Preparation And Properties Of Composite Coatings On High Silicon Aluminum Alloy

Aluminum(Al) is the most abundant metal on the earth.Al and its alloys are widely used in aerospace,transportation,metallurgy,chemical industry,electronic communication and other industries because of their low density,low melting point,good electrical conductivity,thermal conductivity,excellent plastic deformation and easy processing.Under high temperature service conditions,various problems in the application process will appear due to the harsh service environment,which can not meet the requirements of proper thermal conductivity and high corrosion resistance.In this paper,high silicon aluminum alloy was used as the research object,composite aluminum nitride(AlN)and carbon nanotubes(CNTs)ceramic coatings were prepared on the surface of Al-Si alloy by micro arc oxidation(MAO)technology.The effects of electrolyte system,component concentration,additive and electrical parameter on the microstructure and properties of MAO coatings were studied.The microstructure,phase,structure and element composition of the substrate and ceramic coatings were characterized by metallographic microscope,field emission scanning electron microscope(SEM),X-ray diffraction(XRD),Micro Raman spectrometer and energy dispersive spectrometer(EDS).The influence of AlN and CNTs on the corrosion resistance and thermal conductivity of ceramic coatings were studied by electrochemical workstation and thermal conductivity instrument.The effects of AlN and CNTs on the morphology,formation,growth,microstructure,corrosion resistance and thermal conductivity ceramic coatings on high silicon aluminum alloy were investigated.The results show that the ceramic coatings prepared in Na₂Si O₃electrolyte system have good appearance and relatively high corrosion resistance.The coatings are mainly composed of?-Al₂O₃ and?-Al₂O₃ phases.The micropores on the surface of the coatings can be used as the"channels"for various ions and particle groups in the electrolyte to participate in the inward and outward growth,transportation and discharge of the coatings.The consist of 13 g/L Na₂Si O₃ and 2 g/L Na OH was used as the basic electrolyte system,then the different contents of AlN and CNT were added into the system.The results showed that the best addition of AlN is 6 g/L,the coatings and the substrate combined well,the corrosion resistance was the best.The value of electrochemical impedance R_P(2.917×10⁵?·cm²)was the highest,which was three orders of magnitude higher than that of Al-Si alloy substrate R_P(3.117×10²?·cm²).The average thermal conductivity of the AlN composite ceramic coatings was 39.312 W/m·K,which was increased by 88.456%compared with the sample without AlN composite ceramic coatings.AlN can be absorbed and embedded into the coatings and some of them took part in the plasma electrolytic oxidation reaction and transformed into Al₂O₃ phase.Some of them existed on the surface and insided of the coatings in the form of particles,which changed the surface microstructure and internal phase structure of the coatings,improved the quality of the coatings,further affected the corrosion resistance and heat conduction mechanism,therefore the corrosion resistance and thermal conductivity of the ceramic coatings can be controlled.Raman analysis results showed that CNTs could enter the ceramic coatings by surface adsorption,molten inclusion and reaction fusion.The surface micro-morphology of the ceramic coatings was significantly changed after the CNTs entered the coatings.CNTs can form Si C phase by plasma electrolytic oxidation reaction with Si of the substrate and Si O₃^2-ions in the electrolyte.The optimal addition of CNTs was 1.5 g/L.When the CNTs content was less than1.5 g/L,the electrochemical impedance R_P of the composite CNTs composite ceramic coatings increased gradually.When the CNTs content was 1.5 g/L,the maximum R_P value was1.144×10⁷?·cm².Further increasing the CNTs content,the electrochemical impedance R_P of the CNTs composite coatings decreased significantly.The change of the electrochemical performance of the coatings was related to the electrolyte conductivity,degree of plasma reaction,control of the microstructure compactness of the coatings caused by CNTs.Because CNTs failed to form thermal conductive channels through the coatings or did not form a continuous distribution along the axial direction,the average thermal conductivity of the composite CNTs coatings was 26.184 W/m·K.Compared with the sample without the composite CNTs coatings,the thermal conductivity only increased by 25.522%.The results showed that AlN and CNTs could control the microstructure,corrosion resistance and thermal conductivity of the MAO ceramic coatings.