Mn. Gardos, Magneli phases of anion-deficient rutile as lubricious oxides. Part I. Tribological behavior of single-crystal and polycrystalline rutile (TinO2n-1), TRIBOL LETT, 8(2-3), 2000, pp. 65-78
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.