Single- and multiphase samples of the n-type half-Heusler NbCoSn were prepared by directional solidification using the optical floating zone melting method, and the thermoelectric properties of these samples were evaluated. NbCoSn has an excellent thermoelectric power which exceeds −250μVK−1 at around 900K and a relatively high carrier concentration, 4.82×1026m−3. A metalliclike temperature dependence of the electrical resistivity indicates that NbCoSn is a degenerate semiconductor. NbCoSn also shows an excellent power factor, 2.5mWm−1K−2 at about 650K, even without any tuning of the electrical properties which are susceptible to coexisting metallic phases.
A study has been made to determine the dynamic stability of an imperfect circular cylindrical shell subject to a step loading in the axial direction. In the analysis, the radial displacement of the shell is approximated by a finite degree-of-freedom system. The dynamic analysis includes not only the effect of the radial inertia, but also, in an approximate manner, that due to the axial inertia. The critical loads are determined by numerical integration of the equation of motion. Compared with the static case, there is a significant reduction of the dynamic buckling load for the high wave number range of the radial modes. It is concluded that due to frequency coupling between axial and radial motions, the axial inertia plays an essential role in characterizing the dynamic instability of a finite length shell.
Oxy-nitriding is a widely used industrial process aiming to improve the tribological properties and performance of components. Previous studies have shown the effectiveness of the treatment with friction and wear performance, but very few have focussed on optimising this behaviour. The lubrication properties of several EP and AW additives were examined to investigate their effectiveness in improving the tribological properties of the layers formed after treatment. Previous studies showed the presence of an oxide layer on the sample could improve the effectiveness of the sulphurised olefin (SO) and tricresyl phosphate (TCP) additives. The friction and wear behaviour of oxy-nitrided samples were analysed using a tribometer and surface profiler. Scanning electron microscope, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were employed to identify the morphologies and chemical compositions of the treated surface before and after testing. No real effect on friction was observed when using the SO or TCP additives, mostly due to lack of interaction with the less reactive iron nitride layer and their roles as anti-wear additives. However, when the zinc dialkyldithiophosphatecontaining lubricant was used, a higher friction coefficient was observed. Greater improvements in anti-wear properties with the presence of additives in comparison with only using base oil were reported, with the TCP additive producing the lowest wear rates. The study effectively demonstrated that the additive package type used could impact the tribological and tribochemical properties of oxy-nitrided surfaces.
a b s t r a c tDiamond-like carbon (DLC) and Boron Carbide (B 4 C) coatings are promising candidates for slidingrolling contact parts, especially gears to increase scuffing load capacity and prolong lifetime. Generally, commercially available fully formulated oils are compatible with ferrous surfaces, but normally have not been optimised for coatings. In this study, the tribological properties of tungsten carbide containing DLC (WC-DLC), hydrogenated DLC (H-DLC) and B 4 C coatings against steel ball with two fully formulated oils in boundary lubrication conditions have been evaluated. They were compared with that of a Steel/Steel tribocouple. Interactions between lubricants and sliding surfaces were also investigated using surface characterisation techniques such as XPS and Raman Spectroscopy. It was found that Steel/H-DLC system provided the lowest friction coefficient for both oils.
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