-solid extrusion following vacuum infiltration（LEVIS）.The surface modification technology was studied.The microstructure,interface structure and matrix grain size of the composite were analyzed,and the effects of carbon fiber surface modification on properties of composites（tensile properties,thermal expansion behavior,hardness of the matrix alloy）were explored.The main research contents and results of this paper were as follows:The influence of process parameters on the morphology and microstructure of PyC coating was studied.After investigating the influence of deposition temperature,gas flow volume and holding time on the surface morphology of PyC coating,it can be found that the higher the temperature,the coarser the PyC coating and less uniform deposition.The greater the gas flow volume,the greater the thickness of the PyC coating.The longer the holding time,the rougher the surface of the coating and the greater the thickness.It was determined that the optimal technological parameters for PyC coating preparation under current experimental conditions:CH₄ flow volume was 1.0m³/h.The deposition temperature was 1000℃,and the deposition time was 4h.The effect of PyC coating thickness on the mechanical properties of carbon fiber was studied.Carbon fiber surface was successfully coated by CVD method,and the PyC coatings thickness is about 100,150,260 and 320nm,respectively.The deposition rates at 3h,4h,5h and6h were 33.3,37.5,52 and 53.3nm/h,respectively,and the deposition rates increased with the increase of deposition time.The structure,chemical properties and thickness of coating have a strong correlation with deposition time.When the deposition time increased from 3h to 6h,the residual stress of PyC coating first increased and then decreased,in which the sp³ bond content decreased from 0.186 to 0.172.The coating roughness increased with the increase of deposition time,and the increased roughness was conducive to increasing the mechanical meshing action at the interface between carbon fiber and matrix.C_f-PyC has better oxidation resistance than uncoated-C_f.C_f-PyC with the coating thickness of 100nm maintained about 69.4%of the original carbon fiber strength,while C_f-PyC with the coating thickness of 320nm had the lowest strength.The thicker the coating,the greater the dispersion of C_f-PyC tensile strength.The effects of C_f-PyC/AZ91D composite interface on mechanical properties and thermal expansion behavior were investigated.The optimal PyC coating thickness is about 150nm with low texture.C_f-PyC has higher surface roughness and graphitization,which is beneficial to reduce harmful interfacial reactions.Compared with C_f/AZ91D composite,the maximum ultimate tensile strength of C_f-PyC/AZ91D composite can reach 416MPa,which is about 54.9%higher.The increase in mechanical properties of C_f-PyC/AZ91D composites can be attributed to the increase in C_f surface roughness,which resulting in a moderate interfacial bonding strength between carbon fiber and the matrix.Moreover,PyC coating can prevent the interfacial reaction between carbon fiber and aluminum in the matrix to form the brittle phase of Al₄C₃.Both C_f-PyC/AZ91D and C_f/AZ91D composites have lower thermal expansion coefficients.The C_f-PyC/AZ91D composites have slightly higher CTE than C_f/AZ91D composite.The C_f/AZ91D composites have strong interfacial bonding and high interfacial sliding resistance,which leads to the strong constraint of carbon fiber on the matrix and the decrease of CTE.The CTE of PyC is about 2.5×10^-6/℃,which is between the carbon fiber and AZ91D matrix,and can regulate the interfacial bond between the carbon fiber and AZ91D matrix.Therefore,the interfacial reaction of C_f-PyC/AZ91D composite material is less and the bond is moderate,and the restraint of carbon fiber on the matrix is weaker than that of C_f/AZ91D composite.The microstructure and interface structure of C_f-SiCNWs/AZ91D composites were studied.SiCNWs is mainly distributed in the matrix alloy between the carbon fiber layers with uniform dispersion and no agglomeration,which can also enhance the matrix alloy between the carbon fiber layers.The gap between SiCNWs was fully impregnated with AZ91D alloy and no defects were found.SiCNWs can protect carbon fiber and improve the oxidation resistance of carbon fiber.There are a lot of dislocations near the SiCNWs/AZ91D interface in C_f-SiCNWs/AZ91D composites,and O and Al elements are mainly concentrated in the SiCNWs region.By inhibiting harmful interfacial reaction products of Al₄C₃,C_f-SiCNWs/AZ91D composite can provide a better interface.The effects of SiCNWs on the tensile properties,thermal expansion behavior and microhardness of C_f-SiCNWs/AZ91D composites were investigated.The tensile strength of C_f-SiCNWs/AZ91D composite is 356.9MPa,which is about 32.9%higher than that of C_f/AZ91D composite.The fracture morphology showed that the fibers were pulled out as bundles,and a large number of SiCNWs were pulled out from the matrix too,which indicating that the interface strength of the composite was moderate after introducing SiCNWs,and both carbon fiber and SiCNWs can play an enhanced role.As the CTE of SiCNWs(3.5-5×10^-6/K)is higher than the negative CTE of T700 carbon fiber in its axial direction,the introduction of SiCNW will increase the CTE of C_f-SiCNWs/AZ91D composite.In addition,SiCNWs can help C_f-SiCNWs/AZ91D composite obtain a moderate interfacial bonding strength,which weakens the restraint of carbon fiber on the matrix,and ultimately leads to a larger CTE of C_f-SiCNWs/AZ91D composite than the C_f/AZ91D and C_f-PyC/AZ91D composites.Compared with the hardness of AZ91D alloy in C_f/AZ91D and C_f-PyC/AZ91D composites,the hardness of AZ91D alloy in C_f-SiCNWs/AZ91D composite is higher,and the hardness increased from61.66 HV to 84.34 HV,which increased by about 36.78%.This is because the introduction of SiCNWs can refine matrix grains,which is conducive to the formation of nanocrystalline Mg₁₇Al₁₂ in the matrix and plays a dispersion strengthening role in the matrix.Compared with the composites without SiCNWs,the composite with SiCNWs has the improved tensile properties and microhardness of the matrix,but the thermal expansion coefficient is slightly increased.