Method for improving super-hydrophobicity of nickel-titanium alloy by laser composite machining

Abstract

A method for improving superhydrophobicity of nickel-titanium alloy by laser composite processing: selecting a sample for pretreatment: selecting a nickel-titanium alloy sample and polishing to remove edge burrs; cleaning the sample; drying the sample; laser composite inducing the surface of the pretreated nickel-titanium alloy sample, using femtosecond laser and nanosecond laser to texture the surface of the nickel-titanium alloy in turn, forming a multilevel nested grid-like micro-nano composite structure; configuring a fluorosilane / ethanol solution for reducing the surface energy of the laser composite induced nickel-titanium alloy sample; chemical modification of the surface of the laser composite induced nickel-titanium alloy sample: placing the nickel-titanium alloy sample in the fluorosilane / ethanol solution, sealing and standing, and then drying. The present application obtains a multi-scale and multi-level micro-nano structure by femtosecond and nanosecond laser texturing on the surface of the nickel-titanium alloy sample, effectively increases the surface roughness of the nickel-titanium alloy sample, and obtains excellent superhydrophobicity and improved corrosion resistance.

Description

TECHNICAL FIELD

[0001] The application relates to a method for improving the superhydrophobicity of a nickel-titanium alloy. BACKGROUND

[0002] Nickel-titanium alloy has excellent super-elasticity, shape memory effect, biocompatibility and damping characteristics, and has been widely evaluated in the medical field such as orthodontics, brackets and bone implants. With the further development of science and technology, the application of nickel-titanium alloy in various fields has also been expanded. Especially when it is applied to the fields of aerospace, electricity and energy, it is exposed to high temperature and harsh environment, which provides new challenges for nickel-titanium alloy. It is well known that the corrosion resistance of nickel-titanium alloy is comparable to that of stainless steel, and the stable and dense titanium oxide passivation film formed on the surface of the material can prevent further corrosion of the matrix, thereby providing excellent corrosion resistance. At present, the rough surface manufactured by various technologies provides more "air holes", which effectively improves the superhydrophobicity of the material surface. At the same time, the dense micro-nano structure formed by laser ablation completely isolates the nickel-titanium alloy substrate from the corrosion medium, further improving the corrosion resistance. Therefore, improving the surface superhydrophobicity is the basis for the sustainable use of nickel-titanium alloy.

[0003] With the continuous exploration of micro-nano technology, constructing surface micro-nano structure has become an effective method to enhance the performance of the surface, especially nanosecond and femtosecond laser processing technology, which is often used for surface treatment of stainless steel, nickel-titanium memory alloy, magnesium alloy and aluminum alloy. At present, there is no related report on the use of femtosecond and nanosecond laser composite processing to improve the superhydrophobicity of nickel-titanium alloy.

[0004] A new type of laser processing method, i.e. composite femtosecond and nanosecond laser processing technology. By femtosecond and nanosecond laser, a multi-level nested grid-shaped micro-nano composite structure is formed on the surface of the nickel-titanium alloy, which further increases the surface roughness of the material. At the same time, the use of fluorosilane / ethanol solution reduces the surface free energy of the nickel-titanium alloy. Therefore, the formation of rough surface and the reduction of surface energy comprehensively make the nickel-titanium alloy obtain excellent superhydrophobicity. SUMMARY

[0005] The technical problem to be solved by the present application is to overcome the shortcomings of the prior art and provide a method for improving the superhydrophobicity of nickel-titanium alloy by laser composite processing of composite femtosecond and nanosecond laser processing technology.

[0006] The technical solution adopted by the present application is: a method for improving the superhydrophobicity of a nickel-titanium alloy by laser composite processing, comprising the following steps:

[0007] 1) Selecting the sample for pretreatment, including: selecting a nickel-titanium alloy sample, polishing the sample, removing the edge burrs; cleaning the sample; drying the sample;

[0008] 2) Laser composite induced on the surface of the pretreated nickel-titanium alloy sample, using femtosecond laser and nanosecond laser to texture the surface of the nickel-titanium alloy in turn, forming a multi-level nested grid-like micro-nano composite structure;

[0009] 3) Configuring a fluorosilane / ethanol solution for reducing the surface energy of the laser composite induced nickel-titanium alloy sample;

[0010] 4) Chemical modification of the surface of the laser composite induced nickel-titanium alloy sample, including: placing the nickel-titanium alloy sample in the fluorosilane / ethanol solution, sealing and standing for 1-2 h, and then drying.

[0011] Step 1) specifically includes:

[0012] (1) Selecting a nickel-titanium alloy and cutting it into more than 4 nickel-titanium alloy samples;

[0013] (2) Using a metallographic sample grinder and a polisher to polish each nickel-titanium alloy sample to a mirror surface one by one and remove the edge burrs formed by cutting;

[0014] (3) Laying the polished nickel-titanium alloy sample in a beaker, pouring acetone reagent to cover the sample surface, and using an ultrasonic cleaner to clean; after finishing, pouring out the acetone reagent, pouring anhydrous ethanol solution into the beaker, and using an ultrasonic cleaner to clean; after finishing, pouring out the ethanol solution, pouring deionized water into the beaker, and using an ultrasonic cleaner to clean.

