Sliding member and piston ring

Abstract

The present invention provides a sliding member and a piston ring that have a DLC coating having excellent wear resistance even when used in a boundary lubrication environment. A sliding member according to the present invention has a base material and a DLC coating that is substantially free of hydrogen and formed on the surface of the base material. The center wave number of the G band in the Raman spectrum of the DLC coating obtained by Raman spectroscopy using a visible region laser as excitation light is 1585 cm-1 or more. The hardness HIT of the DLC coating displayed in units of GPa obtained by the nanoindentation method is 15-35 GPa.

Description

Sliding member and piston ring

[0001] The present invention relates to a sliding member and a piston ring.

[0002] Piston rings are a type of sliding component that are attached to the piston of an internal combustion engine to maintain airtightness between the piston and the cylinder. Two or three piston rings are attached near the head of the piston, sealing it to prevent combustion gases generated in the cylinder from leaking out toward the crankshaft. They also have the function of controlling the thickness of the engine oil film that forms on the inner wall of the cylinder. They also have the function of releasing the combustion heat received by the piston back into the cylinder.

[0003] In addition to these basic functions, piston rings are also required to be durable. In particular, the outer surface of the piston ring must be wear-resistant because it rubs against the inner wall of the cylinder as the piston reciprocates. In order to reduce the sliding friction between the piston ring and the inner wall of the cylinder, studies have been conducted to form a hard carbon coating on the surface of the piston ring substrate.

[0004] For example, Patent Document 1 describes an invention of a member in which a hard carbon layer made of diamond-like carbon (hereinafter sometimes referred to as "DLC") is formed on the surface of a substrate, and when the hard carbon layer is measured by Raman spectroscopy, ID / IG, which is the area intensity ratio of the D band and G band peaks in the Raman spectrum, is 1 to 6. Furthermore, Patent Document 2 describes an invention of a member in which hard carbon layers made of two types of DLC with different properties are alternately laminated on the surface of a substrate. All of these hard carbon layers according to the prior art were formed while applying a bias voltage to the substrate and simultaneously heating the substrate with a heater.

[0005] JP 2018-123431 A International Publication No. 2017 / 104822

[0006] In the prior art, a method for increasing the durability of piston rings has been to form a DLC coating with as high a hardness as possible on the surface of the piston ring substrate. By forming a DLC coating with a high hardness on the surface of the piston ring, wear of the DLC coating on the outer circumferential surface of the piston ring that rubs against the inner wall of the cylinder can be effectively prevented. This has been thought to extend the product life of the piston ring, thereby reducing the frequency of piston ring replacement.

[0007] In recent automobile engines, in order to meet demands for higher power output and stricter exhaust gas regulations, there is a trend toward increasing the combustion temperature of gasoline in the cylinder, using engine oil with lower viscosity than before, and reducing the amount of engine oil used. Along with these new technological trends, the lubrication between the outer surface of the piston ring and the inner wall of the cylinder is changing from a fluid lubrication environment, in which a continuous oil film made of engine oil is formed, to a boundary lubrication environment, in which direct metal-to-metal contact occurs locally in areas where the oil film is thin.

[0008] According to the research of the inventors, when a piston ring having a hard DLC film on its surface is used in a boundary lubrication environment, it has been found that the wear of the DLC film on the outer surface of the piston ring is more likely to progress than in a hydrodynamic lubrication environment. This suggests that for piston rings used in a boundary lubrication environment, it is difficult to ensure the necessary wear resistance for the piston ring by simply increasing the hardness of the DLC film, as has been done in the past.

[0009] Furthermore, in the prior art, when attempting to improve the durability of piston rings by using a DLC coating with high hardness, the thickness of the DLC coating must be increased to a certain extent, which poses a problem of increased manufacturing costs for the piston rings because it takes time to form the DLC coating.

[0010] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a sliding member and a piston ring provided with a DLC coating that has excellent wear resistance even when used in a boundary lubrication environment.

[0011] The gist and configuration of the present invention are as follows.

