SURFACE TREATMENT METHOD FOR METAL PRODUCT AND METAL PRODUCT

Abstract

 Provided is 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. In the present invention, a substantially spherical spray powder having a median diameter of 1-20 µm 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 in a range from the surface of the metal product to a predetermined depth along the surface, 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.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for surface treatment of a metal article and to a metal article subjected to surface treatment by the method. In particular, the present invention relates to a surface treatment method to strengthen a surface of a metal article by ejecting fine particles against the metal article under predetermined conditions to make a crystal structure in the vicinity of the surface of the metal article to a nano-crystal structure, and to a metal article having a surface strengthened by such a method.

BACKGROUND OF THE INVENTION

[0002] The strength of a metal material being inversely proportional to the square root of crystal grain diameter is known as a Hall-Petch relationship. Micronization of the crystal grain diameter to give such an effect is also utilized in surface strengthening of metal articles.

[0003] In particular, a metal article having a crystal grain diameter in the vicinity of the surface micronized to nano crystal grain diameter not only has dramatically increased surface hardness, but also has been reported to achieve improved wear resistance and corrosion resistance.

[0004] As methods for nano-crystallization of metal articles enabling such strengthening of the surface, successful examples by ball milling, falling weight processing, particle colliding processing, and shot peening has been reported. Especially, nano-crystallization by shot peening is attracting particular attention due to being a low cost and easy method.

[0005] Note that although there is still insufficient understanding of the mechanism underlying the creation of a nano-crystal structure by shot peening, examples of surface treatment are introduced in Patent Document 1 and Non-Patent Document 1. The respective conditions therein are surface treatment by ejecting shot made from high speed steel (SKH59) with an average particle diameter 45 µm at 0.5 MPa for 30 seconds against a soft material, in this case SS400 steel (HV 1.20 GPa (HV122)); and surface treatment by shot peening under the same conditions against a hard material, in this case SCr420 carburized and quenched steel (initial hardness HV 7.55 GPa (HV770)) in Patent Document 1 and Non-Patent Document 1. There are also descriptions therein of large differences between the nano-crystal structures formed in each example (see Patent Document 1 and Non-Patent Document 1).

[0006] Note that in the present specification conversion between HV (GPa) and HV (no units) is computed by "HV(no units) ≈ HV (GPa) × 102" (see Table 1 of JIS R 1610(2003)).

RELATED ARTS

[Patent Document]

[0007] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2007-297651

[Non-Patent Document]

[0008] [Non-Patent Document 1] "Formation of Nanocrystalline Structure by Fine Particle Bombarding" by Shinichi Takagi and Masao Kumagai, published in the Journal of the Japan Society for Precision Engineering, Vol. 72, No.9, 2006, pp 1079 to 1082.

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0009] As stated above, in Patent Document 1 and Non-Patent Document 1, when attempting to generate nano-crystallization in the surface structure of a metal article using shot peening, it has been reported that there is a significant difference between nano-crystal structures (lamellar processing structures) created by treating a metal article configured from a soft material, and nano-crystal structures (not accompanied with lamellar processing structures) created by treating a metal article configured from a hard material.

[0010] From among these structures, nano-crystal structures created in the surface of a metal article made from a hard material (SCr420 carburized and quenched steel) are reported to be generated as nano-crystal structures by a physical state and formed uniformly along the surface in a zone extending to a particular depth from the surface.

[0011] However, with a metal article made from a soft material (SS400 steel), significant indentations and protrusions are formed on the surface in the initial stage of shot peening by colliding with ejection particles, as illustrated in Fig. 1. This is then followed by the protrusions, from out of the indentations and protrusions formed by colliding with ejection particles, being folded over toward the inside of the material so as to be penetrated into the material. Repeatedly folding over of the protrusions from out of further indentations and protrusions formed by subsequent colliding then forms the lamellar processing structures, which have a layered structure resulting from multiple folded over layers. The density of dislocations (strains) is increased significantly in such lamellar processing structures, and this is interpreted as nano-crystallization when it exceeds a critical point.

