Solid lubricating coating film, sliding member and method for producing solid lubricating coating film

Abstract

The present invention provides a solid lubricating coating film which has high durability. This solid lubricating coating film, which is formed on a base material, comprises a mixture layer that contains a metal oxide and an oxide derived from the base material. If the first depth position is specified, in the depth direction, by a position at which the content ratio of oxygen atoms is equal to the value that is obtained by multiplying the content ratio of a metal element that forms the metal oxide by the valence of the metal element, and subsequently dividing the product by the valence of the oxygen atoms, and the second depth position is specified, in the depth direction, by a position at which the content ratio of the metal element is equal to the content ratio of an element that forms the oxide derived from the base material, the mixture layer is defined as the extent between the first depth position and the second depth position. The thickness of the mixture layer is larger than 4.4 nm.

Description

Solid lubricating coating, sliding member, and method for manufacturing solid lubricating coating

[0001] The present invention relates to a solid lubricating coating, a sliding member, and a method for producing a solid lubricating coating.

[0002] One way to save energy is to reduce the frictional force of sliding members. For the purpose of reducing friction, it is known to form a coating made of a solid lubricant (solid lubricant coating) on ​​the surface of the sliding member. The following Patent Documents 1 and 2 disclose a zinc oxide coating as a solid lubricant coating.

[0003] International Publication No. WO 2012 / 039264 International Publication No. WO 2016 / 190375

[0004] The coefficient of friction of a solid lubricating coating can generally increase over time due to oxidation by oxygen in the air or oil. To suppress this increase in the coefficient of friction over time, it is desirable for the solid lubricating coating to contain a metal oxide. In addition to maintaining the coefficient of friction, it is also desirable to improve the durability of the solid lubricating coating. Therefore, there is a demand for a solid lubricating coating with high durability, a sliding member having such a solid lubricating coating, and a method for producing such a solid lubricating coating.

[0005] A solid lubricating coating according to one embodiment is a solid lubricating coating formed on a substrate. The solid lubricating coating has a mixed layer containing a metal oxide and an oxide derived from the substrate. When a first depth position is defined by a position in the depth direction where the content ratio of oxygen atoms is equal to the value obtained by multiplying the content ratio of the metal elements constituting the metal oxide by the valence of the metal element and dividing the result by the valence of the oxygen atom, and a second depth position is defined by a position in the depth direction where the content ratio of the metal elements is equal to the content ratio of the elements constituting the oxide derived from the substrate, the mixed layer is defined as the area between the first depth position and the second depth position. The thickness of the mixed layer is greater than 4.4 nm.

[0006] A sliding member according to one embodiment includes the above-described solid lubricating coating and a substrate on which the solid lubricating coating is formed.

[0007] A method for producing a solid lubricating coating according to one embodiment includes a film-forming step of forming a metal oxide film on a substrate, and the method includes at least one of: performing the film-forming step in an oxygen-containing atmosphere at least at the start of film formation; oxidizing the surface of the substrate before the film-forming step; and diffusing oxygen atoms in the metal oxide after the film-forming step.

[0008] Fig. 1 is a schematic diagram showing an example of a sliding member formed with a solid lubricant coating according to the first embodiment. Fig. 2 is a graph showing the results of measurements of the relationship between the depth and the ratio of elements constituting the zinc oxide coating according to Example 1. Fig. 3 is a graph showing the results of measurements of the relationship between the depth and the ratio of elements constituting the zinc oxide coating according to Reference Example 1. Fig. 4 is a laser microscope photograph of the surfaces of the substrates in Example 1 and Reference Example 1 after a reciprocating friction test. Fig. 5 is a graph showing the results of measurements of the friction coefficients of the surfaces of the substrates in Example 1 and Reference Example 1 during a reciprocating friction test.

[0009] Hereinafter, embodiments will be described with reference to the drawings. In the following drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic and the ratios of the dimensions may differ from those of the actual parts.

[0010] First Embodiment First, a description will be given of a slide member according to First Embodiment. Fig. 1 is a schematic diagram showing an example of a slide member on which a solid lubricant coating according to First Embodiment is formed.

