Piston ring and method for manufacturing same, method for assembling internal combustion engine, and method for selecting piston ring

Abstract

The present invention comprises a base material and a hard coating film provided so as to cover at least a portion of an outer peripheral surface of the base material. The ratio (Y / X) of Vickers hardness Y of the hard coating film after heat treatment to the Vickers hardness X of the hard coating film is 0.7 to 1.1, and the wear index IW of the hard coating film is 1.2 x 10-2 μm / MPa or less. The wear index IW is the sum of the first wear index IW1 obtained by a first wear test implemented using base oil and the second wear index IW2 obtained by a second wear test implemented using a sulfuric acid aqueous solution.

Description

Piston ring and its manufacturing method, method for assembling an internal combustion engine, and method for selecting a piston ring.

[0001] This disclosure relates to piston rings and methods for manufacturing the same, methods for assembling internal combustion engines, and methods for selecting piston rings.

[0002] Piston rings are used in internal combustion engines such as those found in automobiles. Piston rings are fitted into grooves provided on the outer surface of the piston. Piston rings are required to have the function of properly supplying oil to the inner wall of the cylinder bore and the function of suppressing gases from the combustion chamber from entering the crankcase. If too much oil is supplied to the combustion chamber, it leads to increased oil consumption. On the other hand, if the sealing function of the piston ring is insufficient, the amount of gas leaking from the combustion chamber through the piston ring into the crankcase increases, leading to decreased fuel efficiency. Patent Document 1 discloses a piston ring made of a resin composition with an elastic modulus of 0.3 to 50 GPa, and it is said that this piston ring can reduce oil consumption.

[0003] International Publication No. 2019 / 124389

[0004] In recent years, hydrogen engines, which use hydrogen as at least part of their fuel, have attracted attention. Hydrogen engines that use hydrogen alone as fuel can achieve zero emissions by not emitting carbon dioxide. However, according to the inventors' studies, the environment inside a hydrogen engine is different from the environment inside an engine that uses conventional fossil fuels, so conventional piston rings cannot necessarily be applied as is. In other words, hydrogen has a lower minimum ignition energy and a wider explosion range compared to fossil fuels. For this reason, engine oil scattered in the combustion chamber can act as an ignition source, causing abnormal combustion. Abnormal combustion is more likely to occur when the combustion chamber is hot, and in such an environment, there is concern that water produced by the combustion of hydrogen may promote corrosion of the piston rings. "Abnormal combustion" as used here refers to a combustion phenomenon in which the combustion progress deviates from what is expected under predetermined ignition control, and the timing or location of ignition, or the combustion speed, falls outside the design range, resulting in harmful pressure and temperature behavior such as a sharp pressure rise or high-frequency pressure oscillation. Specific examples include pre-ignition, surface ignition, knocking, flashback, and afterfire.

[0005] This disclosure provides a piston ring for an internal combustion engine in which hydrogen is used as at least part of the fuel, and which can suppress the occurrence of abnormal combustion in the combustion chamber. This disclosure also provides a method for manufacturing a piston ring, a method for assembling an internal combustion engine, and a method for selecting a piston ring.

[0006] This disclosure relates to the following: [1] A piston ring for an internal combustion engine in which hydrogen is used as at least part of the fuel, comprising: a base material and a hard coating provided so as to cover at least a portion of the outer surface of the base material, wherein the hard coating satisfies the following conditions shown in formula (1): 0.7 ≤ Y / X ≤ 1.1 ... (1) where X represents the Vickers hardness of the hard coating, Y represents the Vickers hardness of the hard coating after heat treatment, the heat treatment is a process of heating the piston ring in air at 400°C for 5 hours, and the wear index of the hard coating is I Ｗ is 1.2 × 10 －２ The abrasion index I is less than or equal to μm / MPa. Ｗis the first wear index I obtained by the first wear test carried out using a base oil Ｗ１ and the second wear index I obtained by the second wear test carried out using an aqueous sulfuric acid solution Ｗ２ The sum of, and the first wear index I Ｗ１ is a value obtained by the following formula (2), I Ｗ１ = A Ｗ１ / P Ｃ１ ... (2) In formula (2), A Ｗ１ represents the wear amount (unit: μm) obtained by the first wear test, and P Ｃ１ represents the surface pressure (unit: MPa) of the piston ring at the start of the first wear test. The second wear index I Ｗ２ is a value obtained by the following formula (3), I Ｗ２ = A Ｗ２ / P Ｃ２ ... (3) In formula (3), A Ｗ２ represents the wear amount (unit: μm) obtained by the second wear test, and P Ｃ２A piston ring, wherein the surface pressure (unit: MPa) of the piston ring at the start of the second wear test. [2] The piston ring according to [1], wherein the hard coating is a CrN-based coating formed by physical vapor deposition. [3] The piston ring according to [1], wherein the hard coating is a DLC coating. [4] The piston ring according to [1], wherein the hard coating is a chromium-plated coating. [5] The piston ring according to any one of [1] to [4], wherein the developed interface area ratio Sdr of the outer circumferential surface of the base material is 0.04 to 0.14%. [6] The piston ring according to any one of [1] to [5], wherein the internal combustion engine is a spark-ignition engine. [7] The piston ring according to any one of [1] to [6], wherein the top ring is fitted to the groove closest to the combustion chamber among a plurality of grooves provided in the piston. [8] A method for assembling an internal combustion engine in which hydrogen is used as at least part of the fuel, comprising the steps of: mounting a piston ring described in any one of [1] to [7] into a groove provided in a piston; and housing the piston in a cylinder. [9] A method for selecting a piston ring for an internal combustion engine in which hydrogen is used as at least part of the fuel, wherein the piston ring comprises a base material and a hard coating provided so as to cover at least a part of the outer surface of the base material, comprising the steps of determining whether the hard coating satisfies the conditions shown in the following formula (1), and the wear index I of the hard coating Ｗ 1.2 × 10 －２ The process includes determining whether or not the wear index is less than or equal to μm / MPa, wherein 0.7 ≤ Y / X ≤ 1.1 ... (1) In equation (1), X represents the Vickers hardness of the hard coating, Y represents the Vickers hardness of the hard coating after heat treatment, the heat treatment is a process of heating the piston ring in air at 400°C for 5 hours, and the wear index I Ｗ The first wear index I is determined by a first wear test performed using a base oil. Ｗ１ And the second abrasion index I, which is determined by a second abrasion test conducted using a sulfuric acid solution. Ｗ２ It is the sum of the first wear index I Ｗ１ The value is obtained by the following formula (2), and I Ｗ１ = AＷ１ / P Ｃ１ ...(2) In formula (2), A Ｗ１ This indicates the amount of wear (unit: μm) determined by the first wear test described above, and P Ｃ１ This indicates the surface pressure (in MPa) of the piston ring at the start of the first wear test, and the second wear index I Ｗ２ The value is obtained by the following formula (3), and I Ｗ２ = A Ｗ２ / P Ｃ２ ...(3) In formula (3), A Ｗ２ This indicates the amount of wear (unit: μm) determined by the second wear test described above, and P Ｃ２ This indicates the surface pressure (in MPa) of the piston ring at the start of the second wear test, and the hard coating satisfies the conditions shown in formula (1) above and the wear index of the hard coating is I Ｗ 1.2 × 10 －２ A method for selecting piston rings in which piston rings having a pressure of μm / MPa or less are deemed acceptable.

