Tribochemistry of Phosphoric Acid Sheared between Quartz Surfaces: A Reactive Molecular Dynamics Study

Abstract

Tribochemical processes have profound consequences on tribological performance. In the present paper, the tribochemical mechanism of low friction state in the silica/phosphoric acid system is elucidated by reactive molecular dynamics (ReaxFF) simulations. The friction coefficient is found having strong positive correlation with the number of interfacial hydrogen bonds, which suggests that a weaker interfacial hydrogen bond network would favor a lower friction. The friction reduction mechanisms have been analyzed in two temperature regimes: For 300 ≤ T ≤ 600 K, no indication of tribochemical reaction is observed, and the friction coefficient decreases because of the accelerated molecular rotational and translational motion and the corresponding weakened hydrogen bond network. For 800 K ≤ T ≤ 1400 K, the occurrence of tribochemical reactions leads to a clustering and polymerization of the phosphoric acid molecules and generation of a considerable quantity of water molecules distributed mainly in the sliding interface which could act as lubricant, and a low friction state is reached with a friction coefficient of 0.02.

The ReaxFF reactive force-field: development, applications and future directions

“…39,45 Notable developments on the aqueous branch include water-liquid and proton/anion transfer extensions to a range of transition metals and metal oxides (Fe/Ni/Cu/Zn/Al/Ti/ Ca/Si), 7,15,19,35,46-52 along with C/H/O/N/S/P developments aimed at biomolecules and their interactions with inorganic interfaces. [53][54][55][56][57][58][59][60] Comparison to similar methods Although in this article we focus almost exclusively on ReaxFF and its applications, the ReaxFF method is not unique in its aim: to provide a simulation environment for describing the dynamics of chemical reactions at an atomistic scale with significantly fewer computational resources compared with QM. Purely empirical methods essentially abandon-or simplify-QM concepts, 61 providing significantly more freedom in their choice of functional form.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Despite this, ReaxFF parameters have been developed which can be used to investigate some important lubricant and additive molecules, such as alkanes and alcohols [282,283], phosphoric acids [284], organosulphur compounds [285], and molybdenum disulphide [286]. Using such force-fields, relatively large molecular systems (nm) can be compressed and sheared [284] in a similar manner to conventional confined NEMD simulations ( Fig. 1(a)), and the consequent chemical reactions monitored over reasonable simulations time (ns).…”

Theoretical Approaches for Understanding the Interplay Between Stress and Chemical Reactivity

“…More recently, Harrison used another atomically smooth hydrogen-passivated diamond surface as a model for atomic force microscope tips with different geometries [163]. Luo and coworkers have also used reactive FFs to examine tribochemical processes related to lubrication [164]. Our group has used constant strain conditions to model the friction behavior of hydroxylated alumina surfaces [165,166] and aldehydes bound to these surfaces [167], as well as aldehydes compressed between these surfaces [168].…”