Effect of Prior Exposure at Elevated Temperatures on Tensile Properties and Stress-Strain Behavior of Four Non-Oxide Ceramic Matrix Composites

Author

Sarah M. Wallentine

Date of Award

6-18-2015

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD.

Abstract

Thermal stability of four non-oxide ceramic matrix composites was studied. The materials studied were commercially available composites: SiC/SiNC; C/SiC; C/SiC-B4C (C/HYPR-SiC^TM); and SiC/SiC-B4C (SiC/HYPR-SiC^TM). COI Ceramics manufactured the SiC/SiNC and C/SiC composites using polymer infiltration and pyrolysis (PIP). The C/HYPR-SiC^TM and SiC/HYPR-SiC^TM CMCs were manufactured by Hyper-Therm High-Temperature Composites using chemical vapor infiltration (CVI). All four composites rely on a dense matrix for strength, stiffness, and oxidation protection. Fiber coating provides a fiber-matrix interface that allows fiber-matrix debonding and fiber pullout to occur, imparting fracture toughness to the CMC. The SiC/SiNC, C/SiC, and C/HYPR-SiC^TM composites were heat treated in laboratory air for 10 h, 20 h, 40 h, and 100 h at over-temp (1300°C) and for 100 h at operating temperature (1200°C). The SiC/HYPR-SiC^TM composite was heat treated in laboratory air for 10 h, 20 h, 40 h, and 100 h at over-temp (1400°C) and for 100 h at operating temperature (1300°C). Room-temperature tensile properties of heat treated and virgin material were measured, and effect of prior heat treatment on tensile properties was evaluated. Prior heat treatment caused a reduction of tensile strength of at least 10% for all materials. Both PIP-produced CMCs exhibited increased fiber-matrix bonding due to high temperature exposures, contributing to brittle fracture of clumped fiber bundles and thus reduced tensile strength. Both CVI-produced CMCs exhibited considerable matrix voids due to poor infiltration during fabrication. Void prevalence and the associated stress concentrations contributed to premature matrix cracking and composite failure. The CVI CMCs were susceptible to degraded tensile properties and brittle composite fracture due to strengthened fiber-matrix interfaces and fiber degradation caused by prior heat treatment.

AFIT Designator

AFIT-ENY-MS-15-J-048

DTIC Accession Number

ADA623154

Recommended Citation

Wallentine, Sarah M., "Effect of Prior Exposure at Elevated Temperatures on Tensile Properties and Stress-Strain Behavior of Four Non-Oxide Ceramic Matrix Composites" (2015). Theses and Dissertations. 208.
https://scholar.afit.edu/etd/208