Method for manufacturing section TEM sample with hot-dip galvanized steel coating structure

Abstract

The invention relates to a manufacturing method of a cross section TEM sample with a hot-dip galvanized steel coating structure. The method comprises the following steps: S1, cutting a sample; s2, oppositely adhering the test sample; s3, grinding and polishing the sample; s4, adhering the sample to a ring; and step S5, pit thinning: the sample polished in the previous step is subjected to side pit thinning by using a pit instrument, and then is subjected to thinning by using an ion thinning instrument so as to obtain a nano-scale thin area, and the sample is taken as the sample for cross section TEM characterization. The TEM imaging effect is good, and zinc coating structure, element analysis, phase analysis and high-resolution observation can be rapidly achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of surface coating evaluation of hot-dip galvanized steel materials, and in particular relates to a method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure. Background Art

[0002] Steel materials have become important structural materials due to their excellent mechanical properties. They are widely used in the fields of automobile manufacturing, construction, bridge construction, etc. Hot-dip galvanizing is usually used to provide corrosion protection. Hot-dip galvanized steel has gradually become an important corrosion-resistant material. The study of the coating structure of hot-dip galvanized steel is crucial to improving its corrosion resistance, improving the mechanical properties of the material, and optimizing the production process. By deeply analyzing the microstructure of the coating, we can better understand the impact of different process conditions on the quality of the coating, and provide a theoretical basis for improving the service life and safety of the product.

[0003] Transmission electron microscopy (TEM) can provide nanometer-level resolution and can clearly observe the microstructure of the coating, including the phase composition, grain size, interface structure, etc. of the coating, which can provide accurate data for a deep understanding of the bonding characteristics between the coating and the substrate. Using TEM to observe the interface between the coating and the substrate can reveal the element diffusion, interaction, and possible defects at the interface, which is crucial for optimizing the adhesion and corrosion resistance of the coating. Taking advantage of the high resolution of TEM, the structure of the hot-dip galvanized steel coating can be characterized from the cross section, which can not only analyze the coating thickness and uniformity, evaluate the interface structure and bonding strength, and analyze the hierarchical structure and phase composition, but also provide defect information and damage mechanisms, thereby providing an important basis for improving material properties, optimizing production processes, and extending service life.

[0004] Regarding the preparation of transmission electron microscopy samples containing a complete surface zinc coating, if the ordinary metal material transmission sample preparation method is used, it mainly involves the following main steps: cutting, grinding and polishing, punching (cutting round pieces with a diameter of 3 mm that fit the sample rod loading slot of the transmission electron microscope) and ion thinning or electrolytic double-spray thinning. When grinding and polishing to reduce the sample thickness, it is easy to damage the zinc coating. On the one hand, since the zinc coating is very thin and relatively soft, it is easy to be damaged during the sample grinding and polishing process, or the sample is easy to bend, reducing the selectivity of TEM shooting; on the other hand, when preparing ordinary cross-sectional samples, it is difficult to accurately locate the bonding interface between the zinc coating and the substrate, and a large number of samples need to be prepared to increase the probability of finding the photographable area. The patent "Method for Preparing Transmission Electron Microscope Cross-Section Samples" (patent number: 202210107000.6) invented by Qian Ying et al. invented a method for preparing galvanized steel transmission samples using FIB cutting, which directly uses ion beam milling to obtain thin area samples at the desired observation position; however, when using FIB cutting, ion beam marks, namely the "curtain effect", will be generated. Ion beam marks will affect the TEM imaging quality, and in severe cases, the thin area of ​​the TEM sample will be damaged and cannot be observed and analyzed; in addition, the observable area of ​​the sample prepared by FIB cutting is limited, generally 10*10 microns in length and width, and the area of ​​the galvanized layer and interface layer that can be analyzed is limited. It is impossible to guarantee that sufficient analysis area can be obtained to analyze the overall structural characteristics and element distribution, and it is not conducive to crystallographic orientation analysis and defect analysis of each layer, which limits the revelation of related formation laws and process improvements. The industry is in urgent need of a cross-sectional TEM sample method for hot-dip galvanized steel coating structure. With the help of this method, the interface structure, element distribution, defect distribution and other related information of the coating can be fully and accurately reflected. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure in view of the limitations of the current technology. The method adopts a method of gluing two galvanized steel plates on their galvanized sides, grinding them thin, and then gluing them together to fix them, and finally obtains a galvanized steel transmission electron microscope sample containing a relatively large galvanized layer and an interface layer with uniform thickness. The TEM imaging effect of the present invention is good, and the structure, elemental analysis, phase analysis and high-resolution observation of the galvanized layer can be quickly realized.