[0011] The sliding member 100 may have a substrate 200 and a solid lubricating coating 300. The solid lubricating coating 300 is formed on the substrate 200.

[0012] The material constituting the substrate 200 is not particularly limited, but is preferably a metallic material containing an element that forms a metal oxide by reacting with oxygen. The material constituting the substrate 200 may contain at least one element belonging to Groups 3 to 14. Preferably, the material constituting the substrate 200 may contain at least one element belonging to Groups 3 to 11.

[0013] More preferably, the material constituting the substrate 200 contains at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Ni, and Cu. Even more preferably, the material constituting the substrate 200 contains Fe. Such a material may be, for example, iron or an alloy containing iron.

[0014] 1, the solid lubricating coating 300 may have a base layer 310 facing the substrate 200 and a surface layer 320 facing the opposite side from the substrate 200. However, it should be noted that the base layer 310 and the surface layer 320 do not have to be separated by a clear boundary.

[0015] The solid lubricating coating 300 can be formed by a film formation technique such as sputtering. In this case, the base layer 310 and the surface layer 320 of the solid lubricating coating 300 may be layers formed under different film formation conditions.

[0016] Although not shown, the solid lubricating coating 300 may include a layer formed, for example, under different deposition conditions, between the base layer 310 and the surface layer 320. Alternatively, instead of the embodiment shown in Fig. 1, the base layer 310 and the surface layer 320 may be composed of the same layer that is indistinguishable from each other. In other words, the solid lubricating coating 300 may be composed of a single layer formed under the same deposition conditions.

[0017] The solid lubricating coating 300 contains a metal oxide as a main component. ２ O, Al ２ O ３ , Bi ２ O ３ , CaO, Ce ２ O ３ , Cr ２ O ３ , CrO, CrO ３ , Cu ２ O, CuO, Fe ２ O ３ , Fe ３ O ４ , FeO, In ２ O ３ , Li ２ O, MgO, MnO, Nb ２ O５ , NiO, Sb ２ O ３ , Sb ２ O ５ , SiO ２ , SnO ２ , Ta ２ O ５ , TiO ２ , W.O. ３ , ZnO, and ZrO ２ It is preferable that the metal oxide contains at least one selected from the group consisting of: More preferably, the metal oxide contains ZnO.

[0018] The solid lubricating coating 300 has a mixed layer containing a metal oxide and an oxide derived from the substrate. The mixed layer is formed at the interface between the solid lubricating coating 300 and the substrate. The metal oxide that makes up the mixed layer is the aforementioned metal oxide that makes up the main component of the solid lubricating coating 300.

[0019] The oxide derived from the substrate is an oxide formed by oxidation of the material constituting the substrate 200. Specifically, the oxide derived from the substrate may be an oxide of at least one element belonging to Groups 3 to 14. Preferably, the oxide derived from the substrate may be an oxide of at least one element belonging to Groups 3 to 11. More preferably, the oxide derived from the substrate is an oxide of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Ni, and Cu. Here, the oxide derived from the substrate contained in the mixed layer may be an oxide of a main component element among the metal elements constituting the substrate, or may be an oxide of an element different from the main component element among the metal elements forming the substrate.

[0020] The mixed layer is defined as the portion of the solid lubricating coating 300 between a first depth position and a second depth position. In this specification, the first depth position is defined as the position in the depth direction where the content ratio (at %) of oxygen atoms matches the value obtained by multiplying the content ratio (at %) of the metal element that forms the metal oxide that is the main component of the solid lubricating coating 300 by the valence of the metal element and dividing the result by the valence of the oxygen atoms. For example, if the metal oxide that is the main component of the solid lubricating coating 300 is Ag, ２If the metal oxide is O, the first depth position is defined as the position in the depth direction where the oxygen atom content (at %) equals the Ag content (at %) multiplied by 1 and divided by 2. For example, if the metal oxide is ZnO, the first depth position is defined as the position in the depth direction where the oxygen atom content (at %) equals the Zn content (at %). The first depth position corresponds to the point where the oxygen atom content reaches the maximum ratio of oxygen atoms that can bond with the metal elements that form the metal oxide that is the main component of the solid lubricating coating 300.

