Method for preparing magnesium fluoride coating on surface of magnesium alloy by utilizing direct coupling ultrasound

Abstract

The invention relates to a method for preparing a magnesium fluoride coating on the surface of a magnesium alloy by using direct coupling ultrasound, which is characterized by comprising the following steps of: tightly connecting the magnesium alloy with an ultrasonic oscillation generation part of a direct coupling ultrasound reaction jig, immersing the magnesium alloy into a closed plastic beaker filled with a hydrofluoric acid solution, and carrying out ultrasonic treatment for 5 hours; and then respectively carrying out ultrasonic cleaning on the treated sample for three times by using absolute ethyl alcohol and distilled water in sequence, and blow-drying. According to the direct coupling ultrasonic reaction jig and the application method of the direct coupling ultrasonic reaction jig, an ultrasonic vibration source and a metal sample are rigidly coupled, and a compact nonporous magnesium fluoride (MgF2) coating is rapidly prepared in a fluorine-containing chemical medium.

Description

Technical Field

[0001] This invention belongs to the field of biomedical materials technology, and more specifically, relates to a method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound. Background Technology

[0002] Currently, magnesium alloys are attracting widespread attention from researchers as a promising candidate material for implants due to their biodegradability. Magnesium is the fourth most abundant element in the human body and an essential trace element. Numerous studies have confirmed that magnesium can accelerate fracture healing. Compared to traditional non-biodegradable 316L stainless steel, cobalt-chromium (Co-Cr) alloys, and titanium alloys, magnesium alloys possess excellent bioabsorbability. For patients, absorbable fracture fixation devices mean they can avoid the secondary surgical trauma of removing the fixation device, reducing their economic and psychological burden. Furthermore, magnesium alloys have a density of 1.74-1.84 g / cm³. 3 The density of human bone is 1.75-2.1 g / cm³. 3 Magnesium alloys are extremely similar to aluminum alloys, with their elastic modulus (10-30 GPa) being closer to that of titanium alloys (approximately 110 GPa) and stainless steel (approximately 200 GPa) than that of human bone (approximately 18 GPa), making them less prone to stress shielding effects. Compared to other biodegradable materials such as polylactic acid (PLA), poly(lactic-co-glycolic acid) (PLGA), and zinc-based alloys, magnesium alloys exhibit better mechanical properties and high damping, effectively withstanding the pressure of fixing the affected area. Simultaneously, magnesium alloys possess stiffness and high specific strength (approximately 130 kNm / kg) comparable to aluminum alloys, providing better mechanical support to the affected area while reducing the feeling of a foreign body.

[0003] Unfortunately, magnesium alloys degrade too rapidly, a rate that does not match the clinical rate of bone healing. Worse still, the hydrogen gas produced during degradation can accumulate around soft tissues, potentially triggering inflammatory responses or tissue damage. Therefore, controlling the excessively rapid degradation rate of magnesium alloys has become a top priority.

[0004] Currently, surface modification is an effective solution to mitigate the excessively rapid degradation rate of magnesium alloys. Fluoride conversion coatings have been proven to be an effective solution for addressing this issue. Magnesium fluoride (MgF2) coatings, due to their excellent corrosion resistance, biocompatibility, and ability to effectively regulate the degradation behavior of magnesium alloys, have been widely used in the field of biomedical devices, showing significant potential, particularly in biodegradable bone repair materials and cardiovascular stents. Currently, the mainstream preparation methods include direct hydrofluoric acid immersion (HF) and ultrasound-assisted fluorination (HFU).

[0005] The HFU method involves immersing the sample in an HF solution and using ultrasonic cleaning equipment to induce cavitation, thereby improving the reaction rate and coating adhesion. However, in practical applications, this traditional process still faces several technical bottlenecks, primarily: ① Low acoustic energy transfer efficiency: Traditional HFU equipment places the sample in an HF solution, while ultrasonic waves must traverse multiple interfaces (such as cleaning tank water and acid). Acoustic impedance mismatch leads to severe reflection and absorption, resulting in significant energy loss. ② Uncontrollable coating quality: Due to equipment configuration limitations, the sample's position within the tank varies, leading to uneven sound field intensity and inconsistent coating quality. ③ Severe temperature rise issues: Ultrasonic cleaners accumulate heat during long-term operation, and the lack of an effective temperature control mechanism can easily cause localized overheating, damaging materials or disrupting reaction kinetics. ④ High energy consumption and acid consumption: Tank-type equipment is large, powerful, and consumes a large amount of reaction solution, limiting its application in high-precision and green manufacturing. Published literature has introduced the concept of "ultrasonic-assisted fluorination," but it still relies on a "medium-transmitted sound" structure and does not address the aforementioned issues of energy utilization and coating density. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound. This relates to metal surface treatment technology and is a direct-coupled ultrasonic reaction fixture and its application method for rapidly preparing a dense, non-porous magnesium fluoride (MgF2) coating in a fluorine-containing chemical medium by rigidly coupling an ultrasonic vibration source to a metal sample.

[0007] This invention provides a method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound. The magnesium alloy is tightly connected to the ultrasonic oscillation generation part of a direct-coupled ultrasound reaction fixture, and then the magnesium alloy is immersed in a sealed plastic beaker containing a hydrofluoric acid solution. Figure 1 The samples were ultrasonically treated for 5 hours, and then ultrasonically cleaned three times each with anhydrous ethanol and distilled water, and then dried.

[0008] Furthermore, the ratio of the hydrofluoric acid solution to the surface area of ​​the magnesium alloy is 3-6 ml / cm². 2 .

[0009] Furthermore, the concentration of the hydrofluoric acid solution is 40-55 wt%.

[0010] Furthermore, the direct-coupled ultrasonic reaction fixture includes a rear metal block, a transducer, a power supply, and a front metal block connected in sequence, with a trapezoidal amplitude transformer connected to the front end of the front metal block.

[0011] Furthermore, the transducer uses Pb(Zr,Ti)O3 ceramic sheets.

[0012] Furthermore, the power of the directly coupled ultrasonic reactive fixture during processing is 40 W, 68 W, and 96 W.

[0013] Furthermore, the transducer operates at a frequency sweep of 30-45kHz, with a pulse duty cycle of 30-70% and a peak amplitude of 2-8μm; temperature control is achieved through thermocouple / thermometer detection, and the controller automatically adjusts the power by comparing the set value to realize closed-loop regulation, keeping the temperature fluctuation within ±1℃.

[0014] The sample serves as the sound source. A self-developed direct-coupled ultrasonic reaction fixture was created using a direct-coupled energy transfer mechanism. The idea of ​​using a magnesium alloy sample as the sound source was proposed for the first time. The magnesium alloy sample is connected to the end of the amplitude transformer, and the ultrasonic energy acts directly on the sample surface with almost no attenuation. The cavitation effect is concentrated and the efficiency is greatly improved.

[0015] The "band-aid-like self-healing coating" is the first to propose explaining the synergistic mechanism of "ultrasonic venting" and "cavitation-promoted redeposition" as a dynamic self-healing process. That is, in the initial stage, the pores rapidly release hydrogen gas under the action of ultrasonic bubbles, and then new MgF2 is deposited to fill the pores, eventually forming a non-porous, continuous, and dense coating.

[0016] Frequency sweep: Frequency sweeping allows for alternating coverage of multiple minute resonant points, avoiding the formation of "cavity dead zones" at fixed frequencies and improving the uniformity of cavitation distribution. Pulse vibration: High peak amplitude is provided during the "on" phase to achieve strong cavitation; during the "off" phase, the liquid and sample momentarily come to a standstill and dissipate heat, facilitating the removal of bubbles and renewal of the reaction solution. Temperature control: The reaction chamber temperature is monitored in real time (thermocouple / thermometer), and the controller compares it with the set value to automatically adjust the ultrasonic power, maintaining fluctuations within ±1℃. The synergy of these three elements ensures maximum energy utilization without loss of control, which is the key control strategy of this patented process.

[0017] Experimental results show that the direct-coupled ultrasonic device of this invention can transmit ultrasonic energy to the surface of magnesium alloy samples with almost no attenuation. Its effective sound pressure level is significantly higher than that of the traditional HFU device, and the amplitude attenuation rate is reduced by more than 80%, thereby significantly improving the deposition rate and density of the MgF2 coating. SEM image analysis reveals that the coating prepared by DCUF exhibits significantly better continuity and non-porosity than traditional methods, fully verifying the technical advantages of this invention in terms of acoustic energy transmission efficiency. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a directly coupled ultrasonic reaction fixture.

[0019] Figure 2 This is a schematic diagram of the data acquisition and fitting process for a digital model.

[0020] Figure 3The coating formation of HF coating, HFU coating and DCUT coating at 0.5h, 1h, 2h, 3h, 4h and 5h are shown. Figure 4 The groups are: blank group (a1), HF coating group (b1-b4), HFU coating group (c1-c4), and DCUT-E1 coating group (d1-d4). Field emission scanning electron microscope (FE-SEM) images of the upper and lower regions of the DCUT-E2 coating group (e1-e4) and the DCUT-E3 coating group (f1-f4) at 500x and 2000x magnification; Figure 5 Schematic diagrams of coating formation mechanism and sound wave propagation attenuation under different treatment methods: (a) Coating formation mechanism of conventional hydrofluoric acid immersion method (HF); (b1-b4) Sound wave attenuation and coating formation process in conventional ultrasonic-assisted method (HFU); (c1-c4) Sound wave propagation and coating formation mechanism in direct coupled ultrasonic method (DCUT); (d) Schematic diagram of the effect of ultrasonic standing waves on the pore structure of coating and the "band-aid self-healing" mechanism of DCUT.

