Density of lubricant at solid-liquid interface measured by neutron reflectometry (Part 1) - In the case of DLC films having various surface energies and water

Density of lubricant near the solid surface is measured by neutron reflectometry and discussed in the series of studies. In this paper, three kinds of DLC films having different surface energies (normal DLC, hydrophilic DLC and hydrophobic DLC) were prepared as base materials, and water was selected as a lubricant. The instrument we used was a neutron reflectometer 'MINE' in Japan Atomic Energy Agency (JAEA). The DLC films were soaked in water in a specially-designed sample holder, and the neutron reflectivity profiles from the interface between DLC films and water were directly obtained. Through an analytical fitting approach with Parratt's theory to the obtained profiles, authors conclude that: (1) the thickness of hydrophilic layer (O-2 plasma treated layer) on the hydrophilic DLC is 15nm, (2) the thickness of dense water layer infiltrating in hydrophilic DLC is 2nm, and (3) the thickness of low-density water layer on the surface of hydrophobic DLC is 3nm, and its density is almost half of normal water.