<p>Phosphate esters
have a wide range of industrial applications, for example in tribology where
they are used as vapour phase lubricants and antiwear additives. To rationally
design phosphate esters with improved tribological performance, an atomic-level
understanding of their film formation mechanisms is required. One important
aspect is the thermal decomposition of phosphate esters on steel surfaces,
since this initiates film formation. In this study, ReaxFF molecular dynamics
simulations are used to study the thermal decomposition of phosphate esters with
different substituents on several ferrous surfaces. On Fe<sub>3</sub>O<sub>4</sub>(001)
and α-Fe(110),
chemisorption interactions between the phosphate esters and the surfaces occur
even at room temperature, and the number of molecule-surface bonds increases as
the temperature is increased from 300 to 1000 K. Conversely, on hydroxylated,
amorphous Fe<sub>3</sub>O<sub>4</sub>, most of the molecules are physisorbed, even
at high temperature. Thermal decomposition rates were much higher on Fe<sub>3</sub>O<sub>4</sub>(001)
and particularly α-Fe(110) compared to hydroxylated, amorphous Fe<sub>3</sub>O<sub>4</sub>.
This suggests that water passivates ferrous surfaces and inhibits phosphate
ester chemisorption, decomposition, and ultimately film formation. On Fe<sub>3</sub>O<sub>4</sub>(001),
thermal decomposition proceeds mainly through C-O cleavage (to form surface
alkyl and aryl groups) and C-H cleavage (to form surface hydroxyls). The onset
temperature for C-O cleavage on Fe<sub>3</sub>O<sub>4</sub>(001) increases in
the order: tertiary alkyl < secondary alkyl < primary linear alkyl ≈ primary
branched alkyl < aryl. This order is in agreement with experimental
observations for the thermal stability of antiwear additives with similar
substituents. The results highlight surface and substituent effects on the
thermal decomposition of phosphate esters which should be helpful for the
design of new molecules with improved performance.</p>
