Advisory Committee Chair
Advisory Committee Members
Gregory B Thompson
Date of Award
Degree Name by School
Doctor of Philosophy (PhD) College of Arts and Sciences
Superhard materials are catching a lot of attention due to their unique properties and practical applications. When the hardness value of the material exceeds 40 GPa by the Vickers hardness test, the material is called superhard. Diamond is the hardest known material with a hardness value of 70-100 GPa. Although diamond is electrically insulating and thermally conductive, it has significant limitations. In addition to its high cost, it is prone to oxidation at temperatures above 800 °C and unsuitable for high-speed machining of ferrous alloys because of chemical reactions with iron group elements. Thus, diamond is unaffordable and unfavorable for many applications, which leads us to search for new novel superhard materials. Materials based on the light elements of carbon, nitrogen, oxygen, and boron (referred as the ‘CNOB’ system) comprise some of the hardest known materials. These light elements can form short bond lengths with each other and are inclined to form directional covalent bonds, making the structures they form difficult to compress or distort. The goal of this dissertation is to develop novel superhard materials from the CNOB system in low-temperature plasmas and develop an understanding of the formation process and resulting material properties. Reactant gases for growth of the coatings from CNOB system will include H2, CH4, N2, and B2H6. However, little is known of the plasma chemical species and underlying spectroscopy associated with these gas iv precursors in a microwave plasma chemical vapor deposition (MPCVD) environment leading to novel superhard materials. In the context of synthesizing superhard materials from the CNOB system, the investigation of excited state plasma species through optical emission spectroscopy (OES) helps to identify the growth species responsible for the formation of the desired materials. By studying the emission spectra over a range of wavelengths, it is possible to identify specific atomic or molecular species that are present in the plasma during the synthesis process. Microwave power and chamber pressure was systematically varied to investigate their role on growth species. Optimal conditions determined through the investigation of OES was utilized to synthesize the materials from CNOB system, including boron-rich boron carbide (B50C2) and carbon-rich BC10N. The effect of applying a DC substrate bias was also investigated and found to enhance sp3-bonded cubic boron nitride (cBN) and wurtzite boron nitride (wBN). Our work demonstrates the first successful synthesis of a metastable wurtzite boron nitride (wBN) phase made by using a low-temperature plasma method. The compressive stress induced by external DC bias is believed to be responsible in converting B-N bonding from sp2 to sp3. This is supported by our studies involving the effect that argon ions in feedgas mixture. This dissertation thoroughly examines MPCVD coatings to gain a detailed understanding of their properties and structures. X-ray photoelectron spectroscopy (XPS) was employed to analyze all synthesized samples' composition and bonding environment. This technique allowed for identifying and quantifying elements present in the coatings, providing valuable insights into their chemical makeup. Following XPS analysis, x-ray diffraction (XRD) measurements were performed to determine the crystal structure. Raman v spectroscopy was employed to investigate molecular vibrations and lattice dynamics within the materials. Chemical composition and molecular structure were performed by Fourier Transform infrared spectroscopy (FTIR). FTIR allow us to identify functional groups, chemical bonds, and molecular structure present in the coatings. Scanning electron microscopy and atomic force microscopy was also used for morphological analysis. Finally, the mechanical properties of the coatings were evaluated through nanoindentation hardness measurements. Nanoindentation is a technique that involves applying a controlled force on a small area of the sample surface using an indenter, typically a diamond tip. The resulting load-depth curve provides information about the hardness, elastic modulus, and other mechanical properties of the material. The measurements revealed that all synthesized samples, except for hexagonal boron nitride (hBN), were ‘superhard’, confirming their potential for various applications. Overall, this dissertation contributes to our understanding of superhard coatings in low temperature plasma, offering insight into their synthesis, properties, and applications across various fields of research.
Chakrabarty, Kallol, "Microwave Plasma CVD Sythesis of Superhard CNOB Materials" (2023). All ETDs from UAB. 439.