JP7178020B2 - Surface cleaning method and bonding method for thin metal plate - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applies Published on October 15, 2017 at "The 10th Asian Workshop on Micro/Nano Forming Technology"

The present invention relates to a technique for cleaning the surfaces of thin metal sheets, and more particularly, to a technique for cleaning (activating) the surface of each thin stainless steel sheet when diffusion bonding a plurality of thin stainless steel sheets.

Metallic micromechanical elements, typified by micropumps, have complicated internal structures in order to improve their functional performance.
For example, in the case of a micropump, a multi-layered structure of (2+6) layers such as a driving plate + solution reservoir, valves for suction/sweep out, a liquid volume adjustment part, an inlet/outlet part, etc. is required.
Even if each of these is produced by a cutting process or chemical etching, it is necessary to finally join and integrate them.

For austenitic stainless steel materials that are widely used in general, a technology for joining a plurality of members by a diffusion bonding method that performs solid phase bonding at a temperature below the melting point has been put into practical use.
In this method, the metal plates are heated to a specified temperature in a vacuum, the surfaces to be joined are pressurized, and if they are held for a while, they are joined by the diffusion phenomenon of the metal. , is a technique that can reliably bond the bonding surfaces (see Non-Patent Document 1).
What is diffusion bonding (thermocompression bonding) Internet URL: http://www.yama-tech.com/kakusan/ Search date: March 12, 2018

In order to secure the necessary bonding strength by diffusion bonding, it is required to remove the passive film (mainly chromium oxide) on the surface of the stainless steel plate in advance.
For this reason, in the conventional diffusion bonding, as shown in FIG. Temperatures above 900° C. have been chosen (see Non-Patent Document 2).
Effect of Surface Composition on Diffusion Bonding of Stainless Steel/Journal of Japan Institute of Metals, Vol.56, No.5 (1992), pp.579-585, Osamu Ohashi, Seiichiro Suga

As described above, in the conventional electric heating method, it is necessary to hold the pressure load at a high temperature for a relatively long time (for example, 2 hours) in addition to holding the heat at 900 ° C. or higher. There are many problems such as deformation and material change inside, distortion due to uneven heating, deformation during cooling, increase in residual stress, increase in takt time for one product production.

In particular, in the case of austenitic stainless steel, grains grow with recrystallization while the temperature is maintained at 900° C. or higher. Therefore, if it is desired to maintain the strength and ductility of the base material before and after diffusion bonding, it is desired to develop a technique that can lower the diffusion bonding temperature as much as possible.

Patent Documents 1 and 2 have so far reported on lowering the diffusion bonding temperature of austenitic stainless steel.
Patent No. 6082866 JP 2016-140883

In Patent Document 1, diffusion bonding is performed by using a WC material that has been subjected to a reduction of 90% in advance to SUS304, which is a general austenitic stainless steel (Example 6 of the same document).
In this case, as shown in FIG. 15, a comparison at the point of "peeling strength: 2 KN" reveals that the bonding temperature of the SOL material to which the strain was removed and the solution heat treatment was applied was 800°C, whereas the above-mentioned In the case of WC material, it has been confirmed that the temperature can be lowered to nearly 700°C.

However, in this method, the sample is previously buffed with emery paper to remove the passive film and improve the surface roughness.
Also, although the results were obtained with a bonding time of 30 minutes, it is expected that a heating and holding time of 30 minutes or more will be required under practical conditions.
Furthermore, it must be said that it is lacking in convenience because it is necessary to prepare a dedicated material for strong reduction.

Patent Document 2 discloses a technique of boiling SUS304 with an organic acid such as formic acid or citric acid to form and activate an organic acid salt film on the surface of a stainless steel material.
As a result, as shown in FIG. 16, the peel strength of the two samples after diffusion bonding was about 300 N at 1073 K (800 ° C.) for both formic acid (FA) and citric acid (CA). , and achieves nearly three times the strength of an untreated product (as polished).
However, forming the organic acid salt film on the surface of the thin metal plate itself is a lot of trouble, and is also lacking in simplicity.

As can be seen from these examples, it is believed that no process has been established so far to efficiently remove the passivation film on the surface of austenitic stainless steel at low temperatures while also lowering the diffusion bonding temperature. is common.
The present invention has been devised to solve the problems of the prior art, and aims to establish a technology that enables efficient diffusion bonding between thin metal plates at a relatively low temperature. .

In order to achieve the above object, the method for cleaning the surface of a thin metal plate described in claim 1 generates plasma from a mixed gas of hydrogen and argon on at least one side of a thin metal plate to be diffusion-bonded, and cleans the metal. It is characterized by removing at least part of the passive film formed on the surface of the thin plate.

