JP7610226B2 - Etching method and etching apparatus - Google Patents

Description

The present invention relates to an etching method and etching processing apparatus used in wet etching of solid surfaces for the purposes of observing the crystal structure of metal surfaces and processing the surfaces of various materials.

Generally, metals used as structural materials are polycrystalline bodies in which crystal grains (crystal grains) made up of metal atoms are bonded to each other via grain boundaries, and it is known that the strength and other properties of the structural material vary greatly depending on the type of metal atoms and the morphology of the metal structure formed by the polycrystalline body. For this reason, one of the methods for evaluating the soundness of various metallic materials has traditionally been to observe the crystalline structure contained in the metallic material.

For example, Non-Patent Document 1 describes a technique for observing the structure of steel materials and the like constituting various plants, as one of the non-destructive inspections. As described in Non-Patent Document 1, when observing the structure of a metal, the surface to be observed is roughly polished and then finished to a mirror finish, and then etched (corroded) using various chemicals to reveal the structure to be observed and enable clear observation of the structure.
The above-mentioned etching process is carried out for the purpose of taking advantage of various material differences contained in the surface to be observed, such as between the parent phase and the precipitated phase, between crystal grains and crystal grain boundaries, and differences in crystal grain orientation, and emphasizing the differences by producing irregularities or coloring in accordance with the differences, thereby clarifying and facilitating subsequent observation under a microscope, etc.

When etching the polished surface of the above metal, an etching solution is generally used, which is made by dissolving the components that produce the desired etching in water or an organic solvent such as alcohol. The etching solution is brought into contact with the polished surface of the metal under specified conditions, and a chemical reaction occurs between the polished surface of the metal and the etching solution, resulting in the appearance of the desired structure. For this reason, in order to etch the metal surface uniformly, it is necessary to ensure that the chemical reaction that occurs between the polished surface of the metal and the etching solution progresses uniformly.

In other words, as described above, the method of contacting the polished metal surface with an etching solution can be, for example, by contacting absorbent cotton soaked in the etching solution with the polished surface, as described in Non-Patent Document 1. On the other hand, for example, in a system in which the progress of etching is strongly influenced by the components of the etching solution that contacts the polished surface, a mechanism is needed to uniformly advance the chemical reaction that causes the etching phenomenon.

As a means for uniformly progressing the above-mentioned etching, for example, Patent Document 1 describes a technique for propagating ultrasonic waves through the etching solution to remove bubbles generated on the surface of the object to be etched. Patent Document 2 describes a technique for suppressing uneven corrosion caused by etching by including microbubbles in the etching solution in which the sample to be etched is immersed and applying a predetermined ultrasonic wave, and describes that by applying ultrasonic waves while microbubbles are present in the etching solution, a predetermined resonance state is formed and ultrasonic waves are transmitted roughly evenly to the entire etching surface, and hydrogen gas, a reaction product of etching, is absorbed by the microbubbles and smoothly removed.

Patent Document 3 also describes an etching device that stirs the etching liquid around the object to be etched by spraying the circulated etching liquid onto the surface of the etching liquid using a spray nozzle, and describes how this mechanism can reduce unevenness in etching compared to stirring or ejecting the etching liquid from inside the etching liquid.

In addition, in systems in which a solid-phase reaction product is generated due to a chemical reaction between the metal surface and the etching solution, it is known that the reaction product adheres to the metal surface and acts as a mask against the etching, impairing the uniformity of the etching. For this reason, as described in Non-Patent Document 1, Patent Documents 4 and 5, etc., a method is known in which the reaction product generated by the etching process is appropriately removed by mechanical action using a buffing cloth or the like, and the etching process is continued.

In addition, wet etching of solid surfaces is not limited to pretreatment for observing the crystal structure of the above-mentioned metals, but is also used for the purpose of smoothing or roughening the surfaces of metals, glass, etc., as described in, for example, Patent Documents 6 and 7, and is expected to efficiently advance the desired etching in both cases.

Japanese Unexamined Patent Publication No. 60-39174 JP 2012-247382 A JP 2017-168633 A Japanese Unexamined Patent Publication No. 2-141590 Japanese Unexamined Patent Publication No. 62-238380 Japanese Unexamined Patent Publication No. 2-149685 JP 2004-190043 A

Journal of the Japan Welding Society, Vol. 79 (2010), No. 8, pp. 760-764

As described above, various means are provided for etching metal surfaces that have been polished or the like in accordance with the purpose, such as for observing the structure of the metal, but proposals for new etching methods, etc. are required due to changes in the level of quality required for etching treatments and the desire for improved efficiency, etc. In addition, etching treatments of solid surfaces are also used for the purpose of smoothing or roughening the solid surfaces of various metals, glass materials, etc., and in both cases, it is expected that the intended etching treatments will proceed efficiently.
An object of the present invention is to provide a novel etching method that enables simple and highly efficient etching of a solid surface, and an etching processing apparatus for use in said etching method.

