Coating for slide surface, slide member, and method for forming coating to slide surface

Abstract

To extend a fatigue life.SOLUTION: A coating for a slide surface has the composition including 15 atom% or more P, 45 atom% or more O, and 13 atom% or less Fe which is specified by hard X-ray photoelectron spectroscopy, where a bound energy value of the primary peak of 1s peak related to P is within a range of 2146 eV-2152 eV.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a coating for a sliding surface, a sliding member, and a method for forming a coating on a sliding surface. [Background technology]

[0002] Local stresses such as surface shear force are applied to rolling and sliding surfaces, such as gear tooth surfaces or bearings of transaxles used in BEVs (Battery Electric Vehicles) and PHEVs (Plug-in Hybrid Electric Vehicles). For this reason, additives such as extreme pressure agents and anti-wear agents are generally used on the sliding surfaces together with lubricants. A tribo-coating is formed on the sliding surfaces by chemical reactions of phosphorus-based compounds and sulfur-based compounds contained in the additives. The formation of the tribo-coating can alleviate the stress applied to the sliding surfaces.

[0003] Patent Document 1 discloses that a coating is formed on a sliding surface by reacting the sliding surface with at least one compound selected from the group consisting of organic phosphorus compounds, organic sulfur compounds, organic chlorine compounds, and organic metal compounds. According to Patent Document 1, it is possible to provide a rolling bearing with a long life even when used in a location where the lubrication conditions are severe. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-29409 Summary of the Invention [Problem to be solved by the invention]

[0005] While it was known that sliding surfaces could be protected by coatings, it was completely unknown what type of coating was superior in terms of the protective effect of the sliding surface. In view of the above-described circumstances, an object of one embodiment of the present disclosure is to provide a coating for a sliding surface, a sliding member, and a method for forming a coating on a sliding surface that can improve fatigue life. [Means for solving the problem]

[0006] The present disclosure, which achieves the above-mentioned objectives, includes the following. <1> A coating on a sliding surface that is identified by hard X-ray photoelectron spectroscopy as having a composition of 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron, and in which the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV. <2> the calcium content in the coating determined by hard X-ray photoelectron spectroscopy is 0.01 atomic % to 0.3 atomic %; <1> The coating of the sliding surface described in 1. <3> the sulfur content in the coating determined by hard X-ray photoelectron spectroscopy is 0.01 atomic % to 0.6 atomic %; <1> or <2> The coating of the sliding surface described in 1. <4> An oil-based tribo-coating containing a lubricating oil and a compound represented by the following formula (1): <1> ~ <3> 1. A coating for a sliding surface according to any one of the above 1. In the following formula (1), R is a hydrocarbon group.

[0007] [ka]

[0008] <5> <1> ~ <4> A sliding member having the coating film on the sliding surface according to any one of the above items. <6> The sliding surface is a gear tooth surface, a contact portion between a bearing and a shaft, or a gear constituting a transaxle. <5> The sliding member according to claim 1. <7> A method for forming a coating on a sliding surface, comprising sliding a sliding member in the presence of an oil species containing a lubricating oil, and forming a coating on the sliding surface of the sliding member, the coating being identified by hard X-ray photoelectron spectroscopy as having a composition of 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron, and having a binding energy value of a major peak among the phosphorus 1s peaks in the range of 2146 eV to 2152 eV. <8> The coating has a calcium content of 0.01 atomic % to 0.3 atomic % as determined by the hard X-ray photoelectron spectroscopy analysis. <7> 1. A method for forming a coating on a sliding surface according to claim 1. <9> The coating has a sulfur content of 0.01 atomic % to 0.6 atomic % as determined by the hard X-ray photoelectron spectroscopy analysis. <7> or <8> 1. A method for forming a coating on a sliding surface according to claim 1. <10> The oil species includes a compound represented by the following formula (1): <7> ~ <9> 10. The method for forming a coating on a sliding surface according to any one of the above.

