"Internal Friction Mechanisms in Ceramics for Vibrational Energy Dissipation at High Temperatures" 

Benjamin Lam, MSE PhD Candidate 

Advisors: Professors Carlos Martinez & Rod Trice

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ABSTRACT

Mechanical vibrations arise from many different sources, including turbulent flow of air over aircraft wings, uneven roads when driving, and rhythmic detonations in a rotating detonation engine (RDE). RDEs operate at a frequency of several thousand hertz and reach temperatures of 1000°C or higher. Because of these high temperatures ceramics are being considered for RDEs, but due to their inherent brittleness they are generally sensitive to vibrations. In response, there has been an interest in either toughening ceramics for this application or finding a way to dampen vibrations in ceramics. One way to dampen vibrations is through internal friction, which is defined as the energy that is absorbed through internal means. In ceramics, the two primary mechanisms for internal friction that have been observed are reversible grain boundary sliding and long-range segmental motion of glass near the glass transition temperature (Tg). These have been extensively studied in Si3N4 where the glass surrounds the grains. Because the grains are surrounded by glass, when the temperature increases beyond the Tg, extensive grain boundary sliding occurs and mechanical properties suffer. Discreet glass phases have been proposed as a possible solution, since they do not surround the grains with glass, but still have the benefit of internal friction through long-range segmental motion.