Oxycarbide composites

Belot V., Corriu R.J.P., Leclercq D., Mutin P.H., Vioux A. Organosilicon gels containing silicon-silicon bonds, precursors to novel silicon oxycarbide compositions. J. Non-Cryst. Solids 1992 144 287-297... 

In one other example, Raman spectroscopy was employed along with FTIR spectroscopy, XPS, elemental analysis, TGA, SEM and transmission electron microscopy (TEM) to follow the compositional and structure variations of polymethylsilsesquioxane samples pyrolysed at different temperatures in an atmosphere of nitrogen [56]. At 900°C the main product was silica, with formation too of some silica oxycarbide and amorphous carbon, with Raman spectroscopy showing complementary evidence for presence of both the minor species. 

In another attempt to obtain diamond from SiC, no evidence for diamond growth was found by means of XRD and SEM after hydrothermal hydrolysis of P SiC powder at 140 MPa and 800°C [57]. Formation of quartz, a second acicular phase, graphite and maybe nanosized diamond crystals was reported. TEM analysis of whiskers of the second acicular phase revealed lattice spacing of 0.9 nm, a <7-value of 0.56nm and a composition of SiOi 6Co.2- A very sharp band at 1330cm was found in a Raman spectrum and assigned to the silicon oxycarbide, SiOi 6C0.2, rather than to diamond. 

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. 

It is thus clear that the processing atmosphere strongly affects the resulting composition and microstructure of both SiOC and SiC coatings, and that vacuum is the only environment which allows the synthesis of both silicon carbide and silicon oxycarbide thin films [51],... 

Fiber reinforced ceramic matrix composites (CMCs) are under active consideration for large, complex high temperature structural components in aerospace and automotive applications. The Blackglas resin system (a low cost polymer-derived ceramic [PDC] technology) was combined with the Nextel 312 ceramic fiber (with a boron nitride interface layer) to produce a sihcon oxycarbide CMC system that was extensively characterized for mechanical, thermal, and electronic properties and oxidation, creep mpture, and fatigue. A gas turbine tailcone was fabricated and showed excellent performance in a 1500-hour engine test. 

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. 

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