Sliding system

Abstract

A sliding system includes a pair of sliding members having sliding surfaces that can relatively move while facing each other and a lubricant oil interposed between the sliding surfaces facing each other. At least one of the sliding surfaces includes a coating surface of a crystalline Cr plating film. The lubricant oil contains an oil-soluble molybdenum compound comprising a trinuclear Mo structure. In particular, considerably low friction properties can be developed by a combination of the Cr plating film, in which at least one of three types of peak area intensity ratios (P1 to P3) as obtained by X-ray diffraction falls within a predetermined range (P10.015, P20.02, P30.03), and the lubrication oil which contains the trinuclear Mo structure.

Claims

1. A sliding system comprising: a pair of sliding members having sliding surfaces that can relatively move while facing each other; and a lubricant oil interposed between the sliding surfaces facing each other, wherein the lubricant oil comprises an oil-soluble molybdenum compound that has a trinuclear Mo chemical structure, wherein at least one of the sliding surfaces comprises a coating surface of a crystalline chromium plating film, wherein a peak area I(hkl) of a crystal plane (hkl) of the chromium plating film as obtained by X-ray diffraction using an X-ray of Cu-K and 2 of 30-150 satisfies at least one of expressions below:
P1={I(110)+I(211)}/I(222)0.015;
P2={I(110)+I(310)}/I(222)0.02; and
P3={I(110)+I(211)+I(310)}/I(222)0.03.

2. The sliding system as recited in claim 1, wherein the oil- soluble molybdenum compound includes a molecular structural skeleton comprised of at least one of Mo.sub.3S.sub.7 or Mo.sub.3S.sub.8.

3. The sliding system as recited in claim 1, wherein the lubricant oil contains the oil-soluble molybdenum compound with a mass ratio of Mo of 25-900 ppm with respect to the lubricant oil as a whole.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph for comparing XRD profiles of Cr plating films of samples.

(2) FIG. 2A is a bar graph for comparing friction coefficients of the Cr plating films.

(3) FIG. 2B is a bar graph for comparing changes of friction coefficients due to presence or absence of Mo-trinuclear in the lubricant oil.

(4) FIG. 3A is a graph illustrating the relationship between a first peak area intensity ratio (P1) and the friction coefficient of the Cr plating films.

(5) FIG. 3B is a graph illustrating the relationship between a second peak area intensity ratio (P2) and the friction coefficient of the Cr plating films.

(6) FIG. 3C is a graph illustrating the relationship between a third peak area intensity ratio (P3) and the friction coefficient of the Cr plating films.

(7) FIG. 4A is a graph illustrating the relationship between a Mo/Cr peak intensity ratio obtained using TOF-SIMS and the friction coefficient.

(8) FIG. 4B is a graph illustrating the relationship between a Ca/Cr peak intensity ratio obtained using TOF-SIMS and the friction coefficient.

(9) FIG. 5 is a molecular structure diagram illustrating an example of Mo-trinuclear.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(10) One or more features freely selected from the present description can be added to the above-described features of the present invention. The contents described in the present description can be applied not only to the sliding system as a whole according to the present invention but also to sliding members and lubricant oil which constitute the sliding system. Features regarding a method can also be features regarding a product. Which embodiment is the best or not is different in accordance with objectives, required performance, and other factors.

(11) Cr Plating Film

(12) (1) The Cr plating film according to the present invention is crystalline and has a specific crystal structure in which at least one of the above-described peak area intensity ratios (P1 to P3) as obtained by X-ray diffraction (XRD) analysis is within a predetermined range. It suffices that at least one of P1 to P3 of the Cr plating film according to the present invention is within a desired range, but it is more preferred that two or more of P1 to P3 be within respective desired ranges.

(13) The first peak area intensity ratio (P1) may be 0.015 or more in an embodiment, 0.018 or more in another embodiment, and 0.04 or more in still another embodiment. The second peak area intensity ratio (P2) may be 0.02 or more in an embodiment, 0.023 or more in another embodiment, and 0.025 or more in still another embodiment. The third peak area intensity ratio (P3) may be 0.03 or more in an embodiment, 0.037 or more in another embodiment, and 0.05 or more in still another embodiment. In a Cr plating film in which all the peak area intensity ratios (also referred to as area ratios in a simple term) are unduly small, the reduction in friction coefficient is insufficient under a lubricant oil that contains Mo-trinuclear. The upper limit of each area ratio is not limited, but the strongest peak area I(222) is considerably large, so suffice it to say that the upper limit may be 0.3 in an embodiment, 0.2 in another embodiment, and 0.1 in still another embodiment.

