Control of the nanostructure and microtribology of magnetron sputtered surfaces
The technological importance of hard thin films is well established. There is growing recognition that nanometre-scale surface structures can be controlled to the benefit of function. In-process structuring brings these ideas together. This study explores how the morphology, especially the surface topography, and microtribological behaviour of Cr-N films can be controlled during unbalanced magnetron (UBM) sputtering. Experiments varying the sputter power, bias voltage, temperature, total pressure and Ar/N2 ratio during UBM sputtering generated different compositions, crystallite orientations and microstructures, and six associated topography types: pyramidal (type P), grain-like (G), crater-like (CR), ribbon-like (R), conical (C) and hillock-like (H). A new empirical zone model consistently relates these topography types to process parameters. The feature dimensions are also controlled by the deposition parameters. The films have closely reproducible topographical and mechanical properties. The microtribological behaviour for three topography types (P, C and CR) is studied under different conditions relevant to unlubricated contacts, lubricated contacts, and humid environments. Nanostructured surfaces show significantly lower friction than smooth ones, with actual reductions depending on the topography type. Friction strongly correlates with summit density (Ssd). Low friction (Ssd ~ 3 #/µm2) was measured on all type CR surfaces, but only by increasing the lateral dimensions of types P and C. Unlubricated friction is attributed principally to solid-solid adhesion, influenced by the density and curvature of summits. Wear is influenced by the density, shape and size of the surface features and by the mechanical properties of the film. Boundary lubrication reduced friction, with slight dependence on lubricant. Alongside the summits parameters, friction correlates with core fluid retention index. Applying hydrophobic and hydrophilic treatments shows that appropriate nanostructuring reduces the dependence of friction on humidity and sliding velocity, principally by controlling the summit density. In-process structuring is clearly very useful for creating nanostructures in order to enhance the microtribological behaviour of surfaces. Further investigations are recommended into friction optimization by nanostructuring.