JP2018135943A - Sliding system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slide system that can enhance friction reduction effect of a friction modifier added to lubricant.SOLUTION: A slide system includes: lubricant containing MoDTC as an additive agent and of which base oil is constituted of full synthetic oil free from mineral oil; and a first slide member and a second slide member sliding on each other in the lubricant. The first slide member has an uppermost layer part formed of a steel material containing 13% or more of Cr and including at least a slide surface.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a sliding system.

  In recent years, as oil used in automobile engines and the like, lower viscosity lubricating oil has been demanded in order to reduce viscous resistance in a fluid lubrication region. For example, if a general-purpose engine oil with a viscosity at 80 ° C. of 7.5 mPa · s is changed to a lubricating oil with a viscosity at 80 ° C. of 1.5 mPa · s, the 10.15 mode fuel efficiency is improved by 3.8%. (For example, refer nonpatent literature 1).

  However, the lubricating oil having a low viscosity has a reduced friction in the fluid lubrication region, but is easily discharged from the friction surface under high load conditions. For this reason, it has been pointed out that friction is increased by contact between surfaces under frictional conditions that become a boundary lubrication region, such as when operating at the top dead center of an engine piston or at a low speed (for example, non-patent literature). 2). Furthermore, under such friction conditions, the seizure load is reduced, and there is a possibility that seizure between metals or acceleration of wear may occur, which is a problem.

Conventionally, one of the most frequently used lubricating oil additives is a friction modifier, MoDTC (molybdenum dithiocarbamate). About this MoDTC, the mechanism of the reaction process is estimated from the difference in the effect by the combination of sliding conditions and the additive of lubricating oil (for example, refer nonpatent literature 3). Moreover, as a mechanism for obtaining the friction reducing effect of MoDTC, a reaction in which MoDTC is decomposed during sliding to form low friction MoS ₂ (molybdenum disulfide) has been estimated (for example, see Non-Patent Document 4). ). Furthermore, when the sliding members are different, the mechanism is not specified, but it is empirically known that the low friction effect of MoDTC is different (for example, see Non-Patent Document 5).

Okuyama, Y., et.al., “Study of Low-Viscosity Engine Oil on Fuel Economy and Engine Reliability”, SAE International, 2011.04.12, No. 2011-01-1247 Ushioda, N., et al., “Effect of Low Viscosity Passenger Car Motor Oils on Fuel Economy Engine Tests”, SAE International, 2013.10.14, No. 2013-01-2606 A. Morina and A. Neville, “Tribofilms: aspects of formation, stability and removal”, Journal of Physics D: Applied Physics, 2007, Vol. 40, No. 18, p.5476-5487 M. I. De Barros Bouchet et al., “Mechanisms of MoS2 formation by MoDTC in presence of ZnDTP: effect of oxidative degradation”, Wear, June 2005, Vol. 258, p.1643-1650 Gondo, Yamamoto, “Effects of surface film formation mechanism and friction surface material by molybdenum dithiocarbamate (MoDTC)”, tribologist, 1991, 36, 3, p.242-248

  As described in Non-Patent Document 2, the lower the lubricating oil, the higher the friction in the boundary lubrication region such as top dead center and low rotation, so that the sliding that can maintain low friction in the boundary lubrication region In order to develop the mechanism, it is necessary to first elucidate the mechanism by which additives in lubricating oil form a low-friction reaction film. As described in Non-Patent Document 3, the mechanism of the reaction process of the additive has been estimated from the difference in effect due to the sliding condition and the combination of additives, but the combination of the sliding member and additive, that is, the addition There has been no study on a sliding member that makes use of the friction reducing effect of the agent.

  Then, an object of this invention is to provide the sliding system which can heighten the friction reduction effect of the friction modifier added to lubricating oil.

  The present inventors pay attention to how reaction films formed on the friction surfaces of various sliding members are formed, based on the knowledge that sliding characteristics in lubricating oil containing friction modifiers differ depending on the type of sliding member. The present inventors have found that a sliding member in which a Cr passivation film is formed on the surface layer easily produces low-friction molybdenum disulfide, and has led to the present invention.

  That is, the sliding system according to the present invention includes a lubricating oil that includes MoDTC as an additive and whose base oil does not include mineral oil, and a first sliding that slides on each other in the lubricating oil. The first sliding member is characterized in that at least the surface layer portion including the sliding surface is made of a steel material containing 13% or more of Cr.

