Oxidative and tribological properties of amorphous and quasicrystalline approximant Al-Cu-Fe thin films.

The origins of the tribological properties and corrosion resistance of amorphous and quasicrystalline approximant alloys have been studied by comparing their properties in thin Al-Cu-Fe alloy films with compositions lying near the quasicrystalline region of the ternary compositional phase diagram. Six sputtered thin films of an Al-Cu-Fe alloy were studied using X-ray diffraction, X-ray photoemission spectroscopy (XPS), and an in situ ultrahigh vacuum (UHV) tribometer. The films were annealed in UHV to induce the formation of orthorhombic, rhombohedral, and amorphous bulk structures. The properties of these thin films were then determined in the same UHV apparatus without exposing the films to air. The rates of surface oxidation by H2O and O2 were measured using XPS. Although the oxidation rates and oxide thicknesses were dependent on the oxidant, they were not sensitive to the structures of the films. Friction was measured between identical samples in sliding contact. The friction coefficients (micros = 0.36 +/- 0.11 to 0.56 +/- 0.08) were comparable to those observed in other experiments using quasicrystals and approximants in UHV; however, there was no strong correlation between the friction coefficients and either the film structure or the degree of surface oxidation. These results suggest that the tribological and corrosion resistance properties of these quasicrystalline approximant alloys are not directly connected to crystalline structure.

Imaging of fullerene-like structures in CNx thin films by electron microscopy; sample preparation artefacts due to ion-beam milling.

The microstructure of CN(x) thin films, deposited by reactive magnetron sputtering, was investigated by transmission electron microscopy (TEM) at 200kV in plan-view and cross-sectional samples. Imaging artefacts arise in high-resolution TEM due to overlap of nm-sized fullerene-like features for specimen thickness above 5nm. The thinnest and apparently artefact-free areas were obtained at the fracture edges of plan-view specimens floated-off from NaCl substrates. Cross-sectional samples were prepared by ion-beam milling at low energy to minimize sample preparation artefacts. The depth of the ion-bombardment-induced surface amorphization was determined by TEM cross sections of ion-milled fullerene-like CN(x) surfaces. The thickness of the damaged surface layer at 5 degrees grazing incidence was 13 and 10nm at 3 and 0.8keV, respectively, which is approximately three times larger than that observed on Si prepared under the same conditions. The shallowest damage depth, observed for 0.25keV, was less than 1nm. Chemical changes due to N loss and graphitization were also observed by X-ray photoelectron spectroscopy. As a consequence of chemical effects, sputtering rates of CN(x) films were similar to that of Si, which enables relatively fast ion-milling procedure compared to carbon compounds. No electron beam damage of fullerene-like CN(x) was observed at 200kV.

Cross-linked nano-onions of carbon nitride in the solid phase: existence of a novel C(48)N(12) aza-fullerene.

We report a new fullerenelike material consisting of cross-linked nano-onions of C and N. Growth of the onion shells takes place atom by atom on a substrate surface and yields thin solid films during magnetron sputter deposition. Electron microscopy and energy loss spectroscopy show that the core shell contains up to 20 at. % N corresponding to C(48)N(12) aza-fullerene composition. Nanoindentation of this nanostructured material gives high resilience with hardness 7 GPa, Young's modulus 37 GPa, and complete elastic recovery after loading with 0.5 mN to a depth of 75 nm. Total energy calculations show the stability of C(60-2n)N(2n) aza-fullerenes and suggest the existence of a novel C(48)N(12) molecule.