Effect of mGr:mnano-SiO2 on coating propertiesThe SEM images of the coatings prepared by mGr:mnano-SiO2 at 4:6, 6:4, and 8:2 are shown in Fig. 2 when mAPU:mPDMS is 4:6 and the total filler amount is 30 wt%. The surface micro-structure of the coating varies significantly depending on the mass ratios of graphene to nano-SiO2. When the mass ratio of graphene to nano-SiO2 increases, the surface particle characteristics formed by nano-SiO2 agglomeration significantly weaken, the lamellar structure characteristics of multilayer graphene enhance, and the coating surface becomes more compact. When mGr:mnano-SiO2 is 4:6, the flake graphene is evenly divided into the resin and nano-SiO2, and a large amount of nano-SiO2 is distributed on the surface of the coating, leading to a rough surface texture with a surface roughness of 2–6 μm. When mGr:mnano-SiO2 is 6:4, its dispersion in the resin matrix is more uniform due to the large specific surface area of graphene and its good compatibility with APU-modified PDMS25,26. This reduces the surface roughness of the coating and increases its regularity. When mGr:mnano-SiO2 is 8:2, the graphene content is more, the nano-SiO2 content is less, and the graphene is fully dispersed in the resin, the coating surface structure is dense, and the rough structure is almost eliminated.Fig. 2SEM images of coatings with varying mGr:mnano-SiO2, (a, d) 4:6, (b, e) 6:4, and (c, f) 8:2.Figure 3 shows the WCAs of the coatings with varying mGr:mnano-SiO2 of 4:6, 6:4, and 8:2. It can be seen that as the mass ratio of graphene to nano-SiO2 increases the micro-nano rough structure and the WCA decreases. When mGr:mnano-SiO2 is 4:6, the WCA reaches 149.5°, owing to the extensive distribution of nano-SiO2 on the coating surface. There are more mastoid-shaped micro-nano rough structural units constructed by resin matrix and nano-SiO2 on the surface. A large number of mastoid-shaped micro-nano rough structures will increase the contact between the liquid phase and gas phase, thereby increasing the WCA between solid and liquid27. When mGr:mnano-SiO2 is 6:4, the roughness of the coating surface is reduced, and the high specific surface area of graphene increases the surface energy of the coating, thus reducing the WCA.Fig. 3WCAs of coatings with varying mGr:mnano-SiO2, (a) 4:6, (b) 6:4, and (c) 8:2.Figure 4 displays the near-infrared reflectance spectra of the coatings at various mass ratios of graphene to nano-SiO2. The near-infrared reflectivity of coatings with the same total filler addition amount is lower than 10% in the tested near-infrared range. Moreover, the reflectivity of the coating gradually decreases with the increase of mGr:mnano-SiO2. The near-infrared reflectance at 1.06 μm decreases from 8.4% at mGr:mnano-SiO2 = 4:6 to 6.4% at mGr:mnano-SiO2 = 8:2. The reduction of nano-SiO2 content causes the coating surface to become more regular. In addition, the graphene microstructure is composed of a large number of benzene ring structures, and it has an extremely long and complex conjugated system. Therefore, it strongly absorbs near-infrared light, and its incorporation into the coating can achieve ultra-low near-infrared reflectivity. Additionally, as the graphene content increases, it becomes fully dispersed in the resin matrix, resulting in better coverage of the nano-SiO2 particles. This enables the coating to efficiently absorb near-infrared light and reduce near-infrared reflectivity.Fig. 4Near-infrared reflectance spectra of the coatings with varying mGr:mnano-SiO2 ratios.Table 1 displays the adhesion strength, flexibility, and impact strength of the coatings at various mGr:mnano-SiO2 ratios. The adhesion strength, flexibility, and impact strength of the coating are affected by varying mass ratios of graphene and nano-SiO2. Overall, the mechanical characteristics of the coating improved as the mGr:mnano-SiO2 ratio increased. When mGr:mnano-SiO2 = 4:6, the coating demonstrated poor adhesion strength, flexibility, and impact resistance. The reason for this phenomenon is that when the nano-SiO2 content is high in the coating, the resin matrix cannot completely cover the filler particles, resulting in ineffective formation of the coating. Furthermore, the coating contains a significant amount of micro-pores, leading to the presence of a weaker boundary layer within the coating, ultimately resulting in poor mechanical properties28,29. As mGr:mnano-SiO2 increases, the special planar network structure, the coordination relationship between carbon atoms, and the special chemical bonding properties of graphene enhance the chemical bonding energy between the resin matrix and the filler. Consequently, greater force is required to damage the coating structure. As a result, the coating’s mechanical properties are gradually enhanced30,31. Considering the near-infrared