1 micro-arc oxidation ceramic layer
Micro-arc oxidation (MAO), also known as micro-plasma oxidation (MPO), is to grow ceramic film mainly composed of matrix metal oxide on the surface of aluminum, magnesium, titanium and their alloys through the combination of electrolyte and corresponding electrical parameters. Because chemical oxidation, electrochemical oxidation and plasma oxidation coexist in the process of micro-arc oxidation, the micro-arc oxidation process introduces the working area into the high-voltage discharge area, which greatly improves the comprehensive properties of the film. The micro-arc oxidation film has the advantages of firm combination with the substrate, compact structure, high toughness, good wear resistance, corrosion resistance, high temperature impact resistance and electrical insulation. This technology is simple in operation, easy to realize the function adjustment of the film layer, uncomplicated in process and will not cause environmental pollution. It is a brand-new surface treatment technology of environmental protection materials, which has broad application prospects in aerospace, machinery, electronics, decoration and other fields.
Copper and magnesium are beneficial to micro-arc oxidation, while silicon is not. Hou Chaohui et al. [1] studied the technological conditions, film structure and film-forming process of ZL aluminum alloy with silicon content of 8% ~ 12%. The results show that a fine, uniform, thick and high microhardness ceramic oxide film can be obtained by micro-arc oxidation of cast aluminum alloy in water glass composite system. In the electrolyte system of micro-arc oxidation, water glass can make the micro-arc oxidation of cast aluminum alloy go smoothly; The combination of Na2WO4 and EDTA disodium can improve the hardness of the membrane. The technological conditions for obtaining the micro-arc oxidation film of ZL 109 alloy are as follows: NaOH: 2 ~ 4g/L, water glass: 5 ~ 7ml/L, Na2WO4: 2 ~ 4g/L, disodium EDTA: 2 ~ 4g/L, micro-arc oxidation current density of 30 ~ 40a/dm2, and solution temperature of 30 ~ 40℃. In addition, Gong Jianfei and others [2] also studied the micro-arc oxidation of ZL 109, and obtained a uniform oxide ceramic membrane with a dense layer thickness of more than 76μm and a microhardness of HV 1600.
ADC 12 die-casting aluminum alloy is widely used in piston, pulley and other parts and structural parts in automobile, motorcycle and instrument industries. Zhang Jinbin et al. [3] studied the influence of electrolyte composition and electrical parameters on the properties of black ceramic film prepared by microplasma oxidation on the surface of ADC 12 aluminum alloy. The results show that the low concentration of sodium phosphate has a rough surface, and it is easy to precipitate salt film when the concentration is too high. The optimal concentration is12 ~15g/L; The greater the specific gravity of metal oxide K in additive M 1 and M2 component in the film, the more stable the black saturation of the film, and the optimal concentrations are10.0 ~1.0g/L and15.0 ~18.0g/respectively. The optimum pH value of uniform black film is 8.0 ~ 9.0; The optimum current density for forming saturated black is 3.0 ~ 4.0A/dm2；; The black film prepared by the best electrolyte formula has a thickness of 20 ~ 30μ m, a hardness of HV 500 ~ 700 and a black saturation of 0.8 ~ 1.0.
Wang Zongren et al. [4] applied PECC technology to the surface strengthening treatment of Y 1 12 die-casting aluminum alloy, and formed α-Al2O3 and γ-Al2O3 ceramic films on its surface. It is said that the performance of the film is better than that of Teflon coating.
Jin Ling et al [5] made micro-arc oxidation on the surfaces of ZL 109 alloy and SiCp/ZL 109 composite. The results show that both ZL 109 alloy and SiCp/ZL 109 composite can be subjected to micro-arc oxidation, and the micro-arc oxidation layer consists of two layers, namely, a loose layer and a dense layer. The micro-arc oxidation layer of ZL 109 alloy is mainly composed of Al2O3 phases with different structures, and the micro-arc oxidation layer of SiCp/ZL 109 composite is composed of Al2O3 and MgAl 13O40.
