oxide composites produced by the internal crystallization method

CompositesScience and Technology PII:

ELSEVIER

SO266-3538(97)00063-8

51(1997) 1363-1367 0 1997 Elsevier Science Limited Printed in Northern Ireland. AU rights reserved 0266-3538/97/$17.00

OXIDE/OXIDE COMPOSITES PRODUCED BY THE INTERNAL CRYSTALLIZATION METHOD S. T. Mileiko, V. I. Kazmin,* V. M. Kiiko & A. M. Rudnev Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142 432, Russia

(Received 2 December 1996; accepted 26 February 1997)

can provide a greater variety of component properties and so amplifies the potential to develop tough ceramic-fibre/ceramic-matrix composites. The internal crystallization method for making fibrous composites,3*4 which consists of the infiltration of channels pre-made in the matrix with fibre material melt and subsequent crystallization of the melt to produce high-strength fibres, is now applied to fabricate ceramic-fibre/ceramic-matrix composites. The first stage of the work described in the present paper was aimed only at showing the possibility of making ceramic-matrix composites by the internal crystallization method. Hence, composite specimens obtained were not optimized with respect to the microstructure of the fibre, matrix and fibre/matrix interface. However, the real possibility of obtaining oxide/oxide composites by using the internal crystallization method has been demonstrated. Possible ways of improving the mechanical properties of the composites are now clear: the corresponding speculations are also given.

Abstract The internal crystallization method for making fibrous composites, which consists of the infiltration of channels made in the matrix with fibre material melt and subsequent crystallization of the melt to produce highstrength fibre, is now applied to fabricate ceramiccomposites. Composites thus fibre/ceramic-matrix obtained were not optimized with respect to the microstructure of the fibre, matrix and jibre/matrix interface. However, the possibility of making oxide/oxide composites by using the internal crystallization method has been demonstrated. Speculations about possible ways of improving mechanical properties of the composites are presented. 0 1997 Elsevier Science Limited Keywords: A. ceramic-matrix

fracture toughness; crystallization

composites; A. oxides; B. C. elastic properties; internal

1 INTRODUCTION

2 FABRICATION

Ceramic-matrix composites are normally obtained by either consolidation of pre-made fibre and matrix or by growing fibres in the matrix. Methods of the first kind include a variety of routes based on powdermetallurgy methods, liquid or chemical vapour infiltration of the matrix material in the fibre preform, etc. The second group of methods includes either the wellknown unidirectional solidification of eutectic mixtures’ or a less known ‘chemical mixing process’, which is a modification of the powder-metallurgy scheme aimed at the formation of whiskers in situ. The latter process involves a stage of whiskerization reaction in a mixture of the powder of matrix material and necessary reagents to form the whiskers. It is well known that mechanical properties, especially the fracture toughness, of ceramic-matrix composites are very sensitive to the properties of the components and the interface.’ Enlarging the fabrication possibilities to produce ceramic-matrix composites

The method of internal following steps:4*5

crystallization

includes the

1. formation of continuous cylindrical channels in the matrix; 2. infiltration of the channels in the matrix with a melted fibre material; and 3. crystallization of the fibres in the channels. In the case of ceramic-matrix composites, formation of the channels in the matrix can the performed in a number of ways. For a sapphire matrix, a method consisting of preliminary preparation of a molybdenum-wire/oxide-matrix composite’ and subsequent burning off of the molybdenum at a temperature of about 1150-1350°C is suitable. A general view of a sample of the matrix obtained in such a way is shown in Fig. 1. Easy infiltration of the matrix channels with the fibre material melt can be done if the matrix is wetted by the melt. In the present study of a preliminary

* Deceased. 1363

1364

S. T. Mileiko

et al.

Fig. 2. Longitudinal section of an Al,O, + ZrO,( + Y,O$ A1203 composite ( X 100). Fig. 1. Sapphire matrix specimen with cylindrical channels left after burning out molybdenum wires. nature, wetting was provided by careful selection of the fibre/matrix combination. In particular, if the matrix material is A (or an M+N eutectic) then the fibre material is an A+B eutectic (or an M+N+P eutectic). Obviously, to prevent severe dissolution of the matrix, it is necessary to keep the infiltration temperature just above the eutectic point. Also it should be noted that with such a choice of the components, the fibre/matrix interface is expected to be strong and effects such as delamination, fibre pullout, etc., are not to be expected to play an essential role in enhancing the composite fracture toughness. Obviously, a procedure can be included in the fabrication route to control the microstructure and properties of the interface and thus optimize composite behaviour. The crystallization rate of the fibre is chosen do be as high as possible and the channels must be fed with the melt to compensate the volume effect at crystallization. In the case under consideration, a withdrawal rate of about 30 cm min-’ appeared to satisfy that condition. 3 MICROSTRUCTURE