In order to solve the above problems, the present invention provides the following means.
(1) An etching method for performing wet etching using an etching solution on a solid surface as an etching surface, in which ultrasonic waves are applied to an etching solution holding material impregnated with the etching solution while the etching solution holding material is brought into contact with the etching surface.
(2) An etching treatment method in which the etching solution holding material is impregnated with the etching solution by being immersed in the etching solution held inside an etching tank.
(3) The etching method, wherein the etching solution holding material contains a material having an official moisture content of 3% or more.
(4) The solid is a steel material, and the etching method is aimed at revealing the crystal structure of the steel material.
(5) An etching treatment apparatus that performs wet etching using an etching liquid on a solid surface as an etching surface, the etching treatment apparatus having an etching liquid holding material capable of holding the etching liquid therein and an ultrasonic application means capable of applying ultrasonic waves to the etching liquid holding material, and capable of applying ultrasonic waves to the etching liquid holding material while the etching liquid holding material impregnated with the etching liquid is in contact with the etching surface.
(6) An etching treatment apparatus having an etching tank for holding the etching solution, the etching solution holding material being placed inside the etching tank.
(7) An etching treatment device, wherein the etching solution holding material contains a material having an official moisture content of 3% or more.

The present invention enables efficient etching of solid surfaces. In particular, in a system in which a solid-phase reaction product is produced by an etching reaction, it is possible to proceed with the etching process while suppressing adhesion of the reaction product to the solid surface, enabling efficient etching.

FIG. 2 is a schematic diagram showing the mechanism of an etching process according to the present invention. 1 is a schematic diagram showing an example of an etching processing apparatus according to the present invention; 1 is a schematic diagram showing an example of an etching processing apparatus according to the present invention; FIG. 1 is a schematic diagram showing an overview of an apparatus used for evaluating an etching process according to the present invention. 1 is an optical microscope photograph of the surface of a steel material (S45C) that has been subjected to an etching treatment according to the present invention. 1 is an optical microscope photograph (comparative example) of a surface of a steel material (S45C) after etching treatment. 1 is an optical microscope photograph (comparative example) of a surface of a steel material (S45C) after etching treatment. 1 is an optical microscope photograph (comparative example) of a surface of a steel material (S45C) after etching treatment. 1 is an optical microscope photograph of the surface of a steel material (S45C) that has been subjected to an etching treatment according to the present invention. 1 is an optical microscope photograph of the surface of a steel material (SCr415H) that has been subjected to an etching treatment according to the present invention. 1 is an optical microscope photograph (comparative example) of a surface of a steel material (SCr415H) after etching treatment. 1 is an optical microscope photograph (comparative example) of a surface of a steel material (SCr415H) after etching treatment. 1 is an optical microscope photograph of the surface of a steel material (SCM435) that has been subjected to an etching treatment according to the present invention. 1 is an optical microscope photograph of the surface of a steel material (SCM435) that has been subjected to an etching treatment according to the present invention. 1 is a photograph showing the state of a nonwoven fabric used in an etching treatment according to the present invention.

Etching, which is carried out for the purpose of observing the structure of metals, etc., is carried out for the purpose of highlighting various material differences contained in the surface of metals, such as differences between parent phases and precipitated phases, crystal grains and grain boundaries, and differences in crystal grain orientation, or of using these differences to give the surface of metals unique characteristics. Etching also makes use of chemical changes between the surface of the metal, etc. and the etching solution, and the components and concentration of the etching solution, etching temperature, etching time, etc. are adjusted so that chemical changes occur according to the purpose of the etching.

On the other hand, the degree of progress of the chemical reaction between the surface of the metal or the like and the etching solution varies depending on, for example, the concentration of each component contained in the etching solution. As the chemical reaction progresses, the concentration of the components involved in the chemical reaction decreases, and the concentration of the reaction products of the chemical reaction increases, which inevitably impedes the progress of the etching process. As a result, if a small amount of etching solution remains on the surface of the metal or the like, it is thought that a phenomenon will occur in which the desired etching reaction does not progress due to changes in the components of the etching solution.

The inventions described in the above-mentioned Patent Documents 1 and 2 aim to prevent this phenomenon and perform good etching processing. In an environment where a sufficient amount of etching solution is used to complete the etching of the object to be etched, the etching solution is stirred and circulated by various methods, thereby mitigating the degree of change in the composition and non-uniformity of the etching solution, particularly on the surface to be etched, and thereby performing uniform etching.

