The microstructure of Al–Cu–Fe thin films obtained by magnetron sputtering undergoes significant evolution depending on the deposition conditions and subsequent heat treatment. The initially amorphous or nanocrystalline structure is transformed into an icosahedral phase (i-phase) with varying degrees of perfection.
When deposited on low-temperature substrates, amorphous films are formed. Increasing the substrate temperature or subsequent annealing leads to crystallization and formation of the i-phase. The heating rate and holding time at the annealing temperature significantly affect the grain size and degree of order of the i-phase. Slow heating promotes the formation of larger and more perfect grains.
Transmission electron microscopy (TEM) was used to study the microstructural changes in thin quasicrystalline films. Thin layers were produced by magnetron sputtering on sodium chloride crystals, which were then dissolved in water to produce free-standing films. The studies were carried out on the sample immediately after deposition, as well as on samples annealed at 400 °C in an argon atmosphere and at 500 °C in air.
Nanocrystalline structures were observed in the initial sample. After annealing at 400 °C, the films underwent a transformation to a metastable cubic β-phase, which became dominant. Secondary θ- and ω-phases were also observed, appearing as small islands and deposits on the surface, inside the matrix and at grain boundaries, showing a certain orientation relative to the β-phase. Further annealing at 500 °C led to the transformation of the metastable phase to an icosahedral ψ-phase with residual aluminum-rich material, including the λ-phase.
TEM images combined with electron diffraction data revealed various features associated with phase changes during the transition of the material to a quasi-crystalline state. Alumina in the film formed an amorphous layer. Some grains in the film acted as “sacrificial” grains, promoting the growth of other grains in icosahedral phases. Elements near the boundaries of these “sacrificial” grains diffused, forming the ψ phase and leaving fragments in the center of the grains. The role of “sacrificial” grains, diffusion of elements and the mechanism of the phase transition are discussed. Weak adhesion to quasi-crystalline films is also noted.
Small deviations from the stoichiometric composition of Al–Cu–Fe can lead to the formation of secondary phases such as Al2Cu or FeAl. Crystal lattice defects such as dislocations and point defects also affect the stability of the i-phase and its properties.
Quasicrystalline Al–Cu–Fe thin films have unique properties such as high hardness, low friction coefficient and good corrosion resistance. This makes them promising for use as protective coatings, thermoelectric materials and other fields. Further research aimed at optimizing the production conditions and improving the microstructure will make it possible to fully realize the potential of these materials.
Author: E. J. Widjaja, L. D. Marks
Institute: Department of Materials Science and Engineering, Northwestern University, Cook Hall, 2225 North Campus Drive 2036, Evanston, IL 60208-3108, USA