[10] A method for manufacturing piston rings for an internal combustion engine in which hydrogen is used as at least a part of the fuel, comprising the step of forming a hard coating so as to cover at least a part of the outer surface of a base material, wherein the hard coating is formed so as to be an acceptable product in the piston ring selection method described in [9].

[0007] This disclosure provides a piston ring for an internal combustion engine in which hydrogen is used as at least part of the fuel, and which can suppress the occurrence of abnormal combustion in the combustion chamber. This disclosure also provides a method for manufacturing a piston ring, a method for assembling an internal combustion engine, and a method for selecting a piston ring.

[0008] Figure 1 is a schematic cross-sectional view showing one embodiment of the piston ring of this disclosure. Figure 2 is a schematic cross-sectional view showing a modified example of the piston ring shown in Figure 1. Figure 3 is a schematic diagram showing the test method for the wear test. Figure 4 is a graph showing the rotational speed profile in the wear test.

[0009] The embodiments of this disclosure will be described in detail below with reference to the drawings. However, the present invention is not limited to the following embodiments.

[0010] <Piston Ring> Figure 1 is a schematic cross-sectional view showing a piston ring according to this embodiment. The piston ring 10 shown in this figure is applied to an internal combustion engine in which hydrogen is used as at least part of the fuel, and may also be applied to an internal combustion engine in which hydrogen is used as the sole fuel. The piston ring 10 is fitted into a groove formed on the side surface of the piston. The piston ring 10 is used as a top ring. The top ring refers to a compression ring fitted into the groove closest to the combustion chamber of the internal combustion engine. The piston ring 10 may also be used as a second ring. The second ring refers to a compression ring fitted into the groove next closest to the combustion chamber after the groove in which the top ring is fitted. It should be noted that the piston ring 10 can also be applied to conventional internal combustion engines that use fossil fuels.

[0011] The piston ring 10 comprises a base material 1, a hard coating 5 provided to cover the outer circumferential surface 1F of the base material 1, side surfaces 10a and 10b, and an inner circumferential surface 10c. The sliding surface 10F of the piston ring 10 is formed by the surface of the hard coating 5. The piston ring 10 is annular and has an inner diameter corresponding to the size of the piston it is installed in. Here, "annular" does not necessarily mean a closed circle, and the piston ring 10 may have a joint. The piston ring 10 may be perfectly circular or elliptical in plan view.

[0012] As shown in Figure 1, the sliding surface 10F has a rounded shape with the center bulging outwards. This shape is called a barrel face shape. The sliding surface 10F may bulge above or below the center. This shape is called an eccentric barrel face shape. The cross-sectional shape of the piston ring 10 may be a rectangle in which the sliding surface 10F is not rounded. The sides 10a and 10b may be parallel, or they may not be parallel. When the sides 10a and 10b are parallel, this cross-sectional shape is called rectangular. The piston ring 11 shown in Figure 2 is a modified example of the piston ring 10, in which the sides 11a and 11b are not parallel. The sides 11a and 11b are inclined so that they approach each other as you move from the sliding surface 11F toward the inner circumferential surface 11c. This shape is called a keystone shape. The shape may be such that only one of the sides 11a and 11b is inclined. This shape is called a half-keystone shape.

[0013] [Base Material] Base material 1 is a metal annular member. The shape of base material 1 is the same as the shape of the piston ring 10. Base material 1 is made of a heat-resistant alloy. Specific examples of alloys include martensitic stainless steel and spring steel. Of these alloys, base material 1 is made of martensitic stainless steel, which provides higher heat resistance than spring steel.

