Synergistic effect of nanodiamonds on the adsorption of tricresyl phosphate on iron oxide surfaces

Abstract

Nanodiamonds (NDs) have potential uses in many applications, including as additives for liquid lubricants, where they may be combined with more traditional chemicals to form protective films on sliding surfaces. It has been shown that NDs can function synergystically with tri-cresyl phosphate (TCP) to facilitate film formation on air baked iron. Here, reactive molecular dynamics simulations of TCP and NDs on an amorphous iron oxide surface reproduce experimental observations of the temperature at which film formation begins with NDs present and the effect of NDs on film composition. Analysis of chemical bonding in the simulations shows that the film formed in the presence of NDs is comprised of NDs and TCP that are both directly and indirectly bonded to the surface. Notably, the amount of phosphorous in the film, which is important for surface protection, is increased by TCP molecules that are indirectly bonded to the surface via NDs, which suggests that indirect bonding is one mechanism by which NDs facilitate film growth. The synergy of NDs and TCP has important implications for the development of NDs as emerging lubricant additives which must function with existing additives such as TCP in many applications.

Substituent Effects on the Thermal Decomposition of Phosphate Esters on Ferrous Surfaces

“…67 The ReaxFF parameter set has previously been applied to study the thermal decomposition of TCP on a-Fe3O4-OH surfaces 36 as well as TCP interactions with nanodiamonds on the same surface. 37…”

“…Initially, the energy of each system was minimised using the conjugate gradient method, followed by equilibration for 0.2 ns at 300 K. Then, the temperature was linearly increased from 300 to 1000 K over 1.4 ns (heating rate of 0.5 K ps -1 ). 37 The selected temperature range is representative of that experienced by phosphate esters when they are used as VPLs under UHV conditions 9 and antiwear additives under EP conditions. 80 Previous ReaxFF studies of hydrazine molecules showed that, as the heating rate was increased from 50 to 200 K ps -1 , the onset temperature for decomposition increased from 1350 to 1690 K. 62 Although the heating rate used here is relatively low by ReaxFF simulation standards, it is still several orders of magnitude higher than that applied experimentally (ca.…”

“…62 The results shown below represent mean values between three independent simulations with the initial configurations of the phosphate ester molecules shifted by 0.5 nm in either the x or y direction. 37 Chemical bonding was thus effectively monitored for 48 molecules for each case, which is sufficient to capture multiple possible reactions. 36 Videos of the thermal decomposition simulations for some representative systems, TNBP on Fe3O4(001), α-Fe(110), and a-Fe3O4-OH, are provided in the Supporting Information.…”

“…36 It has been applied to study the thermal decomposition of TCP on hydroxylated, amorphous iron oxide surfaces. 36,37 In this study, the thermal decomposition of phosphate esters on ferrous surfaces is investigated using a combination of ReaxFF MD simulations and DFT calculations. In the MD simulations, phosphate esters with a wide range of alkyl and aryl substituents are compared on nascent and passivated ferrous surfaces.…”

Substituent Effects on the Thermal Decomposition of Phosphate Esters on Ferrous Surfaces

“…Initially, the energy of each system was minimised using the conjugate gradient method, followed by equilibration for 0.2 ns at 300 K. Then, the temperature was linearly increased from 300 to 1000 K over 1.4 ns (heating rate of 0.5 K ps -1 ). 37 The selected temperature range is representative of that experienced by phosphate esters when they are used as VPLs under UHV conditions 9 and antiwear additives under EP conditions. 80 Previous ReaxFF studies of hydrazine molecules showed that, as the heating rate was increased from 50 to 200 K ps -1 , the onset temperature for decomposition increased from 1350 to 1690 K. 62 Although the heating rate used here is relatively low by ReaxFF simulation standards, it is still several orders of magnitude higher than that applied experimentally (ca.…”