Optimum tribological improvement of aluminum using oxygen plasma source ion implantation
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and its alloys show poor tribological properties. Oxygen plasma source ion implantation is an emerging technology for the improvement of the surface mechanical properties of these materials. We found an optimum O ion dose, corresponding to 35 at.% O, for which we were able to obtain nanohardness enhancements by factors of 2× and 3× for pure and alloyed (AA7075) Al, respectively. Nanoscratch test results showed reductions in the scratch depths and the friction coefficients by nearly the same factors. It is also important to control the process temperature (∼160 °C). These improvements are due to the formation of a smooth, stiff, but nonbrittle metal–oxide (Al–Al2O3) nanocomposite.
Aluminum could be especially valuable for some microelectromechanical applications because of its good electrical conductivity (contrary to silicon) and its low mass density, allowing fast accelerations at low power. However, in spite of their good specific mechanical properties, aluminum and its alloys are seldom used in mechanisms because of poor tribological properties, leading to friction losses and wear. The hardness of pure Al is only approximately 0.2 GPa and that of the higheststrength Al alloy (AA7075) is approximately 1.5 GPa,1,2 and the macroscopic friction coefficients in air are 艌1.2 Oxygen-ion-beam implantation has been shown to produce significant surface hardening on Al.3 However, ionbeam implantation is neither practical nor cost effective for most foreseeable applications. Here we report work in which an industrially scalable process, oxygen plasma source ion implantation,4,5 is used to treat both pure Al and AA7075T6. The high ion current provided by our PSII system permits quick (∼10 min), convenient (allowing nonplanar workpieces), and inexpensive treatments, compared with ion-beam implantation. We also note that this vacuum process is compatible with the usual microelectromechanical system processes inspired by semiconductor technology. A detailed nanotribological study of the properties of the treated materials is reported. The emphasis of this work is on the optimization of the properties in terms of ion dose. The objective was not to produce a continuous alumina coating, which can be obtained by easier means and leads to a brittle and
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 8, Aug 2003
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electrically insulating layer, but to synthesize an Al– oxide nanocomposite with a hard but tough and conducting surface. The oxygen-ion implantations were performed using our electron cyclotron resonance (ECR) plasma source. Our concept incorporates several of the ideas of plasma immersion ion implantation6 though it differs significantly in several aspects, as previously described.5,7 In particular, a grounded grid is interposed between the plasma production zone and the implantation zone, and it is the plasma source that is pulsed instead of the high voltage. Briefly, the plasma is generated in the sourc
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