[0021] The second depth position is defined as the position where the content (at %) of the metal elements that form the metal oxide that is the main component of the solid lubricating coating 300 matches the content (at %) of the main component element among the metal elements that form the substrate. In other words, the second depth position corresponds to the boundary between the region where the main component of the solid lubricating coating 300 is predominant and the region where the main component of the substrate is predominant.

[0022] In this specification, a mixed layer is a layer in which the oxygen atom content is excessive relative to the metal elements that form the metal oxide that is the main component of the solid lubricating coating 300, and in which the metal elements that are the main components of the solid lubricating coating 300 are present more predominantly than the material derived from the substrate. As will be described later, it is believed that such a mixed layer can be easily formed by performing a process that can actively introduce oxygen atoms from the outside or by diffusing the oxygen atoms in the solid lubricating coating 300.

[0023] The thickness of the mixed layer is, for example, greater than 4.4 nm or 4.8 nm. Preferably, the thickness of the mixed layer may be, for example, 5.0 nm or greater, 5.2 nm or greater, 5.4 nm or greater, 5.6 nm or greater, or 5.8 nm or greater. Here, the thickness of the mixed layer is defined by the distance between the first depth position and the second depth position. The thicker the mixed layer, the thicker the region where the metal oxide, the main component of the solid lubricating coating 300, and the oxide derived from the substrate are mixed. This is thought to improve adhesion between the solid lubricating coating 300 and the substrate 200, and to increase the durability of the solid lubricating coating 300. From the perspective of the durability of the solid lubricating coating 300, a thicker mixed layer is thought to be preferable.

[0024] There is no particular upper limit to the thickness of the mixed layer from the viewpoint of durability of the solid lubricating coating 300. The thickness of the mixed layer may be, for example, 60.0 nm or less, 50.0 nm or less, 40.0 nm or less, 30.0 nm or less, 20.0 nm or less, 18.0 nm or less, 16.0 nm or less, 14.0 nm or less, 12.0 nm or less, or 10.0 nm or less. The thickness of the mixed layer may be within a range that combines any of the above upper and lower limits.

[0025] The solid lubricating coating 300 can be formed by any deposition technique capable of depositing a metal oxide film on a substrate (deposition step). One example of such a deposition technique is sputtering. The method for producing the solid lubricating coating 300 may include at least one of: performing the deposition step in an oxygen-containing atmosphere at least at the start of deposition; oxidizing the surface of the substrate before the deposition step; and diffusing oxygen atoms in the metal oxide after the deposition step.

[0026] The sputtering method will be described below as one preferred method for producing the solid lubricant coating 300. First, a target material that will form the main components of the solid lubricant coating 300 is prepared.

[0027] The target material may be a metal or metal oxide that forms the metal oxide that constitutes the main component of the solid lubricant coating 300. The types of metal elements that form the metal oxide are as described above. Preferably, the target material may be a material made of the metal oxide that constitutes the main component of the solid lubricant coating 300. If the metal oxide that constitutes the main component of the solid lubricant coating 300 is ZnO, the target material is preferably made of Zn or ZnO, and more preferably made of ZnO. If the target material is a metal oxide, oxygen atoms are also scattered by sputtering the target material, making it easier to form oxides of materials derived from the substrate. This makes it easier to increase the thickness of the mixed layer described above.

[0028] The film-forming step is preferably carried out in an oxygen-containing atmosphere at least at the start of film formation. Sputtering in an oxygen-containing atmosphere facilitates the formation of oxides of elements derived from the substrate, allowing the mixed layer to have a larger thickness. As a result, the mixed layer of a solid lubricating coating formed by sputtering in an oxygen-containing atmosphere will be thicker than the mixed layer of a solid lubricating coating formed by sputtering in a rare gas atmosphere substantially free of oxygen.

[0029] The oxygen-containing atmosphere may be, for example, a mixed gas of oxygen gas and a rare gas. The oxygen partial pressure ratio of the oxygen-containing atmosphere is not particularly limited as long as the desired thickness of the mixed layer can be achieved. From the viewpoint of forming the mixed layer, the oxygen partial pressure ratio in the oxygen atmosphere may be, for example, 1.5% or more, 3% or more, 5% or more, 8% or more, or 10% or more.