[0021] Figure 6 EDS spectra of the upper and lower regions of the coatings in the blank group, HF coating group, HFU coating group, DCUT-E1 coating group, DCUT-E2 coating group, and DCUT-E3 coating group; Figure 7 EDS element distribution images of the upper and lower regions of the coatings in the blank group, HF coating group, HFU coating group, DCUT-E1 coating group, DCUT-E2 coating group, and DCUT-E3 coating group; Figure 8 EDS line scan images of the surface of the upper and lower areas of the bare, HF, HFU, DCUT-E1, DCUT-E2, and DCUT-E3 coatings.

[0022] Figure 9 Surface images of each coating after scratch testing; Figure 10 AFM morphology images of the blank group (a1-a2), HF coating group (b1-b2), HFU coating group (c1-c2), DCUT-E1 coating group (d1-d2), DCUT-E2 coating group (e1-e2), and DCUT-E3 coating group (f1-f2); Figure 11 The contact angles of deionized water, diiodomethane, and surface energy of each group of samples are given. Figure 12The mass loss of each group of samples after soaking in NS for 1 day, 3 days, 7 days, and 14 days is represented. Figure 13 The pH changes of each group of samples after 28 days of immersion in NS; Figure 14 Comparison of three-dimensional digital models of each group of samples after immersion in NS for 14 days, demonstrating the influence of different coating treatments on the corrosion morphology of magnesium alloys.

[0023] Figure 15 Stress-strain curves of three-point bending tests were performed on each group of samples before immersion in NS (a) and 7d (b) and 14d (c) after immersion. Figure 16 The absorbance (a) and relative cell viability (b) of MC3T3-E1 cells at 450 nm wavelength were measured using a CCK-8 assay kit after culturing in the extract for 72 h in each group of samples. (*: p < 0.05, **: p < 0.01, ***: p < 0.005, ****: p < 0.001). Figure 17 The surface morphology of each group of implant samples; Figure 18 Micro-CT images of each group of implants 14 and 35 days after implantation in a rat femoral defect model. Detailed Implementation The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation on the present invention in any way. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods, performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Unless otherwise specified, the materials, reagents, etc. used in the following embodiments are commercially available.

[0024] Example 1 1. Sample-sound source direct coupling The magnesium alloy sample is fixed to the end of the trapezoidal amplitude transformer by a snap-fit ​​method, so that it and the Pb(Zr,Ti)O3 ceramic plate (transducer: realizes electrical energy → ultrasonic vibration) form a coaxial integrated sound source, making the sample the end extension of the sound source.

[0025] 2. Microreaction chamber A closed microreaction chamber made of PFA / PTFE material is used, containing 40-55 wt% HF solution, with the HF solution volume to sample surface area ratio being 3-6 mL·cm². - ².

[0026] 3. Frequency sweep-pulse drive and temperature control The transducer operates at a frequency sweep of 30-45kHz, with a pulse duty cycle of 30-70% and a peak amplitude of 2-8μm. The control board monitors the amplitude and cavity temperature in real time and adjusts them in a closed-loop manner. Temperature control: maintained at 25±1°C.

[0027] Frequency sweeping: By sweeping the frequency, multiple tiny resonant points can be swept over in turn, avoiding the formation of "dead zones" in the acoustic cavity at fixed frequencies and improving the uniformity of cavitation distribution. Pulse vibration: High peak amplitude is provided in the "on" segment to achieve strong cavitation; in the "off" segment, the liquid and sample are momentarily still and dissipate heat, which is beneficial for removing bubbles and refreshing the reaction solution. Temperature control: The temperature of the reaction cavity is detected in real time (thermocouple / thermometer), and the ultrasonic power is automatically adjusted by comparing it with the set value through the controller, keeping the fluctuation within ±1℃. The synergy of these three factors ensures maximum energy utilization without loss of control, which is the key control strategy of this patented process.

[0028] 4. After completion, the sample was ultrasonically cleaned with anhydrous ethanol and distilled water in sequence and then dried to obtain a dense, non-porous magnesium fluoride coating.

[0029] Example 2 I. Materials This experiment used a commercially available AZ31 magnesium alloy sheet, the composition of which is shown in Table 1. It was laser-cut into magnesium alloy strips (35 mm long, 4 mm wide, and 2 mm thick), magnesium alloy discs (16 mm in diameter and 2 mm thick), and magnesium alloy cylindrical implants (2 mm long and 1 mm in diameter). The strip samples will be used for surface feature observation, corrosion experiments, and mechanical testing; the disc samples will be used for cell experiments and surface energy experiments; and the cylindrical samples will be used as implants to simulate fixation in animal experiments.

[0030] Table 1. Element content of commercial AZ31 magnesium alloy used in the experiment.

[0031]

[0032] II. Coating Preparation 1. Sample preparation All samples were progressively sanded up to 1200# using SiC sandpaper in anhydrous ethanol. The samples were then immediately ultrasonically cleaned in anhydrous ethanol and dried. At this point, the uncoated samples were defined as the blank group.

[0033] 2. Preparation of hydrofluoric acid (HF) conversion coating The sample was immersed in a sealed plastic beaker containing a 49 wt% hydrofluoric acid solution and soaked at room temperature for 5 h, wherein the ratio of HF solution to sample surface area was 5 ml / cm². 2The treated samples were then ultrasonically cleaned three times each with anhydrous ethanol and distilled water, and then dried. This group of samples was defined as the HF group.

[0034] 3. Preparation of ultrasonic hydrofluoric acid (HFU) conversion coating The sample was immersed in a sealed plastic beaker containing a 49 wt% hydrofluoric acid solution, wherein the ratio of HF solution to sample surface area was 5 ml / cm². 2 Plastic beakers were placed in an ultrasonic cleaner (SAEHAN SH-2100; ultrasonic frequency = 28 kHz; output power = 100 W) and ultrasonically treated for 5 h at room temperature. Subsequently, the treated samples were ultrasonically cleaned three times each with anhydrous ethanol and distilled water, and then dried. This group of samples was defined as the HFU group.

[0035] 4. Preparation of hydrofluoric acid conversion coating using direct coupled ultrasonic treatment technology (DCUT) The direct-coupled ultrasonic reaction fixture has three power levels: E1=40 W, E2=68 W, and E3=96 W. The sample is tightly connected to the ultrasonic oscillation generation part of the fixture, and is immersed in a sealed plastic beaker containing a 49 wt% hydrofluoric acid solution, where the ratio of HF solution to sample surface area is 5 ml / cm². 2 The samples were ultrasonically treated for 5 hours at powers of E1, E2, and E3, respectively. Subsequently, the treated samples were ultrasonically cleaned three times each with anhydrous ethanol and distilled water, and then dried. The samples treated at different powers were designated as DCUT-E1, DCUT-E2, and DCUT-E3 groups, respectively.

[0036] III. Methods 1. Surface Feature Analysis Magnesium alloy strips were cut into upper and lower sections using a low-speed cutting machine (HQG-100D, Bangyi Precision Measuring Instrument, China). The surface and cross-sectional morphology of the upper and lower sections were observed using a field emission scanning electron microscope (FE-SEM, JEOL-JSM-6700F, Japan), followed by energy-dispersive spectroscopy (EDS) to determine the elemental distribution on the surface and cross-section. The surface composition of the samples was measured using X-ray diffraction (XRD, Rigaku Ultima IV, Japan), with Cu-Kα line lengths of 40 kV and 30 mA, a 2THETA range from 20° to 80°, and a scan rate of 1°min. 1The functional groups of the upper and lower coatings of the samples were determined using Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer Spectrum 100 FTIR spectrometer). The roughness of the upper and lower coatings of the samples was determined by atomic force microscopy (AFM), including the mean roughness (Ra) and recess volume (Vvv) of the samples. A 9.0 nm diameter microscope was used. silicon Morphology measurements were performed on the tip, with an analysis area of ​​20 × 20 μm. Finally, the contact angles of each group of magnesium alloy disk samples were determined using deionized water (Aladdin, China) and diiodomethane (Aladdin, China), and the corresponding surface energies were calculated. Coating adhesion strength was determined according to ISO 2409.

[0037] 2. Corrosion Behavior Experiment (1) Mass loss experiment Magnesium alloy strip samples (n=8) were used in the experiment. Physiological saline (NS) was used as the corrosive medium to visually assess the coating's resistance to chloride ions. The samples were vertically immersed in the corrosive medium at 37 °C and maintained in a constant temperature incubator at 37 ± 1 °C for 1, 3, 7, and 14 days. According to ASTM-G31, the NS volume ratio of the sample area was 20 mL / cm³. 2 Fresh NS was used daily. After 1, 3, 7, and 14 days, the samples were ultrasonically cleaned for 1 minute using chromic acid cleaning solution. Then, the samples were rinsed with anhydrous ethanol and deionized water, respectively, and dried. The percentage of mass loss was calculated using the following formula:

[0038] Where M0 = mass of the sample before testing, and M1 = mass of the sample after testing. Eight samples were tested in each group, and the mass loss is expressed as mean ± standard deviation (SD). (2) Evaluation of digital models A 3D scanner (3 Shape E3, Denmark) was used to digitally model the surface of the samples before and after immersion. Each sample will be modeled with its body center as the reference point. Figure 2 A baseline parallel to the long axis of the sample is drawn as shown. The model above and below the baseline is scanned separately, and the output is an STL format file. The scan data is then fitted using Control X to visually represent the corrosion morphology of the sample in a three-dimensional digital model.

