Indentation creep: New machine and test methodology development at various length scales, high temperatures and low deformation rates

在第一期资助期内，研制了一种新型高温高负荷纳米压头，该压头在真空条件下在扫描电子显微镜（SEM）内工作，并成功安装在德国达姆施塔特工业大学。新系统的工作温度范围从RT到1100°C，动态压痕实验与max。负载可达1n。在第一阶段，该系统成功地应用于玻璃、Mo和Ni，并在整个温度范围内成功地确定了材料的弹性模量、硬度、速率灵敏度和活化体积等性能。此外，新的测试方案，如恒定接触压力法已经开发出来，用于确定蠕变特性，并且已经成功地使用动态压痕测试分析了脆性到韧性的转变。然而，高温合金的高温纳米压痕实验仍然具有挑战性，这主要是由于合金在高温下的高硬度以及尖端材料与样品的强化学相互作用。这导致非常高的尖端磨损和潜在的不可靠的数据采集。然而，我们能够证明，通过使用大压痕深度和/或短接触时间的测试程序，采用新的阶跃载荷蠕变方法，即使是Ni等关键材料也可以在超高温下成功测试。在第二个资助期内，计划通过开发一个用户工具箱来解决上述问题，用于测试相关的高温材料，如镍基合金，在其应用温度或接近其应用温度。该工具箱将被设计为应用指南，以选择适当的测试程序，尖端几何形状以及适合各自目的的样品和尖端材料的合适组合。这里的重点将是为非锥体压头几何形状开发新的加载协议，以表征不同长度和时间尺度上的材料参数，特别是用于测试相关的镍基高温合金。因此，不同类型的压痕实验将在具有已知高温力学性能的参考材料（ni -固溶体，NiAl, IN718, ERBO1A）上进行，而新尖端几何形状的数据评估将由有限元建模支持。此外，将在项目中寻找和测试新的更稳定的尖端材料。因此，采用扩散偶法分析临界样品与潜在新尖端材料之间的化学相互作用。然后使用化学惰性材料作为新的尖端材料，研究其在高温压痕测试中的性能。

In the first funding period, a new high temperature and high load nanoindenter, which is operated inside a scanning electron microscope (SEM) under vacuum condition has been developed and successfully installed at TU Darmstadt. The operational temperature of the new system ranges from RT up to 1100°C and dynamic indentation experiments with max. loads of up to 1 N are possible. During the first period, the system was successfully applied to glass, Mo and Ni and material properties like modulus of elasticity, hardness as well as rate sensitivity and activation volume have been successfully determined in the whole temperature range. Furthermore, new test protocols, like the constant contact pressure method have been developed for determination of creep properties and the brittle to ductile transition has been successfully analyzed using dynamic indentation testing. High temperature nanoindentation experiments on alloys for high temperature application remains however challenging, mainly due to the high hardness of the alloys at elevated temperature and the strong chemical interaction of the tip material with the samples. This leads to very high tip wear and potentially unreliable data acquisition. Nevertheless, we were able to show that by applying test procedures using large indentations depths and/or short contact times, with a new step load creep method, even critical materials such as Ni can be successfully tested at ultra-high temperatures. During the second funding period it is planned to address the issues mentioned above by developing a user toolbox for testing relevant high temperature materials like Ni-based alloys at or close to their application temperature. This toolbox will be designed as an application guide to select the appropriate test procedure, tip geometry as well as a suitable combination of sample and tip material for the respective purpose.The focus here will be on the development of new loading protocols for non-pyramidal indenter geometries for characterizing material parameters on different length and time scales, in particular, for testing relevant Ni-based HT alloys. Therefore, different type of indentation experiments will be performed on reference materials (Ni-solid solutions, NiAl, IN718, ERBO1A) with well-known mechanical properties at elevated temperatures, whereas the data evaluation of new tip geometries will be supported by Finite Element modelling. Furthermore, new and more stable tip materials will be sought and tested within the project. Therefore, a diffusion couple approach is used to analyze the chemical interaction between critical sample and potential new tip materials. Chemically inert materials are then used as new tip materials, studying their performance during indentation testing at elevated temperature.