BN-coated fibers

Fig. 8.15 Changes in interface microstructure in SiC fiber-reinforced BMAS glass-ceramic composites induced by exposure to high temperature oxidizing environments, (a) After tensile stress-rupture experiment at 1100°C, the 90° fibers show a distinct dual layer at the BN coating-fiber interface, (b) After thermal aging for 500 h at 1200°C, a subtle double layer appears at the same site, (c) Near the composite surface, the effects of thermal aging (and oxidation) are more pronounced.24...

Fig. 5.33. Load-displaccmcnt records of monolithic mullitc, and SiC fiber reinforced mullitc composites with and without BN coating. After Singh (1988).

SEM micrograph of BN-coated Hi-Nicalon fiber for use as a reinforcement of a SiC/SiC-composite. (Source Nippon Carbon Co Ltd Japan). 

BN coating on SiC fibers or Al203 fibers reduces the thermal mismatch between fiber and matrix and also reduces fiber-matrix interfacial shear strength in ceramic matrix composites leading to higher overall strength and toughness. 

R. W. Rice, BN Coating of Ceramic Fibers for Ceramic Fiber Composites, U.S. Patent 4,642,271, February 10,1987. 

Hollow BN fibers are made by depositing a coating of BN on carbon fibers, heating in an oxidizing atmosphere to form CO or CO2, and leaving a BN coating. The reaction between BF3 and NH3 at 1200-1400°C may be used . Yarn and BN/BN composites can be made with similar techniques (see 17.3.5.6). 

FIGURE 6-6 Depth of coating oxidation in Hi-Nicalon fiber-reinforced SiC-Si matrix composites prepared by melt infiltration. The BN coating had a nominal thickness of 0.5 mm (0.02 mils). Source Brun and Luthra, 1997. 

The oxidation resistance of non-oxide fiber coatings must be improved, from the current calculated value of a few minutes (at 1,200°C [2,192°F]) for 1 pm (0.04 mils) BN coatings to the maximum possible calculated value of 100 hours for 1 pm (0.04 mils) siliea-forming coatings. 

Carbon is usually deposited by cracking hydrocarbons, such as CH4 and C2H2 at temperatures of 1,000 to 1,800°C (1,832 to 3,272°F). BN is usually deposited using a boron trichloride precursor with ammonia and a nitrogen carrier gas at temperatures of 800° to 1,600°C (1,472 to 2,912°F). Other precursors, such as borozine and BF3, have also been used. The crystallinity and deposition rate of BN increases with temperature. However, the coatings also become less uniform as temperature is increased. The non-uniformity is expeeted to be less prevalent when fiber tows are coated and most prevalent when three-dimensional composite lay-ups are coated. The chemistry of the BN coatings depends upon the... 

Pak, S.S., and R.A. Kimel. 1994. Fabrication of Y3Al50,2 fibers from water soluble polymers. U.S. PATENT 5,352,642, 1994 October 4. Parthasarathy T., C.A. Folsom, and L.P. Zawada. 1997. Combined effects of exposure to salt (NaCl) water and oxidation on the strength of uncoated and BN-coated Nicalon fibers. Unpublished work. 

Fabrication by Liquid Silicon Infiltration (reaction bonding) (LSI) A leading candidate for use in industrial gas turbine engine is a SiC matrix composite named toughened Silcomp [175]. It is produced by melt infiltration of molten silicon into a porous preform containing carbon as well as BN-coated SiC fibers (e.g. Textron SCS - 6). The composites thus produced consist of a fully dense matrix of SiC + Si, reinforced with continuous SiC fibers. Moreover, the melt infiltration process is net shape and fast. Ultimate strength and strain at ultimate strength are 220 MPa and 0.8 /o, respectively at room temperature (LSI-SiC/SiC Si). 

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