[0039] (3) pH measurement This experiment used magnesium alloy strip samples (n=8). The corrosive medium and immersion conditions were the same as in the mass loss experiment, and the samples were kept in a constant temperature incubator at 37±1 ℃ for 28 days. pH measurements were performed using a pH meter (PHS-3CT, Dapu Instrument, China), once a day, with each sample measured three times and the average value taken.

[0040] 3. Mechanical Test (1) Hardness test For hardness testing, magnesium alloy strip samples (n=3) were used. A hardness tester (HM-JR-3010, Hongmeng Instrument, China) was used to measure the Knoop and Vickers hardness of different samples. The test conditions were: Knoop hardness test load of 0.15 kg and extrusion time of 40 s; Vickers hardness test load of 0.2 kg and extrusion time of 20 s. Each sample was measured 5 times, and the average value was taken.

[0041] (2) Three-point bending test To verify the effect of the coating's resistance to pitting corrosion on mechanical properties, we tested the bending strength of each group of samples in NS before immersion and at 3, 7, and 14 days after immersion. The three-point bending test used magnesium alloy strip samples (n=3), and the immersion test was conducted in the same manner as described above. A universal testing machine (WD1000N, JIMTEC, China) was used to perform the bending test at a constant beam speed of 1 mm / min, and the force-deformation curves, stress-strain curves, and bending strength were recorded simultaneously.

[0042] 4. Biocompatibility (1) Preparation of alloy extract After high-temperature and high-pressure sterilization, the disc samples were immersed in a-MEM medium with a surface area to volume ratio of 1 / ml and cultured for 3 days in an incubator at 37°C containing 5% carbon dioxide to obtain the extract.

[0043] (2) Cell viability Using extracts from different samples as culture medium, MC3T3-E1 pre-osteoblasts (2 × 10⁶ cells per well) were cultured. 3 Cells were seeded in 96-well plates, with cells cultured in α-MEM medium supplemented with 10% FBS and 1% penicillin-streptomycin serving as the control group. The cell culture medium was changed daily. After 2 days of culture, 10 μL of a mixture of CCK-8 and 90 μL of α-MEM medium was added to each well, and the plates were incubated at 37°C with 5% CO2 for another 2 hours. Finally, the optical density (OD) at 450 nm was measured using a microplate reader.

[0044] (3) Staining of the cytoskeleton of phalloidin MC3T3-E1 cells were seeded into 24-well plates containing extract (1 × 10⁶ cells per well). 4 After 24 hours of adhesion, the extraction medium was changed. Following 48 hours of incubation, the cytoskeletal protein F-actin and the cell nucleus were stained with fluorescein isothiocyanate (FITC)-phalloidin for 40 minutes and 4,6-diamidinyl-2-phenylindole (DAPI) for 10 minutes, respectively. Finally, the stained cells were observed using a fluorescence microscope.

[0045] 5. In vivo experiments (1) Study on rat femoral fracture model and magnesium alloy implants Adult male Wistar rats, weighing 180-220 g, were used in the experiment. They were housed under constant 12-hour diurnal light and shadow, relative humidity of 50%-70%, and temperature of 22-26℃, with free access to food and water for one week. The experiment was divided into 7 groups, with 2 animals in each group: sham surgery group (no sample implantation), Bare group, HF group, HFU group, DCUT-E1 group, DCUT-E2 group, and DCUT-E3 group.

[0046] The surgery was performed under aseptic conditions and general anesthesia. Anesthesia was induced in animals using 5% isoflurane inhalation, and maintained at a 2% isoflurane concentration during the operation. The rat's right leg was shaved and disinfected with povidone. A blunt incision was then made along the longitudinal axis of the quadriceps femoris muscle to expose the femoral shaft. Under normal saline irrigation, a hole approximately 2.5 mm in diameter and 4 mm deep was created in the central femoral shaft using a dental drill at low speed. A pre-sterilized bone screw was inserted into the femur, and the wound was carefully sutured. All animals were housed under identical conditions and provided with adequate nutrition. Two animals from each group were sacrificed at 7, 14, 28, and 35 days, and the implant, along with tissue from adjacent or surgical areas, was removed for control purposes.

[0047] Miniature CT Analysis Twenty-eight days post-operation, degradation and bone formation of magnesium alloy samples within the implantation defect area in rats were monitored using a micro-CT (micro-CT, SCANCO VivaCT80, Switzerland) imaging system. Cross-sections of the samples were imaged using scanning parameters of 10 μm voxel size, 50 keV X-ray energy, and 800 ms exposure time. After high-resolution reconstruction, the resulting grayscale images had an isotropic voxel size of 25 μm (pixel matrix: 2018 × 2048). The grayscale images were further reconstructed and analyzed using Scanco software. Pre- and post-implantation implants were also used for surface area and volume changes analysis, and in vivo degradation rates were assessed using formulas.

[0048] V represents the reduced volume of the implant, A represents the surface area of ​​the implant, and t represents the implantation time. 6. Statistical Analysis All statistical analyses were performed using IBM SPSS Statistics for Windows, version 26.0 (IBM, Armonk, NY, USA). Statistical analysis of the results was performed using one-way ANOVA and Tukey's post-hoc test. A p-value < 0.05 was considered significant.

[0049] III. Results and Analysis 1. Formation rate of fluorinated coating Figure 3 The images show the coating formation speed of different surface treatment techniques on magnesium alloy surfaces over a period of 0-3 hours (after 3 hours, the coating color is difficult to distinguish with the naked eye). It can be observed that the Direct Coupled Ultrasonic Treatment (DCUT) group achieved uniform coating formation on the magnesium alloy surface within 1-1.5 hours; the HFU group achieved uniform coating formation on the magnesium alloy surface in about 2 hours; and the HF group achieved uniform coating formation on the magnesium alloy surface in about 3 hours. DCUT appears to allow magnesium fluoride coatings to deposit on magnesium alloys more quickly.

[0050] 2. Surface morphology (1) Surface and cross-sectional micromorphology Figure 4 The microstructure images of the sample surfaces at different magnifications (500×, 2000×) using FE-SEM are presented. After coating treatment, the HF, HFU, DCUT-E1, DCUT-E2, and DCUT-E3 groups all exhibited a smoother appearance than the bare group. At 500× magnification, the upper ( Figure 4 (b1)) and bottom ( Figure 4 (b2) Both exhibited relatively large pores with a diameter of approximately 3.11 ± 1.25 μm. This is largely consistent with the previous findings of Sun et al. (10.1088 / 2051-672X / ab1679). This is because the reaction of magnesium with hydrofluoric acid occurs according to the following equation:

[0051] A large amount of hydrogen gas is released and escapes from the formed magnesium fluoride film, creating escape channels. These pores often become the source of corrosive ions (such as Cl-) in corrosive media. - The Al intrusion into the pores of the metal substrate exacerbates the corrosion of magnesium alloys. Furthermore, the Al in the AZ31 alloy... x Mn yThe particles were also shown to dissolve in HF, forming pitted structures. Compared to the HF group, the HFU group had fewer pores in the coating, both in the upper and lower parts, with a diameter of approximately 1.16 ± 0.74 μm. Compared to the HF and HFU groups, the DCUT-E1, DCUT-E2, and DCUT-E3 groups ( Figure 4 The .df) exhibits a more uniform appearance, with only a very small number of pore channels (0.06±0.02 μm in diameter) observed on the surface of the DCUT-E1 coating, and the bottom group ( Figure 4 (d2)-(f2)) is greater than the upper group ( Figure 4 (d1)-(f1)) shows a more uniform and smoother appearance, and the number of pores in the bottom part of DCUT-E1 is sharply reduced compared to the upper part.

[0052] There are three possible reasons for the decrease in the number and area of ​​pores in the HFU group: First, under the ultrasonic field generated by the ultrasonic cleaner, the bubbles inside the hydrofluoric acid undergo cavitation and generate instantaneous high temperature (~5000 K), high pressure, jets, and shock waves, thereby promoting the formation of magnesium fluoride (MgF2) crystals and the nucleation rate. The results of studies by Li et al. and Sun et al. have confirmed this. Meanwhile, Shen et al.'s study on the formation of micro-arc oxidation (MAO) coatings by ultrasound found that the micro-jet and shock waves generated by ultrasonic vibration can also destroy hydrated hydrogen ions and increase the number of charged particles in the electrolyte, thereby promoting more efficient ion exchange and accelerating coating growth. Furthermore, the oscillation effect induced by ultrasound can increase the position and frequency of micro-discharges in the electrolyte, thus promoting more uniform coating growth, resulting in smaller and more uniformly distributed pores in the final coating. Second, the acoustic flow effect and mechanical effect of ultrasound promote the transport rate of solutes in the liquid by driving liquid flow, as shown in the study by Xu et al. Furthermore, the hydrofluoric acid cavitation bubbles formed by the ultrasonic field and the microjets generated in the viscous boundary layer around the bubbles will also renew the liquid medium around the magnesium alloy to a certain extent, which will be beneficial to the forward reaction. Third, as the bubbles continue to grow, the grown bubbles condense under the action of Bjerknes and Bernoulli forces and rupture due to the loss of surface tension of the bubble walls. This phenomenon will greatly reduce the slow escape of hydrogen from the inside of the coating, achieving the effect of degassing and sealing the surface of the magnesium alloy. Tian et al. also found that at the moment of bubble collapse, the local temperature will rise significantly, providing additional energy for coating formation and accelerating the reaction.

