JP4087349B2 - Pulse laser surface treatment method - Google Patents

Description

  The present invention irradiates the surface of a member made of high speed steel (high speed), die steel, carburized steel, nitrided steel, bearing steel, cemented carbide, etc. The present invention relates to a pulse laser surface treatment method for adjusting material properties such as thickness and residual stress.

Machine elements and members such as tools, molds, and bearings are exposed to severe operating conditions such as friction, stress, and impact, so heat resistance, impact resistance, wear resistance, plastic deformation resistance, fracture resistance, High durability such as chipping is required.
However, since a hard material is brittle, there is a trade-off problem that when wear resistance and plastic deformation resistance are improved, chipping resistance and chipping resistance are reduced.

Since the surface treatment technique can impart material characteristics different from those of the member to the surface of the member, it is possible to solve such a technical problem of trade-off.
For example, rust prevention of steel materials by painting or plating, improvement of fatigue strength by shot peening, prevention of stress corrosion cracking (SCC), etc. are also examples of application of surface treatment technology.

  When a TiN film, TiC film, TiCN film, TiAlN film, etc. are coated on the surface of a member, the wear resistance, seizure resistance, corrosion resistance, etc. of the member are drastically improved. Therefore, many industrial products such as tools and molds are used. Has been applied.

  These coatings are often deposited by physical methods such as PVD (Physical Vapor Deposition), or chemical vapor deposition (CVD), and Vickers hardness (Hv) of 2,000-3,000. This is called a hard film.

Although the hard film can be formed at a relatively low temperature of about 600 ° C., a high compressive residual stress is inherent in the hard film after the film formation due to the difference in thermal expansion between the hard film and the member.
It is known that in a tool or a die coated with a hard film, when this compressive residual stress is large, excellent performance in terms of fracture resistance and thermal shock resistance is exhibited.

However, when the compressive residual stress inherent in the hard film exceeds the limit and is too high, the hard film cannot withstand the stress during use and may break down, and the hard film may peel off.
As a result, the surface of the parts of these tools and molds is exposed, and not only the performance such as wear resistance, welding resistance and thermal shock resistance is impaired, but also products manufactured using these tools and molds. It is also expected that there will be secondary damage such as mass production of defective products, breakage of tools and molds, breakage of production equipment due to breakage of broken pieces scattered and breakage of production lines.

  For this reason, the coating conditions are adjusted to the extent that peeling does not occur, but at present, it has reached the point of completely suppressing peeling due to the effects of coating lot variations, member shapes, and surface conditions. Absent.

  In some cases, the coating film thickness may be reduced so that peeling does not occur easily. In that case, the film thickness is so thin that its life is limited by wear, and as a tool and mold. There was a tendency for performance to decline, and no satisfactory one was obtained.

In addition, Vickers hardness (Hv) of hard films such as TiN film is as large as 2,000 to 3,000, but since the Vickers hardness (Hv) of the member is 1,000 or less, the hard film and member are also deformed by deformation due to external force. In the case where a mismatch occurs at the interface and the contact with other sharp members is repeated, there is a drawback that the interface easily peels off.
Further, as means for improving the durability of the member surface, means for increasing the hardness of the member surface by a diffusion penetration method, a surface quenching method, a mechanical impact method, or the like has been put into practical use.

  In these cases, the material property mismatch as clear as that at the hard film and member interface of the coating method does not occur, but there is a problem that the surface layer having high hardness is similarly peeled off from the base material depending on the use conditions.

  The present invention has been made in order to solve the above-described problems. For a member having different surface characteristics, the surface layer is peeled off from the member or a crack is generated without damaging the surface of the member. It is an object of the present invention to provide a pulsed laser surface treatment method capable of improving the durability performance such as wear resistance, welding resistance, impact resistance, etc. required for a member and extending the life of the member. To do.

In order to achieve the above object, the invention described in claim 1 is a pulse laser surface treatment method for adjusting material characteristics by irradiating a surface treatment member disposed in a liquid atmosphere with a pulse laser. Then, a surface-treated member is formed by forming a hard film containing titanium on the surface of the metal member by a coating method and having a hardness higher than that of the metal member. placed in a liquid atmosphere having a transparent property Te, the output of the pulse laser is irradiated to the object to be surface treated member, small and the metal than the pressure of the shock wave to be generated is the yield stress of the hard film by elevation Parusure the irradiation after having controlled largely Do so that than the yield stress of the member, and irradiating the pulsed laser.