In light of the above technical background, the present inventors have investigated etching methods for materials that are difficult to etch, and have unexpectedly discovered that by using a cloth or the like that can be impregnated with an etching liquid and capable of holding the etching liquid, and applying ultrasonic waves while the cloth or the like holding the etching liquid is in contact with the surface to be etched, it is possible to reduce the time required for etching, as compared to the case of simply applying ultrasonic waves to an etching liquid, as described in Patent Document 1, for example.

In addition, in particular in systems in which etching produces solid-phase reaction products, the reaction products adhere to the surface being etched, preventing uniform etching. However, by performing etching in the above-described manner, the reaction products are prevented from adhering to the surface being etched, making it unnecessary to interrupt the etching process to remove the reaction products, and it has been found that this makes it possible to perform an efficient etching process.

As described above, the etching process according to the present invention enables uniform etching, and the mechanism by which efficient etching is possible by preventing the adhesion of solid reaction products to the surface being etched is believed to be as follows.

Figure 1 shows a schematic diagram of the state near the surface to be etched when etching is performed by the etching process according to the present invention. For example, when a cloth 3 or the like used to hold an abrasive in buffing is used and the cloth 3 or the like is impregnated with etching liquid 2 and brought into contact with the surface of the object to be etched 1, as shown in Figure 1, it is considered that some of the fiber elements 4 constituting the cloth 3 are in contact with the surface of the object to be etched 1. On the other hand, the etching liquid 2 impregnated in the cloth 3 or the like is present so as to fill the gaps in the fiber elements 4 constituting the cloth 3, and as a result, it is considered that a state is formed in which both the fiber elements 4 and the etching liquid 2 are in contact with the surface of the object to be etched 1.

In the above state, ultrasonic waves are applied to the cloth 3 and the etching solution 2 using an appropriate means, which induces vibrations inside the etching solution 2 and also induces vibrations 20 in the fiber elements 4. In particular, it is believed that the parts of the fiber elements 4 that come into contact with the surface of the object to be etched 1 generate vibrations of a predetermined amplitude while maintaining contact with the surface of the object to be etched 1.

It is generally known that at the interface between a solid surface and a fluid, a boundary layer is formed on the fluid side of the interface, through which the fluid has difficulty flowing. If the etching process progresses within this boundary layer and the composition of the etching solution changes, the progress of the subsequent etching process is thought to be hindered. The thickness of the boundary layer is thought to be able to be reduced by increasing the flow speed of the fluid or by cavitation caused by the application of ultrasonic waves, and therefore the techniques described in the above Patent Documents 1 to 3 are thought to promote the progress of the etching process by reducing the thickness of the boundary layer on the surface of the surface to be etched by agitating the etching solution by various means.

On the other hand, in the etching process of the present invention, the fiber elements 4 inside the etching solution 2 vibrate while maintaining contact with the surface of the object 1 to be etched, mechanically stirring the area near the surface of the object 1 to be etched, and the boundary layer within the etching solution is continuously destroyed, homogenizing the etching solution, suppressing changes in the composition of the etching solution near the surface to be etched, and improving the etching efficiency.

In other words, when wet etching a solid surface using a specific etching solution, the present invention uses an etching solution holding material such as a cloth capable of holding the etching solution inside, and while the etching solution holding material is impregnated with the etching solution and holds the etching solution, it is brought into contact with the solid surface to be etched, and in this state ultrasonic waves are applied to the etching solution holding material and the etching solution to perform the etching process on the solid surface.

2 and 3 show an example of the configuration when performing the etching process according to the present invention. As shown in FIG. 2, the etching according to the present invention can be performed by placing an etching solution holding material 5 capable of holding an etching solution inside in contact with the surface to be etched, impregnating and holding the etching solution 2 in the etching solution holding material 5, and applying ultrasonic waves generated by an ultrasonic generating means 7 to the etching solution holding material 5 and the etching solution 2 via a transducer 6 having appropriate acoustic characteristics. When performing the etching process in the form shown in FIG. 2, it is also preferable to provide a wall 9 or the like to prevent leakage of the etching solution.

The etching method according to the present invention can also be carried out by placing the object 1 to be etched in an etching tank 8 holding an etching solution 2 so that the surface to be etched is in contact with the etching solution holding material 5, as shown in FIG. 3, and applying ultrasonic waves to the etching tank 8 by an ultrasonic generating means 7 or the like. In this case, as shown in FIG. 3(a), the surface to be etched may be placed on the upper surface of the etching solution holding material 5, so that the etching solution holding material 5 and the surface to be etched are brought into contact by the weight of the object 1 to be etched, or as shown in FIG. 3(b), the etching solution holding material 5 may be placed on the upper surface of the surface to be etched, and a weighted body 11 may be used to bring the etching solution holding material 5 and the surface to be etched into contact.

