JP2024111338A - Aluminum-based member and manufacturing method thereof - Google Patents

Abstract

[Problem] To provide an aluminum-based member having an anodized film that is excellent in crack resistance at high temperatures and adhesion to an upper layer (anchor effect). [Solution] The present invention is an aluminum-based member in which at least a portion of the surface of an aluminum base is coated with an anodized film. The anodized film has a main hole extending in a substantially straight tube shape from the aluminum base side and a sub-hole opening in a portion of the side wall of the main hole. The anodized film has a porosity of 18 to 65% as determined by observing the top surface, for example. The main hole has an opening diameter of 50 to 1000 nm, for example. It is preferable that the sub-holes are distributed more on the opening side of the main hole than on the bottom side. Such an anodized film can be obtained, for example, by passing an electric current under specified conditions to an aluminum base in contact with an electrolyte containing phosphoric acid. [Selected Figure] Figure 1

Description

The present invention relates to an aluminum-based component having an anodized film on at least a portion of its surface.

Anodizing is performed on aluminum-based members (Al-based members) to ensure corrosion resistance, insulation, etc. Anodizing is performed by passing electricity through an aluminum base material (Al base material) immersed in an electrolytic solution bath (sulfuric acid bath, oxalic acid bath, etc.) as an anode. As a result, an anodic oxide film (anodized film) made of aluminum oxide (Al ₂ O ₃ , etc.) is formed on the surface of the Al base material (treated surface). Anodizing differs from plating and the like in that it involves oxidation of the Al base material itself.

An anodic oxide film usually consists of a dense, thin barrier layer (active layer) and a porous layer (growth layer) grown on top of it. In a typical anodic oxide film, the majority of the film is made up of a porous layer consisting of roughly straight, tubular micropores (groups) that open on the surface side. An anodic oxide film intended to improve corrosion resistance can also be subjected to a sealing process to close the openings in the porous layer or a sealing process to fill the holes.

Incidentally, in addition to ensuring corrosion resistance, anodized films are also used as underlayers (bases) that ensure the adhesion and bonding properties of upper layers (e.g., resin layers such as coatings) formed on the surface. For example, the following patent documents contain descriptions related to such anodized films:

U.S. Patent 4,085,012 Patent Publication 2008-13805 Patent Publication No. 2013-76118 Patent Publication No. 2021-75763

In Patent Document 1, an anodic oxide film is formed by passing electricity (3 to 25 V) through pure aluminum that is in contact with an electrolyte (65 to 95°F/18 to 35°C) containing phosphoric acid (3 to 20%) (see TABLE 1 in Patent Document 1). Patent Document 1 does not disclose anything about the form of the anodic oxide film.

In Patent Document 2, an anodic oxide film containing an organic compound component is formed by passing a current (2 A/ ^dm2 , 14 V) through an Al-Mn-based substrate (A3004) in contact with an electrolyte (50°C) containing sodium phosphate (5 wt%) and tannic acid (1.25 wt%). This anodic oxide film has a thickness of 20 nm, a water content of 2%, and a porosity of 3%, and is very thin and dense overall (see Example 5 of Patent Document 2).

In Patent Document 3, an anodic oxide film is formed by passing a current (2-4 A/cm ² , 20-80 V) through an Al-Mg-Si-based substrate (A6061) in contact with an electrolyte (15-25°C) consisting of an aqueous phosphoric acid solution (20% by weight). This anodic oxide film has columnar cells consisting of holes extending in the axial direction and branch holes extending vertically from the inner peripheral surface (see FIG. 1 of Patent Document 3). However, Patent Document 3 does not include any microscopic photographs, and the specific form is unclear. Since the substrate containing Si is anodized, it is presumed that the branch holes are very small and were formed in the process in which the porous layer grew while avoiding the Si.

In Patent Document 4, a first oxide film is formed by passing electricity (15 V) through an Al-Mg-based substrate (A5052) in contact with an electrolyte (15°C) consisting of an aqueous phosphoric acid solution (1.46 mol/L), and then the substrate is further brought into contact with an electrolyte (20-23°C) consisting of an aqueous sulfuric acid solution (3.67 mol/L) and electricity (12.5 V) is passed through to form a second oxide film. The film formed in this way has a complex shape that differs from an anodic oxide film with pores that extend in a roughly straight tube shape (see Figure 3 in Patent Document 4).

