in-situ and porous composites · in-situ synthesis and characterization of sic 20 - 3 composite a1...

In-Situ and Porous Composites

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Innovative Processing and Synthesis of Ceramics, Glasses and Composites IX Edited by J. P. Singh, Narottam P. Bansal, Balakrishnan G. Nair,

Tatsuki Ohji and Antonio R. de Arellano López Copyright © 2006. The American Ceramic Society

IN-SITU SYNTHESIS AND CHARACTERIZATION OF SiC - A1203 COMPOSITES

L. N. Satapathya, P.D. Ramesh b, Dinesh Agrawalb and Rustum Roy b

a Ceramic Technological Institute, BHEL,, Bangalore 560012 INDIA b Microwave Processing and Engineering Center, Materials Research Institute The Pennsylvania State University, University Park, PA 16802, USA

ABSTRACT Alumina-silicon carbide composites have been synthesized in-situ in a microwave reactor by

two different methods, namely, polymer infiltration and pyrolysis ( PIP) and direct pyrolysis of a alumina-polymer mixture (DPP). The percentage of silicon carbide in alumina has been limited to a maximum of 10 Vol. %. In the PIP method, polycarbosilane ( PCS) is converted to SiC in an alumina porous compact during the sintering process. On the other hand, the DPP method allows easy conversion of PCS to SiC in an alumina powder matrix. The reacted materials have been characterized by various analytical techniques. The PIP method can produce very fine, homogenous porosity distribution inside alumina compacts that can be potentially useful for gas separation applications.

INTRODUCTION Ceramic matrix composites (CMCs) are developed to overcome the intrinsic brittleness and

lack of reliability of monolithic ceramics, with a view to introduce ceramics in structural parts used in severe environments, such as rocket and jet engines, gas turbines for power plants, heat shields for space vehicles, fusion reactor first wall, aircraft brakes, heat treatment furnaces, etc. The effectivess of a ceramic matrix composite depends on the choice of reinforcement material and the method of preparation.

Silicon carbide, as a reinforcement in alumina, finds diverse applications at high temperature. It controls alumina grain growth and improves the strength of alumina-alumina grain boundaries, resulting in a tough ceramic matrix composite. However, the amount of silicon carbide in alumina is very limited to achieve these properties, and lies below 10 vol. %, as suggested by Ferroni and Pezzoti1. The high hardness, strength and corrosion resistance properties of alumina are improved by the small amount of SiC additions.

The widespread applications of such structural ceramic composites are largely hindered by difficulties in processing . This results in non-homogeneous dispersion of the SiC particles in the alumina matrix, which leads to detrimental effects on densification and mechanical properties due to agglomeration. The solid state mixing of SiC in alumina leads to such problems. Alternative preparation techniques are based on sol-gel3, self-propagating high temperature synthesis4, microwave drying of the coated gels5 and microwave combustion synthesis6, which were used to avoid agglomeration problems. The in-situ formation of silicon carbide in alumina matrix is beneficial for maintaining homogeneity of second phase in the matrix. Further, this method is suitable for maintaining unique microstructure, and reduction of contamination during processing. This method, also in many cases, leads to short processing time and reduction in processing temperature resulting in overall cost reduction. There is hardly any report on this preparation technique in the literature. We recently reported the in-situ synthesis of high volume silicon carbide in alumina using direct carburization process in a microwave reactor7. Further, the in-situ synthesis can also be achieved by many other methods, namely, polymer infiltration and pyrolysis ( PIP) and

Innovative Processing and Synthesis of Ceramics, Glasses and Composites IX 137

Innovative Processing and Synthesis of Ceramics, Glasses and Composites IX Edited by J. P. Singh, Narottam P. Bansal, Balakrishnan G. Nair,

Tatsuki Ohji and Antonio R. de Arellano López Copyright © 2006. The American Ceramic Society

direct pyrolysis of precursor mixed alumina ( DPP). These two methods are used in the present study to generate SiC from its precursor inside an alumina matrix.

