A preferred embodiment of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the embodiment described herein.
A hard carbon coating layer according to this embodiment of the present invention can be applied to slide members such as machine components used under an unlubricated condition.
The test specimens 11 were prepared by forming the hard carbon coating 13 on the substrate 12 using the conditions (Al content, applied bias voltage, and conformation of the gradient layer) shown in Table 1.
The hard carbon coating layer 13 was formed by depositing a diamond-like carbon (DLC) layer on the substrate 12 using an unbalanced magnetron sputtering (UBMS) method. The UBMS method is a film forming method having the following features. Magnetic poles disposed behind a target are intentionally unbalanced in magnetic strength between at the center and periphery of the target to produce a nonequilibrium state, thereby causing a part of the magnetic line of force from the magnetic pole at the periphery of the target to extend to a substrate. This, in return, helps plasma conventionally concentrated in the vicinity of the target to diffuse along the extended magnetic line of force. Therefore, the substrate 12 can be irradiated with an increased amount of ions during the formation of the hard carbon coating layer 13, thereby enabling the denser coating layer 13 to be formed on the substrate 12.
After forming the hard carbon coating layer (henceforth referred to as “coating layer”) 13, it was evaluated for the adhesiveness by examining for peeling of the coating layer 13 in an indentation test using a Rockwell diamond indenter. The surface of the coating layer 13 was also evaluated by a nano indentation method (ISO14577), and was measured for the peel load by a friction test under an unlubricated condition. On the other hand, the coating layer formed on a crown surface of a cam lifter for automotive engines was measured for the friction torque under an oil lubricated condition to calculate reduction rate of the torque, and the coating layer 13 was also examined for peeling after a durability test under the oil lubricated condition.
In the adhesiveness evaluation by an indentation test using a Rockwell diamond indenter, a conical Rockwell diamond indenter having a tip diameter of 200 μm was pushed into the test specimen at a force of 1471 N (150 kgf). A trace produced by the pushing was observed for cracking and peeling of the coating layer 13 around the trace with an optical microscopy.
In the nano indentation (ISO14577) evaluation, a triangular pyramidal Berkovich indenter having a dihedral angle of 115 degrees was forced into the surface of the coating layer 13 for 10 seconds up to a maximum load of 3 mN, which was maintained for 1 second and then the force was unloaded over a period of 10 seconds. The indentation hardness and the indentation Young's modulus were calculated from this evaluation.
The tester 21 is provided with a work table 23 fixed to a rotating shaft 22. On the work table 23 is loaded the test specimen 11, on the upper side of which is placed a 6 mm-diameter metal ball (high carbon chromium bearing steel ball) 24 as the counterpart of the test specimen 11. Here, a metal material used for the metal ball 24 is not limited to the high carbon chromium bearing steel, but may be any steel used for bearings.
The tester 21 is so configured that a spring 25 is used to increase the applied load stepwise by 98 N every one minute from 98 to 2452 N. Here, the metal ball 24 is fixed to a holder 26 to prevent the rotation of the ball. The rotating shaft 22 driven by a motor 27 was rotated at a sliding velocity of approximately 34 mm/sec relative to the metal ball 24, and the torque corresponding to the friction force generated between the metal ball 24 and test specimen 11 was measured by a load cell 28 to calculate the coefficient of friction. In addition, one metal ball 24 is disposed 8 mm from the rotation center and a load of P is applied to it as shown in
The disk substrate 12 of an alloy (chromium molybdenum steel) containing Fe, Cr and Mo was carburized to have a Rockwell C scale surface hardness (HRC) of 58 or more, and was dressed to have an average surface roughness (Ra) of 0.1 μm or less. Then, a coating layer 13 was formed by an UBMS method while introducing inert and hydrocarbon gases so as to have an Al content of 1.88 at. %.
The formed coating layer 13 was evaluated for the adhesiveness by forcing the Rockwell diamond indenter into the coating layer at a load of 1471 N. The result showed excellent adhesiveness between the substrate 12 and the coating layer 13 because no peeling was observed around the trace as shown in
The coating layer 13 of Example 1 is not subject to any peeling from the substrate 12 so that the substrate 12 is never exposed, thus enabling efficient use of the low friction properties of the coating layer 13. When the coating layer 13 of Example 1 was subject to an unlubricated sliding movement, the coefficient of friction was 0.1 or less, meaning that the coefficient of friction could be reduced by approximately 88 to 96% compared to steel without the coating layer 13 or with conventional surface treatments, and could be reduced by approximately 62% compared to conventional coating layers. Moreover, the peel load of the coating layer 13 is as large as 490 N or more, thereby enabling efficient use of the low friction properties of the coating layer 13 even under higher load condition than usual.