[0053] It is noteworthy that the fluorinated coatings prepared by Li et al. and Sun et al. using ultrasonic cleaning were nearly non-porous, while the HFU group in this study showed a porous appearance in both the upper and lower parts. This contradictory result initially puzzled us. However, the results of the DCUT-E1, DCUT-E2, and DCUT-E3 groups may explain this. Figure 4 As shown in the dashed boxes (e3), (e4), and (f3), the upper portions of both DCUT-E2 and DCUT-E3 exhibit structures resembling "scars." Therefore, we speculate that the formation of the non-porous structure seems to be related to the coating thickness, i.e., ultrasound may help accelerate the healing of potential pores on the coating surface. There are three reasons for this: First, the attenuation pattern of the sound wave will affect the surface morphology of the coating. In studies of ultrasound propagation in rarefied bubble media (such as water and hydrofluoric acid), many scholars have focused on the attenuation of ultrasound in the medium. Louisnard and Trujillo proposed and optimized a nonlinear model to establish a sound field calculation model and sound attenuation, concluding that the intensity of the cavitation effect is directly related to the sound pressure level in the liquid. When the ultrasound generated by the sound source is a harmonic, and the liquid is a homogeneous medium such as hydrofluoric acid, the sound field distribution can be solved using this nonlinear Helmholtz equation, i.e.:

[0054] Where, p is the sound pressure level. Because liquids such as hydrofluoric acid have high acoustic impedance, there is sound wave attenuation due to liquid viscosity and cavitation effects, and k is the complex wave number. 2 The real and imaginary parts are represented as follows:

[0055]

[0056] in,, ω=2πf The ultrasonic angular frequency, c These are the speed of sound variables. A 0 and B 0 can be obtained by calculating the bubble oscillation process using the Keller-Mikisis equation:

[0057]

[0058]

[0059]

[0060]

[0061]

[0062] For the physical meanings of the variables in Eq. (4-1)-(5-3), please refer to the relevant literature by Trujillo et al. The formula indicates that ultrasound will inevitably attenuate in hydrofluoric acid, and this attenuation will have two aspects—cavitation of bubbles and attenuation due to liquid viscosity and acoustic impedance. Wu et al. found that at an ultrasonic frequency of around 20 kHz, the energy of the sound wave decreases instantaneously with spatial distance. When ultrasonically treating hydrofluoric acid conversion coatings, we hope that the sound wave energy will be converted into bubble cavitation as much as possible rather than attenuated in the liquid. Figure 5 As shown, Figure 5 (a) shows the mechanism of coating formation by immersion in ordinary hydrofluoric acid for 5 hours. Figure 5 (b1)-(b4) show the mechanism of sound wave attenuation and coating formation under the action of ultrasonic cleaning machine. Figure 5 (b2) demonstrates that during sound wave formation, the sound waves are first reflected when encountering obstacles such as beakers; secondly, a large amount of energy is absorbed during transmission in hydrofluoric acid; and finally, some sound waves are sputtered when encountering H2 bubbles generated by the reaction, resulting in insufficient energy for cavitation. Furthermore, cavitation bubbles continuously form during ultrasonic wave transmission, and most of these cavitation bubbles are far from the magnesium alloy, thus failing to function effectively. Therefore, magnesium fluoride formation on the coating surface is slow, and the bubbles cannot be effectively eliminated, resulting in a porous structure. Figure 5 (b4)

[0063] It is generally believed that the frequency and power of ultrasound directly affect the volume of cavitation bubbles, thus directly influencing coating formation. Therefore, when ultrasonically treating hydrofluoric acid conversion coatings, we prefer that the acoustic energy be converted into bubble cavitation as much as possible rather than attenuated in the liquid, as this would increase the difficulty of frequency and power control, leading to inconsistent sample quality and unnecessary energy waste. Regarding frequency control, Fatimah et al. studied the effect of ultrasonic frequency on the structure of MAO coatings formed on magnesium alloys, finding that frequencies around 40 kHz typically produce larger and more stable cavitation bubbles that guide electrolyte particle movement and coating deposition. However, they also found that at higher frequencies, the bubble volume decreases, the bubble collapse effect is milder, and the generated mechanical energy is lower. Combined with the research of Poulain et al., it was shown that the cavitation bubble flow rate is related to the bubble size. The formula is:

[0064] Where u is the cavitation bubble velocity, R b Let be the volume of the bubble. Furthermore, Shen et al. and Poulain et al. proposed a formula for the internal energy of a bubble, namely:

[0065] in It is the internal pressure of the bubble; It is the difference in bubble volume; It is the heat capacity of water; It is the molar mass of water; It is the temperature of the outer bubble surface; It is the thickness of the thermal boundary layer. From Eq(7), it can be concluded that the reduction in bubble volume may decrease the cavitation bubble flow rate, thereby reducing the ultrasonic transmission range. Regarding power control, studies have shown that although the mechanical effect generated by an appropriate amount of ultrasonic power can apply a moderate mechanical force between the electrolyte and the substrate, effectively weakening or eliminating internal stress, reducing the formation of microcracks, and improving the uniformity and density of the coating, when the ultrasonic power is further increased, the surface structure of the coating begins to deteriorate, manifested as more irregular protrusions, larger pores, and denser cracks. This is because excessive ultrasonic energy causes violent vibration of molecules and atoms in the electrolyte, which may also trigger local overheating, thereby destroying the structure of the oxide film and forming more defects. All of the above illustrates that the quality of commonly used ultrasonically treated coatings is often difficult to control.

[0066] Figure 5 (c1)-(c4) show the mechanism of sound wave attenuation and coating formation under DCUT treatment, that is, the magnesium alloy substrate itself will actively generate ultrasonic waves as a sound source. Figure 5 (c2) In this case, the sound source is very close to the reaction contact surface, so it is minimally affected by the attenuation of ultrasonic waves, greatly reducing the attenuation of ultrasonic waves in liquid media and obstacles. Almost 100% of the ultrasonic energy is converted into cavitation bubbles, accelerating coating formation. Compared to the vibration frequency of the ultrasonic cleaner (approximately 28 kHz), the vibration frequency of the directly coupled ultrasonic reaction fixture is higher (approximately 40 ± 10 kHz). Furthermore, the cavitation bubbles are generated almost entirely on the magnesium alloy surface, resulting in high utilization efficiency and eliminating the need to consider ultrasonic attenuation. Simultaneously, the magnesium alloy sample will undergo a "pendulum-like motion" during vibration. Figure 5 (c2)), which will form vortices ( Figure 5 (c3)) This achieves automatic removal of minute oxide layers or other contaminants from the substrate surface while accelerating liquid renewal on the magnesium alloy surface, thus speeding up coating formation. Furthermore, due to the "pendulum-like motion" of the magnesium alloy, the amplitude in the bottom region is greater than that in the upper region ( Figure 5 (c2)), therefore the scouring and eddy current effects are stronger, resulting in a denser and more uniform coating appearance. Figure 5(c4)-(c5)). Meanwhile, Tang et al. conducted Eckart acoustic flow studies in a heptagonal chamber using multiple acoustic transducers and found that more vortices were generated in the liquid as the ultrasonic frequency increased. Therefore, as the ultrasonic power increased, the amplitude of the magnesium alloy gradually increased, and we also observed that the coating became more uniform with increasing power on FE-SEM. Cai et al. proposed that as the ultrasonic power increased, the energy accumulated inside the bubble increased. The greater the accumulated energy of the bubble, the higher the heat energy generated by its collapse, which accelerates the reaction. Rahimi et al. and Sajjadi et al. have also verified that increasing the ultrasonic power can increase the turbulence intensity and the volume fraction of bubbles in the liquid. These pieces of evidence support the phenomenon that the number of pores in DCUT-E1, DCUT-E2, and DCUT-E3 decreases with increasing power. Furthermore, Shen et al.'s study on the formation of MAO coatings on aluminum surfaces using ultrasound found that as the ultrasonic power increased, the residual compressive stress in the coating was released after ultrasonic treatment, especially under longer ultrasonic treatment, the residual stress changed from compressive stress to tensile stress. This demonstrates that a moderate increase in ultrasonic power significantly reduces the risk of coating cracking and improves coating integrity. It's important to note that while the ultrasonic cleaner has a higher power (approximately 100 W), explaining why the fluorine content in the HFU group samples is higher than that in the DCUT group, as mentioned earlier, its low cavitation effect utilization and significant acoustic energy loss are major reasons for its slower coating formation and less than ideal coating morphology. Furthermore, the potential for localized overheating that might occur if the power is not strictly controlled can be mitigated to some extent in our proposed new process. By allowing the metal sample and the solution to vibrate together, the uniform distribution of heat can be accelerated synergistically, reducing the accumulation of heat energy in the liquid medium. This allows for better temperature control and avoids coating quality problems caused by excessively high temperatures.