The synthesis of materials using microwave energy has been found promising because of lower energy requirement, cycle time reduction and environment friendliness of the process8. A wide range of materials has been synthesized in the last few years and has been comprehensively reviewed by many authors9"11. However, except our report7, there is no other report of in-situ formation of silicon carbide in alumina using microwave energy available in the literature. The synthesis of alumina-silicon carbide composites with silicon carbide content of 2, 5 and 10 vol. % could not be achieved by direct carburization process as reported elsewhere7. This is clearly evident from the time-temperature curves of these reactions (figure 1), which indicates that it was not possible to maintain the a constant reaction temperature of 1300° C for these compositions.

— A l u m i n a * 10 %( Si > 2C ) — AiumlPS ♦ & % f Si-»-2C ) — A l a m l r - a ♦ 2 % ( O t » 2 C )

0 1000 2000 3000 4000 5000 6000 7000 [Time ( soc.H

Figure 1 :Temperature vs. Time for alumina+ x vol.% SiC ( X<10 vol.%)

Keeping this in view, the direct pyrolysis of polymer was carried out to achieve the formation of these composites. Further, the polymer infiltration was also carried out in an alumina compact of varying porosity and in-situ conversion to silicon carbide using microwave energy was performed.

In this study, we report the details of PIP and DPP processes applied for in-situ synthesis of alumina-silicon carbide composites using microwave energy. The sintering studies of the composites have been carried out in a 6 kW microwave furnace and the data have been compared with that obtained by simple SiC mixed alumina powders.

EXPERIMENTAL The raw materials used in this study were polycarbosilane (Starfire Chemical Co., NJ ,

USA,) and CR-6 grade of high purity aluminum oxide ( Baikaowski International Corporation, NC, USA, d50 = 0.6 urn, BET surface area = 6 m2 /g ). In the PIP process, the alumina powder was sieved through a fine mesh and mixed with 2 % polyvinyl alcohol (PVA) binder solution. The powder was then used for pressing cylinders of -12 mm diameter and -3 mm thickness by uniaxial pressing using a stainless steel die. The pressure on the sample was varied in order to achieve different green densities. The variation in green density reflects the variation in initial porosities in

138 Innovative Processing and Synthesis of Ceramics, Glasses and Composites IX

the compacts. The pressed compacts were then calcined at 1050°C for one hour in air to achieve sufficient strength for handling. The green density of the compacts was measured by dimensional method (geometrical). Five different levels of initial porosities were obtained in these compacts. Thereafter, the samples were dipped a polycarbosilane (PCS) solution kept in a glass beaker. The container containing the solution and samples were placed inside a vacuum dessiccator and kept under vacuum for exactly two minutes. The infiltrated samples were removed from the solution and dried under an infrared (IR) lamp. The infiltration process was repeated on these samples again for few more times. The dried samples were sintered in a 6 kW microwave furnace in nitrogen atmosphere. The sintering temperature was 1580°C with a soaking time of lh . The temperature was measured regularly using an optical pyrometer. The sintered compacts were characterized by x-ray diffraction ( SCINTAG Inc., Cupertino, CA), scanning electron microscope (Model S-3500N, Hitachi Ltd., Tokyo) , Mercury porosimetry (Pascal 140 & Pascal 440, Italy) and density/porosity by boiling water method.

In the direct pyrolysis (DPP) method, stoichiometric amount of polycarbosilane (PCS) solution was mixed with alumina powder using an agate mortar and pastle. The estimation of SiC formation from PCS has been taken as 75% yield. The amount of PCS added corresponded to the effective SiC contents of 2, 5 and 10 Vol.% in alumina after pyrolysis. The mixed powder was filled in an alumina boat, which was then placed at the center of a 2 kW tubular microwave furnace (Model RC/20SE, Amana Refrigeration Inc., Amana, CA,USA). The details of microwave furnace and its operation have been reported elsewhere12. Figure 2 describes the flow chart of both the PIP and DPP processes. All the synthesis experiments were carried out in Ar atmosphere. A constant flow rate of 200 ml /min was maintained throughout the experiments. Temperatures were measured using a single wavelength infrared (IR) pyrometer (Model MA2SC, Raytek co., Santa Cruz, CA,USA). The synthesis was carried out at 1573 K for 30 minutes. After synthesis, the product powders were kept in an alumina crucible and calcined at 923 K in air for eight hours, to remove excess carbon in the powder mixture.