[0006] The technical solution of the present invention is: A method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure, the method comprising the following steps: Step S1, sample cutting: 2-6 samples of the same shape are processed from the hot-dip galvanized steel sheet by a precision cutting machine, and the cut samples are ultrasonically cleaned with anhydrous ethanol and blown dry; Step S2, bonding the samples: using thermosetting adhesive to bond two samples of equal size parallel to the direction of the galvanized layer, with the galvanized side on the inside; The thermosetting adhesive is Gatan's G1 adhesive; Step S3, sample grinding and polishing: firstly, the sample is thinned by mechanical polishing, and polished to 200-300 μm; Step S4, sample ring bonding: fix the sample after bonding and thinning on a support ring that matches the size of the TEM sample rod, protect the side with the support ring, and continue to polish and thin the side with the sample until the sample thickness is 30-50 μm, and the total thickness including the support ring and the adhesive layer is 50-70 μm; The support ring can be a molybdenum ring or a copper ring with an outer diameter of 3mm and an inner diameter of 0.8-1.5mm; Step S5, pit thinning: the sample polished in the previous step is pitted at the center of the sample using a pit instrument, during the pitting process, the support ring is ensured to be at the bottom and kept intact, and then the center of the sample is further thinned using an ion thinning instrument to obtain a nanometer-scale thin area, and the sample is used as the sample for the cross-sectional TEM characterization; Among them, the thinning process adopts a decreasing process of 5KeV and 7-5°, and the time is set as: 7° for 10~30min, 6° for 20~60min, and finally 5° to bombard the sample surface until "holes" are broken. After the holes are thinned out, low voltages of 1.5KeV-1KeV-0.5KeV are used to decrease in sequence, and the amorphous particles are purged for 5~15 minutes respectively.

[0007] The size of the pit is set to 10-20 μm, and the thickness of the sample after the pit is completed is 20-30 μm, and the total thickness including the support ring and the adhesive layer is 30-50 μm.

[0008] Preferably, in the above-mentioned cross-sectional TEM characterization method of the hot-dip galvanized steel coating structure: In step S1, the length of the sample ranges from 1 mm to 2 mm, and the thickness ranges from 0.5 mm to 1 mm.

[0009] In step S2, the heating temperature of the thermosetting adhesive can be selected to be 60-80°C, and the heating time is 4-8 hours. During the heating process, clamps are used to fix the upper and lower surfaces of the sample to align the sample position without sliding.

[0010] In step S3, the sample may be mechanically polished with sandpaper, the sandpaper granularity grade decreases from coarse to fine, 800# sandpaper is used for rough polishing, 1500# sandpaper is used for medium polishing, and 2000# sandpaper is used for fine polishing; In step S4, the bonded sample and the support ring may be bonded with epoxy resin AB glue, and the curing time is about 30 minutes to 60 minutes.

[0011] In step S5, an ion thinning instrument is used to bombard the adhesive seams of the sample from high angle and high voltage to low angle and low voltage until holes appear, and then a low voltage of 1.5KeV-1KeV-0.5KeV is used to purge and clean in sequence to remove the amorphous material that may be produced by the previous ion bombardment.

[0012] The essential features of the present invention are: In the current technology, it is necessary to go through the steps of cutting, grinding, punching (cutting discs with a diameter of 3 mm that fit the sample rod loading slot of the transmission electron microscope), and ion thinning or electrolytic double-spray thinning to finally obtain a transmission electron microscope sample. However, in the process, in order to reduce the thickness of the sample, if both sides of the duplex steel and the galvanized layer are ground, the galvanized layer can be easily damaged. If only the duplex steel side is ground, the sample can easily bend due to the fragility of the galvanized layer itself. In addition, it is also difficult to accurately locate the thin area at the bonding interface between the galvanized layer and the substrate.

