Silicon oxycarbide fibers

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Ceramic fibers of the nonoxide variety such as silioon carbide, silicon oxycarbide such as Nicalon, silicon nitride, boron carbide, etc. have become very important because of their attractive combination of high stiffiiess, high strength and low density. We give brief description of some important nonoxide fibers. 

This work reports the development of a polymeric/sol-gel route for the deposition of silicon carbide and silicon oxycarbide thin films for applications such as heat-, corrosion-, and wear-resistant coatings, coatings on fibers for controlling the interaction with the matrix in ceramic matrix composites, or films in electronic and optoelectronic devices. This method, in which the pre-ceramic films are converted to a ceramic coating either by a conventional high temperature annealing or by ion irradiation, is alternative to conventional methods such as chemical or physical vapor deposition (CVD, PVD), molecular beam epitaxy, sputtering, plasma spray, or laser ablation, which are not always practical or cost efficient. 

But CMCs will be commercially successful only when they are produced cost-effectively. Polymer-derived ceramic (PDC) technology is one of the most promising low cost fabrication methods for ceramic matrix composites, particularly for large, complex shapes. In PDC technology, a silicon-based polymer (siloxane, carbosilane, silazane, etc) with fiber or particle reinforcement is shaped and cured in the polymer condition and then pyrolyzed in a controlled atmosphere to form a stable silicon-based ceramic, such as silicon carbide, sihcon nitride, silicon oxycarbide, or silicon oxynitride. 

This chapter will describe the processing and properties of an oxide fiber reinforced ceramic matrix composite with a silicon oxycarbide matrix based on a PDC technology, introduced by AlliedSignal (now Honeywell International) under the trademark of Blackglas ceramic. The oxide fiber in this CMC system is the Nextel 312 fiber (3M, Inc.) that has been treated to form a boron nitride surface coating. The information that follows was primarily developed from Low Cost Ceramic Matrix Composites (LC ) program funded by DARPA from 1991-1997. 

The highest modulus of a given substrate is obtained with a single crystal structure. Single crystal CVD-SiC whiskers (578 GPa) have a stiffen more highly ordered, structure than polycrystalline CVD-SiC fibers (190-400 GPa), and sapphire whiskers and fibers (415 GPa) are stiffer than slurry spun polycrystalline alumina fibers such as Fiber FP (380 GPa). Superimposed upon this relationship is a compositional factor. Fiber modulus and structural order generally also decrease with increasing compositional complexity, e.g., silicon carbide is intrinsically stiffer than silicon oxycarbide such as Nicalon, and slurry spun alumina fibers are stiffer than sol-gel or melt spun aluminate fibers. 

The major deficiency of carbon fibers is their sensitivity to oxidation even at relatively low temperatures. Although silicon carbide (SiC) fibers are also sensitive to oxidation, their oxidation starts at higher temperatures and yields a protective silica coating. In an oxidative environment, SiC and Si-C-0 fibers are generally more useful than carbon fibers [1-3]. Large diameter silicon carbide fibers are obtained by chemical vapor deposition (Chapter 4). Small diameter silicon carbide and oxycarbide fibers are derived from solid polydimethylsilazane precursor fibers (this chapter). 

As the temperature is raised to domain 3, hydrogen corresponding to the residual C-H bonds from the Si-CHrSi backbone is progressively released with a continuous but slow density increase. Above about 1000°C, clusters of free carbon and nanocrystals of p-SiC are formed. At the end of domain 3, ceramic grade Nicalon Si-C-0 fibers consist of a dispersion of free carbon clusters and SiC nanocrystals in an amorphous Si-C-0 matrix [14]. As the temperature is increased to domain 4, growth of p-SiC nanocrystals and decomposition of ternary silicon oxycarbide occurs in the pyrolytic residue, and evolution of CO and SiO is accompanied by further weight loss. 

Thus, the fibers resulting from the pyrolysis at 1200-1300°C of oxygen cured Yajima type PCS precursor fibers in an inert atmosphere are far from consisting of pure SiC since the molar fraction of SiC is about 50%. They also contain significant amounts of partly hydrogenated free carbon and silicon oxycarbide. It Is noteworthy that the value, or perhaps mean value [14] [64], of the term 1-x/2 = 0.447 is dose to 0.5 in Nicalon NL-200 (Figure 7) and that the main tetrahedral units are therefore close to SiOiCz [15]. 

This mechanism accounts for the evolution of a gaseous SiO CO mixture, the growth of the SiC crystals and the deaease in the amounts of free cartxin and silicon oxycarbide. The relative amounts of CO and SiO which are formed and the composition of the final residue (i.e., SiC or SiC + C) depend upon the relative amounts of free caitton and silicon oxycarbide in the fiber. For the Nicalon fiber (NL 200), the main species in the gas phase is CO and the solid residue is SiC. There is therefore enough SiO formed by decomposition of the silicon oxycarbide phase (Equation 9) to consume all the free carbon by Equation 10 [73]. This mechanism also accounts for one of the processes used to produce oxygen-free nearly-stoichiomefric SiC fibers [38] [54]. Since CO and SiO diffuse and escape from the fiber, the decomposition starts near its surface and the decomposition front moves radially towards the fiber axis, yielding a skin/core microstructure [16]. 

