Preparation and characterization of multilayered ZrO2 coatings on silicon carbide fibers for SiC/SiC composites

Article abstract of DOI:10.1134/S0020168511090238

We have studied the surface morphology, phase composition, and oxidation resistance of multilayered tetragonal zirconia coatings produced on silicon carbide fibers by a sol-gel process and measured the tensile strength of individual fibers as a function of the number of layers in the coating. SiC-fiber-reinforced silicon carbide minicomposites have been prepared through pyrolysis of an organosilicon polymer, and their fracture surfaces have been examined. Using microindentation, we have determined the critical fiber-matrix debonding stress. The results demonstrate that the ZrO₂ coating on the fibers has the form of uniform, weakly bonded layers. The presence of a multilayered ZrO₂ interphase alters the fracture behavior of the SiC/SiC composites. The fiber debond stress in the composites markedly decreases with an increase in the number of layers in the interphase.

Full text of DOI:10.1134/S0020168511090238

ISSN 0020ꢀ1685, Inorganic Materials, 2011, Vol. 47, No. 10, pp. 1066–1071. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.V. Utkin, A.A. Matvienko, A.T. Titov, N.I. Baklanova, 2011, published in Neorganicheskie Materialy, 2011, Vol. 47, No. 10, pp. 1176–1181.
Preparation and Characterization of Multilayered ZrO Coatings
2
on Silicon Carbide Fibers for SiC/SiC Composites
a
a
b
a
A. V. Utkin , A. A. Matvienko , A. T. Titov , and N. I. Baklanova
a
Institute of SolidꢀState Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
Joint Institute of Geology, Geophysics, and Mineralogy, Siberian Branch, Russian Academy of Sciences,
b
pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia
eꢀmail: utkinalex@hotmail.com
Received January 21, 2011
Abstract—We have studied the surface morphology, phase composition, and oxidation resistance of multilayꢀ
ered tetragonal zirconia coatings produced on silicon carbide fibers by a sol–gel process and measured the
tensile strength of individual fibers as a function of the number of layers in the coating. SiCꢀfiberꢀreinforced
silicon carbide minicomposites have been prepared through pyrolysis of an organosilicon polymer, and their
fracture surfaces have been examined. Using microindentation, we have determined the critical fiber–matrix
debonding stress. The results demonstrate that the ZrO coating on the fibers has the form of uniform, weakly
2
bonded layers. The presence of a multilayered ZrO interphase alters the fracture behavior of the SiC/SiC
2
composites. The fiber debond stress in the composites markedly decreases with an increase in the number of
layers in the interphase.
DOI: 10.1134/S0020168511090238
INTRODUCTION
which possesses high stability in oxidizing atmoꢀ
spheres at elevated temperatures. Yttriaꢀstabilized tetꢀ
ragonal ZrO2 offers high fracture toughness owing to
Advances in aerospace engineering are highly
dependent on the development of novel oxidationꢀ
resistant highꢀtemperature ceramic materials possessꢀ
ing high thermomechanical performance. Among
such materials are SiCꢀfiberꢀreinforced silicon carꢀ
bide composites. An effective approach for raising the
the
ТꢀZrO2
МꢀZrO2 phase transition induced by
the mechanical stress of a growing crack and accomꢀ
panied by an increase in volume [7].
In this paper, we report a coating design that comꢀ
fracture resistance of SiC/SiC composites is to proꢀ bines the advantages of the two approaches above,
duce an interfacial layer between the fiber and matrix namely, a multilayered structure made up of poorly
(
interphase), which weakens the fiber–matrix bondꢀ bonded, thin layers consisting of one, oxidationꢀresisꢀ
ing, thereby raising the fracture toughness of the mateꢀ tant material.
rial [1].
The best results were obtained with coatings conꢀ
sisting of pyrolytic carbon, hexagonal BN, or alternatꢀ
ing C/SiC layers, which markedly improved the
mechanical performance of SiC/SiC composites
owing to their layered structure and low debond stress
EXPERIMENTAL
Zirconia coatings were produced on Nicalon siliꢀ
con carbide fibers (13–15 m in diameter, ceramic
µ
grade, Nippon Carbon Vo., Japan). Before the coating
process, the sizing was removed by etching for 24 h in
a 1 : 1 mixture of ethanol and acetone, followed by
roomꢀtemperature drying and heat treatment at 450°С
for 1 h in air.
[
2–4]. At the same time, the low oxidation resistance
of such coatings considerably lowers the maximum
working temperature of the composites [5].
An alternative approach to producing an appropriꢀ
ate interphase for SiC/SiC composites is to use oxyꢀ
genꢀcontaining compounds (refractory oxides or salts)
Coatings were applied by immersing fiber tows in a
capable of weakening the fiber–matrix bonding and filmꢀforming sol, followed by drying at room temperꢀ
raising the fracture toughness of the composite owing ature. Next, the fibers were heated in vacuum to 950
to some specific features of their structure or the posꢀ and held there for 1 h. The sol was prepared by dissolvꢀ
sibility of various stressꢀinduced processes (interlayer ing zirconyl chloride octahydrate, ZrOCl2 8H O, in
shear, twinning, and phase transitions) [6]. A candiꢀ an 8 : 1 mixture of ethanol and water with additions of
°С
·
2
date material for this application is zirconia, ZrO2
,
polyethylene glycol and rareꢀearth nitrates. To proꢀ
1066