[0012] [1] A sliding member having a substrate and a DLC coating that is substantially free of hydrogen and formed on the surface of the substrate, wherein the central wave number of the G band in the Raman spectrum of the DLC coating obtained by Raman spectroscopy using a visible laser as excitation light is 1585 cm －１ The hardness H of the DLC coating obtained by nanoindentation, expressed in gigapascals, is ＩＴ [2] The elastic modulus of the DLC coating obtained by nanoindentation, expressed in gigapascals, is E ＩＴ When ＩＴ Elastic modulus E ＩＴ Ratio to H ＩＴ / E ＩＴ [3] The slide member according to [1], wherein an intermediate layer made of one or more elements selected from the group consisting of Cr, Ti, Co, V, Mo, Si, and W, or carbides, nitrides, or carbonitrides thereof is disposed between the substrate and the DLC coating. [4] A piston ring made of the slide member according to any one of [1] to [3].

[0013] According to the present invention, the amount of sliding wear of the DLC coating when a slide member having a DLC coating slides against a counter member is reduced compared to the prior art. This improves the wear resistance of the slide member. Furthermore, the thickness of the DLC coating can be made thinner than in the past.

[0014] FIG. 1 is a schematic diagram showing an example of the cross-sectional shape of a sliding member. FIG. 2 is a schematic diagram showing an example of the cross-sectional shape of a piston ring substrate. FIG. 3 is a schematic diagram showing the structure of a sliding wear tester. FIG. 4 is a graph showing the relationship between the central wavenumber of the G band in the Raman spectrum of a DLC film obtained by Raman spectroscopy and the sliding friction amount ratio. FIG. 5 is a graph showing the relationship between the hardness and the sliding friction amount ratio of a DLC film obtained by nanoindentation. FIG. 6 is a graph showing the relationship between the ratio of the hardness and the elastic modulus of a DLC film and the sliding friction amount ratio. FIG. 7 is a graph showing the relationship between the area intensity ratio ID / IG of the peaks of the D band and the G band in the Raman spectrum of a DLC film obtained by Raman spectroscopy and the sliding friction amount ratio.

[0015] Hereinafter, embodiments of the present invention will be described in detail.

[0016] [Slide Member] In one embodiment, the present invention relates to a slide member. FIG. 1 is a schematic diagram showing an example of a cross-sectional shape of a slide member according to the present invention. Slide member 100 includes a substrate 10 and a DLC coating 12 that substantially does not contain hydrogen and is formed on the surface of substrate 10. Because slide member 100 according to the present invention has excellent wear resistance, it can be applied to various products, such as piston rings, pistons, piston pins, tappets, valve lifters, shims, rocker arms, cams, camshafts, timing gears, timing chains, etc., which are used in sliding parts of internal combustion engines where lubricating oils such as engine oil are present, and vanes, injectors, plungers, cylinders, etc., which are used in fuel supply systems.

[0017] [Substrate] The slide member according to the present invention has a substrate and a DLC coating that is substantially free of hydrogen and formed on the surface of the substrate. The substrate is a component that forms the main body of the slide member. The material and shape of the substrate included in the slide member according to the present invention may be any material and may be any shape, as long as the material and shape allow for the formation of a DLC coating on the surface.

[0018] [DLC Coating] The slide member according to the present invention has a DLC coating formed on the surface of a substrate. The DLC coating is a coating made of diamond-like carbon, which is amorphous carbon that exhibits properties similar to those of diamond. The DLC coating of the slide member according to the present invention may be formed directly on the surface of the substrate, or may be formed on the surface of one or more intermediate layers formed on the surface of the substrate.

[0019] The DLC coating of the slide member according to the present invention is substantially free of hydrogen. In this specification, "a DLC coating substantially free of hydrogen" means that the amount of hydrogen contained in the diamond-like carbon that constitutes the DLC coating is 2.0% or less in atomic percentage. The DLC coating softens when it contains hydrogen. In the present invention, by making the DLC coating of the slide member substantially free of hydrogen, the hardness required for the DLC coating defined in the present invention, as described below, can be ensured. The amount of hydrogen contained in the DLC coating is preferably 1.0% or less in atomic percentage, and more preferably 0.5% or less.

