SLIDING SYSTEM

Abstract

A sliding system includes a pair of sliding members having opposing sliding surfaces adapted to move relative to each other, and a lubricating oil interposed between the opposing sliding surfaces. Here, at least one of the sliding surfaces is a covered surface covered with a specific chromium nitride film. The lubricating oil contains an oil-soluble molybdenum compound formed of a molybdenum dialkyldithiocarbamate. In the chromium nitride film, when an amount of atoms constituting the entire chromium nitride film is set as 100 atom %, there is 32 atom % to 47 atom % of N, the balance is Cr, and a relative area which is an area ratio of the (111) plane with respect to the (200) plane obtained when the chromium nitride film is analyzed by X-ray diffraction is 20% to 30%.

Claims

1. A sliding system including a pair of sliding members having opposing sliding surfaces adapted to move relative to each other and a lubricating oil interposed between the opposing sliding surfaces, wherein at least one of the sliding surfaces is a covered surface covered with a chromium nitride film, in the chromium nitride film, when an amount of atoms constituting the entire chromium nitride film is set as 100 atom %, there is N of 32 atom % to 47 atom %, and the balance is Cr, and a relative area which is an area ratio of a (111) plane with respect to a (200) plane obtained when the chromium nitride film is analyzed by X-ray diffraction is 20% to 30%, the lubricating oil contains a molybdenum dialkyldithiocarbamate which is an oil-soluble molybdenum compound, and a mass ratio of Mo of the molybdenum dialkyldithiocarbamate with respect to the entire lubricating oil is 50 ppm to 800 ppm.

2. The sliding system according to claim 1, wherein the mass ratio of Mo of the molybdenum dialkyldithiocarbamate with respect to the entire lubricating oil is 200 ppm to 700 ppm.

3. The sliding system according to claim 1, wherein the chromium nitride film contains Cr.sub.2N together with CrN.

4. The sliding system according to claim 1, wherein the lubricating oil contains at least one of a phosphorus compound, a sulfur compound, and a nitrogen compound, wherein a mass ratio of P of the phosphorus compound with respect to the entire lubricating oil is 200 ppm to 1,500 ppm, a mass ratio of S of the sulfur compound with respect to the entire lubricating oil is 500 ppm to 3,000 ppm, and a mass ratio of N of the nitrogen compound with respect to the entire lubricating oil is 200 ppm to 2,000 ppm.

5. The sliding system according to claim 1, wherein the sliding system is a hybrid vehicle internal combustion engine or a drive transmission device including a transmission.

6. A sliding system including a pair of sliding members having opposing sliding surfaces adapted to move relative to each other and a lubricating oil interposed between the opposing sliding surfaces, wherein at least one of the sliding surfaces is a covered surface covered with a chromium nitride film, in the chromium nitride film, when an amount of atoms constituting the entire chromium nitride film is set as 100 atom %, there is 32 atom % to 47 atom % of N, the balance is Cr and 1 atom % or less of a doping element, and a relative area which is an area ratio of a (111) plane with respect to a (200) plane obtained when the chromium nitride film is analyzed by X-ray diffraction is 20% to 30%, the lubricating oil contains a molybdenum dialkyldithiocarbamate which is an oil-soluble molybdenum compound, and a mass ratio of Mo of the molybdenum dialkyldithiocarbamate with respect to the entire lubricating oil is 50 ppm to 800 ppm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

[0022] FIG. 1A shows profiles of chromium nitride films according to samples obtained by X-ray diffraction;

[0023] FIG. 1B is an enlarged view of an X-ray diffraction profile of a chromium nitride film according to a 3;

[0024] FIG. 2A is a bar graph in which coefficients of friction of samples are compared;

[0025] FIG. 2B is a bar graph in which wear depths of samples are compared;

[0026] FIG. 3 is a scatter diagram showing a relationship between surface roughness and coefficients of friction of samples;

[0027] FIG. 4 is a scatter diagram showing a relationship between relative areas and coefficients of friction of samples;

[0028] FIG. 5 is a graph showing a relationship between Mo amounts of a Mo-DTC and coefficients of friction according to the sample 3 and a sample C1; and

[0029] FIG. 6 is a schematic diagram showing an example of a molecular structure (R: alkyl group) of a Mo-DTC.