[0009] [ka]

[0010] In the above formula (1), R is a hydrocarbon group. [Effects of the Invention]

[0011] The coating on the sliding surface according to one embodiment of the present disclosure exhibits a characteristic composition by hard X-ray photoelectron spectroscopy analysis, and achieves an improvement in the fatigue life of the sliding surface. A slide member according to another embodiment of the present disclosure has a coating film whose characteristic composition is revealed by hard X-ray photoelectron spectroscopy analysis, and achieves an improvement in fatigue life. A method for forming a coating on a sliding surface according to yet another embodiment of the present disclosure can improve the fatigue life of the sliding surface by forming a coating on the sliding surface whose characteristic composition is revealed by hard X-ray photoelectron spectroscopy analysis. [Brief explanation of the drawings]

[0012] [Figure 1]FIG. 1 is a characteristic diagram showing the results of a comparison of fatigue life when sliding tests were conducted using various types of oil. [Figure 2] FIG. 1 is a characteristic diagram showing the results of analyzing the spectrum of the 1s peak related to phosphorus. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present disclosure will be described. The description is for illustrating the embodiments and is not intended to limit the scope of the present disclosure.

[0014] In this specification, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of a numerical range may be replaced with a value shown in the examples.

[0015] In this specification, when an embodiment is described with reference to drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings. Furthermore, the sizes of components in each drawing are conceptual, and the relative size relationships between components are not limited to these.

[0016] In this specification, each component may contain a plurality of corresponding substances. When referring to the amount of each component in the composition in this embodiment, if a plurality of substances corresponding to each component are present in the composition, the amount refers to the total amount of the plurality of substances present in the composition unless otherwise specified.

[0017] [Coating on sliding surface] The sliding surface coating of the present disclosure is a coating that is identified by hard X-ray photoelectron spectroscopy as having a composition of 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron. In the sliding surface coating of the present disclosure, the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV.

[0018] The sliding surface coating of the present disclosure has the specified composition as described above, and therefore can prevent damage such as pitting from occurring on the sliding surface for a long period of time, thereby improving fatigue life. The fatigue life can be evaluated as the timing at which pitting occurs on the sliding surface subjected to a sliding test. Pitting is damage that occurs when fatigue cracks occur in an area where surface pressure and sliding act simultaneously, such as the tooth root side of the pitch circle on the gear tooth surface, and these cracks propagate and lead to peeling. The sliding test can be performed using the roller test described in Toyota Central R&D Labs. R&D Review, Vol. 34, No. 4, pp. 25-34 (December 1999).

[0019] This roller test, also known as the twin-cylinder test method, involves contacting a 24mm diameter test roller with a 96mm diameter mating roller at a contact width of 4mm under a predetermined pressure. The test roller and mating roller are driven by the same motor via different gears. Driving the test roller and mating roller with the same motor via different gears allows for the desired slip ratio between the test roller and mating roller. The test roller and mating roller are driven while a lubricant of a predetermined temperature is supplied to the contact point between the test roller and mating roller. In this roller test, vibrations of the testing machine associated with pitching are detected by an acceleration sensor. The fatigue life is measured from the start of the test to the detection of vibration. The specific test conditions are a contact pressure of 3.5GPa between the test roller and mating roller, a slip ratio of 41%, and a lubricant temperature of 120°C. The slip ratio is defined as 2*(v2-v1) / (v2+v1), where v1 is the peripheral speed of the test roller and v2 is the peripheral speed of the mating roller.

[0020] The coating on the sliding surface according to the present disclosure is a coating whose effect of improving the fatigue life of the sliding surface has been confirmed by the roller test described above, and whose composition has been identified by hard X-ray photoelectron spectroscopy analysis, which will be described below.

