TitleThe evolution and implication of boron carbide microstructural variations and transformations during powder processing
NameMaiorano, Daniel W. (author), Haber, Richard A (chair), Niesz, Dale E (internal member), Mayo, William E (internal member), Domnich, Vladyslav (internal member), Rafaniello, William (outside member), Behler, Kristopher (outside member), Rutgers University, Graduate School - New Brunswick,
SubjectMaterials Science and Engineering,
DescriptionBoron carbide is a material of key interest in structural ceramic applications due to its extremely favorable physical properties including low theoretical density, high hardness, high strength, and excellent wear resistance. However, boron carbide microstructures often exhibit the presence of secondary phases, most typically composed of carbon. Under very high stress and strain rate conditions, boron carbide has been shown to have reduction in compressive strength. It has been postulated that a ~3 nm wide region of amorphous material is the cause of this reduction in properties and appears to be the result of a pressure induced structural transformation. The impact of both carbon inclusions and the amorphous bands on bulk physical properties is not easy to resolve due to the complex crystal structures of the boron carbide solid solution from 8-20 at% carbon. As these variations have only been noted to date following application of high stress upon the boron carbide with no examination of powders prior to high stress, this dissertation attempts to determine whether these variations are inherent to boron carbide powders or introduced during typical powder processing. Most boron carbides are produced by a large scale carbothermic reduction process within an electric arc furnace. While it remains the most cost effective production method at this time, carbothermic reduction suffers from uneven heating rates across the reactive melt which makes it difficult to maintain a chemically uniform product. Boron carbide melted ingots are crushed, followed by comminution and sorting based upon size. To determine the extent of preexisting variations, commercially available powders were compared with segments taken directly from the carbothermic reduction ingot via X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Raman microspectroscopy. All powders were then subjected to various levels of comminution to examine the level of pressure induced by common powder processing. Analyses showed that due to the nature of the carbothermic reduction process, significant stoichiometry variations of over 2 at% carbon are present across the length of segments classified as “good” boron carbide visually. These stoichiometric variations correspond with a large presence of secondary phase carbon, including unreacted precursor material, deposited thin films, and graphite products. The comminution process impacted the relative exposed carbon content of the boron carbides, resulting in powders that were coated with carbonaceous materials. At high comminution energies in a lab scale jet mill, resulting surfaces exhibited evidence of amorphized boron carbide through Raman and FTIR analysis. Attempts at reversing the processing induced variations proved ineffective and suggest inherent difficulties in altering the structure of boron carbide produced via carbothermic reduction.
NoteIncludes bibliographical references
Noteby Daniel W. Maiorano
CollectionGraduate School - New Brunswick Electronic Theses and Dissertations
Organization NameRutgers, The State University of New Jersey
RightsThe author owns the copyright to this work.