Coatings and bearings

Abstract

To provide a coating that can suppress generation of electric corrosion in a steel stock and has practically sufficient adhesion with respect to a steel stock.SOLUTION: A coating for a steel stock includes: a ground layer; and a surface layer formed on an upper side of the ground layer. The ground layer comprises Cr and has a thickness of 100 nm or more. The surface layer comprises diamond-like carbon and has plastic deformation hardness of 12-20 GPa.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a coating for steel materials and a bearing having this coating.

Background Art

[0002] Currently, in order to suppress the generation of greenhouse gases, in fields such as automobiles, the drive device is globally shifting from an internal combustion engine to an electric motor such as a motor. Along with this, in order to shorten the charging time of the rechargeable battery for the electric motor and increase the output of the electric motor, the drive voltage of the electric motor is being increased. In the future, if the drive voltage of the motor continues to increase, it is predicted that the frequency of so-called electric corrosion will increase in bearings such as ball bearings that support the rotating shaft of the motor.

[0003] Electric corrosion is a phenomenon in which when an electric current passes through the inside of a rotating bearing, a spark occurs at the contact part between the raceway ring and the rolling element of the bearing, and the surfaces of the raceway ring and the rolling element melt and are damaged at the contact part.

[0004] As methods for suppressing the occurrence of this electric corrosion, there are a method of preventing an electric current from flowing through the bearing and a method of preventing the bearing itself from conducting an electric current. As a method of preventing an electric current from flowing through the bearing, there is a method of bypassing with an earth ring or the like. As a method of preventing the bearing itself from conducting an electric current, there are a method of using an insulator such as ceramic for the rolling element and a method of coating the outer ring that constitutes the raceway ring together with the inner ring with an insulating film. As other methods, there is also a method of injecting a highly conductive lubricating oil between the raceway ring and the rolling element so that a spark does not occur inside the bearing.

[0005] As a method to prevent current from flowing through a bearing, for example, Patent Document 1 discloses a coating method for a bearing in which at least one of the outer ring and inner ring of the bearing, which are arranged concentrically with each other, is made of metal and is coated with an insulating film. This method is characterized by performing thermal spraying using a mixed thermal spray material obtained by adding silicon carbide powder with an average particle size of 2 μm to less than 10 μm and / or aluminum nitride powder with an average particle size of 2 μm to less than 10 μm in a total amount of 2 to 40% by mass to aluminum oxide powder with an average particle size of 1 μm or more and less than 30 μm, in an amount of 2 to 40% by mass, thereby forming the insulating film on the outer ring and / or the inner ring.

[0006] Patent Document 2 discloses a method for manufacturing an insulating rolling bearing for preventing electrochemical corrosion, comprising a metal outer ring with an outer ring raceway formed on its inner circumferential surface, a metal inner ring disposed inside the outer ring and having an inner ring raceway formed on its outer circumferential surface, and a plurality of rolling elements, each made of metal, provided to roll freely between the outer ring raceway and the inner ring raceway, and having an insulating coating formed thereon, characterized in that, of the surfaces of the outer ring excluding the outer ring raceway, an electrolytic fusion material granulated to have a particle size distribution of 10 to 70 μm and containing zirconia in a ratio of 5% to 40% by mass of alumina is thermally sprayed to form a thermal spray coating, and then the pores of the thermal spray coating are filled with a sealing agent to perform a sealing treatment.

[0007] Furthermore, Patent Document 3 aims to provide a rolling bearing that can prevent electrolytic corrosion even when used in a motor that generates shaft voltage, in a manner that does not cause a decrease in rotational accuracy, and describes a steel outer ring with a surface resistance of 10 6 A rolling bearing is disclosed, characterized by having a diamond-like carbon (DLC) layer of Ω or greater thickness. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2017-53481 [Patent Document 2] Japanese Patent Publication No. 2015-212576 [Patent Document 3] Japanese Patent Publication No. 2006-226500 [Overview of the project] [Problems that the invention aims to solve]

[0009] The insulating film disclosed in Patent Document 1 and the insulating coating disclosed in Patent Document 2 are formed by thermal spraying of raw materials. Coatings formed by thermal spraying have the problem of being brittle and difficult to handle. On the other hand, the DLC layer disclosed in Patent Document 3 is mainly formed by sputtering and is easier to handle than coatings formed by thermal spraying.