DETAILED DESCRIPTION OF EMBODIMENTS

[0030] One, two or more components that are arbitrarily selected in this specification may be added to the above components of the present disclosure. Content described in this specification can be appropriately applied to not only the entire sliding system of the present disclosure but also to sliding members and a lubricating oil constituting the sliding system.

<<Lubricating Oil>>

[0031] As long as a lubricating oil according to an embodiment of the present disclosure contains a Mo-DTC (refer to FIG. 6), the type of base oil, the inclusion of other additives, and the like are not limited. When an amount of the Mo-DTC is too small, it is difficult for low friction characteristics to be exhibited, but if an amount of the Mo-DTC is excessive, there is no problem. However, because Mo is a type of rare metal, a smaller total amount of Mo contained is preferable. Therefore, regarding an amount of the Mo-DTC according to an embodiment of the present disclosure, a mass ratio of Mo of the Mo-DTC with respect to the entire lubricating oil is 50 ppm to 800 ppm, preferably 200 ppm to 700 ppm, and more preferably 300 ppm to 600 ppm. Here, when a mass ratio of Mo with respect to the entire lubricating oil is expressed as ppm, it is also appropriately expressed as ppmMo. When a Mo compound other than the Mo-DTC or the like is contained in the lubricating oil, an upper limit of a total amount of Mo is preferably 400 ppmMo and more preferably 300 ppmMo with respect to the entire lubricating oil.

[0032] The lubricating oil may contain a compound other than the Mo-DTC. For example, at least one of a phosphorus compound having a mass ratio of P that is preferably 200 ppm to 1,500 ppm and more preferably 400 ppm to 1,200 ppm with respect to the entire lubricating oil, a sulfur compound having a mass ratio of S that is preferably 500 ppm to 3,000 ppm or more preferably 1,000 ppm to 2,700 ppm with respect to the entire lubricating oil, and a nitrogen compound having a mass ratio of N that is preferably 200 ppm to 2,000 ppm and more preferably 500 ppm to 1,500 ppm with respect to the entire lubricating oil may be contained in the lubricating oil.

[0033] Examples of such a phosphorus compound include a molybdenum dialkyldithiophosphate (Mo-DTP), a zinc dialkyl dithiophosphate (Zn-DTP), a phosphate ester and amine salts thereof, a phosphite and amine salts thereof, and a thiophosphate ester. Examples of the sulfur compound include metal sulfonates other than Mo-DTCs, Mo-DTPs, and Zn-DTPs, metal phenates, metal salicylates, sulfide olefins, sulfides, sulfurized fats and oils, thiadiazoles, thiocarbamates, and thiocarbonates. Examples of the nitrogen compound include succinic acid imide, succinic acid ester, aliphatic amines, aromatic amines, thiadiazoles, triazoles, imidazoles, and thiocarbamates. Even in such a lubricating oil containing an (oil-soluble) compound other than a Mo-DTC, the Mo-DTC preferentially acts on a sliding surface (covered surface) covered with a chromium nitride film, and is thought to contribute to forming, for example, a molybdenum sulfide compound (such as MoS2) that can reduce a coefficient of friction.

<<Chromium Nitride Film>>

[0034] A film formation method of a chromium nitride film according to an embodiment of the present disclosure is not limited. A desired chromium nitride film may be efficiently formed by, for example, an arc ion plating (AIP) method, a sputtering (SP) method, and specifically, a physical vapor deposition (PVD) method such as an unbalanced magnetron sputtering (UBMS) method.