[0067] Furthermore, in traditional ultrasonic treatment, the formation of standing waves affects the surface morphology of the coating. Standing waves refer to two waves with the same frequency but opposite propagation directions. In this experiment, the HFU group formed standing waves because the sound waves easily reflected upon reaching the magnesium alloy surface. Although standing waves can accelerate bubble collapse and enhance scrubbing ability to some extent, they should still be considered a negative energy loss. Tian et al. also studied the effect of ultrasonic treatment on the formation of MAO coatings on Mg-Li alloy surfaces. Their study found that standing waves cause a large number of bubbles to be compressed at the interface between the coating and the solution, resulting in smaller but more numerous pores. Figure 5 (d) . ...

[0068] In summary, we propose a "band-aid coating repair mechanism" to explain the formation mechanism of the non-porous coating in the new process. The formation of the non-porous coating not only depends on the venting effect of ultrasound, but is also a dynamic self-healing process. This mechanism can be explained from the following two aspects: First, ultrasound promotes the rapid expulsion of hydrogen gas, reducing the formation of initial pores. In this process, the release of hydrogen gas often creates pores during coating formation, affecting the quality of the final coating. However, the cavitation effect and mechanical effect of ultrasound mentioned above significantly increase the rate at which hydrogen gas escapes from the coating surface, allowing bubbles in the coating to be expelled in time, reducing the number and size of pores. It is this effect that makes the ultrasonically treated coating significantly reduce the number and area of ​​pores compared to the HF group, improving the density of the coating; Second, although ultrasound accelerates the gas expulsion, incompletely sealed pores may still remain on the surface in the early stages of coating growth. Figure 5 (d) However, these pores are not merely simple defects, but can further react with fluoride ions in hydrofluoric acid in the proposed DCUT to generate new magnesium fluoride deposits, thereby gradually filling and sealing these pores. Figure 5 (d) This process is similar to a band-aid covering a wound, repairing the pores on the coating surface through continuous and faster replenishment of the substance. Figure 5 (d) The coating formation rate is positively correlated with the defect filling rate. That is, under higher ultrasonic power, more magnesium fluoride deposition can rapidly fill the pores, thus promoting the final dense, non-porous coating structure. For the non-porous coatings prepared by Li et al. and Sun et al., the ultrasonic treatment time in their experiments was relatively long, about 24 h to 72 h. This explains why the HFU group still exhibited a porous structure, while the DCUT-E3 group showed a non-porous appearance. The long treatment time allowed the coating structure to undergo a complete process of "initial nucleation - pore formation - ultrasonic-promoted repair - final sealing", while the proposed DCUT significantly shortened the treatment time.

[0069] (2) Coating elemental analysis (FTIR+EDS) According to EDS energy dispersive spectroscopy analysis ( Figure 6The elemental distribution on the surface of the six magnesium alloy samples showed significant differences. After HF treatment, different amounts of fluorine were detected on the surface of the magnesium alloys in each group, confirming that the fluorination treatment successfully induced the formation of the MgF2 coating. The overall fluorine content of the DCUT group was higher than that of the HF group, and with the increase of ultrasonic power, the overall fluorine content of the coating in the E3 group was 12%, nearly 4% higher than that of the HF group. Interestingly, we found that EDS spectroscopy showed that the fluorine content of the HFU group reached 20%, much higher than the other groups. This may be because the HFU coating contains more pores, and the looser surface adsorbs free fluoride ions in the solution, thus increasing the surface fluorine content. However, these fluoride ions have no effect on corrosion performance. Unlike the HFU group, which uses water and hydrofluoric acid as the propagation medium, the DCUT group directly uses the magnesium alloy sample as the ultrasonic source, forcing it to vibrate. This reduces the loss of ultrasonic waves during propagation in the solution and generates a high shear rate flow field, which promotes the rapid detachment of H2 bubbles, inducing stress field densification, thereby producing a coating with less porosity and no adsorbed fluoride ions on its surface. SEM images confirm this. Furthermore, according to the results of AFM, SEM, and mechanical tests, the DCUT forms a relatively thick coating, but it can form a dense coating in a short time. The lower density coating exhibits better performance, which is similar to the research results of Fatimah et al.

[0070] Fluorine plays a crucial role in coatings. Firstly, the magnesium fluoride (MgF2) coating formed after fluorination significantly improves the corrosion resistance of magnesium alloys. The stable crystal structure of MgF2 effectively fills microscopic defects in the coating, reducing porosity. In this experiment, the coating density of the DCUT group was significantly better than that of the HF and HFU groups, mainly attributed to the enhanced crystallinity and uniformity of the coating due to the introduction of fluorine.

[0071] It is worth noting that the fluorine content of the coatings in the DCUT-E1, DCUT-E2, and DCUT-E3 groups exhibits a gradient change along the axial direction, with the bottom portion showing a higher fluorine content. - The content increased by 1.48 wt% - 2.30 wt% compared to the upper fraction. This gradient change occurred in... Figure 7This phenomenon is more pronounced in the lower region. It may be related to the enhanced local mass transfer induced by cavitation, where the bottom region is more prone to enrichment of active fluoride ions, leading to spatial heterogeneity in the non-equilibrium deposition rate. Cavitation refers to the transient cavitation bubbles excited by ultrasonic vibration in solution; their collapse generates microjets and shock waves that can disrupt the diffusion boundary layer at the solid-liquid interface. In the upper region near the vibration source, the cavitation intensity is higher, resulting in a significant increase in the mass transfer rate of HF molecules to the magnesium matrix. However, intense cavitation also exacerbates the local consumption of HF solution and gas-liquid mixing in the upper region, triggering concentration polarization, which in turn limits the continued progress of the coating formation reaction. This phenomenon results in unique structural and performance characteristics in different regions of the coating. Specifically, the bottom region, due to the enrichment of fluoride ions, forms a denser and more uniform coating structure, significantly improving its corrosion resistance. While the upper region has a higher mass transfer rate due to the enhanced cavitation effect, the coating formation rate is somewhat limited by the local consumption of HF solution and concentration polarization, resulting in a thinner coating compared to the bottom region. This structural difference did not adversely affect the overall performance of the coating. On the contrary, by optimizing the microstructure of the coating, it enabled the coating to exhibit different functional characteristics in different regions.

[0072] To further investigate the growth and elemental distribution of the coating from a cross-sectional perspective, we performed EDS line scan analysis (…). Figure 8The results showed that all fluorination groups formed a clear magnesium fluoride (MgF2) coating on the magnesium alloy substrate surface, but their thickness and elemental distribution gradients differed significantly. The coating thicknesses of the HF and HFU groups were relatively similar, approximately 3 μm, and the fluorine signal intensity was low, indicating that coating growth was limited by reaction kinetics and that structural defects might form due to hydrogen accumulation. In contrast, the coatings of the DCUT-treated groups exhibited superior thickness and density. With increasing ultrasonic power, the coating thickness gradually increased, especially in the bottom region of the DCUT-E3 group, where the coating thickness reached approximately 4.5 μm, and the fluorine signal intensity was significantly higher than in other groups. Furthermore, EDS line scans showed that in the DCUT group, fluorine exhibited a steep gradient distribution near the substrate interface, subsequently maintaining a high intensity within the coating, indicating a continuous coating structure and good bonding with the substrate. This structure is beneficial for hindering the penetration of corrosive media, thereby improving corrosion resistance. Notably, in the DCUT group, the coating thickness and fluorine signal intensity in the bottom region were generally higher than those in the upper region, which is highly consistent with the morphological characteristics observed by SEM and AFM. We believe this phenomenon is closely related to the DCUT-specific "sample-sound source" direct-coupled vibration mode. In this mode, the sample acts as a sound source, undergoing a "pendulum motion," resulting in a larger amplitude, stronger cavitation effect, and more frequent liquid flow renewal in the bottom region, thereby promoting fluoride ion mass transfer and accelerated coating deposition. This vibration-induced cavitation microfluidic flow can effectively disrupt the diffusion boundary layer at the solid-liquid interface, enhance ion migration efficiency, and thus achieve rapid and uniform coating growth. The aforementioned cross-sectional EDS analysis not only confirms the advantages of DCUT technology in improving coating thickness and uniformity but also provides a microstructural explanation for its excellent performance in corrosion resistance and mechanical properties.

[0073] This gradient variation in fluorine content offers important insights for future coating design and applications. First, this phenomenon demonstrates that the fluorine content in a coating can be precisely controlled by adjusting the power and duration of ultrasound, thereby optimizing the coating's performance distribution. For example, in areas requiring high corrosion resistance, fluoride ion enrichment can be increased; while in areas requiring high biocompatibility, the fluorine content can be appropriately reduced to promote cell adhesion and proliferation. Second, this gradient fluorine content distribution provides theoretical support for the design of multifunctional coatings. By rationally designing the fluorine content gradient in a coating, multiple functions can be synergistically optimized within a single coating, such as balancing corrosion resistance, biocompatibility, and mechanical properties. Furthermore, this gradient structure can also ensure the long-term stability of the coating, as the performance differences in different regions can complement each other, resulting in superior overall performance in complex biomedical environments.

