The effect of microstructure in aluminum oxide ceramics on acoustic loss mechanisms
Creators: name: Bottiglieri, Stephen
Publisher: name: No Publisher Supplied
Year: 2012

description: Ceramic materials have become increasingly popular for use in a wide range of applications due to their favorable mechanical, electrical, and thermal properties.  Specifically, aluminum oxide, or alumina (Al2O3), is one of the most studied and widely used ceramic materials available today.  Along with their superior physical properties, alumina-based materials are suitable for many functions due to the variety of processing techniques available, the ability to create tailored microstructures with multiple secondary phases, their shear abundance, and their relatively low cost.  Regardless of what an alumina-based material is used for, its ability to perform to its peak potential is directly dependent on its microstructure.  Microstructural variations or heterogeneities throughout large sample volumes can severely degrade the physical properties of alumina-based materials.  Variations and heterogeneities in this case refer to variations in density and grain size, unreacted sintering additives, unwanted secondary phases, or large porosity.  The ability to predict the performance of not just alumina-based materials but any ceramic material will be dictated by the ability to understand the type, size, and concentration of all features present throughout the bulk microstructure.   Ultrasound nondestructive testing was chosen as the method for characterizing the microstructures of dense, polycrystalline, high hardness, aluminum oxide materials.  The acoustic waves interact with all aspects of a material’s microstructure through frequency dependent mechanisms.  These interactions include those with grains, secondary phases, solid inclusions, and porosity.  Ultrasonic nondestructive testing is a volumetric technique meaning that a single point contains information relating to billions of features throughout the bulk material.  This testing is also performed over large sample areas such that C-Scan maps of elastic properties, sonic velocities, or the degree of acoustic energy loss may be obtained. The work performed in this thesis involves deconvoluting temporal sample surface reflections seen in an oscilloscope to obtain frequency-based attenuation coefficient spectra.  These spectra contain all microstructural information relating to the size scale in which the acoustic wave is capable of directly interacting with.  Further testing includes defining frequency regimes where one of two possible dominant acoustic loss mechanisms is operable: scattering or absorption.  Within the overarching categories of acoustic scattering and absorption lie specific types of loss mechanisms related to each.  For scattering there are three regimes, Rayleigh, stochastic, and diffuse, which describe how acoustic energy is lost depending on wavelength and scatterer size.  Absorption is the general term used for multiple types of loss mechanisms which rely on converting sound energy into heat.  The primary loss mechanisms seen in the alumina-based materials studied in this thesis were intraparticle thermoelastic absorption, Rayleigh scattering, and stochastic scattering. An analytical solution relating the amount of acoustic energy which is lost due to thermoelastic absorption was derived along with a functional form to predict secondary phase grain size distributions through the use of acoustic spectroscopy.  Determination of material constants, used to relate attenuation-based measurements to grain size based on acoustic scattering, allowed for the prediction of average alumina grain sizes.  Four custom engineered alumina-based sample series were created with the intention of testing and validating the absorption and scattering concepts behind acoustic spectroscopy.  Acoustic techniques were developed and applied to single points and large sample areas to obtain microstructural information relating to secondary phase size distributions as well as Al2O3 average grain sizes.  Comparisons to results obtained through the use of conventional microstructural testing, including Field Emission Scanning Electron Microscopy (FESEM) imaging and X-ray Diffraction (XRD), showed strong correlations between measured and predicted results.  The testing performed in this thesis expounds upon the knowledge of ultrasonic interaction in dense, polycrystalline, ceramics and offers novel characterization methods using acoustic spectroscopy which can be proliferated for use with a wide range of other high density materials.