Interface control and design for SiC fiber-reinforced titanium aluminide composites
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The effectiveness of several coating systems which were used as a diffusion barrier for the SiC fiber-reinforced titanium aluminide composites was investigated. TaC, TiC, TiB 2 , B, C/graded TiB 2 , and Ag/Ta were applied to the SiC fiber via chemical vapor deposition and physical vapor deposition. The interfacial compatibility, interfacial stability, thermal residual stress, interfacial bond strength, and the transverse fracture behavior of the composites with coated fibers were characterized and determined. The results show that none of the above coating systems can satisfy the requirements for a strong, tough, and damage-tolerant SiC fiber-reinforced titanium aluminide composite. Several multilayer, multifunctional coating systems are proposed.
I. INTRODUCTION SiC fiber-reinforced titanium aluminide (Ti3Al) composites have recently received extensive attention due to their promising potential as an advanced gas turbine engine material. These composites are superior to conventional titanium-alloy-based composites in many aspects such as better high-temperature specific stiffness, strength, and improved high-temperature environmental resistance. However, before these materials can be used in structures extensively, a number of critical issues must be resolved. A major problem associated with these composites is the incompatibility between the ceramic fiber and the intermetallic matrix. A previous study1 showed that excessive chemical reactions occurred at the fiber-matrix interface during the consolidation process, leading to the formation of a brittle, multiphase reaction zone around the fiber. This brittle layer is detrimental to the mechanical properties of the composites due to premature cracking in this layer upon loading. Depending on the interfacial bond strength, loading condition, and service environment, the microcrack formed at the interface will propagate toward the fibers and/or matrix, which may lead to the catastrophic failure of the composite. Furthermore, due to the large mismatch of the coefficients of thermal expansion (CTE) between the fiber and matrix, a significant residual stress will be induced during the consolidation process. The residual stress, coupled with the limited ductility of the matrix (1 to 2% at room temperature), can lead to the premature matrix cracking near the interface during processing and thermal cycling. Another critical mechanical characteristic of interface is the fiber/matrix bonding. The transverse properties of the SCS-6/Ti3Al composite are shown to be worse than those of the monolithic matrix alloy. The 2040
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 8, Aug 1993
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weak transverse properties will severely restrict the loadcarrying capability of the off-axis piles. Therefore, to tailor a composite based upon the brittle titanium aluminide matrix with satisfactory strength and toughness, preventing the interfacial chemical interaction as well as controlling the bonding and stress state in the interface are the key issues. One
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