Sliding member and production process thereof

[0001] The present application is related to the following applications: U.S. patent application Ser. No. 09/545,181 (based on Japanese Patent Application Hei-11-102205 filed on Apr. 9, 1999); U.S. patent application Ser. No. 10/468,713, which is the national phase of PCT Application No. JP02/10057 (based on Japanese Patent Application 2001-117680 filed on Apr. 17, 2001); U.S. patent application Ser. No. 10/355,099 (based on Japanese Patent Application 2002-45576 filed on Feb. 22, 2002); U.S. patent application Ser. No. 10/682,559 (based on Japanese Patent Application No. 2002-302205 filed on Oct. 16, 2002); and U.S. patent application Ser. No. 10/692,853 (based on Japanese Patent Application 2002-322322 filed on Oct. 16, 2002).

[0002] The invention relates to a sliding member, and more particularly to a sliding member having a thin coating of hard carbon, such as diamond-like carbon, to show excellent low-friction characteristics and durability in the presence of a specific lubricant. The invention also relates to a process for producing the sliding member.

[0003] Global environmental problems, such as global warming and ozone layer destruction, are coming to the fore. The global warming is significantly affected by CO₂ emission, and the setting of CO₂ emission standards to reduce CO₂ emission has become a big concern to each country. In order to reduce CO₂ emission, it is important to improve vehicle fuel efficiency. The reduction of friction in a vehicle engine is thus desired to obtain a direct improvement in fuel efficiency.

[0004] There are some conceivable ways to reduce the engine friction. One way to reduce the engine friction is to provide engine sliding members with lower friction coefficients and higher wear resistance under extreme friction/wear conditions. For example, it is proposed to apply hard carbon coating materials to the cam follower portions of engine sliding members (such as a valve lifter and a lifter shim) and to use a so-called roller rocker arm equipped with a roller needle bearing. As it has been proved that a diamond-like carbon (DLC) coating shows a lower friction coefficient in the air than those of titanium nitride (TiN) film and chromium nitride (CrN) film, the DLC coating is expected to be useful for the engine sliding members. Another way to reduce the engine friction is to improve the properties of a lubricating oil applied to the sliding members. It is proposed to lower lubricating oil viscosity so as to reduce viscous resistance in hydrodynamic lubrication regions and agitation resistance in mechanical sliding portions. It is also proposed to provide a lubricating oil blended with a suitable friction modifier and other additives so as to reduce engine friction losses under mixed lubrication conditions and boundary lubrication conditions. Many studies have been made on various friction modifiers including organomolybdenum compounds e.g. molybdenum dithiocarbamate (MoDTC) and molybdenum dithiophosphate (MoDTP), and the lubricating oil containing such an organomolybdenum friction modifier is proved to be effective in reducing the friction between steel sliding members in the early stages of use.

[0005] The low-friction characteristics of the DLC coating and the friction modifying properties of the organomolybdenum compound are reported in Japan Tribology Congress 1999. 5, Tokyo, Proceeding Page 11-12, KANO et al. and World Tribology Congress 2001. 9, Vienna, Proceeding Page 342, KANO et al.

[0006] The DLC coating however cannot always show a low friction coefficient in the presence of a lubricating oil. Even in the presence of a lubricating oil containing an organomolybdenum friction modifier, the DLC coating does not show its low friction coefficient properly.

[0007] Further, the adhesion of the DLC coating to a base material is susceptible to improvement when the DLC coating is low in hydrogen content. When the DLC coating has no hydrogen content, it is hard to increase coating thickness. The durability of the DLC coating is then susceptible to improvement.

[0008] It is therefore an object of the present invention to provide a sliding member having a thin coating of hard carbon to show excellent low-friction characteristics and durability in the presence of a lubricant, so as to obtain a further improvement in fuel efficiency when used in a vehicle engine. It is also an object of the present invention to provide a process for producing the sliding member.

[0009] As a result of extensive researches, it is found by the present inventors that a thin coating of hard carbon having a certain structure shows excellent low-friction characteristics and durability by combination with a specific lubricant. The present invention has been accomplished based on the above finding.

[0010] According to a first aspect of the invention, there is provided a sliding member, comprising: a base body; and a hard carbon coating formed on the base body to define a sliding surface for sliding contact with an opposing member under lubrication, the hard carbon coating having an outermost surface portion lower in hydrogen content than a remaining portion thereof.

[0011] According to a second aspect of the invention, there is provided a sliding member, comprising: a base body; and a hard carbon coating formed on the base body to define a sliding surface for sliding contact with an opposing member under lubrication, the hard carbon coating having two or more coating layers laminated together in a thickness direction thereof, the laminated coating layers including an outermost coating layer lower in hydrogen content than at least one other coating layer.

[0012] According to a third aspect of the invention, there is provided a process for producing a sliding member, comprising: providing a base body of the sliding member; and forming a hard carbon coating on the base body in such a manner that the hard carbon coating has an outermost surface portion lower in hydrogen content than a remaining portion thereof.

[0013] According to a fourth aspect of the invention, there is provided a process for producing a sliding member, comprising: providing a base body of the sliding member; and forming, on the base body, a hard carbon coating in such a manner that the hard carbon coating an outermost coating layer lower in hydrogen content than at least one other coating layer.

