PRODUCTION OF OXIDE CERAMIC MATRIX COMPOSITES BY A PREPREG TECHNIQUE

GUGLIELMI, P.O.1,a; NUNES G.F.2; HABLITZEL M.2; HOTZA, D.2,b; JANSSEN, R.1,c

1Technische Universität Hamburg-Harburg (TUHH), Institute of Advanced Ceramics, Hamburg, Germany

2Universidade Federal de Santa Catarina, Núcleo de Pesquisa em Materiais Cerâmicos e Vidros, Florianópolis, Brazil

apaula.guglielmi@tu-harburg.de, bjanssen@tu-harburg.de, cdhotza@gmail.com

Keywords: Oxide ceramic matrix composites (OCMC), prepreg, paraffin suspension, RBAO, alumina

Abstract: Ceramic matrix composites (CMCs) were developed to overcome the intrinsic brittleness

and lack of reliability of monolithic ceramics. Their major advantages include high temperature

capability, light weight, corrosion resistance and adequate damage tolerance. All-oxide Ceramic

Matrix Composites (OCMCs) offer essential advantages with respect to long time stability in

oxidizing atmospheres, when compared to their non-oxide counterparts. Nevertheless, there is at

present almost no production concept which meets the requirements in view of cost and

performance for these materials. This work aims at producing OCMCs by means of a more flexible

production route. This is achieved by integrating well-known powder metallurgy routes with the

prepreg technique, used at present for producing commercial high performance polymer matrix

composites. The processing consists of the following steps: (a) infiltration of commercial alumina

fiber fabrics (3M NextelTM

610) with a liquid suspension of the matrix material; (b) lamination of

the pre-infiltrated fiber textiles with a paraffin-based suspension for the formation of prepregs; (c)

layup of prepregs; (d) warm-pressing for the consolidation of the green body; (e) debinding and (f)

reaction bonding and/or sintering for synthesis of the oxide matrix. Pure alumina or Reaction

Bonded Aluminum Oxide (RBAO) can be used as matrix materials and damage tolerance is

achieved by the porous, weak-matrix approach. Microstructural analysis of a pure alumina

composite fabricated by this route show good infiltration of fiber bundles and proves the good

adhesion of prepregs during processing. Average strength value of 199 MPa in fiber direction is in

good agreement with values presented in the literature for OCMCs produced by other techniques.

Introduction

Ceramic matrix composites (CMCs) are interesting materials for thermomechanical applications

due to their damage tolerant fracture behavior. This is the result of toughening mechanisms, such as

crack deflection into fiber-matrix interface, as well fiber pullout and bridging [1, 2]. All-oxide

CMCs have recently been in the focus of research [2-4] because of their inherent high oxidation

resistance compared to their non-oxide counterparts. This is particularly interesting at high

temperature applications in oxidizing environments, such as gas turbines. However, despite the

considerable interest in these materials over the past decades, there are still barely production

concepts which meet requirements in view of cost and performance. Development of low-cost

techniques suitable for series production, as well as new design concepts that enable the joining of

different CMC parts to more complex shaped components are some of the main remaining

challenges in this field [5].

In this work, a new production route for all-oxide CMCs is proposed. This comprises the integration

of conventional powder metallurgy routes and well-known production concepts already used for

manufacturing polymer matrix composites, in an approach based on the prepreg technology. As

schematically shown in Fig. 1, the proposed route consists of the following steps: (a) infiltration of

commercial oxide fiber fabrics with a liquid suspension of the matrix material; (b) lamination of the

Materials Science Forum Vols. 727-728 (2012) pp 556-561Online available since 2012/Aug/24 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.727-728.556

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-14/11/14,01:41:22)

pre-infiltrated fiber textiles with a paraffin-based suspension for the formation of prepregs; (c)

layup of prepregs; (d) warm-pressing for the consolidation of the green body; (e) burn out of the

organic binders (debinding) and (f) reaction bonding and/or sintering for synthesis of the oxide

matrix.

Fig. 1: Schematic representation of the prepreg processing route for OCMCs proposed in this work.

A great advantage of using paraffin-based suspensions for the lamination of prepregs is the

possibility of joining CMC parts in the green state. This is favored by the thermoplastic nature of

the paraffin-based prepregs, which can be welded together by locally heating the joining surfaces.

