Friction Measurements of Ultra-Thin Carbon Overcoats in Air

The friction force as a function of humidity was measured between thin carbon films coated onto mica surfaces. The friction force was found to be proportional to the area of contact. The shear stress at 0 percent, 33 percent, and 100 percent relative humidity was measured to be $26 MPa±5 MPa,$$12 MPa±2 MPa,$ and $5 MPa±0.5 MPa,$ respectively, and was independent of the applied pressure for pressures less than 20 MPa. Water acts as a lubricant decreasing the friction between the carbon surfaces. The shear stress at 0 percent relative humidity corresponds to the shear stress of a solid paraffin film, and suggests that the shear may be dominated by a thin organic film adsorbed from air, at least at the pressures less than 20 MPa and a velocity of 1 μm/s. At 100 percent relative humidity, the shear stress for carbon coated surfaces was about double that for mica surfaces, indicating a stronger influence of the water for the more hydrophilic mica surface than the more hydrophobic carbon surface. The friction between one uncoated mica and one carbon coated mica surface resulted in immediate damage and generation of wear debris.