colloidal filtration of silicon nitride aqueous slips ... · non-oxide ceramics and, in particular,...

Colloidal Filtration of Silicon NitrideAqueous Slips, Part II: Slip Castingand Pressure Casting PerformanceRodrigo Morenoay Arturo Salomoni,b Ivan Stamenkovicb

and Sonia Mello Castanho,a*aInstituto de CeraÂ mica y Vidrio, C.S.I.C., Crtra. Valencia Km. 24,300, 28500 Arganda del Rey, Madrid, SpainbItalian Ceramic Center, Via Martelli 26, 40138 Bologna, Italy

(Received 4 April 1998; accepted 16 June 1998)

Abstract

In a previous part, the rheological behaviour of sili-con nitride aqueous slips was optimized by dispersingwith TMAH up to pH 11.5 using di�erent mixingprocedures and including di�erent concentrations ofsintering aids (Al2O3 and Y2O3). In this part, theobtention of pressureless sintered silicon nitride bod-ies by colloidal ®ltration techniques is studied. Thekinetics of the di�erent compositions is studied forboth slip casting and pressure casting. The pressurecasting kinetics is up to 20 times faster than that ofslip casting, which allows the scale-up of the processfor a low cost production, The obtained green densityis slightly >58%th for slip casting and 57±55%thfor pressure casting, depending on the applied pres-sure. This small di�erence does not in¯uence sintereddensity. At 1750�C/2h, a ®nal density around 96%this obtained. The sintering conditions are studiesconsidering the time, temperature, atmosphere andsintering bed. The best results are obtained when thesintering bed has the same composition to that of thesample to be sintered. The room temperature prop-erties of the sintered materials show a KIC valuehigher than 6MPam1/2, comparable to those foundin the literature for pressure sintered materials.# 1998 Elsevier Science Limited. All rights reserved

Keywords: Si3N4, suspensions, slip casting, pressurecasting, colloidal ®ltration

1 Introduction

Non-oxide ceramics and, in particular, siliconnitride, have excellent thermomechanical properties

for structural and heat engine applications, asreported elsewhere.1±4 Pressure sintering methodshave been used for the manufacture of densesilicon nitride parts. However, the high cost ofpressure assisted sintering seriously limit the on-line application of the process. The developmentof economical production processes for themanufacturing of complex parts of silicon nitridefor structural and heat engine applications is achallenge from a technological point of view.In this context the use of colloidal forming pro-

cesses followed by pressureless sintering are anattractive procedure for the obtention of siliconnitride parts with good mechanical properties andrelatively high density.5,6 Slip casting is a well-recognized method for the obtention of dense,defect-free ceramics.7,8 On the other hand, pressureslip casting is not yet as well controlled as slipcasting, but it has many clear advantages for alarge-scale fabrication of complex and thickerparts.9±11 A basic requirement for both shapingtechniques is the preparation of a stable, well-dispersed slip able to consolidate during ®ltrationin a dense and homogeneous green body.In the ®rst part of this series,12 the rheological

behaviour of castable silicon nitride aqueous slipswas described. The stability of the slips was studiedas a function of a number of processing para-meters, such as the type and concentration ofde¯occulant or pH-adjuster, the mixing/millingprocedure and homogenization time, the pre-sence of di�erent concentration of sintering aids(Al2O3 and Y2O3), etc. On the basis of the rheolo-gical studies performed in the ®rst part, this secondpart aims to study the parameters involved in theslip casting and pressure casting forming focusingthe kinetics and the characteristics of the greencasts. Pressureless sintering is studied as a functionof di�erent parameters, such as the heating schedule,

Journal of the European Ceramic Society 19 (1999) 49±59

# 1998 Elsevier Science Limited

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P I I : S 0 9 5 5 - 2 2 1 9 ( 9 8 ) 0 0 1 3 7 - X 0955-2219/98/$Ðsee front matter

49

*Permanent address: IPEN-CNEN/SP, PB11049, CidadeUniversitaÂ ria-USP, SaÃ o Paulo, Brazil.yTo whom correspondence should be addressed.

the N2 ¯ow, the sintering bed, etc. The relation-ships between rheology, casting kinetics and thecharacteristics of the green and the sintered sam-ples are discussed. The results presented here arealways related to the slip characteristics referredto in Part I.12