In addition, the production of CMCs is flexibilized by the proposed route, since the majority of the

required equipment is already available in the market. In this paper, the processing route shown in

Fig. 1 is explained in detail and validated by microstructural analysis and strength values of pure

alumina CMCs.

Materials

Alumina fiber fabrics (3M NextelTM

610) are used in this work as reinforcement for the oxide

CMCs. A fine grade, pure alumina (Ceralox HPA 0.5, average particle size of 0.5 µm, RWE-DEA

AG für Mineralöl und Chemie, Germany) is used as a reference matrix material, in order to validate

the processing route. Further investigation will focus on using Reaction Bonding Aluminum Oxide

(RBAO) as matrix, due to its attractive properties such as very low shrinkage after sintering and

transient stress relaxation during synthesis, which both enhance the ability of producing defect-free

CMC matrices [6]. Toughness is achieved in this work by the porous matrix approach (~ 30 vol%

matrix porosity), which enables crack deflection, fiber pullout and bridging by the resulting weak

interface between fibers and matrix.

Prepare processing route

Infiltration of alumina fiber fabrics by an aqueous alumina slurry

NextelTM

610 alumina fiber fabrics were infiltrated by an aqueous alumina slurry containing 45

vol% of solids (76,5 wt%) in deionized water. DOLAPIX CE 64 (Zschimmer und Schwarz,

Germany) was added as a deffloculant in an amount of 0.6 wt% over the solid content. Ammonium

hydroxide (NH4OH) was used to adjust the pH of the slurry to ~ 10. This allowed repulsive

interaction forces to act between the alumina particles, providing an adequate viscosity that

facilitates the flow of particles into the fiber bundles.

Materials Science Forum Vols. 727-728 557

The fiber textile was cut into 90 mm x 50 mm pieces and then dipped into the alumina slurry during

20 min for impregnation. The infiltrated clothes were dried in a muffle furnace at 150 °C for 2 h, so

as to avoid any hydrophobic interaction with the paraffin suspension used subsequently for the

lamination of the prepregs.

Lamination of Prepregs with a paraffin-based suspension

A paraffin-based suspension containing 58 vol% (85,8 wt%) of alumina powder was produced for

the lamination of prepregs. For that, a binder system first developed at TUHH for the production of

alumina and RBAO parts by Low Pressure Injection Moulding (LPIM) [7, 8] was used. Paraffin

wax was chosen as the suspension medium because of its low melting temperature range. In order

to produce suspensions with high solid contents and appropriate rheological properties, different

polar surfactants were used, which promote the sterical stabilization of the suspension by

counteracting the attractive forces among particles [7].

For the preparation of the suspension, all organic ingredients were mixed and preheated at 100 °C in

a glass beaker. The alumina powder was then stepwise added, while mixing the suspension with a

glass stirrer. Further homogenization was achieved by passing the suspension several times through

a three roller mill, whose rolls were preheated at 95 °C. After solidification, the suspension was

granulated using a manual calendar, so as to facilitate its handling during the subsequent processing

steps.

The lamination of prepregs was carried out in a way similar to the industrial double-belt technique.

In a lab scale, this was done by introducing a pre-impregnated alumina textile between layers of the

granulated paraffin-based alumina suspension and placing it onto a heating plate at 100 °C for the

softening of the suspension. The lamination was then manually performed by rolling a dense, 80

mm diameter alumina roll onto the textile-suspension sandwich, as schematically shown in Fig. 1.

Aluminum foils were used here as backing papers, to prevent the molten suspension to adhere to the

heating plate and alumina roll. This procedure resulted in 2-directional prepregs, which were stable

and easy to handle after solidification of the paraffin.

Layup of prepregs and warm-pressing

Consolidation of the green oxide CMC plates were performed by warm-pressing. For this purpose,

8 prepreg layers were laid up by hand and placed between two flat stainless steel plates, which

subsequently served as the pressing moulds. The system was preheated in a muffle furnace at 120

°C for 20 min, so as to soften the paraffin suspension. After heating, the system was quickly

transferred to a uniaxial press, where it was consolidated by a low pressure (~ 1 MPa), in order to

avoid fiber damage. The green oxide CMC bodies were let to cool on mould before handling them

to the subsequent debinding procedure.