  According to the pulse laser surface treatment method of the present invention, it is possible to prevent the surface layer from peeling off or cracking from a member having different surface characteristics without damaging the surface of the member. Further, durability performance such as wear resistance, welding resistance and impact resistance required for the member can be improved, and the life of the member can be extended.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a view showing a pulse laser surface treatment method according to a first embodiment of the present invention, and 1 is a surface treatment member (hereinafter simply referred to as a treatment member) made of metal to be subjected to surface treatment. Not placed in a liquid 2 such as water filled in a container.

Reference numeral 3 denotes a pulse laser oscillated from a laser oscillator (not shown), which is irradiated onto the surface of the member 1 to be processed in the liquid atmosphere via an optical condensing device 4 such as a focus lens.
In such a pulse laser surface treatment method, when the peak output density of the pulse laser 3 exceeds the plasma generation threshold of the member 1 to be processed (approximately 0.1 to 10 TW / m ^{2 in} the case of metal), the object to be irradiated with the pulse laser 3 is irradiated. The extreme surface layer (1 μm or less) of the processing member 1 is instantly evaporated and plasma 5 is generated on the surface.

In the liquid 2, since the inertia of the liquid 2 works momentarily strongly, the plasma 5 hardly expands, and the energy of the pulse laser 3 is concentrated in a narrow region. For this reason, the pressure of the plasma 5 reaches 10 to 100 times that in the air or in vacuum.
If the peak output density of the pulse laser 3 irradiated to the member 1 is I (TW / m ² ), the pressure P (GPa) of the generated plasma 5 is approximately P = (0.2 × I) ^0.5 . Can be sought.

Accordingly, if the peak output density of the pulse laser 3 is 1 to 100 TW / m ² , the pressure of the plasma 5 is approximately 450 MPa to 4.5 GPa, and the surface of the member 1 to be processed is plastically deformed by the irradiation of the pulse laser 3 to obtain material characteristics. Can be adjusted.

The liquid atmosphere in the present invention preferably has a property transparent to the pulse laser 3, and in addition to the water described above, liquids other than water such as oil, alcohol, ammonia water, boric acid water, etc. can be used. . In that case, the pressure of the plasma 5 can be obtained by P = (0.2 × I × k) ^0.5 using the following coefficient k.
k = (acoustic impedance of liquid) / (acoustic impedance of water)
Here, since the acoustic impedance of the liquid is (the density of the liquid) × (the speed of sound in the liquid), if the liquid is the above-described liquid, the result is not significantly different from the case of water.

  The high-pressure plasma 5 generated by the pulse laser surface treatment method shown in FIG. 1 instantaneously compresses the surface of the member 1 to be processed, and the displacement of the surface of the member 1 to be processed due to the compression becomes a shock wave 6. Propagate in the depth direction.

At this time, if the pressure of the shock wave 6 exceeds the yield stress of the member 1 to be processed, local plastic deformation occurs, and as a result, material characteristics such as residual stress and hardness change.
Here, which part (depth) undergoes plastic deformation is determined by the distribution of material characteristics (yield stress) in the member 1 to be processed and the state of propagation (attenuation) of the shock wave 6.

  FIG. 2 shows the member to be processed including the pressure (P1) of the shock wave 6 applied to the member 1 to be processed on the vertical axis and the yield stress (S) of the member 1 to be processed, and the range (D1) on which the plastic deformation extends on the horizontal axis. It is explanatory drawing which showed the relationship with 1 depth (D).

  As shown in FIG. 2, when the shock wave 6 propagates through the member 1 to be processed having a constant initial yield stress (Sb), the plastic deformation is caused by the pressure (P1) of the shock wave 6 from the outermost surface of the member 1 to be processed. It extends to a range (D1) up to a depth lower than the yield stress (Sb) of the processing member 1.

  FIG. 3 shows an example of a member 1 whose surface is coated with a hard film. When the yield stress (S) is not constant, the pressure of the shock wave 6 (P1), the yield stress (S) of the member 1 to be processed, It is explanatory drawing which showed the relationship of the range (D1) which plastic deformation reaches.

