Plasma cleaning technology is an emerging surface modification technique in recent years, which uses particles (electrons, ions, and neutral atoms) to excite, ionize, or destroy reactant molecules, producing a series of etching, polymerization, cross-linking, and other complex physical and chemical effects to improve material surface properties. The main principle is to impact the surface of materials with low-pressure gas molecules (such as oxygen, nitrogen, etc.) through glow discharge, forming new active functional groups on the surface, thereby changing the properties of the material surface. At present, plasma cleaning technology has been applied to various materials that require surface modification, such as magnesium alloys and carbon fibers.

The Effect of Mixed Plasma Cleaning on the Wettability of Magnesium Alloy Surface

In the exploration of mixed plasma surface cleaning technology, plasma mixtures of O2, N2, and Ar were used to clean the surface of magnesium alloys. The contact angle measurement results of magnesium alloy surfaces at different cleaning times are shown in Figure 1 (a). Similarly, the surface of magnesium alloy was subjected to a mixed plasma treatment for up to 180 seconds, and the contact angle of its surface was measured every 30 seconds. As the cleaning time of O2/N2 mixed plasma, O2/Ar mixed plasma, and Ar/N2 mixed plasma increases, the contact angle on the magnesium alloy substrate gradually decreases, indicating that the wettability of the substrate surface is improved after cleaning with mixed plasma. From the measurement results, it can be found that after 120 seconds of mixed plasma cleaning, the contact angle of the magnesium alloy surface decreased to below 2.0 °, and the contact angle image showed that the droplet morphology on the magnesium alloy surface remained basically horizontal, showing a spreading state. Therefore, the optimal cleaning time can be determined as 120 seconds based on the influence of mixed plasma on the surface wettability of magnesium alloys. At the same time, the effect of mixed plasma on the surface wettability of magnesium alloys is also time-dependent. The variation of surface contact angle of magnesium alloy with aging time is shown in Figure 1 (b). After mixed plasma cleaning, the surface contact angle of magnesium alloy gradually increases with the prolongation of aging time. When the cleaned magnesium alloy is placed in air for 12 hours, its surface wetting angle returns to its pre cleaning state.

Figure 1 Surface contact angle of magnesium alloy under different mixed plasma activation and aging times

The influence of mixed plasma cleaning on the surface roughness of magnesium alloys

Figure 2 (a) shows the surface roughness measurement results of magnesium alloy after 120 seconds of mixed plasma cleaning. After cleaning with O2/N2 mixed plasma, O2/Ar mixed plasma, and Ar/N2 mixed plasma, the surface roughness of magnesium alloy increased, and the effect of O2/N2 plasma was the most significant. By extracting the surface characteristic contour lines of the magnesium alloy in Figure 2 (b) after mixed plasma cleaning, in Figure 2 (c), the contour lines of the magnesium substrate surface are rounded and smooth after mixed plasma cleaning. At this time, the microstructure scanned by atomic force microscopy also presents a "hill" morphology. After extending the mixed plasma treatment time to 10 minutes, it was found that the spherical nanostructure on the surface was more pronounced, as shown in Figure 2 (d). This is similar to the effect of a single plasma on the microstructure of magnesium alloy surfaces. After mixed plasma cleaning, the roughness of the magnesium alloy surface can also be improved, and its microstructure can be made uniform. This will be more conducive to increasing the contact area between the substrate and the coating, further enhancing the connection strength of the interface.

Figure 2: Surface roughness and microstructure of magnesium alloy before and after mixed plasma cleaning (a) roughness; (b) Contour line; (c) Microstructure; (d) Microstructure activated for 10 minutes

The influence of mixed plasma cleaning on the surface chemical state of magnesium alloys

The functional group test results on the surface of magnesium alloy after mixed plasma cleaning are shown in Figure 3. After mixed plasma cleaning with three different components, the absorption peaks of hydrophilic functional groups - OH on the surface of magnesium alloy were significantly enhanced, indicating an increase in the number of hydrophilic functional groups on its surface. Meanwhile, as the plasma treatment time was extended from 30s to 120s, the results in Figure 3 (b) - (c) showed that the number of hydrophilic functional groups - OH on the surface of magnesium alloy gradually increased after mixed plasma treatment for different times.

