Weapons that use the direct irradiation of lasers to kill targets are called laser weapons. Laser weapons have the characteristics of high transmission speed, fast transfer of firepower, recoilless launch, high launch accuracy, high lethality, and strong anti-interference ability. They have become the deadly killer of various weapons such as aircraft. Among the major military countries in the world, the United States is at the forefront of laser weapon research. In January 2009, an output of 105kW was achieved, reaching the target of 100kW output power in the third stage of laser weapon application. 100kW power has always been regarded as the threshold for weapon-grade high-energy lasers. High-energy laser weapons have gradually become an important means of combating military powers’ anti-missile, anti-satellite, anti-aircraft and ground targets.

　　As high-energy laser weapons gradually enter the actual combat application stage, laser protection technology research has attracted much attention. Laser protection through material technology includes thin films, structures and coatings. The film type is mainly for the anti-laser protection of satellite optical systems and photoelectric sensors, mainly using the method of coating the surface of the optics and the sensor; the structure type mainly refers to the integration of structure and function through structural design to achieve the purpose of laser protection, such as the United States in 1998 It is reported that some sensitive parts of the aircraft are designed as a sheet plus a metal mesh to achieve laser protection. The metal mesh can be designed as multiple layers according to requirements; coating materials are the most traditional form of protection and special functions for aircraft, because they can be used Simple air spraying or brushing process for construction, so it is one of the most suitable protective materials with the lowest application cost. However, due to the extremely high requirements for the material performance of the laser-resistant coating materials, the laser protective coating material technology research is at home and abroad All belong to a completely new field. This work started with the preparation process of general protective coatings, and through formulation design and process optimization, a coating material with excellent performance was prepared for laser ablation resistance.

　　1. Experimental materials and methods

　　1.1 Coating preparation

　　Use silicone resin and polycarbosilane as binder, add Al2O3, SiC, ZrO2, SiO2, BN, glass powder, carbon fiber and other fillers to form a mixed slurry. The mixed slurry is ground on a sand mill to a fineness of 40-50μm . Then, a coating is prepared on the surface of a 30CrMnSiA steel plate with a size of 50mm×100mm×2mm and a phosphating treatment on the surface. The coating preparation adopts the ordinary air spraying method, the air pressure is 0.4MPa, the spray gun is provided by the German SATA company, and the caliber is 0.8-1mm. The specific spraying process is divided into several passes according to the required thickness. The vertical cross spraying is regarded as one pass. A certain time interval is required between each pass to achieve a certain degree of surface drying and prevent flow. After spraying, leave it at room temperature for 48 hours, and then bake at 200°C for 2 hours to solidify into a film.

　　1.2 Performance characterization

　　The cured coating is first baked in a muffle furnace, heated from room temperature to 600°C, kept for 2 hours, and cooled to room temperature with the furnace, referring to the standard GB/T9286-1998. The cross-hatch method was used to test the adhesion of the paint film, and the coating surface was observed with a stereo microscope for preliminary performance evaluation. Then ablate under oxygen-acetylene to observe the ablation morphology of the coating after ablation for 4s at different flame temperatures. Finally, an anti-laser ablation test was carried out under laser beam irradiation. The laser irradiation time was (4±0.2) s. During the laser irradiation, a thermocouple was welded on the back to monitor the temperature rise process.

　　The thermocouple uses a nickel-chromium-nickel-silicon alloy K-type thermocouple, and the long-term temperature measurement limit is 1300℃. When the temperature is too high and the connection between the thermocouple and the backplane becomes loose or falls off, the resulting temperature curve will jitter. The various temperature curves in this work are reliable and reliable measurements for thermocouple welding. The laser ablation test was carried out in Chengdu Precision Optical Engineering Center. The laser wavelength was 1064nm and the power range was 101~4000W.

　　2. Experimental results and analysis

　　2.1 Coating temperature resistance test results

　　Prepare the coating according to the formula shown in Table 1, the total thickness of the coating is 40-50μm, of which the 3# coating is a two-layer structure, and the upper and lower layers use different formulas. The upper filler contains BN and the thickness is about 1/5 of the total thickness. Placed in a 600 ℃ muffle furnace for 2 hours, the coating condition after high temperature heating is shown in Figure 1. According to the results shown in Figure 1, it can be seen that due to the addition of carbon fiber, the 2# coating has more obvious cracks after heating than the 1# coating, indicating that the thermal stress of the coating increases in the system with carbon fiber.

 

 

 

　　In contrast, 3# coating compared with 1# coating, the surface cracks are significantly improved after heating. This is because during the heating process of the double-layer structure, the evenly dispersed BN powder on the upper layer has good thermal conductivity, which can avoid local overheating of the paint film, and at the same time, it is also conducive to heat dissipation to the surrounding environment, which relieves the thermal stress to a certain extent and improves the paint. Film heat resistance. At the same time, the upper layer of glass powder is added more, which can also reduce the pulverization phenomenon and make the upper and lower layers better merge.

