Principle of microplasma oxidation
The principle of microplasma oxidation is a complex process involving electrochemistry, plasma physics, and materials chemistry. Its core is that under the action of a high-voltage electric field, plasma discharge is generated at the interface between the metal surface and the electrolyte, triggering a series of physical and chemical reactions that ultimately form a ceramic oxide film. The entire process can be divided into four stages: initial film formation, spark discharge, micro-arc discharge, and stable film formation. The reaction characteristics and product structure of each stage are different. In the initial film formation, the metal workpiece is immersed in the electrolyte as the anode. When a low voltage ( 50-100V ) is applied, a thin oxide film (similar to an anodic oxide film) first forms on the surface. At this time, the main electrolytic reaction occurs, and metal ions (such as Al³⁺ and Mg²⁺ ) migrate to the cathode and combine with anions in the electrolyte (such as O²⁻ and SiO₃²⁻ ) to form oxides.
The spark discharge stage is a key feature that distinguishes microplasma oxidation from traditional anodizing. When the voltage reaches a certain threshold (typically 100-200V), the surface oxide film breaks down, generating numerous tiny sparks, marking the onset of plasma discharge. The spark discharge has a diameter of approximately 1-100μm and reaches temperatures as high as 3000-10000K. This creates a localized high-temperature and high-pressure region, causing the oxide film to melt and undergo a chemical reaction. During this process, the metal substrate surface is melted, reacting with elements in the electrolyte (such as Si, P, and O) to form complex oxides or composite oxides. For example, when treating aluminum alloys in a silicate electrolyte, the spark discharge region generates a molten phase of aluminosilicate and aluminum oxide, which forms a dense ceramic structure upon cooling. Simultaneously, gases generated by the discharge (such as H₂ and O₂ ) form bubbles, which promote the diffusion of the melt and provide a source of material for film growth.
The micro-arc discharge stage is the primary phase of rapid film growth. As the voltage increases further (200-600V), the spark discharge transforms into a sustained micro-arc discharge, resulting in a larger and more evenly distributed arc. The film thickness increases at a rate of 1-5μm per minute. During this stage, the high temperatures generated by the plasma discharge continuously melt and sinter the oxide film, forming a layered structure from the inside out: an inner, dense layer (directly bonded to the substrate, 1-5μm thick) composed of metal oxides, offering high hardness and excellent bonding strength; and an outer, loose layer (70%-90% of the total film thickness), containing numerous micropores and electrolyte components, which provide wear and corrosion resistance. During the micro-arc discharge process, the electrolyte continuously replenishes ions into the discharge area, participating in reactions and cooling the molten oxide film, ensuring structural stability while the film grows. For example, when treating titanium alloy in a phosphate electrolyte, micro-arc discharge will cause titanium to react with phosphorus and oxygen to form a composite oxide film containing titanium phosphate, in which the inner layer is a dense TiO₂ layer and the outer layer is a loose titanium phosphate layer.
The chemical reactions during microplasma oxidation are complex and diverse, including oxidation, decomposition, and synthesis. The composition of the final product depends on the metal substrate and the type of electrolyte. For aluminum alloys, the primary oxidation reaction occurs: 2Al + 3H₂O → Al₂O₃ + 3H₂ ↑. Simultaneously, it reacts with silicates in the electrolyte to form aluminosilicates: Al₂O₃ + SiO₃²⁻ → Al₂(SiO₃)₃ + O²⁻ . For magnesium alloys, in addition to magnesium oxidation ( 2Mg + O₂ → 2MgO ), it also reacts with fluorides in the electrolyte to form MgF₂ , which inhibits corrosion. For titanium alloys, the primary reaction is TiO₂ , which may react with calcium and phosphorus in the electrolyte to form compounds such as CaTiO₃ and Ti₃(PO₄)₄ , imparting bioactivity to the film. These chemical reactions are accelerated in the high temperature and high pressure environment of plasma discharge, allowing ceramic phases that originally required high-temperature sintering to form to be generated at room temperature, greatly improving film formation efficiency.
The formation mechanism of microplasma oxide films also involves the physical effects of the plasma, including thermal effects, shock wave effects, and sputtering effects, which have a significant impact on the film structure and properties. The thermal effect causes the material in the discharge region to melt and evaporate, promoting atomic diffusion and chemical reactions, and forming a uniform ceramic structure. The shock wave effect, caused by the expansion of the gas generated by the discharge, forces the molten material to flow to the surrounding area, filling pores and increasing the density of the film. The sputtering effect ejects some of the molten material from the discharge region and deposits on the film surface, participating in new reactions and growth. These physical effects synergize with the chemical reactions to promote film growth and densification. For example, under the action of shock waves, the porosity of the oxide film on the surface of aluminum alloy can be reduced from an initial 20% to below 5% during the stable phase, significantly improving its corrosion resistance. Research has shown that by controlling the intensity and distribution of the plasma discharge (e.g., using a pulsed power supply), these physical effects can be effectively manipulated to optimize the film’s microstructure and properties, providing theoretical guidance for the practical application of microplasma oxidation technology.