[0074] In previous studies of PEO coatings, high-voltage-induced spark and plasma discharges led to the formation of pores and microcracks on the coating surface. In the early stages, discharge channels formed, and local temperature and pressure rapidly increased. Molten oxides, under high pressure, were ejected from the discharge channels via a volcano-like movement, forming fine molten particles that rapidly cooled and solidified, creating porous, pancake-like regions. This rapid and intense ejection process resulted in typically deep pores, which, upon analysis, were found to lack magnesium and were primarily composed of oxides and electrolyte reaction products. As the discharge continued, the molten oxides gradually ejected inward, forming a dense inner layer structure. This inward ejection mechanism made the coating gradually denser, reducing pore formation. However, the lack of magnesium within the pores made it difficult for subsequent reactions to fill them after formation, resulting in persistent pores that negatively impacted the coating's corrosion resistance. Similarly, although the MAF coating improves the chemical stability of the coating by introducing fluoride ions, the magnesium element inside the pores is also consumed during the discharge process, resulting in insufficient reactants inside the pores and making it difficult to achieve complete pore filling.

[0075] In contrast, while the DCUT coating also forms pores during its formation, these pores still contain a certain amount of magnesium. Under ultrasonic vibration, this unreacted magnesium continues to react with hydrofluoric acid, generating MgF2. This dynamic reaction process allows the coating to continuously fill the pores during formation, ultimately achieving complete densification. The high shear rate flow field of the ultrasound not only accelerates coating formation but also promotes the rapid detachment of H2 bubbles, inducing stress field densification. This effectively prevents electrolytes from entering the substrate, further improving the coating's protective performance.

[0076] The unique contribution of the DCUT coating mechanism lies in its rational explanation of the pore formation and filling process. Through the physical action of ultrasound and the dynamic equilibrium of chemical reactions, DCUT coating not only fills a gap in traditional coating technology but also provides new scientific evidence for the surface modification of magnesium alloys. This mechanism not only explains the pore formation process but also demonstrates how to achieve complete densification of pores through dynamic reactions. DCUT coating utilizes the high shear rate flow field and vibration effect of ultrasound, enabling magnesium elements within the pores to continuously participate in the reaction, forming a denser and more uniform coating structure. This mechanism emphasizes the continuous reactivity of magnesium elements within the pores, making the coating formation process more rational and efficient. In this way, DCUT coating exhibits significant scientific value in terms of corrosion resistance and biocompatibility, providing a solid foundation for the application of magnesium alloys in the biomedical field.

[0077] (3) Adhesion test Depend on Figure 9 As shown, each coating exhibits a relatively complete morphology after adhesion, fully meeting the application standards for clinical implants. This is because the formation of the fluorinated coating involves the reaction Eq.(2). There is a strong chemical bond between the magnesium substrate and the magnesium fluoride coating. The strong adhesion strength gives the fluorinated coating a greater potential for application in bone implants.

[0078] (4) Surface roughness Table 2 Roughness

[0079] Surface roughness is typically related to factors such as surface modification methods. Coating roughness affects various properties, including fatigue strength, corrosion resistance, and cell adhesion after implantation. An overly smooth implant surface hinders cell adhesion, reducing biocompatibility, while an overly rough surface with many pores leads to decreased corrosion resistance. The primary parameter for evaluating surface roughness is Ra. Data in the table shows that the Ra values ​​for the DCUT group are lower than those for HF and HFU. The sample surface roughness, from highest to lowest, is HF > Bare > HFU > DCUT-E2 > DCUT-E1 > DCUT-E3. The lower surface roughness of the DCUT group indicates higher coating quality and fewer morphological defects (such as pores and voids). This improves overall performance and reliability. Furthermore, a more uniform and smooth surface also contributes to improved corrosion resistance. When focusing on the upper and lower parts of the sample surface, we find that in the bare, HF, and HFU groups, surface roughness is not specifically related to the location of the sample. In all three DCUT groups, the Ra of the sample bottom surface was smaller than that of the upper surface, indicating that the larger amplitude bottom roughness and the coating was more dense and uniform. (AFM results) Figure 10 This result also visually demonstrates the uniqueness and innovation of DCUT.

[0080] For magnesium alloys, pitting corrosion is one of the main forms of corrosion and a major obstacle limiting their applications. Pores on the coating surface provide channels for corrosive ions to penetrate the magnesium alloy substrate, significantly increasing the likelihood of pitting corrosion. If pitting occurs, it leads to stress localization, resulting in a transformation from pits to cracks and weakening the overall corrosion resistance of the alloy. Since defects such as pores are concentrated in the valleys of the coating, the void volume (Vvv) in the valleys can effectively reflect the number of defects. The coating pore volume decreases sharply from E1 onwards, and is an order of magnitude lower than that of bare and HF / HFU. This indicates that the coating prepared by DCUT is more dense and uniform, with a lower probability of pitting corrosion, significantly overcoming a limitation in the application of magnesium alloys.

[0081] Furthermore, the pore volume and Ra value of HFU are both lower than those of HF. This may be because the standing waves formed by the ultrasonic waves emitted during HFU preparation and the ultrasonic waves reflected from the sample surface cause the loose protrusions of the coating to be sheared off, ultimately resulting in a lower roughness for the HFU coating compared to HF. The cavitation effect of ultrasound enhances the diffusion kinetics of electrolyte ions and causes bubbles to collapse and close rapidly, reducing the number of bubbles and preventing their aggregation in the coating, thereby reducing the pore volume. This is consistent with previous research. Li et al. investigated the process of ultrasonic-assisted electrodeposition of Ni-Co alloy coatings and found that the cavitation effect of ultrasound significantly improved the diffusion kinetics of the electrolyte and reduced the porosity in the coating. Zhang et al. prepared a CoMo / Al2O3 composite coating with a minimum surface roughness of 0.193 μm under 100 W ultrasonic power, exhibiting a denser surface morphology. Sheng et al. used ultrasonic stirring to reduce the roughness of Ni-Co coatings and improve their hardness and corrosion resistance.

[0082] (5) Wettability and surface energy The surface energy and hydrophilicity of orthopedic implants have a significant impact on their performance. Appropriate surface energy promotes cell adhesion and proliferation, accelerating osseointegration; therefore, for orthopedic implants, a surface energy of >50 mJ / m² is typically required. 2 As shown in the figure, the surface energy of magnesium alloys increased after surface modification, exceeding 50 mJ / m². 2 Furthermore, the surface energy of the DCUT group was higher than that of the HF and HFU groups, indicating that its wettability, adhesion, and chemical reactivity were further enhanced.

[0083] However, excessively high surface energy may cause an overactive inflammatory response. Therefore, the optimal range for the surface energy of magnesium alloy orthopedic implants is 60-80 mJ / m². 2 .from Figure 11As can be seen, the DCUT group perfectly meets this requirement. It is worth noting that through surface modification, the water contact angle of the magnesium alloy decreased from over 60° initially to below 40°, significantly improving the hydrophilicity of the magnesium alloy surface. The possible reasons why the hydrophilicity of the HF and HFU groups, both being magnesium fluoride coatings, is lower than that of the DCUT group are as follows: First, some magnesium fluoride decomposes during the preparation process, forming hydrophobic magnesium oxides (such as MgO), resulting in impurities in the coating. Second, the surface roughness of HF and HFU is higher than that of the DCUT group, causing cavitation in the micro-nano structures of their coating surfaces, hindering complete liquid spread. The low-roughness surface of the DCUT group makes complete wetting easier. Ultimately, this manifests as a decrease in roughness and an increase in hydrophilicity. Furthermore, in the early stages of magnesium alloy degradation, the DCUT coating has a small water contact angle, and the hydrophilic surface promotes cell adhesion and reduces protein denaturation. In the later stages of degradation, the hydrophilicity of the coating decreases, and the increased hydrophobicity reduces the immune response. This improves its biocompatibility and functionality as an orthopedic implant.

[0084] 3. Corrosion behavior (1) Mass loss test Figure 12The changes in mass loss of bare Mg alloys with different hydrofluoric acid conversion coatings under NS immersion are shown. For the same immersion time, lower mass loss indicates better corrosion resistance. As shown in the figure, the mass loss of each sample gradually increases with immersion time, highlighting the biodegradability of magnesium alloys. The uncoated bare Mg alloy showed a mass loss exceeding 55% after 14 days of immersion, indicating excessively rapid degradation and poor corrosion resistance. In contrast, the fluorinated coating samples immersing for the same corrosion time all showed mass losses below 25%. This represents a 50% reduction in mass loss compared to bare Mg alloys, significantly improving the corrosion resistance of magnesium alloys, consistent with results presented by Sun et al. Among the fluorinated coating samples studied in this paper, the self-developed DCUT coating exhibited better corrosion resistance than the previously developed HFU and HF coatings. This advantage gradually became apparent starting from 7 days of immersion, with the mass loss of DCUT-E3 being only 1 / 3 that of the HF and HFU coatings, and the mass loss of DCUT-E2 being only 1 / 2 that of the HF and HFU coatings. Furthermore, the advantages of the DCUT coating become more pronounced with prolonged immersion time. As can be seen, on day 14, the mass loss of the DCUT-E1 sample was less than one-third that of the HF sample, the DCUT-E2 sample less than one-quarter, and the DCUT-E3 sample less than one-fifth. Similarly, for ultrasonic fluorination coatings, the mass loss of the DCUT coating was only 15%–52% of that of the HFU coating. This is because the addition of ultrasound enhances the diffusion kinetics of ions in the solution, accelerating the HF renewal rate near the sample; the standing waves formed by the emitted ultrasound and the ultrasound reflected from the sample surface reduce the surface roughness, both contributing to the formation of a finer and more uniform coating and improving corrosion resistance. DCUT changes the traditional application of ultrasound; the improved method of generating ultrasound from the sample into the solution further increases the efficiency of ultrasound and enhances the cavitation effect, resulting in a coating with better corrosion resistance than traditional HFU coatings.