[0030] The upper limit of the oxygen partial pressure ratio in the oxygen-containing atmosphere is not particularly limited. The oxygen partial pressure ratio in the oxygen atmosphere may be, for example, 90% or less, 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, or 30% or less. If the target material during sputtering deposition is a metal oxide, a relatively thick mixed layer can be easily formed even if the oxygen partial pressure ratio during sputtering is low.

[0031] Preferably, in the film-forming step, the target material is a metal oxide, and the sputtering is performed in an atmosphere containing oxygen at least at the start of film formation. In this case, more oxygen atoms than contained in the metal oxide may be scattered toward the substrate at the start of film formation. This makes it easier for oxides derived from the substrate to form at the interface between the substrate and the solid lubricating coating. This is thought to result in an increased thickness of the mixed layer containing the metal oxide as the main component of the solid lubricating coating and oxides derived from the substrate.

[0032] The thickness of the mixed layer can be adjusted by the sputtering conditions, such as the oxygen partial pressure ratio in the atmosphere. The thickness of the mixed layer can also be adjusted by heating the substrate during sputtering to promote oxygen diffusion into the substrate. The thickness of the mixed layer can also be adjusted by placing a mesh material between the target material and the substrate, allowing oxygen atoms to preferentially reach the substrate at the beginning of sputtering.

[0033] Instead of sputtering in an oxygen-containing atmosphere at the start of film formation, the thickness of the mixed layer can also be increased by the following techniques. For example, the substrate surface may be oxidized before sputtering the metal oxide film. The oxidation of the substrate surface can be achieved by annealing, oxygen plasma treatment, ozone treatment, immersion in an oxidizing solution, and / or anodization. If the substrate surface is oxidized in advance, the thickness of the mixed layer containing the metal oxide as the main component of the solid lubricating coating and oxide derived from the substrate is thought to be increased during metal oxide film formation.

[0034] Alternatively, or in addition, a diffusion treatment may be performed after the film-forming step to diffuse oxygen atoms in the metal oxide that is the main component of the solid lubricating coating. Such a diffusion treatment can be performed by annealing. The annealing treatment can be performed by heating the substrate on which the metal oxide has been formed in a desired atmosphere. The desired atmosphere may be an oxygen-containing atmosphere or an oxygen-free atmosphere. This allows the oxygen atoms in the metal oxide to diffuse after the metal oxide film has been formed and form an oxide with the elements that make up the substrate. This is thought to increase the thickness of the mixed layer containing the metal oxide that is the main component of the solid lubricating coating and the oxide derived from the substrate.

[0035] Example 1 Next, a solid lubricant coating according to Example 1 will be described. In Example 1, a zinc oxide coating is provided on a substrate as a solid lubricant coating. The substrate is soft iron (SPCC). As shown in FIG. 1, the zinc oxide coating includes an underlayer 310 and a surface layer 320. The underlayer and surface layers of the zinc oxide coating were formed by a sputtering deposition method using an "Intervac-type sputtering device SIH-300 manufactured by ULVAC, Inc." The target material was zinc oxide.

[0036] In Example 1, the deposition gas used to deposit the underlayer of the zinc oxide coating was a mixture of 80% argon partial pressure and 20% oxygen partial pressure (see Table 1). In Example 1, the deposition gas used to deposit the surface layer was argon, which contained substantially no oxygen. The flow rate of the deposition gas was 50 ml / min. The deposition temperature was 25°C, and the deposition pressure was 0.5 Pa. The discharge power used to deposit the underlayer was 224 W, and the discharge power used to deposit the surface layer was 2236 W. The thickness of the underlayer was 60 nm, and the thickness of the surface layer was 1700 nm. The above-mentioned film thicknesses were measured by spectroscopic ellipsometry (the same applies below).

