Resonance Shear Measurement of Confined Alkylphenyl Ether Lubricants

The tribological properties of four phenyl ether lubricants, monoalkydiphenyl ether (MADE), dialkyldiphenyl ether (DADE), m-phenoxyphenoxy m-biphenyl (m-4P2E), and m-bis(m-phenoxyphenoxy) benzene (m-5P4E), confined between mica surfaces have been investigated using the resonance shear measurement (RSM). The RSM study revealed that the viscous parameter of these lubricants increased when they were confined in a nano-space. The viscous parameter b ₂ values of these lubricants confined in a gap smaller than ca. 2–3 nm were in the order of MADE > DADE > m-4P2E ≈ m-5P4E, while their bulk viscosities were in the order of m-5P4E > m-4P2E > DADE > MADE. Further, the MADE, which has better hydrodynamic lubrication property because of the lowest bulk viscosity, was relatively easily squeezed out from the gap compared to others. These results have demonstrated that phenyl ether lubricants confined in a nano-space behave quite differently from those of the bulk phase, and the direct characterization of confined lubricants is imperative for designing efficient lubricants, especially for boundary lubrication.