Alumina Powder

PCS Hytio,,

PCSmbcod Alumina powder

i Pyrolysis in a micrtowove roacf or in Ar

T

Consolidation of thé powder

\ . Sintering in a N Mleronravo

Furnace in NL, I at 1380» C.


Figure 2: Flow chart of the PIP and DPP processes

The synthesized powders were characterized by various analytical techniques, namely, particle size analysis, x-ray diffraction and scanning electron microscopic analysis coupled with energy dispersive x-ray analysis ( EDS). The powders were consolidated using a stainless steel die and sintered using a 6 kW microwave furnace at 1580° C for lh. The density/porosity was measured by boiling water method. For the densification study, the powders obtained by different methods were consolidated initially by uniaxial pressing, followed by cold isostatic pressing (CIP) at 210 MPa. The compacts were sintered in nitrogen atmosphere in a 6 kW microwave furnace at 1580°C for lh. The direct mixing of alumina and silicon carbide powders has been carried out by solid state mixing method in isopropyl alcohol medium. The sintering aid, MgO, was added in the form of Mg(N03)2.6H20 to the alumina-silicon carbide mixture. The details of such process are described elsewhere .

RESULTS AND DISCUSSION Direct Polymer Pyrolysis

In the DPP method, wherein, the polycarbosilane was pyrolyzed to provide required amount of SiC, the powders were agglomerated having an average particle size of ~0.8 um for all the three compositions (Fig. 3). Both 2% and 10% compositions showed a bimodal particle distribution, which was not observed in the case of 5% SiC composition. Pure alumina shows a trimodal distribution (not shown) and considering the small quantity of SiC present, it is expected that the composites should show a similar behavior. The abnormal behavior in the particle size distribution of the composites must have been originated from the insufficient deagglomeration prior to testing. This is further confirmed from the scanning electron micrograph of a diluted slurry of alumina + 10 % SiC ( Fig. 4).

Particle Diameter ( nm )

Figure 3: Particle size of AI2O3 + xSiC powder (Left) Figure 4: SEM of AI2O3 + x SiC compacts sintered using a 2.45 GHz microwave facility

From Fig. 4 it is clear that the average particle size of the slurry is about 0.2 urn, which is similar to that of pure untreated alumina. This analysis indicated that polycarbosilane has been pyrolyzed to SiC, formed a coating over alumina particles, resulting in a similar particle size distribution and average particle size like alumina. This is also further clear from the change in


color of the treated powder from white to off-white for 2% composition and to grey for 10% composition.

The phase-formation of SiC during the pyrolysis has been confirmed by XRD. Though, it was not possible to distinguish the SiC peaks in 2% and 5% compositions, they become clearly evident in the 10% composition. Fig. 5 demonstrates the XRD patterns of powders synthesized by two different methods: direct carburization (DCP) process and direct polymer pyrolysis (DPP) process; both the processes were expected to produce the same (AI2O3+ 10% SiC) final composition after reaction. In the DCP method, appropriate amounts of Si + C mixture has been added to AI2O3 and treated in a MW field to yield SiC +AI2O3 composite.

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3500

^ 3000

^ 2500

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1000

500

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,

j

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Figure 5: XRD of AI2O3 + 10% SiC mixture produced by directly carburization and direct polymer pyrolysis

70 80 90

Figure 6 : (a) XRD of PCS- heated at 1300 C/30 min. (b) SEM of PCS powder after heat treatment

The absence of any form of silica and /or mullite phase suggests that SiC particles were not oxidized during the reaction and reaction with alumina did not occur. In a subsequent experiment, pure polycarbosilane liquid was taken in an alumina boat and heated in Ar in the 2 kW microwave reactor. After reacting for 30 min at 1300°C, phase pure SiC powder has formed, as revealed by


XRD ( Fig. 6a). However, the scanning electron micrograph ( Fig. 6b) of the resultant black SiC powder revealed a flaky microstructure.

