Microwave Heated Chemical Vapor Infiltration: DensificationMechanism of SiCf/SiC Composites

David Jaglin, Jon Binner,w and Bala Vaidhyanathan

Institute of Polymer Technology and Materials Engineering, University of Loughborough, LoughboroughLE11 3TU, UK

Calvin Prentice and Bob Shatwell

QinetiQ Ltd., Cody Technology Park, Farnborough GU14 0LX, UK

David Grant

Division Materials, School of Mechanical, Materials, Manufacturing Engineering and Management, University ofNottingham NG7 2RD, UK

Silicon carbide fiber-reinforced silicon carbide matrix compos-ites (SiCf/SiC) have been produced using microwave heatedchemical vapor infiltration. Preferential densification of thecomposite from the inside out was clearly observed. Althoughan average relative density of only 55% was achieved in 24 h,representative of an B26% increase over the initial fiber vol%,the center of the preform densified to 73% of the theoretical.The densification mechanisms were investigated using X-rayabsorptiometry and scanning electron microscopy. The initialinverse temperature profile obtained, which was found to resultin the efficient filling of the intratow porosity, although not theintertow porosity, flattened out after approximately 6 h as thedensification front moved outward toward the edges. Althoughnot investigated directly, the evidence suggested that this wascaused by changes in both the thermal conductivity and micro-wave absorption characteristics as the samples densified.

I. Introduction

CHEMICAL vapor infiltration (CVI) has received considerableattention for the fabrication of fiber-reinforced ceramic ma-

trix composites (FRCMCs).1–7 The main benefit is its ability toproduce materials with attractive mechanical properties at rel-atively low processing temperatures2,8–10 and pressures,2,4,11,12

which reduce fiber stress and damage. Other benefits include thepotential to tailor the fiber–matrix interface as an initial step viapre-coating the fibers with C, BN, or SiC,2,13–15 the latter re-sulting in improved fiber pullout and hence composite toughnessas well as protecting the fibers from oxidation; the ability tocontrol the matrix microstructure via manipulation of the dep-osition conditions3,16,17; and the fact that CVI is a near-netshape process, which aids the production of irregular-shapedcomponents for a wide range of applications.6,18,19

The process is based on the decomposition or chemical reac-tion of a gas phase to yield a solid matrix within a heated porouspreform, the latter typically formed from ceramic or carbonfibers. Three distinctly different variants have been studied, iso-thermal CVI (ICVI) being the most widely used commercially.It involves heating the preform very slowly to minimize the

temperature gradients that tend to develop between the hot sur-faces and cooler interior,1,2 the latter resulting in preferentialdeposition or ‘‘crusting’’ at the exterior of the preform. Crustingreduces the ingress of the gas and results in the need to stop theprocess repeatedly to reopen the channels by machining if highresidual porosity is to be avoided. The combination of the needto reduce the temperature profiles to a minimum and the factthat diffusion is the species transport mechanism means thatlong processing times of 600–2000 h are typically required toreach high matrix densities.4,15,20

In an attempt to overcome this problem, an approach basedon the forced flow of gases, combined with the deliberate cre-ation of a temperature gradient across the preform (FCVI), hasbeen developed.21,22 However, although uniformity and efficien-cy are enhanced compared with ICVI,23 disadvantages includethe need for complex temperature and gas flow control, some-thing that is magnified when processing multiple parts andgeometries. The use of a pulsed-pressure CVI process (PCVI)in which the pressure is modulated24,25 has also yielded fasterinfiltration rates, improved levels of densification, and greaterinfiltration uniformity as a result of increased mass transportcompared with ICVI.26 However, PCVI displays low efficiencyin terms of conversion of gas to solid phase and the capitalequipment costs are very high, making scaleup difficult. Hence,it is primarily used for the final densification of composites thatare produced by other routes.2

The use of microwave radiation is a potentially attractive al-ternative due to its potential for generating a controllable in-verse temperature profile during the heating of a ceramic fiberperform.27–29 Volumetric heating of the dielectric coupled withsurface heat losses results in the center of the component beinghotter than the surface, potentially by up to several hundreddegrees centigrade. This allows the microwave (heated) CVI(MCVI) process to initiate at the center and then the reactionfront to move toward the surface as the dielectric characteristicsof the preform change.27,30,31 Premature pore closure due tocrusting is completely avoided, potentially allowing densificationto occur an order of magnitude faster than with ICVI.30,32 Ex-perimental work has yielded this inside-out densification patternin a number of different ceramic systems, including SiC, Si3N4,Al2O3, and ZrO2.

