Piston ring and method for manufacturing same

Abstract

Provided is a piston ring having excellent low-friction properties and abrasion resistance manufactured without the need for precision control using an ordinary film formation device that does not have a special function. A piston ring obtained by coating an amorphous carbon film on the surface of a ring-shaped substrate, the piston ring being configured so that the amorphous carbon film is formed by CVD, an increase region, in which the ratio sp.sup.2/sp.sup.3 of the sp.sup.2 bond to the sp.sup.3 bond continuously increases from the substrate surface toward the film surface, and a decrease region, in which the ratio sp.sup.2/sp.sup.3 continuously decreases, are formed in alternating fashion, a soft film in which the ratio sp.sup.2/sp.sup.3 is low and a hard film in which the ratio sp.sup.2/sp.sup.3 is high are formed so as to be layered in alternating fashion by continuous variation of the ratio sp.sup.2/sp.sup.3 in the boundary between the increase region and the decrease region, and the decrease regions are formed in equal number to or with one region less than the number of increase regions.

Claims

1. A piston ring, comprising: a ring-shaped substrate; and an amorphous carbon film, coated on a surface of the ring-shaped substrate, wherein the amorphous carbon film is formed using a CVD method, and increasing regions in which an sp.sup.2/sp.sup.3 ratio that is a ratio of sp.sup.2 bonding to sp.sup.a bonding continuously increases and decreasing regions in which the sp.sup.2/sp.sup.3 ratio continuously decreases are alternately formed from the substrate surface toward a film surface, and the sp.sup.2/sp.sup.3 ratio continuously changes on borders between the increasing regions and the decreasing regions so that soft films having a low sp.sup.2/sp.sup.3 ratio and hard films having a high sp.sup.2/sp.sup.3 ratio are formed in an alternately layered manner; and the decreasing regions are configured to be equal in number to or one region fewer than the increasing regions.

2. The piston ring according to claim 1, wherein hydrogen content in the amorphous carbon film is equal to or more than 5 atom % in a location having the lowest sp.sup.2/sp.sup.3 ratio.

3. The piston ring according to claim 2, wherein the amorphous carbon film is formed on an outer circumferential sliding surface and upper and lower surfaces of the substrate.

4. The piston ring according to claim 1, wherein the amorphous carbon film is formed on an outer circumferential sliding surface and upper and lower surfaces of the substrate.

5. A method for manufacturing a piston ring, in which the piston ring according to claim 1 is manufactured using a CVD method, the method comprising: forming an amorphous carbon film on a substrate by alternately providing a first step of forming a film under a condition in which a temperature of the substrate rises and a second step of forming a film under a condition in which the temperature of the substrate falls.

6. The method for manufacturing a piston ring according to claim 5, wherein the CVD method is performed using a plasma CVD apparatus.

7. The method for manufacturing a piston ring according to claim 6, wherein the plasma CVD apparatus is a PIG plasma CVD apparatus.

8. The method for manufacturing a piston ring according to claim 5, wherein the amorphous carbon film having distribution of different sp.sup.2/sp.sup.3 ratios is formed with respect to each substrate at the same time by causing each of a plurality of substrates to have a difference in thermal exhaust ability in the CVD method.

9. The method for manufacturing a piston ring according to claim 5, wherein the amorphous carbon film having distribution of different sp.sup.2/sp.sup.3 ratios inside the substrate is formed at the same time by causing the substrate to internally have a difference in thermal exhaust ability in the CVD method.

10. A piston ring, comprising: a ring-shaped substrate; and an amorphous carbon film, coated on a surface of the ring-shaped substrate, wherein the amorphous carbon film is formed using a CVD method, and an ID/IG ratio that is a ratio of a peak area in a D-peak position to a peak area in a G-peak position of a Raman spectrum continuously changes such that high-wavenumber shift regions in which the G-peak position shifts to a high wavenumber and low-wavenumber shift regions in which the G-peak position shifts to a low wavenumber are alternately formed from the substrate surface toward a film surface, and the ID/IG ratio continuously changes on borders between the high-wavenumber shift regions and the low-wavenumber shift regions so that soft films having a low ID/IG ratio and hard films having a high ID/IG ratio are formed in an alternately layered manner; and the low-wavenumber shift regions are configured to be equal in number to or one region fewer than the high-wavenumber shift regions.