This experiment confirms that polycarbosilane is converted to SiC during the DPP process. It is known that during pyrolysis upto 1200°C, PCS is converted into an inorganic amorphous solid by demethanation and dehydrogenation. At temperatures beyond 1200°C, formation of ß-SiC occurs with the evolution of CO and SiO gases. The ß-SiC diffraction peaks were broadened as reported by many authors13'14, which was also reported in this study. Further, as reported by Danko et al.14, the size of ß-SiC nanocrystal increases as the processing temperature increases. This has been also demonstrated earlier1 by Zhu et al, wherein the authors concluded that the heating rate and the peak pyrolysis temperature can have a strong influence on the ceramic yield and composition.

Polymer Infiltration and Pyrolysis (PIP) In the PIP method, the initial amount of porosity in the alumina compacts was varied from

49% to 56% (Table 1). On sintering, this has resulted in only a partial densification with porosities in the range of 26 to 34 %. The XRD of the composition containing the highest initial porosity confirmed the presence of small amount of SiC in the sintered compact. It was however, difficult to determine the SiC content in other compositions by XRD, though the EDS analysis confirmed the presence of SiC in these compacts. With earlier results on the pyrolysis of polycarbosilane, it is confirmed that some amount of SiC has formed. It is interesting to note from Table 1 that the reduction of porosity due to sintering followed a definite pattern; higher initial porosity always leads to a higher porosity content after sintering. The sample with 34% porosity was cut in the middle and observed through SEM. It was observed that fine particles coexist with very fine, uniformly distributed pores ( Fig. 7) .

Sample Id

1

2

3

4

Gieen Density (9/cc)

174

1.83

1.M

2.05

Apparent fofosJty

56

54

52

4?

Bulk Density ( 9/cc)

2.34

2.45

2.5Î

2.43

Open fwosify

34

33

31

U

Table 1 : Relation between green density to sintered density of PCS infiltrated compacts Figure 7 : SEM/EDS of PCS infiltrated and sintered material

The EDS analysis (insert to Fig. 7) indicated the presence of SiC. Also, as noted in Table 2, compositions of Al and Si remained almost the same throughout the entire cross section of the compact. This analysis indicated that the infiltration of PCS has occurred uniformly throughout in the porous alumina compacts. The EDS results also revealed that there is a slight variation in elemental compositions depending on the amount of initial porosity present. Higher the porosity,


better is the infiltration and higher is the amount of elemental Si present. The mercury porosimetry experiment carried out on the cut sample indicated a very fine and homogeneously distributed pores exist in the sintered body. The average pore size obtained by this method was ~ 40 nm. The most important feature, however, is the very narrow pore size distribution (Fig. 8).

Bernent

Silicon

Aluminum

Oxygen

Sample No. 1

Edgel Middle Edge 2 [ Atomic %)

5.2 5.0 5.1

33.8 34.0 33.8

61.0 61.0 61.0

Sample No. 2

Edgel Middle Edge 2 ( Atomic %)

4.7 4.5 4.7

320 321 320

61.3 613 61.3

Table 2 : EDS data for Table 1 samples Figure 8 : Mercury porosimetry graph of PCS infiltrated sintered, cut sample

This effect is significant since the nanoporosity has developed only during the microwave processing stage. This is first such report presented on this new concept of creating nano-porous materials using microwaves. Earlier, Kakimoto et al.13 reported the pore size distribution of pyrolysis of PCS compacts in the range of 10-30 urn range with a non-narrow distribution. Similar results on less dense alumina-silicon carbide powders have been reported earlier6. The samples containing -30% porosity, and a very narrow pore size distribution, and with nanopores find immense applications in gas separation technology. The interest in SiC based porous ceramics has recently been reported by Zhu et al.16, in which, the authors have reported the use of PCS as a bonding agent of SiC in fabricating highly porous materials. The high temperature properties of the alumina-silicon carbide composite add the advantage of the material for similar kinds of applications.

