Selective Modification of the Tribological Properties of Aluminum Through Temperature and Dose Control in Oxygen Plasma
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nts in the tribological properties of pure aluminum and “aeronautical” alloy AA7075-T651 were obtained by oxygen-ion implantation [(0.7 to 5) × 1017 O/cm2, 30 keV] using our pulsed electron cyclotron resonance plasma source. This oxygen plasma source ion implantation process produced oxide nanoprecipitates that enhanced the hardness up to three times in the surface layer and caused reductions in the scratch depths and the friction coefficients by similar factors. A spectrum of tribological properties was obtained depending on temperature and ion dose. Temperature measurement and control were obtained through an integrated thermocouple and by changing the duty-cycle of the microwave source. The oxygen content and the depth-resolved chemical composition were measured and optimized using x-ray photoelectron spectroscopy (XPS) combined with Ar-ion etching. The tribological properties were investigated by (i) depth-sensing nanoindentation for hardness and Young’s modulus, (ii) scratching and scratch-depth measurement via atomic force microscopy (AFM), and (iii) friction force measurements using AFM. Low-temperature (艋160 °C) implantations with optimal O-ion doses produced, in both pure and alloyed Al, an approximately 50-nm-thick, smooth, and extremely fine-grained metal–alumina nanocomposite. The resulting surface was hard and stiff but nonbrittle and displayed high scratch resistance and low friction. High-temperature (∼430 °C) implantation had different effects on pure Al and AA7075. On pure Al, it produced a very hard but brittle Al2O3 layer for which yield points (displacement excursions) were observed at critical load values in the nanoindentation force–displacement curves. On AA7075, XPS chemical profiling revealed an effect of extreme Mg surface segregation and complete Al surface depletion; MgO crystallites formed a rather rough but surprisingly thick layer (>100 nm). The resulting AA7075 surface showed a hardness increase that was substantial but slightly smaller than that obtained at low temperature.
I. INTRODUCTION
Aluminum components have numerous engineering applications, such as aerospace materials. A promising new application is microelectromechanical systems,1 especially high-speed low-actuation-voltage electrostatic actuators, because of the high electrical conductivity of aluminum. However, the poor performance of Al parts in sliding contact due to their low surface strength is a serious disadvantage. Ohira et al.2,3 investigated the improvement of Al surface mechanical properties by ion implantation. They used nitrogen ions to form a hard surface layer, but which delaminated easily due to the
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 12, Dec 2003
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high residual stress at the Al/AlN interface. Oxygenion implantation was then studied by the Sandia Labs group4,5 toward avoiding this undesirable effect by forming a hard nanocomposite produced by oxide precipitation. The synthesis
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