Study On The Preparation And Heat Treatment Of Al-Al₂O₃ Composite Coating

Although particulate reinforced metal matrix composites attract more attention due to their improved physical and mechanical properties, both the complexity of the production and high cost restrict its application. In many cases, the cheap material coated by metal matrix composite coating can work like bulk metal matrix composite material. This can save expensive resources at the same time reduce the cost. Electrochemical codeposition is an appropriate technique to easily fabricate composite coatings on substrates with various shapes at low cost. At present, the study on codeposition of metal and particles still widely use aqueous solution as electrolyte. This restricts the codeposition of metal that can not be electrodeposited from aqueous solution and particles.In this study, Al-Al2O3 composite coating was prepared successfully from acid AlCl3-1-ethyl-3-methylimidazolium chloride (EMIC) with 2:1 in molar ratio containing 0-50g/L Al2O3 particles. The deposition was conducted under galvanostatic mode. Influences of Al2O3 codeposition on Al deposit were analyzed by XRD, SEM and so on. Al2O3 particles can achieve stable dispersion in AICl3-EMIC with 2:1 in molar ratio. Content of Al2O3 in composite coating first increases and latter reduces owe to the agglomeration of Al2O3 with the increasement of Al2O3 concentration in electrolyte. Preferred (200) crystallographic orientation of deposited Al is strengthened due to the codeposition of Al2O3. As a result, composite coating appears flat and smooth. Crystallite size of Al in the coating decreases with Al2O3 concentration in electrolyte. Microhardness of composite coating is improved due to the cooperation of Al2O3 dispersion strengthening and Al grain refinement strengthening. In addition, the codeposition of Al2l3 leads to the lattice distortion of deposited Al and reduces the deposition rate of Al.The heat treatment results suggested that Al2O3 particles can impede the diffusion of Al at temperature that is whether higher or lower than melting point of Al. At 670℃, that Al2O3 particles can impede the diffusion of Al atoms is because they decrease the activity of some Al atoms. Despite that composite coating would diffuse into substrate completely with increasing heat time. By contrast, at 600℃, that Al2O3 particles can impede the diffusion of Al atoms is because they gather on the interface of original coating and alloy layer:when Al diffuses into 1Cr17, Al2O3 left would gather on the interface of coating and alloy layer, which would impede the diffusion of Al more effectively. Moreover, there would be more and more Al2O3 gathering on the interface with the diffusion of Al into 1Cr17 and finally would form AlO3-rich layer. Therefore, the diffusion of Al becomes more and harder with the conduct of heat treatment.