Preparation And Characterization Of Boron Nitride Coating On Silicon Nitride Fibers

In this dissertation,aiming at the high temperature resistant,load bearing and electromagnetic transparence properties of high-performance wave transparent materials,the boron nitride(BN)coating on surface of silicon nitride fiber was prepared by liquid precursor impregnation and pyrolysis(PIP)process with polyborazine as the precursor.The microstructure and mechanical properties of the coated fibers were investigated and evaluated.The following research was designed and conceived,the specific content is as follows:First of all,Polypropylene was a kind of colorless and transparent solution that prepared by direct reaction of boron trichloride(BCl3)with hexamethyldisilazane(HMDZ)at low temperature.The synthesis,cross linking and pyrolysis of BN precursors were investigated.It can be seen from the characterization that with the increase of temperature,the molecular weight of the polymerization becomes larger,the chemical bond were broken and rearranged,and cross-linked into a network structure.With the increase of pyrolysis temperature,the ceramic yield was decreased,the crystalline and the stability of pyrolytic product increased.The ceramic yield of the polyborazine polymer was about 34%under the condition of ammonia pyrolysis at 900 ?.Secondly,the influence of heat treatment temperature and times on the mechanical properties of silicon nitride fiber was analyzed by mechanical properties test.The morphological changes of the fiber before and after heat treatment were characterized by SEM.The results show that the fiber surface was smooth and had no obvious defects.With the increase of temperature,the mechanical properties of the fiber was reduced and the retention rate of the fiber were 87.01%after 900 ? for 1h.Under the condition of 900?-1h,the times of heat treatment had little effect on the mechanical properties of silicon nitride fiber.The BN coating was prepared on the surface of silicon nitride fiber by the PIP process.The morphology and mechanical properties of the coated fiber were evaluated.It was found that the non-polymerized liquid phase precursor and the silicon nitride fiber had good wettability,and the surface coating was smooth,thick but local aggregation.The tensile strength of the coated fiber decreased with the increase of the pyrolysis temperature,the tensile strength were 560MPa after pyrolysis at 900 ?,and the retention rate were 53.94%.With the increase of the pyrolysis temperature,the reactive groups in the BN precursor reacted with the reactive groups such as the hydroxyl groups on the surface of the fiber,and the coating on the surface of the fiber under radial and axial shrinkage,resulting in the strength of decline for the fibers.In order to further improve the mechanical properties of the coated fiber and optimize the coating morphology,the polyborazine was polymerized into a solid phase precursor,and it was formulated into solution.Then,the multi-stage impregnation process of the low concentration precursor solution was used,to reduce the interface damage of fibers.After Optimized,the optimum coating conditions were using a precursor concentration of 5(wt)%with 2 cycles of the PIP process.The thickness of the coating was about 500 nm and the coated fibers were smooth and uniform without spalling.Compared with that of the blank fibers under the same process,the tensile strength of the obtained coated fibers were 676MPa,corresponding to a strength retention of 74.84%.