28,31,33

The work presented here investigates the densification mech-anisms for SiC fiber-reinforced SiC matrix composites (SiCf/SiC)processed by MCVI using principally scanning electron micros-copy (SEM) and X-ray absorptiometry (XRA). The primarygoal was to investigate whether the consequence of the inversetemperature gradient created by microwave heating would bethe efficient densification of the preform from the inside out.

Journal

J. Am. Ceram. Soc., 89 [9] 2710–2717 (2006)

DOI: 10.1111/j.1551-2916.2006.01127.x

r 2006 The American Ceramic Society

2710

T. Besmann—contributing editor

Supported by QinetiQ, formerly the Defence Evaluation Research Agency (DERA), andEPSRC, both in the UK.

wTo whom all correspondence should be addressed. e-mail: j.binner@lboro.ac.uk

Manuscript No. 20265. Received March 4, 2005; approved March 1, 2006.

II. Experimental Procedure

A forced-flow CVI system with microwave heating was specif-ically developed to tackle the problems encountered in conven-tional CVI for the fabrication of FRCMCs. The apparatus wasdesigned so that a pressure gradient forced the reagent stream toflow through a preform subjected to a microwave-generatedinverse temperature gradient.

(1) Materials

The fibers used were polycarbosilane-derived Si–C–O NicalonNL-202 (Nippon Carbon, Tokyo, Japan). The latter were wovenby Sigmatex Ltd., UK, into a two-dimensional plain weave clothaccording to the NP-1616 standard (6.3 tows/cm). The averagenumber of fibers per tow was 500, with a fiber diameter of 1577mm. The fiber cloth was initially heat treated in air for 2 h at6001C to remove the sizing agent covering the fibers and then cutinto disks measuring 48 mm in diameter. Each fiber preform wasmanufactured by stacking 30 disks of the fabric in randomorientations, which were then compressed and stitched togetherusing cotton yarn. The average thickness for the 30-layer pre-forms was 10.070.5 mm. Fifteen percent dense alumina ceramicfoams (Hi-Por Ceramics Ltd., Sheffield, UK) measuring 10 mmthick by 48 mm diameter were positioned above and below thesample to reduce the temperature profile developed within thepreforms by limiting the heat losses occurring at the top andbottom surfaces.

Methyltrichlorosilane (MTS), CH3SiCl3, was used as the pre-cursor gas source because it decomposes to yield stoichiometricSiC over a wide range of operating conditions23,34,35 accordingto the following equation:

CH3Cl3SiðgÞ þ nH2ðgÞ �!�1000�C

SiCðsÞ þ 3HClðgÞ

þ nH2ðgÞ (1)

The MTS (Sigma-Aldrich, Gillingham, UK) and ultrahighpurity hydrogen used as a carrier gas were mixed together beforereaching the reaction cavity. The MTS vapor concentration wasset by either controlling the temperature of the MTS reservoir orby diluting the gas mixture with additional hydrogen. Argonwas used as a purge gas.

(2) Microwave Apparatus and Gas System

Themicrowave energy was generated at 2.45GHz by a continuous-wave magnetron (Model YJ1600, Philips, Eindhoven, the Nether-lands), the output power being adjustable in the range 0.5–5 kW.A purpose-built, mild steel, cylindrical, multi-mode applicatorwas constructed with a diameter and height of 600 mm (Fig. 1).A mode stirrer was used to perturb the field continuously andhence generate as uniform an electromagnetic field in the sampleas possible. Without a stirrer, a multimode cavity would tend togenerate multiple, localized hot spots. The SiC fiber preform waspositioned at least 60 mm above the water-cooled bottom baseof the cylindrical cavity to avoid microwave wall effects. Thefiber specimen was fitted horizontally inside a tight-fitting quartztube. A quartz bell jar was sealed with Viton O-rings on thecavity base to isolate the sample from the applicator. Reflectedpower was diverted via a three-port circulator connected to awater load while a four-stub tuner was used to match the im-pedance of the source and minimize the reflected power. Theexhaust plumbing provided vacuum, pressure control, andscrubber functions via a liquid nitrogen cold trap.