In the method for cleaning the surface of a thin metal plate described in claim 2, the mixing ratio of hydrogen and argon is adjusted to optimize the physical activation action of argon and the chemical reduction action of hydrogen. is characterized by

A method for cleaning the surface of a thin metal plate according to claim 3 is characterized by arranging a plurality of thin metal plates at predetermined intervals and generating plasma between both surfaces of each of the thin metal plates.

A method for joining thin metal plates according to claim 4 is provided by laminating a plurality of thin metal plates subjected to the surface cleaning treatment according to any one of claims 1 to 3 and applying a predetermined temperature and pressure for a predetermined time. It is characterized by diffusion bonding of the surfaces of the respective thin metal plates.

In the method for joining thin metal plates according to claim 5, a laminate in which a plurality of thin metal plates are laminated is arranged in a vacuum chamber, and an insulating material having excellent thermal conductivity is placed on the upper and lower surfaces of the laminate. A heating material made of an induction-heatable metal material is placed on the surface of each release material, and a heat insulating material made of an insulating material with low thermal conductivity is placed on the surface of each heating material. The thin metal plates are diffusion-bonded by applying pressure to the surface of the heat insulating material at the same time that a high-frequency current is applied to a dielectric heating coil arranged outside the vacuum chamber.

In the method for cleaning the surface of the thin metal plate described in claims 1 and 2, a method of removing the passivated film of the thin metal plate by the physical activation action of argon and the chemical reduction action of hydrogen is adopted. Therefore, processing at relatively low temperatures is possible.
As a result, it is possible to efficiently remove the passivation film of the metal thin plate in a short time while suppressing distortion, deformation, etc. of the metal thin plate.

In the method for cleaning the surfaces of thin metal plates, it is possible to efficiently remove passivation films formed on both surfaces of a plurality of thin metal plates in one treatment.

In the case of the method for joining thin metal plates according to claim 4, since the thin metal plates from which the passivation film has been removed in advance by plasma treatment are diffusion-bonded, joining is performed at a lower temperature than in the conventional joining method. becomes possible. Alternatively, higher bonding strength can be achieved when bonding at a high temperature comparable to the conventional one.

In the case of the method for joining thin metal plates according to claim 5, by the principle of so-called high-frequency induction heating (IH), the stack of thin metal plates is efficiently heated with a heating material, and the space between the plurality of thin metal plates is heated once. can be diffusion bonded to

FIG. 1 is a schematic diagram showing the structure of an RF-DC plasma device 10 used for surface cleaning of a thin metal plate. and an RF oscillator 18 located outside the vacuum chamber 12 .
Although not shown, outside the vacuum chamber 12, a control device for the RF oscillator 18, a control device for the DC bias 14, and a cooling device are installed.

Inside the vacuum chamber 12, a plurality of thin metal plates 22 are vertically suspended by a clothesline-shaped jig 20 arranged horizontally.
Each thin metal plate is made of an austenitic stainless steel sheet such as SUS304, and a predetermined distance is secured between them.
As shown in FIG. 2, a pair of jigs 20 are arranged side by side in the vacuum chamber 12, and four thin metal plates 22 are suspended from each jig. Therefore, a total of eight metal sheets 22 are set at once.

Next, after the inside of the vacuum chamber 12 is evacuated to a predetermined reference pressure, a mixed gas of argon and hydrogen is introduced and maintained at a predetermined pressure, and an RF voltage and a DC bias voltage are applied to ignite plasma. At this time, external heating is not performed.
FIG. 3 is a photograph showing the state of plasma generation, and it can be confirmed that plasma 24 is induced between each of the thin metal plates 22 arranged in the vertical direction, and both surfaces are cleaned at the same time.

As shown in FIG. 4(a), a passivation film 26 made of chromium oxide or the like is formed on the surface of the base material of the thin metal plate 22. However, the physical action of argon sputtering and the chemical action of hydrogen By this action, the passivation film 26 can be made thin and discontinuous at a low temperature of 750° C. or less (see FIG. 4(b)).

Thus, the present invention utilizes physical surface activation by argon and chemical reduction by hydrogen.
Since 100% argon lacks a chemical effect and 100% hydrogen cannot be expected to have a physical effect, it is necessary to consider the optimum ratio of both.

Here, the flow rate of hydrogen is increased by diluting argon, plasma diagnosis is performed at each concentration, and the intensity of excited argon nuclides (wavelength: 740 nm) and excited hydrogen nuclides (wavelength: 610 nm) are analyzed by spectroscopic measurement diagnosis. Using the ratio as a parameter, we investigated its change.

As a result, as shown in FIG. 5, the intensity of hydrogen was maximized at a hydrogen concentration of 20%. This maximizes the reducing action of hydrogen against the physical sputtering effect of argon.
However, the present invention is not limited to a hydrogen concentration of 20%.

