SURFACE TREATMENT METHOD FOR METAL PRODUCT AND METAL PRODUCT

Abstract

A surface treatment method capable of continuously forming a uniform nanocrystalline structure along the surface of a metal product regardless of whether the metal product is hard or soft. A substantially spherical spray powder that has a median diameter of 1-20 m and a fall velocity in the air of 10 sec/m or more is sprayed onto a metal product at a spray pressure of 0.05-0.5 MPa. Thus, even when the metal product is made of a soft material, it is possible to form a uniform continuous nanocrystalline structure layer in which nanocrystals are micronized to an average crystal grain size of not more than 300 nm, preferably not more than 100 nm, without forming a laminar worked structure, impart a high compression residual stress of from about 180 MPa up to the order of 1200 MPa, and strengthen the surface of the metal product.

Claims

1. A method for surface treatment of a metal article comprising: ejecting substantially spherical ejection particles having a median diameter d50 of from 1 m to 20 m and a falling time through air of not less than 10 sec/m against a metal article at an ejection pressure of from 0.05 MPa to 0.5 MPa; forming a nano-crystal structure layer continuously along a surface of the metal article in a zone to a prescribed depth from the surface of metal article by uniform micronization to nano-crystals having an average crystal grain diameter of not greater than 300 nm; and imparting compressive residual stress to the surface of the metal article.

2. The method for surface treatment of the metal article according to claim 1, wherein the ejection velocity of the ejection particles is not less than 80 m/sec.

3. The method for surface treatment of the metal article according to claim 1, wherein the material of the metal article is either aluminum or an aluminum alloy, and the crystal grain diameter of the nano-crystal structure layer is micronized to a crystal grain diameter of not greater than 100 nm.

4. The method for surface treatment of the metal article according to claim 1, wherein: the metal article is a machining tool, and a region to be treated is a cutting-edge of the machining tool and the vicinity of the cutting-edge; and dimples having an equivalent diameter of from 1 m to 18 m and a depth of from 0.02 m to 1.0 m or less than 1.0 m are formed on the region to be treated by ejecting the ejection particles, such that a projected surface area of the dimples occupies not less than 30% of a surface area of the region to be treated.

5. The method for surface treatment of the metal article according to claim 1, wherein: the metal article is a sliding member; at least a sliding portion of the sliding member is a region to be treated; and dimples having an equivalent diameter of from 1 m to 18 m and a depth of from 0.02 m to 1.0 m or less than 1.0 m are formed on the region to be treated by ejecting the ejection particles, such that a projected surface area of the dimples occupies not less than 30% of a surface area of the region to be treated.

6. A metal article comprising: a base metal having a hardness not greater than HV714; and a nano-crystal structure layer formed continuously along a surface of the metal article in a zone to a prescribed depth from the surface of metal article by uniform micronization to nano-crystals having an average crystal grain diameter of not greater than 300 nm; and a compressive residual stress being imparted to the surface of the metal article.

7. The metal article according to claim 6, wherein the metal article is configured from either aluminum or an aluminum alloy, and a crystal grain diameter of the nano-crystal structure layer is not greater than 100 nm.

8. The metal article according to claim 6, wherein: the metal article is a machining tool; the nano-crystal structure layer is formed on a surface of a region to be treated including a cutting-edge and a vicinity of the cutting-edge; and dimples having an equivalent diameter of from 1 m to 18 m and a depth of from 0.02 m to 1.0 m or less than 1.0 m are formed such that a projected surface area of the dimples occupies not less than 30% of a surface area of the region to be treated.

9. The metal article according to claim 6, wherein: the metal article is a sliding member; the nano-crystal structure layer is formed on a surface of a sliding portion of the sliding member that makes sliding contact with another member; and dimples having an equivalent diameter of from 1 m to 18 m and a depth of from 0.02 m to 1.0 m or less than 1.0 m are formed such that a projected surface area of the dimples occupies not less than 30% of a surface area of the region to be treated.

10. The method for surface treatment of the metal article according to claim 2, wherein the material of the metal article is either aluminum or an aluminum alloy, and the crystal grain diameter of the nano-crystal structure layer is micronized to a crystal grain diameter of not greater than 100 nm.

11. The method for surface treatment of the metal article according to claim 2, wherein: the metal article is a machining tool, and a region to be treated is a cutting-edge of the machining tool and the vicinity of the cutting-edge; and dimples having an equivalent diameter of from 1 m to 18 m and a depth of from 0.02 m to 1.0 m or less than 1.0 m are formed on the region to be treated by ejecting the ejection particles, such that a projected surface area of the dimples occupies not less than 30% of a surface area of the region to be treated.

12. The method for surface treatment of the metal article according to claim 2, wherein: the metal article is a sliding member; at least a sliding portion of the sliding member is a region to be treated; and dimples having an equivalent diameter of from 1 m to 18 m and a depth of from 0.02 m to 1.0 m or less than 1.0 m are formed on the region to be treated by ejecting the ejection particles, such that a projected surface area of the dimples occupies not less than 30% of a surface area of the region to be treated.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0045] FIG. 1 is an explanatory diagram illustrating a mechanism by which lamellar processing structures are formed in a soft material.

[0046] FIG. 2 are explanatory diagrams illustrating an example of application to a cutting-edge of a machining tool: (A) illustrates a state before treatment, and (B) illustrates a state after treatment.

[0047] FIG. 3 is an explanatory diagram of a portion (pressure receiving surface) where compressional force acts when collided by an ejection particle.

[0048] FIG. 4 is a Von Mises stress analysis image using FEM (5 m ejection particles).

[0049] FIG. 5 is a Von Mises stress analysis image using FEM (10 m ejection particles).

[0050] FIG. 6 is a Von Mises stress analysis image using FEM (20 m ejection particles).

[0051] FIG. 7 is a Von Mises stress analysis image using FEM (50 m ejection particles).

[0052] FIG. 8 is a Von Mises stress analysis image using FEM (100 m ejection particles).

[0053] FIG. 9 is a graph illustrating a relationship between particle diameter of ejection particles and stress.

[0054] FIG. 10 is a graph illustrating a relationship between particle diameter of ejection particles and depth of maximum stress generation.

[0055] FIG. 11 is a graph illustrating relationships between ejection pressure and dynamic hardness.

[0056] FIG. 12 are SIM images of pre-hardened steel (NAK 80, manufactured by Daido Steel Co., Ltd): (A) illustrates a state before treatment, and (B) illustrates a state after the treatment of the present invention.

