WO2015145236A1 - Method of forming carbon coating - Google Patents

Method of forming carbon coating Download PDF

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Publication number
WO2015145236A1
WO2015145236A1 PCT/IB2015/000372 IB2015000372W WO2015145236A1 WO 2015145236 A1 WO2015145236 A1 WO 2015145236A1 IB 2015000372 W IB2015000372 W IB 2015000372W WO 2015145236 A1 WO2015145236 A1 WO 2015145236A1
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Prior art keywords
graphite powder
substrate
carbon coating
graphite
carbon
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PCT/IB2015/000372
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French (fr)
Inventor
Noritaka Miyamoto
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Toyota Motor Corp
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Toyota Motor Corp
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C24/00—Coating starting from inorganic powder
    • C23C24/02—Coating starting from inorganic powder by application of pressure only
    • C23C24/04—Impact or kinetic deposition of particles

Definitions

  • the present invention relates to a method of forming a carbon coating on a substrate by blowing graphite powder thereto.
  • a carbon coating is formed on, for example, a metal substrate.
  • a method of forming a carbon coating generally, PVD or CVD is used to form a carbon coating on a surface of a substrate.
  • PVD or CVD is used to form a carbon coating on a surface of a substrate.
  • a carbon coating cannot be easily formed over a wide range of a surface of a substrate.
  • JP 2009-179847 A discloses a method of forming a carbon coating on a surface of a substrate by blowing graphite powder thereto using an aerosol deposition method.
  • a carbon coating is formed on a surface of a copper substrate by causing nanocarbon particles having an average particle size of 10 nm to 50 nm formed of carbon black or the like to collide against the copper substrate at 10 m/min to lOOO m/min.
  • the carbon coating has a structure such as a green compact in which particles constituting the graphite powder are deposited on the soft substrate in a state of being stuck therein.
  • the strength of the carbon coating during the formation is not sufficient, and thus, even if nanocarbon particles are further blown to the surface of the substrate, only a brittle carbon coating having a small thickness of 100 nm or less can be obtained.
  • the carbon coating formed as described above is sufficient for use as a member in which heat dissipation of a substrate is required. However, in many cases, this carbon coating has a lower strength than that of a substrate and is insufficient for use as a sliding member or a structural member.
  • the invention has been made to provide a method of forming a carbon coating in which a high-strength carbon coating can be formed by blowing graphite powder.
  • a method of forming a carbon coating including: forming a carbon coating which is formed of carbon constituting graphite powder by blowing the graphite powder to a substrate such that a portion of graphite constituting the graphite powder is modified into amorphous carbon.
  • graphite particles constituting the graphite powder are attached to and deposited on a substrate or graphite particles which are coated on the substrate.
  • a surface of a portion (graphite particles) of the graphite powder which is blown to the substrate or a portion of the graphite powder which is crushed by being blown is modified into amorphous carbon due to collision energy, and this amorphous carbon is present between the graphite particles deposited on the substrate.
  • a carbon coating is formed by causing the graphite particles to collide against the substrate with high collision energy capable of modifying the graphite particles into amorphous carbon. Therefore, unlike carbon coatings in the related art, a dense and high-strength carbon coating is formed.
  • the graphite of the graphite particles functions as a solid lubricant. Therefore, the carbon coating can exhibit low friction, and the wear resistance of the carbon coating can be improved by increasing the amount of amorphous carbon (by modifying a graphite structure derived from the graphite particles into a diamond structure).
  • the graphite powder is a material which is likely to be negatively charged in triboelectric series.
  • graphite is more likely to be negatively charged than a material of a nozzle of a coating forming device, and a substrate to which the graphite powder is blown is more likely to be positively charged than the graphite. Accordingly, the graphite powder is more likely to be negatively charged at a nozzle tip end during triboelectric charging, and when colliding, tends to be attracted to the substrate which is positively charged. As a result, a dense film is likely to be formed.
  • a dense carbon coating is also formed under a condition of applying collision energy so as to modify graphite of graphite powder into amorphous carbon. Therefore, coating forming conditions are not particularly limited.
  • graphite powder graphite powder having an average particle size of 1.0 ⁇ to 10 ⁇ may be used, and the carbon coating may be formed by blowing the graphite powder at a flying speed of 12000 m/min to 42000 m/min to the substrate heated to a temperature within a range from 80°C to 350°C.
  • the graphite powder is blown to the substrate under conditions including: the above-described range of the average particle size of the graphite powder (graphite particles constituting the graphite powder); the above-described range of the flying speed of the graphite powder (graphite particles constituting the graphite powder); and the above-described range of the heating temperature of the substrate.
  • a portion of the graphite particles is suitably modified into amorphous carbon, and this amorphous carbon is likely to be present between the graphite particles.
  • Carbon coating which is formed of carbon constituting graphite powder described in the invention refers to "carbon coating which is formed of carbon derived from the graphite powder" and, as described above, refers to the carbon coating which contains the amorphous carbon.
  • the average particle size of the graphite powder when the average particle size of the graphite powder is less than 1.0 ⁇ , the average particle size of the graphite powder is excessively small. Therefore, sufficient collision energy may not be applied to the graphite particles during the collision, and thus a carbon coating may not be formed.
  • the average particle size of the graphite powder when the average particle size of the graphite powder is more than 10 ⁇ , the average particle size of the graphite powder is excessively large. Therefore, it is considered that the graphite powder rebounds from the substrate during the collision and thus is not likely to be attached to the substrate. In addition, the attached graphite powder may be blasted by the flown particles.
  • the substrate may be heated within a range of 80°C to 350°C.
  • the temperature of the substrate is lower than 80°C, the negative charge amount of the carbon coating may not be sufficiently reduced.
  • the temperature of the substrate is higher than 350°C, the temperature of the graphite after being heated during the collision of the graphite powder approaches the combustion temperature (500°C) thereof. Therefore, a carbon coating may not be formed.
  • the graphite powder when the graphite powder is blown from a nozzle under a condition where the graphite powder is modified into amorphous carbon, the graphite powder is likely to be negatively charged due to friction with the nozzle.
  • the graphite powder may be blown to the substrate.
  • the quartz glass is more likely to be positively charged (rubbed) than graphite, and the graphite powder is more likely to be negatively charged than the quartz glass.
  • the quartz glass is arranged to be closer to the positive side than steel in the triboelectric series and thus is more likely to be positively charged than steel.
  • the contact surface of the nozzle with the graphite powder is formed of the quartz glass, and thus the graphite powder ejected from the nozzle is likely to be negatively charged.
  • the graphite powder which is negatively charged is likely to be attracted to the substrate.
  • a dense carbon coating can be formed.
  • a high-strength carbon coating can be formed by blowing graphite powder.
  • FIG. 1 is a diagram showing an example of a method of forming a carbon coating according to an embodiment of the invention
  • FIGS. 2 A to 2D are diagrams showing a mechanism for forming a carbon coating using the coating forming method shown in FIG. 1 , in which FIG. 2 A shows a state before a graphite particle collides against a substrate, FIG. 2B shows a state where the graphite particle collides against the substrate, FIG. 2C shows a state where the graphite particle is crushed after the collision, and FIG. 2D shows a state where the graphite particle further collides against the substrate;
  • FIG 3 is a schematic cross-sectional view showing a carbon coating which is formed using the coating forming method shown in FIG. 1 ;
  • FIG 4 shows conditions for forming carbon coatings according to Examples 1 to 8 and Comparative Examples 1-1 to 16 and the evaluation results of the coating formability
  • FIG. 5A is a cross-sectional image showing a carbon coating according to Example 3.
  • FIG. 5B is a diagram showing the results of measuring the hardness of the carbon coating according to Example 3.
  • FIG. 6A is a waveform chart showing the results of analyzing graphite powder and the carbon coating used in Example 3 by Raman spectroscopy;
  • FIG. 6B is a diagram showing a relationship between a ratio (D/G) of a peak intensity of D band to a peak intensity of G band and a full width at half maximum of G band when the graphite powder and the carbon coating of Example 3 (the number of samples for each of the graphite powder and the carbon coating: 10) are analyzed by Raman spectroscopy;
  • FIG 7 is a diagram showing the results of a friction test according to Example 3 and Reference Example;
  • FIG 8 is a diagram showing a relationship between the temperature of a substrate and the thickness of a carbon coating which corresponds to each of Examples 1 to 8 and Comparative Examples 4-3, 5-3, 15, and 16; and
  • FIG. 9 shows conditions for forming carbon coatings according to Example 9 and Comparative Examples 17-1 to 20-2 and the evaluation results of the coating formability
  • FIG. 10 shows conditions for forming carbon coatings according to Examples 10-1 to 12-4 and the evaluation results of the coating formability
  • FIG. 11 shows conditions for forming carbon coatings according to Examples 13-1 to 16-3 and Comparative Examples 21-1 to 24-2 and the evaluation results of the coating formability;
  • FIG 12 is a diagram showing images of surfaces after a coating forming process which correspond to Examples 13-1 , 14, 15, and 16-1 and Comparative Examples 21-1, 21-2, 22-1, 22-2, 23-1, 23-2, 24-1, and 24-2.
  • FIG. 1 is a diagram showing an example of the method of forming a carbon coating according to the embodiment of the invention.
