Reactivity and interfacial properties of phosphonium phosphate ionic liquid on steel surfaces

In the last two decades, ionic liquids (ILs) have emerged as a new class of functional materials with potential applications in several fields, including lubrication. Compared to traditional non-ionic lubricant additives (e.g., zinc dialkyl dithiophosphate – ZDDP), the unique physical properties of ILs, such as high thermal stability, low vapor pressure, and low flammability, together with their ability to lubricate a variety of materials have made them promising additives that could potentially replace environmentally-hazardous molecules currently used in oil formulations to reduce friction and wear. While numerous studies on ILs have demonstrated the good tribological performance of ILs, a fundamental understanding of the underpinning lubrication mechanism(s) and its dependence on various external factors (e.g., impurities, temperature, humidity, etc.) is still lacking. This has effectively limited the broad implementation of ILs in real-world tribological applications. Among the ILs that have been investigated for their promising tribological behaviors, trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate ([P₆,₆,₆,₁₄][DEHP]) has drawn extensive attention due to its high miscibility with many synthetic oils currently used as base stocks in oil formulations. Recent macroscale tribological studies indicated that [P₆,₆,₆,₁₄][DEHP] lubricates steel and cast iron surfaces by forming a sacrificial and lubricious layer at sliding interfaces. This layer is the result of the stress-assisted, thermally-activated chemical reaction of [P₆,₆,₆,₁₄][DEHP] on iron surfaces. This finding contradicts nanoscale tribological studies of ILs, which suggested that the lubricity of ILs derives from their layered structure when nanoconfined between solid surfaces. To shed light on the origin of this discrepancy, this dissertation aims to elucidate the lubrication mechanism of [P₆,₆,₆,₁₄][DEHP] on steel surfaces by combining pressure-dependent in situ atomic force microscopy (AFM) nanoscale tribological testing, macroscale tribological testing, and multiple advanced ex situ surface characterization methods. In situ AFM nanoscale tribological experiments at single-asperity sliding contacts revealed the existence of two distinct lubrication mechanisms of [P₆,₆,₆,₁₄][DEHP] when used to lubricate steel/diamond-like carbon sliding contacts: at a normal pressure below 5.5 ± 0.3 GPa, the mechanical sliding induces a change in the ionic arrangement of [P₆,₆,₆,₁₄][DEHP] and leads to the formation of a solid-like interfacial layer that reduces friction and prevents wear of the surfaces in relative motion. Based on previously published molecular simulations of [P₆,₆,₆,₁₄]⁺-based ILs subjected to hydrostatic pressure, the formation of the interfacial layer observed in our study is proposed to be the result of the enhanced interactions between the cations and anions in [P₆,₆,₆,₁₄][DEHP], which leads to a more organized arrangement of the charged chemical groups of the ions while the apolar alkyl groups mediate the ionic interaction. In situ AFM experiments also indicated that this interfacial layer is not stable at normal pressures above 5.5 ± 0.3 GPa, as it is mechanically removed from the contact and wear of the underlying steel substrate occurs. Under these conditions, a second mechanism leads to the reduction of nanoscale friction, as suggested by synchrotron-based X-ray photoemission electron microscopy (X-PEEM), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and low-energy electron microscopy (LEEM) measurements. The mechanical removal of the native oxide layer from steel exposes metallic iron to [P₆,₆,₆,₁₄][DEHP]. The higher surface coverage of [DEHP]⁻ anions adsorbed on metallic iron leads to the generation of a brush-like structure of the boundary layer, where the charged phosphate end groups of the [DEHP]⁻ anions anchor to the steel surface with the alkyl groups orienting farther away from the surface. This compact and well-organized arrangement of the ions is proposed to be a key contributing factor to reducing nanoscale friction. Within this dissertation, we also, for the first time, systematically investigated the dependence of the lubricating properties of [P₆,₆,₆,₁₄][DEHP] on the concentration of halides, which are commonly present in ILs as impurities from the synthesis procedures. Tribological testing of [P₆,₆,₆,₁₄]⁺-based ILs with different Br⁻-to-[DEHP]⁻ ratios at steel/steel interfaces showed that the IL’s lubrication performance depends on the balance between corrosivity and surface reactivity of the IL. XPS and ToF-SIMS measurements highlight the corrosion-inhibition effect of the adsorbed [DEHP]⁻ anions on steel, which form a protective layer on the sliding surfaces and largely suppress pitting corrosion of steel in the presence of a significant amount of Br⁻. On the other hand, the complete absence of [DEHP]⁻ anions can result in pitting corrosion of steel, which generates the alkaline conditions necessary for the hydrolysis of the [P₆,₆,₆,₁₄]⁺ cations. ToF-SIMS results provided evidence that this reaction forms a phosphorus-rich surface layer that lowers friction by decreasing adhesion at steel/steel contacts and the interfacial shear strength. The outcomes of this dissertation provide novel insights into the lubrication mechanisms of [P₆,₆,₆,₁₄][DEHP] and its dependence on the presence of Br⁻. The latter also demonstrates that [P₆,₆,₆,₁₄][DEHP] can be utilized with a high tolerance of Br⁻ without the necessity of achieving ultra-high purity, thus opening the path for the cost-effective implementation of this IL in tribological applications.