(3) Temperature and Pressure Control

Temperature was experimentally measured using a 3 mm diam-eter, shielded, inconel alloy, type-K thermocouple inserted half-way through the thickness of the sample via a predrilled 3 mmdiameter hole in the center of one of the flat surfaces. The in-ternal temperature of the sample was controlled by a feedbackcontrol system between the temperature output and the micro-

wave unit, the temperature being maintained within 751C ofthe initial set point. The possibility of electromagnetic interac-tion with the thermocouples was investigated by comparing themeasurements with values obtained via an optical thermometer(Model 100 Accufiber, Luxtron, Santa Clara, CA). Experimentsshowed that the temperature difference between the two was lessthan 201C, a result also observed by Grellinger and Janney36

using alumina and zirconia ceramics at temperatures up to14001C. During preliminary experiments of up to a 12-h dura-tion, the temperature difference between two points 15 mmapart was monitored, the center of the preform acting as thecontrol point.

The reactor pressure was monitored and controlled by twopiezoresistive pressure transducers (Models CVC2 and DVR2,Vacuubrand, Wertheim, Germany) with a 72 mbar accuracyof measurement; they were placed upstream and downstream ofthe reaction cavity, respectively. The pressure was set at 70.070.5 kPa using a solenoid valve (Model VCC6, Vacuubrand)connected to the CVC2 controller following preliminary workaimed at investigating the optimum system pressure. The systemwas operated at reduced pressure to increase mass transport andavoid sealing off the reactant flow stream as the densificationproceeded. A safety valve was installed to release gases in theevent of an overpressurized atmosphere.

(4) Experimental Procedure

All the preform infiltrations of SiC into the Nicalon cloth stackswere performed using the same MCVI process conditions; theseare summarized in Table I. Once the sample and ceramic foaminsulation disks were in place inside the quartz tube, the quartzbell jar was positioned over the top and the reaction vessel at-mosphere roughed out using a rotary pump. Argon was flushedthrough the system for 15 min before a 30 min purge with a 95%Ar–5% H2 mixture. After the cavity pressure was set at the de-sired value, theMTS was carried to the quartz bell jar by passinghydrogen through the reservoir containing the MTS. Prelimi-nary experiments indicated that B0.5 kW of microwave powerwas required to heat the green samples to a control temperatureof 1273 K, with the power level being steadily increased duringthe heating sequence until the central sample temperaturereached the desired set point. It should be noted that whenpartially infiltrated samples were reheated, there was generally aneed to use higher microwave power levels, although they neverexceeded 2 kW. After infiltration, the MTS-H2 stream andmicrowave power were shut off and argon was allowed toflow through the reaction vessel for a minimum of 2 h until

REACTANTS

EXHAUST

WAVEGUIDE

BELLJAR

MODESTIRRER

BASE PLATE

QUARTZTUBE

THERMOCOUPLES

ALUMINAFOAMS

SAMPLE

MTS

H

Ar

REACTANTS

WATER-COOLEDPLATE

Fig. 1. Schematic diagram of the microwave chemical vapor infiltrationexperiment setup.

September 2006 Microwave Heated Chemical Vapor Infiltration 2711

the sample had cooled down to room temperature and the toxicby-products were completely evacuated and neutralized.

The reproducibility of infiltration rates was checked by per-forming a minimum of two runs under the same conditions. Thelonger infiltrations were divided into runs of 4 or 8 h and theaverage densification rate was calculated on the basis of twospecimens processed for up to 24 h.

(5) Structural Characterization

The main purpose was to determine how the densification proc-ess progressed within the sample during the MCVI process. Fordensity and porosity analysis, attempts using metallurgical sec-tioning and stereological measurements proved inconclusive,primarily because the small number of pores, coupled with thecharacteristically strong anisotropy, required an inordinatenumber of sections to obtain good statistics, a view supportedby Kinney et al.37 Thus, a non-destructive characterization tech-nique, XRA, was used on sequentially infiltrated samples beforetechniques involving sectioning of the samples were carried out.This had the advantage of allowing a sample to be characterizednon-destructively before further infiltration on the same sample.