Figure 3 Infrared spectroscopy test results of magnesium alloy surface before and after mixed plasma activation at different cleaning times

After fitting the XPS spectrum in Figure 4, it was found that the Mg-O bond content on the surface of the magnesium alloy increased after mixed plasma cleaning of the three components. And the two oxygen-containing mixed plasma components, O2/Ar and O2/N2, will directly oxidize the surface of the magnesium substrate due to their oxidation effect, resulting in a significant increase in the relative content of Mg-O bonds to 58.9% and 59.9%, respectively. This is because, on one hand, oxygen containing plasma directly oxidizes the magnesium alloy substrate, and on the other hand, high-energy plasma acts on the surface of Mg, causing energy injection and placing the substrate in a thermodynamic metastable state. At this point, the surface is easily adsorbed with oxygen, resulting in a secondary oxidation process. The cleaned samples are usually stored in an atmospheric environment due to the scheduling of XPS testing cycles, which can also lead to secondary oxidation processes on the highly active Mg surface. Moreover, after treatment with the three mixed plasma components, MgCO3 was decomposed on the surface of the magnesium alloy, and the formation of MgO also led to an increase in the Mg-O bond content. The decomposition of MgCO3 on the surface of magnesium substrate may be caused by plasma etching generated by ion collision.

Figure 4 XPS test results of Mg element on the surface of magnesium alloy before and after mixed plasma cleaning (a) untreated; (b) Ar/N2 plasma; (c) O2/Ar plasma; (d) O2/N 2 plasma

The mechanism of plasma cleaning on the surface of magnesium alloy

Plasma dissociation produces highly active O atoms, which can adsorb and recombine with an oxygen molecule or oxidize the metal when they reach the metal surface. And it is accelerated to the surface of the sample in the plasma sheath, bringing energy to the sample surface through collision, affecting the surface structure, morphology, and roughness, and providing cleaning energy for surface diffusion. Firstly, both single plasma and mixed plasma have the effect of decomposing MgCO3 on the surface of magnesium alloys, as shown in Figure 5 (a). After plasma cleaning, high-energy ions bombard the surface of magnesium alloy and collide with the substrate. MgCO3 is decomposed into magnesium oxide and carbon dioxide, giving the magnesium alloy substrate a cleaner surface. The reaction process is shown in equation (3-1). Meanwhile, due to the etching effect of plasma on the substrate, the surface roughness of the cleaned magnesium alloy increases, and the surface microstructure becomes more rounded due to plasma etching, improving surface uniformity and obtaining a nanoscale surface structure. Moreover, both single plasma and mixed plasma can improve the wettability of magnesium alloy surfaces, increasing the number of hydrophilic functional groups - OH on the substrate surface. After plasma cleaning, the relative content of Mg-O dangling bonds on the surface of magnesium alloy increased significantly. Under atmospheric conditions, it is easier to absorb moisture from the air and form hydrophilic Mg OH structures, which modify the surface of magnesium alloys with more - OH hydrophilic functional groups. This is the modification effect of plasma on the chemical functional groups on the surface of magnesium alloys. On the other hand, the decomposition of MgCO3 produces MgO, which further absorbs water molecules in the air, thereby generating Mg (OH) 2 and increasing the amount of surface OH (Equation 3-2). The effects of these actions vary depending on the plasma composition.

Figure 5: The effects of different plasmas on the surface of magnesium alloys (a) MgCO3 decomposition; (b) O2 plasma; (c) N2 plasma; (d) Ar plasma; (e) Ar/N2 plasma; (f) O2/Ar plasma; (g) O2/N 2 plasma