A method for preparing SEM-DIC nanospeck patterns based on femtosecond laser surface reconstruction

Abstract

This invention belongs to the field of photomechanics, specifically disclosing a method for preparing SEM-DIC nanospeckles based on femtosecond laser surface reconstruction. The preparation steps of SEM-DIC nanospeckles are as follows: First, determine the femtosecond laser surface reconstruction parameters (pulse width ≤ 1 ps, pulse frequency 1–1000 kHz, pulse spacing 1–10 μm, wavelength 250–1064 nm, scanning irradiation output power 1–500 mW, circular polarization, spot diameter 5–50 μm, and scanning speed 0.1–10 mm / s); then, use a triaxial femtosecond laser processing system to scan and irradiate the sample to obtain nanospeckles. The preparation method disclosed in this invention is simple, rapid, and does not damage the sample. The nanospeckles are small, uniform, and have high contrast, without any detachment issues, and can be prepared on microscale samples and curved surfaces.

Description

Technical Field

[0001] This invention relates to the field of photomechanics, and in particular to a method for preparing SEM-DIC nanospeck patterns based on femtosecond laser surface reconstruction. Background Technology

[0002] Digital image correlation (DIC) technology is widely used in displacement and deformation measurement due to its advantages of being non-contact and highly accurate. DIC technology involves preparing speckle patterns on the sample surface and taking images before and after deformation. The region of interest is then divided into subsets, and the position of each subset is matched to obtain the displacement field. The strain field is further calculated from these subsets.

[0003] By combining speckle density (DIC) with scanning electron microscopy (SEM), strain measurements at the micro- and nano-scale can be achieved. Further integration with EBSD technology, coupling crystallographic information and strain fields, helps to better understand the mechanical behavior of materials and structures at the microscopic level. A crucial aspect of successfully applying SEM-DIC technology is the preparation of high-quality speckle patterns. Ideally, SEM-DIC speckles should be small and uniform in size, isotropic, not interfere with EBSD characterization, and be able to deform with the sample without detaching. The preparation process should be time-efficient, easily repeatable, and should not affect sample properties or damage the sample surface.

[0004] Existing speckle patterns include microgrid methods, focused ion beam (FIB) methods, ultrafast laser etching methods, nanofilm reconstruction methods, and nanoparticle methods. Among these, the microgrid method uses a tiny grid as a template to deposit a grid-like speckle pattern on the sample surface. For example, Chinese invention patent (publication number: CN 110057632A) discloses a method for preparing micron-sized speckles based on an optical and scanning electron microscope platform. This method first places a 2000-mesh electron microscope grid on the area to be tested on the sample surface, deposits a layer of platinum, and then removes the grid to obtain a grid-like speckle pattern. The speckle size prepared by this method is directly related to the grid size and cannot prepare smaller speckles below the micrometer level.

[0005] Focused ion beam (FIB) methods use an FIB system to directly etch or deposit speckle patterns on the sample surface. For example, Chinese invention patent (publication number: CN 101832759A) discloses a method for fabricating micro- and nano-scale speckle patterns, which uses a computer to generate speckle patterns and then etches them onto the sample surface to obtain the desired speckle patterns. This method damages the sample surface structure, affecting EBSD characterization, and the FIB system is expensive, making it difficult to apply on a large scale.

[0006] Ultrafast laser etching uses a laser to directly etch speckle patterns onto the sample surface. For example, Chinese invention patent (publication number: CN 110006935A) discloses a method for preparing finely differentiated speckle patterns using ultrafast lasers based on DIC micro-area dynamic strain testing. This method uses an ultrafast laser combined with a five-axis machining platform to process lattice, orthogonal, or finely differentiated speckle patterns on the sample surface. However, this method can only prepare speckles of at least several hundred nanometers in size, and laser etching can damage the sample surface layer and affect EBSD characterization.

[0007] The nanofilm reconstruction method involves depositing a nanofilm of metal onto the sample surface and then reconstructing the film at a higher temperature to obtain fine speckle patterns. For example, Chinese invention patent (publication number: CN112485280A) discloses a speckle pattern preparation method for high-resolution local strain analysis of metallic materials. This method involves depositing a gold film tens of nanometers thick on the surface of a metal sample and heat-treating it in water vapor at 200–300°C for 10–120 minutes to obtain speckles with a size of approximately tens of nanometers. However, the noble metal used in this method can block electron signals, thus preventing EBSD characterization, and the heat treatment may affect the sample's microstructure.

