Hertzian stress contribution to low friction behavior of thin MoS 2 coatings

Abstract

Friction coefficients were measured for bearing materials slid in dry air against sputter-deposited MoS2-coated substrates. Ball versus flat tests were performed over a wide range of initial Hertzian pressures (200-1500 MPa) by varying loads (1-50 N), elastic moduli (70-615 GPa), and ball diameters (1.6-12.7 mm). The friction coefficient μ decreased as load L increased according to μ∝ L(-0.32), in agreement with the Hertzian contact model. Regression analysis of over 600 data points for friction coefficient versus Hertzian pressure (PH), fitted to μ=(S 0/PH)+α, gave mean values of the shear strength S0=24.8 MPa±0.5 and α= 0.001±0.001, with S 0 in good agreement with values in the literature.