[0015] (4) Laying the cleaned nickel-titanium alloy sample on a beaker or dust-free paper and drying it in a drying oven at 20-60℃ to complete the pretreatment of the nickel-titanium alloy sample.

[0016] Step 2) includes: first using a femtosecond laser to draw a grid pattern with a minimum unit of 20-30μm on each nickel-titanium alloy sample; then using a nanosecond laser to draw a grid pattern with a minimum unit of 100-300μm on each nickel-titanium alloy sample; specifically as follows:

[0017] (1) Placing the pretreated nickel-titanium alloy sample on the three-dimensional motion platform of the femtosecond laser, using MarkingMate control software to adjust the femtosecond laser focus to the surface of the nickel-titanium alloy sample, and drawing a grid pattern with a minimum unit of 20-30μm;

[0018] (2) Put the femtosecond laser textured nickel-titanium alloy sample on the nanosecond laser processing platform, adjust the laser focus to the surface of the nickel-titanium alloy sample, and use Han's LASER Marking control software to adjust the nanosecond laser to draw a grid pattern with a minimum unit of 100-300 μm.

[0019] The center wavelength of the femtosecond laser is 1064 nm, the pulse width is 340 fs, the maximum output power is 56 W, and the minimum focused spot diameter is 10 μm; the femtosecond laser processing parameters are set as follows: average power 8-12 W, scanning speed 200-400 mm / s, and scanning times 5-10; the center wavelength of the nanosecond laser is 1064 nm, the pulse width is 50 ns, the maximum output power is 20 W, the repetition frequency is 20 kHz, and the minimum focused spot diameter is 50 μm; the nanosecond laser processing parameters are set as follows: average power 12-14 W, scanning speed 400-600 mm / s, and scanning times 10-15.

[0020] Step 3) comprises:

[0021] (1) Take two measuring cylinders to measure 62.7 mL of anhydrous ethanol and 19.2 mL of deionized water, respectively, and pour them into a beaker in sequence; place the beaker containing the mixed solution on a balance, and slowly drop 0.5 g of fluoroalkylsilane reagent into the beaker using a rubber dropper;

[0022] (2) Take the beaker off, put a magnetic stirrer in the beaker, and then place it on a magnetic stirrer for stirring for 12 h or more;

[0023] (3) After standing for 1-2 h, it is ready for use. The mixed solution is called fluoroalkylsilane / ethanol solution.

[0024] Step 4) comprises:

[0025] (1) Place the laser composite induced nickel-titanium alloy sample in a glassware, with the processing surface facing up, pour the fluoroalkylsilane / ethanol solution into the glassware until the surface of the nickel-titanium alloy sample is covered, seal it and stand for 1-2 hours;

[0026] (2) Take out the nickel-titanium alloy sample using tweezers, lay it on a dust-free paper, and place it in a drying oven at 100-120°C for 1-2 hours.

[0027] The method for improving super-hydrophobicity of nickel-titanium alloy by laser composite machining can give full play to the advantages of femtosecond laser manufacturing technology, such as high resolution, high precision and repeatability, and the advantages of nanosecond laser, such as flexibility and high efficiency, and can design micro-nano structures with super-hydrophobic function in combination with the bionic manufacturing principle. The fluorosilane / ethanol solution is combined to reduce the surface free energy of the material. The combination of laser composite machining and silanization treatment process comprehensively makes the surface of the nickel-titanium alloy have excellent super-hydrophobicity, which has great application value in the fields of aerospace and industry.

[0028] The method for improving super-hydrophobicity of nickel-titanium alloy by laser composite machining can give full play to the advantages of femtosecond laser manufacturing technology, such as high resolution, high precision and repeatability, and the advantages of nanosecond laser, such as flexibility and high efficiency, and can design micro-nano structures with super-hydrophobic function in combination with the bionic manufacturing principle. The fluorosilane / ethanol solution is combined to reduce the surface free energy of the material. The combination of laser composite machining and silanization treatment process comprehensively makes the surface of the nickel-titanium alloy have excellent super-hydrophobicity, which has great application value in the fields of aerospace and industry. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is a schematic diagram of a femtosecond laser machining system and a laser scanning path;

[0030] Figure 2 is a schematic diagram of a nanosecond laser machining system and a laser scanning path;

[0031] Figure 3 is a schematic diagram of the configuration process of a fluorosilane / ethanol solution;

[0032] Figure 4 is a schematic diagram of the preparation process of a laser-induced nickel-titanium alloy sample to obtain a super-hydrophobic surface;

[0033] Figure 5 is a scanning electron microscope image of a laser-induced nickel-titanium alloy sample;

[0034] Figure 6 is the measurement result of the contact angle and the rolling angle of the surface of a nickel-titanium alloy sample; DETAILED DESCRIPTION

[0035] The method for improving super-hydrophobicity of nickel-titanium alloy by laser composite machining will be described in detail below in combination with embodiments and drawings.