PREPARATION AND CHARACTERIZATION OF MULTILAYERED ZrO COATINGS
1067
2
duce multilayered coatings, the procedure was
repeated several times.
The surface morphology and microstructure of the
coatings were examined by highꢀresolution scanning
electron microscopy (SEM) on an LEO 1430VP (LEO
Electron Microscopy Ltd, UK). Raman spectra were
taken with a SPEX Triplemate spectrometer (France)
equipped with a CCD detector and microscope. The
–1
spectra were measured from 40 to 1700 cm . As the
excitation source, we used a 488ꢀnm laser beam,
which was focused by a lens to a 2ꢀ m spot diameter
µ
on the fiber surface. Uncoated and coated fibers were
tested in tension on an FMꢀ27 tensile testing machine
(
Hungary) at room temperature.
10 µm
Uncoated and coated fibers were oxidized in air at (а)
1000°С for 35 h in a KOꢀ14 muffle furnace (Roemhild,
Germany). To determine the sample weight, the fibers
were withdrawn from the furnace, cooled to room
temperature in a desiccator, and weighed on an AX
Fiber
Coating
1
0
20 electronic balance (Shimadzu, Japan), having
.1ꢀmg divisions.
The silicon carbide matrix was prepared through
vacuum
pyrolysis polydimethylsilethyne,
of
(
⎯
(CH ) Si–C
2
≡C–)n, at 950°С. Uncoated and coated
3
fibers were immersed in a polymer suspension in chloꢀ
roform. The resultant samples were dried at room
temperature, and then the polymer was pyrolyzed for
1
h. The procedure was performed twice in order to fill
the spaces between the fibers. The volume fraction of
the filler in the unidirectional composites was
40%.
Ӎ
1 µm
(
b)
Vickers microhardness was measured at room temꢀ
perature using the MPH 100 diamond microindenter
of a NEOPHOTꢀ21 optical microscope (Carl Zeiss
Jena). The fiber debond stress was measured as
described by Marshall [8]. A fiber was indented with a
diamond pyramid. Above a critical load, the fiber debꢀ
onded from the matrix and slid into the composite.
From analysis of indents on the fiber and matrix, we
evaluated the critical fiber–matrix debonding stress.
1 µm
Indented minicomposite surfaces were imaged by
scanning electron microscopy (TMꢀ1000, Hitachi,
Japan and LEO 1430VP, LEO Electron Microscopy
Ltd, UK).
Ridge
RESULTS AND DISCUSSION
(c)
Sol
SEM examination of the surface morphology and
microstructure of the coated fibers showed that the
coating had the form of dense, uniform layers ranging
in thickness from 50 to 100 nm (Figs. 1a, 1b). No
bonding between fibers through the coating was
detected, but some of the fibers had elongated ridges
Fig. 1. Coated fibers: (a) general view of a fiber tow, (b) surꢀ
face of an individual fiber, (c) surface ridge and schematic
illustrating ridge formation.
on their surface. This was because the sol was retained reduce the density of flaws the initial total salt concenꢀ
between the closely spaced fibers, leaving ridges after tration in the sol was lowered to 0.1 M, and polyethylꢀ
heat treatment (Fig. 1c). The retained sol volume ene glycol was added to the sol in order to maintain its
depended on the viscosity of the sol. In view of this, to filmꢀforming properties unchanged.
INORGANIC MATERIALS Vol. 47
No. 10 2011

