A weapon that uses a direct illumination of a laser to kill a target is called a laser weapon. Laser weapons have the characteristics of high propagation speed, fast transfer of firepower, low recoil, low launch accuracy, high killing power, strong anti-interference ability, etc., which have become the deadly killers of various weapons such as aircraft. Among the world's major military countries, the United States is at the forefront of research on laser weapons. In January 2009, the output of 105kW was achieved, achieving the goal of the third phase of 100kW output power for laser weapon applications. 100kW power has always been regarded as the threshold of weapon-grade high-energy lasers. High-energy laser weapons have gradually become an important means of combating anti-missile, anti-satellite, anti-aircraft and ground targets in various military powers.
As high-energy laser weapons gradually enter the practical application stage, research on laser protection technology has received much attention. Laser protection through material technology includes films, structures and coatings. The film mainly focuses on the anti-laser protection of satellite optical systems and photoelectric sensors, mainly adopting the method of plating thin films on optical and sensor surfaces; the structure mainly refers to the integration of structural functions through structural design to achieve laser protection purposes, such as the United States in 1998. Some of the sensitive parts of the aircraft are designed to be laser-protected by adding metal mesh to the plate. The metal mesh can be designed as multiple layers according to requirements. The coating material is the most traditional protection of the aircraft and the material form for special functions. Simple air spraying or brushing process is one of the most suitable protective materials for application. However, due to the extremely high material performance requirements of coating materials against laser, laser protective coating material technology research at home and abroad All belong to a new field. This work started from the preparation process of general protective coatings, and through the formulation design and process optimization, the anti-laser ablation coating material with excellent performance was prepared.
1. Experimental materials and methods
1.1 Coating preparation
Using silicone resin and polycarbosilane as binder, adding Al2O3, SiC, ZrO2, SiO2, BN, glass powder, carbon fiber and other fillers to form a mixed slurry, and the mixed slurry is ground on a sand mill to achieve a fineness of 40-50 μm. . The coating was then prepared on the surface of a 30 cm MnSiA steel sheet having a size of 50 mm × 100 mm × 2 mm and having a phosphating surface. The coating was prepared by ordinary air spraying method, the air pressure was 0.4 MPa, and the spray gun was supplied by German SATA company with a diameter of 0.8 to 1 mm. The specific spraying process is divided into several sections according to the required thickness, and is considered as one in a vertical cross-spraying process. Each lane needs a certain time interval to achieve a certain degree of surface drying to prevent runny. After spraying, it was allowed to stand at room temperature for 48 hours, and then baked at 200 ° C for 2 hours to form a film.
1.2 Performance characterization
The fully cured coating is first baked in a muffle furnace, heated from room temperature to 600 ° C, held for 2 h, and cooled to room temperature with the furnace, with reference to standard GB/T 9286-1998. The adhesion of the paint film was tested by the cross-hatch method, and the surface of the coating was observed by a stereo microscope to evaluate the performance. Then ablation was performed under oxy-acetylene to observe the ablation morphology of the coating after ablation for 4 s at different flame temperatures. Finally, the laser ablation test was carried out under laser beam irradiation. The laser irradiation time was （4±0.2） s. During the laser irradiation, the thermocouple was welded on the back side to monitor the temperature rise process.
The thermocouple uses a K-type thermocouple of nickel-chromium-nickel-silicon alloy, and the long-term temperature limit is 1300 °C. When the temperature is too high, the thermocouple is loose or detached from the backing plate, and the resulting temperature profile will be shaken. The various temperature profiles in this work are robust measurements of thermocouple welding. The laser ablation test was carried out at the Chengdu Precision Optical Engineering Center with a laser wavelength of 1064 nm and a power range of 101 to 4000 W.
2. Experimental results and analysis
2.1 coating temperature test results
The coating was prepared according to the formulation shown in Table 1. The total thickness of the coating was 40-50 μm. The 3# coating was a two-layer structure. The upper and lower layers were made of different formulations. The upper layer filler contained BN and the thickness was about 1/5 of the total thickness. It is placed in a muffle furnace at 600 ° C for 2 h, and the coating condition after high temperature heating is shown in Fig. 1. According to the results shown in Fig. 1, it can be seen that, due to the addition of carbon fibers, the crack of the 2# coating is more pronounced after heating than the coating of #1, indicating that the system in which carbon fibers are present increases the thermal stress of the coating.
In contrast, the 3# coating was significantly improved in surface cracks after heating compared to the ## coating. This is because the double-layer structure in the heating process, the upper layer uniformly dispersed BN powder has a good thermal conductivity, can avoid local overheating of the paint film, but also facilitate the dissipation of heat to the surrounding environment, to some extent alleviate thermal stress and improve the paint. Film heat resistance. At the same time, the amount of the upper glass powder is increased, which can also reduce the pulverization phenomenon and can better fuse the upper and lower layers.
