DLC film and a member coated therewith

Abstract

To provide a DLC film in which a compressive residual stress is not large, and exfoliation or breakage of the film hardly occurs, while having a high density.SOLUTION: Concerning a DLC film comprising diamond-like carbon (DLC) not containing substantially hydrogen, and having a film thickness of 50 nm-1.5 μm, in optical measurement with a wavelength of 550 nm by spectroscopic ellipsometry, a refractive index thereof is 2.5-3.0, and an extinction coefficient is in the range of 0.75-1.20. A component coated therewith is also provided. A compressive residual stress of the thin film is 0.5-2.0 GPa.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a DLC film which is a carbon-based thin film having characteristics close to diamond and a member coated therewith. It has high hardness and excellent peel resistance, and can also be used for sliding members such as mechanical parts, wear-resistant members such as tools, electronic member applications, and biological members.

Background Art

[0002] For carbon-based thin films used for sliding members and wear-resistant members, diamond or a material generally referred to as DLC (Diamond-like-carbon) is used. The diamond thin film, which is the hardest material, is limited in use as a sliding member because its crystal is strong, its surface roughness is rough, and it is difficult to mirror-polish.

[0003] On the other hand, DLC is classified into four types, ta-C, a-C, ta-C:H, and a-C:H, according to the standard ISO20523. This classification is based on two factors: the ratio of the SP 3 hybrid orbital bond of diamond and the SP 2 hybrid orbital bond of graphite in the C-C bond in DLC, and the amount of hydrogen contained in DLC. Specifically, DLC with an SP 3 hybrid orbital bond ratio of 50% or more is classified as ta-C and ta-C:H, and DLC with a ratio of 50% or less is classified as a-C and a-C:H. Also, DLC with a hydrogen content of 5% or less is classified as ta-C and a-C, and those containing 5 to 50% are classified as ta-C:H and a-C:H. That is, the DLC closest to diamond has a hydrogen content of 5% or less and an SP 3 / (SP 2 +SP 3) is classified as ta-C with 50% or more. Some ta-C exhibits a high hardness of 70 GPa or more as a mechanical property close to diamond. For ta-C, it is necessary to have a diamond bond dominant over a graphite bond. Therefore, in the production method of ta-C, usually carbon ions are collided with a substrate at high energy, resulting in a large residual stress in the formed carbon film, low adhesion strength, and brittleness, so there were problems of short life when applied to sliding members etc.

[0004] Also, as a classification method for DLC, a classification method based on optical property evaluation has been proposed by ISO, and studies on standardization of tests by spectroscopic ellipsometry are underway. In the evaluation by spectroscopic ellipsometry, light with a wavelength of 550 nm (wavelength in the range of 450 to 950 nm) is used, and the DLC film is classified according to the refractive index and extinction coefficient as optical properties of the DLC film. Generally, the refractive index has a close relationship with density, and the same can be considered for DLC if the composition is the same. Also, regarding the extinction coefficient, graphite showing black has a large value, and the extinction coefficient of transparent diamond is almost zero. It is also said to be related to the ratio of sp 2 hybrid orbital bond and sp 3 hybrid orbital bond in the C-C bond of DLC.

[0005] In Non-Patent Document 1, a DLC classification proposal based on the refractive index n and extinction coefficient κ by the spectroscopic ellipsometry evaluation method proposed and studied by ISO is shown as in the optical classification diagram of Fig. 1. The numerical ranges of the refractive index n and extinction coefficient κ for each classification are shown below. For ta-C, the range is 2.56 < n < 3.0, 0 < κ < 0.75, for a-C, the range is 2.04 < n < 2.42, 0.53 < κ < 0.86, for ta-C:H, the range is 2.42 < n < 2.56, 0 < κ < 0.75, and for a-C:H, the range is 2.04 < n < 2.42, 0 < κ < 0.86.

[0006] In Patent Document 1, a DLC film and a DLC-coated mold limited in range by the refractive index n and the attenuation coefficient κ are shown, and in Patent Document 2, a DLC film and an article coated with a DLC film numerically limited by the refractive index n and the attenuation coefficient κ are shown. Both aim to improve durability as a protective film for molds and articles. In Patent Document 1, the range is 2.5 < n < 2.8 and κ < 0.2, and in Patent Document 2, the range of 2.5 < n < 3.0 and 0.05 < κ < 0.4 is shown to be excellent. That is, it shows that ta-C with a large refractive index and a small attenuation coefficient of κ < 0.4 is a highly durable film.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] The present invention relates to a thin-film carbon material used for sliding members such as machine parts and wear-resistant members such as tools, and is a DLC (Diamond-like-carbon) thin-film material excellent in peel resistance and wear resistance and having excellent durability. Ta-C in the ISO classification has a high refractive index and high density, and is classified as the DLC closest to diamond, and is positioned in a classification with a small attenuation coefficient and high transparency. In the current manufacturing method, in order to produce a high-density DLC film (ta-C), high-energy ions accelerated at a high voltage are bombarded onto carbon. The acceleration voltage is much higher than the C-C bond energy of diamond, 7.2 eV, and is 50 to 150 V or higher. Due to the high-energy impact, the density of the thin-film carbon material can be made close to the density of diamond, and high density and high hardness can be achieved. However, at the same time, strong compressive residual stress remains in the thin film, and there has been a problem that minute defects and film peeling are likely to occur.

