Piston ring

Abstract

This piston ring includes: a piston ring base material having a joint part; and a film covering at least an outer peripheral surface of the piston ring base material. The thermal resistance value of the film is 2.0 × 10-6 to 12.0 × 10-6 m2•K / W.

Description

Piston rings

[0001] The present disclosure relates to piston rings.

[0002] In recent years, awareness of the global environment has grown, and there is a strong demand for low fuel consumption and low emissions in engines. To improve combustion efficiency, there is a trend toward higher compression ratios and higher loads in engine specifications. The resulting higher furnace temperatures lead to thermal settling of piston rings, resulting in a decrease in piston ring tension and a deterioration in sealing performance. As a means of solving this problem, Patent Document 1 discloses replacing piston rings from conventional spring steel or carbon steel with chromium-based stainless steel, which has better heat resistance.

[0003] Japanese Unexamined Patent Publication No. 56-81243

[0004] According to the study of the present inventors, it has been found that piston rings made of chromium-based stainless steel may increase the amount of blow-by gas, a type of emission, even under conditions where thermal settling does not occur. The present disclosure provides a piston ring that is useful for sufficiently reducing the amount of blow-by gas.

[0005] The inventors focused on the fact that the amount of blow-by gas and fuel economy vary depending on the material that constitutes the outer peripheral surface of a piston ring. As a result of repeated experiments in which the material and thickness of the coating were changed, they found that the thermal resistance value of the coating correlates with fuel economy and blow-by gas, that is, when the thermal resistance value of the coating is within a predetermined range, the amount of blow-by gas can be sufficiently reduced. The thermal resistance value is a value expressed by the following formula 1, and the thermal resistance value and thermal conductivity are related by the following formula 2. Thermal resistance value = film thickness / (specific gravity x specific heat x thermal diffusivity) (Formula 1) Thermal resistance value = film thickness / thermal conductivity (Formula 2)

[0006] One aspect of the present disclosure relates to a piston ring having a coating. That is, the piston ring according to the present disclosure includes a piston ring substrate having a butt joint and a coating covering at least an outer peripheral surface of the piston ring substrate, and the coating has a thermal resistance value of 2.0×10 －６ ~12.0 x 10 －６ m ２・K / W.

[0007] The piston ring described above can sufficiently reduce the amount of blow-by gas. The inventors speculate that the main reason for this is as follows: The temperature of the piston ring rises as the engine operates, and the piston ring expands accordingly, narrowing the gap at the butt end of the piston ring. The thermal resistance of the coating is 2.0 x 10 －６ m ２ When the thermal resistance of the coating is 12.0×10 －６ m ２ When the value is equal to or less than K / W, the temperature of the piston ring base material can be prevented from rising excessively, and thermal settling of the piston ring, which causes an increase in the amount of blow-by gas, can be prevented.

[0008] According to the present disclosure, a piston ring is provided that is useful for substantially reducing the amount of blow-by gas.

[0009] Fig. 1 is a plan view schematically showing one embodiment of a piston ring according to the present disclosure. Fig. 2 is a cross-sectional view taken along line II-II shown in Fig. 1. Fig. 3 is a cross-sectional view schematically showing another embodiment of a piston ring according to the present disclosure. Fig. 4 is a graph showing the results of examples and comparative examples.

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiments.

[0011] <Piston Ring> FIG. 1 is a plan view schematically showing a piston ring according to this embodiment. FIG. 2 is a cross-sectional view taken along line II-II in FIG. 1. As shown in FIG. 1, the piston ring 10 is annular and has a butt portion 11. The piston ring 10 has a pair of side surfaces 12, 13, an inner circumferential surface 14, and an outer circumferential surface 15. The side surfaces 12, 13 intersect the inner circumferential surface 14 and are, for example, substantially perpendicular to the inner circumferential surface 14. The outer diameter d1 of the piston ring 10 is, for example, 40 to 300 mm. The gap S1 of the butt portion 11 is, for example, 0.15 to 0.60 mm. The piston ring 10 may be perfectly circular or elliptical in plan view.

