Chapter 10 Ceramic Matrix Composites
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Manufacturing Technology for Aerospace Structural Materials Monolithic ceramic materials contain many desirable properties, such as high moduli, high compression strengths, high temperature capability, high hardness and wear resistance, low thermal conductivity, and chemical inertness. As shown in Fig. 10. 1, the high temperature capability of ceramics makes them very attrac- tive materials for extremely high temperature environments. However, due to their very low fracture toughness, ceramics are limited in structural applications While metals plastically deform due to the high mobility of dislocations (i.e slip), ceramics do not exhibit plastic deformation at room temperature and are prone to catastrophic failure under mechanical or thermal loading. They have a very low tolerance to crack-like defects, which can result either during fabri cation or in-service. Even a very small crack can quickly grow to critical size leading to sudden failure hile reinforcements such as fibers, whiskers, or particles are used to strengthen polymer and metal matrix composites, reinforcements in ceramic matrix composites are used primarily to increase toughness. Some differences in polymer matrix and ceramic matrix composites are illustrated in Fig. 10.2. The toughness increases afforded by ceramic matrix composites are due to energy dissipating mechanisms, such as fiber-to-matrix debonding, crack deflection, fiber bridging, and fiber pull-out. A notional stress-strain curve for a monolithic ramic and a ceramic matrix composite is shown in Fig. 10.3. Since the area under the stress-strain curve is often considered as an indication of toughness, the large increase in toughness for the ceramic matrix composite is evident. The CFRP A SIALON 800 2001600 emperature(°F) Fig. 10. 1. Relative Material Te
Manufacturing Technology for Aerospace Structural Materials Monolithic ceramic materials contain many desirable properties, such as high moduli, high compression strengths, high temperature capability, high hardness and wear resistance, low thermal conductivity, and chemical inertness. As shown in Fig. 10.1, the high temperature capability of ceramics makes them very attractive materials for extremely high temperature environments. However, due to their very low fracture toughness, ceramics are limited in structural applications. While metals plastically deform due to the high mobility of dislocations (i.e., slip), ceramics do not exhibit plastic deformation at room temperature and are prone to catastrophic failure under mechanical or thermal loading. They have a very low tolerance to crack-like defects, which can result either during fabrication or in-service. Even a very small crack can quickly grow to critical size leading to sudden failure. While reinforcements such as fibers, whiskers, or particles are used to strengthen polymer and metal matrix composites, reinforcements in ceramic matrix composites are used primarily to increase toughness. Some differences in polymer matrix and ceramic matrix composites are illustrated in Fig. 10.2. The toughness increases afforded by ceramic matrix composites are due to energy dissipating mechanisms, such as fiber-to-matrix debonding, crack deflection, fiber bridging, and fiber pull-out. A notional stress-strain curve for a monolithic ceramic and a ceramic matrix composite is shown in Fig. 10.3. Since the area under the stress-strain curve is often considered as an indication of toughness, the large increase in toughness for the ceramic matrix composite is evident. The t- t-" L__ .u_ k.J tll 0"1 - 400 800 1200 1600 2000 Temperature (~ F) Fig. 10.1. Relative Material Temperature Limits 2400 2800 460
Ceramic Matrix Composites Polymer and Metal Matrix Composites Ceramic Matrix C Strengthening tannen ○○○○○ ○○○○○ ○○○○○ ○○○○○ High Strength Fiber High Strength Fiber Strong Interfacial Bond Weak Interfacial Bond Low Strength/Low Modulus Matrix High Strength/High Modulus Matrix Fig. 10.2. Comparison of Polymer and Metal with Ceramic Matrix Composites Fracture Initiation of Monolithic Ceramic Fiber Pull-out Tension strain Fig. 10.3. Stress-Strain for Monolithic and Ceramic Matrix Composites
Ceramic Matrix Composites Polymer and Metal Matrix Composites Strengthening Ceramic Matrix Composites Toughening 00000 00000 0 12 0 ,iber 00000 00000 O. C ~PDO tren~/ ;i;it;r;:i ii~:ir ix Fig. 10.2. Comparison of Polymer and Metal with Ceramic Matrix Composites Fiber Fracture Further Matrix ~ ~ / Cracking-Crack ~" ~ ig D/f/tction at \ ~" I1." Matrix Cracking "t /.', ~ono,,..,c /,I ~ C;;a~i~ .... I ' Fiber / Pull-out Tension Strain Fig. 10.3. Stress-Strain for Monolithic and Ceramic Matrix Composites 461
Manufacturing Technology for Aerospace Structural Materia Debonding Fiber pull-out 100p Fig. 10. 4. Crack Dissipation Mechanisms mechanisms of debonding and fiber pull-out are shown in Fig. 10.4. For these mechanisms to be effective, there must be a relatively weak bond at the fibe to-matrix interface. If there is a strong bond, the crack will propagate straight through the fibers, resulting in little or no energy absorption. Therefore, proper control of the interface is critical. Coatings are often applied to protect the fibers during processing and to provide a weak fiber-to-matrix bond