FIG. 6 is an SEM image showing the effect of plasma cleaning treatment.
First, FIG. 6(a) shows the surface properties of a thin metal plate 22 having a thickness of 10 μm after being subjected to a standard chemical cleaning treatment. can be seen.
On the other hand, FIG. 6(b) shows the surface properties of a similar thin metal plate 22 after being subjected to the above-described plasma cleaning treatment. I can confirm. Moreover, when the maximum roughness was measured, it decreased by about 100 nm.

Next, the pair of metal thin plates 22 subjected to the above plasma cleaning treatment were superimposed and joined under the conditions of temperature: 700° C., heating time: 30 minutes, and pressure: 12 MPa.
A SEM image of the cross section of the sample after this bonding experiment is shown in FIG. 7(a).
The boundary line between the two metal sheets 22 is still slightly recognizable as black dotted residual voids (non-bonded portions) 28, but in the other portions, new recrystallized grains are formed at the interface and become the base material. I understand.

FIG. 7(b) shows a cross-section of the joint between the thin metal plates 22 whose surfaces have not been cleaned, and it can be qualitatively understood that a larger number of residual voids 28 are present.

As described above, since the passivation film 26 is thinned and intermittent by the plasma cleaning treatment, unnecessary elements (O and C) are diffused into the base material of the thin metal plate even at a low temperature of about 700°C. Therefore, it is considered that easy bonding can be achieved.

Incidentally, when the residual void ratio on the boundary line was measured by image processing, it was 50% or less when the surface was cleaned, and it was confirmed that even in this state, the bonding area required by the micropump was satisfied.

Next, the eight thin metal plates 22 whose surfaces were cleaned under the processing conditions shown in FIG. 8 were simultaneously joined together using the diffusion bonding apparatus 40 shown in FIG.
This diffusion bonding apparatus 40 includes a pressure rod 44 arranged in a glass vacuum chamber 42, an upper heat insulating material 46, an upper heating material 48, an upper mold release material 50 and a lower mold release material 52, and a lower heating material. 54 , lower insulation 56 and base 58 .
Reference numeral 60 in the drawing denotes a pin for connecting the upper projection 46a of the upper heat insulator 46 and the lower recess 44a of the pressure rod 44. As shown in FIG.

The upper heat insulating material 46 and the lower heat insulating material 56 are made of an insulating material with relatively low thermal conductivity.
Also, the upper heating material 48 and the lower heating material 54 are made of a metal material such as iron or stainless steel that can be heated by high-frequency induction heating.
Also, the upper release material 50 and the lower release material 52 are made of an insulating material such as aluminum nitride or silicon carbide having excellent thermal conductivity.

As shown in FIG. 10, the laminate 62 of the thin metal plates 22 to be joined is interposed between the upper release member 50 and the lower release member 52 .
At this time, the pins 66 are inserted into the pair of through holes 64 formed in the laminate 62 of the thin metal plates 22, and both ends of each pin 66 are fitted into the recesses 68 formed in the upper heating member 48 and the lower heating member 54. Positioning is performed by

A circular coil 70 for high-frequency induction heating (IH) and a ceramic cylindrical cover 72 are arranged around the object to be joined (laminated body 62 of thin metal plates 22) outside the vacuum chamber 12 .
Circular coil 70 is fixed to the outer peripheral surface of cylindrical cover 72 .

Here, when a high-frequency power source (not shown) is turned on and a high-frequency current (for example, 400 kHz) is passed through the circular coil 70, an eddy current is generated inside the upper heating material 48 and the lower heating material 54, and as a result, both are heated. Material heats up. The heating temperature can be controlled by passing thermocouples through holes 56a and 58a that penetrate through the central portions of the lower heat insulating material 56 and the pedestal 58, and measuring the temperature of the lower release material 52 or the laminate 62 of the thin metal plates 22. Heating at temperature is done.
At the same time, the pressing rod 44 applies pressure to the stack 62 of the thin metal plates 22 for a predetermined time (eg, 30 minutes) at a predetermined pressure (eg, 30 MPa), so that the contact surfaces of the respective thin metal plates 22 are diffusion-bonded.

Next, the effect of increasing the diffusion bonding strength by plasma cleaning was confirmed.
That is, with respect to the thin metal plates 22 joined by the above process, experiments were conducted under three conditions of 750° C., 770° C., and 800° C., using the heating temperature as a parameter.
The sample size is 20 mm square, the number of laminated sheets is 8, and the size of the upper release material 50 and the lower release material 52 is 12 mm square.
After diffusion bonding, as shown in FIG. 11, the unbonded part of the boundary surface of the fourth and fifth 20 mm square samples was bent at a position of 4 mm from the end, and each end was subjected to a tensile tester. After being set on the chucks 80 and 82, it was peeled off at a speed of 5 mm/s, and the maximum strength was taken as the peeling strength.