2 Experimental

The starting Si3N4 powder and the sintering aids(Al2O3 and Y2O3), were characterized in Part I, aswell as the de¯occulating conditions and the slippreparation methods. Compositions labelled asSN1, SN2 and SN3 consisted of pure Si3N4,Si3N4+3wt% Al2O3+3wt% Y2O3, and Si3N4+6wt% Al2O3+6wt% Y2O3, respectively. Aqueousslips of these compostions were prepared accordingto three mixing/milling procedures: A, using a highshear mixer; B, using a plastic ball mill; and C,using a centrifugal mill. The slips were preparedusing tetramethylammonium hydroxide (TMAH)as pH-adjuster to a solid content of 65wt%, whichrepresents volume contents of 36, 36.3 and 35.8%for SN1, SN2 and SN3 compositions. Someexperiments were carried out using a previouslyattrition milled Y2O3

Slip casting experiments were performed onplaster of Paris moulds prepared with a ratio ofplaster to water of 100/70 to obtain solid disks with3 cm in diameter, cylindrical bars with 0.6 cm indiameter and 8 cm in length or plates with 12�6�0.4 cm. The cast wall thickness was determined bymeasuring the time required to cast di�erent slipvolumes by ®ltering in one direction. Volumes upto 6 cm3 were cast for this test putting a non-porous open cylinder onto a plaster plate, whichproduced uniaxial ®ltering.Pressure slip casting was performed using a

laboratory uniaxial press (Gabbrielli, Italy) equippedwith both displacement and pressure transducers

and a computerized data acquisition system. Figure1 illustrates the pressure casting system used in thiswork. Although the system allows to operate undereither constant displacement or constant pressure,all the experiments were made at constant pressureconditions. Pressure casting was performedapplying di�erent pressures, usually around 1.7,3.4, 6.8 and 10.2MPa. Moulds consisted of poroussintered stainless steel disks with 4.5 cm internaldiameter, which were covered with ®lter paper forcasting. It has been proven that the permeabilityof the ®ltering system is two or three orders ofmagnitude higher than that of the formed cakeand hence, this can be neglected for kineticscalculations.The kinetics of the casting has been quanti®ed as

a ®ltration process on the basis of the Darcy'sequation, which allows to obtain the ¯ux of a ®l-trate volume through a porous bed. By applyingthe mass balance to the press system, and integrat-ing the Darcy's equation, the following equation isobtained relating the thickness of the cake (x) withcasting time (t):

�2 � 2k�p

�

�0�c ÿ �0

� �t

where k is the permeability of the porous bed, �the viscosity of the suspending media, p the appliedpressure, �0 the slip density and �c the cast density.The green cast samples were left in air 48 h for

drying and subsequently characterized. Densitieswere measured by Hg immersion. The as-cast den-sity was recalculated from the dry green densityconsidering the drying shrinkage.Surface oxidation state was evaluated by XPS

according to the procedure described in Part I forslip cast and pressure cast green samples. The ana-lysis were made in each case for samples takenfrom the bulk and samples taken from the externalsurfaces of the pieces, that is, those in contact withthe mould.Sintering experiments were made at 1750�C

under N2 ¯owing atmosphere introducing thesamples into a BN crucible containing di�erent bedmaterials, including powder of silicon nitride, siliconnitride bed for samples coated with BN spray, andmixtures of silicon nitride with the sintering aids inthe proportions corresponding to compositionsSN2 and SN3.Microstructural analysis was performed by opti-

cal and scanning electron microscopy on greenand sintered specimens in order to evaluate thehomogeneity of the casts obtained at di�erentconditions. The sample preparation for SEMobservations was also studied. Chemical etchingwas performed using an alkaline mixture at

Fig. 1. Schematic representation of the pressure castingequipment.

50 R. Moreno et al.

275�C for di�erent etching times. Plasma etchingwas also performed using CF4 on polished surfacesfor 40 s after treating the samples with hot N2 for10min.The behaviour of sintered samples was evaluated

measuring hardness and toughness at room tem-perature by Vickers indentation with applied loadsof 10±50 kg for 15 s.