absorption properties, hydrophobic properties, and mechanical properties of the coating, the optimum mass ratio of graphene to nano-SiO2 in the coating is determined to be 6:4.Table 1 Mechanical properties of coatings with varying mGr:mnano-SiO2.Effect of total filler addition amount on coating performanceFigure 5 shows SEM images of the coatings prepared under the curing temperature of 120 °C, APU to PDMS mass ratio of 4:6, graphene to nano-SiO2 mass ratio of 6:4, and total filler amount of 10 wt%, 30 wt%, 40 wt%, and 50 wt%. It shows that the roughness of the coating surface increases with total filler content. At the total filler amount of 50 wt%, the surface roughness of the coating can reach 3–8 μm. In addition, the surface porosity of the coating increases while its density decreases. When the filler content in the coating is 10 wt%, the composite fillers composed of graphene and nano-SiO2 are scattered throughout the coating and cannot cover the entire coating, there are more resins between the functional fillers, and the surface is very smooth. As the composite filler content increased to 30 wt%, the density of graphene and nano-SiO2 distributed in the coating increased significantly, but the coating still had a large and thick resin matrix. When the total filler content reaches 40 wt%, a significant number of micro-nano rough structural units composed of nano-SiO2 and resin matrix develop on the coating surface. These rough structural units enhance the coating’s wetting resistance, leading to improved hydrophobic performance. In addition, as the amount of composite fillers in the coating increases, the content of graphene also increases and becomes more evenly dispersed in the resin and nano-SiO2. This increased dispersion improves the coating’s ability to absorb incident near-infrared light, resulting in relatively low reflectivity at 1.06 μm, thereby meeting the requirements of laser stealth. When the coating contains an excessive amount of composite fillers (Fig. 5d, h), the nano-SiO2 particles push out each other, increasing the surface roughness of the coating and causing a large number of pores to appear, significantly weakening the mechanical properties of the coating.Fig. 5SEM images of coatings with varying total filler additions, (a, e) 10 wt%, (b, f) 30 wt%, (c, g) 40 wt%, and (d, h) 50 wt%Figure 6 displays the WCAs of the coatings with varying total filler additions. It shows that the WCA of the coating increases significantly with the increase in the total filler content. When the total fill content is 10 wt%, the WCA is only 98.5°, indicating a poor hydrophobic effect. This is because the surface of the coating is mainly a resin matrix and relatively thick, preventing the hydrophobic nano-SiO2 from spreading on the surface of the coating, resulting in a low hydrophobicity of the coating. With the increase of the total filler content, the roughness of the coating surface also increases, and the micro-nano rough structural units with mastoid structure characteristics increase significantly, resulting in lower surface energy and larger WCA. When the total filler content is 50 wt%, the coating has a WCA of 152.5°, indicating super-hydrophobic properties.Fig. 6WCAs of coatings with varying total filler additions, (a) 10 wt%, (b) 30 wt%, (c) 40 wt%, and (d) 50 wt%Figure 7 shows the near-infrared reflectance spectra of the coatings with varying total filler additions. The reflectivity of the coating at 1.06 μm decreases from 7.7% with a total filler addition of 10 wt% to 4.2% with a total filler addition of 20 wt% and then increases to 13.6% with a total filler addition of 50 wt%. The reason for this phenomenon is that graphene has excellent near-infrared absorption properties21,22. Even with a total filler proportion of only 10 wt%, a small amount of uniformly dispersed graphene in the coating can still efficiently absorb near-infrared light. This reduces the coating’s reflectivity at 1.06 μm to as low as 7.7%. When the proportion of total filler increased to 20 wt%, the absolute content of graphene in the coating also increased and remained evenly dispersed. This enhanced the coating’s absorption of near-infrared light and reduced reflectivity to as low as 4.2% at 1.06 μm. However, as the total filler content increased, the reflectivity of the coating to near-infrared light increased. The primary factor is that as the total filler amount in the coating continues to rise, so does the absolute content of nano-SiO2, which not only has no absorption effect on near-infrared light, but also has a certain reflection effect. In addition, when the proportion of total filler is too large, it will not only increase the gap on the surface of the coating, but also easily cause a large number of nano-SiO2 to gather on the surface of the coating and weaken the absorption