[0085] Wang et al. applied ultrasound to superhydrophobic calcium carbonate coatings. The addition of ultrasound increased the nucleation rate of CaCO3 crystals and promoted the growth of smaller crystals, resulting in a denser, thicker coating with fewer defects, thus improving corrosion resistance. Furthermore, the packing effect induced by ultrasound can improve the coating's adhesion and hardness. Wang et al. found that introducing ultrasonic vibration during the MAO process can significantly improve the coating's corrosion resistance. In Liu et al.'s study, the coating's corrosion resistance continuously improved with increasing ultrasonic power up to 50 W. Wekwejt et al. also confirmed that the application of ultrasound can significantly increase the coating thickness. Zhang et al. also enhanced the corrosion resistance of Cu-incorporated TiO2 coatings by introducing ultrasound. Our DCUT-modified samples also showed similar results; the corrosion resistance and thickness of the coating increased with increasing ultrasonic power.

[0086] Not only that, Cl This corrosive ion has excellent penetrating power, easily penetrating magnesium alloy substrates, promoting the decomposition of the surface film and triggering localized corrosion of Mg-Al alloys. Higher concentrations result in deeper pitting corrosion and more significant damage to magnesium alloys. The corrosive medium used in this experiment was neutral sulfide (NS), which avoids the effects of additives such as MgSO4, Na2HPO4, NaHCO3, and NaH2PO4 in simulated body fluids on preventing MgF2 coating degradation. This reduces the risk of contamination from microbial growth, minimizes interactions between components, and reduces sensitivity to experimental conditions. Compared to other corrosive solutions, the basic corrosion reaction in NS is (Main, accompanied by hydrogen evolution reaction [HER]).

[0087] (Secondary, accompanied by Oxygen reduction reaction (ORR) The main corrosion products are Mg(OH)₂ and MgO. The insufficient formation of protective corrosion products allows for a better reflection of the material's intrinsic information. However, it is precisely this lack of protective products that makes NS highly corrosive to AZ31. In previous studies, the high corrosiveness of NS often prevented the observation of the long-term corrosion status of the samples. However, the experimental results show that after immersion in NS for 14 days, the sample largely retained its structural integrity under the protection of our newly developed DCUT coating, better demonstrating the effect of the fluorinated coating on Cl₂. The corrosion resistance of magnesium alloys demonstrates extreme Cl... Corrosion resistance of magnesium alloys under certain conditions.

[0088] (2) pH test pH has been shown to play a regulatory role in bone mineralization and fracture healing. However, the dynamic changes in pH in in vitro corrosive solutions are mainly attributed to the release of Mg from the continuous corrosion of the magnesium matrix. 2+ And OH during corrosion process - Gradient accumulation. For example... Figure 13 As shown, the blank group experienced a rapid increase in pH early on and completed the reaction after approximately 21 days. This is because the rapid chemical reaction between magnesium and chloride ions leads to a burst of hydroxide ion release, resulting in localized over-alkalization, which is detrimental to osteogenic differentiation of osteoblast progenitors. While the HF and HFU groups showed an overall decreasing pH trend, it was not significant. This may be related to the micropores on their surfaces. Although the magnesium fluoride film provides some barrier effect against chloride ions, unfortunately, this film is rapidly dissolved by NS within just one day under extensive defect exposure. However, in the later stages, with the formation of corrosion products such as magnesium hydroxide, these precipitates adhere to the magnesium substrate, forming a barrier effect and reducing the corrosion rate. The DCUT group, on the other hand, showed a significant decreasing pH trend. This is because the coating formation rate increased, and a large number of pores were "healed" by magnesium fluoride, significantly reducing the possibility of chloride ions penetrating the magnesium substrate. Notably, the DCUT-treated group exhibited a pH evolution characteristic significantly different from other experimental groups: its solution pH was negatively correlated with ultrasonic power, indicating that the coating formed by DCUT has a significant protective effect on the magnesium alloy and improves its corrosion resistance. Under long-term corrosive conditions, the protective effectiveness of the coating shows a decreasing trend, which may be due to the corrosive Cl... - Gradually replace F in the coating - This is caused by localized defects in the coating structure. The pH value of the Mg, HF, and HFU solutions showed a trend of first increasing and then decreasing. This may be because after most of the samples degraded, the exposed area of ​​the remaining samples was too small, and the Mg released during continued corrosion after solution replacement... 2+ The pH value actually decreased after the amount of water was reduced.

[0089] Yan et al. applied a fluoride coating similar to that used in the HF group of this experiment to the surface of AZ31B magnesium alloy. Their results showed that in stimulated blood plasma, the pH changes of the fluorinated magnesium alloy and bare AZ31B solution were not significantly different; however, in neutral sulfide (NS), the pH of the fluorinated magnesium alloy solution was significantly lower than that of the bare magnesium alloy, exhibiting better corrosion resistance. This may be due to the higher Cl content in NS. -Concentration can cause MgF2 crystals to dissolve, damaging the MgF2 layer. However, in simulated plasma, the presence of other additives such as MgSO4, Na2HPO4, NaHCO3, and NaH2PO4 effectively prevents the dissolution of the MgF2 coating. Therefore, the pH of the fluorinated magnesium alloy solution remains stable in simulated plasma. Thus, NS can expose coating defects to the greatest extent, making it an ideal in vitro simulated solution for testing coating corrosion resistance.

[0090] (3) Digital model analysis Corrosion morphology can effectively reflect coating defects. Figure 14 Digital model images of each group of samples before corrosion (0 days) and after 14 days of corrosion are presented. The significant differences in corrosion behavior among the six groups are clearly visualized. The blank group exhibits a significant and widespread pitting corrosion morphology. Clearly, without the coating acting as a barrier layer, AZ31 Mg alloy loses its mechanical integrity in NS, which is extremely poor. In contrast, the HF and HFU groups show some protective effect from the coating in terms of corrosion morphology. However, the integrity of the HFU group samples is significantly greater than that of the HF group. This is because the numerous defects in the HF group provide abundant chloride ion entry channels, and the defect concentration areas are high-risk sites for corrosion, prone to pitting corrosion and rapid expansion of the corrosion area. The DCUT group shows a relatively intact morphology overall, which will greatly improve their mechanical integrity and clinical applicability. Interestingly, most of these corrosion areas appear in the upper area of ​​the samples, which is precisely the possible concentration site of defects mentioned in the surface and cross-sectional micromorphology section. Furthermore, the corrosion phenomenon shows a significant decreasing trend with increasing ultrasonic power, and there is a tendency to shift from pitting corrosion to uniform corrosion, indicating that reasonable control of ultrasonic parameters helps to improve the density and protective performance of the coating.

[0091] This experiment used a 3D scanner to acquire three-dimensional images. 3D scanners are currently the most commonly used digital model acquisition tool in dentistry, especially in prosthodontics and orthodontics. Compared to intraoral scanners, extraoral scanners offer a wider field of view and higher precision. Compared to the 75 μm voxel size of CBCT, dental 3D scanners have a smaller voxel size, approximately 4 μm, and eliminate the potential damage to the sample caused by radiation, making them an ideal method for three-dimensional visualization. Scanning accuracy consists of two parts: trueness and precision. Trueness is the deviation in size between the digital model in the dental 3D scanner scan file and the actual sample. According to ISO 5725-1 [I. 5725e1: Accuracy (trueness and precision) of measurement methods and results Part 1: general principles and definitions (ISO 5725e1:1994), 1994.], precision is the deviation between multiple repeated scans (including repeated scans using the same equipment and scans performed by different personnel). The dental 3D scanner used in this experiment has been verified by Zhuang et al. to have high realism and accuracy, both within clinically acceptable ranges. Therefore, the model dimensions in this experiment are highly reliable.

[0092] This experiment has preliminarily verified the applicability of dental 3D scanning in corrosion morphology assessment. Furthermore, digital models are commonly used in dentistry for finite element analysis and 3D fitting comparisons. If these functions are applied to magnesium alloy research in the future, their high precision will allow for non-invasive, refined fitting and comparison of coatings in the early stages of corrosion or those difficult to observe, based on subtle structural differences before and after corrosion, thereby revealing potential corrosion mechanisms. This provides a rapid and economical assessment method. Moreover, digital models are commonly used in bone implant research; using an extraoral scanner to create digital models of samples will avoid errors caused by manual model construction, representing a groundbreaking innovation. We also plan to apply the above scheme in subsequent experimental designs to complete related experimental studies and further verify its feasibility.

[0093] 4. Mechanical properties (1) Hardness test The table shows the Knoop and Vickers hardness for each group. The Knoop and Vickers hardness of bare magnesium were 57.3 and 61.2, respectively, while the Knoop and Vickers hardness of the DCUT-E1 group increased to 73.2 and 61.9, respectively. With increasing power, the hardness also increased, reaching 79.8 Hk and 65.5 Hv for the DCUT-E3 group. Considering the slight increase in coating hardness in each group, and given that coating uniformity and density affect hardness, the DCUT-formed coating is more uniform and dense, resulting in higher hardness. Increased hardness improves the implant's resistance to bending and compression, making it less prone to deformation or damage under physiological loads. Simultaneously, a harder bone implant surface better resists wear, reducing wear particles generated during use.

