(677h) Molecular Dynamics Study of the Degradation of Alkylsilane Monolayers Under Shear

Chemisorbed alkylsilane monolayer coatings have been shown to reduce the friction, adhesion, and wear of surfaces under shearing contact; however, long-term lubrication is hindered by film degradation. This degradation is thought to proceed through the rupture of interfacial bonds between individual chains and the substrate and the removal of these chains from the monolayer [1]. As such, conditions which alter monolayer structure, such as normal load, surface roughness, and chain length are likely to influence monolayer degradation. Molecular dynamics (MD) simulations provide a very useful tool for examining how these conditions influence tribological behavior at the molecular level. However, monolayer wear typically occurs on the order of hours [2], which is inaccessible by MD, and typical classical forcefields do not allow for dynamic bond breakage/formation. One strategy to address these issues using classical simulations, is to model monolayer degradation through the random removal of chains and use the reduced monolayer density to sample various stages of the wear process [3,4]. However, Booth et al. [2] have shown that film stability is promoted by increasing monolayer chain length and proposed that the interchain dispersion interactions facilitate longevity; as such, the presence of these broken chains may in itself influence stability.  

Here, we use classical MD simulations to examine the degradation of monolayers by randomly eliminating interfacial bonds (i.e., breaking the chains). This allows unbound alkylsilane chains to remain in the system and their mobility to be used as a metric to study wear behavior. Furthermore, previous MD simulations [5] have revealed a twofold increase in friction coefficient for systems that feature realistic surface roughness, suggesting substrate features play a key role in monolayer wear. To explore this, we examine monolayers with defects on both crystalline and amorphous silica at various representative surface roughnesses. In agreement with Booth, et al., we find increasing chain length reduces free-chain mobility, promoting monolayer stability through a reduction in chain-chain energy. We observe enhanced free-chain mobility for monolayers attached to rough, amorphous substrates and also find irregular chain arrangements promote mobility by reducing nematic order.  Additionally, we find negligible normal load effects on the wear behavior of amorphous systems as compared to crystalline systems, where shear-induced order is found to promote monolayer stability.

[1]    B. D. Booth, S. G. Vilt, C. McCabe, and G. K. Jennings, “Tribology of Monolayer Films: Comparison between n-alkanethiols on Gold and n-alkyl Trichlorosilanes on Silicon.,” Langmuir, vol. 25, no. 17, pp. 9995–10001, Sep. 2009.

[2]    B. D. Booth, S. G. Vilt, J. Ben Lewis, J. L. Rivera, E. A. Buehler, C. McCabe, and G. K. Jennings, “Tribological Durability of Silane monolayers on Silicon.,” Langmuir, vol. 27, no. 10, pp. 5909–17, May 2011.

[3]    P. T. Mikulski and J. A. Harrison, “Packing-Density Effects on the Friction of n-Alkane Monolayers,” J. Am. Chem. Soc., vol. 123, no. 30, pp. 6873–6881, Jun. 2001.

[4]    M. Chandross, E. B. Webb, M. J. Stevens, G. S. Grest, and S. H. Garofalini, “Systematic Study of the Effect of Disorder on Nanotribology of Self-Assembled Monolayers,” Phys. Rev. Lett., vol. 93, no. 16, p. 166103, Oct. 2004.

[5]    J. E. Black, C. R. Iacovella, P. T. Cummings, and C. McCabe, “Molecular Dynamics Study of Alkylsilane Monolayers on Realistic Amorphous Silica Surfaces,” Langmuir, vol. 31, pp. 3086–3093, Feb. 2015.