On the other hand, as a test specimen under an oil lubricated condition, the coating layer 13 was applied to a crown surface 53 (carburized to have an HRC value not less than 58 and dressed to have an Ra value not more than 0.1 μm) of a motor vehicle cam lifter 52 as shown in
Applying the coating layer 13 of Example 1 to the motor vehicle cam lifter 52 reduces the torque applied thereto, thus reducing the slide load between the cam lifter 52 and a cam 54. Furthermore, there is produced no peeling of the coating layer 13, thus enabling continuous use of the low friction properties of the coating layer 13. This can provide a motive power engine (such as an automotive engine) capable of maintaining a high energy efficiency due to a low mechanical loss over a long period of time. The coating layer 13 of Example 1 is not subject to any peeling from the substrate 12 (the crown surface 53 of the cam lifter 52 for automotive engines) so that the substrate 12 is never exposed, thus enabling efficient use of the low friction properties of the coating layer 13.
The coating layer 13 of Example 1 is a hard carbon coating layer containing both sp2 bonded carbon typified by the carbon bond of graphite and sp3 bonded carbon typified by the carbon bond of diamond. This can provide the coating layer 13 having both abrasion resistance and low friction properties. A hard carbon coating layer is made of amorphous carbon or hydrogenated carbon, and is called amorphous carbon, hydrogenated amorphous carbon (a-C:H), diamond-like carbon (DLC) or the like. The method for forming the hard carbon coating layer includes: plasma CVD (chemical vapor deposition) in which hydrocarbon gas is decomposed by plasma to form a film; vapor phase synthesis such as ion beam deposition using carbon or hydrocarbon ions; ion plating in which graphite or the like is vaporized by arc discharge to form a film; and sputtering in which a target is sputtered in an inert atmosphere to form a film.
The coating layer of Example 1 combines excellent adhesiveness, abrasion resistance and low friction properties, thereby rendering the coating applicable to slide members. Accordingly, this can provide a slide member capable of reducing load under an unlubricated condition as well as under water, organic solvent, fuel and oil lubricated conditions. In addition, a machine component normally sliding in a lubricating oil can maintain its reliability even when the lubricating oil runs short, thus reducing the amount of lubricating oil.
In the present invention, the substrate 12 having the coating layer 13 formed thereon preferably contains at least one element selected from the group made up of V, Cr, Fe, Co, Ni, Zr, Nb, Mo, Ta, W, Ir and Pt. Furthermore, because the temperature rises during the formation of the coating layer 13, it more preferably contains a high melting point metal (in particular, Fe, Co and Ni) to prevent the transformation of the substrate. The Cr intermediate layer 41 is also formed before forming the hard carbon coating layer. Therefore, the substrate preferably contains Cr to improve adhesiveness between the substrate 12 and the Cr intermediate layer 41.
The gradient layer 42 formed between the Cr intermediate layer 41 and outermost surface layer 43 preferably has a continuously decreasing Cr concentration and continuously increasing C concentration from the Cr intermediate layer 41 side to the outermost surface layer 43 side. In the case where the gradient layer 42 is formed by stacking layers of a different composition (different Cr C contents), each layer preferably has a thickness of 15 nm or less. If Cr carbide, one of the materials composing the gradient layer 42, is expressed as CrxCy, its composition can be gradually varied from the Cr intermediate layer 41 side to the outermost surface layer 43 side by gradually varying the ratio of x and y. Therefore, it prevents abrupt change in film characteristic in the thickness direction of the gradient layer 42.
When a thickness of the coating layer 13 is less than 0.15 μm, the layer is more prone to be worn away by sliding movement. While, a thickness of the coating 13 is more than 6.0 μm, the layer has a higher internal stress and is thereby more prone to peel off. Therefore, the coating layer 13 preferably has a thickness within a range of 0.15 to 6.0 μm. Since the coating layer 13 has a high surface hardness, the greater the average surface roughness is above 0.1 μm, the more likely it is to wear away the counterpart. Therefore, the coating layer 13 preferably has an average surface roughness of 0.1 μm or less. Additionally, when the thicknesses of the gradient layer 42 and the outermost surface layer 43 are denoted by dG and dC respectively, it is preferred that dC/dG≦1.
The coating layer 13 is preferably formed by sputtering, plasma CVD, ion plating or the like, more preferably formed by sputtering or ion plating. In addition, the coating layer 13 contains Al in the outermost surface layer 43. Its content is preferably within a range of 0.5 to 4.5 at. %, more preferably within a range of 0.6 to 1.9 at. %. The Al is contained in the form of at least one material selected from the group made up of Al metal, Al boride, Al carbide, Al nitride, Al oxide and Al hydroxide, and preferably in the form of Al oxide and/or Al hydroxide.