  • FIGS. 2A to 2D are diagrams showing a mechanism for forming a carbon coating using the coating forming method shown in FIG. 1.
  • FIG. 2 A shows a state before a graphite particle collides against a substrate
  • FIG. 2B shows a state where the graphite particle collides against the substrate
  • FIG. 2C shows a state where the graphite particle is crushed after the collision
  • FIG. 2D shows a state where the graphite particle further collides against the substrate
  • FIG. 3 is a schematic cross-sectional view showing a carbon coating which is formed using the coating forming method shown in FIG 1.
  • FIG 1 shows a coating forming device 1 that forms a carbon coating 30 which is formed of carbon constituting graphite powder 3 (which is formed of carbon derived from the graphite powder) by blowing the graphite powder 3 to a surface of a substrate 20 using a powder jet deposition method.
  • the carbon coating 30 is formed using the powder jet deposition method.
  • a coating forming method such as an aerosol deposition method or a cold spray method may be used.
  • the coating forming device 1 includes: a first nozzle 16 that guides supply gas Gl ; and a second nozzle 15 that guides accelerating gas G2 so as to accelerate the graphite powder 3 which is carried by the supply gas Gl .
  • a part of the second nozzle 15 having a greater diameter than that of the first nozzle 16 is coaxially arranged so as to cover a tip end of the first nozzle 16 in a blowing direction, and these nozzles are arranged in a housing 11 so as to supply the accelerating gas G2 from an outer periphery of the first nozzle 16.
  • the first nozzle 16 is a stepped nozzle, and when compressed gas flows through the inside of a stepped flow path, a negative pressure is generated at a portion where the inner diameter changes. At this portion where the negative pressure is generated, a supply port 13 of the graphite powder 3 which is contained in a container 14 is provided such that the graphite powder 3 can be carried to the inside of the first nozzle 16 along with the supply gas Gl.
  • the graphite powder 3 which is carried to the inside of the first nozzle 16 can be blown to the substrate 20 at a desired flying speed.
  • the graphite powder 3 is a material which is likely to be negatively charged in the triboelectric series.
  • a contact surface with the graphite powder 3 (for example, the entire surface of the second nozzle) is formed of quartz glass or steel.
  • the quartz glass is arranged to be closer to the positive side than steel in the triboelectric series and thus is more likely to be positively charged than steel.
  • the specific gravity of the graphite powder 3 is preferably 1.4 to 2.4, and the average particle size thereof is preferably 1.0 ⁇ to 10 ⁇ . Within such ranges, the carbon coating 30 shown in FIG 3 described below can be obtained.
  • the average particle size of the graphite powder 3 is less than 1.0 ⁇ , the average particle size of the graphite powder 3 is excessively small. Therefore, sufficient collision energy cannot be applied to graphite particles 3 a during the collision, and thus a carbon coating cannot be formed.
  • the average particle size of the graphite powder 3 is more than 10 ⁇ , the average particle size of the graphite powder is excessively large.
  • the graphite powder rebounds from the substrate during the collision and thus is not likely to be attached to the substrate.
  • this average particle size is a median size (D50) which is measured using a microtrack method
  • the average particle size of carbon black is a catalog value (size which is measured by microscopy).
  • the graphite powder 3 is an aggregate of powder which is formed of graphite particles containing graphite as a major component, and examples thereof include graphite powder formed of flaky particles and spherical graphite powder which is produced through pulverizing and spheroidizing.
  • the graphite powder 3 can be flown at a flying speed of 12000 m/min to 42000 m/min.
  • flying speed is based on the gas flow rate which is analyzed under the condition of use of a structure of a blowing device shown in an example described below (analyzed under conditions including use software: SCRYU/Tetra (registered trademark) Ver. 9, manufactured by Software Cradle Co., Ltd., density condition: ideal gas, turbulence model: standard k- ⁇ model) and is equivalent to the actual flying speed. It was experimentally verified that the flying speed of the actual particles is substantially equivalent to the analyzed gas flow rate.
  • the blowing angle of the graphite powder is 90° (perpendicular) with respect to the surface of the substrate 20.
  • the present inventors found from an experiment described below that, even when this blowing angle is further inclined by 30° (when the blowing angle is 60° with respect to the surface of the substrate), the carbon coating 30 shown in FIG. 3 can be formed at the above-described flying speed range. Accordingly, when the blowing angle is 60° to 90° with respect to the surface of the substrate 20, the carbon coating 30 can be formed at the above-described flying speed range.
  • the carbon coating 30 can be formed by heating the substrate 20 and blowing the graphite powder 3 to the substrate 20 heated to a temperature within a range from 80°C to 350°C.
  • the graphite powder 3 is likely to be negatively charged, and even if the charged graphite powder is deposited as a carbon coating during the formation, electrons by which the carbon coating is negatively charged directly flow to the substrate by heating the substrate, and the negative charge amount of the carbon coating can be reduced. As a result, a dense carbon coating can be formed.
  • the temperature of the substrate 20 is lower than 80°C, the negative charge amount of the carbon coating cannot be sufficiently reduced.
  • the temperature of the substrate 20 is higher than 350°C, the temperature of the graphite after being heated during the collision of the graphite powder 3 approaches the combustion temperature (500°C) thereof. Therefore, a carbon coating 30 may not be formed.
  • the substrate 20 is formed of a material (for example, a metal material such as steel or aluminum, or glass such as quartz glass) which is more likely to be positively charged than the graphite.
  • a material for example, a metal material such as steel or aluminum, or glass such as quartz glass.
  • the graphite particle 3 a is blown to the substrate 20 as shown in FIG 2 A, the graphite particle 3 a collides against the substrate 20 and is cracked as shown in FIG 2B, and the graphite particle 3 a is crushed as shown in FIG. 2C.
  • a portion of the graphite particle 31 is attached to the substrate 20, a crushed portion is modified into amorphous carbon 32, and this amorphous carbon is attached to the substrate 20.
  • the graphite particle is likely to be negatively charged (rubbed).
  • the substrate 20 is likely to be positively charged (rubbed) during the collision, the blown graphite particle 3a is attached to the substrate 20.
  • the substrate when the substrate is formed of a metal material or glass as described above, this phenomenon is particularly significant.
  • the contact surface of the second nozzle 15 with the graphite powder 3 is formed of the quartz glass, and thus the graphite particle 3 a is likely to be negatively charged. Further, by heating the substrate 20 in the above-described range, even if the carbon coating is negatively charged during the formation, electrons are likely to be ejected from the substrate due to this charging.
  • the graphite particles 31 are dispersed, and the dense carbon coating 30 in which the amorphous carbon 32 is present between the graphite particles 31 can be obtained. Further, in the obtained carbon coating 30, the graphite of the graphite particles 31 functions as a solid lubricant. Therefore, the carbon coating 30 can exhibit low friction, and the wear resistance of the carbon coating 30 can be improved by increasing the amount of amorphous carbon 32 (by modifying a graphite structure derived from the graphite particles into a diamond structure).
  • a carbon coating was formed with a powder jet deposition method using the device shown in FIG. 1.
  • spherical graphite powder having an average particle size of 1 ⁇ which was obtained by pulverizing spherical graphite powder (CGB10, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 10 ⁇ using a mill for 10 hours was prepared as the graphite powder.
  • an aluminum alloy casting JIS standard: AC8A having a size of 30 mmx30 mmx30 mm was prepared as a substrate.
  • a carbon coating was formed on a surface of the substrate under conditions shown in Table 1 and FIG. 4.
  • the graphite powder was blown to the substrate such that the number of times of lamination was 25 times under conditions including a accelerating gas pressure: 0.95 MPa, a supply gas blowing pressure: 0.05 MPa, a blowing angle (with respect to the surface of the substrate): 90°, a gap between the substrate and a nozzle tip end: 1 mm, a supply rate of the graphite powder: 3 g/min, a nozzle moving speed: 1.0 mm/sec, and a pitch: 1 mm.
  • the flying speed of the graphite powder was 12000 m/min.
  • the substrate was heated to 80°C and was held at this temperature.
  • Carbon coatings were formed with the same method as that of Example 1.
  • Examples 2 to 8 were different from Example 1 , in that at least one of the particle size of the graphite powder and the temperature of the substrate was changed as shown in FIG 4.
  • the spherical graphite powder (CGB10, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 10 ⁇ was used as the graphite powder.
  • the heating temperature of the substrate was 150°C.
  • Example 4 the heating temperature of the substrate was 150°C, and the graphite powder of Example 2 was used.
  • the heating temperatures of the substrates were 250°C and 350°C, respectively.
  • Examples 6 and 8 the heating temperatures of the substrates were 250°C and 350°C, respectively, and the graphite powder of Example 2 was used.
  • Example 1 Carbon coatings were formed using a powder jet deposition method as in the case of Example 1.
  • Example 1 was different from Comparative Examples 1-1 to 5-3 and 6, mainly in that the substrate was not heated.
  • Comparative Examples 1-1 to 5-3 and 6 the average particle size of the graphite powder and the flying speed thereof were changed as shown in the conditions of FIG. 4.
  • Comparative Example 4-3 the average particle size of the graphite powder and the flying speed thereof were the same as that of Example 1 , but at least one condition of the average particle size of the graphite powder and the flying speed thereof was different from that of Example 1.
