Method for forming silica-based multilayer coating on substrate from polysilazane-containing compostions

A method for forming a coating comprising:

• a first step of applying a first solution comprising a polysilazane consisting of perhydropolysilazane to a surface of a metal substrate, heating the first solution to make the first solution undergo conversion into silica and form a first coating having a crack or an open pore on a surface thereof, and
• a second step of applying a second solution comprising a polysilazane consisting of perhydropolysilazane to the surface of the first coating to fill the crack or the open pore, and heating the second solution at a temperature lower than a heating temperature in the first step to make the second solution undergo conversion into silica and form a second coating on the surface of the first coating.