Silicon doped boron carbide for armour

214 Pages·2012·47.47 MB·English

IMPERIAL COLLEGE LONDON
Department of Materials

Silicon doped boron carbide for armour

by
Cyril Besnard

October 2017

A thesis submitted for the degree of Doctor of Philosophy (PhD)

Declaration

I hereby confirm that this thesis is my own work. Works by collaborators and from the literature
have been acknowledged and referenced.
Cyril Besnard

‘The copyright of this thesis rests with the author and is made available under a Creative
Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy,
distribute or transmit the thesis on the condition that they attribute it, that they do not use it for
commercial purposes and that they do not alter, transform or build upon it. For any reuse or
redistribution, researchers must make clear to others the licence terms of this work.’

i | P a ge
Abstract
Boron carbide is a popular candidate armour ceramic. During high velocity impact, however,
amorphous bands form, leading to the collapse of the structure, and reducing the usefulness of
boron carbide in such applications. Recent experimental data from the literature suggests that
silicon (Si) doping of boron carbide nanowires reduces this amorphisation. This project focuses on
creating a process for Si doping of boron carbide that could potentially be up-scaled to commercial
quantities. As shown in this thesis, as long as free carbon, or, in fact, carbon-rich boron carbide is
present, Si additions react with carbon to form silicon carbide. Therefore, three methods for
reducing the carbon content in boron carbide powders were investigated: plasma cleaning,
oxidation/reduction, and annealing in the presence of amorphous boron (B). This resulted in a range
of boron carbide powders with various carbon contents covering a wide range of the phase diagram.
The effect on the structure of the powder will be discussed.
Si was mixed with these boron carbides, and evidence for Si-doped boron carbide phase B (C,Si,B)
12 3
was found in many of the powders produced, as well as with additional phases. An enhancement of
the doping correlated with a reduction in initial carbon content for a comparable concentration of Si.
A promising result of the reduction of the amorphisation on the doped powder was confirmed for
one condition after high pressure diamond anvil testing. Similarly, the reduction of amorphisation
was also confirmed by indentation at the interface of a diffusion couple formed from a wafer of Si
annealed at 1400 °C between two pieces of boron carbide. The boron carbide at the interface
exhibited Raman features similar to the Si-doped powder. These results of powder and interface
suggest that a new type of lightweight armour material could be produced that overcomes one of
the biggest challenges of this ceramic: the amorphisation.

ii | P a ge
Acknowledgments
Firstly, I would like to thank my sponsors: the Defence Science and Technology Laboratory (DSTL),
and Peter Brown, and also my supervisors, for giving me the opportunity to undertake this project. I
am very grateful to Luc Vandeperre and Finn Giuliani for all their support, discussions, help, and
guidance throughout the past few years. I would also like to thank Garry Stakalls for all his help with
the equipment needed for this project, and Mahmoud Ardakani and Ecaterina Ware for all their
training and support with the electron microscopy facility.
There are also many people who assisted me with various data analyses: I would like to thank Ayan
Bhowmik for his help with the TEM/FIB analysis, and Ben Britton, Catriona M. McGilvery, Angela E.
Goode, Hendrix Demers, and Dominique Drouin for their help with the Quanta, EDS Bruker, and
Monte Carlo study using CASINO. I am also very grateful to Anna Regoutz for the XPS analysis, John
E. Proctor for the diamond anvil tests and Raman analysis, Tommaso Giovannini for the in situ
indentation, and Maria Parkes and Joao Piroto Duarte for their help with the Raman analysis and
impact tests. I would like to thank Yatish Patel, Adam Tudball, Rick Fitzgerald, Richard Sweeney, and
Victoria Bemmer for their discussions on and help with the furnaces and XRD during my PhD. I
appreciate the hard work of Haomin Wang, my MSc student, and Markus Wohlfahrt, my
undergraduate student during their studies, and their contribution to my project.
I would like to thank all the people at the Centre for Advanced Structural Ceramics (CASC), Imperial
College, for their support and help throughout this time, and Ayan, Angela, Jane, and my brother for
their help with the writing of my thesis. Jane, thank you very much for accompanying me in
discovering and experiencing new things outside of doing research. Finally, I would like to thank all
my friends, and my sister, my parents, and my family. I would not have been able to achieve this
without their kind support and warm help with every aspect of my life in London.

iii | P a ge
Table of Contents
1 Introduction .................................................................................................................................... 1
1.1 Structure of the thesis ............................................................................................................ 3
2 Literature Review ............................................................................................................................ 5
2.1 Boron carbide .......................................................................................................................... 5
2.2 Production ............................................................................................................................... 9
2.3 High pressure amorphisation ................................................................................................ 10
2.4 Production of silicon doped boron carbide .......................................................................... 20
2.5 Removal of free carbon and carbon from the structure ....................................................... 24
2.6 Oxidation of boron carbide ................................................................................................... 29
2.7 Densification ......................................................................................................................... 31
2.8 Conclusion ............................................................................................................................. 33
3 Materials and Methods ................................................................................................................. 35
3.1 Raw materials ........................................................................................................................ 35
3.2 Characterisation techniques ................................................................................................. 43
4 Reduction of carbon ...................................................................................................................... 64
4.1 Introduction .......................................................................................................................... 64
4.2 Conclusion ........................................................................................................................... 102
5 Si-doped boron carbide powder ................................................................................................. 104
5.1 Introduction ........................................................................................................................ 104
5.2 Experimental procedures .................................................................................................... 104
5.3 Results ................................................................................................................................. 106
5.4 Conclusion ........................................................................................................................... 145
6 Establishing the viability of silicon doping of boron carbide ...................................................... 146
6.1 Introduction ........................................................................................................................ 146
6.2 Experimental procedures .................................................................................................... 146
6.3 Results ................................................................................................................................. 147
iv | P a ge
6.4 Discussion ............................................................................................................................ 164
6.5 Conclusion ........................................................................................................................... 166
7 Conclusion ................................................................................................................................... 168
8 Future work ................................................................................................................................. 169
References .......................................................................................................................................... 172
Appendix ............................................................................................................................................. 189