11. The piston ring according to claim 10, wherein hydrogen content in the amorphous carbon film is equal to or more than 5 atom % in a location having the lowest ID/IG ratio.

12. The piston ring according to claim 11, wherein the amorphous carbon film is formed on an outer circumferential sliding surface and upper and lower surfaces of the substrate.

13. The piston ring according to claim 10, wherein the amorphous carbon film is formed on an outer circumferential sliding surface and upper and lower surfaces of the substrate.

14. A method for manufacturing a piston ring, in which the piston ring according to claim 10 is manufactured using a CVD method, the method comprising: forming an amorphous carbon film on a substrate by alternately providing a first step of forming a film under a condition in which a temperature of the substrate rises and a second step of forming a film under a condition in which the temperature of the substrate falls.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view illustrating an overview of a cathodic PIG plasma CVD apparatus used in a method for manufacturing a piston ring according to an embodiment of the present invention.

(2) FIG. 2 is a graph illustrating a result of measurements of an ID/IG ratio of an amorphous carbon film in a depth direction in Example 1.

(3) FIG. 3 is a graph illustrating a result of measurements of a G-peak position of the amorphous carbon film in the depth direction in Example 1.

(4) FIG. 4 is a graph illustrating a relationship between the G-peak position and the ID/IG ratio in Example 1.

(5) FIG. 5 is a graph illustrating a result of measurements of the ID/IG ratio of the amorphous carbon film in the depth direction in Example 2.

(6) FIG. 6 is a graph illustrating a result of measurements of the G-peak position of the amorphous carbon film in the depth direction in Example 2.

(7) FIG. 7 is a graph illustrating a relationship between the G-peak position and the ID/IG ratio in Example 2.

DESCRIPTION OF THE EMBODIMENTS

(8) Hereinafter, the present invention will be specifically described based on an embodiment.

(9) 1. Configuration of Piston Ring

(10) A piston ring according to the present embodiment includes a substrate having a ring shape, and an amorphous carbon film provided on at least an outer circumferential sliding surface and upper and lower surfaces of the substrate. The piston ring is mounted in a ring groove formed on an outer circumferential surface of a piston and slides on an inner circumferential surface of a cylinder in accordance with reciprocating motion of the piston.

(11) (1) Substrate

(12) A piston ring main body which has been used in the related art can be employed as the substrate, and its material is not particularly limited. Examples of the material include a stainless steel material, a casting material, a cast steel material, and a steel material. In addition, its surface may be subjected to nitriding or may be coated with chromium plating or CrN coating.

(13) (2) Amorphous Carbon Film

(14) In the present embodiment, in the amorphous carbon film, while having an sp.sup.2/sp.sup.3 ratio that is a ratio of sp.sup.2 bonding to sp.sup.3 bonding as an index, increasing regions in which the sp.sup.2/sp.sup.3 ratio continuously increases and decreasing regions in which the sp.sup.2/sp.sup.3 ratio continuously decreases are alternately formed from the substrate surface toward a film surface. Moreover, the sp.sup.2/sp.sup.3 ratio also continuously changes on borders between the increasing regions and the decreasing regions. The decreasing regions are configured to be equal in number to or one region fewer than the increasing regions. When the decreasing regions are one region fewer, both the surface of the substrate and the surface of the amorphous carbon film become the increasing regions.

(15) Accordingly, as described above, a soft film is formed in a part having a low sp.sup.2/sp.sup.3 ratio. On the other hand, a hard film is formed in a part having a high sp.sup.2/sp.sup.3 ratio. Then, the soft films and the hard films are alternately layered. In this case, since the sp.sup.2/sp.sup.3 ratio continuously changes from the substrate surface to the film surface, mechanical properties continuously change not only inside the hard films and inside the soft films but also in interfaces of the hard films and the soft films. As a result, it is possible to sufficiently ensure adhesion in the interfaces between the hard films and the soft films and to prevent occurrence of separation in the interfaces. In addition, since soft films and hard films are alternately formed, it is possible to relax internal stress and to easily thicken an amorphous carbon film.