1068
UTKIN et al.
Raman spectroscopy data for the surface of the
coated fibers demonstrate that the coating consists
predominantly of tetragonal zirconia (Fig. 2). SEM
images of some fibers having threeꢀ to fiveꢀlayer coatings
showed local peeling of the outermost layer (Fig. 3). This
was caused by the mechanical stress arising during
cooling because of the thermal expansion mismatch
between SiC and ZrO2. The peeling of the outermost
layer of the coating from the underlying layers points
to poor adhesion between the layers in the multilayꢀ
ered coating.
TꢀZrO2
A similar multistep sol–gel coating process was
reported by Fair et al. [9–11], but they obtained
monolithic coatings, in contrast to the poorly bonded
layers in this study, and the only effect of each processꢀ
ing step was to increase the thickness of the coating.
The likely reason for this was that the layers were not
sufficiently well crystallized before the next coating
cycle and were thus well bonded to one another. In this
study, the fibers were heatꢀtreated at high temperature
after each coating cycle, so each layer was deposited on
the surface of a dense, wellꢀcrystallized layer, without
firmly adhering to it. Moreover, the loss of water and ethꢀ
anol molecules during heating reduces the thickness of
the growing layer, resulting in spaces between the layers,
which further reduce the interlayer bonding.
1
00
200
300
400
500
600
700
–1
Raman shift, cm
Fig. 2. Raman spectrum of the surface of a coated fiber.
Peeling
To assess the coating effect on the mechanical
properties of the fibers, we measured the tensile
strength of all the types of fibers. The experimental
tensile strength data were analyzed in terms of the
Weibull statistics. The results were used to determine
the average tensile strength for each type of fiber.
According to data in the literature, the strengths of the
unprocessed and heatꢀtreated fibers are 2.0 and
1
.8 GPa, respectively [12]. In our measurements, the
tensile strength of all the coated fibers was
1.6 GPa.
Thus, the first coating layer reduces the mechanical
strength of the fiber by 10%, which is probably due to
Fig. 3. Peeling of the outermost layer in a multilayered
coating.
Ӎ
Ӎ
the formation of aboveꢀmentioned surface ridges on
the coated fibers. Analysis of SEM images of the end
portions of the coated fibers suggests that surface
ridges may initiate fiber fracture. The strength of the
fibers coated with two or more layers is also 1.6 GPa,
despite the larger number of surface ridges. It seems
likely that the strength of the fibers is only influenced
by the flaws in the first coating layer, which is wellꢀ
bonded to the fiber. The ridges in the next layers do not
reduce the mechanical strength of the fiber because of
the poor bonding between the layers.
2
2
1
1
0
.5
.0
.5
.0
.5
Uncoated fiber
Singleꢀlayer coating
Threeꢀlayer coating
Fiveꢀlayer coating
Oxidation testing results showed that, in the initial
stage of oxidation (less than 10 h), all of the coated
fibers had enhanced oxidation stability (Fig. 4). At
longer oxidation times, the oxidation rate of the fibers
having threeꢀ and fiveꢀlayer coatings was slower. It
seems likely that the microstructure of the singleꢀlayer
coating was not dense enough to prevent atmospheric
0
5
10 15 20 25 30 35
Time, h
40
Fig. 4. Weight gain as a function of fiber oxidation time.
INORGANIC MATERIALS Vol. 47
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2011