[0035] The AIP method is a method in which, for example, in a reaction gas (processing gas), a metal target (evaporation source) is arc discharged as a negative electrode, metal ions generated from the metal target react with reaction gas particles, and a dense coating is formed on a coated surface to which a bias voltage (negative pressure) is applied. In the present embodiment, for example, the target may be metal Cr, and the reaction gas may be N.sub.2 gas. In addition, in the case of a chromium nitride film containing Cr and a doping element other than N, a target or a reaction gas containing the doping element may be used. Furthermore, in addition to adjusting components of the target and the reaction gas, a composition, a structure, and the like of the chromium nitride film can be adjusted by adjusting a gas pressure of the reaction gas. For example, if a monolayer film made of CrN can be obtained by adjusting a pressure of N.sub.2 gas, it is possible to obtain a composite film made of CrN and Cr.sub.2N.

[0036] The SP method is a method in which a target is set as a negative electrode side, a coated surface is set as a positive electrode side, a voltage is applied, inert gas atomic ions generated by a glow discharge collide with a target surface, and released target particles (atoms and molecules) are deposited on the coated surface to form a coating. In the present embodiment, for example, sputtering is performed using metal Cr as a target and Ar gas as an inert gas, released Cr atoms (ions) react with N.sub.2 gas, and thereby a chromium nitride film can be formed on the sliding surface.

<<Applications>>

[0037] A type, a form, a sliding form, and the like of sliding members according to an embodiment of the present disclosure are not limited as long as they have sliding surfaces adapted to move relative to each other with a lubricating oil interposed therebetween. Specific forms and applications of a sliding system including such sliding members are not limited, and the sliding system can be widely applied to various machines and devices for which a reduction in sliding resistance and a reduction in mechanical loss due to sliding are required. For example, the sliding system of the embodiment of the present disclosure is suitably used for a drive system unit (such as an engine and a transmission) of an automobile and the like. Examples of the sliding member constituting such a sliding system include a cam, a valve lifter, a follower, a shim, a valve, and a valve guide constituting a valve system and a piston, a piston ring, a piston pin, a crankshaft, a gear, a rotor, and a rotor housing.

SUMMARY

[0038] A plurality of test components (sliding members) covered with a chromium nitride film and a plurality of lubricating oils with varying blending amounts of a Mo-DTC (oil-soluble molybdenum compound) were prepared, and a Block-on-Ring friction test was performed on the various combinations. The embodiment of the present disclosure will be described in more detail based on the test results.

Production of Samples

(1) Substrate

[0039] A plurality of block type (6.3 mm15.7 mm10.1 mm) substrates made of a quenched steel material (JISSUS440C) were prepared. Surfaces (coated surfaces) of the substrates were mirror-finished (surface roughness Ra: 0.08 m).

[0040] As a comparative sample (a sample C1 in Table 1) not covered with a chromium nitride film, a steel material (JISSCM420) that was simply carburized was prepared. The carburizing surface (hardness HV of 700) was also mirror-finished at a similar surface roughness.

(2) Film Formation of Chromium Nitride Film

[0041] Test components (samples 1 to 5) in which various chromium nitride films shown in Table 1 were formed on the surfaces of the substrates were prepared. The film formation of the chromium nitride film was performed by the arc ion plating (AIP) method or the sputtering (SP) method.

[0042] In the film formation according to the arc ion plating method, in N2 gas (reaction gas) adjusted to 0.3 Pa to 6 Pa, a target made of metal Cr was arc discharged. Film formation of a chromium nitride film containing O was performed using a gas mixture of N.sub.2 gas and O.sub.2 gas as a reaction gas. Here, a proportion of an amount of O in that case was 0.1 volume % with respect to the entire gas mixture. In addition, film formation of a chromium nitride film containing B was performed using a Cr-B alloy (Cr-5 mass % B) for a target.

[0043] Film formation according to the sputtering method was performed by sputtering a target made of metal Cr using Ar gas, and reacting released Cr atoms (ions) with N.sub.2 gas. In this case, N.sub.2 gas was 0.5 Pa to 6 Pa.

<<Measurement and Analysis of Chromium Nitride Film>>

[0044] (1) Film composition and film properties

[0045] Film compositions of the samples were quantified by EPMA (JXA-8200 commercially available from JEOL Ltd.). The film hardness was measured by a nanoindenter testing machine (TRIBOSCOPE commercially available from HYSITRON). The film thickness was specified from a wear mark obtained by Calotest commercially available from CMS. Table 1 shows the obtained film compositions and film properties of the samples. Here, the surface shape (roughness) according to this example was measured by a white interference method non-contact surface shape measuring machine (NewView5022 commercially available from Zygo).

