Slurry infiltration and consolidation is the most prevalent process used to make glass and glass-ceramic composites, mainly because the processing temperatures used for glass and glass-ceramics are lower than those used for crystalline ceramics. The melting point of crystalline ceramics is so high, that even fibers with interfacial coatings would be either dissolved or severely degraded. Another problem is the large temperature difference between extremely high processing temperatures and room temperature, which can result in shrinkage and matrix cracking. In addition, crystalline ceramics heated past their melting points have such high viscosities that the infiltration of preforms is very difficult, if not impossible.
Glass-ceramics start as amorphous glasses that can be formed into a shape and then converted to crystalline ceramics by a high temperature heat treatment. During heat treatment, small crystallites (~ 1 nm) nucleate and grow until they impinge on adjacent crystallites. On further heating, very fine (< 1 ^m) angular crystallites are formed. The resulting glass-ceramic is a fine polycrystalline material in a glassy matrix with a crystalline content as high as 95-98%.
It should be noted that only certain compositions of glasses are capable of forming glass-ceramics. For example, lithium aluminosilicate (LAS) is a glass-ceramic with the composition Li2O-Al2O3-SiO2. Titanium dioxide (TiO2) is added as a nucleating agent. When this glass is heated to 1400° F for 1.5 h, TiO2 precipitates nucleate in the glass matrix. When the temperature is further raised to 1750° F, crystallization of the glass matrix initiates at the TiO2 precipitate particles.
In slurry infiltration, fiber tows or a preform are impregnated in a tank containing the liquid slurry matrix, as shown in Fig. 10.9. The slurry consists of the matrix powder, an organic binder, and a liquid carrier such as water or alcohol. The slurry composition is very important. Variables such as the powder content, particle size distribution, type and amount of binder, as well as the carrier medium, will have a significant impact on part quality. For example, using a matrix powder that is smaller than the fiber diameter will help in thorough impregnation, thereby reducing porosity. Wetting agents can also be used to help infiltration into the fiber tows or preform.
After infiltration, the liquid carrier is allowed to evaporate. The resulting prepreg can then be layed-up on a tool for consolidation. Prior to consolidation, the organic binder must be burned out. Consolidation is normally accomplished in a hot press; however, HIP is an option if complex shapes are required. Consolidation parameters (time, temperature, and pressure) will also affect part quality. While high temperatures, long times, and high pressures may help to reduce porosity, fiber damage can result from either high pressures (mechanical damage) or high temperatures and long times (interfacial reactions).13
The slurry infiltration process generally yields a composite with a fairly uniform fiber distribution and low porosity. The main disadvantage is that it is restricted to relatively low melting or low softening point matrix materials.
10.8 Polymer Infiltration and Pyrolysis (PIP)1'16'17
The PIP process is very similar to the processes used to make polymer matrix composites. Either a fiber reinforced prepreg is made with a matrix material that can be converted to a ceramic on heat treatment (pyrolysis) or a dry preform is infiltrated multiple times with a liquid organic precursor that can be converted by pyrolysis to a ceramic. In the case of prepreg, after the initial conversion to a ceramic, subsequent infiltrations are conducted with a liquid precursor material. Instead of a conventional thermoset resin (e.g., epoxy), an organometallic polymer is used. The polymer infiltration and pyrolysis process is shown schematically in Fig. 10.10. The process consists of:
• Infiltration of the preform with the polymer,
• Consolidate the impregnated preform,
• Cure of the polymer matrix to prevent melting during subsequent processing,
• Pyrolysis of the cured polymer to convert it to a ceramic matrix, and
• Repeating the infiltration and pyrolysis process "N" times to produce the desired density.
This section covers three types of PIP processes, the Space Shuttle carboncarbon process, the conventional PIP process, and the sol-gel infiltration and pyrolysis process.
Space Shuttle C-C. The fabrication process1 for the carbon-carbon Space Shuttle nose cap and wing leading edge components (Fig. 10.11) is a multi-step process, typical of the infiltration and pyrolysis technology used to produce C-C composites. As shown in Fig. 10.12, initial material lay up is similar to that used for thermoset composite parts. Plain weave carbon fabric, impregnated
Nose Cap and Chin Panel
Nose Cap and Chin Panel
Wing Leading Edge
Fig. 10.11. Space Shuttle Carbon-Carbon Applications
Wing Leading Edge
Fig. 10.11. Space Shuttle Carbon-Carbon Applications
with phenolic resin, is layed up on a fiberglass/epoxy tool. Laminate thickness varies from 19 plies in the external skin and web areas up to 38 plies at the attachment locations. The part is vacuum bagged and autoclave cured at 300° F for 8 h. The cured part is rough trimmed, X-rayed, and ultrasonically inspected. The part is then post-cured by placing the part in a graphite restraining fixture, loading it into a furnace, and heating it to 500° F very slowly to avoid distortion and delamination. The post-cure cycle alone can take up to 7 days.
