JOURNAL OF MATERIALS SCIENCE 33 (1998) 4287—4295

Effects of HF treatments on tensile strength ofHi-Nicalon fibres

N. P. BANSALNational Aeronautics and Space Administration, Lewis Research Center, Cleveland,OH 44135-3191, USA

Tensile strengths of as-received Hi-Nicalon fibres and those having a dual BN—SiC surfacecoating, deposited by chemical vapour deposition, have been measured at roomtemperature. These fibres were also treated with HF for 24 h followed by tensile strengthmeasurements. Strengths of uncoated and BN—SiC coated Hi-Nicalon fibres extracted fromcelsian matrix composites, by dissolving away the matrix in HF for 24 h, were alsodetermined. The average tensile strength of uncoated Hi-Nicalon was 3.19^0.73 GPa witha Weibull modulus of 5.41. The Hi-Nicalon—BN—SiC fibres showed an average strength of3.04^0.53 GPa and Weibull modulus of 6.66. After HF treatment, the average strengths ofthe uncoated and BN—SiC coated Hi-Nicalon fibres were 2.69^0.67 and 2.80^0.53 GPa andthe Weibull moduli were 4.93 and 5.96, respectively. The BN—SiC coated fibres extractedfrom the celsian matrix composite exhibited a strength of 2.38^0.40 GPa and a Weibullmodulus of 7.15. The strength of the uncoated Hi-Nicalon fibres in the composite was soseverely degraded that they disintegrated into small fragments during extraction with HF.The uncoated fibres probably undergo mechanical surface damage during hot pressing ofthe composites. Also, the BN layer on the coated fibres acts as a compliant layer, whichprotects the fibres from mechanical damage during composite processing. The elementalcomposition and thickness of the fibre coatings were determined using scanning Augeranalysis. Microstructural analyses of the fibres and the coatings were done by scanningelectron microscopy and transmission electron microscopy. Stengths of fibres calculatedusing average and measured fibre diameters were in good agreement. Thus, the strengths offibres can be evaluated using an average fibre diameter instead of the measured diameter ofeach filament. Q 1998 Kluwer Academic Publishers

1. IntroductionHi-Nicalon fibre [1] has a desirable combination oftensile strength, elastic modulus, density and retentionof these properties at temperatures up to approxim-ately 1200 °C. Thus it is suitable for a variety ofaerospace applications as a reinforcement in highperformance structural composites with ceramic,metal and polymer matrices. The Hi-Nicalon fibresurface needs to be protected with appropriateceramic coating(s) in order to alleviate chemical reac-tions with ceramic and metal matrices during com-posite processing and use, and also, to provide a weakfibre—matrix interface for toughened ceramic com-posites.

In a recent study [2] it was observed that the0.75BaO—0.25SrO—Al

2O

3—2SiO

2(BSAS) celsian matrix

composite reinforced with Hi-Nicalon (uncoated)fibres showed low flexure strength along with catas-trophic failure. In contrast, the composite reinforcedwith BN—SiC coated Hi-Nicalon fibres was strong,tough and showed graceful failure. Fibre push-through tests and microscopic examinations indicateda weak fibre—matrix interface in both composites,

0022—2461 ( 1998 Kluwer Academic Publishers

and no chemical reaction was observed between thefibres and the matrix. It was postulated that the lowstrength of the Hi-Nicalon (uncoated)— celsian com-posite was due to degradation of fibre strength duringcomposite processing.

The primary objective of the present research was toverify the postulate of this earlier study. Another ob-jective was to carry out microstructural and chemicalanalyses of the fibres and the coatings. Room temper-ature tensile strengths of the as-received Hi-Nicalonfibres and those coated with a dual BN—SiC surfacelayer were measured. In order to investigate fibrestrength degradation during composite processing,the strengths of fibres extracted from celsian matrixcomposites, by leaching away the oxide matrix in 40%HF for 24 h at room temperature, were also measured.The strengths of fibres that had been treated with 40%HF for 24 h at room temperature were also deter-mined. Weibull statistical parameters were determinedfor each type of fibre. Elemental compositions andthicknesses of the fibre coatings were determined byscanning Auger analysis. Microstructural analyses ofthe fibres and the coatings were done by scanning

4287

electron microscopy (SEM) and transmission electronmicroscopy (TEM).