[0014] From the viewpoint of adhesion of the hard coating 5 to the outer surface 1F of the base material 1, the developed interface area ratio (hereinafter referred to as "Sdr") of the outer surface 1F is, for example, 0.04 to 0.14%. The lower limit of Sdr may be 0.07%, and the upper limit may be 0.10% or 0.07%. When the value of Sdr is 0.04% or higher, the adhesion of the hard coating 5 to the base material 1 tends to improve, resulting in the effect of improving the durability of the piston ring 10. On the other hand, when the value of Sdr is 0.14% or lower, it tends to be easier to form a uniform hard coating 5, resulting in the effect of improving the wear resistance and corrosion resistance of the hard coating 5. Sdr may also be 0.04 to 0.10%, 0.04 to 0.07%, 0.07 to 0.14%, or 0.07 to 0.10%. The Sdr value referred to here is the value measured using a laser microscope within a rectangular area centered on a point on the center line of the outer surface 1F of the substrate 1, with an axial length of 75% of the nominal width (h1 dimension), using a Gaussian filter, with the S-filter cutoff wavelength being 5 μm and the L-filter cutoff wavelength being 0.1 mm.

[0015] The height of the protruding peaks or protruding valleys on the outer surface 1F is, for example, 0.1 to 1.2 μm, and may be 0.1 to 1.0 μm, 0.1 to 0.9 μm, 0.5 to 1.2 μm, 0.5 to 1.0 μm, or 0.5 to 0.9 μm. When the height of the protruding peaks and protruding valleys of the base material 1 is 0.1 to 1.2 μm, the adhesion of the hard coating 5 to the base material 1 is improved, the durability of the piston ring 10 is improved, and it becomes easier to obtain a hard coating 5 with fewer defects, resulting in improved wear resistance and corrosion resistance of the hard coating 5. The height of the protruding peaks and protruding valleys of the base material 1 referred to here means the value measured under the same conditions as the measurement of Sdr described above.

[0016] [Hard coating] The hard coating 5 constitutes the sliding surface 10F and provides the piston ring 10 with properties such as wear resistance and corrosion resistance. The hard coating 5 only needs to cover at least a part of the outer circumferential surface 1F of the base material 1, and may cover the entire outer circumferential surface 1F of the base material 1.

[0017] The thickness of the hard coating 5 is, for example, 3 to 100 μm, and may be 3 to 70 μm, 5 to 100 μm, or 5 to 70 μm, or 100 μm or more. A thickness of 3 μm or more for the hard coating 5 improves the durability of the piston ring 10 and tends to suppress engine oil scattering for a longer period of time, while a thickness of 100 μm or less ensures high productivity of the hard coating 5.

[0018] The Vickers hardness of the hard coating 5 only needs to be higher than that of the substrate, for example, it may be 600 to 3500 HV. The Vickers hardness of the hard coating 5 may be 800 HV or higher. In this case, the effects of improved wear resistance and corrosion resistance are achieved. The Vickers hardness of the hard coating 5 may be 800 to 3500 HV. The Vickers hardness referred to here means the value obtained by measuring with a Vickers hardness tester under the condition of a test force of 0.980 N. Note that the test force may be lowered to, for example, 0.490 N depending on the film thickness.

[0019] (Vickers hardness) The hard coating 5 has the ability to maintain sufficient hardness even when the piston ring 10 is subjected to a high-temperature environment of approximately 400°C (a temperature at which abnormal combustion may occur in a hydrogen engine). That is, the hard coating 5 satisfies the condition shown in the following formula (1): 0.7 ≤ Y / X ≤ 1.1 ... (1) In formula (1), X represents the Vickers hardness of the hard coating 5, Y represents the Vickers hardness of the hard coating 5 after heat treatment, and the heat treatment is a process of heating the piston ring 10 in air at 400°C for 5 hours. The heat treatment is carried out, for example, by a heating furnace. The time of the heat treatment means the time from when the temperature reaches 400°C until the cooling begins, that is, the period during which the temperature is maintained at 400°C. The lower limit of the value of Y / X may be 0.8, 0.82, or 0.85. The upper limit of the value of Y / X may be 1.0 or 0.9. The range of Y / X may be 0.7 to 1.0, 0.7 to 0.9, 0.8 to 1.1, 0.8 to 1.0, 0.8 to 0.9, 0.82 to 1.1, 0.82 to 1.0, 0.82 to 0.9, 0.85 to 1.1, 0.85 to 1.0, or 0.85 to 0.9. Note that a Y / X value greater than 1 indicates that the Vickers hardness will increase as a result of the above heat treatment.

[0020] (Abrasion index I Ｗ ) Abrasion index I of hard coating 5 Ｗ is 1.2 × 10 －２ It is less than or equal to μm / MPa. Abrasion index I Ｗ The upper limit is 0.9 × 10 －２ It is μm / MPa and 0.6 × 10 －２ It may also be μm / MPa. Abrasion index I Ｗ The lower limit is, for example, 0.0 × 10 －２ It is μm / MPa and 0.1 × 10 －２ μm / MPa or 0.3 × 10⁻⁶ －２ It may also be μm / MPa. Abrasion index I Ｗ The range is 0 or greater, 1.2 × 10 －２ μm / MPa or less, 0 or more 0.9×10 －２ μm / MPa or less, 0 or more 0.6×10 －２ μm / MPa or less, 0.1×10 －２The above 1.2 x 10 －２ μm / MPa or less, 0.1×10 －２ The above is 0.9 x 10 －２ μm / MPa or less, 0.1×10 －２ The above is 0.6 x 10 －２ μm / MPa or less, 0.3×10 －２ μm / MPa or more 1.2×10 －２ μm / MPa or less, 0.3×10 －２ The above is 0.9 x 10 －２ μm / MPa or less or 0.3 × 10⁻⁶ －２ The above is 0.6 x 10 －２ It is also acceptable if it is less than μm / MPa. The wear index I referred to here is Ｗ This is the first wear index I, which will be described later. Ｗ１ and the second wear index I Ｗ２ It is the sum of the wear index I. Ｗ１ , I Ｗ２ These values ​​are obtained from the measurements of the first and second abrasion tests, which are performed under the conditions described in Table 1. The initial surface pressure indicates the surface pressure when the upper limit test load is applied to the test piece S at the start of the first or second abrasion test.

