Microscopic evaluation of the fracture behaviour of additively manufactured polymer–polymer interfaces under tensile load

This paper aims to give insights towards the microscopic fracture behaviour and fracture strength of single- and multi-material interfaces from samples manufactured by fused filament fabrication (FFF). For this purpose, multi-material tensile test specimens—polyetherimide (PEI) and polyphenylsulfone (PPSU)—were compared to single-material PEI reference specimens. As a first parameter study, the nozzle temperature was varied between prints. To achieve reliable testing of the interface strength between layers, the authors modified a single-wall testing geometry to force fracture between two specific layers. To investigate the fracture path on microscopic scale, the tensile testing setup was miniaturised and tests were performed underneath an optical microscope. The resulting fracture surfaces were evaluated with low-voltage scanning electron microscopy and energy dispersive X-ray spectroscopy. The results show a softened fracture behaviour for the multi-material samples compared to the single-material PEI specimen, introduced by the softer PPSU. Apart from degradation of the PEI at 420 °C, no trend in tensile strength was observed for pure PEI samples. For PEI printed onto PPSU at 380 °C, fracture strength was close to the single-material specimen, but was greatly reduced at 360 °C and showed degradation within the PEI at 420 °C. All samples fractured cohesively, close to the layer interface with the fracture switching between materials for the multi-material specimens. The investigations show the potential of miniaturised single-wall tests to reliably determine interlayer bond strength in FFF and give an outlook towards further improvement for this technique.