JP7722810B2 - DLC layer forming method and mold manufacturing method - Google Patents

Description

The present invention relates to a method for forming a DLC (Diamond-Like Carbon) layer on a substrate surface.

Conventionally, a hard layer has been formed on the surface of press and forging dies to improve the durability of the die and reduce wear of the workpiece due to friction between the die surface and the workpiece. DLC layers, known for their high hardness, excellent durability, and low coefficient of friction, are increasingly being used as hard layers to form on die surfaces. Known methods for depositing DLC layers on die surfaces include plasma CVD, as described in Patent Document 1, and unbalanced magnetron sputtering (hereinafter referred to as UBM sputtering), as described in Patent Document 2.

JP 2015-178670 A JP 2013-079445 A

In recent years, in an effort to reduce the weight of automobiles, the material used for automotive parts has been replaced from steel to aluminum materials such as pure aluminum and aluminum alloys. The molding process for the workpiece is carried out in an environment where lubricant is present between the workpiece and the mold surface (a so-called wet environment). However, with molds on which a DLC layer has been formed using conventional methods, the coefficient of friction between the workpiece and the DLC layer tends to increase even in wet environments. When the workpiece is aluminum, the increased coefficient of friction tends to generate wear powder from the workpiece, which can stick to the surface of the mold. If molding is carried out under such conditions, there is a concern that the workpiece may be scratched.

The present invention was made in consideration of the above circumstances, and aims to provide a method for forming a DLC layer that can suppress an increase in the coefficient of friction between the surface of the DLC layer and a mating member in contact with the surface in a wet environment.

The inventors came up with the idea that by forming recesses on the surface of a DLC layer, oil reservoirs can be created, which serve to retain and supply lubricating oil, thereby suppressing an increase in the coefficient of friction between the DLC layer and a mating member that comes into contact with the surface of the DLC layer. They then focused on the UBM sputtering method as a DLC layer deposition method that can form recesses on the surface of the DLC layer and conducted extensive research.

As a result, they discovered that they could utilize the so-called reverse sputtering phenomenon, in which the deposited DLC layer is sputtered by the collision of argon ions. Furthermore, since supplying only argon gas as the deposition gas would result in too much collision energy of argon ions with the DLC layer, they discovered that the above objective could be achieved by supplying hydrocarbon gas and controlling its flow rate ratio, which led to the completion of this invention.

One aspect of the present invention that solves the above-mentioned problems is a method for forming a DLC layer on the surface of a substrate by unbalanced magnetron sputtering using a carbon target, the method comprising adjusting the flow rates of argon gas and hydrocarbon gas supplied into a chamber where the film formation process is performed so that the flow rate ratio (flow rate of argon gas/flow rate of hydrocarbon gas) between the argon gas and the hydrocarbon gas is 35 to 60, and applying a pulsed bias voltage of 190 to 275 V to the substrate.

In another aspect, the present invention is characterized by using the above-mentioned DLC layer deposition method to manufacture a mold in which the DLC layer is deposited on the surface of the substrate.

It is possible to form a DLC layer that can suppress an increase in the coefficient of friction with a mating member that comes into contact with the surface of the DLC layer in a wet environment. Note that in this specification, the coefficient of friction with a mating member that comes into contact with the surface of the DLC layer in a wet environment refers to the coefficient of friction measured using a ball-on-disk test, with polyalphaolefin interposed between the sample and the ball.

1 is a diagram showing an outline of a layered structure of a DLC coating material according to one embodiment of the present invention. 1 is a diagram showing a schematic configuration of a film forming apparatus as a UBM sputtering apparatus according to an embodiment of the present invention. FIG. 2 is a diagram showing the substrate shape of a test piece. FIG. 1 is a schematic diagram of a ball-on-disk test. 1 shows the results of a ball-on-disk test (wet environment) for Example 1 and Comparative Example 1. 10 shows the results of a ball-on-disk test (wet environment) for Example 2. 10 shows the results of a ball-on-disk test (wet environment) for Example 3. 10 shows the results of a ball-on-disk test (wet environment) for Example 4. 10 shows the results of a ball-on-disk test (wet environment) for Example 5. 1 shows the results of a ball-on-disk test (dry environment) for Example 1. 10 shows the results of a ball-on-disk test (dry environment) for Example 2. 10 shows the results of a ball-on-disk test (dry environment) for Example 3. 10 shows the results of a ball-on-disk test (dry environment) for Example 4. 10 shows the results of a ball-on-disk test (dry environment) for Example 5. 1 is an SEM image of the surface of the DLC layer of Example 1. 1 is an SEM image of the surface of the DLC layer of Comparative Example 1.