[0037] Reference Example 1 A solid lubricating coating according to Reference Example 1 will be described. In Reference Example 1, a zinc oxide coating was provided on a substrate as a solid lubricating coating. The zinc oxide coating according to Reference Example 1 was formed under the same conditions as in Example 1, except for the components of the deposition gas used in depositing the underlayer. In Reference Example 1, the deposition gas used in depositing the underlayer and surface layer was argon, and contained substantially no oxygen (see Table 1). In Reference Example 1, the thickness of the underlayer was 60 nm, and the thickness of the surface layer was 1700 nm.

[0038] (Table 1)

[0039] The ratios of elements contained in the zinc oxide coatings of Example 1 and Reference Example 1 were measured by X-ray photoelectron spectroscopy. The apparatus and conditions used for X-ray photoelectron spectroscopy are as follows: Apparatus: VersaProbe II (ULVAC-PHI) Excitation X-ray: Monochromatic AlKα ray (1486.6 eV) X-ray diameter: 200 μm X-ray power: 50 W / 15 KV Photoelectron detection angle: 45°

[0040] In addition, in order to calculate the element ratio at each position in the depth direction of the zinc oxide film, the zinc oxide film was scraped off by argon ion sputtering, and the element ratio was measured by X-ray photoelectron spectroscopy.

[0041] Fig. 2 is a graph showing the results of measuring the relationship between the depth and the ratio of elements forming the zinc oxide coating according to Example 1. Fig. 3 is a graph showing the results of measuring the relationship between the depth and the ratio of elements forming the zinc oxide coating according to Reference Example 1.

[0042] 2 and 3, the horizontal axis represents the position in the depth direction of the zinc oxide coating, and the vertical axis represents the elemental ratio (at%) of each element. In Example 1 and Reference Example 1, the zinc and oxygen content ratios decrease with increasing depth in the zinc oxide coating. On the other hand, the iron content ratio, which is a material derived from the substrate, increases with increasing depth in the zinc oxide coating.

[0043] In Example 1 and Reference Example 1, the position where the content ratios of zinc element and iron element in the depth direction are the same corresponds to the "second depth position" defined above, while the position where the content ratios of zinc element and oxygen element in the depth direction are the same corresponds to the "first depth position" defined above.

[0044] The mixed layer described above is defined as a region between the first depth position and the second depth position. In the zinc oxide coating of Example 1, the mixed layer had a thickness of 7.7 nm. On the other hand, in the zinc oxide coating of Reference Example 1, the mixed layer had a thickness of 4.4 nm. In Example 1, at the start of film formation, i.e., during film formation of the underlayer, sputtering was performed in an oxygen-containing atmosphere. This is thought to have resulted in the formation of a large amount of oxide derived from the substrate, i.e., iron oxide, near the interface between the zinc oxide coating and the substrate. As a result, the thickness of the mixed layer in Example 1 is thought to be greater than the thickness of the mixed layer in Reference Example 1.

[0045] In Example 1, the sputtering film was formed using zinc oxide as a target material in an oxygen-containing atmosphere at the start of film formation, and therefore, more oxygen atoms than zinc atoms were scattered toward the substrate, which is thought to have resulted in the formation of a larger amount of oxide derived from the substrate, i.e., iron oxide.

[0046] A reciprocating friction test was conducted on the sliding members having the zinc oxide coatings of Example 1 and Reference Example 1. In the reciprocating friction test, a ball made of steel (SUJ-2) having a diameter of 0.5 inches was moved back and forth on the surface of the substrate on which the zinc oxide coating was formed. Here, the reciprocating friction test was conducted with machine oil applied to the zinc oxide coating.

[0047] The steel ball was pressed against the substrate with a load of 3 kgf and moved back and forth on the substrate. The temperature during the test was room temperature. The stroke width of the steel ball was 20 mm, and the stroke speed was 10 mm / s. The steel ball was also moved back and forth on the slide member 100 having a zinc oxide coating.

[0048] Fig. 4 shows laser microscope photographs of the surfaces of the substrates in Example 1 and Reference Example 1 after the reciprocating friction test. Fig. 4 shows that the zinc oxide coating in Example 1 hardly peeled off even after the reciprocating friction test. In contrast, the zinc oxide coating in Reference Example 1 partially peeled off after the reciprocating friction test. This result shows that the durability of the zinc oxide coating in Example 1 is improved.