[0020] [Method for Measuring the Amount of Hydrogen Contained in a DLC Film] The hydrogen content of a DLC film can be measured by Rutherford Backscattering Spectrometry (RBS) / Hydrogen Forward Scattering Spectrometry (HFS) for DLC films formed on flat sliding surfaces or surfaces with a sufficiently large curvature. RBS / HFS is a known method for analyzing film composition, but cannot be applied to the analysis of non-flat surfaces. Therefore, for DLC films formed on non-flat sliding surfaces, such as the outer peripheral surface of a piston ring, the hydrogen content can be measured by combining RBS / HFS and secondary ion mass spectrometry (SIMS). The procedure is described below.

[0021] First, a DLC film to be measured as a reference value is formed on a mirror-polished flat test piece (a quenched SKH51 disk, φ25 × thickness 5 mm, hardness HRC 60-63) as a reference sample with a flat surface. The film is formed on the reference sample using a reactive sputtering method with C as the atmospheric gas. ２ H ２ , Ar, H ２ Then, we introduce H ２ Flow rate and / or C ２ H ２ By changing the flow rate, the amount of hydrogen contained in the DLC film can be adjusted. In this way, DLC films composed of hydrogen and carbon and with different hydrogen contents are formed, and the hydrogen and carbon contents of these films are evaluated using RBS / HFS.

[0022] Next, the sample is analyzed by SIMS to measure the secondary ion intensities of hydrogen and carbon. SIMS analysis can also be used to measure DLC coatings formed on uneven surfaces, such as the outer peripheral surface of a piston ring. Therefore, for the same DLC coating on a reference sample, an empirical formula (metric curve) is calculated showing the relationship between the hydrogen content and carbon content (unit: atomic %) obtained by RBS / HFS and the ratio of the hydrogen and carbon secondary ion intensities obtained by SIMS. In this way, the hydrogen content and carbon content can be calculated from the SIMS secondary ion intensities of hydrogen and carbon measured on the outer peripheral surface of an actual piston ring. The value of the secondary ion intensity measured by SIMS is the average value of the secondary ion intensities of each element observed at least 20 nm deep from the surface of the DLC coating and within a 50 nm square area.

[0023] [Method for forming DLC ​​film] In the present invention, to form a DLC film that is substantially free of hydrogen on the surface of the substrate of a sliding member, the DLC film may be formed using a PVD method such as ion plating by vacuum arc discharge using a carbon target. The PVD method can form a DLC film that is almost free of hydrogen, has high hardness, and is excellent in wear resistance. It is preferable to use a filtered cathodic vacuum arc (FCVA) method that includes a filter that removes carbon microparticles. However, the method for forming a DLC film that is substantially free of hydrogen is not limited to these methods.

[0024] The DLC coating may be formed on at least a portion of the surface of the substrate, or may be formed over the entire surface of the substrate. When the DLC coating is formed on a portion of the surface of the substrate, it is preferable to form the DLC coating on a portion of the surface of the sliding member that is particularly required to have wear resistance. In this case, the surface of the formed DLC coating constitutes the sliding surface of the sliding member.

[0025] [Central Wavenumber of G Band] The DLC coating of the sliding member according to the present invention has a central wavenumber of G band of 1585 cm in the Raman spectrum of the DLC coating obtained by Raman spectroscopy using a visible laser as excitation light. －１ That's all. The Raman spectrum of a DLC coating can be obtained by measuring a slide member having a DLC coating formed on its surface using Raman spectroscopy. Laser light in the visible range is used as the excitation light when measuring the Raman spectrum. If it is desired to measure the Raman spectrum for each part of the slide member, a microscopic Raman spectrometer can be used.