[0010] In wear-resistant materials used for machine parts, tools, etc. including DLC films, high-hardness materials are often considered to be high-wear-resistant materials. In thin-film materials, an indentation hardness tester is used to measure hardness. The indentation hardness tester is a method of measuring deformation resistance, and when there is compressive residual stress in the film, the hardness is likely to be measured high. In the case of a DLC film, in order to increase the diamondization rate of the carbonaceous film, a larger compressive residual stress remains than in a normal thin film due to the impact of high-energy ions during film formation. In addition, since DLC is a material with excellent lubricity, the bonding force with other materials is weak, that is, the DLC film has a weak adhesion to the substrate, and thus has a property of being strongly affected by the film peeling force due to compressive residual stress.

[0011] If the condition of a DLC film with suppressed compressive residual stress is satisfied, it can also be said that for wear resistance, a film with higher hardness, that is, closer to diamond, is preferable. The density of diamond is 3.5 g / cm 3 while the density of graphite is 2.2 g / cm 3 . As an index of the durability of the DLC film, it is extremely important to know how close the film is to diamond based on the film density, etc., without emphasizing the hardness of the film from the viewpoint of compressive residual stress.

[0012] The present invention has been made in view of such points, and an object thereof is to provide a DLC film that has a high density, a small amount of compressive residual stress, and is less likely to cause film peeling or breakage. Another object is to achieve a longer service life of a member coated with the DLC film.

Means for Solving the Problems

[0013] The invention according to claim 1 is a diamond-like carbon (DLC) film having a thickness of 50 nm to 1.5 μm that is substantially free of hydrogen, and in optical measurement at a wavelength of 550 nm by spectroscopic ellipsometry, its refractive index is in the range of 2.5 to 3.0 and its extinction coefficient is in the range of 0.75 to 1.20. The invention according to claim 2 is characterized in that the compressive residual stress of the DLC film according to claim 1 is 0.5 to 2.0 GPa. The invention according to claim 3 is a member coated with the DLC film according to claim 1 or 2.

Effects of the Invention

[0014] The DLC film according to claim 1 has a refractive index in the range of 2.5 to 3.0 and an extinction coefficient in the range of 0.75 to 1.20. It has a high density, a small amount of compressive residual stress, and is less likely to cause film peeling or breakage. The DLC film according to claim 2 has a low compressive residual stress in the film and is difficult to break, so it has excellent peel resistance. A member coated with the DLC film according to claim 1 or 2 can be used as a sliding member such as a mechanical part where low friction and high wear resistance are desired, or a wear-resistant member such as a tool, and the service life of the member can be extended.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0016] As a classification method for DLC, a standard proposal as shown in FIG. 1 based on the refractive index n and extinction coefficient κ by spectroscopic ellipsometry evaluation method has been proposed and examined by ISO (Non-Patent Document 1 NEW DIAMOND No. 136 (2020), January issue, pages 3-8). This is a standard proposal shown from Japan to ISO as an optical classification method for DLC films, and shows the classification of DLC films using the refractive index n and extinction coefficient κ at a wavelength of 550 nm. The refractive index n is a value obtained by dividing the speed of light in vacuum by the speed of light in the substance, and is known as a method for easily knowing the difference in film density from the difference in refractive index for the same substance. The refractive index of diamond is greater than 2.4, and that of a graphite film is 2 or less. On the other hand, the extinction coefficient κ is a parameter indicating the transmission of visible light through a substance, and its value increases as the visible light transmittance deteriorates. The DLC film is an amorphous film in which two types of C-C bonds (the SP 3 hybrid orbital bond of diamond and the SP 2 hybrid orbital bond of graphite) coexist. It is considered that the extinction coefficient increases when the dangling bond part where the bond between C-C bonds is interrupted absorbs light. Also, in the case of translucent ceramics, it is known that the transparency increases when the size of the ceramic crystal particles is smaller than the wavelength of light, and it becomes opaque when the crystal size is larger than the wavelength. When high wear resistance and durability are desired in sliding members, mechanical parts, etc., a DLC film close to diamond is preferable. Also, in order to exhibit high durability, it is desirable to minimize the residual stress in the film. For this purpose, it is desirable that the two types of C-C bond structures of the DLC film are uniformly distributed in an appropriate size and there is little strain in the film.