Here, when the yield stress of the hard film is Sc and the yield stress of the workpiece 1 is Sb, and the pressure P1 of the shock wave 6 satisfies the relationship Sc>P1> Sb, the hard film has the pressure of the shock wave 6 (P1 ), And only the member 1 to be processed is subjected to adjustment of material characteristics by plastic deformation.
That is, under such conditions, the hard film on the surface of the member to be processed 1 is not subjected to any characteristic change, and only the member to be processed 1 immediately below the surface is subjected to the characteristic change within the depth (D1) range. it can.

More precisely, since the surface of the member 1 to be processed is plastically deformed by the irradiation of the pulse laser 3 and the work hardening proceeds, the distribution of the yield stress (S) in FIG. 2 and FIG. Will change.
Therefore, by controlling the number of pulse lasers 3 irradiated to one place of the member 1 to be processed, the change in characteristics inside the member 1 to be processed can be adjusted.

  4 and 5 show a specific example of the pulse laser surface treatment method according to the present invention when a pulse laser 3 having a pulse energy of 200 mJ is focused to a spot diameter of 0.8 mm and irradiated to austenitic stainless steel (SUS304) in water. It is the figure which showed the change of the depth (D) of the to-be-processed member 1, residual stress, and hardness.

The pulse width of the used pulse laser is 8 ns, and the peak output density is 50 TW / m ² . According to the above formula, the pressure P of the plasma 5 is about 3 GPa, which is sufficiently higher than the yield stress (several hundred MPa) of austenitic stainless steel (SUS304).
In this example, a 20 mm × 20 mm region on the surface of the member is uniformly irradiated with a pulsed laser, but 18 pulses are irradiated on one surface of the material.

  As shown in FIGS. 4 and 5, the residual stress distribution 9 and the hardness distribution 10 after irradiation with the pulse laser have a compressive residual stress up to a depth of about 1 mm as compared with the residual stress distribution 7 and the hardness distribution 8 before the pulse laser irradiation. Formed, the hardness rises to about 0.7mm from the surface.

The residual stress on the surface of the member to be processed was determined by X-ray diffraction, and the depth distribution was determined by repeating electrolytic polishing and residual stress measurement.
The hardness distribution was measured with a micro Vickers hardness meter after polishing the cross section of the member to be treated.

  Next, a pulse laser surface treatment method will be described in which a material to be treated is adjusted by irradiating a member to be treated whose surface layer hardness has been increased in advance by nitriding treatment using a diffusion permeation method.

  FIG. 6 shows the depth (D) and Vickers hardness of the processed member before and after the pulse laser irradiation for the processed member obtained by soft nitriding the quench-tempered material of hot die tool steel (SKD61). It is explanatory drawing which showed the distributions 11 and 12 of (Hv).

  Pulse laser irradiation conditions were a pulse energy of 200 mJ and a spot diameter of 0.6 mm, and 28 pulses were irradiated per spot in water. The area irradiated with the pulse laser is 20 mm × 20 mm.

  As shown in FIG. 6, the Vickers hardness distribution 11 of the nitrocarburized material before the pulse laser irradiation has a Vickers hardness (Hv) exceeding 1,000 within a range of about 0.1 mm from the surface. It can be seen that there is an extreme difference in hardness (Hv = 400-500).

On the other hand, the Vickers hardness distribution 12 of the nitrocarburized material after the pulse laser irradiation is a gentle curve with increasing hardness in the range of 0.1 to 0.6 mm from the surface.
Thus, it is understood that the hardness distribution which is one of the material characteristics can be adjusted by the pulse laser irradiation.

In order to confirm the effect of adjusting the material characteristics (hardness distribution), a thermal fatigue test was performed on the nitrocarburized material subjected to pulse laser irradiation and the material not irradiated with the pulse laser.
As a result, peeling of the surface layer started in about 16,000 shots for the parts not irradiated with the pulse laser, but peeling or cracking of the surface layer did not occur even after exceeding 30,000 shots for the parts subjected to the pulse laser irradiation, and the hardness distribution It was confirmed that the durability performance of the nitrocarburized material can be improved by adjusting the above.

  Next, a pulse laser surface treatment method that adjusts material properties by irradiating a target laser whose surface layer hardness has been increased in advance by shot peening, which is one of mechanical shock methods, is irradiated with a pulse laser. To do.

