As technology scales shrink, tribological interactions play increasingly dominant roles. This can be problematic (e.g., high friction and wear in micro/nano devise), or beneficial (e.g., using adhesion to drive the formation of nanostructures). The biggest challenge in exploring and exploiting these issues is that the interface between two materials is hidden from view. Recent advances in in situ methods have enabled tribological mechanisms at previously inaccessible interfaces to be studied with unprecedented resolution and insight. I will discuss the science revealed by two in situ methods to develop new, physically-based insights into tribological processes.

I will first discuss using atomic force microscopy (AFM) to study the formation of anti-wear tribofilms derived from additives in lubricating liquids [1,2]. AFM is conducted in additive-containing oils to monitor the growth and properties of tribofilms in situ during sliding of a small-scale contact. We show that the famous additive molecule zinc dialkyldithiophosphate (ZDDP) functions via surface-based nucleation, growth, and saturation of ZDDP antiwear tribofilms [1,3]. New results show that this works not only for pure ZDDP, but also for fully-formulated oils containing ZDDP and other additives. We then show that 5 nm diameter zirconia (ZrO₂) nanoparticles also form protective films on surfaces, opening a pathway for more environmentally-friendly additives to enable energy-saving low viscosity lubricants [4,5].

I will then discuss new insights into nanoscale adhesion and wear from in situ transmission electron microscopy (TEM) studies. We see a strong, reversible, sliding-history dependence of adhesion between silicon nanoasperities, attributed to shear-induced adsorbates removal [6,7]. I will also preview new results applying the technique to the study of contact between two-dimensional materials including MoS₂.

Robert Carpick

Appearances