effect as a result of the phase transformation in the zirconia. At the same time, microcracking of the matrix yields some improvement in crack resistance (Table 1). When the matrix is obtained by crystallization of a eutectic mixture of two oxides, its microstructure is of a composite nature (Fig. 6(b)). The colonies of nearly constant orientations of the structure which are clearly seen in Fig. 6(c), together with smaller formations of constant orientations that could be called subcolonies (Fig. 6(b)), can have various orientations of the fibrous phase. The same type of microstructure is characteristic for the fibre/matrix interface (Fig. 6(d)) which is certainly always a eutectic composed of the oxides contained in the fibre and matrix. No doubt, such a structure of the interface yields a high interface strength, so a macrocrack in the composite does not trigger specific mechanisms of toughening in ceramicmatrix composites (fibre pull-outs, crack bridging,

AND PROPERTIES

Micrographs presented in Figs 2-6 show characteristic microstructures of the composites. First, we note that the fibres are not perfectly aligned in a longitudinal section. Second, rather a high degree of interaction at the fibreimatrix interface takes place in all cases. Then one can see that the partial stabilization of zirconia, as a component of the eutectics to form the fibre material, leads to a regular microstructure without any microcracking (Fig. 2). When zirconia in the fibre is not stabilized the sapphire matrix undergoes microcracking (Fig. 5), almost certainly owing to a volume

Fig. 3. Longitudinal section of an A&O3+ ZrO, + Y3A15012/ A&O, + ZrOZ composite ( X 100).

Oxide/oxide ceramics by internal crystallization

1365

When an oxide with a low thermal expansion coefficient, such as mullite, is introduced in the fibre material then neither fibre nor matrix undergoes microcracking (Fig. 4).

4 FIBRE CONTAINING

Fig. 4.

Longitudinal section of an AlzO, + ZrO, + mullitel A&O3+ ZrO, composite ( X 100).

TIAL205

The complex oxide Ti02*A1,05 appears to be an attractive candidate for use as a component of an oxide/oxide composite because of its low thermal expansion coefficient (about 2 X 10F6 K-‘). On the other hand, its Young’s modulus is low (about 50 GPa). An even more essential drawback of this oxide is its instability in the temperature interval between 700 and 1300”C6 Moreover, the stoichiometry of titanium-containing oxides is known to be easily lost on heating in vacuum. Hence, dealing with TiA1205 in both fabrication and service stages requires extreme

interface delamination, etc.). Some evidence of such behaviour of the composites can be seen in Fig. 7. If an oxide which is known to stabilize zirconia is present in the fibre, then a zirconia-containing matrix appears to remain untracked (Fig. 3): however, the mechanical properties of such composites are low (Table 1).

Fig. 5. Longitudinal section of an A1,0,+ZrOZIA1203 composite ( X 100).

Fig. 6. Scanning electron mlcrographs of the cross-section of an A1203+ ZrOz + Y3A1501JA120s+ ZrOz( + MgO) composite: (a) general view, (b) the matrix, (c) a fibre and surroundings, (d) the fibrelmatrix interface.

Table 1. Strength and fracture toughness of composites

Fibre material A&O3+ ZrO, A1203+ ZrOz A1203+ ZrO, + Y3A15012 A1,03 + ZrO, + mullite

Matrix material

Fibre volume fraction

Bending strength (MPa)

Y203 + A&O3 A&O, A1203+ ZrO, A1203+ ZrO,

0.40 0.40 0.40 0.40

l@-300 160 150

220

Critical stress intensity factor (MPa V/m) 0.9 5-6

3-4 -

S. T. Mileiko

1366

accuracy. It seems interesting, nevertheless, to study composites with a TiA1,OS component. Composite specimens with an A1201 matrix were obtained by infiltration of the matrix channels with an oxide mixture corresponding to a eutectic composition in the Al,O,iTiA1,OS system. After infiltration in vacuum and crystallization of the fibre, the composite was annealed at 1400-1500°C for 5 h in air to restore the stoichiometry of titanium-containing oxides. X-ray phase analysis revealed both A1,03 and TiAl*O, in specimens after annealing. To observe decomposition of TiAl*O, into A&O, and Ti02, additional annealing at temperatures between 1100 and 1300°C was performed. X-ray phase analysis of a specimen which had undergone the additional annealing at 1200°C for

Fig. 7. Failure

surface

of an A&O, +ZrO,( composite.