[0010] Patent Document 3 discloses, as an example, the relationship between the plastic deformation hardness and volume resistivity of a DLC layer. However, Patent Document 3 does not disclose the specific thickness, area resistivity, volume resistivity, or voltage at which the DLC layer is destroyed by sparks (hereinafter also referred to as "dielectric breakdown voltage") for any of the DLC layers used in the examples and comparative examples used to investigate the presence or absence of galvanic corrosion. Therefore, the specific conditions for preventing galvanic corrosion were not clear. Furthermore, while Patent Document 3 discloses the provision of a chromium underlayer and a chromium carbide intermediate layer between the film to be deposited and the DLC layer, the roles of these layers, their influence on galvanic corrosion, and their specific thicknesses are not disclosed.

[0011] Furthermore, the inventors' investigations revealed that the conditions under which electrolytic corrosion occurs vary depending on factors such as the voltage applied to the motor, the environment in which the motor and bearings are located, and the capacity of the battery connected to the motor. Therefore, it is difficult to specifically determine the required thickness of the DLC layer and the dielectric breakdown voltage to prevent electrolytic corrosion. Additionally, it was found that even when a coating with some kind of underlayment is provided between the DLC layer and the bearing surface, the adhesion of the DLC layer and the coating including the underlayment to the bearing is practically insufficient depending on the type and thickness of the underlayment.

[0012] This invention has been made in view of the above problems, and aims to provide a coating that can suppress the occurrence of galvanic corrosion in steel materials and has practically sufficient adhesion to steel materials. It also aims to provide a bearing having such a coating. [Means for solving the problem]

[0013] As a result of various studies, the inventors have found that the above objective can be achieved by the following invention.

[0014] A coating according to one aspect of the present invention is a coating for steel materials, It has a base layer and a surface layer formed on the upper side of the base layer, The aforementioned underlayer consists of Cr and has a thickness of 100 nm or more. The aforementioned surface layer is made of diamond-like carbon and has a plastic deformation hardness of 12 to 20 GPa.

[0015] In another aspect of the present invention, the bearing has the above-mentioned coating formed on at least a portion of its surface. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a coating that can suppress the occurrence of galvanic corrosion in steel materials and has practically sufficient adhesion to steel materials. Furthermore, it is possible to provide a bearing having such a coating. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 is a schematic diagram of a coating according to an embodiment of the present invention. [Figure 2] Figure 2 is a cross-sectional view of a bearing according to an embodiment of the present invention. [Figure 3] Figure 3 is a schematic diagram of the apparatus used to form the underlying layer. [Figure 4] Figure 4 is a schematic diagram of the apparatus used to form the surface layer. [Figure 5]FIG. 5 is a schematic diagram of a dielectric breakdown voltage measuring device used for measuring the dielectric breakdown voltage. [Figure 6] FIG. 6 is a graph showing the relationship between the plastic deformation hardness of the surface layer and the dielectric breakdown voltage per 1 μm thickness. [Figure 7] FIG. 7 is a graph showing the relationship between the thickness of the surface layer and the dielectric breakdown voltage. [Figure 8] FIG. 8 is a graph showing the relationship between the thickness of the underlying layer and the dielectric breakdown voltage. [Figure 9] FIGS. 9(a) to (d) are photographs showing the state of the underlying layer after the scratch test.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, a film and a bearing according to an embodiment of the present invention will be described with reference to the drawings.

[0019] 〈Configuration of the Film〉 FIG. 1 is a schematic diagram of the film according to the present embodiment. The film 1 according to the present embodiment is a film for steel materials, and has an underlying layer 2 and a surface layer 3 formed on the upper side of the underlying layer. The underlying layer 2 is made of Cr and has a thickness of 100 nm or more, and the surface layer 3 is made of diamond-like carbon and has a plastic deformation hardness of 12 to 20 GPa. By providing the film 1 on the surface of the steel material, the occurrence of electrolytic corrosion in the steel material can be suppressed. Further, the film 1 has practically sufficient adhesion to the steel material.