　　2.2 Test results of coating resistance to oxyacetylene flame ablation

　　Prepare the coating according to the formula shown in Table 1, with a coating thickness of 500-600 μm, and then simulate laser ablation with oxyacetylene flames at different temperatures for 4 seconds. It can be seen from Figure 2 that there is no obvious change in the back of the substrate after the ablation of the 1# coating, but deep cracks appear around the ablation area.

 

 

　　After ablation at 2000°C, the center of the ablation formed hills (see Figure 3), and after ablation at 3000°C, the center of the hills formed a depression, which is due to the effect of ZrO2 in the formula. ZrO2 is a typical phase change ceramic. The low-temperature stable phase is a monoclinic phase, which transforms into a tetragonal phase at about 1000°C, and a cubic phase at about 2370°C. The phase change occurs in the opposite direction during the cooling process from high temperature to low temperature. From low temperature to high temperature, each phase change is accompanied by volume shrinkage, and from high temperature to low temperature, each phase change is accompanied by volume expansion. As the temperature of the coating in the ablation zone increases, ZrO2 undergoes a phase change and shrinks in volume, so hills are formed in the ablation center. Due to the small specific heat and thermal conductivity of ZrO2, the periphery of the ablation zone did not quickly heat up to a phase change, which resulted in deep ravines around the central zone.

 

 

　　The ablation morphology of 2# coating is very similar to that of 1#, except that the surface of the coating around the ablation area is more severely curled. Fall off, reducing its adhesion performance.

 

 

　　The back of the substrate did not change significantly after the 3# coating was ablated. Compared with the 1# sample, the carbonization and blackening of the surface are serious, and it is covered with a loose structure that is easy to fall off (see Figure 6). This is due to the obvious oxidation of BN in the upper layer when the temperature in the air reaches above 800°C, and the B2O3 formed by oxidation begins to volatilize in a large amount in gaseous state when the temperature reaches 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, which has a certain blocking effect on the intrusion of oxygen into the material, according to literature reports, the compatibility of B2O3 and ZrO2 is not good, so it is difficult to effectively isolate it. 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 coating performance. 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 formula should be reasonably determined according to specific requirements.
　　2.3 Coating laser ablation resistance test results
　　 According to the results of oxyacetylene flame simulation laser ablation, 1# coating and blank steel plate were selected for laser ablation, and the results were compared to verify the laser ablation resistance of the coating. The coating thickness is 900～1000μm, and the irradiation parameters are given in Table 2.

 

 

　　It can be seen from Figure 7 that under laser irradiation, the blank steel plate has obvious ablation damage areas on the front and back, while the steel plate coated with 1# coating on the surface is only burned by the laser on the front coating, and no obvious change is found on the back. This means that the laser has not penetrated the coating to cause damage to the substrate. The coating has excellent resistance to laser radiation and effectively protects the substrate. Figure 8 shows the temperature change curve of the back of the blank steel plate and the coated steel plate during the ablation process, which can further confirm the anti-laser ablation effect of the coating. The two curves in the figure represent the temperature changes on the back of the steel plate during the ablation process. The blank steel plate has no coating protection. After being irradiated by the laser, the temperature rises rapidly, and the peak temperature reaches 1387℃. After the irradiation is stopped, the temperature drops rapidly; for the steel plate coated with 1# coating, the temperature rises after being irradiated by the laser. It is relatively slow, with 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 resistance and heat insulation effect. The thickness is in the range of 900-1000 μm, and the heat insulation effect on the surface of 1mm thick 30CrMnSiA steel reaches 1000°C or more.

 

 

 

　　

　　3. Conclusion

　　(1) Using silicone resin, polycarbosilane and glass powder as binders, adding Al2O3, BN, SiC, ZrO2, SiO2 and carbon fiber and other heat-resistant fillers to prepare anti-laser ablation coatings, the prepared coatings have good properties Resistance to laser ablation and heat insulation.

　　(2) The prepared coating is in the thickness range of 900～1000μm, irradiated for 4s under the laser power density of 531W/cm2, and the heat insulation effect on the surface of 1mm thick 30CrMnSiA steel reaches above 1000℃.

　　(3) In the temperature range below 1000°C, BN can effectively improve the heat resistance of the coating and reduce thermal cracking of the coating. In the high temperature thermal ablation stage above 1000°C, the lower oxidation temperature of BN leads to its ablated surface The formation of a loose structure can not protect the substrate; and the carbon fiber does not play a role in reinforcing the coating during the entire heating temperature range, but increases the tendency of the coating to crack and fall off. ZrO2 with low thermal conductivity plays a key role in the fight against laser ablation.