Relationship Between Composition,Microstructure And Properties Of KD Series Continuous Silicon Carbide Fibers

Continuous silicon carbide?SiC?fibers have broad application prospects in the fields of advanced weaponry,aviation,aerospace and nuclear industry.But at present,the understanding of the composition,microstructure and properties of SiC fibers is not deep enough,and in particular,the researches on how the composition and microstructure of the fibers affect their properties remain inadequate.In this study,the composition and microstructure of KD series SiC fibers were systematically characterized.Then we investigated the relationships among the composition,microstructure and properties of SiC fibers,including mechanical properties at room temperature,high-temperature resistance under an inert environment and oxidation resistance in air.On the basis of these results,the performance of the SiC fibers was optimized by controlling the composition and microstructure,and by surface coating modification.The composition and microstructure of SiC fibers were analyzed and characterized by elemental analysis,EPMA,XPS,AES,SEM,AFM,XRD,HRTEM,Raman spectroscopy,SAXS and so on.The SiC fibers have surface layers,typically several to tens of nanometers thick,which are rich in carbon,oxygen,or even nitrogen.KD-12fibers,a first-generation fiber with a chemical composition of SiC_1.27O_0.44,are composed of a small amount of?-SiC microcrystallites with an average diameter of 2.5nm,amorphous SiC_xO_y,free carbon and SiO₂.KD-22 fibers,a second-generation fiber with a chemical composition of SiC_1.41O_0.04,consist of?-SiC microcrystallites with an average diameter of 8.0 nm,free carbon and a small amount of amorphous SiC_xO_y.KD-32 fibers,a third-generation fiber with a chemical composition of SiC_1.05O_0.04,contain?-SiC microcrystallites with an average diameter of 8.0 nm,a small amount of free carbon and amorphous SiC_xO_y.Pores with an average size of several nanometers are also found in the SiC fibers.The porosity of thte KD-12,KD-22 and KD-32 fibers is1.12%,6.91%and 4.59%,respectively.Fracture mirror analysis shows that the fracture of the SiC fibers originate mostly from surface defects and interior defects.For KD-22 fibers,51.4%of the fibers rupture due to surface defects,and 37.1%of fiber breakage is caused by interior defects,while the other 11.5%of the fibers show no obvious defects in their fracture surfaces.The common surface critical defects are particle inclusions,cracks,pores and strumaes,and the interior critical defects mainly include particle inclusions,individual pores and pore clusters.The strength of the SiC fibers is related to the critical defect sizes;larger defect sizes result in lower strength.The critical defect size is proportional to the mirror radius,and determination of the mirror radius is ultimately an easier procedure than the measurement of the critical defect size.Thus,the mirror radius can be used to predict fiber strength,and the mirror constant of KD-12,KD-22 and KD-32 fibers is 2.11 MPa·m^1/2,2.79 MPa·m^1/2 and 2.86 MPa·m^1/2,respectively.In addition,there is a significant negative correlation between the fiber strength and the surface roughness on one hand,and the porosity on the other hand,indicating that the increment of surface roughness and/or porosity decreases the fiber strength.The grain size also affects the fiber strength.The grain growth will strengthen the fiber strength when the grain size is small,while the grain growth will reduce the fiber strength when the grain size is large,and the fibers with grain sizes of 5⁹nm usually exhibit higher strength.The effect of oxygen content on the high temperature resistance of SiC fibers in an inert environment is significant.A higher oxygen content in SiC fibers leads to poorer high-temperature resistance because of the instability of the SiC_xO_y phase at high temperatures.The pores that are formed by the decomposition of SiC_xO_y phase are the main cause of the strength degradation in SiC fibers with high oxygen content at high temperature.The SiC fibers with high oxygen content exhibit different high temperature behavior in Ar,N₂ and vacuum environments.Compared with Ar,N₂ can inhibit the decomposition of the SiC_xO_y phase and improve the high temperature resistance of SiC fibers,while a vacuum environment can accelerate the decomposition of the SiC_xO_y phase and reduce the high temperature resistance of the fibers.The KD-12 fibers completely lose their strength after treatment at 1400? in vacuum and maintain strength of about 1.0 GPa after treatment in Ar;on the other hand,these fibers retain a strength of up to 1.6 GPa in N₂.The surface composition can influence the high-temperature resistance of SiC fibers with low oxygen content in Ar by affecting the formation of large SiC grains on the fiber surfaces.Large SiC grains are formed on the surfaces of SiC fibers with oxygen-enriched surface layers during the process of high-temperature treatment due to gas phase reactions,and the formation of the large grains is the main cause for the degradation of the fiber strength.However,annealing under N₂,a nitrided case is formed on the fiber surfaces which suppresses the pyrolysis of the SiC_xO_y phase inside the fibers and inhibits the outward transport of the pyrolytic gases.On the other hand,the decomposition of the SiC_xO_yN_z phase in the fiber surfaces is also restricted because of ambient N₂.Therefore,the formation of large surface grains is limited due to the lack of SiO when the heat-treatment is carried out in N₂,which causes that the effect of surface composition on the high temperature resistance of SiC fibers is not significant.For the second generation of SiC fibers,with low oxygen and high carbon content,the strength begins to decline rapidly at temperatures above 1500?,which is attributed to the?-SiC grain growth,the residual stresses caused by thermal expansion mismatch between the?