[0014] The other objects and features of the invention will also become understood from the following description.

[0015] FIG. 1A is a plan view of a piston ring according to one exemplary embodiment of the present invention.

[0016] FIG. 1B is a partial section view of the piston ring of FIG. 1A.

[0017] FIG. 1C is an enlarged view of the encircled portion of FIG. 1B.

[0018] FIG. 2 is a schematic view of a reciprocating friction/wear test unit.

[0019] The present invention will be described below in detail. In the following description, all percentages (%) are by mass unless otherwise specified.

[0020] A sliding member according to one exemplary embodiment of the present invention (hereinafter just referred to as a “sliding member”) has a base body, a thin coating of hard carbon formed on the base body to define a sliding surface for sliding contact with an opposing member, and a film of lubricant applied to the sliding surface so that the sliding member slides on the opposing member with the lubricant film interposed between the sliding member and the opposing member.

[0021] Although the sliding member has a multitude of uses, it is desirable to use the sliding member under extreme friction/wear conditions so as to make the most of excellent low-friction characteristics and durability of the sliding member. For example, the sliding member can be formed into a piston ring, such as a top ring and/or an oil ring, for an automotive engine, as shown in FIGS. 1A, 1B and 1C. The hard carbon coating is preferably formed on the opposite ring faces 1 of the base body of the piston ring, which come into contact with the piston-ring groove of a piston (as the opposing member, not shown in the drawings), so as to not only reduce the friction between the piston and the piston ring effectively but also improve the seizure resistance of the piston. Also, the hard carbon coating is preferably formed on the outer circumferential face 2 of the body portion of the piston ring, which comes into sliding contact with the cylinder bore of an engine block (as the opposing member, not shown in the drawings), so as to reduce the friction between the piston ring and the cylinder wall effectively. It is alternatively possible to cover the whole of the piston ring with the hard carbon coating and possible to form a hard carbon coating on the cylinder wall.

[0022] The base body is formed of any known base material, such as an iron-based (steel) material or aluminum-based (aluminum alloy) material, and may be given surface treatment before being covered with the hard carbon coating.

[0023] The hard carbon coating is generally made of amorphous carbon in which carbon elements exist in both sp² and Sp³ hybridizations to form a composite structure of graphite and diamond. Specific examples of the amorphous carbon include hydrogen-free amorphous carbon (a-C), hydrogen-containing amorphous carbon (a-C:H) and/or metal carbide or metal carbon (MeC) that contains as a part a metal element of titanium (Ti) or molybdenum (Mo). The hydrogen-free amorphous carbon and the amorphous carbon low in hydrogen content are also called “diamond-like carbon (DLC)”.

[0024] According to a first embodiment of the present invention, the hard carbon coating has an outermost surface portion lower in hydrogen content than a remaining portion thereof. With such a hydrogen content distribution, it is possible to improve the adhesion of the hard carbon coating to the base body and possible to increase the thickness of the hard carbon coating to a sufficient degree, thereby securing or improving the low-friction characteristics, wear resistance and durability of the sliding member. Herein, the “outermost surface portion” of the hard carbon coating is defined as a portion below the sliding surface, for example, extending within a range of 5% of the coating thickness, or extending within a range of 1.0 μm in thickness.

[0025] In order to obtain a larger friction reducing effect, it is desirable to minimize the hydrogen content of the outermost surface portion of the hard carbon coating in the first embodiment. The average hydrogen content of the outermost surface portion of the hard carbon coating is preferably controlled to 20 atomic % or less, more preferably 10 atomic % or less, still more preferably 5 atomic % or less, yet more preferably 0.5 atomic % or less, and most preferably substantially zero.

[0026] The hydrogen content distribution of the hard carbon coating is not particularly restricted in the first embodiment so long as the outermost surface portion of the hard carbon coating is lower in hydrogen content than the remaining portion. In view of the procedures and conditions of the coating process and the production cost, the hydrogen content distribution of the hard carbon coating can be varied appropriately in accordance with the low-friction characteristics, wear resistance and durability desired of the hard carbon coating. It is however desirable that the hydrogen content of the hard carbon coating gradually decreases in a coating thickness direction from a base body side to a sliding surface side (i.e. an outermost coating surface side). When the hard carbon coating is formed of such a functionally gradient material having a continuous hydrogen content gradient, the internal stress of the hard carbon coating becomes relieved. This makes it possible to prevent the occurrence of cracking in the hard carbon coating and to thereby further improve the durability of the hard carbon coating. Alternatively, the hydrogen content of the hard carbon coating may become decreased in a stepwise manner.

[0027] According to a second embodiment of the present embodiment, the hard carbon coating has two or more layers laminated together in a coating thickness direction and including an outermost layer lower in hydrogen content than at least one other layer. Preferably, the outermost layer of the hard carbon coating is made lower in hydrogen content than any other layer or layers. With such a layer structure, it is also possible to improve the adhesion of the hard carbon coating to the base body and possible to increase the thickness of the hard carbon coating to a sufficient degree, thereby securing or improving the low-friction characteristics, wear resistance and durability of the sliding member. Herein, the “layer” of the hard carbon coating is defined as a portion having a substantially uniform hydrogen content throughout its thickness.

