Material Properties of Hard Coatings Developed for High Damping

What is it about?

Some hard coatings used for such purposes as increasing resistance to erosion or corrosion, or for thermal protection, can also dissipate vibrational energy. By doing so, such hard coatings have potential for multifunctional roles in such challenging environments as are encountered by blades in turbojet engines. The vibratory energy can be dissipated by several different mechanisms. Ceramic, metallic, and composite coatings have been considered.1-3 When a coating is applied for the purpose of reducing the vibrational amplitude of a structure or component, the most relevant material property is the loss modulus, defined as the total energy dissipated in a unit volume of material during one complete cycle of oscillation at a particular combination of temperature and amplitude of oscillatory strain. If the design process is to include an evaluation of the influence of the coating on the resonant frequencies of the system, it is also necessary to know the density and the storage modulus of the material, defined as the peak energy stored in a unit volume of material during a completely reversed cycle of oscillation with uniform values of temperature and strain. These moduli are analogous to the real and imaginary parts of a complex Young's modulus. As the desired moduli are not directly observable, it is necessary to deduce them by first measuring their influences on the system level response of a vibrating system. The extent to which the resonant frequency of a test sample is changed by the addition of the coating provides a link to the storage modulus. Likewise, a change in the system loss factor (proportional to the ratio of dissipated to stored energy) resulting from the addition of a coating provides a means of determining the energy dissipated by that coating. From this a loss modulus may be extracted. To identify possible dependencies on frequency and temperature, tests should be conducted in several modes (frequencies), and at least one elevated temperature, as well as at room temperature.

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