[0012] As shown in Fig. 2, the piston ring 10 includes a piston ring substrate 1 having a butt joint 11, a coating 5 constituting an outer peripheral surface 15 of the piston ring 10, and an intermediate layer 3. The intermediate layer 3 is provided between the piston ring substrate 1 and the coating 5, and serves to, for example, enhance the adhesion of the coating 5 to the piston ring substrate 1. The cross section of the piston ring 10 is substantially rectangular, and the outer peripheral surface 15 of the piston ring 10 may be rounded and bulge outward. The outer peripheral surface 15 is a surface that slides against the inner surface of a cylinder liner.

[0013] The piston ring 10 is a compression ring for an internal combustion engine (e.g., a heavy-duty diesel engine or an automobile engine). The compression ring is mounted, for example, in a ring groove formed on the side of the piston. The compression ring is exposed to an environment with high thermal load in the engine. One of the functions of the compression ring is gas sealing. The compression ring suppresses gas (blow-by gas) leaking from the combustion chamber side to the crankcase side. An increase in blow-by gas leads to engine oil deterioration, reduced fuel efficiency, and increased environmental load. The butt joint of the piston ring is one of the leakage paths for blow-by gas. From the perspective of suppressing blow-by gas, it is desirable to make the gap at the butt joint as small as possible when the engine is running.

[0014] Another function of the compression ring is to cool the piston by dissipating heat. The piston is heated by heat from the combustion chamber, and the heat from the piston is dissipated to the cylinder liner through the compression ring. If the piston temperature rises excessively, the piston material will adhere, significantly reducing the engine's fuel efficiency. On the other hand, if there is excessive heat dissipation via the piston and piston ring, the engine's thermal efficiency will deteriorate, leading to reduced fuel efficiency. Therefore, from the perspective of fuel efficiency, the thermal resistance of the piston ring must be within a specified range.

[0015] (Piston ring base material) The piston ring base material 1 is made of a heat-resistant alloy. Specific examples of the alloy include martensitic stainless steel, such as chromium-based stainless steel, tool steel, and heat-resistant steel. By making the piston ring base material out of martensitic stainless steel, thermal settling of the piston ring base material is suppressed, and a piston ring that can maintain good gas sealing performance even when used at high temperatures is realized.

[0016] (Intermediate Layer) As described above, the intermediate layer 3 serves to improve the adhesion between the piston ring substrate 1 and the coating 5. The thickness of the intermediate layer 3 is preferably 0.1 to 3.0 μm, and more preferably 0.5 to 2.5 μm. When the thickness of the intermediate layer 3 is 0.1 μm or more, the effect of improving adhesion by alleviating the internal stresses that the piston ring substrate 1 and the coating 5 each have is achieved. On the other hand, if the intermediate layer 3 is excessively thick, the effect of alleviating the internal stress becomes saturated, while the film formation cost tends to increase. Therefore, the thickness of the intermediate layer 3 is preferably 3.0 μm or less.

[0017] The intermediate layer 3 may be composed of, for example, chromium, or may be composed of chromium containing Si, Al, or the like. When the intermediate layer 3 is composed of chromium containing Si, the Si content in the intermediate layer 3 is preferably 2.0 to 10.0 at%. A Si content of 2.0 at% or more increases the thermal resistivity of the intermediate layer 3, thereby increasing the total thermal resistivity of the intermediate layer 3 and the coating 5, further contributing to suppression of the amount of blow-by gas. On the other hand, a Si content of 10.0 at% or less maintains the effect of the intermediate layer 3 in alleviating internal stress. The lower limit of the Si content may be 2.5 at% or 3.0 at%. The upper limit of the Si content may be 8.5 at% or 6.0 at%. When the intermediate layer 3 is composed of chromium containing Al, the Al content in the intermediate layer 3 is preferably 10.0 to 50.0 at%. An Al content of 10.0 at% or more increases the thermal resistivity of the intermediate layer 3, and the combined thermal resistivity of the intermediate layer 3 and the coating 5, thereby further contributing to suppressing the amount of blow-by gas. On the other hand, an Al content of 50.0 at% or less maintains the effect of alleviating internal stress by the intermediate layer 3. The lower limit of the Al content may be 13 at% or 15 at%. The upper limit of the Al content may be 25 at% or 20 at%.

[0018] When the intermediate layer 3 is made of chromium containing Si, the coating 5 is preferably made of chromium nitride containing Si. By making the intermediate layer 3 and the coating 5 out of these materials, interdiffusion occurs at the interface between the intermediate layer 3 and the coating 5, improving the adhesion between the intermediate layer 3 and the coating 5. This allows the piston ring 10 to maintain the effects of improving fuel economy and suppressing the amount of blow-by gas for a sufficiently long period of time, even when used in a harsh environment.

