Mechanochemistry describes a wide range of phenomena in which mechanical force influences chemical reactivity. This includes reactions in ball mills, where energy is transferred from moving balls to the milled medium during collisions of the balls with each other and the jar walls. Mechanochemical reactions can be performed without solvents or heating, which can provide considerable environmental benefits over conventional routes. The choice of ball and jar materials are known to be affect reactivity for a range of processes, such as cocrystallisation and cycloaddition, emphasizing the important role of the solid material. In some cases, the milling balls themselves are the catalyst of the reaction, which is known at direct mechanocatalysis. Another example and particularly high-value application of mechanochemistry is lubricant additives that reduce wear in tribological contacts. The most popular antiwear additive for automotive lubricants, zinc dialkyldithiophosphate (ZDDP), forms protective tribofilms on rubbing surfaces through mechanochemical processes. The tribofilm growth of ZDDP is strongly dependent on the nature of the rubbing surfaces. Surfaces with a high elastic modulus, such as tungsten carbide, require lower loads to form tribofilms than steel, due to higher contact stresses. In experiments using aluminium-silicon alloys, thicker ZDDP tribofilms form on the silicon phase than the aluminium matrix, which is also due to the higher elastic modulus and thus stress. As well as the mechanical properties of the rubbing surfaces, the surface energy can also affect ZDDP adsorption and tribofilm growth. ZDDP forms thicker tribofilms on surfaces to which it readily adsorbs, such as steel, silicon nitride, and tungsten carbide surfaces than those where minimal adsorption occurs, such as diamond-like carbon (DLC) and silicon carbide. Ionimplanting alloying elements into steel can affect ZDDP tribofilm growth, nickel promotes tribofilm formation, molybdenum and chromium slows growth, while vanadium and tungsten have no significant effect. ZDDP forms thicker tribofilms on aluminiummagnesium alloys than on either of the pure metals due to the stronger adsorption of the ZDDP molecules on the alloy. A recent study revealed how the interplay of mechanical and chemical factors determine the tribological behaviour of ZDDP on DLC, with hydrogen-free coatings of moderate elastic modulus being most wear-resistant while still allowing ultralow friction. Surface chemistry can also affect other mechanochemical processes inside rubbing contacts. For example, the rate of α-pinene tribopolymerisation is higher on palladium, copper oxide, nickel oxide, and chromium oxide than gold, silicon oxide, aluminium oxide, and DLC surfaces. Moreover, the α-pinene tribopolymerisation rate is higher on bare silica surfaces than hydroxylated silica. This is due to the fact that stress can be more efficiently transferred from the sliding surfaces to additive molecules that are strongly adsorbed. Phosphate esters lubricant additives have numerous industrial applications due to their stability, particularly in the presence of oxygen. Their versatility in structure, including the ability to incorporate a variety of substituent groups, has led to their application as lubricant additives to improve their antiwear, boundary lubrication, antioxidant, anti-corrosion, anti-foaming, and metal deactivation performance. Phosphate esters come in various forms, including aromatic, alkyl, thiophosphates, and metal-containing esters, and are readily accessible for commercial use. One of the most popular of these additives is tricresyl phosphate (TCP), which has been widely employed in aviation lubricants for gas turbine engines for decades. The molecular structure of TCP is shown in Figure . Like ZDDP, TCP is an effective antiwear additive for steel contacts. Another benefit of TCP for aviation lubricants is that the tribofilms it forms on metal surfaces slow the degradation of synthetic neopentyl polyol ester base stocks. It was initially suggested that the ability of TCP to reduce wear on steel surfaces originated from polishing of the surfaces through the formation of iron phosphide. However, later studies showed that the major component of the tribofilms formed from TCP on steel was iron phosphate, which acted as a protective layer, rather than polishing the surfaces. It has also been shown that either organic polyphosphates or iron polyphosphates are eventually