High-hardness metal skin film

摘要

A high-hardness metal skin film is formed of an aggregate of metal crystals. The area rate A of pyramid-shaped metal crystals in a surface of the skin film is in a range of A>/=40%. The aggregate contains at least one element selected from boron (B), carbon (C) and hydrogen (H) as an added element, wherein the B content is in a range of B>/=0.01% by weight, the C content is in a range of C>/=0.03% by weight, and the H content is in a range of H>/=0.01% by weight. The skin surface takes on an intricate aspect due to the presence of a large number pyramid-shaped metal crystals and hence, has a good oil retention. In addition, the wear of each pyramid-shaped metal crystal is inhibited by an increase in hardness produced with the containment of boron and/or carbon and/or hydrogen and therefore, the oil retention is maintained.