(2) Structure of Film

[0046] The chromium nitride films of the samples were analyzed by X-ray diffraction. The profiles obtained accordingly are superimposed in FIG. 1A. In addition, FIG. 1B is an enlarged view of the profile according to the sample 3.

[0047] Based on the profile shown in FIGS. 1A and 1B, a relative area of the (111) plane with respect to the (200) plane was obtained according to the above-described method. The relative areas of the samples calculated in this manner are shown in Table 1. As can be understood from the profiles of the samples, in all of the samples, Cr was not detected and CrN was mainly detected. However, only in the sample 3, Cr.sub.2N was detected in addition to CrN. Table 1 also shows whether Cr.sub.2N was detected or not.

<<Lubricating Oil>>

[0048] An engine oil was assumed as a lubricating oil used for a friction test. A plurality of sample oils shown in Table 2 were prepared. In order to prepare the sample oil, additives shown in Table 1 were added to a base oil (hydrocarbon base oil/YUBASE8 commercially available from SK lubricants), and the mixture was then heated and stirred at 60 C. for 30 minutes. The additives used at that time were as follows. [0049] Mo-DTC: Adeka Sakura Lube 165 commercially available from ADEKA [0050] Zn-DTP: 1371 commercially available from Lubrizol (Secondary alkyl type/antiwear agent and antioxidant) [0051] Overbased calcium sulfonate: 6477C commercially available from Lubrizol (Base number: 300 mgKOH) [0052] Polybutenyl succinimide: 6412 commercially available from Lubrizol (Ashless dispersant)

[0053] Representative element contents contained in the sample oils are shown in Table 2. Contents of contained elements were obtained from data of contents of elements contained in additives and blending proportions of additives shown in Table 1.

<<Block-On-Ring Friction Test>>

[0054] (1) The test components and a sample oil D were combined and a Block-on-Ring friction test (simply referred to as a friction test) was performed. In the friction test, the test components were formed into block test pieces with a sliding surface width of 6.3 mm, and an S-10 standard test piece (a hardness HV of 800, a surface roughness of 1.7 to 2.0 mRzjis, commercially available from FALEX) made of a carburized steel material (AISI4620) was used as a ring test piece (an outer diameter of 35 mm and a width of 8.8 mm). In this case, the friction test was performed at a test load of 133 N (Hertz surface pressure: 210 MPa), a sliding speed of 0.3 m/s, and an oil temperature of 40 C. (constant) for 30 minutes. A average value for 1 minute immediately before the test ended was set as a coefficient of friction of each sliding surface in this test.

[0055] FIG. 2A shows a bar graph in which coefficients of friction of the test components are compared. In addition, the sliding surfaces of the test component after the friction test were measured by the above non-contact surface shape measuring machine, and the wear depth and surface roughness thereof were obtained. FIG. 2B shows a bar graph in which the wear depths are compared. FIG. 3 shows a relationship between the surface roughness and the coefficient of friction according to the test components. In addition, FIG. 4 shows a relationship between a coefficient of friction of the test components and a relative area of the (111) plane with respect to the (200) plane of the chromium nitride film of the test components.

[0056] (2) The test component of the sample 3 or C1 and one of the sample oils A to D were combined and the above friction test was performed in the same manner, and the coefficient of friction in each case was obtained. FIG. 5 shows a relationship between a Mo amount of the Mo-DTC to which each of the sample oils was added and the coefficient of friction of the test components.

<<Evaluation>>

(1) Friction Characteristics

[0057] As can be clearly understood from FIG. 2A, it was found that, when a lubricating oil containing a Mo-DTC (Mo amount: 500 ppm) was used, despite the low oil temperature of 40 C., the coefficient of friction of only the sample 3 was lower than those of the other samples. In addition, as can be clearly understood from FIG. 2B, it was confirmed that the wear depth of the sample 3 was much smaller than that of the sample C1, and the sample 3 exhibited excellent wear resistance.