[0021]

[0022] (First wear index I Ｗ１ ) First wear index I Ｗ１ [μm / MPa] is the first wear amount A, which is determined by a first wear test performed using the base oil. Ｗ１ [μm] and the surface pressure (initial surface pressure) P of the piston ring at the start of the first wear test. Ｃ１ It is calculated using [MPa] by the following formula (2). Ｗ１ = A Ｗ１ / P Ｃ１ ... (2) I Ｗ１ The upper limit is, for example, 0.3 × 10 －２ It is μm / MPa and 0.2 × 10 －２ It may also be μm / MPa. Ｗ１ The lower limit is, for example, 0.0 × 10 －２ It is μm / MPa and 0.1 × 10 －２ The first wear index I may be μm / MPa. Ｗ１The range is 0 or more and 0.3×10 －２ μm / MPa or less, 0 or more and 0.2×10 －２ μm / MPa or less, 0.1×10 －２ or more and 0.3×10 －２ μm / MPa or less, or 0.1×10 －２ or more and 0.2×10 －２ μm / MPa or less may be used. Hereinafter, the method for conducting the first wear test will be described while referring to FIG. 3.

[0023] First, a test piece S is obtained by cutting the piston ring 10 to be tested. Specifically, about 10 mm is cut near the opposite part of the ring joint of the piston ring 10 (the part rotated 180° from the joint part around the rotation axis of the piston ring 10 when the piston ring 10 is viewed in plan view). Next, the test piece S is installed in the fixing jig of the sliding fatigue tester 50. The surface pressure is calculated using Hertz's formula (the following formula (A)) based on the physical properties and shape of the test piece S and the mating material 51, and the test load.

[0024] E １ represents the longitudinal elastic modulus of the mating material 51, and E ２ represents the longitudinal elastic modulus of the test piece S. v １ represents the Poisson's ratio of the mating material 51, and v ２ represents the Poisson's ratio of the test piece S. P represents the concentrated load (test load). p ０ represents the maximum stress (surface pressure) generated at the center of the contact surface. R １ represents the major radius of curvature at the axial contact point of the mating material 51, and R´ １ represents the major radius of curvature at the circumferential contact point of the mating material 51, and R ２ represents the major radius of curvature at the axial contact point of the test piece S, and R´ ２ represents the major radius of curvature at the circumferential contact point of the test piece S. 2a represents the major diameter of the contact surface circle of the mating material 51, and 2b represents the major diameter of the contact surface circle of the test piece S. The values of m and n are given in the following table with cosθ = B / A.

[0025]

[0026] Next, a wear test is conducted under the conditions described in Table 1. The rotational speed of the mating material 51 is changed according to the profile shown in FIG. 4. A series of changes in the rotational speed shown in FIG. 4 is defined as one cycle, and the wear test is performed by repeating the number of cycles described in Table 1. The first wear amount A Ｗ１ is calculated as the difference between the sliding portion and the non-sliding portion of the sliding surface 10F of the test piece S after the wear test using a laser microscope. Note that "0.5 m / s" described on the vertical axis of FIG. 4 means the maximum speed (peripheral speed) of the mating material 51 in the first wear test, and "(0.25 m / s)" means the maximum speed (peripheral speed) of the mating material 51 in the second wear test.

[0027] (The second wear index I Ｗ２ ) The second wear index I Ｗ２ [μm / MPa] is obtained by the second wear amount A Ｗ２ [μm] obtained by the second wear test performed using an aqueous sulfuric acid solution and the surface pressure (initial surface pressure) P Ｃ２ [MPa] at the start of the second wear test, using the following formula (3). I Ｗ２ = A Ｗ２ / P Ｃ２ ... (3) The upper limit of I Ｗ２ is, for example, 1.0 × 10 －２ μm / MPa, and it may be 0.9 × 10 －２ μm / MPa or 0.8 × 10 －２ μm / MPa. The lower limit of I Ｗ２ is, for example, 0.1 × 10 －２ μm / MPa, and it may be 0.2 × 10 －２ μm / MPa. The range of the second wear index I Ｗ２ is 0.1 × 10 －２ or more and 1.0 × 10 －２ μm / MPa or less, 0.1 × 10 －２ or more and 0.9 × 10 －２ μm / MPa or less, 0.1 × 10 －２ or more and 0.8 × 10 －２ μm / MPa or less, 0.2 × 10 －２ or more and 1.0 × 10 －２ μm / MPa or less, 0.2 × 10 －２ or more and 0.9 × 10 －２μm / MPa or less or 0.2 × 10⁻⁶ －２ The above is 0.8 x 10 －２ It may be less than μm / MPa.

[0028] The second wear test is conducted in the same manner as the first wear test, except that it is carried out under the conditions for the "second wear test" instead of the conditions for the "first wear test" as described in Table 1. From the second wear test, the surface pressure P of the piston ring 10 is determined. Ｃ２ [MPa] and second wear amount A Ｗ２ You can obtain this.

