Structure–mechanical properties relationships of co-sputter deposited iron–magnesium coatings

Fe–Mg coatings were co-sputter-deposited under different pressure and bias conditions on glass slides and on steel substrates. Mg resputtering occurs for substrate bias between − 75 and − 100 V, with strong side effects which lead to Mg content gradients between the centre and the edge of the substrates and a rather important Ar implantation. Coatings deposited around 0.2 Pa with a substrate bias of − 50 V conserve a dense structure with intrinsic compressive stress up to about 20 at.% Mg and no significant presence of Ar. With increasing Mg content, the coatings remain partly bcc with an increasing fraction of amorphous phase from about 17 at.% Mg, and become wholly amorphous over about 37 at.% Mg. Hardness measurements show that the Mg enrichment up to about 7 at.% increases the hardness due to solid solution effects. For higher Mg enrichment, the hardness decreases conformably with its progressive amorphisation and stabilises for amorphous coatings. Crystallisation of amorphous coatings occurs around 200 °C in a mixture of supersaturated bcc solid solution deriving from Fe and of hcp Mg. Increasing the annealing temperature decreases the bcc phase lattice parameter, and hence, its Mg supersaturation. It is finally shown that even low Mg content coatings (3 at.% Mg) present both bcc Fe and hcp Mg diffraction lines after annealing at 300 °C, which confirms the insolubility of Mg into Fe under thermodynamical equilibrium conditions.