2. Experimental procedure2.1. MaterialsHi-Nicalon fibre from Nippon Carbon Co. was usedin the present study. The properties of the as-receivedfibre are shown in Table I. These fibres containnanocrystalline (10—20 nm) b-SiC along with 30—40at % excess carbon, which is also reflected in its lowelastic modulus. A large variation (10—18 lm) in thefibre diameter was observed, although the manufac-turer reports an average value of approximately14 lm.

The polyvinyl alcohol (PVA) sizing on the as-received Hi-Nicalon fibres was burned off in a Bunsenburner flame. Single filaments were carefully separatedfrom the fibre tow for testing. All the coatings on thefibres were applied by 3M Co. using a continuouschemical vapour deposition (CVD) reactor. The BNcoating was deposited at approximately 1000 °C util-izing a proprietary precursor and was amorphous topartly turbostratic in nature. A thin overcoating ofSiC was also applied to the BN-coated fibres. Thenominal coating thickness was 0.4lm of BN and0.3lm of SiC. Uncoated and BN/SiC coated fibreswere extracted from the celsian matrix composites bydissolving away the oxide matrix in 40% HF for 24 h.The BN—SiC coated and uncoated Hi-Nicalon fibreswere also treated with 40% HF for 24 h to determine ifHF leaching affected fibre strength.

2.2. Electron microscopySurfaces of the fibres were examined using SEM andTEM. For cross-sectional analysis, fibres were moun-ted in a high temperature epoxy and polished beforeexamination. Fibre cross-sectional thin foils for TEMwere prepared using a procedure developed for ce-ramic fibres, which involves epoxy potting, slicing,polishing, dimple grinding and Ar ion-beam milling.A thin carbon coating was evaporated onto the TEMthin foils and SEM specimens for electrical conductiv-ity prior to analysis. The thin foils were examined ina Philips EM400T operating at 120 keV. SEM wasperformed using a Jeol JSM-840A operating at15 keV.

2.3. Scanning Auger analysisThe elemental compositions of the fibre near thesurface and of the fibre surface coatings were analysed[3] with a scanning Auger microprobe (FisonsInstruments Microlab Model 310-F). The fibresfor this analysis were mounted on a stainless steelsample mount by tacking the ends with colloidalgraphite. Depth profiling was performed by sequentialion-beam sputtering and Auger analysis. The ionetching was done with 3 keV Argon ions rastered overan approximately 1 mm2 area on the specimen. Theetch rate in Ta

2O

5under these conditions was

0.05 nm s~1.

4288

Auger electron spectroscopy (AES) analysis wasperformed using an electron beam current of approx-imately 1.5 nA. The beam was rastered over a 2] 20lm2 area of the fibre with the long axis of the areaaligned with the long axis of the fibre. Spectra wereacquired in integral (as opposed to derivative) form ata beam energy of 2 keV and depth profiles were cre-ated by plotting peak areas against ion etch time. Theatomic concentrations were calculated by dividing thepeak areas by the spectrometer transmission functionand the sensitivity factors for each peak, then scalingthe results to total 100%. The sensitivity factors werederived from spectra of ion etched Ag, Si, B, Ti, SiC,BN and TiO

2standards. The sensitivity factors used

for each element should not be trusted to better than$20%. The depth scale is from the Ta

2O

5calib-

ration and no attempt has been made to adjust for theactual etch rate for each material. Only the fibres witha smooth surface coating, rather than those havingthick and rough coating morphologies, were used forAuger analysis.

2.4. Tensile strength measurementRoom temperature tensile strengths of the individualfilaments were measured in ambient atmosphere witha screw driven Micropull fibre test frame equippedwith pneumatic grips. A 1000 g load cell, Sensotecmodel 34, was used. Measurements were carried out ata constant crosshead speed of 1.261 mmmin~1.A single filament was mounted on a 20 lb paper tabusing Hardman extra fast setting epoxy. The sideportions of the tab were cut with a hot wire beforeapplication of the load, producing a fibre gauge lengthof 2.54 cm. Twenty filaments of each type of fibre weretested. For the as-received fibres, the actual diameterof each filament tested was measured using a ZeissAxioskop microscope with a Sony DXC-960MDvideo camera attached to a Boeckeler VIA-150 videomeasurement system. Magnification range from X100to X800, depending upon the size of the fibre. Weibullparameters for tensile strength of as-received fibreswere calculated using the measured diameters andalso an average diameter of 13.5lm. For all otherfibres tested, tensile strengths were calculated basedonly on an average filament diameter of 13.5lm.

