Ph.D., Faculty of Engineering and the Built Environment, University of the Witwatersand, 2012 / This thesis focuses on the boron suboxide B6O, a boron-rich super-hard ceramic material.
With hardness values previously reported between 24 GPa and 45 GPa, B6O is one of
the hardest known materials. Although first reports on boron suboxides date back as
far as 1909 (Weintraub E., Transactions of the American Electrochemical Society, 16
(1909) 165), it is the B6O-based composites that have attracted considerable interest in
recent years due to their enormous technological potential, especially as an alternative
to polycrystalline diamond and polycrystalline cubic boron nitride for wear and abrasive
applications. Investigations into the properties of B6O itself appear to have been neglected
in favour of the improvement of densification and fracture toughness of the composites.
The B6O samples used for the work reported in this work was hot-pressed was prepared
under an argon environment at 1800 C and 50 MPa for 20 minutes followed metallographical
preparation. The density of the hot-pressed compacts measured 2.44 g/cm3. The
starting B6O powder material was supplied from the Fraunhofer Institute for Ceramic
Technologies and Systems, Dresden.
This thesis primarily seeks to generate and report as much practical data for polycrystalline
B6O materials prepared by uniaxial hot-pressing as possible from a variety of characterization
techniques. Firstly, the Raman spectra of hot-pressed B6O, which was until now
poorly understood, was investigated using using a 514.5 nm green Ar+ laser excitation
source. Secondly, the fundamental nature of some mechanical properties of hot-pressed
B6O were investigated by means of Vickers and Berkovich indentation techniques. New
approaches for data analysis, especially the investigations of the nanomechanical properties
of hot-pressed B6O by Berkovich nanoindentation, were also suggested. Thirdly,
the intrinsic hardness of hot-pressed B6O was investigated by means of a comprehensive
inter-model comparison study. Fourthly, a combined experimental and simulation approach
for determining mechanical properties of hot-pressed B6O by nanoindentation was also
carried out, based on the outcomes of the study, the deformation response of the material
under dynamic indentation was investigated at di erent stages using a custom developed
finite element model. Finally, based on the preliminary ab initio density functional calculations
of the structural properties of B6O conducted by Lowther showing that the presence
of a high electronegativity interstitial in the B6O structure could enhance the strength
of the bonding in B6O, fluorine ion implantation into B6O were conducted. The e ects
to the structural and the nanomechanical properties of radiation damage induced by ion
implantation was investigated. The possible formation of novel nanostructures in the
ion-irradiated B6O matrix near-surface was also investigated.
Results obtained from this study provides a vast amount of practical data for hot-pressed
B6O materials as well as a number of novel analysis approaches for the extraction useful
properties from the measured raw data. Firstly, using an automated background subtraction
method, observable first- and second-order Raman spectra of B6O were obtained. A
comparative analysis with previously reported spectra of other -rhombohedral boron-rich
ceramic materials demonstrate a good agreement. Results also confirm the existence
of highly resolved Raman modes measured at ambient conditions using a green Ar+
excitation source which is contrary to the conventional understanding. Secondly, results
from the micro-indentation investigations indicate the measured microhardness exhibits
indentation load dependence. A model inter-comparison study of indentation size e ects
in the microhardness measurements of hot-pressed B6O is comprehensively discussed.
Thirdly, the intrinsic hardness value of 30 GPa was deduced. Fourthly, a quantitative
analysis approach was developed to simulate multi-cycling loading load-displacement
curves from a single measured load-displacement nanoindentation curve. Based on the
results, the nature of the indentation size e ect in the nanoindentation hardness as well as
the intrinsic nanomechanical properties of hot-pressed B6O were established. Fifthly, a
combined experimental and finite element method simulation approach for determining
mechanical properties of hot-pressed B6O by nanoindentation was developed. Based on the
outcomes of the combined experimental and simulation studies, the deformation response
of the material under dynamic indentation was also investigated at di erent stages using a
custom developed finite element model. Finally, results from the structural characterization
of the ion implanted B6O material demonstrates the formation of novel nanostructures by
means of the ion bombardment of B6O. In addition, the study presented here also seeks to
investigate the e ects of the fluorine ion implantation on the near-surface nanomechanical
properties of hot-pressed B6O.
The principal conclusions that the study provide are both comprehensive practical data
for B6O materials prepared by uniaxial hot-pressing. A number of properties, including
the Raman spectra, the intrinsic hardness, and the radiation resistance and the e ects of
radiation damage are reported in the thesis.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/11980 |
| Date | 25 September 2012 |
| Creators | Machaka, Ronald |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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