One embodiment of the present invention will be described below with reference to the drawings. Note that in this specification and drawings, elements that have substantially the same functional configuration will be assigned the same reference numerals, and redundant description will be omitted.

<Layer structure of DLC coating material>
1 is a diagram showing an example of a layer structure of a DLC coating material obtained by the manufacturing method of the present invention, in which a DLC layer is formed on the surface of a substrate. The DLC coating material 1 of this embodiment shown in FIG. 1 includes a substrate 2, an intermediate layer 3 formed on the substrate 2, a DLC intermediate layer 4 formed on the intermediate layer 3, and a DLC layer 5 formed on the DLC intermediate layer 4.

The substrate 2 is typically made of a steel material such as alloy tool steel for mold applications, such as SKD11, or high-speed tool steel, but is not limited to these steel materials.

The intermediate layer 3 is an optional layer formed to improve adhesion between the substrate 2 and the DLC layer 5. The intermediate layer 3 has a hardness intermediate between that of the substrate 2 and that of the DLC layer 5. The intermediate layer 3 preferably contains elements contained in the chemical composition of the substrate 2 to improve compatibility with the substrate 2 and adhesion thereto. For example, the intermediate layer 3 preferably contains one or more elements selected from the group consisting of Cr, W, Ti, V, and Mo, which may be contained in the chemical composition of steel. Furthermore, when the substrate 2 is a steel material such as alloy tool steel or high-speed tool steel for mold applications, the intermediate layer 3 preferably contains chromium, which is abundant in such steel materials. Furthermore, the intermediate layer 3 preferably has a hardness that increases from the substrate 2 side toward the DLC layer 5 side. The intermediate layer 3 may have a multi-layer structure.

The DLC intermediate layer 4 is a layer made of carbon that is deposited as needed to improve adhesion between the intermediate layer 3 and the DLC layer 5, and has a hardness intermediate between that of the intermediate layer 3 and that of the DLC layer 5. It is also preferable that the hardness of the DLC intermediate layer 4 increases from the substrate 2 side toward the DLC layer 5 side.

The DLC intermediate layer 4 of this embodiment has a two-layer structure including a first DLC intermediate layer 4a formed on the intermediate layer 3 and a second DLC intermediate layer 4b formed on the first DLC intermediate layer 4a and denser than the first DLC intermediate layer 4a. The second DLC intermediate layer 4b preferably has a higher hardness than the first DLC intermediate layer 4a. Note that the DLC intermediate layer 4 is not limited to a two-layer structure, and does not have to have a multi-layer structure.

<Method of manufacturing DLC coating material>
A method for manufacturing the DLC coated material 1 in which the DLC layer 5 is formed on the surface of the substrate 2 will be described. The DLC coated material 1 is formed by UBM sputtering.

(Film forming equipment)
A deposition apparatus used to manufacture DLC coating materials includes a sputtering pulse power supply that supplies power to the target, multiple magnetic poles arranged behind the target, a bias pulse power supply that applies a negative bias voltage to the substrate in pulses, and gas inlets that supply argon gas and hydrocarbon gas. The UBM sputtering method is a deposition method that uses an ion-assisted effect in such an apparatus, which increases the plasma density near the substrate by imbalancing the magnetic poles and extending some of the magnetic field lines to the vicinity of the substrate, and also increases the collision energy of argon ions with the substrate by applying a bias voltage to the substrate.

Figure 2 shows an example of a film formation apparatus as a UBM sputtering device. The film formation apparatus 10 shown in Figure 2 includes a chamber 11 in which a substrate 2 is housed and a film formation process is performed, a stage 12 on which the substrate 2 is placed, and a bias pulse power supply 13 connected to the stage 12 and applying a bias voltage to the substrate 2 via the stage 12. A gas inlet 14 is provided on the side wall of the chamber 11 to supply the film formation gas used in forming the intermediate layer 3, DLC intermediate layer 4, and DLC layer 5. The gas inlet 14 is connected to a gas supply source 15 equipped with a control unit for adjusting the flow rate of the film formation gas. A gas exhaust pipe 16 is provided on the bottom wall of the chamber 11 to exhaust the atmospheric gas from the chamber 11 and is connected to a vacuum pump (not shown). During the film formation process, the pressure within the chamber 11 is controlled by appropriate evacuation via the gas exhaust pipe 16. Various targets 17 used in forming the intermediate layer 3, DLC intermediate layer 4, and DLC layer 5 are arranged above the stage 12. The target 17 is connected to a sputtering pulse power supply 18 located outside the chamber 11. A magnetic pole 19 is provided on the back side of the target 17, and when the magnetic pole 19 becomes unbalanced, the generated plasma 20 reaches the vicinity of the substrate 2 placed on the stage 12.

