Oxidation Behaviors Of Silicon Carbide Fibers At High Temperature

The polycarbosilane-derived continuous silicon carbide fibers?SiC_f?have high strength,high modulus,excellent high temperature performance and excellent weaving ability,which have been used as reinforcement in ceramic matrix composites for application in the fields of aviation,aerospace,defence equipments,nuclear energy and many other high temperature fields.As the SiC_f-CMCs was often exposed in environments containing oxygen and water,the oxidizing medium could diffuse through the matrix defects to SiC fiber surface,resulting in the fiber oxidation and strength degradation.In this work,choosing Nicalon and Hi-Nicalon as references,domestic three types of precursor-derived SiC fibers,KD-A,KD-B and KD-C fibers were selected as the research objects.KD-A fiber was Si-C-O fibers of the first generation through curing in air,KD-B fiber was SiC fibers of the second generation fibers by anaerobic curing,and KD-C fiber was near-stoichiometric SiC fiber with pyrolysis process under hydrogen.The stability of KD fibers was characterized,and the evolutions of the structures and properties of KD fibers during the heat treatment in air and simulated environment of aeroengine combustion chamber were studied systematically.Also,the effect of water vapor on the oxidation process of KD fibers was analyzed,and the differences of oxidation resistance between KD fibers and Nicalon fibers were compared as well.Firstly,the diameter,element content,monofilament tensile strength and Young's modulus of KD-A,KD-B and Nicalon fibers were characterized.The results showed that KD-B fiber exhibited the best stability and smallest fluctuations,while the fluctuation of KD-A and Nicalon fibers was relatively large.The oxidation kinetics of KD-B fiber was analyzed and a certain correspondence between changes in weight gain ratio and oxide layer thickness was witnessed after the heat treatment in air.With the increment of heat treatment time,the weight gain ratio and the silica film thickness increased mildly below 1300?,while the changes became magnificent at 1400?.Using the Arrhenius equation and oxidation rate,the activation energy of oxidation of the three types fibers during the diffusion controlled process was calculated as E_KD-A=131KJ/mol,E_KD-B=103KJ/mol,E_Nicalon=158KJ/mol,respectively.Secondly,KD-A,KD-B,Nicalon and Hi-Nicalon fibers were exposed in air at1000-1500?for 1-100h.The results showed that the strength degradation of KD-B and Hi-Nicalon fibers gradually slowed down with extending heat treatment time.Heat treatment in oxidizing environment below 1500?had little effect on the?-SiC crystallization,while the crystallization from amorphous silica into?-cristobalite was greatly affected by both temperature and time of the heat treatment.The KD-C fiber was selected to study the influence of the oxidizing time and temperature on the oxidation of the fiber.By raising heat treatment temperature,the energy of oxidation system increased,leading to acceleration of the oxidation rate and reduction of the fiber strength.With extending heat treatment time,the oxide film became thicker and the diffusion resistance of oxidative medium increased.Since the oxide film on the surface could prevent the inner fiber from further oxidization,the oxidation rate and the degradation of strength were reduced.Due to the different thermal expansion coefficients of the oxide film and the fiber core,stress formed and cracks occurred in silica layer when the oxide film became thick enough.So,the diffusion resistance of oxidative medium reduced and oxidation rate slightly increased.Finally,using our own experimental platform ofP_{O2:H2 O:N2}?28?8:14:78KPa to simulate the environment of aeroengine combustor,the performances of KD-B,KD-C and Hi-Nicalon fibers in the simulated environment were analyzed.The results showed that KD-B and Hi-Nicalon fibers exhibited similar tendency of strength degradation after heat treatment in the water-oxygen environment,and that the residual strength of KD-B fiber was about 0.2GPa lower than that of Hi-Nicalon fiber.Compared with the heat treatments in dry air,being exposed in the simulated environment,KD-B fiber performed slightly lower tensile strength,while Hi-Nicalon and KD-C fibers exhibited higher strength.KD-B and KD-C fibers possessed higher oxygen content,thicker silica film,and larger SiO₂ grain after being oxidized in the water-oxygen environmentr.The active energies of oxidation of the KD-C fiber in water-oxygen and dry air were calculated as E_wet=200KJ/mol and E_dry=135KJ/mol,indicating accelerated oxidation of the fibers,promoted crystallization and growth of?-cristobalite,and increased active energy by water vapor in the simulated environment.Compared with the heat treatment in dry air,more gases and larger internal stress in the oxide film were induced by the reaction under water vapor,so the cracks were more likely to occur in the oxide film.As the residue strength of SiC fibers was dramatically affected by oxide film morphology,thickness and crystallization,the effect of water vapor on the performance of SiC fibers during the oxidation was multifaceted and complicated.