JP6757769B2 - piston ring - Google Patents

Description

The present invention relates to piston rings that can be used for internal combustion engines.

The sliding surface of the piston ring is covered with a hard carbon coating to improve low friction and wear resistance. The hard carbon film is also called a DLC (diamond-like carbon) film, and various developments have been made.

According to Patent Document 1, in a piston ring in which a DLC coating is coated on a sliding surface, the DLC coating is a laminated coating in which two or more layers having different hardness are laminated, and the hardness difference between the two layers is different. Is disclosed to be 500 to 1700 HV.

Patent Document 2 describes a hard carbon film formed on at least the outer peripheral sliding surface of the piston ring base material, wherein the sp ² component ratio measured by the TEM-EELS spectrum is 40 to 80%, and the hard carbon. A hard carbon coating is disclosed in which the area ratio of macroparticles on the coating surface is 0.1% to 10%.

Japanese Unexamined Patent Publication No. 2012-202522 International Publication No. 2014/133095

Various developments have been made regarding the DLC coating, and in addition to the wear resistance that suppresses the wear of the DLC coating against sliding, the sliding surface of the cylinder bore is not worn, that is, the aggression of the sliding surface of the cylinder bore is low. Is also required.
An object of the present invention is to provide a piston ring coated with a DLC coating having low cylinder bore sliding surface aggression in addition to wear resistance.

The present inventors have made extensive studies to solve the above problems, and have come up with the idea that the DLC coating having reduced defects in the internal structure is a coating having excellent wear resistance and low cylinder bore sliding surface aggression. , Completed the invention.

The present invention is a piston ring whose sliding surface is covered with a DLC coating.
The DLC coating includes a piston ring in which the number of pits observed in a cross-sectional SEM image (× 10,000) in the thickness direction is 15 or less per 3 μm × 10 μm, and the hardness is 1000 HV or more and 2000 HV or less. ..

Further, the DLC coating has a refractive index measured by a spectroscopic ellipsometer of 2.3 or more at a wavelength of 550 nm, which tends to have no pits in the internal structure, which is preferable.
The hardness of the DLC coating is preferably 1000 HV or more and 1700 HV or less, and the friction coefficient of the sliding surface surface is preferably 0.10 or less. From the viewpoint of cylinder bore sliding surface aggression, reciprocating In the wear amount measurement test by the dynamic friction tester, the wear amount of the mating material is preferably 0.6 μm or less.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a piston ring coated with a DLC coating having low cylinder bore sliding surface aggression in addition to wear resistance.

It is a schematic diagram which shows the structure of the reciprocating dynamic friction tester. The cross-sectional SEM image of the DLC coating produced in Example 1 is shown (drawing substitute photograph). The cross-sectional SEM image of the DLC coating produced in Example 2 is shown (drawing substitute photograph). A cross-sectional SEM image of the DLC coating produced in Comparative Example 1 is shown (drawing substitute photograph). A cross-sectional SEM image of the DLC coating produced in Comparative Example 2 is shown (drawing substitute photograph).

One embodiment of the present invention is a piston ring whose sliding surface is covered with a DLC coating, and the DLC coating has a number of pits observed in a cross-sectional SEM image (× 10,000) in the thickness direction thereof. The number is 15 or less per 3 μm × 10 μm, and the hardness is 1000 HV or more and 2000 HV or less.

In the present embodiment, the DLC coating has reduced defects in its internal structure. Internal defects of the DLC coating can be confirmed by observing the cross-sectional SEM image of the DLC coating in the thickness direction. Since defects may not be observed in the cross-sectional SEM image when the magnification is small, in the present embodiment, the internal defects of the DLC coating are confirmed by the number of pits observed when the image is 10,000 times. ..
The DLC coating has a number of pits of 15 or less, 10 or less, 5 or less, and 3 or less in a region of 3 μm × 10 μm in a cross-sectional SEM image (× 10,000). It may be 2 or less, and may be 1 or less. By reducing the number of pits in the cross-sectional observation of the DLC coating, the DLC coating becomes a coating having excellent wear resistance and low cylinder bore sliding surface aggression.