The manufacturing method for DLC coating material 1 will be explained below in order of steps.

(Substrate cleaning process)
Before forming the intermediate layer 3, the DLC intermediate layer 4, and the DLC layer 5 on the substrate 2, an argon bombardment process is performed as necessary to clean the surface of the substrate 2.

(Intermediate layer film formation process)
To improve adhesion between the substrate 2 and the DLC layer 5, a film formation process for the intermediate layer 3 is performed as needed. The film formation conditions are not particularly limited as long as an intermediate layer 3 having a hardness intermediate between that of the substrate 2 and that of the DLC layer 5 can be formed. Furthermore, from the viewpoint of improving adhesion to the substrate 2, the intermediate layer 3 may be formed by appropriately changing the power supplied to the target 17 so that the intermediate layer 3 contains one or more elements selected from the group consisting of Cr, W, Ti, V, Mo, and C that may be contained in the chemical composition of the substrate 2. For example, when the steel material of the substrate 2 is SKD11, it is preferable to form an intermediate layer containing Cr, W, and C to a thickness of 1 μm or more by UBM sputtering. Furthermore, from the viewpoint of improving adhesion between the substrate 2 and the DLC layer 5, the voltage of the sputtering pulse power supply may be changed continuously or stepwise to form the intermediate layer 3 so that its hardness increases from the substrate 2 side to the DLC layer 5 side. Furthermore, the film formation process may be performed so that the intermediate layer 3 has a multi-layer structure as needed.

(DLC intermediate layer formation process)
In order to improve the adhesion between the intermediate layer 3 and the DLC layer 5, a film-forming step of the DLC intermediate layer 4 is carried out as necessary.

The deposition conditions are not particularly limited as long as they allow deposition of a DLC intermediate layer 4 having a hardness intermediate between that of the intermediate layer 3 and that of the DLC layer 5. Furthermore, from the perspective of improving adhesion between the intermediate layer 3 and the DLC layer 5, the power supplied to the carbon target during deposition of the DLC intermediate layer 4 is set lower than the power supplied to the carbon target during deposition of the DLC layer 5, for example, so that the DLC intermediate layer 4 has a lower hardness than the DLC layer 5. Furthermore, from the perspective of improving adhesion between the intermediate layer 3 and the DLC layer 5, the bias voltage during deposition is increased continuously or stepwise, changing the density of the film so that the hardness increases from the substrate 2 side toward the DLC layer 5 side.

If necessary, the DLC intermediate layer 4 may be deposited so that it has a multi-layer structure. For example, after depositing a first DLC intermediate layer 4a, a second DLC intermediate layer 4b, which is denser than the first DLC intermediate layer 4a, may be deposited. When depositing such a DLC intermediate layer 4, for example, the bias voltage may be kept constant during deposition of the first DLC intermediate layer 4a, and the bias voltage may be gradually increased during deposition of the second DLC intermediate layer 4b.

(DLC layer formation process)
Argon gas and hydrocarbon gas are supplied to the chamber 11 as deposition gases for the DLC layer 5. Examples of hydrocarbon gases include methane ( _CH4 ), acetylene ( _C2H2 ), ethylene ( _C2H4 ), ethane ( _{C2H6 )} , benzene _{( C6H6 )} , and toluene _{( C7H8} ). From the standpoints of ease of handling and cost, it is preferable to use methane gas or acetylene gas. When supplying deposition gases to the chamber 11, _a single type of hydrocarbon gas may be supplied, or a mixture of multiple types of hydrocarbon gases may be supplied.

