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 463Ceramic 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 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 (0.054-0.072 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, oxida￾tion 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