(16) In this case, as described above, the hydrogen content in an amorphous carbon film is preferably equal to or more than 5 atom % in a location having the lowest sp.sup.2/sp.sup.3 ratio and more preferably ranges from 5 to 60 atom %.

(17) In forming such a carbon film, as described above, in place of the sp.sup.2/sp.sup.3 ratio, an ID/IG ratio obtained by measuring Raman scattering light (Raman spectrum) using a Raman spectrometer can be employed as an index. An increase of the ID/IG ratio and a shift of a G-peak position to a high wavenumber are behaviors of a case in which a decrease of hydrogen and an increase of the sp.sup.2/sp.sup.3 ratio occur at the same time, and a decrease of the ID/IG ratio and a shift of the G-peak position to a low wavenumber are behaviors of a case in which an increase of hydrogen and a decrease of the sp.sup.2/sp.sup.3 ratio occur at the same time, thereby leading to further limited conditions. However, there are few practical problems.

(18) 2. Method for Manufacturing Piston Ring

(19) Next, a method for manufacturing a piston ring according to the present embodiment will be described.

(20) (1) Cathodic PIG Plasma CVD Apparatus

(21) To begin with, a cathodic PIG plasma CVD apparatus used in the present embodiment will be described.

(22) FIG. 1 is a view illustrating an overview of a cathodic PIG plasma CVD apparatus used in the present embodiment. As illustrated in FIG. 1, a cathodic PIG plasma CVD apparatus 1 includes a film forming chamber 11, a plasma chamber 12, an exhaust port 13, a pulsed power supply 14, an electrode 15, a Ti sputtering source 16, and a substrate holding tool 17. The reference sign W indicates a substrate.

(23) A cooling device (not illustrated) for supplying cooling water is provided in the substrate holding tool 17, and a heater (not illustrated) is provided in the film forming chamber 11.

(24) The heater and the cooling device heat and cool the substrate holding tool 17, respectively. Accordingly, the substrate W is indirectly heated and cooled. Here, the heater is configured to be temperature-controllable. On the other hand, the cooling device is configured to have an adjustable speed of supplying the cooling water. Specifically, the cooling device is configured to supply the cooling water to the substrate holding tool 17 when cooling is performed and to stop supplying the cooling water when cooling stops.

(25) (2) Manufacturing Piston Ring

(26) Next, a specific procedure of manufacturing a piston ring (substrate) by forming a carbon film, in which the sp.sup.2/sp.sup.3 ratio (ID/IG ratio) continuously changes, on a surface of the piston ring using the cathodic PIG plasma CVD apparatus will be described.

(27) (a) Preparation Before Forming Carbon Film

(28) First, the substrate W subjected to degrease cleansing is set in the substrate holding tool 17 of the cathodic PIG plasma CVD apparatus 1 and is installed inside the film forming chamber 11. The substrate W and the substrate holding tool 17 are electrically connected to the electrode 15, and the pulsed power supply 14 applies a pulse voltage thereto.

(29) Next, the insides of the plasma chamber 12 and the film forming chamber 11 are evacuated through the exhaust port 13 using an exhaust pump (not illustrated). Thereafter, argon (Ar) is introduced into the plasma chamber 12 and the film forming chamber 11 as discharge gas, and the pressure is adjusted. The substrate W is held by the substrate holding tool 17 and revolves inside the film forming chamber 11 while rotating on its axis on the electrode 15 until a series of processing steps of forming a film ends.

(30) Thereafter, Ar plasma is generated by discharging a direct current between a hot filament and an anode (not illustrated) inside the plasma chamber 12. The generated Ar plasma is transported to the inside of the film forming chamber 11. The surface of the substrate W, to which a bias voltage (pulse voltage) is applied by the pulsed power supply 14, is irradiated with Ar ions. Then, cleaning processing is performed by etching.

(31) Next, discharging inside the plasma chamber 12 stops, and sputtering is performed with Ti from the Ti sputtering source 16 under predetermined sputtering conditions. Then, a Ti layer having a thickness ranging from 0.1 to 2.0 m is formed on the surface of the substrate W.