[0028] The layer structure of the hard carbon coating is not particularly restricted in the second embodiment so long as the outermost coating layer is lower in hydrogen content than any other coating layer. In view of the procedures and conditions of the coating process and the production cost, the layer structure of the hard carbon coating can be determined appropriately in accordance with the low-friction characteristics, wear resistance and durability desired of the hard carbon coating. It is however desirable that the hydrogen content of the hard carbon coating gradually decreases layer by layer from a base body side to a sliding surface side. Alternatively, the hydrogen content of the hard carbon coating may become decreased in a stepwise manner.

[0029] It is also desirable to minimize the hydrogen content of the outermost layer of the hard carbon coating in the second embodiment, in order to obtain a larger friction reducing effect. The hydrogen content of the outermost layer of the hard carbon coating is preferably controlled to 20 atomic % or less, more preferably 10 atomic % or less, still more preferably 5 atomic % or less, yet more preferably 0.5 atomic % or less, and most preferably substantially zero.

[0030] In each of the first and second embodiments, the hard carbon coating can be formed by a physical vapor deposition (PVD) process, a chemical vapor deposition (CVD) process or a combination thereof. To lower the hydrogen content of the outermost surface portion or outermost layer of the hard carbon coating, it is effective to reduce the amount of hydrogen in a coating atmosphere while forming the hard carbon coating by e.g. a PVD arc ion plating process.

[0031] Further, the base body may preferably be given undercoating by either a chromium plating process, a chromium nitride process, a nitriding process or a combination thereof before being covered with the hard carbon coating, so as to increase the adhesion of the hard carbon coating to the base body for improvement in durability.

[0032] Either of the following lubricating oil and lubricating agent is desirably used as the lubricant.

[0033] The lubricating oil is predominantly composed of a base oil, and preferably contains therein at least one of an ashless fatty-ester friction modifier, an ashless aliphatic-amine friction modifier, polybutenyl succinimide, a derivative of polybutenyl succinimide and zinc dithiophosphate. Especially, the lubricating oil containing either or both of the ashless fatty-ester friction modifier and the ashless aliphatic-friction modifier produces a greater friction reducing effect on the sliding friction between the sliding member covered with the hard carbon film according to the present embodiment and the opposing member made of iron- or aluminum-based material.

[0034] The base oil is not particularly limited, and can be selected from any commonly used base oil compounds, such as mineral oils, synthetic oils and fats.

[0035] Specific examples of the mineral oils include normal paraffin oils and paraffin-based or naphthene-based oils prepared by extracting lubricating oil fractions from petroleum by atmospheric or reduced-pressure distillation, and then, purifying the obtained lubricating oil fractions with any of the following treatments: solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, hydro-refining, wax isomerization, surfuric acid treatment and clay refining. Although the lubricating oil fraction is generally purified by hydro- or solvent-refining, it is preferable to use the mineral oil prepared by purifying the lubricating oil fraction through the deep hydrocracking or the GTL (Gas-to-Liquid) wax isomerization process for reduction of an aromatics content in the oil.

[0036] Specific examples of the synthetic oils include: poly-α-olefins (PAO), such as 1-octene oligomer, 1-decene oligomer and ethylene-propylene oligomer, and hydrogenated products thereof; isobutene oligomer and a hydrogenated product thereof; isoparaffines; alkylbenzenes; alkylnaphthalenes; diesters, such as ditridecyl glutarate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate and dioctyl sebacate; polyol esters, such as trimethylolpropane esters (e.g. trimethylolpropane caprylate, trimetylolpropane pelargonate and trimethylolpropane isostearate) and pentaerythritol esters (e.g. pentaerythritol-2-ethyl hexanoate and pentaerythritol pelargonate); polyoxyalkylene glycols; dialkyl diphenyl ethers; and polyphenyl ethers. Among these synthetic oil compounds, preferred are poly-α-olefins, such as 1-octene oligomer and 1-decene oligomer, and hydrogenated products thereof.

[0037] The above-mentioned base oil compounds can be used alone or in combination thereof. In the case of using as the base oil a mixture of two or more of the base oil compounds, there is no particular limitation to the mixing ratio of the base oil compounds.

[0038] The sulfur content of the base oil is not particularly restricted, and is preferably 0.2% or less, more preferably 0.1% or less, still more preferably 0.05% or lower, based on the total mass of the base oil. It is desirable to use the hydro-refined mineral oil or the synthetic oil because the hydro-refined mineral oil and the synthetic oil each have a sulfur content of not more than 0.005% or substantially no sulfur content (not more than 5 ppm).

[0039] The aromatics content of the base oil is not also particularly restricted. Herein, the aromatics content is defined as the amount of an aromatics fraction determined according to ASTM D2549. In order for the lubricating oil to maintain low-friction characteristics suitably for use in an internal combustion engine over an extended time period, the aromatics content of the base oil is preferably 15% or less, more preferably 10% or less, and still more preferably 5% or less, based on the total mass of the base oil. The lubricating oil undesirably deteriorates in oxidation stability when the aromatics content of the base oil exceeds 15%.