The etching solution holding material 5 can be any material that has a structure capable of holding the etching solution by the action of the surface tension of the etching solution due to the presence of minute voids, such as woven fabrics, knitted fabrics, nonwoven fabrics, raised fabrics, paper-like materials, cotton-like materials (fiber masses) and the like made up of various fiber elements 4, as well as porous materials such as sponges made up of sea sponges and synthetic polymers.

Fiber elements 4 constituting the etching solution holding material 5 can be cellulose fibers derived from plants such as cotton thread, regenerated cellulose fibers made from cellulose, fibers made from polylactic acid, natural fibers made from proteins such as wool and silk thread, and fibers made from synthetic polymers such as polyester and PET resin. When using an aqueous solution in which various reactive components are dissolved as the etching solution, it is preferable to use an etching solution holding material 5 made from a material such as cellulose fiber, regenerated cellulose fiber, or synthetic polymer fiber that exhibits high hydrophilicity by hydrating with water, in order to improve the retention of the etching solution.

The affinity of fibers, etc. for water can be evaluated, for example, by the degree of weight change (moisture content) that various fibers exhibit when absorbing moisture in a room temperature atmosphere. In the present invention, when an etching solution holding material is described as being hydrophilic, this means that the material constituting the etching solution holding material has an official moisture content of 3% or more under standard conditions (20°C, humidity 65%), and in particular, by being composed of a material with a moisture content of 5% or more, or 10% or more, the etching solution holding material can hold the aqueous etching solution well.

From the viewpoint of moisture content, for example, by using an etching solution holding material containing natural fibers such as wool (official moisture content: about 16%), silk, hemp (about 9%), cotton (about 7%), or regenerated cellulose fibers such as rayon and cupra (about 13%), the aqueous etching solution is particularly well held in the voids within the etching solution holding material, and both the fiber elements that make up the etching solution holding material and the etching solution can be brought into contact with the surface to be etched.

In addition, the fiber diameter of the regenerated cellulose fibers such as rayon and cupra can be freely set during spinning, and it is possible to make fine fibers with a fiber diameter of tens of μm or less, especially 20 μm or less, and even 10 μm or less, and it is possible to make an etching solution holding material that exhibits high water retention by effectively using the surface tension of the etching solution. In addition, by making the fibers that make up the etching solution holding material fine, it is possible to reduce the average distance between adjacent fiber elements, and even when applying ultrasonic waves with a particularly high frequency and limited amplitude, it is possible to suppress unevenness that occurs when the fiber elements come into contact with the etched surface and sweep it.

The etching solution holding material can be made of multiple types of materials, and can have a structure suited to the purpose, for example, a structure that combines a part that mainly comes into contact with the surface to be etched and contributes to the flow of the etching solution near the surface to be etched with a part that mainly contributes to holding the etching solution.

On the other hand, since friction occurs between the etching solution holding material 5 and the object to be etched due to ultrasonic vibration during the etching process, it is desirable to use an etching solution holding material 5 made of a material that is sufficiently less hard than the object to be etched in order to prevent scratching the surface to be etched, particularly when using the etching method of the present invention as a pretreatment for metal structure observation.

For example, as shown in FIG. 3, when etching is performed inside an etching tank 8 holding an etching solution 2, since there is no risk of leakage of the etching solution from the etching solution holding material 5, it is possible to select the etching solution holding material 5 mainly from the viewpoints of ease of use and availability. For example, it is possible to perform the etching using absorbent cotton or nonwoven fabric made of regenerated cellulose fibers.

On the other hand, when etching is performed mainly using an etching solution impregnated in the etching solution holding material 5 as shown in FIG. 2, the etching solution holding material 5 is made of a material that exhibits good hydrophilicity, such as fine regenerated cellulose fibers with a fiber diameter of tens of μm or less, preferably 10 μm or less, and has high water retention, making it possible to perform etching on surfaces to be etched that are not horizontal, for example.

As described above, in the etching process according to the present invention, the fibrous elements 4 constituting the etching solution holding material 5 are used to agitate the vicinity of the surface to be etched, thereby disrupting the boundary layer formed within the etching solution and performing an efficient etching process. It is preferable to use an etching solution holding material 5 that is flexible overall and can easily come into uniform contact with the surface to be etched.

