Method for modifying surface of piston for internal combustion engine, and piston for internal combustion engine

A method for modifying a surface of a piston for an internal-combustion engine by using a shot made of an iron-based alloy as a particle to be ejected having a diameter of 20 μm to 200 μm, a thermal conductivity of 30 W/m·k or less at a temperature of 25° C., and a specific gravity of 7.5 g/cm3 or greater, comprising:

• a first ejection treatment for ejecting the particle to be ejected onto a surface of a piston in a space in which oxygen exists under conditions such that an arc height value is set within a range from 0.07 mm (N) to 0.13 mm (N), and the piston being made of an aluminum-silicon alloy or an aluminum-copper-based alloy and obtained by casting and forging;
• a second ejection treatment for ejecting the particle to be ejected onto the surface of the piston to which the first ejection treatment has been applied in a space in which oxygen exists and under conditions such that an arc height value is set within a range from 0.13 mm (N) to 0.22 mm (N);
• and
• a heating treatment subsequently applying to the piston for 1.5 hours or longer at a temperature of 170° C. to 190° C. in a space in which oxygen exists.