2.2 Coating oxygen-resistant acetylene flame ablation test results
Coatings were prepared according to the formulation shown in Table 1, with a coating thickness of 500-600 μm, and then simulated by laser ablation for 4 s with different temperatures of oxy-acetylene flame. It can be seen from Fig. 2 that the 1# coating does not show significant changes in the back of the substrate after ablation, but deep cracks appear around the ablated area.
After ablation at 2000 °C, the ablation center forms a hill （see Figure 3）, and after ablation at 3000 °C, the center of the hill forms a depression due to the role of ZrO2 in the formulation. ZrO2 is a typical phase change ceramic. The low temperature stable phase is a monoclinic phase. It transforms into a tetragonal phase at around 1000 °C, and becomes a cubic phase at about 2370 °C. The reverse phase transition occurs from high temperature to low temperature. From low temperature to high temperature, each phase change is accompanied by volumetric shrinkage. From high temperature to low temperature, each phase change is accompanied by volume expansion. As the coating temperature of the ablated region increases, ZrO2 undergoes a phase change and a volume shrinks, thus forming a hill at the ablation center. Due to the small specific heat and thermal conductivity of ZrO2, the periphery of the ablated region does not rapidly heat up to a phase transition, resulting in deep gullies around the central region.
The ablation morphology of the 2# coating is very similar to that of 1#, except that the surface coating of the ablation area is more severely curled, which indicates that the addition of carbon fiber does not enhance the effect as expected, but rather increases the coating. Shedding, reducing its adhesion properties.
After the 3# coating was ablated, there was no significant change in the back of the substrate. Compared with the 1# sample, the surface carbonization is severely black and covered with a loose structure that is easy to fall off （see Figure 6）. This is because the BN in the upper layer is obviously oxidized when the temperature in the air reaches 800 ° C or higher, and the B 2 O 3 formed by oxidation starts to volatilize in a large amount in a gaseous state at a temperature of 1000 ° C, which weakens the protective effect of the coating. On the other hand, although B2O3 can form a liquid film on the surface of the material, it has a certain hindrance to the intrusion of oxygen into the interior of the material. However, according to reports in the literature, the compatibility of B2O3 and ZrO2 is not good, so it is difficult to isolate it well. The role of oxygen.
Combined with the temperature resistance test, it is not difficult to find that the addition of BN is a double-edged sword for improving the performance of the coating. On the one hand, BN coating can effectively reduce the high temperature cracking of the paint film and improve the temperature resistance of the coating; on the other hand, BN will reduce the ablation resistance of the coating. Therefore, in practical applications, the effects of these two aspects should be considered comprehensively, and the coating formulation should be reasonably determined according to specific requirements.
2.3 coating laser ablation test results
According to the results of simulated ablation of oxy-acetylene flame, laser ablation was performed using 1# coating and blank steel plate, and the results were compared to verify the laser ablation resistance of the coating. The coating thickness was 900-1000 μm, and the irradiation parameters were given in Table 2.
It can be seen from Fig. 7 that under the laser irradiation, the blank steel plate has obvious ablation and destruction zones on the front and back sides, while the steel plate coated with 1# coating on the surface only has the frontal coating burned by laser, and no obvious change is found on the back surface. It means that the laser does not penetrate the coating and causes damage to the substrate. The coating has excellent tolerance to laser irradiation and effectively protects the substrate. Figure 8 shows the temperature profile of the back side of the blank steel plate and the coated steel plate during the ablation process, which can further confirm the laser ablation resistance of the coating. The two curves in the figure represent the temperature changes on the back side of the steel plate during ablation. The blank steel plate has no coating protection. After the laser irradiation, the temperature rises rapidly, the peak temperature reaches 1387 °C, and the temperature drops rapidly after the irradiation stops. For the steel plate coated with the 1# coating, the temperature rises after the laser irradiation. It is slower and has a peak temperature of 246 ° C. After the irradiation is stopped, the temperature drop rate is also slower than that of the blank steel plate. It can be seen that the coating has a significant ablation and heat-insulating effect, the thickness is in the range of 900-1000 μm, and the heat-insulating effect on the surface of the 1 mm-thick 30CrMnSiA steel is above 1000 °C.
（1） Anti-laser ablation coating was prepared by using silicone resin, polycarbosilane and glass powder as binder, and adding heat-resistant fillers such as Al2O3, BN, SiC, ZrO2, SiO2 and carbon fiber. The prepared coating has good coating. Resistance to laser ablation and thermal insulation.
（2） The prepared coating is irradiated for 4 s at a laser power density of 531 W/cm 2 in a thickness range of 900 to 1000 μm, and the heat insulating effect on the surface of a 1 mm thick 30CrMnSiA steel is 1000 ° C or more.
（3） In the temperature range below 1000 °C, BN can effectively improve the heat resistance of the coating and reduce the thermal cracking of the coating. In the high temperature thermal ablation stage above 1000 °C, the lower oxidation temperature of BN leads to the ablated surface. The formation of a loose structure does not protect the substrate; while the carbon fiber does not reinforce the coating as expected throughout the heated temperature range, but increases the tendency of the coating to be thermally cracked and the coating to fall off. A key to the laser ablation is the low thermal conductivity of ZrO2.