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Cordierite-ZrO2 and cordierite-Al2O3 composites obtained by sonocatalytic methods
J O U R N A L OF
Journal of Non-Crystalline Solids 147&148 (1992) 523-531 North-Holland
NON-CRYSTALLINE SOLIDS
Cordierite-ZrO 2 and cordierite-A1203 composites obtained by sonocatalytic methods M. Pifiero, M. Atik a n d J. Zarzycki Laboratoire de Science des Matdriaux Vitreux, University of Montpellier II, Case 069, Place E. Bataillon, Montpellier, France
Cordierite-matrix-based composites were prepared by the sol-gel process involving sonocatalysis during preliminary chemical reactions. ZrO 2 and A1203 ceramic fibers were employed as reinforcing phases. Sintering was accomplished by hot-pressing techniques. Devitrification of sintered cordierite sonogels showed that, when ~x- and c~-cordierite forms appear, a substantial increase in the mechanical strength is observed. Addition of the nucleant agent TiO 2 further improves mechanical properties.
1. Introduction
2. Experimental procedure
Sol-gel processing is a relatively new method of preparing ceramic composites. Reinforcing phases are infiltrated with a low viscosity solution which may be helped by the application of an external pressure. This promotes formation of an intimate interface between the matrix and reinforcing phase after gelation, resulting in a high interracial bond strength which should improve mechanical performances. Recent studies on the application of ultrasonic radiation to the sol-gel process [1] have shown that high density 'sonogels' with a larger specific surface than those observed for 'classic gels' [2,3] could be obtained by this method, this would constitute a great advantage for the sintering process, since densification may be carried out at lower temperatures. Using the sonocatalytic approach, composites in the SiOz-SiO 2 system were obtained by introducing fine silica particles (Aerosil) into a SiO 2 sonosol [4]. In the present work sonogels of cordierite (5Si2-2A1203-2MgO) , a low expansion ceramic compound, were used in the preparation of the matrix for ceramic-ceramic composites. Reinforcing phases used were ceramic industrial A1203 and ZrO 2 fibers (Zircar Products).
2.1. Preparation of pure cordierite matrix Tetraethoxysilane, Si(OEt) 4 (TEOS), (Fluka) was used as a source of silica, aluminium secbutoxide Al(OBuS)4 (ASB) for alumina and magnesium acetate tetrahydrate Mg(Ac)4H20 for magnesia. The solvent used was 2-methoxyethanol (2-MeOEtOH). Because hydrolysis and polymerization of aluminium alkoxide is faster than that of TEOS, 0.6 tool of acetic acid per mole ASB was added drop by drop under the effect of ultrasound to a solution containing ASB and 20 vol.% of 2methoxyethanol. The TEOS was then added and the resulting solution (A) was homogenized under the effect of ultrasound. This solution was left for 24 h at room temperature. After this time, a separately prepared solution (B) containing magnesium acetate which was dissolved under sonication in 2-methoxyethanol was slowly added to the solution (A). Precipitates of aluminium hydroxide appeared and were redissolved with another ultrasonic dose until a clear and transparent sonosol (C) was obtained which was left at room temperature for another 24 h. The last step was the
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
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M. Pifiero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
addition of water of pH = 2 to the solution (C), such that [ H 2 0 ] : [ T E O S ] = 33 : 1. The reaction was sonocatalyzed and the final solution was gelled at room temperature. The gelation time observed was about 30 min and the resulting sonogel was transparent. This route is shown schematically in fig. l(a). In other cases, cordierite gels were obtained containing 7 - 1 1 % mol of TiO 2. Titanium oxide was added as tetrabutyl orthotitanate Ti(OBun)4 (TBOT) to the final sonosol of cordierite before gelation. The resulting solution was submitted to an additional dose of ultrasound ( = 100 J cm-3). An alternative route (see fig. l(b)) to prepare cordierite gels was used in order to reduce processing time from about 3 days previously described to a few minutes.
(a)
In this route, propanol-2 was used as a solvent for TEOS and 5.3 mol of 2-PrOH per mote TEOS was employed. Mixing was carried out under ultrasound. This solution is designated as solution (A'). A stoichiometric quantity of alumina was added in the form of a solution of ASB containing 1 mol of acetylacetone (CH3COCH 2COCH 3) per mole ASB. The resulting solution (B') was homogenized with another dose of ultrasound. Immediately afterwards, the magnesium acetate dissolved in 55 mol of distilled water per mole magnesium was added under ultrasonic radiation to promote hydrolysis. The gelation time of this solution was about 3 min. Organic residues trapped in this gel were eliminated by thermal treatment according to the
(b) ASB
2-MeOEtOH AcH
I
2-PrOH
Us= 120 J cm-3
l
1 Us=300Jcm-3 ASB MeCOMeCOMe
,.
SOL A
I AC2Mg 2MeOEtOH [
~
24 hours room tern ~erature
Us = 125Jcm-3 SOL C 24 hours room temp. H20 ~
1
s =48Jcm-3
SOL A'
]
t
Us=300Jcm-3 Ac2Mg, H20
SOL B'
I Us=60Jcm-3 Us=90 Jcm-3
--y l
CORDIERITE SONOSOL
CORDIERITE I SONOSOL Fig. 1. Schematic routes for preparing cordierite sonogels.
