Polymer infiltration

Fiber reinforced ceramics such as C/SiC, SiC/SiC can be manufactured by the polymer infiltration and pyrolysis technique at reasonable cost. The developed production technique allows the manufacturing of large and complex structures comparable to fiber-reinforced plastics. The material has excellent high temperature resistance, low density, and good damage tolerance, and is therefore well... 

In Section 3.4.2, we introdnced the concept of chemical vapor infiltration, CVI, in which a chemical vapor deposition process is carried out in a porous preform to create a reinforced matrix material. In that section we also described the relative competition between the kinetic and transport processes in this processing technique. In this section we elaborate npon some of the common materials used in CVI processing, and we briefly describe two related processing techniques sol infiltration and polymer infiltration. 

Polymer Infiltration. Polymer infiltration is similar to sol infiltration. The polymer is introduced in liqnid form nnder vacuum and coats the fibers. The polymer is then dried and pyrolyzed to yield the desired composition. Multiple infiltration cycles... 

Development has been conducted to fabricate SiC-based matrix/Si-C-based fiber composites by polymer infiltration. Nicalon fibers are used, and various polymers such as polysilazanes, polysiloxanes, and polycarbosilanes are used to yield matrices of SiCO, SiNC, and SiC. As with CVI of SiC/SiC composites, carbon acts as an interface layer but does not result in stability at high temperatures. 

This polymer precursor (1) requires relatively inexpensive starting materials, (2) is quite stable in air, (3) offers good processability for polymer infiltration processing of composites, (4) provides excellent SiC ceramic yields and (5) high purity with controllable microstructures. However, one important drawback is the use of costly LiAlHzt. This polymer is now available commercially (Starfire Inc., NY). 

Furthermore, not all applications will need or benefit from the availability of phase pure SiC. For example, in polymer infiltration and pyrolysis processing of composites, the reinforcing material frequently is oxidized at the surface. Thus, an SiC precursor that produces excess carbon may be required to ensure that the oxide surface layer is reduced off during processing so that good interfaces are obtained. 

Besides the continuous fibers, application of metallorganic polymers to heat-resistant coatings, dense ceramic moldings, porous bodies, and SiC matrix sources in advanced ceramics via polymer infiltration pyrolysis (PIP) have been developed. Novel precursor polymers have been synthesized and investigated for ceramics in addition to PCS (Table 19.1). For SiC ceramics, various Si-C backbone polymers have been synthesized. Their polymer nature (e.g., viscosity, stability, cross-linking mechanism, and ceramic yield) are, however, fairly different from PCS. On the other hand, polysilazane, perhydropolysilazane, polyb-orazine, aluminum nitride polymers, and their copolymers have been investigated... 

Yoshino, K., Satoh, S., Shimoda, Y. et al.. Tunable optical properties of conducting polymers infiltrated in synthetic opal as photonic crystal, Synthet. Met., 121, 1459, 2001. 

Coakley K. M. and McGehee M. D. (2003), Photovoltaic cells made from conjugated polymers infiltrated into mesoporous titania , Appl. Phys. Lett. 83, 3380-3382. 

K. W. Chew, A. Sellinger, R.M. Laine, processing aluminium nitride-silicon carbide composites via polymer infiltration and pyrolysis of polymethyl silane, a precursor to stoichiometric silicon carbide, J. Am. Ceram. Soc., 82, 857-866 (1999)... 

A further technological advantage of applying the polymer route for composite matrices is that no mechanical damage of the reinforcing fibers is caused. The scheme in Fig. 13 demonstrates the steps for making composites by the liquid polymer infiltration process [250, 251]. 

Fig. 13. Liquid polymer infiltration route for fabrication of SiC composite structures fi-om Si-precursors [fi om 251]...

LPI-liquid polymer infiltration CVI-chemical vapor infiltration... 

Table 7 contains a comparison of mechanical properties achieved for the three unconventional routes described to make SiC, composite matrices by LPI (liquid polymer infiltration), CVI (chemical vapor infiltration) and Si-infiltration [277]. 

Due to its solubility and the adjustable viscosity, PBS-Me is suitable for the polymer infiltration process used in the production of CMCs (ceramic matrix... 

Polymer infiltration and various chemical infiltrants 800 to 1,200 = C( 1,472 to multiple steps with 100-... 

Fabrication by Liquid Polymer Infiltration (LPI) In the first step of the LPI process, a carbon-fiber preform is infiltrated with resin (e.g. polycarbosilane), to bind the fibers together. Then the polymer is pyrolized to form SiC. These process steps are repeated a number of times until the pores are narrow enough that further... 

To address the temperature issues related to excess silicon, all the same constituents in the N24-C system are used for potential N26 CMC generations, but remaining open pores in the CVI SiC matrix are filled by silicon-free ceramics, rather than by melt infiltration of silicon. In particular, for the N26-A CMC system, a SiC-yielding polymer from Starfire Inc. [13] is infiltrated into the matrix porosity at room temperature and then pyrolyzed at temperatures up to 2912°F (1600°C). This polymer infiltration and pyrolysis (PIP) process was repeated a few times until composite porosity was reduced to 14 vol,%, At this point, the total CMC system is then thermally treated at NASA to improve its thermal conductivity and creep-resistance. Thus although more porous than the other CMC systems, the N26-A system has no free silicon in the matrix, thereby allowing long-time structural use at 2600°F... 

Liquid Polymer Infiltration (LPI) or Polymer Infiltration and Pyrolysis (PIP)... 

FIGURE 11. The technique for making C/SiC components by liquid polymer infiltration (LPI)... 

SiC- Sic and SiC-C (Continuous Fiber-Reinforced SiC Matrix Composites) Three different processes are commonly used to manufacture carbon fiber-reinforced SiC materials (i) chemical vapor infiltration (CVI) [340] (ii) liquid polymer infiltration (LPI also termed polymer infiltration and pyrolysis, PIP) [341]) and (iii) melt infiltration or liquid silicon infiltration (MI/LSI) [342]. 

The strength and tensile behavior, at room and high temperatures, as well as the structure of three dimensional earbon fiber/SiC composites, fabricated by the slurry pulse/CVI combined process, were eharacterized by Suzuki et al [207-209]. Carbon fiber preforms, constructed with 4-step braid, 4-step/axial braid, 2-step braid and orthogonal weave, were used as reinforcements of the composites. The composites were fabrieated by a process consisting of slurry and dissolved organosilicon polymer infiltrations, followed by the application of pulse CVI. 

Ziegler et al [249] prepared carbon fiber reinforced ceramic matrix composites by infiltration of fiber preforms using the polymer infiltration pyrolysis (PIP) technique. 

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