In actual production, effective physical and chemical modification of glass surfaces can be conducted to improve the interfacial bonding strength. Traditional methods such as mechanical roughening, chemical cleaning and coupling agent treatment have respective limitations including introducing microcracks, failing to alter intrinsic inertness, as well as complex processes and poor uniformity. As a highly efficient interfacial modification method, plasma cleaning can synchronously regulate the surface physical structure and chemical composition at the nanoscale. By bombarding the material surface with high-energy active particles, it can simultaneously achieve deep surface cleaning, form a uniform micro-nano rough morphology through physical etching, and introduce high-density polar functional groups at the nanoscale. This enhances both mechanical interlocking and chemical bonding while significantly increasing the surface energy. Characterized by high efficiency, cleanliness, environmental friendliness and no substrate damage, plasma cleaning has become a new approach to improving the reliability of interfacial adhesion.

Regulating the wettability of glass surfaces and increasing the glass surface energy by plasma treatment are the key to enhancing the glass bonding performance, as they can promote mechanical interlocking and chemical bonding between glass and other interfaces.

In this study, atmospheric plasma cleaning equipment was adopted for the plasma surface treatment of glass surfaces, with the equipment power set at 530 W and the working pressure at standard atmospheric pressure. The experiments were carried out in a constant temperature and humidity laboratory (25 °C, 40% relative humidity) to minimize the influence of environmental and component fluctuations on the consistency of plasma surface modification. The pre-cleaned samples were fixed on the workpiece holder. The glass surface modification rate was controlled by adjusting the moving speed of the plasma nozzle, which was set to 9, 7, 5, 3 and 1 mm/s respectively. The surface modification rate is inversely proportional to the surface modification intensity, i.e., a lower speed means a longer treatment time per unit area and a higher modification intensity.

Effect of Plasma Cleaning and Activation on the Wettability of Lithium Aluminosilicate Glass Surfaces

The results of static contact angle tests on lithium aluminosilicate glass surfaces (Figure 1(a)) show that plasma cleaning and activation effectively alter the glass surface properties. With the decrease in surface modification rate, the treatment time per unit area of the sample surface is prolonged and the modification intensity is enhanced accordingly. The static contact angle exhibits a trend of first decreasing and then increasing, and the corresponding surface energy (Figure 1(b)) shows a characteristic of first increasing and then decreasing. Surface energy consists of polar and dispersive components: the polar component dominates the hydrophilicity of materials, while the dispersive component reflects hydrophobic properties. Data in Table 2 show that after treatment at a surface modification rate of 5 mm/s, the water contact angle decreases from 53.00° to 15.00°, and the surface energy increases from 48.77 mN/m to 70.78 mN/m. Among them, the polar component rises from 24.52 mN/m to 42.87 mN/m, confirming the increase in the density of oxygen-containing functional groups; the dispersive component remains relatively stable, indicating no significant change in the non-polar intermolecular interactions.

Figure 1 Contact angle and surface energy of glass surfaces at different plasma cleaning speeds

Effect of Plasma Cleaning Modification on Surface Morphology

To analyze the mechanism underlying the improvement of wettability of lithium aluminosilicate glass surfaces, SEM was used to characterize the surface morphology after plasma cleaning, with the results shown in Figure 2. As can be seen from Figure 2(a), the untreated sample surface is smooth and dense with almost no concave-convex structures. Gradient morphological evolution is observed at different surface modification rates, the fundamental mechanism of which lies in the physicochemical coupling effect between high-energy particles in plasma and the glass surface. At a surface modification rate of 9 mm/s, the particles act on the sample surface for a short time with limited treatment intensity, resulting in shallow etching marks on the surface (Figure 2(b)). At a surface modification rate of 5 mm/s, high-energy particles preferentially break the Si—O—Si network bonds on the glass surface, prompting the fracture points to transform into high-density polar sites accompanied by microscopic volume adjustment. Thus, uniformly distributed and appropriately sized nano-scale etched structures are self-organized on the surface (Figure 2(d)). Such regular microstructures effectively increase the specific surface area, providing an ideal micro-morphological basis for interfacial bonding, and the structures themselves are rich in active functional groups, serving as an active platform for strengthening interfacial chemical bonding.