of the coating to near-infrared light. When the total filler content is too high, the near-infrared light absorption of the coating is reduced due to the combined action of the above factors.Fig. 7Near-infrared reflectance spectra of the coatings with varying total filler additions.Table 2 shows the adhesion strength, flexibility, and impact strength of the coatings under different proportions of total fillers. It shows that the adhesion strength is reduced from grade 1 with a total filler volume of 10 wt% to grade 2 with a total filler volume of 50 wt%, and the flexibility is reduced from 3 mm with a total filler volume of 10 wt% to 8 mm with a total filler volume of 50 wt%. The impact strength decreased from 40 kg × cm with a total packing volume of 10 wt% to 5 kg × cm with a total packing volume of 50 wt%. This phenomenon is primarily due to the fact that the polyurethane resin contains strong reactive polar groups, such as isocyanate groups, which can form a good interface bond with the filler. When the total filler is only 10 wt%, the content of polyurethane resin is higher, and it has a strong binding force on the filler, resulting in the coating having excellent mechanical properties under these conditions. However, because the resin used in the coating contains PDMS with low surface energy, its binding to the filler is weak, resulting in suboptimal flexibility and impact strength of the coating when the total filler content is low. As the proportion of total filler increases, the content of resin decreases, as does the wrapping strength of resin on the filler and the corresponding binding force. Considering the near-infrared absorption, hydrophobic, and mechanical properties of the coating, the proportion of total fillers in the coating is determined to be 40 wt%.Table 2 Mechanical properties of coatings with varying total filler additions.Effect of KH560 on coating propertiesFigure 8 shows SEM images of the coatings prepared under the following conditions: curing temperature of 120 °C, mass ratio of APU to PDMS of 4:6, mass ratio of graphene to nano-SiO2 of 6:4, total filler addition of 40 wt%, and KH560 addition of resin mass of 0 wt%, 4 wt%, and 8 wt% According to the morphology of the coating surface, the addition of KH560 has little effect on the micro-structure of the coating. The resin in the coating is evenly mixed with graphene and nano-SiO2 under different KH560 dosages, the resin layer thickness is suitable, and the graphene is evenly dispersed in the coating. Uniform and slightly agglomerated nano-SiO2 particles can be seen on the surface of the coating, their arrangement is regular, and the structure is tight, leading to a clear micro-nano rough structure.Fig. 8SEM images of coatings with varying KH560 additions, (a, d) 0 wt%, (b, e) 4 wt%, and (c, f) 8 wt%Figure 9 shows the WCAs of the coatings prepared with the addition of KH560 at 0 wt%, 4 wt%, and 8 wt% of the resin mass. It shows that the WCA can be increased from 149° without adding KH560 to 153° when adding 4 wt%, resulting in a coating with super-hydrophobic properties. As the amount of KH560 added to the coating increases, the influence of the coating’s WCA is minimal. The reason is that the silico-oxygen bond in KH560 can chemically react with the hydroxyl group on the surface of the inorganic material, forming a hydrophobic Si–O–Si group. This reaction lowers the surface energy of the coating, further enhancing its hydrophobic properties32. The specific action mechanism and process are shown in Fig. 10.Fig. 9WCAs of coatings with varying KH560 additions, (a) 0 wt%, (b) 4 wt%, and (c) 8 wt%Fig. 10Effect mechanism of KH560 on micro-structure of the coating.Figure 11 displays the near-infrared reflectance spectra of the coatings with varying KH560 additions. It can be seen that the modification of KH560 has little effect on the near-infrared reflectivity of the coating at 1.06 μm, with a variation of only 0.6%. The main reason is that when the ratio of fillers, the proportion of total fillers, and the ratio of resin remain constant, the addition of a small amount of KH560 has no significant effect on the micro-structure of the coating, and the graphene that absorbs near-infrared light is still evenly and widely distributed between the nano-SiO2 and the resin. Even if the silicon-oxygen bond of KH560 interacts with PU, PDMS, and some inorganic fillers to increase the binding force between the adhesive and the filler, the resin thickness, the void between the filler and the resin, and the overall micro-structure of the coating change slightly. These minor changes have little effect on the near-infrared absorption capacity of the coating, allowing it to maintain outstanding near-infrared low reflectivity properties.Fig. 11Near-infrared reflectance spectra of coatings with varying KH560 