The study on the integrated treatment of micro-arc oxidation and blackening of aluminum alloy surface under the condition of constant current of AC power supply shows that vanadate plays a decisive role in the blackening effect of micro-arc oxidation ceramic membrane; Black ceramic film has stable color and high microhardness, which can provide effective corrosion protection for base metal. The main elements of black ceramic film include O, Al, Si, V and P. The compounds in the film mainly exist in amorphous and/or microcrystalline forms, and only a few γ-Al2O3 and ε-Al2O3 crystals are found. The black ceramic membrane is a loose single-layer structure, with rough and uneven surface, micron-sized dense micropores, obvious high-temperature sintering traces and microcracks. The microstructure of black ceramic film is related to its formation mechanism.
The growth of micro-arc oxidation ceramic film of ZL 10 1 cast Al-2Si alloy can be divided into three stages. At the initial stage of oxidation, the current density is high, but the film growth is slow. In the rapid growth stage of the membrane, the growth rate of the membrane reaches the maximum; After the film growth entered a stationary period, it remained basically unchanged, the external size of the sample no longer increased, and the film gradually turned to the internal growth of the matrix. The influence of alloying element silicon is mainly manifested in the obstacle to the growth of thin films in the early stage of oxidation; After micro-arc oxidation treatment, the corrosion current of cast aluminum alloy is greatly reduced and the polarization resistance is increased by several orders of magnitude. Thin micro-arc oxidation film also greatly improves the corrosion resistance of Al-Si alloy.
The micro-arc oxidation ceramic film of high strength cast aluminum alloy ZL205 was studied by neutral salt spray corrosion test [8]. The results show that micro-arc oxidation treatment can significantly improve the corrosion resistance of ZL205. With the increase of thickness, the corrosion resistance of ceramic membrane is improved, but the corrosion resistance of ceramic membrane is not improved obviously after the thickness reaches a certain value. With the increase of thickness, the surface morphology and phase structure of micro-arc oxidation film change, which leads to the change of corrosion resistance of micro-arc oxidation film.
2 electrodeposited layer
Electrodeposition is the process of electrochemical deposition of metal or alloy from its compound aqueous solution, non-aqueous solution or molten salt. It is the basis of metal electrolytic smelting, electrolytic refining, electroplating and electroforming. These processes are carried out under specific electrolyte and operating conditions. The difficulty of metal electrodeposition and the morphology of deposits are related to the properties of deposited metals, as well as the composition of electrolyte, pH value, temperature, current density and other factors. Wu et al. [9] studied the corrosion resistance of electrodeposited Ni2SiC composite coating on ZL 105 aluminum alloy by electrochemical method. The results show that the surface morphology of Ni2SiC composite coating is completely different from that of pure Ni coating, and its corrosion resistance is better than that of pure Ni coating. After heat treatment at 300℃×2h, the corrosion resistance was further improved.
3 multi-arc ion plating
Multi-arc ion plating is a kind of vacuum chamber, which uses gas discharge or partial ionization of evaporated substances to deposit evaporated substances or reactants on the substrate while gas ions or evaporated substance particles bombard. Ion plating organically combines glow discharge, plasma technology and vacuum evaporation, which can not only obviously improve the quality of thin films, but also expand the application scope of thin films.
Its advantages are strong film adhesion, good diffraction performance and wide film materials. There are many kinds of ion plating, such as resistance heating, electron beam heating, plasma electron beam heating and high frequency induction heating. Multi-arc ion plating uses arc discharge instead of glow discharge of traditional ion plating for deposition. Simply put, the principle of multi-arc ion plating is to use the cathode target as the evaporation source, and evaporate the target through arc discharge between the target and the anode shell, thus forming plasma in space and depositing the substrate. Ti-Cr-N coating was deposited on ZL20 1 aluminum alloy by multi-arc ion plating technology, and lipid film was prepared on Ti-Cr-N coating [10]. The results show that Cr in Ti-Cr-N coating exists in TiN crystal in the form of solid solution, and no single CrN phase is formed. The coating can effectively improve the salt spray corrosion resistance of ZL20 1 aluminum alloy.