3. Results and discussion3.1. MicrostructureSEM micrographs showing the surface of as-received,flame desized Hi-Nicalon fibres are presented inFig. 1. The fibre surface appears to be fairly smoothand featureless. Energy dispersive spectroscopy (EDS)compositional spectra (Fig. 2) taken from the polishedcross-section of the fibre indicate the presence of onlySi and C along with a small amount of oxygen, whichis in qualitative agreement with the manufacturer’sdata (Table I). The surface of HF-treated Hi-Nicalonfibres is also fairly smooth and featureless as seenfrom the SEM micrograph in Fig. 3. SEM micro-graphs from the surface and polished cross-section ofthe BN—SiC coated Hi-Nicalon fibres are given in

Figure 1 SEM micrographs showing surface of desized Hi-Nicalonfibres.

Figure 2 EDS spectra from the cross-section of desized Hi-Nicalonfibre at 3 kV.

Fig. 4a, b. The coating on most of the fibres is smoothand uniform, but some fibres have a quite granularcoating. The BN coating is often nodular as can beseen in Fig. 5. The nodules in the BN layer remain or,often, are enhanced by the subsequent overcoatingof SiC. The number density of the nodules varies

TABLE I Properties of Hi-Nicalon fibres!

Property Value

Density, g cm~3 2.74Diameter,lm 14Filaments per tow 500Denier, g (9000m)~1 1800Tensile strength, GPa 2.8Elastic modulus, GPa 269Thermal expansion coefficient, 3.5 (25—500) "

10~6K~1

Specific heat, J g~1K~1 0.67 (25) "

1.17 (500) "

Thermal conductivity, Wm~1K~1 7.77 (25) "

10.1 (500) "

Electrical resistivity, ) cm 1.4Chemical composition, wt% 63.7 Si, 35.8C, 0.5 OC/Si, atomic ratio 1.3—1.4Temperature capability, °C 1200

!Data from Nippon Carbon Co."Values in parentheses are in degrees celcius.

Figure 3 SEM micrographs showing surface of Hi-Nicalon fibreafter treatment with HF.

considerably from filament to filament, but is moreconsistent along an individual fibre. Within a tow, thefibres on the outside had thicker and more granularcoatings than those on the inside. Overall, the coatingthickness was reasonably uniform. Generally, the BNcoating thickness was 0.2—0.4 lm, but could be much

4289

Figure 4 SEM micrographs from (a) surface and (b) polished cross-section of BN—SiC coated Hi-Nicalon fibres.

Figure 5 SEM micrograph from polished cross-section of BN—SiCcoated Hi-Nicalon fibres showing nodular coating. Some of thenodules are indicated by arrows.

higher at the nodules. The total coating thickness ona fibre could be as high as 7lm when a granularcluster is attached (Fig. 5). Typically, the SiC coatingthickness varied between 0.2—0.3 lm. Sharp contrastis observed between the fibre, BN coating andSiC coating in polished cross-sections (Fig. 6). The

4290

Figure 6 SEM micrograph at high magnification from polishedcross-section of BN—SiC coated Hi-Nicalon fibres showing sharpcontrast between the fibre and the coatings.

Figure 7 TEM bright field image showing cross-section of theas-received BN—SiC coated Hi-Nicalon fibre; four distinct sub-layers are visible within the BN coating.

microstructure of these coatings is quite similar tothose of BN—SiC coatings on HPZ ceramic fibres [3].

TEM analysis of the loose, BN—SiC coated Hi-Nicalon fibres in cross-section revealed four distinctlayers in the BN coating (Fig. 7). While there was nodetectable variation in composition between thelayers, there was a subtle variation in crystallographicstructure. All four layers were turbostratic in struc-ture, but layers 1 and 3 (counting from the fibreoutward) were more directional. The basic structuralunits of the BN are more aligned parallel to the fibresurface, giving sharper cusps in microelectron diffrac-tion patterns (Fig. 8). The SiC layer on the as-coatedfibres typically ion milled away before electron trans-parent thin regions were obtained, but appeared to beadherent to the BN layers.