Manufacturing Technology for Aerospace Structural Materials Debonding Fiber Pull-out Fig. 10.4. Crack Dissipation Mechanisms mechanisms of debonding and fiber pull-out are shown in Fig. 10.4. For these mechanisms to be effective, there must be a relatively weak bond at the fiberto-matrix interface. If there is a strong bond, the crack will propagate straight through the fibers, resulting in little or no energy absorption. Therefore, proper control of the interface is critical. Coatings are often applied to protect the fibers during processing and to provide a weak fiber-to-matrix bond. 462
Ceramic Matrix Composites Carbon-carbon(C-C)composites' are the oldest and most mature of the ceramic matrix composites. They were developed in the 1950s by the aerospace industry for use as rocket motor casings, heat shields, leading edges, and thermal protection. It should be noted that C-C composites are often treated as a sep- arate material class from other ceramic matrix composites, but their usage and fabrication procedures are similar and overlap other ceramic matrix composites A relative comparison of C-C with other ceramic matrix composites is given in Table 10. 1. For high temperature applications carbon- carbon composites offer exceptional thermal stability(4000 F) in non-oxidizing atmospheres, along with low densities(0.054-0072 lb /in. ). Their low thermal expansion and range of thermal conductivities provides high thermal shock resistance. In vacuum and inert gas atmospheres, carbon is an extremely stable material, capable of use to temperatures exceeding 4000 F. However, in oxidizing atmospheres, it starts oxidizing at temperatures as low as 950 F. Therefore, C-C composites for elevated temperature applications must be protected with oxidation resistant coating systems, such as silicon carbide that is over-coated with glasses. The silicon carbide coating provides the basic protection, while the glass over-coat melts and flows into coating cracks at elevated temperature. In addition, oxida- tion inhibitors, such as boron, are often added to the matrix to provide additional rotection Ceramic matrix materials include the element carbon, glasses, glass-ceramics oxides(e. g, alumina-Al2O3)and non-oxides(e. g, silicon carbide -SiC). The majority of ceramic materials are crystalline with predominately ionic bonding, Table 10.1 Carbon-Carbon and Ceramic Composite Comparison Carbon-Carbon Continuous Cmcs Discontinuous CMc Exceptional High Temp Mech Excellent High Temp mech Excellent High Temp Mech Properties High Specific Strength and High Specific Strength and Lower Specific Strength Low to moderate Toughness Low to Moderate Toughness Dimensional Stabl Dimensional Stability Low Thermal Expansion Low Thermal Expansion ood but lower than High Thermal Shock Resistance Good Thermal Shock hermal Shock Resistance Graceful Failure Modes Resistance Graceful Failure wer than Continuous CMCs Tailorable Properties Oxidation Resistance Amendable to Lower Cost Machinability ing Mor ventional Processes Poor Oxidation Resistance Processing More Complicated Machining Expensive d Expensive 463
Ceramic Matrix Composites Carbon-carbon (C-C) composites 1 are the oldest and most mature of the ceramic matrix composites. They were developed in the 1950s by the aerospace industry for use as rocket motor casings, heat shields, leading edges, and thermal protection. It should be noted that C-C composites are often treated as a separate material class from other ceramic matrix composites, but their usage and fabrication procedures are similar and overlap other ceramic matrix composites. A relative comparison of C-C with other ceramic matrix composites is given in Table 10.1. For high temperature applications carbon-carbon composites offer exceptional thermal stability (>4000 ~ F) in non-oxidizing atmospheres, along with low densities (0.054-0.072 lb/in.3). Their low thermal expansion and range of thermal conductivities provides high thermal shock resistance. In vacuum and inert gas atmospheres, carbon is an extremely stable material, capable of use to temperatures exceeding 4000 ~ F. However, in oxidizing atmospheres, it starts oxidizing at temperatures as low as 950 ~ F. Therefore, C-C composites for elevated temperature applications must be protected with oxidation resistant coating systems, such as silicon carbide that is over-coated with glasses. The silicon carbide coating provides the basic protection, while the glass over-coat melts and flows into coating cracks at elevated temperature. In addition, oxidation inhibitors, such as boron, are often added to the matrix to provide additional protection. Ceramic matrix materials include the element carbon, glasses, glass-ceramics, oxides (e.g., alumina- A1203) and non-oxides (e.g., silicon carbide- SIC). The majority of ceramic materials are crystalline with predominately ionic bonding, Table 10.1 Carbon-Carbon and Ceramic Composite Comparison Carbon-Carbon Continuous CMCs Discontinuous CMCs Exceptional High Temp Mech Properties High Specific Strength and Stiffness Low to Moderate Toughness Dimensional Stability Low Thermal Expansion High Thermal Shock Resistance Graceful Failure Modes Tailorable Properties Machinability Poor Oxidation Resistance Excellent High Temp Mech Properties High Specific Strength and Stiffness Low to Moderate Toughness Dimensional Stability Low Thermal Expansion Good Thermal Shock Resistance Graceful Failure Modes Oxidation Resistance Machining More Difficult Processing More Complicated and Expensive Excellent High Temp Mech Properties Lower Specific Strength and Stiffness Fracture Toughness Good but Lower than Continuous CMCSs Thermal Shock Resistance Lower than Continuous CMCs Amendable to Lower Cost Conventional Processes Machining Expensive 463
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