[0040] The kinematic viscosity of the base oil is not particularly restricted. To use the lubricating oil in an internal combustion engine, the kinematic viscosity of the base oil is preferably 2 mm²/s or higher, more preferably 3 mm²/S or higher, and, at the same time, is preferably 20 mm²/s or lower, more preferably 10 mm²/S or lower, still more preferably 8 mm²/s or lower, as measured at 100° C. When the kinematic viscosity of the base oil is less than 2 mm²/s at 100° C., there is a possibility that the lubricating oil fails to provide sufficient wear resistance and causes a considerable evaporation loss. When the kinematic viscosity of the base oil exceeds 20 mm²/s at 100° C., there is a possibility that the lubricating oil fails to provide low-friction characteristics and deteriorates in low-temperature performance. In the case of using two or more of the base oil compounds in combination, it is not necessary to limit the kinematic viscosity of each base oil compound to within such a specific range so for as the kinematic viscosity of the mixture of the base oil compounds at 100° C. is in the above-specified range.

[0041] The viscosity index of the base oil is not particularly restricted, and is preferably 80 or higher, more preferably 100 or higher, most preferably 120 or higher, to use the lubricating oil in an internal combustion engine. When the base oil has a higher viscosity index, the lubricating oil can attain improved oil-consumption performance as well as low-temperature viscosity properties.

[0042] As the fatty-ester friction modifier and the aliphatic-amine friction modifier, there may be used fatty acid esters and aliphatic amines each having C₆-C₃₀ straight or branched hydrocarbon chains, preferably C₈-C₂₄ straight or branched hydrocarbon chains, more preferably C₁₀-C₂₀ straight or branched hydrocarbon chains. When the carbon number of the hydrocarbon chain of the friction modifier is not within the range of 6 to 30, there arises a possibility of failing to produce a desired friction reducing effect. Specific examples of the C₆-C₃₀ straight or branched hydrocarbon chains of the fatty-ester and aliphatic-amine friction modifiers include: alkyl groups, such as hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl and triacontyl; and alkenyl groups, such as hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, octadecenyl, nonadecenyl, icosenyl, heneicosenyl, docosenyl, tricosenyl, tetracosenyl, pentacosenyl, hexacosenyl, heptacosenyl, octacosenyl, nonacosenyl and triacontenyl. The above alkyl and alkenyl groups include all possible isomers.

[0043] The fatty acid esters are preferably exemplified by esters of fatty acids having the above C₆-C₃₀ hydrocarbon groups and monohydric or polyhydric aliphatic alcohols. Specific examples of such fatty acid esters include glycerol monooleate, glycerol dioleate, sorbitan monooleate and sorbitan dioleate.

[0044] The aliphatic amines are preferably exemplified by aliphatic monoamines and alkylene oxide adducts thereof, aliphatic polyamines, imidazolines and derivatives thereof each having the above C₆-C₃₀ hydrocarbon groups. Specific examples of such aliphatic amines include: aliphatic amine compounds, such as laurylamine, lauryldiethylamine, lauryldiethanolamine, dodecyldipropanolamine, palmitylamine, stearylamine, stearyltetraethylenepentamine, oleylamine, oleylpropylenediamine, oleyldiethanolamine and N-hydroxyethyloleylimidazolyne; alkylene oxide adducts of the above aliphatic amine compounds, such as N,N-dipolyoxyalkylene-N-alkyl or alkenyl (C₆-C₂₈) amines; and acid-modified compounds prepared by reacting the above aliphatic amine compounds with C₂-C₃₀ monocarboxylic acids (such as fatty acids) or C₂-C₃₀ polycarboxylic acids (such as oxalic acid, phthalic acid, trimellitic acid and pyromellitic acid) so as to neutralize or amidate the whole or part of the remaining amino and/or imino groups. Above all, N,N-dipolyoxyethylene-N-oleylamine is preferably used.

[0045] The amount of the fatty-ester friction modifier and/or the aliphatic-amine friction modifier contained in the lubricating oil is not particularly restricted, and is preferably 0.05 to 3.0%, more preferably 0.1 to 2.0%, and most preferably 0.5 to 1.4%, based on the total mass of the lubricating oil. When the amount of the fatty-ester friction modifier and/or the aliphatic-amine friction modifier in the lubricating oil is less than 0.05%, there is a possibility of failing to obtain a sufficient friction reducing effect. When the amount of the fatty-ester friction modifier and/or the aliphatic-amine friction modifier in the lubricating oil exceeds 3.0%, there is a possibility that the solubility of the friction modifier or modifiers in the base oil becomes so low that the lubricating oil deteriorates in storage stability to cause precipitations.

[0046] As the polybutenyl succinimide, there may be used compounds represented by the following general formulas (1) and (2).

[0047] In the formulas (1) and (2), PIB represents a polybutenyl group derived from polybutene having a number-average molecular weight of 900 to 3500, preferably 1000 to 2000, that can be prepared by polymerizing high-purity isobutene or a mixture of 1-butene and isobutene in the presence of a boron fluoride catalyst or aluminum chloride catalyst. When the number-average molecular weight of the polybutene is less than 900, there is a possibility of failing to provide a sufficient detergent effect. When the number-average molecular weight of the polybutene exceeds 3500, the polybutenyl succinimide tends to deteriorate in low-temperature fluidity. The polybutene may be purified, before being used for the production of the polybutenyl succinimide, by removing trace amounts of fluorine and chlorine residues resulting from the polybutene production catalyst with any suitable treatment (such as adsorption process or washing process) in such a manner as to control the amount of the fluorine and chlorine residues in the polybutene to 50 ppm or less, desirably 10 ppm or less, more desirably 1 ppm or less.