  • Comparative Examples 1-1 to 1-3 carbon black (#3400B, manufactured by Mitsubishi Chemical Corporation) having an average particle size of 20 ran was prepared as the graphite powder.
  • Comparative Examples 2-1 to 2-3 carbon black (#3050B, manufactured by Mitsubishi Chemical Corporation) having an average particle size of 50 nm was prepared as the graphite powder.
  • Comparative Examples 3-1 to 3-3 spherical graphite powder having an average particle size of 0.2 ⁇ which was obtained by pulverizing spherical graphite powder (CGB10, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 10 ⁇ using a mill for 30 hours was prepared as the graphite powder.
  • Comparative Examples 4-1 to 4-3 the same graphite powder as that of Example 1 was used.
  • Comparative Examples 5-1 to 5-3 the same graphite powder as that of Example 2 was used.
  • Comparative Example 6 spherical graphite powder (CGB50, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 50 ⁇ was prepared as the graphite powder.
  • graphite powder having the same average particle size as described above was prepared with the same method as described above. Further, in order to control the flying speed to be 1000 m/min and 6000 m/min as shown in FIG 4, the accelerating gas pressures in the coating forming device used in Example 1 were set to 0.25 MPa and 0.6 MPa, respectively.
  • Comparative Examples 7-1 to 7-3 were different from Example 1, mainly in that the average particle size of the graphite powder was 0.2 ⁇ . Comparative Examples 7-1 and 7-2 were further different from Example 1, in that the flying speeds were 1000 m/min and 6000 m/min.
  • Comparative Examples 10-1 to 10-3 were different from Example 1, mainly in that the average particle size of the graphite powder was 50 ⁇ . Comparative Examples 10-1 and 10-2 were further different from Example 1 in the flying speed.
  • Comparative Examples 11-1 to 11-3 were different from Example 1, mainly in that the temperature of the substrate was 150°C, and the average particle size of the graphite powder was 20 ran. Comparative Examples 11-1 and 11-2 were further different from Example 1, in that the flying speeds were 1000 m/min and 6000 m/min.
  • Comparative Examples 12-1 to 12-3 were different from Example 1, mainly in that the temperature of the substrate was 150°C, and the average particle size of the graphite powder was 50 ran. Comparative Examples 12-1 and 12-2 were different from Example 1, in that the flying speeds were 1000 m/min and 6000 m/min.
  • Carbon coatings were formed using a powder jet deposition method as in the case of Example 1.
  • Comparative Examples 13-1 and 13-2 were different from Example 1, in that the temperature of the substrate was 150°C, and the flying speeds of the graphite powder were 1000 m/min and 6000 m/min.
  • Comparative Examples 14-1 and 14-2 were different from Example 1, in that the temperature of the substrate was 1 0°C, the average particle size of the graphite powder was 10 ⁇ ⁇ , and the flying speeds of the graphite powder were 1000 m/min and 6000 m/min.
  • Carbon coatings were formed using a powder jet deposition method as in the case of Example 1.
  • Comparative Example 15 was different from Example 1 in that the temperature of the substrate was 400°C
  • Comparative Example 16 was different from Example 1 in that the temperature of the substrate was 400°C and the average particle size of the graphite powder was 10 ⁇ .
  • the carbon coating was formed by blowing the graphite powder having an average particle size of 1 ⁇ to 10 ⁇ to the substrate heated to a temperature within a range from 80°C to 350°C at a flying speed of 12000 m/min.
  • Comparative Examples 1-1 to 1-3, 11-1 to 11-3, and 12-1 to 12-3 the carbon coating was formed but was a green compact in a state where the graphite powder was deposited. Accordingly, a dense carbon coating was not formed unlike Examples 1 to 8.
  • the thickness of the carbon coating was 0.2 ⁇ or less, and the carbon coating equivalent to that of Example 1 was formed over time.
  • the state of the carbon coating was different from those of Comparative Examples 11-1 to 12-3.
  • FIG. 5A is a cross-sectional image showing the carbon coating according to Example 3, and the hardness was measured from the surface of the carbon coating according to Example 3 using a nanoindenter. The results are shown in FIG. 5B.
  • the carbon coating of Example 3 was formed in a dense state.
  • the hardness was Hv950 in terms of the Vickers hardness which was harder than graphite.
  • Example 3 was analyzed by Raman spectroscopy. The results are shown in FIGS. 6A and 6B.
  • FIG 6A is a waveform chart showing the results of analyzing the graphite powder and the carbon coating used in Example 3 by Raman spectroscopy
  • FIG. 6B is a diagram showing a relationship between a ratio (D/G) of a peak intensity of D band to a peak intensity of G band and a full width at half maximum of G band when the graphite powder and the carbon coating of Example 3 (the number N of samples for each of the graphite powder and the carbon coating: 10) are analyzed by Raman spectroscopy.
  • Example 2 In order to confirm an effect on the seizure resistance of a test piece according to Example 2, a block-on-disc test was performed. A gray cast iron (FC 230) was prepared as a ring test piece. Under oil (GF-5) lubrication, the ring test piece was rotated while pushing a peripheral surface of the ring test piece against a block test piece of Example 2. The friction coefficient was measured while increasing a load of the ring test piece applied to the block test piece. For comparison to Example 2, Reference Example was prepared by performing the friction test on a substrate on which a carbon coating was not formed. The results are shown in FIG 7.
  • Example 3 the carbon coating containing amorphous carbon and graphite was formed, and thus, unlike Reference Example, low friction was exhibited even under a high load, and seizure resistance and wear resistance were improved.
  • FIG. 8 shows a relationship between the thickness of the carbon coating and the temperature of the substrate in each of Examples 1 to 8 and Comparative Examples 4-3, 5-3, 15 and 16 which were examples in which the average particle sizes were 1 ⁇ and 10 ⁇ and the flying speed was 12000 m/min during the coating formation.
  • a carbon coating was formed using an aerosol deposition method under conditions shown in Table 2.
  • the same substrate and graphite powder as those of Example 1 were prepared.
  • the aerosolization chamber pressure was set to 40 kPa such that the flying speed of the graphite powder was 12000 m/min, nitrogen gas was carried at 8 L/min, a gap between the substrate and a nozzle tip end was 20 mm, a jet angle was 30° (an angle which was inclined by 30° from the right angle to the surface of the substrate, that is, a blowing angle was 60°), and the traverse speed of the substrate was 1 mm/sec.
  • the graphite powder was blown while moving the nozzle such that the number of times of lamination was 10 times.
  • Carbon coatings were formed using an aerosol deposition method as in the case of
  • Comparative Examples 17-1 to 17-3 were different from Example 9, mainly in that the average particle size of the graphite powder was 20 nm, and the substrate was not heated (the temperature of the substrate: 20°C). Comparative Examples 17-1 and 17-2 were further different from Example 9, in that the flying speeds were 1000 m/min and 6000 m/min. Comparative Example 17-1 corresponds to the coating forming method disclosed in JP 2009-179847 A.
  • Comparative Examples 18-1 to 18-3 were different from Example 9, mainly in that the average particle size of the graphite powder was 20 ⁇ . Comparative Examples 18-1 and 18-2 were further different from Example 9, in that the flying speeds were 1000 m/min and 6000 m/min.
  • Comparative Examples 19-1 to 19-3 were different from Example 9, mainly in that the substrate was not heated (the temperature of the substrate: 20°C). Comparative Examples 19-1 and 19-2 were further different from Example 9, in that the flying speeds were 1000 m/min and 6000 m/min.
  • Comparative Examples 20-1 and 20-2 were different from Example 9, mainly in that the flying speeds were 1000 m/min and 6000 m min.
  • Carbon coatings were formed using an aerosol deposition method as in the case of
  • Example 9 The conditions of Example 10-3 were the same as those of Example 9.
  • Example 10-1 was different from Example 9, in that the temperature of the substrate was 80°C.
  • Example 10-2 was different from Example 9, in that the temperature of the substrate was 80°C, and the average particle size of the graphite powder was 10 ⁇ .
  • Example 10-4 was different from Example 9, in that the average particle size of the graphite powder was 10 ⁇ .
  • Examples 11-1 to 11-4 Carbon coatings according to Examples 11-1 to 11-4 were formed under conditions corresponding to Examples 10-1 to 10-4 in this order. Examples 11-1 to 11-4 were different from Examples 10-1 to 10-4, in that the nozzle was formed of glassy carbon.
  • Examples 12-1 to 12-4 Carbon coatings according to Examples 12-1 to 12-4 were formed under conditions corresponding to Examples 10-1 to 10-4 in this order.
  • Examples 12-1 to 12-4 were different from Examples 10-1 to 10-4, in that the nozzle was formed of quartz glass.
  • FIG. 10 shows the evaluation results of coating formability.
  • "O” represents that the carbon coating having a sufficient thickness was formed
  • " ⁇ " represents that time was required to form the carbon coating having a sufficient thickness.
  • the thickness of the formed carbon coating increases in order of quartz glass, stainless steel, and glassy carbon.
  • the order of the materials matches with the order (positive triboelectric series) of the materials in which the materials are more likely to be positively charged than graphite.
  • the graphite powder can be actively negatively charged, and the carbon coating can be more efficiently formed.
  • the carbon coating is more likely to be negatively charged as compared to other materials. Therefore, it is considered that the graphite powder is further attached to the substrate, and a denser carbon coating can be formed.