[0026] The G band in the Raman spectrum of the DLC film is formed by sp bonds of carbon atoms contained in the DLC film. ２ It is believed that the central wave number of the G band is related to the bonding component and changes depending on the change in the crystal structure of graphite. The central wave number of the G band in the present invention was determined by the following procedure. First, the central wave number of the G band was measured in the range of 900 to 1900 cm for the obtained Raman spectrum. －１ After background subtraction in the range of 1350 cm －１The peak near the D band is 1550 cm －１ The peak near the center of the spectrum was assumed to be the G band, and fitting was performed using two Gaussian functions. Next, the wavenumber at the peak position of the separated G band spectrum was determined, and this wavenumber was taken as the central wavenumber of the G band.

[0027] In the present invention, the central wave number of the G band in the Raman spectrum of the DLC coating is 1585 cm －１ When the DLC coating on the sliding member satisfies this condition, the amount of wear when sliding friction occurs between the sliding member and the mating member is reduced compared to a sliding member having a DLC coating according to the prior art, which serves as the standard for comparison.

[0028] The amount of wear of the DLC film on the sliding member when sliding friction occurs between the sliding member and the mating member is measured when the central wave number of the G band in the Raman spectrum of the DLC film is 1585 cm －１ Although the reason for the decrease when the value is above this level is not clear, the following is thought to be the likely cause. As mentioned above, it is thought that the central wavenumber of the G band changes depending on the change in the crystalline structure of the graphite contained in the DLC coating. It is speculated that the load and heat caused by sliding make it easier for a protective film called a tribofilm to form on the surface of the DLC coating, or that the wear phenomenon known as abrasive wear, in which the surface is scraped away by foreign matter present between the friction surfaces, is suppressed.

[0029] The central wavenumber of the G band is 1585 cm －１ There is no particular upper limit to the G band shift amount associated with the change in the crystal structure of graphite. Therefore, the central wave number of the G band is substantially 1610 cm －１ Limited to the following:

[0030] [Hardness of DLC Film] The sliding member according to the present invention has a hardness H of the DLC film obtained by nanoindentation, expressed in gigapascals. ＩＴThe hardness is 15 GPa or more and 35 GPa or less. In this specification, "nanoindentation method" refers to a measurement method based on the nanoindentation hardness test method specified in the international standard ISO 14577. This method involves pressing a tiny diamond indenter with a regular triangular pyramid (Berkovich type) tip into the surface of a sample, and measuring the load-displacement curve at that time to measure the hardness and elastic modulus of a micro-area. To avoid the influence of the substrate, which is the base of the DLC coating, the indentation load is set so that the maximum indentation depth of the diamond indenter is 1 / 10 or less of the film thickness. Furthermore, as a pre-treatment for the test, the test area is mirror-polished to reduce the variability of the test values.

[0031] Hardness H of the DLC film measured by nanoindentation ＩＴ It is known that the hardness H of the DLC film varies greatly depending on the conditions for forming the DLC film. For example, the hardness of graphite is about 4 GPa, and the hardness of diamond is 90 to 100 GPa. The hardness H of the DLC film of the sliding member according to the present invention is ＩＴ lies between these values.

[0032] DLC film hardness H ＩＴ If the hardness H of the DLC film is less than 15 GPa, the amount of wear of the DLC film increases when sliding friction occurs with a sliding member. ＩＴ If the hardness H of the DLC film is greater than 35 GPa, abrasive wear is likely to occur due to the generation of wear particles, and the amount of wear of the DLC film also increases. ＩＴ If the hardness H of the DLC film is too small or too large, the amount of wear of the DLC film increases. ＩＴ When the modulus of elasticity is in the range of 15 GPa or more and 35 GPa or less as specified in the present invention, the wear amount of the DLC film is reduced to less than half of that of the DLC film according to the conventional technology, which is the standard for comparison.

[0033] [Thickness of DLC Coating] In a preferred embodiment, the thickness of the DLC coating on the sliding member according to the present invention is not particularly limited, but is preferably 1.0 μm or more and 30 μm or less. If the thickness is 1.0 μm or more, the durability of the DLC coating will not be insufficient, and if the thickness is 30 μm or less, the adhesion to the base material will not be insufficient and peeling will not occur. A more preferred thickness of the DLC coating is 5.0 μm or more and 20.0 μm or less. The thickness of the DLC coating can be measured using a calotest or by observing the cross section of a resin-embedded coating.