[0019] When the intermediate layer 3 is made of chromium containing Al, the coating 5 is preferably made of chromium nitride containing Al. By making the intermediate layer 3 and the coating 5 out of these materials, interdiffusion occurs at the interface between the intermediate layer 3 and the coating 5, improving the adhesion between the intermediate layer 3 and the coating 5. This allows the piston ring 10 to maintain the effects of improving fuel economy and suppressing the amount of blow-by gas for a sufficiently long period of time, even when used in a severe environment.

[0020] (Coating) As described above, the coating 5 constitutes the outer peripheral surface 15 of the piston ring 10. The thickness of the coating 5 is preferably 3 to 100 μm, and more preferably 5 to 70 μm. When the thickness of the coating 5 is 3 μm or more, not only can the durability of the coating 5 due to sliding contact between the outer peripheral surface 15 of the piston ring 10 and the cylinder liner be increased, but also an effect of improving fuel efficiency and suppressing the amount of blow-by gas due to an increase in the thermal resistivity of the piston ring 10 is achieved. On the other hand, when the thickness is 100 μm or less, an excessive rise in the temperature of the piston ring base material 1 can be suppressed, and not only can thermal settling of the piston ring 10, which causes an increase in the amount of blow-by gas, be prevented, but also an effect of ensuring high productivity is achieved.

[0021] The coating 5 is made of, for example, a metal nitride such as chromium nitride or titanium nitride, or an amorphous carbon film. The coating 5 may be a single layer or a laminate of two or more layers, as long as it has a predetermined overall thermal resistance. An example of a laminate of two or more layers is a laminate of a chromium nitride layer and a carbon-containing chromium nitride layer.

[0022] By forming the surface (outer peripheral surface 15) of the coating 5 from metal nitride or amorphous carbon, the progression of wear of the coating 5 can be suppressed, and the functionality of the coating 5 can be maintained for a sufficiently long period of time. The coating 5 may further contain Si, Al, C, H, etc. When the coating 5 contains Si or Al, blow-by gas can be more effectively suppressed.

[0023] When the coating 5 is made of chromium nitride containing Si, the Si content in the coating 5 is preferably 1.0 to 10.0 at%. Chromium nitride with a Si content of 1.0 at% or more is composed of fine crystal grains and has good crack resistance and peeling resistance. On the other hand, chromium nitride with a Si content of 10.0 at% or less is likely to form an appropriate amorphous phase and has good crack resistance. Therefore, a coating 5 made of chromium nitride containing Si within the above range can maintain the effects of improving fuel economy and suppressing the amount of blow-by gas even in more severe operating environments. Furthermore, chromium nitride containing Si within the above range has excellent wear resistance. Therefore, by forming a sliding surface using this chromium nitride, the effects of improving fuel economy and suppressing the amount of blow-by gas can be maintained for a longer period of time. The lower limit of the Si content may be 1.5 at% or 2.0 at%. The upper limit of the Si content may be 8.0 at% or 6.0 at%.

[0024] When the coating 5 is made of chromium nitride containing Al, the Al content in the coating 5 is preferably 10 to 50 at%. An Al content of 10 at% or more can sufficiently reduce the thermal conductivity of the material constituting the coating 5, thereby imparting heat-shielding properties to the coating 5. In addition, the wear resistance of the coating 5 can be enhanced, allowing the effects of improving fuel economy and suppressing blow-by gas to be maintained for a longer period of time. On the other hand, an Al content of 50 at% or less allows Al to be sufficiently dissolved in the chromium nitride, resulting in a uniform coating 5. Furthermore, chromium nitride containing Al within the above range has excellent wear resistance. Therefore, by forming the sliding surface using this chromium nitride, the effects of improving fuel economy and suppressing blow-by gas can be maintained for a longer period of time. The lower limit of the Al content may be 13 at% or 15 at%. The upper limit of the Al content may be 25 at% or 20 at%.