After treatment with HF, the BN—SiC surface coat-ing was cracked, and spalled off at some places on thefibres, as seen in the SEM micrographs in Fig. 9. SEMmicrographs showing surfaces of the uncoatedBN—SiC coated fibres extracted from the celsiancomposite are given in Figs. 10 and 11, respectively.

Figure 8 Microelectron diffraction patterns typical for BN layers inBN—SiC coated Hi-Nicalon fibres: (a) layers 1 and 3 and (b) layers2 and 4.

Figure 9 SEM micrographs showing surface of BN—SiC coatedHi-Nicalon after treatment with HF.

Matrix particles are still sticking to many of the fibresand cracks in the coating are observed on those fibreshaving a thick granular coating.

3.2. Scanning Auger analysisElemental composition depth profiles, as obtainedfrom scanning Auger analysis of as-received, but de-sized, Hi-Nicalon fibres and those having BN—SiCsurface coatings are shown in Fig. 12a, b. Depth pro-files of the uncoated and coated fibres, after treatmentwith HF, are also presented in Fig. 12c, d. The elemen-tal composition (atomic percent) of the as-receivedfibre is approximately 42% Si and 56% C with C/Si

Figure 10 SEM micrographs showing surface of uncoated Hi-Nicalon fibres extracted from celsian matrix composite by dissolv-ing away the matrix in HF.

Figure 11 SEM micrograph showing surface of BN—SiC coatedHi-Nicalon fibres extracted from celsian matrix composite bydissolving away the matrix in HF.

atomic ratio of 1.33. A trace amount of oxygen is alsodetected. These results are in good agreement with thefibre composition supplied by the manufacturer(Table I). The composition of the HF treated Hi-Nicalon fibre is found to be approximately 43% Si,54%C and a trace of oxygen. The Hi-Nicalon—BN—SiC fibre has approximately 130 nm thick layer ofSi-rich SiC followed by approximately 500 nm thicklayer of B-rich BN. However, the coating thicknesswas non-uniform from filament to filament as seenfrom the SEM micrographs in Figs. 4 and 5. The fibres

4291

Figure 12 Surface depth profiles of various elements obtained usingscanning Auger microprobe for (a) Hi-Nicalon, (b) Hi-Nicalon—BN—SiC, (c) HF treated Hi-Nicalon and (d) HF treatedHi-Nicalon—BN—SiC fibres.

on the outside of a tow had thicker and more granularcoatings than those on the inside. The BN coatingis contaminated with oxygen and contains a highconcentration of carbon. In fact, the concentration of

4292

carbon is larger than those of boron and nitrogen. Theresults for the HF treated Hi-Nicalon—BN—SiC fibreare quite similar to those for the untreated fibre. How-ever, a trace amount of F is also detected in the BNlayer of the treated fibre.

3.3. Tensile strengthThe strength of ceramic fibres is determined by thestatistical distribution of flaws in the material. Thetensile strength of ceramic fibres is generally analysedon the basis of the well known Weibull statistics [4].An excellent review of the subject was provided byVan der Zwaag [5]. The Weibull modulus describesthe distribution of strength in materials that fail atdefects according to weakest link statistics. The prob-ability of survival of a material is given by the empiri-cal equation

P4"1!P(r)"expM!»[(r!r

6)/r

0].N (1)

where P4is the survival probability (P(r)"1!P

4is

the failure probability) of an individual fibre at anapplied stress of r, » is the fibre volume, r

6is the

stress below which failure never occurs, r0is the scale

parameter, and m is the shape or flaw dispersionparameter. For a given material, the Weibull modulusm and r

0are constant. Assuming r

6"0 and uniform

fibre diameter along the length, ¸, corresponding tothe volume, », Equation 1 can be rearranged as

lnln(1/P4)"lnlnM1/[1!P (r)]N"m lnr# constant

(2)

The experimental data are ranked in ascending orderof strength values and the cumulative probabilityP (r

*), is assigned as

P (r*)"i/(1#N) (3)

where i is the rank of the tested fibre in the rankedstrength tabulation and N is the total number of fibrestested. A least-squares linear regression analysis isthen applied to a plot of [ln ln1/P