[0017] The manufacturing methods of DLC films include the CVD method (chemical vapor deposition method) and the PVD method (physical vapor deposition method). Generally, the PVD method is used for manufacturing DLC films that do not contain hydrogen. To produce a DLC film with a density close to that of diamond and without hydrogen, methods such as accelerating ionized carbon at a high voltage of 50 - 150 V or more (in some cases, several kV) and bombarding the substrate with high energy, or accelerating argon ions, etc. at a high voltage and bombarding with high energy simultaneously with the deposition of carbon onto the substrate to change the C - C bonds into diamond - type bonds are used. However, the DLC films produced by such manufacturing methods are subjected to a much higher energy impact than the C - C bond energy of diamond (7.2 eV), so the density approaches that of diamond, but there is a problem that strong compressive residual stress remains, and microdefects and film peeling are likely to occur. To reduce excessive residual stress, it is advisable to lower the acceleration voltage of the ions to the C - C bond energy level of diamond. However, to obtain a denser DLC film, by significantly increasing the number of impacts by low - energy ions, a DLC film showing high density and low residual stress can be produced.

[0018] The extinction coefficient κ by spectroscopic ellipsometry evaluation method is a parameter indicating the transmission of visible light through a substance. For a crystalline material, if the crystal grains are smaller than visible light, it shows a small value. The DLC film of the present invention is in a region not included in the optical classification diagram of FIG. 1 and is in a region with a high refractive index and a large extinction coefficient like the shaded part in FIG. 2 (the optical evaluation region of the DLC film of the present invention). This is a high - density DLC film produced by bombarding with low - energy ions close to the C - C bond energy level of diamond (7.2 eV) many times more than usual, and although it is amorphous, it has an SP 2 hybrid orbital bond and SP 3 hybrid orbital bond, and the tissue structure composed of them is homogeneously dispersed. It is presumed that a large number of dangling bonds or structural sizes existing in the tissue structure strongly absorb visible light, thus increasing the extinction coefficient. That is, it is considered that a high - quality DLC film that is homogeneous, has little residual stress, and has high density can be produced by carefully bombarding the C - C bonds with low - energy ions in a carpet - bombing manner.

[0019] In a DLC film excellent in slidability, wear resistance, and durability, it is desired that the density is high and the compressive residual stress is small. The preferable ranges of the refractive index n and the extinction coefficient κ (the optical evaluation region of the DLC film of the present invention) at a wavelength of 550 nm by spectroscopic ellipsometry are shown in the shaded portion of FIG. 2. That is, the refractive index is in the range of 2.5 to 3.0 and the extinction coefficient is in the range of 0.75 to 1.20, which was not known in the conventional DLC classification method. Further, since the DLC film of the present invention has a high density and exhibits a small residual stress, it is an optimal film. The compressive residual stress value can be measured from the amount of deformation of the Si substrate before and after film formation, and is preferably 0.5 to 2.0 GPa, which is smaller than 2.5 to 7.0 GPa of the conventional DLC film. The compressive residual stress is more preferably 0.5 to 1.5 GPa.

[0020] The film thickness of the DLC film of the present invention is preferably 50 nm to 1.5 μm. When the film thickness is greater than 1.5 μm, the compressive stress becomes too strong, and in applications such as sliding members and mechanical parts that are stressed, film peeling is likely to occur. When the film thickness is thin, film peeling is less likely to occur, so a film thickness of 1 μm or less is more preferable. In the production of the DLC film of the present invention, since low-energy ions are carefully bombarded in a carpet bombing manner, the surface of the DLC film is also excellent in flatness. Therefore, the friction coefficient is small, and excellent sliding characteristics are exhibited.

[0021] The DLC film of the present invention substantially does not contain hydrogen. FIG. 3 shows the hydrogen analysis results in the DLC film of the present invention by glow discharge optical emission spectrometry (GD-OES method), where the vertical axis represents the intensity of the emission line of the detected element and the horizontal axis represents the analysis time. The region from 2 s to 30 s of the analysis time is the thin film region, the region from 30 s to 40 s is the interface region between the thin film and the Si substrate, and the region after 40 s is the Si substrate region. Hydrogen is detected from around 2 s to 6 s at the initial stage of the analysis start, and this is considered to be the detection of moisture adhering to the film surface during storage after film formation. Thus, although hydrogen adsorbed on the film surface can be confirmed, hydrogen is almost absent in the film. The inside of the DLC film is substantially composed of only carbon.