[0021] Hard X-ray photoelectron spectroscopy (HAXPES), like conventional X-ray photoelectron spectroscopy (XPES), is an analytical method for calculating the concentration of elements constituting the surface of a test sample with high sensitivity. By increasing the incident X-ray energy to approximately 8 keV, HAXPES can achieve an analysis depth of several tens of nanometers, an order of magnitude deeper than XPS. This allows for comprehensive information on the coating that improved fatigue life formed in the roller test described above. In contrast, XPS, with its shallow analysis depth, can only obtain information on the coating surface, and the influence of components such as oil remaining on the surface cannot be eliminated. As the X-ray energy increases, the ionization cross-section decreases, resulting in a decrease in the number of photoelectrons that can be collected, and this leads to a decrease in sensitivity. However, high-sensitivity detection is possible by using the high-brilliance synchrotron X-rays from the Super Photon Ring 8 GeV (SPring-8).

[0022] The coating on the sliding surface of the present disclosure contains 15 atomic % or more of phosphorus as determined by hard X-ray photoelectron spectroscopy, preferably 15 atomic % to 30 atomic %, and more preferably 20 atomic % to 30 atomic %.

[0023] Furthermore, the coating on the sliding surface of the present disclosure has an oxygen content of 45 atomic % or more as determined by hard X-ray photoelectron spectroscopy, but it is particularly preferable that the oxygen content be 45 atomic % to 70 atomic %, and more preferably 55 atomic % to 70 atomic %.

[0024] Furthermore, the coating on the sliding surface of the present disclosure has an iron content of 13 atomic % or less as determined by hard X-ray photoelectron spectroscopy, but in particular, 0.01 atomic % to 13 atomic % is preferable, and 0.1 atomic % to 10 atomic % is even more preferable.

[0025] In the sliding surface coating of the present disclosure, the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV. Here, the main peak refers to the peak with the highest intensity in the phosphorus 1s peak spectrum. The binding energy value of the main peak being in the range of 2146 eV to 2152 eV means that in the sliding surface coating of the present disclosure, phosphorus is present mainly as phosphate. Furthermore, in the sliding surface coating of the present disclosure, phosphorus is present only in an extremely small amount as a phosphide such as iron phosphide.

[0026] The sliding surface coating of the present disclosure can improve the fatigue life of the sliding surface by having the phosphorus, oxygen, and iron contents determined by hard X-ray photoelectron spectroscopy within the above-mentioned ranges and the binding energy value of the main peak of the phosphorus 1s peak within the range of 2146 eV to 2152 eV. In particular, the sliding surface coating of the present disclosure is excellent in protecting the sliding surface from local stresses, such as surface shear stress, applied to the sliding surface. A coating in which the phosphorus, oxygen, and iron contents determined by hard X-ray photoelectron spectroscopy within the above-mentioned ranges and the binding energy value of the main peak of the phosphorus 1s peak within the range of 2146 eV to 2152 eV can be obtained by adjusting the type and amount of a compound (for example, a compound represented by formula (1) described below) added to the lubricating oil.

[0027] In particular, the coating of the sliding surface of the present disclosure preferably has a calcium content of 0.01 atomic % to 0.3 atomic %, and more preferably 0.01 atomic % to 0.25 atomic %, as determined by the hard X-ray photoelectron spectroscopy analysis, in addition to the above-mentioned specifications regarding the phosphorus, oxygen, and iron compositions. By ensuring that the calcium content of the coating of the sliding surface of the present disclosure is within the above-mentioned range as determined by the hard X-ray photoelectron spectroscopy analysis, the fatigue life of the sliding surface can be further improved.

[0028] Furthermore, in addition to the above-mentioned phosphorus, oxygen, and iron compositions, the coating for the sliding surface of the present disclosure preferably has a sulfur content of 0.01 atomic % to 0.6 atomic %, and more preferably 0.01 atomic % to 0.5 atomic %, as determined by the hard X-ray photoelectron spectroscopy analysis. By ensuring that the sulfur content of the coating for the sliding surface of the present disclosure is within the above-mentioned range as determined by the hard X-ray photoelectron spectroscopy analysis, the fatigue life of the sliding surface can be further improved.