3 Results and Discussion

3.1 Slip castingAs demonstrated in Part I, the pH value plays acritical role in the suspension stability and rheology.For this reason, the following results are referred toslips de¯occulated with TMAH at pH=111.3�0.2for any composition and mixing procedure.Figure 2 shows the cast wall thickness evolution

versus casting time of the three considered compo-sitions prepared according to mixing route B,which presented the best characteristics from arheological point of view. The casting rate of pureSN1 compositions is faster than that of slipscontaining sintering aids. This fact must be relatedwith the di�culties to maintain the desired pH.The long mixing time promotes adsorption of theTMAH but also some volatilization can occur. It

has been proven that when pH is readjusted beforecasting the resulting kinetics is similar to thatobtained for the other compositions.Similarly, Fig. 3 plots the slip casting curves of

the slips prepared by homogenizing route C (cen-trifugal milling). In this case, a quite similar kineticsis obtained for all three compositions. Comparingwith Fig. 2, it must be noted that the compositionwithout additives SN1 exhibits the same castingrate for both mixing routes, and the di�erences areonly observed in the slips containing sintering aids.These compositions behave similarly in each case,but the casting rate is much lower when slips areprepared by route B. This is associated with theshort mixing time in route C which is not enoughto assure the particles to reach an equilibrium atthe surface. The ®nal pH of the slips, just prior tocasting, is summarized in Table 1. Slips preparedby route C maintain a higher pH, thus suggestingthat adsorption occurs to a lower level. Taking intoaccount the strong dependency of viscosity withpH, it could be expected that slips prepared byroute C should present lower casting rates becauseof the higher pH. The fact that route B-slips havelower casting rates, even presenting lower pHsclearly demonstrates that a higher homogenizationdegree is achieved, which is in complete agreementwith the rheological behaviour described in Part I.Homogenization route A was not considered for

kinetics studies due to the lower reliability and theobservation of bubbles during casting.The green densities of the slip cast samples are

presented in Table 2. These values con®rm therheological and casting behaviour described before.Route B produces once more better bodies withdensities higher than those obtained by the othertwo routes. The maximum reachs a value of near59% th, which is quite satisfactory. The lowestdensities correspond to the slips prepared by routeC, as could be predicted from the rheological stu-dies shown in the ®rst part of this paper.The general morphology of samples cast from

slips prepared according to the di�erent mixingprocedures was observed by optical microscopy onpolished surfaces after sintering at 1750�C l h-1

under N2 ¯ow. Figure 4(a) shows a general pictureof a SN2 sample prepared by route A. As can beobserved, big holes due to air bubbles are always

Fig. 2. Slip casting kinetics of slips prepared according tomixing route B.

Fig. 3. Slip casting kinetics of slips prepared by mixing route C.

Table 1. Final pH values of slips after mixing according toroutes B and C

Composition Mixing route

B C

SN1 11 11.4SN2 11.2 11.2SN3 11.l 11.4

Colloidal ®ltration of silicon nitride aqueous slips, Part II. 51

present, as a consequence of the high mechanicalshearing. These bubbles could not be removed evenafter a further low speed agitation. This factexplains why route A was rejected for the obtention

of homogeneous, defect-free bodies. The sameaspect is observed for the other compositions pre-pared by shear mixing. The aspect of sintered castSN2 samples prepared by routes B and C can beseen in Fig. 4(b) and (c), respectively. As expectedfrom the rheology and from the casting kinetics,route B allows the obtention of more uniformmicrostructures with smaller pore sizes for anycomposition. In samples prepared by route Coccasional big pores can be also detected.

3.2 Pressure slip castingPressure slip casting experiments were performedfor the three compositions after preparation of theslips by routes B and C, since route A was observedto give heterogeneous microstructures withremaining big pores.The wall thickness fomiation rate is plotted in

Figs 5 and 6 for the slips prepared by route B andC, respectively for an applied pressure of 6.8MPa.The relative behaviour of the di�erent composi-tions for each mixing route is similar, the castingrate being lower when sintering aids are introduced,similarly to the slip casting behaviour. It can bestated the strong di�erence in the kinetics of slipcasting and pressure casting, in this case surpassingat least one order of magnitude. It can be observedonce more, that pressure casting rate is higher forslips prepared by route C, as observed for slipcasting, this being a consequence of the higherviscosity of the slips.During pressure casting, an experimental problem

is related to the fact that there is a certain time inwhich the piston has started the movement beforethe ®nal pressure is achieved. When the ®nal pres-sure is reached, ®ltering has started and a thin cakeis already formed. The uncontrolled ®rst step ofcasting can be the responsible for the di�erencesfound in the casting behaviour. If the values regis-tered before reaching the ®nal pressure areremoved, the casting kinetics at constant pressureconditions are identical for the three compositions.This can be observed in Fig. 7, which representsthe casting behaviour of the curves plotted in Fig. 5considering only the part in which the pressure is

Table 2. Green densities of slip cast samples

Mixing route A B C

Composition (g cmÿ3) (% th) (g cmÿ3) (% th) (g cmÿ3) (% th)

SN1 1.66�0.01 52.2�0.3 1.78�0.01 56.5�0.4 1.59�0.02 50.4�0.7SN2 1.72�0.01 53.0�0.3 1.91�0.01 58.8�0.1 1.69�0.02 51.0�0.2SN3 1.76�0.01 52.8�0.3 1.93�0.01 58.1�0.3 1.70�0.01 50.9�0.5SN2 - milled Y2O3 1.74�0.01 53.5�0.1 1.89�0.01 58.1�0.3 Ð Ð

Theoretical densities (g cmÿ3): SN1, 3.18; SN2, 3.25; SN3, 3.33.