[0029] The second abrasion test can be said to be a test conducted under more severe conditions than the first abrasion test, and the second abrasion index I Ｗ２ This is the first wear index I Ｗ１ It tends to be a larger value than I. Ｗ１ and I Ｗ２ In comparison, the wear index in the second wear test, which is generally a more severe wear environment, is I. Ｗ２ However, I Ｗ１ It tends to be larger than the second wear index I. Ｗ２ and the first wear index I Ｗ１ The difference (I Ｗ２ -I Ｗ１ For example, 1.5 × 10 －２ It is less than or equal to μm / MPa and 1.0 × 10 －２ μm / MPa or less or 0.5 × 10 －２ It may be less than or equal to μm / MPa, and 0.1 × 10 －２ The abrasion index I may be greater than or equal to μm / MPa. Ｗ２ and the first wear index I Ｗ１ The difference (I Ｗ２ -I Ｗ１ The range of ) is 0.1 × 10 －２ The above 1.5 x 10 －２ μm / MPa or less, 0.1×10 －２ The above 1.0 x 10 －２ μm / MPa or less or 0.1 × 10⁻⁶ －２ The above 0.5 × 10 －２ It may be less than μm / MPa.

[0030] (Material) The hard coating 5 may be a coating made of a single material or a coating made of multiple materials. The hard coating 5 may be, for example, a coating formed by physical vapor deposition (PVD-treated coating), a DLC coating (diamond-like carbon coating), or a Cr-plated coating (chromium-plated coating). If the hard coating 5 is a PVD-treated coating, the hard coating 5 has excellent abrasion resistance and corrosion resistance. If the hard coating 5 is a DLC coating, the hard coating 5 has excellent corrosion resistance and abrasion resistance, as well as a low coefficient of friction and excellent lubricity. If the hard coating 5 is a Cr-plated coating, the hard coating 5 has excellent abrasion resistance and heat resistance. If the hard coating 5 is a coating made of multiple materials, the hard coating 5 may have a single-layer structure or a multi-layer structure, as long as the hard coating 5 as a whole can satisfy the requirements of the hard coating 5. If the hard coating 5 has a multilayer structure, the outermost layer (the layer constituting the sliding surface 10F of the piston ring 10) may be a PVD-treated coating, a DLC coating, or a Cr-plated coating.

[0031] The material constituting the PVD-treated film may be CrN-based. The PVD-treated film may contain only one type of material or two or more types. For example, it may contain at least one material selected from the group consisting of Ti, Cr, Zr, V, Hf, Al, Si, and N. When the PVD-treated film contains two or more of the above materials, the strength of the hard film 5 is improved by solid solution strengthening.

[0032] DLC coatings use diamond bonds (sp) as the carbon bonds. ３ Bonds) and graphite bonds (sp ２ It is a mixture of bonded and unbonded particles. sp in DLC coating ３ The ratio is, for example, 30-80%, and may be 30-75%, 30-60%, 35-80%, 35-75%, or 35-60%. Especially sp ３ Increasing the ratio tends to increase the hardness of the coating. Note that "sp" here refers to ３ The ratio is the sp in DLC film. ２ Bonding and sp ３ sp for bonding ３ Bond ratio (sp ３ / (sp ２ +sp ３This indicates the value calculated based on the spectrum obtained by electron energy loss spectroscopy (EELS).

[0033] DLC coatings have a hydrogen content of 0 atomic% or more and less than 5 atomic%. DLC coatings may also have a hydrogen content of 0 atomic% or more and less than 2 atomic% or 0 atomic% or more and less than 1 atomic%, and may not contain substantially any hydrogen. It has been confirmed that if the hydrogen content of a DLC coating is less than 5 atomic%, the dangling bonds of the carbon atoms on the surface of the DLC coating are not terminated with hydrogen, so the oily agent molecules containing OH groups in the lubricating oil are easily adsorbed onto the surface of the DLC coating, resulting in an extremely low coefficient of friction. The hydrogen content of a DLC coating can be measured by Rutherford Backscattering Spectrometry (RBS) or Hydrogen Forward Scattering (HFS).

[0034] A chromium-plated film is, for example, a film obtained by electroplating using a chromium-containing solution (plating solution) with the substrate as the anode. The chromium-plated film may be amorphous, crystalline, or a mixture of both. The crystallinity of the chromium-plated film can be improved, for example, by heat treatment. High crystallinity of the chromium-plated film improves the hardness of the hard film 5, and the wear index I of the hard film 5 is improved. Ｗ１ There is a tendency for it to become smaller.

[0035] <Method for selecting piston rings> The method for selecting piston rings includes, for example, the following steps: (A) A step of determining whether the hard coating 5 satisfies the conditions shown in formula (1) above. (B) The wear index I of the hard coating 5. Ｗ 1.2 × 10 －２ The method for selecting a piston ring to determine whether or not the wear index is less than or equal to μm / MPa is such that the hard coating 5 satisfies the conditions shown in formula (1) above and the wear index of the hard coating 5 is I Ｗ 1.2 × 10 －２Piston rings with a pressure of μm / MPa or less are deemed acceptable, meaning they are piston rings that can suppress oil splatter into the combustion chamber when incorporated into an internal combustion engine that uses hydrogen as fuel.

[0036] Both process (A) and process (B) are processes for selecting piston rings that can suppress the deterioration of performance such as oil control performance or wear resistance in the environment to which piston rings incorporated into hydrogen-fueled internal combustion engines are exposed when the internal combustion engine is in operation. Specifically, process (A) evaluates indicators for high-temperature environments, and process (B) evaluates indicators for sliding and corrosive environments.

[0037] <Method for Manufacturing Piston Rings> The piston ring 10 is manufactured by, for example, the following steps: (a) a step of cleaning the surface of the base material 1; (b) a step of forming a hard coating 5 so as to cover at least the outer peripheral surface 1F of the base material 1.

[0038] (a) Step (a) is a step to clean the surface of the substrate 1 prior to the formation of the hard coating 5. For example, degreasing or cleaning by shot blasting may be performed. In addition, bombard cleaning may be performed inside the chamber. The Sdr of the outer peripheral surface 1F after cleaning may be within the above range, and the height of the protruding peaks or protruding valleys may be within the above range.

