Mainak Saha1,2
1Department of Metallurgical and Materials
Engineering, National Institute of Technology, Durgapur-713209, India
2Department of Metallurgical and Materials
Engineering, Indian Institute of Technology Madras, Chennai-600036,
India
Abstract - While descending through different layers of
atmosphere with tremendously high velocities, hypersonic re-entry
nosecones fabricated using ultra-high temperature ceramic matrix
composites (UHTCMCs) are subjected to repeated thermal shocks. This
necessitates extensive investigations on the cyclic oxidation behaviour
of UHTCMCs at temperatures ranging from 1100°C to
1300°C (service temperature of the nosecones). To this
end, the present work is aimed at investigating the cyclic oxidation
behaviour of ZrB2 -20 vol.%MoSi2 (ZM20) UHTCMC (a very widely
investigated ZM CMC) by carrying out cycles for 6h, at 1cycle/h and
estimating oxidation kinetic law. This has been followed by extensive
characterisation using X-Ray Diffraction (XRD) to indicate the phases
formed during oxidation and Scanning electron microscopy-energy
dispersive spectroscopy (SEM-EDS), in order to determine the chemical
composition of the oxides formed between 1100°C and
1300°C.
Keywords- Borides; ceramic composites; cyclic oxidation;
kinetics; oxide layer
Among UHTCs, ZrB2-based ceramics have been reported to be potential
candidates for the
manufacture of reusable Thermal Protection Systems (TPS) in Hypersonic
re-entry nosecones,
due to very high thermal conductivity and relatively low density
[1,2]. However, the low
fracture toughness and poor thermal shock resistance of these ceramics
pose major obstacles
to their use in extreme environment [3].
Moreover, the poor oxidation resistance of ZrB2 at temperatures above
1200°C, due to
formation of B2O3, and a non-protective porous scale of ZrO2 [4],
poses restrictions to its use
at elevated temperatures, especially above 1200°C. Thus, it becomes
extremely important to
find materials, which may highly enhance the oxidation resistance of
ZrB2 [5-8]. A significant
amount of work has already been done in that direction [8-12].
Besides, a significant amount of research has been done on reinforcing
diborides like ZrB2,
HfB2 and TiB2 with SiC, MoSi2, or ZrSi2 for enhanced oxidation
resistance beyond 800°C [3-
23]. However, a limited amount of study has been made on cyclic
oxidation of ZM20 at
temperatures exceeding 1100°C, which is not at all unlikely, in the
context of Hypersonic
nosecones, during a high velocity descent through different layers of
atmosphere. Thus, the
scope of the present study is to investigate the cyclic oxidation
behaviour of ZM20 between
1100 and 1300 °C.
The important conclusions drawn from the results and discussions of this
study have been
elucidated. Cyclic oxidation behaviour of ZrB2-20 vol.% MoSi2 composite
have been studied at 1100 °C, 1200 °C, 1250 °C and 1300 °C for 6hrs.
Monitoring weight change and examining oxide scales draw following
conclusions:
(i) Weight gain for both the composites increased with increasing
temperature and time.
(ii) Weight gain occurred due to
formation of ZrO2 and SiO2, at elevated
temperatures.
(iii) The main oxidation products were
ZrO2, MoO3 and SiO2.
(iv) At 1200 °C and above, the
presence of SiC particles markedly improves the resistance to
oxidation of the composite due to the formation of borosilicate glass.
(v) Due to formation of oxide layer on the surface, the hardness of the
samples i.e. its
mechanical properties decreased from center to surface.
(vi) The cyclic oxidation of the samples follow linear oxidation
kinetics from 1100 to 1250 ºC
while at 1300 ºC it follows parabolic oxidation kinetics due to the
protective action of SiO2
above 1250 ºC.
The results of the present study and their analyses lead to the
following directions for future
work:
(i) The oxidation kinetics of the samples beyond 1300 ºC can be
studied.
(ii) Residual strain calculations can be carried out.
(iii) Mathematical modelling study of the oxidation kinetics can be
carried out.
(iv) TEM study of the samples can be carried out for more precarious
measurements.
(v) Carrying out diffusion studies on oxide
layer.AcknowledgementThe authors are grateful to the
Department of Metallurgical and Materials Engineering, NIT
Durgapur and Central Research Facility(CRF), IIT Kharagpur, for their
support to carry out
the work and hereby declare no conflict of interest.