[0057] FIG. 13 are SIM images of an alloy tool steel (SKD11): (A) illustrates a state before treatment, and (B) illustrates a state after treatment of the present invention.

[0058] FIG. 14 are SIM images of an aluminum alloy (A7075): (A) illustrates a state before treatment, and (B) illustrates a state after treatment of the present invention.

[0059] FIG. 15 is a grain diameter distribution diagram for pre-hardened steel (NAK 80, manufactured by Daido Steel Co., Ltd) treated by the method of the present invention.

[0060] FIG. 16 is a grain diameter distribution diagram for alloy tool steel (SKD11) treated by the method of the present invention.

[0061] FIG. 17 is a graph of measurement results of residual stress in pre-hardened steel (NAK 80, manufactured by Daido Steel Co., Ltd).

[0062] FIG. 18 is graph of measurement results of residual stress in alloy tool steel (SKD11).

[0063] FIG. 19 is a graph of measurement results of residual stress in aluminum alloy (A7075).

[0064] FIG. 20 is a graph of measured changes in friction with respect to elapsed time.

DESCRIPTION OF EMBODIMENTS

[0065] Next, explanation follows regarding an embodiment of the present invention, with reference to the appended drawings.

[0066] Object to be Treated

[0067] A metal article subjected to treatment by the surface treatment method of the present invention may be any article made from metal, and, as well as application to ferrous metals, application may also be made to metal articles made from non-ferrous metals and alloys thereof.

[0068] Moreover, the metal article to be treated is not limited to a metal article configured from a hard base metal, and application may be made to a range of metals from comparatively soft metals of about HV20 to HV400 such as aluminum and alloys thereof, pre-hardened steels (NAK 80, manufactured by Daido Steel Co., Ltd: HV400) and the like, up to high hardness steels, such as SKD11 (HV697).

[0069] In particular, the method of the present invention is able to treat metal articles made from soft materials, in which hitherto it has been impossible to form a nano-crystal structured layer uniformly and continuously due to the formation of lamellar processing structures as explained with reference to FIG. 1. From among such soft materials, it has been confirmed that the method can achieve a nano-crystal structure layer formed with an extremely fine crystal grain diameter, this being a crystal grain diameter of 100 nm or less, when metal articles made from aluminum and aluminum alloys, which have a particularly low hardness, are treated. A large surface strengthening effect can be obtained as a result.

[0070] Note that there are no particular limitations to the usage application of the treated metal article, and application may be made to metal articles employed in various applications requiring surface strengthening. However, a preferable application of the surface treatment method of the present invention is application to a cutting-edge of a machining tool such as cutting tool, or to the vicinity of the cutting-edge. This is due to not only being able to strengthen the cutting-edge portion, but also being able to prevent the material to be processed from accumulating to the cutting-edge.

[0071] When performing treatment on a cutting-edge of a machining tool in this manner, ejection particles described later are ejected to the region to be treated where the ejection particles are ejected and caused to be collided thereto, i.e., a portion of the cutting-edge (edge) as illustrated in FIG. 2 where shearing starts when cutting or shearing, and a range of at least 1 mm from the cutting-edge, and preferably a range of at least 5 mm from the cutting edge (the range from the cutting-edge indicated by the double-dashed broken lines in the drawings). Dimples are also formed in this region accompanying the formation of a nano-crystal structure layer on the surface of this portion, as illustrated in FIG. 2(B).

[0072] In the present embodiment, inclined faces on either side of the cutting-edge may be employed as the region to be treated. However, the region to be treated may be solely provided on the inclined face that bears the greatest frictional resistance during cutting, or solely provided on the inclined face on the side that cut material might be accumulated thereto.

[0073] Furthermore, when performing the surface treatment method of the present invention with the objective of surface strengthening and improving the slidability of a sliding member employed to slide against another member, such as a bearing, shaft, or gear, the region to be treated referred to above is at least a portion of the sliding member that slides against the other member.

[0074] Note that the surface of the metal article to be treated may be in a burred state, or may be in a state in which processing marks such as tool marks remain formed thereon. However, preferably pre-polishing is performed in advance to polish to surface roughness having an arithmetic mean roughness (Ra) of 3.2 m or less.

[0075] There are no particular limitations to the method by which such pre-polishing is performed, and polishing may be performed by manual lapping or buffing. However, such pre-processing is preferably performed by blasting using an elastic abrasive.

[0076] Such an elastic abrasive is an abrasive having abrasive particles dispersed in an elastic body such as a rubber or an elastomer, or is an abrasive having abrasive particles supported on the surface of an elastic body. Such an elastic abrasive can be caused to slide across the surface of a metal article by being ejected at an inclination thereto, or the like. The surface of the metal article can thereby be comparatively simply polished to a mirror finish, or polished to a state close to a mirror finish.

[0077] The abrasive particles dispersed in, or supported by, the elastic body of the elastic abrasive may be appropriately selected according to the surface state of the metal article etc. An example of abrasive particles that may be employed therefor are silicon carbide and diamond abrasive particles of from 1000 grit to 10000 grit.

[0078] Surface Treatment

[0079] Substantially spherical ejection particles are ejected against the regions described above of the surface of the metal article where surface strengthening is to be performed, and are caused to collide these regions.

[0080] Examples of the ejection particles, ejection apparatus, and ejection conditions employed when performing the above surface treatment are given below.

[0081] (1) Ejection Particles

[0082] For the substantially spherical ejection particles employed in the surface treatment method of the present invention, substantially spherical means that they do not need to be strictly spherical, and ordinary shot may be employed therefor. Particles of any non-angular shape, such as an elliptical shape and a barrel shape, are included in substantially spherical ejection particles employed in the present invention.

[0083] Materials that may be employed for the ejection particles include both metal-based and ceramic-based materials. Examples of materials for metal-based ejection particles include steel alloys, cast iron, high-speed tool steels (HSS) (SKH), tungsten (W), stainless steels (SUS), boron (B), chromium boron steels (FeCrB), and the like. Examples of materials for ceramic-based ejection particles include alumina (Al.sub.2O.sub.3), zirconia (ZrO.sub.2), zircon (ZrSiO.sub.4), hard glass, glass, silicon carbide (SiC), and the like.