[0020] The film 1 is formed on the surface of the steel material 4. The use and chemical composition of the steel material are not particularly limited. When the film 1 is used for a bearing such as a ball bearing, SUJ2 to SUJ5 defined in JIS G 4805:2019 (high-carbon chromium bearing steel material) can be used as the steel material.

[0021] The base layer 2 is made of Cr (chromium) and has a thickness of 100 nm or more. The base layer 2 is placed between the steel material 4 and the surface layer 3. The base layer 2 has high adhesion to both the steel material 4 and the surface layer 3. Therefore, the coating 1 according to this embodiment has higher adhesion to the steel material 4 compared to the case where the surface layer 3 is formed directly on the surface of the steel material 4.

[0022] The thickness of the base layer 2 should be 100 nm or more. If the thickness of the base layer 2 is less than 100 nm, sufficient adhesion of the coating 1 cannot be obtained. Furthermore, from the viewpoint of increasing the dielectric breakdown voltage of the coating 1, the thickness of the base layer 2 is preferably 200 nm or less, and more preferably close to 100 nm.

[0023] The surface layer 3 is made of diamond-like carbon (DLC) and has a plastic deformation hardness of 12 to 20 GPa. By setting the plastic deformation hardness of the surface layer 3 to 12 to 20 GPa, the dielectric breakdown voltage per 1 μm thickness of the surface layer 3 can be set to 35 V / μm or more. The inventors have found that by setting the dielectric breakdown voltage per 1 μm thickness of the surface layer 3 to 35 V / μm or more, the occurrence of galvanic corrosion in the steel material 4 on which the coating 1 is formed can be suppressed.

[0024] The plastic deformation hardness of the surface layer 3 is preferably 13 GPa or higher. This further suppresses the occurrence of galvanic corrosion in the steel material 4 on which the coating 1 is formed. The plastic deformation hardness of the surface layer 3 is more preferably 14 GPa or higher. This further suppresses the occurrence of galvanic corrosion in the steel material 4. Furthermore, the plastic deformation hardness of the surface layer 3 is preferably 17 GPa or lower. This further suppresses the occurrence of galvanic corrosion in the steel material 4. The plastic deformation hardness of the surface layer 3 is more preferably 15 GPa or lower. This further suppresses the occurrence of galvanic corrosion in the steel material 4.

[0025] Furthermore, if the plastic deformation hardness of the surface layer 3 is insufficient, the surface layer 3 may be damaged when, for example, a bearing with the coating 1 formed on it is assembled into a bearing support member or motor rotating shaft located inside an automobile or the like. However, the plastic deformation hardness of the surface layer 3 in this embodiment is 12 GPa or higher, which is harder than the 8.5 GPa plastic deformation hardness of SUJ2, thus suppressing such damage.

[0026] In this embodiment, "plastic deformation hardness" is a value calculated from the load applied to the indenter and the depth of indentation when the object to be measured is plastically deformed by pressing an indenter of a predetermined shape against it with a specified load, and can be measured by nanoindentation testing.

[0027] The thickness of the surface layer 3 should be determined according to the actual usage environment of the product on which the coating 1 is formed. A thickness of 1 to 10 μm is preferred for the surface layer 3. If the thickness of the surface layer 3 is 1 μm or more, the surface layer 3 will have a sufficient dielectric breakdown voltage of 60 V or more. A thicker surface layer 3 is preferable because the dielectric breakdown voltage of both the surface layer 3 and the coating 1 increases with thickness. By having a surface layer 3 with a sufficient dielectric breakdown voltage, the insulating performance of the coating 1 is improved, and damage to the coating 1 can be suppressed even if electrolytic corrosion occurs on the steel material 4. However, the thicker the surface layer 3 is, the longer it takes to form the surface layer 3, so a thickness of 10 μm or less is preferred for the surface layer 3.

