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

用弯曲振动自由衰减法研究了硅基铝薄膜在180~360K温度范围内0.2~2.0微米厚的内耗随应变幅值的变化规律。根据本构方程，可以从薄膜-基片复合材料的数据中单独计算薄膜中的内耗。在应变范围内，铝膜的内耗与幅值有关，约比块体铝的内耗高两个数量级。根据微塑性理论，变幅内耗可转化为位错运动有效应力的塑性应变。由此得到的铝膜的力学响应表明，随着应力的增加，10^&lt；-9&gt；数量级的塑性应变呈非线性增加。随着薄膜厚度的减小和温度的降低，这些曲线倾向于向更高的应力方向移动，这都表明微塑性变形受到了抑制。在所考察的所有温度下，在恒定塑性应变水平下的微观流动应力与薄膜厚度成反比，这与宏观屈服强度的变化是定性一致的。从实用的角度来看，本文的内耗分析方法适用于粘结在衬底上的薄膜的应力应变响应的无损评估，并可直接与从衬底上剥离的独立薄膜的单轴拉伸测试相比较。

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.