Preparation of ZrO2 and ZrO2-Y2O 3 coatings on silicon carbide fibers

Article abstract of DOI:10.1134/S0020168506060136

The formation of ZrO₂ and ZrO₂-Y₂O ₃ coatings from hydrous metal oxide sols on Nicalon NLM 202 silicon carbide fibers is investigated in detail. The results indicate that the microstructure of the oxide layer and the surface morphology of the coatings depend on the physicohemical properties of the sol. Kinetic studies of the oxidation of uncoated and coated fibers at different Y₂O₃ contents demonstrate that the oxidation rate of silicon carbide fibers decreases with increasing coating thickness. The effect of oxidation on the phase composition of Nicalon cloth samples coated with ZrO₂ and ZrO ₂-Y₂O₃ is examined. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0020168506060136

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 6, pp. 647–653. © Pleiades Publishing, Inc., 2006.
Original Russian Text © T.M. Zima, N.I. Baklanova, E.I. Belyaeva, N.Z. Lyakhov, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 6, pp. 716–723.
Preparation of ZrO and ZrO –Y O Coatings
2
2
2
3
on Silicon Carbide Fibers
T. M. Zima, N. I. Baklanova, E. I. Belyaeva, and N. Z. Lyakhov
Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
Received May 22, 2005
Abstract—The formation of ZrO and ZrO –Y O coatings from hydrous metal oxide sols on Nicalon NLM
2
2
2
3
2
02 silicon carbide fibers is investigated in detail. The results indicate that the microstructure of the oxide layer
and the surface morphology of the coatings depend on the physicochemical properties of the sol. Kinetic studies
of the oxidation of uncoated and coated fibers at different Y O contents demonstrate that the oxidation rate of
2
3
silicon carbide fibers decreases with increasing coating thickness. The effect of oxidation on the phase compo-
sition of Nicalon cloth samples coated with ZrO and ZrO –Y O is examined.
2
2
2
3
DOI: 10.1134/S0020168506060136
INTRODUCTION
Samanta et al. [9]. The oxidation rate of silicon carbide
during coating with oxides was shown to depend on the
chemical nature and concentration of the precursor. The
reduced oxidation rate of SiC at temperatures above
SiC-fiber-reinforced ceramic-matrix composites
have been recently the subject of intense studies. The
thermomechanical properties of fiber-reinforced
ceramics are known to be highly dependent on the
microstructure of the interfacial layer between the
fibers and matrix [1]. In view of this, the microstructure
of such layers has been the subject of special attention.
1
000°C was attributed to the formation of secondary
phases (mullite and zirconium silicate) with low oxy-
gen diffusion coefficients [9].
Yttria-stabilized zirconia is among the materials the
most widely used in structural ceramics [10]. The stabi-
lization of zirconia with Y O additions enhances its
Silicon carbide fibers are most frequently coated
with thin pyrolytic carbon or hexagonal BN layers [2–
]. These materials have layered hexagonal structures,
which are incapable of ensuring strong mechanical
bonding between fibers and matrices. Moreover, coat-
ings from these materials have low oxidation resistance
at temperatures above 450°C. In connection with this,
there is wide research interest in silicon carbide fibers
coated with refractory metal oxides.
2
3
mechanical properties, thermal stability, and fracture
4
toughness [11–16]. The influence of Y O is known to
2
3
depend on its concentration and distribution in the
material [12, 14, 15]. Among oxide coatings on silicon
carbide fibers, ZrO containing 2–3 mol % Y O is
2
2
3
expected to offer the most attractive combination of
mechanical properties. Earlier results suggest that,
upon the formation of microcracks in such coatings, the
Oxide-coated silicon carbide fibers have been stud- tetragonal-to-monoclinic phase transformation of in
ied by many groups [5–9]. According to Lee et al. [5], zones around cracks will produce compressive stress.
silicon carbide fibers protected by SiO /ZrO /SiO
2
2
2
The increase in the unit-cell volume of ZrO upon the
2
multilayer coatings remain stable at 960°C for 10 h, tetragonal-to-monoclinic phase transition must reduce
without interacting with the coating. Good mechanical the crack size or eliminate the crack.
properties are offered by silicon carbide fibers coated
The most attractive approach to producing multi-
component oxide coatings is sol–gel processing [17,
with porous Al O prepared by metalorganic sol–gel
2
3
processing [6]. It is believed that such coatings are
capable of dissipating the energy of microcracks gener-
ated in the matrix under service conditions. Studies of
1
8], which offers the possibility of preparing materials
with a particle distribution uniform on the molecular
scale and controlling their chemical and phase compo-
sitions and microstructure (porosity, grain size, etc.).
