Magneli phases of anion-deficient rutile as lubricious oxides. Part I. Tribological behavior of single-crystal and polycrystalline rutile (TinO2n-1)

In part I of this paper series, wide temperature range SEM-tribometric resu lts generated in vacuum and various partial pressures of oxygen are combine d with relevant literature data to examine a hypothesis correlating the oxy gen stoichiometry of the TinO2n-1 Magneli phases of the rutile polymorph of titania with their tribological behavior. Single-crystal and polycrystalli ne rutile specimens of narrow stoichiometry ranges were sliding against alp ha-SiC and themselves. The surface shear strength changes were determined a s a function of the thermal-atmospheric test environment, and the shear str ength values were estimated by the coefficients of friction, the real area of contact and the published yield strength of rutile. The data appear to b e sufficient and sufficiently reliable to confirm the accuracy of the hypot hesis. The tendency of the rutile stoichiometry (ergo the friction) to shif t as a function of temperature and partial pressure of oxygen causes this m aterial to be thermo-oxidatively unstable for tribological applications in extreme environments. In part II, a study is described to formulate oxidati vely more stable Magneli phases by Cu-doping, and test the new materials by SEM tribometry using a procedure used for pure rutile here in part I. By e mploying this doping methodology similar to creating high-temperature super conductive oxides in part II, some feasibility of producing oxidatively sta ble, lubricious oxides with acceptably low wear rates is indicated.