[0048] Further, n represents an integer of 1 to 5, preferably 2 to 4, in the formulas (1) and (2) in view of the detergent effect.

[0049] The production method of the polybutenyl succinimide is not particularly restricted. For example, the polybutenyl succinimide can be prepared by reacting a chloride of the polybutene, or the polybutene from which fluorine and chlorine residues are sufficiently removed, with maleic anhydride at 100 to 200° C. to form polybutenyl succinate, and then, reacting the polybutenyl succinate with polyamine (such as diethylene triamine, triethylene tetramine, tetraethylene pentamine or pentaethylene hexamine).

[0050] As the polybutenyl succinimide derivative, there may be used boron- or acid-modified compounds obtained by reacting the polybutenyl succinimides of the formula (1) or (2) with boron compounds or oxygen-containing organic compounds so as to neutralize or amidate the whole or part of the remaining amino and/or imide groups. Among others, boron-containing polybutenyl succinimides, especially boron-containing bis(polybutenyl)succinimide, are preferably used. The content ratio of nitrogen to boron (B/N) by mass in the boron-containing polybutenyl succinimide compound is usually 0.1 to 3, preferably 0.2 to 1.

[0051] The boron compound used for producing the polybutenyl succinimide derivative can be a boric acid, a borate or a boric acid ester. Specific examples of the boric acid include orthoboric acid, metaboric acid and tetraboric acid. Specific examples of the borate include: ammonium salts, such as ammonium borates, e.g., ammonium metaborate, ammonium tetraborate, ammonium pentaborate and ammonium octaborate. Specific examples of the boric acid ester include: esters of boric acids and alkylalcohols (preferably C₁-C₆ alkylalcohols), such as monomethyl borate, dimethyl borate, trimethyl borate, monoethyl borate, diethyl borate, triethyl borate, monopropyl borate, dipropyl borate, tripropyl borate, monobutyl borate, dibutyl borate and tributyl borate.

[0052] The oxygen-containing organic compound used for producing the polybutenyl succinimide derivative can be any of C₁-C₃₀ monocarboxylic acids, such as formic acid, acetic acid, glycolic acid, propionic acid, lactic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, undecylic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, oleic acid, nonadecanoic acid and eicosanoic acid; C₂-C₃₀ polycarboxylic acids, such as oxalic acid, phthalic acid, trimellitic acid and pyromellitic acid, and anhydrides and esters thereof; C₂-C₆ alkylene oxides; and hydroxy(poly)oxyalkylene carbonates.

[0053] The amount of the polybutenyl succinimide and/or polybutenyl succinimide derivative contained in the lubricating oil is not particularly restricted, and is preferably 0.1 to 15%, more preferably 1.0 to 12%, based on the total mass of the lubricating oil. When the amount of the polybutenyl succineimide and/or polybutenyl succinimide derivative in the lubricating oil is less than 0.1%, there is a possibility of failing to attain a sufficient detergent effect. When the amount of the polybutenyl succineimide and/or polybutenyl succinimide derivative in the lubricating oil exceeds 15%, the lubricating oil may deteriorate in demulsification ability. In addition, it is uneconomical to add such a large amount of the polybutenyl succineimide and/or polybutenyl succinimide derivative in the lubricating oil.

[0054] As the zinc dithiophosphate, there may be used compounds represented by the following general formula (3).

[0055] In the formula (3), R⁴, R⁵, R⁶ and R⁷ each represent C₁-C₂₄ hydrocarbon groups. The C₁-C₂₄ hydrocarbon group is preferably a C₁-C₂₄ straight- or branched-chain alkyl group, a C₃-C₂₄ straight- or branched-chain alkenyl group, a C₅-C₁₃ cycloalkyl or straight- or branched-chain alkylcycloalkyl group, a C₆-C₁₈ aryl or straight- or branched-chain alkylaryl group or a C₇-C₁₉ arylalkyl group. The above alkyl group or alkenyl group can be primary, secondary or tertiary. Specific examples of R⁴, R⁵, R⁶ and R⁷ include: alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, heneicosyl, docosyl, tricosyl and tetracosyl; alkenyl groups, such as propenyl, isopropenyl, butenyl, butadienyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, octadecenyl (oleyl), nonadecenyl, icosenyl, heneicosenyl, docosenyl, tricosenyl and tetracosenyl; cycloalkyl groups, such as cyclopentyl, cyclohexyl and cycloheptyl; alkylcycloalkyl groups, such as methylcyclopentyl, dimethylcyclopentyl, ethylcyclopentyl, propylcyclopentyl, ethylmethylcyclopentyl, trimethylcyclopentyl, diethylcyclopentyl, ethyldimethylcyclopentyl, propylmethylcyclopentyl, propylethylcyclopentyl, di-propylcyclopentyl, propylethylmethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl, ethylcyclohexyl, propylcyclohexyl, ethylmethylcyclohexyl, trimethylcyclohexyl, diethylcyclohexyl, ethyldimethylcyclohexyl, propylmethylcyclohexyl, propylethylcyclohexyl, di-propylcyclohexyl, propylethylmethylcyclohexyl, methylcycloheptyl, dimethylcycloheptyl, ethylcycloheptyl, propylcycloheptyl, ethylmethylcycloheptyl, trimethylcycloheptyl, diethylcycloheptyl, ethyldimethylcycloheptyl, propylmethylcycloheptyl, propylethylcycloheptyl, di-propylcycloheptyl and propylethylmethylcycloheptyl; aryl groups, such as phenyl and naphthyl; alkylaryl groups, such as tolyl, xylyl, ethylphenyl, propylphenyl, ethylmethylphenyl, trimethylphenyl, butylphenyl, propylmethylphenyl, diethylphenyl, ethyldimethylphenyl, tetramethylphenyl, pentylphenyl, hexylphenyl, heptylphenyl, octylphenyl, nonylphenyl, decylphenyl, undecylphenyl and dodecylphenyl; and arylalkyl groups, such as benzyl, methylbenzyl, dimethylbenzyl, phenethyl, methylphenethyl and dimethylphenethyl. The above hydrocarbon groups include all possible isomers. Above all, preferred are C₁-C₁₈ straight- or branched-chain alkyl group and C₆-C₁₈ aryl or straight- or branched-chain alkylaryl group.

