Mechanical Property Control And Fabrication Of Al₂O₃ Composite With Micro/Nano Al₂O₃ Particles

Al₂O₃ ceramic particulate reinforced aluminum matrix composite has been extensively studied due to their high specific strength,high specific modulus and wear resistance,etc.Generally,Al₂O₃ particles show different particle size,nanoparticles and micro particles.The addition of micro Al₂O₃ particles can improve the strength and modulus,but reduce the ductility of the aluminum matrix composite.Nanoparticles can significantly improve their strength and ductility,but it is easy to agglomerate,and the volume fraction should be lower.To overcome the above problems,micro and nanoparticles are incorporated simultaneously,in which the microparticles could make the distribution of nanoparticles uniform,and effectively increase the volume fraction of ceramic particles.It is useful to improve the strength,modulus and ductility by coupling of the double scale particles.In this paper,the micro/nano Al₂O₃ particles reinforced aluminum matrix composites were prepared by high energy ball milling and spark plasma sintering（SPS）.The thermal expansion coefficient,friction and wear properties and dynamic impact compression behaviors were studied systematically,and revealing the underline mechanism of nano/micro particles on the matrix.Main conclusions are as follows:（1）The effects of milling time and sintering temperature on the microstructure of Al₂O₃/Al composites were studied.Three kinds of powders are mixed by high energy ball milling,and the optimal process was obtained as follows.Firstly,the nano Al₂O₃ reinforced particles are mixed with the Al powder,and then the micro Al₂O₃ reinforced particles are added for mixing,which could obtain the mixed powders with uniform dispersion.Both nanoparticles and microparticles uniformly dispersed in as-fabricated composites after SPS.The optimized milling time and sintering temperature are 15h and 560 ^oC,respectively.（2）Nano/micro Al₂O₃ particles play an important role on the thermal conductivity and thermal expansion coefficient of Al₂O₃/Al composite,this study revealed their conduction mechanism.It shows that the thermal expansion coefficient and thermal conductivity of Al₂O₃/Al composite decrease with the increase of Al₂O₃ content.The thermal expansion coefficient decreases from 2.2×10^-6 to 1.9×10^-6K^-1 if the volume fraction of Al₂O₃ increases from 10 to 20 vol.%,and the thermal conductivity decreases from 192.6 to 82.3 W/（m·K）.Compared with the microparticles,the nanoparticles are more effective to decrease the thermal expansion coefficient and thermal conductivity.（3）The friction and wear behaviors were studied to specify the effect mechanisms of micro/nano Al₂O₃ particles.The results show that the Al₂O₃ particles can improve the friction resistance,and the microparticles are more effective to improve the wear resistance than nanoparticles.The wear mechanisms of as-fabricated composite at room temperature were abrasive wear,adhesion wear and slight oxidation wear,while the oxidation wear is the main wear mechanism at high temperature.The main wear mechanism includes adhesive wear,micro cutting and delamination if only introduced microparticles,and the wear resistance was greatly enhanced by coupling of micro and nano particles at high temperature.（4）The effects of micro/nano particles on the mechanical behaviors of Al₂O₃/Al composite were studied to reveal the strengthen and toughen mechanism.The nanoparticles play a strengthen effect to improve the maximum yield strength and tensile strength up to 280 and 482MPa at room temperature,respectively.Interestingly,the yield strength increases firstly and then decreases with the increase of nanoparticles content at 493 K.As nanoparticles,the strengthening mechanism is mainly thermal mismatch strengthen at room temperature,and Orowan strengthen at high temperature.The strengthen effect of micro/nano Al₂O₃/Al composite is more effective than that of single-scale particle reinforcement.The optimized mechanical properties can be obtained for the composites containing 2 vol.%microparticles and 1 vol.%nanoparticles at room temperature,while 1 vol.%microparticles and 2 vol.%nanoparticles at high temperature.（5）The dynamic compression behaviors of Al₂O₃/Al composite at high strain rates were studied,and the compression deformation mechanism was revealed.The results show that the dynamic flow stress is higher than the static compression.It increases with the increase of strain rate.The adiabatic shear band of as-fabricated composites is clear with only microparticles,and it’s a gradual evolution process for the adiabatic shear band.The as-fabricated composites with high strength could induce localized plastic deformation,which produced critical strain and critical strain rate,and the critical strain shows obvious“size effect”.Meanwhile,the critical strain of local adiabatic shear is smaller if the particle size is smaller under the same conditions.