Polymer infiltration and pyrolysis

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... 

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. 

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)... 

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)... 

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]. 

As for other classes of composite materials, there are many processes that can be used to make CMCs. Key considerations in process selection are porosity and reactions among reinforcements, reinforcement coatings, and matrices. The most important processes for making CMCs at this time are chemical vapor infiltration, melt infiltration, preceramic polymer infiltration and pyrolysis (PIP), slurry infiltration, sol-gel, hot pressing, and hot isostatic pressing. In addition, there are a number of reaction-based processes, which include reaction bonding and direct metal oxidation ( Dimox ),... 

Ivekovic, A., Drazic, G., Novak, S. (2011). Densification of a SiC-matrix by electrophoretic deposition and polymer infiltration and pyrolysis process. Journal of the European Ceramic Society, 31,833-840. doi 10.1016/j.jeurceramsoc.2010.11.021. 

Fig. 1. Scheme of the CMC manufacturing process of the infiltration and pyrolysis of Si-polymers... 

Compared with conventional techniques for fabricating ceramic-matrix composites, such as hot pressing (HP), reactive melt infiltration (RMI) and polymer impregnation and pyrolysis (PIP), CVI techniques have distinct advantages, which can be summarised as follows [8, 9] ... 

Silicon carbide (SiC) matrix composites have been fabricated by chemical vapor infiltration (CVl), polymer impregnation and pyrolysis (PIP), and reaction sintering (RS). The RS process can be recognized as an attractive technique, because it offers a high density and good thermal conductivity, compared to those of CVl and PIP process. In general, the fabrication of fiber reinforced SiC matrix composites by reaction sintering involves melt infiltration (Ml) or liquid silicon infiltration (LSI). However, the fabrication of continuous fiber reinforced SiC matrix composites by RS focused in melt infiltration (Ml) such as liquid silicon infiltration (LSl) Vapor silicon infiltration was rarely used for SiC matrix composites. 

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... 

In the fabrication process of three dimensional carbon fiber reinforced SiC matrix composite, Suzuki and Nakano [207] applied PCVI as the final densification process for the specimen, which was made by the joint process of slurry infiltration and organosilicon polymer pyrolysis. The open porosity and bulk density of the specimen changed from 5.3% and 2.63 g-cm (relative density of 94%) to 3.5% and 2.67 g cm (relative density of 95%) by the apphcation of PCVI (1173 1223K, total 90,000 pulses). The flexural strength of the specimen increased over 20% (mean value =153 MPa, maximum value = 174 MPa). 

For the further increase of toughness, continuous hber reinforcement has been employed. In this category, silicon carbide reinforced with silicon carbide hbers, which is usually fabricated by the densification process using chemical vapor infiltration (CVl) or impregnation and pyrolysis of organosilicon polymers, is one of the most attractive materials. In these materials, the interface is also essentially important. As shown in Figure 9.1.12, appropriate thickness of carbon interface introduced by CVl process leads to a nonlinear fracture. 

Ceramic Matrix Composites (CMCs) 11 in the broad category of technical ceramics [I]. Unlike monolithic ceramics where surface and sub-surface flaws are known to be clearly detrimental from a tensile and durability [2] point of view, the effect of defects in CMCs is not as clear. The range of porosity (key defect) foimd in oxide/oxide is 25% [3], melt infiltrated nonoxide CMCs is 2% [3], polymer infiltrations pyrolysis ntm-oxide CMCs is 5% [3] and chemical vapor infiltrated non-oxide CMCs is 12% [4]. The properties vary widely between and wifliin these overall CMC classes. The above percent porosity for these classes of CMCs covers the conventional expectations from fabrication and does not consider local variations or unexpected processing concerns. Within all these systems, there is a range of durability behavior seen (both fatigue and creep). 

A systematic effort is therefore required to investigate the effect of porosity < i the durability performance of ceramic matrix composites. A Polymer Infiltration Pyrolysis (PIP) CMC system was chosen for this effort based on past work of the authors [5-7]. The PIP system also should allow different porosity levels as most PIP systems require multiple infiltrations. By stopping the manufacturing process at different infiltration cycles, different porosity levels will be produced in the material. The results of mechanical and durability testing on composite material produced at four different PIP cycles are explored in this paper. 

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