Densification study The densification study was carried out on samples, which were processed by four different

methods. A) Direct mixing of alumina and SiC ( developed in our laboratory earlier 17); B) Direct mixing of alumina and SiC with 250 ppm MgO additive ; C) Powder obtained by direct polymer pyrolysis and D) Direct mixing of stoichiometric amount of Polycarbosilane in alumina and sinter the compacts

Figure 9 shows the density trend of A, B, C and D, which were fired at 1580°C for lh in flowing nitrogen. As pointed out earlier7, MgO is an effective sintering aid for alumina containing low amount of SiC. This is evident from the higher density values recorded for all the three compositions with MgO (curve B) in comparison to curve A.. Also, it has clearly been observed from SEM (not shown) that the grain size of alumina could be reduced with increase in SiC content.


This result confirms the earlier observation that SiC pins the grain growth of alumina. This effect assists in improving the mechanical properties of alumina.

In an other experiment, when the polymer is first pyrolyzed in a microwave reactor and then allowed to sinter in a microwave field, the pellets result in a better density (curve C) in contrast to simultaneous pyrolysis and sintering of the same ( curve D). This is because, in the latter experiment, first the pyrolysis of the polymer occurred followed by sintering within the limited time and thus the densification is delayed ( curve D). The preheating effect of the solution inside the microwave reactor promotes the uniformly distribution of SiC particles in alumina matrix.

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3.6

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Sintering condition: 1680 C /1h

A: Dir«* mixing of alumina and SC - without sintering aid B. Direct mixing of alumina and SIC -With 200 ppm MgO C. PCS mixod with alumina • Synthosizod in Ar. At 1300 C/30 min. D. Direct m being of PCS solution with Alumina

0 2 4 6 8 10 12

SiC content ( Vol. %)

Figure 9: Density plots of AI2O3 + x SiC mixture prepared by various processes

However, the densities obtained in both the cases are lower than that achieved by direct mixing of fine SiC with alumina powder because the pyrolysis of polymer resulted in coarse, agglomerated powder, which was relatively difficult to sinter. The densification study indicated that the sintering of alumina-silicon carbide is more effective with fine alumina and SiC powders and further enhanced with suitable sintering aids. Similar results have also been reported earlier4, where the authors could not sinter an alumina-37.1 % SiC composite material even at 1700° C. However, pressure assisted densification like hot pressing is suitable for such materials.

CONCLUSIONS The synthesis of 2, 5 and 10 % silicon carbide in alumina has been carried out by in-situ

pyrolysis of polycarbosilane in alumina in a microwave reactor in argon atmosphere. The resultant powders were consolidated and sintered in a microwave furnace along-with the compacts of PCS mixed alumina . It was observed that though both the processes yielded porous bodies though the densities of the latter was lower. In another experiment, the in-situ pyrolysis of PCS was also carried out inside an alumina compact after infiltrating the same during sintering at 1580°C /lh in nitrogen. Albeit full densification could not be achieved, the samples resulted uniform distribution of the second phase inside alumina matrix. Further, these samples yielded very narrow pore size distribution of the order of 40 nm , which is potentially beneficial for gas separation applications.


ACKNOWLEDGEMENT The present work was carried out at the Pennsylvania State University, USA and one of the author (L. N. Satapathy) acknowledges the financial support received as a BOYSCAST fellow of Department of Science and Technology, Govt. of India and the sponsorship from Bharat Heavy Electricals Limited, New Delhi, India.

