Sol–gel derived, nanostructured oxide lubricant coatings

A number of studies have shown that the introduction of lubricious solid films, especially metal oxides, can improve elevated temperature tribological performance. Two such studies have identified a class of oxides that offer exceptional tribological performance at elevated temperatures. Certain combinations of ion-implanted titanium and nickel yielded coefficients of friction (COF) of 0.06–0.09 at 800°C. Examination of the surface layers by Auger spectroscopy indicated that the low friction coefficients were obtained on surfaces with a titanium oxide film that had a substantial nickel content. The second study demonstrated a dramatic reduction in the wear of silicon nitride by adding 15 vol.% titanium carbide. Reductions in the COF by up to 50% were also obtained. Auger spectroscopy of the wear tracks identified a mixed titanium and silicon oxide as the lubricious species in the titanium carbide-containing silicon nitride. We have developed wet chemical methods of depositing the titanium oxide materials believed to be responsible for the improved tribological performance observed in previous studies. Deposition of the oxides as nanostructured materials should promote lubricious behavior at sub-ambient temperatures. The microstructure was examined by X-ray diffraction (XRD) of powders, energy dispersive spectroscopy (EDS) in a scanning electron microscope (SEM), and thermal analysis (TGA/DSC). COF was measured by a reciprocating ball-on-plate at 25 and 500°C. Wear tracks were observed with surface profilometry, optical microscopy and SEM. Titania/nickel oxide coatings reduced the COF by at least 50% at both temperatures and demonstrated the best wear resistance. This paper will discuss these results with respect to microstructure and processing parameters.