[0029] Furthermore, in addition to the above-mentioned specifications regarding the phosphorus, oxygen, and iron compositions, the coating of the sliding surface of the present disclosure preferably has a chromium content of 0.1 atomic % or less, and ideally 0 atomic %, as determined by the hard X-ray photoelectron spectroscopy analysis. By ensuring that the chromium content of the coating of the sliding surface of the present disclosure is within the above-mentioned range as determined by the hard X-ray photoelectron spectroscopy analysis, the fatigue life of the sliding surface can be further improved.

[0030] One embodiment of the coating on the sliding surface of the present disclosure is a tribo-coating of an oil type containing a lubricating oil and a compound having a specific structure. The tribo-coating refers to a protective coating formed by the adsorption of additives contained in the lubricating oil onto the surface of a friction member or by a chemical reaction of the additives. Here, the compound having the specific structure can be a compound represented by the following formula (1):

[0031] [ka]

[0032] In the above formula (1), R is a hydrocarbon group. The hydrocarbon group is preferably an alkyl group, more preferably an alkyl group having 5 or less carbon atoms.

[0033] The tribo-coating produced using a lubricating oil and the compound represented by formula (1) as one embodiment of the sliding surface coating of the present disclosure is characterized in that phosphorus, oxygen, and iron, as determined by hard X-ray photoelectron spectroscopy, are present in the above-mentioned ranges, and the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV. Therefore, the tribo-coating produced using a lubricating oil and the compound represented by formula (1) as one embodiment of the sliding surface coating of the present disclosure can prevent damage such as pitting on the sliding surface for a long period of time and improve the fatigue life of the sliding surface.

[0034] In a tribo-coating produced using a lubricating oil and the compound represented by formula (1), which is shown as one embodiment of the coating for a sliding surface of the present disclosure, calcium and / or sulfur, as determined by hard X-ray photoelectron spectroscopy, are present within the above-mentioned ranges. Therefore, the tribo-coating produced using a lubricating oil and the compound represented by formula (1), which is shown as one embodiment of the coating for a sliding surface of the present disclosure, can more reliably prevent damage such as pitting on the sliding surface over a long period of time, and can further improve the fatigue life of the sliding surface.

[0035] [Sliding member] The sliding member of the present disclosure has the above-described coating on the sliding surface of the present disclosure. That is, the sliding member of the present disclosure has a sliding surface on which the above-described coating is formed. The sliding member of the present disclosure has an improved fatigue life of the sliding surface due to the coating.

[0036] The term "sliding member" is not particularly limited, but refers to one or both of a pair of members that slide at a contact portion. Examples of the sliding member of the present disclosure include a pair of gears, specifically gears that constitute a transaxle. The tooth surfaces of the pair of gears are sliding surfaces that come into contact with each other and are subjected to rolling and sliding friction when rotated. The pair of gears, which is one embodiment of the sliding member of the present disclosure, are prevented from damage such as pitting over a long period of time and have an improved fatigue life by forming the above-described coating on their tooth surfaces. Examples of the sliding member of the present disclosure include a bearing and a shaft. The bearing and the shaft are subjected to rolling or sliding friction at their contact portion when the shaft is rotated. The bearing and the shaft, which are one embodiment of the sliding member of the present disclosure, are prevented from damage such as pitting over a long period of time and have an improved fatigue life by forming the above-described coating on their contact portion.

[0037] [Film formation method] The coating formation method of the present disclosure is a method for forming the above-described coating on a sliding surface. That is, the coating formation method of the present disclosure involves sliding a sliding member in the presence of an oil containing a lubricating oil, and forming a coating on the sliding surface of the sliding member, the coating being determined by hard X-ray photoelectron spectroscopy to have a composition of 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron, and the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV. According to the coating formation method of the present disclosure, by forming the coating on the sliding surface, damage such as pitting in the sliding member having the sliding surface can be prevented for a long period of time, and the fatigue life of the sliding member can be improved.

