Fundamental study of a refractory-based carbon fiber reinforced composite made by reactive melt infiltration for hypersonic applications

Ceramic matrix composites are excellent candidates for ultrahigh temperature applications due their good physical properties, which are a combination of a chemically stable matrix and tough fiber reinforcement. This work is a fundamental study of a carbon fiber reinforced zirconium carbide composite (Cf/ZrC). The background chapter reviews reactive melt infiltration, which is the processing method used to make the Cf/ZrC composite. The first chapter discusses the microstructural characterization and development of Cf/ZrC. A formation mechanism of the unique matrix microstructure is proposed based on the characterization results.;In the second chapter the mechanical properties of Cf/ZrC were determined. The fracture toughness at room temperature is obtained with a standard four point bend test, while flexural strength of Cf/ZrC is obtained to the ultra high temperature regime. For high temperatures a testing rig was modified to operate in inert atmosphere and tests were conducted at 1100 °C, 1350 °C and 1650 °C. Correlation is made between the flexural strength and fiber coatings of two different composite types. In situ compression tests were performed a modified SEM. Digital image correlation was used to monitor strains during compression. The stress-strain information is correlated to surface deformation.;The environmental durability and oxidation behavior of Cf/ZrC and ZrC is detailed in the third and fourth chapters. The oxidation and shock behavior of Cf/ZrC were observed under both slow and rapid heating rates to ultra high temperatures. For rapid heating rates a panel was subjected to heating at steady-state and non-steady state heat flux. For slow heating rates specimen coupons were heated at 2000 °C in a bottom-loading furnace. Specimens were characterized post-test by x-ray diffraction, electron microscopy, electron probe microanalysis, and optical microscopy.;The oxidation kinetics of Cf/ZrC composites and ZrC powders (45 micron and 60 nanometer particle sizes) were determined at 1600 °C, 1500 °C, 1400 °C, 1300 °C and 1200 °C using thermogravimetric analysis (TGA). The environment chosen was a low oxygen concentration (0.5 ppm oxygen in argon). Oxidized composite coupons were examined post-test: cross-sections were observed by SEM and oxide scales were analyzed by x-ray diffraction. Changes in ZrC lattice parameter were correlated to the formation of oxycarbide during oxidation. The kinetics of the two powder types are compared. Particle sizes pre- and post-test were analyzed by SEM and determined by x-ray diffraction using a modified form of Scherrer's equation.;The dissertation concludes with suggestions for future work. There are still many technical challenges related to reactive melt infiltration processing and prohibitive costs of raw materials. There are also many experimental opportunities to study the oxidation of these materials as they relate to hypersonic applications.