[0008] The nanoparticle method generates speckle patterns by attaching nanoparticles to the sample surface. For example, Chinese invention patent (publication number: CN 109580677A) discloses a method for preparing speckle patterns and a method for characterizing the micro-region deformation of materials. This method involves preparing nanoparticles into a suspension, allowing it to stand and centrifuging, then placing the substrate to be tested in the subsoil and drying it to obtain speckle patterns. This method produces fine speckle patterns that do not affect EBSD, but it is time-consuming and the nanoparticles may detach.

[0009] In summary, existing SEM-DIC speckle preparation methods often suffer from problems such as excessively large speckle size, complex processes, long processing times, and incompatibility with EBSD. Therefore, providing a method for preparing SEM-DIC nano-speculiar speckles that avoids these issues is a pressing problem to be solved in this field. Summary of the Invention

[0010] In view of this, the present invention provides a method for preparing SEM-DIC nanospeck patterns based on femtosecond laser surface reconstruction, in order to solve the problems of existing speckle preparation methods such as excessive size, complex process, long time consumption, and incompatibility with EBSD.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] A method for preparing SEM-DIC nanospeckled patterns based on femtosecond laser surface reconstruction includes the following steps:

[0013] 1) Determine the femtosecond laser surface reconstruction parameters;

[0014] 2) The sample was scanned and irradiated using a triaxial femtosecond laser processing system to obtain nano-speckle patterns;

[0015] The femtosecond laser surface reconstruction parameters include: pulse width ≤ 1ps, pulse frequency 1~1000kHz, pulse spacing 1~10μm, wavelength 250~1064nm, scanning irradiation output power 1~500mW, circular polarization, spot diameter 5~50μm, and scanning speed 0.1~10mm / s.

[0016] Preferably, the sample also includes a pretreatment step before scanning irradiation.

[0017] Preferably, the pretreatment step includes sequentially grinding, polishing and cleaning the surface of the sample;

[0018] The polishing includes mechanical polishing or electrochemical polishing.

[0019] Preferably, determining the femtosecond laser surface reconstruction parameters in step 1) further includes: performing femtosecond laser scanning on the surface of the sample, gradually reducing the scanning irradiation output power until the surface of the sample no longer changes, and using the minimum scanning irradiation output power that enables surface reconstruction as the scanning irradiation output power; then gradually reducing the pulse spacing, using the maximum spacing with uniform surface structure as the pulse spacing.

[0020] Preferably, the scanning path used for scanning illumination in step 2) is one of serpentine scanning, raster scanning, close-packed serpentine scanning, and close-packed raster scanning.

[0021] Preferably, the structure of the nanospeckled pattern includes nanoparticles, nanoprotrusions, and nanopores, with a size ≤100nm.

[0022] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The method of the present invention is simple and the process is fast. For flat surfaces, only one step of laser scanning is required to produce nano-speckle patterns, which takes no more than 5 minutes.

[0024] 2. The femtosecond laser pulse duration used in this invention is on the order of femtoseconds, and the scanning irradiation output power can be adjusted so that the peak energy density is only slightly higher than the ablation threshold of the material, thus avoiding thermal diffusion and preventing damage to the sample;

[0025] 3. This invention is a non-contact speckle preparation method, where the sample is not subjected to force, and speckle can be prepared on microscale samples and curved surfaces;

[0026] 4. The nanospeckles prepared by this invention have small and uniform size and high contrast, with an average size of approximately 30 nm, and can achieve 100 × 100 nm. 2SEM-DIC analysis under a subset, without affecting EBSD characterization;

[0027] 5. The speckle pattern originates from the sample surface structure, can deform along with the sample, does not have the problem of falling off, and has the same coefficient of thermal expansion as the sample, so it can adapt to tests in various environments such as high temperature and low temperature. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the laser scanning path in this invention, where the length a is the pulse interval.

[0030] Figure 2 This is a nanospeck pattern prepared on the surface of a copper sample using femtosecond laser surface reconstruction in Example 1 of the present invention;

[0031] Figure 3 This is a nanospeck pattern prepared on the surface of a copper sample using femtosecond laser surface reconstruction in Example 2 of the present invention;

[0032] Figure 4 This refers to the nanospeck pattern prepared on the surface of a copper sample using femtosecond laser surface reconstruction in Example 3 of the present invention.