[0028] <Bearing configuration> Figure 2 is a cross-sectional view of the bearing according to this embodiment. The bearing 30 is a so-called rolling bearing, specifically a ball bearing. The bearing 30 has an inner ring 31 and an outer ring 33 that constitute a raceway, and spherical rolling elements 32 sandwiched between the raceway. The bearing 30 is used to support the rotating shaft of a motor inside an automobile or the like. The outer circumferential surface 33a of the outer ring 33 of the bearing 30 is supported by a support member (not shown) provided inside the automobile or the like. The inner ring 31 of the bearing 30 supports the rotating shaft (not shown) of the motor.

[0029] In this embodiment, the bearing 30 has the above-mentioned coating formed on at least a portion of its surface. In the bearing 30, the location where the coating is formed can be any location, but it is preferable to form it on the surface of the outer ring 33. When formed on the surface of the outer ring 33, the coating may be formed on the entire surface of the outer ring 33. Alternatively, the coating may be formed on the outer circumferential surface 33a of the outer ring 33. By forming the coating on at least the outer circumferential surface 33a of the outer ring 33, leakage current flowing from the motor to the internal support member of an automobile or the like can be blocked, and electrolytic corrosion can be suppressed between the inner ring 31 and outer ring 33 constituting the raceway and the spherical rolling element 32 sandwiched between the raceway.

[0030] <Method for manufacturing the coating> The method for manufacturing the coating according to this embodiment will now be described. However, the method for manufacturing the coating is not limited to the following method.

[0031] Figure 3 is a schematic diagram of an apparatus for forming the base layer, and Figure 4 is a schematic diagram of an apparatus for forming the surface layer. The base layer of the coating according to this embodiment can be manufactured using the unbalanced magnetron sputtering (UBMS) apparatus 10 shown in Figure 3. The UBMS apparatus 10 comprises a chamber 11, a cathode 13 and a stage 16 arranged inside the chamber 11, a sputtering power supply 12 connected to the cathode 13, and a bias power supply 15 connected to the stage 16. The chamber 11 has an inlet 11a into which process gas 18 is introduced and an exhaust port 11b from which exhaust gas 19 is discharged. The cathode 13 has a plurality of magnets 13a, 13b, and 13c arranged to be exposed side by side on the surface of the cathode 13, and a plate-shaped target 14 made of Cr is placed on the surface of the cathode 13. A substrate 17 made of steel is placed on the stage 16.

[0032] In the UBMS apparatus 10, when voltage is applied to the sputtering power supply 12 and the bias power supply 15, and the process gas 18, which is argon gas, is introduced, Ar atoms 18a collide with the target 14, causing Cr ions 14a to be ejected. These Cr ions 14a are then adsorbed onto the substrate 17, forming a base layer.

[0033] The conditions for forming the underlayer can be set as follows, for example: the power of the sputtering power supply 12 can be 2kW, the bias voltage applied to the bias power supply 15 can be 50V, and the pressure of the process gas 18 (argon gas) in the chamber 11 can be 0.6Pa. These values ​​are just examples and can be changed according to the thickness of the underlayer to be formed, the size and shape of the substrate, etc.

[0034] The surface layer of the coating according to this embodiment can be formed using the plasma CVD (Chemical Vapor Deposition) apparatus 20 shown in Figure 4. The plasma CVD apparatus 20 comprises a chamber 21, a high-frequency application electrode 23 and a stage 26 arranged inside the chamber 21, a high-frequency power supply 22 connected to the high-frequency application electrode 23, and a bias power supply 25 connected to the stage 26. The chamber 21 has an inlet 21a into which the raw material gas 28 is introduced and an exhaust port 21b from which the exhaust gas 29 is discharged. A substrate 27 on which a base layer 27a is formed is placed on the stage 26. When forming a surface layer made of diamond-like carbon according to this embodiment, hydrocarbon gases such as CH4 (methane), C2H2 (acetylene), and C7H8 (toluene) can be used as the raw material gas 28.

[0035] In the plasma CVD apparatus 20, when voltage is applied to the high-frequency power supply 22 and the bias power supply 25 and the raw material gas 28 is introduced, the raw material gas 28 is converted into plasma, and the plasma 28a is adsorbed onto the underlayer 27a of the substrate 27, forming a surface layer made of diamond-like carbon on the underlayer 27a.