A negative bias voltage is applied to the substrate 2. In this specification, when comparing bias voltages, the bias voltage with a smaller absolute value is referred to as the "small bias voltage." In other words, the larger the bias voltage, the more argon ions generated by the plasma are attracted to the substrate 2, and the stronger the energy with which they collide with the substrate 2. In the DLC layer 5 deposition process, the bias voltage is set to 175 to 400 V. Applying a bias voltage to the substrate 2 within this range facilitates the formation of recesses on the surface of the DLC layer 5. If the bias voltage is less than 175 V, the argon ions are attracted to the substrate 2, and the energy with which they collide with the substrate 2 is weak, preventing the formation of recesses on the surface of the DLC layer 5. If the bias voltage exceeds 400 V, the argon ions are attracted to the substrate 2, and the energy with which they collide with the substrate 2 is too strong, preventing the formation of the DLC layer 5. Note that the bias voltage may vary during the deposition of the DLC layer 5 as long as it is within the range of 175 to 400 V. Additionally, the bias voltage is preferably 300V or less.

The DLC layer 5 is formed when argon ions collide with the carbon target, causing the sputtered carbon to deposit on the substrate 2. Meanwhile, while a bias voltage is applied to the substrate 2, argon ions are attracted to the substrate 2 and collide with the formed DLC layer 5. In other words, while a bias voltage is applied to the substrate 2, the formation of the DLC layer 5 and the collision of argon ions with the DLC layer 5 occur simultaneously. While the application of the bias voltage to the substrate 2 is stopped, the argon ions mainly collide with the carbon target, and the sputtered carbon deposits on the substrate 2, thereby progressing the formation of the DLC layer 5.

It is believed that the time for which the bias voltage is applied to the substrate 2 affects the number of recesses formed on the surface of the DLC layer 5. The voltage application time of the bias pulse power supply 13 can be controlled by adjusting the duty ratio, which is the voltage application time per cycle. The duty ratio is calculated as follows:
Duty ratio [%] = 100 x application time (ON time) / {application time (ON time) + application stop time (OFF time)}

The duty ratio is preferably set to 30-80%. When the duty ratio is 30% or higher, recesses are more easily formed on the surface of the DLC layer 5 than when the duty ratio is less than 30%, allowing for a larger number of recesses. When the duty ratio is 80% or lower, the DLC layer 5 is more easily formed than when the duty ratio exceeds 80%. Therefore, to achieve a high level of both DLC layer 5 formation and recess formation, the duty ratio is preferably set to 30-80%. For example, when the bias voltage is 175-300V, a duty ratio of 60-80% is preferred.

In the DLC layer 5 deposition process, simply setting the bias voltage to 175-400 V may result in the energy of argon ions colliding with the substrate 2 being too strong, preventing the DLC layer 5 from being deposited. Therefore, in the DLC layer 5 deposition process, the flow rates of the argon gas and hydrocarbon gas supplied into the chamber 11 are adjusted so that the flow rate ratio (argon gas flow rate/hydrocarbon gas flow rate) is maintained within a range of 20-60. Because the argon gas and hydrocarbon gas are supplied while the chamber 11 is being evacuated, the flow rate ratio (argon gas flow rate/hydrocarbon gas flow rate) within the chamber 11 is also 20-60. For example, when the bias voltage is 175-300 V, the flow rate ratio (argon gas flow rate/hydrocarbon gas flow rate) of the argon gas and hydrocarbon gas is preferably 30 or greater. Furthermore, the flow rate ratio is preferably 50 or less. The flow rates of the gases are "mass flow rates" and are controlled by a mass flow controller. In this specification, "mass flow rate" refers to a "flow rate converted to standard conditions" that is independent of the actual gas temperature and pressure used, with "standard conditions" being 101.3 kPa (1 atm) and 0°C.

By setting the flow rate ratio of argon gas to hydrocarbon gas (argon gas flow rate/hydrocarbon gas flow rate) between 20 and 60, ionization of the argon gas is suppressed, and the number of argon ions attracted to the substrate 2 is reduced. This prevents the energy of argon ions colliding with the DLC layer 5 from becoming too strong, making it possible to form recesses in the surface of the DLC layer 5 while depositing the DLC layer 5. The flow rate of argon gas supplied to the chamber 11 is preferably between 100 and 450 ml/min, and the flow rate of hydrocarbon gas is preferably between 5 and 15 ml/min. Experiments conducted by the inventors have confirmed that the DLC layer 5 is not deposited if hydrocarbon gas is not supplied.

The sputtering power used in the DLC layer 5 deposition process is preferably 2.5 to 12 kW. "Sputtering power" refers to the power supplied to the target 17. The sputtering power is more preferably 3 to 10 kW, and even more preferably 5 to 6 kW.