[0049] 5 is a graph showing the results of measuring the friction coefficients of the surfaces of the substrates in Example 1 and Reference Example 1 during the reciprocating friction test. It can be seen from Fig. 5 that in Example 1, the friction coefficient remained stable even as the number of sliding movements of the steel ball increased. In contrast, in Reference Example 1, the dynamic friction coefficient increased sharply after the 20th or so sliding movement, reaching approximately 0.16. This value is close to the dynamic friction coefficient of the substrate, and it is believed that the zinc oxide coating in Reference Example 1 had partially peeled off after the 20th or so sliding movement.

[0050] As described above, it is clear that the zinc oxide coating of Example 1 has higher durability than the zinc oxide coating of Reference Example 1. This is thought to be because the increased thickness of the mixed layer containing zinc oxide and iron oxide improves the adhesion between the zinc oxide coating and the substrate.

[0051] From the above perspective, it is believed that the durability of the zinc oxide coating increases as the mixed layer thickness increases, and from this perspective, it is believed that a mixed layer thickness of greater than 4.4 nm can provide a solid lubricating coating with greater durability than the solid lubricating coating of Reference Example 1.

[0052] As described above, the contents of the present invention have been disclosed through embodiments and examples, but the descriptions and drawings that form part of this disclosure should not be understood to limit the present invention. Various alternative embodiments, examples, and operating techniques will become apparent to those skilled in the art from this disclosure. Therefore, the technical scope of the present invention is defined only by the inventive features of the claims that can be reasonably understood from the above description.

[0053] This application claims priority based on Japanese Patent Application No. 2023-036077, filed on March 8, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A solid lubricating film formed on a substrate, The solid lubricating film has a mixed layer containing a metal oxide and an oxide derived from the substrate, When the first depth position is defined by a position in the depth direction where the oxygen atom content ratio matches the value obtained by multiplying the content ratio of the metal element forming the metal oxide by the valence of the metal element and dividing by the valence of the oxygen atom, and the second depth position is defined by a position in the depth direction where the metal element content ratio matches the content ratio of the element forming the oxide derived from the substrate, the mixed layer is defined between the first depth position and the second depth position. A solid lubricating film having a thickness greater than 4.4 nm of the aforementioned mixed layer.

2. The solid lubricating film according to claim 1, wherein the thickness of the mixed layer is greater than 4.8 nm and less than or equal to 20.0 nm.

3. The metal oxide is Ag ２ O, Al ２ O ３ 、Bi ２ O ３ 、CaO, Ce ２ O ３ 、Cr ２ O ３ 、CrO, CrO ３ 、Cu ２ O, CuO, Fe ２ O ３ 、Fe ３ O ４ 、FeO, In ２ O ３ 、Li ２ O, MgO, MnO, Nb ２ O ５ 、NiO, Sb ２ O ３ 、Sb ２ O ５ 、SiO ２ 、SnO ２ 、Ta ２ O ５ 、TiO ２ 、WO ３ 、ZnO, and ZrO ２ The solid lubricating coating according to claim 1, comprising at least one selected from the group consisting of

4. The solid lubricating film according to claim 1, wherein the oxide derived from the substrate is an oxide of at least one element belonging to groups 3 to 14.

5. The solid lubricating film according to claim 1, wherein the oxide derived from the substrate is an oxide of at least one element belonging to groups 3 to 11.

6. The solid lubricating film according to claim 1, wherein the oxide derived from the substrate is an oxide of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Ni, and Cu.

7. A solid lubricating film according to any one of claims 1 to 6, A sliding member having a substrate on which the solid lubricating film is formed.

8. A method for manufacturing a solid lubricating film according to any one of claims 1 to 6, The process includes a film formation step of forming a metal oxide film on a substrate, A method for producing a solid lubricant film, comprising at least one of the following: performing the film formation step in an atmosphere containing oxygen at least at the start of film formation; oxidizing the surface of the substrate before the film formation step; and diffusing oxygen atoms in the metal oxide after the film formation step.

9. The method for producing a solid lubricant film according to claim 8, wherein the film formation step includes sputtering a metal or metal oxide as a target material in an oxygen-containing atmosphere at least at the start of film formation.