525
M. Pihero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
results obtained from a thermogravimetric analysis previously effected. The sequence of steps followed is given in fig. 2. Differential thermal analysis (DTA) and X-ray diffraction studies were performed. Hot-pressing of cordierite sonogels was carried out at 20, 30 and 40 bar pressure at temperatures between 700 and 1000°C for 15-60 rain. Fully densified cordierite samples were obtained in the form of a disk 26 mm in diameter and 2 - 4 mm thick.
2.2. Preparation of composites Composites were made by dispersing the reinforcing phase in sonosolutions of cordierite and cordierite-TiO 2 before gelation. A high speed rotatory blender (Ultraturrax TP 18/10) operating at 20000 rpm was used to produce a dispersion of fibers. In some cases, the reinforcing phase was dispersed in the matrix sonosolution under the effect of a heavy dose of ultrasound. Gelation occurred in a few seconds to 3 - 4 min. After gelation, composite preforms were aged for 24-48 h in hermetically sealed containers and then dried following the same thermal schedule as for cordierite sonogets without reinforcement. Monolithic pieces of calcined composites were submitted to conventional sintering by soaking at
800
different temperatures from 900 to 1400°C for 1-24 h. In other cases, the sample was crushed and the sintering of composite powders was accomplished by hot-pressing, in the same conditions as already mentioned for pure cordierite sonogels. A complete study of the densification process under pressure was made both for cordierite sonogels and cordierite based composites, using the Murray, Rodgers and Williams treatment [5] deduced from the Mackenzie and Shuttleworth model [6] for viscous flow sintering. According to Murray et al. [5], the densification kinetics are expected to obey eq. (1): ln(1 - D )
=
3PA 4/x
t + ln(1 - D i ) ,
where PA is the applied pressure, D is the relative density, D i the starting relative density and/x the shear viscosity. Since the hot-pressing experiments were carried out at constant heating rate, eq. (1) is transformed into eq. (2): d In(1 - D ) dT
1 3Pa -
v 4/x'
750°C 650°C
Heating r a t e : 1 1 °C/min 450°C 4 h
400 300°C 2oooc
200
/
/
h
140°CJ " ~4 h
4h
t (hours) i
(2)
where v = dT/dt is the constant heating rate. Viscosity values, /x(T), during sintering were obtained from the slope of the L n ( 1 - D) vs. T
T (°C)
600
(1)
J
0 10 20 30 Fig. 2. Sequence ofstepsfollowed forthe elimination oforganicresiduesofcordieritesonogelsand composites.
526
M. Pi~ero et a L / Cordierite-ZrO 2 and cordierite-Al203 composites
Table 1 Physical properties of cordierite ceramics obtained from hot-pressing of sonogel powders Composition
Matrix form
Relative density
Strength (MPa)
15 15 15 30 20
glass glass glass ix-cord a-cord
0.65 0.75 0.96 0.99 0.97
6.30 25.60 53.00 93.20 70.00
30 30 30 30
15 15 15 15
glass glass ix-cord Ix-cord
0.70 0.80 0.99 0.99
9.70 18.70 65.00 96.70
30
20
a-cord
0.98
43.10
Hot-pressing T (°C)
P (bar)
t (min)
750 820 900 950 1000
30 30 30 30 30
Cord (ll%TiO 2)
750 850 900 950
Cord (7%TIO 2)
1150
Cordierite
plot. F r o m t h e s e v a l u e s , a n d a d m i t t i n g A r r h e nius-type dependance, the activation energy, E*, o f t h e d e n s i f i c a t i o n p r o c e s s by v i s c o u s f l o w f o r cordierite ceramics, cordierite-ZrO 2 and c o r d i e r i t e - A 1 2 0 3 c o m p o s i t e s w a s also o b t a i n e d . T h e s e v a l u e s a r e v e r y s i m i l a r a n d r a n g e d f r o m 70 k c a l / m o l f o r p u r e c o r d i e r i t e to 76 k c a l / m o l f o r cordierite-A120 3 composites.