[0039] (b) The hard coating 5 in step (b) can be formed by, for example, physical vapor deposition or plating. Examples of physical vapor deposition methods include ion plating and sputtering. All of these physical vapor deposition methods are carried out in a vacuum chamber, and in the case of PVD, for example, the pressure in the vacuum chamber is set to a range of 1 to 6 Pa. Also, the bias voltage is set to a range of 0 to -100 V.

[0040] The composition of the hard coating 5 can be changed by changing the target used in the physical vapor deposition method. For PVD, for example, Ti-Al may be used as the target. After creating a vacuum atmosphere in the chamber, for example, 1.0 × 10 －３By replacing Pa with an inert gas such as argon gas, a hard coating 5 containing only cathode-derived atoms can be formed. Alternatively, by introducing a gas containing, for example, oxygen, nitrogen, or hydrogen into the chamber, a hard coating 5 containing each of these atoms can be formed.

[0041] To obtain a hard coating 5 with a Y / X value in the range of 0.7 to 1.1, in the case of DLC, the pressure inside the vacuum chamber should be set to a lower value within the above range, and the bias voltage should be in the range of 0 to -50V. Also, the wear index I Ｗ 1.2 × 10 －２ To obtain a hard coating 5 with a density of μm / MPa or less, sp ２ The film deposition should be carried out under conditions with a relatively low ratio. That is, the film deposition temperature should be set to a relatively low level, for example, around 100 to 200°C, and the vacuum level in the vacuum chamber should also be set to a relatively low level.

[0042] Examples of plating methods include electroplating and electroless plating. Both of these plating methods are carried out in a solution, and for example, the solution temperature is set in the range of 50 to 90°C. The composition of the hard film 5 can be changed by changing the solution used in the plating method. According to the inventors' studies, in order to obtain a hard film 5 with the above-mentioned Y / X value in the range of 0.7 to 1.1, the hard film 5 may be formed by electroplating. Also, the wear index I Ｗ 1.2 × 10 －２ To obtain a hard coating 5 with a density of μm / MPa or less, the hard coating 5 may also contain Cr and N.

[0043] <Internal Combustion Engines> The internal combustion engines to which the piston ring according to this embodiment is applied are those that use hydrogen as at least part of their fuel. Examples of internal combustion engines include automobile engines and marine engines. Applying the piston ring 10 to a spark-ignition engine is particularly useful. This is because, compared to other types of engines, spark-ignition engines are more prone to abnormal combustion when using hydrogen as fuel, and the piston ring 10 can significantly suppress the occurrence of abnormal combustion.

[0044] An internal combustion engine (not shown) comprises, for example, a combustion chamber, a cylinder, a piston, and a groove provided in the piston, in which a piston ring is fitted. An internal combustion engine fitted with the piston ring according to this embodiment can suppress oil splashing into the combustion chamber. The number of grooves provided in the piston and the number of piston rings fitted in the grooves are not particularly limited, but for example, the number may be two or three. If the number is two or more, the internal combustion engine only needs to have at least one piston ring 10 of this disclosure, or it may have multiple piston rings 10.

[0045] In internal combustion engines, oil is used to improve the sliding properties between the cylinder and piston rings. The three main factors contributing to oil consumption are: (1) scattering of oil into the combustion chamber in the form of droplets; (2) evaporation of oil from the cylinder wall into the combustion chamber; and (3) movement of mist-like oil into the combustion chamber. Here, "droplets" refers to liquid particles with a diameter of approximately 10 μm or more, while "mist" refers to liquid particles with a diameter of approximately 5 μm or less (for example, 0.01 μm to 5 μm). Droplets corresponding to (1) above often have a diameter of 10 μm or more. After entering the combustion chamber, such droplets remain on the piston crown surface and are heated, potentially forming carbon deposits of a certain size or larger. Since these deposits can act as ignition sources, an increase in droplet scattering increases the frequency of abnormal combustion. On the other hand, vapor or mist diffuses easily and uniformly within the combustion chamber, making it difficult to form large deposits. Therefore, among the factors contributing to oil consumption, the amount of droplet scattering is considered to have a strong influence on the occurrence of abnormal combustion. The scattering of droplets described in (1) above occurs, for example, as follows: Oil present in the gap between the outer circumference of the piston ring and the inner wall of the cylinder, or at the joint of the piston ring, is retained in the form of droplets, and as the piston moves up and down, it is scraped up by the piston ring and moves towards the combustion chamber.

[0046] <Method for assembling an internal combustion engine> The method for assembling an internal combustion engine according to this disclosure comprises, for example, the steps of mounting a piston ring 10 in a groove provided in a piston, and housing the piston in a cylinder. The mounting of the piston ring 10 can be performed, for example, using a piston ring expander or a piston ring compressor.

[0047] The present disclosure will be described in more detail below based on examples. However, the present invention is not limited to the following examples.

[0048] The following rings were prepared as the base material for use in the examples and comparative examples. Material: SP-3 (martensitic stainless steel) Ring dimensions: φ190.0 mm × ｈ１ 3.5 mm × ａ１ 5.7 mm (h1 represents the distance from side surface 11a to side surface 11b on the outer circumference of the ring, and a1 represents the distance from the sliding surface 11F to the inner circumference 11c of the ring (see Figure 2). Outer circumference shape: Eccentric barrel face Cross-sectional shape: Keystone shape (see Figure 2))

[0049] <Example 1> PVD-1 The piston ring according to Example 1 was manufactured through the following process.