[0038] Furthermore, the coating formation method of the present disclosure can form a coating in which, in addition to the above-mentioned specifications regarding the phosphorus, oxygen, and iron compositions, the calcium content in the coating as determined by hard X-ray photoelectron spectroscopy is 0.01 atomic % to 0.3 atomic %, preferably 0.01 atomic % to 0.25 atomic %, or ideally 0 atomic %. The coating formation method of the present disclosure can further improve the fatigue life of the sliding surface by forming a coating in which the calcium content as determined by hard X-ray photoelectron spectroscopy is in the above-mentioned range.

[0039] Furthermore, the coating formation method of the present disclosure can form a coating in which, in addition to the above-mentioned phosphorus, oxygen, and iron composition specifications, the sulfur content in the coating as determined by hard X-ray photoelectron spectroscopy is 0.01 atomic % to 0.6 atomic %, preferably 0.01 atomic % to 0.5 atomic %, or ideally 0 atomic %. The coating formation method of the present disclosure can further improve the fatigue life of the sliding surface by forming a coating in which the sulfur content as determined by hard X-ray photoelectron spectroscopy is in the above-mentioned range.

[0040] One embodiment of the coating formation method of the present disclosure is to form a tribo-coating on a sliding surface using the lubricating oil and the compound represented by formula (1) described above. This allows the formation of a coating on the sliding surface, in which phosphorus, oxygen, and iron, as determined by hard X-ray photoelectron spectroscopy, are present within the above-described ranges, and the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV, as described above. Therefore, according to one embodiment of the coating formation method of the present disclosure, damage such as pitting on the sliding surface can be prevented for a long period of time, and the fatigue life of the sliding surface can be improved.

[0041] Furthermore, in one embodiment of the coating formation method of the present disclosure, in which a lubricating oil and the compound represented by formula (1) are used, a tribo-coating can be formed in which calcium and / or sulfur, as determined by hard X-ray photoelectron spectroscopy, are present within the above-mentioned ranges. Thus, according to one embodiment of the coating formation method of the present disclosure, damage such as pitting on the sliding surface can be more reliably prevented over a long period of time, and the fatigue life of the sliding surface can be further improved.

[0042] [Other embodiments] As described above, it has been explained that the fatigue life of a sliding surface can be improved if the coating has the characteristics that phosphorus, oxygen, and iron, as determined by hard X-ray photoelectron spectroscopy, are present in the above-mentioned ranges and the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV. Therefore, if these characteristics of the coating formed on the sliding surface are used as indicators, it is possible to screen for compounds that can be used together with lubricating oils and that improve the fatigue life of the sliding surface.

[0043] According to the present disclosure, a method for screening compounds that can be used with a lubricating oil and that improve the fatigue life of a sliding surface can be provided. The screening method of the present disclosure involves performing a sliding test using an oil type containing a test compound and a lubricating oil, and measuring the phosphorus, oxygen, and iron of a coating formed after the sliding test by hard X-ray photoelectron spectroscopy. If the measurement results from hard X-ray photoelectron spectroscopy show that the coating has a composition specified as having 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron, and that the binding energy value of the main peak of the phosphorus 1s peak is in the range of 2146 eV to 2152 eV, the test compound can be screened as a compound that improves the fatigue life of a sliding surface.

[0044] As mentioned above, the sliding test here can be the roller test described in Toyota Central R&D Labs. R&D Review, Vol. 34, No. 4, pp. 25-34 (December 1999). Specific test conditions include contacting the test roller and the mating roller at a surface pressure of 3.5 GPa, setting the slip ratio to 41%, and setting the temperature of the supplied oil to 120°C.

