Fiber-Reinforced Alumina-Based Composites Using Nonwoven ...

ADVANCED CERAMICS

Figure 2

Homogeneous alumina grains.

Figure 4

Native textile randomly distributed fibers.

Figure 5

Native textile different morphology

Fiber-Reinforced Alumina-Based Composites Using Nonwoven Cellulose Fabrics Fibrous alumina has been fabricated using vacuum infiltration of aqueous alumina slurry into two-dimensional nonwoven cellulose fabric. M. Awaad and S.M. Naga Ceramics Dept., National Research Center, Dokki, Cairo, Egypt T. Khalifa Textile Dept., Faculty of Applied Arts, Helwan University, Cairo, Egypt P. Greil and O. Russina Dept. of Material Science, University of Erlangen-Nuenberg, Erlangen, Germany N.A. Ibrahim Textile Research Div., National Research Center, Dokki, Cairo, Egypt

ost continuous-fiber ceramic composite systems (CFCCs) are based on SiC fibers with either oxide or non-oxide matrices. SiC-based systems have durability limitations. Therefore, interest in all-oxide CFCCs has increased.1 All-oxide CFCCs are immune to oxidation. They have been increasingly explored during the past decade.2–5

M

CFCCs with suitably tailored interfaces exhibit inelastic deformation characteristics.

©The American Ceramic Society

American Ceramic Society Bulletin

Therefore, they retain strength in the presence of holes and notches.6 Damage tolerances and inherent refractoriness of CFCCs have enabled them to emerge as candidates for many high-temperature structural applications, such as combustors.7 Levi et al.5 have developed an all-oxide ceramic composite based on a stable matrix that consists of mullite and alumina mixtures in combination with polycrystalline alumina–mullite fibers. They have manufactured the composite using conventional slurry infiltration methods. The mechanical performance of the produced materials is comparable with that of other fiber-dominated CFCCs, such as SiC/carbon and carbon/carbon. The important advantages of carbon-matrix materials are their superior oxidative stability and creep response. Mattoni et al.8 have demonstrated that the properties of porous-matrix CFCCs are sensitive to the level and distribution of matrix porosity. As the matrix porosity decreases, the tensile strength and damage tolerance decrease and the extent of

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all-oxide fibers with submicron ceramic particles results in a homogenous powder matrix with a high particle-packing density.

Figure 6

Figure 8

Figure 9

In the present study, a nonwoven cellulose fabric is used.The nonwoven manufacturing process is12,13 fiber preparation (cleaning and opening), web formation (fibers formed into a web), web bonding (web layers bonded to each other) and posttreatment (according to end use).

Vascular vessel pores of infiltrated alumina bodies.

Web bonding can be categorized into two main categories:14–16 mechanical bonding (needle punching or hydroentanglement) and chemical bonding (adhesives or thermoplastic fibers). Needle punching is based on a mutual mechanical binding for the fiber web. Premade fibrous web is introduced to barbed needles that oscillate vertically, on a slant, or in both directions to the surface of the web. These barbs catch the fibers on the surface of the batt and push them into center. This causes the structure to densify and produces strength through entanglement.14–16

Open pore channels with a dense matrix.

A novel and much simpler and lesstime-consuming CFCC fabrication method is reported here. The produced material porosity, pore size and pore distribution as well as its microstructure and phase composition have been studied.

Matrix cracks.

correlated fiber failure increases. To produce fiber composites, ceramic particles are packed around the fibers within a fiber perform using pressure filtration.4,5,9 The slurry must be formulated such that the particles are repulsive with respect to themselves and the fibers. The particles also must be much smaller than the fiber diameter to ensure good particle packing.10 Haslam et al.11 have used a mullite powder that does not begin to shrink until ~1300ºC. This practice prevents large cracklike voids from developing within the matrix. The infiltration of


Starting Materials The materials used in this investigation were nonwoven viscose rayon fabric and chemically pure aluminum nitrate nonahydrate (ANN) of >98% purity (S.d. fine, Chem Ltd., India).

Processing and Characterization Tetragonal specimens with dimensions of 25 25 mm were cut from pieces of native nonwoven textile fabric. Specimens were immersed in solutions of various concentrations


(1:1, 1:2 and 1:4 ANN:H2O) under vacuum for 2 h. The soaked specimens were dried slowly at 50ºC for 6 h, at 80ºC for 6 h and then at 110ºC overnight. The specimens were fired at 600ºC under a flowing air stream. This accelerated the oxidation of the formed carbon produced by the cellulose fiber and removed gases that resulted from nitrate disintegration. A slow heating rate (2ºC/min from room temperature to 600ºC) was adopted at low temperature. This prevented body cracking during the burnout process. The slow heating rate was followed by a higher rate (5ºC/min to maximum temperature). The specimens were sintered for 2 h at temperatures between 1500 and 1700ºC. This permitted assessment of densification behavior and crystalline phase development. Bulk density and apparent porosity of the fired specimens were evaluated using the Archimedes method (ASTM C-20) with xylene as the liquid medium. Changes in the microstructure of the specimens were elucidated using SEM (Model XL 30, Philips, Eindhoven, Netherlands) observations of fractured specimens. XRD analysis (CuKα radiation) was used to identify crystalline reaction products. Pore size and pore-size distribution were obtained using high-pressure mercury porosimetry (Model 200 Porosometer, Carlo Erba, Milan, Italy). Thermal expansion behavior and coefficients were recorded between room temperature and 1000°C at a heating rate of 5°C/min (Netzsch Genätebau). Bending strength was measured using a three-point bending mode on a universal testing machine (Model 4204, Instron). The permeability of air through specimens was measured at 1 atm of the gas pressure difference in a homemade permeation test cell. A 10 mm diameter iris was used for the measurements. Permeability was determined according to Darcy’s law.