[0056] The zinc dithiophosphate compounds are preferably exemplified by zinc diisopropyldithiophosphate, zinc diisobutyldithiophosphate, zinc di-sec-butyldithiophosphate, zinc di-sec-pentyldithiophosphate, zinc di-n-hexyldithiophosphate, zinc di-sec-hexyldithiophosphate, zinc di-octyldithiophosphate, zinc di-2-ethylhexyldithiophosphate, zinc di-n-decyldithiophosphate zinc di-n-dodecyldithiophosphate, and zinc diisotridecyldithiophosphate.

[0057] The amount of the zinc dithiophosphate contained in the lubricating oil is not particularly restricted. In order to obtain a larger friction reducing effect, the zinc dithiophosphate is preferably contained in an amount of 0.1% or less, more preferably in an amount of 0.06% or less, most preferably in a minimum effective amount, in terms of the phosphorus element based on the total mass of the lubricating oil. When the amount of the zinc dithiophosphate in the lubricating oil exceeds 0.1%, there is a possibility of inhibiting the friction reducing effect of the ashless fatty-ester friction modifier and/or the ashless aliphatic-amine friction modifier, particularly on the sliding friction between the DLC-coated sliding member and the iron-based opposing member.

[0058] The production method of the zinc dithiophosphate is not particularly restricted, and the zinc dithiophosphate can be prepared by any known method. For example, the zinc dithiophosphate may be prepared by reacting alcohols or phenols having the above R⁴, R⁵, R⁶ and R⁷ hydrocarbon groups with phosphorous pentasulfide (P₂O₅) to form dithiophosphoric acid, and then, neutralizing the dithiophosphoric acid with zinc oxide. It is noted that the molecular structure of zinc dithiophosphate differs according to the alcohols or phenols used as a raw material for the zinc dithiophosphate production.

[0059] The above zinc dithiophosphate compounds can be used alone or in the form of a mixture of two or more thereof. In the case of using two or more of the zinc dithiophosphate compounds in combination, there is no particular limitation to the mixing ratio of the zinc dithiophosphate compounds.

[0060] In order to improve the properties of the lubricating oil for use in an internal combustion engine, the lubricating oil may further contain any other additive or additives, such as a metallic detergent, an antioxidant, a viscosity index improver, a friction modifier other than the above-mentioned fatty-ester friction modifier and aliphatic-amine friction modifier, an ashless dispersant other than the above-mentioned polybutenyl succinimide and polybutenyl succinimide derivative, an anti-wear agent or extreme-pressure agent, a rust inhibitor, a nonionic surfactant, a demulsifier, a metal deactivator and/or an anti-foaming agent.

[0061] The metallic detergent can be selected from any metallic detergent compound commonly used for engine lubricants. Specific examples of the metallic detergent include sulfonates, phenates and salicylates of alkali metals, such as sodium (Na) and potassium (K), or alkali-earth metals, such as calcium (Ca) and magnesium (Mg); and a mixture of two or more thereof. Among others, sodium and calcium sulfonates, sodium and calcium phenates, and sodium and calcium salicylates are suitably used. The total base number and amount of the metallic detergent can be selected in accordance with the properties desired of the lubricating oil. The total base number of the metallic detergent is usually 0 to 500 mgKOH/g, preferably 150 to 400 mgKOH/g, as measured by perchloric acid method according to ISO 3771. The amount of the metallic detergent is usually 0.1 to 10% based on the total mass of the lubricating oil.

[0062] The antioxidant can be selected from any antioxidant compounds commonly used for engine lubricants. Specific examples of the antioxidant include: phenolic antioxidants, such as 4,4′-methylenebis(2,6-di-tert-butylphenol) and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; amino antioxidants, such as phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine and alkyldiphenylamine; and mixtures of two or more thereof. The amount of the antioxidant is usually 0.01 to 5% based on the total mass of the lubricating oil.