[0013] The present invention can accurately locate the thin area position observed by electron microscope to the galvanized layer or the bonding surface of the zinc layer and the substrate mainly through the steps of cutting, cross-section bonding, grinding, ring bonding, thinning, etc. Among them, through the step of cross-section bonding, the protection of the galvanized layer and the bonding interface of the zinc layer and the substrate can be achieved; through the step of ring bonding, the sample bonding structure can be supported and protected to prevent the sample from bending or cracking. After we improve the above steps, the thin area position to be observed can be accurately located to the galvanized layer or the bonding surface of the galvanized layer or the zinc layer and the substrate. In addition, since the bonding gap is in the center of the sample, the position of the thinning hole is on the bonding gap, so after the sample comes out of the hole, there will be thin areas at both ends of the hole parallel to the bonding gap, and since both sides of the bonding gap are the galvanized layer sections of the duplex steel, 4 thin areas for observing the galvanized layer and the interface can be obtained in the end. Finally, the sample bonding structure prepared according to this method is stable, not easy to bend and crack, and is also convenient for subsequent storage and repeated shooting.

[0014] The beneficial effects of the present invention are: The present invention provides a method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure. The method is original in that the double galvanized steel is bonded and ground thin before being bonded and fixed, and has the following beneficial effects: it solves the problem that the thin area of ​​the galvanized layer is easy to curl, the quality of the thin area is not high, and it is not conducive to high-resolution characterization such as TEM. In addition, it avoids the high cost of FIB processing and the problem of small characterization area. The prepared sample coating area has a large thin area range, small curvature, and good TEM imaging effect. It can quickly realize the structure, elemental analysis, phase analysis and high-resolution observation of the galvanized layer, and provides powerful and scientific technical support for the characterization and quality evaluation of the galvanized layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A flowchart of a specific embodiment of the present invention; Figure 2 This is a typical TEM morphology photo of the sample of the present invention; Figure 3 A typical selected area electron diffraction pattern of the zinc coating of the sample of the present invention; Figure 4 This is a mapping information diagram of the element Fe in the example of the present invention; Figure 5 This is a mapping information diagram of the element Zn of the example of the present invention; Figure 6 This is a mapping information diagram of the element Al of the example of the present invention; Figure 7 It is a mapping information diagram of the element O element of the example of the present invention; Figure 8 This is the mapping information diagram of the element Mn of the example of the present invention. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention.

[0017] Embodiment 1: A group of hot-dip galvanized dual-phase steel DP980 produced by Tangshan Iron and Steel Group was selected as the experimental material. The preparation methods of different samples are the same and applicable. Taking one group of hot-dip galvanized dual-phase steel DP980 samples as an example, the specific operation steps of the method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure of the present invention are as follows: Step S1, sample cutting, the original plate is first cut into 10mm*10mm*5mm by wire cutting, and then finely cut with a diamond wire saw, cutting two 2mm*2mm*1mm blocks, and retaining one side of the galvanized layer when cutting in the thickness direction. The effect of cutting in two steps is smooth incision, less oxide scale, and less loss of galvanized layer. Put the sample in anhydrous ethanol, clean it with an ultrasonic machine for 3 minutes, then take it out and blow dry. Select two samples of the same size for each group of samples, one sample with the galvanized layer surface facing up and one sample with the galvanized layer facing down, respectively placed on filter paper, and the remaining samples are reserved.

[0018] Step S2, specimen bonding: prepare thermosetting adhesive (Gatan's G1 adhesive) and a fixture. In order to protect the galvanized layer from being damaged during the sample preparation process, evenly apply thermosetting adhesive on one of the samples with the galvanized layer surface facing upwards, and then stack another sample with the galvanized layer facing downwards in parallel on top, ensuring that the galvanized surfaces of the two samples are adjacent and fixed with thermosetting adhesive. Keep the relative positions of the bonded samples unchanged, place them in the fixture, add weighted clips on the upper and lower surfaces, heat the fixture as a whole, use a heating table, set the temperature to 80°C, and heat for 6 hours. After heating, remove the sample and check the bonded sample. Check if the cross section is flat and the bonding is strong. Figure 1 As shown in (b) in .