The next step is initial pyrolysis. The part is loaded in a graphite restraining fixture and placed in a steel retort, which is packed with calcined coke. The part is slowly heated to 1500° F and held for 70 h to facilitate conversion of the phenolic resin to a carbon state. During pyrolysis, the resin forms a network of interconnected porosity that allows the escape of volatiles. This stage is extremely critical. Adequate volatile escape paths must be provided, and sufficient times must be employed, to allow the volatiles to escape. If the volatiles become entrapped and build up internal pressure, massive delaminations can occur in the relatively weak matrix. After this initial pyrolysis cycle, the carbon is designated reinforced carbon-carbon-0 (RCC-0), a state in which the material is extremely light and porous with a flexural strength of only 3000-3500 psi.
Densification is accomplished in three infiltration and pyrolysis cycles. In a typical cycle, the part is loaded in a vacuum chamber and impregnated with furfural alcohol. It is then autoclave cured at 300° F for 2 h and post-cured at 400° F for 32 h. Another pyrolysis cycle is then conducted at 1500° F for 70 h. After three infiltration/pyrolysis cycles, the material is designated RCC-3, with a flexural strength of 18 000 psi.
To allow usage at temperatures above 3600° F in an oxidizing atmosphere, it is necessary to apply an oxidation resistant coating system. The coating system consists of two steps: (1) applying a SiC diffusion coating to the C-C part, and (2) applying a glassy sealer to the SiC diffusion coating. The coating process (Fig. 10.13) starts with blending of the constituent powders, consisting of 60% silicon carbide, 30% silicon, and 10% alumina. In a pack cementation process, the mix is packed around the part in a graphite retort. The retort is loaded into a vacuum furnace, where it undergoes a 16 h cycle that includes drying at 600° F and then a coating reaction up to 3000° F in an argon atmosphere. During processing, the outer layers of the C-C are converted to SiC. The silicon carbide coated C-C part is removed from the retort, cleaned, and inspected. During cool down from 3000° F, the silicon carbide coating contracts slightly more than the C-C substrate, resulting in surface crazing (coating fissures). This crazing, together with the inherent material porosity, provides paths for oxygen to reach the C-C substrate.
To obtain increased life, it is necessary to add a surface sealer. The surface sealing process involves impregnating the part with TEOS (tetraethylorthosil-icate). The part is covered with a mesh, placed in a vacuum bag, and the bag is filled with liquid TEOS. A five cycle TEOS impregnation process is
then performed on the bagged part. After the fifth impregnation cycle, the part is removed from the bag and oven cured at 600° F to liberate hydrocarbons. This process leaves silica (SiO2) in all of the microcracks and fissures, greatly enhancing the oxidation protection.
Conventional PIP Processes. For ceramic matrices other than carbon, silicon based organometallic polymers are the most common precursors, including Si-C, Si-C-O, Si-N, Si-O-N, and Si-N-C-O precursors. Al-O and BN have also been studied.16 The polymeric precursor should produce a high char yield to obtain the desired density in as few a polymer infiltration and pyrolysis cycles as possible. Polymers containing highly branched and crosslinked structures, and those with high percentages of ring structures, are good candidates. Those that contain long chains tend to break up into low weight volatiles and are poor choices. Polymer branching and crosslinking increases the ceramic yield, by inhibiting chain scission and the formation of volatile silicon oligomers during pyrolysis. Initial pyrolysis produces an amorphous ceramic matrix, while high temperature treatments lead to crystallization and shrinkage as the amorphous matrix develops small domains of crystalline structure.16 Fillers, such as silica or ceramic whiskers, can improve matrix properties by reducing and disrupting the matrix cracks that form during shrinkage. Typical filler loadings are 15-25 volume percent of the matrix.
If prepreg is going to be used, the first step is to impregnate the reinforcement with the precursor matrix material. Another alternative is to use a dry preform and to infiltrate the precursor resin directly into the preform before pyrolysis.
Resin transfer molding, vacuum impregnation, fiber placement, and filament winding have all been used to impregnate and form preceramic preforms. The composite part is initially autoclave cured at 300-500° F and 50-100 psi. The part is then put through the first pyrolysis cycle to convert the precursor matrix to ceramic. Pyrolysis in either argon or nitrogen atmospheres of at least 1300° F are required, with typical processing temperatures in the range of 1700-2200° F. During pyrolysis, large amounts of organic volatiles such as H2 and CO are released. Therefore, pyrolysis cycles must be done slowly to allow the volatiles to escape without causing part delamination. As a result of pore formation and growth, pyrolysis gases can produce both micro- and macro-cracking. In addition, there is an extremely large reduction in volume during the matrix conversion from polymers to ceramics.18 Cycles as long as 1-2 days are not uncommon.
Subsequent infiltrations are conducted with a low viscosity prepolymer. Infiltration is best done by vacuum impregnation in a vacuum bag.16 After the first pyrolysis cycle, the matrix is amorphous and highly porous, with multiple matrix cracks and a void content of 20-30%. The infiltration and pyrolysis process is repeated, often between 5 and 10 times to reduce the porosity, fill the cracks, and obtain the desired density. After the last pyrolization cycle, the part can be heat treated at higher temperatures to convert the amorphous matrix to a crystalline phase, relieve residual stresses, and provide final consolidation.
