Friction measurements of nanometer-thick lubricant films using ultra-smooth sliding pins treated with gas cluster ion beam

- We used GCIB to smooth sliding pins for friction tests of nanometer-thick PFPE films.
- An ultra-smooth sliding pin with Rp less than monolayer film thickness was achieved.
- We used γ and h/σ to optimize the interplay of surface roughness and film thickness.
- High γ induces high friction coefficient. Low h/σ leads to high friction coefficient.
- Both γ and h/σ should be considered as design parameters for optimum design of HDDs.

Friction properties of nanometer-thick lubricant films confined between two ultra-smooth solid surfaces are crucial to the practical performance of technologically advanced mechanical devices such as micro-electro-mechanical systems and hard disk drives. In this work, we applied argon gas cluster ion beam (Ar-GCIB) treatments to obtain ultra-smooth sliding pins for pin-on-disk tests of nanometer-thick perfluoropolyether (PFPE) lubricant films coated on magnetic disk surfaces. The GCIB treatments effectively smoothed the pin surfaces, and increases in the Ar dose decreased surface roughness. An ultra-smooth surface with a maximum peak height (Rp) less the monolayer lubricant film thickness was achieved when the Ar dose was increased to 8 × 1016 ions/cm2. We observed that both surface roughness and film thickness affected the friction coefficients of the PFPE films. To quantitatively describe the interplay of surface roughness and film thickness, we introduced two roughness characteristics: the ratio of film thickness to the surface's root-mean-square roughness (h/σ), and a surface-pattern parameter (γ), defined as the ratio of correlation lengths in two orthogonal directions. We infer that a fixed γ and higher h/σlead to lower friction coefficients, while a fixed h/σand higher γ induce higher friction coefficients.