Debinding and sintering

Debinding was performed thermally, according to the literature for the binder system used [7, 8]. In

this procedure, the organic components of the paraffin suspension were eliminated by a combining

effect of capillarity and pyrolysis, in a three-step heating cycle up to 330 °C. For that, the green

oxide CMC bodies were placed in an alumina powder bed (average particle size of 200 µm) and

heated in a muffle furnace. After debinding, the oxide CMCs were sintered at 1200°C for 30 min.

Composite Characterization

Microstructural analysis of the sintered alumina CMCs was carried out by scanning electron

microscopy (SEM) (Gemini/Zeiss, Leo 1530 FESEM). In order to assess the fiber-dominated

composite properties, in-plane 4-point bending tests were carried out according to the ASTM C

1341-00 standard [9]. Bending bars were cut out of the oxide CMC plates by means of a metal-

bonded diamond cutting disc. Specimens were 50 mm long and 33 mm wide and were tested using

a load span of 10 mm. The fracture surfaces of the samples were subsequently analyzed by SEM, in

order to observe pullout of fibers.

558 Advanced Powder Technology VIII

Results and Discussion

The microstructure of a pure alumina composite produced by the prepreg technique reported in this

work is shown in Fig. 2. “IT” stands for inter-textile matrix (resulting from the paraffin suspension)

and “IB” indicates intra-bundle matrix (resulting from the impregnation of textiles with the aqueous

slurry. Fig. 2 (c) clearly shows the boundary between these two regions.

Fig. 2: Microstructure of a pure alumina ceramic matrix composite produced in this work. “IT”

indicates the inter-textile matrix, resulting from the paraffin suspension. “IB” stands for intra-

bundle matrix, resulting from the liquid slurry infiltration. The cracks present in “IT” are typical for

conventional matrix systems.

A good impregnation of fiber bundles was achieved by the liquid slurry. No delamination occurred

in the samples, even after debinding and sintering, giving evidence of a good adhesion between the

pre-infiltrated textiles and the paraffin suspension used for the prepreg lamination. In addition, the

absence of delamination also proves the effectiveness of the warm-pressing for the consolidation of

the green bodies. The transverse cracks present in the inter-textile matrix (IT) (Fig. 2 (a)) are

common in conventional ceramic matrix composites and result from the constrained shrinkage of

the matrix during sintering. This problem will be overcome by replacing conventional alumina by

RBAO for the CMC matrix, due to its low-shrinkage and superplastic transient behaviour.

Fig. 3: Representative curve of the 4-point bending tests performed on the alumina CMCs produced

in this work. The inset shows schematically the fiber orientation during mechanical testing.

Materials Science Forum Vols. 727-728 559

A representative force-deflection curve of the 4-point bending tests performed on the alumina

CMCs fabricated in this work is presented in Fig. 3. The inset shows schematically the sample

geometry and fiber orientation during the mechanical tests. A straight dashed line was added in

order to evidence the non-linear behavior of the loading curve.

The loading curve shows an initial linear-elastic behavior, followed by a slight nonlinear

deformation until the ultimate strength (c) of the composite is achieved. At this point, final failure

occurs and a reduced load can still be carried by some intact fibers, which are subsequently pulled

out of the matrix. This load displacement behavior is in good agreement with what is reported in the

literature for porous-matrix ceramic composites tested by bending [10, 11, 12]. According to Cao

[11], the non-linearity characterizes a continuously decrease in the overall stiffness of the

composite, due to the initiation and propagation of cracks in the matrix. Since the matrix-fiber

interfaces are weak, debonding occurs and fibers bridge the matrix cracks. Stresses are transferred

from fibers to matrix by friction and are redistributed by the matrix to other fibers. As load

increases, fiber failure is initiated, until the ultimate strength of the composite is achieved.

Nonetheless, in case of porous matrix CMCs, the amount of load that can be carried by the weak

matrix is limited and therefore the nonlinear behavior is minor [10]. The measured strength value

for the alumina CMCs produced in this work (199 ± 9 MPa) is in the same range as those reported

in the literature for OCMCs produced by classic processing techniques [3, 4, 12, 13].

Fig. 4: Fracture surface of a representative pure alumina composite fabricated in this work.

Fig. 4 shows fracture surfaces of the alumina composites produced here, indicating the desired fiber

pullout during fracture.