[0050] (a) Cleaning of the substrate surface After cleaning the substrate by degreasing and shot blasting, it was placed in the chamber. Next, the inside of the chamber was 1.0 × 10 －２ After creating a vacuum atmosphere at Pa, the chamber was replaced with argon gas, and the argon pressure was set to 1.0 Pa. Then, the substrate was bombarded and cleaned by glow discharge under bias voltage conditions of -700V to -900V.

[0051] The surface roughness of the outer surface of the substrate after the above cleaning was within the following range: Sdr: 0.07–0.10%; Height of protruding peaks: 0.9 μm or less; Height of protruding valleys: 0.9 μm or less.

[0052] (b-1) Formation of a hard coating A CrN-based coating was formed on the outer surface of the substrate by ion plating under the following conditions to obtain the piston ring according to Example 1. Target: Chromium arc Current: 140-170A Bias voltage: ~-10V Deposition temperature: 400℃ Atmosphere: 3Pa, nitrogen gas (100% nitrogen) Film thickness: 30μm

[0053] <Comparative Example 1> PVD-2 A piston ring according to Comparative Example 1 was obtained in the same manner as in Example 1, except that the following film deposition conditions were used. [Film deposition conditions] Target: Titanium Arc current: 140-170A Bias voltage: ~-10V Film deposition temperature: 400℃ Atmosphere: 4Pa, nitrogen gas (100% nitrogen) Film thickness: 30μm

[0054] <Example 2> A piston ring according to Example 2 was obtained in the same manner as in Example 1, except that the surface roughness of the outer surface of the substrate after DLC surface treatment was within the following range, and the following film deposition conditions were used. [Surface roughness of the outer surface of the substrate] Sdr: 0.04 to 0.07% Height of protruding peaks: 0.6 μm or less Height of protruding valleys: 0.5 μm or less [Film deposition conditions] Target: carbon arc Current: 70 A Bias voltage: 0 V Chamber atmosphere: argon gas (2.5 × 10 －２ Pa) Deposition temperature: 180°C Film thickness: 20 μm

[0055] <Example 3> A piston ring according to Example 3 was obtained in the same manner as in Example 1, except that the surface roughness of the outer surface of the substrate after Cr plating surface treatment was within the following range, and a hard film was formed by electroplating instead of ion plating. [Surface roughness of the outer surface of the substrate] Sdr: 0.11 to 0.14% Height of protruding peaks: 1.2 μm or less Height of protruding valleys: 1.2 μm or less [Formation of hard film] A hard film was formed on the outer surface of the substrate by electroplating under the following conditions. Then, the hard film was heat-treated at 400°C for 5 hours in air to obtain the piston ring according to Example 3. [Film formation conditions] Anode: Substrate Cathode: High-density graphite plate Bath temperature: 60°C Current density: 45 A / dm ２ Plating solution: Chromic anhydride 250 g / L, Fluorine silicic acid 3.5 g / L, Sulfuric acid 2 g / L, Trivalent chromium 6 g / L. Film thickness: 100 μm

[0056] <Comparative Example 2> A piston ring according to Comparative Example 2 was obtained in the same manner as in Example 3, except that the copper plating film deposition conditions were as follows, and the subsequent heat treatment conditions were 400°C for 5 hours in air. (Film deposition conditions) Anode: Substrate Cathode: High-density graphite plate Bath temperature: 60°C Current density: 7 A / dm ２pH: 12 Plating solution: Copper 50 g / L, free potassium cyanide 15 g / L, potassium carbonate 30 g / L Film thickness: 100 μm

[0057] <Comparative Example 3> A piston ring according to Comparative Example 3 was obtained in the same manner as in Comparative Example 2, except that a nitrided layer was formed instead of forming a hard film by nitriding electroplating. Specifically, first the substrate was degreased and washed, and then placed in a chamber. Next, nitriding treatment was performed by gas nitriding, with ammonia gas added to the chamber and the nitriding temperature set to 570°C and the treatment time set to 2 hours. The thickness of the nitrided layer was 60 μm.

[0058] <Comparative Example 4> Without forming a hard coating on the raw material substrate, the substrate was used as is for the piston ring according to Comparative Example 4.

[0059] [Measurement of Vickers Hardness] The Vickers hardness X before heat treatment and the Vickers hardness Y after heat treatment of the piston ring coating were measured using a Vickers hardness tester (Futuretech, product name: FLC-50VX). The Y / X value was calculated from these measured values. The measurement conditions were a test force of 0.980 N. For the piston ring in Comparative Example 3, the Vickers hardness was measured on the outer surface of the base material. The results are shown in Table 3.

[0060]

[0061] [Measurement of wear amount] The piston rings of the examples and comparative examples were subjected to a first wear test and a second wear test using a sliding fatigue testing machine, and the wear amount A of the first and second tests was measured. Ｗ１ A Ｗ２ The following measurements were taken. The test methods were those of the first and second abrasion tests described above, and the test conditions were those of the first and second abrasion tests described in Table 1. The surface pressure was calculated using the method described above. The results are shown in Table 4.

[0062]

[0063] Based on the wear measurements from the first and second abrasion tests described above, simulations estimated the amount of oil splatter. The results showed that Example 2 had the least amount of oil splatter (the amount of oil that moved to the combustion chamber as liquid particles with a diameter of 10 μm or more), followed by Example 1 and then Example 3, in that order of decreasing oil splatter. Comparative Examples 1 to 4 showed a higher amount of oil splatter compared to Example 3.

