Raman Investigation Of SiC Fiber Reinforced Titanium Matrix Composites

Owing to its advantages of high specific strength, stiffness and microstructural stability in high temperature environment, continuous chemical vapor deposition（CVD） SiC fiber reinforced titanium matrix composites（TMCs） acquires vast interests of the aviation society, and are considered a new kind of high temperature-resistant structural material with broad application prospects.Since the system of SiC fiber and titanium alloy matrix is chemical non-equilibrium, chemical reaction between SiC fiber and matrix alloy and related microstructural evolution is inevitable, which affects mechanical degradation and service duration of the composites. Traditional research on the microstructure of SiC fiber is mainly grain size measurement and microstructural defects observation, which depends on transmission electron microscope（TEM）. Interfacial reaction products only can be located and identified by TEM and selected area electron diffraction（SAED）. Considering its high demand on sample preparation, TEM research is low efficient and can’t be used to conduct processing parameter improvement. In some researches, Raman spectroscopy is also used in study of SiC fibers. Furthermore, interfacial reaction products of TMCs are Raman active. Moreover, Raman spectroscopy is a kind of method of high efficiency, which requires little sample preparation and does no damage to the samples. Therefore, in this study, Raman spectroscopy is tested as a characterization method to investigate the microstructure of SiC fiber and interfacial reaction products of the TMCs and related evolution after high temperature thermal exposure. At the same time, TEM and scanning electron microscope（SEM） with energy dispersive spectrometer（EDS） are used to assist and testify Raman results.TEM investigation on the tungsten-core SiC fiber reveals that the sequence of W-SiC interfacial reaction products is W/W₂C/W₅Si₃/SiC. This sequence mainly results from CH₃SiCl₃ decomposition during high temperature CVD process and subsequent diffusional reaction. Grain size of the SiC section varies, which increases first then decrease from inner side to outer surface along fiber radius. Moreover, there exists high density of microstructural defects such as stacking faults and twins, whose distribution is almost random in SiC grains. The Raman linescanning result and TEM observation along fiber radius indicates that information of SiC TO（transverse optical） mode can be used to measure variation of grain size and defects. Meanwhile, co-deposition of Si embedded in some areas of SiC and residual stress related peak shift are also detected by Raman spectrometer. Raman investigation along thermal exposed fiber radius indicates that high temperature stability depends on SiC microstructure which varies along fiber radius.It is found that the thickness of carbon coating deposited with two stage CVD process under 900℃ using C₃H₈+C₂H₂+H₂ as reactant is stable（about 3μm）, which can be used to continuous carbon coating manufacturing. Debonding occures between double layer carbon coating during tensile loading of the composite, which is propitious to fiber pullout and can improve toughness and fatigue lifetime of the composites. Raman and TEM analysis indicates that microstructure of pyrolitic carbon made of C₃H₈+C₂H₂+H₂ and C₂H₂+H₂ is similar. The average size of pyrolitic carbon nanocrystal calculated using the ratio of D band and G band is close to that observed through TEM. Meanwhile, Raman results indicate that the microstructure of pyrolitic carbon is stable at the temperature below 900℃.Raman investigation of the interfacial reaction product of SiCf/C/Mo/Ti₆Al₄ V composite reveals that after 200 h thermal exposure under 700-800℃, TiCx is the only reaction product. At the same time free carbon is also detected within the interfacial reaction layer. After 900℃200h thermal exposure, the sequencing of interfacial reaction products become SiC/Ti₅Si₃ /TiCx/ Ti₆Al₄ V, which is consistent with main conclusion of TEM and SAED. However, a thin reaction layer of Ti₃SiC₂ is also detected by TEM. Omission of Ti₃SiC₂ layer results from similarity between Raman spectra of Ti₅Si₃ and Ti₃SiC₂, and the resolution limit of Raman spectrometer. Furthermore, after 800-900℃200h thermal exposure, carbide particles are observed in the matrix, which originates from diffusion of Mo and C atoms. Raman and TEM investigation indicates that the particles can be divided into Ti₃AlC₂ and TiC_x. Moreover, there exist microstructural defects in these particles, and the formation of Ti₃AlC₂ particles is closely related to neighbouring TiC_x particles.Raman result of interfacial reaction products of as-prepared SiC_f/Ti43Al9 V composite is consistent with TEM analysis that the products sequencing is SiC_f/TiC_x/Ti₂AlC/Ti43Al9 V. High resolution TEM observation result indicates that there exists large amount of stacking fault and microtwins in TiC_x grains, which probably activates Raman signal of TiC_x grain. Raman peak width of Ti₂ AlC broadens variously, which originates from microstructural defects in the grains. Raman result indicates that after 800-900℃200h thermal exposure, reaction product gradually transforms to Ti₂ AlC with temperature increasing.