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Frittage de pièces de Polytétrafluoroéthylène (PTFE) compacté : Mécanismes physiques et modèles

Abstract : Polytetrafluoroethylene (PTFE) is a semi-crystalline polymer with many outstand- ing properties including excellent thermal resistance, extremely low friction coefficient and high corrosion resistance. These advantages make it suitable for many applications.It is shaped by processes similar to those used for metal powders. In one of them, the PTFE powder is uniaxially compacted into cylindrical parts. The next step is the compacted powder sintering process, in which the parts are heated above the melting temperature of the polymer. During this thermal cycle, the parts undergo large strains caused by the melting and the recrystallization of PTFE. Thermal gradients coupled with these strains induce mechanical incompatibilities that can lead to damage or even failure of the parts.This study provides a more detailed understanding of the physical mechanisms at work dur- ing the sintering of compacted PTFE through microstructural observations and analyses, and thermal and mechanical characterization tests. The relaxation of residual stresses, the closure of porosity and a secondary crystallization mechanism have thus been highlighted and generate macroscopic stress-free strain (called eigenstrain). The mechanical properties of the material as function of temperature have been also determined during sintering. A model has been devel- oped to take into account these observations. It has been integrated into a thermomechanical simulation of the sintering process of a reference part. The model has been validated by com- paring the simulation results with measurements from a laboratory experiments.Finally, a finite element numerical simulation tool has been developed. It is used to determine the stresses and strains as well as the distribution of crystallization rates and thermomechanical properties within a PTFE part compacted during the sintering cycle. In the long term, this tool could make it possible to optimize the industrial process parameters to reduce sintering time, for example, while ensuring that the parts are undamaged and have the required properties.
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Gabriel Guenoun. Frittage de pièces de Polytétrafluoroéthylène (PTFE) compacté : Mécanismes physiques et modèles. Mécanique des matériaux [physics.class-ph]. Université Paris-Saclay, 2019. Français. ⟨NNT : 2019SACLN054⟩. ⟨tel-02504877⟩

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