[0036] The method for improving superhydrophobicity of nickel-titanium alloy by laser composite machining of the application comprises the following steps:

[0037] The method for improving superhydrophobicity of nickel-titanium alloy by laser composite machining of the application comprises the following steps:

[0038] 1) Selecting a sample for pretreatment, comprising: selecting a nickel-titanium alloy sample, polishing the sample to remove edge burrs, cleaning the sample, and drying the sample, specifically comprising:

[0039] (1) Selecting a nickel-titanium alloy and cutting it into more than four nickel-titanium alloy samples;

[0040] (2) Polishing each nickel-titanium alloy sample to a mirror surface using a metallographic sample grinder and polisher, and removing edge burrs formed by cutting;

[0041] (3) Laying the polished nickel-titanium alloy sample flat in a beaker, pouring acetone reagent to cover the sample surface, and cleaning it with an ultrasonic cleaner for 15 minutes in this embodiment; after the end, pour out the acetone reagent, pour anhydrous ethanol solution into the beaker, and clean it with an ultrasonic cleaner for 15 minutes in this embodiment; after the end, pour out the ethanol solution, pour deionized water into the beaker, and clean it with an ultrasonic cleaner for 15 minutes in this embodiment. Thus, the oil and impurities on the surface of the nickel-titanium alloy sample are removed;

[0042] (4) Laying the cleaned nickel-titanium alloy sample flat on a beaker or dust-free paper, and drying it in a drying oven at 20-60℃ for 30 minutes in this embodiment to complete the pretreatment of the nickel-titanium alloy sample.

[0043] 2) Laser composite induction of the nickel-titanium alloy surface of the pretreated nickel-titanium alloy sample, using femtosecond laser and nanosecond laser to texture the nickel-titanium alloy surface in turn to form a multi-level nested grid-like micro-nano composite structure. Comprising: first, using a femtosecond laser to draw a grid-like pattern with a minimum unit of 20-30μm on each nickel-titanium alloy sample; then using a nanosecond laser to draw a grid-like pattern with a minimum unit of 100-300μm on each nickel-titanium alloy sample; specifically as follows:

[0044] (1) the pretreated nickel-titanium alloy sample is placed on a three-dimensional motion platform of a femtosecond laser, a MarkingMate control software is used to adjust the femtosecond laser focus to the surface of the nickel-titanium alloy sample, and a grid pattern with a minimum unit of 20-30 μm is drawn; the structure pattern of the femtosecond laser texturing is a small grid arranged closely, and the characteristic is that the femtosecond laser performs traversal ablation on the surface of the sample, and all regions of the surface of the nickel-titanium alloy sample are irradiated by the laser;

[0045] (2) the nickel-titanium alloy sample textured by the femtosecond laser is placed on a nanosecond laser processing platform, the laser focus is adjusted to the surface of the nickel-titanium alloy sample, and a Han's LASER Marking control software is used to adjust the nanosecond laser to draw a grid pattern with a minimum unit of 100-300 μm; the structure pattern of the nanosecond laser texturing is a large grid, and the characteristic is that it is composed of a nanosecond laser irradiation region and a non-irradiation region, and the non-irradiation region will be nested with the dense micro-nano structure formed by the femtosecond laser irradiation.

[0046] The model of the femtosecond laser used in the embodiment of the application is FemtoYL-40, the central wavelength is 1064 nm, the pulse width is 340 fs, the maximum output power is 56 W, and the minimum focusing spot diameter is 10 μm; the femtosecond laser processing parameters are set as follows: the average power is 8-12 W, the scanning speed is 200-400 mm / s, and the scanning times are 5-10.

[0047] The model of the nanosecond laser used in the embodiment of the application is IPG-YLP-20, the central wavelength is 1064 nm, the pulse width is 50 ns, the maximum output power is 20 W, the repetition frequency is 20 kHz, and the minimum focusing spot diameter is 50 μm; the nanosecond laser processing parameters are set as follows: the average power is 12-14 W, the scanning speed is 400-600 mm / s, and the scanning times are 10-15.

[0048] 3) a fluorosilane / ethanol solution for reducing the surface energy of the laser composite induced nickel-titanium alloy sample is configured; the solution comprises:

[0049] (1) two measuring cylinders are taken to measure 62.7 mL of anhydrous ethanol and 19.2 mL of deionized water respectively, and the two are sequentially poured into a beaker for mixing; the beaker containing the mixed solution is placed on a balance, and a rubber dropper is slowly dripped into 0.5 g of a fluorosilane reagent;

[0050] (2) the beaker is taken down, a magnetic stirrer is placed in the beaker, and then the beaker is placed on a magnetic stirrer for stirring for 12 h or more;

[0051] (3) the mixed solution is used after being placed for 1-2 h, and the mixed solution is called a fluorosilane / ethanol solution.

[0052] 4) chemically modifying the surface of the laser-composite-induced NiTi alloy sample, including: placing the NiTi alloy sample in a fluorosilane / ethanol solution, sealing and standing for 1-2 h, and then drying; including:

[0053] (1) placing the laser-composite-induced NiTi alloy sample in a glass container with the processed surface facing upward, pouring the fluorosilane / ethanol solution to cover the surface of the NiTi alloy sample, sealing and standing for 1-2 h;

[0054] (2) using tweezers to take out the NiTi alloy sample and lay it on a dust-free paper, and then placing it in a drying oven at 100-120℃ for 1-2 h.

