Date of Completion

6-6-2016

Embargo Period

6-5-2016

Major Advisor

Dr. Steven Suib

Associate Advisor

Dr. Ronald Wikholm

Associate Advisor

Dr. Edward Neth

Associate Advisor

Dr. Alfredo Angeles Boza

Associate Advisor

Dr. Douglas Adamson

Field of Study

Chemistry

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Chemical Vapor Deposition (CVD) was used to deposit a variety of carbide, boride, and nitride interfacial coatings on substrates including silicon carbide based ceramic fibers for use in fabrication of components in Ceramic Matrix Composites (CMCs). Experiments were designed to correlate coating thickness and time required for these coating depositions and the materials fabricated in these experiments were used in various other experimental designs in this work. Coatings were characterized for their thickness and morphology, as well as to identify the phases present in the materials.

A polycarbosilane pre-ceramic polymer was developed using a modified in-situ Grignard reaction with chlorinated silane precursors. This polymer was designed for use as both a precursor to silicon carbide ceramic fibers and for the ceramic matrix material in CMCs fabricated using Polymer Impregnation Pyrolysis (PIP). The polymer was characterized using Fourier Transform Infrared (FT-IR) spectroscopy and 1 H Nuclear Magnetic Resonance (NMR) to identify functional groups and to propose polymer structure, and Thermogravimetric Analysis coupled with Mass Spectrometry (TG-MS)was used to propose a mechanism for ceramic formation. The pyrolyzed ceramic was characterized using X-Ray Diffraction (XRD) to determine crystalline phases present, Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) to examine grain morphology and elemental distribution.

A second pre-ceramic polymer was designed to incorporate titanium and boron into a commercially available polymer (Starfire® SMP-10) for the intention of improving select properties for use in CMCs as both a fiber precursor and as a matrix material. The polymer was designed to form silicon carbide and titanium diboride upon pyrolysis in a two-step process. Titanium was first incorporated into the polymer, followed by a heat-treatment in boron trichloride to boronize the titanium. Both the as-received polymer and the modified polymer were characterized using TG-MS to determine the ceramic yield and mechanism of ceramic formation. Heat treatment studies were performed using XRD to determine the optimum temperature range for boron incorporation during the heat treatment. The ceramic was characterized using SEM and EDS to examine the grain growth inhibition provided by the titanium diboride, as well as to determine elemental distribution in the ceramics. CMCs were fabricated from both the commercially available polymer and the modified polymer and the composites were analyzed for the ultimate Flexural Stress measured before mechanical failure occurred.

Cobalt metal was deposited on a variety of substrates including ceramic fiber and then coated with CVD silicon dioxide and Chemical Vapor Inf iltration (CVI) silicon carbide protective coatings for use in a magnetic ceramic matrix composite. The protective coatings were designed to help increase oxidation resistance so the composite would retain magnetic properties at elevated temperatures under oxidizing conditions. The material was characterized using XRD to determine crystalline phases present, and SEM and EDS were used to examine the coating morphology, adhesion, and elemental distribution in the composite. Superconducting Quantum Interference Device (SQUID) was used to determine the magnetic properties of the material during coating application and oxidation resistance studies. Tensile testing was used to to determine the ultimate tensile strength and the effect on coating application and oxidation on the strength of the composite both with and without the oxidation resistive coatings.

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