  • Example 13-1 a carbon coating was formed with a cold spray method under conditions shown in Table 3.
  • the same graphite powder as that of Example 2 was prepared.
  • As the substrate pure aluminum was prepared.
  • the carrier gas pressure was set to 0.6 MPa using air such that the flying speed of the graphite powder was 12000 m/min.
  • a gap between the substrate and a nozzle tip end was 10 mm, a jet angle was 90°, and the traverse speed of the substrate was 0 mm/sec. Under the above conditions, the graphite powder was blown.
  • Example 13-2 a carbon coating was formed with the same method as that of Example 13-1. As shown in Table 3, Example 13-2 was different from Example 13-1, in that the flying speed of the graphite powder was 42000 m/min during the coating formation. In Example 13-3, a carbon coating was formed with the same method as that of Example 13-1. As shown in Table 3, Example 13-3 was different from Example 13-1, in that the flying speed of the graphite powder was 70000 m/min during the coating formation.
  • Example 14 was different from Example 13-1, in that the substrate was formed of iron (SS41), and Example 15 was different from Example 13-1, in that the substrate was formed of steel (Almen strip A).
  • Carbon coatings were formed with a cold spray method as in the case of Examples 13-1 to 13-3.
  • Examples 16-1 to 16-3 were different from Examples 13-1 to 13-3 corresponding thereto, in that the substrate was formed of quartz glass.
  • Carbon coatings were formed with a cold spray method as in the case of Example 13-1.
  • Comparative Example 21-1 was different from Example 13-1, in that the substrate was not heated (the temperature of the substrate: 20°C).
  • Comparative Example 21-2 was different from Example 13-1, in that the substrate was heated to 60°C.
  • Comparative Example 22-1 was different from Example 13-1, in that the substrate was formed of iron (SS41) and was not heated.
  • Comparative Example 22-2 was different from Example 13-1, in that the substrate was formed of iron (SS41) and was heated to 60°C.
  • Comparative Example 23-1 was different from Example 13-1, in that the substrate was formed of steel (Almen strip) and was not heated.
  • Comparative Example 23-2 was different from Example 13-1, in that the substrate was formed of steel (Almen strip) and was heated to 60°C.
  • Comparative Example 24-1 was different from Example 13-1, in that the substrate was formed of quartz glass and was not heated.
  • Comparative Example 24-2 was different from Example 13-1, in that the substrate was formed of quartz glass and was heated to 60°C.
  • FIG. 11 shows the evaluation results of coating formability.
  • "O” represents that the carbon coating having a sufficient thickness was formed
  • " ⁇ " represents that much time was required to form the carbon coating having a sufficient thickness.
  • a carbon coating can be formed with a cold spray method as long as the average particle size of the graphite powder, the flying speed of the graphite powder, and the temperature of the substrate satisfy the same conditions as those of an powder jet deposition method and an aerosol deposition method.
  • the carbon coating was formed to be thicker at the periphery than at the blowing center. The reason is considered to be that the negatively charged graphite powder was crushed and was attracted to the substrate which was positively charged due to the collision of the graphite powder.
  • the order of the materials in which the materials are more likely to be positively charged than graphite is the order of quartz glass, pure aluminum, and iron (or Almen strip). According to this order, the thickness of the carbon coating increases.
  • the condition of modifying a portion of the graphite constituting the graphite powder into amorphous carbon is specified by specifying the average particle size of the graphite powder, the flying speed of the graphite powder, and the temperature of the substrate; however, the condition is not limited thereto as long as the graphite of the graphite powder can be modified into amorphous carbon due to collision energy.

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Abstract

Provided is a method of forming a carbon coating (30) which is formed of carbon constituting graphite powder (3) by blowing the graphite powder (3) to a substrate (20), in which the graphite powder (3) is blown such that a portion of graphite constituting the graphite powder (3) is modified into amorphous carbon (32).

Description

METHOD OF FORMING CARBON COATING
BACKGROUND OF THE INVENTION 1. Field of the Invention
[0001] The present invention relates to a method of forming a carbon coating on a substrate by blowing graphite powder thereto.
2. Description of Related Art
[0002] A carbon coating is formed on, for example, a metal substrate. As such a method of forming a carbon coating, generally, PVD or CVD is used to form a carbon coating on a surface of a substrate. However, with these methods, a carbon coating cannot be easily formed over a wide range of a surface of a substrate.
]0003] Therefore, for example, Japanese Patent Application Publication No. 2009-179847 (JP 2009-179847 A) discloses a method of forming a carbon coating on a surface of a substrate by blowing graphite powder thereto using an aerosol deposition method. In this coating forming method, a carbon coating is formed on a surface of a copper substrate by causing nanocarbon particles having an average particle size of 10 nm to 50 nm formed of carbon black or the like to collide against the copper substrate at 10 m/min to lOOO m/min.
[0004] However, in the coating forming method disclosed in JP 2009-179847 A, the nanocarbon particles formed of carbon black or the like are crushed by colliding against the substrate, and newly-formed surfaces which are activated by the crushing appear, thereby forming a carbon coating. However, as clearly seen from an experiment by the inventors described below, even when the new-formed surfaces are exposed from the surfaces of the nanocarbon particles, a dense and high-strength carbon coating cannot be obtained.
[0005] Specifically, the carbon coating has a structure such as a green compact in which particles constituting the graphite powder are deposited on the soft substrate in a state of being stuck therein. In addition, the strength of the carbon coating during the formation is not sufficient, and thus, even if nanocarbon particles are further blown to the surface of the substrate, only a brittle carbon coating having a small thickness of 100 nm or less can be obtained.
[0006] The carbon coating formed as described above is sufficient for use as a member in which heat dissipation of a substrate is required. However, in many cases, this carbon coating has a lower strength than that of a substrate and is insufficient for use as a sliding member or a structural member. SUMMARY OF THE INVENTION
[0007] The invention has been made to provide a method of forming a carbon coating in which a high-strength carbon coating can be formed by blowing graphite powder.
[0008] As a result of intensive studies, the present inventors have newly found that, when graphite powder is caused to collide against a substrate with higher kinetic energy than ever before, a portion of the graphite powder is modified into amorphous carbon, and thus a dense film which contains the amorphous carbon and graphite particles (graphite) constituting the graphite powder is formed.
[0009] The invention has been made based on the new finding by the inventors. According to an aspect of the invention, there is provided a method of forming a carbon coating including: forming a carbon coating which is formed of carbon constituting graphite powder by blowing the graphite powder to a substrate such that a portion of graphite constituting the graphite powder is modified into amorphous carbon.
[0010] According to the invention, graphite particles constituting the graphite powder are attached to and deposited on a substrate or graphite particles which are coated on the substrate. A surface of a portion (graphite particles) of the graphite powder which is blown to the substrate or a portion of the graphite powder which is crushed by being blown is modified into amorphous carbon due to collision energy, and this amorphous carbon is present between the graphite particles deposited on the substrate. In addition, a carbon coating is formed by causing the graphite particles to collide against the substrate with high collision energy capable of modifying the graphite particles into amorphous carbon. Therefore, unlike carbon coatings in the related art, a dense and high-strength carbon coating is formed. Further, in the obtained carbon coating, the graphite of the graphite particles functions as a solid lubricant. Therefore, the carbon coating can exhibit low friction, and the wear resistance of the carbon coating can be improved by increasing the amount of amorphous carbon (by modifying a graphite structure derived from the graphite particles into a diamond structure).
[0011] Typically, the graphite powder is a material which is likely to be negatively charged in triboelectric series. When the above-described collision energy acts on graphite powder, typically, graphite is more likely to be negatively charged than a material of a nozzle of a coating forming device, and a substrate to which the graphite powder is blown is more likely to be positively charged than the graphite. Accordingly, the graphite powder is more likely to be negatively charged at a nozzle tip end during triboelectric charging, and when colliding, tends to be attracted to the substrate which is positively charged. As a result, a dense film is likely to be formed.
[0012] In addition, as described above, a dense carbon coating is also formed under a condition of applying collision energy so as to modify graphite of graphite powder into amorphous carbon. Therefore, coating forming conditions are not particularly limited. As the graphite powder, graphite powder having an average particle size of 1.0 μπι to 10 μπι may be used, and the carbon coating may be formed by blowing the graphite powder at a flying speed of 12000 m/min to 42000 m/min to the substrate heated to a temperature within a range from 80°C to 350°C.
[0013] According to the invention, the graphite powder is blown to the substrate under conditions including: the above-described range of the average particle size of the graphite powder (graphite particles constituting the graphite powder); the above-described range of the flying speed of the graphite powder (graphite particles constituting the graphite powder); and the above-described range of the heating temperature of the substrate. As a result, a portion of the graphite particles is suitably modified into amorphous carbon, and this amorphous carbon is likely to be present between the graphite particles. "Carbon coating which is formed of carbon constituting graphite powder" described in the invention refers to "carbon coating which is formed of carbon derived from the graphite powder" and, as described above, refers to the carbon coating which contains the amorphous carbon.
