Patent Application
20190100714
The disclosure of Japanese Patent Application No. 2017-193626 filed on Oct. 3, 2017 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
The present disclosure relates to a sliding system that can reduce a coefficient of friction applied between sliding surfaces according to a new combination of a sliding surface and a lubricating oil.
Many machines include sliding members that configured to move relative to each other while in sliding contact. In a system including such sliding members (referred to as a “sliding system” in this specification, for example, a sliding machine), when a resistance force (sliding resistance) applied to the sliding part is reduced, it is possible to improve performance and reduce an energy required for operation. Such a reduction in sliding resistance is generally achieved by reducing a coefficient of friction acting between sliding surfaces.
The coefficient of friction acting between the sliding surfaces differs according to surface states of the sliding surfaces and a lubrication state between the sliding surfaces. Thus, in order to reduce the coefficient of friction, surface modification of sliding surfaces and improvement of a lubricant (lubricating oil) supplied between sliding surfaces have been studied. Details related thereto are described in, for example, the following patent documents.
In Japanese Patent No. 6114730, a sliding surface covered with a chromium nitride film and a lubricating oil containing an oil-soluble molybdenum compound formed of a Mo trinuclear body are combined, and thus both low friction characteristics and high wear resistance are achieved. However, in Japanese Patent No. 6114730, since a general engine oil is assumed as the lubricating oil, specific friction characteristics only for when the oil temperature is 80° C. are described in Japanese Patent No. 6114730.
The present disclosure provides a sliding system that can reduce a coefficient of friction according to a new combination of a siding coating and a lubricating oil that were previously unknown.
The inventors conducted extensive studies in order to address the above problems, and found that a coefficient of friction between sliding surfaces can be reduced according to a new combination of a specific chromium nitride film and a lubricating oil containing a molybdenum dialkyldithiocarbamate (Mo-DTC). Moreover, they found that both low friction characteristics and high wear resistance can be achieved. This result was expanded to complete the present disclosure to be described below.
(1) A sliding system according to an aspect of the present disclosure is 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. 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 (simply referred to as a “Mo-DTC”) which is an oil-soluble molybdenum compound. A mass ratio of Mo of the Mo-DTC with respect to the entire lubricating oil is 50 ppm to 800 ppm.
(2) A sliding system in which a coefficient of friction between sliding surfaces is reduced is obtained according to a combination of sliding surfaces covered with a specific chromium nitride film and a lubricating oil containing a Mo-DTC. According to the sliding system of the aspect of the present disclosure, it is possible to reduce sliding resistance and friction loss, improve movement performance of various machines, and save energy.
In addition, the chromium nitride film according to the aspect of the present disclosure can exhibit low friction characteristics and also excellent wear resistance. Therefore, the sliding system is suitable for a drive system machine that is operated for a long time under severe conditions from a boundary lubrication (friction) condition to a mixed lubrication (friction) condition.
For example, in the sliding system, even if the temperature (simply referred to as an “oil temperature”) of the lubricating oil is 60° C. or lower or 50° C. or lower, a coefficient of friction between the sliding surfaces may be 0.06 or less or 0.055 or less. In addition, a sliding surface formed of the chromium nitride film has a wear depth, which is an index of wear resistance, of ¼ or less or ⅙ or less, compared to a sliding surface made of a steel material of the related art. Thus, the sliding system of the aspect of the present disclosure is suitable for a hybrid vehicle internal combustion engine or a drive transmission device (such as a transmission and a differential gear) in which the oil temperature rises gently.
(3) A mechanism through which a combination of a specific chromium nitride film and a lubricating oil containing a Mo-DTC according to the aspect of the present disclosure exhibits low friction characteristics and the like is not clearly known, but it is currently thought as follows.
When the sliding system (specifically, a sliding machine) according to the aspect of the present disclosure is activated, on the sliding surface formed of a chromium nitride film, an adsorption reaction of a Mo-DTC contained in the lubricating oil is promoted. Accordingly, other additives which are in a competitive adsorption relationship with the Mo-DTC or its constituent elements (for example, Mo) are easily adsorbed on the sliding surface. As a result, it is thought that a relatively large amount of a molybdenum sulfide compound having a MoS2 structure is (thickly) adsorbed on the covered surface (sliding surface) formed of a chromium nitride film. The molybdenum sulfide compound having a MoS2 structure has a layered structure, and exhibits low shear characteristics. Therefore, it is thought that, even in a wide range of operation situations including boundary friction, the coefficient of friction on the sliding surface formed of a chromium nitride film is reduced.