XRA is based on scanning a specimen with a coherent beamof X-rays and measuring the transmitted beam that has beenattenuated by the sample. The resultant image is a map of pho-ton attenuation, with denser zones represented by a bright areawhile less densified areas appear darker. All other factors beingequal, this can be directly correlated to spatial variations indensity within the sample. The equipment used was a LunarDPX-L DEXA, originally calibrated for measuring mineralbone density. After recalibration using a range of SiC materialsof varying density, it enabled a semi-quantitative, non-destruc-tive assessment of the relative density within the SiCf/SiC pre-forms to be obtained. The XRA resolution was of the order of amillimeter (one pixel from the scan corresponds to a 0.6� 1.2mm rectangular section) and the technique could typically detect0.5%–1% variation in density. Scan enhancement through col-orization was achieved using Scion Image computer software(Scion Corp., Frederick, MD). Phase characterization of thedensified materials was performed on a Siemens Kristalloflex810 D500 diffractometer (Munich, Germany) at a wavelength of1.5418 A using CuKa radiation. Analysis of cross-sectionedsamples was performed using a Philips XL30 ESEM-FEG(Philips, Eindhoven, the Netherlands) with an EDAX CDUleap energy-dispersive X-ray (EDX) analyzer. Secondary (SE)and back-scattered (BSE) images were valuable for differentiat-ing matrix from fiber. EDX analysis also provided elemental dotmapping of the cross sections.

The gas permeability of the sample, K (m2), was calculated bymeasuring the pressure drop as a function of airflow rate usingan arrangement similar to that described elsewhere.38 Samples

were fastened within a tube connected to a compressed airsource and flow rates were gradually increased, with inlet andoutlet pressures recorded. Using Darcy’s law for laminar viscousflow in porous materials,39 the gas permeability of the sample Kwas calculated by plotting the ratio between the pressure differ-ence across the sample thickness versus the airflow.

(6) Dielectric Measurements

A limited number of dielectric measurements as a function oftemperature up to 1373 K were made on sections of sampleswith varying density (72%). These included an initial preform(29% of theoretical density), four densified sections from proc-essed samples that had been infiltrated by varying amounts up to72.5%, and a piece of notionally 100% dense SiC produced bychemical vapor deposition (CVD). The approach used has beendescribed elsewhere40 and is a cavity perturbation-based tech-nique operated at 2.21 GHz. The loss tangent, tan d, was calcu-lated from the ratio of the dielectric loss factor, e00, and dielectricpermittivity, e0.

III. Results

(1) Infiltration Rates and Stoichiometry

Infiltration rates and stoichiometry of the deposited matrix areshown in Table II as a function of the control temperature. Theinfiltration rates in the preform are useful data reflecting MCVIdynamics and are dependent exponentially on the inverse oftemperature in the 1073–1323 K range. For temperatures below1173 K, stoichiometry measurements were not possible as thedeposition layer on the individual fibers was too thin for EDXanalysis. Above 1173 K, the SiC deposited at the specimen cen-ter was carbon rich; C values as high as 57.5 at.% were observedas the temperature increased to 1323 K. For the samples infil-trated for up to 24 h, the stoichiometry of the SiC matrix variedfrom being C rich at the center to Si rich at the periphery; a 1:1Si:C ratio was observed approximately 13 mm from the center ofthe preform in the sample processed for 24 h (S24). X-ray dif-fraction suggested that the matrix SiC deposited in the preformprimarily consisted of polycrystalline cubic b-SiC (3C) withtraces of hexagonal a-SiC (2H). Some free carbon was detect-ed, which may explain the high carbon content of the matrix atthe center of the preforms. The deposited matrix also typicallycontained between 1.2 and 2.5 at.% of oxygen.

(2) Relative Density Variations

The average densification variation for the fabric preforms as afunction of infiltration time is shown in Fig. 2. After 24 h, theinitial 29 vol% fiber preform had been infiltrated up to an av-erage density of 55%. The densification rate in % h�1 is alsorepresented as a function of the process time. Both curves showthat a decrease in densification rate occurred over time.