[0045] Furthermore, as mentioned above, for hard X-ray photoelectron spectroscopy, a method can be applied in which the energy of the incident X-rays is increased to about 8 keV and high-brightness synchrotron radiation X-rays from the Super Photon Ring 8 GeV (SPring-8) are used.

[0046] Furthermore, in the screening method of the present disclosure, when the coating formed in the sliding test is subjected to hard X-ray photoelectron spectroscopy analysis, it can be determined that the fatigue life of the sliding surface can be further improved if the calcium content is 0.01 atomic % to 0.3 atomic %, preferably 0.01 atomic % to 0.25 atomic %, or 0 atomic %, In this case, it can be determined that the test compound is a compound that can more effectively improve the fatigue life of the sliding surface.

[0047] Furthermore, in the coating formation method of the present disclosure, when the coating formed in the sliding test is subjected to hard X-ray photoelectron spectroscopy analysis, it can be determined that the fatigue life of the sliding surface can be further improved when the sulfur content is 0.01 atomic % to 0.6 atomic %, preferably 0.01 atomic % to 0.5 atomic %, or 0 atomic %. In this case, it can be determined that the test compound is a compound that can more effectively improve the fatigue life of the sliding surface. [Example]

[0048] The present disclosure will be described in more detail below using examples, but the technical scope of the present disclosure is not limited to the following examples.

[0049] [Example 1] In this example, a commercially available lubricating oil was prepared by adding the thiophosphate of the following formula (1) as an additive. The commercially available lubricating oil used was e-Transaxle Fluid TE (08886-02605). The amount of additive added was 10 times the amount of sulfur contained in the commercially available lubricating oil. Specifically, the amount of additive was 2.5% by mass per 100% by mass of the commercially available lubricating oil.

[0050] [ka]

[0051] In this example, IRGAL UBE 63 manufactured by BASF was used as the thiophosphate.

[0052] In this example, a sliding test was performed using the roller test described in Toyota Central R&D Labs. R&D Review Vol. 34, No. 4, pp. 25-34 (December 1999). In this roller test, a test roller with a diameter of 24 mm was contacted with a mating roller with a diameter of 96 mm at a contact width of 4 mm and a surface pressure of 3.5 GPa, and a sliding ratio of 41% was set between the test roller and the mating roller. Oil at 120°C was supplied between the test roller and the mating roller. When pitting occurred on a portion of the test surface of the test roller and a vibration meter detected an increase in the vibration of the testing machine, the test was stopped, and this timing was defined as the fatigue life.

[0053] Next, the test piece with the coating formed was subjected to hard X-ray photoelectron spectroscopy, which used the SPring-8 BL16XU (Sunbeam) large-scale synchrotron radiation facility as the light source, with the incident X-ray energy set to approximately 8 keV.

[0054] [Comparative Example 1] A sliding test was conducted in the same manner as in Example 1, except that no additives were used and only commercially available lubricating oil e-Transaxle Fluid TE (08886-02605) was used, and the test piece on which the coating was formed was subjected to hard X-ray photoelectron spectroscopy analysis.

[0055] Comparative Example 2 A sliding test was carried out in the same manner as in Example 1, except that dibenzyl disulfide of the following formula (2) was used as the additive, and the test piece on which the coating was formed was subjected to hard X-ray photoelectron spectroscopy analysis.

[0056] [ka]

[0057] Comparative Example 3 A sliding test was conducted in the same manner as in Comparative Example 2, except that the amount of additive was halved (equivalent to five times the amount of sulfur contained in commercially available lubricating oil), and the test piece on which the coating was formed was subjected to hard X-ray photoelectron spectroscopy analysis.

[0058] Comparative Example 4 A sliding test was carried out in the same manner as in Example 1, except that dipropyl disulfide of the following formula (3) was used as the additive, and the test piece on which the coating was formed was subjected to hard X-ray photoelectron spectroscopy analysis.