The present invention was made in consideration of these circumstances, and aims to provide an aluminum-based component having a new type of anodized film.

As a result of intensive research into solving the above problems, the inventors succeeded in forming an anodic oxide film with sub-holes that penetrate the side walls of a main hole that extends in a substantially straight tube shape. By expanding on this result, the inventors were able to complete the present invention, which is described below.

<Aluminum-based components>
(1) An aluminum-based member (also referred to as "Al-based member") of the present invention is an aluminum-based member in which at least a portion of the surface of an aluminum base is covered with an anodized film, the anodized film having a main hole extending in a substantially straight tubular shape from the aluminum base side and a sub-hole opening in a portion of a side wall of the main hole, and the anodized film has a porosity of 18 to 65% as determined by observing the top surface.

(2) The anodized film according to the present invention has a high overall porosity, and in addition to the main holes that extend in a generally straight tube shape, it also has sub-holes that open into the side walls of the main holes. For this reason, for example, when an upper layer (e.g., a resin layer) is formed on the anodized film, the constituents (e.g., resin) can easily penetrate, fill, and engage not only the main holes but also the sub-holes, and it is expected that the adhesion and bonding properties of the upper layer will be improved due to an increased anchor effect, etc.

The anodized film according to the present invention has a high porosity, with sub-holes in addition to main holes, and is therefore highly flexible. Such an anodized film is less likely to crack even when subjected to impact or thermal stress, and can exhibit excellent impact resistance and heat resistance.

<<Manufacturing methods for aluminum-based components, etc.>>
The present invention can also be understood as a method for producing an aluminum-based member or an anodizing method. For example, the present invention may be a method for producing the aluminum-based member described above, comprising an electrolysis step of applying current to an aluminum base material that has been brought into contact with an electrolytic solution containing phosphoric acid.

When the electrolyte contains phosphoric acid, the formation of the anodic oxide film described above is promoted, and the surface of the anodic oxide film can be coated with an insoluble phosphate film. The phosphate film suppresses natural pore sealing (hydration reaction with moisture in the air) of the anodic oxide film. Therefore, for example, even if the anodic oxide film is left in the air for a long time, the main pores and sub-pores remain open, and the function of the anodic oxide film as a base layer can be stably ensured.
"others"

(1) In this specification, for ease of explanation, the outermost surface side of the anodic oxide film (the opening side of the main hole) is referred to as the upper side or top side. Conversely, the opposite side (the Al substrate side) is referred to as the lower side or bottom side. For convenience, the film thickness direction of the anodic oxide film (usually the growth direction of the main hole) is also referred to as the vertical direction, and the direction approximately perpendicular to that is also referred to as the horizontal direction.

The porosity and opening diameter are evaluated based on an image (observation image) obtained by observing the top side of the anodized film under a microscope. The presence (number) and shape of sub-holes can also be evaluated based on the observation image of the top side, but evaluation based on the observation image of the side (cross-section) of the anodized film (longitudinal cross-section obtained by cutting in the film thickness direction) is preferable. Sub-holes should be evaluated at least in the surface layer region (depth 1 μm × width 10 μm) near the outermost surface of the anodized film.

The observed image can be evaluated, for example, using image analysis software. The number (distribution) of subholes can be counted visually based on the observed image.

The field of view (evaluation range) of the observed image is not important, but it may be, for example, 0.6 x 0.5 μm to 12.7 x 8.8 μm.

Porosity is the ratio of the pore area (total value) to the total area of the anodized film within the observed field of view. The aperture diameter is the average arithmetic value of the circle-equivalent diameter of each pore within the same field of view. The porosity and aperture diameter are calculated, for example, using image analysis software (Image J).

Unless otherwise specified, the thickness of the anodic oxide film is the measured value using a film thickness meter. If appropriate, the film thickness may be determined as the distance (average value) from the bottom surface of the anodic oxide film (the bottom surface of the barrier layer) to the top surface (the top surface of the porous layer) based on the observed image.

(2) Unless otherwise specified, "x to y" in this specification includes a lower limit of x and an upper limit of y. Any number included in the various numerical values or numerical ranges described in this specification may be used as a new lower limit or upper limit to create a new range such as "a to b."

