Manufacturing Technology for Aerospace Structural Materials along with some covalent bonding. These bonds, in particular the strongl directional covalent bond, provide a high resistance to dislocation motion and go a long way in explaining the brittle nature of ceramics. Since ceramics and carbon-carbon composites require extremely high processing temperatures compared to polymer or metal matrix composites, ceramic matrix composites are difficult and expensive to fabricate Reinforcements for ceramic matrix composites are usually carbon, oxide or or pal arily in carbon-carbon composites, while oxide fibers(such as alumina)or non-oxide fibers(such as silicon carbide) are used in glass, glass-ceramic and crystalline ceramic matrices. Most high performance oxide and non-oxide con- tinuous fibers are expensive, further leading to the high cost of ceramic matrix composites. The cost and great difficulty of consistently fabricating high quality ceramic matrix composites has greatly limited their applications to date. 10.1 Reinforcements Fibers used in ceramic matrix composites, classified according to their diameters and aspect ratios, fall into three general categories: whiskers, monofilaments, and textile multifilament fibers. Reinforcements in the form of particulates and platelets are also used. A summary of a number of oxide and non-oxide continuous ceramic fibers is given in Table 10.2, Whiskers are nearly perfect single crystals with strengths approaching the theoretical strength of the material. They are usually 1 um in diameter, or less, and up to 200um long. As reinforcements, it is their size and aspect ratio (length/diameter) that determines their strengthening effect. SiC, Si3N4, and AL2O3 are the most commonly used whiskers for ceramic matrix composites Monofilament SiC fibers are produced by chemical vapor deposition of sili con carbide onto a 1.3 mil diameter amorphous carbon substrate, resulting in large 5.5 mil diameter fiber. A carbon substrate is preferred to a tungsten sub strate, because, above 1500%F, silicon carbide reacts with tungsten, resulting in fiber strength degradation. During manufacture, a l um thick layer of pyrolytic graphite is deposited on a resistance heated carbon substrate to provide a smooth surface and control its electrical conductivity. The coated substrate is then chem ically vapor deposited (CVD) using a mixture of silane and hydrogen gases On exiting the reactor, a thin layer of carbon and silicon carbide is applied to provide improved handleability; act as a diffusion barrier for reducing the reac- tion between the fiber and the matrix; and heal surface flaws for improved fiber trength. Since these monofilaments are large, they can tolerate some surface eaction with the matrix without a significant strength loss. However, their large diameter also inhibits their use in complex structures due to their large diameter and high stiffness, which limits their ability to be formed over tight radi Ceramic textile multifilament fibers, in tow sizes ranging from 500 to fibers, are available that combine high temperature properties with
Manufacturing Technology for Aerospace Structural Materials along with some covalent bonding. These bonds, in particular the strongly directional covalent bond, provide a high resistance to dislocation motion and go a long way in explaining the brittle nature of ceramics. Since ceramics and carbon-carbon composites require extremely high processing temperatures compared to polymer or metal matrix composites, ceramic matrix composites are difficult and expensive to fabricate. Reinforcements for ceramic matrix composites are usually carbon, oxide or non-oxide ceramic fibers, whiskers, or particulates. Carbon fiber is used primarily in carbon-carbon composites, while oxide fibers (such as alumina) or non-oxide fibers (such as silicon carbide) are used in glass, glass-ceramic and crystalline ceramic matrices. Most high performance oxide and non-oxide continuous fibers are expensive, further leading to the high cost of ceramic matrix composites. The cost and great difficulty of consistently fabricating high quality ceramic matrix composites has greatly limited their applications to date. 10.1 Reinforcements Fibers used in ceramic matrix composites, classified according to their diameters and aspect ratios, fall into three general categories: whiskers, monofilaments, and textile multifilament fibers. Reinforcements in the form of particulates and platelets are also used. A summary of a number of oxide and non-oxide continuous ceramic fibers is given in Table 10.2. Whiskers are nearly perfect single crystals with strengths approaching the theoretical strength of the material. They are usually 1 i~m in diameter, or less, and up to 200p~m long. As reinforcements, it is their size and aspect ratio (length/diameter) that determines their strengthening effect. 