A coating is formed on a substrate by sputter ion plating wherein a DC glow discharge is generated under soft vacuum conditions in the presence of the substrate and of a cathode constituted by the coating material thereby to release material at the cathode by ion bombardment, which released material diffuses to the substrate to form the coating thereon, and wherein a potential is applied to the substrate during coating. The coating material comprises a MCrAlSi allow (where M is a single metal or combination of metals selected from the group consisting of nickel, cobalt and iron) and the potential is in the form of an AC bias in order to control the properties of the coating. It is thereby possible to coat uniformly subtrates having sharp edges such as aerofoil components. Moreover, the coatings are found to possess outstanding corrosion resistance properties.