Method for producing piston ring with recess

A method for producing a piston ring for a cylinder that moves in a sliding direction, comprising:

• providing a wire rod being formed by a piston ring base material and having an upper surface, a lower surface, a pre-formed first recess and a cylindrical surface, a first recess extending from the upper surface to the cylindrical surface, the cylindrical surface extending from the first recess to the lower surface, the first recess having a bottom surface parallel to the cylindrical surface and an inclined surface extending from the bottom surface to the cylindrical surface;
• forming a hard film on the bottom surface and the inclined surface of the first recess and on the cylindrical surface at a predetermined thickness, the hard film having a higher hardness than the piston ring base material, an outer surface of the hard film on the bottom surface of the first recess establishing a sliding surface;
• forming a second recess by performing a polishing process on the sliding surface based on at least three steps including:
• removing the hard film formed on the cylindrical surface;
• removing the hard film formed on a part of the inclined surface of the first recess and a second part of the piston ring base material disposed adjacent to the cylindrical surface, to a point substantially flush with the sliding surface, and
• removing a third part of the piston ring base material from a section of the piston ring base material exposed by removal of the second part of the piston ring base material,
• wherein the second recess is directly connecting the first recess and forming a connecting vertex with the inclined surface of the first recess.