[0012] The nano-crystal structures accompanying such lamellar processing structures are not contiguously distributed along the surface of the metal article. Sometimes peripheral work-hardening regions are exposed at the surface, and sometimes the lamellar processing structures (nano-crystal structures) penetrate to positions deeper than the work-hardening regions. Moreover, when bonding during folding is insufficient and inter-layer cracking has occurred to produce a non-uniform structure, this gives rise to a concern that nano-crystallization induced by shot peening might actually result in a deterioration of the fatigue properties of the metal article. This is due to the presence of surface portions where a nano-crystal structure is not formed and due to stress concentrating at portions where cracks have developed, etc.

[0013] Thus considering the point that when treatment is performed on a soft material in this manner, the nano-crystal structures that are created along with the lamellar processing structures do not enable surface strengthening to be performed, the treatment of Patent Document 1 is limited to treat metal articles made from hard steels having an initial hardness exceeding HV 7.0 GPa (HV714). There is no disclosure therein of a method applicable to a soft material for forming a uniform nano-crystal structure continuously along the surface thereof.

[0014] The present invention accordingly solves the deficiencies of the related art described above. A first objective of the present invention is to provide a method for surface treatment of a metal article in which the surface treatment method is capable of forming a uniform nano-crystal structure continuously along the surface of the metal article, without forming the lamellar processing structures described above, even when the metal article is made from a soft material.

[0015] A second objective of the present invention is to provide a surface treatment method of a metal article that is: capable of being applied commonly to metal articles spanning from those made of soft materials to those made of hard materials, irrespective of the hardness of the base metal of the metal article to be treated; and capable of forming a uniform nano-crystal structure continuously along the surface of the metal article.

[0016] Note that in a cutting process performed using a cutting tool, the surface of the workpiece is physically cut into and parted by the cutting-edge of the cutting tool, and a portion of the workpiece is scraped off. Performing cutting by continuously pressing-in the cutting-edge while removing the swarf (chip) generated by such scraping leads to a high pressure being generated between the chip and the rake face of the cutting tool. The accompanying large frictional resistance and associated cutting heat physically and chemically changes the chip such that a portion of the chip accumulates to a leading portion of the cutting-edge. Accumulation formed by the chip accumulated to the cutting-edge of the cutting tool accordingly forms what is referred to as a "built-up edge", which differs from the original cutting-edge.

[0017] Such built-up edge formation is not desirable due to it leading to a dulling of the cutting-edge of the cutting tool, to a reduction in processing precision, and the like.

[0018] The accumulation of material to be processed typified by such a built-up edge is something that is not confined to cutting tools such as drills, end mills, hobs, broaches, milling cutters, and the like. Accumulation of material to be processed also occurs with cutting-edge portions in general of machining tools that include a cutting-edge (edge) for cutting and parting, such as punching tools like punches.

[0019] However, applying the surface treatment method of the present invention to cutting-edge portions of machining tools, as has been tried by the inventors of the present invention, has been demonstrated to improve the mechanical properties of the cutting-edge portions, such as increase the hardness and improve the wear resistance thereof. In addition, the capability of the surface treatment method to prevent material to be processed from accumulating to the cutting-edge portions, such as by suppressing built-up edge generation, has also been confirmed.

[0020] Moreover, it is generally known that for sliding members, the slidability is improved by an effect in which oil is retained in dimples formed by the ejection of, and collision by, particles. However, for dimples formed by treatment using a related treatment method, metal is pushed apart by collision with an abrasive, and the outer peripheries of the dimples are pushed up greatly into protruding shapes.

[0021] These protrusions at the outer peripheries of the dimples result in the initial wear for a sliding member being raised. The protrusions at the outer periphery of the dimples are accordingly undesirable due to causing cut metal to accumulate by initial wear, and due to causing a deterioration in the slidability such as abrasive wear and the like.

[0022] Such a phenomenon is generated for sliding members in general, such as bearings, shafts, gears, etc.

[0023] Applying the treatment of the present invention to sliding members imparts hardness and residual stress to the sliding member. The treatment has, moreover, been confirmed to be a treatment method that improves the slidability, and makes the generation of projections less liable to occur at the outer periphery of dimples which would raise the initial wear of the sliding member.