formed from TCP inside tribological contacts, which are ultimately responsible for its antiwear performance on steel surfaces. At high temperatures, TCP molecules form relatively thick (60-100 nm) thermal films on steel surfaces; 34 however, aryl phosphates like TCP form thinner tribofilms (∼ 20 nm) on rubbing surfaces. This is much thinner that the ZDDP tribofilms that form on rubbing steel surfaces (∼ 100 nm). The thermal decomposition of TCP has been studied on several different substrates. Wheeler and Faut 37 used X-ray photoelectron spectroscopy (XPS) to compare the adsorption and dissociation of TCP on gold and iron surfaces. On both substrates, adsorption saturated at about one monolayer, but adsorption was slower on gold than on iron. TCP underwent non-dissociative adsorption on gold and desorbed upon heating, with no TCP left on the surface at 473 K. On iron, no desorption occurred upon heating and additional interactions were observed between the substrate and the tolyl groups on the TCP molecule. TCP decomposed on iron between 423 K and 473 K with one of the tolyl groups being removed to form iron phosphate. The temperature range for decomposition is close to the temperature at which TCP significantly reduced the friction of rubbing steel surfaces in tribometer experiments. It has since been shown that decomposition of TCP on iron surfaces occurs mainly via P-O bond scission to form methylphenoxy intermediates. When TCP was heated to 800 K on iron, the methylphenoxy intermediates either desorbed as cresol via hydrogenation or decomposed further to generate tolyl intermediates. Some of these tolyl intermediates desorbed as toluene via hydrogenation, but the majority decomposed resulting in hydrogen and carbon monoxide desorption and carbon deposition onto the iron surface. The steel surfaces inside tribological contacts are chemically heterogeneous and comprise multiple iron oxides. From experiments conducted at high temperature (750 K), it was suggested that, due to acid-base reactions, TCP vapor preferentially reacts with iron in higher oxidation states, with reactivity increasing in the order: Fe < FeO < Fe 3 O 4 < Fe 2 O 3 . However, more recent experiments suggested that reactivity towards TCP increases in the order Fe 3 O 4 < Fe < Fe 2 O 3 . In both of these experimental studies, the Fe surfaces will also form oxide layers that react with the TCP, rather than the native metal. In addition to surface oxides, metal carbides are another important component of bearing steels. A previous study of thermal decomposition found that TCP reactivity increases in the order: tungsten carbide < molybdenum carbide < chromium carbide < vanadium carbide. In recent years, experimental studies of TCP adsorption and decomposition have been complemented by molecular simulations. For example, Khajeh et al. used molecular dynamics (MD) simulations with a reactive force field (ReaxFF) to study the adsorption of TCP on hydroxylated amorphous Fe 3 O 4 . They found that, at lower temperatures (≤ 500 K), chemisorption occurred mostly via Fe-C bond formation through the tolyl groups. At higher temperatures (600-700 K), a significant amount of Fe-O and O-P bonding also occurred, indicating chemisorption through the central phosphate group. Ewen et al. compared the chemisorption and dissociation of TCP to trialkyl phosphates on Fe 3 O 4 (001) surfaces. In agreement with previous experiments, they found that the triaryl phosphate TCP was much less reactive compared to the trialkyl phosphates. The MD simulations also showed that, while the main decomposition mechanism for trialkyl phosphates was C-O bond dissociation, the only bonds that broke in TCP were P-O bonds. For the trialkyl phosphates, Ewen et al. also compared the adsorption and dissociation on Fe 3 O 4 (001) to hydroxylated amorphous Fe 3 O 4 and α-Fe surfaces. They found that the reactivity of the trialkyl phosphates increased in the order: hydroxylated amorphous Fe 3 O 4 < Fe 3 O 4 (001) < α-Fe. Ayestarán Latorre et al. studied the mechanochemical decomposition of trialkyl phosphates between sliding iron surfaces using non-equilibrium molecular dynamics (NEMD) simulations with ReaxFF. They found that secondary trialkyl phosphates were much more reactive than primary ones, in agreement with previous experiments on ZDDP. The influence of surface chemistry on the mechanochemical response of organophosphates is yet to be explored either experimentally or through NEMD simulations.