silicon carbide fibers coated with Al O , ZrO , TiO ,
2
3
2
2
and binary mixtures of these oxides by different pro- The rheological properties of the sol (viscosity, cohe-
cesses [7, 8] have shown that the microstructure, sur- sion, and strength of coagulation structures) play an
face morphology, and phase composition of oxide coat- important role in determining the microstructure of the
ings depend not only on the nature of the oxide but also coating. The rheological behavior of sols on substrates
on the preparation procedure. The oxidation resistance is, however, difficult to investigate. We think that the
of silicon carbide during the preparation of M Si O
(
rheological characteristics of sols must be studied
M = Al, Zr, Mg) layers on its surface was studied by before application to the substrate. Moreover, one must
x
y
z
6
47

6
48
ZIMA et al.
Rheological characteristics of freshly prepared hydrous ZrO and ZrO –Y O sols
2
2
2
3
η, mPa s (at the specified strain rate)
wt % Y O
Pcr, Pa
2
3
3
s–1
146 s–1
365 s–1
729 s–1
1312 s–1
0
1
5
0
(ZrO2)
58
72
90
4
2
1.5
6
1
1
1
1
8
3
2.5
7
6
5
11
41
2
370
31.5
18
12
10
Note: η is viscosity, and Pcr is the strength of the coagulation structure.
take into account that the substrate may also have a sig-
nificant effect on the microstructure of the coating.
The rheological properties of the sols were studied
using a Reotest-2.1 rotational viscometer and cells with
coaxial cylinders. The strain rate gradient was varied
The purpose of this work was to study the formation
of ZrO and ZrO –Y O coatings on silicon carbide
–
1
from 3 to 1312 s .
2
2
2
3
fibers. The coatings were produced using hydrous
metal oxide sols.
Phase composition of the samples was determined
by x-ray diffraction (XRD) on a DRON-4 powder dif-
fractometer with ëuK radiation. Electron micrographs
α
were obtained on JEOL JSM T-20 and Tesla BF-350
scanning electron microscopes (SEM) at 5-nm resolu-
EXPERIMENTAL
Oxide coatings were produced using hydrous zirco- tion.
nia sols, with and without yttria. The sols were prepared
electrochemically from appropriate aqueous 1 M metal
chloride solutions, using platinum gauze electrodes
RESULTS AND DISCUSSION
(Fischer electrodes). After heating the solution to 60°C,
The rheological characteristics of freshly prepared
hydrous zirconia sols, with and without yttria, are listed
in the table. As seen, the initial viscosity (at a strain rate
a current was passed through the cell. To obtain homo-
geneous ZrO –Y O sols, the required amount of
2
2
3
yttrium chloride was added during the preparation of
the zirconyl chloride solution. Xerogels were obtained
by drying the sols at room temperature (18–25°C) to a
constant weight. To obtain powder samples, the xero-
–1
of 3 s ) of the sol increases withY O content. Raising
2
3
–
1
the strain rate to above 100 s sharply reduces the vis-
cosity of freshly prepared sols in hydrous systems. The
strength of the coagulation structure of the sol, evalu-
ated from the entire rheological curve as described
in [19, pp. 177–180], increases with increasing stabi-
lizer content.
2
gels were calcined in air or vacuum (10 Pa) at 1000°C
for 2 h.
The sols were applied to Nicalon NLM 202 silicon
carbide cloth. Before coating, the sizing was removed
by etching for 24 h in a 1 : 1 mixture of ethanol and ace-
tone, followed by heat treatment for 2 h in air at 450°C.
The total weight loss in these steps was about 3%.
Figure 1 shows the relative weight change as a func-
tion of the number of immersion cycles for Nicalon
cloth samples coated with ZrO and ZrO –5 wt %Y O .
2
2
2
3
As seen, in producing ZrO coatings on silicon carbide
2
Coatings were produced by immersing the Nicalon
cloth in the sol. To remove the excess sol from the fiber
surface, we used a special procedure similar to that
described by Francis [18]. Next, the samples were dried
in air at room temperature for 24 h and then heated in
fibers, the first two immersion cycles are the most effec-
tive. During subsequent immersion cycles, the weight
of the Nicalon cloth sample increases only slightly.
After the second immersion cycle, the weight of the
cloth sample coated with ZrO –5 wt % Y O increases
considerably more rapidly in comparison with the
ZrO -coated sample.
2
2
2
3
vacuum (10 Pa) to 450°C at a rate of 150°C/h and then
to 960°C at 300°C/h. At the latter temperature, the sam-
ples were held for 1 h, followed by slow cooling to
room temperature in vacuum. To increase the coating
thickness, the procedure was repeated several times.