Figure 2 Micro-morphology of glass surfaces at different plasma cleaning speeds

Testing and Analysis of Chemical Composition after Plasma Surface Modification

XPS was used to characterize the surface chemical properties of the samples, and the changes in chemical element content are shown in Table 3. At a surface modification rate of 5 mm/s, the molar fraction of C element decreases significantly, while the contents of O and N elements increase accordingly. This change confirms the high-efficiency decomposition and removal capacity of high-energy particles in plasma for organic carbon contaminants adsorbed on the glass surface. Among them, the oxygen-to-carbon ratio (O/C), nitrogen-to-carbon ratio (N/C) and oxygen-to-silicon ratio (O/Si) all rise significantly, indicating that the number of surface oxygen-containing functional groups increases and the activity reaches the maximum. Excessively high surface modification rates lead to insufficient impact of high-energy particles, which restricts the generation of oxygen-containing functional groups; excessively low rates will damage the newly generated active groups. Both cases result in a rebound in C element content, a decrease in O and N element contents and key element ratios such as O/C, N/C and O/Si, which further leads to a reduction in surface activity.

To further analyze the regulatory effect of plasma surface modification on surface chemical groups, peak fitting was performed on the C1s and Si2p spectral peaks, with the fitting results shown in Figures 3 and 4 respectively. The changes in the contents of carbon-containing and silicon-containing groups on the glass surface at different surface modification rates are shown in Tables 4 and 5 respectively. The results in Figure 3 show three characteristic peaks at 284.8, 286.3 and 288.3 eV on the glass surface, corresponding to C—C, C—O/C—N and C=O respectively. Quantitative analysis in Table 4 reveals that C element exists mainly in the form of C—C bond structure on the original glass surface with a molar fraction of 77.78% and a low content of oxygen-containing functional groups. After plasma surface modification at 5 mm/s, the content of C—C groups decreases to 60.91%, while oxygen-containing functional groups increase significantly, with C—O/C—N and C=O groups rising to 19.07% and 11.97% respectively. In the process of cleaning organic carbon contaminants on the glass surface, plasma surface modification converts part of the residual carbon into oxygen/nitrogen-containing functional groups through particle bombardment and oxidation, realizing the transformation from a "contaminated surface" to a "clean and chemically activated surface", which enhances the surface hydrophilicity and chemical affinity and achieves chemical activation.

Peak fitting analysis of the Si2p spectral peaks on the lithium aluminosilicate glass surface shows three characteristic peaks at 103.6, 102.7 and 101.5 eV on the sample surface, corresponding to Si—O—Si, Si—OH and Si—O—C bonds respectively. As shown in Table 5, the original glass surface is dominated by the Si—O—Si network structure (64.83%), with a small amount of Si—OH (29.48%) and a trace amount of Si—O—C (5.69%). After plasma surface modification, the content of Si—O—Si decreases, while the contents of Si—OH and Si—O—C show an increasing trend. At a surface modification rate of 5 mm/s, the content of Si—O—Si decreases to 25.93%, the content of Si—OH rises to 62.52%, and the content of Si—O—C increases to 11.55%. Plasma surface modification causes the fracture of the silicon-oxygen network on the surface layer through the bombardment of high-energy particles, which promotes its reaction with water vapor in the environment to generate a large number of polar silanol groups. The sharp increase in its content leads to a significant rise in the polar component of surface energy, which is the chemical driver for the remarkable improvement of glue wettability.

Plasma cleaning and activation is an effective approach to enhancing interfacial bonding performance. Through plasma cleaning modification, the surface energy state and chemical composition of lithium aluminosilicate glass can be effectively regulated, thereby significantly improving its interfacial bonding performance with other materials.