In the etching process according to the present invention, in order to bring the etching solution holding material 5 into good contact with the surface to be etched, it is desirable to place the two in a form in which they are pressed against each other. By setting the pressing force appropriately, it is possible to prevent the etching solution holding material from not contacting part of the surface to be etched due to the tension of the etching solution holding material or the unevenness of the surface to be etched, and to perform a uniform etching process. On the other hand, if the pressing force is excessive, the part of the etching solution holding material that contacts the surface to be etched may be prevented from being displaced between the surface to be etched and the etching solution holding material, which may result in insufficient disturbance of the boundary layer.

The pressure with which the etching solution holding material 5 is pressed against the surface to be etched can be, for example, about 0.1 to 10 g/ ^cm2 , or about 0.5 to 3 g/ ^cm2 , when the surface to be etched is flat. By adjusting the pressure within this range, it is possible to adjust the density at which the fiber elements 4 constituting the etching solution holding material 5 contact the surface to be etched, and the pressure at the contact points.

When performing the etching process in the form shown in FIG. 2, for example, in order to press the etching solution holding material 5 against the surface to be etched, a vibrator 6 having a predetermined mass can be used, or a force can be applied to the vibrator 6 from the outside. When performing the etching process in the form shown in FIG. 3, the object to be etched 1 can be placed on the etching solution holding material 5, so that the weight of the object to be etched 1 brings the etching solution holding material 5 into contact with the surface to be etched. It is also possible to apply a weight to the back surface of the object to be etched 1 to bring the etching solution holding material 5 into strong contact.

The thickness of the etching solution holding material 5 when performing the etching treatment according to the present invention can be appropriately determined depending on the type of the etching treatment, etc.
For example, as shown in Fig. 3, when etching a flat surface to be etched by immersing the etching solution holding material 5 in an etching solution, a cloth-like etching solution holding material having a thickness of about 0.1 to 5 mm, or 0.5 to 2 mm, can be used to bring the etching solution holding material into contact with the surface to be etched. When etching a curved or uneven surface to be etched, a sponge-like etching solution holding material that can absorb the unevenness and come into contact with the entire surface can be used, and an appropriate weight can be applied to the back surface of the object to be etched as necessary, thereby achieving a good etching process.

On the other hand, when performing an etching process using an etching solution held in an etching solution holding material 5 in contact with the surface to be etched in the form shown in FIG. 2, it is desirable to use an etching solution holding material 5 having a thickness sufficient to hold a sufficient amount of etching solution for the etching process. In addition, when performing an etching process in the form shown in FIG. 2, it is also effective to supply an etching solution to the etching solution holding material 5 from the outside as necessary.

In the etching process according to the present invention, the etching solution 2 and the fiber elements 4 constituting the etching solution holding material 5 are mixed in the space to which ultrasonic waves are applied, and the etching process is performed with both in contact with the surface to be etched at the same time. In this state, the ultrasonic propagation mode becomes complex due to differences in the ultrasonic transmission speeds between the fiber elements 4 and the etching solution 2, suppressing the generation of standing waves near the surface to be etched, and in particular, even when the power density of the applied ultrasonic waves is increased, erosion of the material to be etched due to localization of ultrasonic energy can be suppressed.

In particular, when etching a system that generates solid-phase reaction products through chemical reactions associated with etching is performed using the etching process according to the present invention, it has been observed that the reaction products are less likely to adhere to the surface being etched. For example, as described in Patent Documents 4 and 5, particularly in etching processes for iron-based materials, a phenomenon occurs in which solid-phase reaction products precipitate and adhere to the etching surface through chemical reactions with the etching solution, which hinders uniform etching, and therefore it has been necessary to remove the solid-phase reaction products by mechanical polishing such as buffing during the etching process.

On the other hand, according to the etching method of the present invention, adhesion of reaction products to the surface to be etched is suppressed, so that the etching process can be completed without interruption, enabling efficient processing.
As described above, the mechanism by which the etching method of the present invention suppresses adhesion of reaction products to the etched surface is not clear; however, it is thought that the stirring effect of the etching solution holding material 5 as described above suppresses changes in the components of the etching solution near the surface of the etched surface, thereby preventing a local increase in the concentration of reaction products and suppressing the amount of precipitation.

In addition, after performing the etching process of the above system with the etching solution holding material 5 in contact, solid-phase precipitates may be observed inside the etching solution holding material 5. This suggests the existence of a mechanism in which precipitates are mechanically removed from the etched surface by contact with the etching solution holding material 5 in the early stages of precipitation of reaction products, or the presence of the etching solution holding material as a solid phase near the etched surface causes nucleation and precipitation of reaction products on the surface of the etching solution holding material.