v | P a ge
Nomenclature & Abbreviations
a- Amorphous
Al Aluminium
Ar Argon
Ar/10 % H Argon plus 10 % hydrogen
2
As Arsenide
At% Atomic %
B Boron
BCl Boron chloride
3
BSi Backscattered electron image
B O Boron oxide
2 3
B C/B . C Boron carbide
4 43
B O/B O Boron suboxide
6 12 2
B As Boron arsenide
12 2
B P Boron phosphide
12 2
C Carbon
CO Monoxide carbon
Cr Chromium
CVD Chemical vapour deposition
DFT Density functional theory
DTA Differential thermal analysis
EDS Energy dispersive X-ray spectroscopy
EELS Electron energy-loss spectroscopy
FIB Focus ion beam
FT Fourier transform
FTIR Fourier transform infrared
H BO Boric acid
3 3
HP Hot pressed
HR High resolution
IR Infrared spectroscopy
ISE Indentation size effect
MEK Methyl ethyl ketone
Mg Magnesium
Mg(ClO ) Magnesium perchlorate
4 2
MgO Magnesium oxide
NMR Nuclear magnetic resonance
O Oxygen
P Phosphorus
PC Plasma cleaning
pCo Partial pressure of CO
PVA Poly(vinyl alcohol)
R3(cid:3364)m Rhombohedral lattice of trigonal symmetry
sccm Standard cubic centimetres per minute
SEi Secondary electron image
Si Silicon
SiB Silicon triboride
3
SiB Silicon tetraboride
4
SiB Silicon hexaboride
6
vi | P a ge
SiC Silicon carbide
SPS Spark plasma sintering
STEM Scanning transmission electron microscopy
TEM Transmission electron microscopy
TGA Thermal gravimetric analysis
V Vanadium
W Tungsten
Wt% Weight %
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
3D Three-dimensional
ρth Theoretical density

vii | P a ge
List of Figures
Figure 1. A chart of the hardness and density for materials created with the CES EduPack 2015
software [30, 31]. .................................................................................................................................... 2
Figure 2. Flowchart of the studies. ......................................................................................................... 4
Figure 3. (a, b) Structures of boron carbide in the rhombohedral crystal lattice and hexagonal crystal
lattice. (c) The main compositions for the chain [53, 55], with boron atoms in blue and carbon atoms
in orange. The structures were plotted using VESTA [56] with slight modification of the cif file from
Clark et al. [54]. ....................................................................................................................................... 6
Figure 4. Binary phase diagram for boron-carbon in atomic percent (figure was reproduced from [58]
with permission of the rights holder, AIP Publishing LLC). Electron probe microanalysis and chemical
analysis were used to measure the carbon content. .............................................................................. 7
Figure 5. Cumulative mass of debris with a maximum of 4 mm in size plotted as a function of the
impact velocity. The critical zone is in orange (figure reproduced from [41] with permission of the
rights holder, The American Association for the Advancement of Science). ....................................... 11
Figure 6. Comparison of Raman spectra from different samples as reported. Annealing was done
using a laser with a wavelength of 514.5 nm. Figure is credited to [92], with permission of the rights
holder, Elsevier. .................................................................................................................................... 13
Figure 7. Comparison of Raman spectra of nanocrystalline boron carbide after various loads. Figure is
credited to [47], with permission of the rights holder, Nature Publishing Group. ............................... 14
Figure 8. Evolution of Raman spectrum with depth on an indentation at a load of 100 g on a boron
carbide sample. The figure is credited to [91], with permission of the rights holder, Elsevier. ........... 15
Figure 9. (a) 3D representation post-indentation of the amorphisation using Raman spectroscopy
mapping with the filter for the D peak. (b) Another orientation of the representation, and the D peak
was plotted in red. Figure is credited to [91], with permission of the rights holder, Elsevier. ............ 15
Figure 10. Schematic diagram of the formation of B (CCC) from B C (CBC), with eq for equatorial
12 11 eq
and p for polar (reproduced from [22, 23] and plotted with VESTA [56]). ........................................... 16
Figure 11. Schematic diagram of the formation of a-C and a-B from rhombohedra crystal structure
12
of boron carbide B C(CBC) (reproduced from [86] and plotted using VESTA [56]). ............................ 17
11
Figure 12. Comparison of the XRD patterns of B C starting powder and B C infiltrated with Si at 1450
4 4
°C for 20 min. Figure is credited to [115], with permission of the rights holder, Elsevier. ................... 22
Figure 13. Energy-filtered transmission electron spectroscopy chemical maps of C and B elements,
showing graphite within boron carbide grain. Figure is credited to [135], with permission of the
rights holder, John Wiley and Sons. ...................................................................................................... 24
Figure 14. XRD pattern of boron carbide powder with B:C > 4 heated at 1850 °C. Figure is credited to
[143], with permission of the rights holder, Taylor & Francis. ............................................................. 25
Figure 15. The XRD patterns of the mixture 2B O + 4C after heat treatment at different
2 3
temperatures for 20 min. Figure is credited to [130], with permission of the rights holder, Elsevier. 26
viii | P a ge