Manufacturing Technology for Aerospace Structural Materials diameters (0. 4-0. 8 mil), allowing them to be used for a wide range of manu- facturing options, such as filament winding, aving, and braiding. A useful measure of the ability of a fiber to be formed into complex part shapes is the critical bend radius Pcr, which is the smallest radius that the fibers can be bent before they fracture. The critical bend radius Per can be calculated by multiply ing the fiber failure strain by the fiber radius. High strength, low modulus, and small diameters all contribute to fibers that can be processed using conventional textile technology. For example, while SiC monofilaments have a critical bend radii of only 7 mm, many ceramic textile multifilament fibers are less than 1 mm Both oxide and non-oxide fibers are used for ceramic matrix composites Oxide based fibers, such as alumina, exhibit good resistance to oxidizing atmospheres, but, due to grain growth, their strength retention and creep resistance at high temperatures is poor. Oxide fibers can have creep rates of up to two orders of magnitude greater than non-oxide fibers. Non-oxide fibers, such as C and SiC, have lower densities and much better high temperature strength and creep retention than oxide fibers but have oxidation problems at high temperatures Ceramic oxide fibers are composed of oxide compounds, such as alumina (AL2O3) and mullite(3Al2O3-2SiO2). Unless specifically identified as single crystal fibers, oxide fibers are polycrystalline. 3M,'s Nextel family of fibers are by far the most prevalent. Nextel is produced by a sol-gel process, in which a sol-gel solution is dry spun into fibers, dried, and then fired at 1800-2550F. Nextel 3 12, 440, and 550 were designed primarily as thermal insulation fibers Both Nextel 312 and 440 are aluminosilicate fibers containing 14% boria (B,O,) and 2% boria, respectively, which means that both of these fibers contain both crystalline and glassy phases. Although boria helps to retain high temperature short time strength, the glassy phase also limits its creep strength at high tem- peratures. Since Nextel 550 does not contain boria, it does not contain a glassy hase and exhibits better high temperature creep resistance, but lower short time high temperature strength For composite applications, Nextel 610 and 720 de not contain a glassy phase and have more refined a-Al2O3 structures, which allows them to retain a greater percentage of their strength at elevated temper atures. Nextel 610 has the highest room temperature strength due to its fine grained single phase composition of a-AL2O3, while Nextel 720 has better creep resistance due to the addition of sio, that forms a-Al,O/mullite, which reduces grain boundary sliding. As a class, oxide fibers are poor thermal and electrical conductors, have higher CTE, and are denser than non-oxide fibers. Due to the presence of glass phases between the grain boundaries, and as a result of grain growth, oxide fibers rapidly lose strength in the 2200-2400 F rang Ceramic non-oxide fibers are dominated by silicon carbide based compo- sitions. All of the fibers in this category contain oxygen. Nippon's Nicalon series of Sic fibers are the most prevalent Nicalon fibers are produced by a lymer pyrolysis process that results in a structure of ultra fine B-Sic pa ticles (1-2 nm)dispersed in a matrix of amorphous SiO2 and free carbonManufacturing Technology for Aerospace Structural Materials diameters (0.4-0.8 mil), allowing them to be used for a wide range of manu￾facturing options, such as filament winding, weaving, and braiding. A useful measure of the ability of a fiber to be formed into complex part shapes is the critical bend radius Pcr, which is the smallest radius that the fibers can be bent before they fracture. The critical bend radius Per can be calculated by multiply￾ing the fiber failure strain by the fiber radius. High strength, low modulus, and small diameters all contribute to fibers that can be processed using conventional textile technology. For example, while SiC monofilaments have a critical bend radii of only 7 mm, many ceramic textile multifilament fibers are less than 1 mm. Both oxide and non-oxide fibers are used for ceramic matrix composites. Oxide based fibers, such as alumina, exhibit good resistance to oxidizing atmospheres, but, due to grain growth, their strength retention and creep resistance at high temperatures is poor. Oxide fibers can have creep rates of up to two orders of magnitude greater than non-oxide fibers. Non-oxide fibers, such as C and SiC, have lower densities and much better high temperature strength and creep retention than oxide fibers but have oxidation problems at high temperatures. Ceramic oxide fibers are composed of oxide compounds, such as alumina (A1203) and mullite (3A1203-2SIO2). Unless specifically identified as single crystal fibers, oxide fibers are polycrystalline. 3M's Nextel family of fibers are by far the most prevalent. Nextel is produced by a sol-gel process, in which a sol-gel solution is dry spun into fibers, dried, and then fired at 1800-2550 ~ F. Nextel 312, 440, and 550 were designed primarily as thermal insulation fibers. Both Nextel 312 and 440 are aluminosilicate fibers containing 14% boria (B203) and 2% boria, respectively, which means that both of these fibers contain both crystalline and glassy phases. Although boria helps to retain high temperature short time strength, the glassy phase also limits its creep strength at high tem￾peratures. Since Nextel 550 does not contain boria, it does not contain a glassy phase and exhibits better high temperature creep resistance, but lower short time high temperature strength. For composite applications, Nextel 610 and 720 do not contain a glassy phase and have more refined ce-A1203 structures, which allows them to retain a greater percentage of their strength at elevated temper￾atures. Nextel 610 has the highest room temperature strength due to its fine grained single phase composition of ce-A1203, while Nextel 720 has better creep resistance due to the addition of SiO2 that forms ce-A1203/mullite, which reduces grain boundary sliding. 4 As a class, oxide fibers are poor thermal and electrical conductors, have higher CTE, and are denser than non-oxide fibers. Due to the presence of glass phases between the grain boundaries, and as a result of grain growth, oxide fibers rapidly lose strength in the 2200-2400~ F range. Ceramic non-oxide fibers are dominated by silicon carbide based compo￾sitions. All of the fibers in this category contain oxygen. Nippon's Nicalon series of SiC fibers are the most prevalent. Nicalon fibers are produced by a polymer pyrolysis process that results in a structure of ultra fine /3-SIC par￾ticles (~l-2nm) dispersed in a matrix of amorphous SiO 2 and free carbon. 466