In the etching process according to the present invention, etching solutions that are commonly used can be used depending on the material to be etched. For example, in the etching process for steel materials, an aqueous solution of ferric chloride, an ethanol solution of nitric acid, an aqueous solution of picric acid, an ethyl ether solution, etc. can be used. In addition, in the etching process for titanium alloys, an aqueous mixed acid solution of hydrofluoric acid and nitric acid can be used, for aluminum alloys, an aqueous mixed acid solution of hydrofluoric acid, hydrochloric acid, nitric acid, etc., an aqueous caustic soda solution can be used, and for copper alloys, an ethanol solution of hydrochloric acid or an aqueous ammonium persulfate solution can be used. In addition, for materials to be etched other than metals, for example, for glass, etc., an aqueous hydrofluoric acid solution can be used for etching.

In the etching process according to the present invention, any ultrasonic application means can be used to apply ultrasonic waves to the etching solution holding material impregnated with the etching solution, as long as it can excite vibrations with a frequency of about 20 to 200 kHz with a specified output.

For example, when performing an etching process by placing an etching solution holding material 5 impregnated with an etching solution in contact with a surface to be etched as shown in FIG. 2, ultrasonic waves can be applied to the etching solution holding material 5 and the etching solution held in the etching solution holding material by exciting ultrasonic waves to a transducer 6 placed on the back surface of the etching solution holding material 5 using an ultrasonic generating means sold under the name of, for example, an ultrasonic cutter.

In addition, when performing an etching process by immersing the surface to be etched in an etching solution as shown in FIG. 3, for example, an ultrasonic cleaner or other device for applying ultrasonic waves to liquid can be used to place the etching solution and the etching solution holding material inside a tank to which ultrasonic waves can be applied, and the object to be etched is placed so that the surface to be etched is in contact with the etching solution holding material, thereby applying ultrasonic waves to the etching solution and the etching solution holding material 5.

The etching process according to the present invention can be performed by contacting an etching solution holding material for holding the etching solution with the surface to be etched, which is a conventional etching process in which a solid surface is wet-etched using an etching solution to which ultrasonic waves are applied. By applying this to various etching processes for solid surfaces such as metals and inorganic materials, the etching process time can be shortened and the interruption of the etching process can be suppressed by suppressing the adhesion of reaction products.

The etching process according to the present invention can be preferably applied to systems in which the concentrations of various components in the etching solution are kept low or etching is required at low temperatures, for example, to increase the selectivity ratio of the etching reaction on the surface to be etched, and etching takes a long time.

In addition, by applying the etching process according to the present invention to a system in which a solid-phase reaction product is generated by the etching reaction and adheres to the surface to be etched, such as an etching process for observing the structure of an iron-based material, the adhesion of the solid-phase reaction product to the surface to be etched is suppressed, making it possible to perform highly accurate etching efficiently.

In addition to the wet etching process as a pretreatment for observing the crystal structure of the above metals, the etching process according to the present invention can also be applied to etching processes aimed at smoothing or roughening the surfaces of metals, glass, etc., and can be preferably used in systems in which the etching reaction causes significant consumption of the etching solution, systems in which the acceptable range of the composition of the etching solution when causing the desired etching reaction is narrow, and systems in which the etching reaction produces solid-phase reaction products that inhibit the desired etching process.

The etching process according to the present invention will be explained in more detail below, taking as an example the appearance of prior austenite grain boundaries observed in steel materials that have undergone quenching treatment, but the present invention should not be interpreted as being limited to these examples.

Table 1 shows the steel materials used as samples for the etching process according to the present invention, and the heat treatment conditions. All of the steel materials used in the evaluation had a thermal history of a so-called quenching process, in which the steel was heated to a temperature range where an austenite structure was formed, and then rapidly cooled to introduce a martensite structure with high hardness etc., by a non-diffusion phase transformation, and the crystal structure had prior austenite grain boundaries, which were the crystal grain boundaries of the austenite structure.

In steel materials that have been quenched, mechanical properties such as strength, toughness, and corrosion resistance change depending on the size of the crystal grains when the material is austenitized, so prior austenite grain boundaries are observed for the purpose of material management, etc. On the other hand, as described in Patent Document 5, for example, etching to reveal prior austenite grain boundaries in various quenched steel materials is accompanied by various problems, and measures to avoid these problems have been proposed, making this an extremely difficult etching process.

Figure 4 shows a schematic diagram of the apparatus used to evaluate the etching process according to the present invention. The samples used were each steel material cut into a cube with a side length of approximately 1 cm, embedded in epoxy resin with a diameter of φ30 mm and a height of approximately 25 mm so that one side of the cube was exposed, and used as the etched object (sample) 1. The exposed surface of the steel material was mirror-polished using emery paper and buff polishing, and this surface was used as the etched surface for the following etching process.