[0019] Step S3, sample grinding and polishing: turn the bonded sample 90°, and grind and polish the side with the cross-section bonding seam. Use 800# sandpaper for rough polishing, polish until the surface scratches are uniform, there are no diamond wire saw blade marks, the polished surface is flat, and the polishing direction is parallel to the cross-section bonding gap direction; use 1500# sandpaper for medium polishing, polish until the rough scratches of the sandpaper in the previous step are covered, the surface is flat and the scratches are uniform, and the polishing direction is parallel to the cross-section bonding gap direction; use 2000# sandpaper for fine polishing, polish until the rough scratches of the sandpaper in the previous step are covered, the surface is flat and the scratches are uniform, and the polishing direction is parallel to the cross-section bonding gap direction. Figure 1 As shown in (c), the upper and lower surfaces where the cross-section gap can be seen need to be polished. After the sample is finally polished to a thickness of 200 μm (measured with the polished surface facing up), the sample is cleaned with anhydrous ethanol and blown dry for later use.

[0020] Step S4, sample bonding ring: prepare a molybdenum ring with an outer diameter of 3 mm and an inner diameter of 1 mm. After polishing with 2000# sandpaper until both surfaces are flat, clean it with anhydrous ethanol and blow it dry for later use. The thickness of the molybdenum ring is about ~15 μm. Take Fukang 14250 epoxy resin AB glue and mix it. Use a toothpick to dip the AB glue and apply it on the molybdenum ring. Apply it until the glue layer on the ring is uniform and thin, and there is no glue contamination inside the ring. At this time, the total thickness of the molybdenum ring and the glue does not exceed 20 μm. Place the cleaned sample on the molybdenum ring coated with glue with the bonding seam as the symmetry line. The sample is placed in the direction as shown in the figure. Figure 1As shown in (d), let it stand for 40 minutes. After the AB glue is completely cured, protect the side with the molybdenum ring, and continue to polish the other side of the sample with 2000# sandpaper until the sample thickness is ~35μm, and the total thickness including the molybdenum ring and the glue layer is ~50μm.

[0021] Step S5, sample thinning: first use the pit meter to thin the center of the sample, and the pit meter's polishing wheel can be used to thin the sample to a total thickness of about ~35 μm, and then use the ion thinning instrument to bombard the sample from high angle and high voltage to low angle and low voltage to the adhesive seam. The high voltage and high angle are divided into three steps: the voltage is unified to 5KeV, 7° setting time is 20min, 6° setting time is 40min, and then 5° is used to bombard the sample surface until the "hole" is broken. Low voltage and low angle means using a low voltage of 1.5KeV-1KeV-0.5KeV to blow and clean the "hole" for 10 minutes in sequence, and the angle used is unified to 3°. (such as Figure 1 The purpose of using high voltage and high angle is to obtain a wedge-shaped hole with a nanometer-scale thin area around the hole that meets the requirements of transmission electron microscopy observation. The purpose of low-angle and low-voltage purge is to remove the amorphous that may be produced by the previous ion bombardment.

[0022] After sample preparation is completed, transmission observation can be carried out: use a high-resolution transmission electron microscope and energy spectrum analysis to observe the prepared coating cross-section TEM sample.

[0023] Figure 2 This is the cross-sectional TEM observation result of the interface between the coating and the substrate of the hot-dip galvanized dual-phase steel DP980 sample. The blank area in the upper right corner of the picture is the exit hole during ion thinning. The area around the hole with clear sample information and that can be used for characterization and analysis is the above-mentioned "nanoscale thin area". The coating structure obtained by this preparation method is relatively complete and clear, and there is a nanoscale interface layer between the galvanized layer and the dual-phase steel.

[0024] Figure 3 This is a selected area electron diffraction photograph of the zinc coating in this sample. The results show that the selected area is a pure zinc layer.

[0025] Figure 4 The energy spectrum analysis of the Fe element mapping at this location shows that in addition to the Fe-rich layer in the dual-phase steel matrix, an Fe-rich layer is also found at the interface. Combined with the energy spectrum analysis of the Al element, it can be seen that the Fe-rich layer is a Fe-Al inhibition layer.

[0026] Figure 5 The energy spectrum analysis of Zn element mapping shows that Zn element is mainly enriched in the galvanized layer.