The sputtering power density in the film formation process of the DLC layer 5 is preferably 2.5 to 10 W/ ^cm2 . "Sputtering power density" is the value obtained by dividing the sputtering power (W) to the target 17 by the area ( ^cm2 ) of the surface 17a of the target facing the substrate 2. A sputtering power density of 2.5 W/ ^cm2 or higher promotes sputtering of the carbon target, increasing the speed at which the DLC layer 5 is formed and improving production efficiency. Furthermore, a sputtering power density of 10 W/ ^cm2 or lower is desirable because it makes it possible to suppress cracking of the target due to an increase in target temperature.

Through the above steps, the DLC coating material 1 is manufactured. In the DLC coating material 1 of this embodiment, recesses are formed on the surface of the DLC layer 5 during the DLC layer 5 deposition process. These recesses function as oil reservoirs. If the DLC coating material 1 is, for example, a mold, they can steadily supply lubricant to the workpiece and the mold surface, even when the mold is used repeatedly. This makes it possible to suppress an increase in the coefficient of friction between the workpiece and the mold surface in a wet environment. In particular, when the workpiece is aluminum, an increase in the coefficient of friction tends to generate wear particles from the aluminum. However, a mold having a DLC layer 5 with recesses can suppress an increase in the coefficient of friction, thereby significantly suppressing scratches on the workpiece caused by wear particles. In addition to molds, the DLC coating material 1 can also be used for components that slide against other components.

From the perspective of reducing wear and ensuring high durability, it is preferable that the Vickers hardness of the DLC layer 5 be 1800 HV or higher. Furthermore, from the perspective of reducing film stress and improving adhesion, it is preferable that the Vickers hardness of the DLC layer 5 be 4000 HV or lower.

The above explanation focuses on a DLC layer with a low coefficient of friction in a wet environment using lubricant. However, from the perspective of reducing costs and improving the working environment, it is desirable for molding processing of workpieces to be carried out in an environment where no lubricant is present between the workpiece and the mold surface (a so-called dry environment). However, molding processing in a dry environment results in greater friction between the workpiece and the DLC layer than processing in a wet environment, making it a harsh environment for molding materials such as aluminum.

To form a DLC layer with a low coefficient of friction even in such a dry environment, it is preferable to set the flow rate ratio of argon gas to hydrocarbon gas (argon gas flow rate/hydrocarbon gas flow rate) to 35 to 60 and the bias voltage to 190 to 275 V in the DLC layer formation process described above. A DLC layer 5 formed to meet these conditions will have a low coefficient of friction with the workpiece not only in wet environments but also in dry environments. To achieve a DLC layer with a low coefficient of friction even in dry environments, the flow rate ratio of argon gas to hydrocarbon gas is preferably 50 or less, and even more preferably 40 or less. To achieve a DLC layer with a low coefficient of friction even in dry environments, it is more preferable that the bias voltage be 200 V or more. From the same perspective, it is even more preferable that the bias voltage be 250 V or less. Note that, in this specification, the coefficient of friction between the DLC layer surface and a mating member in contact with the surface in a dry environment refers to the coefficient of friction measured by a ball-on-disk test.

Although one embodiment of the present invention has been described above, the present invention is not limited to such an example. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Test pieces were prepared with a DLC layer formed on the surface of the substrate, and their properties were evaluated.

SKD11, a type of alloy tool steel, was prepared and processed to a diameter of 22 mm and a thickness of 7 mm as shown in Figure 3. This was used as the substrate. The deposition system used to deposit the intermediate layer and DLC layer was the UBMS707 UBM sputtering system manufactured by Kobe Steel, Ltd., which has the configuration shown in Figure 2 above.

The substrate was vacuum hardened and tempered. The substrate's coating surface (Figure 3) was then polished until the 10-point average roughness (Rzjis) specified in JIS B 0601:2013 was 0.5 μm or less. The substrate was then immersed in acetone (special grade) manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. and ultrasonically cleaned for 10 minutes. After ultrasonic cleaning, nitrogen was blown onto the substrate to dry it, and the substrate was then loaded into the chamber of the UBM sputtering device.

The chamber was then evacuated to 2.6 × ¹⁰ Pa, and the substrate was heated for 120 minutes using a heater until the substrate reached 180°C. After heating, the surface of the substrate was cleaned by Ar bombardment. After cleaning, a chromium target, a tungsten carbide target, and a carbon target were sputtered by UBM sputtering to form an intermediate layer (thickness 1.7 µm) containing Cr, W, and C as elements.