[0063] As the viscosity index improver, there may be used: non-dispersion type polymethacrylate viscosity index improvers, such as copolymers of one or more kinds of methacrylates and hydrogenated products thereof; dispersion type polymethacrylate viscosity index improvers, such as copolymers of methacrylates further including nitrogen compounds; and other viscosity index improvers, such as copolymers of ethylene and α-olefins (e.g. propylene, 1-butene and 1-pentene) and hydrogenated products thereof, polyisobutylenes and hydrogenated products thereof, styrene-diene hydrogenated copolymers, styrene-maleate anhydride copolymers and polyalkylstyrenes. The molecular weight of the viscosity index improver needs to be selected in view of the shear stability. For example, the number-average molecular weight of the viscosity index improver is desirably in a range of 5000 to 1000000, more desirably 100000 to 800000, for the dispersion or non-dispersion type polymethacrylate; in a range of 800 to 5000 for the polyisobutylene or hydrogenated product thereof; and in a range of 800 to 300000, more desirably 10000 to 200000 for the ethylene/α-olefin copolymer or hydrogenated product thereof. The above viscosity index improving compounds can be used alone or in the form of a mixture of two or more thereof. The amount of the viscosity index improver is preferably 0.1 to 40.0% based on the total mass of the lubricating oil.

[0064] The friction modifier other than the above-mentioned fatty-ester friction modifier and aliphatic-amine friction modifier can be exemplified by ashless friction modifiers, such as boric acid esters, higher alcohols and aliphatic ethers, and metallic friction modifiers, such as molybdenum dithiophosphate, molybdenum dithiocarbamate and molybdenum disulfide.

[0065] The ashless dispersant other than the above-mentioned polybutenyl succinimide and polybutenyl succinimide derivative can be any of polybutenylbenzylamines and polybutenylamines each having polybutenyl groups of which the number-average molecular weight is 900 to 3500, polybutenyl succinimides having polybutenyl groups of which the number-average molecular weight is less than 900, and derivatives thereof.

[0066] As the anti-friction agent or extreme-pressure agent, there may be used: disulfides, sulfurized fats, olefin sulfides, phosphate esters having one to three C₂-C₂₀ hydrocarbon groups, thiophosphate esters, phosphite esters, thiophosphite esters and amine salts of these esters.

[0067] As the rust inhibitor, there may be used: alkylbenzene sulfonates, dinonylnaphthalene sulfonates, esters of alkenylsuccinic acids and esters of polyalcohols.

[0068] As the nonionic surfactant and demulsifier, there may be used: noionic polyalkylene glycol surfactants, such as polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers and polyoxyethylene alkylnaphthylethers.

[0069] The metal deactivator can be exemplified by imidazolines, pyrimidine derivatives, thiazole and benzotriazole.

[0070] The anti-foaming agent can be exemplified by silicones, fluorosilicones and fluoroalkylethers.

[0071] Each of the friction modifier other than the fatty-ester and aliphatic-amine friction modifiers, the ashless dispersant other than the polybutenyl succinimide and polybutenyl succinimide derivative, the anti-wear agent or extreme-pressure agent, the rust inhibitor and the demulsifier is usually contained in an amount of 0.01 to 5% based on the total mass of the lubricating oil, the metal deactivator is usually contained in an amount of 0.005 to 1% based on the total mass of the lubricating oil, and the anti-foaming agent is usually contained in an amount of 0.0005 to 1% based on the total mass of the lubricating oil.

[0072] The lubricating agent is predominantly composed of a compound having a hydroxyl group, which produces a greater friction reducing effect on the sliding friction between the sliding member covered with the hard carbon film according to the present embodiment and the opposing member made of iron- or aluminum-based material. Specific examples of the hydroxyl group containing compound include alcohols. Among various alcohols, either glycerol or ethylene glycol is preferably used as the lubricating agent.

[0073] The present invention will be described in more detail by reference to the following examples. However, it should be noted that the following examples are only illustrative and not intended to limit the invention thereto.

[0074] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was first cut from S45C steel (according to JIS G4051). A DLC coating (as an inner coating layer) having a hydrogen content of 20 atomic % and a thickness of 10 μm was formed by a CVD process on a semicylindrical face of the cut piece. Another DLC coating (as an outer coating layer) having a hydrogen content of 5 atomic % and a thickness of 0.5 μm was subsequently formed by a PVD arc ion plating process, thereby giving a test specimen. The test specimen was then subjected to the following friction/wear test using poly-α-olefin oil with no additives as a lubricating oil.

[0075] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was first cut from S45C steel (according to JIS G4051). A DLC coating (as an inner coating layer) having a hydrogen content of 20 atomic % and a thickness of 10 μm was formed by a CVD process on a semicylindrical face of the cut piece. Another DLC coating (as an outer coating layer) having a hydrogen content of 0.5 atomic % and a thickness of 0.5 μm was subsequently formed by a PVD arc ion plating process, thereby giving a test specimen. The test specimen was subjected to the friction/wear test using poly-α-olefin oil with no additives as a lubricating oil.

[0076] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was first cut from S45C steel (according to JIS G4051). A DLC coating (as an inner coating layer) having a hydrogen content of 20 atomic % and a thickness of 10 μm was formed by a CVD process on a semicylindrical face of the cut piece. Another DLC coating (as an outer coating layer) having a hydrogen content of 0.5 atomic % and a thickness of 0.5 μm was subsequently formed by a PVD arc ion plating process, thereby giving a test specimen. The test specimen was subjected to the friction/wear test using poly-α-olefin oil blended with 1 mass % glycerol monooleate (as an ashless fatty-ester friction modifier) as a lubricating oil.