C o r d i e r i t e - Z r O 2 m o n o l i t h i c s p e c i m e n s sint e r e d in air at 1400°C f o r 24 h w e r e s u b m i t t e d to t h e ' B r a z i l i a n t e s t ' , in w h i c h a c y l i n d r i c a l specim e n is t e s t e d in c o m p r e s s i o n p e r p e n d i c u l a r to t h e axis o f t h e c y l i n d e r . T h e f r a c t u r e stress, ~t, is o b t a i n e d f r o m t h e r e l a t i o n o-t = 2 P / ' r r d L , w h e r e P is t h e l o a d at f a i l u r e , d t h e d i a m e t e r a n d L t h e l e n g t h o f t h e c y l i n d e r . I n o r d e r to m i n i m i z e d e -
Table 2 Physical properties of cordierite-based composite ceramics obtained from hot-pressing of sonogel powders Composition
Hot-pressing
Matrix form
Relative density
Vf
Strength (MPa)
I (°C)
P (bar)
t (rain)
750 950 1000 900 1150
30 30 30 40 -
15 15 15 15 120
glass glass Ix-cord
0.60 0.93 0.99
0.60 0.52 0.49
2.00 38.00 110.15
a-cord
0.98
0.48
93.20
Cord (7%TIO 2) /A1203
1000 1000 1050
40 40 -
20 20 120
px-cord
0.95
0.51
158.0
a-cord
0.92
0.52
106.0
Cord/ZrO 2
750 800 950 980 1000 980 1050
30 30 30 40 30 40 -
15 15 15 60 15 60 120
glass glass glass Ix-cord Ix-cord
0.64 0:87 0.90 0.90 0.91
0.63 0.61 0.62 0.64 0.65
1.40 23:00 45.00 105.20 58.50
a-cord
0.90
0.66
74.00
Cord/A120 3
Vf, volume fraction.
527
M. Pihero et al. / Cordierite-ZrO 2 and cordierite-Al20 s composites
formation on the contacts, thin sheets of plastic were inserted between the cylinder and compression plates. The tests were accomplished with a cross-head speed of 2 m m / m i n . The D values ranged from 0.6 to 1.2 cm and L was about 2 cm.
Other mechanical properties of composites and pure cordierite matrix hot-pressed samples were determined by means of a three-point loading bending test using an Instron 1195 testing instrument with a cross-head speed of 0.5 m m / m i n .
(a) Cordierite 750 °C , I h
/
I
I
t
I
[
J
I
I
l
I ~ _ _ ~
l
Cordierite 780 °C , 2 h
I
I
I
N-Cordierite 1015 °C , 20 20 m i n .
I
bar
I
I
~-Cordierite 1015 °C , 20 b a r + 1120 °C , 2 h.
70 '
6S '
I
, 20
+'o
I
I
t
I
min
5
50 '
45 '
40 '
35 '
3 'o
L
21S
2LO
ILS
]0
28-
Fig. 3. X-ray diffraction spectra for: (a) pure cordierite sonogels; (b) cordierite-ZrO 2 composites; (c) cordierite-Al203 composites. e, ix-cordierite; &, a-cordierite; ©, a-alumina; A, tetragonal zirconia.
M. Pifiero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
528
Differential thermal analysis (DTA) combined with X-ray diffraction showed the presence of ~x-cordierite (hexagonal) (850-980°C) metastable low temperature form and c~-cordierite (orthorhombic) (980-1465°C) stable high temperature form. Figure 3 shows the diffraction patterns obtained for pure cordierite which give the structural evolution of the matrix with temperature. (There exists a third modification, [3-cordierite, obtained only from hydrothermal conditions from the glass at temperatures lower than 830°C which was not observed in our experiments.) Table 3 shows the evolution of final apparent densities of cordierite-based composites obtained by conventional sintering for different temperatures and soaking times.
The dimensions of samples were about 20 mm length, 5 mm width and 2 mm thickness. Cross-sections of composite specimens after failure were examined by scanning electron microscopy (SEM) to determine the fracture morphology.
3. Results
3.1. Physical properties The physical properties of cordierite ceramics obtained from hot-pressing sonogel powders are summarized in table 1, and table 2 gives the results for composites.
(b) ~-Cordierite/Zr02 980 °C , 40 bar 60 min
A
A
I
I
I
I
I
I
1
I
I
I
~
I
L
i
25
20
15
~-Cordierite/Zr0~ 980 °C , 40 bar , 60 min. + 1050 °C , 10 h. A
A
k_ I
I
1
I
70
65
60
i
55
I
50
I
45
a
40
28 Fig. 3. ( c o n t i n u e d ) .
i
35
L
30
10
529
M. PiJTeroet aL / Cordierite-ZrO z and cordierite-Al20 s composites
(c) ~-C0rdierite/Al203 980 °C , 20 b a r 20 min.
• 0
0
oo
o ,
0
I
~-Cordierite/Al20 ~ 980 °C , 20 b a r , 20 min + 1120 °C , 2 h.
I
o
o oo
•
o
"
70
65
oA
•
I
&A
A AUL
J
60
55
50
45
,
40
35
30
25
_ _ J
,
20
15
._
10
28
Fig. 3. (continued). 3.2. M e c h a n i c a l p r o p e r t i e s
Results for cordierite and cordierite-titania ceramics and composites reinforced with fibers show an initial elastic behaviour followed by a catastrophic failure typical of brittle ceramics. Table 3 Evolution of final apparent densities of cordierite-based composites obtained by conventional sintering for different temperatures and soaking times Matrix
Cordierite
Filler
A|20 3 ZrO 2
Mechanical resistance for hot-pressed pure cordierite samples and composites are shown in tables 1 and 2. Hot-pressed cordierite fiber reinforced composites show the same characteristic elastic regime up to the final crack, but higher breaking stresses are observed. The highest strength, ~t, obtained for c o r d i e r i t e - Z r O 2 composites sintered in air at 1400°C is 35-40 MPa for a volume fraction Vf = 0.5 of reinforcing fibers. This value decreases rapidly for higher Vf.