[0034] [Ratio of Hardness to Elastic Modulus of DLC Film] In a preferred embodiment, the elastic modulus of the DLC film obtained by nanoindentation, expressed in gigapascals, is expressed as E ＩＴ When ＩＴ Elastic modulus E ＩＴ Ratio to H ＩＴ / E ＩＴ The elastic modulus E of the DLC film is 0.095 or more. ＩＴ is an index that indicates the resistance of the DLC film to elastic deformation when a load is applied to the DLC film. ＩＴ The smaller the hardness H, the more flexibly the DLC coating deforms when the sliding member having the DLC coating is pressed against the mating member. ＩＴ When comparing when the ratio H ＩＴ / E ＩＴ The larger the value, the more flexibly the DLC film deforms, which is thought to suppress the progression of abrasive wear of the DLC film.

[0035] In a preferred embodiment, the ratio H of the hardness to the elastic modulus of the DLC coating ＩＴ / E ＩＴ The ratio H between the hardness and the elastic modulus of a material is generally 0.095 or more, and there is no particular upper limit. Therefore, for a DLC film that does not substantially contain hydrogen, the ratio H between the hardness and the elastic modulus expressed in the same units is ＩＴ / E ＩＴ is substantially limited to 0.120 or less.

[0036] [Intermediate Layer] In a preferred embodiment, the slide member according to the present invention further comprises an intermediate layer between the substrate and the DLC coating, the intermediate layer comprising one or more elements selected from the group consisting of Cr, Ti, Co, V, Mo, Si, and W, or carbides, nitrides, or carbonitrides thereof. The intermediate layer is provided on at least a portion of the surface of the substrate. The presence of the intermediate layer on the surface of the substrate relieves stress between the substrate and the DLC coating, improving the adhesion of the DLC coating.

[0037] The intermediate layer may cover at least a portion of the surface of the substrate, or may cover the entire surface. When a piston ring is selected as the substrate of the sliding member, for example, the outer peripheral surface of the piston ring may be covered. This is preferable because it improves the adhesion of the DLC coating to the outer peripheral surface. When covering the entire surface of the substrate, the intermediate layer may be left covering the entire surface of the substrate, or after forming the intermediate layer, unnecessary portions of the intermediate layer may be removed by polishing or the like to expose the substrate.

[0038] The thickness of the intermediate layer is preferably 0.1 μm or more and 0.6 μm or less, and more preferably 0.2 μm or more and 0.5 μm or less. A thickness of 0.1 μm or more can improve the adhesion of the DLC coating, and a thickness of 0.6 μm or less makes the intermediate layer less likely to undergo plastic flow during sliding, making the DLC coating less likely to peel off. The intermediate layer can be formed on the substrate by, for example, suitably using a known PVD (physical vapor deposition) method.

[0039] [Piston Ring] In another embodiment, the present invention is directed to a piston ring. The piston ring according to the present invention is a piston ring made of the sliding member according to the present invention. The substrate of the piston ring according to the present invention is made of steel having a ring shape. The steel used for the substrate is not particularly limited as long as it has the properties required for a piston ring, such as strength, elastic modulus, and thermal conductivity. As the steel, carbon steel, spring steel, martensitic stainless steel, etc. are preferably used. The piston ring according to the present invention has a ring shape, and a gap is provided in part of the ring shape. The piston ring is mounted in a groove provided on the outer peripheral surface of the piston with the gap closed and the diameter reduced. Inside the cylinder, the outer peripheral surface of the piston ring slides while being pressed against the inner wall of the cylinder by tension.

[0040] Fig. 2 is a schematic diagram showing an example of the cross-sectional shape of the substrate of a piston ring according to the present invention. In Fig. 2, a DLC coating, which will be described later, is omitted. As shown in Fig. 2, a piston ring 200 includes a substrate 20 whose cross section is approximately rectangular. However, there are many variations in the cross-sectional shape of the substrate 20 constituting the piston ring, and it does not necessarily have to be rectangular.