[0033] Figure 5 This is a BC diagram of the speckle region in Embodiment 1 of the present invention;

[0034] Figure 6 This is a BC diagram of the speckle region in Embodiment 2 of the present invention;

[0035] Figure 7 This is a BC diagram of the speckle region in Embodiment 3 of the present invention;

[0036] Figure 8 This is the BC diagram of Comparative Example 1 of the present invention;

[0037] Figure 9 The BC distribution diagrams are for Embodiments 1-3 and Comparative Example 1 of this invention.

[0038] Figure 10 This is a nanospeck pattern prepared on the surface of a stainless steel sample using femtosecond laser surface reconstruction in Example 4 of the present invention. Detailed Implementation

[0039] This invention provides a method for preparing SEM-DIC nanospeck patterns based on femtosecond laser surface reconstruction, comprising the following steps:

[0040] 1) Determine the femtosecond laser surface reconstruction parameters;

[0041] 2) The sample was scanned and irradiated using a triaxial femtosecond laser processing system to obtain nano-speckle patterns;

[0042] The femtosecond laser surface reconstruction parameters include: pulse width ≤ 1 ps, specifically 0.01 ps, 0.05 ps, 0.1 ps, 0.2 ps, 0.4 ps, 0.5 ps, 0.6 ps, 0.8 ps, preferably 380 fs; pulse frequency 1–1000 kHz, specifically 1 kHz, 5 kHz, 10 kHz, 20 kHz, 50 kHz, 80 kHz, 100 kHz, 200 kHz, 500 kHz, 800 kHz, 1000 kHz, 2000 kHz, 5000 kHz, 8000 kHz; pulse spacing 1–10 μm, specifically 1 μm, 2 μm, 4 μm, 5 μm, 6 μm, 8 μm; and wavelength 250–1064 nm, specifically 280 nm, 300 nm, 350 nm, 400 nm. The wavelengths are m, 500nm, 600nm, 800nm, and 1000nm, preferably 343nm; the scanning irradiation output power is 1–500mW, specifically 1mW, 2mW, 5mW, 7.5mW, 10mW, 12.5mW, 15mW, 20mW, 25mW, 50mW, 80mW, 100mW, 150mW, 200mW, 250mW, and 300mW. 350mW, 400mW, 450mW; circular polarization; spot diameter of 5–50μm, specifically 5μm, 10μm, 20μm, 30μm, 40μm; scanning speed of 0.1–10mm / s, specifically 0.1mm / s, 0.2mm / s, 0.5mm / s, 0.8mm / s, 1mm / s, 2mm / s, 5mm / s, 8mm / s.

[0043] In this invention, the sample also includes a pretreatment step before scanning irradiation.

[0044] In this invention, if the surface to be processed is obtained by methods such as FIB or femtosecond laser processing, and the surface quality is already high, or if it is necessary to retain the original surface, the pretreatment step can be omitted.

[0045] In this invention, the pretreatment step includes sequentially grinding, polishing and cleaning the surface of the sample.

[0046] In this invention, the polishing includes mechanical polishing or electrochemical polishing, and the cleaning is preferably done by alcohol cleaning followed by drying.

[0047] In this invention, determining the femtosecond laser surface reconstruction parameters in step 1) further includes: performing femtosecond laser scanning on the surface of the sample, gradually reducing the scanning irradiation output power until the surface of the sample no longer changes, and using the minimum scanning irradiation output power that enables surface reconstruction as the scanning irradiation output power; then gradually reducing the pulse spacing, using the maximum spacing with uniform surface structure as the pulse spacing.

[0048] In this invention, the scanning path for scanning illumination in step 2) is one of serpentine scanning, raster scanning, close-packed serpentine scanning, and close-packed raster scanning. A schematic diagram of the scanning path for scanning illumination is shown below. Figure 1 As shown.

[0049] In this invention, the structure of the nanospeckled pattern includes nanoparticles, nanoprotrusions, and nanopores with a size ≤100nm, specifically 10nm, 20nm, 30nm, 40nm, 50nm, and 80nm.

[0050] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0051] Example

[0052] Taking copper and 316L stainless steel as examples, SEM-DIC nanospecks were prepared on the sample surface using femtosecond laser surface reconstruction. The specific steps are as follows:

[0053] (1) Sample surface pretreatment: Copper and 316L stainless steel samples were successively ground using #400, #800, #1200, and #2000 sandpaper, and then electrochemically polished at room temperature. The electrolytic polishing solution for copper consisted of phosphoric acid, anhydrous ethanol, and deionized water in a volume ratio of 1:1:2, with an electrochemical polishing voltage of 6V and a time of 30s. The polishing solution for 316L stainless steel consisted of a 10% perchloric acid aqueous solution, with a voltage of 13V and a time of 17s.

