Abstract

Nanoindentation (NI) is a contact method to investigate localized micromechanical properties of materials, whereby NI of semi-crystalline polymers is challenging. The influence of morphological structures such as spherulites or crystal-lamellae on localized NI depth-force behavior is discussed controversially in literature. Hence, the main objective of this study is to determine the influence of crystalline zones on NI results.

Polyoxymethylene (POM) exhibits high crystallinity with the spherulitic structure on the micrometer scale and was therefore chosen to proof the influence of spherulite distribution on NI results concerning modulus. Furthermore, the correspondence between the mean elastic modulus from different NI experiments and macroscopic compression tests will be demonstrated.

A POM tensile bar was investigated by NI with a large sphero-conical and a Berkovich indenter tip at different positions of its cross-section. Here, it was found that regions at the edge of the sample have a lower elastic modulus than regions in the middle of the cross-section. This agrees well with polarized light microscopy results, which reveal a skin layer with less crystallinity close to the sample edge. Therefore, the NI measurements in this edge zone result in a lower elastic modulus compared to the more crystalline middle of the cross-section.

In summary, semi-crystallinity influences the NI results obtained for POM and the mean of the elastic modulus distribution over the cross-section of the POM sample is in good agreement with macroscopic compression test results.

摘要

纳米压痕 (NI) 是一种研究材料局部微机械性能的接触方法，其中半结晶聚合物的 NI 具有挑战性。在文献中有争议地讨论了形态结构（如球晶或晶片）对局部 NI 深度力行为的影响。因此，本研究的主要目的是确定结晶区对 NI 结果的影响。

聚甲醛 (POM) 具有微米级球晶结构的高结晶度，因此被选择来证明球晶分布对关于模量的 NI 结果的影响。此外，还将证明来自不同 NI 实验和宏观压缩测试的平均弹性模量之间的对应关系。

NI 对 POM 拉伸棒进行了研究，该拉伸棒在其横截面的不同位置具有大球锥和 Berkovich 压头尖端。在这里，发现样品边缘区域的弹性模量低于横截面中间区域的弹性模量。这与偏光显微镜结果非常吻合，偏光显微镜结果显示靠近样品边缘的表层结晶度较低。因此，与横截面的结晶度较高的中间相比，该边缘区域的 NI 测量导致较低的弹性模量。

总之，半结晶度影响 POM 获得的 NI 结果，POM 样品横截面的弹性模量分布的平均值与宏观压缩测试结果非常一致。