1 is a SEM image of the upper surface of an anodic oxide film according to a first example. This is an SEM image of the upper side of the cross section of the anodic oxide film. This is an SEM image of the lower side (bottom side) of a cross section of the anodic oxide film. 13 is a SEM image of the upper side of a cross section of an anodic oxide film according to a second embodiment. This is an SEM image of the lower side (bottom side) of a cross section of the anodic oxide film. 13 is a SEM image of the upper surface of an anodic oxide film according to the third example. 13 is a SEM image of the upper surface of an anodic oxide film according to the third example.

The contents described in this specification may apply not only to the products of the present invention (Al-based members, anodized films, etc.) but also to methods (methods for manufacturing Al-based members, anodization methods, etc.) as appropriate. One or more components selected arbitrarily from this specification may be added to the components of the present invention. Which embodiment is best depends on the target, required performance, etc.

<Anodic oxide film>
The anodized film has a main hole extending in a substantially straight tube shape from the Al base side, and a sub-hole opening in a part of the side wall of the main hole.

(1) Main hole The main hole is, for example, a bottomed cylindrical hole extending from the bottom on the Al base side to the opening at the top. The form (shape, size) of the hole may vary slightly along the vertical direction (depth direction, axial direction, or extension direction).

The opening diameter of the main hole is, for example, 50 to 1000 nm, 100 to 500 nm, or 150 to 350 nm. Such opening diameters tend to be larger than conventional columnar holes (cells).

(2) Sub-hole The sub-hole opens in a part of the side wall of the main hole and extends laterally (in a direction substantially perpendicular to the side wall of the main hole). At least a part of the sub-hole may penetrate the side wall of the main hole (connect adjacent main holes).

The distribution of the sub-holes may be uniform or non-uniform along the longitudinal direction of the main hole. If the sub-holes are larger on the upper side (opening side) of the main hole than on the bottom side, the anchoring effect on the upper layer provided on the surface of the anodized film may be increased.

There should be at least one sub-hole in the field of view when observing the anodized film from the side. Furthermore, it is desirable to have at least one surface region (depth 1 μm × width 10 μm) that contains one or more, three or more, or even five or more sub-holes.

(3) Porosity The porosity determined by observing the top surface of the anodic oxide film is, for example, 18 to 65%, 20 to 55%, 25 to 50%, 30 to 45%, or 35 to 40%. Such porosity tends to be higher than that of conventional porous layers consisting of columnar holes (cells). It is believed that this allows the anodic oxide film according to the present invention to exhibit excellent flexibility (crack resistance), adhesion and bonding to upper layers, etc.

(4) Film Thickness The anodized film has a film thickness of, for example, 0.1 to 20 μm, 0.5 to 10 μm, or 1 to 5 μm. The film thickness of the anodized film includes the thickness of the so-called barrier layer. The thickness of the barrier layer is, for example, 10 to 1000 nm, 50 to 500 nm, or 100 to 300 nm. The group of main holes formed on the barrier layer is also referred to as the "porous layer" in this specification as appropriate.

(5) Components The anodized film is mainly made of aluminum oxide (Al ₂ O ₃ ), but may contain components derived from the composition of the electrolyte. For example, if an electrolyte containing phosphoric acid is used, P, H, etc. may be contained in the anodized film. In addition, phosphates (e.g. Al-P-O compounds, Al-P-O-H compounds) may be present near the surface of the anodized film.

<<Aluminum base material>>
As long as an anodized film having main holes and sub-holes is formed, an Al base material may be selected according to the application of the Al-based member. A representative Al base material is, for example, pure aluminum (JIS A1000 series) having an Al content of 98 mass% or more, 98.5 mass% or more, 99 mass% or more, or even 99.6 mass% or more relative to the total. If the Al-based member is a conductive member, an Al base material having a conductivity of 50% IACS (International Annealed Copper Standard) or more, 55% IACS or more, or even 60% IACS or more may be used. Note that "%IACS" is a relative index (ratio) to the conductivity (100%IACS) of annealed standard soft copper (volume resistivity: 1.7241×10 ⁻⁸ Ωm).