(32) Next, a Si-containing diamond-like carbon (DLC) layer having a thickness ranging from 0.1 to 3.0 m is further formed on the formed Ti layer.

(33) Specifically, the process is performed, for example, by supplying hydrocarbon such as acetylene (C.sub.2H.sub.2) and methane (CH.sub.4); and hydrogen (H.sub.2) together with a compound containing Si such as tetramethylsilane (TMS), as source gas in a cathodic PIG plasma CVD method using the cathodic PIG plasma CVD apparatus.

(34) In this manner, a carbon film sufficiently adheres to the substrate surface due to the Ti layer and the Si-containing DLC layer provided as intermediate layers between the substrate surface and the carbon film.

(35) (b) Forming Carbon Film

(36) Next, a carbon film in which the sp.sup.2/sp.sup.3 ratio (ID/IG ratio) continuously changes is formed on the Ti layer and the Si-containing DLC layer.

(37) Specifically, the process is performed by alternately providing a first step of forming a film under the condition in which the temperature of the substrate rises and a second step of forming a film in a step in which the temperature of the substrate W falls and the substrate W is cooled. That is, the sp.sup.2/sp.sup.3 ratio (ID/IG ratio) continuously increases (increasing regions are formed) in the first step, and the sp.sup.2/sp.sup.3 ratio (ID/IG ratio) continuously decreases (decreasing regions are formed) in the second step. The steps are alternately repeated until the carbon film is formed to have a predetermined thickness.

(38) In this case, the temperature of the substrate W is caused to rise using a heater for heating, and the increasing regions of the sp.sup.2/sp.sup.3 ratio (ID/IG ratio) can be formed with high continuity by continuously increasing the substrate temperature.

(39) In place of using the heater for heating, it is possible to utilize a natural rise of the substrate temperature at the time of film forming using a tool having low thermal exhaust ability.

(40) On the other hand, the substrate temperature is caused to fall using a dedicated a cooling device, and the decreasing regions of the sp.sup.2/sp.sup.3 ratio (ID/IG ratio) can be formed with high continuity by continuously decreasing the substrate temperature.

(41) In place of using the dedicated cooling device, it is possible to cause the substrate temperature to fall by adjusting the exhaust heat rate in accordance with the tool shape.

(42) In addition, film forming may be temporarily halted and the substrate temperature may be caused to fall through natural cooling in a cooling step. In this case, since film forming is temporarily halted and the substrate temperature is caused to fall through natural cooling in the cooling step when the sp.sup.2/sp.sup.3 ratio decreases, film forming is performed at substrate temperature that is discontinuously falling between steps before the cooling step and during the cooling step. However, since carbon is likely to achieve the sp.sup.2 bonding (graphite structure), even if the substrate temperature falls discontinuously, the sp.sup.2/sp.sup.3 ratio does not significantly decrease. The sp.sup.2/sp.sup.3 ratio gradually decreases, and discontinuity of the sp.sup.2/sp.sup.3 ratio is suppressed.

(43) In this manner, when the method for manufacturing a piston ring according to the present embodiment is applied, even if an ordinary film forming apparatus having no special function is used, a continuous change of the sp.sup.2/sp.sup.3 ratio (ID/IG ratio) can be controlled by a very simple control method, that is, controlling the substrate temperature. Therefore, it is possible to easily form a thick carbon film having high continuity of mechanical properties between hard films and soft films and to provide a piston ring having excellent low-friction properties and abrasion resistance.

(44) 3. sp.sup.2/sp.sup.3 Ratio (ID/IG ratio) of Amorphous Carbon Film

(45) As described above, it is possible to know the film forming circumstances for an amorphous carbon film formed as above, through the sp.sup.2/sp.sup.3 ratio or the ID/IG ratio. Therefore, a method for checking for the sp.sup.2/sp.sup.3 ratio and the ID/IG ratio in a formed amorphous carbon film will be described below.

(46) (1) Method for Measuring sp.sup.2/sp.sup.3 Ratio

(47) The sp.sup.2/sp.sup.3 ratio can be obtained by calculating the strength of sp.sup.2 and the strength of sp.sup.3 using electron energy-loss spectroscopy (EELS) analysis.

