JP7690730B2 - DLC film and members coated with it - Google Patents

Description

The present invention relates to a DLC film, a carbon-based thin film with properties similar to those of diamond, and to components coated with the DLC film. The DLC film has high hardness and excellent peel resistance, and can be used for sliding components such as machine parts, wear-resistant components such as tools, electronic components, and biomaterials.

Carbon-based thin films used in sliding and wear-resistant components are made of diamond or a material commonly known as DLC (diamond-like carbon). Diamond thin films are the hardest material, but their crystals are strong and their surface roughness is coarse, making it difficult to mirror-finish them, so their use as sliding components is limited.

On the other hand, DLC is classified into four types, ta-C, a-C, ta-C:H, and a-C:H, according to the ISO20523 standard. This classification is based on two factors: the ratio of the SP ³ hybrid orbital bond of diamond to the SP ² hybrid orbital bond of graphite for the C-C bonds in DLC, and the amount of hydrogen contained in DLC. Specifically, DLC with a ratio of SP ³ hybrid orbital bonds 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. DLC with a hydrogen content of 5% or less is classified as ta-C and a-C, and DLC with a hydrogen content of 5 to 50% is classified as ta-C:H and a-C:H. That is, the DLC closest to diamond is classified as ta-C, which has a hydrogen content of 5% or less and an SP ³ /(SP ² +SP ³ ) of 50% or more. Some ta-C exhibit high hardness of 70 GPa or more, which is a mechanical property close to diamond, and it is necessary to make diamond bonds more dominant than graphite bonds. For this reason, in the manufacturing method of ta-C, carbon ions are usually bombarded with a substrate with high energy, so the residual stress of the carbon film formed is large, the adhesion strength is low, and the film is brittle, which leads to a short life when applied to sliding members, etc.

In addition, a classification method based on optical property evaluation has been proposed to ISO as a classification method for DLC, and the standardization of testing by spectroscopic ellipsometry is underway. In the evaluation by spectroscopic ellipsometry, light with a wavelength of 550 nm (wavelength of 450 to 950 nm) is used, and DLC films are classified according to the refractive index and extinction coefficient as the optical properties of the DLC film. In general, the refractive index is closely related to the density, and the same can be considered for DLC if the composition is the same. In addition, the extinction coefficient of black graphite is large, while the extinction coefficient of transparent diamond is almost zero. It is also said to be related to the ratio of SP ² hybrid orbital bonds to SP ³ hybrid orbital bonds in the C-C bonds of DLC.

In Non-Patent Document 1, a proposed classification of DLC based on the refractive index n and extinction coefficient κ, as determined by the spectroscopic ellipsometry evaluation method, which has been proposed to and is being considered by the ISO, is shown in the optical classification diagram in Figure 1. The numerical ranges of the refractive index n and extinction coefficient κ for each classification are as follows: ta-C is in the range of 2.56<n<3.0, 0<κ<0.75, a-C is in the range of 2.04<n<2.42, 0.53<κ<0.86, ta-C:H is in the range of 2.42<n<2.56, 0<κ<0.75, and a-C:H is in the range of 2.04<n<2.42, 0<κ<0.86.

Patent Document 1 shows a DLC film and DLC-coated mold with a range limited by the refractive index n and extinction coefficient κ, while Patent Document 2 shows a DLC film and DLC film-coated article with a numerical limit by the refractive index n and extinction coefficient κ. Both aim to improve durability as protective films for molds and articles. Patent Document 1 recommends the ranges 2.5<n<2.8 and κ<0.2, while Patent Document 2 states that the ranges 2.5<n<3.0 and 0.05<κ<0.4 are excellent. In other words, it is shown that ta-C with a large refractive index and a small extinction coefficient κ<0.4 is a highly durable film.

JP 2008-297171 A International Publication No. 2016-021671

NEW DIAMOND No. 136 (2020) January issue pages 3-8

The present invention relates to a thin carbon film material used for wear-resistant components such as sliding members of machine parts and tools, and is a DLC (diamond-like-carbon) thin film material with excellent peeling resistance, wear resistance, and durability. In the ISO classification, ta-C has a high refractive index and high density, and is classified as the DLC closest to diamond, with a small extinction coefficient and high transparency. In the current manufacturing method, to produce a high-density DLC film (ta-C), carbon is bombarded with high-energy ions accelerated by a high voltage. The acceleration voltage is 50 to 150 V or more, which is significantly higher than the C-C bond energy of diamond, 7.2 eV. The high-energy bombardment brings the density of the thin carbon film material close to that of diamond, achieving high density and high hardness. However, at the same time, there is a problem that strong compressive residual stress remains in the thin film, making it prone to minute chipping and film peeling.