PREPARATION AND CHARACTERIZATION OF MULTILAYERED ZrO COATINGS
1069
2
30 µm
(а)
50 µm
(b)
Fig. 5. Fracture surfaces of SiC/SiC composites: (a) no interphase, (b) threeꢀlayer interphase.
oxygen from reaching the fiber surface. The next layers
closed the pores in the first layer, thereby reducing the
oxidation rate of the fiber.
Interphase
Using SEM, we examined fracture surfaces of
minicomposites reinforced with uncoated and coated
fibers. Micrographs of the composites reinforced with
uncoated fibers point to brittle fracture behavior of the
materials: there is no fiber pullout from the matrix,
and the surface is weakly branched (Fig. 5a). In the
case of the fibers with multilayered coatings, the
appearance of fracture surfaces points to ductile fracꢀ
ture: the fibers are pulled out of the matrix, and the
fracture surfaces have a complex shape (Fig. 5b). A
more detailed examination of fracture surfaces of the
composites having a multilayered interphase indicated
that the fibers debonded from the matrix along an
interface between the layers of the interphase, which
remained bonded to both the fiber and matrix (Fig. 6).
To evaluate the fiber–matrix bonding strength in
the composites, we measured the critical fiber–matrix
Fig. 6. Interphase delamination upon fracture of the
composite.
INORGANIC MATERIALS Vol. 47
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1070
UTKIN et al.
this is that the presence of a multilayered interphase
550
with weak interlayer bonding in the composites leads
to the development of potential surfaces along which
the fiber may slide relative to the matrix. The fiber debꢀ
ond stress then depends on the interlayer bonding
strength, the roughness of the sliding surfaces, and the
density of flaws that impede sliding (Fig. 8a). When
there are flaws or asperities that prevent sliding, an
interphase consisting of several layers enables a change
in sliding plane to obviate the asperity (Fig. 8b),
thereby reducing the fiber debond stress.
3
90
1
35
8
0
1
2
3
5
Layers in the interphase
Under service conditions, the interlayer boundꢀ
aries in an interphase may act as facilitated propagaꢀ
tion paths for microcracks nucleating in the matrix.
Microcrack deflection by the interphase and faciliꢀ
tated fiber sliding in the composite ensure reinforceꢀ
ment integrity during matrix crack propagation and
raise the fracture energy of the composite.
Fig. 7. Fiber debond stress in the composite against the
number of ZrO layers in the interphase.
2
(а)
(b)
CONCLUSIONS
A multilayered interphase design concept was
applied to silicon carbide fiber reinforced composites.
Using a sol–gel process, we produced multilayered
yttriaꢀstabilized zirconia coatings on SiC fibers, with
weak interlayer bonding in the coatings. Reinforcing
SiC composites with the coated fibers, we obtained
composites with ductile fracture behavior. The ductile
fracture behavior is assumed to result from the weakꢀ
ening of the fiber–matrix bonding on account of the
multilayered ZrO2 interphase. The proposed interꢀ
phase design allows one to control the fracture behavꢀ
ior of composites and ensures high thermal stability
and oxidation resistance of the fiber, which makes it
highly attractive for the fabrication of SiC/SiC comꢀ
posites.
Hindered
sliding
Facilitated sliding
Fig. 8. Schematic illustrating fiber sliding relative to the
matrix: (a) sliding hindered by an asperity; (b) a change in
sliding plane to obviate an asperity.
The proposed approach to producing multilayered
coatings can be successfully used to apply other oxide
coatings to various substrates using filmꢀforming sols.
debonding stress. The measurement results are preꢀ
sented in Fig. 7. Indentation of the interphaseꢀfree
composites caused no fiber debonding from the
Therefore, optimizing not only the coating mateꢀ
matrix. Instead, the fiber cracked, and the cracks rial, possessing appropriate physicochemical properꢀ
propagated into the matrix. The singleꢀlayer coatings ties and microstructure, but also the number of layers
failed to ensure the desired weakening of the fiber– in the coating, one can effectively control the proꢀ
matrix bonding. Indentation of such composites cesses at the fiber/matrix interface in SiC/SiC comꢀ
caused partial fiber debonding from the matrix and posites. This, in turn, offers the possibility of tailoring
fiber cracking.
the thermomechanical properties of the entire comꢀ
posite material.
In the case of the fibers coated with two or more
layers, the composites exhibited a different indentaꢀ
tion behavior: the fiber completely debonded from the
SiC matrix and slid relative to it. It is worth pointing
out that the fiber debond stress in the composites
ACKNOWLEDGMENTS
This work was supported by the Chemistry and
markedly decreases with an increase in the number of Materials Science Division of the Russian Academy of
layers in the interphase (Fig. 7). The likely reason for Sciences, grant no. 5.2.1.
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