[0036] The conditions for forming the surface layer can be set as follows, for example: The power frequency of the high-frequency power supply 22 is 300 kHz, the bias voltage applied to the bias power supply 25 is 600 V, and the pressure of the raw material gas 28 in the chamber 21 can be 5 to 10 Pa when the raw material gas 28 is CH4 or C2H2. These values ​​and gas types are just examples and can be changed according to the thickness of the surface layer to be formed, the size and shape of the substrate, etc.

[0037] Furthermore, the plastic deformation hardness of the surface layer, i.e., the diamond-like carbon, can be controlled by the pressure and type of the raw material gas 28 in the chamber 21. Specifically, the surface layer can be made harder by lowering the pressure of the raw material gas 28. Also, the change in the plastic deformation hardness of the diamond-like carbon in response to pressure changes is greater for gas types with a larger number of carbon atoms in the raw material gas 28 compared to gas types with a smaller number of carbon atoms.

[0038] <Summary of disclosed technologies> As described above, this specification discloses various aspects of technology, the main technologies among them are summarized below.

[0039] As described above, a coating according to one aspect of the present invention is a coating for steel materials, comprising a base layer and a surface layer formed on the upper side of the base layer, wherein the base layer is made of Cr and has a thickness of 100 nm or more, and the surface layer is made of diamond-like carbon and has a plastic deformation hardness of 12 to 20 GPa.

[0040] This configuration makes it possible to suppress the occurrence of galvanic corrosion in steel materials and to obtain a coating that has practically sufficient adhesion to the steel material.

[0041] The coating with the above configuration preferably has a surface layer thickness of 1 to 10 μm.

[0042] This configuration allows the coating to have a sufficient dielectric breakdown voltage, and even if electrolytic corrosion occurs, damage to the coating can be suppressed.

[0043] In the above-described coating, it is preferable that the thickness of the underlying layer is 200 nm or less.

[0044] This configuration allows for an increase in the dielectric breakdown voltage of the coating.

[0045] The coating with the above configuration may be formed on a steel bearing.

[0046] Furthermore, as described above, in a bearing relating to another aspect of the present invention, the above-mentioned coating is formed on at least a portion of the surface. [Examples]

[0047] The thickness, plastic deformation hardness, dielectric breakdown voltage, and adhesion of the underlayer and surface layers formed on the surface of the steel material were measured or evaluated.

[0048] <Samples used> Sample A was prepared by directly forming a surface layer of diamond-like carbon on the surface of the steel material; Sample B was prepared by forming a base layer of Cr and a surface layer of diamond-like carbon on the surface of the steel material; and Sample C was prepared by forming only a base layer of Cr on the surface of the steel material.

[0049] Sample A was used to evaluate plastic deformation hardness. Samples A and B were used to evaluate dielectric breakdown voltage. Sample C was used to evaluate adhesion.

[0050] For each sample, a 5mm thick, 50mm diameter steel plate made of SUJ2 as specified in JIS G 4805:2019 was used.

[0051] The underlying layers of samples B and C were formed using the UBMS apparatus shown in Figure 3. The conditions for forming the underlying layers were as follows:

[0052] Sputtering power: 2kW Bias voltage: 50V Process gas: Ar Process gas pressure: 0.6 Pa The surface layers of samples A and B were formed using the plasma CVD apparatus shown in Figure 4. The surface layer formation conditions were as follows: The surface layer of sample A was formed on top of the underlying layer.

[0053] Power frequency: 300kHz Bias voltage: 600V Raw material gas: CH4, C2H2 Pressure of the raw material gas: 5-10 Pa <Measurement and evaluation methods for each characteristic> (Method for measuring thickness) The thickness of the surface layer of sample A and the thickness of the base layer of sample C were measured by the following method: A surface layer or base layer was formed on a steel plate with masking applied to a portion of it, and the masking was removed. The height of the step formed between the area where the masking was removed and the area where the surface layer or base layer was formed was measured using a stylus-type step meter, and this height was defined as the thickness of the surface layer or base layer.

