Soft materials like aluminum and magnesium can be made reasonably wear resistant through the use of hard coatings.  However, unless these coatings are made relatively thick, the lack of stiffness and strength in the substrate leads to fatigue damage accumulation in the soft material, and eventual fracture and delamination of the coating.  If thicker coatings are used, the large residual stresses involved, together with mismatches in thermal expansion coefficient and elastic modulus, can also result in interfacial failure.  For components in closed tribological systems, these failures are generally catastrophic.

Superelastic NiTi can be deposited on such substrate materials, as an intermediate layer situated between the hard coating and substrate, by magnetron sputtering.  They are shown here to be capable of improving hard-coating performance on soft metals by limiting the stresses associated with asperity-contact, and by effectively buffering interfacial CTE and modulus mismatches.  This paper reviews our work on sputtered NiTi and NiTiCu thin films used as interlayers between 6061 aluminum and both CrN and diamond-like carbon (DLC) hard-coatings.  It is shown that dramatic improvements in wear resistance are possible.  These are further shown to be strongly linked to transformational superelastic deformation in the NiTiX interlayers.