| Aluminum alloys tend to be the key candidate packaging materials for heat dissipation substrate of high power electronic devices,due to the combined advantages of low density,high strength and the outstanding thermal conductivity.However,the poor radiative heat dissipation performance owing to low emissivity,as well as the weak corrosion resistance serving under marine environment are the key obstacle that limits the extended applications for high reliability and long service life.How to improve the radiative heat dissipation and corrosion resistance of aluminum alloy used in high-power electronic devices is a key problem in this field.In this thesis,considering the design of composition and structure of coatings,two kinds of composite coating were proposed.Firstly,nano Si C and h BN particles modified composite coatings with high emissivity were fabricated by nanoparticles deposition synchronous microarc oxidation(MAO)methods.Secondly,a multilayer composite coating for radiative heat dissipation,hydrophobicity and corrosion resistance was prepared by microarc oxidation/electrodeposition duplex technique.The relationship between microstructures of coatings and surface emissivity,radiative heat dissipation,hydrophobicity and corrosion resistance performance were explored.The formation mechanism of particle modified microarc oxidation nanocomposite coatings and multilayers composite coatings were revealed.Results show that the intrinsic morphologies of substrate side ‘pits’ and coating side ‘protrusions’ at the interface of microarc oxidation coating were duplicated by alkali treatment and electrochemical detachment.Based on the microstructure results by FIB-TEM,continuous microdischarges induce the "local over-growth" at the coating/substrate interface.Cracks initiate at the bottom of ‘over-growth’ regions between the coating/substrate interface and propagate into aluminum alloy substrate recorded by the in-situ SEM tensile tests and microstructure evolution.This work provides insights to control ‘over-growth’ characteristics of substrate/coating interface by tailoring electrical parameters or electrolytes composition.In order to improve the emissivity of MAO coated aluminum alloy and enhance the radiation heat dissipation property,nanoparticles deposition synchronous microarc oxidation composite coating was proposed based on the optimized structure between coating/substrate interface.Under special parameter(temperature of electrolyte: 70～90°C,applied voltage: >550V),Si C(S-75),h BN(B-42)and Si C/h BN(BS-78)modified nanocomposite coatings with thickness of 75μm,42μm and 78μm were prepared,respectively.Compared with conventional microarc oxidation coating,the growth rate was increased by more than 15 times.TEM and GDOES results indicate that composite coating includes three layers: dense barrier layer,intermediate layer and nanocomposite outer layer.The thickness of dense barrier layer is about 600 nm,while the nanocomposite outer layer is about 1/5～1/3 of the total coating thickness.The growth mechanism of nanoparticles deposition synchronous microarc oxidation was that the negatively charged Si C and h BN particles moved toward the metal substrate when an electric field was generated between the anode and cathode potential by applied voltage.Si C and h BN were deposited on the coating outer layer owing to the concentration potential and plasma assistance.The incorporation of nanoparticles would be partially oxidized to form glassy phases under the present conditions of the process.Thus,Si C and h BN nanoparticles were sintered to bond together on top layer of composite coating by amorphous glassy phases such as Si O2 or boron-phosphorosilicate,generated by the rapid cooling of discharge.To improve the heat radiation dissipation and corrosion resistance of aluminum alloy,firstly,microarc oxidation/spinning coating methods are used to construct a double layer composite coating of aluminua/reduced graphene oxide(Al2O3/r GO).The composite coating consists of an Al2O3 bottom layer with thickness of 14μm and r GO top layer with thickness of 60～80nm.Lamellar r GO can seal the micropores on bottom layer effectively,and decrease the surface roughness slightly according to SEM and 3D contour morphologies.Moreover,a double layer composite coating of alumina/cerium hexadecate(Al2O3/CH)with radiative heat dissipation,superhydrophobic and corrosion resistant was fabricated by microarc oxidation and electrodeposition technique.The micro-papillae microstructural CH with a diameter around 3～6μm was deposited at the porous Al2O3 bottom layer.These micropapillae show a flower-like structure,which are characterized by randomly staggered nanoplatelets with a length of 500～900nm and a width of 70～90nm.Microarc oxidation and nanocomposite coatings can increase the spectral emissivity of aluminum alloy substrate.The emissivity of aluminum alloy is as low as 0.1～0.2 in the range of 3～20μm.Microarc oxidation coating can enchance the emissivity value to 0.8 in the range of 8～20μm,while the value is 0.3 in 3～8μm.The h BN particles modified nanocomposite coating can improve the emissivity over 0.8 in a wide wavelength band of 3～20μm.Besides,Si C and Si C/h BN modified composite coatings significantly promoted the emissivity values up to 0.88～0.9.The increased high emissivity contributes to the strong infrared vibration absorption bands of Si-C,B-N and Si-O-Si(especially in the wavelength of 3～8μm),accompanying with rough surface and microporous structure of coatings.Al2O3/r GO and Al2O3/CH multilayer composite coatings also show good thermal radiation properties with average emissivity of 0.78 and 0.75 in 3～20μm,respectively.It is mainly attributed to the coordination results of the absorption matching of multi-chemical bond,such as Al-O,C-O,C=O and O=C-O-polar bonds,in wide spectrum of Al2O3,r GO and CH.The radiation cooling test of coated aluminum alloy(exposure area size of 7cm2)under 5 W constant power heat source shows that the cooling efficiencies of microarc oxidation coating(MAO-12)and nanocomposite coatings(S-75,B-42 and BS-78)are 13%,21%,17.2% and 22.3%,respectively under a closed environment of 1.3dm3.Besides,the heat dissipation efficiencies of Al2O3/r GO and Al2O3/CH multilayer composite coatings are 15% and 14.1% respectively.The results indicate that the increasing surface emissivity of coating significantly improved the heat dissipation performance.The surfaces of microarc oxidation and nanocomposite coating are hydrophilic.The corrosion resistance results show that the coating can enhance the corrosion potential of aluminum alloy by 60%.The polarization resistance value was one order of magnitude higher than the substrate.For hydrophobic Al2O3/r GO composite coating,the surface contact angle is 114.42°.Compared with the metal substrate,the polarization resistance is increased by one order of magnitude.While,the static water contact angle and sliding angle of Al2O3/CH layer were 165.5° and 5.2°,which exhibits excellent self-cleaning performance.Al2O3/CH composite coating can significantly improve the corrosion resistance of aluminum alloy,and the polarization resistance can be increased by 2～3 orders of magnitude. |
PDF Full Text Request