Ceramic Matrix Composites to oxidation if they are not protected from oxygen exposure. The two pri- mary oxidation protection methods are external coatings and internal oxidation inhibitors. Surface coatings, such as silicon carbide, provide an external barrier to oxygen penetration. The addition of internal oxidation inhibitors acts either nternal barriers to oxygen ingress or as oxygen sinks(forming a protective bar rier). In high temperature oxidizing environments, the time-temperature-cycle capabilities of these oxidation barriers are the primary limit to the temperature capabilities of current carbon-carbon composites Because they can be consolidated at lower temperatures and pressures than poly crystalline ceramic materials, glass-ceramics are potentially attractive matrix mate rials. Since all glass-ceramics contain some residual glass after ceraming, the upper use temperature is controlled by the softening point of the residual glass; however, silica based glass-ceramics can be used for moderate temperature applications. The most common glass-ceramic systems are LAS, MAS, MLAS, CAS, and BMAS where L= Li,O, A=AL,O3, S=SiO2, M=MgO, C= CaO, and B= BaO. 9 Glass-ceramics have the advantage that, being glasses, they can be melted to relatively low viscosities during hot pressing to impregnate the fiber bundle, and the processing temperatures are lower than for traditional crystalline ceramics, thereby reducing fiber degradation. After hot pressing, they are converted to a glass-ceramic by a heat treatment (i.e, ceraming). The amount of crystalline phase can be as high as 95-98%. However, the presence of the residual glassy phase limits their elevated temperature creep resistance Non-oxide matrices include carbon (C), silicon carbide (SiC), and silicon nitride(Si N4). Carbon is extremely refractory and has a low density; however, for elevated temperature applications it must be protected from oxidation. Silicon carbide has a high melting point and excellent mechanical properties at elevated temperature. Silicon carbide is a little less refractory than carbon and has a slightly higher density, with oxidation resulting in the formation of silica(Sio2) however, it can be used up to 2700 F in air. Silico on nitride matrices properties similar to silicon carbide, except that they are less thermally stable nd exhibit lower conductivities Oxide matrices include alumina(Al,O,), mullite(3Al2O2-2SiO2 ) cordierite (2MgO-2AlO3-5SiO2 ) and zirconia(ZrO2). Oxide matrices are of relatively low cost, exhibit rapid sintering at moderate temperatures, and exhibit high temperature oxidation resistance. Limitations include poor thermal expansion matches with many fibers, intermediate strength, and low high-temperature properties. Alumina is the most prevalent oxide matrix and contains the best balance of properties. Mullite has a lower thermal expansion than alumina, processes well with sol-gel methods, and exhibits good toughness. Cordierite has very low thermal expansion and is often used in conjunction with othe oxides, such as mullite for matrices. Ziconia, which has excellent toughness when partially stabilized, loses much of its toughness at elevated temperatures 469
Ceramic Matrix Composites to oxidation if they are not protected from oxygen exposure. The two primary oxidation protection methods are external coatings and internal oxidation inhibitors. Surface coatings, such as silicon carbide, provide an external barrier to oxygen penetration. The addition of internal oxidation inhibitors acts either as internal barriers to oxygen ingress or as oxygen sinks (forming a protective barrier). In high temperature oxidizing environments, the time-temperature-cycle capabilities of these oxidation barriers are the primary limit to the temperature capabilities of current carbon-carbon composites. Because they can be consolidated at lower temperatures and pressures than polycrystalline ceramic materials, glass-ceramics are potentially attractive matrix materials. Since all glass-ceramics contain some residual glass after ceraming, the upper use temperature is controlled by the softening point of the residual glass; however, silica based glass-ceramics can be used for moderate temperature applications. The most common glass-ceramic systems are LAS, MAS, MLAS, CAS, and BMAS where L = Li20, A = A1203, S = SiO 2, M = MgO, C = CaO, and B = BaO. 9 Glass-ceramics have the advantage that, being glasses, they can be melted to relatively low viscosities during hot pressing to impregnate the fiber bundle, and the processing temperatures are lower than for traditional crystalline ceramics, thereby reducing fiber degradation. After hot pressing, they are converted to a glass-ceramic by a heat treatment (i.e., ceraming). The amount of crystalline phase can be as high as 95-98%. However, the presence of the residual glassy phase limits their elevated temperature creep resistance. Non-oxide matrices include carbon (C), silicon carbide (SIC), and silicon nitride (Si3N4). Carbon is extremely refractory and has a low density; however, for elevated temperature applications it must be protected from oxidation. Silicon carbide has a high melting point and excellent mechanical properties at elevated temperature. Silicon carbide is a little less refractory than carbon and has a slightly higher density, with oxidation resulting in the formation of silica (SiO2); however, it can be used up to 2700~ in air. Silicon nitride matrices have properties similar to silicon carbide, except that they are less thermally stable and exhibit lower conductivities. Oxide matrices include alumina (A1203), mullite (3A1203-2SIO2), cordierite (2MgO-2A1203-5SiO2), and zirconia (ZrO2). Oxide matrices are of relatively low cost, exhibit rapid sintering at moderate temperatures, and exhibit high temperature oxidation resistance. Limitations include poor thermal expansion matches with many fibers, intermediate strength, and low high-temperature properties. Alumina is the most prevalent oxide matrix and contains the best balance of properties. Mullite has a lower thermal expansion than alumina, processes well with sol-gel methods, and exhibits good toughness. Cordierite has very low thermal expansion and is often used in conjunction with other oxides, such as mullite for matrices. Ziconia, which has excellent toughness when partially stabilized, loses much of its toughness at elevated temperatures, l~ 469
