| 其他摘要 | Composite materials made of a Mg-based matrix reinforced by carbon fibres appear to be attractive candidate for aerospace applications and automobile industry. Owning to the low density, such materials could effectively combine high specific strength and stiffness with high temperature resistance, high damping capacity and a near-zero coefficient of thermal expansion.
In this dissertation, the physical and chemical compatibility of C/Mg composites was studied; different coatings for carbon fibres and solute elements for Mg-based matrix were investigated in order to improve the wetting ability of C/Mg composites. The effects of processing parameters were discussed during the fabrication of C/Mg composite by a gas pressure infiltration technology. The microstructures and bonding strength of the interface were characterized, and also the relationship between the mechanical property and interface of C/Mg composites was analyzed. The effects of the interfacial reactions and its productions, defects in C/Mg composites and the fibre alignment as well as the thermal residual stress on the mechanical properties of C/Mg composites were also studied.
SiO2 and Al2O3 coatings were prepared on the surface of carbon fibres by a Sol-Gel method. Tetraethyl Orthosilicate and aluminum isopropoxide were chosen as precursors for SiO2 and Al2O3 sol, respectively. The hydrolysis degree of precursors influenced the structure of coatings, while the viscosity of sol influenced the coating process. The thickness of coating obtained by one coating process was affected by the dipping speed besides the viscosity of sol: the quicker of the dipping speed, the thicker of the coatings. A crack-free and homogenous coating could be get by controlling the parameters above. SiO2 and Al2O3 coatings had improved the wetting ability of C/Mg as well as the anti-oxidation ability of carbon fibres (the improvement was ranging from 200 to 300℃). The thickness of coating was 50-100nm after twice coating processes, however, the tensile strength of SiO2 and Al2O3 coated fibres decreased by 10% and 9% respectively. This process was simple, no-pollution and low-cost. Also the processing parameters were easily to be controlled.
C/Mg composites were fabricated by a gas pressure infiltration technology. There existed a threshold value for the infiltration pressure, which was 0.2MPa and 0.3MPa respectively corresponded to C/Mg composites with SiO2 and Al2O3 coatings. The infiltration pressure was divided into four stages: infiltration initiation stage, macro perform infiltration stage, micro perform infiltration stage and pressurized solidification stage. The defects, such as preform delamination or congregation of fibre tows, caused by infiltration could be avoided via this four-stage infiltration pressure control. Shorten the time of carbon fibres staying with molten matrix could decrease the interfacial reaction on the premise that molten matrix could completely fill the fibrous preform. The tensile strength of C/Mg composites increased as the infiltration speed increased ranging from 5-12.5mm/s. The fibrous perform could be fully infiltrated by molten Mg with a gas pressure lower than 2MPa via the gas pressure infiltration technology, while the infiltration pressure was 100-300MPa for the traditional squeeze casting technology.
The macro and micro fracture surface, microstructures of the interface as well as the interfacial reactions of the C/Mg composites with different fibre coatings and different Al content in the Mg-based matrix were characterized by SEM and TEM. It was revealed that the interfacial reactions increased with the increasing of Al content in the matrix, and which caused degradations of carbon fibres. The tensile strength of carbon fibres extracted from C/AZ91 composite had decreased by 62.4%. In the C/Mg composites with Al content lower than 3 wt%, there was only one chemical reaction happened between coatings and Mg. While in the C/Mg composites with Al content higher than 3 wt%, there was one more chemical reaction between C and the Mg-Al matrix. The reaction product was a kind of lath-shaped carbide, which had a thickness of 50nm and mean length of 600nm in C/AZ91 composite. Also the concentration of Al at the interface of C/AZ31 and C/AZ91 composites was detected.
Different reactions and reaction degree in C/Mg composites had enhanced the bonding strength of the interface in different extent and leaded to three kinds of fracture mode of the composites: single fibre pullout, bundle fracture and brittle fracture. The highest tensile strength was obtained when Al content was 1 wt%. C/Mg-1Al composite which had a intermediate bonding strength failed with a bundle-fibres fracture, and the length of pullout fibre was 100-150nm. Al2O3 coating was more effective in preventing interfacial interdiffusion than SiO2 coating, and avoided thick brittle interlayer. By adjusting the Al content of the matrix and fibre coatings, the interface of C/Mg composites was effectively controlled.
The stress distribution around the lath-shaped carbide and the broken end of carbon fibre was analyzed by finite element method. Sheer stress concentration was found in these areas, and which caused notch effect. The sheer stress around the carbide with h=400nm, w=300nm was 28.5 times more than that far away from it.
The effects of fibres alignment in Mg-based matrix were analyzed. A uniform alignment of fibre in the cross-section could achieve by control the process parameters. However, the deviation of fibres from its axial direction was hard to correct, which weaken the reinforcement effect of carbon fibre. It’s one of the main reasons caused the tensile strength of C/Mg composite lower than the Row of Mixture.
The dislocations caused by thermal residual stress were investigated by TEM. The concentration of dislocations at the interface damaged the out layer of fibres and degraded the fibres. It’s also one of the reasons caused the tensile strength of C/Mg composite lower than the Row of Mixture. |
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