[0094] Table 3. Knoop hardness and Vickers hardness for each group

[0095] (2) Three-point bending test After immersion in physiological saline for 0, 7, and 14 days, the stress-strain curves of all groups of samples exhibited typical brittle fracture characteristics. Before immersion, [the samples were prepared by...]. Figure 15 (a) It can be seen that there are no significant differences among the groups, and all groups have high bending strength, indicating that the DCUT coating can also give the bone plate good bending performance. Good bending performance helps to provide good support for the fracture site. After soaking for 7 days, if... Figure 15 (b) The flexural strength of the DCUT group decreased the least compared with that of the immersion for 0 days. The flexural strength of the DCUT-E1, DCUT-E2 and DCUT-E3 groups increased in sequence, reaching 326.01 MPa, 378.27 MPa and 407.73 MPa respectively. The other groups decreased significantly, and there was no significant difference between the groups. Figure 15 (c) shows the stress-strain curves after 14 days of immersion, showing an improvement of nearly 101.9%. This result is likely due to DCUT significantly reducing the corrosion rate of the sample, allowing it to maintain high integrity after 14 days of immersion, thus preserving good flexural strength and effectively preventing implant failure caused by a rapid decrease in mechanical integrity. These results indicate that the coating formed by DCUT can effectively improve the mechanical properties of magnesium alloy samples, such as flexural strength.

[0096] 5. Cell testing (1) Cell viability test As shown in Figure 16 (left), there were significant differences in the OD values ​​of MC3T3-E1 cells after 72 h of culture in the extracts of different groups. Compared with the control group, the OD value of the bare group was significantly lower (p<0.001), indicating strong cytotoxicity. This result is consistent with the study by UM et al., suggesting that excessive Mg may be released during degradation. 2+ This may lead to a local alkaline environment, thereby inhibiting the proliferation of MC3T3-E1 cells. In contrast, the HF, HFU, and DCUT groups showed significantly higher cell viability than the bare group (p<0.001), indicating improved cell compatibility. The results of relative cell viability (Figure 16 right) are consistent with the OD trend. The relative cell viability of the bare group was only about 60%, while the HF and HFU groups showed improvement (about 75–95%, p<0.001 vs. bare), consistent with the study by Pan et al. Notably, the cell viability of both the DCUT-E2 and DCUT-E3 groups exceeded 100%, with DCUT-E3 approaching 120% (p<0.001), showing a significant proliferative effect. This may be due to the dense and uniform coating slowing down the degradation rate of magnesium alloy, maintaining a stable and mild microenvironment, preserving a certain hydrophobic surface, reducing cytotoxicity, and simultaneously... - The uniform release of the substance imparts certain biological activity to the surface, providing a favorable microenvironment for osteoblast adhesion and proliferation.

[0097] (2) Staining of the cytoskeleton of phalloidin Figure 17 The results of F-actin and nuclear fluorescence staining of MC3T3-E1 on the surface of each group of samples are shown. Typically, when cells come into contact with biomaterials, they can alter their morphology to achieve material-cell integration. Sun et al. found that elongated osteoblast morphology often favors osteogenic differentiation and promotes intercellular communication to coordinate their behavior. In the Bare group, the number of cells was low, the F-actin fluorescence signal was weak and sparsely distributed, and the cell morphology appeared atrophic. This phenomenon is mainly attributed to excessive Mg²⁺. +Release of the fluorinated material led to an increase in the pH of the culture medium, thereby inhibiting cell growth. Compared with the Bare group, the HF group showed an increase in cell number, and F-actin stress fibers gradually became visible. However, due to the porous defects in the coating, which provided channels for corrosion ions and exacerbated the corrosion of the magnesium alloy, this limited the spread of the cytoskeleton to some extent. The HFU group showed stronger F-actin signaling and better cell spreadability, indicating that ultrasound-assisted fluorination treatment can improve the compactness of the fluorinated coating and enhance cell compatibility. In the DCUT treatment groups, the cell number in the DCUT-E2 and DCUT-E3 groups was significantly increased compared with the HFU group. The cytoskeleton structure remained intact, almost covering the entire field of view, exhibiting a polygonal morphology, and accompanied by a large number of filamentous pseudopodia and plate pseudopodia. This cytoskeleton structure helps cells adhere firmly to the material surface and enhances their ability to resist blood shear stress. When mammalian cells are cultured on a matrix, they typically undergo a cell adhesion process, including attachment, spread, cytoskeleton development, survival, and proliferation. Therefore, coatings with good cell adhesion can also promote cell proliferation, which is consistent with the experimental results of cell viability. In contrast, the cytoskeleton extensibility of the DCUT-E1 group was not as good as that of the DCUT-E2 and DCUT-E3 groups. This may be due to insufficient ultrasonic power, which resulted in obvious pores in the coating, thus slightly inhibiting cell proliferation.

[0098] 6. Micro-CT Animal Experiments To evaluate the osseointegration and degradation behavior of magnesium alloy implants with different coatings in vivo, we performed micro-CT imaging analysis on samples implanted in a rat femoral defect model. Figure 18 The results showed significant differences in bone healing and material degradation among the experimental groups on days 14 and 35 post-implantation. These differences were highly consistent with the coating performance revealed in previous in vitro studies and had a clear causal relationship. The fracture control group exhibited slow and incomplete bone healing throughout the observation period, providing a necessary pathological baseline for the experiment and confirming that self-healing alone is insufficient for critical-sized bone defects without the mechanical support and bioactive stimulation of the implant. The rapid implant disintegration, severe bone resorption, and repair failure observed in the Bare group directly reflected the uncontrolled degradation kinetics in vivo. This corresponds to the in vitro mass loss and alarming corrosion rate in pH testing. This rapid degradation not only led to premature loss of implant mechanical integrity, failing to provide a stable mechanical environment for fracture healing, but also resulted in excessive hydrogen production, creating a gaseous shadow and accumulating high concentrations of OH. -This creates an alkaline microenvironment unfavorable to osteoblast survival and function, ultimately leading to repair failure. While the bone healing in the HF and HFU groups was better than that in the Bare group, it still had limitations. The fundamental reason lies in the inherent structural defects of the coating: the HF group showed numerous pores in in vitro SEM, which became corrosive media (such as Cl-) in vivo. - The intrusion of tissue fluid (and other substances) leads to a significant decline in its protective performance after 35 days of implantation, accelerating matrix degradation and thus interrupting the bone healing maturation process. The HFU group, benefiting from the relatively dense coating formed in vitro by the ultrasonic cavitation effect, exhibited more continuous new bone formation and a more stable implant structure in vivo than the HF group. However, its "medium-borne sound transmission" mode resulted in energy attenuation and sound field inhomogeneity, limiting its ability to maintain protective efficacy in long-term dynamic physiological environments. The DCUT treatment group of this invention exhibits significant and expected advantages. Its superior in vivo performance is directly attributed to the dense MgF2 coating prepared by the "sample-sound source" direct-coupled vibration mode and the sweep frequency-pulse temperature control strategy. This coating has been shown in vitro to have the lowest porosity, optimal corrosion resistance (lowest mass loss, highest pH stability), and best biocompatibility (cell viability exceeding 100%). In vivo, this high-performance coating effectively regulated the degradation rate of the magnesium alloy. In the first 14 days, the dense coating effectively blocked the erosion of body fluids, creating a stable microenvironment for the DCUT-E2 and E3 groups, allowing for the rapid formation of a large number of dense and well-connected calluses, achieving early bone bridging. At 35 days, the continuous and slow degradation of the coating provided adequate magnesium ions to the newly formed bone, potentially playing an osteogenic induction role, while maintaining structural integrity and avoiding catastrophic damage similar to that in the Bare group. Ultimately, the DCUT-E3 group, due to its highest ultrasound power, formed the most perfect coating structure, achieving optimal synergy between degradation rate and bone regeneration rate, thus achieving near-complete bone healing, with the newly formed bone most closely resembling normal bone tissue in terms of volume, density, and structure.

Claims

1. A method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct coupled ultrasound, characterized in that, The magnesium alloy was tightly connected to the ultrasonic oscillation generation part of the direct-coupled ultrasonic reaction fixture. Then, the magnesium alloy was immersed in a sealed plastic beaker containing hydrofluoric acid solution and ultrasonically treated for 5 hours. Subsequently, the treated sample was ultrasonically cleaned three times with anhydrous ethanol and distilled water respectively and then dried.

2. The method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound according to claim 1, characterized in that, The ratio of the hydrofluoric acid solution to the surface area of ​​the magnesium alloy is 3-6 ml / cm². 2 .

3. The method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound according to claim 1, characterized in that, The concentration of the hydrofluoric acid solution is 40-55 wt%.

4. The method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound according to claim 1, characterized in that, The direct-coupled ultrasonic reaction fixture includes a rear metal block, a transducer, a power supply, and a front metal block connected in sequence, with a trapezoidal amplitude transformer connected to the front end of the front metal block.

5. The method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound according to claim 1, characterized in that, The transducer uses Pb(Zr,Ti)O3 ceramic sheets.

6. The method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct coupled ultrasound according to claim 1, characterized in that, The power of the directly coupled ultrasonic reactive jig during processing is 40 W, 68 W, and 96 W.

7. The method for preparing a magnesium fluoride coating on a magnesium alloy surface using direct-coupled ultrasound according to claim 1, characterized in that, The transducer operates at a frequency sweep of 30-45kHz, with a pulse duty cycle of 30-70% and a peak amplitude of 2-8μm; temperature control keeps temperature fluctuations within ±1℃.

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