Common knowledge and application scope of plasma micro-arc oxidation

Microplasma oxidation (MPO), also known as micro-arc oxidation, is a combination of electrolyte and corresponding electrical parameters. It grows on the surface of aluminum, magnesium, titanium and their alloys by the instantaneous high temperature and high pressure generated by arc discharge Ceramic film mainly composed of base metal oxide. In the process of micro-arc oxidation, chemical oxidation, electrochemical oxidation, and plasma oxidation coexist. Therefore, the formation process of the ceramic layer is very complicated. So far, there is no reasonable model to fully describe the formation of the ceramic layer.
The micro-arc oxidation process introduces the working area from the common anodized Faraday area to the high-voltage discharge area, overcoming the defects of hard anodization and greatly improving the overall performance of the film. The micro-arc oxidation film layer is firmly combined with the substrate, the structure is dense, the toughness is high, and it has the characteristics of good wear resistance, corrosion resistance, high temperature impact resistance and electrical insulation. This technology has the characteristics of simple operation and easy adjustment of the film function, and the process is not complicated and does not cause environmental pollution. It is a brand-new green environmental protection material surface treatment technology, which has the fields of aerospace, machinery, electronics, decoration, etc. Broad application prospects.
The principle and characteristics of micro-arc oxidation technology:
Micro-arc oxidation or micro-plasma surface ceramization technology refers to the use of arc discharge to enhance and activate the reaction occurring on the anode based on ordinary anodization, so that aluminum, titanium, magnesium metal and its alloys are used as materials The method of forming a high-quality reinforced ceramic film on the surface of the workpiece is to apply a voltage to the workpiece with a special micro-arc oxidation power source to interact the metal on the surface of the workpiece with the electrolyte solution to form a micro-arc discharge on the surface of the workpiece. Under the influence of factors, a ceramic film is formed on the metal surface to achieve the purpose of strengthening the surface of the workpiece.
The outstanding features of the micro-arc oxidation technology are: (1) The surface hardness of the material is greatly improved. The micro-hardness is from 1000 to 2000HV, up to 3000HV, which is comparable to cemented carbide and greatly exceeds the high carbon after heat treatment The hardness of steel, high alloy steel and high-speed tool steel; (2) good wear resistance; (3) good heat resistance and corrosion resistance. This fundamentally overcomes the shortcomings of aluminum, magnesium, and titanium alloy materials in application, so this technology has broad application prospects; (4) has good insulation performance, and the insulation resistance can reach 100MΩ. (5) The ceramic film is grown in situ on the substrate, the bonding is firm, and the ceramic film is dense and uniform.

Micro-arc oxidation bath:
Micro-arc oxidation mainly targets valve metals such as aluminum, magnesium, titanium, zirconium, niobium, and thallium (valve metals refer to metals that function as electrolytic valves in the electrolyte). The same liquid can be used for aluminum titanium.
1. Oxidation liquid density: different liquids have different specific gravities, and the specific gravities range from 1.0 to 1.1.
2. Working voltage of oxidizing liquid: 400V-750V.
3. Current density: The current density of the workpiece is different depending on the liquid. Generally about: 0.01-0.1 ampere per square decimetre. But there are also large currents that exceed 8 amperes per square decimeter.
4. Micro-arc oxidation time: 10-60 minutes, the longer the time, the denser the film, but the roughness also increases.
5. Liquid pH: alkaline, PH is usually 8-13
6. Micro-arc oxidation process:
Oil removal —- Washing —- Micro-arc oxidation —- Pure water washing —- Closed
Influencing factors of micro-arc oxidation work
1. Workpiece material and surface condition
(1) Micro-arc oxidation does not have high requirements on aluminum materials, whether it is copper or silicon-containing difficult to anodize aluminum alloys, as long as the proportion of valve metal accounts for more than 40%, it can be used for micro-arc oxidation and can obtain an ideal film Floor.
(2) The surface condition generally does not need to be polished. For rough surfaces, after micro-arc oxidation, it can be repaired flat and smooth; for low roughness (that is, smooth) surfaces, it will increase the roughness.
2. Effect of liquid composition on oxidation
The electrolyte composition is the key factor to obtain a qualified film. The micro-arc oxidation liquid generally selects alkaline salt solutions containing certain metal or non-metal oxides, such as silicate, phosphate, and borate. Under the same micro-arc electrolysis voltage, the greater the electrolyte concentration, the faster the film formation speed, and the slower the temperature rise of the solution. On the contrary, the film formation speed is slower, and the solution temperature rises faster.
3. The effect of temperature on micro-arc oxidation
Micro-arc oxidation is different from anodic oxidation, and the required temperature range is wider. Generally, it is 10-90 degrees. The higher the temperature, the faster the film formation, but the roughness also increases. And the temperature is high, it will form moisture. Generally recommended at 20-60 degrees. Since the micro-arc oxidation is released in the form of heat energy, the temperature of the liquid rises rapidly. The micro-arc oxidation process must be equipped with a large-capacity heat exchange refrigeration system to control the bath temperature.
4. Effect of time on micro-arc oxidation
Micro-arc oxidation time is generally controlled at 10 ~ 60min. The longer the oxidation time, the better the density of the film, but its roughness also increases.
5. Cathode material
The cathode material can be stainless steel, carbon steel, nickel, etc. The above materials can be suspended or used as a cathode tank.
6. Effect of post-treatment on micro-arc oxidation
After micro-arc oxidation, the workpiece can be used directly without task processing, and can also be subjected to subsequent processing such as sealing, electrophoresis, and polishing.
Advantages and disadvantages and scope of use
The ceramic film layer on the aluminum-based surface after micro-arc oxidation treatment has high hardness (HV>1200), strong corrosion resistance (CASS salt spray test>480h), good insulation (film resistance>100MΩ), and the film layer is combined with the base metal Strong, and has good wear resistance and thermal shock resistance. Micro-arc oxidation technology has strong process capability, and can obtain oxide film layers with different characteristics by changing process parameters to meet the needs of different purposes; it can also make the film layer have certain characteristics or show different colors by changing or adjusting the composition of the electrolyte ; The same workpiece can also be subjected to multiple micro-arc oxidation treatments with different electrolytes to obtain multiple layers of ceramic oxide films with different properties.
Since the micro-arc oxidation technology has the above advantages and characteristics, it has extremely broad application prospects in industrial fields such as machinery, automobiles, national defense, electronics, aerospace and civil construction. It can be mainly used for surface strengthening treatment of aluminum-based components with special requirements on wear resistance, corrosion resistance, thermal shock resistance, high insulation, etc.; it can also be used in construction and civil industries with high requirements for decoration, wear resistance and corrosion resistance The surface treatment of the aluminum substrate; can also be used for the surface strengthening treatment of special aluminum-based alloy materials that cannot be treated by conventional anodization. For example, aluminum-based pistons, piston seats, cylinders and other aluminum-based components of automobiles and other vehicles; various aluminum-based molds in the machinery and chemical industries, the inner walls of various aluminum cans, and various aluminum-based zeros in aircraft manufacturing Components such as warehouse floors, roller bars, rails, etc.; and various aluminum-based hardware products, fitness equipment, etc. in the civil industry.
There are still some shortcomings in the micro-arc oxidation technology, such as the study of process parameters and supporting equipment needs to be further improved; the oxidation voltage is much higher than the conventional aluminum anodization voltage, and safety protection measures must be taken during operation; and the electrolyte temperature rises Faster, need to be equipped with larger capacity refrigeration and heat exchange equipment.