This experiment used low-temperature plasma in three atmospheres of air, nitrogen, and oxygen to treat the steel substrate, and a coating was prepared on the surface of the treated sample using pure epoxy resin powder. The bonding strength between the coating and the substrate was quantitatively measured using the peeling method, and the effects of different atmospheres and treatment durations on the bonding strength between the coating and the substrate were analyzed. To prevent interference from other factors, the sample was not sandblasted before spraying. The test results are shown in Figure 1. As shown in Figure 1, after plasma treatment with air, nitrogen, and oxygen, the bonding strength between the substrate and the coating first increases and then decreases with the extension of treatment time. When the treatment time is 2 minutes, the bonding strength of the coatings prepared on the substrate treated with the three atmospheres is the best, which is 2.5 times, 4.0 times, and 4.5 times that of the untreated samples, respectively. As the processing time increases, its strength decreases, but it is still greater than the bonding strength without plasma treatment. Comparing the treatment effects of three types of plasma, oxygen plasma treatment showed the greatest improvement in the bonding strength between the coating and the substrate, followed by nitrogen treatment, and air treatment showed the smallest improvement in bonding strength.

Figure 1 Relationship curve between different plasma treatment times and coating bonding strength

Generally speaking, the bonding force between organic coatings and metal substrates is composed of mechanical embedding force, chemical bond, and intermolecular van der Waals force. The magnitude of these three forces is directly related to the roughness, surface chemical energy, and surface free energy of the substrate surface. In order to analyze the effect of plasma treatment on the bonding strength of coatings prepared by different plasma treatments, this chapter uses AFM, XRD, XPS, infrared spectroscopy, and surface energy testing to analyze and characterize the mechanism of the effect of low-temperature plasma treatment on the bonding strength of coatings on steel substrate surfaces.

The Effect of Plasma Treatment on the Chemical Structure of Coating Substrate Surface

To investigate the effects of low-temperature plasma treatment in three different atmospheres, oxygen, nitrogen, and air, on the surface energy distribution of the samples, XPS spectra were used to test and characterize the surface energy of the samples before treatment and after treatment with different methods. Figure 2 shows the XPS spectra of Q235 steel surface before and after plasma treatment. According to the analysis of XPS spectra, it can be concluded that low-temperature plasma treatment in different atmospheres has little effect on the content of various chemical elements on the surface of the substrate sample before and after treatment. Figure 3 shows the high-resolution spectra of Fe2p on the surface of steel substrate samples before and after low-temperature plasma treatment under different atmospheres, where (a) is the untreated spectrum, and (b), (c), and (d) are the spectra after air, oxygen, and nitrogen plasma treatment, respectively. Peak splitting was performed on each spectrum, and the content of each functional group was calculated based on the area corresponding to each peak. The results are shown in Table 1.

Figure 2 XPS spectra of Q235 steel surface treated with different plasma treatments

Figure 3 High resolution spectra of Fe2p on steel surfaces treated with different plasma treatments (a) Untreated matrix; (b) Low temperature plasma treatment of air; (c) Oxygen low-temperature plasma treatment; (d) Nitrogen low-temperature plasma treatment

According to Figure 3 and Table 1, it can be seen that before low-temperature plasma treatment, the surface Fe of Q235 steel substrate is partially oxidized to form mainly FeO and Fe (OH) 2 oxides. On the XPS spectrum, there are peaks of Fe Fe and Fe-O, Fe (OH) 2 at 706.8eV and 708.6eV, respectively. The main oxidation products are FeO and Fe (OH) 2. After low-temperature plasma treatment, the unoxidized parts of the steel substrate surface and components such as Fe-O and Fe (OH) 2 were re oxidized by the active components in the plasma, and all were converted into Fe2O3 and FeOOH. On the XPS spectrum, the peaks of Fe Fe and Fe-O/Fe (OH) 2 disappeared, while the peaks of Fe2O3 and FeOOH were significantly enhanced, with a larger proportion of FeOOH components. The results of low-temperature plasma treatment with three different atmospheres showed that the FeOOH content on the substrate surface was highest after oxygen plasma treatment, followed by nitrogen atmosphere plasma treatment, and lowest after air plasma treatment. Due to the presence of hydroxyl groups in the FeOOH component on the surface of the substrate, hydrogen bonds can be formed between polar groups in the epoxy resin, significantly improving the bonding strength between the coating and the substrate. The higher the FeOOH content, the more obvious the bonding strength improvement, which is completely consistent with the bonding strength sample data.

