PURPOSE:To improve the adhesive property of a diamond film on a substrate consisting of a sintered hard metal for cutting tools, etc., by electrolytically polishing the metallic substrate in a mineral acid, then flawing the base body with abrasive grains and forming the diamond film at the time of coating the surface of the metallic substrate with the diamond film by a vapor reaction. CONSTITUTION:The surface of the metallic substrate for cutting tools obtd. by sintering the high hardness particles of the carbide, nitride, boride, silicide, etc., of metals with iron family metals, such as Co, Ni and Fe, as a binder is used as an anode and is subjected to electrolytic polishing as a pretreatment in the mineral acid, such as hydrochloric acid, sulfuric acid, and nitric acid, by which numerous projections are formed on the surface. The abrasive grains of diamond, cubic boron nitride, alumina, etc., are then blown onto the surface to flaw the surface. This metallic substrate 4 is put into a reaction furnace 1 and is placed in a region 7 where the gaseous acetylene from a nozzle 2 of a burner 6 is incompletely burned. The diamond film having the high adhesive strength is thus formed on the surface of the metallic substrate 4.
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