2 SiC, Si3N 4, and A120 3 are the most commonly used whiskers for ceramic matrix composites. Monofilament SiC fibers are produced by chemical vapor deposition of silicon carbide onto a 1.3 mil diameter amorphous carbon substrate, resulting in a large 5.5 mil diameter fiber. A carbon substrate is preferred to a tungsten substrate, because, above 1500 ~ F, silicon carbide reacts with tungsten, resulting in fiber strength degradation. During manufacture, a 1 Ixm thick layer of pyrolytic graphite is deposited on a resistance heated carbon substrate to provide a smooth surface and control its electrical conductivity. The coated substrate is then chemically vapor deposited (CVD) using a mixture of silane and hydrogen gases. On exiting the reactor, a thin layer of carbon and silicon carbide is applied to provide improved handleability; act as a diffusion barrier for reducing the reaction between the fiber and the matrix; and heal surface flaws for improved fiber strength. 3 Since these monofilaments are large, they can tolerate some surface reaction with the matrix without a significant strength loss. However, their large diameter also inhibits their use in complex structures due to their large diameter and high stiffness, which limits their ability to be formed over tight radii. Ceramic textile multifilament fibers, in tow sizes ranging from 500 to 1000 fibers, are available that combine high temperature properties with small 464
le 10. 2 Properties of Selected Continuous Ceramic Fibers Tensile Tensile Critical bend Sic on c monofilament 7.0 Nextel 312 62Al2O3-14B2O3-15SO 0.4 Nextel 440 70Al2O3-2B, O3-28Sio Nextel 480 0AL2O3-2B2O3-28SiO 3.05 0.40.5 Nextel 550 73AL2O3-27SiO2 0.48 extel 610 Nextel 720 85A2O,, 99 a-Al,O 2272848082967856 0.5 3.60 85Al2O3-155O2 0400 Nicalon nl20o 57Si3lC-120 Hi-Nicalon Hi· Nicalon-S 689Si-309C0.20 Tyranno LOX M 554Si-324C-10.20-2Ti Tyranno ZM 55.3Si-339C9.80-1Zr 666Si-28.5C-23B-2.ITi0.80-04N 444
Table 10.2 Properties of Selected Continuous Ceramic Fibers Fiber SCS-6 Nextel 312 Nextel 440 Nextel 480 Nextel 550 Nextel 610 Nextel 720 Almax Altex Nicalon NL200 Hi-Nicalon Hi-Nicalon-S Tyranno LOX M Tyranno ZM Sylramic Tonen Si3N 4 Composition Tensile Strength (ksi) Tensile Modulus (msi) Density (g/cm 3) Diameter (mil) SiC on C Monofilament 62A12 O 3-14B203-15SiO 2 70AI 2 O3-2B 2 O3-28SIO 2 70A12 O3-2B 2 O3-28SIO 2 620 250 300 330 62 22 27 32 3.00 2.7 3.05 3.05 5.5 0.4 0.4--0.5 0.4-0.5 0.4--0.5 0.6 0.4-0.5 0.4 0.6 0.6 0.6 0.5 0.4 0.4 0.4 0.4 73A1203-27SIO2 99 a-AI203 85A1203-15SIO2 99 a-AI203 85A1203-15SIO2 57Si-31C-120 62Si-32C-0.50 68.9Si-30.9C-0.20 55.4S i-32.4C- 10.20-2Ti 55.3Si-33.9C-9.80-1Zr 66.6Si-28.5C-2.3 B-2.1Ti-0.80-0.4N 58Si-37N-40 290 425 300 260 290 435 400 375 480 480 465 360 28 54 38 30 28 32 39 61 27 28 55 36 3.03 3.88 3.4 3.60 3.20 2.55 2.74 3.10 2.48 2.48 3.00 2.50 Critical Bend Radii (mm) 7.0 0.48 I 0.48 0.53 0.36 0.27 0.80
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 carbon
Manufacturing Technology for Aerospace Structural Materials diameters (0.4-0.8 mil), allowing them to be used for a wide range of manufacturing 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 multiplying 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 temperatures. 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 temperatures. 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 compositions. 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 particles (~l-2nm) dispersed in a matrix of amorphous SiO 2 and free carbon. 466
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 with
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 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 temperatures up to ~ 2000 ~ F. 3 The oxide based fibers are typically more strength limited at high temperatures than the non-oxide fibers; however, oxide fibers have a distinct advantage in having a greater compositional stability in high temperature oxidizing environments. 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 stability 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