The sliding system according to the present invention is presumed to form a stable reaction film including a molybdenum disulfide (MoS ₂ ) layer by the interaction between the Fe-based material including Cr and MoDTC by the reaction shown in FIG. The That is, as shown in FIG. 1A, in the lubricating oil containing the friction modifier MoDTC as an additive, the first sliding member (Fe / Cr Substrate) made of Fe-based material containing Cr. When the second sliding member is slid, the surface of the first sliding member is exposed at the initial stage of sliding. Further, as described in Non-Patent Document 4, MoDTC is decomposed (MoDTC resolution) at the time of sliding, and MoS ₂ and intermediate product MoS _2-x O _x are generated. At this time, as shown in FIG. 1B, MoS _2-x O _x is reduced by Cr distributed on the exposed surface of the first sliding member, and the surface of the first sliding member is reduced. Cr oxide is produced. Further, by Cr to reduce _{MoS 2-x O x,} generation of MoS ₂ layer is accelerated. Further, it is estimated that the formed passive film prevents oxidation of Fe in the first sliding member and forms a stable reaction film including the MoS ₂ layer.

Thus, the sliding system according to the present invention can form a stable reaction film with low friction, and can enhance the friction reduction effect of the friction modifier of the additive. Friction can be reduced by using a steel material in which at least the surface layer portion including the sliding surface includes 13% or more of Cr as the first sliding member. This is due to the fact that, by containing a large amount of Cr, a low friction MoS ₂ layer is easily formed in the reaction film, the reaction film is formed thin, and the MoS ₂ layer is easily oriented. As a result, it is considered that the friction lowers faster and the friction coefficient can be further reduced.

In the sliding system according to the present invention, the lubricating oil of the base oil does not contain mineral oil. Generally, since mineral oil contains various impurities including metals, it is more susceptible to oxidative degradation than synthetic oil. For this reason, if the lubricating oil contains mineral oil, the impurities in the mineral oil are oxidized, thereby inhibiting the oxidation of Cr on the metal surface and possibly suppressing the generation of the MoS ₂ layer. Conceivable. Moreover, since impurities in the mineral oil are adsorbed on the sliding surface and inhibit the adsorption of MoDTC, the generation of the MoS ₂ layer is also inhibited. From this, the sliding system according to the present invention can efficiently form a stable reaction film including the MoS ₂ layer because the lubricating oil of the base oil does not contain mineral oil.

  In the sliding system according to the present invention, the first sliding member may be a steel material in which at least the surface layer portion including the sliding surface contains 13% or more of Cr, and the portion other than the surface layer portion has a Cr content. It may be 13% or more or less than 13%. Further, the portion other than the surface layer portion may be a steel material or a material other than the steel material. Examples of the steel material containing 13% or more of Cr include martensitic stainless steel, austenitic stainless steel, and chrome plating.

  In the sliding system according to the present invention, the lubricating oil preferably has a base oil made of polyalphaolefin (PAO). In this case, the friction reduction effect of MoDTC can be further enhanced as compared with a mixture of PAO and another base oil.

  In the sliding system according to the present invention, it is preferable that the second sliding member is also made of a steel material. The first sliding member and the second sliding member may be the same type of steel material or may be different. Examples of the steel material include bearing steel and stainless steel.

  The sliding system according to the present invention is preferably configured such that Cr sliding is formed on the sliding surface when sliding in the lubricating oil. In this case, it is preferable that the surface has a large amount of Cr exposed or the surface layer contains a large amount of Cr. Moreover, you may have Cr oxide in the sliding surface previously. As a result, the active Cr exposed on the sliding surface can reduce MoDTC, form a stable reaction film with low friction, and enhance the friction reducing effect of the friction modifier.

In the sliding system according to the present invention, the first friction member and the second sliding member are slid in the lubricating oil to form a low friction formed on the sliding surface of the first sliding member. The reaction film consists of a low friction sliding coating containing 55% or more of Mo (IV) in the total Mo amount by quantitative analysis of XPS and an oxidation formed on the opposite side of the sliding surface of the low friction sliding coating. It is preferable to have a layer. That is, by sliding, the sliding system according to the present invention slides with each other in the lubricating oil, which contains MoDTC as an additive and the base oil is composed of a total synthetic oil not containing mineral oil. A first sliding member and a second sliding member, wherein the first sliding member is made of a steel material in which at least a surface layer portion including the sliding surface includes 13% or more of Cr, and the sliding surface Furthermore, it is preferable to have a low friction sliding coating containing 55% or more of Mo (IV) out of the total Mo, and to have an oxide layer on the side opposite to the sliding surface of the low friction sliding coating. In this case, since the low-friction sliding coating contains 55% or more of Mo (IV), the ratio of low-friction MoS ₂ is increased, and lower friction can be exhibited.

