Date of Award

2007

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Advisor

Vincent Caccese

Second Committee Member

Donald A. Grant

Third Committee Member

Michael L. Peterson

Abstract

High temperature compression tests were administered in an oxidizing atmosphere on a commercially available monolithic (un-reinforced) reaction bonded silicon carbide (RBSiC) and an in-house processed silicon carbide based on a pre-ceramic polymer precursor. Ceramics such as silicon carbide have exceptional mechanical properties for both structural and thermal load applications. High tolerance specimen preparation via diamond wheel cutting and grinding to allow for compression testing on both types of silicon carbide ceramic at temperatures ranging from 20°C (68°F) to 1000°C (1832°F) is also addressed. Two types of specimen geometry were chosen due to the high strength and high stiffness of silicon carbide materials. A reduced gage section specimen geometry, often referred to as a dumbbell shaped geometry, and a constant cross section rectangular prism shaped specimen were used in a direct end loading compression test using custom machined ceramic fixtures. To allow for minimal failure of the custom ground silicon carbide load platens, the dumbbell shaped specimens were loaded to ultimate failure, which in many cases was a brittle, high energy release failure, whereas the constant cross section specimens were loaded only to retrieve strain data for calculation of the specimen compressive modulus of elasticity. In-house processed specimens were tested using only the rectangular prism specimen geometry, due to their lower strength and stiffness; which did not require a dumbbell geometry. Results from compressive testing in an oxidizing (air) atmosphere demonstrate both silicon carbide types, commercial and in-house processed, had significant strength and stiffness dependence on temperature. Failure modes for the commercial grade allowed for a brittle, pulverizing failure at temperatures up to 600°C (1112°F), with a yielding and plastic deformation failure mode at temperatures ranging from 800-1000°C (1472- 1832°F), where the in-house grade only pulverized upon failure. Average compressive strength and modulus of elasticity for the commercial grade ranged from 346 ksi (2385 MPa) and 34.9 msi (241 GPa) at room temperature, respectively, to 88.3 ksi (608 MPa) and 12.4 msi (85 GPa) at 1000°C (1832°F). Density of the reaction bonded commercial grade silicon carbide specimens was consistent at 3.00 g/cm3. For the in-house processed grade, lower strength and stiffness data resulted from testing, due to the porosity of the material and due to flaws and imperfections in the form of cracks from processing. Density of the processed silicon carbide ranged from 2.41 g/cm3 to 2.57 g/cm3. Average compressive strength and modulus of elasticity for the in-house grade ranged from 109.1 ksi (752 MPa) and 25.6 msi (177 GPa) at room temperature, respectively, to 80.6 ksi (556 MPa) and 9.93 msi (69 GPa) at 1000°C (1832°F), respectively.

Download

Share