[0014] Here, when the average particle size of the graphite powder is less than 1.0 μπι, the average particle size of the graphite powder is excessively small. Therefore, sufficient collision energy may not be applied to the graphite particles during the collision, and thus a carbon coating may not be formed. On the other hand, when the average particle size of the graphite powder is more than 10 μηι, the average particle size of the graphite powder is excessively large. Therefore, it is considered that the graphite powder rebounds from the substrate during the collision and thus is not likely to be attached to the substrate. In addition, the attached graphite powder may be blasted by the flown particles.
[0015] When the flying speed of the graphite powder is lower than 12000 m/min, sufficient collision energy may not be applied to the graphite particles during the collision, and thus a carbon coating may not be formed. On the other hand, when the flying speed of the graphite powder is higher than 42000 m/min, a carbon coating may not be efficiently formed due to the flowing of gas which carries the graphite powder, or the attached graphite powder may be blasted by the flown particles.
[0016] Further, even if negatively charged graphite powder is deposited as a carbon coating during the formation, electrons by which the carbon coating is negatively charged flow in contact with the substrate by heating the substrate, and the negative charge amount of the carbon coating can be reduced. Accordingly, the substrate may be heated within a range of 80°C to 350°C. When the temperature of the substrate is lower than 80°C, the negative charge amount of the carbon coating may not be sufficiently reduced. On the other hand, when the temperature of the substrate is higher than 350°C, the temperature of the graphite after being heated during the collision of the graphite powder approaches the combustion temperature (500°C) thereof. Therefore, a carbon coating may not be formed.
[0017] As described above, it is considered that, when the graphite powder is blown from a nozzle under a condition where the graphite powder is modified into amorphous carbon, the graphite powder is likely to be negatively charged due to friction with the nozzle. However, in order to further promote such charging, using a nozzle whose tip end portion has a contact surface which is formed of quartz glass and comes in contact with the graphite powder when the graphite powder is ejected through the nozzle, the graphite powder may be blown to the substrate.
[0018] The quartz glass is more likely to be positively charged (rubbed) than graphite, and the graphite powder is more likely to be negatively charged than the quartz glass. In particular, the quartz glass is arranged to be closer to the positive side than steel in the triboelectric series and thus is more likely to be positively charged than steel. As described above, the contact surface of the nozzle with the graphite powder is formed of the quartz glass, and thus the graphite powder ejected from the nozzle is likely to be negatively charged. As a result, as described above, the graphite powder which is negatively charged is likely to be attracted to the substrate. As a result, a dense carbon coating can be formed.
[0019] According to the invention, a high-strength carbon coating can be formed by blowing graphite powder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
FIG. 1 is a diagram showing an example of a method of forming a carbon coating according to an embodiment of the invention;
FIGS. 2 A to 2D are diagrams showing a mechanism for forming a carbon coating using the coating forming method shown in FIG. 1 , in which FIG. 2 A shows a state before a graphite particle collides against a substrate, FIG. 2B shows a state where the graphite particle collides against the substrate, FIG. 2C shows a state where the graphite particle is crushed after the collision, and FIG. 2D shows a state where the graphite particle further collides against the substrate;
FIG 3 is a schematic cross-sectional view showing a carbon coating which is formed using the coating forming method shown in FIG. 1 ;
FIG 4 shows conditions for forming carbon coatings according to Examples 1 to 8 and Comparative Examples 1-1 to 16 and the evaluation results of the coating formability;
FIG. 5A is a cross-sectional image showing a carbon coating according to Example 3;
FIG. 5B is a diagram showing the results of measuring the hardness of the carbon coating according to Example 3;
FIG. 6A is a waveform chart showing the results of analyzing graphite powder and the carbon coating used in Example 3 by Raman spectroscopy;
FIG. 6B is a diagram showing a relationship between a ratio (D/G) of a peak intensity of D band to a peak intensity of G band and a full width at half maximum of G band when the graphite powder and the carbon coating of Example 3 (the number of samples for each of the graphite powder and the carbon coating: 10) are analyzed by Raman spectroscopy;
FIG 7 is a diagram showing the results of a friction test according to Example 3 and Reference Example;
FIG 8 is a diagram showing a relationship between the temperature of a substrate and the thickness of a carbon coating which corresponds to each of Examples 1 to 8 and Comparative Examples 4-3, 5-3, 15, and 16; and
FIG. 9 shows conditions for forming carbon coatings according to Example 9 and Comparative Examples 17-1 to 20-2 and the evaluation results of the coating formability;
FIG. 10 shows conditions for forming carbon coatings according to Examples 10-1 to 12-4 and the evaluation results of the coating formability;
FIG. 11 shows conditions for forming carbon coatings according to Examples 13-1 to 16-3 and Comparative Examples 21-1 to 24-2 and the evaluation results of the coating formability;
FIG 12 is a diagram showing images of surfaces after a coating forming process which correspond to Examples 13-1 , 14, 15, and 16-1 and Comparative Examples 21-1, 21-2, 22-1, 22-2, 23-1, 23-2, 24-1, and 24-2.
DETAILED DESCRIPTION OF EMBODIMENTS
[0021] A method of forming a carbon coating according to an embodiment of the invention will be described below. FIG. 1 is a diagram showing an example of the method of forming a carbon coating according to the embodiment of the invention. FIGS. 2A to 2D are diagrams showing a mechanism for forming a carbon coating using the coating forming method shown in FIG. 1. FIG. 2 A shows a state before a graphite particle collides against a substrate, FIG. 2B shows a state where the graphite particle collides against the substrate, FIG. 2C shows a state where the graphite particle is crushed after the collision, and FIG. 2D shows a state where the graphite particle further collides against the substrate; FIG. 3 is a schematic cross-sectional view showing a carbon coating which is formed using the coating forming method shown in FIG 1.
[0022] FIG 1 shows a coating forming device 1 that forms a carbon coating 30 which is formed of carbon constituting graphite powder 3 (which is formed of carbon derived from the graphite powder) by blowing the graphite powder 3 to a surface of a substrate 20 using a powder jet deposition method. In the embodiment, the carbon coating 30 is formed using the powder jet deposition method. However, as long as the carbon coating 30 which contains amorphous carbon obtained by modifying a portion of the graphite particles can be formed under coating forming conditions described below, a coating forming method such as an aerosol deposition method or a cold spray method may be used.
[0023] As shown in FIG. 1, the coating forming device 1 includes: a first nozzle 16 that guides supply gas Gl ; and a second nozzle 15 that guides accelerating gas G2 so as to accelerate the graphite powder 3 which is carried by the supply gas Gl . A part of the second nozzle 15 having a greater diameter than that of the first nozzle 16 is coaxially arranged so as to cover a tip end of the first nozzle 16 in a blowing direction, and these nozzles are arranged in a housing 11 so as to supply the accelerating gas G2 from an outer periphery of the first nozzle 16.
[0024] The first nozzle 16 is a stepped nozzle, and when compressed gas flows through the inside of a stepped flow path, a negative pressure is generated at a portion where the inner diameter changes. At this portion where the negative pressure is generated, a supply port 13 of the graphite powder 3 which is contained in a container 14 is provided such that the graphite powder 3 can be carried to the inside of the first nozzle 16 along with the supply gas Gl.
[0025] Further, by adjusting the gas pressure of the accelerating gas G2, the graphite powder 3 which is carried to the inside of the first nozzle 16 can be blown to the substrate 20 at a desired flying speed. Here, the graphite powder 3 is a material which is likely to be negatively charged in the triboelectric series. In addition, in a tip end portion of the second nozzle through which the graphite powder 3 is ejected when being blown to the substrate 20, a contact surface with the graphite powder 3 (for example, the entire surface of the second nozzle) is formed of quartz glass or steel. As a result, the graphite powder 3 is more likely to be negatively charged. In particular, the quartz glass is arranged to be closer to the positive side than steel in the triboelectric series and thus is more likely to be positively charged than steel.
[0026] In addition, the specific gravity of the graphite powder 3 is preferably 1.4 to 2.4, and the average particle size thereof is preferably 1.0 μηι to 10 μιη. Within such ranges, the carbon coating 30 shown in FIG 3 described below can be obtained. In particular, when the average particle size of the graphite powder 3 is less than 1.0 μπι, the average particle size of the graphite powder 3 is excessively small. Therefore, sufficient collision energy cannot be applied to graphite particles 3 a during the collision, and thus a carbon coating cannot be formed. On the other hand, when the average particle size of the graphite powder 3 is more than 10 μπι, the average particle size of the graphite powder is excessively large. Therefore, it is considered that the graphite powder rebounds from the substrate during the collision and thus is not likely to be attached to the substrate. In this specification, when "the average particle size" of the powder is 1 μπι or more, this average particle size is a median size (D50) which is measured using a microtrack method, and the average particle size of carbon black is a catalog value (size which is measured by microscopy).
[0027] Further, the graphite powder 3 is an aggregate of powder which is formed of graphite particles containing graphite as a major component, and examples thereof include graphite powder formed of flaky particles and spherical graphite powder which is produced through pulverizing and spheroidizing.
[0028] Further, by adjusting the gas pressure of the above-described accelerating gas using the coating forming device 1, the graphite powder 3 can be flown at a flying speed of 12000 m/min to 42000 m/min. In this specification, "flying speed" is based on the gas flow rate which is analyzed under the condition of use of a structure of a blowing device shown in an example described below (analyzed under conditions including use software: SCRYU/Tetra (registered trademark) Ver. 9, manufactured by Software Cradle Co., Ltd., density condition: ideal gas, turbulence model: standard k-ε model) and is equivalent to the actual flying speed. It was experimentally verified that the flying speed of the actual particles is substantially equivalent to the analyzed gas flow rate.