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Porosity (%)

Relative pore volume (%)

Cumulative volume (nm3/g)

Pore size distribution

Temperature (°C) Pore diameter (µm) Figure 7

Figure 10 Apparent Porosity of Fired Alumina Bodies

Pore Size Distribution

porous structure with randomly situated pores. Because of the large interstitial spaces, the Firing Thermal Tensile Apparent alumina precursor can penetemperature expansion strength porosity Permeability trate the native fiber more eas(MPa) (%) (10–10) Symbol (°C) (10–6) ily. Excess precursor can be A 1600 5.8 4.60 75 1.28 removed readily. The final alumina ceramic produced after textile infiltration shows large vascuMechanical Properties lar vessel and fine pores (Fig. 6). The pore size ranges between 100 and 4 The variation in specimen bulk denµm with an average pore diameter sity as a function of sintering temof 55.7 µm (Fig. 7). perature has been plotted (Fig. 1). As expected, the density increases with increase in temperature. The speciMicrostructure men sintered at 1700ºC has a fine, uniform and homogeneous grain The microstructure of the material morphology with a mean grain size bridges between the open pore of 3 µm (Fig. 2). channels and dense matrix with no interparticle pores (Fig. 8). This suggests that pronounced sintering No elongated grains are formed. must have occurred. Examination at Therefore, there is no liquid-phase lower magnification reveals the presformed during sintering. Some intraence of a regular pattern of matrix granular porosity is present. This is a cracks caused by the constrained consequence of trapped pores shrinkage of the matrix during dryformed at the triple points, which are ing (Fig. 9). This phenomenon is retained during the final sintering induced by the biaxial constraint stage when grain growth occurs. This imposed by the reinforcement and is phenomenon is typically found in enhanced by the presence of large alumina. unreinforced matrix regions.

Table 1. Permeability, Thermal and Mechanical Properties of the Sintered Specimens

XRD results reveal corundum as the only phase present (Fig. 3). SEM micrographs of the native textile show a random fiber distribution (Figure 4). Flat-strip and sticklike fiber morphologies are easily distinguished (Fig. 5). Native textile exhibits a


Such cracking does not occur in unidirectional composites.4,9 Carelli et al.17 have reported that these features are due to some matrix densification in the matrix segments contained between the cracks along the direction perpendicular to the cracks. Their hypothesis is supported by the


measurements of the composite porosity, which indicates no significant change after thermal treatment. The porosity results of the present study (Fig. 10) are in good agreement with the Carelli et al. findings. The tensile strength of the specimens has been measured at room temperature (Table 1). The measurements show an elastic deformation followed by significant pseudoductility. The relatively low level of the mechanical strength is attributed to the high matrix porosity. The mechanical properties of porousmatrix CFCCs are sensitive to the level and distribution of matrix porosity. As the matrix porosity decreases, the damage tolerance decreases and the extent of correlated fiber failure increases. The low coefficient of thermal expansion values of the textile-based bodies (Table 1) may be due to orientation of the alumina crystals parallel to the textile fibers plane.

Attributes and Applications Nonwoven cellulose fabric can be successfully used as a template for generating ceramic materials that have similar initial textile structure. The produced material has many favorable attributes. •It is possible to manufacture CFCCs using conventional infiltration methods. A fiber coating is not required.


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Low-cost cellulose fibers are used.

4W.C.Tu, F.F. Lange

•There is no evidence of the tertiary matrix-cracking phenomenon that degrades the performance of SiC/SiC composites.18 • A wide range of textile architecture can be used. Novel ceramic composites that have uniform and nonuniform pore structure can be processed. The produced ceramics might be used as high-temperatureresistant exhaust-gas cleaning devices, advanced microreactor systems and immobilization supports for medical and biotechnological processes. ■

and A.G. Evans, “Concept for a Damage-Tolerant Ceramic Composite with Strong Interfaces,” J. Am. Ceram. Soc., 79 [2] 417–24 (1996).

5C.G. Levi, J.Y.Yang, B.J. Dalgleish, F.W. Zok

and A.G. Evans,“Processing and Performance of an All-Oxide Ceramic Composite,” J. Am. Ceram. Soc., 81 [8] 2077–86 (1998). 6A.G. Evans, F.W. Zok

and T.J. Mackin,“The Structure Performance of Ceramic-Matrix Composites”; pp. 1–84 in HighTemperature Mechanical Behavior of Ceramic Composites. Edited by S.V. Nair and K. Jakus. Butterworth–Heinemann, Boston, 1995.

7R.L. Bannister, N.S. Cerwa, D.A. Little

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