[0025] The thermal resistance of the film 5 is 2.0×10 －６ m ２ K / W or more, preferably 2.5 x 10 －６ m ２K / W or more, more preferably 4.0 × 10 －６ m ２ K / W or more, more preferably 5.0 × 10 －６ m ２ K / W or more, most preferably 7.0 x 10 －６ m ２ The thermal resistance of the coating 5 is 2.0×10 －６ m ２ - By having a thermal insulation coefficient of 1000 kJ / W or more, it is possible to impart an appropriate heat-shielding performance to the piston ring 10, and the temperature of the piston ring base material 1 can be sufficiently increased when the engine is running. This reduces the gap at the butt end portion 11, and the amount of blow-by gas can be suppressed. In addition, since the piston ring 10 has heat-shielding performance, it is possible to suppress a temperature drop in the combustion chamber. In other words, since the piston ring 10 has appropriate heat-shielding performance, it is possible to suppress the amount of blow-by gas and realize an engine with good fuel economy characteristics.

[0026] The thermal resistance of the film 5 is 12.0×10 －６ is preferably 11.0×10 －６ m ２ K / W or less, more preferably 10.3 × 10 －６ m ２ K / W or less, more preferably 9.0 × 10 －６ m ２ The thermal resistance of the coating 5 is 12.0 × 10 －６ m ２ By keeping the heat insulating performance of the piston ring 10 at or below 100 K / W, it is possible to prevent the heat insulating performance of the piston ring 10 from becoming excessive. Therefore, it is possible to prevent problems that may occur due to the piston ring 10 being overheated. Such problems include thermal settling of the piston ring base material 1 and adhesion of the piston ring 10 to the piston.

[0027] The term "thermal resistance" here refers to a value determined by laser thermoreflectance. In laser thermoreflectance, a molybdenum (Mo) film is formed on a mirror-finished sample surface. The thin film surface is then periodically heated by irradiating it with a laser beam, and the thermal diffusivity is determined by analyzing the reflectance modulation of the metal surface. More specifically, the sample surface is periodically heated by a laser beam, and heat diffuses from the surface of the metal thin film into the sample. The sample's surface temperature changes depending on the sample's thermal conductivity. Taking advantage of the temperature-dependent reflectance of Mo, a continuous, unmodulated laser beam is irradiated coaxially with the heating beam to obtain reflected light corresponding to the temperature modulation of the sample surface. For samples with fast thermal diffusion, whose temperature changes in accordance with the heating beam cycle, the phase delay of the reflected light relative to the heating beam cycle is small. On the other hand, for samples with slow thermal diffusion, whose temperature changes in accordance with the heating beam cycle, the phase delay of the reflected light relative to the heating beam cycle is large. The thermal diffusivity of the sample is determined by analyzing the phase delay of the reflected light relative to the heating beam. By applying this thermal diffusivity value and the specific heat value obtained by differential scanning calorimetry to Equation 1, the thermal resistance value is calculated.

[0028] Note that the thermal resistance value may be determined by a method other than the laser thermoreflectance method, provided that it can be confirmed that a value equivalent to the thermal resistance value measured by the laser thermoreflectance method can be obtained, or that there is a correlation with the thermal resistance value measured by the laser thermoreflectance method. For example, the thermal diffusivity and specific heat may be determined by the laser flash method, and these may be applied to Equation 1 to calculate the thermal resistance value. In the laser flash method, a disk-shaped sample held in a vacuum is irradiated with a pulsed laser beam for heating, and the thermal diffusivity and specific heat are determined by analyzing the temperature history of the rear surface of the sample. More specifically, first, the disk-shaped sample is placed in a vacuum, and the entire sample is heated to a constant temperature T ０ Next, the entire front surface of the sample is pulse-heated using a laser beam with a pulse width of about 100 μs, and the temperature change on the back surface is measured. At this time, since the front surface of the sample is uniformly heated, heat diffuses one-dimensionally from the front surface to the back surface, and the temperature eventually becomes uniform. Therefore, in Equation 3, the time t required for the entire sample to reach half the uniform temperature is calculated. ｈａｌｆand the thickness L of the sample, the thermal diffusivity can be calculated. ｓ and final temperature T ｓ , and the specific heat C of the reference material ｐ，ｒ , weight W ｒ and final temperature T ｒ The specific heat can be calculated by applying this to Equation 4: Thermal diffusivity = 0.1388 x L ２ / t ｈａｌｆ (Equation 3) Specific heat = C ｐ，ｒ ×W ｒ × (T ｒ -T ０ ) / (W ｓ × (T ｓ -T ０ )) (Formula 4)

[0029] <Manufacturing Method of Piston Ring> The piston ring 10 is manufactured by, for example, the following steps: (a) a step of cleaning the surface of a piston ring base material 1; (b) a step of forming an intermediate layer 3 so as to cover at least the outer peripheral surface of the piston ring base material 1; and (c) a step of forming a coating 5 on the surface of the intermediate layer 3.