2
Figure 2 illustrates the surface morphology of Nica-
lon fibers coated with ZrO and ZrO –Y O . The fibers
2
2
2
3
coated with ZrO using unstabilized hydrous zirconia
The oxidation kinetics of Nicalon cloth before and
2
after coating were examined at atmospheric pressure in sol have a smooth surface (Fig. 2a), with occasional
air at 1000°C. Samples weighing 100–200 mg were rounded or elongated hillocks. The coating contains no
placed in a preheated KO-14 muffle furnace, held there cracks or other macrodefects. The ZrO –1 wt % Y O
2
2
3
for a certain lapse of time, withdrawn from the furnace, coating contains pores (Fig. 2b) Moreover, one can see
and cooled in a desiccator. After weighing with an some defects on the surface of the oxide layer, and part
accuracy of 0.05 mg, the samples were again placed in of the fiber remains uncoated. The oxide layer thickness
the furnace for further oxidation.
determined by SEM is ~100 to 200 nm. The ZrO –5 wt %
2
INORGANIC MATERIALS Vol. 42 No. 6 2006

PREPARATION OF ZrO AND ZrO –Y O COATINGS
649
2
2
2
3
Y O coating (Fig. 2c) is very similar in surface mor-
∆m/m, %
2
3
phology to the ZrO coating produced from unstabi-
2
8
lized hydrous zirconia sol and consists of densely
packed, rounded particles about 50 nm in size, with no
cracks. The coating thickness determined by SEM is
6
4
2
2
~
200 to 300 nm. The coating surface is rougher in com-
parison with the ZrO coating produced from unstabi-
2
lized sol and has occasional elevations.
Figure 3 shows the square of the relative weight
2
change, (∆m/m) , as a function of oxidation time for
1
ZrO -coated Nicalon cloth samples. The oxidation rate
2
of the ZrO -coated fibers is seen to be far slower than
2
that of the uncoated fibers under identical conditions.
The fibers coated in several immersion–calcination
cycles exhibit an induction period in the initial stages of
oxidation. Increasing the number of cycles increases
the induction time.
0
1
2
3
4
5
Immersion cycles
2
Figure 4 plots (∆m/m) against oxidation time for
Fig. 1. Relative weight change as a function of the number
of immersion cycles for Nicalon cloth samples coated with
Nicalon cloth samples coated with ZrO –Y O . The
2
2
3
oxidation kinetics are seen to depend on the stabilizer
(1) ZrO and (2) ZrO –5 wt % Y O .
2 2 2 3
content. Under the conditions of this study, 1000°C in
air, the weight of the sample coated with ZrO –1 wt %
2
Y O increases linearly with time, with no induction
2
3
loss in comparison with the as-prepared xerogels with
the sameY O content. In addition, vacuum calcination
period. At Y O contents above 1 wt %, the oxidation
2
3
2
3
kinetics deviate from linearity. As with ZrO -coated
2
is accompanied by color changes: the ZrO powders
2
fibers, increasing the number of immersion–calcination
cycles increases the induction time.
containing 5 wt %Y O turn grayish, and those contain-
2
3
ing 20 wt % Y O turn dark gray.
2
3
Figure 5 shows XRD patterns taken before and after
oxidation (air, 1000°C, 30 h) of Nicalon silicon carbide
cloth samples coated with ZrO and ZrO –5 wt %Y O
Thus, our experimental data demonstrate that the
formation of oxide layers on Nicalon silicon carbide
fibers from sols of hydrous metal oxides depends on a
large number of factors, including the rheological prop-
erties of the sol, which play an important role in the
process. According to the classification proposed by
Ur’ev [19], freshly prepared hydrous zirconia sols, both
unstabilized and yttria-stabilized, are structured liquid-
like systems exhibiting pseudoplastic flow. The data in
the table indicate that the cohesion forces between par-
ticles of the freshly prepared sols are rather strong.
With increasing Y O content, the degree of sol struc-
2
2
2
3
in five immersion cycles. As apparent from Fig. 5a, oxi-
dation has no effect on the phase composition of the
ZrO -coated sample. The XRD pattern of that sample
2
shows, in addition to reflections from monoclinic ZrO2,
weak reflections from tetragonal ZrO . At the same
2
time, oxidation slightly changes the phase composition
of the sample coated with ZrO –Y O Fig. 5b). Before
2
2
3
oxidation, the XRD pattern of the Nicalon cloth sample
coated with ZrO –Y O shows only reflections from
2
2
3
2
3
tetragonal ZrO . After exposure to air at 1000°C for
2
turing increases. As the strain rate is raised, the viscos-
ity of the sols decreases owing to partial destruction of
the coagulation structure. The coagulation structure of
3
0 h, weak reflections from monoclinic ZrO are dis-
2
cernible.
