High temperature static and dynamic mechanical behavior of advanced ceramic materials

Ceramic matrix composites have emerged as candidate materials that will allow higher operating temperatures in advanced engines. A need therefore exists to determine the stiffness and damping capacity properties of these advanced experimental materials as a function of temperature. The damping behavior can provide a source of energy dissipation that may have profound effects on the materials high cyclic fatigue performance and capability to control noise and vibrations. In this investigation resonance frequency and internal friction measurements were performed on a variety of ceramic composite materials in the audio frequency range (700-2000 Hz) as a function of temperature (20{dollar}spcirc{dollar}C to 1200{dollar}spcirc{dollar}C) in vacuum (10{dollar}sp{lcub}-4{rcub}{dollar} Torr). Similar measurements were conducted on CVD silicon carbide fibers in order to determine effects of the fiber constituents on the overall composite's elastic and anelastic response.; In addition this investigation effects of annealing on, Nextel 610 and Altex continuous polycrystalline alumina fiber, were determined in air for 3, 10, and 100 hr at temperatures of 500 to 1300{dollar}spcirc{dollar}C. The fibers' room temperature strength and microstructure were determined. Also, fiber bundles were heat treated to determine their susceptibility to sintering in the woven state. Changes in fiber tensile creep stress-rupture, and bend stress relaxation (BSR) behavior with annealing was then determined. BSR tests were also used to compare as-received and annealed fibers against other polycrystalline oxide fibers. It was shown that annealing can have a significant effect, particularly on the Altex fiber, resulting in increased creep resistance as reinforcements in composite materials.; Silicon carbide fibers are leading candidates as reinforcements in ceramic matrix composites. In the as-produced condition these fibers such as Textron Specialty fiber SCS CVD SiC as well as Carborundum's coreless sintered {dollar}alpha{dollar}-SiC, have both demonstrated excellent high temperature strength, and creep and oxidation resistance up to 1400{dollar}spcirc{dollar}C. It has been stated in the literature that creep properties of the composite will be predicated on the creep resistance of the fiber reinforcements. The damping of reinforcement fibers is important in determining the vibration and creep response of structural composites. In addition the damping capacities of two types of chemically vapor deposited silicon carbide fibers were measured between {dollar}-{dollar}200 and 800{dollar}spcirc C{dollar}. Measurements were made at frequencies from 50 to 15000 Hz at the lowest flexural modes of individual cantilevered fibers. At least four sources have been identified which contribute to fiber damping, the most significant being grain boundary sliding. Potential microstructural factors controlling all sources are discussed, as is the relationship between grain boundary damping and fiber creep by use of mathematical models.