[0041] The outer peripheral surface 22 is located outside the outer peripheral surface of the piston when the piston ring 200 is fitted into a groove in the piston (not shown), and is a surface that slides in contact with the inner wall of the cylinder. The shape of the outer peripheral surface 22 may be a flat shape as shown in FIG. 2 , a barrel shape with a central portion bulging outward, or a tapered shape with a diameter that changes continuously with position. As described above, the outer peripheral surface 22 is required to have wear resistance that makes it resistant to wear due to sliding against the inner wall surface of the cylinder. In the piston ring according to the present invention, a DLC coating (not shown) is formed on the outer peripheral surface 22.

[0042] The side surfaces 24a and 24b are surfaces where the piston ring 200 comes into contact with a groove in a piston (not shown). An upper side surface 24a and a lower side surface 24b are shown in Figure 2. An inner peripheral surface 28 is located on the opposite side of the outer peripheral surface 22. Outer peripheral edges 26a and 26b are located at the boundaries between the outer peripheral surface 22 and the side surfaces 24a and 24b. Generally, the outer peripheral edges 26a and 26b are chamfered (not shown) to prevent chipping of the base material 20.

[0043] [Sliding Fatigue Test] A method for evaluating the wear amount of a DLC film on a sliding member when sliding friction is generated between the sliding member according to the present invention and a mating material will be described. FIG. 3 is a schematic diagram showing an outline of a method for conducting a sliding fatigue test for this evaluation. A sliding fatigue tester 300 used in the sliding fatigue test has a rotating drum 30. The sliding fatigue tester 300 applies a load repeatedly while pressing the surface of a test piece 32 against the outer circumferential surface of the drum 30 using a drive mechanism (not shown). The wear amount of the DLC film formed on the surface of the test piece 32 is determined by measuring the change in dimension of the test piece 32 before and after the test. This is divided by the wear amount of a standard sample run under the same conditions to determine the sliding wear amount ratio.

[0044] The following are examples of test conditions for conducting a sliding fatigue test. The drum 30, which is the mating member against which the test piece 32 slides, is a cylindrical member with a diameter of 80 mm. The material constituting the drum 30 is preferably a highly wear-resistant steel, such as SUJ2 bearing steel as specified in Japanese Industrial Standard JIS G 4805 "High Carbon Chromium Bearing Steel." The speed of the circumferential surface of the drum 30 rotating relative to the fixed test piece 32 is 10.0 m / s. The rotation of the drum 30 is accelerated from a stationary state to a speed of 10.0 m / s over 30 seconds, held at 10.0 m / s for 20 seconds, and then decelerated to zero over 30 seconds. The rotation direction is then reversed, and the acceleration, hold, and deceleration cycle is repeated. This cycle constitutes one cycle, and 10 cycles are repeated.

[0045] A load is repeatedly applied to the test piece 32 by a drive mechanism (not shown), and the test piece 32 is pressed perpendicularly against the outer circumferential surface of the drum 30. The load magnitude is set to a maximum load of 50 N and a minimum load of 20 N, and the load is changed over time between the maximum load and the minimum load in a sinusoidal wave shape with a frequency of 50 Hz. The surface temperature of the drum 30 is maintained at 80°C by a heater (not shown). Additive-free motor oil is supplied from the tip of the oil supply pipe 34 at a rate of 0.1 cm per minute. ３ The oil is supplied to the surface of the drum 30 at a rate of 1 / 2 sq. m / s. This oil supply rate is insufficient for fluid lubrication, creating a boundary lubrication environment.

[0046] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0047] A substrate made of silicon chromium steel and having a ring shape with a cross section that approximated a rectangle was prepared as the piston ring substrate. The substrate had a nominal diameter of 78 mm, a thickness of 2.5 mm, and a width of 1.2 mm. The outer peripheral surface had a barrel face shape with a slight bulge in the center. The outer peripheral edge of the substrate was chamfered with a width of 0.15 mm. Using a cathodic arc ion plating device (M720, manufactured by Nippon ITF Co., Ltd.), the outer peripheral surface of the substrate was coated with a 0.5 μm thick intermediate layer made of chromium.