[0054] (2) Femtosecond laser surface reconstruction: The sample is fixed on the sample stage of the triaxial femtosecond laser processing system, with the surface to be prepared facing upwards. The femtosecond laser parameters are: pulse width of 380 fs, pulse frequency of 100 kHz, wavelength of 343 nm, circular polarization, spot diameter of 10 μm, and scanning speed of 1 mm / s. Figure 1The closely spaced serpentine scanning path is shown, and the PSO function of the femtosecond laser processing system is used. Other process parameters are shown in Table 1. Examples 1-3 and Comparative Example 1 are copper samples, and Example 4 is a 316L stainless steel sample. Comparative Example 1 was only polished and did not undergo femtosecond laser surface reconstruction; it was used to compare the effects of the femtosecond laser.

[0055] Table 1 Process Parameter Setting Table

[0056]

[0057]

[0058] (3) Obtain SEM images and EBSD data of the samples.

[0059] SEM images of the nanospecks prepared on the copper surface in Examples 1-3 are attached. Figures 2-4 The SEM images show that the speckle pattern is evenly distributed with good contrast, and the size ranges from 10 to 80 nm. It is composed of nanoparticles, protrusions, and pores.

[0060] The BC image can be obtained from EBSD data. A higher BC value results in a brighter BC image, indicating a lower degree of surface defect. (See attached image.) Figures 5-8 The BC images of Examples 1-3 and Comparative Example 1 are shown below. It can be seen that the brightness of the BC images of the samples reconstructed by the femtosecond laser is lower than that of the BC image of Comparative Example 1, with the BC image of Example 2 being relatively brighter. (See attached image.) Figure 9 As shown in the corresponding BC distribution curves, it can be seen that the BC peak positions of the samples after femtosecond laser surface reconstruction are all shifted to the left relative to the unscanned areas. Among them, the peak position of Example 2 has the smallest left shift, only slightly lower than that of Comparative Example 1, indicating that the femtosecond laser surface reconstruction has the least impact on the EBSD test in Example 2.

[0061] SEM images of nanospeckled patterns prepared on stainless steel surfaces in Example 4 are attached. Figure 10 Its speckle distribution is uniform, with good contrast, and the size is 10-50 nm. It is composed of nanoparticles, protrusions and pore structures.

[0062] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0063] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing SEM-DIC nanospeckled patterns based on femtosecond laser surface reconstruction, characterized in that, Includes the following steps: 1) Determine the femtosecond laser surface reconstruction parameters; 2) The sample was scanned and irradiated using a triaxial femtosecond laser processing system to obtain nano-speckle patterns; The femtosecond laser surface reconstruction parameters include: pulse width ≤ 1ps, pulse frequency 1~1000kHz, pulse spacing 1~10μm, wavelength 250~1064nm, scanning irradiation output power 1~500mW, polarization circular polarization, spot diameter 5~50μm, and scanning speed 0.1~10mm / s. The determination of femtosecond laser surface reconstruction parameters in step 1) further includes: performing femtosecond laser scanning on the surface of the sample, gradually reducing the scanning irradiation output power until the surface of the sample no longer changes, and using the minimum scanning irradiation output power that can reconstruct the surface as the scanning irradiation output power; then gradually reducing the pulse spacing, using the maximum spacing with uniform surface structure as the pulse spacing.

2. The method for preparing SEM-DIC nanospeckled patterns based on femtosecond laser surface reconstruction according to claim 1, characterized in that, The sample also undergoes a pretreatment step before scanning irradiation.

3. The method for preparing SEM-DIC nanospeckled patterns based on femtosecond laser surface reconstruction according to claim 2, characterized in that, The pretreatment steps include sequentially grinding, polishing, and cleaning the surface of the sample; The polishing includes mechanical polishing or electrochemical polishing.

4. The method for preparing SEM-DIC nanospeckled patterns based on femtosecond laser surface reconstruction according to claim 1, characterized in that, In step 2), the scanning path for scanning illumination is one of the following: serpentine scanning, raster scanning, close-packed serpentine scanning, and close-packed raster scanning.

5. The method for preparing SEM-DIC nanospeckled patterns based on femtosecond laser surface reconstruction according to claim 4, characterized in that, The structure of the nanospeckled pattern includes nanoparticles, nanoprotrusions, and nanopores, with a size ≤100nm.

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