[0055] In order to verify the superiority of the method for improving the superhydrophobicity of a NiTi alloy by laser composite machining according to the present application, experiments of different methods can be carried out, and the results are measured and characterized as given below after each experiment, and according to the results of the measurement and characterization, it can be determined which method is the most superior.

[0056] Measuring and characterizing the laser-composite-induced NiTi alloy sample, including: measuring the contact angle and rolling angle of the surface of the NiTi alloy sample for more than 5 times, and calculating the average value of the contact angle and rolling angle; obtaining the surface morphology of the NiTi alloy sample; obtaining the surface roughness of the NiTi alloy sample; including:

[0057] (1) using a contact angle measuring instrument (model VCA Optima, AST Products, USA) to measure the contact angle and rolling angle of the surface of the NiTi alloy sample, using a CAM surface analysis measurement system to control the droplet volume to be 5-10 μL, and measuring each NiTi alloy sample for 5 times in turn to calculate the average value of the contact angle and rolling angle;

[0058] (2) characterizing the surface morphology of the NiTi alloy sample by a scanning electron microscope (model SEM TESCAN MI);

[0059] (3) measuring the surface roughness of the NiTi alloy sample by a three-dimensional optical profiler (model Bruker, NPFLEX).

[0060] The superiority of the method for improving the superhydrophobicity of a NiTi alloy by laser composite machining according to the present application is verified by four experimental groups as follows.

[0061] Experimental group one, denoted as NiTi-I: the treatment steps of the NiTi alloy sample are as follows:

[0062] 1) Pretreatment: The nickel-titanium alloy sample was polished and polished, and then sequentially cleaned with acetone, anhydrous ethanol and deionized water for 15 min. After cleaning, it was placed in a drying oven at 35°C and dried for 30 min.

[0063] 2) Measurement and characterization of the nickel-titanium alloy sample: The contact angle and roll angle of the surface of the nickel-titanium alloy sample were measured using a contact angle measuring instrument (model VCA Optima, AST Products, USA). The CAM surface analysis measurement system was used to control the droplet volume to be 6 μL. Each nickel-titanium alloy sample was measured 5 times in turn, and the average value of the contact angle and roll angle was calculated.

[0064] Group two, denoted as NiTi-Ⅱ: The treatment steps of the nickel-titanium alloy sample are as follows:

[0065] 1) Pretreatment: The nickel-titanium alloy sample was polished and polished, and then sequentially cleaned with acetone, anhydrous ethanol and deionized water for 15 min. After cleaning, it was placed in a drying oven at 35°C and dried for 30 min.

[0066] 2) Laser-induced surface modification of the pretreated nickel-titanium alloy sample: Femtosecond laser-induced nickel-titanium alloy sample: The pretreated nickel-titanium alloy sample was placed on the three-dimensional motion platform of the femtosecond laser, and the MarkingMate control software was used to adjust the femtosecond laser focus to the surface of the nickel-titanium alloy and draw a grid pattern with a minimum unit of 25 μm. The femtosecond laser processing parameters were set as follows: average power 10 W, scanning speed 250 mm / s, and scanning times 5.

[0067] 3) Preparation of fluorosilane / ethanol solution for reducing the surface energy of the laser-induced nickel-titanium alloy sample: Two measuring cylinders were used to measure 62.7 mL of anhydrous ethanol and 19.2 mL of deionized water, respectively, and then mixed in a beaker. The beaker containing the mixed solution was placed on a balance, and then a rubber dropper was slowly added to 0.5 g of fluorosilane reagent. The beaker was removed and placed in a magnetic stirrer, and then placed on a magnetic stirrer for 12 h. After standing for 2 h, the mixed solution was ready for use, and the mixed solution was called fluorosilane / ethanol solution.

[0068] 4) Chemical modification of the surface of the laser-induced nickel-titanium alloy sample: The femtosecond laser-induced nickel-titanium alloy sample was placed in a glass container with the processed surface facing up, and the prepared fluorosilane / ethanol solution was poured into the nickel-titanium alloy sample surface until the surface was covered. Seal and stand for 2 h. The nickel-titanium alloy sample was removed with tweezers and laid on a dust-free paper, and placed in a 110°C drying oven for 2 h.

[0069] At this point, the surface free energy of the femtosecond laser-induced nickel-titanium alloy sample has been reduced by the above method, and the surface has obtained super-hydrophobic properties.

[0070] 5) Measurement and characterization of the laser-induced nickel-titanium alloy samples:

[0071] (1) The contact angle and the roll-off angle of the surface of the nickel-titanium alloy samples were measured using a contact angle meter (model VCA Optima, AST Products, USA), using the CAM Surface Analysis Measurement System to control the droplet volume to be 6 μL, and each nickel-titanium alloy sample was measured 5 times in succession, and the average values of the contact angle and the roll-off angle were calculated;

[0072] (2) The surface morphology of the nickel-titanium alloy samples was characterized using a scanning electron microscope (model SEM TESCAN MI);

[0073] (3) The surface roughness of the nickel-titanium alloy samples was measured using a three-dimensional optical profilometer (model Bruker, NPFLEX).