Here, the chromium nitride film according to the aspect of the present disclosure is generally harder than a substrate (for example, a steel material) of the sliding member and is not easily transferred to the sliding surface on the counterpart sliding side. Therefore, according to the sliding system of the present disclosure, in the presence of the above lubricating oil, high wear resistance is also exhibited and excellent low friction characteristics are stably exhibited for a long time.
(4) Other than (inevitable) impurities, the chromium nitride film according to the aspect of the present disclosure may include Cr and N. In the chromium nitride film, Cr and N are mainly present as CrN, but some thereof may be Cr2N (dichromium nitride) or the like. Based on this, in the chromium nitride film according to the aspect of the present disclosure, when an amount of atoms constituting the entire chromium nitride film is set as 100 atom %, preferably, there is 32 atom % to 47 atom % of N, and the balance is Cr. In addition, N may be 35 atom % to 44 atom %. Here, a small amount (for example, 1 atom % or less in total) of a doping element (for example, O, B) may be contained in the chromium nitride film as long as low friction characteristics of the chromium nitride film are not inhibited or the amount is within a range in which the low friction characteristics are improved. Here, the film composition in this specification is specified by an electron beam microanalyzer (EPMA).
In addition, the chromium nitride film according to the aspect of the present disclosure easily exhibits low friction characteristics when it has a specific crystal structure. That is, in the chromium nitride film, a relative area which is an area ratio of the (111) plane with respect to the (200) plane obtained when the film is analyzed by X-ray diffraction is 20% to 30%. In addition, the relative area may be 22% to 28%, or the relative area may be 23% to 27%. Here, the area ratio (relative area) of each surface in this specification is calculated by image analysis based on profiles obtained by X-ray diffraction.
(5) Another aspect of the present disclosure relates to 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. 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. A mass ratio of Mo of the molybdenum dialkyldithiocarbamate with respect to the entire lubricating oil is 50 ppm to 800 ppm.
(1) The “sliding system” in the present disclosure is sufficient as long as it includes a sliding member and a lubricating oil, and is not limited to a complete body as a machine, but includes a combination of mechanical elements constituting a part thereof. The sliding system of the present disclosure may be appropriately referred to as a sliding structure, a sliding machine (for example, an engine and a transmission) or the like.
The covered surface formed of a chromium nitride film according to the present disclosure may be formed on at least one of sliding surfaces of opposing sliding members configured to move relative to each other. Of course, more preferably, both opposing sliding surfaces are covered surfaces formed of a chromium nitride film.
(2) Unless otherwise specified, “x to y” in this specification includes a lower limit x and an upper limit y. A range such as “a to b” may be newly set with various numerical values described in this specification or arbitrary numerical values included in a numerical value range as a new lower limit or upper limit.
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:
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.
As long as a lubricating oil according to an embodiment of the present disclosure contains a Mo-DTC (refer to
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.
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.
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.
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 N2 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 N2 gas, it is possible to obtain a composite film made of CrN and Cr2N.
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 N2 gas, and thereby a chromium nitride film can be formed on the sliding surface.
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.
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.
A plurality of block type (6.3 mm×15.7 mm×10.1 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 μm).
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.
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.
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 N2 gas and O2 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.
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 N2 gas. In this case, N2 gas was 0.5 Pa to 6 Pa.
(1) Film composition and film properties
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).
The chromium nitride films of the samples were analyzed by X-ray diffraction. The profiles obtained accordingly are superimposed in
Based on the profile shown in
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.
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.
(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 μmRzjis, commercially available from FALEX) made of a carburized steel material (AISI4620) was used as a ring test piece (an outer diameter of φ 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 μ average value for 1 minute immediately before the test ended was set as a coefficient of friction of each sliding surface in this test.
(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.
As can be clearly understood from
As can be clearly understood from
As can be clearly understood from
As can be clearly understood from
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, MoS2) 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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-193626 | Oct 2017 | JP | national |