The Al substrate may be, for example, a wrought material, a cast material, a plastically processed material, a thermally sprayed material, etc. The Al substrate may contain small amounts (for example, a total of 1 mass % or less, or even 0.5 mass % or less) of modifier elements such as Mg, Fe, and Si to improve mechanical properties, etc.

《Substrate》
At least a part of the anodized film may have an adherend. The adherend may be, for example, a resin body, a ceramic body, a metal body, etc. The adherend may be a layer (including a film) or a block (bulk). In the case of a resin body, for example, there is a resin layer such as a coating film, a sealant, an adhesive, etc. In this specification, the adherend provided on the surface of the anodized film is appropriately referred to as an "upper layer" regardless of its form.

《Manufacturing method/anodizing method》
The anodic oxide film is formed by an electrolytic process in which an electric current is passed through an aluminum substrate in contact with an electrolyte.

(1) Electrolyte The composition of the electrolyte is not limited as long as the main pores and the sub pores are formed. The electrolyte may be, for example, an inorganic acid solution (such as an aqueous phosphoric acid solution, an aqueous sulfuric acid solution, or an aqueous chromic acid solution) or an organic acid solution (such as an aqueous oxalic acid solution).

When an electrolyte containing phosphoric acid is used, the phosphoric acid concentration may be, for example, 1 to 30%, 2 to 20%, or 3 to 10%. In this specification, the concentration of the electrolyte is the mass percentage relative to the total unless otherwise specified. The temperature of the electrolyte (bath temperature) may be, for example, 15 to 80°C, 20 to 60°C, or 25 to 40°C.

(2) Passing current The electrolysis process is performed, for example, by passing a constant current or a constant voltage. The current (density) or applied voltage may change (fluctuate) during the passage of current. The electrolysis process may be performed by direct current, alternating current, or superimposed AC/DC current in which an AC component and a DC component are combined. The waveform of the alternating current may be a sine wave, a rectangular wave, a pulse wave, or the like.

When the electrolysis process is carried out by direct current, for example, a high voltage of about 30 to 250 V, 40 to 200 V, or even 75 to 175 V may be applied to the Al substrate at least temporarily. There is no restriction on the counter electrode placed in the electrolyte, but a platinum electrode or a graphite electrode is usually used.

<<Uses>>
The Al-based member may be used in any application. When the anodized film is an insulating coating or a part thereof, the Al-based member may be a conductive member (electric wire, element, etc.). In this case, another insulator may be applied onto the anodized film. For example, a resin layer (insulating layer) made of an insulating resin (enamel material) such as polyimide, polyurethane, or epoxy may be provided on the anodized film. The resin layer may contain a filler. The filler may be, for example, a thermally conductive filler, an anti-surge filler, or a dielectric constant reducing filler.

A variety of anodizing treatments were performed under different conditions to produce several types of samples (Al-based components) in which an anodized film was formed on the surface of the Al base material. The present invention will be described in more detail below with reference to these specific examples.

[First Example/Constant Current Electrolysis]
<Sample Preparation>
(1) Al Substrate As an Al substrate to be anodized, a test piece (50 mm square × 2 mm thick) made of pure aluminum (JIS A1050) was prepared.

(2) Electrolysis Step The test piece was subjected to an electrolysis step under the conditions shown in Table 1. An aqueous solution of phosphoric acid, sulfuric acid or oxalic acid was used as the electrolyte. The concentrations shown in Table 1 are mass ratios to the entire aqueous solution.

The entire test piece was immersed in a treatment bath containing an electrolytic solution, and a constant current (density) direct current was applied to the test piece as the anode and the platinum electrode as the cathode while stirring the electrolytic solution. The current density shown in Table 1 is the value obtained by dividing the applied current value by the treated area of the test piece. The test piece was masked, and only a part of its surface (24 ^cm2 ) was designated as the treated surface.

The applied voltage fluctuates during the electrolysis process, but for reference, the measured voltage 10 minutes after the start of current application is also shown in Table 1.

(3) After the electrolysis process, the test pieces were removed from the electrolyte and thoroughly washed with distilled water. They were then sprayed with compressed air to remove moisture and thoroughly dried in the air. The anodized test pieces (samples 1, A1, and B1) obtained in this way were subjected to the measurements and observations described below.

