1. A process for coating a substrate comprising the steps of:providing a substrate having at least one free surface;
depositing a first layer of a first material on the free surface of the substrate;
depositing a second layer of a second material, different from the first material, on the first layer;
depositing a third layer of a third material, different from the first and second materials, on the second layer;
depositing a protective layer of a fourth material, different and separate from the first, second and third materials, on the third layer, the protective layer is an outermost layer deposited on the substrate, the fourth material of the protective layer consists of graphite powder and microcrystalline active carbon; and
performing a reflow of at least the second and third layers from the first, second, and third layers, by transfer of heat through the thermal contact on the protective layer, such that the fourth material of the protective layer does not melt or mix with the first, second, and third layers during reflow and the protective layer prevents oxidation of at least the third layer.