additions.Table 3 shows the adhesion strength, flexibility, and impact strength of the coatings with different KH560 additions. The addition of KH560 did not affect adhesion strength and flexibility of the coating, but it significantly improved impact strength. Even a small amount of KH560 can increase the impact strength of the coating to its optimum level (50 kg × cm). This is attributed to the bridging effect of KH560 as a standard coupling agent. One end of the molecular structure of KH560 can react with groups such as silica bond and hydroxyl group in inorganic fillers to form covalent bonds, and the other end can engage with active groups like amino group, hydroxyl group, and carboxyl group present in the resin binder to establish chemical bonds. These interactions serve to improve the interface bonding strength between materials, and the adhesion strength between resin and substrate, resin and filler. The coating’s impact strength is significantly enhanced in the end33. Considering the near-infrared absorption properties, hydrophobic properties, and mechanical properties of the coating, the optimal addition of KH560 in the coating is determined to be 4 wt%.Table 3 Mechanical properties of coatings with varying KH560 additions.Effects of mAPU:mPDMS on coating propertiesFigure 12 shows SEM images of the coatings prepared under the following conditions: mGr:mnano-SiO2 of 6:4, total filler addition of 40 wt%, KH560 addition of 4 wt%, and mass ratios of APU to PDMS of 1:9, 3:7, 4:6, and 5:5. As can be seen in Fig. 10a, a large mass ratio difference between APU and PDMS results in uneven local coating thickness, disordered and irregular distribution of nano-SiO2, and a coating surface with large voids and high roughness. As mAPU:mPDMS increases (Fig. 12b, c, d), the nano-SiO2 in the coating becomes denser and more evenly distributed, and the graphene is also evenly dispersed, which makes the coating surface more regular and compact.Fig. 12SEM images of coatings with varying mAPU:mPDMS, (a, e) 1:9, (b, f) 3:7, (c, g) 4:6, and (d, h) 5:5Figure 13 shows the WCAs of the coatings with varying mAPU:mPDMS ratios of 1:9, 3:7, 4:6, and 5:5. As shown in the figure, the smaller the mass ratio of APU to PDMS, the larger the WCA of the coating. When the mAPU:mPDMS ratio is 1:9, the coating exhibits a WCA of 158° and excellent super-hydrophobic properties. This occurs because PDMS, the main adhesive in the coating, consists mainly of a large number of non-polar silicon-oxygen-silicon bonds, which results in a very low surface energy. Consequently, the coating also has a very low surface energy, thereby optimizing its hydrophobic properties. As the mass ratio of APU to PDMS gradually increases, the absolute content of PDMS decreases, which reduces the organic content that can reduce the surface energy of the coating. Additionally, the APU utilized acrylic-modified polyurethane, which contains numerous robust polar groups such as hydroxyl and ether bonds that can establish hydrogen bonds with water molecules, enhancing the coating’s wettability. As the mass ratio of APU to PDMS increases, so does the content of APU in the coating, and the wetting effect of the coating on water improves, reducing the WCA of the coating.Fig. 13WCAs of coatings with varying mAPU:mPDMS, (a) 1:9, (b) 3:7, (c) 4:6, and (d) 5:5Figure 14 shows the near-infrared reflectance spectra of the coatings with varying mAPU:mPDMS ratios. The coating has minimal effect on the reflectivity of 1.06 μm near-infrared light at various APU to PDMS mass ratios. The result shows that the adhesive has little effect on the laser stealth effect of the coating, and that graphene, as a near-infrared absorber, is the core factor that determines the ultra-low near-infrared reflectivity of the coating. Firstly, the stoichiometric numbers of graphene and nano-SiO2 decreased with the increase of the mass ratio of APU to PDMS in the coating formulation, but the value change was only 0.04%, indicating that the change in graphene content had little effect on the composition of the whole formulation. Secondly, the change of mAPU:mPDMS has very little effect on the micro-structure of the coating. Although the graphene content in the coating decreases slightly with the increase of mAPU:mPDMS, its near-infrared reflectivity at 1.06 μm remains approximately 10%, maintaining outstanding laser stealth performance.Fig. 14Near-infrared reflectance spectra of coatings with varying mAPU:mPDMS ratios.Table 4 shows the adhesion strength, flexibility, and impact strength of the coatings having different APU to PDMS mass ratios. It shows that the adhesion strength of the coating increases as the absolute content of PDMS decreases, while the flexibility deteriorates. However, the impact strength of the coating has been maintained in the best state due to the