Apparent density (g cm-3) 600°C
1400°C (t h)
1400°C (24 h)
0.56 0.50
0.60 0.70
0.61
4.36
4. Discussion From the D T A diagram, it can be seen that devitrification in cordierite-(7%TiO a) samples occurs at lower temperatures ( 9 3 0 ° 0 than for
530
M. Pihero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
cordierite samples without nucleant agent (970°C). A second peak (1150-1200°C) observed in the thermogram is attributed to the presence of a-cordierite. Relative densities of cordierite hot-pressed sonogels samples increase with sintering temperature, from 0.65 at 700°C to 0.99 at 950°C, and decrease at higher temperatures at the moment of structural change from the ~-cordierite to the a-form. Since the high temperature form of cordierite has larger crystallographic parameters than the ix-form, this induces a volume increase experimentally observed for the samples. The data reported in table 3 show that only cordierite-ZrO 2 composites could be sintered in the absence of an external pressure. The matrix was found to exist in a glassy state in this sample which was sintered for 24 h at 1400°C. Fusion was probably due to the extreme conditions of sintering. The mechanical strength obtained for these sample ( ~ 40 MPa) was increased to 105 MPa when hot-pressed samples technique is used. A chemical incompatibility between the impregnating cordierite sonosol and some impurities on the surface of A120 3 fibers is probably the reason why cordierite-A120 3 composites could not be sintered under the same conditions as cordierite-ZrO 2 samples, and hot-pressing was required to accomplish their densification. The maximum matrix strength (95 MPa) found in a hot-pressed c o r d i e r i t e - l l % TiO 2 sample was increased with the addition of reinforcing phases and hot-pressing up to 105 MPa for a ix-cordierite-ZrO 2 composite, and 160 MPa for a ix-cordierite(7% TiO2)-A120 3 composite. Stress-strain curves obtained in the flexural test indicate that both cordierite-ZrO 2 and cordierite-A120 3 composites seem to fail in the same brittle way as the isolated ceramic matrix. From SEM observations of the surface of failed composites, the absence of porosity can be seen in the matrix for samples with the ix-form and, according to fracture stress data obtained, we estimate that the fracture origin in these samples is caused by some discontinuities created at the matrix-fiber interface during sintering. Moreover a large porosity is observed in composites where a-cordierite is the matrix form, which could ex-
plain the lower fracture stress observed in comparison with the other samples with the ix-form. The highest strength was obtained for composites where TiO 2 was employed to control devitrification of the matrix. This resulted in a decrease of the particle size, leading to an improvement of the intergranular zones in the matrix and of the bonding at the matrix-fiber interface. This also contributed to the elimination of the residual porosity which is apparent in samples without nucleant agent. The highest fracture stress values were obtained for a ix-cordierite (7% TiO2)-AI20 3 composite (160 MPa). Lower stress was however obtained for composites using ZrO z fibers (105 MPa). This is attributed to a dispersion of A120 3 fibers in the starting sonosol better than of ZrO 2 fibers, which provided a better stress distribution in the composite. The ZrO 2 fibers appear as bundles of agglomerated elementary microfibers and this hinders their dispersion. 5. Conclusions
Sonocatalysis in the sol-gel process provides an interesting method in the synthesis of ceramic-ceramic composites. A highly homogeneous and low viscosity sonosolution of the matrix composition can be obtained and its gelling time may be substantially reduced by a special preparation method. After gelation, a gel preform of composite is obtained with excellent interfacial bonding strength characteristics and which can be successfully densified by hot-pressing or conventional techniques. Fracture resistance of pure cordierite and cordierite composites obtained are increased by the addition of 7-11 tool% of nucleant TiO2, in alkoxide precursor form to the starting cordierite sonosol. Hot-pressing substantially improves the mechanical characteristics of these composites. The authors gratefully acknowledge the financial support of the AFOSR under contract No. 89 0533 A and of the project MAT-1022/91.
M. Pifiero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
References [1] M. Tarasevich, in: Proc. 86th Annual Meeting of the American Ceramic Society, Pittsburgh (PA), May 2 (1984); Ceram. Bull. 63 (1984) 500 (abstract only). [2] N. de la Rosa-Fox, L. Esquivias and J. Zarzycki, in: Effects of Modes of Formation on the Structure of Glass. Proc. 2nd Int. Conf., Nashville, TN, June, 1987, ed. R.A. Weeks and D.L. Kinser (Trans. Tech., Aedermannsdorf, 1987) p. 363.
531
[3] N. de la Rosa-Fox, L. Esquivias and J. Zarzycki, Rev. Phys. Appl. 24 Coll. C4 (1989) 233. [4] M. Atik, thesis, University of Montpellier (1990). [5] P. Murray, E.P. Rodgers and A.E. Williams, Trans. Br. Ceram. Soc. 53 (1954) 473. [6] J.D. Mackenzie and R. Shuttleworth, Proc. Phys. Soc. 62 (1949) 833.
