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
[email protected], [email protected], [email protected]
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
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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.
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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
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