In the case of wear-resistant materials used in machine parts and tools, including DLC films, materials with high hardness are often considered to be highly wear-resistant materials. For thin film materials, a hardness tester is used to measure the hardness. A hardness tester is a method for measuring deformation resistance, and if the film has compressive residual stress, it is likely to show high hardness. In the case of DLC films, in order to increase the diamondification rate of the carbonaceous film, the impact of high-energy ions during film formation leaves a larger compressive residual stress than in normal thin films. In addition, because DLC is a material with excellent lubricity, it has a weak bond with other materials, i.e., DLC films have a weak adhesion to the substrate, and are therefore highly susceptible to the effects of film peeling forces caused by compressive residual stress.

If the condition of a DLC film with suppressed compressive residual stress is met, it can be said that a film with higher hardness, i.e., closer to diamond, is preferable for wear resistance. The density of diamond is 3.5 g/ ^cm3 , while that of graphite is 2.2 g/ ^cm3 . As a guideline for the durability of DLC films, it is also extremely important to know how close the film is to diamond, based on film density, etc., without placing too much emphasis on the hardness of the film from the viewpoint of compressive residual stress.

The present invention was made in consideration of these points, and aims to provide a DLC film that is high density, does not have large compressive residual stress, and is resistant to peeling and destruction. Another aim is to achieve a long life for components coated with the DLC film.

The invention of claim 1 is a diamond-like carbon (DLC) film having a film 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, the DLC film has a refractive index of 2.5 to 3.0 and an extinction coefficient 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 third aspect of the present invention is a member coated with the DLC film of the first or second aspect.

The DLC film of claim 1 has a refractive index of 2.5 to 3.0 and an extinction coefficient in the range of 0.75 to 1.20, and while it is high density, the compressive residual stress is not large, making the film less susceptible to peeling or destruction.
The DLC film of claim 2 has low compressive residual stress in the film and is resistant to breakage, and therefore has excellent peeling resistance.
A member coated with the DLC film of claim 1 or 2 can be used as a wear-resistant member such as a sliding member for machine parts or a tool where low friction and high wear resistance are desired, thereby achieving a longer life for the member.

FIG. 1 is a diagram for explaining a method for classifying DLC using optical property evaluation by spectroscopic ellipsometry. FIG. 2 is a diagram showing the range of refractive index and extinction coefficient of the DLC film of the present invention. FIG. 1 is a diagram showing the results of measuring the hydrogen content in the DLC film of the present invention by glow discharge optical emission spectroscopy (GD-OES).

As a classification method for DLC, a proposed standard as shown in FIG. 1 based on the refractive index n and extinction coefficient κ by the spectroscopic ellipsometry evaluation method has been proposed to ISO and is being considered (Non-Patent Document 1 NEW DIAMOND No. 136 (2020) January issue, pp. 3-8). This is a proposed standard presented by 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 the value obtained by dividing the speed of light in a vacuum by the speed of light in a substance, and is known as a method for easily finding the difference in film density from the difference in refractive index if the substance is the same. The refractive index of diamond is greater than 2.4, and that of graphite film is less than 2. On the other hand, the extinction coefficient κ is a parameter that indicates the material transmission of visible light, and the value increases as the visible light transmittance decreases. DLC films are amorphous films that contain two types of C-C bonds (the SP ³ hybrid orbital bonds of diamond and the SP ² hybrid orbital bonds of graphite). It is believed that the extinction coefficient increases when the dangling bonds, where the bonds between C-C bonds are broken, absorb light. In the case of translucent ceramics, it is known that when the ceramic crystal grain size is smaller than the wavelength of light, the transparency increases, and when the crystal grain size is larger than the wavelength, the transparency becomes opaque. When high wear resistance and durability are required for sliding members and machine parts, DLC films, which are similar to diamond, are good. In addition, to achieve high durability, it is desirable to minimize the residual stress of the film. To achieve this, it is desirable for the two types of C-C bond structures in the DLC film to be uniformly distributed at appropriate sizes and to have little strain within the film.