Journal of Non-Crystalline Solids 147&148 (1992) 523-531 North-Holland
NON-CRYSTALLINE SOLIDS
Cordierite-ZrO 2 and cordierite-A1203 composites obtained by sonocatalytic methods M. Pifiero, M. Atik a n d J. Zarzycki Laboratoire de Science des Matdriaux Vitreux, University of Montpellier II, Case 069, Place E. Bataillon, Montpellier, France
Cordierite-matrix-based composites were prepared by the sol-gel process involving sonocatalysis during preliminary chemical reactions. ZrO 2 and A1203 ceramic fibers were employed as reinforcing phases. Sintering was accomplished by hot-pressing techniques. Devitrification of sintered cordierite sonogels showed that, when ~x- and c~-cordierite forms appear, a substantial increase in the mechanical strength is observed. Addition of the nucleant agent TiO 2 further improves mechanical properties.
1. Introduction
2. Experimental procedure
Sol-gel processing is a relatively new method of preparing ceramic composites. Reinforcing phases are infiltrated with a low viscosity solution which may be helped by the application of an external pressure. This promotes formation of an intimate interface between the matrix and reinforcing phase after gelation, resulting in a high interracial bond strength which should improve mechanical performances. Recent studies on the application of ultrasonic radiation to the sol-gel process [1] have shown that high density 'sonogels' with a larger specific surface than those observed for 'classic gels' [2,3] could be obtained by this method, this would constitute a great advantage for the sintering process, since densification may be carried out at lower temperatures. Using the sonocatalytic approach, composites in the SiOz-SiO 2 system were obtained by introducing fine silica particles (Aerosil) into a SiO 2 sonosol [4]. In the present work sonogels of cordierite (5Si2-2A1203-2MgO) , a low expansion ceramic compound, were used in the preparation of the matrix for ceramic-ceramic composites. Reinforcing phases used were ceramic industrial A1203 and ZrO 2 fibers (Zircar Products).
2.1. Preparation of pure cordierite matrix Tetraethoxysilane, Si(OEt) 4 (TEOS), (Fluka) was used as a source of silica, aluminium secbutoxide Al(OBuS)4 (ASB) for alumina and magnesium acetate tetrahydrate Mg(Ac)4H20 for magnesia. The solvent used was 2-methoxyethanol (2-MeOEtOH). Because hydrolysis and polymerization of aluminium alkoxide is faster than that of TEOS, 0.6 tool of acetic acid per mole ASB was added drop by drop under the effect of ultrasound to a solution containing ASB and 20 vol.% of 2methoxyethanol. The TEOS was then added and the resulting solution (A) was homogenized under the effect of ultrasound. This solution was left for 24 h at room temperature. After this time, a separately prepared solution (B) containing magnesium acetate which was dissolved under sonication in 2-methoxyethanol was slowly added to the solution (A). Precipitates of aluminium hydroxide appeared and were redissolved with another ultrasonic dose until a clear and transparent sonosol (C) was obtained which was left at room temperature for another 24 h. The last step was the
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
524
M. Pifiero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
addition of water of pH = 2 to the solution (C), such that [ H 2 0 ] : [ T E O S ] = 33 : 1. The reaction was sonocatalyzed and the final solution was gelled at room temperature. The gelation time observed was about 30 min and the resulting sonogel was transparent. This route is shown schematically in fig. l(a). In other cases, cordierite gels were obtained containing 7 - 1 1 % mol of TiO 2. Titanium oxide was added as tetrabutyl orthotitanate Ti(OBun)4 (TBOT) to the final sonosol of cordierite before gelation. The resulting solution was submitted to an additional dose of ultrasound ( = 100 J cm-3). An alternative route (see fig. l(b)) to prepare cordierite gels was used in order to reduce processing time from about 3 days previously described to a few minutes.
(a)
In this route, propanol-2 was used as a solvent for TEOS and 5.3 mol of 2-PrOH per mote TEOS was employed. Mixing was carried out under ultrasound. This solution is designated as solution (A'). A stoichiometric quantity of alumina was added in the form of a solution of ASB containing 1 mol of acetylacetone (CH3COCH 2COCH 3) per mole ASB. The resulting solution (B') was homogenized with another dose of ultrasound. Immediately afterwards, the magnesium acetate dissolved in 55 mol of distilled water per mole magnesium was added under ultrasonic radiation to promote hydrolysis. The gelation time of this solution was about 3 min. Organic residues trapped in this gel were eliminated by thermal treatment according to the
(b) ASB
2-MeOEtOH AcH
I
2-PrOH
Us= 120 J cm-3
l
1 Us=300Jcm-3 ASB MeCOMeCOMe
,.
SOL A
I AC2Mg 2MeOEtOH [
~
24 hours room tern ~erature
Us = 125Jcm-3 SOL C 24 hours room temp. H20 ~
1
s =48Jcm-3
SOL A'
]
t
Us=300Jcm-3 Ac2Mg, H20
SOL B'
I Us=60Jcm-3 Us=90 Jcm-3
--y l
CORDIERITE SONOSOL
CORDIERITE I SONOSOL Fig. 1. Schematic routes for preparing cordierite sonogels.
