Interface Tailoring In Coated Carbon Fiber Reinforced Magnesium Alloy Composites

Carbon fiber reinforced magnesium alloy composites (C/Mg composites) have exhibitedhigh mechanical properties, good thermal conductivity and zero or near-zero coefficient ofthermal expansion. It has been suggested that composites based on ultra-high modulus carbonfibers possess specific stiffnesses higher than any known metallic material. Unfortunately, theintrinsic disadvantage of C/Mg composites restrict its development, including:(1) Poorwettability of C-Mg system, the fabrication process of C/Mg composites are suffered from thepoor wettability.(2) Harmful interfacial reactions, these reactions between carbon fiber andmagnesium matrix can introduce unexpected interface bonding.In present work, C/Mg composits have been successfully fabricated with optimizated oninterfacial structure using squeeze casting method. The interfacial reactions andmicrostructure in C/Mg composites have been studied. The results showed that the interfacialstructure can be tuned by different fiber coatings. The optimized interfacial structure has beenachieved in TiO2coated C/Mg composites. Further, the effect of Al content in Mg matrix andthickness of fiber coating on the interfacial structures and mechanical properties ofCf-TiO2/Mg composites have been investigated.A specific model of interface structure:“the thin reaction layer interface structure” isproposed to accomplish the improved interface bonding in C/Mg composites. In this novelinterface structure, a diffusion barrier around carbon fiber is existed to protect the fiber fromdegradation for harmful interface reactions. The interface reaction products can form acontinue uniform transition layer under nanoscale between fiber coating and metal matrix,rather than aggregating around interface to form needle-like interphases or particles, which can avoid stress concentration caused by those harmful interphases.Carbon fibers were coated with MgO, SiO2and TiO2films respectively using the sol-gelmethod. The uniform, adherent, crack-free and non-bridging fiber coatings can be achieved bycontrolling sol-gel precursors, coating processes and heating parameters.MgO coated C/Mg composites are tend to progegate infiltration defect due to the poorwettability between Mg and C. flexural strength are86GPa and643MPa. SiO2coating canimprove the wettability of C/Mg composites. The needle-like Mg2Si phase is formed as theinterfacial react product at the same time. The flexural modulus and flexural strength are116GPa and676MPa. The interfacial zone of TiO2coated C/Mg composites is clean and withoutany brittle phase observed. The flexural modulus and flexural strength increase to132GPaand1009Mpa respectively. The TEM results showed that the interfacial structure is formed a20-30nm thick reaction layer. Since Ti is highly compatible with Mg, and the characteristic ofTi and Mg is benefit to control the extent of interface reaction, the designed interfacialmicrostructure is thus achieved. This novel interface structure can improve the mechanicalproperties of C/Mg composites.The interfacial structure and mechanical properties are found on dependence of both Alcontent in Mg-Al alloy matrix and the thickness of TiO2coating.When the Al content increases in Mg alloy from Mg-2%Al matrix to AZ91matrix, theflexural strength decreased from1277MPa in to1009MPa (The thickness of TiO2coating is150nm in this comparison).A minimum thickness of interfacial layeris found existed in C/Mg composites in ourstudy. This minimum thickness is affected by Al content in matrix. The minimumthicknesses of C/Mg-2%Al and C/AZ91composites are30nm and40nmrespectively in the experiments. Fundamentally，this interfacial layer is a barrier layerfor the Al diffusing from Mg matrix to carbon fiber to form Al4C3phase. Thus, aminimum thickness of interfacial layer is required to take into effect. When the thickness of TiO2coating is larger than minimum thickness, the mechanicalstrength is decreasing. For example, it is found if the thickness of TiO2coating inCf-TiO2/Mg-2%Al composites is150nm,60nm and30nm, the flexural strength ofcomposites is1277MPa,1309MPa and1391MPa, respectively.When the thickness of TiO2coating is smaller than minimum thickness, the TiO2fibercoating is too thin to work as diffusion barrier to protect carbon fiber. The brittleAl4C3phase reacted in interface zone decreases mechanical properties of C/Mgcomposites. When the thickness of TiO2coating decreases from50nm to30nm inCf-TiO2/AZ91composites, the flexural strength decreases by21%from1108MPa to874MPa.