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We report an investigation of the friction mechanisms of MoS2 thin films under changing environments and contact conditions using a variety of computational and experimental techniques. Molecular dynamics simulations were used to study the effects of water and molecular oxygen on friction and bonding of MoS2 lamellae during initial sliding. Characterization via photoelectron emission microscopy (PEEM) and Kelvin probe force microscopy (KPFM) were used to determine work function changes in shear modified material within the top few nanometers of MoS2 wear scars. The work function was shown to change with contact conditions and environment, and shown by density functional theory (DFT) calculations and literature reports to be correlated with lamellae size and thickness of the basally oriented surface layer. Results from nanoscale simulations and macroscale experiments suggest that the evolution of the friction behavior of MoS2 is linked primarily to the formation or inhibition of a basally oriented, molecularly thin surface film with long-range order. [ABSTRACT FROM AUTHOR]

Abstract: DNA G‐quadruplex structures were recently discovered to provide reliable scaffolding for two‐dimensional organic frameworks due to the strong hydrogen‐bonding ability of guanine. Herein, 2,7‐diaryl pyrene building blocks with high HOMO energies and large optical gaps are incorporated into G‐quadruplex organic frameworks. The adjustable substitution on the aryl groups provides an opportunity to elucidate the framework formation mechanism; molecular non‐planarity is found to be beneficial for restricting interlayer slippage, and the framework crystallinity is highest when intermolecular interaction and non‐planarity strike a fine balance. When guanine‐functionalized pyrenes are co‐crystallized with naphthalene diimide, charge‐transfer (CT) complexes are obtained. The photophysical properties of the pyrene‐only and CT frameworks are characterized by UV/Vis and steady‐state and time‐resolved photoluminescence spectroscopies, and by EPR spectroscopy for the CT complex frameworks. [ABSTRACT FROM AUTHOR]

Compositionally graded cylinders comprising FeCo-2V, a soft magnetic alloy, and 316L stainless steel, were produced by directed energy deposition incorporating two different gradient lengths. Five distinct regions in the microstructure were observed in both the as-printed and annealed gradients. When normalized for the gradient length, the gradients showed similar grain size and microhardness profiles. In tensile tests, the as-printed samples failed in the 316L region with an effective total "composite" strain of 20-30% and overall strength approaching that of wrought 316L. The annealed samples failed in the FeCo-2V region. Neither exhibited failure in the gradient region. Molecular dynamics simulations were used to calculate tensile strength as a function of composition, showing good correlation to experimental trends but not absolute values. This work demonstrates the tunability of site-specific properties using blown powder directed energy deposition to gradually grade from FeCo-2V to 316L stainless steel in a monolithic component. [Display omitted] [ABSTRACT FROM AUTHOR]