  FIG. 7 shows the distribution 13 of the depth (D) and the residual stress (MPa) of the processed member before and after the pulse laser irradiation for the processed member subjected to the carburized and quenched steel for gears (SNCM420H). It is explanatory drawing which showed 14 and 15. FIG.

  The irradiation conditions of the pulse laser were the following two conditions. That is, (1) pulse energy 80mJ, spot diameter 0.8mm, 50 pulses per spot in water, (2) pulse energy 200mJ, spot diameter 0.8mm, 50 spots per spot in water 2 It is a condition. The area irradiated with the pulse laser is 20 mm × 20 mm.

  As shown in FIG. 7, the residual stress distribution 13 of the carburized and hardened steel for gears (SNCM420H) before pulse laser irradiation shows high compressive residual stress in the range of about 0.2 mm from the surface, and residual stress at a position deeper than 0.2 mm. Since the value is almost 0, there is an extreme difference in the stress value.

On the other hand, in the residual stress distribution 15 of the carburized and quenched steel for gears (SNCM420H) irradiated with the pulse laser under the condition (1), the residual stress is shifted to the compression side in the range of about 0.6 mm from the surface.
Further, in the residual stress distribution 14 of the carburized and hardened steel for gears (SNCM420H) irradiated with the pulse laser under the condition (2), the residual stress is shifted to the compression side in the range of about 1.0 mm from the surface.

As a result, it was found that the pulsed laser irradiation improved the sudden decrease in compressive residual stress value in the vicinity of a depth of 0.2 mm under both conditions, and it was possible to adjust the residual stress distribution, which is one of the material properties. .
Although the hardness distribution is not shown, it has the same tendency as the residual stress distribution, and it has been found that the hardness distribution becomes a gentler curve by the pulse laser irradiation.

  In order to confirm the effect of adjusting the material properties (residual stress and hardness distribution), a chipping test was performed on the shot peened material that was irradiated with the pulse laser and the material that was not irradiated with the pulse laser.

  As a result, chipping started with an impact load of about 380 times for the member not irradiated with the pulse laser, but about 1,900 times for the member to be processed irradiated with the pulse laser under the condition (1), approximately under the condition (2). It was confirmed that chipping did not occur until the impact load of 3,200 times, and that the durability performance of the shot peened material could be improved by adjusting the residual stress and hardness distribution.

In addition, with respect to the processed member whose surface hardness was increased by the surface quenching method, the same tendency as the soft nitriding material (diffusion penetration method) and the shot peening material (mechanical impact method) was obtained. It has been.
That is, it has been confirmed that the material characteristics of the surface-hardened material can be adjusted by pulse laser irradiation, and the durability performance can be improved.

Next, the surface of the pulse laser that adjusts the material properties by irradiating the target member whose surface hardness has been increased in advance by forming a TiN hard film on the surface by PVD, which is one of the coating methods. A processing method will be described.
The surface of high-speed tool steel (SKH51) was coated with TiN having a thickness of about 2 μm by arc ion plating and irradiated with a pulsed laser.

  The temperature of the material to be treated (substrate) during coating is 300 ° C. When the material to be treated is cooled down to room temperature after coating, the coating material is affected by the difference in thermal expansion coefficient between the coating material (TiN) and the material to be treated (SKH51). A large compressive stress is generated in (TiN).

As a result of measuring stress by X-ray diffraction, a compressive stress of about 1 GPa was generated in the coating material (TiN).
On the other hand, since the member to be treated is extremely thick compared to the coating material (TiN), the residual stress value is almost 0, and a large residual stress mismatch has occurred at the interface between the hard film and the member to be treated.

  FIG. 8 shows the residual stresses 16, 17, and 18 of the hard film (TiN) before and after the pulse laser irradiation, the depth of the member to be processed (SKH51) for the high-speed tool steel (SKH51) coated with TiN (D ) And residual stress (MPa) distributions 19, 20, and 21.

  Pulse laser irradiation was performed under the following two conditions. That is, (1) pulse energy of 15 mJ and spot diameter of 0.4 mm, irradiation of 13 pulses per spot in water, (2) pulse energy of 50 mJ and spot diameter of 0.4 mm, irradiation of 13 pulses per spot in water It is a condition. The area irradiated with the pulse laser is 20 mm × 20 mm.

