Mechanical property measurement of titanium/titanium nitride and aluminium–magnesium/magnesium freestanding multilayer foils

The advent of the closed field unbalanced magnetron sputtering (CFUBMS) system has allowed a novel method for the production of ultra-thick (above 50 μm) multilayer coatings, which form freestanding foils when removed from the substrate. Applications for this method range from the production of complex metal/ceramic probe tips to an alternative route for the production of axisymmetric high precision-machined components. In this study the CFUBMS system was developed to grow 50 μm aluminium–magnesium/magnesium (Al–Mg/Mg) and titanium/titanium nitride (Ti/TiN) multilayer combinations. The Ti/TiN foils were deposited using opposing magnetrons, with the TiN layers produced by reactive sputtering. The Al–Mg/Mg foils were deposited from pure Al and Mg targets in a co-planar configuration, conditions having been initially found that produced high hardness/high strength amorphous Al–Mg alloy foils. Both multilayer systems benefit mechanically from the combination of a high strength component (TiN; Al–Mg) and a ductile component (Ti; Mg). Conditions were found that limited the stresses induced in these ultra-thick coatings such that once removed from the substrate, the foils remained flat. Experimental arrays were then constructed to investigate the effect of relative percentage volume fraction, substrate bias and interlayer wavelength against the Youngʹs modulus, tensile strength, ultimate tensile strength and other mechanical properties of the foils. These properties were determined using a modified Hounsfield tensometer. The study found that when in tension, the interlayer wavelength has no effect upon the tensile strength of the foil. However, as expected, tensile strength was strongly dependent upon the relative percentage volume fraction of the high strength component of the foil. The substrate bias was found to have a limited influence upon mechanical properties. Conditions were found that produced a tensile strength-to-film density ratio of 244, approaching that of tool steel.