Study On Preparing Technology Of Black Ceramic Coating And Its Forming Mechanism By Micro-plasma Oxidation On Aluminum Alloy

Aluminum alloy is widely applied to engineering due to its outstanding specific strength, electrical conductivity, thermal conductivity, easily forming and making, etc. However, its lower surface hardness and poor corrosion resistance confine its applied range. So, it is important that its engineering properties are improved by surface treatment on aluminum alloy. The color coating on aluminum alloy is prepared by traditional technology,which include that film of anodic oxidation adsorb organic or inorganic color dye, and color coatings is directly formed by anode oxidation. But the coating obtained by said two technology has labile color, lower wearing resistance and poor corrosion resistance, etc. Currently, micro-plasma oxidation is explored and developed, which is a technology that ceramic film is formed in condition of hyperthermia and heavy pressure produced by micro-plasma on aluminum alloy in given electrolyte. A film with excellent hardness, heat resistant, corrosion resistance, adhesive power and color can be prepared by said micro-plasma oxidation technology. Electrolyte compositions and electrical parameters of Fe-Cr-Co duplex oxidation forming coating and W-P-Mo once oxidation forming film in preparing black ceramic coating by micro-plasma oxidation on LC4 and LY12 aluminum alloy were studied in this article. Furthermore, the results of test at different technology were compared, and shapes of cathode and anode were analyzed. It has been shown that optimal technology parameters of adoption to W-P-Mo once oxidation forming film follows: Na2WO4 11.0g/L, Na₃PO₄ 15.0g/L,(NH₄)₆Mo₇O₂₄ 18.0g/L,PH value 8.0, constant current of DC 3.5A/dm2, surrounding cathode plate, sample of non-sharp corner and non-sharp edge. The film obtained by said W-P-Mo technology has smoothness black, thickness of film:35⁶0μm, micro-hardness:HV800～1200. At the same time, SEM micrographs of surface and section, XRD spectra of black ceramic coating were analyzed, Results show that coatings contain main phase of α-Al₂O₃ and γ-Al₂O₃, another minor compounded phase of WOx and MoOx which mainly were composed with WO3 and MoO3. Simultaneously, forming film mechanism was studied by micro-plasma physics , physical chemistry and material science. Process of forming film follows: (1) creating of surface obstruction film; (2) electron-puncture of obstruction film; (3) growth of amorphous oxide-film; (4) creating of local ceramic oxide-film; (5) electro-puncture of ceramic oxide-film; (6) growth of ceramic oxide-film.