[0024] Thus the present invention also has the objectives of: being utilized as a surface treatment method to prevent material to be processed from accumulating to cutting-edge portions of machining tools; and being utilized as a surface treatment method to raise the hardness and impart residual stress to sliding members, and to improve the slidability of sliding members.

MEANS FOR SOLVING THE PROBLEMS

[0025] In order to achieve the above objectives, a method for surface treatment of a metal article according to the present invention is the method comprising:

ejecting substantially spherical ejection particles having a median diameter d50 of from 1 µm to 20 µm 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.

[0026] "Median diameter d50" refers to the diameter at a cumulative mass 50 percentile, namely, to a diameter that when employed as a particle diameter to divide a group of particles into two, results in the total mass of particles in the group of particles of larger diameter being the same as the total mass of particles in the group of particles of smaller diameter. This is the same definition as "particle diameter at a cumulative 50% point" in JIS R 6001 (1987).

[0027] In the above mentioned method for surface treatment of the metal article, preferably, the ejection velocity of the ejection particles is not less than 80 m/sec.

[0028] Furthermore, the material of the metal article may be either aluminum or an aluminum alloy. In such case, the crystal grain diameter of the nano-crystal structure layer can be micronized to a crystal grain diameter not greater than 100 nm.

[0029] Furthermore, the metal article may be a machining tool, and a region to be treated may be a cutting-edge (edge) of the machining tool and the vicinity of the cutting-edge, preferably, a range of at least 1 mm from the cutting edge, more preferably, a range of at least 5mm from the cutting edge; and
dimples having an equivalent diameter of from 1 µm to 18 µm, preferably, 1 µm to 12 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm may be 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.

[0030] Moreover, the metal article may be a sliding member employed to slide against another member, such as a bearing, shaft, or gear; at least a sliding portion of the sliding member is a region to be treated; and dimples having an equivalent diameter of from 1 µm to 18 µm, preferably, 1 µm to 12 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm may be 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. Note that reference to "equivalent diameter" in the present invention refers to the diameter of a circle determined by converting the projected surface area for a single dimple formed on the region to be treated into a circular surface area ("projected surface area" in the present specification means the surface area of the outline of the dimple).

[0031] Furthermore, a metal article according to the present invention is the metal article comprising: a base metal having a hardness not greater than HV714 (HV 7.0 GPa); 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. Moreover, the metal article according to the present invention 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.

[0032] Furthermore, the metal article may be a machining tool; the nano-crystal structure layer may be 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 µm to 18 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm may be 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.

[0033] Moreover, the metal article may be a sliding member; the nano-crystal structure layer may be 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 µm to 18 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm may be 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.

EFFECT OF THE INVENTION

[0034] By performing the surface treatment with the surface treatment method of the present invention as explained above, a uniform nano-crystal structure layer can be formed continuously even on metal articles made from soft materials, in which hitherto it has not been possible to form a uniform nano-crystal structure layer continuously due to the formation of lamellar processing structures. Moreover, this surface treatment also imparts a high compressive residual stress equal to or higher than that imparted when large ejection particles of comparatively large particle diameter are ejected at high ejection pressure.

[0035] Namely, ejection particles that have a small median diameter of from 1 µm to 20 µm and have a falling time through air of not less than 10 sec/m have a small mass. Although this means that stress is concentrated in the vicinity of the surface of the metal article and does not propagate deeply, the surface deformation of the metal article on being collided can also be made small. Such ejection particles are easily carried on an airflow, and can therefore be propelled at a velocity close to the airflow velocity. This enables such ejection particles to be ejected at similar velocities to the velocity of airflow flowing inside an ejection nozzle, at velocities of 80 m/sec or greater, for example.

[0036] As a result, the colliding energy required to obtain nano-crystal structures can be achieved even when ejecting with a comparatively low ejection pressure of about 0.05 MPa. The surface hardness increasing effect on a metal article is substantially saturated when the ejection pressure is about 0.1 MPa, and there is substantially no further increase in hardness observed from ejecting at ejection pressures of 0.1 MPa and greater. Nano-crystal structures can be obtained irrespective of the base metal hardness of the metal article even with comparatively weak ejection pressures not exceeding 0.5 MPa. Compressive residual stress can also be imparted therewith that is of the same level to when ejection particles of 50 µm or greater are ejected at high pressure as described in the related art.