Measurement and Observation
(1) Film Thickness The anodized surface of each sample (simply referred to as the "treated surface") was measured with an eddy current film thickness meter (SWT-9200, manufactured by Sanko Electronics Laboratory Co., Ltd.). The film thicknesses obtained are also shown in Table 1.

(2) Surface Observation The treated surface (surface/upper surface) of each sample was observed with a scanning electron microscope (SEM/S-5500 manufactured by Hitachi High-Technologies Corporation). The obtained SEM images are shown in FIG.

(3) Cross-section observation The longitudinal section (side section) of each sample was observed by SEM in the same manner. The obtained SEM images are shown in Figures 2A and 2B (collectively referred to as "Figure 2"). Figure 2A shows the upper side (front side) near the treated surface, and Figure 2B shows the lower side (bottom side).

Image Analysis
(1) Porosity Based on the SEM image (see FIG. 1) of the treated surface (upper surface), the porosity of each sample was determined by image analysis using ImageJ (free software). The arithmetic mean value of the porosity of five arbitrarily selected fields of view is also shown in Table 1.

In this case, the field of view for sample 1 was 1280 nm x 890 nm, and the field of view for samples A1 and B1 was 640 nm x 445 nm. The size of each field of view was adjusted so that each field of view contained 10 or more main holes.

The porosity was calculated as follows. First, the image to be analyzed was converted to 8 bits, and then binarized to extract the pores that were open on the surface. Next, each pore was processed by the Watershed Algorithm, and the total opening area was calculated for pores with an opening area of 100 ^nm2 or more. The ratio of the total opening area to the total area of the field of view was taken as the porosity.

(2) Aperture diameter A similar image analysis was performed based on the SEM image of the treated surface (upper surface) to determine the aperture diameter of each sample. Similarly, the arithmetic mean value of the aperture diameters for five fields of view is also shown in Table 1. The aperture diameter was determined as follows. First, as described above, the total value of the aperture area of each extracted hole is divided by the number of extracted holes to determine the average aperture area. The equivalent circle diameter (diameter when the hole is assumed to be a perfect circle) calculated from the average aperture area was used as the aperture diameter. Note that the image of sample 1 includes sub-holes in addition to the main holes, so the maximum diameter (maximum length) of any 10 main holes was measured, and the arithmetic mean value was used as the aperture diameter.

"evaluation"
(1) Anodized film As can be seen from Table 1, sample 1, which was subjected to the electrolysis process by applying a high voltage in an electrolyte containing phosphoric acid, had a significantly larger porosity and opening diameter than the other samples. Also, as is clear from Figures 1 and 2, in the anodized film of sample 1, in addition to the thick main holes extending in a substantially straight tube shape from the Al base side, many thin sub-holes were formed that penetrated into the side walls of the main holes. This type of anodized film had a unique form not seen in the anodized films of other samples.

(2) Sub-holes As can be seen from Fig. 2A, the anodized film of Sample 1 had multiple (approximately 10) sub-holes formed in the surface region (depth 1 μm × width 10 μm). Furthermore, as can be seen from comparing Fig. 2A with Fig. 2B, the sub-holes were larger in the upper region of the anodized film than in the lower region (bottom side, barrier layer side). Moreover, the density of the sub-holes tended to increase from the bottom to the top of the anodized film.

[Second Example/Upper Layer]
<Sample Preparation>
As in the first embodiment, the anodization treatment shown in Table 2 was performed. After repeatedly spraying polyimide varnish onto the obtained anodized film (application step), the anodized film was further heated in an oven (350°C x 1 hour) (thermal curing step). The test pieces (samples 2, A2, and B2) in which a resin layer (upper layer) was thus adhered onto the anodized film were subjected to the measurements and observations described below.

Measurement and Observation
(1) Film Thickness The film thickness of the resin layer of each sample was measured using the eddy current film thickness meter described above. The results are also shown in Table 2. The film thickness of the anodized film itself, which was measured before the coating process, is also shown in Table 2. The film thickness of the resin layer was determined by subtracting the film thickness of the anodized film alone from the total film thickness after the resin layer was formed. In other words, the film thickness of the resin layer shown in Table 2 means the distance from the outermost surface of the anodized film to the outermost surface of the resin layer.

