The application of reactive sputtering in a mixed gas of argon (Ar) and nitrogen (N2) to Al metal resulted in the formation of compositionally gradient film consisting of Al and AlN phases. By changing the mixing gas ratio of N2 to N2+Ar so as to be 0, 0.05, 0.1, and 0.2 in a total gas pressure of 1 Pa, the resulting phase changes in the order of Al, Al+AlN, AlN+Al, and AlN and the thickness of each phase is 14, 23, 18, and 8 μm, respectively, after sputtering for 3.6, 5.4, 7.2, and 10.8 ks, respectively. The Al concentration along the sample thickness changes from 100% to about 50% with an increase of mixing ratio from 0 to 0.2. The lattice parameters of Al and AlN phases remain constant along the sample thickness. The Knoop hardness and the electrical resistivity increase steeply from 100 to 3900 and from 3×10−8 Ω m to an infinite value, respectively, with the phase transition from Al to AlN through Al+AlN and AlN+Al. The load leading to an exfoliation among the layers was measured to be 16 N. Thus, the sputtered Al‐AlN film is concluded to possess functionally gradient characteristics resulting from the compositional and structural gradients.

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