Phase and microstructural evolution in polymer-derived composite systems and coatings
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Polymer-derived ceramics have shown promise as a novel way to process low-dimensional ceramics such as environmental barrier coatings. Composite coatings have been developed as oxidation and carburization barriers on steel using poly(hydridomethylsiloxane) matrix and titanium disilicide as reactive fillers. A systematic study of the phase transformations and microstructural changes in the coatings and their components during pyrolysis in air is presented here. The system evolves from an amorphous polymer filled with a binary metal at room temperature to an inorganic amorphous network of oxidized silicon and titanium at the target temperature of 800 °C. Crystallization of the composite occurs at higher temperatures to reach cristobalite and rutile by 1600 °C. The polymer-to-ceramic conversion occurs between 200 and 600 °C. The oxidation of the expansion agent and the densification of the composite take place between 300 and 800 °C.
I. INTRODUCTION
The formation of ceramics and ceramic composites from polymeric precursors has garnered interest due to several advantages it offers over traditional methods of ceramic processing. Those advantages include lower pyrolysis temperature, tailorable composition, and the ability to use polymeric processing techniques, such as dip and spin coating and injection molding.1 While significant effort has been spent to create bulk and near-set shape parts, the most promising area for the application of these materials has been in low-dimensional products such as coatings, joints, and fibers.2–9 In the area of coatings, preceramic polymers, such as polysilazane, polysilsesquioxane, and polycarbosilane, have been used to form coatings and thin films on a variety of substrates, mainly for thermal and environmental protection of the substrate.2–5 The focus of this research was to implement the novel processing of preceramic polymers to develop a ceramic composite coating on stainless steel as a barrier to oxidation and carburization. The anticipated application is for use in steam–methane-reforming reaction chambers, where steel is exposed to steam and methane gas at temperatures between 800 and 900 °C. Preceramic polymers are organometallics that lose their organic components upon pyrolysis to form an amorphous or nanostructured ceramic. The processing of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0246 J. Mater. Res., Vol. 22, No. 7, Jul 2007
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polymer-derived ceramics (PDCs) can be thought of as occurring in three stages10,11: synthesis of the polymeric precursor; cross-linking of the precursor into a highmolecular-weight compound; and pyrolysis of the crosslinked system into the desired ceramic. A broad range of polymeric precursors are now available commercially. Most preceramic polymers are thermosets and can be liquid or solid at room temperature. Cross-linking, or curing, is the step by which linear polymer chains rearrange, for example, by hydrolysis and condensation
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