As shown in FIG. 8, in the residual stress before the pulse laser irradiation, a mismatch of about 1 GPa occurred at the interface between the hard film 16 and the member 19 to be processed.
However, by irradiating with a pulse laser under the condition (1), the residual stress becomes a compressive stress 17 of about 300 MPa at the interface between the hard film 17 and the member 20 to be processed. The residual stress distribution 20 was almost eliminated.

On the other hand, when the pulse laser was irradiated under the condition (2), the residual stress 18 of the hard film was tensile of about 400 MPa, and the residual stress 21 of the member to be processed showed a compressive stress of about 700 MPa at the interface.
This is because, as described above, the hard film has high hardness and does not undergo plastic deformation under the pressure of the shock wave. As a result of the plastic member being subjected to plastic deformation in the in-plane direction, the member to be processed is subjected to compressive stress. As a reaction, the hard film is considered to have become a tensile stress.
In the case of the condition (1), it was an appropriate condition for eliminating the mismatch of the residual stress before the pulse laser irradiation. However, in the condition (2), it is considered that the plastic deformation of the member to be processed further progressed. .

From these results, it was found that the material characteristics (residual stress) in the vicinity of the interface between the hard film and the member to be processed can be adjusted by controlling the irradiation condition of the pulse laser.
In order to confirm the effect of adjusting material properties (residual stress), high-speed tool steel (SKH51) coated with a hard film with TiN was prepared by applying pulsed laser irradiation and non-irradiated TiN. The peelability of was evaluated.

  The adhesion of the film is generally evaluated by a scratch test. Here, after applying a load of 1,000 N using a Rockwell hardness tester, the film peels and cracks inside and around the indentation. Was observed and evaluated.

As a result, a lot of minute cracks were observed around the indentation in the case where the pulse laser was not irradiated.
On the other hand, no peeling or cracking was observed around the indentation in the case of pulse laser irradiation. This is presumably because, under condition (1), the residual stress mismatch at the interface between the hard film and the member to be processed was eliminated by the pulse laser irradiation.

Further, in condition (2), the hardness of the member to be processed is increased by the plastic deformation of the member to be processed by the pulse laser irradiation, and the main factor is considered that the deformation at the time of pressing the hard ball by the Rockwell hardness tester is reduced. It is done.
From the above, it was confirmed that the durability performance of the TiN coating film and the coating member was improved by irradiating the pulse laser and adjusting the material properties of the member to be processed.

Next, the case where a to-be-processed member is irradiated with a pulse laser in water, oil, alcohol, ammonia water, boric acid water as a liquid atmosphere will be described.
As described above, when the pulse laser 3 is irradiated in the liquid atmosphere, the extreme surface layer of the member 1 to be processed is instantly evaporated and the plasma 5 is generated.

Since the liquid 2 has a high density, the plasma 5 instantaneously collides with liquid molecules. As a result, some liquid molecules become active plasma almost simultaneously with the generation of the plasma 5 and react with the surface of the member 1 to be processed.
Further, the extreme surface layer (several μm) of the member 1 to be processed is heated by the heat radiation from the plasma 5 and the diffusion of atoms into the member to be processed is promoted.

In the experiment, when the object to be treated 1, which is mainly composed of iron, is irradiated with a pulse laser in water, the oxygen produced by the decomposition of iron and water reacts and forms iron trioxide (Fe ₃ O, which is difficult to form in the atmosphere). It was found that the dense surface layer ₄ ) was formed over a thickness of several μm.

The formation of triiron tetroxide (Fe ₃ O ₄ ) is effective for surface deactivation, and the surface of hot die tool steel (SKD61) has a thickness of several μm and is made of iron trioxide (Fe ₃ O _4). ), It was found that the melting loss due to erosion, corrosion, seizure and the like is remarkably reduced.

  Similarly, when the surface of the member 1 containing iron as a main component is irradiated with a pulse laser 3 in oil or alcohol, carbon atoms constituting the oil or alcohol molecules are taken into the member surface to form a carburized layer. I understood that.