The XRD data for as-prepared (with no substrate) the hydrous zirconia sol stabilized with 1 wt % Y
O
2
3
ZrO –Y O (0–20 wt % Y O ) xerogels calcined in air breaks down at lower shear stresses in comparison with
2
2
3
2
3
agree with earlier results (see, e.g., [20, 21]). The calci- the unstabilized hydrous zirconia sol. It may be that, at
3
+
nation at 1000°C in vacuum changes the phase compo- a lowY O content (~1 wt %), theY ions are arranged
2
3
sition of the Y O -containing (0–20 wt % Y O ) xero- in the material at random (disordered distribution) or
2
3
2
3
gels. In particular, the powder containing 5 wt % Y O
are present only in the first coordination shell of the sol
2
3
consists predominantly of tetragonal ZrO , with very particles, which influences the interaction between all
2
of the structural components in the system.
weak reflections from monoclinic ZrO , in contrast to
2
the powder calcined in vacuum.
The structural properties and surface morphology of
At a givenY O content, the weight loss during vac- Nicalon silicon carbide fibers have been the subject of
2
3
uum calcination exceeds that during air calcination. extensive studies (see, e.g., [22–24]). The fibers were
Moreover, the weight loss during calcination increases shown to consist of a metastable, amorphous material
with Y O content. Vacuum calcination of the xerogels formed by silicon carbide, silicon oxycarbide (SiO C ),
2
3
x
y
applied to silicon carbide fibers causes a larger weight and “free carbon.” The fiber surface is covered with thin
INORGANIC MATERIALS Vol. 42 No. 6 2006

6
50
ZIMA et al.
(
a)
1
0 µm
2 µm
(
b)
1
µm
1 µm
(
c)
2
00 nm
1
µm
Fig. 2. SEM micrographs of the surface of Nicalon fibers coated with (a) ZrO , (b) ZrO –1 wt %Y O , and (c) ZrO –5 wt %Y O .
2
2
2
3
2
2 3
INORGANIC MATERIALS Vol. 42 No. 6 2006

PREPARATION OF ZrO AND ZrO –Y O COATINGS
651
2
2
2
3
2
(
several tens of ångströms) SiO and amorphous carbon
2
(∆m/m) , %
layers. It is, therefore, reasonable to assume that the
adhesion of ZrO and ZrO –Y O coatings to the fibers
0
.30
2
2
2
3
is due to the interaction between Si–OH groups, always
1
present on the SiO surface [25], and positively charged
2
polymeric particles of the hydrous oxide sols.
2
3
The larger weight loss during the calcination of the
xerogels on substrates as compared to the as-prepared
xerogels seems to be related to the nature of the surface
of Nicalon silicon carbide fibers. Heat treatment of the
xerogel layer applied to fibers gives rise to the interac-
tion between the phases present in the surface layer of
the fibers and the components of the coating (xerogel).
Clearly, the properties of the resulting interfacial layer
depend strongly on the physicochemical properties of
the sol.
0
.15
4
0
10
20
30
Time, h
SEM results demonstrate that the surface morphol-
ogy of the coatings depend on the Y O content. In
2
3
Fig. 3. Square of the relative weight change as a function of
addition, the surface morphology and coating thickness
are influenced by the viscosity of the freshly prepared
sol. Weakly structured hydrous systems with a low
strength of coagulation structures exhibit a low film-
forming capacity. Interfacial layers forming in such
systems are rapidly broken up by the thermal stress
arising during heating.
oxidation time for (1) uncoated and (2–4) ZrO -coated
2
Nicalon cloth samples: (2) one, (3) three, and (4) six immer-
sion–calcination cycles.
2
(∆m/m) , %
ZrO –1 wt % Y O coatings cause an insignificant
2
2
3
1
0
0
.2
.8
.4
1
decrease in the oxidation rate of silicon carbide fibers in
air as compared to uncoated fibers (Figs. 3, 4): the oxi-
dation-induced weight changes in uncoated cloth sam-
ples differ little from those in samples coated with
ZrO –1 wt % Y O . In both instances, the oxidation
2
2
2
3
3
4
kinetics are linear and are, therefore, controlled by oxy-
gen diffusion through the SiO layer. As shown earlier
2
[
8], using BET measurements, oxidation increases the
specific surface area of uncoated silicon carbide fibers
by 20–25%. The particle and pore sizes in the oxidized
fibers are 20–60 nm, as determined by SEM.