This can offer a hard carbon coating layer having both abrasion resistance and low friction properties. The test specimen 11 formed according to Example 1 combines excellent adhesiveness, abrasion resistance and low friction properties, and is thus applicable to slide members. Accordingly, this can provide a slide member capable of reducing load under an unlubricated condition as well as under water, organic solvent, fuel and oil lubricated conditions. Also, a machine component normally sliding in a lubricating oil can maintain its reliability even when the lubricating oil runs short, thus reducing the amount of lubricating oil.
Al exposed on the surface of the coating layer 13 reacts with oxygen, water or the like in the atmosphere to form an Al oxide or Al hydroxide, thereby also providing a dielectric coating layer with stable electrical properties although it contains metal. As a result, Al contained in such a coating layer 13 is present on its surface in the form of Al oxide and/or Al hydroxide. That is, the coating layer 13 (in particular, the outermost surface layer 43) has a hydrophilic surface, thereby enabling realization of low friction properties in a water lubricated sliding movement. This also permits realization of low friction properties in a sliding movement under the presence of liquid or steam having the same OH-group as water (e.g., alcohols or additives in oil). Example 1 is actually based on the discovery of a phenomenon in which the presence of Al in the coating layer 13 reduces its internal stress, thereby causing it less likely to be peeled away from the substrate 12.
When a surface Al content of the coating layer 13 is less than 0.5 at. %, the outermost surface layer contains less amount of Al oxide and/or Al hydroxide from which the hydrophilicity is derived, and thus cannot be expected to produce a friction reducing effect. It was found from the result of X-ray photoelectron spectroscopy (XPS) that there was possible carburization of Al for the coating layer 13 having Al content more than 5 at. % on its surface (particularly the surface and inside of the outermost surface layer 43). That is, the coating layer 13 having a surface Al content more than 5 at. % produces more Al carbides, making the coating layer 13 more prone to cracking due to the embrittlement of the Al carbides.
A surface hardness of the coating layer 13 is more than 20 GPa. The surface hardness less than 20 GPa lowers the abrasion resistance, thus making the coating layer 13 more prone to abrasion. Addition of Al to the coating layer 13 tends to lower the hardness of the layer compared to the case without such addition. However, addition of Si (silicon) with Al to the layer is effective to increase the hardness. Furthermore, a surface Young's modulus of the coating layer 13 is within a range of 50 to 180 GPa. The Young's modulus is increased by the formation of Al carbides, but when it exceeds 180 GPa, the coating layer 13 is more prone to cracking or peeling. On the other hand, when the Young's modulus is less than 50 GPa, it lowers the load bearing properties in a sliding movement.
In the case where Al contained in the coating layer 13 is present in the form of Al oxide and/or Al hydroxide, the coating layer 13 has a high affinity to liquid or steam with OH-groups, thus enabling realization of low friction properties under a water lubricated condition. The coating layer 13 containing Al can be formed by using an Al target in the case of sputtering or ion plating. On the other hand, in the case of plasma CVD, an organic Al compound typified by trimethylaluminum can be introduced as gas into the chamber to form the coating layer 13 containing Al.
A high adhesiveness to the substrate 12 can be realized by using the hard carbon coating layer 13 in which the gradient layer 42 has a decreasing Cr concentration and increasing C concentration from the Cr intermediate layer 41 side to the outermost surface layer 43 side, and further the outermost surface layer 43 contains Al. Such a coating layer can be used not only under an unlubricated condition, but also under water, organic solvent, fuel and oil lubricated conditions. In addition, the gradient layer 42 preferably has a continuously decreasing Cr concentration and continuously increasing C concentration from the Cr intermediate layer 41 side to the outermost surface layer 43 side. Further, the gradient layer 42 preferably has a thickness within a range of 0.25 to 5.0 μm.
The outermost surface of the coating 13 (the outermost surface layer 43) contains at least Al, or contains both Al and Si. Additionally, the coating layer 13 preferably has a total thickness within a range of 0.5 to 4 μm. A slide member according to a preferred embodiment of the present invention can be applied to applications that require durability, e.g., motor vehicle slide components (in particular, engine cylinders), and vanes for refrigerant compressors or rotary compressors used in refrigerators, air conditioners, etc.
A disk substrate 12 made of JIS SCM415 was carburized to have a surface hardness HRC not less than 58, and was dressed to have an average surface roughness Ra not more than 0.1 μm. Here, “JIS SCM415” is a kind of chromium molybdenum steels and contains 0.13-0.18 wt. % of C; 0.15-0.35 wt. % of Si; 0.60-0.90 wt. % of Mn; 0.90-1.20 wt. % of Cr; 0.15-0.25 wt. % of Mo; not more than 0.030 wt. % of P; not more than 0.030 wt. % of S; not more than 0.25 wt. % of Ni; and not more than 0.30 wt. % of Cu.