(48) Specifically, a spectrum imaging method in a scanning-type TEM (STEM) mode is applied. Under the conditions of 200 kv for the acceleration voltage, 10.sup.9 for the sample absorption current, and 1 nm for the beam spot size, EELS obtained at a pitch of 1 nm is integrated, and a CK absorption spectrum is extracted as average information from a region of approximately 10 nm, thereby calculating the sp.sup.2/sp.sup.3 ratio.

(49) (2) Method for Measuring ID/IG Ratio by Raman Spectroscopy

(50) Due to the time and effort required in the EELS analysis, it is not considered to be easy to calculate the sp.sup.2/sp.sup.3 ratio. Therefore, in place of thereof, the ID/IG ratio may be measured by the Raman spectroscopy as described above.

(51) Specifically, as described above, when Raman scattering light is measured with respect to a carbon film using a Raman spectrometer, a D-peak appears near 1,350 cm.sup.1 and a G-peak appears near 1,570 cm.sup.1. The D-peak is a peak based on a six-membered ring structure of carbon, and the G-peak is a peak based on double bonding of carbon. The ID/IG ratio is calculated from each of the areas ID and IG of peaks. Since the obtained ID/IG ratio has a positive interrelationship with the sp.sup.2/sp.sup.3 ratio, it is possible to indirectly know the sp.sup.2/sp.sup.3 ratio by obtaining the ID/IG ratio.

EXAMPLES

(52) Hereinafter, based on examples, the present invention will be more specifically described. In the description below, the ID/IG ratio is employed as an index.

Example 1

(53) First, a Ti layer and a Si-containing DLC layer were formed on a substrate as an intermediate layer (adhesion layer) using a cathodic PIG plasma CVD apparatus, and a carbon film layer in which the ID/IG ratio continuously changes was formed on a surface layer thereof.

(54) Film forming in the carbon film layer was performed under the conditions of 0.4 Pa for gas pressure, 500 V for substrate bias voltage, and 5 A for discharge current, and by causing 20 ccm of Ar gas and 80 ccm of CH.sub.4 gas to flow. After film forming was performed for 97 minutes, discharging stopped and natural cooling was performed for 90 minutes. After the film forming and cooling were repeated twice, film forming was performed for 97 minutes for the third time, and film forming of the carbon film layer was completed (total thickness of 6.2 m). In this Example, heating was performed utilizing a natural rise of the substrate temperature at the time of film forming using a tool having low thermal exhaust ability, and cool was performed utilizing natural cooling by dropping discharging.

(55) In regard to the formed carbon film layer, FIG. 2 illustrates a measurement result of the ID/IG ratio in the depth direction. In FIG. 2, the vertical axis is the ID/IG ratio and also indicates a relationship with the sp.sup.2/sp.sup.3 ratio (also similar in FIG. 3). In addition, the horizontal axis is the depth (unit: m) and indicates the height from the film surface in a negative value (also similar in FIG. 3).

(56) In addition, FIG. 3 illustrates a relationship between the depth (horizontal axis) and the G-peak position (vertical axis), and FIG. 4 illustrates a relationship between the ID/IG ratio (horizontal axis) and the G-peak position (vertical axis).

(57) From FIG. 2, it is ascertained that the ID/IG ratio continuously changes from the substrate toward the film surface, and the ID/IG ratio continuously rises as film forming proceeds but the ID/IG ratio continuously falls for a while after the cooling step is inserted. It is ascertained that three hard films and three soft films are alternately formed in accordance with the ID/IG ratio repetitively rising and falling.

(58) Similarly, from FIG. 3, it is ascertained that high-wavenumber shift regions in which the G-peak position shifts to a high wavenumber and low-wavenumber shift regions in which the G-peak position shifts to a low wavenumber are alternately formed, and three hard films and three soft films are alternately formed.

(59) From FIG. 4, it is ascertained that there is a positive interrelationship between the shifts of the ID/IG ratio and the G-peak position.

