Fabrication And Performance Of Continuous Fiber-reinforced Silicon Carbide Matrix Composites By Reaction Infiltration

Continuous fiber reinforced silicon carbide matrix composites (Cf/SiC andSiCf/SiC) have been considered as perfect high-temperature structural and functionalmaterials in aerospace and fusion fields due to their high strength, high modulus,excellent thermal stability, good corrosion resistance, oxidation resistance, irradiationresistance and low activity characteristics. Up to date, the studies on the fabricationmethods of continuous fiber reinforced silicon carbide matrix composites are mainlyfocused on the polymer impregnation and pyrolysis (PIP), chemical vapor infiltration(CVI) and nano-infiltrated transient eutectoid (NITE) process, the researches on thereaction infiltration (RI) are rather scarcely reported. In order to seek new fabricationroutes for continuous fiber reinforced silicon carbide matrix composites withoutstanding performance, the investigation for the continuous fiber reinforced siliconcarbide matrix composites fabricated by RI process are developed in this thesis.Firstly, carbon matrix intermediate was designed and fabricated. According tothe modeling and calculating for the critical density and porosity of carbon matrixintermediate, it was found that either resin carbon (RC) or pyrolytic carbon (PyC), thecritical density of SiC and C fiber reinforced carbon matrix intermediate are1.68g/cm3and1.36g/cm3respectively; On the other hand, either SiC fiber or C fiber,the critical porosity of RC and PyC matrix intermediate are19%and26%respectively. The critical density and porosity are favorable for fabricating continuousfiber reinforced silicon carbide matrix composites by RI process.The PyC/SiC multilayer coating with thickness of0.2~0.3μm were fabricated onthe suiface of C fiber and SiC fiber fabric by chemical vapor deposition (CVD)process. This multilayer coating is very continuous, uniform and compact. Moreover,the interface among the coating is very distinct. The PyC coating not only modifiesthe surface state of fibers, but also provides routes for the spread of micro-crackwithin the coating due to the sandwich of PyC. SiC coating protects the fibers becauseof its high oxidation resistance.The carbon matrix intermediate was fabricated by infiltration carbonization (IC)and CVI process. It was found that the solution of phenolic resin synthesized byresorcinol and formaldehyde possess low viscosity (25mPa S), good infiltrationproperty and high efficiency of infiltration/carbonization. Moreover, it also indicatesthat the PyC is very compact with a typical turbostratic structure containing short range order and long range disorder. However, the RC is rather loose, and its structureis distinct disordered.Secondly, the RI process was researched by thermodynamics and dynamicsanalysis. It was found that the fabrication of continuous fiber reinforced siliconcarbide matrix composites by RI process is feasible. The main factors affecting RIprocess comprise physical performance of Si and Si alloy (concentration and masstransfer coefficient of Si vapor, viscosity, density and surface tension of Si alloy),wettability of Si alloy with carbon matrix intermediate, and pore size of carbon matrixintermediate. Increasing the concentration and mass transfer coefficient of Si vapor,decreasing the viscosity, density and surface tension of Si alloy, and improving thewettability of Si alloy are all favorable to enhance reaction velocity and improveeffect of RI process.Finally, continuous fiber reinforced silicon carbide matrix composites werefabricated by RI process. The results show that the properties of continuous fiberreinforced silicon carbide matrix composites fabricated by RI process are the bestwhen the density and porosity of carbon matrix intermediate are close to the criticaldensity and porosity. The maximal flexure strength and fracture toughness of SiCf/SiCfabricated by reaction infiltration Si process (1650℃,1h) using the intermediate ofSiCf/PyC, SiCf/RC and SiCf/PyC+RC are301.2MPa and17.6MPa m1/2,144MPa and4.53MPa m1/2,342MPa and10.2MPa m1/2, respectively. The maximal flexure strengthand fracture toughness of SiCf/SiC fabricated by reaction infiltration Si-Ti, Si-Zr andSi-Zr-B process (1550℃,0.5h) are307.1MPa and17.8MPa m1/2,294MPa and18.3MPa m1/2,292.0MPa and18.1MPa m1/2, repectively.As the RI holding time extended to60min, the density of SiCf/SiC fabricated byreaction infiltration Si-Ti, Si-Zr and Si-Zr-B process increased, the porosity andresidual Si decreased. However, the flexure strength and