Effect Of Thermal And Mechanical Coupling On The Sintering Microstructure And Mechanical Properties Of Alumina Fibers

Continuous alumina fibers have the advantages of light weight and high strength,high temperature resistance and oxidation resistance,and have great application prospects in important fields such as the aerospace and defense industry.Sintering is a key process to regulate the fiber structure and obtain high strength alumina fibers with fine grain and dense microstructure.During the sintering process,alumina fibers undergo a series of complex processes such as phase transformation,grain growth and densification,which make it difficult to effectively regulate the microstructure of the fibers.During the evolution of the alumina fiber structure,the fiber shrinks considerably,and under untensioned conditions,the fiber is susceptible to bending,resulting in a significant reduction in the mechanical properties of the fiber.Tension sintering helps to eliminate bending in fiber morphology,but for alumina fibers,the effect of thermal and mechanical coupling on fiber structural evolution and mechanical properties and the related mechanisms have been rarely reported.This paper adopts the sol-gel method to prepare alumina fibers,studies the evolution of alumina fiber structure during the sintering process,reveal the influence mechanism of sintering schedule and its coupling with tension on the phase transition,grain growth and densification of fibers,optimize the microstructure of alumina fibers,and provide a theoretical foundation for the preparation of high performance alumina fibers The study found:（1）The alumina precursor fibers exhibit thermal decomposition and phase transition during sintering.The total mass loss of alumina precursor fibers during sintering reached 64.61wt%,while the transformation of amorphous Al₂O₃ toγ-Al₂O₃ andγ-Al₂O₃ toα-Al₂O₃ occurred at 844°C and1068°C,respectively,and the alumina fibers sintered at 1200°C for 10 min were completely converted toα-Al₂O₃.The activation energies of phase transition fromγ-Al₂O₃ toα-Al₂O₃ were calculated by the Kissinger method and the FWO method,which were 822.2 k J/mol and 804.6 k J/mol,respectively.（2）The sintering schedule is a key factor affecting the microstructure of alumina fibers.The sintering regime is a key factor affecting the microstructure of alumina fibers.After sintering at 800～1000°C,the fibers have a nano-γ-Al₂O₃ structure with grain size ranging from 10～50 nm and slowly increasing with temperature.After sintering at 1200～1400°C,the fibers have anα-Al₂O₃ structure,with a large number of nano-pores in the fibers,and the average grain size increases rapidly from 118 nm to 275 nm with increasing temperature.The fibers showed a fully dense structure after sintering at 1500°C,but some of theα-Al₂O₃ grains reached several microns.It is difficult to obtainα-Al₂O₃ fibers with a fully dense nanocrystalline microstructure at sintering temperatures of 1200°C and1550°C with regulated sintering times.The two-step sintering process helped to obtain dense nanocrystalline structures,and nearly dense alumina fibers were obtained by sintering at 900°C or 1000°C for 10 min and then at 1550°C for 20 s.The average grain size of the fibers was 156 nm and153 nm,respectively.（3）The thermal and mechanical coupling not only significantly improves the bending problem of continuous alumina fibers,but also affects the evolution of the fiber microstructure.Compared to sintering without tension,applying tension results in straight fibers with good bunching,while inhibiting fiber axial shrinkage and promoting radial shrinkage,facilitating the decomposition and removal of alumina precursor fibers,accelerating the transformation ofγ-Al₂O₃ toα-Al₂O₃,and promote the densification and grain growth of fibers.Dense fine-grained alumina fibers were obtained by using the two-step sintering process of 1000°C in the first section and 1550°C in the second section,while applying a tensile stress of 0.9 MPa to the fibers.The average tensile strength of the fibers reached 1.88 GPa,an increase of 135%relative to fibers sintered under tension-free conditions.