Table 1 Relative content of Fe oxides on the substrate surface after different plasma treatments

Analysis of the influence of plasma treatment on the surface roughness of steel substrate

In order to characterize the effect of low-temperature plasma treatment on the surface morphology of Q235 steel substrate, atomic force microscopy (AFM) was used to compare the microstructure of the substrate surface before and after oxygen plasma treatment. The results are shown in Figure 4 and Table 2. In order to facilitate comparative analysis of the results of low-temperature plasma treatment, the substrate surface was polished smooth with sandpaper before treatment, and the scanning area during AFM analysis was 20 × 20 μ m.

Figure 4: 3D morphology of Q235 steel surface treated with different low-temperature plasma (a) untreated substrate; (b) Air low-temperature plasma treatment (c) oxygen low-temperature plasma treatment; (d) Nitrogen low-temperature plasma treatment

From Figure 4, it can be seen that surface particles are rarely visible in the scanning area of the untreated sample, mainly consisting of parallel single directional scratches. This is caused by smoothing the surface of the sample with sandpaper before plasma treatment. The plasma treatment for min has a significant impact on the surface morphology of the sample. The surface scratches of the sample treated with air plasma become lighter, but still exist significantly. Compared with the untreated sample, some unevenness can be observed on the surface; After nitrogen plasma treatment, the scratches on the substrate surface are no longer visible, and the surface roughness is more pronounced; The surface roughness of the substrate after oxygen plasma treatment is more pronounced, and the particles are larger. The presence of particles causes the surface of the sample to be distributed in an "island" shape, increasing the bonding surface between the coating and the substrate, making it easier to form a large number of mechanical interlocking structures.

Table 2 Surface roughness values of Q235 steel treated with different low-temperature plasma

According to Table 2, the average surface roughness value (RaRoughness) of Q235 steel substrate polished with sandpaper without low-temperature plasma treatment is 27.69nm, and the root mean square roughness value (RMSRoughness) is 38.05nm. After 2 minutes of air plasma treatment, the Ra value of the steel substrate surface increases significantly, reaching 38.62nm, and the RMS value increases to 49.39nm. After 2 minutes of nitrogen plasma treatment, compared with the sample treated with air plasma, the surface Ra value continues to increase, reaching 50.60nm, and the RMS value increases to 65.63nm. The sample treated with oxygen plasma for 2 minutes has an increase in surface Ra value compared with nitrogen plasma treatment, reaching 54.94nm, and the RMS value increases to 76.73nm. The results of surface roughness improvement after three plasma treatments are: oxygen plasma treatment>nitrogen plasma treatment>air plasma treatment, and this data trend is completely consistent with the shape of Figure 4AFM.

From the analysis of the above experimental results, it can be concluded that the bonding strength between the steel substrate treated with low-temperature plasma and the epoxy resin composite coating is significantly improved, and the increase amplitude shows a trend of oxygen plasma treatment>nitrogen plasma treatment>air plasma treatment. As the processing time increases, the binding strength shows a trend of first increasing and then decreasing. This is because after low-temperature plasma treatment of Q235 steel surface, the surface of the steel substrate becomes cleaner. At the same time, high-energy plasma causes oxidation of the steel substrate surface, generating Fe2O3 and FeOOH. On the one hand, the generation and accumulation of oxides improve the surface roughness of the coating, increase the bonding area between the substrate and the coating, and make the mechanical interlocking effect between the substrate and the coating more obvious, increasing the mechanical interlocking force; On the other hand, the FeOOH generated by oxidation contains hydroxyl groups, which significantly enhance the chemical bonding strength between polar groups in epoxy coatings and the substrate, thereby improving the bonding strength of the coating.