engleski

Al-W amorphous thin films

Aluminum-tungsten amorphous alloys seem to be promising materials according to the specific properties of their respective components. However, their thermal stability is essential for any application at sustained elevated temperatures. We have investigated Al-W amorphous alloys thermal stability and variation of mechanical, electrical and corrosion properties up to the crystallization temperature. The Al-W thin films were prepared by simultaneous d.c. sputtering of both pure Al and pure W in a two source sputtering apparatus. The films with composition in the range from Al82W18 to Al62W38 were completely amorphous. Thermal stability of amorphous films was investigated by the continuous in situ electric resistance measurements in a vacuum better than 10-5 mbar. The initial and final structure of the film submitted to the up/down temperature cycle were determined by the XRD method. The Al-W amorphous alloys exhibit high crystallization temperatures (520oC-620oC) within a whole compositional range of amorphicity. A dominant crystallization product is always Al4W. The Al-W films are also rather stable against prolonged annealing at temperatures slightly below crystallization temperature. However, the amorphous films exhibit a pronounced structural relaxation upon first heating at temperatures between about 100oC to 500oC. The relaxation was more pronounced in aluminum-rich alloys, which might be due to specific electronic structure of aluminum. Hall effect measurements support the importance of the sp-d electronic bands hybridization in the examined films. Attempt to minimize relaxation by the increase of substrate temperature during deposition yielded a two-phase films, and the SEM examination revealed a dense population of whiskers at the surface of samples. Microhardness of the as-deposited Al-W amorphous films is about 11 GPa and only slightly depends upon composition. It only marginally increases upon annealing in vacuum up to the crystallization temperatures, while rise of about 20-30% is observed after crystallization. Corrosion resistance of amorphous Al-W alloys was investigated in chloride acid solution and artificial saliva solution. The Al-W thin films are inherently passive materials, and the increase of tungsten content in the alloy increases the resistance against pitting corrosion and lowers the overall rate of corrosion. Thermally crystallized Al-W films exhibit somewhat lower corrosion resistance in comparison with amorphous films. In conclusion, Al-W amorphous thin films are rather thermally stable and exhibit physical properties which make them suitable for certain applications.

aluminum; tungsten; amorphous; thin film

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