[0037] Moreover, about 60% of the hardness and compressive residual stress that resulted from an ejection pressure of 0.1 MPa could also be confirmed at an ejection pressure of 0.05 MPa.

[0038] As a result, the lamellar processing structures such as those explained with reference to Fig. 1 are not formed even for metal articles made from soft materials such as aluminum alloys. This thereby enables a nano-crystal structure layer to be formed uniformly and continuously. This is thought to enable a nano-crystal structure layer to be formed uniformly and continuously using a lower ejection pressure than the ejection pressure indicated in the related art documents, even for a metal article made from a hard material.

[0039] Moreover, due to being able to perform surface treatment on metal articles under the same treatment conditions irrespective of the hardness of the base metal of the metal article, as described above, this enables nano-crystallization to be performed without ascertaining in advance the hardness or the like of the metal article to be treated. This enables surface treatment to be performed continuously, such as on a conveyor line conveying plural types of metal article made from different materials etc.

[0040] Moreover, the surface treatment method of the present invention enables a uniform nano-crystal structure layer to be formed continuously along a surface without forming the lamellar processing structures described above, even for metal articles made from aluminum or aluminum alloys, which have particularly low hardness from among metal materials. Due to being able to achieve a finer crystal grain diameter of 100 nm or less for the nano-crystal structure layer formed when treating aluminum or an aluminum alloy, a higher degree of surface strengthening effect can be obtained.

[0041] Moreover, consider an example in which the region to be treated is a cutting-edge (edge) of a machining tool such as a cutting tool and in the vicinity of the cutting-edge, and the equivalent diameter of dimples formed by the ejection of ejection particles onto the region to be treated is from 1 µm to 18 µm, and preferably from 1 µm to 12 µm, the depth of such dimples is from 0.02 µm to 1.0 µm or less than 1.0 µm, and the projected surface area of such dimples is not less than 30% of the surface area of the region to be treated. In such an example, not only is the generation of built-up edge or the like at the cutting-edge prevented, and the cutting-edge of the treated machining tool strengthened, but the material to be processed can also be prevented from accumulating to the cutting-edge.

[0042] Thus employing the method of the present invention to treat a sliding member enables the height of protrusions formed at the outer peripheries of the dimples to be suppressed, and enables the slidability to be improved by preventing abrasive wear and accumulation of abraded powder, etc. due to reducing initial wear.

BRIEF DESCRIPTION OF DRAWINGS

[0043] 

Fig. 1 is an explanatory diagram illustrating a mechanism by which lamellar processing structures are formed in a soft material.

Figs. 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.

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

Fig. 4 is a Von Mises stress analysis image using FEM (5 µm ejection particles).

Fig. 5 is a Von Mises stress analysis image using FEM (10 µm ejection particles).

Fig. 6 is a Von Mises stress analysis image using FEM (20 µm ejection particles).

Fig. 7 is a Von Mises stress analysis image using FEM (50 µm ejection particles).

Fig. 8 is a Von Mises stress analysis image using FEM (100 µm ejection particles).

Fig. 9 is a graph illustrating a relationship between particle diameter of ejection particles and stress.

Fig. 10 is a graph illustrating a relationship between particle diameter of ejection particles and depth of maximum stress generation.

Fig. 11 is a graph illustrating relationships between ejection pressure and dynamic hardness.

Figs. 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.

Figs. 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.

Figs. 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.

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.

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

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

Fig. 18 is graph of measurement results of residual stress in alloy tool steel (SKD11).

Fig. 19 is a graph of measurement results of residual stress in aluminum alloy (A7075).

Fig. 20 is a graph of measured changes in friction with respect to elapsed time.

DESCRIPTION OF EMBODIMENTS

[0044] Next, explanation follows regarding an embodiment of the present invention, with reference to the appended drawings.

Object to be treated

[0045] 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.

[0046] 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).