In the present embodiment, the pit refers to a V-shaped defect existing in a circled portion in the SEM images shown in FIGS. 2 to 5. Due to the existence of many such pits inside the DLC coating, irregularities remain on the surface of the DLC coating when the sliding surface is formed, so that the aggression to the mating material, specifically, the aggression to the cylinder bore sliding surface. The present inventors presume that the amount of wear of the cylinder bore material increases. In the DLC coating of the present embodiment, the number of pits existing inside the DLC coating is reduced, so that a sliding surface without unevenness can be formed, and the aggression to the mating material can be reduced.

The number of pits is measured at 3 μm × 10 μm, but considering the thickness of the DLC coating normally formed on the piston ring, the width of 3 μm is set in the thickness direction of the DLC coating, and the width of 10 μm is set in the circumferential direction of the piston ring. It is typical to do. However, on the contrary, the measurement width may be set, or the measurement width may be set in the oblique direction.
In the present embodiment, it is sufficient that at least a part of the DLC coating has a region having 15 or less pits per 3 μm × 10 μm, but the number of pits per 3 μm × 10 μm is 50% or more of the DLC coating. The number of regions is preferably 15 or less, more preferably 80% or more, and further preferably 90% or more.

For the DLC coating with a reduced number of pits, the filtered arc ion plating method (FCVA) is used in the manufacturing process, the pulse bias voltage is increased, and the hardness of the DLC coating is not excessively increased. Can be formed.

In the present embodiment, the hardness of the DLC film is 1000 HV or more and 2000 HV or less, may be 1700 HV or less, and may be 1500 HV or less. Generally, considering wear resistance, it is preferable that the film has a high hardness, but if the film is too hard, the cylinder bore sliding surface aggression tends to be high, and the DLC film is the outer circumference of the piston ring. Since the film is formed on the surface, the film is broken when it is deformed during assembly work on the piston, so it is preferable to set the above range so as not to be excessively hard in this embodiment.

The DLC coating preferably has a refractive index measured by a spectroscopic ellipsometer of 2.3 or more at a wavelength of 550 nm. When the refractive index is above this range, the pits in the DLC coating tend to be reduced. The refractive index may be 2.35 or higher and may be 2.40 or higher. As the spectroscopic ellipsometer for measuring the refractive index, for example, a spectroscopic ellipsometer UVISEL (manufactured by HORIBA, Ltd.) can be used.

Since the DLC coating according to the present embodiment has low mating material aggression, it is preferable that the friction coefficient of the sliding surface of the DLC coating is a relatively low value. Specifically, it is preferable that the friction coefficient μ of the sliding surface surface of the DLC coating covering the piston ring is 0.10 or less. The coefficient of friction μ may be less than 0.10, 0.095 or less, and 0.090 or less.

The DLC film according to the present embodiment is a film mainly composed of amorphous carbon, but may contain hydrogen or may contain other unavoidable impurities. The hydrogen contained in the DLC film is usually 5 at% or less, may be 3 at% or less, may be 2 at% or less, may be 1 at% or less, or may be 0.5 at% or less. ..
The DLC coating with an electron energy loss spectroscopy (EELS) the measured sp ² component ratio is generally 40% or more by, may be 50% or more, it may be 60% or more, and usually 80% or less Is.

The method for producing the DLC coating according to the present embodiment is not particularly limited as long as the DLC coating having the above physical properties can be produced. One example is a method of forming a film using a filtered arc ion plating method (FCVA). In the FCVA method, the entire DLC film may be formed in one step, or the DLC film may be formed by forming the DLC film multiple times without changing the applied pulse bias or changing the pulse bias. Good.
The pulse bias to be applied is not particularly limited, but may be -500 V or less, -800 V or less, -1000 V or less, -1500 V or less, or -2000 V or less. Further, it may be -3000V or more.