Conclusion

The results presented here confirm the potential of adapting the commercially established polymer

prepreg technology for the production of oxide CMCs, by incorporating some powder metallurgy

concepts to it. The production of CMCs is in this way flexibilized, since the required equipment is

already available in the market. This can lead to a substantial reduction on the necessary capital

investment for production. Microstructure of pure alumina composites fabricated by the presented

technique showed good infiltration of fiber bundles and good adhesion between prepregs during

processing. The measured value of strength in fiber direction meets the values presented in the

literature for OCMCs fabricated by conventional techniques. Since pure alumina was used in this

study as a reference matrix, typical transverse matrix cracks were generated during sintering. This

problem will be overcome when RBAO is used as the matrix material since shrinkage-related

problems during sintering are reduced.

Acknowledgements

The authors gratefully acknowledge the financial support of the German Research Foundation

(DFG) under the project number JA 655/23-1 and the Brazilian “Coordenação de Aperfeiçoamento

de Pessoal de Nível Superior” (CAPES) under the project 015/09.

560 Advanced Powder Technology VIII

References

[1] K.K. Chawla: Ceramic Matrix Composites. London: Chapman & Hall 1993.

[2] D.J. Green: An introduction to the mechanical properties of ceramics. 1 ed., Cambridge

Cambridge University Press 1998.

[3] M.A. Mattoni, J.Y. Yang, C.G. Levi and F.W. Zok: J. Am. Ceram. Soc. Vol. 84 (11) (2001),

p. 2594.

[4] M. Schmücker and P. Mechnich: All-Oxide Ceramic Matrix Composites with Porous

Matrices, in: Ceramic Matrix Composites, W. Krenkel, Editor. WILEY-VCH: Weinheim

2008. p. 205.

[5] M. Schmücker and S. Reh: Ingenieur-Werkstoffe, Konstruktion Zeitschift Juni Vol. 6 (2010),

p. 8.

[6] S.M. Goushegir et al.: J. Am. Ceram. Soc. (2011). accepted for publication

[7] J.J. Coronel, R. Janssen and N. Claussen: Bol. Soc. Esp. Ceram. Vol. 43(5) (2004), p. 843.

[8] M. Leverköhne et al.: Adv. Eng. Mater. Vol. 3(12) (2001), p. 995.

[9] ASTM C 1341-00 (2000): ASTM International.

[10] D. Koch: Microstructural Modeling and Thermomechanical Properties, in: Ceramic Matrix

Composites, W. Krenkel, Editor. 2008: Weinheim. p. 231.

[11] H. Cao and M.D. Thouless: J. Am. Ceram. Soc. Vol. 73(7) (1990), p. 2091.

[12] J.J. Haslam, K.E. Berroth and F.F. Lange: J. of the Eur. Ceram. Soc. Vol. 20 (2000), p. 607.

[13] R.A. Simon: Int. J. of Appl. Ceram. Technol. Vol. 2 (2) (2005), p. 141.

Materials Science Forum Vols. 727-728 561

Advanced Powder Technology VIII 10.4028/www.scientific.net/MSF.727-728 Production of Oxide Ceramic Matrix Composites by a Prepreg Technique 10.4028/www.scientific.net/MSF.727-728.556

DOI References

[3] M.A. Mattoni, J.Y. Yang, C.G. Levi and F.W. Zok: J. Am. Ceram. Soc. Vol. 84 (11) (2001), p.2594.

http://dx.doi.org/10.1111/j.1151-2916.2001.tb01059.x [7] J.J. Coronel, R. Janssen and N. Claussen: Bol. Soc. Esp. Ceram. Vol. 43(5) (2004), p.843.

http://dx.doi.org/10.3989/cyv.2004.v43.i5.411 [8] M. Leverköhne et al.: Adv. Eng. Mater. Vol. 3(12) (2001), p.995.

http://dx.doi.org/10.1002/1527-2648(200112)3:12<995::AID-ADEM995>3.0.CO;2-D [12] J.J. Haslam, K.E. Berroth and F.F. Lange: J. of the Eur. Ceram. Soc. Vol. 20 (2000), p.607.

http://dx.doi.org/10.1016/S0955-2219(99)00259-9 [13] R.A. Simon: Int. J. of Appl. Ceram. Technol. Vol. 2 (2) (2005), p.141.

http://dx.doi.org/10.1111/j.1744-7402.2005.02016.x