薄膜断裂韧性的微桥拉伸法研究

The testing of the hardness of a thin film has not been a problem since the nanoindentation method was introduced by Oliver and Pharr in 1992. However, up until now, there is still no international standard or agreed methodology for the evaluation of the toughness of a thin film, which has been troubling researchers for decades..Tensile testing for a standalone thin film is straightforward and reliable, like the corresponding tensile testing for a bulk material. However, it is horribly difficult in clamping and direct loading of a freestanding thin film for the testing. A micro bridge tensile testing method was proposed to solve the problems by the applicant in 2012, in which the tensile testing for a thin film is realizes through conducting a tensile loading on the substrate. The method has received wide attention as it has been successfully used for the testing of the fracture toughness for a thin film. In the project, cyclic loading of the micro bridge tensile testing method will be realized firstly so that the testing of fatigue toughness for a thin film can be achieved. Then a great endeavor will be made on the standardization of the micro bridge tensile testing method. The optimization of sample preparation procedure, the updating of the testing stage, as well as the amendment of the calculation model for fracture toughness are all to be carried out. At the same time, a series of Zr-based metallic glass multilayer thin films will be tested to assess the reliability of the method. The precision accuracy and the error of the standard testing method will also be analyzed. .The standardization of the micro bridge tensile testing method is aimed to provide a standard testing methodology for the fracture toughness, as well as for the fatigue toughness of a thin film, which is still not reachable so far.

纳米压入方法的提出实现了薄膜硬度的常规化测试，但薄膜的韧性还没有一个标准化测试方法。采用拉伸法实现薄膜韧性的量化表征，是近年来薄膜力学性能实验表征研究领域关注的热点，难点在于薄膜钳制和拉伸的微力度。申请人近年来在薄膜的微拉伸实验表征方法方面开展了深入研究，提出了通过基体变形的方式实现其表面薄膜微桥样品的拉伸测试，巧妙地避开了这两个难点，引起国际上同行广泛关注。本项目拟结合Zr-基非晶合金薄膜的韧性测试，开展该方法的标准化研究。通过对样品制备、加载测试、模型计算等关键环节进行重点突破，实现薄膜断裂韧性和疲劳韧性的标准化测试。以期这一方法得到广泛应用，从而填补这一空白。

断裂韧性是结构材料的基本力学性能之一。对于块体材料，已经建立了标准化的测试方法和测试规范。但对于薄膜材料，受限于厚度的微小尺寸， 迄今为止还缺乏有效的测试方法。本项目提出开展薄膜断裂韧性的微桥拉伸测试方法研究，建立规范化的测试样品制备、测试装置以及计算模型，实现典型薄膜体系的标准化测试。.本项目在微桥法研究方面，开展了7个方面的研究内容，包括紧凑拉伸基体样品结构的标准化、单晶硅片基体初始尖锐裂纹的有效制备、拉伸测试装置的升级改造、薄膜拉伸应变的计算模型、薄膜断裂韧性的计算模型、标准化测试规程和表格化的测试结果处理、拉伸位移载荷的循环加载；在典型薄膜体系测试方面，完成了2个方面的工作，包括CuZr非晶合金薄膜体系和C-基非晶复合薄膜体系的制备、结构表征以及断裂韧性测试。取得以下重要结果：.1）实现了微桥法薄膜断裂韧性的标准化测试，包括搭建了标准化的测试装置、建立了标准化的测试样品制备方法、建立了薄膜应变及断裂韧性计算模型。.2）揭示了CuZr非晶合金薄膜断裂韧性的尺寸效应：在亚微米尺度范围内，随着薄膜厚度的减小，薄膜断裂韧性逐渐降低。CuZr薄膜厚度为247.7nm时，断裂韧性为2.04MPa·m1/2；厚度为887.4nm时，断裂韧性为3.39MPa·m1/2，是前者的1.7倍。.3）发现C-基非晶/纳米晶纳米复合薄膜的硬度和断裂韧性存在不同的变化趋势，但存在一个共同优化区域。此外，H/E和H3/E2抗塑性变形指数存在与断裂韧性相类似的变化规律。.4）CuZr非晶合金薄膜和C-基非晶复合薄膜的断裂韧性测试值KIC与薄膜预制裂纹长度（af/Wf）有关，随着裂纹长度增加，测量值增大，表明薄膜断裂韧性的测试值与测试样品的几何尺寸参数有关。