[0064] The above simulation was performed in accordance with the analysis methods described in References 1 and 2 below, under the following conditions. Reference 1: Kazuya Mochizuki, et al., “Prediction and Experimental Verification for Oil Transport Volume around Three-Piece Type Oil Control Ring Affecting Lubricating Oil Consumption”, March 29, 2022, SAE paper 2022-01-0522. Reference 2: Chiba, et al., "Development of an MBD Tool for Piston Ring Performance Prediction," Society of Automotive Engineers of Japan, October 2024, pp. 1-6. (Conditions) Oil: SAE 10W-30 Piston ring shape (top ring): BF shape (barrel face, arc shape) Piston ring shape (second ring): Tapered shape Piston ring shape (oil ring): 2-piece type, straight outer circumference Cylinder shape: No bore deformation (circular bore with temperature distribution applied and thermally expanded shape) Cylinder material: FC250 Temperature (initial): Cylinder temperature upper part 144°C, middle 106°C, lower part 106°C Pressure: Maximum in-cylinder pressure (combustion chamber pressure) is approximately 7.5 MPa (calculated assuming constant atmospheric pressure in the crankcase and under the condition of 2000 rpm - WOT (Wide Open Throttle, full load)) Operating conditions: Firing conditions

[0065] 1...Base material, 1F...Outer surface, 5...Hard coating, 10, 11...Piston ring, 10a, 10b, 11a, 11b...Side surfaces, 10c, 11c...Inner surfaces, 10F, 11F...Sliding surfaces

Claims

1. A piston ring for an internal combustion engine in which at least a part of fuel is hydrogen, comprising a base material and a hard film provided so as to cover at least a part of the outer peripheral surface of the base material, the hard film satisfies the condition represented by the following formula (1), 0.7 ≦ Y / X ≦ 1.1... (1) In the formula (1), X represents the Vickers hardness of the hard film, Y represents the Vickers hardness of the hard film after heat treatment, and the heat treatment is a treatment in which the piston ring is heated in the atmosphere at 400 ° C. for 5 hours, and the wear index I of the hard film Ｗ is 1.2 × 10 －２ μm / MPa or less, and the wear index I Ｗ is the first wear index I Ｗ１ obtained by a first wear test performed using a base oil and the second wear index I Ｗ２ obtained by a second wear test performed using an aqueous sulfuric acid solution, and the first wear index I Ｗ１ is a value obtained by the following formula (2), I Ｗ１ = A Ｗ１ / P Ｃ１ ... (2) In the formula (2), A Ｗ１ represents the wear amount (unit: μm) obtained by the first wear test, and P Ｃ１ represents the surface pressure (unit: MPa) of the piston ring at the start of the first wear test. The second wear index I Ｗ２ is a value obtained by the following formula (3), I Ｗ２ = A Ｗ２ / P Ｃ２ ... (3) In the formula (3), A Ｗ２ represents the wear amount (unit: μm) obtained by the second wear test, and P Ｃ２ represents the surface pressure (unit: MPa) of the piston ring at the start of the second wear test. A piston ring.

2. The piston ring according to claim 1, wherein the hard coating is a CrN-based coating formed by physical vapor deposition.

3. The piston ring according to claim 1, wherein the hard coating is a DLC coating.

4. The piston ring according to claim 1, wherein the hard coating is a chrome plating coating.

5. The piston ring according to claim 1, wherein the developed interface area ratio Sdr of the outer circumferential surface of the base material is 0.04 to 0.14%.

6. The piston ring according to claim 1, wherein the internal combustion engine is a spark-ignition engine.

7. The piston ring according to claim 1, wherein the top ring is fitted into the groove closest to the combustion chamber among a plurality of grooves provided in the piston.

8. A method for assembling an internal combustion engine in which hydrogen is used as at least part of the fuel, comprising the steps of: fitting a piston ring according to any one of claims 1 to 7 into a groove provided in a piston; and housing the piston in a cylinder.

9. A method for selecting a piston ring for an internal combustion engine in which hydrogen is used as at least part of the fuel, wherein the piston ring comprises a base material and a hard coating provided so as to cover at least a part of the outer surface of the base material, and the steps of determining whether the hard coating satisfies the conditions shown by the following formula (1) and the wear index I of the hard coating Ｗ 1.2 × 10 －２ The process includes determining whether or not the wear index is less than or equal to μm / MPa, wherein 0.7 ≤ Y / X ≤ 1.1 ... (1) In equation (1), X represents the Vickers hardness of the hard coating, Y represents the Vickers hardness of the hard coating after heat treatment, the heat treatment is a process of heating the piston ring in air at 400°C for 5 hours, and the wear index I Ｗ The first wear index I is determined by a first wear test performed using a base oil. Ｗ１ And the second abrasion index I, which is determined by a second abrasion test conducted using a sulfuric acid solution. Ｗ２ It is the sum of the first wear index I Ｗ１ The value is obtained by the following formula (2), and I Ｗ１ = A Ｗ１ / P Ｃ１ ...(2) In formula (2), A Ｗ１ This indicates the amount of wear (unit: μm) determined by the first wear test described above, and P Ｃ１ This indicates the surface pressure (in MPa) of the piston ring at the start of the first wear test, and the second wear index I Ｗ２ The value is obtained by the following formula (3), and I Ｗ２ = A Ｗ２ / P Ｃ２ ...(3) In formula (3), A Ｗ２ This indicates the amount of wear (unit: μm) determined by the second wear test described above, and P Ｃ２ This indicates the surface pressure (in MPa) of the piston ring at the start of the second wear test, and the hard coating satisfies the conditions shown in formula (1) above and the wear index I of the hard coating. Ｗ 1.2 × 10 －２ A method for selecting piston rings in which piston rings with a pressure of μm / MPa or less are considered acceptable.

10. A method for manufacturing a piston ring for an internal combustion engine in which hydrogen is used as at least part of the fuel, comprising the step of forming a hard coating so as to cover at least part of the outer surface of a base material, wherein the hard coating is formed so as to be a product that meets the standards for selecting a piston ring according to claim 9.

Images