Table I. Standard MCVI Process Conditions and a Selectionof Infiltrated Samples

Preform control temperature (K) 1073–1323Total pressure (kPa) 70MTS reservoir temperature (K) 298Hydrogen gas flow rate (cm3/min) 300MTS fraction at inlet (vol%) 1.3

Sample code MCVI time (h)

S00 0S02 2S04 4S06 6S08 8w

S24 24z

wTwo runs of 4 h. zThree runs of 8 h. MCVI, microwave chemical vapor

infiltration.

Table II. Average Infiltration Rate and Stoichiometry of theSiC Deposited

Temperaturew (K)

Infiltration

ratez (mg/min)

Matrix atomic composition (at.%)

C Si O

1073 2.3 y y y

1123 5.8 y y y

1173 12.8 49.4 49.4 1.21223 15.1 51.9 46.8 1.31273 19.5 55.2 42.3 2.51323 22.1 57.5 41.2 1.3

wControl temperature at the center. zBased on a 2-h MCVI run on a fresh

preform. yDeposition layer too thin for EDX measurement. MCVI, microwave

chemical vapor infiltration; EDX, energy-dispersive X-ray.

2712 Journal of the American Ceramic Society—Jaglin et al. Vol. 89, No. 9

(3) Infiltration Evolution

SEM on polished sections at low and high magnification of theraw preforms and densified samples after 2, 4, 6, 8, and 24 h ofthe MCVI process provided information on the extent of dens-ification within the fiber tows (Fig. 3). The fabric layers initiallyappear as longitudinal fiber tows and sectioned transverse fibertows. Intertow porosity could be as large as 250 mm whereasintratow pores typically varied in the range 10–15 mm. In theearly stages of the infiltration, deposition of the matrix tookplace around the individual cylindrical filaments with relativelylittle merging of the deposited material occurring. As the dep-osition became more extensive, the matrix began to link up untilthe intratow porosity appeared to become isolated after 8 h, al-though this was difficult to confirm due to the two-dimensionalnature of the sections. A 24 h-infiltrated sample was character-ized across its diameter; Fig. 4 shows the microstructure 0, 10,and 20 mm from the center axis. Although the central region ofthe preform contained little residual porosity, the extent of dep-osition decreased rapidly toward the edge. At the preform cen-ter, the layer of deposited matrix could be up to 20 mm thickafter 24 h, while only 2–3 mm was deposited at the preform edgeafter the same infiltration time. It was also noted that while themajority of deposition occurred within the tows, the intertowpores could be reduced from an average width ofB250 toB100mm at the center of the preform, Fig. 4, when no delamination ofthe layers occurred.

Figure 5 presents the variation of pressure difference betweenthe outlet and inlet sides of the reaction cavity as a function ofthe infiltration time. The permeability K measured on samplesinfiltrated for different times is also plotted, and shows thatdensification from 29% to 55% average density only decreasedthe permeability by less than a factor of 2.

Figure 6 represents the raw XRA scans and their color en-hancements for the samples listed in Table I.z From a uniformgreen preform, the densification clearly progressed from insideout with the average density rising from 29% to B37% after6 h, with the region around the center axis reaching B60% oftheoretical in the same time. However, during the remainder ofthe 24 h infiltration period, densification became more uniformacross the preform, with the average density increasing fromB37% to 55% while the center increased from B60% to 73%.

The temperature difference between the center and a point15 mm away was recorded for the first 12 h and is shown inFig. 7. The temperature gradient remained relatively unchanged

at B1201C for the first 6 h before decreasing linearly with in-filtration time, suggesting that the radius of the hot zone beganto increase at this point as the temperature at the center of thepreform was controlled and fixed. These results are in line withthe XRA scans.