(14) Combinations other than the above can also be considered as crystal planes for selecting the area ratio. For example, I(110)/I(222) and I(211)/I(222) also appear to have a correlation similar to the area ratios P1 to P3 with regard to the change in friction coefficient. According to the research studies made by the present inventors, however, it has been revealed that the area ratios P1 to P3 selected in the present invention exhibit the most definite correlation with the friction coefficient and the dispersion among respective data is the smallest. In addition, no clear correlation has been found between I(200)/I(222) and I(310)/I(222) and the friction coefficient.

(15) (2) The Cr plating film according to the present invention may be formed using a dry process such as PVD and CVD, but may be particularly preferably formed using a wet process. The plating bath to be used in the wet process can be a Sargent bath that contains chromic acid and sulfuric acid as main components, a fluoride bath that contains chromic acid and hydrofluosilicic acid as main components, or other appropriate bath. The Sargent bath preferably contains, for example, 100-400 g/L of chromic anhydride and 1-4 g/L of sulfuric acid. The fluoride bath preferably contains, for example, 230-250 g/L of chromic anhydride, 0.7-1.5 g/L of sulfuric acid, and 2-5 g/L of hydrofluosilicic acid. The (electric) plating bath may contain an activator such as organic sulfonic acid. The plating conditions are preferably, for example, a bath temperature of 40-60 C. and a current density of 15-60 A/dm.sup.2. The thickness of the Cr plating film is preferably 3-30 m in an embodiment and 5-15 m in another embodiment. The hardness Hv of the Cr plating film is preferably 500 to 11,000 in an embodiment and 600 to 900 in another embodiment.

(16) Lubricant Oil

(17) The lubricant oil according to the present invention is not limited in the type of a base oil and may further contain additives other than the Mo-trinuclear, provided that the lubricant oil contains the Mo-trinuclear and the other additives do not interfere with the reduced friction. In general, lubricant oil such as engine oil contains various additives including S, P, Zn, Ca, Mg, Na, Ba, Cu, etc. It can be considered that, even in such lubricant oil, the Mo-trinuclear according to the present invention preferentially acts on the sliding surface (coating surface) coated with the Cr plating film and generates a molybdenum sulfide compound (e.g. MoS.sub.2, Mo.sub.3S.sub.7, Mo.sub.3S.sub.8, Mo.sub.2S.sub.6) which can reduce the friction coefficient, thereby contributing to the reduced friction. The lubricant oil according to the present invention may contain other Mo-based compounds (such as MoDTC) than the Mo-trinuclear, but the total amount of the contained Mo may preferably be small because Mo is a kind of rare metal.

(18) While an unduly large amount of the Mo-trinuclear may not cause any problem, a very small amount of the Mo-trinuclear may be enough for reducing the friction. For example, the mass ratio of Mo to the total lubricant oil is preferably 25-900 ppm in an embodiment, 50-800 ppm in another embodiment, 60-500 ppm in still another embodiment, and 70-200 ppm in a further embodiment. When the mass ratio of Mo to the total lubricant oil is represented in ppm, it will be denoted by ppmMo. When the lubricant oil contains Mo-based compounds and the like other than the Mo-trinuclear, the upper limit of the total amount of Mo is preferably 1,000 ppmMo in an embodiment and 400 ppmMo in another embodiment to the total lubricant oil.

(19) Use Application

(20) The present invention is not limited in the use and the like. Examples of the sliding system include engine units and drive system units (such as transmission) for vehicles such as cars. Examples of the sliding members that constitute the sliding system include components, such as a cam, valve lifter (e.g., the sliding surface is a contacting surface with a cam), follower, shim, valve and valve guide, which constitute a dynamic valve system; piston (e.g., the sliding surface is a piston skirt); piston ring; piston pin; crankshaft; gear; rotor; rotor housing; valve; valve guide; and pump.

