Structural properties and thermal stability of Fe/Al 2 O 3 multilayers
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Iron/alumina multilayers have been deposited on sapphire wafers using RF magnetron sputtering. To study the interdiffusion, the multilayers were annealed in a tubular furnace under a mbar vacuum, and the samples examined by using a combination of classical diffractometry (8/28) and Grazing Incidence Scattering (GIS) for the phase determination, and Small Angle X-ray Scattering (SAXS) for the superstructure of the multilayers. In all cases, in the as-deposited state the alumina is amorphous and the iron is crystalline in the bcc phase. Thermal anneals at temperatures between 573 and 873 K give evidence for segregation to the interfaces. At higher temperatures, interdiffusion occurs, leading to the formation of different phases. The Fe-A1203 interdiffusion coefficient has been evaluated in the temperature range from 873 to 1273 K.
1. INTRODUCTION
The properties and stability of interfaces are of great interest in a large field of applications. From a metallurgical point of view, ceramic coatings are very efficient for the high temperature protection of iron (used, for instance, in some matrices). In this case, one has to make sure that thermal cycles do not change the properties of the interfaces. Such a problem has been investigated by several groups,',2 but at a millimeter or micrometer scale (thick deposits or droplets). However, the crucial point of characterizing interdiffusion in the metal-ceramic system is not yet resolved. Multilayers are good systems in which to study the properties governing the stability of metal-ceramic interfaces. Small Angle X-ray Scattering (SAXS) is an appropriate technique to check the stability of multilayer structures; for example, in the case of W/C x-ray mirrors, the formation of interfacial WC alloy, which acts as an interdiffusion barrier, has been found to improve the thermal stability of the multilayers.? Moreover, SAXS experiments on multilayers where interdiffusion occurs may lead to the determination of interdiffusion coefficients as low as ~ r n ' / s . ~Some other groups, such as Greer et aL5 have studied the interdiffusion in multilayers for transition metals systems which lead to amorphous alloys like Ni-Zr. Their approach to the problem is the same as ours: interdiffusion coefficients are deduced from the evolution of SAXS spectra during isothermal anneals. However, investigations of interdiffusion in multilayers have mainly been focused on metal-metal or metalsemiconductor systems. In this paper, we report on the
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J. Mater. Res., Vol. 10, No. 12, Dec 1995
http://journals.cambridge.org
Downloaded: 13 Dec 2015
first study where interdiffusion coefficients are obtained in a metal-ceramic multilayer system: iron-alumina. This system has already been studied in the form of unheated samples (as-deposited state)6; it has been found to be a good candidate for the study of metal-ceramic interfaces as it is easy to obtain high quality as-deposited multilayers. As presented in previous paper^,^^^ hightemperature annealing
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