(2) Surface Observation The surface (resin layer) of each sample was visually observed to check for the presence or absence of cracks. The results are shown in Table 2.

(3) Cross-Section Observation The longitudinal sections of each sample were observed by SEM. The obtained SEM images are shown in Figures 3A and 3B (collectively referred to as "Figure 3"). Figure 3A shows the upper side near the treated surface, and Figure 2B shows the lower side.

"evaluation"
(1) Crack Resistance As shown in Table 2, no cracks were observed on the surface of sample 2, indicating that it had excellent crack resistance (heat resistance). On the other hand, cracks were observed on the surface of the other samples. Such cracks are thought to be caused by thermal stress reflecting the difference in thermal expansion coefficient between the Al base material (metal) and the anodized film (ceramics). The anodized film of sample 2 has a high porosity similar to that of sample 1 and is highly flexible, so it is thought to have been less likely to crack.

(2) Adhesion As can be seen from Figure 3, in the case of sample 2, polyimide penetrated into the anodized film, filling the main holes and also being retained in the sub-holes. This situation was hardly observed in the anodized films of the other samples. The reason why the main holes in sample 2 were not filled with polyimide is thought to be because the polyimide broke and fell off when the test piece was cut.

[Third Example/Constant-potential electrolysis]
<Sample Preparation>
The test piece made of the above-mentioned Al base material was subjected to an electrolysis process using a constant voltage direct current under the conditions shown in Table 3. The other treatment conditions were the same as those in the first example.

Measurement and Observation
The test pieces (samples 31 to 36) of each sample after anodization treatment were measured in the same manner as in Example 1. The measurement results of the porosity and opening diameter are also shown in Table 3. However, one visual field of samples 31 to 33 was 2560 nm × 1780 nm, and one visual field of samples 34 to 36 was 1280 nm × 890 nm.

SEM images of the test piece surface (top surface) of each sample are shown together in Figures 4A and 4B (both figures collectively referred to as "Figure 4"), as in the first example.

"evaluation"
As can be seen from Table 3 and FIG. 4, when an electrolyte containing phosphoric acid was used, the porosity and opening diameter increased as the temperature (bath temperature) increased or the applied voltage increased.

From the above, it was confirmed that the present invention produces an anodic oxide film with a unique shape, which has crack resistance at high temperatures and can exert a strong anchor effect on the upper layer.

Claims (14)

An aluminum-based member in which at least a part of the surface of an aluminum base is coated with an anodized film,
the anodic oxide film has a main hole extending in a substantially straight tube shape from the aluminum base material side and a sub-hole opening in a part of a side wall of the main hole,
The anodized film is an aluminum-based member having a porosity of 18 to 65% as determined by observing the top surface. The aluminum-based component according to claim 1, wherein the main pores have an opening diameter of 50 to 1000 nm. The aluminum-based component according to claim 1, wherein the opening side of the sub-hole is larger than the bottom side of the main hole. The aluminum-based component according to claim 3, wherein the sub-hole is present in one or more of the surface regions (depth 1 μm × width 10 μm) of the observed side surface. The aluminum-based component according to claim 1, wherein at least a portion of the sub-hole penetrates the side wall of the main hole. The aluminum-based member according to claim 1, wherein the anodized film has a thickness of 0.1 to 20 μm. The aluminum-based member according to claim 1, wherein the aluminum base contains 98% by mass or more of Al. The aluminum-based member according to claim 1, wherein the aluminum base material has a conductivity of 50% IACS or more. The aluminum-based component according to claim 1, further comprising an upper layer that covers at least a portion of the anodized film. The aluminum-based member according to claim 1, wherein at least a portion of the anodized film is coated with a resin. The aluminum-based member according to claim 1, which is a conductive member. The method includes an electrolysis step of applying a current to an aluminum substrate that is in contact with an electrolytic solution containing phosphoric acid,
A manufacturing method for obtaining the aluminum-based member according to any one of claims 1 to 11. The method for manufacturing an aluminum-based component according to claim 12, wherein the electrolysis step is performed with the electrolytic solution at a temperature of 15 to 80°C. The method for manufacturing an aluminum-based component according to claim 12, wherein the electrolysis step is performed by applying 30 to 250 V to the aluminum base material.