Manufacturing Technology for Aerospace Structural Materials Table 10.3 Select Ceramic Matrix Materials Matrix Modulus of Modulus of Fracture Density Thermal Rupture (ksi) Elasticity Toughness (g/cm) Expansion Point( F) Pyrex Glass 8 7 007 3,24 2285 LAS Glass AL,O 3720 3.30 5.76 56-70 4867 3.21 3600 72-120 3.19 Zr.o 250-7.705.56-5.75792-13.5 Note: Values depend on exact composition and processing the matrix during processing can consume the entire reinforcement. Likewise large differences in thermal expansion between the fibers and matrix can result in large residual stresses and matrix cracking. Several important matrix materials are listed in Table 10.3 Carbon is an exceptionally stable material in the absence of oxygen, capable of surviving temperatures greater than 4000F in vacuum and inert atmospheres. In addition, carbon is lightweight with a density of approximately 0.072 Ib/in. However, monolithic graphite is brittle, of low strength, and can- not be easily formed into large complex shapes. To overcome these limita- tions, carbon-carbon composites were developed, in which high strength carbon fibers are incorporated into a carbon matrix. For high temperature applica tions, carbon-carbon composites offer exceptional thermal stability(>4000 F) ith low de (0.0540.0721b/in.3) Carbon-carbon composites are used in rocket nozzles, nosecones for reentry vehicles, leading edges, cowlings, heat shields, aircraft brakes, brakes for racing vehicles, and high temperature furnace setters and insulation. These applications utilize the following nominal properties of carbon-carbon composites(which depends on fiber type, fiber architecture, and matrix density) Ultimate Tensile Strength >40 ksi Modulus of Elasticity >10 msi Thermal Conductivity=0.9-19 Btu- in. /(s-ft"-F Thermal Expansion= 1. I ppm/K Density <0.072 1b/in. 3 The low thermal expansion and range of thermal conductivities give carbon carbon composites high thermal shock resistance. As previously mentioned, the one major shortcoming of carbon-carbon composites is their oxidation suscepti bility. At temperatures above 9500F, both the matrix and the fiber are vulnerable
Manufacturing Technology for Aerospace Structural Materials Table 10.3 Select Ceramic Matrix Materials Matrix Modulus of Modulus of Fracture Density Thermal Melting Rupture (ksi) Elasticity Toughness (g/cm 3) Expansion Point (~ F) (msi) (ksi ~/~-n. ) (10-6/~ C) Pyrex Glass 8 7 0.07 2.23 3.24 2285 LAS Glass- 20 17 2.20 2.61 5.76 - Ceramic A1203 70 50 3.21 3.97 8.64 3720 Mullite 27 21 2.00 3.30 5.76 3360 SiC 56-70 48-67 4.50 3.21 4.32 3600 Si3Ni 4 72-120 45 5.10 3.19 3.06 3400 Zr203 36-94 30 2.50-7.70 5.56-5.75 7.92-13.5 5000 Note: Values depend on exact composition and processing. the matrix during processing can consume the entire reinforcement. Likewise, large differences in thermal expansion between the fibers and matrix can result in large residual stresses and matrix cracking. Several important matrix materials are listed in Table 10.3. Carbon ~ is an exceptionally stable material in the absence of oxygen, capable of surviving temperatures greater than 4000~ in vacuum and inert atmospheres. In addition, carbon is lightweight with a density of approximately 0.072 lb/in. 3. However, monolithic graphite is brittle, of low strength, and cannot be easily formed into large complex shapes. To overcome these limitations, carbon-carbon composites were developed, in which high strength carbon fibers are incorporated into a carbon matrix. For high temperature applications, carbon-carbon composites offer exceptional thermal stability (> 4000 ~ F) in non-oxidizing atmospheres along with low densities (0.054-0.0721b/in.3). Carbon-carbon composites are used in rocket nozzles, nosecones for reentry vehicles, leading edges, cowlings, heat shields, aircraft brakes, brakes for racing vehicles, and high temperature furnace setters and insulation. These applications utilize the following nominal properties of carbon-carbon composites (which depends on fiber type, fiber architecture, and matrix density): Ultimate Tensile Strength > 40 ksi Modulus of Elasticity > 10 msi Thermal Conductivity - 0.9-19 Btu-in./(s-ft 2-~ F) Thermal Expansion -- 1.1 ppm/K Density < 0.072 lb/in. 3 The low thermal expansion and range of thermal conductivities give carboncarbon composites high thermal shock resistance. As previously mentioned, the one major shortcoming of carbon-carbon composites is their oxidation susceptibility. At temperatures above 950 ~ F, both the matrix and the fiber are vulnerable 468