(DLC intermediate layer film formation process)
Next, a first DLC intermediate layer and a second DLC intermediate layer were formed on the intermediate layer composed of chromium, tungsten, and carbon by unbalanced magnetron sputtering. The conditions for forming the DLC intermediate layer are shown in Table 1 below. In the following explanation, "sputtering power" refers to the power supplied to the target. "Sputtering power density" refers to the power (W) supplied to the carbon target divided by the area (cm2) of the carbon target's surface facing the substrate.

(First DLC Intermediate Layer Forming Process)
Argon gas was supplied into the chamber at a flow rate of 300 ml/min, and methane gas was supplied at a flow rate of 8 ml/min. The bias voltage, which is a negative voltage applied to the substrate, was set to 100 V, the sputtering power of the carbon target was set to 6 kW, and the duty ratio of the bias voltage was set to 75%, and sputtering of the carbon target was carried out for 7.4 minutes.

(Second DLC Intermediate Layer Forming Process)
Subsequently, the carbon target was sputtered while the bias voltage was gradually increased from 100 V to 200 V over a period of 36.6 minutes.

(DLC layer deposition process)
Next, a DLC layer was formed on the second DLC intermediate layer by unbalanced magnetron sputtering. The DLC layer formation conditions are shown in Table 2 below.

Example 1
In this example, the argon gas flow rate and methane gas flow rate during the DLC layer deposition process were 300 ml/min and 8 ml/min, respectively. The flow rate ratio of the argon gas to the hydrocarbon gas (argon gas flow rate/hydrocarbon gas flow rate) supplied into the chamber was 37.5, within the aforementioned range (20 to 60). While adjusting the flow rates of the argon gas and hydrocarbon gas in this manner, the pulsed bias voltage applied to the substrate was set to 200 V, with a bias voltage duty ratio of 75%, and sputtering of the carbon target was performed for 264 minutes. This resulted in a 1.0 μm DLC layer being deposited on the second DLC intermediate layer. Through the above steps, a test piece having a DLC layer deposited on the surface of a substrate was prepared as Example 1 of the present invention.

Example 2
A test piece on which a DLC layer of 1.0 μm was formed was prepared under the same conditions as in Example 1, except that the pulsed bias voltage applied to the substrate in the DLC layer forming step was set to 250 V.

Example 3
A test piece on which a DLC layer of 1.0 μm was formed was prepared under the same conditions as in Example 1, except that the pulsed bias voltage applied to the substrate in the DLC layer forming step was set to 300 V.

Example 4
A test piece on which a DLC layer of 1.0 μm was formed was prepared under the same conditions as in Example 1, except that the pulsed bias voltage applied to the substrate in the DLC layer forming step was set to 175 V.

Example 5
Test pieces on which a 1.0 μm DLC layer was formed were prepared under the same conditions as in Example 1, except that the flow rate of methane gas was changed to 10 ml/min in the DLC layer formation process and the flow rate ratio of argon gas to hydrocarbon gas (flow rate of argon gas/flow rate of hydrocarbon gas) was set to 30.

(Comparative Example 1)
Test pieces were prepared under the same conditions as in Example 1, except that the DLC intermediate layer deposition process was not performed and a DLC layer was deposited by plasma CVD on the surface of the intermediate layer made of chromium, tungsten, and carbon. The DLC layer of Comparative Example 1 was deposited under conditions in which only acetylene gas was supplied to the chamber at a flow rate of 1000 ml/min. In the DLC layer deposition process of Comparative Example 1, a pulsed bias voltage applied to the substrate was set to 800 V, with a bias voltage duty ratio of 30%, and the deposition process was performed for 130 minutes, resulting in the deposition of a 1.5 μm DLC layer.

A ball-on-disk test was performed on the test specimens of Examples 1 to 5 and Comparative Example 1 to evaluate their resistance to adhesion to aluminum materials. A hardness measurement test was also performed to evaluate the hardness of the DLC layer. Furthermore, FE-SEM observation was performed on the test specimens of Example 1 and Comparative Example 1 to observe the surface of the DLC layer.