[0029] When the flying speed of the graphite powder 3 is lower than 12000 m/min, sufficient collision energy cannot be applied to the graphite particles during the collision, and thus a carbon coating cannot be formed. On the other hand, when the flying speed of the graphite powder 3 is higher than 42000 m min, a carbon coating may not be efficiently formed due to the flowing of gas which carries the graphite powder 3.
[0030] In addition, in FIG. 1, the blowing angle of the graphite powder is 90° (perpendicular) with respect to the surface of the substrate 20. However, the present inventors found from an experiment described below that, even when this blowing angle is further inclined by 30° (when the blowing angle is 60° with respect to the surface of the substrate), the carbon coating 30 shown in FIG. 3 can be formed at the above-described flying speed range. Accordingly, when the blowing angle is 60° to 90° with respect to the surface of the substrate 20, the carbon coating 30 can be formed at the above-described flying speed range.
[0031] Further, the carbon coating 30 can be formed by heating the substrate 20 and blowing the graphite powder 3 to the substrate 20 heated to a temperature within a range from 80°C to 350°C. As described above, the graphite powder 3 is likely to be negatively charged, and even if the charged graphite powder is deposited as a carbon coating during the formation, electrons by which the carbon coating is negatively charged directly flow to the substrate by heating the substrate, and the negative charge amount of the carbon coating can be reduced. As a result, a dense carbon coating can be formed.
[0032] When the temperature of the substrate 20 is lower than 80°C, the negative charge amount of the carbon coating cannot be sufficiently reduced. On the other hand, when the temperature of the substrate 20 is higher than 350°C, the temperature of the graphite after being heated during the collision of the graphite powder 3 approaches the combustion temperature (500°C) thereof. Therefore, a carbon coating 30 may not be formed.
[0033] In addition, since the graphite powder 3 is likely to be negatively charged, it is preferable that the substrate 20 is formed of a material (for example, a metal material such as steel or aluminum, or glass such as quartz glass) which is more likely to be positively charged than the graphite. By forming the substrate 20 as described above, the negatively charged graphite powder is actively attracted to the substrate 20, and a denser carbon coating can be formed.
[0034] When the graphite powder 3 is blown to the substrate 20, for example, under the above-described carbon coating conditions, the graphite particle 3 a is blown to the substrate 20 as shown in FIG 2 A, the graphite particle 3 a collides against the substrate 20 and is cracked as shown in FIG 2B, and the graphite particle 3 a is crushed as shown in FIG. 2C. A portion of the graphite particle 31 is attached to the substrate 20, a crushed portion is modified into amorphous carbon 32, and this amorphous carbon is attached to the substrate 20. Here, the graphite particle is likely to be negatively charged (rubbed). Further, since the substrate 20 is likely to be positively charged (rubbed) during the collision, the blown graphite particle 3a is attached to the substrate 20. In particular, when the substrate is formed of a metal material or glass as described above, this phenomenon is particularly significant. [0035] In particular, the contact surface of the second nozzle 15 with the graphite powder 3 is formed of the quartz glass, and thus the graphite particle 3 a is likely to be negatively charged. Further, by heating the substrate 20 in the above-described range, even if the carbon coating is negatively charged during the formation, electrons are likely to be ejected from the substrate due to this charging.
[0036] In this way, as shown in FIG. 3, the graphite particles 31 are dispersed, and the dense carbon coating 30 in which the amorphous carbon 32 is present between the graphite particles 31 can be obtained. Further, in the obtained carbon coating 30, the graphite of the graphite particles 31 functions as a solid lubricant. Therefore, the carbon coating 30 can exhibit low friction, and the wear resistance of the carbon coating 30 can be improved by increasing the amount of amorphous carbon 32 (by modifying a graphite structure derived from the graphite particles into a diamond structure).
[0037] Hereinafter, examples of the invention will be described.
[0038] (Example 1)
Under blowing conditions shown in Table 1, a carbon coating was formed with a powder jet deposition method using the device shown in FIG. 1. Specifically, spherical graphite powder having an average particle size of 1 μηι which was obtained by pulverizing spherical graphite powder (CGB10, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 10 μιη using a mill for 10 hours was prepared as the graphite powder. Further, an aluminum alloy casting (JIS standard: AC8A) having a size of 30 mmx30 mmx30 mm was prepared as a substrate.
[0039] Next, a carbon coating was formed on a surface of the substrate under conditions shown in Table 1 and FIG. 4. Using nitrogen gas, the graphite powder was blown to the substrate such that the number of times of lamination was 25 times under conditions including a accelerating gas pressure: 0.95 MPa, a supply gas blowing pressure: 0.05 MPa, a blowing angle (with respect to the surface of the substrate): 90°, a gap between the substrate and a nozzle tip end: 1 mm, a supply rate of the graphite powder: 3 g/min, a nozzle moving speed: 1.0 mm/sec, and a pitch: 1 mm. At this time, the flying speed of the graphite powder was 12000 m/min. In addition, during the coating formation, the substrate was heated to 80°C and was held at this temperature.
[Table 1]
Figure imgf000013_0001
[0040] (Examples 2 to 8)
Carbon coatings were formed with the same method as that of Example 1. Examples 2 to 8 were different from Example 1 , in that at least one of the particle size of the graphite powder and the temperature of the substrate was changed as shown in FIG 4. Specifically, in Example 2, the spherical graphite powder (CGB10, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 10 μπι was used as the graphite powder. In Example 3, the heating temperature of the substrate was 150°C. In Example 4, the heating temperature of the substrate was 150°C, and the graphite powder of Example 2 was used. In Examples 5 and 7, the heating temperatures of the substrates were 250°C and 350°C, respectively. In Examples 6 and 8, the heating temperatures of the substrates were 250°C and 350°C, respectively, and the graphite powder of Example 2 was used.
[0041] (Comparative Examples 1-1 to 6)
Carbon coatings were formed using a powder jet deposition method as in the case of Example 1. Example 1 was different from Comparative Examples 1-1 to 5-3 and 6, mainly in that the substrate was not heated. In Comparative Examples 1-1 to 5-3 and 6, the average particle size of the graphite powder and the flying speed thereof were changed as shown in the conditions of FIG. 4. In Comparative Example 4-3, the average particle size of the graphite powder and the flying speed thereof were the same as that of Example 1 , but at least one condition of the average particle size of the graphite powder and the flying speed thereof was different from that of Example 1.
[0042] In Comparative Examples 1-1 to 1-3, carbon black (#3400B, manufactured by Mitsubishi Chemical Corporation) having an average particle size of 20 ran was prepared as the graphite powder. In Comparative Examples 2-1 to 2-3, carbon black (#3050B, manufactured by Mitsubishi Chemical Corporation) having an average particle size of 50 nm was prepared as the graphite powder. In Comparative Examples 3-1 to 3-3, spherical graphite powder having an average particle size of 0.2 μπι which was obtained by pulverizing spherical graphite powder (CGB10, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 10 μιη using a mill for 30 hours was prepared as the graphite powder. In Comparative Examples 4-1 to 4-3, the same graphite powder as that of Example 1 was used. In Comparative Examples 5-1 to 5-3, the same graphite powder as that of Example 2 was used. In Comparative Example 6, spherical graphite powder (CGB50, manufactured by Nippon Graphite Industries, Ltd.) having an average particle size of 50 μιη was prepared as the graphite powder. In the following Examples and Comparative Examples, graphite powder having the same average particle size as described above was prepared with the same method as described above. Further, in order to control the flying speed to be 1000 m/min and 6000 m/min as shown in FIG 4, the accelerating gas pressures in the coating forming device used in Example 1 were set to 0.25 MPa and 0.6 MPa, respectively.
[0043] (Comparative Examples 7-1 to 7-3)
Carbon coatings were formed using a powder jet deposition method as in the case of Example 1. As shown in FIG. 4, Comparative Examples 7-1 to 7-3 were different from Example 1, mainly in that the average particle size of the graphite powder was 0.2 μιη. Comparative Examples 7-1 and 7-2 were further different from Example 1, in that the flying speeds were 1000 m/min and 6000 m/min.
[0044] (Comparative Examples 8- 1 , 8-2, 9- 1 , and 9-2) Carbon coatings were formed using a powder jet deposition method as in the case of Example 1. As shown in FIG. 4, Comparative Examples 8-1 and 8-2 were different from Example 1, in that the flying speeds of the graphite powder were 1000 m/min and 6000 m/min. As shown in FIG. 4, Comparative Examples 9-1 and 9-2 were different from Example 1, in that the average particle size of the graphite powder was 10 μιη, and the flying speeds of the graphite powder were 1000 m/min and 6000 m/min.
[0045] (Comparative Examples 10-1 to 10-3)
Carbon coatings were formed using a powder jet deposition method as in the case of Example 1. As shown in FIG 4, Comparative Examples 10-1 to 10-3 were different from Example 1, mainly in that the average particle size of the graphite powder was 50 μηι. Comparative Examples 10-1 and 10-2 were further different from Example 1 in the flying speed.