[0030] The step (a) is a step for cleaning the surface of the piston ring substrate 1 prior to the formation of the intermediate layer 3. For example, cleaning treatment such as degreasing or shot blasting may be performed. In addition, bombardment cleaning may be performed in a chamber.

[0031] The formation of the intermediate layer 3 and the coating 5 in steps (b) and (c) can be carried out by physical vapor deposition. 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 the pressure in the vacuum chamber is set, for example, in the range of 1 to 6 Pa. The bias voltage is set, for example, in the range of -1 to 18 V.

[0032] The composition of the intermediate layer 3 and the coating 5 can be changed by changing the target used in the physical vapor deposition method. For example, an alloy of Si or Al and Cr, or a Cr ingot, etc. may be used as the target. After the chamber is evacuated, for example, 1.0 × 10－３ By replacing the atmosphere in the chamber with an inert gas such as argon gas at 100 Pa, it is possible to form the intermediate layer 3 and the coating 5 containing only atoms derived from the cathode. Alternatively, by introducing a gas containing, for example, oxygen, nitrogen, or hydrogen into the chamber, it is possible to form the intermediate layer 3 and the coating 5 containing each of the atoms. When forming the intermediate layer 3 and the coating 5, a common target may be used, and the gas introduced into the chamber may be changed. This makes it possible to obtain an intermediate layer 3 and a coating 5 with good adhesion and reduce the manufacturing cost of the piston ring 10.

[0033] Although the embodiments of the present disclosure have been described in detail above, the present invention is not limited to the above embodiments. In the above embodiments, the piston ring 10 including the intermediate layer 3 has been exemplified. However, as long as sufficient adhesion of the coating 5 to the piston ring substrate 1 can be ensured, the intermediate layer 3 need not be provided. In the piston ring 20 shown in FIG. 3, the coating 5 is formed directly on the outer peripheral surface of the piston ring substrate 1. Note that the piston ring substrate 1 may be one that has been subjected to a surface treatment on at least the outer peripheral surface.

[0034] The present disclosure relates to the following: [1] A piston ring comprising: a piston ring base material having a butt joint; and a coating covering at least an outer peripheral surface of the piston ring base material, wherein the coating has a thermal resistance value of 2.0×10 －６ ~12.0 x 10 －６ m ２ - A piston ring made of K / W. [2] The piston ring according to [1], wherein the coating contains Al or Si. [3] The piston ring according to [1] or [2], further comprising an intermediate layer formed on the surface of the piston ring base material, wherein the coating is formed on the surface of the intermediate layer. [4] The piston ring according to [3], wherein the intermediate layer contains Al or Si. [5] The piston ring according to [3] or [4], wherein the coating is made of chromium nitride containing Si, and the intermediate layer is made of chromium containing Si. [6] The piston ring according to any of [3] to [5], wherein the coating is made of chromium nitride containing Al, and the intermediate layer is made of chromium containing Al.

[0035] The present disclosure will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0036] As a piston ring substrate, a ring having the following composition equivalent to SUS440 and having a gap of 0.40 mm was prepared: Fe: 80.4% by mass, C: 0.85% by mass, Cr: 17.0% by mass, Si: 0.5% by mass, Mn: 0.5% by mass, and other elements: balance.

[0037] Comparative Example 1 A nitrided layer was formed on the surface of a piston ring substrate by nitriding under the following conditions. That is, first, the piston ring substrate was degreased and cleaned, and then placed in a chamber. Next, ammonia gas was added to the chamber by gas nitriding, and nitriding was performed under the conditions of a nitriding temperature of 570°C and a treatment time of 2 hours. The thickness of the nitrided layer was 60 μm.

[0038] <Examples 1 to 4 and Comparative Examples 2 to 4> Piston rings according to Experimental Examples 1 to 4 and Comparative Examples 2 to 4 were produced through the following three steps. The film-forming time in each step was adjusted appropriately depending on the target thermal resistance value of the coating or the film thickness of the intermediate layer.

