Evaluating the effect of powder oxygen content on the microstructure and mechanical properties of silicon carbide densified by spark plasma sintering

Silicon carbide (SiC) is an important material in industry due to its favorable mechanical, thermal, chemical, and electrical properties. While it has been mainly used as an abrasive material in the past, more modern applications like armor and other structural applications, often require densified ceramic bodies. SiC powders can be densified in a number of ways, but one common method is solid-state sintering, either with or without applied pressure. It is well known that in the presence of oxygen, pure SiC will form a passivating oxide layer of silica (SiO2) on its surface. This poses a problem in sintering as SiO2 can inhibit the densification of solid state sintered SiC. This thesis examines the effects of varying oxygen content levels in silicon carbide powders on the microstructure and mechanical properties of the resulting densified bodies after solid state sintering via the spark plasma sintering (SPS) method. Two commercial SiC powders were obtained, characterized, and treated to introduce a range of different oxygen content levels. These powders were then densified via the spark plasma sintering method using boron carbide and carbon additives to produce dense samples. Three series of samples were made using each powder, one varying the amount of carbon added as a particulate, one varying the amount of carbon added as a liquid resin, and one where the oxygen content of the powder was directly manipulated by HF washing, aging, or heat treating. The dense SiC samples were then characterized to determine the effect of the powder’s oxygen content on the microstructure and mechanical properties. The samples were examined using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), Knoop microhardness testing, and nondestructive ultrasonic evaluation techniques including acoustic spectroscopy and conventional NDE methods. SEM and EBSD analysis revealed that changes in the powder oxygen content can result in a number of microstructural effects. At intermediate oxygen levels, exaggerated grain growth can occur resulting in large plate-like grains, accompanied by a transformation from the 6H to 4H SiC polytype. At higher oxygen levels, densification may be inhibited and at very high oxygen contents formation of an oxygen rich secondary phase can occur. Varying the oxygen content of the SiC powder also significantly affects the mechanical properties of the dense ceramic. Ultrasonic measurements of the elastic properties showed a clear decrease in the elastic moduli as the oxygen content is increased. Knoop microhardness measurements show similar behavior with a reduction in hardness with increased powder oxygen content.Ph.D.Includes bibliographical referencesby Vincent Alexander DeLucc