(4) Microwave Absorption

Figure 8 shows the variation in microwave power used duringthe MCVI process. It had to be gradually increased over each ofthe three 8 h cycles comprising the 24 h processing periodin order to maintain the required control temperature. AfterB18 h, the power needed to be increased substantially, causingthe infiltration to be terminated after 24 h because of the exces-sive microwave power used in comparison with the small samplesize; other parts of the system began to absorb microwaves aswell as the sample. This implies that as the samples densified,their microwave absorption characteristics decreased, a resultconfirmed by the dielectric data presented in Fig. 9.y,41 For

0

10

20

30

40

50

60

0 10 20 30

Time / h

Rel

ativ

e d

ensi

ty /

%

0

0.5

1

1.5

2

2.5

3

Den

sifi

cati

on

rat

e / %

h−1

Relative density Densification rate

Fig. 2. Average density variation of fiber preforms as a function ofmicrowave chemical vapor infiltration processing time.

Fig. 3. Back-scattered electron micrographs showing (a) the initial30-layer Nicalon fiber preform and the effect of densification near thecentral axis after (b) 2 h, (c) 4 h, (d) 8 h, and (e) 24 h of infiltration.

zThe central bright area observed in some preforms is the tip of the thermocouple thatbroke off when the sample was removed from the system due to chemical attack by theMTSand chlorine by-products; however, the density calculations took this into account.

yIt should be noted that it is very difficult to obtain accurate dielectric values for highloss materials; hence, while the trend in these results is almost certainly valid, the absolutevalues must be treated with care.

September 2006 Microwave Heated Chemical Vapor Infiltration 2713

values of tan d below 0.01, a material may be effectively regard-ed as non absorbing; hence, it may be seen that while uninfil-trated SiC fiber preforms are readily microwave absorptive atroom temperature, infiltration with a SiC matrix resulted in thecomposite becoming steadily less microwave absorptive as theirdensification progressed.

IV. Discussion

The inverse nature of the temperature profile generated by theuse of microwave heating allowed the infiltration of the SiC fiberpreforms from the inside-out. Problems related to ‘‘crusting’’and poor inner infiltration encountered in ICVI were thereforeovercome as the porous structure remained accessible for infil-tration. However, in this experimental MCVI system, an aver-age density of just 55% was achieved in 24 h, although the

center reached densities of 73% of the theoretical. The overallrelative efficiency of the process was therefore lower than hasbeen achieved in some FCVI systems, although it should benoted that the infiltration conditions were not optimized andinfiltration had to be terminated after 24 h due to the need forexcessive microwave power as the sample size was small com-pared with the size of the microwave cavity. Several teamsworking on conventionally heated FCVI have accomplisheduniform densities above 80% across specimens42,43 and, in par-ticular, Matlin et al.12 achieved a final density of B90% in just10 h using a two-step FCVI process. Nevertheless, if optimiza-tion of the MCVI process is to be attempted, then a clear un-derstanding of the densification mechanism is important andthis is discussed below.

The deposition rate shown in Fig. 2 initially decreased fairlyrapidly as infiltration progressed, under constant gas streamconditions, from an initial high value of B1.7%/h to B1%/h

Fig. 4. Back-scattered electron micrographs showing the state of densification of preform in three locations after 24 h of infiltration: (a) at the center,(b) 10 mm from the center, and (c) 20 mm from the center (preform radius was 24 mm).

2714 Journal of the American Ceramic Society—Jaglin et al. Vol. 89, No. 9

after B6 h; however, the rate of decrease then slowed so thatafter 24 h it was still B0.8%/h. Although overall, these resultscompare favorably with the ICVI process,44,45 as indicatedabove they are slower than the FCVI process. One of the rea-sons was observed from the SEM analysis. This showed thatwhile the large intertow porosity remained only partly filled, themajority of the intratow porosity in the initial hot zone regionthat developed during the first 6 h was successfully filled withdeposited SiC. The consequence of this will have been to reducesignificantly the total surface area for deposition by at leastpartially sealing the tow surfaces. This also resulted in a match-ing decrease in the gas permeability (Fig. 5).

