Fiber-Reinforced Tubular Composites by Chemical Vapor Infiltration*
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FIBER-REWFORCED TUBULAR COIPSiTIS BY CHEMICAL VAPOR WNIFULTRATION 0 D. P. Stinton, R. A. Lowden, and T. M. Besmann Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
ABSTRACT A forced-flow thermal-gradient chemical vapor infiltration process has been developed to fabricate composites of thick-walled tubular geometry common to many components. Fibrous preforms of different fiber architectures (3-dimensionally braided and filament wound) have been investigated to accommodate components with different mechanical property requirements. This paper will discuss the fabrication of tubular, fiber-reinforced SiC matrix composites and their mechanical properties. INTRODUCTION Fiber-reinforced SiC-matrix composites appear promising for gas turbine applications because of their high strength at elevated temperature, light weight, thermal shock resistance, damage tolerance, and oxidation and corrosion resistance. However, incorporation of continuous ceramic fibers into ceramic matrices without significant damage to the fibers is difficult. Hot-pressing of fiber-reinforced composites is impractical because the extremes of temperature and pressure weaken the continuous fibers. Cold-pressing and sintering routes to the fabrication of composites are of no value because typical sintering temperatures greatly exceed the temperature limit of the fibers [1,2]. Because of these limitations several novel impregnation processes have been developed. One such process impregnates a fibrous preform with liquid precursors that transform to ceramic materials on heat treating [3-5]. A second process impregnates a fibrous preform with extremely fine metallic silicon which converts to silicon nitride when reacted with nitrogen gas at elevated temperatures [6,7]. The greatest success has been achieved by vapor-phase processing, leading to a class of techniques termed chemical vapor infiltration (CVI). Two distinctly different vapor phase processes have been used to fabricate matrices: isothermal CVI, [8-11] which is used commercially, and forced CVI under development at Oak Ridge National Laboratory (ORNL) [12-15]. Fiber-reinforced composites are being considered for combustors, burner tubes, heat exchangers, headers, hot-gas filters and even rotors for stationary gas turbine engines. Unfortunately, neither of the CVI processes described above has demonstrated the ability to fabricate thick-walled tubular shapes appropriate for turbine engines. Isothermal CVI is ideal for the fabrication of thin-walled structures including complex shapes, however, infiltration times become extremely long for thick cross sections. The forced CVI process has been developed for thick-wall plates, however, very few tubular shapes have been infiltrated. Therefore, the focus of this investigation was the development of the forced CVI process for the fabrication of thick-walled tubular composites.
FIBROUS PREFORMS Nicalon fiber preforms of a tubular geometry (2.5 to 3.8 cm ID; 0.6 cm wall thickness; =15 cm long) were fabricated with different fiber architectures.
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