Abstract: The invention relates to a method and to apparatus for treating the surface of a substrate, e.g. by depositing a coating thereon by plasma flux spraying, the method being of the type in which an electric arc is established inside a chamber between a cathode and an anode, in which an inert gas is injected into the chamber so that it is ionized on passing through the electric arc, thereby forming a high temperature plasma, and in which the plasma is ejected from the chamber through an ejection nozzle whose outlet orifice is in the form of a slit so as to project particles of the material for constituting the coating onto the substrate, the particles melting on contact with the plasma, thereby forming a coating on the substrate.