Advancing the near-surface mechanical characterization of polytetrafluoroethylene by indentation techniques

The near-surface mechanical behavior of semi-crystalline thermoplastic polytetrafluoroethylene (PTFE) is reported at room temperature using the instrumented indentation technique (IIT). A set of four PTFE samples was tested depending on the fabrication process. First, a non-melting technique, i.e., spark plasma sintering (SPS), is used to consolidate a highly viscous PTFE micro-powder at two heating rates (100 and 160 °C/min). Second, PTFE samples prepared by conventional melting techniques, i.e., molding and extrusion, are considered for comparison. This study aims to assess how the mechanical metrics (hardness, H , and Young's modulus, E ) depend on the fabrication process. The PTFE samples are characterized using XDR, FTIR, and Raman spectroscopy to identify relationships between microstructure/processing route. Next, the force-indentation depth curves are analyzed using the Berkovich technique at the nano- and microscale. Both the loading and unloading parts are successfully described by the Meyer law and the Oliver-Pharr model, respectively. The newly inverted approach is also tested, and the results agree with those obtained from the Oliver-Pharr model. Additionally, the elastic/plastic contributions during deformation are analyzed for the PTFE samples. For H and E , the results are consistent with the observed increase as the applied displacement decreases. This behavior, called the indentation size effect (ISE), is quantified according to the Meyer law. This work also evaluates the correlations between the local and bulk properties extracted from tensile data. The effect of heating rate is found to be more pronounced in the tensile behavior than in the indentation data. The key takeaway concerns the efficiency of the SPS process, opening the possibility of fabricating product-friendly microstructures that can achieve specific target performance.