Both the XRA results and the data depicting temperatureprofile variations, Figs. 6 and 7, suggest that the initial centralhot zone remained stable in size for approximately the first 6 hand then gradually expanded across the diameter of the samplein the form of a ‘‘moving front,’’ which could be consideredto be similar in principle to the ‘‘reaction front’’ found inFCVI.5,23,37,42 Although the reasons for this were not investi-gated directly, they are probably a result of a combination offactors including the change in the sample thermal conductivityand dielectric characteristics with increasing sample density as afunction of time. For example, in FCVI, the thermal conduc-tivity has been shown to play an important role,2 with the regionof the preform that is hot enough to promote deposition chang-ing as a result of changes in the local thermal conductivity,which in turn occur because the density increases as a result ofdeposition. Eighty percent dense SiCf/SiC composites producedby CVI commonly exhibit thermal conductivities of

B25 W � (m �K)�1 in-plane (and B7 (W �K)�1 across theplies)46,47 compared with values of only 1–3 W � (m �K)�1 in-plane at room temperature for the uninfiltrated performs.48 It isprobable that a similar effect occurred in the present work and isone of the factors behind the broadening of the hot zone overtime, especially as the density at the center of the compositereached 73% of the theoretical.

The dielectric characteristics of the preform and subsequentcomposite will also have had an effect on the absorption of mi-crowave energy and hence the evolution of the temperature pro-file.49,50 As shown in Fig. 9, as the sample densifies the losstangent decreases, i.e., the sample becomes less microwave ab-sorptive. Examination of the separate real and imaginary partsof the data, not presented here to save space, actually shows thatthis is because the material becomes more reflective of micro-waves. The reason is that the b-SiC deposited onto the NicalonSiC fibers has a much higher dielectric loss and hence theamount of microwave energy reflected increases, preventingthe fibers in the deposited regions of the preform from beingas effectively heated.31 This results in a local decrease in tem-perature within the infiltrated region,28 or a need to increase themicrowave power to maintain the same temperature. The grad-ual increase in microwave power required to maintain the cen-tral control temperature at 1273 K during infiltration supportsthis concept (Fig. 8).

Table II indicates that when the control temperature at thepreform center was 1173 K, the SiC deposited was approxi-mately stoichiometric while when higher control temperatureswere used it was carbon rich. There are two possible explana-tions for this. The first is that it is a result of very high depletionof the MTS,4,51 whereas the second is that the temperatures weremuch higher than those measured; it is known that carbon-richSiC is observed at high temperatures.45 If the latter is true, themost likely explanation is that the local temperature at the sur-face of the fibers was actually somewhat hotter than the bulkvalue measured by the thermocouple. Evidence for extremelyhigh, very localized temperatures on particle surfaces, leading infact to plasma discharges, have been observed using in situ ultra-high resolution video microscopy during microwave sintering.52

Radial variations in stoichiometry were also observed, withthe center being C rich and the outer regions Si rich. Devlinet al.53 also found stoichiometric variations and postulated thatthey could also have influenced the local microwave absorptioncharacteristics. They proposed that a Si-rich SiC matrix wouldabsorb microwave energy to a greater degree as the extra siliconatoms would lower the matrix conductivity. This would suggestthat microwave absorption would be enhanced toward the edgeof the sample; however, there was no evidence for this in thepresent work. Nevertheless, a potential concern that arises fromthe variation in stoichiometry is that it could result in non-

0

5

10

15

20

25

30

35

40

0 10 20 30Time / h

Pre

ssu

re d

iffe

ren

ce /

kPa

0

0.5

1

1.5

2

Per

mea

bili

ty /

10−1

1 m

2

Measured permeabilityPressure difference

Fig. 5. Pressure differential and permeability as a function of theprocessing time.

Fig. 6. X-ray absorptiometry scans for preforms infiltrated by microwave chemical vapor infiltration for 0, 2, 4, 6, 8, 16, and 24 h.

September 2006 Microwave Heated Chemical Vapor Infiltration 2715

uniformity of the component properties. One solution would beto control the inverse temperature gradient so that it was lesssteep across the preform, for example using a hybrid microwave-conventional heating system. This will be discussed furtherbelow.

Despite the efficient filling of the intratow porosity within8 h, the intertow pores remained up to 100 mm wide even after24 h (Fig. 4). This is a common problem in the fabrication ofFRCMCs and has been addressed before.35 For example,Skamser et al.43 obtained an 82% dense composite with resid-ual interpores as large as 0.3–0.6 mm in width and spanning tensof millimeters through the specimen. With fiber preforms pro-duced from the lay-up of fiber sheets, interpores are mainlycaused by the boundaries between groups of tows. They are dif-ficult to eliminate and are the most harmful defect as they sig-nificantly reduce the mechanical properties and also provide apath for corrosive agents.