[0084] Regarding the particle diameter of the ejection particles employed, particles having a median diameter (d50) in a range of from 1 m to 20 m may be employed. Iron-based ejection particles that may be employed have a median diameter (d50) in a range of from 1 m to 20 m, and preferably in a range of from 5 m to 20 m. Ceramic-based ejection particles that may be employed have a median diameter (d50) in a range of from 1 m to 20 m, and preferably in a range of from 4 m to 16 m.

[0085] For fine powder ejection particles having a median diameter from 1 m to 20 m, the ejection particles can be imparted with the property of having a long falling time through air (caused to float in air) by selecting a material density of the ejection particles. Ejection particles having such properties readily ride on an airflow, and can be propelled with a velocity similar to that the airflow velocity.

[0086] In the surface treatment method of the present invention, the ejection particles employed have a falling time in still air conditions of 10 sec/m or greater. This enables the ejection particles to be ejected at substantially the same velocity as the velocity of an airflow being ejected from an ejection nozzle of a blasting apparatus.

[0087] With regard to the falling speed, for the same particle diameter, the falling time is longer, the lower the density of the material configuring the ejection particles. For iron-based ejection particles having a relative density (specific gravity) of 7.85, the falling time is 10.6 sec for a particle diameter of 20 m, and 41.7 sec for a particle diameter of 10 m. For ceramic-based ejection particles having a relative density of 3.2, the falling time is 26.3 sec for a particle diameter of 20 m, and 100 sec for a particle diameter of 10 m.

[0088] Note that the ejection particles employed are preferably ejection particles of a material having a hardness equivalent to or greater than that of the base metal of the metal article to be treated. When ceramic-based ejection particles are employed, the ejection particles have a higher hardness than substantially all metal articles. The density of ceramic-based ejection particles is also low, and the falling time as described above is long. This means that ceramic-based ejection particles are preferably employed due to being able to obtain a high ejection velocity.

[0089] (2) Ejection Apparatus

[0090] A known blasting apparatus for ejecting abrasive together with a compressed gas may be employed as the ejection apparatus to eject the ejection particles described above toward the surface of region to be treated.

[0091] Such blasting apparatuses are commercially available, such as a suction type blasting apparatus that ejects abrasive using a negative pressure generated by ejecting compressed gas, a gravity type blasting apparatus that causes abrasive falling from an abrasive tank to be carried by compressed gas and ejected, a direct pressure type blasting apparatus in which compressed gas is introduced into a tank filled with abrasive and the abrasive is ejected by merging the abrasive flow from the abrasive tank with a compressed gas flow from a separately provided compressed gas supply source, and a blower type blasting apparatus that carries and ejects the compressed gas flow from such a direct pressure type blasting apparatus with a gas flow generated by a blower unit. Any one of the above may be employed to eject the ejection particles described above.

[0092] (3) Treatment Conditions

[0093] Substantially spherical ejection particles configured from one of the materials described above or the like, and having a median diameter d50 of from 1 m to 20 m and a falling time through air of not less than 10 sec/m are ejected against the metal article as described above at an ejection pressure of from 0.05 MPa to 0.5 MPa.

[0094] Confirmation of Optimum Conditions

[0095] (1) Diameter of Ejection Particle

[0096] (1-1) Concept

[0097] As described above, the lamellar processing structures as explained with reference to FIG. 1 need to be suppressed from being generated in order to form a uniform nano-crystal structure layer continuously along a surface of a metal article made from a soft material. In order to suppress the generation of such lamellar processing structures, deformation of the metal article surface needs to be suppressed from occurring when collided by the ejection particles.

[0098] On the other hand, it is considered that strain exceeding a critical value needs to be imparted in the vicinity of the surface of the metal article in order to generate nano-crystal structures, and that a large colliding force needs to be imparted to the surface of the metal article by collision of the ejection particles in order to impart strain exceeding the critical value.

[0099] However, the larger the colliding force imparted to the surface of the metal article, the larger the amount of deformation at the surface of the metal article, and the more readily the lamellar processing structures explained with reference to FIG. 1 are generated. This makes it difficult for a metal article made from a soft material to generate a uniform nano-crystal structure layer continuously across the surface without being accompanied by lamellar processing structures.

[0100] The inventors of the present invention have accordingly investigated treatment conditions that enable these conflicting demands to be satisfied, i.e. the need to reduce the colliding force received by the metal article surface when collided by the ejection particles to suppress deformation of the surface of the metal article, with the need to also impart strain exceeding the critical value required to generate the nano-crystal structures.

[0101] (1-2) Deformation Amount by Collision

[0102] The deformation amount generated at the metal article surface when collided by the ejection particles has been investigated.

Particles having a median diameter d50 of from 20 m to 40 m were caused to collide a surface, and the volume of protrusions on the surface was measured using a profile analyzing laser microscope. A comparison was then made between the protrusion volume and the ease of generation of the lamellar processing structures formed by folding. This was done because it was thought that the larger the protrusion volume, the larger the amount of folding that would be generated when collided by the particles.

[0103] A profile analyzing laser microscope (VK-X250, manufactured by Keyence Corporation) was employed as the measuring method, and measurements were taken of the surface at a measurement magnification of 1000.

[0104] The measured data was analyzed using a Multi-File Analysis Application (manufactured by Keyence Corporation).

[0105] The Multi-File Analysis Application is software that uses data measured by a laser microscope to perform various measurements, such as surface roughness, flatness measurements, profile measurements, volume/area measurements, etc.

[0106] In measuring, first the image processing function was used to set the reference plane (however, in cases in which the surface shape is a curved plane, the reference plane is set after the curved plane has been corrected to a flat plane by using plane shape correction). Then, the measurement mode was set to protrusion in the volume/area measurement function of the application, protrusions were measured with respect to the set reference plane, and the average value of the volume in the protrusion measurement results was set as a dimple protrusion volume.

[0107] Note that the reference plane described above was computed from height data using a least squares method.

[0108] These results are given in the following table (Table 1). The particle diameter of 20 m of the scope of the present invention resulted in a protrusion volume that was about 70% less than that resulting from a 40 m diameter in related art. It was thought that this extremely small deformation amount was a reason for the uniform nano-crystal structure formation.

TABLE-US-00001 TABLE 1 Ejection Particle Diameter and Protrusion Volume Example Comparative Example Ejection particle 20 40 diameter (D50) m Protrusion volume m.sup.3 932 2738

[0109] (1-3) Investigation of Colliding Force F

[0110] When the above treatment conditions were investigated, the relationship between the colliding force F and the ejection particle diameter was re-investigated based on computation equations to compute the colliding force F imparted to the surface of the metal article by colliding with an ejection particle (1 particle).