(Ball-on-disk test)
The test machine is CSM
A Tribometer manufactured by Instruments was used. The disk was a test specimen from any of Examples 1 to 5 and Comparative Example 1. A pure aluminum ball with a diameter of 6 mm was used as the ball to be contacted with the disk. Under a wet environment with a temperature of 23-24°C, a humidity of 21%, and a drop of polyalphaolefin applied to the disk as a lubricant, the aluminum ball was brought into contact with the disk as shown in Figure 4. A load of 13 N was applied to the aluminum ball, and the disk was rotated at a sliding speed of 0.05 m/s. The contact point between the aluminum ball and the disk was a point 5 mm radius from the center of the disk. The friction force between the disk and the aluminum ball was measured at 0.2-second intervals until the sliding distance between the disk and the aluminum ball reached 100 m, and the friction coefficient was measured using TRIBOX, the software provided with the testing machine. The test was stopped when the friction coefficient reached 0.5 or greater. The ball-on-disk test was also conducted in a dry environment without applying lubricant to the disk. The method for the ball-on-disk test in a dry environment is similar to the method for the ball-on-disk test in a wet environment, except that no lubricant is applied to the disk.

The results of the ball-on-disk tests are shown in Figures 5 to 14 and Table 2 above. Note that Figures 5 to 9 show test results in a wet environment, while Figures 10 to 14 show test results in a dry environment. The lines in the graphs in each figure connect the measured values of the friction coefficient measured at 0.2 second intervals.

<Wet environment>
As shown in Figure 5 and Table 2, the coefficient of friction of the test piece of Example 1 varied between 0.003 and 0.093 until the sliding distance reached 100 m, but no significant increase in the coefficient of friction was observed, demonstrating excellent adhesion resistance to the aluminum material. Figure 5 also shows the results of Comparative Example 1. The coefficient of friction of the test piece of Comparative Example 1 rose to 0.5 or more immediately after the start of sliding, and the test was stopped.
As shown in FIG. 6 and Table 2, the friction coefficient of the test piece of Example 2 changed between 0.027 and 0.122 until the sliding distance reached 100 m, but no significant increase in the friction coefficient was observed, and the test piece had excellent adhesion resistance to the aluminum material.
As shown in FIG. 7 and Table 2, the friction coefficient of the test piece of Example 3 changed between 0.033 and 0.155 until the sliding distance reached 100 m, but no significant increase in the friction coefficient was observed, and the test piece had excellent adhesion resistance to the aluminum material.
As shown in FIG. 8 and Table 2, the friction coefficient of the test piece of Example 4 changed between 0.007 and 0.093 until the sliding distance reached 100 m, but no significant increase in the friction coefficient was observed, and the test piece had excellent adhesion resistance to the aluminum material.
As shown in FIG. 9 and Table 2, the friction coefficient of the test piece of Example 5 changed between 0.089 and 0.147 until the sliding distance reached 100 m, but no significant increase in the friction coefficient was observed, and the test piece had excellent adhesion resistance to the aluminum material.

These results show that in Examples 1 to 5, the coefficient of friction with the aluminum material in a wet environment was extremely low at 0.2 or less, and a DLC layer was formed in which an increase in the coefficient of friction was suppressed. Furthermore, in Examples 1 to 5, the DLC layer was formed at a high deposition rate of 0.2 μm/hr or more. In other words, the DLC layer deposition method of the present invention makes it possible to obtain a DLC layer with a low coefficient of friction in a wet environment in a short period of time.

<Dry environment>
As shown in FIG. 10 and Table 2, the friction coefficient of the test piece of Example 1 changed between 0.135 and 0.296 until the sliding distance reached 100 m, but no significant increase in the friction coefficient was observed, and the test piece had excellent adhesion resistance to the aluminum material.
As shown in FIG. 11 and Table 2, the friction coefficient of the test piece of Example 2 changed between 0.099 and 0.269 until the sliding distance reached 100 m, but no significant increase in the friction coefficient was observed, and the test piece had excellent adhesion resistance to the aluminum material.
As shown in FIG. 12 and Table 2, the coefficient of friction of the test piece of Example 3 rose to 0.5 or more immediately after the start of sliding, and the test was stopped.
As shown in FIG. 13 and Table 2, the coefficient of friction of the test piece of Example 4 rose to 0.5 or more immediately after the start of sliding, and the test was stopped.
As shown in FIG. 14 and Table 2, the coefficient of friction of the test piece of Example 5 rose to 0.5 or more immediately after the start of sliding, and the test was stopped.
As shown in Table 2, the coefficient of friction of the test piece of Comparative Example 1 rose to 0.5 or more immediately after the start of sliding, and the test was stopped.