presence of KH560. When the absolute content of PDMS is high, the binding between resin, filler, and substrate is primarily determined by the silicon-oxygen bond in PDMS, which has high bond energy and very low binding force. As the absolute content of PDMS decreases, the absolute content of PU increases gradually, and the content of amino, hydroxyl, and carboxyl groups also increases. These groups readily establish covalent and hydrogen bonds with oxygen atoms or hydroxyl groups on the substrate’s surface, consequently enhancing the adhesion strength of the coating. In addition, as the absolute content of APU increases, so do the strong reactive groups such as the isocyanate group and hydroxyl group, resulting in a significant increase in the cross-linking density. Consequently, the coating’s hardness and brittleness increase, ultimately reducing flexibility. The impact strength of the coating is also always optimal thanks to the presence of KH560. To maintain near-infrared reflectivity at 1.06 μm below 10%, and achieve both super-hydrophobic and good mechanical properties, the optimal mAPU:mPDMS ratio is determined to be 3:7.Table 4 Mechanical properties of coatings with varying mAPU:mPDMS ratios.Effect of DOP on coating propertiesFigure 15 shows SEM images of the coatings prepared with DOP additions of 0 wt%, 1 wt%, and 3 wt% under the conditions of mAPU:mPDMS of 3:7, mGr:Mnano-SiO2 of 6:4, the total filler addition amount of 40 wt%, and the KH560 addition amount of 4 wt%. It can be seen that the DOP-modified coating has little effect on the surface micro-structure of the coating. Without or with a small amount of DOP, the graphene in the coating is uniformly dispersed, and the nano-SiO2 is widely distributed, neatly arranged and compact. The bonding between the filler and the resin is appropriate, and the resin thickness is consistent. The size and arrangement of rough structural units on the surface of the coating are uniform.Fig. 15SEM images of coatings with varying DOP additions, (a, d) 0 wt%, (b, e) 1 wt%, and (c, f) 3 wt%Figure 16 shows the WCAs of the coatings with DOP additions of 0 wt%, 1 wt%, and 3 wt%. It can be seen that DOP has a certain wetting effect on the coating, as the WCA of the coating decreases from 155° when DOP is not added to the coating to 149° when 3 wt% is added. The reason for this change could be that the addition of DOP introduces some polar ester groups, which increases the surface energy of the coating and thus reduces the WCA. However, even with a 1 wt% DOP addition, the coating maintains a good super-hydrophobic property, with a WCA 152°.Fig. 16WCAs of coatings with varying DOP additions, (a) 0 wt%, (b) 1 wt%, and (c) 3 wt%Figure 17 shows the near-infrared reflectance spectra of the coatings with varying DOP additions. The addition of DOP has a slightly positive impact on the absorption of the coating at 1.06 μm near-infrared light, but the resulting change in reflectivity is only 1.356%. The unique molecular structure of DOP contains both polar ester groups and non-polar carbon chains, which enables easy insertion into the resin’s molecular chain. This weakens inter-molecular forces within the resin, reduces fine gaps in the coating, and improves filler dispersion. These are conducive to enhancing the absorption of graphene to near-infrared light, so as to achieve the effect of reducing the near-infrared reflectivity of the coating.Fig. 17Near-infrared reflectance spectra of coatings with varying DOP additions.Table 5 shows the adhesion strength, flexibility, and impact strength of the coatings with different DOP additions. It can be seen that the addition of DOP significantly improved the flexibility of the coating, from the original 4 mm without DOP to 2 mm (optimal flexibility) when the DOP addition is 5 wt%. The addition of KH560 has maintained the impact strength of the coating at its best, DOP has not influenced the adhesion strength of the coating, which remains at grade 2. The primary factor is that the DOP molecular structure consists of polar ester groups and non-polar carbon chains, facilitating their insertion into the resin molecular chains. This action reduces the inter-molecular force of the resin, the degree of cross-linking between the resin chains after curing, and the brittleness of the coating, thereby greatly enhancing the flexibility of the coating. Based on the above consideration, it is determined that the DOP addition amount is 1 wt%. The optimal DOP addition results in a coating with 9.3% near-infrared reflectivity, 152° WCA, grade 2 adhesion strength, 3 mm flexibility, and 50 kg × cm impact strength. The coating meets the design requirements of ultra-low near-infrared reflectivity, super-hydrophobicity, and outstanding mechanical properties.Table 5 Mechanical properties of coatings with varying DOP additions.