  ADVANTAGE OF THE INVENTION According to this invention, the sliding system which can raise the friction reduction effect of the friction modifier added to lubricating oil can be provided.

(A) A state in which the surface of the first sliding member is exposed at the initial stage of sliding in the sliding system according to the present invention. It is explanatory drawing of the state by which Cr oxide is produced | generated on the surface of a moving member and the production | generation of MoS ₂ layer is accelerated | stimulated. It is a graph which shows the relationship between the frequency | count of friction of the ball | bowl obtained by the ball-on-disk friction test of the sliding system of embodiment of this invention, and friction coefficient (micro | micron | mu). The transmission electron microscope of the reaction film formed on the sliding member after the ball-on-disk friction test shown in FIG. 2 with respect to the Cr amount in the sliding member of the sliding system of the embodiment of the present invention (TEM) Photograph and elemental analysis result. (A) A graph showing a quantitative analysis result by the photoelectron spectrometer (XPS) of the wear scar after the ball-on-disk test shown in FIG. 2 of the sliding system according to the embodiment of the present invention, (b) a total Mo amount It is a graph which shows the relationship between the ratio of Mo (IV) which occupies, and a friction coefficient. The first sliding system and reaction film of the first sliding member Fe-17% Cr (SUS440C) of the sliding system according to the embodiment of the present invention after the ball-on-disk test shown in FIG. (A) Transmission electron microscope (TEM) photograph near the boundary with (b) Graph showing elemental analysis results by energy dispersive X-ray analysis (EDX), (c) Electron energy loss spectroscopy (EELS) analysis It is a spectrum.

A sliding system according to an embodiment of the present invention includes a lubricating oil containing MoDTC as an additive and a base oil composed of PAO, and a first sliding member and a second sliding that slide with each other in the lubricating oil. The first sliding member is made of a steel material containing 13% or more of Cr.
Hereinafter, based on the results of the ball-on-disk test performed on various sliding members including the first sliding member and the elemental analysis of the formed reaction film, the sliding system according to the embodiment of the present invention. Will be described.

[Ball-on-disk test]
A ball on disk test was performed in which a ball fixed so as not to rotate was brought into contact with the rotating disk to measure the frictional resistance. The ball-on-disk test was performed by bringing a disk made of each sliding member into contact with a steel (SUJ2) ball and immersing it in a lubricating oil containing a friction modifier MoDTC and having a base oil of PAO8. . The pressing force of the ball was 10 N, and the frictional force was measured by circular movement at a speed of 0.5 m / s. The temperature of the lubricating oil was adjusted to be constant at 80 ° C. In the sliding system according to the embodiment of the present invention, the first sliding member corresponds to a disk, and the second sliding member corresponds to a ball.

  In the test, each sliding member constituting the disk is made of a steel material mainly composed of iron and contains 1.0% Cr (Fe-1.0% Cr; SCr440) and 1.5% (Fe -1.5% Cr; SUJ2), containing 12% (Fe-12% Cr), containing 14% (Fe-14% Cr), containing 17% (Fe-17% Cr; SUS440C), and steel Six types of Cr plating (Cr plating; 99% Cr content on the surface) were used.

  FIG. 2 shows the relationship between the number of rotations of the ball and the friction coefficient μ obtained by using each sliding member. As shown in FIG. 2, it was confirmed that as the Cr content increases, the friction coefficient μ decreases more rapidly and the decreased friction coefficient μ tends to be lower. In particular, in a sliding member containing 14% or more of Cr, the friction coefficient μ decreases to about 0.05 to 0.06 after 2000 to 3000 times, and stabilizes at the reduced friction coefficient μ after 3000 times. Was confirmed.

[Elemental analysis of reaction film]
In order to grasp the structure of the reaction film formed on the sliding surface after the ball-on-disk test, elemental analysis of the reaction film cross section was performed in a transmission electron microscope (TEM). For elemental analysis, energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) were used. The analysis was performed by classifying the sliding members into four types: those with a Cr amount of 0 to 1%, those with 1 to 13%, those with 13 to 20%, and those with 20% or more.