fracture toughness declinedto188.9MPa and9.2MPa m1/2,178.9MPa and8.8MPa m1/2,184.1MPa and7.0MPa m1/2, respectively. It indicates that the decompositon of SiC fiber wasenhanced due to extending the holding time, therefore the SiC fiber was severelydamaged, the property of SiC fiber decreased sharply, accordingly the mechanicalproperties of SiCf/SiC decreased.The TiC phase was formed in the SiCf/SiC fabricated by reaction infiltrationSi-Ti at the temperature of1450℃for0.5h.The ZrC phase was formed in theSiCf/SiC fabricated by reaction infiltration Si-Zr and Si-Zr-B only as the temperatureexceeding1600℃. The content of SiC, TiC and ZrC in the SiCf/SiC fabricated by reaction infiltration Si-Ti, Si-Zr and Si-Zr-B gradually increased as the temperaturerised, but the mechanical properties first increased, then decreased. The mechanicalproperties are the worst as the temperature is1650℃, the flexure strength and fracturetoughness were61.6MPa and2.8MPa m1/2,38.1MPa and1.6MPa m1/2,38.6MPa and1.8MPa m1/2. The reason is that the SiC fiber was damaged by exothermal reaction,which caused the SiC fiber acutely decomposed.The properties of Cf/SiC fabricated by reaction infiltration process areremarkably improved after the carbon matrix intermediate being graphitized. Theporosity of Cf/SiC fabricated by reaction infiltration Si decreased from1.8%to0.7%,the flexure strength increased from170.7MPa to250MPa, and the fracture toughnessincreased from10.8MPa m1/2to13.3MPa m1/2. Additionally, the porosity of Cf/SiCfabricated by reaction infiltration Si-Ti, Si-Zr and Si-Zr-B decreased from6.8%,7.2%and7.0%to1.6%,1.8%and1.4%; the flexure strength increased from195MPa,185MPa and190.1MPa to294MPa,252MPa and250.4MPa; the fracture toughnessslightly decreased from24.5MPa m1/2,22.6MPa m1/2and18.8MPa m1/2to20.1MPa m1/2,19.2MPa m1/2and17.7MPa m1/2. It also reveales that after the carbonfiber reinforced carbon matrix intermediate being graphitized, the ZrC phase wasformed in the Cf/SiC fabricated by reaction infiltration Si-Zr and Si-Zr-B at thetemperature of1550℃for0.5h. The results show that the graphitizing not onlyimproved the porosity of the carbon matrix intermediate, increased the pore specificsurface area, thereby increased the density of composites; but also improved thestructural order of carbon, enhanced the reaction activity, sequentially more SiC, TiCand ZrC were formed.The morphology, crystal structure and interfacial component of the continuousfiber reinforced silicon carbide matrix composites fabricated by RI process wereexamined by SEM, TEM, HRTEM, EDX and SAED. It was found that the interior ofSiC fiber crystalled at the temperature of1550℃, and free carbon layer existed on thesurface of SiC fiber. Moreover, PyC coating remained undamaged, and exhibitedshort range order. However, SiC coating combined with the SiC matrix, which iscomposed of high degree crystallized large-grains with different size-scale andnano-crystal, therefore, the properties of SiCf/SiC fabricated by reaction infiltration Sialloy at the temperature of1550℃are the best. It was also found that SiC fibercompletely decomposed and became nano-crystal at the temperature of1650℃.Furthermore, the free carbon on the surface of SiC fiber and the PyC coating becamecrystal. Additionally, SiC coating also combined with the reaction formed matrix, the grain size of the matrix increased, as a result, the properties of SiCf/SiC fabricated byreaction infiltration Si alloy at the temperature of1650℃are the worst.The RI process could be divided into four steps: Firstly, solid Si and Si alloy aremelted into Si vapor and liquid Si alloy, which diffuse into the pore of carbon matrixintermediate; Secondly, Si vapor and liquid Si alloy contact and react with carbonmatrix, which form continuous and compact SiC layer until Si vapor and liquid Sialloy are detached to carbon matrix. Thirdly, the fomation of SiC, TiC and ZrC carryout by the diffusion of Si, Ti, Zr and C atom through the SiC layer. Finally, pore sizeof carbon matrix intermediate decrease as the forming of SiC, TiC and ZrC, and theresidual pore is filled through condensing of Si vapor and liquid Si alloy. The time ofSi vapor and liquid Si alloy contact and react with carbon matrix is very short, thediffusion of Si, Ti, Zr and C atom through the SiC layer dominate the formation ofSiC, TiC and ZrC. Improving the reaction temperature is helpful to increase thediffusion velocity and reduce the bonding force of Si-Ti, Si-Zr of Si alloy, whichcould enhance the degree of reaction. Accordingly, the content of SiC, TiC and ZrCwould be increased.