J. Am. Ceram. Soc., 92 [7] 1574–1579 (2009)
DOI: 10.1111/j.1551-2916.2009.03049.x
r 2009 The American Ceramic Society
ournal
J
Sintering Mechanisms of Zirconium and Hafnium Carbides
Doped with MoSi2
Laura Silvestroni,w,z,y,z Diletta Sciti,z Jens Kling,y Stefan Lauterbach,y and Hans-Joachim Kleebey
zCNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy
yTUD-IAG, Institute of Applied Geosciences, SchnittspahnstraXe 9, D-64287 Darmstadt, Germany
zDepartment of Industrial Chemistry and Materials, University of Bologna, Via Risorgimento 4, I-40136 Bologna, Italy
The microstructure of two pressureless-sintered ultra-high-tem-
perature ceramics, namely ZrC120 vol% MoSi2 and HfC120
vol% MoSi2, was characterized by scanning and transmission
electron microscopy. With regard to the ZrC–MoSi2 system,
ZrxSiy compounds and SiC were detected. In the HfC–MoSi2
system, a mixed phase was detected at the triple points and
identified as (Mo,Hf)5Si3. For both the systems investigated, the
high wettability of the silicide-based phases on the matrix grains
suggests that sintering is assisted by a liquid phase. This con-
tribution reports for the first time on the sintering mechanisms of
early transition metal carbides doped with MoSi2 as a sinter
additive, on the basis of the microstructural evolution observed
upon sintering and in the light of phase diagrams and thermo-
dynamical calculations.
effective densification mechanisms. A thorough literature anal-
ysis revealed that neither detailed TEM work nor reports on
densification mechanisms are available for this class of materi-
als, which, however, are essential to optimize the sintering aids
utilized and the processing parameters applied.
In the present study, the microstructure development of ZrC–
MoSi2 and HfC–MoSi2 composites was carefully analyzed in
order to gain an insight into the densification mechanisms oc-
curring during pressureless sintering.
II. Experimental Procedure
The compositions under investigation are indicated in Table I.
Details on the powder processing and sintering conditions are
reported elsewhere.9,10 For the sake of clarity, the impurities
present in the commercial powders are also reported in Table II.
To identify the crystalline phases formed, all samples were
examined using X-ray diffraction (Siemens D500, Karlsruhe,
Germany), with CuKa radiation, a stepsize of 0.04, and a 1 s
counting rate. The microstructures were analyzed on polished
and fractured surfaces by scanning electron microscopy (SEM,
Cambridge S360, Cambridge, U.K.) and energy-dispersive X-
ray spectroscopy (EDS, INCA Energy 300, Oxford instruments,
Tubney Woods, Abingdon, Oxfordshire, U.K.). In order to limit
lateral spreading of the electron beam, EDS analysis was carried
out at acceleration voltages r6 keV. TEM samples were pre-
pared by cutting 3 mm disks from the sintered pellets. These
were mechanically ground down to about 20 mm and then fur-
ther ion beam thinned until small perforations were observed by
an optical microscope. The detailed phase analysis was per-
formed using TEM operating at 120 kV (FEI CM12 STEM,
Eindhoven, the Netherlands) equipped with an energy-disper-
sive detector system (EDAX Genesis 2000, Amtek GmbH,
Wiesbaden, Germany). Quantitative calculations of the micro-
structural parameters, like residual porosity and secondary
phase content, were carried out via image analysis with a com-
mercial software package (Image Pro-plus 4.5.1. Media Cyber-
netics, Silver Springs, MD).
I. Introduction
IRCONIUM and hafnium carbide (ZrC and HfC) are candi-
date materials for ultra-high-temperature structural appli-
Z
cations due to their high melting point of 41731 and 36931C,
respectively,1 and their capability to withstand temperatures
exceeding 16001C in an aggressive environment. They possess
interesting engineering properties, such as high hardness (ZrC:
B25.5 GPa; HfC: B20.0 GPa),2 high electrical conductivity, and
a high elastic modulus (ZrC: 400–440 GPa; HfC: 425 GPa).2,3
ZrC is suitable for electronic applications, as thermoionic emit-
ters, and nuclear applications, as a diffusion barrier for fission
metals in the coatings of nuclear fuels. HfC is presently consid-
ered as a potential candidate material for aerospace applications
as well as for a variety of applications including cutting tools,
high-temperature shielding, field emitter tips, and arrays.4,5
Despite possessing useful properties, the use of monolithic
carbides is strongly limited because of their poor sinterability
and high machining costs. Several attempts have been made in
order to decrease the sintering temperature, the applied pressure,
and the amount of secondary phase required to achieve full
density. Recently, it has been demonstrated that amounts of
MoSi2 ranging from 5 to 20 vol% promote the sinterability of
TiB2,6 ZrB2,7,8 HfB2,8 ZrC,9 and HfC10 at temperatures in the
range 18001–20001C, even without the application of pressure.
However, the densification mechanisms are still unclear and
under debate.
III. Results
In Fig. 1, the fractured surfaces of pure ZrC and HfC materials
are presented for comparison. The monoliths of undoped ZrC
and HfC were pressureless sintered under identical sintering
conditions as composites prepared with the addition of MoSi2;
however, their density retained values of approximately 70% of
the theoretical density.
Transmission electron microscopy (TEM) is a powerful tool
to explore microstructures on a small length scale to disclose the
R. Cutler—contributing editor
(1) ZrC- 20 vol% MoSi2
Manuscript No. 25624. Received December 11, 2008; approved February 20, 2009.
This work was supported by The Air Force Research Laboratory through the research
grant FA8655-09-M-4002, in particular the contract monitor, Dr. Joan Fuller.
This material showed good sinterability and the final density
was 6.22 g/cm3. The crystalline phases detected after sintering
were cubic ZrC, tetragonal MoSi2, and traces of b-SiC
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