ICMCTF1999 Session E1-2: Reduction of Friction Through Coating/Surface Modification

A number of studies have shown that the intentional introduction of lubricious solid films, especially metal oxides, can improve elevated temperature tribological performance. Two studies performed at the Southwest Research Institute (SwRI) have identified a class of oxides that offer exceptional tribological performance at elevated temperatures. Certain combinations of titanium and nickel yielded coefficients of friction (COF) of 0.06 to 0.09 at 800 °C. Examination of the surface layers by Auger Electron Spectroscopy (AES) indicated that the low friction coefficients were obtained on surfaces with a titanium oxide film that had substantial nickel content. The second study involved modifying the surface of silicon nitride with titanium carbide (TiC). Examination of the wear surfaces by profilometry revealed a dramatic reduction in wear brought about by the TiC particles. Reductions in the COF of up to 50% were also obtained. AES studies of the wear tracks identified a mixed titanium and silicon oxide as the lubricious species in the TiC-containing materials.

Building on this work, we have developed wet chemical methods of depositing the titanium oxide materials believed to be responsible for the improved tribological performance observed in the SwRI studies. Deposition of the oxides as nanostructured materials should promote lubricious behavior at sub-ambient temperatures. Coating microstructure was examined by X-ray diffraction (XRD) from powders, energy dispersive spectroscopy (EDS) in a scanning electron microscope (SEM), and thermal analysis (TGA/DSC). COF was measured by reciprocating ball-on-plate at 25 °C and 800 °C. Wear tracks were examined by surface profilometry and SEM. Titania-nickel oxide and silica-titania coatings lowered the COF significantly and demonstrated the best wear resistance. This paper will discuss these results with respect to microstructure and processing parameters.

Hydrocarbon materials have traditionally been used to prevent the friction and wear of mechanical components in sliding contact. One important example of this is the use of oil in conventional combustion engines. When very thin layers of hydrocarbon liquids are confined between plates, experiments have shown that they no longer behave like liquids and atomic scale interactions are paramount in determining the behavior of the confined material. The advent of chemical vapor deposition technology has piqued interest in the use of solid hydrocarbons as lubricants. These solid lubricants might prove useful in extreme environments, such as outer space. In both of these situations, a detailed knowledge of the molecular-scale mechanisms responsible for lubrication would be invaluable in the design of novel lubricants.

With these things in mind, we are using molecular dynamics to examine the atomic-scale phenomena governing the tribology of hydrocarbon-containing systems. Because liquid hydrocarbons and boundary layer lubricants, such as self-assembled monolayers, are to be studied, the potential energy function must include intermolecular interactions. Brenner's reactive empirical bond-order potential¹ has been adapted to include these interactions without compromising the reactivity of the potential by introducing the dispersion and intermolecular repulsion interactions through a novel adaptive algorithm. With these improvements, the new adaptive intermolecular reactive empirical bond-order potential (AIREBO)² can simulate reactive and non-reactive processes in a wide range of environments, including graphite, liquid hydrocarbons, and self-assembled monolayers. Simulations of tribology in some of these systems will be discussed.

^* Supported by ONR and AFOSR. ¹ D.W. Brenner, Phys. Rev. B 42 (1990) 9458. ² Steven J. Stuart and Judith A. Harrison, submitted.

The anisotropic properties of carbon can lead to different carbon surfaces with a given chemical, mechanical and electrical properties.

The sliding of metal against carbon induces many surface modifications such as orientation and structure modifications according the carbon grain size, the adsorbed environment gas affinity, the mechanical sliding parameters and according the relaxation time of surface.

Carbon surface modifications enhanced during friction, as we have shown by Scanning Electron Microscopy, X -ray analysis and by Diffraction Electron Microscopy will be presented and discussed versus the transition favourable parameters,