AFM as a tool to study tribology
During my interview talk for my postdoctoral fellowship at ExxonMobil, I presented a slide on the importance of studying the liquid-solid interface since much of my graduate research was about STM of self-asssembled monolayers at the liquid solid interface. Items on that list included corrosion and tribology. One of my interviewers (a mechanical engineer by the name of Martin Webster) asked me in our 1:1 session “So, what is tribology?” I did not know!
This was especially bad because I was interviewing for a position in the lubrication sciences section. Fortunately, not only is Martin one of the most knowledgeable tribologists I ever had the pleasure of working with, he is also a very decent person. He smiled and said “no problem, let me explain it to you.”
So why was it bad? Because tribology is the study of interacting surfaces that are moving relative to one another and is basically the heart of lubrication. Specifically, tribology involves the study of friction and wear, two incredibly important phenomena that govern the performance of most moving machinery like engines, gears, hydraulics, wind turbines – think anything that you would put a lubricant on!
My postdoc ended up being in the field of nanotribology – the study of friction and wear but simply on the nanoscale. The nano component can take on many forms. In my case, I was studying the field of boundary lubrication where very thin inorganic solid films are formed (<100nm thick) in high temperature and pressure conditions to prevent wear and reduce friction. Nanotribology also includes the tribology of nanomaterials like graphene and 2D materials. All these studies can of course take place in air, or better yet, in the presence of liquid lubricants (if liquid lubrication is required.)
Atomic force microscopy has become an essential tool for the field of nanotribology! Not only can it characterize the topography and mechanical properties of these thin films that we have all known to grow and love, but it actually has the ability to measure friction on the nanoscale. It does this through a mode called lateral force mode (LFM) which is based on one of the original AFM modes called contact mode. In this mode, the tip is simply dragged across a surface at a load (force) set by the user. The key in LFM is to drag (scan) the tip in a direction perpendicular to the cantilever axis, because in this way the side to side torque induced on the cantilever can be measured and then related to friction.
Measuring actual friction on the nanoscale is not easy, and requires a very careful measurement with challenging calibrations and careful analysis that eliminates topographic artifacts (which will definitely convolute frictional measurements.) But it’s possible! The relationship between friction measured on the nano and macro scale is an interesting topic and subject of much research; in many systems they are comparable and in others they are quite different.
Robert Carpick, one of the pioneers of the field since his graduate school days where he developed the initial calibration methods for LFM, continues to do very exciting work in this field. Below is a film that his research group generated using an AFM tip, giving a new dimension to the term nanotribology. In this work published in ACS Applied Materials and Interfaces in 2018, the tribofilm was formed with a dispersion of zirconia nanoparticles in the presence of a low viscosity oil.
A Tribofilm, Carpick et al., ACS Appl Mater Interfaces, 2018, 10 (46), pp. 40335
Want to learn more about nanotribology? One of the best places to start is the Society for Tribology and Lubrication Engineers which typically hosts a weeklong symposium on nanotribology at its May annual meeting. This fall’s ACS meeting will also have a tribology symposium including a nano-session and is hosted in the Division of Colloid and Surface Chemistry.