[0074] Group three, denoted as NiTi-III: the treatment steps for the nickel-titanium alloy samples were as follows:

[0075] 1) Pretreatment: the nickel-titanium alloy samples were polished and polished, and then ultrasonically cleaned with acetone, anhydrous ethanol and deionized water for 15 min, and then placed in a drying oven at 35°C for 30 min.

[0076] 2) Laser induction on the surface of the pretreated nickel-titanium alloy sample: nanosecond laser-induced nickel-titanium alloy sample: the pretreated nickel-titanium alloy sample was placed on the processing platform of the nanosecond laser, the laser focal point was adjusted to the surface of the nickel-titanium alloy sample, and the Han's LASER Marking control software was used to draw a grid pattern with a minimum unit of 200 μm, and the nanosecond laser processing parameters were set as follows: average power 12 W, scanning speed 500 mm / s, and scanning times 10.

[0077] 3) Preparation of a fluorosilane / ethanol solution for reducing the surface energy of the laser-induced nickel-titanium alloy sample: two measuring cylinders were used to measure 62.7 mL of anhydrous ethanol and 19.2 mL of deionized water, respectively, and then mixed in a beaker. The beaker containing the mixed solution was placed on a balance, and then a rubber dropper was slowly added to 0.5 g of fluorosilane reagent. The beaker was removed and placed in a magnetic stirrer, and then placed on a magnetic stirrer for 12 h or more. After standing for 2 h, the mixed solution was ready for use, and the mixed solution was called a fluorosilane / ethanol solution.

[0078] 4) Chemical modification of the laser-induced nickel-titanium alloy sample surface: Place the nanosecond-induced nickel-titanium alloy sample in a glassware, with the processed surface facing up, pour the prepared fluorosilane / ethanol solution to cover the surface of the nickel-titanium alloy sample, seal and stand for 2 h. Take out the nickel-titanium alloy sample using tweezers and lay it on a dust-free paper, and place it in a 110°C drying oven for 2 h.

[0079] By the above method, the surface free energy of the nanosecond laser-induced nickel-titanium alloy sample is reduced, and the surface obtains super-hydrophobic properties.

[0080] 5) Measurement and characterization of the laser-induced nickel-titanium alloy sample:

[0081] (1) Measure the contact angle and roll-off angle of the surface of the nickel-titanium alloy sample using a contact angle measuring instrument (model VCA Optima, AST Products, USA), use the CAM surface analysis measurement system to control the droplet volume to be 6 μL, measure each nickel-titanium alloy sample for 5 times in turn, and calculate the average value of the contact angle and roll-off angle;

[0082] (2) Characterize the surface morphology of the nickel-titanium alloy sample using a scanning electron microscope (model SEM TESCAN MI);

[0083] (3) Measure the surface roughness of the nickel-titanium alloy sample using a three-dimensional optical profilometer (model Bruker, NPFLEX).

[0084] Experiment group four, NiTi-IV: The treatment steps of the nickel-titanium alloy sample are as follows:

[0085] 1) Pretreatment: polish and polish the nickel-titanium alloy sample, and ultrasonically clean it with acetone, anhydrous ethanol and deionized water in turn for 15 min, and then place it in a drying oven at 35°C for 30 min after cleaning.

[0086] 2) Laser-induced surface of the pretreated nickel-titanium alloy sample: composite laser-induced nickel-titanium alloy sample: Place the pretreated nickel-titanium alloy sample on the three-dimensional motion platform of the femtosecond laser, adjust the femtosecond laser focus point to the surface of the nickel-titanium alloy using the MarkingMate control software and draw a grid pattern with a minimum unit of 25 μm, set the femtosecond laser processing parameters: average power 10 W, scanning speed 250 mm / s, scanning times 5. Then place the femtosecond laser-induced nickel-titanium alloy sample on the processing platform of the nanosecond laser, adjust the laser focus point to the surface of the nickel-titanium alloy sample, draw a grid pattern with a minimum unit of 200 μm using the Han's LASER Marking control software, set the nanosecond laser processing parameters: average power 12 W, scanning speed 500 mm / s, scanning times 10.

[0087] 3) Preparation of fluorosilane / ethanol solution for reducing the surface energy of laser-induced NiTi alloy sample: Take two graduated cylinders to measure 62.7 mL of anhydrous ethanol and 19.2 mL of deionized water respectively, and pour them into a beaker in sequence. Place the beaker containing the mixed solution on a balance, then take a rubber dropper and slowly drop 0.5 g of fluorosilane reagent into it. Take the beaker off, put a magnetic stirrer in it, and then place it on a magnetic stirrer for 12 h or more. After standing for 2 h, the mixed solution is ready for use, which is called fluorosilane / ethanol solution.

[0088] 4) Chemical modification of the surface of the laser-induced NiTi alloy sample: Place the nanosecond-induced NiTi alloy sample in a glassware with the processed surface facing up, pour the prepared fluorosilane / ethanol solution into it until it covers the surface of the NiTi alloy sample, seal it and stand for 2 h. Take out the NiTi alloy sample using tweezers and lay it on a dust-free paper, and then place it in a 110°C drying oven for 2 h.

