PRESSURE DEPENDENCE OF SHEAR STRENGTHS OF THIN FILMS ON METAL SURFACES MEASURED IN ULTRAHIGH VACUUM

FENG GAO; OCTAVIO JAVIER FURLONG; PETER V. KOTVIS; WILFRED T. TYSOE

TRIBOLOGY LETTERS

SPRINGER/PLENUM PUBLISHERS

Año: 2008 vol. 31 p. 99 - 106

1023-8883

The friction coefficient is measured for systems consisting of a thin potassium chloride film deposited onto a variety of clean, flat metal substrates, namely Pb, Sn, Au, Ag, Cu, Pd, Fe, Ta, and two types of steel, which are rubbed by a tungsten carbide pin in an ultrahigh vacuum. The friction coefficients are plotted versus 1/HS, the inverse of the substrate hardness, where two regimes are found. In the first regime, where deformation at the asperity tips is suggested to be plastic, the observed variation in friction coefficient with substrate hardness is rationalized by assuming that the shear strength S for sliding on a KCl film varies with contact pressure P as S = S0 + aP, yielding values for a of 0.14 ± 0.02 and S0 of ~60–70 MPa. In the second regime, it is proposed that the softer, film-covered Pb and Sn substrates are closer to being in conformal contact with the rough tribopin. These values of S0 and a, along with the measured surface asperity height distribution of the tribopin and the value of the friction coefficient for a KCl monolayer on the metal, are used to rationalize the observed increase in friction coefficient with increasing film thickness.