[0046] (Comparative Examples 11-1 to 11-3 and 12-1 to 12-3)
Carbon coatings were formed using a powder jet deposition method as in the case of Example 1. As shown in FIG 4, Comparative Examples 11-1 to 11-3 were different from Example 1, mainly in that the temperature of the substrate was 150°C, and the average particle size of the graphite powder was 20 ran. Comparative Examples 11-1 and 11-2 were further different from Example 1, in that the flying speeds were 1000 m/min and 6000 m/min.
[0047] As shown in FIG. 4, Comparative Examples 12-1 to 12-3 were different from Example 1, mainly in that the temperature of the substrate was 150°C, and the average particle size of the graphite powder was 50 ran. Comparative Examples 12-1 and 12-2 were different from Example 1, in that the flying speeds were 1000 m/min and 6000 m/min.
[0048] (Comparative Examples 13-1, 13-2, 14-1, and 14-2)
Carbon coatings were formed using a powder jet deposition method as in the case of Example 1. As shown in FIG. 4, Comparative Examples 13-1 and 13-2 were different from Example 1, in that the temperature of the substrate was 150°C, and the flying speeds of the graphite powder were 1000 m/min and 6000 m/min. As shown in FIG. 4, Comparative Examples 14-1 and 14-2 were different from Example 1, in that the temperature of the substrate was 1 0°C, the average particle size of the graphite powder was 10 μη , and the flying speeds of the graphite powder were 1000 m/min and 6000 m/min.
[0049] (Comparative Examples 15 and 16)
Carbon coatings were formed using a powder jet deposition method as in the case of Example 1. As shown in FIG. 4, Comparative Example 15 was different from Example 1 in that the temperature of the substrate was 400°C, and as shown in FIG. 4, Comparative Example 16 was different from Example 1 in that the temperature of the substrate was 400°C and the average particle size of the graphite powder was 10 μιιι.
[0050] <Evaluation of Coating Formability>
Regarding the above-described Examples and Comparative Examples, a case where the carbon coating was formed was evaluated as "O", and the thickness of the carbon coating was measured. The results are shown in FIG. 4. In addition, a case where the carbon coating was not formed was evaluated as "X". In addition, a case where the carbon coating was formed on the substrate but was a green compact in a state where the graphite powder was merely deposited (state where the carbon coating was easily deformed when being pressed by a nail) and a case where the graphite powder was stuck in the substrate were evaluated as "Δ".
[0051] It is considered from the results of FIG 4 that the carbon coating was formed by blowing the graphite powder having an average particle size of 1 μπι to 10 μπι to the substrate heated to a temperature within a range from 80°C to 350°C at a flying speed of 12000 m/min. In addition, in Comparative Examples 1-1 to 1-3, 11-1 to 11-3, and 12-1 to 12-3, the carbon coating was formed but was a green compact in a state where the graphite powder was deposited. Accordingly, a dense carbon coating was not formed unlike Examples 1 to 8. In Example 8, the thickness of the carbon coating was 0.2 μπι or less, and the carbon coating equivalent to that of Example 1 was formed over time. However, the state of the carbon coating was different from those of Comparative Examples 11-1 to 12-3. [0052] <E valuation of Hardness>
A cross-section of the carbon coating according to Example 3 was observed by FE-SEM. This result is shown in FIG. 5A. FIG. 5A is a cross-sectional image showing the carbon coating according to Example 3, and the hardness was measured from the surface of the carbon coating according to Example 3 using a nanoindenter. The results are shown in FIG. 5B. As shown in FIG. 5A, the carbon coating of Example 3 was formed in a dense state. As shown in FIG 5B, the hardness was Hv950 in terms of the Vickers hardness which was harder than graphite.
[0053] <Carbon Structure of Carbon Coating>
A carbon structure between the graphite powder and the carbon coating according to
Example 3 was analyzed by Raman spectroscopy. The results are shown in FIGS. 6A and 6B. FIG 6A is a waveform chart showing the results of analyzing the graphite powder and the carbon coating used in Example 3 by Raman spectroscopy, and FIG. 6B is a diagram showing a relationship between a ratio (D/G) of a peak intensity of D band to a peak intensity of G band and a full width at half maximum of G band when the graphite powder and the carbon coating of Example 3 (the number N of samples for each of the graphite powder and the carbon coating: 10) are analyzed by Raman spectroscopy. The plotted points are the average values of the graphite powder (N=10) and the carbon coating (N=10) in Example 3, and the range of ±1σ of the measured data is shown in the elliptical region.
[0054] As shown in FIG. 6A, the peak of D band of the carbon coating was higher than that of the graphite powder, and it is obvious from the results of FIG 6B that a portion of the graphite powder was modified into amorphous carbon during the coating formation. On the other hand, it was verified that, in Comparative Examples 1-1 to 1-3, 11-1 to 11-3, and 12-1 to 12-3, the carbon coating was formed, but the carbon structure of the carbon coating was the same as the carbon structure of the graphite powder. That is, in Comparative Examples 1-1 to 1-3, 11-1 to 11-3, and 12-1 to 12-3, amorphous carbon was not formed. It is considered from the above results that, in Examples 1 to 8, as shown in FIG. 3, the carbon coating was formed due to the above-described mechanism shown in FIGS. 2A to 2D.
[0055] <Friction Test>
In order to confirm an effect on the seizure resistance of a test piece according to Example 2, a block-on-disc test was performed. A gray cast iron (FC 230) was prepared as a ring test piece. Under oil (GF-5) lubrication, the ring test piece was rotated while pushing a peripheral surface of the ring test piece against a block test piece of Example 2.The friction coefficient was measured while increasing a load of the ring test piece applied to the block test piece. For comparison to Example 2, Reference Example was prepared by performing the friction test on a substrate on which a carbon coating was not formed. The results are shown in FIG 7.
[0056] It can be said from the above results that, in Example 3, the carbon coating containing amorphous carbon and graphite was formed, and thus, unlike Reference Example, low friction was exhibited even under a high load, and seizure resistance and wear resistance were improved.
[0057] It is considered that, in the carbon coating according to Example 3, the graphite of the graphite particles functioned as a solid lubricant; therefore, the carbon coating exhibited low friction, and the wear resistance of the carbon coating was improved by the amount of amorphous carbon being increased.
[0058] Relationship between Thickness and Temperature of Substrate>
FIG. 8 shows a relationship between the thickness of the carbon coating and the temperature of the substrate in each of Examples 1 to 8 and Comparative Examples 4-3, 5-3, 15 and 16 which were examples in which the average particle sizes were 1 μηι and 10 μηι and the flying speed was 12000 m/min during the coating formation.
[0059] It is considered from the above results of Comparative Examples 15 and 16 that, when the substrate was heated to 400°C, the temperature of the graphite after being heated during the collision of the graphite powder approached the combustion temperature (500°C) thereof; therefore, a carbon coating was not able to be formed. In addition, it is considered from the results of Comparative Examples 4-3 and 5-3 that the flying graphite powder was negatively charged, the formed carbon coating was also negatively charged, and thus the negative charge amount of the carbon coating was not able to be sufficiently decreased; as a result, a carbon coating was not able to be formed.
[0060] (Example 9)
A carbon coating was formed using an aerosol deposition method under conditions shown in Table 2. The same substrate and graphite powder as those of Example 1 were prepared. The aerosolization chamber pressure was set to 40 kPa such that the flying speed of the graphite powder was 12000 m/min, nitrogen gas was carried at 8 L/min, a gap between the substrate and a nozzle tip end was 20 mm, a jet angle was 30° (an angle which was inclined by 30° from the right angle to the surface of the substrate, that is, a blowing angle was 60°), and the traverse speed of the substrate was 1 mm/sec. Under the above conditions, the graphite powder was blown while moving the nozzle such that the number of times of lamination was 10 times.
[Table 2]
Figure imgf000019_0001
[0061] (Comparative Examples 17-1 to 20-2)
Carbon coatings were formed using an aerosol deposition method as in the case of
Example 9. Comparative Examples 17-1 to 17-3 were different from Example 9, mainly in that the average particle size of the graphite powder was 20 nm, and the substrate was not heated (the temperature of the substrate: 20°C). Comparative Examples 17-1 and 17-2 were further different from Example 9, in that the flying speeds were 1000 m/min and 6000 m/min. Comparative Example 17-1 corresponds to the coating forming method disclosed in JP 2009-179847 A.
[0062] Comparative Examples 18-1 to 18-3 were different from Example 9, mainly in that the average particle size of the graphite powder was 20 μιη. Comparative Examples 18-1 and 18-2 were further different from Example 9, in that the flying speeds were 1000 m/min and 6000 m/min.
[0063] Comparative Examples 19-1 to 19-3 were different from Example 9, mainly in that the substrate was not heated (the temperature of the substrate: 20°C). Comparative Examples 19-1 and 19-2 were further different from Example 9, in that the flying speeds were 1000 m/min and 6000 m/min.
[0064] Comparative Examples 20-1 and 20-2 were different from Example 9, mainly in that the flying speeds were 1000 m/min and 6000 m min.