As a consequence of the macropores, it is known that it is theelimination of the last 30% of the porosity that is financially

costly because the infiltration time is extended during this peri-od. In order to produce CMCs by any form of CVI at a com-mercially acceptable cost, the processing time must be kept asshort as possible.30 Despite the significant reduction in infiltra-tion time achieved by FCVI, and potentially MCVI, two-stageCVI processes and the pre-impregnation of the macropores by aprocess other than CVI are realistic approaches that have beeninvestigated. In the former, microvoids within the tows can beinfiltrated in the first stage of the process, followed by a changeof conditions to increase the deposition rate, leading to rapidinfiltration of the macropores between tows.44 An alternativeapproach based on the impregnation of SiC powder within thefiber structure provides impregnated preforms that can be sub-sequently densified efficiently by CVI. Such an approach hasbeen investigated in combination with MCVI by the presentauthors and the results are being presented separately.54 It issufficient to say here that faster infiltration rates were achieved.An additional approach would be the development of a hybridfacility where a combination of both conventional radiative andmicrowave heating is used to achieve more rapid densification.One could imagine a system whereby the center of the preform isinitially infiltrated using microwave heating and then there was agradual transition to conventional heating to cause the surfaceregions of the parts to be infiltrated. Dual frequency microwavesystems could also be envisaged to allow greater control of thetemperature profile as the absorption of microwaves in materialsis frequency dependent.30,55

V. Conclusions

Inverse temperature profiles were successfully produced via mi-crowave heating, resulting in preferential densification of theSiCf/SiC composites from the inside out. ForB29% dense fiberpreforms, an average relative density of B55% was achievedafter 24 h, although the center was densified up to 73% of thetheoretical. Further densification was limited by the small phys-ical size of the samples compared with the size of the microwavecavity used, which resulted in excessive microwave power beingneeded. This is not a fundamental limitation of the process.

In the central hot zone region, the intratow porosity wasefficiently filled with matrix within 6–8 h, although limited

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 4 8 12 16 20 24

Infiltration time / h

Tem

per

atu

re /

K (

x103 )

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Mic

row

ave

po

wer

/ kW

Microwave power (W)Temperature (K)

Fig. 8. Heating behavior and microwave power variation for a stand-ard 30-layer Nicalon fiber sample during the microwave chemical vaporinfiltration process (three runs of 8 h).

0

20

40

60

80

100

120

140

0 5 10 15

Infiltration Time / h

Tem

per

atu

re d

iffe

ren

ce /

K

Fig. 7. Temperature difference between the center of the preform anda point 15 mm away.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

273 473 673 873 1073 1273

Temperature (K)

Tan

δ

Green preform42.50%52.50%62.50%72.50%CVD grown SiC

Fig. 9. tan d values for an uninfiltrated SiC fiber preform, partiallyinfiltrated SiCf–SiC preforms of different density, and a dense piece ofSiC produced by chemical vapor deposition (CVD).

2716 Journal of the American Ceramic Society—Jaglin et al. Vol. 89, No. 9

deposition occurred in the intertow pores even after 24 h. Theprogression of the densification showed that there was a ‘‘reac-tion front’’ as the filling of the porosity moved outward towardthe edges of the preform. Although not investigated directly, thethermal conductivity and microwave absorption characteristicsof the deposited matrix are believed to have been the primarysource of this effect. XRA has also been shown to be a veryuseful non-destructive tool for investigating the infiltration ofceramic matrix composites and has been able to reveal theinverse deposition profile very clearly.

The development of a hybrid microwave-conventional heatedCVI system and the use of a preliminary pre-impregnation stepto fill the large intertow porosity partially are potential solutionsto reduce the non-uniformity of deposition and stoichiometry inaddition to offering the potential to reduce infiltration timessignificantly.

Acknowledgment

The authors acknowledge Dr. Pye of the Medical School, University ofNottingham, UK, for his help with the X-ray absorptiometry.

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