[0077] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was first cut from S45C steel (according to JIS G4051). A DLC coating (as an inner coating layer) having a hydrogen content of 20 atomic % and a thickness of 10 μm was formed by a CVD process on a semicylindrical face of the cut piece. Another DLC coating (as an outer coating layer) having a hydrogen content of 0.5 atomic % and a thickness of 0.5 μm was subsequently formed by a PVD arc ion plating process, thereby giving a test specimen. The test specimen was then subjected to the friction/wear test using glycerol as a lubricating agent.

[0078] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was cut from S45C steel (according to JIS G4051). A coating of chrome plating having a thickness of 50 μm was formed on a semicylindrical face of the cut piece, thereby giving a test specimen. The test specimen was then subjected to the friction/wear test using poly-α-olefin oil with no additives as a lubricating oil.

[0079] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was cut from S45C steel (according to JIS G4051). A coating of titanium nitride (TiN) having a thickness of 20 m was formed on a semicylindrical face of the cut piece, thereby giving a test specimen. The test specimen was subjected to the friction/wear test using poly-α-olefin oil with no additives as a lubricating oil.

[0080] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was cut from S45C steel (according to JIS G4051). A coating of chromium nitride (CrN) having a thickness of 20 μm was formed on a semicylindrical face of the cut piece, thereby giving a test specimen. The test specimen was subjected to the friction/wear test using poly-α-olefin oil with no additives as a lubricating oil.

[0081] A substantially semicylindrical piece (as a base body) having a dimension of 8×12×40 mm was cut from S45C steel (according to JIS G4051). A DLC coating having a hydrogen content of 20 atomic % and a thickness of 10 μm was formed by a CVD process on a semicylindrical face of the cut piece, thereby giving a test specimen. The test specimen was then subjected to the friction/wear test using poly-α-olefin oil blended with 1 mass % glycerol monooleate (as an ashless fatty-ester friction modifier) as a lubricating oil.

[0082] The friction/wear test was conducted under the following conditions using a reciprocating friction/wear tester. In the friction/wear tester, the test specimen (10) of each of Examples 1 to 4 and Comparative Examples 1 to 4 was set as shown in FIG. 2 so as to reciprocate in directions S and T while sliding the semicylindrical portion (10a) of the test specimen (10) on the area A of the plate-shaped opposing specimen (11) under a load L. Herein, the opposing specimen was made of FC250 iron casting (according to JIS G5501). During the test, the coefficient of friction between the test specimen (10) and the opposing specimen (11) was measured at a turning end of the area A. Also, the wear amount of the test specimen (10) was measured after the test. The measurement results are shown in TABLE. In TABLE, the wear amounts of the test specimens of Examples 1, 2 and 4 and Comparative Examples 1 to 4 are indicated with reference to the wear amount (1.0) of the test specimen of Example 3.

[Test Conditions]

• [0083] Test specimen (10): A semicylindrical-shaped member formed with a S45C steel base body and a coating(s) thereon and having a dimension of 8×12×40 mm
• [0084] Opposing specimen (11): A plate-shaped member formed of FC250 iron casting and having a dimension of 40×60×7 mm
• [0085] Test unit: Reciprocating friction/wear tester
• [0086] Reciprocating motion: 600 cycles per minute
• [0087] Test temperature: 25° C.
• [0088] Load (P) applied: 98 N
• [0089] Test time: 60 min

[0090] The sliding members of Examples 1 to 4 had lower friction coefficients and smaller wear amounts than those of Comparative Examples 1 to 4, and the unexpected results of the present invention are clearly demonstrated in TABLE. Among others, Example 4 seems to provide the most favorable results at the present moment in view of the fact that the sliding member of Example 4 had a lower friction coefficient and a smaller wear amount that those of Examples 1 to 3.

[0091] As described above, the sliding member shows excellent low-friction characteristics and durability by combination of a certain structured hard carbon coating and a specific lubricant according to the present invention. It is therefore possible to obtain a greater improvement in fuel efficiency by the use of the sliding member of the present invention in an internal combustion engine, than that obtained by the use of a steel sliding member lubricated with a lubricating oil containing an organomolybdenum compound according to the earlier technology.

[0092] The entire contents of Japanese Patent Application No. 2003-206809 (filed on Aug. 8, 2003) and No. 2004-225411 (filed on Aug. 2, 2004) are herein incorporated by reference.

[0093] Although the present invention has been described with reference to specific embodiments of the invention, the invention is not limited to the above-described embodiments. Various modification and variation of the embodiments described above will occur to those skilled in the art in light of the above teaching. The scope of the invention is defined with reference to the following claims.

	TABLE
	Friction coefficient	Wear amount
Example 1	0.07	1.5
Example 2	0.06	1.2
Example 3	0.04	1.0
		(Reference)
Example 4	0.02	1.0
Comparative Example 1	0.18	5.4
Comparative Example 2	0.17	3.5
Comparative Example 3	0.15	2.0
Comparative Example 4	0.13	1.8