[0054] The thickness of the base layer and surface layer of sample B was measured using the following method: First, a base layer was applied to a steel plate with masking tape in two places. Then, one of the masking tapes was removed, and a surface layer was applied to the steel plate and the base layer. The height of the step formed between the area where the remaining masking tape was removed and the area where only the base layer was formed, and the area where only the surface layer was formed, was measured, and this height was defined as the thickness of the base layer or surface layer.

[0055] (Method for measuring plastic deformation hardness) The plastic deformation hardness was measured using sample A as described above, by nanoindentation testing. The nanoindentation testing machine used was manufactured by Elionix Co., Ltd. A triangular pyramidal Berkovich indenter was used, and the load was set to 10 mN.

[0056] (Method for measuring dielectric breakdown voltage) The dielectric breakdown voltage was measured using samples A and B described above, in accordance with JIS C 2110-1:2016 (Solid electrical insulating materials - Test methods for dielectric breakdown strength - Part 1: Test by application of commercial frequency AC voltage).

[0057] Figure 5 is a schematic diagram of the dielectric breakdown voltage measuring device used to measure the dielectric breakdown voltage. The dielectric breakdown voltage measuring device 40 includes a stage 41 made of a metal plate, a metal sphere 45, a weight 42 provided on the upper part of the metal sphere 45 via a metal rod 46, an electrode 43 connected to the stage 41, and an electrode 44 connected to the metal rod 46. The sample S in the figure is either sample A or sample B, and is placed between the stage 41 and the metal sphere 45 with the surface facing the metal sphere 45. The voltage applied to the sample S placed between the stage 41 and the metal sphere 45 was increased at a boosting rate of 10 V / s, and the voltage when the current value reached 1 mA was defined as the dielectric breakdown voltage.

[0058] (Method for evaluating adhesion) Adhesion was evaluated using sample C, as described above, by a scratch test. Specifically, sample C was placed on a stage with the substrate layer facing upwards, and a needle was placed on the substrate layer. The needle was moved over the substrate layer at a speed of 10 mm / min, and the load on the needle was increased from 0 N to 100 N per minute. The load at which the substrate layer began to peel off (hereinafter referred to as the "peeling initiation load") and the observed peeling state of the substrate layer were used to evaluate adhesion. The peeling initiation load was calculated from the length of the scratch marks made by the needle.

[0059] <Measurement and evaluation results for each characteristic> (Surface thickness, plastic deformation hardness, and dielectric breakdown voltage of sample A) For Sample A, multiple samples (Sample No. A1 to A11) were prepared, each having a surface layer made of diamond-like carbon formed by varying the type and pressure of the raw material gas. Table 1 shows the type of gas used, pressure, and measurement results of each characteristic for each sample. The surface layer was considered acceptable if the dielectric breakdown voltage per 1 μm thickness was 35 V / μm or higher, which is sufficient to suppress the occurrence of electrolytic corrosion in the coating.

[0060] [Table 1]

[0061] Figure 6 is a graph showing the relationship between the plastic deformation hardness of the surface layer and the dielectric breakdown voltage per 1 μm thickness, obtained from the results shown in Table 1. From Table 1 and Figure 6, it was found that if the plastic deformation hardness of the surface layer is 12 to 20 GPa, an acceptable dielectric breakdown voltage of 35 V / μm or higher can be obtained for a surface layer thickness of 1 μm.

[0062] Furthermore, it was found that by setting the plastic deformation hardness of the surface layer to 13 GPa or higher, or 17 GPa or lower, the dielectric breakdown voltage per 1 μm thickness can be set to 60 V / μm or higher. In addition, it was found that by setting the plastic deformation hardness of the surface layer to 14 GPa or higher, or 15 GPa or lower, the dielectric breakdown voltage per 1 μm thickness can be set to 80 V / μm or higher.

[0063] Furthermore, it was found that the change in plastic deformation hardness of the surface layer due to changes in gas pressure is greater in C2H2 compared to CH4.