EXAMPLES

(21) Surfaces of base materials were coated with Cr plating films to produce a plurality of samples. The friction coefficient when each Cr plating film was used as the sliding surface was measured under lubricant oil. The present invention will be more specifically described in reference to such examples.

(22) Test Piece

(23) (1) Base Material

(24) A plurality of block-like base materials (6.3 mm15.7 mm10.1 mm) was prepared, each comprising stainless steel (JIS SUS440C). The mirror-finished surface of each base material (surface roughness: Ra 0.08 m) was used for the coating surface.

(25) (2) Film Formation

(26) A Cr plating film was formed by wet electroplating to provide the coating surface of each base material. A Sargent bath containing chromic acid and sulfuric acid as main components was used as the plating bath. This Sargent bath contains about 250 g/L of chromic anhydride and about 2.5 g/L of sulfuric acid.

(27) The plating process was carried out at a bath temperature of about 50 C. and a current density of 15-60 A/dm.sup.2. At that time, the current density was changed to form the Cr plating films having different crystal structures on the surfaces of the base materials. Test pieces were thus obtained, having the Cr plating films as the sliding surfaces (Samples 1 to 6).

(28) All the Cr plating films were not heat-treated. The film thickness was about 5 m and the hardness was about 850 Hv. The film thickness of each Cr plating film was obtained by measuring the dimensional change before and after the film formation using a micrometer. The composition of the Cr plating film was, for example, O: 0.4 mass %, H: 0.05 mass %, and the balance: Cr.

(29) (3) Comparative Sample

(30) A test piece was prepared as a comparative sample (Sample C0) by mirror-finishing the carburized surface of a steel material (JIS SCM420) (surface roughness: Ra 0.08 hardness HV 600) to provide the sliding surface.

(31) Lubricant Oil

(32) Engine oil (motor oil SN 0W-20 available from TOYOTA MOTOR CORPORATION) having a viscosity grade of 0W-20 and corresponding to ILSAC GF-5 standard was prepared as the lubricant oil to be used for a friction test. This engine oil (simply referred to as standard oil) is free from molybdenum dithiocarbamate (MoDTC) and molybdenum dithiophosphate (MoDTP).

(33) Mo-trinuclear denoted as Trinuclear in the disclosed documentation Molybdenum Additive Technology for Engine Oil Applications available from Infineum International Limited was added to the standard oil so that the Mo content in the oil as a whole would be 80 ppmMo equivalent. This oil will be referred to as Mo-trinuclear-containing oil.

(34) For reference to the standard oil, oil free from MoDTC and MoDTP but containing 130 ppmMo of a Mo-based antioxidant (referred to as a Mo-trinuclear-free oil) was prepared.

(35) Measurement of Cr Plating Film

(36) The Cr plating film of each sample was analyzed using an X-ray diffractometer (available from Rigaku Corporation). Used X-ray was Cu-K line and 2 was 30-150. Profiles thus obtained were illustrated in FIG. 1 in an overlapping manner. On the basis of the profiles illustrated in FIG. 1, area ratios (P1 to P3) with respect to the strongest peak area I(222) were obtained, which are listed in Table 1. The peak area of each crystal plane was obtained using XRD analysis software (JADES) available from Materials Data, Inc.

(37) Friction Test

(38) Block-on-ring friction test (simply referred to as a friction test) was carried out for a combination of each test piece and each oil to measure the friction coefficient () of each sliding surface. The friction test was performed by sliding a block-like test piece of each sample (sliding surface width: 6.3 mm) and a ring-like standard test piece of a carburized steel material (AISI4620) (S-10 available from FALEX CORPORATION, hardness: HV800, surface roughness Rzjis: 1.7-2.0 m, outer diameter 35 mmwidth 8.8 mm) on each other under the presence of each oil. The sliding surface of test piece of each sample was preliminarily polished using #2000 emery paper before the test to have surface roughness Ra of 0.01-0.04 m.Test conditions were a test load of 133 N (Hertz contact pressure: 210 MPa), a sliding speed of 0.3 m/s, an oil temperature of 80 C. (fixed), and a test time of 30 minutes.

(39) The average value of the friction coefficient () measured for one minute immediately before completion of the friction test was employed as the friction coefficient in this test. The friction coefficient thus obtained of each sample is listed in Table 1 and illustrated as a bar graph for comparison in FIG. 2A and FIG. 2B (these figures will be collectively referred to as FIG. 2 in a simple term).