Manufacturing Technology for Aerospace Structural Materials In selecting a fiber and matrix combination for a ceramic matrix composite, several factors need to be considered. First, the constituents need to be compat- ible from the standpoint of CtE. If the Cte of the matrix is greater than th radial CTE of the fiber, the matrix, on cooling from the processing temperature, will clamp the fibers resulting in a strong fiber-to-matrix bond and will exhibit brittle failures in service. on the other hand. if the cte of the matrix is less than the radial CtE of the fibers, the fibers may debond from the matrix on cooling Chemical compatibility of the constituents is also an important factor. Due to the high processing temperatures for ceramic matrix composites, reactions between the fiber and matrix resulting in reduced fiber strengths are an ever prese concern. For example, SiC fibers react with silica based glass-ceramics, so the fibers must be coated with a protective interfacial coating For temperatures exceeding 1800 F, candidate matrices are carbon, silicon carbide, silicon nitride, and alumina. Although these compositions are possible, e performance requirement that the matrix material have a CtE very close to that of the commercially available carbon, silicon carbide, and alumina based fibers effectively eliminates silicon nitride and alumina as matrix choices for the ilicon carbide fibers and silicon carbide and silicon nitride as matrix for the alumina based fibers. The lack of high thermal conductivity availability of oxide based fibers that are creep resistant for long times 1800 F are two factors currently limiting the commercial viability of oxide/ox ceramic matrix composites. 10.3 Interfacial Coatings Interfacial, or interphase, coatings are often required to: (1) protect the fibers from degradation during high temperature processing, (2)aid in slowing oxida- tion during service, and(3) provide the weak fiber-to-matrix bond required for toughness. The coatings, ranging in thickness from 0. I to 1.0 um, are applied rectly to the fibers prior to processing usually by CVD. CVd produces coatings of relatively uniform thickness, composition and structure, even with preforms of complex fiber architecture. Carbon and boron nitride(bn) are typi cal coatings, used either alone or in combination with each other. Frequently, in addition to the interfacial coatings, an over-coating is also applied, such as a thin ayer(0.5 um) of SiC that becomes part of the matrix during processing. The SiC over-coating helps to protect the interfacial coating from reaction with the matrix during processing. Since C and bn interfacial coatings will degrade in an ambient environment, the over-coating is usually applied immediately after the interfacial coating. During service, the over-coating also acts to protect the fibers and interfacial coatings from aggressive environments, such as oxygen and water vapor. The interfacial and over-coating are sometimes repeated as multilayer coatings to provide environmental protection layers in the presence of in-service generated matrix cracks
Manufacturing Technology for Aerospace Structural Materials In selecting a fiber and matrix combination for a ceramic matrix composite, several factors need to be considered. First, the constituents need to be compatible from the standpoint of CTE. If the CTE of the matrix is greater than the radial CTE of the fiber, the matrix, on cooling from the processing temperature, will clamp the fibers resulting in a strong fiber-to-matrix bond and will exhibit brittle failures in service. On the other hand, if the CTE of the matrix is less than the radial CTE of the fibers, the fibers may debond from the matrix on cooling. 9 Chemical compatibility of the constituents is also an important factor. Due to the high processing temperatures for ceramic matrix composites, reactions between the fiber and matrix resulting in reduced fiber strengths are an ever present concern. For example, SiC fibers react with silica based glass-ceramics, so the fibers must be coated with a protective interfacial coating. For temperatures exceeding 1800 ~ F, candidate matrices are carbon, silicon carbide, silicon nitride, and alumina. Although these compositions are possible, the performance requirement that the matrix material have a CTE very close to that of the commercially available carbon, silicon carbide, and alumina based fibers effectively eliminates silicon nitride and alumina as matrix choices for the silicon carbide fibers, and silicon carbide and silicon nitride as matrix choices for the alumina based fibers. The lack of high thermal conductivity and the availability of oxide based fibers that are creep resistant for long times above 1800 ~ F are two factors currently limiting the commercial viability of oxide/oxide ceramic matrix composites. 10.3 Interfacial Coatings Interfacial, or interphase, coatings are often required to: (1) protect the fibers from degradation during high temperature processing, (2) aid in slowing oxidation during service, and (3) provide the weak fiber-to-matrix bond required for toughness. The coatings, ranging in thickness from 0.1 to 1.0~m, are applied directly to the fibers prior to processing, usually by CVD. CVD produces coatings of relatively uniform thickness, composition and structure, even with preforms of complex fiber architecture. Carbon and boron nitride (BN) are typical coatings, used either alone or in combination with each other. Frequently, in addition to the interfacial coatings, an over-coating is also applied, such as a thin layer (~0.5 ~m) of SiC that becomes part of the matrix during processing. The SiC over-coating helps to protect the interfacial coating from reaction with the matrix during processing. Since C and BN interfacial coatings will degrade in an ambient environment, the over-coating is usually applied immediately after the interfacial coating. During service, the over-coating also acts to protect the fibers and interfacial coatings from aggressive environments, such as oxygen and water vapor. The interfacial and over-coating are sometimes repeated as multilayer coatings to provide environmental protection layers in the presence of in-service generated matrix cracks. 470