DLC films are produced by chemical vapor deposition (CVD) and physical vapor deposition (PVD). Hydrogen-free DLC films are generally produced by the PVD method. To produce hydrogen-free DLC films with a density close to that of diamond, a method is used in which ionized carbon is accelerated at a high voltage of 50 to 150 V or more (several kV in some cases) and bombarded with high energy on the substrate, or a method is used in which argon ions are accelerated at a high voltage and bombarded with high energy while carbon is being deposited on the substrate, thereby changing the C-C bonds to diamond-type bonds. However, DLC films produced by such methods have a density close to that of diamond due to the energy bombardment that is significantly higher than the C-C bond energy of diamond, 7.2 eV, but there is a problem that strong compressive residual stress remains, making it easy for minute chipping and film peeling to occur. In order to reduce excessive residual stress, it is effective to lower the ion acceleration voltage to the C-C bond energy level of diamond, but to obtain an even denser DLC film, it is possible to create a DLC film that exhibits high density and low residual stress by significantly increasing the number of bombardments with low-energy ions.

The extinction coefficient κ measured by the spectroscopic ellipsometry evaluation method is a parameter indicating the material transmission of visible light, and if the material is crystalline, it will show a small value if the crystal grains are smaller than visible light. 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 that shows a large extinction coefficient while having a high refractive index, as shown in the shaded area of FIG. 2 (optical evaluation region of the DLC film of the present invention). This is presumed to be because the DLC film is a high-density DLC film produced by bombardment several orders of magnitude more than usual with low-energy ions close to the C-C bond energy level of diamond, and the tissue structure consisting of SP ² hybrid orbital bonds and SP ³ hybrid orbital bonds is homogeneously dispersed even though it is amorphous, and the visible light is strongly absorbed due to the large number of dangling bonds or structure size present in the tissue structure, thereby increasing the extinction coefficient. In other words, it is believed that a homogeneous, low-residual-stress, high-density, high-quality DLC film could be produced by carefully carpet-bombarding the C-C bonds with low-energy ions.

For DLC films with excellent sliding properties, wear resistance, and durability, it is desirable to have high density and small compressive residual stress. The preferred range of refractive index n and extinction coefficient κ (optical evaluation range of the DLC film of the present invention) measured optically at a wavelength of 550 nm by spectroscopic ellipsometry is shown in the shaded area in Figure 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. In addition, the DLC film of the present invention is an optimal film because it has high density and shows small residual stress. The compressive residual stress value can be measured from the deformation amount of the Si substrate before and after film formation, and is preferably 0.5 to 2.0 GPa, which is smaller than the 2.5 to 7.0 GPa of conventional DLC films. The compressive residual stress is more preferably 0.5 to 1.5 GPa.

The thickness of the DLC film of the present invention is preferably 50 nm to 1.5 μm. If the film thickness is greater than 1.5 μm, the compressive stress becomes too strong, and film peeling is likely to occur in applications such as sliding members and machine parts that are subject to stress. As film peeling is less likely to occur as the film thickness becomes thinner, a film thickness of 1 μm or less is more preferable. The DLC film of the present invention is produced by carefully bombarding the film with low-energy ions in a carpet bombardment manner, so the surface of the DLC film also has excellent flatness. This results in a small coefficient of friction and excellent sliding properties.

The DLC film of the present invention does not substantially contain hydrogen. Figure 3 shows the results of hydrogen analysis of the DLC film of the present invention by glow discharge optical emission spectrometry (GD-OES), with the intensity of the detected element emission line on the vertical axis and the analysis time on the horizontal axis. The analysis time from 2 s to 30 s is the thin film region, from 30 s to 40 s is the interface region between the thin film and the Si substrate, and from 40 s onwards is the Si substrate region. Hydrogen was detected from 2 s to around 6 s at the start of the analysis, which is thought to be due to the detection of moisture adhering to the film surface during storage after film formation. In this way, hydrogen adsorbed on the film surface can be confirmed, but there is almost no hydrogen in the film. The inside of the DLC film is essentially composed of only carbon.

