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 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, 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.

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 , 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 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 , 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.   
     
     
         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 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.   
     
     
         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 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.