REFERENCES 'L. P. Ferroni and G. Pezzotti, Evidence for Bulk Residual Stress - Strengthening in AbOß/SiC Nanocomposites , J. Am. Ceram. Soc, 85 2033-2038 (2002). 2M. Sternitzke, Review: Structural Ceramic Nanocomposites, J. Eur. Ceram. Soc., 17 1061-1082 (1997). 3U. S. Hareesh, M. Sternitzke, R. Janssen and K. G. K. Warrier, Processing and Properties of Sol-Gel-Derived Alumina/Silicon Carbide Nanocomposites ,J. Am. Ceram. Soc., 87 1024-1030 (2004). 4J. H. Lee, C. Y. An, C. W. Won, S. S. Cho and B. S. Chun, Characteristics of A1203 - SiC composite powder prepared by the self-propagating high-temperature synthesis process and its sintering behavior, Mat. Res. Bull., 35 945-954 (2000). 5S. Ananthakumar, U. S. Hareesh. A. D. Damodaran and K. G. K. Warrier, Microwave processing of boehmite coated SiC composite precursors for alumina-silicon carbide nanocomposites, J. Mat. Sei. Lett., 17 145-148 (1998). 6R. H. G. A. Kiminami, M. R. Morelli, D. C.Folz and D. E.Clark, Microwave combustion synthesis of AI2O3 / SiC powders, in Microwaves: Theory and applications in Materials processing V, ed by D. E. Clark, J. G. P. Binner and D. A. Lewis, Ceram. Trans., Vol. 111 (2001), 189-196. 7L. N. Satapathy, P. D. Ramesh, Dinesh Agrawal and Rustum Roy, Microwave Synthesis and sintering studies on alumina -silicon carbide composites, Proceedings of the 4th world congress on Microwave and RF applications, Austin, Texas, USA ( In press) 8K. J. Rao, B. Vaidyanathan, M. Ganguli and P. A. Ramakrishnan, Synthesis of inorganic solids using microwaves, Chem. Mater., 11, 882-885, (1999). 9R. Roy, D. Agrawal, J. P.Cheng and S. Gedavanishvili,, Full sintering of powdered metal bodies in a microwave field, Nature (London), 399 668-670 (1999). 10 R. D. Peelamedu, M. fleming, D. K. Agrawal and R. Roy, Preparation of Titanium Nitride: Microwave induced carbothermal reaction of titaniun dioxide, J. Am. Ceram. Soc, 85, 117-122, (2002). 11 G. Swaminathan, A. B. Datta and L. N. Satapathy, Microwave sintering of abrasion resistant alumina liner tiles, Proceedings of the 4th world congress on Microwave and RF applications, Austin, Texas, USA ( In press) 12 B. Vaidhyanathan, D. K. Agrawal and R. Roy, Novel synthesis of nitride powders by microwave-assisted combustion, J. Mater. Res., 15, 974-981 (2000). 13 K. Kakimoto, F. Wakai, J. Bill and F. Aldinger, Fabrication of polycarbosilane derived SiC bulk ceramics by carbothermal reduction- Effect of green density on crystallinity of pyrolyzed compacts, Nano str. Mat!., 12, 175-178, (1999). 14 G. A. Danko, R. Silberglitt, P. Colombo, E. Pippel and J. Woltersdorf, Comparison of microwave hybrid and conventional heating of preceramic polymers to form silicon carbide and silicon oxycarbide ceramics, J. Am. Ceram. Soc, 83, 1617-1625, ( 2000).


,5D. Bahloul, M. Pereira, P. Goursat, N. S. Choong Kwet Yive, and R. J. P. Corriu,"Preparation of Silicon Carbonitrides from an Organosilicon Polymer: I, ThermalDecomposition of the Cross-linked Polysilazane," J. Am. Ceram. Soc, 76, 1156-62(1993). I6S. Zhu, S. ding, H. Xi and R. Wang, Low temperature fabrication of porous SiC ceramics by preceramic polymer reaction bonding, Mater. Lett., 59(5), 595-597 (2004).

L. N. Satapathy, P. D. Ramesh, D.Agrawal and R. Roy- Microwave assisted synthesis of Silicon carbide powder from Si and C , Mater. Res. Bull., 40(10), 1871-1882 (2005).


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