[0111] When the mass of an ejection particle (1 particle) is m (kg), the velocity of the ejection particle before impact is v1 (m/sec), the velocity of the ejection particle after impact is v2 (m/sec), and the coefficient of restitution on impact is assumed to be 1.0, then a momentum M1 of the ejection particle before impact, and the momentum M2 after impact, are given by the following equations:

M1=m.Math.v1 (kgm/s)Equation 1

M2=m.Math.v2 (kgm/s)Equation 2

[0112] Thus a change in momentum M of the ejection particle between before and after impact is:

M=M1M2=m.Math.v1(m.Math.v1)=2m.Math.v1 (kgm/s)Equation 3

[0113] The change in momentum M here is equivalent to the impulse Ft (wherein t is the duration of impulse).

Ft=MEquation 4

[0114] Thus, the colliding force F imparted to the surface of the metal article when collided by the ejection particle (1 particle) is:

F=M/t=2mv1/t(N)Equation 5

[0115] According to the colliding force F of Equation 5, the colliding force F changes in proportional to a mass m of the ejection particle, and so the colliding force F gets larger as the ejection particle diameter increases.

[0116] (1-4) Ejection Particle Diameter and Pressure Receiving Surface

[0117] As the particle diameter of the ejection particle increases, the colliding force F also increases, as described above. Thus if the particle diameter of the ejection particles employed is large and the colliding force F is large, the surface area of the portion of the metal article surface undergoing deformation (the portion indicated by the reference sign S in FIG. 3) also increases when the surface of the metal article is collided with the ejection particles.

[0118] The way in which the surface area (pressure receiving surface S) acted on by compressional force on the metal article surface changes was investigated by changing the particle diameter of such ejection particles.

[0119] Taking the surface of the metal article where interaction with the ejection particles occurs (a circular shape horizontal plane) as a pressure receiving surface S, then relationships expressed by Equation 6 and Equation 7 below are satisfied between a radius a of the pressure receiving surface S, a radius r of the ejection particles, and a depth X of the depressions:

a.sup.2+(rX).sup.2=r.sup.2Equation 6

a.sup.2=r.sup.2(rX).sup.2=r.sup.2(r.sup.22rX+X.sup.2)=2rXX.sup.2Equation 7

[0120] Wherein, taking a as a ratio of a depth X of the depressions to a diameter d of the ejection particles, then:

X=2rEquation 8

[0121] Thus substituting Equation 8 for X in Equation 7 gives:

a.sup.2=2r(2r)(2r).sup.2Equation 9

[0122] Since 2r=d:

a.sup.2=d.sup.2d.sup.2.sup.2=d.sup.2(1)Equation 10

[0123] Thus, a surface area S (m.sup.2) of the pressure receiving surface is given by Equation 10.

S=.sup.2=d.sup.2(1)Equation 11

[0124] Equation 11 shows that the surface area of the pressure receiving surface S increases in proportional to the square of the diameter of the ejection particles.

[0125] With regard to the lamellar processing structures explained above with reference to FIG. 1, indentations and protrusions are formed during colliding, and then the protrusions from out of these indentations and protrusions are folded over to form the lamellar processing structures. These protrusions are formed by base metal at the depression portions explained with reference to FIG. 3 (the shaded portion in FIG. 3) being pushed out when collided by an ejection particle.

[0126] Thus, as the surface area of the pressure receiving surface S described above increases, the protrusions formed become larger, and this is postulated to facilitate the formation of the lamellar processing structures.

[0127] (1-5) Ejection Particle Diameter and Ejection Velocity

[0128] From the above equation of colliding force F (Equation 5), the colliding force F does not only increase with an increase in mass m of the ejection particles, but also increases with an increase in the ejection velocity v1.

[0129] The ejection velocity was computed with reference to ejection velocity computation equations in a paper regarding how the ejection velocity changes with respect to changes in particle diameters of ejection particles: Measurement and Analysis of Shot Velocity in Pneumatic Shot Peening by Ogawa, Asano, et al (Transactions of the Japan Society of Mechanical Engineers, Edition C. Volume 60. No. 571, 1994-3).

[0130] (1-6) Predicting Optimum Particle Diameters for Ejection Particles

[0131] The above computation equations and the like were employed, and the change in colliding conditions to changes in particle diameter for steel ejection particles (relative density of 7.85) as an example, are summarized in Table 2 and Table 3, below.

TABLE-US-00002 TABLE 2 Change in Colliding Conditions with Changes to Ejection Particle Diameter (Ejection Pressure: 0:5 MPa) Colliding Pressure force F/ Ejection Receiving Pressure Number of Particle Velocity Impulse Surface Receiving Ejection Colliding diameter Mass m V1 Duration t Colliding Area S Surface Particles Energy (m) (g) (m/sec) (s) force F (kgf) (mm.sup.2) Area S (per kg) (J) 5 0.0005 245 10 2.57 10.sup.6 2.35 10.sup.7 10.9 1.95 10.sup.15 3.00 10.sup.7 10 0.0041 245 20 1.03 10.sup.5 9.40 10.sup.7 10.9 2.43 10.sup.14 3.00 10.sup.7 20 0.0329 198 40 3.17 10.sup.6 3.76 10.sup.6 8.4 3.04 10.sup.13 1.79 10.sup.7 50 0.5138 150 100 1.57 10.sup.4 2.35 10.sup.5 6.7 1.95 10.sup.12 1.13 10.sup.7 100 4.1103 130 200 5.45 10.sup.4 9.40 10.sup.5 5.8 2.43 10.sup.11 8.45 10.sup.6

TABLE-US-00003 TABLE 3 Change in Collision Conditions with Ejection Particle Diameter (Ejection Pressure: 0.05 MPa) Pressure Colliding Number Ejection Receiving force F/ of Particle Velocity Impulse Colliding Surface Pressure Ejection diameter Mass m V1 Duration t force F Area S Receiving Particles Colliding (m) (g) (m/sec) (s) (kgf) (mm.sup.2) Surface Area S (per kg) Energy (J) 5 0.0005 112 10 1.17 10.sup.6 2.35 10.sup.7 5.0 1.95 10.sup.15 6.27 10.sup.6 10 0.0041 112 20 4.47 10.sup.6 9.40 10.sup.7 5.0 2.43 10.sup.14 6.27 10.sup.6 20 0.0329 86 40 1.44 10.sup.5 3.76 10.sup.6 3.8 3.04 10.sup.13 3.70 10.sup.6 50 0.5138 61 100 6.40 10.sup.5 2.35 10.sup.5 2.7 1.95 10.sup.12 1.86 10.sup.6 100 4.1103 47 200 1.97 10.sup.4 9.40 10.sup.5 2.1 2.43 10.sup.11 1.10 10.sup.6

[0132] As is apparent from Table 2 and Table 3, the colliding force F increases the larger the particle diameter of the ejection particles, however, accompanying such increases, the pressure receiving surface area S also increases. As a result, large protrusions are formed on the surface of the metal article being collided with the ejection particles. This is thought to facilitate generation of the lamellar processing structures, which are thought to be generated by folding such protrusions.