[0089] At this point, the surface free energy of the laser-induced NiTi alloy sample has been reduced by the above method, and the surface has obtained super-hydrophobic properties.

[0090] 5) Measurement and characterization of the laser-induced NiTi alloy sample:

[0091] (1) Use a contact angle measuring instrument (model VCA Optima, AST Products, USA) to measure the contact angle and roll angle of the surface of the NiTi alloy sample, use the CAM surface analysis measurement system to control the droplet volume to be 6 μL, measure each NiTi alloy sample 5 times in sequence, and calculate the average value of the contact angle and roll angle;

[0092] (2) Use a scanning electron microscope (model SEM TESCAN MI) to characterize the surface morphology of the NiTi alloy sample;

[0093] (3) Use a three-dimensional optical profilometer (model Bruker, NPFLEX) to measure the surface roughness of the NiTi alloy sample.

[0094] Next, the measurement and characterization results of NiTi-I, NiTi-II, NiTi-III and NiTi-IV are analyzed:

[0095] Figure 1 The schematic diagram of the femtosecond laser processing system and the laser scanning path is shown, Figure 2The processing system and laser scanning path schematic diagram of the nanosecond laser are shown. Two different types of lasers are used to manufacture multi-level nested grid-like micro-nano composite structures on the surface of a nickel-titanium alloy sample. The femtosecond laser and nanosecond laser parameters are shown in Table 1. The femtosecond laser processing parameters are set to an average power of 10 W, a scanning speed of 250 mm / s, a scanning number of 5 times, and a minimum grid unit of 25 μm. The nanosecond laser processing parameters are set to an average power of 12 W, a scanning speed of 500 mm / s, a scanning number of 10 times, and a minimum grid unit of 200 μm.

[0096] Table 1 Processing parameters of femtosecond laser and nanosecond laser

[0097]

[0098] Figure 3 The configuration process schematic diagram of the fluoro-silane / ethanol solution is shown, Figure 4 The preparation process schematic diagram of the laser-induced nickel-titanium alloy sample to obtain a super-hydrophobic surface is shown. The key steps for the preparation of the super-hydrophobic surface are to immerse the femtosecond laser-induced nickel-titanium alloy sample, the nanosecond laser-induced nickel-titanium alloy sample, and the laser-composite-induced nickel-titanium alloy sample in a fluoro-silane / ethanol solution for 2 hours, and then dry them in a 110°C drying oven for 2 hours for subsequent testing and analysis.

[0099] Figure 5 The scanning electron microscope images of the laser-induced nickel-titanium alloy sample are shown. Figure 5 Figure (a) in the figure shows the microstructure and nanostructure obtained by femtosecond laser induction. Since the minimum grid unit size is only 25 μm and the focus point size is 18 μm, the femtosecond laser ablates the entire area of the nickel-titanium alloy surface, presenting a continuous corrugated microstructure. Figure 5 Figure (b) in the figure shows the microstructure manufactured by the nanosecond laser. It can be seen that under the ablation of the nanosecond laser, the surface of the nickel-titanium alloy forms a large-depth groove, and the size of the minimum grid unit is greater than 200 μm, while the size of the focus point is about 50 μm, so a neat and clear grid structure is presented. Figure 5 Figure (c) in the figure shows the continuous processing of the femtosecond laser and the nanosecond laser, i.e. Figure 5 Figure (a) and figure (b) in the figure are superimposed, thereby presenting a multi-level micro-nano structure. Figure 5 Figure (d) in the figure is a partial enlarged view corresponding to Figure 5 Figure (a) in the figure, clearly showing that the continuous corrugated microstructure is composed of neatly arranged shallow pits. Figure 5 Figure (e) in the figure shows a corresponding Figure 5A magnified view of a portion of Figure (b) shows the smallest grid cell refined into three parts: a groove with a perimeter width of approximately 50 μm, a ridge formed by the superposition of the groove and the unprocessed area, and the unprocessed area. Furthermore, due to the intense ablation by the nanosecond laser, a large number of micro / nanoparticles are sputtered and solidified in the unprocessed area. Figure 5 Figure (f) in the middle shows the relationship with Figure 5 The smallest element in the magnified image corresponding to (c) is... Figure 5 The grooves and ridges in diagram (d) are similar. A notable feature is the internal structure of the grid ( Figure 5 The unprocessed area in Figure (d) consists of shallow pits formed by femtosecond laser ablation. Laser-sputtered micro / nanoparticles are solidified on the ridges of these pits. Figure 5 Image (g) is a magnified view of a portion of image (d), clearly showing the wavy structure with "fingerprint" characteristics within the shallow pit. Figure 5 The middle (h) image is a magnified view of the unprocessed area in the (e) image, showing a large number of irregular sputtered micro / nanoparticles solidifying in this planar region. Figure 6 Image (i) is a magnified view of the femtosecond laser texture region in image (f). It can be seen that in this layered region, most of the micro / nanoparticles sputtered by nanosecond laser ablation are solidified on the ridges formed by femtosecond laser ablation, while only a small number of micro / nanoparticles are solidified in the pits, which are composed of a large number of dense nanoparticles.