525
M. Pihero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
results obtained from a thermogravimetric analysis previously effected. The sequence of steps followed is given in fig. 2. Differential thermal analysis (DTA) and X-ray diffraction studies were performed. Hot-pressing of cordierite sonogels was carried out at 20, 30 and 40 bar pressure at temperatures between 700 and 1000°C for 15-60 rain. Fully densified cordierite samples were obtained in the form of a disk 26 mm in diameter and 2 - 4 mm thick.
2.2. Preparation of composites Composites were made by dispersing the reinforcing phase in sonosolutions of cordierite and cordierite-TiO 2 before gelation. A high speed rotatory blender (Ultraturrax TP 18/10) operating at 20000 rpm was used to produce a dispersion of fibers. In some cases, the reinforcing phase was dispersed in the matrix sonosolution under the effect of a heavy dose of ultrasound. Gelation occurred in a few seconds to 3 - 4 min. After gelation, composite preforms were aged for 24-48 h in hermetically sealed containers and then dried following the same thermal schedule as for cordierite sonogets without reinforcement. Monolithic pieces of calcined composites were submitted to conventional sintering by soaking at
800
different temperatures from 900 to 1400°C for 1-24 h. In other cases, the sample was crushed and the sintering of composite powders was accomplished by hot-pressing, in the same conditions as already mentioned for pure cordierite sonogels. A complete study of the densification process under pressure was made both for cordierite sonogels and cordierite based composites, using the Murray, Rodgers and Williams treatment [5] deduced from the Mackenzie and Shuttleworth model [6] for viscous flow sintering. According to Murray et al. [5], the densification kinetics are expected to obey eq. (1): ln(1 - D )
=
3PA 4/x
t + ln(1 - D i ) ,
where PA is the applied pressure, D is the relative density, D i the starting relative density and/x the shear viscosity. Since the hot-pressing experiments were carried out at constant heating rate, eq. (1) is transformed into eq. (2): d In(1 - D ) dT
1 3Pa -
v 4/x'
750°C 650°C
Heating r a t e : 1 1 °C/min 450°C 4 h
400 300°C 2oooc
200
/
/
h
140°CJ " ~4 h
4h
t (hours) i
(2)
where v = dT/dt is the constant heating rate. Viscosity values, /x(T), during sintering were obtained from the slope of the L n ( 1 - D) vs. T
T (°C)
600
(1)
J
0 10 20 30 Fig. 2. Sequence ofstepsfollowed forthe elimination oforganicresiduesofcordieritesonogelsand composites.
526
M. Pi~ero et a L / Cordierite-ZrO 2 and cordierite-Al203 composites
Table 1 Physical properties of cordierite ceramics obtained from hot-pressing of sonogel powders Composition
Matrix form
Relative density
Strength (MPa)
15 15 15 30 20
glass glass glass ix-cord a-cord
0.65 0.75 0.96 0.99 0.97
6.30 25.60 53.00 93.20 70.00
30 30 30 30
15 15 15 15
glass glass ix-cord Ix-cord
0.70 0.80 0.99 0.99
9.70 18.70 65.00 96.70
30
20
a-cord
0.98
43.10
Hot-pressing T (°C)
P (bar)
t (min)
750 820 900 950 1000
30 30 30 30 30
Cord (ll%TiO 2)
750 850 900 950
Cord (7%TIO 2)
1150
Cordierite
plot. F r o m t h e s e v a l u e s , a n d a d m i t t i n g A r r h e nius-type dependance, the activation energy, E*, o f t h e d e n s i f i c a t i o n p r o c e s s by v i s c o u s f l o w f o r cordierite ceramics, cordierite-ZrO 2 and c o r d i e r i t e - A 1 2 0 3 c o m p o s i t e s w a s also o b t a i n e d . T h e s e v a l u e s a r e v e r y s i m i l a r a n d r a n g e d f r o m 70 k c a l / m o l f o r p u r e c o r d i e r i t e to 76 k c a l / m o l f o r cordierite-A120 3 composites.
C o r d i e r i t e - Z r O 2 m o n o l i t h i c s p e c i m e n s sint e r e d in air at 1400°C f o r 24 h w e r e s u b m i t t e d to t h e ' B r a z i l i a n t e s t ' , in w h i c h a c y l i n d r i c a l specim e n is t e s t e d in c o m p r e s s i o n p e r p e n d i c u l a r to t h e axis o f t h e c y l i n d e r . T h e f r a c t u r e stress, ~t, is o b t a i n e d f r o m t h e r e l a t i o n o-t = 2 P / ' r r d L , w h e r e P is t h e l o a d at f a i l u r e , d t h e d i a m e t e r a n d L t h e l e n g t h o f t h e c y l i n d e r . I n o r d e r to m i n i m i z e d e -
Table 2 Physical properties of cordierite-based composite ceramics obtained from hot-pressing of sonogel powders Composition
Hot-pressing
Matrix form
Relative density
Vf
Strength (MPa)
I (°C)
P (bar)
t (rain)
750 950 1000 900 1150
30 30 30 40 -
15 15 15 15 120
glass glass Ix-cord
0.60 0.93 0.99
0.60 0.52 0.49
2.00 38.00 110.15
a-cord
0.98
0.48
93.20
Cord (7%TIO 2) /A1203
1000 1000 1050
40 40 -
20 20 120
px-cord
0.95
0.51
158.0
a-cord
0.92
0.52
106.0
Cord/ZrO 2
750 800 950 980 1000 980 1050
30 30 30 40 30 40 -
15 15 15 60 15 60 120
glass glass glass Ix-cord Ix-cord
0.64 0:87 0.90 0.90 0.91
0.63 0.61 0.62 0.64 0.65
1.40 23:00 45.00 105.20 58.50
a-cord
0.90
0.66
74.00
Cord/A120 3
Vf, volume fraction.