[0133] Moreover, the larger the particle diameter of the ejection particles, the larger the value of the colliding force F. However, the surface area of the pressure receiving surface S increases in proportional to the square of the diameter d of the ejection particles, as stated above. This means that when the colliding force F per unit surface area of the pressure receiving surface S (colliding force F/pressure receiving surface area S) is considered, then the force imparted per unit surface area actually decreases.

[0134] With regard to colliding energy, in cases in which each of the particles in Table 1 are ejected at 0.5 MPa, if the colliding energy when the particle diameter d50=50 m is taken as 1, then the particles with d50=10 m and 20 m can be projected against a surface with high energies of about two to three times this energy.

[0135] Thus when ejection particles of large diameter are employed, not only is the surface of the metal article is more readily deformed, facilitating the generation of lamellar processing structures as explained with reference to FIG. 1, but this is also conjectured to make it difficult to obtain a strain exceeding the critical value required to obtain nano-crystal structures.

[0136] A simulation of Von Mises stress was accordingly performed by analysis using a finite element method (FEM) (referred to below as FEM analysis) based on the computed values given in Table 2 and Table 3. These results are illustrated in FIG. 4 to FIG. 8.

[0137] Moreover, the results obtained from this simulation are illustrated as a graph in FIG. 9 of a relationship between change in stress and ejection particle diameter, and as a graph in FIG. 10 of a relationship between depth at which the maximum stress is generated and ejection particle diameter.

[0138] FEM analysis is a numerical analysis method for use in cases difficult to solve by analytical methods such as complex geometric models. In FEM analysis, an area is divided into finite elements, simple formulae are established at the element level, and a solution for the whole system is obtained by using interpolation functions between elements to make an approximation thereof. Femap with NX Nastran (sold by NST Co., Ltd.) was employed as analysis software.

[0139] Moreover, Von Mises stress is equivalent stress based on shear strain energy theory. Von Mises stress is expressed as a scalar value without directionality, and in a stress field where complex loading acts in in plural directions, the Von Mises stress is a value for uniaxial tension or compressive stress.

[0140] The Von Mises stress is referenced as an indicator to determine whether or not a given material will yield. This means that there is no need to look at stress in other directions when comparing against yield stress, and yield determination is made using a single Von Mises stress. This was utilized to simulate stress arising from colliding with the ejection particles.

[0141] It is apparent from looking at the simulation results between particle diameter of ejection particles and a depth where stress is applied (generated), that a high stress is applied to extremely shallow layers at the surface as the particle diameter of the ejection particles gets smaller. It is also apparent that although stress is input to deeper layers as the particle diameter gets larger, this stress is lower.

[0142] In particular, it is apparent from FIG. 4 to FIG. 8 that the depth and intensity of the stress input to the surface of the metal article changes at a turning point of an ejection particle diameter of 20 m. The intensity of the stress is greatly decreased when the ejection particle diameter exceeds 20 m.

[0143] Namely, in the contour diagrams of FIG. 4 to FIG. 8, the center of the portions where a crescent shape can be seen represents the portion input with highest intensity stress. An extremely high stress was imparted to portions in the vicinity of the surface in the simulation of ejection particles of 20 m or less. However, stress is spread out and dispersed deeply as the particle diameter increases, resulting in a weaker intensity of stress (see FIG. 9 and FIG. 10).

[0144] From the above results it is thought that indentations and protrusions (in particular protrusions), which are the cause of lamellar processing structure formation as explained with reference to FIG. 1, are not liable to be formed on the surface of the metal article when ejection particles of 20 m or less are employed. Moreover, employing such ejection particles is thought to result in an effect by which compositional strain exceeding the critical value required to generate the nano-crystal structures is concentrated and generated in the vicinity of the surface of the metal articles.

[0145] Ferrous alloy ejection particles having a median diameter d50 of 20 m were ejected against regions of 6 mm5 mm on test strips made from an alloy tool-steel (SKD11), a pre-hardened steel (NAK80, manufactured by Daido Steel Co., Ltd), and an aluminum alloy (A7075). Changes in surface hardness (dynamic hardness) were measured for each of the test strips.

[0146] In order to derive the ejection pressure suitably applied to each of hard materials and soft materials, test strips were produced for each of the materials and treated at different ejection pressures. The dynamic hardness was measured at 30 points in the regions of 6 mm5 mm on the test strips, and the found hardness taken as the surface hardness (dynamic hardness) of each test strip.

[0147] A graph of these measurement results is illustrated in FIG. 11.

[0148] Note that the dynamic hardness (DHT) is a hardness measured by indentation, and the conditions of measurement are as follows. [0149] Test Instrument: Dynamic Ultra Micro Hardness Tester DUH-W210, manufactured by Shimadzu Corporation [0150] Indentation Load: 3 gf (A7075), 5 gf (NAK80), 10 gf (SKD11) [0151] Time Held: 5 seconds [0152] Shape of Indenter: Triangular pyramid diamond indenter (115) [0153] Computation Method DHT=P/(D.sup.2)

[0154] Note that in the above equation DHT is the dynamic hardness, a is an indenter shape coefficient (3.8584), P is the indentation load (mN), and D is the indentation depth.

[0155] Hitherto, it has been thought that raising the ejection pressure is effective when attempting to impart intense stress to the surface of a metal article using shot peening.

[0156] However, from the measurement results of the dynamic hardness (DHT) illustrated in FIG. 11, even employing the ejection particles of the present invention having a median diameter of 20 m or less, an increase in surface hardness (dynamic hardness) of a metal article according to the rise in ejection pressure has been confirmed to be achieved in a range of ejection pressures from more than 0 MPa to 0.1 MPa. It was also confirmed that a further rise in surface hardness was no longer seen for ejection pressures exceeding 0.1 MPa, regardless of whether the test strip was made from a high hardness material or a low hardness material, i.e. the hardness raising effect became saturated in the vicinity of an ejection pressure of 0.1 MPa.

