Text

theses

Silicon carbide has a high melting point, high mechanical and elastic properties and excellent chemical stability. Boron carbide is a non-metallic material with exceptional physical and chemical properties. Boron carbide is third hardest material after diamond and cubic boron nitride. Besides a high hardness, boron carbide also has a high melting point, high strength, high neutron cross-section, and low density. Due to the their outstanding properties both materials can be used in spray nozzles, turbine engines, heat conducting tubes, and in the defense industry as armor plates. Since silicon carbide and boron carbide are both strongly covalent bonded, the densification of these materials is extremely difficult. High sintering temperature and fine powder sizes are critical to achieve a high density. Fine starting powders also add the presence of oxygen. The residual oxide layer forms as SiO2 on the SiC surfaces and as B2O3 on the B4C surfaces. These oxide layers inhibit high density and cause grain coarsening.
The goals of this thesis were to produce high density SiC-B4C composites, optimize the mechanical properties of SiC-B4C composites, and understand the role of excess oxygen content on sinterability. To achieve these goals, the oxygen content was managed using two methods; a safe and effective laboratory scale acid etching process developed to reduce the oxygen content of SiC and additional carbon used to remove the residual oxide layer. Then, SiC- B4C composites with varying amounts of C powders were mixed and sintered by spark plasma sintering method (SPS).
The dense composites were characterized to evaluate the effect of the oxygen content and residual carbon on the microstructure and mechanical properties. The composite samples’ microstructure was characterized using The Zeiss Sigma emission scanning electron microscopy. The phase of the composites was determined using X-ray diffraction. Poisson’s ratio, Young’s modulus, shear modulus, and bulk modulus were measured by ultrasound analyses. Since the densification of a ceramic affects the mechanical properties, the Archimedes method was used to determine the density of the sintered composite samples. Polished samples were used for hardness testing using a Vickers diamond tipped (9.8 N load) LECO-M-400-G3 and Berkovich nano-hardness (100 mN-500 mN load).
The results showed that the oxygen content and the addition of carbon should be matched to achieve high density and high mechanical properties. Carbon also played a role on the mechanical properties as well as the oxygen since modifying the oxygen content by adding varying amounts of carbon caused surplus carbon. The presence of excess carbon decreased the elastic modulus and hardness of the composite.

ETD_9405

Ph.D.

Includes bibliographical references

by Zeynep Aygüzer Yaşar

doi:10.7282/t3-1s5q-0n15

ETD doctoral

The author owns the copyright to this work.