[0064] Figure 7 is a graph showing the relationship between surface layer thickness and dielectric breakdown voltage obtained from the results shown in Table 1. From Figure 7, it was found that when the plastic deformation hardness of the surface layer is between 13 GPa and 17 GPa, the dielectric breakdown voltage of the surface layer is proportional to the thickness of the surface layer. Furthermore, from the approximate straight line of measurement results shown in Figure 7, it was found that when the thickness of the surface layer is 1 μm or more, the dielectric breakdown voltage is 60 V or more.

[0065] (Thickness of the surface and sublayer and dielectric breakdown voltage of sample B) For sample B, multiple samples (samples No. B1 to B4) were prepared by varying the thickness of the Cr-based underlayer formed on the steel plate surface. The surface layer of sample B was formed using C2H2 as the raw material gas and a raw material gas pressure of 10 Pa. Table 2 shows the measurement results for each sample, including the thickness of the underlayer, the thickness of the surface layer, the total thickness of the underlayer and surface layer, the combined dielectric breakdown voltage of the underlayer and surface layer, the dielectric breakdown voltage per 1 μm of surface layer thickness, and the dielectric breakdown voltage per 1 μm of the total thickness of the underlayer and surface layer.

[0066] [Table 2]

[0067] Figure 8 is a graph showing the relationship between the thickness of the substrate and the dielectric breakdown voltage obtained from the results shown in Table 2. From Figure 8, it can be seen that in the range of substrate thickness of 200 nm or less, the thinner the substrate, the higher the dielectric breakdown voltage. Also, from Table 2, it can be seen that in the range of substrate thickness of 200 nm or less, a dielectric breakdown voltage per 1 μm of surface thickness that satisfies the acceptable value can be obtained, and that the thinner the substrate, the higher the dielectric breakdown voltage per 1 μm of surface thickness. The same applies to the dielectric breakdown voltage per 1 μm of the total thickness of the surface and substrate layers.

[0068] (Adhesion of the underlying layer of sample C) For sample C, multiple samples (samples No. C1 to C4) were prepared by varying the thickness of the Cr-based underlayer. Table 3 shows the measurement results for the underlayer thickness and peeling initiation load for each sample. Figure 9 shows photographs of the underlayer condition of each sample after the scratch test. Figure 9(a) is a photograph of sample No. C1, Figure 9(b) is a photograph of sample No. C2, Figure 9(c) is a photograph of sample No. C3, and Figure 9(d) is a photograph of sample No. C4.

[0069] [Table 3]

[0070] Table 3 shows that there was no significant difference in the delamination initiation load depending on the thickness of the substrate layer. On the other hand, when the substrate layer was 100 nm or thicker, as shown in Figures 9(b) to 9(d), no significant delamination was observed around the scratch marks. In contrast, when the substrate layer was less than 100 nm thick, as shown in Figure 9(a), large, scaly delamination was clearly observed around the scratch marks, as indicated by the area enclosed by ellipse A. These results indicate that the substrate layer thickness needs to be 100 nm or thicker to obtain sufficient adhesion to the steel material.

[0071] Measurement results for samples B and C showed that while a thinner substrate allows for a higher dielectric breakdown voltage per 1 μm of surface layer thickness, the substrate needs to be at least 100 nm thick to ensure proper adhesion. [Explanation of symbols]

[0072] 1 coating 2 Base layer 3 Surface layer 4 Steel material 30 bearings

Claims

1. A coating for steel materials, It has a base layer and a surface layer formed on the upper side of the base layer, The aforementioned underlayer consists of Cr and has a thickness of 100 nm or more. The aforementioned surface layer is made of diamond-like carbon and has a plastic deformation hardness of 12 to 20 GPa. A coating having a dielectric breakdown voltage of 35 V / μm or more per 1 μm of surface thickness.

2. The coating according to claim 1, wherein the thickness of the surface layer is 1 to 10 μm.

3. The coating according to claim 1, wherein the thickness of the underlying layer is 200 nm or less.

4. The coating according to claim 1, formed on a steel bearing.

5. A bearing having a coating according to any one of claims 1 to 4 formed on at least a portion of its surface.

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