Silicon carbide fibers coated with ZrO and ZrO –
2
2
0
10
20
30
Time, h
Y O (5 and 20 wt %Y O ) display more complex oxi-
2
3
2
3
dation kinetics. According to SEM results, such coat-
ings are denser and contain no defects. Vacuum calcina-
tion of such xerogels on substrates markedly increases
the density of anion (oxygen) defects in the oxide layer
Fig. 4. Square of the relative weight change as a function of
oxidation time for Nicalon cloth samples coated with ZrO2–
Y O : (1) 1, (2, 4) 5, (3) 20 wt % Y O ; (1–3) one immer-
2
3
2 3
sion–calcination cycle, (4) five cycles.
[
26]. During subsequent oxidation, these defects adsorb
oxygen molecules from air. This accounts for the slow
weight gain in the initial stages of oxidation. With
increasing oxidation time, the oxygen vacancies
become filled, and the oxidation rate of the samples
coated with ZrO –5 wt %Y O and ZrO –20 wt %Y O
color of the ZrO –Y O powders containing 5 and
2
2
3
2
0 wt %Y O . During air dehydration of xerogels with
2 3
these compositions, atmospheric oxygen molecules
take part in the process, gradually filling the forming
vacancies, so that the color of the powders remains
unchanged.
2
2
3
2
2
3
increases. As reported by Jang et al. [27], anion defects
can be produced in ZrO not only by vacuum calcina-
2
tion but also by introducing di- and trivalent cations
forming substitutional solid solutions. Clearly, the
The oxidation rate of silicon carbide fibers can be
vacancy concentration in ZrO –Y O coatings reduced via coating with zirconium silicate, ZrSiO4.
2
2
3
3
+
increases with Y concentration. The formation of Tsai et al. [28] assume that, at 1000°C, oxygen diffu-
anion defects in ZrO during vacuum calcination is evi- sion in ZrSiO is much slower than that in ZrO . The
2
4
2
denced, albeit indirectly, by the observed changes in the formation of zirconium silicate via the reaction
INORGANIC MATERIALS Vol. 42 No. 6 2006

6
52
ZIMA et al.
(
a)
(b)
2
2
1
1
1
0
20
30
θ, deg
10
20
30
2θ, deg
2
Fig. 5. XRD patterns of Nicalon cloth samples coated with (a) ZrO and (b) ZrO –5 wt %Y O : (1) before oxidation, (2) after expo-
2
2
2 3
sure to air at 1000°C for 30 h.
between ZrO and SiO at temperatures above 970°C
phases at Elevated Temperatures in Air, J. Am. Ceram.
Soc., 1997, vol. 80, no. 8, pp. 2029–2042.
2
2
was reported by Luecke et al. [29]. In this study, how-
ever, no XRD evidence for the presence of this phase in
coatings was found (Fig. 5).
5. Lee, W.Y., Lara-Curzio, E., and More, K.L., Multilay-
ered Oxide Interphase Concept for Ceramic-Matrix
Composites, J. Am. Ceram. Soc., 1998, vol. 81, no. 3,
pp. 717–720.
CONCLUSIONS
6
. Verdenelli, M., Parola, S., Chassagneux, F., et al., Sol–
Gel Preparation and Thermo-Mechanical Properties of
Porous xAl O –ySiO Coatings on SiC Hi-Nicalon
Oxide coatings produced on Nicalon silicon carbide
fibers by sol–gel processing using hydrous zirconia sols
with and without Y O additions adhere well to the
2
3
2
Fibres, J. Eur. Ceram. Soc., 2003, vol. 23, no. 8,
pp. 1207–1213.
2
3
fibers and are homogeneous on the molecular scale.
The surface morphology, thickness, and porosity of the
coatings depend on the physicochemical properties of
the sols and the number of immersion–calcination
cycles. The structural properties of the fiber surface
influence the formation of the oxide layer, its chemical
behavior, and its oxidation resistance at 1000°C.
7
. Baklanova, N.I., Zima, T.M., Naimushina, T.M., and
Kosheev, S.V., The Formation of Refractory Oxide Coat-
TM
ings on Nicalon
Fiber by Sol–Gel Process, J. Eur.
Ceram. Soc., 2004, vol. 24, no. 10/11, pp. 3139–3148.
8. Zima, T.M., Baklanova, N.I., and Titov,A.T., The Behav-
TM
ior of the Oxide Coated Nicalon Fibers Exposed toAir
at High Temperatures, J. Eur. Ceram. Soc., 2005, vol. 25,
no. 11, pp. 1943–1952.
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2
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