Ceramic Matrix Composites Fiber manufacture consists of synthesizing a spinnable polymer; spinning the polymer into a precursor fiber; curing the fiber to crosslink it so that it will then pyrolyzing ber into a ceramic fiber. Nicalon's high oxygen content( 12%)causes an instability problem above 2200F, by producing gaseous carbon monoxide. Therefore, a low oxygen content(0.5%)variety, called Hi-Nicalon, was developed that has improved thermal stability and creep resistance. The oxygen content is reduced by radiation curing using an electron beam in a helium atmosphere. Their latest fiber, Hi-Nicalon-S, has an even lower oxygen content(0.%)and a larger grain size(20-200nm)for enhanced creep resistance. 6 Another SiC type fiber with TiC in its structure is Tyranno, produced by Ube Industries. It contains 2 weight percent titanium to help inhibit grain growth at elevated temperatures. In the Tyranno ZM fiber, zirconium is used instead of titanium to enhance creep strength and improve the resistance to salt corrosion A new silicon carbide fiber, Sylramic-iBN, contains excess boron in the fib which diffuses to the surface where it reacts with nitrogen to form an in boron nitride coating on the fiber surface. The removal of boron from fiber bulk allows the fiber to retain its high tensile strength while significantly improving its creep resistance and electrical conductivity. Although the creep strengths of the stoichiometric fibers, such as Hi-Nicalon-S, Tyranno SA, and Sylramic, are better than that of the earlier non-stoichiometric silicon carbide fibers, their moduli are 50% higher and their strain-to-failures are 1/3 lower, which adversely impacts their ability to toughen eramic matrices. However, of the commercial fibers currently available, the dvanced Nicalon and Tyranno fibers are the best in terms of as-produced strength, diameter, and cost for ceramic matrix composites for service tempera tures up to~2000°F3 The oxide based fibers are typically more strength limited at high tempera tures than the non-oxide fibers; however, oxide fibers have a distinct advantage in having a greater compositional stability in high rature oxidizing ronments. While fiber creep can be a problem with both oxide and non Is a o, it is generally a bigger problem with the oxide fibers. Fiber grain is a compromise, with small grains contributing to higher strength, while large grains contribute to better creep resistance 10.2 Matrix Materials The selection of a ceramic matrix material is usually governed by thermal sta bility and processing considerations. The melting point is a good first indication of high temperature stability. However, the higher the melting point, the more difficult it is to process. Mechanical and chemical compatibility of the matrix with the reinforcement determines whether or not a useful composite can be fabricated. For some hisker reinforced ceramics, even moderate reactions withCeramic Matrix Composites Fiber manufacture consists of synthesizing a spinnable polymer; spinning the polymer into a precursor fiber; curing the fiber to crosslink it so that it will not melt during pyrolysis; and then pyrolyzing the cured precursor fiber into a ceramic fiber. 5 Nicalon's high oxygen content (12%) causes an instability problem above 2200 ~ F, by producing gaseous carbon monoxide. Therefore, a low oxygen content (0.5%) variety, called Hi-Nicalon, was developed that has improved thermal stability and creep resistance. The oxygen content is reduced by radiation curing using an electron beam in a helium atmosphere. Their latest fiber, Hi-Nicalon-S, has an even lower oxygen content (0.2%) and a larger grain size (20-200 nm) for enhanced creep resistance. 6 Another SiC type fiber with TiC in its structure is Tyranno, produced by Ube Industries. It contains 2 weight percent titanium to help inhibit grain growth at elevated temperatures. In the Tyranno ZM fiber, zirconium is used instead of titanium to enhance creep strength and improve the resistance to salt corrosion. A new silicon carbide fiber, Sylramic-iBN, contains excess boron in the fiber, which diffuses to the surface where it reacts with nitrogen to form an in situ boron nitride coating on the fiber surface. The removal of boron from the fiber bulk allows the fiber to retain its high tensile strength while significantly improving its creep resistance and electrical conductivity. 7 Although the creep strengths of the stoichiometric fibers, such as Hi-Nicalon-S, Tyranno SA, and Sylramic, are better than that of the earlier non-stoichiometric silicon carbide fibers, their moduli are 50% higher and their strain-to-failures are 1/3 lower, which adversely impacts their ability to toughen ceramic matrices. 8 However, of the commercial fibers currently available, the advanced Nicalon and Tyranno fibers are the best in terms of as-produced strength, diameter, and cost for ceramic matrix composites for service tempera￾tures up to ~ 2000 ~ F. 3 The oxide based fibers are typically more strength limited at high tempera￾tures than the non-oxide fibers; however, oxide fibers have a distinct advantage in having a greater compositional stability in high temperature oxidizing envi￾ronments. While fiber creep can be a problem with both oxide and non-oxide fibers, it is generally a bigger problem with the oxide fibers. Fiber grain size is a compromise, with small grains contributing to higher strength, while large grains contribute to better creep resistance. 10.2 Matrix Materials The selection of a ceramic matrix material is usually governed by thermal sta￾bility and processing considerations. The melting point is a good first indication of high temperature stability. However, the higher the melting point, the more difficult it is to process. Mechanical and chemical compatibility of the matrix with the reinforcement determines whether or not a useful composite can be fabricated. For some whisker reinforced ceramics, even moderate reactions with 467