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The aim of this thesis is to advance the field of solid lubrication science by developing coatings that provide reliable performance in ambient conditions, work on rough surfaces, and are amenable to industrial size and design complexities. Two different coating systems, Ti3C2Tx-MoS2 and Ti3C2Tx-Graphene Oxide blends, were studied in this work. The Ti3C2Tx-MoS2 nanocomposites were spray-coated onto rough 52100-grade steel surfaces, and their tribological performance was evaluated in a ball-on-disk configuration in a unidirectional sliding mode. The test results indicate that Ti3C2Tx-MoS2 coatings achieved superlubricity, which has not been previously reported for either pristine material under macroscale sliding conditions. The observed synergistic mechanism enabled the superlative performance, which was explained by the in-situ formation of a robust tribolayer responsible for sustained lubricity even at high contact pressures (>1.1 GPa) and sliding speeds (0.1 m/s). Processing, structure, and property correlation studies were conducted to understand the underlying phenomena. Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to reveal the formation of the tribolayer. The Ti3C2Tx-Graphene Oxide blends were also spray-coated onto rough-bearing steel surfaces, and their tribological assessment was carried out in ambient environmental conditions and high contact pressures in a ball-on-disc experimental setup. The coatings led to substantial friction reduction compared to uncoated and single-component-coated surfaces, with a friction coefficient as low as 0.065 at 1 GPa contact pressure and 100 mm/s sliding speed, surpassing the state-of-the-art. The coatings also provided excellent protection against wear loss of the substrate and counter-face. The results were explained based on the observations from Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and nanoindentation measurements. The in-operando formation of a dense, hard, and stiff tribolayer was observed, which was responsible for the sustained lubricity even at high test loads and sliding speeds. This thesis presents a holistic exploration and correlation of structure-property-processing for the advancement of solid lubrication science. It provides insights into the development of solid lubricant materials and their tribological performance, which can be useful for various industrial applications.

Two-dimensional materials find use in the areas of electronics, energy storage, sensors, and catalysis due to their unique properties near the monolayer limit. The production and general application of certain 2D materials is bottlenecked by the inability to scale production. Discovery and optimization of alternate synthesis methods is paramount to bring emerging materials into technological maturity Currently, powdered materials use cost-intensive production methods with low yields and thin-film materials range in domain size from nanometer to micrometer which in insufficient for applications. In this dissertation, we explore the novel and direct synthesis of molybdenum, tungsten, and vanadium-based MXenes(2D metal carbides/nitrides)/MXene-like material carbon foam composites as well as the wafer-scale growth of transition metal dichalcogenides(TMDs). We studied the factors that influenced the morphology of carbon foams made via a sol-gel method with reducing sugars and metal precursors. We determine the conditions that are most amenable to producing suitable foam resins for pyrolysis. From there, the carbon scaffold was functionalized with catalytic metal nanoparticles for the synthesis of carbon nanotubes. This sol-gel method was then applied to molybdenum, tungsten, and vanadium oxides to produce MXenes. Studies affecting the full reduction, carburization, and resulting crystal polytype were also conducted. X-ray diffraction and EDX confirmed the synthesis of hexagonal Mo2C required no reductive gases while the tungsten and vanadium compounds required it. Various crystal polytypes were noted and the study was concluded with attempts to form metal nitride variants. To achieve wafer-scale coverage of MoS2 and WS2, we employed a chelant-enhanced chemical vapor deposition technique. Metal thiols were combined with chelating agents of varying chain length in a DMSO solution, spun coat onto a SiO2/Si wafer, and annealed under vacuum. Molybdenum was found to form films while tungsten proved more difficult. The samples were characterized via Raman spectroscopy, SEM, AFM, and STEM. In summary, this work produced two alternative synthetic routes for MXenes and TMDs and elucidated the factors/conditions that affect reduction and carburization (MXenes) and wafer-scale growth (TMDs). The sol-gel and chelant enhanced methods can be employed to produce compounds in the same material family.