These results show that in Examples 1 and 2, a DLC layer with a low friction coefficient was formed not only in wet environments but also in dry environments.

(hardness measurement)
Hardness measurements were performed using the nanoindentation method using a Fischer Instruments PICODENTOR (registered trademark) Hm500. Specifically, a Berkovich indenter was pressed into the test specimen with a maximum indentation load of 5 mN, and the indentation depth was continuously measured. From the obtained indentation depth measurement data, Martens hardness and Vickers hardness converted from Martens hardness were calculated using Fischer Instruments' software "Product Name: WIN-HCU (registered trademark)." The calculated Vickers hardness was displayed on the screen of the measuring device, and this value was treated as the hardness of the film at the measurement point. In this example, the Vickers hardness was measured at 20 arbitrary points on the surface of each test specimen, and the average of the obtained hardness values was recorded as the hardness of the film. Note that when pressing the indenter into the test specimen, the indentation load may propagate up to approximately 10 times the maximum indentation depth of the indenter. Therefore, if the propagation of the indentation load reaches the substrate of the test specimen, the influence of the substrate may be included in the hardness measurement results. Therefore, in order to measure the hardness of a pure hard film, it is necessary to satisfy the following condition: "thickness of hard film>maximum indentation depth of indenter×10".

As shown in Table 2, the test piece of Example 1 had a sufficient hardness of 2629 HV in Vickers hardness.
The test piece of Example 2 had a sufficient hardness of 2535 HV in Vickers hardness.
The test piece of Example 3 had a sufficient hardness of 2682 HV in Vickers hardness.
The test piece of Example 4 had a sufficient hardness of 2404 HV in Vickers hardness.
The test piece of Example 5 had a sufficient hardness of 2215 HV in Vickers hardness.
The test piece of Comparative Example 1 had a sufficient hardness of 2052 HV in Vickers hardness.

(FE-SEM surface observation)
Next, the surface of each test piece, i.e., the surface of the DLC layer of each test piece, was observed using an FE-SEM. Fig. 15 is an observation image of the DLC layer surface of Example 1. The DLC layer of Example 1 had recesses formed in places. Fig. 16 is an observation image of the DLC layer surface of Comparative Example 1. The DLC layer of Comparative Example 1 had minute protrusions formed on the surface, but unlike Example 1, no recesses were formed.

These results indicate that the reason the test piece of Example 1 has a lower coefficient of friction with the aluminum material than the test piece of Comparative Example 1 is because recesses are formed on the surface of the DLC layer. Furthermore, this type of DLC layer is formed by setting the bias voltage to a high value and specifying the flow rate ratio of argon gas to hydrocarbon gas (argon gas flow rate/hydrocarbon gas flow rate) within a specific range during the DLC film formation process.

The present invention can be used, for example, to form a hard film on the surface of a mold used to mold aluminum material.

1 DLC coating material 2 Base material 3 Intermediate layer
4 DLC intermediate layer 4a First DLC intermediate layer 4b Second DLC intermediate layer 5 DLC layer 10 Film forming apparatus 11 Chamber 12 Stand 13 Bias pulse power supply 14 Gas inlet 15 Gas supply source 16 Gas exhaust pipe 17 Target 17a Surface facing substrate 18 Sputtering pulse power supply 19 Magnetic pole 20 Plasma

Claims (5)

A method for forming a DLC layer on a surface of a substrate by unbalanced magnetron sputtering using a carbon target, comprising:
adjusting the flow rates of the argon gas and the hydrocarbon gas so that the flow rate ratio (flow rate of the argon gas/flow rate of the hydrocarbon gas) of the argon gas and the hydrocarbon gas supplied into the chamber where the film formation process is performed is 35 to 60;
A method for forming a DLC layer, comprising applying a bias voltage of 190 to 275 V in pulses to the substrate. The method for forming a DLC layer according to claim 1 , wherein the flow rate ratio is 50 or less. 3. The method for forming a DLC layer according to claim 1 , wherein the duty ratio of the bias voltage is set to 30 to 80%. 4. The method for forming a DLC layer according to claim 1 , wherein the hydrocarbon gas is methane gas. A method for manufacturing a mold, comprising manufacturing a mold having a DLC layer formed on a surface of a substrate by using the method for forming a DLC layer according to any one of claims 1 to 4 .