(40) Surface Analysis

(41) Sliding surfaces of the test pieces of Samples 4-6 were analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS/a TOF-SIMS apparatus available from Ion-Tof) after the friction test. At that time, high resolution spectrum measurement was performed for a region of 100 m100 m using a Bi.sup.+ beam of 30 keV as the primary ions. Mo/Cr peak intensity ratios and Ca/Cr peak intensity ratios were calculated on the basis of the obtained results. Relationships between these peak intensity ratios and the friction coefficients of respective samples are illustrated in FIG. 4A and FIG. 4B (these figures will be collectively referred to as FIG. 4 in a simple term).

(42) Evaluation

(43) (1) Friction Coefficient

(44) As apparent from Table 1 and FIG. 2, under the Mo-trinuclear-containing oil, the friction coefficient of Sample C0, in which the sliding surface is not coated with a Cr plating film, is higher than 0.1 while the friction coefficient of each of Samples 1-6, in which the sliding surfaces are coated with Cr plating films, is specifically lower than that of Sample C0. In particular, the friction coefficient of each of Samples 3-6 is lower than 0.04. It has thus been found that the Cr plating film selectively develops considerably low friction properties.

(45) As apparent from FIG. 2B, under the Mo-trinuclear-free oil, the friction coefficient of Sample C0, in which the sliding surface is not coated with a Cr plating film, is comparable with that of Sample 4, in which the sliding surface is coated with the Cr plating film.

(46) It has also been found that, under the Mo-trinuclear-containing oil, the friction coefficient of Sample 4 is considerably reduced even though the friction coefficient of Sample C0 increases. In other words, it has been revealed that Sample C0 and Sample 4 exhibit opposite sliding properties under the Mo-trinuclear-containing oil.

(47) (2) Crystal Structure of Cr Plating Film

(48) As apparent from the XRD profiles illustrated in FIG. 1, the Cr plating films of Samples 1-6 are all crystalline. However, as listed in Table 1 and illustrated in FIG. 3A to FIG. 3C (these figures will be collectively referred to as FIG. 3 in a simple term), the area ratios of Cr plating films of the samples are different.

(49) As apparent from FIG. 3, in all of the Cr plating films of Samples 3-6 which develop low friction under the Mo-trinuclear-containing oil, the area ratios P1 to P3 fall within the ranges as defined by the present invention.

(50) Consideration

(51) The reason that the combination of the specific Cr plating film and the Mo-trinuclear-containing oil can develop low friction is estimated as below.

(52) As found from Table 1 and FIG. 3, it can be considered that the interaction between the crystal structure of the Cr plating film, in particular, its crystalline orientation property and the Mo-trinuclear develops the considerably low friction properties.

(53) As found from FIG. 4A, it is found that the friction coefficient decreases as the Mo/Cr peak intensity ratio increases. From this fact, it can be estimated that, originating from the Mo-trinuclear, a layered structural body (boundary film) similar to MoS.sub.2 and the like is generated on the sliding surface and its excellent low shear property allows the development of the considerably low friction.

(54) As found from FIG. 4B, it is found that the friction coefficient increases as the Ca/Cr peak intensity ratio increases. In other words, Ca is considered to interfere with the reduced friction of the sliding surface comprising the Cr plating film. From this, it can be considered that the Cr plating film according to the present invention preferentially adsorbs or react with Mo rather than Ca under the Mo-trinuclear-containing oil to develop the low friction. Ca is considered to be a component originated from a cleaning agent, which is generally added to engine oil.

(55) TABLE-US-00001 TABLE 1 Friction coefficient Peak area intensity Containing Free from Sample Sliding ratio by XRD Mo- Mo- No. surface P1 P2 P3 trinuclear trinuclear 1 Cr 0.005 0.001 0.013 0.077 2 Plating 0.013 0.016 0.021 0.052 3 film 0.045 0.029 0.059 0.037 4 0.060 0.043 0.081 0.037 0.085 5 0.019 0.025 0.038 0.033 6 0.047 0.024 0.054 0.026 C0 SCM420 0.104 0.086 carburized material P1 = {I(110) + I(211)}/I(222) P2 = {I(110) + I(310)}/I(222) P3 = {I(110) + I(211) + I(310)}/I(222)