The etching solution 2 is generally used for revealing prior austenite grain boundaries, and is made by dissolving 2 g of picric acid, 0.15 g of sodium chloride, 0.07 g of sodium sulfate, and 5 g of surfactant in 100 g of water. Each sample was immersed in a glass beaker (volume: 500 ml) as an etching tank 8 containing approximately 250 ml of the etching solution at room temperature for a specified period of time to perform the etching process.

In the etching process, when the etching solution holding material 5 was brought into contact with the surface to be etched, a nonwoven fabric (Bencotto (registered trademark)) made of regenerated cellulose fibers (cupra) with an average fiber diameter of about 10 μm was laid on the bottom of the beaker as the etching solution holding material 5 (thickness in dry state: about 1 mm), and the sample was placed on the etching solution holding material with the surface to be etched facing downward, and the surface to be etched and the etching solution holding material were brought into contact with each other under the weight of the sample. In addition, when the etching solution holding material 5 was not used, the sample was immersed in the etching solution with the surface to be etched facing upward.

In the treatment in which ultrasonic waves were applied during etching, the etching tank 8 in which the sample was immersed was quickly placed in an ultrasonic cleaner 12 (Bransonic 1210J type: Emerson Japan, Ltd.) containing tap water as a vibration transmitter 11, and ultrasonic waves were applied for a predetermined time at room temperature. In the treatment in which ultrasonic waves were not applied during etching, the etching tank 8 in which the sample was immersed was left stationary for a predetermined time to perform the etching treatment.
After etching for a specified time, the samples were removed and washed with water. If necessary, inclusions adhering to the etched surface were washed off with absorbent cotton soaked in diluted neutral detergent and running water, and then the samples were dried with a blower and observed under an optical microscope.

Figures 5A to 5E show optical microscope photographs of the etched surface of the S45C material after etching under various conditions. In Figures 5A to 5D, the surface to be etched in which ultrasonic waves were applied during the etching process is indicated as "US," and the surface to be etched in which the etching solution holding material was brought into contact is indicated as "BF," with the immersion time in the etching solution also indicated.

As shown in Figure 5A, in the sample where ultrasonic waves were applied while the etched surface was in contact with the etching solution holding material, a contrast that was thought to be a prior austenite grain boundary that separates prior crystal grains of about several tens of microns was obtained after 5 minutes of etching. In addition, no scratches or other defects that were thought to be caused by the etching solution holding material that was in contact with the sample during etching were observed on the surface of the sample.

On the other hand, in the sample to which ultrasound was applied without using an etching solution retaining material (Figure 5B), contrast derived from crystalline structure such as prior austenite grain boundaries was not obtained by the 5-minute etching process. Moreover, in the sample that was left in a state where the etched surface was in contact with the etching solution retaining material (Figure 5C), contrast derived from crystalline structure was not obtained by the 5-minute etching process. Furthermore, in the sample that was treated by immersing in the etching solution for 30 minutes (Figure 5D), contrast derived from crystalline structure was not obtained.

In addition, in a sample in which ultrasonic waves were applied for 10 minutes while the etched surface was in contact with an etching solution holder (Figure 5E), contrast was observed to occur mainly within the crystal grains, compared to a sample etched for 5 minutes (Figure 5A), suggesting that etching for about 5 minutes is sufficient to reveal prior austenite grain boundaries.

These results show that applying ultrasonic waves to the surface to be etched while an etching solution holding material impregnated with etching solution is in contact with the surface to be etched can reveal clear prior austenite grain boundaries in a short period of time. This result is believed to be due to the fact that applying ultrasonic waves while the etching solution holding material is in contact with the surface to be etched effectively stirs the boundary layer of the etching solution that forms near the surface of the surface to be etched, suppressing changes in the composition of the etching solution near the surface of the surface to be etched, and allowing the chemical reaction between the surface to be etched and the etching solution to continue to progress.

Figures 6A to 6C show optical microscope photographs of the etched surface of the above-mentioned SCr415H material after etching under various conditions. The descriptions of "US", "BF", immersion time in the etching solution, etc. in Figures 6A to 6C are the same as those in Figure 1.

As shown in Figure 6A, the etching process according to the present invention, in which ultrasonic waves are applied while an etching solution holding material is in contact with the surface to be etched, produced a contrast believed to be a prior austenite grain boundary that divides grain sizes of about tens of microns, even in SCr415H material, after a 5-minute etching process. On the other hand, in the sample to which ultrasonic waves were applied without using an etching solution holding material (Figure 6B) and the sample that was left stationary while in contact with the etching solution holding material (Figure 6C), contrast believed to be a prior austenite grain boundary was observed in some places, but the clarity was not as clear as in Figure 6A.