[0059] [ka]

[0060] [result] The results of measuring the fatigue life for Example 1 and Comparative Examples 1 to 4 are shown in Figure 1, and the results of hard X-ray photoelectron spectroscopy analysis of the coatings are shown in Table 1. Furthermore, the results of measuring the 1s peak related to phosphorus by hard X-ray photoelectron spectroscopy analysis are shown in Figure 2.

[0061] [Table 1]

[0062] Compared to Comparative Examples 1 to 4, Example 1 achieved an extremely excellent fatigue life, as shown in Figure 1. Furthermore, as shown in Table 1, hard X-ray photoelectron spectroscopy analysis of the coating formed in Example 1 revealed that the coating had a composition of 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron. Furthermore, as shown in Figure 2, analysis of the phosphorus-related 1s peak by hard X-ray photoelectron spectroscopy revealed that the binding energy value of the main peak of the coating formed in Example 1 was in the range of 2146 eV to 2152 eV.

[0063] In the coatings formed in Comparative Examples 1 to 3, a slight peak was observed near 2143 eV in the analysis results of the phosphorus 1s peak by hard X-ray photoelectron spectroscopy. This indicates that the phosphorus in the coating exists not as a phosphate but as a phosphide such as iron phosphide. In contrast, no peak was observed near 2143 eV in the coating formed in Example 1.

[0064] As described above, it was confirmed that the fatigue life can be improved by forming a coating on the sliding surface whose composition is determined by hard X-ray photoelectron spectroscopy analysis to be 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron, and whose main peak among the phosphorus 1s peaks has a binding energy value in the range of 2146 eV to 2152 eV.

Claims

1. A coating on a sliding surface, which is identified by hard X-ray photoelectron spectroscopy as having a composition of 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron, and the binding energy value of the main peak among the phosphorus 1s peaks is in the range of 2146 eV to 2152 eV.

2. 2. The coating for a sliding surface according to claim 1, wherein the calcium content in the coating determined by the hard X-ray photoelectron spectroscopy analysis is 0.01 atomic % to 0.3 atomic %.

3. 2. The coating for a sliding surface according to claim 1, wherein the sulfur content in the coating determined by the hard X-ray photoelectron spectroscopy analysis is 0.01 atomic % to 0.6 atomic %.

4. 2. The coating for a sliding surface according to claim 1, which is an oil-based tribo-coating containing a lubricating oil and a compound represented by the following formula (1): 【Chemical 1】 [In the above formula (1), R is a hydrocarbon group.]

5. A sliding member having the coating film on the sliding surface according to any one of claims 1 to 4.

6. 6. The sliding member according to claim 5, wherein the sliding surface is a gear tooth surface, a contact portion between a bearing and a shaft, or a gear constituting a transaxle.

7. A method for forming a coating on a sliding surface, comprising sliding a sliding member in the presence of an oil species containing a lubricating oil, and forming a coating on the sliding surface of the sliding member, the coating being identified by hard X-ray photoelectron spectroscopy as having a composition of 15 atomic % or more of phosphorus, 45 atomic % or more of oxygen, and 13 atomic % or less of iron, and having a binding energy value of a major peak among phosphorus 1s peaks in the range of 2146 eV to 2152 eV.

8. 8. The method for forming a coating on a sliding surface according to claim 7, wherein the coating has a calcium content of 0.01 atomic % to 0.3 atomic % as determined by the hard X-ray photoelectron spectroscopy analysis.

9. 8. The method for forming a coating on a sliding surface according to claim 7, wherein the coating has a sulfur content of 0.01 atomic % to 0.6 atomic % as determined by the hard X-ray photoelectron spectroscopy analysis.

10. The method for forming a coating on a sliding surface according to claim 7 , wherein the oil type contains a compound represented by the following formula (1): 【Chemistry 2】 [In the above formula (1), R is a hydrocarbon group.]

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