-SiC grains and the free carbon,the large grains on the fiber surfaces as well as the pores formed by decomposition of the SiC_xO_y phase.When the fibers with a low C/Si ratio are treated in Ar at high temperature,a large number of large grains are generated on the fiber surfaces and cause serious degradation of fiber strength.These grains are mainly caused by the formation of a SiC–Si eutectic when free silicon is present in the fibers.N₂ can suppress the formation of large grains to a certain extent below 1600?,but it forms numerous large grains at temperatures above 1600? because of the decomposition of Si₃N₄,and these grains lead to a rapid decrease in the fiber strength.When the heat treatment is conducted in vacuum,the formation of large grains on the fiber surfaces does not occur because of the volatility of the gas-phase decomposition products and the eutectic liquid phase;however,a porous carbon layer forms on the fiber surfaces,and the presence of this layer and the grain growth cause the fiber strength to decrease slowly.Near-stoichiometric fibers shows the best high-temperature performance in vacuum;for instance,KD-32 fibers can maintain a strength of about 1.5GPa after treatment in vacuum at 1800?,but they totally lose their strength after treatment at 1800? in N₂,and they completely lose their strength after treatment only at 1600? in Ar.The presence of silicon in the fibers along with the existence of oxygen enriched surface layers are responsible for the rapid strength decrease of the near-stoichiometric fibers after high-temperature treatment in Ar;hence,these parameters need to be carefully controlled during the fiber preparation process.Passive oxidation of SiC fibers occurs when they are heated in the air,accompanied by the formation of a SiO₂ layer,and the oxidation process is controlled by gas diffusion.There is a several-hundred-nm-thick transition zones at the interface between the oxide layer and the fiber,and the fibers are partly oxidized in the transition region.There is a high correlation between the crystalline and the thickness of the oxide layer,and the oxide layer usually starts to crystallize when the oxide thickness is in the range of1.0¹.5?m.The fiber strength decreases as the oxide layer thickness increases,which is mainly because the thicker oxide layer results in larger stress in the oxide layer and a higher probability of large defects in the oxide layer and at the interface between the oxide layer and the fiber.However,the fibers can maintain high strength after the removal of the oxide layer.The oxidation rate constant of KD-12,KD-22 and KD-32 at 1400? in air is 2689nm²/min,1288 nm²/min and 1340 nm²/min,respectively.The oxygen content has an adverse effect on the oxidation resistance of the fibers,since the decomposition of the SiC_xO_y phase leads to the formation of pores in the oxide layer and at the interface between the oxide layer and the fiber.Generally,the effect of carbon content on the oxidation resistance of the fibers is small,because the free carbon does not preferentially oxidize,and the presence of carbon can inhibit the crystallization of the oxide layer to some extent.Moreover,the formation of an oxide layer can inhibit the decomposition of the SiC_xO_y phase;for example,the KD-12 fibers that are oxidized at1600? retain an oxygen content of 9wt%after removal of the oxide layer,while the oxygen content of KD-12 fibers almost reaches zero after treatment at 1600? in Ar.In the case of the carbon-enriched fibers with a low degree of crystallinity,only surface oxidation of the fibers occurs.However,in the case of the carbon-enriched fibers with a high degree of crystallinity,the internal areas of the fibers are also oxidized as well as the fiber surfaces.Free carbon is distributed continuously in the grain boundaries of the fibers with a high degree of crystallinity,and this free carbon can be oxidized preferentially to form many pores along grain boundaries;therefore,the oxygen can diffuse into fibers through the interconnected pores and cause internal oxidation.However,for the near-stoichiometric fibers with high crystallinity,oxidation does not occur inside the fibers.Moreover,grain growth can improve the oxidation resistance of SiC fibers if internal oxidation does not occur in the fibers.Therefore,the near stoichiometric fibers with high crystallinity show excellent oxidation resistance.The composition and microstructure of the fibers evolve during the firing process.As the firing temperature increases,the surface oxygen content gradually decrease accompanied by the formation of surface carbon-rich layers and a gradual increasing in grain size and crystallinity of the fibers.The porosity also changes with increasing firing temperature,and the fibers fired at 1150? have the highest porosity?12.13%?.The firing environment also affects the surface composition of the fibers,and the surface oxygen content is the lowest when firing in vacuum.In addition,the thickness of the carbon layer on the fiber surfaces can be adjusted by pre-oxidation of the cured fibers.There are 140-nm-thick carbon-rich layers in the surfaces of SiC fibers that are pre-oxidized at 185? and fired in vacuum at 1300?.Carbon coatings were successfully prepared on the surfaces of SiC fibers using a CVD process,and the optimal deposition parameters were selected as follows:a temperature of 1000?,a pressure of 5kPa,a C₃H₆/N₂ volume ratio of 1:3,and a flow rate of 100 sccm.The deposition rate is up to 2.7?m/h,and the prepared coatings are smooth,with a microcrystalline graphite structure.Carbon coatings with an appropriate thickness can improve the tensile strength of SiC fibers,and reduce the fiber strength dispersion.Carbon coatings can also improve the high-temperature resistance of SiC fibers in Ar because they can suppress the decomposition of the oxygen-containing phase and inhibit the formation of large grains on the fiber surfaces at high temperatures.