[0157] It is accordingly thought to be possible by the method of the present invention to impart the energy required to raise the hardness of the surface of the metal article (and therefore to cause nano-crystallization thereon) by treatment with an ejection pressure of 0.05 MPa or greater. It was confirmed that it was possible to perform surface treatment of both hard materials and soft materials by using a comparatively low ejection pressure of not more than 0.5 MPa.

[0158] Moreover, due to being able to perform treatment employing such fine ejection particles with a comparatively low ejection pressure, the deformation of the metal article surface is suppressed to a minimum even when treating a metal article made from a soft material, and it is thought that this enables the lamellar processing structures explained with reference to FIG. 1 to be suppressed from being generated.

[0159] In this manner, it is thought that the reason why a low ejection pressure can be employed in the method of the present invention is because, although generally when particles in the air are caused to settle out under gravity the particles settle out due to weight (external force) when the particle diameter is large, the particles are readily carried on an airflow and have the property of not being liable to settle out when the particle diameter is small.

[0160] Namely, such ejection particles of small particle diameter have a small mass and the influence of inertia is small. There is accordingly no need for a large force to move such particles, and these ejection particles are easily carried on an ejected airflow even when the pressure of the transport gas is a low pressure. This enables the ejection particles to be ejected from the ejection nozzle easily with a velocity close to that of the compressed gas since the distance until the maximum velocity is achieved is short.

[0161] As a result, employing ejection particles that are easily carried on an airflow as stated above eliminates a large difference between the ejection velocities of the ejection particles when ejected at an ejection pressure of 0.1 MPa and when ejected at an ejection pressure of 0.5 MPa. This is accordingly thought to lead to being able to obtain a similar increase in hardness to that at an ejection pressure of 0.5 MPa even when the ejection pressure is 0.1 MPa.

[0162] Moreover, a hardness that is not less than 60% of the hardness at 0.1 MPa can still be imparted even when the pressure is 0.05 MPa.

[0163] However, even with ejection particles having a median diameter of 20 m or less, those having a large mass are more readily influenced by inertia, are less liable to be carried on an airflow, and arrive at the surface of the metal article prior to reaching the maximum velocity.

[0164] Thus the iron-based ejection particles having a median diameter of 20 m employed in the above tests have a falling time through air (inverse of terminal velocity according to Stokes' Law or Stokes' equation) that is 10.6 sec/m. In the tests employing such ejection particles, a good rise in surface hardness (dynamic hardness) could be obtained for ejection pressures within the range of from 0.05 MPa to 0.5 MPa.

[0165] It is accordingly thought that the required ejection velocity can be achieved as long as the falling time through air is longer than that of these ejection particles so that the ejection particles are readily carried on an airflow, enabling nano-crystallization to be obtained at the surface of the metal article.

[0166] From the results described above, the ejection particles employed in method of the present invention are determined to be ejection particles having a median diameter of not greater than 20 m, and having a falling time through air of not less than 10 sec/m.

[0167] Note that, as seen from Table 2 and Table 3, the ejection velocity is not less than 80 m/sec for the above described iron-based ejection particles having a particle diameter of 20 m. Thus in the surface treatment method of the present invention, the ejection particles are preferably ejected at an ejection velocity of not less than 80 m/sec.

[0168] Advantageous Effect Confirmation Tests

[0169] (1) Tests Objective

[0170] Performing shot peening under the treatment conditions obtained from the results of the tests and simulations performed to derive the treatment conditions as described above confirmed that a uniform nano-crystal structure formation could be continuously formed along the surface of both metal articles made from hard materials and metal articles made from soft materials. It was also confirmed that a high residual stress could be imparted to the surface of the metal article.

[0171] (2) Test Method

[0172] The surface treatment of the method of the present invention was performed on the test strips made from a pre-hardened steel (NAK80, manufactured by Daido Steel Co., Ltd), an alloy tool-steel (SKD11), and an aluminum alloy (A7075). The surface treatment conditions are listed in Table 4 below.

TABLE-US-00004 TABLE 4 Test Conditions NAK80 SKD311 A7075 Surface Blasting method SF SF SF Treatment Ejection particle Ferrous alloy Ferrous alloy Ferrous alloy material and median (Median diameter (Median diameter (Median diameter diameter D50 (m) D50: 20 m) D50: 20 m) D50: 20 m) Ejection pressure 0.5 0.5 0.5 (MPa) Nozzle diameter (mm) 7 7 7 Ejection time (sec) 30 30 30

[0173] (3) Observation Method

[0174] Each of the test strips that had been surface treated under the conditions described above was observed by the following method.

[0175] (3-1) SIM Observation

[0176] A scanning ion microscope (SIM) (SMI3050SE, manufactured by Hitachi High-Tech Science Corporation) was employed to observe changes in crystal structure in the vicinity of the surface of each test strip.

[0177] (3-2) EBSD Observation

[0178] Electron back scatter diffraction analysis was employed (using an Electron Back Scatter Diffraction instrument manufactured by TSL Solutions Corporation) to observe crystal structure in the vicinity of the surface of each test strip, and to observe the crystal grain diameter and a crystal grain distribution therein.

[0179] (3-3) Residual Stress Measurements

[0180] A portable X-ray residual stress analyzer (p-X360 manufactured by Pulsetech Industrial Co., Ltd) was employed to measure the residual stress at the outermost surface layer of each of the test strips.

[0181] (4) Test Results

[0182] (4-1) Results of SIM Observations

[0183] FIG. 12 to FIG. 14 illustrate SIM images for each of the test strips. FIG. 12 is an SIM image for a pre-hardened steel (NAK80), FIG. 13 is an SIM image for an alloy tool-steel (SKD11). FIG. 14 is an SIM image for an aluminum alloy (A7075). In each of the respective drawings, the figure appended with A was captured for test strips before treatment, and the figure appended with B was captured for test strips after treatment.

[0184] It could be confirmed for the test strips of all of the materials that the metal structure was clearly micronized in a zone down to about 3 m from the surface layer after the surface treatment according to the method of the present invention had been performed. The crystal grains after micronization were all confirmed to have a nano-crystal structure.

[0185] The nano-crystal structures were formed continuously along the surface of the test strips within the field of view of SIM mages (about 10 m), and the formation of a continuous nano-crystal structure layer was confirmed.

