Interface and matrix processing evaluation for non-oxide ceramic matrix composites

Date of Completion

January 2009

Keywords

Chemistry, Inorganic

Degree

Ph.D.

Abstract

The aim of the reported research is to develop and characterize chemical processes leading to the fabrication of high temperature structural materials which experience temperatures above 1000°C. Ceramic based materials are chosen due to high melting points and potentially greater then-no-mechanical stability compared with metals. Ceramic Matrix Composites (CMCs) represent a significant improvement in toughness over monolithic type ceramics which allows for their incorporation as structural materials. The synthesis of multiphase material systems was investigated to exploit synergistic enhancement which composite like materials exhibit, especially non-brittle mechanical behavior as ceramic components. ^ Three specific areas of study will be discussed: Chemical Vapor Deposition (CVD) of ceramic interface coatings, ceramic preform and pre-ceramic polymer synthesis for composite processing, and environmental/thermal barrier coatings for oxide and non-oxide substrates. Processing of boron nitride-silicon nitride coatings for SiC based ceramic fibers was studied to identify deposition parameters, microstructure and the resultant composite properties utilizing the coatings in as-received and in wet oxidative conditions above 1000°C. The second area concerns the development and characterization of composites utilizing novel, low cost ceramic preforms and a synthesized polycarbosilazane pre-ceramic polymer compared with commercially available material systems. 2-D 8 ply laminate CMCs via a modified Polymer Impregnation and Pyrolysis (PIP) method were investigated. Commercially available fibers based on SiC were used including Nicalon™ and Tyranno™. Analysis from the first section involving interface coatings was applied to the CMC processing. The third area of consideration is the CVD deposition of refractory/transition metal silicides and oxides of MoSi 2/Al2O3 as barrier coatings for the advanced protection of the metal and non-oxide CMCs substrates. Conditions for the atmospheric pressure deposition of the bi-layer coating are discussed. Total CMC fabrications and the impact of the individual component processing developed in this research will be compared to currently available systems for efficacy of the discussed fabrication routes. ^

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