The tensile strength of Si-C-0 fibers decreases after exposure to elevated temperatures. When Nicalon NL 200 fibers are exposed for 1 hour to 1300 C in argon (P = 100 kPa), their mean tensile strength and scale parameter, Co, decrease by 45% while their Weibull modulus remains unchanged [80-83]. Fibers exposed to more severe conditions (e.g., for 5 hours in a vacuum at 1500°C) are so weak that they cannot be tested. Finally, the fact that oxygen-free fibers maintain their tensile strength under similar conditions relates to the absence of silicon oxycarbide and its decomposition process. 

Conversely, the creep curves for fibers tested in COrich atmospheres also display a large steady state domain (secondary creep) [59]. This linear domain is related to the fact that, under such conditions, the silicon oxycarbide and the fibers are stable. For higher test temperatures (Ti >1400"C), the fibers are no longer stable for Pco <100 kPa and the linear domain can no longer be observed. The strain rate t Jtime data in the steady state domain obey the Dorn relationship ... 

The dramatic change observed in the electrical behavior of SiC based fibers with increasing pyrolysis temperature is related to the formation or/and the organization of free carbon around the SiC crystals. In Si-C-0 fibers it occurs above 1200°C. Below 1200°C, the microstructure of the fibers is either amorphous or nanocrystalline, and carbon exists as isolated small BSUs. The material has semiconducting properties (E. = 0.4 eV). Above 1200 C, decomposition occurs with formation of jJ-SiC crystals and free carbon. The increase in electrical conductivity might be related to the removal of the glassy silicon oxycarbide and the formation of a continuous network of carbon around the SiC crystals [18]. 

Si-C-N(O) fibers derived from HPZ precursor fibers are nanoporous and heterogeneous with a skin/core structure. The composition changes from SiOxCy in the external porous surface to SiNxC, in the core. The molecuiar formuia of this fiber is close to 4 mol. >4 SiOa, 81 mol.% SiNxCy (x = 1.02, y = 0.23) and 15 mol.% free C [22]. The presence of complex tetrahedral units is supported by the Si NMR spectrum which shows a broad signal covering the chemical shift region expected for silicon oxycarbide, siiicon oxynitride and silicon carbonitride units [21]. The occurrence of free carbon, expected from the nature of the precursor, is supported by the C Is XPS pattern [22]. 

The interface between HM carbon fiber and a Pyrex borosilicate glass matrix was Analyzed by Bleay and Scott [111,112] and found to be some 100 nm thick and believed to comprise Na enriched silicon oxycarbide, showing that some reaction had taken place during fabrication. Measurement of the interlaminar shear strength of the composite indicated that this layer was not a source of weakness. Substantial fiber pull-out had occurred, however, exposing clean fiber surfaces and smooth sockets. It was concluded that the interfacial shear process was confined to the outer layer of the fiber. Heat treatment of the composite in air caused preferential oxidation of the fiber, the rate being higher parallel to the fiber axis than perpendicular to it. 

Balaba WM, Weirauch DA, Perrotta AJ, Armstrong GH, Anyalebechi PN, Kauffman S, MacInnes AN, Winner AM, Barron AR, Effect of sUoxane spin-on-glass and reaction bonded silicon oxycarbide coatings with a self-propagating interfacial reaction treatment (aspire) in the synthesis of carbon/graphite fiber-reinforced A1 metal matrix composites, J Mater Res, 8(12), 3192-3201, 1993. 

The presence of amorphous oxycarbide (Si-O-C) phase was also revealed in the Nicalon fibers at the interface between SiC core and surface SiOa layer (Pumpuck, 1980 Lipowitz, 1987 Laffon, 1989 Porte, 1989), although any thermodynamically stable compounds have not been found in the Si-O-C system. The formation of such an amorphous silicon oxycarbide phase was also suggested during the pyrolysis of organics-substituted polysiloxane gels to form SiC (Zhang, 1990 Babonneau, 1990 Bums, 1992). 

Kamiya K., Yoko T., Tanaka K., Takeuchi M. Thermal evolution of gels derived from CH3Si(OC2Hs)3 by the sol-gel method. J. Non-Cryst. Solids 1990b 121 182-187 Kamiya K., Katayama A., Matsuoka J., Nasu H. Preparation of silicon oxycarbide glass fibers by the sol-gel method (in Japanese). New Glass 1994 9 4-14 Kamiya K. Fibers from sol-gel. Ceram. Trans. 1995 55 371-382... 

Above-mentioned facts promoted the research work on the oxycarbide glasses prepared from organically modified alkoxysilanes by the sol-gel method. Also, thermal decomposition process of gels made from alkoxides of other metals than silicon has been applied to produce carbide fibers, films and powders. In this chapter, the research works on sol-gel-derived oxycarbide glasses and crystalline carbides are reviewed. 

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