Then, a coating layer 13 was formed by an UBMS method while introducing inert and hydrocarbon gases. Al was not added to the coating layer 13 of Comparative example 1. When forming a gradient layer 42 positioned between a Cr intermediate layer 41 and an outermost surface layer 43 of the coating layer 13, the powers input to the Cr and C targets and the bias voltage applied to the substrate were controlled as shown in
The formed coating layer 13 was evaluated for the adhesiveness by forcing the Rockwell diamond indenter into the coating layer at a load of 1471 N. The result showed a poor adhesiveness between the substrate 12 and the coating layer 13 because some peeling was observed around the trace in the coating layer 13 as shown in
A disk substrate 12 made of JIS SCM415 was carburized to have a surface hardness HRC not less than 58, and was finish processed to have an average surface roughness Ra not more than 0.1 μm. Then, a coating layer 13 was formed by an UBMS method while introducing inert and hydrocarbon gases. Al was not added to the coating layer 13 of Comparative example 2. When forming a gradient layer 42 positioned between a Cr intermediate layer 41 and an outermost surface layer 43 of the coating layer 13, the powers input to the Cr and C targets and the bias voltage applied to the substrate were controlled as shown in
The formed coating layer 13 was evaluated for the adhesiveness by forcing the Rockwell diamond indenter into the coating layer 13 at a load of 1471 N. The result showed an excellent adhesiveness between the substrate 12 and the coating layer 13 because no peeling was observed around the trace as shown in
On the other hand, a coating layer 13 was applied to a crown surface 53 (carburized to have an HRC value not less than 58 and finish processed to have an Ra value not more than 0.1 μm) of a motor vehicle cam lifter 52 made of JIS SCM415 by an UBMS method while introducing inert and hydrocarbon gases similarly to the case of the substrate 12. The result of the friction torque measurement in durability test under an oil lubricated condition was that in the case of Comparative example 2, the torque could be reduced by approximately 43% compared to a typical torque for motor vehicle cam lifters without the coating layer 13.
However, after the test, peeling and roughness were observed in a part of the coating layer 13 formed on the crown surface 53 as shown in
When the coating layer 13 of Comparative example 2 is applied to the cam lifter 52 for automotive engines, it is expected to be subject to peeling or increase in the coefficient of friction over time, thus preventing continuous use of the low friction properties of the coating layer 13. This fails to provide a motive power engine (such as an automotive engine) capable of maintaining a high energy efficiency over a long period of time.
A disk substrate 12 made of JIS SCM415 was carburized to have a surface hardness HRC not less than 58, and was finish processed to have an average surface roughness Ra not more than 0.1 μm. Then, a coating layer 13 was formed by an UBMS method while introducing inert and hydrocarbon gases. The coating layer 13 of Comparative example 3 was formed to contain 0.65 at. % of Al. When forming a gradient layer 42 positioned between a Cr intermediate layer 41 and an outermost surface layer 43 of the coating layer 13, the powers input to the Cr and C targets and the bias voltage applied to the substrate were controlled as shown in
The formed coating layer 13 was evaluated for the adhesiveness by forcing the Rockwell diamond indenter into the coating layer 13 at a load of 1471 N. The result showed an excellent adhesiveness between the substrate 12 and the coating layer 13 because no peeling was observed around the trace as shown in
On the other hand, a coating layer 13 was applied to a crown surface 53 (carburized to have an HRC value not less than 58 and finish processed to have an Ra value not more than 0.1 μm) of a motor vehicle cam lifter 52 made of JIS SCM415 by an UBMS method while introducing inert and hydrocarbon gases similarly to the case of the substrate 12. The result of the friction torque measurement in durability test for Comparative example 3 under an oil lubricated condition was that the torque could not be reduced but increased by approximately 3% compared to a typical torque for motor vehicle cam lifters without the coating layer 13. In addition, after the test, some peeling was observed in the coating layer 13 formed on the crown surface 53 as shown in
When the coating layer 13 of Comparative example 3 is applied to the cam lifter 52 for automotive engines, the torque increases, and therefore the slide load between the coating layer 13 and a cam 54 cannot be reduced. Further, some peeling of the coating layer 13 occurs, thus preventing continuous use of the low friction properties of the coating layer 13. This fails to provide a motive power engine (such as an automotive engine) with a high energy efficient.
The present invention is a slide member comprising a hard carbon coating layer with excellent abrasion resistance and low friction properties and can be applied to machine components such as internal combustion engines which are used under an oil lubricated condition.
Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.
Number | Date | Country | Kind |
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2006-259866 | Sep 2006 | JP | national |