[0100] Figure 6 The contact angle and roll-off angle measurements of the nickel-titanium alloy sample surface are shown. The surface wettability of the laser-induced nickel-titanium alloy sample was investigated by measuring the contact angle and roll-off angle. The droplet volume was controlled at 6 μL using a CAM surface analysis measurement system. To ensure accuracy, the contact angle and roll-off angle were measured at five different locations on the nickel-titanium alloy sample, and the average of the data was used as the final result. ​It can be seen that the surface of NiTi-I is hydrophilic with a contact angle value of 56.3 ± 5° and a roll-off angle over 90°. The laser-induced surfaces of NiTi-II, NiTi-III, and NiTi-IV are superhydrophobic with contact angle values of 157.7 ± 2.5°, 158.1 ± 2°, and 158.9 ± 2°, respectively, and roll-off angle values of 7.1 ± 2°, 6.7 ± 1.5°, and 6.2 ± 1.5°, respectively. It can be clearly seen that the contact angle values of the superhydrophobic surfaces are all close to 160° and the roll-off angle values are all less than 10°. These results indicate that the nickel-titanium alloy surface modified by laser processing and fluoro-silane / ethanol solution has good superhydrophobic properties. In addition, the superhydrophobic properties of the NiTi-IV surface induced by femtosecond laser and nanosecond laser are superior to those of the NiTi-III surface induced by only nanosecond laser and those of the NiTi-II surface induced by only femtosecond laser. This phenomenon indicates that the surface roughness has an effect on the wettability of the nickel-titanium alloy surface. The Wenzel model theoretically reveals the conversion mechanism of surface wettability: the increase of surface roughness will reduce the contact angle value of the intrinsic hydrophilic surface and increase the contact angle value of the intrinsic hydrophobic surface, as shown in equation (1).

[0101] cos θ ω = r cos θ f (1)

[0102] where r is the roughness factor, defined as the ratio of the physical surface area to the projected area of the solid surface. Obviously, the value of r is greater than 1 in the case of a rough surface. θ f and θ ω represent the intrinsic static contact angle and the rough surface contact angle of the solid surface, respectively. Equation (1) indicates that the increase of surface roughness will lead to the enhancement of surface wettability, which means that with the increase of roughness, the intrinsic hydrophilic surface is converted into a more hydrophilic surface and the intrinsic hydrophobic surface is converted into a more hydrophobic surface. Therefore, the nickel-titanium alloy surface, which is essentially hydrophilic, is converted into a superhydrophilic surface after laser processing. However, after laser processing and fluoro-silane / ethanol solution modification, the superhydrophilic surface is converted into a superhydrophobic surface due to the reduction of surface energy.

[0103] According to the characteristics of femtosecond laser and nanosecond laser processing, the surface ablated by nanosecond laser usually has a greater roughness. The results of surface roughness are shown in Table 2.

[0104] Table 2 Surface roughness of nickel-titanium alloy samples

[0105]

[0106] The surface roughness value of the NiTi-I sample without any treatment is only 0.3 μm. The surface roughness of the NiTi-II sample after femtosecond laser ablation slightly increases, and the surface roughness value is 2.9 μm. The surface roughness of the NiTi-III sample after nanosecond laser ablation greatly increases, and the surface roughness value rises to 16.7 μm. The surface roughness of the NiTi-IV sample etched by femtosecond and nanosecond lasers is about 17 times that of the NiTi-II sample and 3 times that of the NiTi-III sample, and the surface roughness value is 51.3 μm. This shows that the composite microstructure and nanostructure obtained by femtosecond and nanosecond laser processing effectively increase the surface roughness of the nickel-titanium alloy. The increase of the roughness makes the inherent hydrophilic surface more hydrophilic, and after the surface energy is reduced, it is converted into a more hydrophobic surface. Therefore, the theoretical analysis result of formula (1) is consistent with the experimental measurement result, and the surface roughness and superhydrophobicity of the nickel-titanium alloy are ranked as NiTi-IV > NiTi-III > NiTi-II > NiTi-I.

[0107] The beneficial effects of the embodiment are: a method for preparing a superhydrophobic surface of a nickel-titanium alloy sample by laser composite induction is proposed, which combines femtosecond laser, nanosecond laser composite process and fluorosilane / ethanol solution modification. Morphological characteristics and wettability measurement show that the surface modified by irradiation combined with fluorosilane / ethanol solution has good superhydrophobic performance. At the same time, the superhydrophobic performance is ranked as NiTi-IV > NiTi-III > NiTi-II. This is because the multi-level micro-nano structure increases the surface roughness of the material, and at the same time provides a large number of "air holes", and finally obtains the excellent superhydrophobic performance of the NiTi-IV surface. Compared with the use of femtosecond laser or nanosecond laser alone, this laser composite manufacturing method can further improve the superhydrophobicity of the surface of the nickel-titanium alloy. In addition, the present application has a more comprehensive and profound understanding of the preparation method of the superhydrophobic surface through the unique combination of femtosecond and nanosecond laser composite induction and chemical modification, and can provide some reference for the efficient preparation of the superhydrophobic surface from the perspective of microstructure and composite processing method.