The biggest advantage of the polymer infiltration and pyrolysis process is the use of the familiar methods employed in organic matrix composite fabrication. However, the multiple infiltration and pyrolysis cycles required to obtain high density parts are expensive and the lead times are long. In addition, it is almost impossible to fill all of the fine matrix cracks, which degrades mechanical properties.
Sol-Gel Infiltration. Sol-gel infiltration is a lower temperature infiltration and pyrolysis process that can be used for oxide based ceramic matrix composites. Densification still requires pressure and high temperatures, but the densification temperatures are usually less than those required in the slurry infiltrated matrices. Sol-gel techniques require minimal exposures to temperatures above 1800° F, helping to reduce thermal damage to the fibers. Infiltration can be performed at temperatures less than 600° F, using either vacuum infiltration or autoclave molding.9 In addition, sintering aids, such as boria, are not required due to the lower processing temperatures.
In sol-gel infiltration, a chemical precursor is hydrolyzed, polymerized into a gel, and then dried and fired to produce a glass or ceramic composition. The precursors range from mixtures of water, alcohols, and metal oxides to commercially available stabilized colloids containing discrete ceramic particles.17 Hydrolysis reactions form an organometallic solution, or sol, composed of polymer-like chains containing metallic ions and oxygen. Amorphous oxide particles form from the solution, producing a rigid gel. The gel is then dried and fired to provide sintering and densification of the final ceramic part.
As an example, in the sol-gel process for alumina, an organometallic precursor is hydrolyzed, converted to colloidal solution (i.e., peptized) with hydrochloric acid, and then fired at increasing temperatures to produce first y-alumina and eventually a-alumina as follows:17
Al(OC4H9)3 + H2O ^ Al(OC4H9)2 + C4H9OH (Hydrolysis) 2 Al(OC4H9)2 (OH) + H2O_HCl ^ C4H9O-AlOH-O-AlOH-OC4H9 + 2C4H9OH (Peptization)
The ideal sol should have as high a ceramic content as possible (>30 weight percent if possible), a low viscosity (15-20 cP), and a small particle size (<30 nm if possible). A neutral pH helps minimize fiber degradation. The sol should be capable of being processed at room temperature for several hours and should be stable enough to allow shipping and storage.17
Again, fillers can be used to help reduce shrinkage and subsequent matrix cracking. Ceramic filler powders can (1) reduce shrinkage and matrix cracking as it loses water and alcohols, (2) provide sites for the nucleation of grains, and (3) maximize matrix densification during the infiltration. Typically, silica particles are added to silica sols and alumina particles to alumina sols. However, loading the liquid precursor with powder increases the viscosity considerably and hinders the reimpregnation step.19
Impregnation of fiber tows can be accomplished by several methods: (1) the tows can be passed through a bath containing the sol and then hydrolyzed in humid air; (2) the tows can be passed through a bath containing a partially hydrolyzed sol and then wet wound; or (3) the tows can be dry wound and then pressure infiltrated with the sol. For woven 2-D and 3-D fiber architectures, it is normal practice to weave the fabric first and then infiltrate the woven preform with the sol. For 2-D woven fabrics that will require lay-up, polymeric binders are often used to provide tack and drape but require burn-off prior to densification.
A typical fabrication sequence would be to weave a preform and then impregnate it with the sol. Impregnation can be conducted by immersing the preform in the sol and pulling a vacuum to facilitate impregnation. An autoclave can also be used to provide positive pressure, resulting in better impregnation. The sol is then gelled by heating. Temperatures in the range of 200-400° F will remove water and alcohols, while higher temperatures (550-750° F) can be used to drive off any organics.
The infiltration and gelling cycle is repeated until the desired density is obtained. Processing under vacuum pressure usually yields a composite with about 20% porosity. Lower porosity levels can be achieved by using 50-100 psi autoclave pressure. After the desired density is achieved through multiple infiltration cycles, the part is fired at high temperatures to obtain the final ceramic microstructure.
While lower processing temperatures can be used in the sol-gel process, disadvantages are high shrinkage which results in matrix cracking, low yield which requires multiple infiltrations, and high precursor costs. High shrinkage of the gelled sols results from the large volume of water and alcohols that must be removed, often resulting in significant levels of porosity and extensive matrix micro- and macro-cracking. Excess water is typically used to insure complete hydrolysis, but large amounts of water also reduce the yield of the dried matrix obtained per infiltration cycle. In some cases, the polymeric precursor chemicals are very expensive, and most metal-organic compounds are very sensitive to moisture, heat, and light.18
10.9 Chemical Vapor Infiltration (CVI)20'21
Chemical vapor deposition is a well-established industrial process for applying thin coatings to materials. When it is used to infiltrate and form a ceramic matrix, it is called chemical vapor infiltration (CVI). In CVI, a solid is deposited within the open volume of a porous structure by the reaction or decomposition of gases or vapors. A porous preform of fibers is prepared and placed in a high temperature furnace, as shown in Fig. 10.14. Reactant gases or vapors are
Interfacial and Overcoatings
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