4] versus ln(r), whose

slope is the Weibull modulus m.The ln ln (1/P

4) versus lnr Weibull probability plots

for room temperature tensile strength of uncoatedHi-Nicalon fibres using measured and average dia-meters of the filaments are presented in Fig. 13 and thevalues of Weibull parameters obtained from linearregression analysis are summarized in Table II. Thetensile strength of as-received Hi-Nicalon fibre is3.30$0.57 GPa and the Weibull modulus, m, is 7.03using measured diameter for each filament tested. Theaverage value of fibre diameter was found to be13.5 lm. Using this average diameter, values of fibretensile strength and m were calculated to be3.19$0.73 GPa and 5.41, respectively. The manufac-turer’s information data sheet [1] reports a value of2.8 GPa for the room temperature tensile strength ofthese fibres. The strengths of fibres obtained frommeasured and average fibre diameters are in goodagreement. Thus, an average fibre diameter, instead ofthe measured diameter of each fibre, can be used indetermining fibre strength. Weibull moduli calculated

Figure 13 Weibull probability plots for room temperature tensilestrengths of Hi-Nicalon fibres using (a) measured and (b) averagediameters of the filaments: s, a; h, b.

from measured and average filament diameters arealso in reasonable agreement. Similar results have alsobeen reported very recently by Petry et al. [6] fromtensile strength measruements of Nicalon andalumina—yttrium aluminum garnet eutectic fibres. TheWeibull parameters for the tensile strengths of otherfibres tested in the present study were evaluated usingan average filament diameter of 13.5 lm.

Weibull plots for the uncoated Hi-Nicalon fibresand those having a dual BN—SiC surface coating areshown in Fig. 14. Similar Weibull plots for the un-coated and BN—SiC coated Hi-Nicalon fibres, whichhave been treated with 40% HF for 24 h, and thoseextracted from the celsian matrix composite are givenin Fig. 15. Values of Weibull parameters obtainedfrom linear regression analysis for various fibres aresummarized in Table II. The BN—SiC coated Hi-Nicalon fibres show an average tensile strength of3.04$0.53 GPa with an m value of 6.66 indicating nostrength degradation during fibre coating by CVD.After HF treatment, both the as-received and theBN—SiC coated fibres show strength degradation ofapproximately 8—15%. The tensile strength of the

Figure 14 Weibull probability plots for room temperature tensilestrengths of fibres: (a) Hi-Nicalon and (b) BN—SiC coatedHi-Nicalon: s, a; h, b.

BN—SiC coated fibres extracted from the celsianmatrix composite by dissolving away the matrix in HFhas degraded to 2.38$0.40 GPa with an m value of7.15. Exposure of the fibres to high temperature dur-ing hot pressing of the composites could probablyaccount for this strength degradation. In contrast, thefibres extracted from the Hi-Nicalon (uncoated)celsian composite were so weak that they disinteg-rated into small pieces during extraction with HF. Thestrengths of these uncoated fibres probably degradedue to mechanical surface damage during compositehot pressing. The BN layer in the BN—SiC coatedfibres acts as a compliant layer protecting the fibresfrom mechanical damage during composite process-ing. SEM micrographs showing typical fracture surfa-ces of uncoated and BN—SiC coated Hi-Nicalon fibresin the celsian matrix composite are shown in Fig. 16.Fracture surfaces of the two fibres are seen to be quitedifferent. The fracture of the uncoated fibre is feature-less, whereas fracture mirror and heckle are present inthe fracture surface of the coated fibre. UncoatedHi-Nicalon fibres extracted from lithium aluminosili-cate (LAS) glass-ceramic matrix composites [7], which

TABLE II Weibull parameters for room temperature tensile strength of Hi-Nicalon fibres (Gauge length"2.54 cm; crossheadspeed"1.261mmmin~1)

Fibre treatment Fibre Average Standard Weibulldiameter strength deviation# modulus(lm) (GPa) (GPa) (m)

As-received, flame desized Hi-Nicalon Actual! 3.30 0.57 7.03As-received, flame desized Hi-Nicalon 13.5 (average) 3.19 0.73 5.41As-received, flame desized Hi-Nicalon; 13.5 (average) 2.69 0.67 4.93treated with 40% HF, 24 hBN—SiC coated Hi-Nicalon 13.5 (average) 3.04 0.53 6.66BN—SiC coated Hi-Nicalon; treated 13.5 (average) 2.80 0.53 5.96with 40% HF, 24 hUncoated, extracted from composite — b — bBN—SiC coated Hi-Nicalon, extracted 13.5 (average) 2.38 0.40 7.15from celsian composite by dissolving away matrixin 40% HF, 24h

!Actual fibre diameter measured for each filament tested."Fragmented into small pieces during extraction with HF (could not measure strength).#20 filaments tested for each fibre type.