[0186] Moreover, this nano-crystal structure, even for the test strip to be treated made from the aluminum alloy (A7075) which is a soft material, was confirmed to be formed as a uniform nano-crystal structure without cracks or the like occurring in the structure, and without being accompanied by the formation of the lamellar processing structures explained with reference to FIG. 1.

[0187] It was confirmed that in these test strips there was a region with significant fine-crystallization (nano-crystallization) in a zone down to 3 m from the surface layer. There was also some micronization observed in a deeper zone of increased depth from the surface layer, and micronization was particularly significant in the test strip made from aluminum alloy.

[0188] The results of observations using SIM confirmed that the surface treatment method of the present invention was capable of forming a uniform nano-crystal structure layer continuously along the surface, without being accompanied by the formation of the lamellar processing structures, in a zone of a particular depth (about 3 m) from the surface for both test strips made from hard materials and test strips made from soft materials.

[0189] The test strips formed in this manner with a nano-crystal structure layer in the vicinity of surface had, as explained with reference to FIG. 11, a surface hardness (dynamic hardness) is increased by about 100 to 200 compared to untreated test strips (indicated at ejection pressure 0 MPa in FIG. 11). This confirmed that the effectiveness as a method for strengthening surfaces of metal articles formed from various materials from soft materials through to hard materials.

[0190] (4-2) Results of EBSD Observations

[0191] The results obtained from EBSD analysis indicated a crystal grain diameter distribution in the vicinity of the surface of the pre-hardened steel (NAK80) test strip as illustrated in FIG. 15, and a crystal grain diameter distribution in the vicinity of the surface of the alloy tool-steel (SKD11) test strip as illustrated in FIG. 16.

[0192] The results of observations using EBSD confirmed that the crystal grain diameter of the nano-crystal structure layer in the pre-hardened steel (NAK80) was in the range of from 100 nm to 500 nm. Moreover, the average crystal grain diameter in the crystal grain diameter distribution of this nano-crystal structure layer was found to be 240 nm (see FIG. 15).

[0193] In the alloy tool-steel (SKD11), the crystal grain diameter of the nano-crystal structure layer was confirmed to be in the range of from 100 nm to 500 nm. Moreover, the average crystal grain diameter in the crystal grain diameter distribution of this nano-crystal structure layer was found to be 223 nm (see FIG. 16).

[0194] Note that in the aluminum alloy (A7075) test strip, the generated crystal grain diameter was much smaller than the resolution of EBSD. Thus, although crystallite analysis could not be performed by EBSD, due to the highest resolution by EBSD being 30 nm, since the finest crystal grains were observed in the test strips by SIM imaging, most of the crystal grains can logically be presumed to mainly be smaller than the 30 nm, which is the highest resolution of EBSD, in the nano-crystal structure layer formed on the surface of the aluminum alloy (A7075). The crystal grain diameter of the nano-crystal structure layer formed on the surface of the aluminum alloy (A7075) is accordingly thought to be 100 nm or less.

[0195] (4-3) Residual Stress Measurement Results

[0196] The results of measurements of residual stress at the outermost surface layer of each of the test strips are summarized by graphs illustrated in FIG. 17 to FIG. 19.

[0197] In each of the test strips, residual stresses in the untreated state that had positive values (tensional stress) flipped to negative values (compressional stress). The surface treatment method of the present invention was accordingly confirmed to be capable of imparting a high compressive residual stress.

[0198] From among these test strips, the stress in the pre-hardened steel (NAK80) illustrated in FIG. 17 and the stress in the aluminum alloy (A7075) illustrated in FIG. 19 showed hardly any change by changes of the ejection pressure. This confirmed that sufficient compressive residual stress could be imparted by ejection at comparatively low ejection pressures of not more than 0.5 MPa, as long as an ejection pressure of 0.1 MPa or above was achieved as stated above.

[0199] The residual stress of the aluminum alloy (A7075) is illustrated in the graph of FIG. 19. This graph shows as a Comparative Example the results of residual stress measurements when ejection particles having a median diameter of 40 m, this being larger than the range of the present invention, were ejected at an ejection pressure of 0.5 MPa.

[0200] Thus, in the example (Comparative Example) with ejection particles having a comparatively large particle diameter to the ejection particles employed in the present invention, although a compressive residual stress could be imparted, the residual stress that could be imparted thereby was or less in compared with the residual stress imparted by the method of the present invention. Therefore, when the method of the present invention was employed to perform surface treatment, a higher surface strengthening effect was obtained.

[0201] Note that in the test results for alloy tool-steel (SKD11) (see FIG. 18), although an increase in residual stress with increasing ejection pressure was observed, sufficient residual stress was still imparted even in cases in which ejection was performed at the lowest ejection pressure (0.1 MPa).

[0202] Moreover, in the example in which surface treatment of the present invention was performed at an ejection pressure of 0.1 MPa, although the residual stress was slightly lower than that of the Comparative Example (ejection particles having a median diameter of 40 m at an ejection pressure of 0.5 MPa), a residual stress similar to or surpassing that of the Comparative Example was imparted at ejection pressures of 0.3 MPa and 0.5 MPa.

[0203] Application to Cutting-Edge of Machining Tool

[0204] (1) Test Method

[0205] Blanking punches made from SKD11 and having cutting-edge portions treated with the surface treatment method of the present invention (Examples 1 and 2), a blanking punch made from untreated SKD11 (untreated punch), and a blanking punch made from SKD11 surface treated under treatment conditions deviating from the treatment conditions of the present invention (Comparative Example 1) were employed for punch processing. The states of the cutting-edge portions were respectively observed after processing.

[0206] (2) Surface Treatment Conditions

[0207] Surface treatment was performed under the conditions listed in Table 5 below on a cutting-edge portion (the cutting-edge and a region up to 5 mm from the cutting-edge) of each of the punches (length 3 cm, diameter 0.5 cm) for punch-processing made from SKD11.

TABLE-US-00005 TABLE 5 Surface Treatment Conditions of a Punch for Punch-Processing Comparative Example 1 Example 2 Example 1 Surface Ejection method SF SF SF treatment Ejection particle HSS (Median Aluminum (Median HSS (Median Median diameter diameter D.sub.50: diameter D.sub.50: diameter D.sub.50: D.sub.50 (m) 15 m) 16 m) 80 m) Ejection pressure (MPa) 0.3 0.05 0.3 Nozzle diameter (mm) 7 7 7 Ejection duration (sec) 30 30 30

[0208] Note that SF for Ejection method in Table 5 indicates a suction ejection method employing a SFK-2 manufactured by Fuji Manufacturing Co., Ltd. as the blasting apparatus in these test examples.