[0039] (a) Step of Cleaning the Surface of the Base Material First, the piston ring base material was degreased and cleaned, and then placed in a chamber. Next, the chamber was filled with 1.0 × 10 －２ After creating a vacuum atmosphere of 1.0 Pa, the chamber was purged with argon gas until the argon pressure was 1.0 Pa. Then, glow discharge was performed under the condition of a bias voltage of −700 V, thereby bombarding and cleaning the piston ring substrate.

[0040] (b-1) Step of forming an intermediate layer by ion plating In Examples 2 to 4 and Comparative Examples 3 and 4, an intermediate layer was formed on the outer peripheral surface of the cleaned piston ring substrate by chamber ion plating under the following conditions: (Film formation conditions) Arc current: 150 A Bias voltage: -10 V Film formation temperature: 500°C Atmosphere: Argon gas, 1.0 x 10 －１Pa. Target: chromium. In Example 1 and Comparative Example 2, the step of forming an intermediate layer was not carried out.

[0041] (c-1) Step of forming a coating by ion plating For Example 1 and Comparative Example 2, a coating was formed on the surface of the piston ring substrate by chamber ion plating under the following conditions. For Examples 2 to 4 and Comparative Examples 3 and 4, a coating was formed on the surface on which the intermediate layer had been formed by chamber ion plating under the following conditions. (Coating conditions) Arc current: 150 A Bias voltage: -10 V Coating temperature: 500°C Atmosphere: nitrogen gas, 4.0 Pa Target: chromium

[0042] <Examples 5 and 6> Piston rings according to Examples 5 and 6 were produced in the same manner as in Example 2, except that the following step (c-2) was carried out instead of step (c-1). (c-2) Step of forming a coating in which two types of layers are laminated by an ion plating method In the film formation by the ion plating method, the layers were alternately used so that CrN(C) and CrN were deposited peripherally. The film formation time was adjusted so that the lamination ratio of CrN(C) to CrN was 1:2.

[0043] <Examples 7 and 8 and Comparative Examples 7 and 8> Piston rings according to Examples 7 and 8 and Comparative Examples 7 and 8 were produced in the same manner as in Example 2, except that in the step (c-1), the film was formed under the following conditions: (Film-forming conditions) Arc current: 70 A Bias voltage: 0 V Chamber atmosphere: argon gas, 1.0 × 10 －３ Pa Film formation temperature: 300°C Target: carbon

[0044] Examples 9 and 10 and Comparative Examples 9 and 10 In step (c-1), the atmosphere in the chamber was filled with acetylene gas, 1.0×10 －1 Piston rings according to Examples 9 and 10 and Comparative Examples 9 and 10 were produced in the same manner as in Example 7, except that the tensile strength was set to Pa.

[0045] <Example 11> Piston rings according to Example 11 were produced in the same manner as Example 1, except that a Cr-Al alloy (Al content: 50 at %) was used as the target in the step (c-1). <Examples 12 and 13> Piston rings according to Examples 12 and 13 were produced in the same manner as Example 2, except that a Cr-Al alloy (Al content: 50 at %) was used as the target in the steps (b-1) and (c-1).

[0046] <Example 14> Piston rings according to Example 14 were produced in the same manner as Example 1, except that a Cr-Si alloy (Si content: 10 at %) was used as the target in the step (c-1). <Examples 15 to 17 and Comparative Examples 5 and 6> Piston rings according to Examples 15 to 17 and Comparative Examples 5 and 6 were produced in the same manner as Example 2, except that a Cr-Si alloy (Si content: 10 at %) was used as the target in the steps (b-1) and (c-1).

[0047] (Calculation of Thermal Resistance Value) The thermal resistance values ​​of the coatings of the piston rings according to Examples 1 to 17 and Comparative Examples 1 to 10 were calculated by applying the film thickness, specific gravity, specific heat, and thermal diffusivity measured by the methods described below to Equation 1. The obtained thermal resistance values ​​are shown in Table 1.

[0048] The thickness of the coating was measured by cross-sectional observation using the following method. First, the piston ring was cut perpendicular to the coating surface and embedded in resin so that the cut surface was exposed. Next, the surface where the cut surface was exposed was wet-polished with waterproof abrasive paper, and then finish-polished with diamond film #8000 to obtain a sample for cross-sectional observation. Then, the surface of the obtained sample was magnified and observed using an optical microscope, and the thickness of the coating was measured.

