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