Development of novel nanocomposite PVD coatings to improve wear and corrosion resistance of magnesium alloys

The main aim of this research was to develop novel nanocomposite PVD coatings for magnesium alloys, to improve their wear and corrosion resistance – and thereby explore the potential to extend the use of such alloys to moving parts for light-weighting of tribological components, where the potential for cumulative weight savings is immense if key parts can be made from magnesium, but the alloys cannot currently be used successfully due to their poor wear and corrosion behaviour under dynamic loading. The work comprises two main stages. The first stage was to produce a base layer for subsequent PVD ceramic nitride (or nitrogen-doped hard metallic) coating deposition. The second stage was to deposit a nanocomposite coating with improved tribological performance, by introducing sequentially nitrogen reactive gas, subsequent to the base layer preparation step.
In the first stage, sixteen AlCuMoMgZrB PVD coating layers were prepared by pulsed direct current closed-field unbalanced magnetron sputtering. Four deposition runs were carried out, with substrate negative bias voltages of 50 V, 60 V, 75 V and 100 V being applied. For each deposition run, four proprietary WE43 magnesium alloy substrates were placed at different positions (P1-P4) on the substrate holder, between AlMgB and ZrMoCu composite sputter targets mounted at 90° to each other. Investigations into composition, microstructure, mechanical and electrochemical properties were then carried out, to select the most suitable base layer. The P1-60 layer (i.e. deposited at P1 position, closest to the AlMgB composite target, with substrate negative bias of 60 V) was chosen as the most suitable candidate amongst the sixteen AlCuMoMgZrB coating layers due to its superior mechanical properties, electrochemical properties, and amorphous microstructure.
In the second stage, four novel AlCuMoMgZrB(N) nanocomposite PVD coatings with different nitrogen reactive gas flow rates (i.e. 5 sccm, 10 sccm, 15 sccm and 20 sccm), introduced partway through the sputter deposition process, were produced sequentially, on top of the selected P1-60 base layer. Further detailed investigations into composition, microstructure, mechanical, tribological and electrochemical properties were performed to evaluate the improved wear and corrosion resistance.
For practical applications, P1-60-15sccm (46.27 at.% Al, 8.71. at.% Mg, 5.35 at.% Cu, 3.63 at.% Mo, 1.30 at.% Zr, 2.65 at.% B and 32.08 at.% N) seems a likely candidate to provide an optimal combination of wear and corrosion resistance – in terms of the best and the second-best performance in micro-abrasion and in corrosion tests, respectively.