Carmichael’s Concise Review
were performed ex situ . However, previous theoretical work had predicted that the tribofilm formation may be driven by direct pressure in the area of contact. For the first time, Gosvami et al. conducted in situ single-asperity studies of ZDDP tribofilm growth by immersing an AFM tip in the additive-infused oil, heating it up, and sliding in contact to form that tribofilm at nanoscale resolution. In situ single- asperity studies have several advantages: the formation of the tribofilm can be imaged directly at each step; the contact loads and geometries can be controlled and quantified; and the local tribofilm properties can be measured concurrently. The results are also ideal for comparison with atomistic simulations.
Among the many findings from this study, the authors showed that in AFM experiments under high stresses the growth rate of the tribofilms increased exponentially and reached a limiting value above a critical stress. An expo- nential increase of the tribofilm growth rate was observed as a function of temperature. For each given temperature and load, the volume of the tribofilms reached a constant value as a result of prolonged sliding. Within the subnano- meter vertical resolution of their instrument, the tribo- films formed without any observable wear of the substrate, which in this case was iron oxide or silicon, in contrast to theories of growth that assume significant damage to and mixing in of the iron substrate to form the tribofilm. It was determined that the energy barrier for the relevant
tribochemical reaction would be lower where the local stress is higher. The growth rate increased exponentially with either applied compressive stress or temperature, consistent with a thermally activated stress-assisted reaction rate model. This means that the self-limiting growth of the films can be explained by stress: as the film gets thicker, it “cushions” the applied load, leading to lower stress, and slowing growth of a less strongly bonded film. This stress effect would lead to a graded film, and eventually the stress is too low to promote growth.
Gosvami et al. suggested that their in situ approach can be directly applied to better understand molecular-level tribochemical phenomena and functionality such as the behavior of other important lubricant additives (for example, friction modifi ers) or fi lms formed in vapor-phase lubrication. T is breakthrough study will lead the way to future studies that will improve the effi ciency of internal combustion engines and other machine components and devices. T e global economic impact of improved understanding of lubricant additives used in practical applications will be enormous!
References [1] NN Gosvami et al., Science 348 (2015) 102–06. [2] T e author gratefully acknowledges Drs. Robert Carpick and Nitya Nand Gosvami for reviewing this article.
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