[0047] 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 100nm 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.

[0048] 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.

[0049] 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).

[0050] 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.

[0051] 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.

[0052] 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 µm or less.

[0053] 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.

[0054] 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.

[0055] 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.

Surface Treatment

[0056] 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.

[0057] Examples of the ejection particles, ejection apparatus, and ejection conditions employed when performing the above surface treatment are given below.

(1) Ejection Particles

[0058] 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.

[0059] 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 (Al₂O₃), zirconia (ZrO₂), zircon (ZrSiO₄), hard glass, glass, silicon carbide (SiC), and the like.

[0060] Regarding the particle diameter of the ejection particles employed, particles having a median diameter (d50) in a range of from 1 µm to 20 µm may be employed. Iron-based ejection particles that may be employed have a median diameter (d50) in a range of from 1 µm to 20 µm, and preferably in a range of from 5 µm to 20 µm. Ceramic-based ejection particles that may be employed have a median diameter (d50) in a range of from 1 µm to 20 µm, and preferably in a range of from 4 µm to 16 µm.

[0061] For fine powder ejection particles having a median diameter from 1 µm to 20 µm, 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.

[0062] 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.

[0063] 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 µm, and 41.7 sec for a particle diameter of 10 µm. 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 µm, and 100 sec for a particle diameter of 10 µm.

[0064] 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.

(2) Ejection Apparatus

[0065] 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.

[0066] 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.

(3) Treatment Conditions

[0067] Substantially spherical ejection particles configured from one of the materials described above or the like, and having a median diameter d50 of from 1 µm to 20 µm 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.

Confirmation of Optimum Conditions

(1) Diameter of Ejection Particle

(1-1) Concept

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

(1-2) Deformation Amount by Collision

[0072] The deformation amount generated at the metal article surface when collided by the ejection particles has been investigated.

[0073] Particles having a median diameter d50 of from 20 µm to 40 µm 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.

[0074] 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 1000X.

[0075] The measured data was analyzed using a Multi-File Analysis Application (manufactured by Keyence Corporation).

[0076] 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.

[0077] 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.

[0078] Note that the reference plane described above was computed from height data using a least squares method.

[0079] These results are given in the following table (Table 1). The particle diameter of 20 µm of the scope of the present invention resulted in a protrusion volume that was about 70% less than that resulting from a 40 µm diameter in related art.

[0080] It was thought that this extremely small deformation amount was a reason for the uniform nano-crystal structure formation.

[0081] Table 1

Ejection Particle Diameter and Protrusion Volume
	Example	Comparative Example
Ejection particle diameter (D50) µm	20	40
Protrusion volume µm3	932	2738

(1-3) Investigation of Colliding Force F

[0082] 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).

[0083] 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 ε 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:

[0084] Thus a change in momentum ΔM of the ejection particle between before and after impact is:

[0085] The change in momentum ΔM here is equivalent to the impulse FΔt (wherein Δt is the duration of impulse).

[0086] Thus, the colliding force F imparted to the surface of the metal article when collided by the ejection particle (1 particle) is:

[0087] 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.

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

[0088] 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.

[0089] 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.

[0090] 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:

[0091] Wherein, taking α as a ratio of a depth X of the depressions to a diameter d of the ejection particles, then:

[0092] Thus substituting Equation 8 for X in Equation 7 gives:

[0093] Since 2r = d:

[0094] Thus, a surface area S(m²) of the pressure receiving surface is given by Equation 10.

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.

[0095] 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.

[0096] 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.

(1-5) Ejection Particle Diameter and Ejection Velocity

[0097] 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.

[0098] 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).