(60) In this manner, the hard films and the soft films are alternately formed due to the following reason. When the substrate temperature keeps on rising, graphitization proceeds (the amount of hydrogen decreases) and a hard film is formed. On the other hand, when the substrate temperature falls, graphitization is suppressed (the amount of hydrogen increases) and the surface layer returns to a soft film which is the original film properties. If the soft film is formed once, the soft film does not change in quality to the hard film unless the temperature becomes higher. Therefore, there is no change in quality at the maximum substrate temperature of 230 C. when forming a hard film in this Example, thereby realizing a structure in which the hard films and the soft films are alternately formed in order through film forming performed by repeating rising and falling of the substrate temperature. However, since such an orderly structure cannot be formed when the maximum temperature at the time of forming a hard film reaches the temperature at which a soft film changes in quality, there is a need to be cautious.

Example 2

(61) First, a Ti layer and a Si-containing DLC layer were formed on a substrate as an intermediate layer (adhesion layer) using a cathodic PIG plasma CVD apparatus having a size larger than the apparatus used in Example 1, and a carbon film layer in which the ID/IG ratio continuously changes was formed on a surface layer thereof.

(62) Film forming in the carbon film layer was performed under the conditions of 0.4 Pa for gas pressure, 500 V for substrate bias voltage, and 10 A for discharge current, and by causing 40 ccm of Ar gas, and 150 ccm of C.sub.2H.sub.2 gas to flow. After film forming was performed for 77 minutes, discharging stopped and natural cooling was performed for 30 minutes. After the film forming and cooling were repeated twice, film forming was performed for 28 minutes for the third time, and film forming of the carbon film layer was completed (total thickness of 9.6 m). In this Example as well, heating was performed utilizing a natural rise of the substrate temperature at the tune of film forming using a tool having low thermal exhaust ability, and cool was performed utilizing natural cooling by dropping discharging.

(63) In regard to the formed carbon film layer, FIG. 5 illustrates a measurement result of the ID/IG ratio in the depth direction. In FIG. 5, the vertical axis is the ID/IG ratio and also indicates a relationship with the sp.sup.2/sp.sup.3 ratio (also similar in FIG. 6). In addition, the horizontal axis is the depth (unit: m) and indicates the height from the film surface in a negative value (also similar in FIG. 6).

(64) In addition, FIG. 6 illustrates a relationship between the depth (horizontal axis) and the G-peak position (vertical axis), and FIG. 7 illustrates a relationship between the ID/IG ratio (horizontal axis) and the G-peak position (vertical axis).

(65) From FIGS. 5 to 7, it is ascertained that this Example is also similar to Example 1.

(66) That is, from FIG. 5, it is ascertained that the ID/IG ratio continuously changes from the substrate toward the film surface, and the ID/IG ratio continuously rises as film forming proceeds but the ID/IG ratio continuously falls for a while after the cooling step is inserted. It is ascertained that two hard films and three soft films are alternately formed in accordance with the ID/IG ratio repetitively rising and falling.

(67) Similarly, from FIG. 6, it is ascertained that high-wavenumber shift regions in which the G-peak position shifts to a high wavenumber and low-wavenumber shift regions in which the G-peak position shifts to a low wavenumber are alternately formed, two hard films and three soft films are alternately formed.

(68) From FIG. 7, it is ascertained that there is a positive interrelationship between the shifts of the ID/IG ratio and the G-peak position.

(69) In this manner, similar to the case of Example 1, the hard films and the soft films are alternately formed due to the following reason. When the substrate temperature keeps on rising, graphitization proceeds (the amount of hydrogen decreases) and a hard film is formed. On the other hand, when the substrate temperature falls, graphitization is suppressed (the amount of hydrogen increases) and the surface layer returns to a soft film which is the original film properties. If the soft film is formed once, the soft film does not change in quality to the hard film unless the temperature becomes higher. Therefore, there is no change in quality at the maximum substrate temperature of 210 C. when forming a hard film in this Example, thereby realizing a structure in which the hard films and the soft films are alternately formed in order through film forming performed by repeating rising and falling of the substrate temperature. However, since such an orderly structure cannot be formed when the maximum temperature at the time of forming a hard film reaches the temperature at which a soft film changes in quality, there is a need to be cautious.

(70) Hereinabove, the present invention has been described based on the embodiment. However, the present invention is not limited to the embodiment. It is possible to add various changes with respect to the embodiment within the same range as the present invention and a range equivalent thereto.