Chemical strengthening treatment unit for applying chemical strengthening treatment to glass substrates used in information recording media application

A method for producing a hard disk, the method comprising:

• providing the hard disk comprising a glass substrate having a peripheral surface and a recording surface;
• polishing the peripheral surface of the glass substrate;
• lapping polishing the recording surface of the glass substrate;
• precision polishing the recording surface;
• supporting the glass substrate in a holder;
• and
• performing chemical strengthening treatment for forming a compressive stress layer on the surfaces by immersing the glass substrate into a molten solution of a prescribed salt, wherein said molten solution is contained within a tank;
• and forming at least one of a primary film, a magnetic film, a protective film;
• and a lubricant film on a surface of the glass substrate, wherein at least one of the holder and the tank comprises a nickel alloy that contains nickel as a major constituent.