[0022] Hydrogen easily binds to the dangling bonds of carbon and tends to form C-H bonds. This C-H bond tends to hinder the formation of diamond bonds in C-C bonds and has a low density and a low refractive index. The formation of DLC films classified as ta-C with more than 50% diamond bonds is usually carried out under high vacuum to avoid moisture. However, since the water molecules adhering to the walls of the vacuum furnace decompose to generate hydrogen, a trace amount of hydrogen of 0.5% or less may remain in the film. Also, after film formation, air and moisture may adhere to the surface of the DLC film when stored in air. There are two types of C-C bonds (the SP 3 hybrid orbital bond of diamond and the SP 2 hybrid orbital bond of graphite) mixed in the DLC film, and the amount was measured using NEXAFS of synchrotron radiation. The DLC film of the present invention had 50% or more of SP 3 bonds.

[0023] Since the DLC film of the present invention does not contain hydrogen in the film, the CVD method using hydrocarbon gas as a raw material gas as a carbon source is not suitable. As the carbon source, a raw material containing no hydrogen such as sputtering from a carbon target or fullerene C60 is used. Also, since homogeneous substrate bombardment with a large amount of ion sources is required, it is preferable to use an ion beam such as argon or argon clusters.

[0024] The substrate used for the coating member of the present invention varies depending on the use of the coating member. For sliding members and machine parts, metals such as high-carbon steel and die steel, and cemented carbide are selected. Also, for electronic members, Si, ceramics, and heat-resistant resins are selected, and for biological materials, ceramics such as Ti or Ti alloys and apatite are selected. Also, when forming the DLC thin film of the present invention on the surface of various substrate materials, in order to increase the adhesion strength to the substrate, an intermediate layer film composed of one or more of silicon, chromium, tungsten, titanium, and their carbides can be provided between the substrate and the DLC thin film. The film thickness of the intermediate layer is not particularly limited, but is preferably not more than the film thickness of the DLC thin film.

[0025] Hereinafter, based on the test results of the present invention, examples of the present invention will be shown and described in more detail. Of course, the present invention is not limited to the following examples, and it goes without saying that various embodiments can be taken more specifically.

Example

[0026] By irradiating the substrate with an argon ion beam in parallel with the deposition of the carbonaceous material on the Si substrate surface, a DLC thin film with a thickness of 150 nm was fabricated. The film formation conditions are shown in Table 1. The acceleration energy of argon atoms and the ratio of the number of carbon atoms to the number of argon atoms were used as variables to fabricate Inventive Samples 1 to 4 and Comparative Samples 1 to 3. Also, for each sample, optical measurement was performed at a wavelength of 550 nm by spectroscopic ellipsometry, and the refractive index n and extinction coefficient κ of each sample were measured. In addition, the compressive residual stress value was determined from the measurement of the deformation amount of the Si substrate before and after the formation of the DLC film.

[0027]

Table 1

[0028] All of Inventive Samples 1 to 4 showed refractive indices and extinction coefficients within the scope of the present invention, and the compressive residual stress showed 2 GPa or less. In Comparative Sample 1, although the number of argon atoms was large relative to the number of carbon atoms, the acceleration voltage was excessively large compared to the C-C bond energy of diamond, 7.2 eV, so large stress remained in the film and a high-hardness coating was obtained. Comparative Sample 2 shows an example of fabricating a DLC film by the arc ion plating method. An acceleration voltage higher than the C-C bond energy of diamond, 7.2 eV, was applied. It showed a hardness close to that of diamond, a high refractive index, and a small extinction coefficient value. However, the residual stress was large, and there was a problem with peel resistance. In Comparative Sample 3, the number of carbon atoms was small relative to the number of argon atoms, and no coating was formed.

Industrial Applicability

[0029] The DLC film according to the present invention, having a refractive index in the range of 2.5 to 3.0 and an attenuation coefficient in the range of 0.75 to 1.20, is of high density and has low compressive residual stress, making it difficult for film peeling or breakage to occur. It can be utilized for mechanical parts, tool members, etc. that are desired to have low friction, high wear resistance, and long life. It can also be used for electronic member applications and biological member applications.

Claims

**Claim 1** A diamond-like carbon (DLC) with a film thickness of 50 nm to 1.5 µm that substantially does not contain hydrogen, and in optical measurement at a wavelength of 550 nm by spectroscopic ellipsometry, its refractive index is in the range of 2.5 to 3.0 and the extinction coefficient is in the range of 0.75 to 1.

20. **Claim 2** The DLC film according to Claim 1, wherein the compressive residual stress of the film is 0.5 to 2.0 GPa. **Claim 3** A member coated with the DLC film according to Claim 1 or 2.

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