Morphological characterization of semi-crystalline POM using nanoindentation

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.