4 chemical composite coating
The process of adding water-insoluble solid particles into the plating solution and depositing together with the main metal is called composite plating. If the electroplating process is adopted, it is called composite electroplating; If the electroless plating process is adopted, it is called composite electroless plating. The coating obtained is called composite coating. In principle, any metal that can be electroplated can be used as the main metal, but several metals such as nickel, chromium, cobalt, gold, silver and copper are widely studied and applied. There are mainly two kinds of solid particles, one is high hardness and high melting point particles to improve the wear resistance of the coating; One is solid lubricant particles to improve the self-lubricating characteristics of the coating. Ni-P- diamond electroless composite coating [1 1] was prepared on the surface of cast aluminum. The results show that ceric sulfate can promote diamond particles to enter the coating, and the stability of the plating solution is greatly improved with the increase of ceric sulfate content, and then tends to be stable. The wear resistance of Ni-P- diamond composite coating is better than that of Ni-P coating. After adding 2mg/L ceric sulfate, the wear resistance is further significantly improved than that of Ni-P coating.
5 chemical conversion film
Chemical conversion film is to make the metal contact with a specific corrosive liquid, and under certain conditions, a chemical reaction occurs, forming an insoluble product film with good adhesion on the metal surface. These films can protect the base metal from water and other corrosive media, or improve the adhesion and aging resistance of organic coatings, or give the surface other properties. Because the chemical conversion film is formed by the direct participation of the substrate metal in the film-forming reaction, the bonding force between the chemical conversion film and the substrate is much greater than that between the electroplated layer and the electroless plating layer. Almost all metals can be transformed in the selected medium to obtain chemical conversion films with different application purposes, but steel, aluminum, zinc, copper, magnesium and their alloys are widely used in industry at present. Chemical conversion film is different from other coatings on metals (for example, electrodeposition of metals). Its formation must be directly involved by the base metal, and anions in the medium generate self-conversion products (MmAn), so it can also be said that the formation of chemical conversion film can actually be regarded as a controlled metal corrosion process. Chemical conversion film can be divided into oxide film, phosphate film, chromate film, oxalate film and so on.
Aluminum alloy is vulnerable to intergranular corrosion in atmospheric environment. At present, high-strength cast aluminum alloy generally contains elements such as silicon, copper and magnesium, and the addition of these elements increases the corrosion sensitivity of the alloy. Secondly, the surface hardness is low, easy to wear, and the appearance gloss can not be maintained for a long time, which requires higher protective measures. Among them, the formation of chemical conversion film on aluminum alloy surface has the advantages of simple equipment, low cost and less investment. Peng Liang et al. [12] used chromate method to produce chemical transformation on Y 1 12 alloy.
The experimental results show that the conversion film has high corrosion resistance and beautiful golden yellow outer surface.
The corrosion potential of the chemical oxidation film [13] on the surface of aluminum alloy with manganate and zirconium salt as main salts is about 0.45V higher than that of aluminum alloy samples, and the corrosion current density is only 0.286 μ a/cm2. The impedance value at the low frequency end of AC impedance spectrum is one order of magnitude larger than that of aluminum alloy sample. The appearance of chemical oxide film on aluminum alloy surface is golden yellow with regular columnar growth structure.
Ge Shengsong et al. [14] prepared black conversion film on the surface of cast aluminum alloy by chromium-free chemical method, and evaluated the corrosion resistance of the film by drop test. Scanning electron microscope and electron probe were used to observe the morphology of the film and determine its composition elements. Finally, the formation mechanism and corrosion resistance mechanism of black film are put forward.