[0027] Figure 6By mapping the energy spectrum of the Al element at this location, it can be found that the interface layer is mainly an Al-enriched Fe-Al inhibition layer, and tends to be continuous.

[0028] Figure 7 For O element mapping energy spectrum analysis, the location where O element is enriched is generally where oxides are concentrated. Combined with the distribution map of Mn element, the precipitation level of manganese oxides in hot-dip galvanized duplex steel can be determined, thus helping to evaluate the performance of hot-dip galvanizing.

[0029] Figure 8 For Mn element mapping spectrum analysis, Figure 7 , Figure 8 Combined with the analysis, we can see that MnO is generated at the interface and inside the dual-phase steel. Based on this characterization result, we can evaluate that the coating structure of the hot-dip galvanized DP980 dual-phase steel is complete, the Fe-Al inhibition layer at the interface is continuous, and the hot-dip galvanizing performance is good.

[0030] It can be clearly seen from the TEM photos, selected area electron diffraction photos, and mapping information collection photos and results of the embodiments that the samples prepared by the hot-dip galvanized steel coating structure cross-section TEM sample preparation method provided by the present invention have a large thin area for sample observation and good TEM imaging effect, and good observation and evaluation results can be obtained, providing technical support for scientifically reflecting information such as coating quality.

[0031] Matters not covered by the present invention are known technologies.

Claims

1. A method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure, characterized in that the method comprises the following steps: Step S1, sample cutting: 2-6 samples of the same shape are processed from the hot-dip galvanized steel sheet by a precision cutting machine, and the cut samples are ultrasonically cleaned with anhydrous ethanol and blown dry; Step S2, bonding the samples: using thermosetting adhesive to bond two samples of equal size parallel to the direction of the galvanized layer, with the galvanized side on the inside; Step S3, sample grinding and polishing: firstly, the sample is thinned by mechanical polishing, and polished to 200-300 μm; Step S4, sample bonding ring: fix the bonded and thinned sample on a support ring that matches the size of the TEM sample rod, and continue to polish and thin the side with the sample until the sample thickness is 30-50 μm, and the total thickness including the support ring and the adhesive layer is 50-70 μm; Step S5, pit thinning: the sample polished in the previous step is pitted at the center of the sample using a pit instrument, during the pitting process, the support ring is ensured to be at the bottom and kept intact, and then the center of the sample is further thinned using an ion thinning instrument to obtain a nanometer-scale thin area, and the sample is used as the sample for the cross-sectional TEM characterization; in, During the thinning process, 5KeV and 7-5° are used in a decreasing process. The time is set as follows: 7° for 10~30min, 6° for 20~60min, and finally 5° is used to bombard the sample surface until a "hole" is broken. After the hole is thinned out, a low voltage of 1.5KeV-1KeV-0.5KeV is used to decrease in sequence, and the amorphous is purged for 5~15 minutes respectively.

2. The method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure according to claim 1, characterized in that: The area can be found symmetrically at four locations at both ends of the hole and on both sides of the bonding seam. Each location includes the galvanized layer and the bonding interface between the zinc layer and the duplex steel. The size of the thin area can be determined depending on the specific operating conditions of the specific sample, and the field of view size is approximately 2*2μm to several hundred microns.

3. The method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure according to claim 1, wherein In step S1, the length of the sample ranges from 1 mm to 2 mm, and the thickness ranges from 0.5 mm to 1 mm.

4. The method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure according to claim 1, wherein In step S2, the thermosetting adhesive is Gatan's G1 adhesive; the heating temperature of the thermosetting adhesive can be selected to be 60-80° C., and the heating time is 4-8 hours. During the heating process, clamps are used to fix the upper and lower surfaces of the sample to align the sample position without sliding.

5. The method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure according to claim 1, wherein In step S3, the sample may be mechanically polished with sandpaper, the sandpaper particle size grade decreases from coarse to fine, 800# sandpaper is used for coarse polishing, 1500# sandpaper is used for medium polishing, and 2000# sandpaper is used for fine polishing.

6. The method for preparing a cross-sectional TEM sample of a hot-dip galvanized steel coating structure according to claim 1, wherein In the step S4, the bonded sample and the support ring may be bonded with epoxy resin AB glue, and the curing time is about 30 minutes to 60 minutes.

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