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Tribology and sliding electrical contact resistance of e-beam hard Au: Effects of annealing

Wear

Mogonye, J.E.; Argibay, Nicolas A.; Goeke, Ronald S.; Kotula, Paul G.; Scharf, T.W.; Prasad, Somuri V.

Nanocomposite Au-ZnO thin films in the dilute oxide (<5.0 vol%) regime were synthesized by electron beam (e-beam) evaporation, as alternatives to electroplated Au hardened with Ni. Tribological measurements of e-beam hard Au were made while passing current through sliding contacts; electrical contact resistance (ECR) and friction data were simultaneously acquired during the test. The friction, wear and ECR behaviour were studied for the as-deposited film condition, and after annealing at 250 °C and 350 °C in air. The study revealed that the 250 °C annealed Au-2 vol% ZnO film exhibited the lowest, stable friction coefficient s (µ~0.25) and ECR (~35 mΩ) during sliding. Furthermore, the wear rate of this 250 °C annealed ZnO hardened Au nanocomposite film was an order of magnitude lower at 1.5×10−5 mm3/N m than for a typical Ni hardened, electroplated Au film at 1.3×10−4 mm3/N m. Cross-sectional transmission electron microscopy studies inside the wear surfaces revealed that the extremely stable, low friction coefficients and wear rate of annealed Au-2 vol% ZnO film was due to partial coverage of the wear surface with a ZnO tribofilm that reduced the adhesive contact contribution to wear with minimal impact on ECR. The potential implications of this study in the search for an environmentally friendly alternative to widely used electroplated hard Au are discussed.

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TYPE Conference Poster YEAR 2017
DOIOSTIScopus

Relating adatom emission to improved durability of Pt-Pd diesel oxidation catalysts

Journal of Catalysis

Johns, Tyne R.; Goeke, Ronald S.; Ashbacher, Valerie; Thune, Peter C.; Niemantsverdriet, J.W.; Kiefer, Boris; Kim, Chang H.; Balogh, Michael P.; Datye, Abhaya K.

Sintering of nanoparticles is an important contributor to loss of activity in heterogeneous catalysts, such as those used for controlling harmful emissions from automobiles. But mechanistic details, such as the rates of atom emission or the nature of the mobile species, remain poorly understood. Herein we report a novel approach that allows direct measurement of atom emission from nanoparticles. We use model catalyst samples and a novel reactor that allows the same region of the sample to be observed after short-term heat treatments (seconds) under conditions relevant to diesel oxidation catalysts (DOCs). Monometallic Pd is very stable and does not sinter when heated in air (T ≤ 800°C). Pt sinters readily in air, and at high temperatures (≥800°C) mobile Pt species emitted to the vapor phase cause the formation of large, faceted particles. In Pt-Pd nanoparticles, Pd slows the rate of emission of atoms to the vapor phase due to the formation of an alloy. However, the role of Pd in Pt DOCs in air is quite complex: at low temperatures, Pt enhances the rate of Pd sintering (which otherwise would be stable as an oxide), while at higher temperature Pd helps to slow the rate of Pt sintering. DFT calculations show that the barrier for atom emission to the vapor phase is much greater than the barrier for emitting atoms to the support. Hence, vapor-phase transport becomes significant only at high temperatures while diffusion of adatoms on the support dominates at lower temperatures.

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TYPE Journal Article YEAR 2015
DOIOSTIScopus
Results 26–50 of 101
Results 26–50 of 101