The thickness of the DLC coating to be produced is not particularly limited, but when the coating is formed on the sliding surface of the piston ring, it is usually 1 μm or more, 3 μm or more, 5 μm or more, and 20 μm. It may be less than or equal to, and may be 15 μm or less.

The manufactured DLC coating has a very reduced mating material aggression. The aggression of the mating material can be evaluated by a wear amount measurement test using a reciprocating friction tester. In the test, the amount of wear of the cast iron cylinder bore material as the mating material after sliding for 60 minutes is preferably 0.6 μm or less, preferably 0.5 μm or less, and 0.4 μm or less. It may be 0.3 μm or less.

The wear amount measurement test by the reciprocating friction tester is as follows.
This is performed using a pin-on plate type reciprocating friction tester whose outline is shown in FIG. The upper test piece used in the reciprocating friction test is a piston ring or a member that looks like the outer peripheral part of the piston ring. The material of the pin is the base material for the piston ring, and the entire surface of the pin is the piston ring as needed. A nitriding treatment is performed in the same manner as the base metal to form a nitrided layer. Then, various DLC films are formed on the surface of the nitrided layer.
The pin used for producing the upper test piece has a diameter of 8 mm, and the tip of the pin (sliding surface) is mirror-finished so that the radius of curvature is 18 mm. Further, as the lower test piece, a plate (FC250 equivalent material: HRB100, A type graphite 75%, carbide precipitation 0.5% or less, Rz 1.0 μm, polishing finish with # 600 emery paper) is used as a cast iron cylinder bore.

In the reciprocating dynamic friction test, lubricating oil was supplied to the sliding interface between the upper test piece and the lower test piece using a tubing pump or an air dispenser. The test conditions at this time are shown below.
<Test conditions>
・ Stroke: 50 mm
・ Load: 50N
-Speed: 300 cycles / min
・ Temperature of lower test piece: 80 ℃
Lubricating oil: 0W-20 (150 μl)
・ Test time: 60min

A DLC film was produced by the following method.
(Example 1)
With the piston ring base material set in the filtered arc ion plating device, the inside of the device was evacuated to reduce the pressure, and then the base material was heated. After that, ion bombarding was performed with argon ions in a state where a pulse bias voltage was applied to the substrate in the range of −500 to -1500V.
Next, using a sputtering method, the bias voltage was set in the range of −50 to −300 V with respect to the piston ring base material, and then a Ti film was formed on the piston ring base material as an adhesive layer.
Next, the first amorphous carbon layer and the second amorphous carbon layer were alternately formed and laminated on the Ti film. Here, the first amorphous carbon layer is formed by using a sputtering method and applying a bias voltage to the piston ring base material in the range of -50 to -300 V in an argon gas atmosphere using a carbon target. Membrane. Further, the second amorphous carbon layer was formed by using a filtered arc method and using a carbon target in a state where a pulse bias voltage was applied to the base material in the range of −500 to -1500V.
The film formation of the first amorphous carbon layer and the second amorphous carbon layer was carried out without introducing a gas containing hydrogen into the film formation chamber so that hydrogen would not be taken into the film. The thickness of the first amorphous carbon layer was 2 to 8 nm, and the thickness of the second amorphous carbon layer was 350 to 400 nm. Then, one set of the first amorphous carbon layer and one layer of the second amorphous carbon layer were formed into a set of two layers, and the set was repeatedly laminated in units of two layers to obtain a DLC film having a thickness of 5.0 μm. ..

(Example 2)
With the piston ring base material set in the filtered arc ion plating device, the inside of the device was evacuated to reduce the pressure, and then the base material was heated. After that, ion bombarding was performed with argon ions in a state where a pulse bias voltage was applied to the substrate in the range of −500 to -1500V.
Next, using a sputtering method, the bias voltage was set in the range of −50 to −300 V with respect to the piston ring base material, and then a Ti film was formed on the piston ring base material as an adhesive layer.
Next, an amorphous carbon layer was formed on the Ti film. The amorphous carbon layer was formed by using a carbon target with a pulse bias voltage applied to the substrate in the range of −2000 to −3000 V. The film formation of the amorphous carbon layer was carried out without introducing a gas containing hydrogen into the film formation chamber so that hydrogen would not be taken into the film. The thickness of one amorphous carbon layer was 350 to 400 nm, and the process was repeated 13 times to obtain a DLC film having a thickness of 5.0 μm.