  The transmission electron microscope (TEM) photograph and elemental analysis result of the reaction film of each sliding member are shown in FIG. FIG. 3 shows the results of the material containing no Cr (Fe-0% Cr; maraging steel), representing the Cr amount of 0 to 1% among the sliding members shown in FIG. As a representative of the Cr content of 1-13%, the results of 1.5% Cr (Fe-1.5% Cr; SUJ2), the Cr content of 13-20% The result of 17% Cr (Fe-17% Cr; SUS440C), with the Cr content representing 20% or more, Cr plated on the surface of the steel (Cr plating; The content is 99%).

As shown in FIG. 3, the thickness of the reaction film is 200 nm or more when the Cr amount is 0 to 1%, 50 to 100 nm when the Cr amount is 1 to 13%, and the Cr amount is 13 to 13%. It was confirmed that the thickness was 20-50%, 30-50 nm, the Cr content was 20% or more, 10-20 nm, and the thinner the Cr content, the thinner. The reaction film has a Cr content of 1% or more, and has an iron or molybdenum oxide layer (Fe, Mo oxide) on the sliding member side and a molybdenum disulfide layer (MoS ₂ ) on the surface side. It was confirmed that Further, it was confirmed that the molybdenum disulfide layer became thinner and the orientation became clearer as the Cr content increased. It was confirmed that when the Cr content was 0 to 1%, the reaction film was made of iron oxide crystals (Fe oxide) and no molybdenum disulfide layer was formed.

FIG. 4A shows the spectrum of Mo obtained by photoelectron spectroscopic analysis (XPS) for the wear marks after the ball-on-disk test shown in FIG. 2 of each sliding member. As shown in FIG. 4 (a), the spectrum of Mo was confirmed to have two peaks of Mo (IV) indicating molybdenum disulfide and two peaks of Mo (VI) indicating molybdenum oxide. It was. FIG. 4 (b) shows the ratio of Mo (IV) to the total amount of Mo determined from the peak intensity and the correlation with the friction coefficient plotted. As shown in FIG. 4B, it was confirmed that the coefficient of friction decreases as the Mo (IV) / Mo on the horizontal axis is larger, that is, as the Mo (IV) generation ratio is larger. In particular, in the region where the friction coefficient μ is 0.06 or less, the ratio of Mo (IV) is 55% or more, and it was confirmed that more MoS ₂ is generated contributes to low friction.

FIG. 5 shows the result of detailed analysis of the vicinity of the boundary between the sliding member and the reaction film for the Fe-17% Cr (SUS440C) sliding member. As shown to Fig.5 (a), the 3-5 nm layer was recognized between the sliding member and the oxide layer. When this boundary layer was analyzed by EDX, it was confirmed that a relatively large amount of Cr and O was contained as shown in FIG. Further, as shown in FIG. 5C, when the sliding member, the boundary layer, and the oxide layer are analyzed by EELS, the sliding member and the boundary layer have _two spectra of Cr-L ₂ and Cr-L ₃ in the spectrum. A peak was observed. From these two peaks, it was confirmed that the sliding member and the boundary layer were Cr metal and Cr ₂ O ₃ , respectively. In the oxide layer, no peak was observed in the spectrum, and it was confirmed that the oxide layer was amorphous.

From the results of the above ball-on-disk test and elemental analysis, the following was confirmed. That is, the sliding system according to the embodiment of the present invention includes a low friction MoS ₂ layer by sliding in a lubricating oil that includes MoDTC as an additive and the base oil does not include mineral oil. Can be formed, and the friction reducing effect of the friction modifier of the additive can be enhanced. In addition, since the amount of MoS ₂ produced is large and the MoS ₂ layer is easily oriented, the frictional reduction is advanced quickly and the friction coefficient can be further reduced. Moreover, the oxidation of Fe in the first sliding member can be prevented by the passive film of Cr oxide formed on the surface of the first sliding member.

Claims (4)

A lubricating oil comprising MoDTC as an additive and a base oil comprising a total synthetic oil not containing mineral oil;
A first sliding member and a second sliding member that slide relative to each other in the lubricating oil;
The sliding system is characterized in that the first sliding member is made of a steel material in which at least a surface layer including a sliding surface contains 13% or more of Cr.   The first sliding member has a low friction sliding coating containing 55% or more of Mo (IV) in the Mo on the sliding surface, and is opposite to the sliding surface of the low friction sliding coating. The sliding system according to claim 1, further comprising an oxide layer.   The sliding system according to claim 1 or 2, wherein the lubricating oil includes a base oil made of polyalphaolefin (PAO). The sliding system according to any one of claims 1 to 3, wherein the second sliding member is made of a steel material.