[0209] (3) Punch-Press Processing Conditions and Observation Method

[0210] Punches respectively surface treated with the methods of Example 1, Example 2, and Comparative Example 1, and an untreated punch, were respectively employed to perform punch-press processing successively for 9000 cycles on a workpiece made from SS steel. The surface state of each of the punches after the punch-press processing had been performed was then observed by eye and with a microscope, and the state of wear noted.

[0211] (4) Observation Results

[0212] The surface state of each of the punches after the punch-press processing is as listed in Table 6 below.

TABLE-US-00006 TABLE 6 Punch Surface State After Punch-press Processing Treatment Conditions Surface State Example 1 Hardly any observable damage. No occurrences of accumulation of material to be processed. Example 2 Hardly any observable damage. No occurrences of accumulation. Comparative Multiple scratches having a striation shape. Example 1 along the length direction observed. Some accumulations of material to be processed were occurred. Untreated punch Unusable after 1800 cycles.

[0213] (5) Interpretation

[0214] In the present invention, performing the surface treatment of the present invention on punches made from SKD11 was seen to raise hardness, from a surface hardness of about 750 Hv when untreated to a hardness of about 950 Hv after surface treatment by the treatment of Example 1, that is, an uplift in hardness of about 21%.

[0215] Moreover, the treatment of Example 2 was seen to raise hardness to about 870 Hv, that is, an uplift in hardness of about 16%.

[0216] Such an uplift in hardness is thought to have been achieved due to the formation of the nano-crystal structure layer described above.

[0217] Moreover, the punches treated with the surface treatment method according to the present invention (Examples 1 and 2) were capable of preventing material to be processed from accumulating to the cutting-edge as described above. This is thought to be a reason why good punching performance was exhibited over a prolonged period of time, and a reason why the lifespan of the punches was raised.

[0218] The mechanism obtaining the effect of preventing accumulation of cut material is not entirely clear. However, fine dimples (see FIG. 2(B)) having an equivalent diameter of from 1 m to 18 m and a depth of from 0.02 m to 1.0 m or less than 1.0 m were formed on the surface of the metal article treated by the surface treatment method of the present invention. The projected area of these dimples is at least 30% of the surface area of the region to be treated. It is thought that the effect of preventing accumulation of cut material is obtained because these dimples are served as oil reservoirs.

[0219] Note that the diameter (equivalent diameter) and depths of the dimples were measured using a profile analyzing laser microscope (VK-X250 manufactured by Keyence Corporation). Measurements of the metal article surface were made directly in cases in which direct measurement was possible. In cases in which direct measurement was not possible, methyl acetate was dripped onto a cellulose acetate film to cause the cellulose acetate film to conform to the metal article surface, and after subsequently drying and peeling off the cellulose acetate film, measurement was performed based on the inverted dimples transferred to the cellulose acetate film. Surface image data imaged by the profile analyzing laser microscope (or, image data processed to invert captured images measured by employing the cellulose acetate film) was analyzed using a Multi-File Analysis Application (VK-H1XM by Keyence Corporation) to perform the measurements.

[0220] The Multi-File Analysis Application is an application that uses data measured by a laser microscope to measure surface roughness, line roughness, height and width, etc. The application analyzes the equivalent circular diameter, depth, and the like, sets a reference plane, and is capable of performing image processing such as height inversion.

[0221] In measuring, first the image processing function is used to set the reference plane (however, in cases in which the surface shape is a curved plane, the reference plane is set after the curved plane has been corrected to a flat plane by using plane shape correction). Then, the measurement mode is set to indentation in the volume/area measurement function of the application, indentations are measured with respect to the set reference plane, and the average depth in the indentation measurement results and the average value of the results for equivalent circular diameter are set as the depth and equivalent diameter of the dimples.

[0222] Note that the reference plane described above was computed from height data using a least squares method.

[0223] Moreover, the equivalent circular diameter and the equivalent diameter mentioned above are measured as the diameter of a circle determined by converting the projected surface area measured for an indentation (dimple) into a circular projected surface area.

[0224] Note that the reference plane described above indicates a flat plane at the origin (reference) measurement for height data, and is employed mainly to measure depth, height, etc. in the vertical direction.

[0225] Application to Sliding Member

[0226] (1) Test Method

[0227] Three types of flat sheets of SUS304, size 40 mm40 mm and thickness 2 mm, were prepared: sheets treated by the present invention (Example 3): untreated sheets having a mirror finish (Comparative Example 2); and sheets treated by related art (Comparative Example 3). The slidability of the sheets was then evaluated by friction-wear tests.

TABLE-US-00007 TABLE 7 Surface Treatment Conditions Example 3 Comparative Example 3 Ejection method SF SF Ejection particle Ferrous alloy HSS (Median diameter Median diameter (Median diameter D.sub.50: 40 m) D.sub.50 (m) D.sub.50: 20 m) Ejection pressure (MPa) 0.1 0.3 Nozzle diameter (mm) 7 7 Ejection duration (sec) 20 20

[0228] (2) Evaluation Method

[0229] Ball-on-disc tests were performed on the SUS304 sheets treated under the conditions described above until a friction coefficient of 2.0 was achieved. The times until this occurred were measured and compared to evaluate slidability.

TABLE-US-00008 TABLE 8 Friction-Wear Test Conditions Test Instrument FPR-2000 Load (g) 10 Rotation diameter (mm) 4 Rotation speed (rpm) 200 Lubrication None Gauge head 3/16 inch SUS304 ball

[0230] A ball-on-disc friction-wear tester was employed. A ball of 3/16 inch diameter made from SUS304 was employed therein.

[0231] (3) Evaluation Results

[0232] A graph of measured changes to friction with respect to elapsed time is illustrated in FIG. 20.

[0233] As is apparent from these measurement results, when treatment was performed with the conditions of Example 3, the durability was about 5 times high in comparison with the untreated one (Comparative Example 2), or about 3 times high in comparison with the one treated with the conditions of Comparative Example 3.

[0234] (4) Interpretation

[0235] It could be presumed that the testing had been performed with a commensurately low friction due to obtaining the durability of about 5 times high in comparison with the Comparative Example 2 and about 3 times high in comparison with Comparative Example 3. Thus performing the treatment of the present invention is thought to obtain about 3 times high in the slidability.