Carbon fiber-boron nitride matrix

Boron nitride derived from the borazine oligomer has been successfully used to replace the carbonaceous matrix in carbon fiber/carbon matrix composites (C/C) [15,18]. The carbon fiber/boron nitride matrix composites (C/BN) exhibit improved oxidation resistance over the C/C. Excellent mechanical strength and toughness have also been observed in the C/BN. As shown in Figure 2.16, both the flexural strength and modulus of pitch carbon fiber/BN specimens actually increase with extended heat treatment [18]. 

Seghi, S. Lee, J. and Economy, J. High density carbon fiber/boron nitride matrix composites Fabrication of composites with exceptional wear resistance. Carbon, 2005, 43, 2035-2043. 

Recent research has explored a wide variety of filler-matrix combinations for ceramic composites. For example, scientists at the Japan Atomic Energy Research Institute have been studying a composite made of silicon carbide fibers embedded in a silicon carbide matrix for use in high-temperature applications, such as spacecraft components and nuclear fusion facilities. Other composites that have been tested include silicon nitride reinforcements embedded in silicon carbide matrix, carbon fibers in boron nitride matrix, silicon nitride in boron nitride, and silicon nitride in titanium nitride. Researchers are also testing other, less common filler and matrix materials in the development of new composites. These include titanium carbide (TiC), titanium boride (TiB2), chromium boride (CrB), zirconium oxide (Zr02), and lanthanum phosphate (LaP04). 

Wider use of fiber-reinforced ceramic matrix composites for high temperature structural applications is hindered by several factors including (1) absence of a low cost, thermally stable fiber, (2) decrease in toughness caused by oxidation of the commonly used carbon and boron nitride fiber-matrix interface coatings, and (3) composite fabrication (consolidation) processes that are expensive or degrade the fiber. This chapter addresses how these shortcomings may be overcome by CVD and chemical vapor infiltration (CVI). Much of this chapter is based on recent experimental research at Georgia Tech. 

Prior to the matrix infiltration one ore more layers of pyrolytic carbon or boron nitride are usually applied to the fibers to provide a means of fiber debonding and toughening by pull-out and crack bridging. 

The fiber/matrix interfece is a key area of study since the interface controls if the CMC behaves as a composite or not [4]. To assure composite behavior a weak bond is desired and this is the reason Carbon or Boron Nitride is used as the fiber interface coating [S]. The interfacial shear stress is a key property since it controls (influences) the prevalent damage mechanism and the resulting non-linearity [6-7]. 

Multi-component ceramics allow the optimization of various physical properties. These include ceramics which form multi-component oxides as well as fiher-rein-forced ceramic matrix composites. However, the oxidation behavior of these materials is complex compared with the pure materials. The leading fiber-reinforced composites are silicon-based and contain continuous SiC fibers with coatings of graphitic carbon or hexagonal boron nitride. The oxidation of the fiber coating at intermediate temperatures is a major issue and models of this process are discussed for both carbon and boron nitride coatings. 

An interface material is deposited on the fibers. This acts as a debond layer between the fiber and the matrix. This interlayer consists essentially of Pyrocarbon, Boron Nitride or a multilayer ((PyC/SiC)n or (BN/SiC) n sequences). The gas precursor is CH4 for carbon, BCI3 andNHs for boron nitride. Multilayered interphases may be deposited via pulsed CVl. 

The common commercially available fibers used in composites are fiberglass, graphite (carbon), aramid, polyethylene, boron, silicon carbide, and other ceramics such as silicon nitride, alumina, and alumina silica. Many matrix choices are available, both thermosetting and thermoplastic. Each type has an impact on the processing technique, physical properties, and environmental resistance of the finished composite. The most common resin matrices include polyester, vinyl esters, epoxy, bismaleimides, polyimides, cyanate ester, and triazine. 

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