Processing and Deformation of ZrB2
Author(s)
Wang, Jianye
Type
Thesis
Abstract
Zirconium diboride, ZrB2, based materials have been proposed for structural applications at ultra-high temperatures (>2000 [degrees] C). However, their
mechanical behaviour at such temperatures is only poorly documented. In
this work, the processing and the deformation behaviour of ZrB2 at temperatures up to 2000 [degrees] C is investigated.
Densification of zirconium diboride based materials is difficult and most
reported routes use a combination of high pressures and high temperatures
to obtain a high density. However, it had been reported that with the
aid of carbon, boron carbide and silicon carbide, pressureless sintering of
ZrB2 is possible. Further work in this thesis shows that the key factor to
obtain successful sintering is to limit the oxidation of the raw materials. It
is shown also that dense materials can be obtained from relatively coarse
powders with only carbon as the sintering additive. Adding silicon carbide
or boron carbide does allow the grain growth at the sintering temperature
to be limited.
Mechanical characterisation of these materials was performed firstly using
small-scale hardness measurements by nano-indentation at moderate temperatures (25-300 [degrees] C). The indentations were carried out at strain rates
in the range 10-4 and 10-1 s-1. An analysis to extract the Peierls stress
(6.6 ± 0.7 GPa) and activation energy (2.56 ± 1:6 x 10-19 J) for lattice resistance controlled plastic flow is presented. Additional mechanical characterisation consisted in measuring the self-contact hardness at temperatures
from 900-2000 [degrees] C. These measurements clarify that the initial rapid decrease in hardness at room temperature is followed by a region of more or
less constant hardness before further decreases in hardness become apparent at the highest temperatures. A TEM investigation of the deformation
mechanisms shows that near room temperature, extensive dislocation flow
occurs underneath indentations, whereas at the highest temperatures measured in this work, dislocations either anneal out or do not partake in the
deformation. The available data was then summarised through proposing a
deformation mechanism map for ZrB2.
mechanical behaviour at such temperatures is only poorly documented. In
this work, the processing and the deformation behaviour of ZrB2 at temperatures up to 2000 [degrees] C is investigated.
Densification of zirconium diboride based materials is difficult and most
reported routes use a combination of high pressures and high temperatures
to obtain a high density. However, it had been reported that with the
aid of carbon, boron carbide and silicon carbide, pressureless sintering of
ZrB2 is possible. Further work in this thesis shows that the key factor to
obtain successful sintering is to limit the oxidation of the raw materials. It
is shown also that dense materials can be obtained from relatively coarse
powders with only carbon as the sintering additive. Adding silicon carbide
or boron carbide does allow the grain growth at the sintering temperature
to be limited.
Mechanical characterisation of these materials was performed firstly using
small-scale hardness measurements by nano-indentation at moderate temperatures (25-300 [degrees] C). The indentations were carried out at strain rates
in the range 10-4 and 10-1 s-1. An analysis to extract the Peierls stress
(6.6 ± 0.7 GPa) and activation energy (2.56 ± 1:6 x 10-19 J) for lattice resistance controlled plastic flow is presented. Additional mechanical characterisation consisted in measuring the self-contact hardness at temperatures
from 900-2000 [degrees] C. These measurements clarify that the initial rapid decrease in hardness at room temperature is followed by a region of more or
less constant hardness before further decreases in hardness become apparent at the highest temperatures. A TEM investigation of the deformation
mechanisms shows that near room temperature, extensive dislocation flow
occurs underneath indentations, whereas at the highest temperatures measured in this work, dislocations either anneal out or do not partake in the
deformation. The available data was then summarised through proposing a
deformation mechanism map for ZrB2.
Date Issued
2012-09
Date Awarded
2013-05
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Vandeperre, Luc
Giuliani, Finn
Alford, Neil
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)