+Y,O,)/AI,O,

et al. 10 h revealed A1,03 and TiOz, and did not reveal TiA120s. Hence, after the second annealing, the fibre consists of a mixture of two simple oxides. The Laue patterns showed that the fibre contains large volumes of single crystalline materials. Optical micrographs of specimens after the first and the second annealing are similar to that shown in Fig. 5, with an intensive matrix microcracking. The phase composition and microstructure of the composites are also indicated by the elastic characteristics determined on cubic specimens by measurements of ultrasonic wave velocities. The cubic specimens with an edge size of 1 cm were prepared in such a way as to make one side to be normal to the fibre direction (i.e. the x1 direction). The wave dispersion within the frequency interval used (0.5-5 MHz) was marked, so the characteristic wave velocities were obtained by extrapolating the velocity/frequency dependences to zero frequency. Results of the calculation of the components of the elastic tensor, C,,, C,, and &, according to the well-known formulae, are presented in Table 2. We see that, first, the decomposition of TiAl,OS into A120, and TiOz after low-temperature annealing yields a large increase in all of the elastic constants. The difference between the values of C,, and C,, is certainly caused by the non-hexagonal arrangement of the fibres in the plane normal to the fibre direction. The irregular microcracking of the matrix makes it difficult to compare possible theoretical evaluations of the elastic characteristics with the measured values. The dependences of bending strength of the composites on annealing regimes are presented in Figs 8 and 9. The dependences can be considered to be a result of the kinetics of two processes, the first being changes in the composition of oxides approaching stoichiometry during annealing in air, and the second being the formation of alumina and rutile as a result of the decomposition of TiA120,. Comparing the two sets of data presented in Fig. 8, we may suppose that annealing of the as-produced composites at 1400°C for 5 h is in sufficient to restore the stoichiometry as annealing at 1500°C yields higher strength composites. It is then obvious (see also data on the elastic moduli and phase composition) that the strength of the TiA1,OS fibre is very low: the second annealing at 1300°C leads to a drastic drop in the strength (Fig. 8). The more complete the decomposition of TiA120s, the higher the strength of the composite. However, there should still be a third process yielding strength increase during the second annealing. In fact, the Xray phase analysis does not reveal the presence of TiAl*O, in the specimens of highest strength achieved, but looking at the temperature/strength and time/strength dependences shown we see that a maximum in strength has not been reached. It is impossible to identify the third process (or processes) without

Oxide/oxide ceramics by internal crystallization Table 2. Elastic characteristics

of the composites

Specimen characterization Annealing at 1500°C for 5 h in air, specimen 1 The same, specimen 2 Annealing at 1500°C for 5 h in air plus annealing 1200°C for 10 h in air, specimen 2

special experiments. We may simply speculate on possible recrystallization of alumina and/or rutile phases leading to their strengthening. 100 -

0

at

1367

containing ‘II&O5

Cl1 (GRa)

C,, (GRa)

C,, (GPa)

72.85 86.33 32463

19508 184.39 406.32

158.99 184.39 479.27

5 CONCLUSION The

internal crystallization method, which was initially developed to produce metal-matrix composites, can now be used to obtain oxide/oxide fibrous composites. The complicated structure of such materials calls for an optimization

procedure,

which remains

to achieve a satisfactory combination properties of the composite.

< 80%

to be done,

of mechanical

“a 60 ACKNOWLEDGEMENTS

40 -

The

1100

1150

1200

T/O

1C?50

1300

Fig. 8. Bending

strength of A120s-TiA1,05/A1203 composites versus the temperature of the second annealing stage. The annealing time is 10 h.

work was partially supported by the Russian Foundation for Basic Research under grant 93-013-16742. The authors are thankful to Dr V. F. Degtereva for the X-ray phase analysis of composites and Dr N. V. Kondrashova for scanning electron microscopy.

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100

I

1

I

1

I

-

ilk

s8o. u

01

;: 60D . 40 -

1. Rabinovich, M., Stohr, J. F., Khan. T. and Bibring, H., Directionally solidified composites for application at high temperature. In Fabrication of Composites (Handbook of Composites, Vol. 4) ed. A. Kelly and S. T. Mileiko. NorthHolland, Amsterdam, 1983, pp. 295-372. 2. Evans, A. G. and Zok, E W., The physics and mechanics of fibre-reinforced brittle matrix composites. J. Muter. Sci. 1994,29,3857-3896. 3. Mileiko, S. T. and Kazmin, V. I., Crystallization of fibres inside a matrix: a new way of fabrication of composites. J. Mater. Sci. 1992,27,2165-2172.

2 Fig. 9. Bending

4

6t/h

8

strength of Al@-TiA1205/Alz03 ites versus the time of the second annealing annealing temperature is 1200°C.

10

composstage. The

4. Mileiko, S. T. and Kazmin, V. I., Structure and mechanical properties of oxide fibre reinforced metal matrix composites produced by the internal crystallization method. Compos. Sci. Technol. 1992,45,209-220. 5. Kazmin, V. I., Mileiko, S. T. and Tvardovsky, V. V., Strength of ceramic matrix-metal fibre composites. Compos. Sci. Technol. 1990,38,69-84.

6. Hennicke, H. M., J. de Phys. 1986,47, Cl-533.