Use of Eyring Equation to Explore the Frictional Responses of a –CF3 and a –CH3 Terminated Monolayers Self-Assembled on Silicon Substrate

• Published in: Tribology letters Vol. 32; no. 1; pp. 1 - 11
• Main Authors: Subhalakshmi, K., Devaprakasam, D., Math, S., Biswas, S. K.
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01.10.2008; Springer Nature B.V
• Subjects: Activation energy; Chemistry and Materials Science; Coefficient of friction; Coordination; Corrosion and Coatings; Friction; Materials Science; Monolayers; Nanotechnology; Original Paper; Physical Chemistry; Self-assembly; Shear; Silicon nitride; Silicon substrates; Silicon wafers; Sliding; Surface energy; Surfaces and Interfaces; Theoretical and Applied Mechanics; Thin Films; Tribology; Friction mechanisms; Fluorocarbons; Automotive industry; Boundary lubrication friction
• ISSN: 1023-8883; 1573-2711
• DOI: 10.1007/s11249-008-9353-y

Lateral force microscopy is used to measure the frictional forces generated in sliding a silicon nitride tip on perflurooctyl trichlorosilane (FOTS) and octadecyltrichlorosilane (ODTS) monolayers self-assembled (SAM) on silicon wafer. The work is motivated by a need to rationalize the high friction of FOTS in comparison to that found for ODTS, inspite of the former having a low surface energy compared to that of the latter. Having first established that the tribology here is a thermally activated process, we use the Eyring equation to estimate the energetic barrier height to sliding motion, system activation energy, shear coordination and pressure activation volumes. For a molecular species when the velocity and temperature are unchanged in a sliding experiment and the activation energy remains unchanged the change in shear coordination with increasing normal load controls the friction coefficient. In comparing the performance of the two test molecules the friction coefficient of the FOTS is found to be three times greater than that of the ODTS; the corresponding difference in barrier height is about 10%. Our results indicate that when the two molecules are well ordered, the shear coordination modulates the frictional differential but it is the difference in their system activation energy, principally determined by electrostatic repulsion between the neighbouring molecules which is a dominant influence on their friction differential.