Richard R. Chromik, McGill University, Department of Mining and Materials Engineering, Montreal, QC,
Canada
Abstract:
At macroscopic length scales, solid lubricant coatings provide friction control and wear resistance by the
formation of a stable transfer films. However, the microtribology is not nearly as well understood,
especially when considering possible instability of transfer films in a ‘mesoscopic’ environment with
local roughness. For this study, diamond-like carbon, MoS2 and metal (Ti or Au) doped-MoS2 coatings
have been studied in microscale sliding contacts using a nanoindentation instrument. Atomic force
microscopy (AFM) was used to characterize the tip shape of different spherical diamond tips (radii
between 2 and 100 m). Area functions for the tips from AFM data were then used to best determine
the contact area during microtribology testing. Interfacial shear strengths for microtribology,
determined from tests with varying contact pressures, were in most cases similar to those found in the
literature for macroscopic experiments. However, in the latter portion of the talk, a direct comparison
between the tribological performance of a Ti-MoS2 coating at the two length scales will be presented. A
‘real time’ study of the transfer film behavior and velocity accommodation modes (VAMs) at the macroscale (contact radii  50 m) was conducted with an in situ tribometer, while on the micro-scale (contact
radii  1 m) , transfer films were analyzed ex situ on the counterface with AFM. Higher friction was
observed with microtribology compared to macrotribology and was attributed to, in some cases,
different VAMs as compared to macroscopic scales. For dry sliding, the behavior of microscopic contacts
on Ti-MoS2 deviates only slightly from macroscopic results, showing higher limiting friction and
microplowing. For humid sliding, microscopic contacts deviate significantly from macroscopic behaviour,
showing plowing behaviour and absence of transfer films. The possible mechanisms for the scaling
effects on the tribological performance of Ti-MoS2 will be discussed in the context of local roughness,
the effect of humidity on MoS2 coatings, and contact size and pressure.
Biography:
Richard R. Chromik is an Assistant Professor of Materials Engineering at McGill University where he leads
research efforts in the McGill Nanomechanics and Tribology Laboratory. Richard joined McGill in
September 2006. Prior to this, he was a post-doctoral scientist at the U.S. Naval Research. He completed
his Ph.D. in Chemical Physics at Binghamton University in 2001. Throughout his career, Richard has
studied many aspects of thin films and coatings, including phase transformations, magnetic, electrical,
mechanical and tribological properties. He has published 45 peer reviewed articles and proceedings
papers. Currently, his research focuses on using novel in situ test methods to examine third bodies in
tribological contacts and development of multi-scale indentation testing protocols for mechanical
properties of coatings and thin films.