[0049] The specific gravity of the film was calculated from the volume determined from the film thickness and the area of ​​the surface of the piston ring to be treated, and from the change in weight of the piston ring before and after the film formation treatment.

[0050] The specific heat of the coating was measured using a differential scanning calorimeter (apparatus name: DSC-50, manufactured by Shimadzu Corporation). The measurement was carried out under conditions in accordance with JIS R1672:2006, using a thin film prepared on a piston ring under the same conditions as the coating as a sample.

[0051] The thermal diffusivity of the coating was measured by a laser thermoreflectance method after preparing a measurement sample using the following method. Specifically, the piston ring was first cut perpendicular to the coating surface and embedded in resin so that the cut surface was exposed. The exposed cut surface (measurement surface) was then wet-polished with waterproof abrasive paper, followed by finish polishing using diamond film #8000 until the cut surface of the piston ring was polished to a mirror finish. Furthermore, a 50 nm Mo film was prepared on the measurement surface by a sputtering method. The resin was then cut into small pieces using a fine cutter, and the measurement surface was then machined to a size of 10 mm x 10 mm or less and a height of 3 mm or less using a surface grinder to obtain a measurement sample for laser thermoreflectance measurement. The thermal diffusivity of the obtained measurement sample was measured by a laser thermoreflectance method. Measurements were performed using a thermal microscope (device name: Thermal Microscope TM3, manufactured by Bethel Co., Ltd.) under the following conditions. The measurement position was set so that the outer peripheral surface of the coating and the piston ring substrate were within the measurement range, and the average value of 30 measurements was taken as the measurement result. (Measurement conditions) Heating laser shape: 28 μm × 25 μm ellipse Heating laser wavelength: 808 nm Modulation frequency: modulation frequency of 1 MHz Detection laser shape: circle with diameter of 3 μm Detection laser wavelength: 658 nm Measurement range: width 120 μm × depth 60 μm Measurement pitch: 2 μm

[0052] (Measurement of Blow-by Gas) An engine having the following performance was prepared as an internal combustion engine to which piston rings were applied: Bore diameter: φ120 mm Displacement: 10 L Fuel: diesel

[0053] The piston rings according to Examples 1 to 17 and Comparative Examples 1 to 10 were attached to the engine, and the amount of blow-by gas was measured when the engine was operated under the following conditions. The relative values ​​of the amount of blow-by gas for each Example and Comparative Example, with the amount of blow-by gas for Comparative Example 1 taken as 100, are shown in Table 1 and Figure 4. Engine speed: 2000 rpm Cooling water temperature: 90°C Oil temperature: 100°C

[0054]

[0055] A reduction in blow-by gas was observed in Examples 1 to 10 compared to Comparative Example 1. In Examples 1 to 10, a tendency was observed where the fuel economy improved as the thermal resistance value increased. A more significant reduction in blow-by gas was observed in Examples 11 to 17 compared to Examples 1 to 10, which had similar thermal resistance values. It is presumed that the reason for the large amount of blow-by gas in Comparative Examples 4, 6, 8, and 10 was that the thermal resistance of the coating was large, causing thermal settling of the piston rings.

[0056] DESCRIPTION OF SYMBOLS 1... piston ring substrate, 3... intermediate layer, 5... coating, 10, 20... piston ring, 11... butt joint, 12, 13... side surface, 14... inner peripheral surface, 15... outer peripheral surface

Claims

1. A piston ring comprising: a piston ring base material having a butt joint; and a coating covering at least an outer peripheral surface of the piston ring base material, wherein the thermal resistance of the coating is 2.0×10 －６ ~12.0 x 10 －６ m ２ ・K / W piston rings.

2. The piston ring according to claim 1, wherein the coating contains Al or Si.

3. The piston ring according to claim 1, further comprising an intermediate layer formed on a surface of said piston ring base material, said coating being formed on the surface of said intermediate layer.

4. The piston ring according to claim 3, wherein the intermediate layer contains Al or Si.

5. A piston ring according to claim 3 or 4, wherein said coating is made of chromium nitride containing Si and said intermediate layer is made of chromium containing Si.

6. A piston ring according to claim 3 or 4, wherein said coating is made of chromium nitride containing Al, and said intermediate layer is made of chromium containing Al.

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