[0048] Next, the substrates were degreased and cleaned, and then set in a cathodic arc ion plating device with their sides in contact. The surfaces were cleaned by ion bombardment, and then the outer periphery of the substrate was coated with a DLC film using an ion plating method with vacuum arc discharge. After the DLC film was formed, the DLC film that had spread around to the side surfaces was removed using a honing device and a lapping device, yielding a plurality of piston rings for Examples 1 to 5 and Comparative Examples 1 to 9 shown in Table 1. The DLC film formation conditions are shown in Table 1.

[0049]

[0050] The set bias shown in Table 1 is the bias voltage of the substrate set on the device during film formation. Regardless of the set bias value, the bias voltage holding circuit of the device functions to maintain the bias voltage at the set bias value. The measured bias shown in Table 1 is the bias voltage of the substrate actually monitored during film formation. Even when the set bias is set to zero, the substrate is charged by the charge carried by the carbon ions, and a non-zero bias voltage is measured. This is sometimes called the self-bias voltage. When a self-bias voltage occurs, the carbon ions are accelerated and their energy increases, making the temperature of the substrate more likely to rise even when not heated by a heater.

[0051] As shown in Examples 1 to 4, when the absolute value of the self-bias voltage is greater than the absolute value of the set bias, the measured bias voltage is dominated by the self-bias voltage and is not affected by the set bias. Furthermore, when the set bias is the same, the measured bias changes depending on the magnitude of the arc current.

[0052] The "floating potential" in Example 5 means that the bias voltage holding circuit of the device is cut off and the bias voltage is not controlled. In this case, too, the substrate is charged by the electric charge of the carbon ions.

[0053] Regarding the film formation rates shown in Table 1, "fast" indicates a film formation rate faster than the conditions of Comparative Example 1, which is used as a reference; "same" indicates a film formation rate equal to the conditions; and "slow" indicates a slower film formation rate.

[0054] The piston ring samples thus obtained were subjected to the following evaluations. First, the Raman spectrum of the DLC coating on the outer peripheral surface of the piston ring was measured using a Raman spectrometer (inViaReflex, manufactured by Renishaw). Measurements were performed under conditions of an Ar ion excitation laser wavelength of 532.0 nm, a laser output of 50 mW, an objective lens of 100x magnification, and light passed through an attenuator. From the obtained Raman spectrum data, the central wavenumber of the G band and ID / IG, which is the area intensity ratio of the D band and G band peaks, were determined.

[0055] Next, the hardness H of the DLC film was measured by a nanoindentation method conforming to the international standard ISO 14577 using a nanoindenter (manufactured by Elionix, model number ENT-1100a). ＩＴ and elastic modulus E ＩＴ From the measured values, the ratio H between hardness and elastic modulus was calculated. ＩＴ / E ＩＴ was calculated.

[0056] Next, using the sliding fatigue tester shown in Figure 3, the sliding wear amount of the DLC coating on the piston ring was measured under the above-mentioned test conditions. For the measurement, a test piece having a length of about 20 mm including the outer peripheral surface cut from the piston ring sample was used. In the sliding wear test, the wear amount after 10 cycles of 1 hour test was measured, and the ratio of this wear amount to the wear amount of Comparative Example 1, which served as a benchmark, was calculated and used as the sliding wear amount ratio. These evaluation results are shown in Table 2. In addition, the central wave number of the G band and hardness H ＩＴ , ratio H ＩＴ / E ＩＴ The relationship between the ratio ID / IG and the sliding wear amount ratio is shown in Figures 4 to 7, respectively.

[0057]

[0058] According to Table 2 and Figure 4, the central wave number of the G band is 1585 cm －１ Among the piston rings having the above values, those of Examples 1 to 5 had significantly less sliding wear than those of Comparative Example 1 having a DLC coating according to the conventional technology, which was used as the standard for comparison. The central wave number of the G band was 1585 cm －１ The sliding wear amounts of the piston rings of Comparative Examples 2 to 7, which did not exceed 100%, were larger than those of Comparative Example 1, or even if they were slightly smaller, no significant improvement was observed. Furthermore, in Comparative Examples 8 and 9, the hardness of the DLC coating, which will be described later, was insufficient, and the sliding wear amounts increased compared to Comparative Example 1.