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

[0099] 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 2

Change in Colliding Conditions with Changes to Ejection Particle Diameter
					(Ejection Pressure: 0.5 MPa)
Particle diameter (µm)	Mass m (µg)	Ejection Velocity V1 (m/sec)	Impulse Duration Δt (µs)	Colliding force F (kgf)	Pressure Receiving Surface Area S (mm2)	Colliding force F/ Pressure Receiving Surface Area S	Number of Ejection Particles (per kg)	Colliding Energy (J)
5	0.0005	245	10	2.57 × 10-6	2.35 × 10-7	10.9	1.95 × 1015	3.00 × 107
10	0.0041	245	20	1.03 × 10-5	9.40 × 10-7	10.9	2.43 × 1014	3.00 × 107
20	0.0329	198	40	3.17 × 10-6	3.76 × 10-6	8.4	3.04 × 1013	1.79 × 107
50	0.5138	150	100	1.57 × 10-4	2.35 × 10-5	6.7	1.95 × 1012	1.13 × 107
100	4.1103	130	200	5.45 × 10-4	9.40 × 10-5	5.8	2.43 × 1011	8.45 × 106

Table 3

Change in Collision Conditions with Ejection Particle Diameter
					(Ejection Pressure: 0.05 MPa)
Particle diameter (µm)	Mass m (µg)	Ejection Velocity V1 (m/sec)	Impulse Duration Δt (µs)	Colliding force F (kgf)	Pressure Receiving Surface Area S (mm2)	Colliding force F/ Pressure Receiving Surface Area S	Number of Ejection Particles (per kg)	Colliding Energy (J)
5	0.0005	112	10	1.17 × 10-6	2.35 × 10-7	5.0	1.95 × 1015	6.27 × 106
10	0.0041	112	20	4.47 × 10-6	9.40 × 10-7	5.0	2.43 × 1014	6.27 × 106
20	0.0329	86	40	1.44 × 10-5	3.76 × 10-6	3.8	3.04 × 1013	3.70 × 106
50	0.5138	61	100	6.40 × 10-5	2.35 × 10-5	2.7	1.95 × 1012	1.86 × 106
100	4.1103	47	200	1.97 × 10-4	9.40 × 10-5	2.1	2.43 × 1011	1.10 × 106

[0100] 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.

[0101] 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.

[0102] 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 µm is taken as 1, then the particles with d50 = 10 µm and 20 µm can be projected against a surface with high energies of about two to three times this energy.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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 µm. The intensity of the stress is greatly decreased when the ejection particle diameter exceeds 20 µm.

[0111] 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 µm 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).

[0112] 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 µm 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.

[0113] Ferrous alloy ejection particles having a median diameter d50 of 20 µm were ejected against regions of 6 mm × 5 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.

[0114] 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 mm × 5 mm on the test strips, and the found hardness taken as the surface hardness (dynamic hardness) of each test strip.

[0115] A graph of these measurement results is illustrated in Fig. 11.
Note that the dynamic hardness (DHT) is a hardness measured by indentation, and the conditions of measurement are as follows.
Test Instrument: Dynamic Ultra Micro Hardness Tester "DUH-W210", manufactured by Shimadzu Corporation
Indentation Load: 3 gf (A7075), 5 gf ("NAK80"), 10 gf(SKD11)
Time Held: 5 seconds
Shape of Indenter: Triangular pyramid diamond indenter (115°)
Computation Method DHT = α × P / (D²)
Note that in the above equation DHT is the dynamic hardness, α is an indenter shape coefficient (3.8584), P is the indentation load (mN), and D is the indentation depth.

[0116] 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.

[0117] 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 µm 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] However, even with ejection particles having a median diameter of 20 µm 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.

[0125] Thus the iron-based ejection particles having a median diameter of 20 µm 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.

[0126] 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.

[0127] 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 µm, and having a falling time through air of not less than 10 sec/m.

[0128] 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 µm. 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.

Advantageous Effect Confirmation Tests

(1) Tests Objective

[0129] 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.

(2) Test Method

[0130] 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).

[0131] The surface treatment conditions are listed in Table 4 below.

Table 4

Test Conditions
		NAK80	SKD311	A7075
Surface Treatment	Blasting method	SF	SF	SF
	Ejection particle material and median diameter D50 (µm)	Ferrous alloy (Median diameter D50: 20 (µm)	Ferrous alloy (Median diameter D50: 20 µm)	Ferrous alloy (Median diameter D50: 20 µm)
	Ejection pressure (MPa)	0.5	0.5	0.5
	Nozzle diameter (mm)	ϕ7	ϕ7	ϕ7
	Ejection time (sec)	30	30	30

(3) Observation Method

[0132] Each of the test strips that had been surface treated under the conditions described above was observed by the following method.

