High temperature thermal protection application of micro-arc oxidation coating

       The application range of micro-arc oxidation technology under high temperature conditions is also particularly wide. Curran et al. found that the MAO coating has a low Young's modulus, which can reduce the spalling caused by thermal stress, and that the oxide film has a large number of micropores and amorphous phases, and its thermal conductivity is much lower than that of bulk crystal oxides. Therefore, MAO coating becomes a potential and attractive thermal protection coating, which has significant effects on improving the oxidation resistance, surface heat radiation characteristics and high temperature friction and wear properties of the base metal.



(1) High temperature oxidation resistance



Metals (Ti, Al, Zr, Nb, etc.) and their alloys are often used as high-temperature structural materials for aircraft casings or engine parts. However, for the shell of an ultra-high-speed aircraft, the pressure and friction existing between it and the atmosphere generate aerodynamic heat, and the surface temperature increases sharply, which intensifies the metal oxidation rate and affects the performance of the shell. Micro-arc oxidation technology surface treatment, the generated MAO coating can be used as a barrier layer to isolate the oxygen in the atmosphere from contact with the substrate and reduce the oxygen diffusion rate.



Xie Niansuo et al. prepared a micro-arc oxidation coating on the surface of TC4 titanium alloy. The high-temperature oxidation test results showed that the oxidation weight of TC4 titanium alloy was 30.51mg/cm2 after oxidation at 750°C for 100 hours, while the oxidation increased after micro-arc oxidation treatment. The weight is reduced to 7.8mg/cm2. Wang et al. compared and analyzed the effects of one-step and two-step micro-arc oxidation on the high-temperature oxidation resistance of MAO coatings on Ti alloy surfaces. The oxidation kinetic curve shows that the two-step method can reduce the oxidation weight gain rate and the high-temperature friction coefficient. Reduced by more than 0.1.



(2) Surface heat radiation characteristics



For the shell of an ultra-high-speed aircraft, the pressure and friction existing between it and the atmosphere generate aerodynamic heat, and the surface temperature increases sharply. The surface heat radiation characteristics of metal materials are extremely poor. The surface emissivity value in the mid-infrared band (3~20um) at room temperature is lower than 0.1. When the temperature rises to 400~600℃, the emissivity value only increases to 0.2 ~0.3, unable to quickly radiate energy to outer space through thermal radiation. The metal oxide coating produced by micro-arc oxidation has strong polar bonds, which is beneficial to increase the surface emissivity value.



Wang et al. prepared a micro-arc oxidation coating on the surface of Ti alloy. The coating is mainly composed of rutile and anatase TiO2. The emissivity of the Ti alloy tested at 700℃ in the 3-20um band is about 0.1-0.2, and as the Ti alloy thermally oxidizes to produce an oxide film, it slowly increases to 0.2-0.3; after the micro-arc oxidation treatment, the surface The emissivity is increased to 0.8 on average, and with the increase of the micro-arc oxidation treatment time, the emissivity of the coating can be further improved. Wang et al. also tried micro-arc oxidation treatment on the surface of Al alloy and Mg alloy, and the obtained MAO coating also had a higher emissivity value. The emissivity of 2024 aluminum alloy tested under 500℃ is between 0.2~0.3, and the surface oxide film produced by thermal oxidation makes the emissivity around 0.3; after the treatment of micro-arc oxidation technology, the emissivity of 3~8um band is in Above 0.4, the surface emissivity in the 8-20um band can be increased to above 0.8, and with the increase of the micro-arc oxidation time, the emissivity in the 3-8um band will increase significantly. The emissivity of AZ91D Mg alloy at 350℃ is between 0.1 and 0.2; after micro-arc oxidation treatment, the emissivity of 3~8um band is above 0.4, and the emissivity of 8-20um band is increased to above 0.7, which is the high emissivity The film can be used on the heat sink of the LED lamp to improve the heat dissipation performance.



(3) High temperature friction and wear performance



As Al is softer under high temperature conditions and is prone to adhesive wear, micro-arc oxidation technology can be used to change the wear mechanism and reduce the wear rate. Arslan et al. used micro-arc oxidation technology to improve the friction and wear resistance of Al alloy at 200°C, and the wear rate was reduced by 1/3. Liu et al. studied the friction and wear characteristics of the 2219 aluminum alloy surface micro-arc oxidation coating under different temperature conditions. The results showed that as the friction and wear environment temperature increased from the test room temperature to 160℃, the friction coefficient of the aluminum alloy substrate increased from 0.31 liters. As high as 0.56, the friction coefficient of the alloy with micro-arc oxidation coating is reduced from 0.64 to 0.42, and its wear mechanism is not abrasive wear rather than adhesive wear. Chen et al. tried to prepare TiO2-Nb205-Zr02 coating on the surface of Ti-39Nb-6Zr alloy. The wear mechanism of the base alloy at room temperature is abrasive wear, and it transforms into a mixed mechanism of abrasive wear, adhesive wear and oxidative wear at 400℃; while the wear mechanism at room temperature and high temperature after MAO treatment is always abrasive wear, and the friction coefficient is also somewhat different. reduce.