[0059] According to Table 2 and FIG. 5, the hardness H of the DLC film ＩＴAmong the piston rings having a compressive strength of 15 GPa or more and 35 GPa or less, those of Examples 1 to 5 had significantly less sliding wear than those of Comparative Example 1. As described above, Comparative Examples 3, 4, and 6 had a central wave number of the G band of 1585 cm －１ Therefore, the sliding wear amount was not significantly smaller than that of Comparative Example 1. From the above results, it is clear that the sliding member according to the present invention is superior in wear resistance to that of the prior art, and that the film formation speed can also be increased by appropriately adjusting the film formation conditions.

[0060] Next, according to FIG. 6, the ratio H between hardness and elastic modulus ＩＴ / E ＩＴ A good correlation was observed between the ratio of the amount of sliding wear and the hardness of the sample, except for Comparative Example 5, which had an extremely high hardness. ＩＴ / E ＩＴ This indicates that when DLC films having the same hardness are compared, the elastic modulus E ＩＴ This suggests that the smaller the value, the more flexibility the DLC film has against repeated loads, thereby reducing the amount of sliding wear.

[0061] On the other hand, according to Table 2 and FIG. 7, no clear correlation was found between ID / IG, which is the area intensity ratio of the D band and G band peaks described in Patent Document 1, and the sliding wear amount ratio.

[0062] As described above, the DLC coatings of Examples 1 to 5, which exhibited good wear resistance, were all formed without heating the substrate with a heater under conditions where the set bias was 0 V or a floating potential and the self-bias voltage was about 0 V to -35 V. In contrast, the DLC coating of Comparative Example 4, in which the set bias was set to a non-zero value similar to conventional film formation conditions, had a central wavenumber of the G band of 1585 cm －１ The DLC film formed with a non-zero bias setting had a hard sp ３ This is thought to be due to the tendency for clusters to increase.

[0063] Regarding heater heating, it is preferable to adjust the temperature of the substrate only by heating with arc current, without using heater heating. This is thought to be because heater heating tends to increase the adhesion of foreign matter such as macroparticles from the coating formed on locations other than the substrate during film formation, leading to an increase in the amount of wear. Thus, in the sliding member according to the present invention, the central wave number of the G band and the hardness H ＩＴ By adjusting the DLC film formation conditions shown in Table 1 so that the above properties fall within the predetermined ranges, excellent sliding friction properties can be obtained.

[0064] REFERENCE SIGNS LIST 100 Sliding member 10 Substrate 12 DLC coating 200 Piston ring 20 Substrate 22 Outer peripheral surface 24a, 24b Side surface 26a, 26b Outer peripheral edge 28 Inner peripheral surface 300 Sliding fatigue tester 30 Drum 32 Test piece 34 Oil supply pipe

Claims

1. A sliding member having a substrate and a DLC coating substantially free of hydrogen formed on the surface of the substrate, wherein the central wave number of the G band in the Raman spectrum of the DLC coating obtained by Raman spectroscopy using a visible laser as excitation light is 1585 cm －１ The hardness H of the DLC coating obtained by the nanoindentation method, expressed in gigapascals, ＩＴ A sliding member characterized in that the compressive strength is 15 GPa or more and 35 GPa or less.

2. The elastic modulus of the DLC film obtained by the nanoindentation method, expressed in gigapascals, is defined as E ＩＴ When ＩＴ Elastic modulus E ＩＴ Ratio to H ＩＴ / E ＩＴ The sliding member according to claim 1 , wherein the σ is 0.095 or more.

3. The sliding member according to claim 1, further comprising an intermediate layer between the substrate and the DLC coating, the intermediate layer being made of one or more elements selected from the group consisting of Cr, Ti, Co, V, Mo, Si and W, or a carbide, nitride or carbonitride thereof.

4. A piston ring comprising the sliding member according to any one of claims 1 to 3.

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