DOI: 10.1002/adem.201300261
Oxidation Behavior at 1600 °C of Si-SiC-ZrB
2
Composites
Produced by Si Reactive Infiltration**
By Giuseppe Claudio D’Amico,* Alberto Ortona, Sara Biamino, Paolo Fino, Claudio Badini
and Claudio D’Angelo
Dense silicon carbide materials are commonly employed in
air at high temperatures because their outer SiO
2
scale, under
passive oxidation conditions, has the lowest permeability to
oxygen among common oxides.
[1]
By “dense SiC” one usually
refers to ceramics without open/close porosity produced by
techniques such as sintering (pressureless or pressure assisted)
and chemical vapor deposition (CVD). The drawback of these
techniques is that they are expensive, they present limitations
in product thickness or shape and, in the case of pressureless
sintering, may contain sintering aids (typically carbon
[2]
and
boron
[2]
) which, under particular operating conditions,
decrease oxide scale viscosity and thus enhance oxygen
diffusion.
[3]
Silicon carbide ceramics produced by silicon
reactive infiltration (SRI) were first obtained by Hillig et al.
[4]
They infiltrated carbonaceous material with molten Si under
10
2
mbar residual pressure at temperatures ranging from 1450
to 1600 °C. This process allows the shaping and consolidation
of preforms made of ceramic powders or fibers that are bound
by a polymer with a high carbon yield after pyrolysis. The
major drawback of this technique is that SiC grains are
interpenetrated with a continuous phase made of unreacted
silicon which melts at 1423 °C. Liquid silicon may reduce the
thermo-mechanical properties of the composite and their
resistance to oxygen. In thermal protection system (TPS),
however, are not required high mechanical strength so that for
this application have been proposed materials such as SiC,
ZrB
2
, and their composites that in the conditions of re-entry
into Earth’s atmosphere may give rise to oxidizing vitreous
phases that become progressively more fluid with increasing
temperature.
SiC has been added to transition metal diborides (ZrB
2
and
HfB
2
) for aerospace applications to increase the relative poor
oxidation resistance of pure ZrB
2
and HfB
2
at temperatures
above 1600 °C.
[5]
At these temperatures several researchers
reported the formation of an oxide scale of borosilicate glass
layer (BSZ) containing SiO
2
, ZrO
2
, and B
2
O
3
and the formation
of an intermediate region, between the oxides and the
unreacted material, characterized by SiC depletion.
[6–8]
Recently Willams et al.
[9]
studied the oxidation performance
of ZrB
2
–SiC composites with a significantly higher amount of
SiC than the standard 20–30 vol%. They showed that oxidized
samples containing >50 vol% of SiC did not present a SiC
depleted region because their larger SiC reservoir did not allow
SiO (g) to be transported from the SiC depleted region to the
SiO
2
layer.
In a recent work, we presented Si–SiC–ZrB
2
composites
produced by SRI.
[10]
Both works showed similar oxide layers
and the absence of a SiC depleted region. The main difference,
mainly due to the presence of silicon, stands in the thickness of
the outer borosilicate layer. This work presents a detailed study
of the mutual interactions between the Si–SiC–ZrB
2
constituent
materials during their processing and of their chemical activity
with oxygen at high temperatures in the prospect of using this
material as a matrix of a continuous fiber reinforced composite.
As silicon is the more reactive component in Si–SiC–ZrB
2
composites, we also developed a technique to quantify its
amount from the analysis of the silicon XRD peaks. Si wt% was
inferred from a curve that was drawn on the basis of data from
XRD acquisitions of Si–SiC powder samples with a known
composition.
1. Experimental
1.1. Materials
The composites were produced by mixing ceramic powders
with a plastic binder. The ceramic powders and the plastic
binder employed in the present work were, respectively:
(i) a-Silicon carbide (Grade UF 05, Stark Ag, Goslar, D) with
an average particle size d50 of 1.4 mm and a specific area
of 4–6m
2
g;
(ii) Zirconium diboride (grade A, Stark Ag, Goslar, D), the
particle size d50 is 0.3–5.0 mm;
(iii) Micronized (64 mm diameter) phenolic novolac powder
(Momentive, Columbus, OH, USA).
Different compositions, (Table 1), were prepared in order to
study the effect of the constituent materials on the ceramic
microstructure before and after their oxidation.
[*] G. C. D’Amico, S. Biamino, P. Fino, C. Badini
Politecnico di Torino, Dipartimento di Scienza dei Materiali e
Ingegneria Chimica, Corso Duca degli Abruzzi 24, 10129
Torino, Italy
E-mail: giuseppe.damico@polito.it
A. Ortona, C. D’Angelo
ICIMSI-SUPSI, Strada Cantonale, CH-6928 Manno, Switzerland
[**] The authors are deeply grateful to Erbicol SA for having
performed reactive infiltrations on the preforms and for the
sample cutting and milling.
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