The graph of FIG. 12 shows the relationship between the peeling strength and the bonding temperature.
In the case of "no" plasma cleaning, the bond strength remained at 79.4N even at 800°C. Also, it drops to 30.4N at 770°C and 11.1N at 750°C.
On the other hand, when the plasma cleaning was "yes", the strength was 24.7 N at 750° C., which was 2.2 times higher. At 770° C., the strength increases to 90.1 N, which is three times the strength. Furthermore, at 800° C., it increased to 337.7 N, showing 4.3 times the strength.
That is, it was shown that the difference in peel strength increased as the bonding temperature increased, confirming the effect of surface cleaning to remove at least a portion of the passivation film of the thin metal sheet from plasma generation.

FIG. 13 shows the results of SEM observation of the peeled sample surface.
From this result, when comparing the ductile fracture structure of the material structure on the surface of the sample, it is clear that the sample with plasma cleaning “with” is more ductile than the sample with plasma cleaning “without”. Many tissues are broken.
This tendency becomes remarkable when the bonding temperature reaches 800°C, and when the plasma cleaning is "yes", it is found that almost the entire surface has a ductile fracture structure, which corresponds to the peel strength measurement results in Fig. 12. ing.

1 is a schematic diagram showing the structure of an RF-DC plasma apparatus used for surface cleaning of a thin metal plate. FIG. 4 is a photograph showing a metal thin plate jig in an RF-DC plasma apparatus. 4 is a photograph showing a state of plasma generation in an RF-DC plasma apparatus; FIG. 2 is a conceptual diagram showing how a passivated film of a thin metal plate is thinned and intermittent due to surface cleaning treatment. 4 is a graph showing the relationship between hydrogen gas concentration and hydrogen intensity. Fig. 2 is an SEM image showing the contrast between the surface texture of a metal sheet subjected to a standard chemical cleaning treatment and the surface texture of a metal sheet subjected to a plasma cleaning treatment; Fig. 10 is an SEM image showing a comparison between the state of bonding between thin metal plates subjected to plasma cleaning treatment and the state of bonding between thin metal plates not subjected to surface cleaning treatment. 4 is a table showing an example of processing conditions for joining a plurality of thin metal plates. 1 is a cross-sectional view showing a configuration of a diffusion bonding apparatus used for bonding thin metal plates; FIG. It is an exploded perspective view showing the internal structure of the diffusion bonding apparatus. FIG. 4 is a schematic diagram showing how peel strength of joined thin metal plates is measured. 4 is a graph showing the relationship between peeling strength and bonding temperature. It is an SEM image of the peeled sample surface. 4 is a graph showing the relationship between the concentration of the constituent elements of the stainless steel material and the heating temperature, described in Non-Patent Document 2. FIG. 1 is a graph showing the relationship between bonding strength and heating temperature between stainless steel materials that have been rolled down by 90% in advance, described in Patent Document 1. FIG. 2 is a graph showing the relationship between bonding strength and heating temperature between stainless steel materials that have been subjected to boiling treatment with an organic acid in advance, described in Patent Document 2. FIG.

10 RF-DC plasma equipment
12 Vacuum chamber
14 DC bias
16 RF electrodes
18 RF oscillator
20 Jig
22 Sheet metal
24 Plasma
26 Passive film
28 Residual voids
40 Diffusion bonding equipment
42 Vacuum chamber
44 pressure rod
46 Top Insulation
48 Top Heater
50 Top release material
52 Bottom release material
54 Lower Heater
56 Bottom Insulation
58 Pedestal
60 pins
62 Laminates of sheet metal
70 circular coils
72 cylindrical cover
80, 82 Tensile tester chuck

Claims (1)

At least one side of the thin metal plate to be diffusion-bonded is subjected to a surface cleaning treatment that removes at least a portion of the passivation film formed on the surface of the thin metal plate by generating plasma with a mixed gas of hydrogen and argon. A laminate is formed by laminating a plurality of thin metal plates,
A method of bonding thin metal plates by applying a predetermined temperature and pressure to the upper and lower surfaces of the laminate for a predetermined time to diffusion bond the surfaces of the respective thin metal plates,
While arranging the laminate in a vacuum chamber,
A release material made of an insulating material with excellent thermal conductivity is placed on the upper and lower surfaces of this laminate,
A heating material made of an induction-heatable metal material is placed on the surface of each release material,
A heat insulating material made of an insulating material with low thermal conductivity is placed on the surface of each heating material,
A high-frequency current is passed through a coil for dielectric heating disposed outside the vacuum chamber, and at the same time pressure is applied to the surface of the heat insulating material, thereby diffusion bonding the metal thin plates. Joining method.

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