It is understood that when the pulse laser 3 is irradiated in the ammonia water, nitrogen atoms constituting the ammonia molecules react with the surface of the member 1 to be processed, and a surface layer mainly composed of iron nitride (Fe ₄ N) is formed. It was.
Further, when the pulse laser 3 is irradiated in boric acid water, boron atoms react with the surface of the member 1 to be processed, and a surface layer mainly composed of iron boride (Fe ₂ B and FeB) is formed. I understood.

When these triiron tetroxides (Fe ₃ O ₄ ), carburized layers, iron nitride (Fe ₄ N), and iron borides (Fe ₂ B and FeB) are formed on the surface of the member to be treated 1, the durability of the member to be treated 1 Although it is known that the performance is improved, it is usually formed by the diffusion permeation method using the thermochemical equilibrium state, so a processing temperature exceeding several hundred degrees to one thousand degrees is required. I must.
Further, such a high temperature treatment is not preferable because it often causes adverse effects such as coarsening of the metal structure.

  As described above, according to the surface treatment method by pulse laser irradiation according to the present invention, a high-temperature container is unnecessary and continuous treatment is possible, which greatly contributes to improvement in durability performance and extension of life of machine elements and members. .

The front view for demonstrating the pulse laser surface treatment method by the 1st Embodiment of this invention. Explanatory drawing which shows the relationship between the range of the pressure of the shock wave, the yield stress of a to-be-processed member, and plastic deformation in the pulse laser surface treatment method by the 1st Embodiment of this invention. Explanatory drawing which shows the plastic deformation range in case the yield stress of the member in the pulse laser surface treatment method by the 1st Embodiment of this invention is not constant. Explanatory drawing which shows the change of a residual stress when SUS304 in other embodiment of this invention is irradiated with a pulse laser in water. Explanatory drawing which shows the change of hardness when SUS304 in other embodiment of this invention is irradiated with a pulse laser in water. Explanatory drawing which shows the change of the hardness when the SKD61 nitrocarburizing material in other embodiment of this invention is irradiated with a pulse laser in water. Explanatory drawing which shows the residual stress change when the pulse laser is irradiated to the SNCM420H shot peening material in other embodiment of this invention in water. Explanatory drawing which shows the residual stress change when the TiN coating material of SKH51 in other embodiment of this invention is irradiated with a pulse laser in water.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Member, 2 ... Liquid, 3 ... Pulse laser, 4 ... Optical condensing device, 5 ... Plasma, 6 ... Shock wave, 7 ... Residual stress distribution (SUS304) before laser irradiation, 8 ... Hardness distribution before laser irradiation (SUS304), 9 ... Residual stress distribution after laser irradiation (SUS304), 10 ... Hardness distribution after laser irradiation (SUS304), 11 ... Hardness distribution before laser irradiation (SKD61), 12 ... Hardness after laser irradiation Distribution of thickness (SKD61), 13 ... Residual stress distribution before laser irradiation (SNCM420H), 14 ... Residual stress distribution after laser irradiation (SNCM420H, condition (1)), 15 ... Residual stress distribution after laser irradiation (SNCM420H, condition) (2)), 16 ... residual stress before laser irradiation (TiN), 17 ... residual stress after laser irradiation (TiN, condition (1)), 18 ... residual stress after laser irradiation (TiN, condition (2)) 19 ... Residual stress distribution before laser irradiation (SKH51), 20 ... Residual stress after laser irradiation Cloth (SKH51, condition (1)), 21 ... residual stress distribution after laser irradiation (SKH51, condition (2)).

Claims (2)

A pulse laser surface treatment method for adjusting a material property by irradiating a surface treatment member disposed in a liquid atmosphere with a pulse laser,
A surface-treated member is formed by forming a hard film containing titanium by a coating method on the surface of the metal member and having a hardness higher than that of the metal member,
This surface-treated member is disposed in a liquid atmosphere having a property transparent to the pulse laser,
The output of the pulse laser is irradiated to the object to be surface treated member, the pressure of the shock wave to be generated is controlled in so that Do greater than the yield stress of the small and the metal member than the yield stress of the hard film by elevation Parusure The irradiation after, the pulse laser surface treatment method, which comprises irradiating a pulsed laser.   The pulse laser surface treatment method according to claim 1, wherein the liquid atmosphere is formed of at least one of water, oil, alcohol, ammonia water, and boric acid water.

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