Abstract
The early-stage sintering of thin layers of micron-sized polystyrene (PS) particles, at sintering temperatures near and above the glass transition temperature Tg (~ 100°C), is studied utilizing 3D tomography, nanoindentation and confocal microscopy. Our experimental results confirm the existence of a critical particle radius (r crit ~ 1 μm) below which surface forces need to be considered as additional driving force, on top of the usual surfacetension driven viscous flow sintering mechanism. Both sintering kinetics and mechanical properties of particles smaller than r crit are dominated by contact deformation due to surface forces, so that sintering of larger particles is generally characterized by viscous flow. Consequently, smaller particles require shorter sintering. These experimental observations are supported by discrete particle simulations that are based on analytical models: for small particles, if only viscous sintering is considered, the model under-predicts the neck radius during early stage sintering, which confirms the need for an additional driving mechanism like elastic-plastic repulsion and surface forces that are both added to the DEM model.
Highlights
Recent developments in 3D printing and Additive Manufacturing [1] enable the fabrication of individualized serial products based on powders and grains
The experimental results are correlated with Discrete Element Method (DEM) simulations to calibrate a temperature– and pressure–dependent sintering model that includes the contribution of surface forces
The contact area initially grows much faster for particles with rp < rcrit than predicted by the classical sintering models [8, 9], which neglect the contribution of surface forces as well as the resultant plastic and elastic contact deformation in the early stages of sintering
Summary
Recent developments in 3D printing and Additive Manufacturing [1] enable the fabrication of individualized serial products based on powders and grains. None of the conventional techniques like, e.g., selective laser sintering allow the production of interconnected porous polymer scaffolds with a variety of pore sizes and reproducible morphology, which are commonly used to stimulate the formation of new tissue [2]. This forces the attention of scientists to the initial stage of sintering, where the porous structure is characterized by the formation of necks between individual particles. The experimental results are correlated with Discrete Element Method (DEM) simulations to calibrate a temperature– and pressure–dependent sintering model that includes the contribution of surface forces
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