4293

Figure 15 Weibull probability plots for room temperature tensilestrengths of chemically treated fibres: (a) HF treated Hi-Nicalon, (b)BN—SiC coated Hi-Nicalon treated with HF and (c) BN—SiC coatedHi-Nicalon extracted from celsian matrix composite by leachingaway matrix in HF: s, a; h, b; n, c.

Figure 16 SEM micrographs showing typical fracture surfaces of (a)as-received and (b) BN—SiC coated Hi-Nicalon fibres in celsianmatrix composites.

were hot pressed at 1360 °C for 40min, showed only20—25% reduction in tensile strength. This is becausethe LAS composites were densified for a short timejust above the matrix melting point, making use of theviscous flow of glass.

4294

3.4. Summary of resultsRoom temperature tensile strengths of as-received Hi-Nicalon fibres and those having a dual BN—SiC sur-face coating have been determined. Tensile strengthsof these fibres following treatment with HF for 24hhave also been measured. Room temperature tensilestrengths of uncoated and BN—SiC coated Hi-Nicalonfibres extracted from celsian matrix composites, byleaching away the matrix in HF for 24 h, were alsodetermined. The average tensile strength of as-re-ceived Hi-Nicalon was 3.19$0.73GPa with aWeibull modulus of 5.41. Also, tensile strengths ofuncoated fibres calculated using average and meas-ured fibre diameters were in good agreement. TheBN—SiC coated Hi-Nicalon fibres showed an averagestrength of 3.04$0.53GPa and Weibull modulus of6.66. After HF treatments, the average strengths of theuncoated and BN—SiC coated Hi-Nicalon fibres were2.69$0.67 and 2.80$0.53GPa and the Weibullmoduli were 4.93 and 5.96, respectively. The BN—SiCcoated fibres extracted from celsian matrix compositeexhibited a strength of 2.38$0.40GPa and Weibullmodulus of 7.15. The strength of the uncoated Hi-Nicalon fibres in the composite had been so severelydegraded that they disintegrated into small piecesduring extraction with HF. This strength degradationis probably caused by mechanical surface damage tothe uncoated fibres during hot pressing of the com-posites. The elemental composition and thickness ofthe fibre coatings were determined using scanningAuger analysis. HF leaching had no effect on thecomposition of the fibres. The BN coating, consistingof four distinct layers, was amorphous to partly turbo-stratic and contaminated heavily with carbon andsomewhat with oxygen. The silicon carbide coatingwas crystalline, non-stoichiometric, non-uniform andsometimes flaky. Microstructural analyses of the fibresand the coatings gave no evidence that HF treatmentdamaged the fibres or that surface damage to thefibres (excluding the coatings) occurred.

4. ConclusionsThe BN coating in the BN—SiC coated Hi-Nicalonfibres acts as a compliant layer protecting the fibresfrom mechanical damage during composite hot press-ing. Tensile strengths of fibres can be evaluated usingan average diameter instead of the measured diameterof each filament. This should circumvent the amountof time and effort spent in the measurement of dia-meters of individual filaments.

AcknowledgementsThanks are due to Dan Gorican for fibre strength mea-surements, John Setlock for SEM, Rob Dickerson forTEM, and Darwin Boyd for scanning Auger analysis.

References1. M. TAKEDA, J. SAKAMOTO, A. SAEKI and H.

ICHIKAWA, Ceram. Eng. Sci. Proc. 16 (1995) 37.

2. N. P. BANSAL and J. I . ELDRIDGE, J. Mater. Res. 13(1998) 1530.

3. N. P. BANSAL and R. M. DICKERSON, Mater. Sci. Eng.A222 (1997) 149.

4. W. A. WEIBULL, J. Appl. Mech. 18 (1951) 293.5. S. VAN DER ZWAAG, J. ¹esting Eval. 17 (1989) 292.

6. M. D. PETRY, T.-I . MAH and R. J. KERANS, J. Amer. Ceram.Soc. 80 (1997) 2741.

7. W. K. TREDWAY, Ceram. Eng. Sci. Proc. 17 (1996) 291.

Received 3 Apriland accepted 16 June 1998

4295