527
M. Pihero et al. / Cordierite-ZrO 2 and cordierite-Al20 s composites
formation on the contacts, thin sheets of plastic were inserted between the cylinder and compression plates. The tests were accomplished with a cross-head speed of 2 m m / m i n . The D values ranged from 0.6 to 1.2 cm and L was about 2 cm.
Other mechanical properties of composites and pure cordierite matrix hot-pressed samples were determined by means of a three-point loading bending test using an Instron 1195 testing instrument with a cross-head speed of 0.5 m m / m i n .
(a) Cordierite 750 °C , I h
/
I
I
t
I
[
J
I
I
l
I ~ _ _ ~
l
Cordierite 780 °C , 2 h
I
I
I
N-Cordierite 1015 °C , 20 20 m i n .
I
bar
I
I
~-Cordierite 1015 °C , 20 b a r + 1120 °C , 2 h.
70 '
6S '
I
, 20
+'o
I
I
t
I
min
5
50 '
45 '
40 '
35 '
3 'o
L
21S
2LO
ILS
]0
28-
Fig. 3. X-ray diffraction spectra for: (a) pure cordierite sonogels; (b) cordierite-ZrO 2 composites; (c) cordierite-Al203 composites. e, ix-cordierite; &, a-cordierite; ©, a-alumina; A, tetragonal zirconia.
M. Pifiero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
528
Differential thermal analysis (DTA) combined with X-ray diffraction showed the presence of ~x-cordierite (hexagonal) (850-980°C) metastable low temperature form and c~-cordierite (orthorhombic) (980-1465°C) stable high temperature form. Figure 3 shows the diffraction patterns obtained for pure cordierite which give the structural evolution of the matrix with temperature. (There exists a third modification, [3-cordierite, obtained only from hydrothermal conditions from the glass at temperatures lower than 830°C which was not observed in our experiments.) Table 3 shows the evolution of final apparent densities of cordierite-based composites obtained by conventional sintering for different temperatures and soaking times.
The dimensions of samples were about 20 mm length, 5 mm width and 2 mm thickness. Cross-sections of composite specimens after failure were examined by scanning electron microscopy (SEM) to determine the fracture morphology.
3. Results
3.1. Physical properties The physical properties of cordierite ceramics obtained from hot-pressing sonogel powders are summarized in table 1, and table 2 gives the results for composites.
(b) ~-Cordierite/Zr02 980 °C , 40 bar 60 min
A
A
I
I
I
I
I
I
1
I
I
I
~
I
L
i
25
20
15
~-Cordierite/Zr0~ 980 °C , 40 bar , 60 min. + 1050 °C , 10 h. A
A
k_ I
I
1
I
70
65
60
i
55
I
50
I
45
a
40
28 Fig. 3. ( c o n t i n u e d ) .
i
35
L
30
10
529
M. PiJTeroet aL / Cordierite-ZrO z and cordierite-Al20 s composites
(c) ~-C0rdierite/Al203 980 °C , 20 b a r 20 min.
• 0
0
oo
o ,
0
I
~-Cordierite/Al20 ~ 980 °C , 20 b a r , 20 min + 1120 °C , 2 h.
I
o
o oo
•
o
"
70
65
oA
•
I
&A
A AUL
J
60
55
50
45
,
40
35
30
25
_ _ J
,
20
15
._
10
28
Fig. 3. (continued). 3.2. M e c h a n i c a l p r o p e r t i e s
Results for cordierite and cordierite-titania ceramics and composites reinforced with fibers show an initial elastic behaviour followed by a catastrophic failure typical of brittle ceramics. Table 3 Evolution of final apparent densities of cordierite-based composites obtained by conventional sintering for different temperatures and soaking times Matrix
Cordierite
Filler
A|20 3 ZrO 2
Mechanical resistance for hot-pressed pure cordierite samples and composites are shown in tables 1 and 2. Hot-pressed cordierite fiber reinforced composites show the same characteristic elastic regime up to the final crack, but higher breaking stresses are observed. The highest strength, ~t, obtained for c o r d i e r i t e - Z r O 2 composites sintered in air at 1400°C is 35-40 MPa for a volume fraction Vf = 0.5 of reinforcing fibers. This value decreases rapidly for higher Vf.