(3-1) SIM Observation

[0133] 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.

(3-2) EBSD Observation

[0134] 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.

(3-3) Residual Stress Measurements

[0135] A portable X-ray residual stress analyzer ("µ-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.

(4) Test Results

(4-1) Results of SIM Observations

[0136] 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.

[0137] 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 µm 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.

[0138] The nano-crystal structures were formed continuously along the surface of the test strips within the field of view of SIM mages (about 10 µm), and the formation of a continuous nano-crystal structure layer was confirmed.

[0139] 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.

[0140] It was confirmed that in these test strips there was a region with significant fine-crystallization (nano-crystallization) in a zone down to 3 µm 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.

[0141] 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 µm) from the surface for both test strips made from hard materials and test strips made from soft materials.

[0142] 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.

(4-2) Results of EBSD Observations

[0143] 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.

[0144] 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).

[0145] 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).

[0146] 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.

(4-3) Residual Stress Measurement Results

[0147] 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.

[0148] 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.

[0149] 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.

[0150] 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 µm, this being larger than the range of the present invention, were ejected at an ejection pressure of 0.5 MPa.

[0151] 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 1/5 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.

[0152] 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).

[0153] 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 µm 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.

Application to Cutting-Edge of Machining Tool

(1) Test Method

[0154] 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.

(2) Surface Treatment Conditions

[0155] 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 5

Surface Treatment Conditions of a Punch for Punch-Processing
		Example 1	Example 2	Comparative Example 1
Surface treatment	Ejection method	SF	SF	SF
	Ejection particle Median diameter D50 (µm)	HSS (Median diameter D50: 15µm)	Aluminum (Median diameter D50: 16µm)	HSS(Median diameter D50: 80µm)
	Ejection pressure (MPa)	0.3	0.05	0.3
	Nozzle diameter (mm)	7	7	7
	Ejection duration (sec)	30	30	30

[0156] 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.

(3) Punch-Press Processing Conditions and Observation Method

[0157] 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.

(4) Observation Results

[0158] The surface state of each of the punches after the punch-press processing is as listed in Table 6 below.

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 Example 1	Multiple scratches having a striation shape along the length direction observed. Some accumulations of material to be processed were occurred.
Untreated punch	Unusable after 1800 cycles.

(5) Interpretation

[0159] 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%.

[0160] Moreover, the treatment of Example 2 was seen to raise hardness to about 870 Hv, that is, an uplift in hardness of about 16%.

[0161] Such an uplift in hardness is thought to have been achieved due to the formation of the nano-crystal structure layer described above.

[0162] 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.

[0163] 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 µm to 18 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm 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.

[0164] 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.

[0165] 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.

[0166] 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.

[0167] Note that the reference plane described above was computed from height data using a least squares method.

[0168] 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.

[0169] 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.

Application to Sliding Member

(1) Test Method

[0170] Three types of flat sheets of SUS304, size 40 mm × 40 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 7

Surface Treatment Conditions
	Example 3	Comparative Example 3
Ejection method	SF	SF
Ejection particle Median diameter D50 (µm)	Ferrous alloy (Median diameter D50 :20µm)	HSS (Median diameter D50 :40µm)
Ejection pressure (MPa)	0.1	0.3
Nozzle diameter (mm)	7	7
Ejection duration (sec)	20	20

(2) Evaluation Method

[0171] 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 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

[0172] A ball-on-disc friction-wear tester was employed. A ball of 3/16 inch diameter made from SUS304 was employed therein.

(3) Evaluation Results

[0173] A graph of measured changes to friction with respect to elapsed time is illustrated in Fig. 20.

[0174] 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.

(4) Interpretation

[0175] 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.

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 µm to 20 µm 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 or 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.

4. The method for surface treatment of the metal article according to claim 1 or 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 µm to 18 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm 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 or 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 µm to 18 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm 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 µm to 18 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm 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 µm to 18 µm and a depth of from 0.02 µm to 1.0 µm or less than 1.0 µm 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.

Drawing