Advanced Silicate-based Lubricant Additive Induced Diamond-like Carbon Structured Restoration Layer

An advanced silicate based lubricant additive has been employed in long-term pin-on-disk tribological experiments. The worn steel/steel surfaces were characterized using nano-indentation, SEM, XPS, and Raman spectroscopy for their physical, mechanical, and chemical properties. The average nano-hardness of the repaired layers on the disk and the pin is 10.2 GPa and 16.7 GPa respectively, which is substantially higher than that of the disk (HV 221, or 0.71 GPa) and the pin (HRC55, or 1.8 GPa) before tribological tests, forming super hard surfaces on the contact pair surfaces. Combined Raman spectroscopy and XPS studies suggest the formation of diamond-like carbon based restoration layers. A new formation mechanism of the restoration DLC layer contributing to hard and smooth contact surfaces is proposed

Strong pinning in the hole-doped pnictide superconductor La0.34_{0.34}Na0.66_{0.66}Fe2_2As2_2

We present magnetization studies as a function of time, temperature and magnetic field for $H$ $\parallel$ c-axis, in a hole-doped pnictide superconductor, La$_{0.34}$Na$_{0.66}$Fe$_2$As$_2$, with, $T_c$ $\approx$ 27 K. The obtained vortex phase-diagram shows that the magnetic irreversibility line is very close to the mean-field superconducting transition line, similar to the low $T_c$ superconductors, evidencing a strong pinning behavior. The irreversibility line does not follow a power law behavior with ($T_c$-$T$), however, it is well described using an expression developed in the literature considering the effect of disorder in the system. The critical current density estimated using the Bean's critical-state model is found to be of the order of 10$^5$ A/cm$^2$ below 12 K in the limit of zero magnetic field. A plot of the normalized pinning force density as a function of the reduced magnetic field at different temperatures shows a good scaling and the analysis suggests that the vortex pinning is due to normal point like pinning centers. The temperature dependence of the critical current density suggests that the pinning due to the variation in charge carrier mean free path alone is not sufficient to explain the experimental data. Magnetic relaxation rate as a function of temperature and magnetic field is also studied.Comment: 19 pages, 6 Figures, Accepted for Publication in the "Journal of Applied Physics 2019

Tracking the nematicity in cuprate superconductors: a resistivity study under uniaxial pressure

Overshadowing the superconducting dome in hole-doped cuprates, the pseudogap state is still one of the mysteries that no consensus can be achieved. It has been suggested that the rotational symmetry is broken in this state and may result in a nematic phase transition, whose temperature seems to coincide with the onset temperature of the pseudogap state $T^*$ around optimal doping level, raising the question whether the pseudogap results from the establishment of the nematic order. Here we report results of resistivity measurements under uniaxial pressure on several hole-doped cuprates, where the normalized slope of the elastoresistivity $\zeta$ can be obtained as illustrated in iron-based superconductors. The temperature dependence of $\zeta$ along particular lattice axis exhibits kink feature at $T_{k}$ and shows Curie-Weiss-like behavior above it, which may suggest a spontaneous nematic transition. While $T_{k}$ seems to be the same as $T^*$ around the optimal doping and in the overdoped region, they become very different in underdoped La$_{2-x}$Sr$_{x}$CuO$_4$. Our results suggest that the nematic order, if indeed existing, is an electronic phase within the pseudogap state.Comment: 6 pages, 4 figure

Corrosion behavior and residual stress of microarc oxidation coated AZ31 magnesium alloy for biomedical applications

Thesis (Ph.D.) University of Alaska Fairbanks, 2012Mg alloys are potentially new biomaterials for bone repair or replacement. Appropriate coating is, however, needed to make the Mg alloy more resistant to corrosion. In this research, protective microarc oxidation (MAO) coatings were produced on AZ31 Mg alloys in sodium phosphate electrolyte. The coatings were produced under varying pulse frequency, applied voltage, oxidation time and electrolyte concentrations. This research analyzed the effects of the above four MAO process control parameters on the residual stresses and the corrosion behavior. Optimization of the MAO control parameters would allow production of AZ31 Mg alloy with high corrosion resistance. It is well accepted that residual stress and corrosion behavior are two significant factors in the development of AZ31 Mg alloys. The residual stresses in the MAO coatings were evaluated by the X-ray diffraction (XRD)-sin²ψ method. A predictive model of the residual stresses is proposed and a principal components analysis (PCA) was conducted to determine the contribution of the MAO control parameter on the residual stresses. Long-term corrosion behavior of MAO-coated Mg alloys was evaluated by the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. The porosity of the samples after various immersion durations was evaluated by the potentiodynamic polarization method. The pre- and post- corrosion microstructures and the phase composition of MAO-coated samples were studied. Post-corrosion phase identification showed that hydroxyapatite (HA) was formed on the surface of the samples. The ratio of Ca/P in HA was determined by the X-Ray Fluorescence (XRF) technique. The degradation of the MAO-coated AZ31 alloys is reduced due to the MAO coating and the formation of a corrosion product layer. A predictive model of the corrosion current density is proposed and a PCA was conducted to determine the contributions of the individual MAO control parameter on the corrosion rate. The corrosion process and mechanism of MAO-coated AZ31 alloys in SBF were modeled based on the electrochemical corrosion results and the pre- and post-corrosion surface analysis. It is believed that under optimized control parameters, the MAO-coated AZ31 Mg alloy is superior implant material for biomedical applications

Nematic Quantum Critical Fluctuations in BaFe2−x_{2-x}Nix_xAs2_2

We have systematically studied the nematic fluctuations in the electron-doped iron-based superconductor BaFe$_{2-x}$Ni$_x$As$_2$ by measuring the in-plane resistance change under uniaxial pressure. While the nematic quantum critical point can be identified through the measurements along the (110) direction as studied previously, quantum and thermal critical fluctuations cannot be distinguished due to similar Curie-Weiss-like behaviors. Here we find that a sizable pressure-dependent resistivity along the (100) direction is present in all doping levels, which is against the simple picture of an Ising-type nematic model. The signal along the (100) direction becomes maximum at optimal doping, suggesting that it is associated with nematic quantum critical fluctuations. Our results indicate that thermal fluctuations from striped antiferromagnetic order dominate the underdoped regime along the (110) direction. We argue that either there is a strong coupling between the quantum critical fluctuations and the fermions, or more exotically, a higher symmetry may be present around optimal doping.Comment: 5 pages, 3 figures. This Letter only contains parts of the former version 1511.0578