Claims

1. A method for improving the superhydrophobicity of nickel-titanium alloys through laser composite processing, characterized in that, Includes the following steps: 1) Select samples for pretreatment, including: selecting nickel-titanium alloy samples, grinding and polishing the samples to remove edge burrs; cleaning the samples; and drying the samples. 2) Laser composite induction was performed on the surface of the pretreated nickel-titanium alloy sample. Femtosecond laser and nanosecond laser were used to texture the surface of the nickel-titanium alloy in sequence. First, the femtosecond laser was used to texture the surface of the nickel-titanium alloy sample to form a grid-like microstructure of the smallest unit. Then, the nanosecond laser was used to texture the laser-induced surface to form a grid-like microstructure of the smallest unit larger than the smallest unit of the femtosecond laser, thus forming a multi-level nested grid-like micro-nano composite structure. 3) Prepare a fluorosilane / ethanol mixed solution to reduce the surface energy of laser-induced nickel-titanium alloy samples; 4) Chemical modification of the surface of the laser-induced nickel-titanium alloy sample, including: placing the nickel-titanium alloy sample in a fluorosilane / ethanol mixed solution, sealing and standing for 1-2 hours, and then drying.

2. The method for improving the superhydrophobicity of nickel-titanium alloys through laser composite processing according to claim 1, characterized in that, Step 1) specifically includes: (1) Select a nickel-titanium alloy and cut it into more than 4 nickel-titanium alloy samples; (2) Use a metallographic sample grinding machine and polishing machine to grind and polish each nickel-titanium alloy sample to a mirror finish and remove the edge burrs formed by cutting; (3) Place the polished nickel-titanium alloy sample flat in a beaker, pour in acetone reagent until it covers the sample surface, and clean it with an ultrasonic cleaner; after that, pour out the acetone reagent, pour anhydrous ethanol solution into the beaker, and clean it with an ultrasonic cleaner; after that, pour out the ethanol solution, pour deionized water into the beaker, and clean it with an ultrasonic cleaner. (4) Spread the cleaned nickel-titanium alloy sample flat on a beaker or dust-free paper and dry it in a drying oven at 20-60℃ to complete the pretreatment of the nickel-titanium alloy sample.

3. The method for improving the superhydrophobicity of nickel-titanium alloys through laser composite processing according to claim 1, characterized in that, Step 2) includes: first, using a femtosecond laser to draw a grid pattern with a minimum unit size of 20–30 μm on each nickel-titanium alloy sample; then, using a nanosecond laser to draw a grid pattern with a minimum unit size of 100–300 μm on each nickel-titanium alloy sample; specifically as follows: (1) Place the pretreated nickel-titanium alloy sample on the three-dimensional motion platform of the femtosecond laser, use MarkingMate control software to adjust the femtosecond laser focus to the surface of the nickel-titanium alloy sample, and draw a grid pattern with a minimum unit of 20-30 μm. (2) Place the femtosecond laser-textured nickel-titanium alloy sample on the nanosecond laser processing platform, adjust the laser focus to the surface of the nickel-titanium alloy sample, and use Han's LASER Marking control software to adjust the nanosecond laser to draw a grid pattern with a minimum unit of 100-300 μm.

4. The method for improving the superhydrophobicity of nickel-titanium alloys through laser composite processing according to claim 3, characterized in that, The femtosecond laser has a center wavelength of 1064 nm, a pulse width of 340 fs, a maximum output power of 56 W, and a minimum focused spot diameter of 10 μm. The femtosecond laser processing parameters are set as follows: average power 8–12 W, scanning speed 200–400 mm / s, and 5–10 scans. The nanosecond laser has a center wavelength of 1064 nm, a pulse width of 50 ns, a maximum output power of 20 W, a repetition frequency of 20 kHz, and a minimum focused spot diameter of 50 μm. The nanosecond laser processing parameters are set as follows: average power 12–14 W, scanning speed 400–600 mm / s, and 10–15 scans.

5. The method for improving the superhydrophobicity of nickel-titanium alloys through laser composite processing according to claim 1, characterized in that, Step 3) includes: (1) Take two graduated cylinders and measure 62.7 mL of anhydrous ethanol and 19.2 mL of deionized water respectively, and pour them into a beaker and mix them. Place the beaker containing the mixed solution on a balance and slowly add 0.5 g of fluorosilane reagent with a dropper. (2) Remove the beaker, place a magnetic stir bar in the beaker, and then place it on a magnetic stirrer to stir for more than 12 hours; (3) After standing for 1-2 hours, the mixed solution is called fluorosilane / ethanol mixed solution.

6. The method for improving the superhydrophobicity of nickel-titanium alloys through laser composite processing according to claim 1, characterized in that, Step 4) includes: (1) Place the laser-induced nickel-titanium alloy sample in a glass container with the processed side facing up, pour in the fluorosilane / ethanol mixed solution until it covers the surface of the nickel-titanium alloy sample, seal it and let it stand for 1 to 2 hours; (2) Use tweezers to take out the nickel-titanium alloy sample and lay it flat on a dust-free paper. Place it in a drying oven at 100-120℃ and dry for 1-2 hours.

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