Two dimensional magnetic materials hold the promise of helping to achieve beyond CMOS computing tasks. 2D magnetic materials can be used in fabricating magnetic tunnel junctions with higher tunnel magnetoresistance which can then be applied to making new neuromorphic computing architectures primarily geared towards artificial intelligence and machine learning applications. In this work I summarize my synthesis and investigation of the properties of Cr₂C which belongs to the group of 2D transition metal carbides or nitrides called MXenes. Cr₂C has been predicted to have intrinsic half metallic ferromagnetic behaviors. These magnetic behaviors can be tuned based on the level of functionalization of the surface of the material. I show different parameters such as etchant, reaction temperature, and molar concentration that I have tuned in order to optimally derive Cr₂C from its parent MAX phase Cr₂AlC by removing the Al layer with a fluoride salt and hydrochloric acid. I also show how magnetic tunnel junctions (MTJs), which are two ferromagnetic layers with a tunnel barrier in between, can be used to make a synapse which is a neuromorphic computing primitive. The synapse circuit that I have proposed displays spike timing dependent plasticity which is an integral component of learning and memory in the brain. I show how different delay conditions between the presynaptic signal and the postsynaptic signal lead to currents of different magnitudes flowing through the ferromagnetic layer of the magnetic tunnel junction synapse. I also show how these currents move the domain wall both in micromagnetic simulation and using a domain wall MTJ Spice model that has been developed. I went on to wire four of these synapses together to observe the temporal dynamics of the system. My results show that the lower the delay between the presynaptic pulse and the postsynaptic pulse, the higher the current through the MTJ synapse and hence the larger the domain wall displacement. These studies pave the way for empirical understanding of the Cr₂C MXene, including its potential magnetic properties, as well as doing online machine learning classification tasks with arrays of magnetic synapses

The dream device for electrochemical energy storage (EES) should have both high energy density (as in batteries) and high power density (as in supercapacitors). Pseudocapacitors are a promising type of EES technologies toward that dream. The power density of a pseudocapacitor is mainly determined by the ion transport at the interface and inside the electrode. Therefore, atomistic understanding of the ion intercalation and transport is needed for rational design of high-performance pseudocapacitors. In this dissertation, we have investigated the ion transport, intercalation mechanism, interfacial structure, and spectroscopic features of two typical pseudocapacitive systems: MXenes (2D metal carbides and nitrides) and tungsten oxide hydrates. In the first part of the dissertation, we employed density functional theory (DFT) and first-principles molecular dynamics (FPMD) to study the ion insertion and transport in MXene electrodes. We first studied the proton dynamics in MXene-confined water to understand the relation between proton diffusion and surface redox behavior under confinement. Then, we studied the Li+ insertion into MXene in water-in-salts electrolyte. Our DFT calculation agrees well with the experimental electrochemical quartz crystal microbalance (EQCM) and in-situ X-ray diffraction (XRD) results, suggesting the desolvation-free Li+ (de)intercalation mechanism. Next, we investigated MXene edge structure by integrating the DFT calculation and structural search methods. The predicted most stable structure in vacuum exhibits an interlayer connection at the edge, while the configurational search of hydrated edge suggests the spontaneous CH4 formation, which was observed in previous experiments. In the second part of the dissertation, we studied the protonation sites and proton diffusion in tungsten oxides hydrates. We first investigated proton diffusion in tunnel h-WO3 and reported the optimal linear water density of 4 water/nm for highest proton conductivity. Next, we compared the calculated vibrational densities of states (VDOS) with inelastic neutron scattering (INS) measurement, to pinpoint proton at the bridging oxygen site in monoclinic WO3·H2O. Finally, we studied the proton adsorption in the H2W2O7 and revealed low proton adsorption energies at terminal oxygen sites. In summary, our simulation work provided significant mechanistic understanding on the pseudocapacitive behavior of MXenes and tungsten oxides hydrates from both thermodynamic and kinetic perspective, which will facilitate the development of high-performance pseudocapacitors for EES.