Absolute Evaluation of Mechanical Properties of Thin Films Based on the Microplasticity Theory of Internal Friciton.

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Internal friction in aluminum thin films 0.2 to 2.0mum thick on silicon substrates has been investigated between 180 and 360 K as a function of strain amplitude by means of a free-decay method of flexural vibraion. According to the constitutive equation, the internal friction in the film alone can be evaluated separately from the data on the film-substrate composite. The amplitude-dependent internal friction in aluminum films is found in the strain range approximately two orders of magnitude higher than that for bulk aluminum. On the basis of the microplasticity theory, the amplitude-dependent internal friction can be converted into the plastic strain as a function of effective stress on dislocation motion. The mechanical responses thus obtained for aluminum films show that the plastic strain of the order of 10^<-9> increases nonlinearly with increasing stress. These curves tend to shift to a higher stress with decreasing film thickness and also with decreasing temperature, both indicating a suppression of the microplastic deformation. At all temperatures examined, the microflow stress at a constant level of the plastic strain varies inversely with the film thickness, which qualitatively agrees with the variation in the macroscopic yield strengths. From a practical standpoint, the present analytical approach of internal friction is applicable for non-destructive evaluation of stree-strain responses in thin films bonded to a substrate, and is directly comparable with uniaxial tensile testing of free-standing films detached from a substrate.