7A and 7B show optical microscope photographs of the etched surface of the SCM435 material after the etching process according to the present invention. The notations "US", "BF", and immersion time in the etching solution in Fig. 7A and 7B are the same as those in Fig. 5.
In particular, as shown in FIG. 7A, it was observed that by subjecting SCM435 material to the etching process according to the present invention, a contrast believed to be a prior austenite grain boundary clearly appeared even after etching for a short period of time, such as about 2 minutes.

The appearance of prior austenite grain boundaries and the like by etching is influenced by the individual structure of the grain boundaries, and in particular, it is believed that the ease with which prior austenite grain boundaries are revealed varies depending on the alloy components present as precipitates at the grain boundaries, and that the etching selectivity with respect to the grain interior varies. From this, it was inferred that the degree of appearance of prior austenite grain boundaries and the like varies depending on the alloy components contained in the steel material and the heat treatment history, and that the degree of effect of the etching treatment according to the present invention varies depending on the steel material used as the sample.

On the other hand, as described above, the etching process according to the present invention makes it possible to reveal prior austenite grain boundaries even in materials where this is difficult, and it has been shown that prior austenite grain boundaries can be clearly revealed by a short etching process.

Figure 8 shows (photograph) the state of the etching solution holding material (nonwoven fabric) after a sample containing the S45C material was etched for five minutes using the etching solution by applying ultrasonic waves while the etching solution holding material was in contact with the surface to be etched. After etching steel materials such as the S45C material using the etching process according to the present invention, it was observed that black stains, as shown in Figure 8, adhered to the etching solution holding material (nonwoven fabric) immersed in the etching bath 8 at all points in contact with the surface to be etched of the sample.

As described in Non-Patent Document 5 and other publications, when etching steel materials using an etching solution containing picric acid or the like, reaction products adhere to the sample surface, and it is known that in order to achieve a uniform etching process, the products must be removed mechanically in a timely manner. On the other hand, in the etching process according to the present invention, it was observed that there was significantly less adhesion of reaction products to the sample surface after etching was completed.

From the above, it was inferred that the dirt adhering to the nonwoven fabric shown in Figure 8 was the result of products that precipitated on the sample surface during etching being mechanically removed by the nonwoven fabric used as an etching solution holding material, or that the products precipitated on the fiber elements that make up the nonwoven fabric.

For this reason, the etching process according to the present invention suppresses the adhesion of reaction products generated by the etching process to the surface to be etched, and prevents the reaction products from acting as a mask and causing the etching process to become non-uniform, making it possible to perform a high-quality etching process. Furthermore, even in etching processes that generate a large amount of reaction products, the suppression of the adhesion of reaction products to the surface to be etched makes it possible to perform continuous etching, thereby making the process more efficient.

The present invention enables efficient etching of solid surfaces, and by applying it to the quality control and surface treatment of various metal structures for which etching has traditionally been used, it is possible to improve efficiency and quality.

Reference Signs List 1: Object to be etched 2: Etching solution 3: Cloth 4: Fiber element 5: Etching solution holding material 6: Vibrator 7: Ultrasonic wave generating means 8: Etching tank 9: Etching solution leakage prevention wall 10: Weighted body 11: Vibration transmitting object (water)
12 Ultrasonic cleaner 20 Vibration

Claims (7)

1. An etching method for performing wet etching using an etching solution on a solid surface as an etching target surface to reveal a crystal structure of the etching target surface , comprising the steps of:
An etching method comprising applying ultrasonic waves to an etching solution holding material impregnated with an etching solution while the etching solution holding material is in contact with the surface to be etched. The etching method according to claim 1, characterized in that the etching solution holding material is impregnated with the etching solution by being immersed in the etching solution held inside the etching tank. The etching method according to claim 1 or 2, characterized in that the etching solution holding material contains a material with an official moisture content of 3% or more. The etching method according to any one of claims 1 to 3, characterized in that the solid is a steel material and the purpose is to reveal the crystal structure of the steel material. An etching processing apparatus that performs wet etching using an etching solution on a solid surface as an etching target surface to reveal a crystal structure of the etching target surface ,
An etching solution holding material capable of holding an etching solution therein;
an ultrasonic wave applying means capable of applying ultrasonic waves to the etching solution holding material;
1. An etching treatment apparatus, characterized in that ultrasonic waves can be applied to an etching solution holding material impregnated with an etching solution while the etching solution holding material is in contact with a surface to be etched. An etching tank for holding the etching solution,
6. The etching processing apparatus according to claim 5, wherein the etching solution holding member is placed inside the etching tank. 7. The etching processing apparatus according to claim 5, wherein the etching solution holding material contains a material having an official moisture content of 3% or more.

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