[0065] <Evaluation of Coating Formability>
In FIG. 9, as in the case of FIG. 4, the coating formability was evaluated as "O", "Δ", and "X". The following was found from the results. As in the case of the powder jet deposition method shown in FIG. 4, even with the aerosol deposition method, a carbon coating equivalent to that of Example 1 can be formed when the blowing angle is within a range of 60° to 90° with respect to the surface of the substrate 20 under conditions including the average particle size of the graphite powder: 1 μιη to 10 μπι, the flying speed of the graphite powder: 12000 m/min or higher, and the temperature of the substrate: 150°C or higher.
[0066] (Examples 10-1 to 10-4)
Carbon coatings were formed using an aerosol deposition method as in the case of
Example 9. The conditions of Example 10-3 were the same as those of Example 9. Example 10-1 was different from Example 9, in that the temperature of the substrate was 80°C. Example 10-2 was different from Example 9, in that the temperature of the substrate was 80°C, and the average particle size of the graphite powder was 10 μπι. Example 10-4 was different from Example 9, in that the average particle size of the graphite powder was 10 μπι.
[0067] (Examples 11 - 1 to 11 -4)
Carbon coatings according to Examples 11-1 to 11-4 were formed under conditions corresponding to Examples 10-1 to 10-4 in this order. Examples 11-1 to 11-4 were different from Examples 10-1 to 10-4, in that the nozzle was formed of glassy carbon.
[0068] (Examples 12-1 to 12-4)
Carbon coatings according to Examples 12-1 to 12-4 were formed under conditions corresponding to Examples 10-1 to 10-4 in this order. Examples 12-1 to 12-4 were different from Examples 10-1 to 10-4, in that the nozzle was formed of quartz glass.
[0069] <Evaluation of Coating Formability>
FIG. 10 shows the evaluation results of coating formability. In FIG. 10, "O" represents that the carbon coating having a sufficient thickness was formed, and "Δ" represents that time was required to form the carbon coating having a sufficient thickness. It can be seen from the results of FIG. 10 that the thickness of the formed carbon coating increases in order of quartz glass, stainless steel, and glassy carbon. The order of the materials matches with the order (positive triboelectric series) of the materials in which the materials are more likely to be positively charged than graphite. It can be said that, when the nozzle is formed of quartz glass or stainless steel, the graphite powder can be actively negatively charged, and the carbon coating can be more efficiently formed. In particular, when the nozzle is formed of quartz glass, the carbon coating is more likely to be negatively charged as compared to other materials. Therefore, it is considered that the graphite powder is further attached to the substrate, and a denser carbon coating can be formed.
[0070] (Examples 13-1 to 13-3)
In Example 13-1, a carbon coating was formed with a cold spray method under conditions shown in Table 3. The same graphite powder as that of Example 2 was prepared. As the substrate, pure aluminum was prepared. In a state where the substrate was heated to 250°C and was held at this temperature, the carrier gas pressure was set to 0.6 MPa using air such that the flying speed of the graphite powder was 12000 m/min. A gap between the substrate and a nozzle tip end was 10 mm, a jet angle was 90°, and the traverse speed of the substrate was 0 mm/sec. Under the above conditions, the graphite powder was blown.
[0071] In Example 13-2, a carbon coating was formed with the same method as that of Example 13-1. As shown in Table 3, Example 13-2 was different from Example 13-1, in that the flying speed of the graphite powder was 42000 m/min during the coating formation. In Example 13-3, a carbon coating was formed with the same method as that of Example 13-1. As shown in Table 3, Example 13-3 was different from Example 13-1, in that the flying speed of the graphite powder was 70000 m/min during the coating formation.
[Table 3]
Figure imgf000022_0001
[0072] (Examples 14 and 15)
Carbon coatings were formed with a cold spray method as in the case of Example 13-1. Example 14 was different from Example 13-1, in that the substrate was formed of iron (SS41), and Example 15 was different from Example 13-1, in that the substrate was formed of steel (Almen strip A).
[0073] (Examples 16-1 to 16-3)
Carbon coatings were formed with a cold spray method as in the case of Examples 13-1 to 13-3. Examples 16-1 to 16-3 were different from Examples 13-1 to 13-3 corresponding thereto, in that the substrate was formed of quartz glass.
[0074] (Comparative Examples 21 - 1 and 21 -2)
Carbon coatings were formed with a cold spray method as in the case of Example 13-1. Comparative Example 21-1 was different from Example 13-1, in that the substrate was not heated (the temperature of the substrate: 20°C). Comparative Example 21-2 was different from Example 13-1, in that the substrate was heated to 60°C.
[0075] (Comparative Examples 22-1 and 22-2)
Carbon coatings were formed with a cold spray method as in the case of Example 13-1. Comparative Example 22-1 was different from Example 13-1, in that the substrate was formed of iron (SS41) and was not heated. Comparative Example 22-2 was different from Example 13-1, in that the substrate was formed of iron (SS41) and was heated to 60°C.
[0076] (Comparative Examples 23-1 and 23-2)
Carbon coatings were formed with a cold spray method as in the case of Example 13-1. Comparative Example 23-1 was different from Example 13-1, in that the substrate was formed of steel (Almen strip) and was not heated. Comparative Example 23-2 was different from Example 13-1, in that the substrate was formed of steel (Almen strip) and was heated to 60°C.
[0077] (Comparative Examples 24-1 and 24-2)
Carbon coatings were formed with a cold spray method as in the case of Example 13-1. Comparative Example 24-1 was different from Example 13-1, in that the substrate was formed of quartz glass and was not heated. Comparative Example 24-2 was different from Example 13-1, in that the substrate was formed of quartz glass and was heated to 60°C.
[0078] <Evaluation of Coating Formability>
Surfaces after a coating forming process were observed, the surfaces corresponding to Examples 13-1, 14, 15, and 16-1 and Comparative Examples 21-1, 21-2, 22-1, 22-2, 23-1, 23-2, 24-1, and 24-2. The results are shown in FIG. 12. FIG. 11 shows the evaluation results of coating formability. In FIG. 11, "O" represents that the carbon coating having a sufficient thickness was formed, and "Δ" represents that much time was required to form the carbon coating having a sufficient thickness.
[0079] As can be seen from the results from Examples 13-1, 14, 15, and 16-1, it is considered that a carbon coating can be formed with a cold spray method as long as the average particle size of the graphite powder, the flying speed of the graphite powder, and the temperature of the substrate satisfy the same conditions as those of an powder jet deposition method and an aerosol deposition method.
[0080] In addition, as clearly seen from the results of Examples 13-1, 14, 15, and 16-1 shown in FIG 12, the carbon coating was formed to be thicker at the periphery than at the blowing center. The reason is considered to be that the negatively charged graphite powder was crushed and was attracted to the substrate which was positively charged due to the collision of the graphite powder. The order of the materials in which the materials are more likely to be positively charged than graphite is the order of quartz glass, pure aluminum, and iron (or Almen strip). According to this order, the thickness of the carbon coating increases. That is, it is considered that the negatively charged graphite powder was attracted due to an electrostatic attractive force, this electrostatic attractive force was larger in the above-described order of the materials, and this electrostatic attractive force was more dependent on the thickness of the carbon coating rather than the hardness of the substrate (refer to the hardness of the substrate of FIG. 12).
[0081] When the temperature of the substrate was 20°C (the substrate was not heated) or 60°C, the carbon coating which was negatively charged during the coating formation maintained its charged state. Therefore, it is considered that, even if the substrate was positively charged, the attractive force was weakened due to the electrostatic attractive force, and a carbon coating was not substantially formed (for example, only a pattern was formed as shown in Comparative Example 21-1, and there was substantially no thickness).
[0082] Accordingly, as shown in Examples 13-1, 14, 15, and 16-1, when a carbon coating is formed while heating the substrate to 250°C, electrons of the carbon coating which is negatively charged during the coating formation can be released to the substrate. As a result, it is considered that new graphite powder which is negatively charged is attracted to the substrate, and the thickness of the carbon coating increases.
[0083] Further, it is considered that, when the flying speed of the graphite powder is 12000 m/min to 42000 m/min, a carbon coating is likely to be efficiently formed; and when the flying speed of the graphite powder is 70000 m/min, time is required to form a carbon coating.
[0084] Hereinabove, the embodiment of the invention has been described. However, the invention is not limited to the above-described embodiment, and various design modifications can be made thereto within a range not departing from the concepts of the invention.
[0085] In the embodiment, the condition of modifying a portion of the graphite constituting the graphite powder into amorphous carbon is specified by specifying the average particle size of the graphite powder, the flying speed of the graphite powder, and the temperature of the substrate; however, the condition is not limited thereto as long as the graphite of the graphite powder can be modified into amorphous carbon due to collision energy.

Claims

CLAIMS:
1. A method of forming a carbon coating comprising:
forming a carbon coating which is formed of carbon constituting graphite powder by blowing the graphite powder to a substrate such that a portion of graphite constituting the graphite powder is modified into amorphous carbon.
2. The method according to claim 1, wherein
as the graphite powder, graphite powder having an average particle size of 1.0 μηι to 10 μπι is used, and
the carbon coating is formed by blowing the graphite powder at a flying speed of 12000 m/min to 42000 m/min to the substrate heated to a temperature within a range from 80°C to 350°C.
3. The method according to claim 1 or 2, wherein
using a nozzle whose tip end portion has a contact surface which is formed of quartz glass and comes in contact with the graphite powder when the graphite powder is ejected through the nozzle, the graphite powder is blown to the substrate.
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