(2) Surface Roughness

[0058] As can be clearly understood from FIG. 3, there was no particular correlation between the surface roughness and the coefficient of friction of the sliding surface. This can be clearly understood when comparing the sample 1 and the sample 3 whose coefficients of friction were greatly different despite the fact that the surface roughnesses of the sliding surfaces were substantially the same. Thus, low friction characteristics of the sample 3 were not thought to be caused by smoothing of the sliding surface.

(3) Structure of Chromium Nitride Film

[0059] As can be clearly understood from FIG. 4, it was found that, unlike the other samples, in the sample 3 exhibiting low friction characteristics, a relative area of the (111) plane with respect to the (200) plane of the chromium nitride film was within a limited specific range. That is, it was found that low friction characteristics were exhibited more in the chromium nitride film in which the (111) plane was mixed to some extent (20% to 30%) than in the chromium nitride film in which the orientation of the (200) plane was strong. The reason for this is inferred to have been the fact that, in the chromium nitride film having a relative area within a certain range, a sulfur compound of Mo was more likely to be formed on the film due to competitive adsorption of the Mo-DTC and other oil additives.

(4) Mo-DTC Amount (Mo Equivalence)

[0060] As can be clearly understood from FIG. 5, when an amount of the Mo-DTC was too small, the coefficient of friction of the sample 3 was not much different from the coefficient of friction of the sample C1. However, it was found that, when an amount of the Mo in Mo-DTC was 50 ppm or more or preferably 200 ppm or more, the sample 3 having a specific chromium nitride film clearly exhibited low friction characteristics unlike the sample C1.

(5) Consideration

[0061] Based on the above results, it can be clearly understood that, when a specific structure chromium nitride film having a relative area of the (111) plane with respect to the (200) plane within a predetermined range and a lubricating oil containing a predetermined amount or more of the Mo-DTC were combined, low friction characteristics were exhibited also in a low temperature range. The reason for this is inferred to have been that, only in the case in which the sliding surface was formed of a specific chromium nitride film, the Mo-DTC adsorbed on or reacted with the sliding surface, and a molybdenum sulfide compound (for example, MoS.sub.2) exhibiting low shear characteristics was formed. Therefore, this was thought to reduce the boundary friction coefficient when the molybdenum sulfide compound was directly in contact with the sliding surface and thus the coefficient of friction of the entire macro contact part was reduced.

TABLE-US-00001 TABLE 1 Film structure (X-ray diffraction results) Film properties Relative Film composition Film Film area Sample Production (atom %) hardness thickness (111)/(200) No. Name method Cr N O B (GPa) (m) (%) Cr.sub.2N CrN Cr Note 1 B SP method 62.4 37.6 24.1 2 3.4 B 2 T AIP 50.7 49.3 16.9 13 34 A 3 N method 60.9 39.1 21.5 8 24.7 A A 4 T-O 49.7 40.5 9.8 22.4 13 28.5 A 5 N-B 48.7 50.0 1.3 21.0 9 75.8 A C1 (Carburized (HV SCM material) 700) 420 , , and shown in X-ray diffraction results are as follows. : there was no substantial diffraction peak. A: there was a strong diffraction peak B: there was a weak diffraction peak

TABLE-US-00002 TABLE 2 Blending amount of additives Name (the balance: base oil) (mass %) of Overbased sample calcium Polybutenyl Contents of contained elements (ppm) oil Mo-DTC Zn-DTP sulfonate succinimide Mo S P Zn Ca N A 0 0.8 2.0 6.0 0 2,036 800 884 2,400 900 B 0.06 25 2,064 800 884 2,400 925 C 0.17 100 2,147 800 884 2,400 1,000 D 0.56 500 2,591 800 884 2,400 1,400

[0062] Between the pair of sliding members, a sliding surface of one sliding member may be the sliding surface of the embodiment, or both sliding surfaces may be the sliding surface of the embodiment. When the sliding surface of one sliding member was used as the sliding surface of the embodiment, a sliding member having the same configuration except that the chromium nitride film of the embodiment is not provided may be used as the other sliding member.