Chemically Based Strategies in Tribology
Michael N. Gardos, Raytheon Electronic Systems, Engineering Service Center,
El Segundo, CA 90245 USA
The temperature and atmospheric environment heavily influence the adhesion, friction and wear of tribosystems consisting of bearing material counterfaces separated by a thin layer of solid lubricant film. These layers can range from sacrificial films depo-sited by a variety of techniques and bonding schemes, transfer films formed from self-lubricating bearing materials sliding/rolling against an initially unlubricated metal or ceramic counterface, to lubricious surface layers generated by tribo-thermochemically reacting bare bearing materials with the surrounding atmosphere.
The present paper offers a review of the author's decade-long attempts to examine the surface chemistry-induced friction and wear changes with a variety of such bearing material and solid lubricants combinations as a function of atmospheric environment and temperature. A unique pin-on-oscillating-flat-type scanning electron microscope (SEM) tribometer was specially designed and constructed in the late '80's to fill the gap between an atomic force microscope and a conventional, bench-top friction and wear tester. Its primary purpose has been to reveal, with the help of some ex-situ surface analyses, changes in the tribochemical behavior of bearing materials and solid lubricants influ-enced mainly by elevated temperatures in moderate vacuum and in low partial pressures of selected gases, under realistic engineering (Hertzian) contact stresses.
A variety of model experiments have been performed with the SEM tribometer on (a) carbon-graphite seal materials vs. polycrystallinea-SiC in vacuum and partial pressures of H2 and He, (b) graphite intercalated with various metal chlorides vs. polycrystalline a-SiC in vacuum and in low partial pressures of air, (c) MoS2 deposited on steel counterfaces operated in vacuum and in 1 atmosphere of N2, (d) rutile (lubricious TiO2-x) oxides sliding against polycrystalline a-SiC and themselves in vacuum and in various partial pressures of O2, and, more recently, (d) polycrystalline diamond, poly-crystalline a-SiC and various Si crystallinities sliding against themselves in vacuum and various low partial pressures of H2 and O2 and in temperatures ranging from room ambient to 1000 ˚C. The experiments were completed under Hertzian stresses in the GPa to MPa (from many thousands to hundreds of psi) range, depending on the modulus, Poisson's ratio and the wear rate of the counterface materials.
As exemplified by a brief overview of these experiments, the particular friction and wear mechanism changes influenced by the thermal-atmospheric environments have yielded both fundamental insights and practical solutions to the solid lubrication of a variety of moving mechanical assemblies.