MS Thesis Presentation by Samuel R. Martin
Monday, April 26, 2004

( Dr. Min Zhou, Chair)

"Experimental Characterization of the Effect of Microstructure on Dynamic Behavior of SiC"

Abstract

For roughly the past 15 years the military has increased its efforts to utilize the properties of ceramics for armor applications. Current high-performance ceramics have extremely high compressive strengths and low densities. Silicon carbide is shown to be highly resistant under ballistic impact. Further, it has been found that even within the silicon carbides, those manufactured by certain methods and those with certain microstructural properties have advantages over others. In an initiative to understand the microstructural reasons behind improved ballistic properties, plate impact tests were conducted on two sintered silicon carbides with slightly different microstructures.


Two variations of a silicon carbide with the trade name Hexoloy SA were obtained through Saint Gobain. Regular Hexoloy (RH) and Enhanced Hexoloy (EH) are pressureless sintered and share exactly the same chemistries. EH went through additional powder processing prior to sintering, producing a final product with a slightly different morphology than RH. Samples of each were characterized microstructurally before plate impact experiments including morphology (grain size, aspect ratio), density, porosity, elastic moduli, Vickers microhardness, fracture toughness, and flexure strength. The characterization revealed differences in percent area fraction of elongated grains, size of largest pores, and flexure strength, the latter two believed to be directly related. In the literature it has been demonstrated that elongated grains as seen in EH could lead to higher fracture toughness. However, in this case, subsequent testing proved that the elongated grains had no effect on material performance. Instead it was determined that the porosity distribution in EH had fewer large pores leading to an 18% increase in flexural strength over that for RH.


The focus of the mechanics of materials community concerning dynamic material behavior is to pin down what exactly is happening microstructurally during ballistic events. Several studies have been conducted where material properties of one ceramic type are varied and the dynamic behavior is tested and analyzed. Usually, from one variation to the next, several properties are different making it hard to isolate the effect of each property. For this study, the only difference in the materials was porosity distribution leading to an 18% increase in the four point flexure strength of EH.


Plate impact experiments were conducted at ARL using the gas gun facilities within the Impact Physics Branch where a VISAR was utilized to measure surface velocities. Tests were performed on each material to determine the Hugoniot Elastic Limit (HEL) and spall strength. Spall strength was measured as a function of impact stress, and pulse duration. Results show very little difference in the HEL and spall strength of RH and EH samples. Within the variability of the test results, the spall strengths were the same, independent of pulse width, and showed a trend similar to that found in other studies for SiC. The materials demonstrated a spall strength above the HEL. Finally, it was found that the HELs of the two types of samples were quite similar.