Hydrogen easily bonds with dangling bonds of carbon and easily forms C-H bonds. This C-H bond tends to prevent 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 diamond bonds of 50% or more is usually performed under high vacuum because they dislike moisture. However, water molecules adhering to the walls of the vacuum furnace decompose to generate hydrogen, so a trace amount of hydrogen of 0.5% or less may remain in the film. In addition, air or moisture may adhere to the surface of the DLC film by storing it in air after film formation. The DLC film contains two types of C-C bonds (SP ³ hybrid orbital bonds of diamond and SP ² hybrid orbital bonds of graphite), and the amount of these bonds was measured using NEXAFS of synchrotron radiation. The DLC film of the present invention had SP ³ bonds of 50% or more.

Since the DLC film of the present invention does not contain hydrogen in the film, the CVD method using a hydrocarbon gas as a raw material gas as a carbon source is not suitable. For the carbon source, a raw material that does not contain hydrogen, such as sputtering from a carbon target or fullerene C60, is used. In addition, since a homogeneous bombardment of the substrate with a large amount of ion source is required, it is preferable to use an ion beam such as argon or argon cluster.

The substrate used for the coated member of the present invention varies depending on the application of the coated member. For sliding members and machine parts, metals such as high carbon steel and die steel, and cemented carbide are selected. For electronic components, Si, ceramics, and heat-resistant resins are selected, and for biomaterials, Ti or Ti alloys and ceramics such as apatite are selected. When forming the DLC thin film of the present invention on the surface of various substrate materials, an intermediate layer film made of one or more of silicon, chromium, tungsten, titanium, and carbides thereof can be provided between the substrate and the DLC thin film in order to increase the adhesion strength with the substrate. The thickness of the intermediate layer is not particularly limited, but is preferably equal to or less than the thickness of the DLC thin film.

Below, examples of the present invention will be shown based on the test results of the present invention, and the invention will be described in more detail. Of course, the present invention is not limited to the following examples, and various more specific embodiments are possible.

A DLC thin film with a thickness of 150 nm was produced by irradiating the substrate with an argon ion beam in parallel with the deposition of carbonaceous material on the surface of the Si substrate. The film formation conditions are shown in Table 1. Samples 1 to 4 of the present invention and comparative samples 1 to 3 were produced using the acceleration energy of argon atoms and the ratio of the number of carbon atoms to the number of argon atoms as variables. In addition, optical measurements were performed on each sample using spectroscopic ellipsometry at a wavelength of 550 nm to measure the refractive index n and extinction coefficient κ of each sample. In addition, the compressive residual stress value was obtained by measuring the amount of deformation of the Si substrate before and after the formation of the DLC film.

All of the inventive samples 1 to 4 exhibited refractive index and extinction coefficient within the range of the inventive method, and the compressive residual stress was 2 GPa or less. In comparative sample 1, the number of argon atoms was high relative to the number of carbon atoms, but the accelerating voltage was excessively higher than the C-C bond energy of diamond, 7.2 eV, so a large stress remained in the film and the film became very hard. Comparative sample 2 shows an example of a DLC film produced by the arc ion plating method. An accelerating voltage higher than the C-C bond energy of diamond, 7.2 eV, was applied. It showed 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 peeling resistance. In comparative sample 3, the number of carbon atoms relative to the number of argon atoms was low, so no film was formed.

The DLC film according to the present invention, which has a refractive index of 2.5 to 3.0 and an extinction coefficient in the range of 0.75 to 1.20, has high density and small compressive residual stress, and is less susceptible to peeling or destruction, making it suitable for use in machine parts and tool components that require low friction, high wear resistance, and a long life. It can also be used in electronic components and biomaterials.

Claims (3)

A diamond-like carbon (DLC) film with a thickness of 50 nm to 1.5 μm that is substantially hydrogen-free, and in optical measurements at a wavelength of 550 nm using spectroscopic ellipsometry, the DLC film has a refractive index of 2.5 to 3.0 and an extinction coefficient in the range of 0.75 to 1.20. The DLC film of claim 1, in which the compressive residual stress of the film is 0.5 to 2.0 GPa. A component coated with the DLC film of claim 1 or 2.

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