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

本文用弯曲振动自由衰减方法研究了硅基片上0.2 ~ 2.0 μ m厚铝薄膜在180 ~ 360 K温度范围内的内耗随应变振幅的变化。根据本构方程，单独的膜中的内摩擦可以从膜-基底复合材料上的数据单独评估。在应变范围内，铝膜的振幅依赖性内耗比体铝高出两个数量级。根据细观塑性理论，将与振幅有关的内耗转化为位错运动时有效应力的函数，即塑性应变。由此获得的铝膜的力学响应表明，10^数量级的塑性应变<-9>随应力的增加而非线性地增加。这些曲线倾向于随着膜厚度的减小和温度的降低而向更高的应力偏移，这两者都表明微塑性变形的抑制。在所有的温度下，在一个恒定的水平的塑性应变的微流应力与膜的厚度，这定性同意与宏观屈服强度的变化成反比。从实际的角度来看，本分析方法的内部摩擦适用于非破坏性评价的应力应变响应的薄膜粘结到基板上，并直接与单轴拉伸测试的独立膜从基板上分离。

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.