(Comparative Example 1)
A piston ring base material was prepared, and a CrN layer having a thickness of 10 μm was coated using an arc-type PVD device. Then, a Cr intermediate layer having a thickness of 0.2 μm was coated. While heating the heater to 245 ° C, arc discharge was performed at a bias voltage of -700 V and an arc current of 40 A for 10 minutes, and then arc discharge was performed at a bias voltage of -170 V and an arc current of 40 A to achieve a black color with a total thickness of 0.5 μm. After forming a hard layer and a white hard layer, the mixture was once cooled to 125 ° C.
After that, an arc discharge is performed at a bias voltage of −1000 V and an arc current of 40 A for 90 seconds to form an adhesion layer made of white hard carbon, and then an arc discharge is performed again at a bias voltage of −170 V and an arc current of 40 A to 245. The heater is heated to ℃ to form a black hard layer with a total thickness of 0.5 μm and a white hard layer. The temperature rise and cooling are repeated eight times to form a DLC film with a total thickness of 5.0 μm. A film was formed.

(Comparative Example 2)
With the piston ring base material set in the arc ion plating device, the inside of the device was evacuated to reduce the pressure, and then the base material was heated. After that, with a bias voltage applied to the substrate in the range of −500 to −1000 V, a Cr target was used to discharge the substrate at an arc current of 50 to 100 A, and Cr ion bombard was performed.
Next, in arc ion plating, a bias voltage is applied to the piston ring base material in the range of -10 to -100 V, and a Cr target is used to discharge the piston ring with an arc current of 50 to 100 A to form an adhesive layer. A Cr coating was formed on the piston ring base material.
Next, an amorphous carbon layer was formed on the Cr film. The amorphous carbon layer is formed by discharging the amorphous carbon layer with an arc current of 50 to 100 A using a carbon target with a bias voltage applied to the base material in the range of 0 to -100 V to increase the thickness of the amorphous carbon layer. A 5.0 μm DLC film was obtained.

With respect to the DLC coatings obtained in Examples 1 and 2 and Comparative Examples 1 and 2, the number of pits (pieces / 3 × 10 μm ² ) by cross-sectional SEM, refractive index by spectroscopic ellipsometer, hardness (Hv), sp ² component ratio ( %), The friction coefficient (μ) by the reciprocating dynamic friction test, the DLC coating wear amount (μm), and the mating material wear amount (μm) were measured. The coefficient of friction 1 minute after the start of the reciprocating dynamic friction test was taken as the coefficient of friction. The results are shown in Table 1. In addition, in this specification, Example 1 is read as Reference Example 1.

100 Upper test piece 110 Lower test piece 120 Movable block 122 Heater

Claims (4)

A piston ring whose sliding surface is covered with a DLC coating.
The DLC coating is a piston ring in which the number of pits observed in a cross-sectional SEM image (× 10,000) in the thickness direction is 3 or less per 3 μm × 10 μm, and the hardness is 1000 HV or more and 1500 HV or less. The piston ring according to claim 1, wherein the DLC coating has a refractive index measured by a spectroscopic ellipsometer of 2.3 or more at a wavelength of 550 nm. The piston ring according to claim 1 or 2, wherein the DLC coating has a friction coefficient of 0.10 or less on the surface of the sliding surface. According to any one of claims 1 to 3, the DLC coating has a wear amount of 0.6 μm or less of a test piece corresponding to a cast iron cylinder bore as a mating material in a wear amount measurement test using a reciprocating friction tester. The described piston ring.