Apparent density (g cm-3) 600°C
1400°C (t h)
1400°C (24 h)
0.56 0.50
0.60 0.70
0.61
4.36
4. Discussion From the D T A diagram, it can be seen that devitrification in cordierite-(7%TiO a) samples occurs at lower temperatures ( 9 3 0 ° 0 than for
530
M. Pihero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
cordierite samples without nucleant agent (970°C). A second peak (1150-1200°C) observed in the thermogram is attributed to the presence of a-cordierite. Relative densities of cordierite hot-pressed sonogels samples increase with sintering temperature, from 0.65 at 700°C to 0.99 at 950°C, and decrease at higher temperatures at the moment of structural change from the ~-cordierite to the a-form. Since the high temperature form of cordierite has larger crystallographic parameters than the ix-form, this induces a volume increase experimentally observed for the samples. The data reported in table 3 show that only cordierite-ZrO 2 composites could be sintered in the absence of an external pressure. The matrix was found to exist in a glassy state in this sample which was sintered for 24 h at 1400°C. Fusion was probably due to the extreme conditions of sintering. The mechanical strength obtained for these sample ( ~ 40 MPa) was increased to 105 MPa when hot-pressed samples technique is used. A chemical incompatibility between the impregnating cordierite sonosol and some impurities on the surface of A120 3 fibers is probably the reason why cordierite-A120 3 composites could not be sintered under the same conditions as cordierite-ZrO 2 samples, and hot-pressing was required to accomplish their densification. The maximum matrix strength (95 MPa) found in a hot-pressed c o r d i e r i t e - l l % TiO 2 sample was increased with the addition of reinforcing phases and hot-pressing up to 105 MPa for a ix-cordierite-ZrO 2 composite, and 160 MPa for a ix-cordierite(7% TiO2)-A120 3 composite. Stress-strain curves obtained in the flexural test indicate that both cordierite-ZrO 2 and cordierite-A120 3 composites seem to fail in the same brittle way as the isolated ceramic matrix. From SEM observations of the surface of failed composites, the absence of porosity can be seen in the matrix for samples with the ix-form and, according to fracture stress data obtained, we estimate that the fracture origin in these samples is caused by some discontinuities created at the matrix-fiber interface during sintering. Moreover a large porosity is observed in composites where a-cordierite is the matrix form, which could ex-
plain the lower fracture stress observed in comparison with the other samples with the ix-form. The highest strength was obtained for composites where TiO 2 was employed to control devitrification of the matrix. This resulted in a decrease of the particle size, leading to an improvement of the intergranular zones in the matrix and of the bonding at the matrix-fiber interface. This also contributed to the elimination of the residual porosity which is apparent in samples without nucleant agent. The highest fracture stress values were obtained for a ix-cordierite (7% TiO2)-AI20 3 composite (160 MPa). Lower stress was however obtained for composites using ZrO z fibers (105 MPa). This is attributed to a dispersion of A120 3 fibers in the starting sonosol better than of ZrO 2 fibers, which provided a better stress distribution in the composite. The ZrO 2 fibers appear as bundles of agglomerated elementary microfibers and this hinders their dispersion. 5. Conclusions
Sonocatalysis in the sol-gel process provides an interesting method in the synthesis of ceramic-ceramic composites. A highly homogeneous and low viscosity sonosolution of the matrix composition can be obtained and its gelling time may be substantially reduced by a special preparation method. After gelation, a gel preform of composite is obtained with excellent interfacial bonding strength characteristics and which can be successfully densified by hot-pressing or conventional techniques. Fracture resistance of pure cordierite and cordierite composites obtained are increased by the addition of 7-11 tool% of nucleant TiO2, in alkoxide precursor form to the starting cordierite sonosol. Hot-pressing substantially improves the mechanical characteristics of these composites. The authors gratefully acknowledge the financial support of the AFOSR under contract No. 89 0533 A and of the project MAT-1022/91.
M. Pifiero et al. / Cordierite-ZrO 2 and cordierite-Al203 composites
References [1] M. Tarasevich, in: Proc. 86th Annual Meeting of the American Ceramic Society, Pittsburgh (PA), May 2 (1984); Ceram. Bull. 63 (1984) 500 (abstract only). [2] N. de la Rosa-Fox, L. Esquivias and J. Zarzycki, in: Effects of Modes of Formation on the Structure of Glass. Proc. 2nd Int. Conf., Nashville, TN, June, 1987, ed. R.A. Weeks and D.L. Kinser (Trans. Tech., Aedermannsdorf, 1987) p. 363.
531
[3] N. de la Rosa-Fox, L. Esquivias and J. Zarzycki, Rev. Phys. Appl. 24 Coll. C4 (1989) 233. [4] M. Atik, thesis, University of Montpellier (1990). [5] P. Murray, E.P. Rodgers and A.E. Williams, Trans. Br. Ceram. Soc. 53 (1954) 473. [6] J.D. Mackenzie and R. Shuttleworth, Proc. Phys. Soc. 62 (1949) 833.