Fig. 4. Optical micrographs of slip cast SN2 samples preparedaccording to mixing routes (A) A, (B) B, and (C) C after sin-

tering at 1750�C/1 h.

52 R. Moreno et al.

constant. The possible di�erences in the kineticscan be only associated to the ®rst seconds of theprocess, in which the control is very limited. ThepH of these suspensions was readjusted prior tocasting, to avoid pH shifting, which modi®es therheology and then, the casting behaviour.The green densities of the samples pressure

cast at 6.8MPa are shown in Table 3. Higherdensities are obtained for mixing route B, asexpected. However, the green densities are alwayslower than those obtained by slip casting, due tothe much higher formation rate which di�cultsthe arrangement of particles during forming.The e�ects of the magnitude of the applied

pressure on both the kinetics and the greenmicrostructure of SN2 compositions was describedin a previous work.13 The higher is the appliedpressure the faster is the wall thickness formationrate. This statement is also valid during all the cast-ing time, independently of the formation of the ®rstcake when the ®nal pressure is not yet reached. It canbe noticed that when the applied pressure is higherthe time required to reach the ®nal pressure is lower,and for all the pressures it was observed that theformed ®rst layer had the same thickness (1.8mm).The green density was very similar for any

applied pressure (1.8 g cm-3).From the casting kinetics at di�erent pressures,

the wall cast thickness can be represented as a

function of the applied pressures for constant cast-ing times, as plotted in Fig. 8. This plot is useful asa control in production cycles, where the feasibilityof this manufacturing technique is limited by thee�ective pressure that can be applied with theavailable press.All these experiments were made from the as-

received powders. However, the as-received Y2O3

powder had a mean particle size of 3.5�m, fourtimes higher than that of the other components inthe mixture, producing in some cases heterogeneitiesin the microstructure. Once the processing condi-tions facing the best rheological behaviour and themaximum casting control were established, pres-sure casting tests were also performed using a mil-led Y2O3 powder with a mean particle size of0.8�m. Figure 9 shows the evolution of castthickness with time at the four considered appliedpressures for SN2 slips prepared with the milledY2O3. The casting curves are quite similar to thoseobtained using the as-received yttria powder,13 butit can be detected a small tendency to increase thecasting rate when the milled powder is used. This isin good agreement with the small increase in visc-osity reported in the rheological considerations.The corresponding green densities are shown inTable 4, where the densities obtained for pressurecast samples formed from the as-received powdersare also included for comparison purposes. Whenmilled yttria is used, the green density of the cast isinversely related to the applied pressure. Amaximumrelative density of 57.1% is obtained for 1.7MPa,decreasing up to 55.0% for 10.2MPa. This is theexpected variation of density with pressure which

Fig. 7. Pressure casting kinetics of slips prepared by mixingroute C from the point in which the maximum pressure is

achieved.

Fig. 5. Pressure casting kinetics of slips prepared by mixingroute B.

Fig. 6. Pressure casting kinetics of slips prepared by mixingroute C.

Table 3. Green densities of pressure cast samples prepared bymixing routes B and C and pressed at 6.3MPa

Mixing route Density (g cmÿ3)

SN1 SN2 SN3

B 1.67�0.03 1.78�0.03 1.77�0.01C 1.59�0.03 1.66�0.02 1.70�0.02


contrasts with the abnormal independency found forthe mixtures prepared from as-received yttria, whereno clear tendencies could be stated. This is probablydue to microstructural unhomogeneities promotedby the coarse, agglomerated starting yttria.

3.3 Surface characterizationIn previous works14,15 it has been stated that theaddition of TMAH as dispersant in aqueous siliconnitride slip inhibits a progressive surface oxidationbecause of the formation of a protective partiallyoxydized screen. Surface analysis were also per-formed by XPS for pressure cast green samples andthe results compared with those obtained for slipcasting.The bending energy of core electrons determined

by XPS for SN2 green compacts obtained by bothcolloidal ®ltration techniques (slip casting, SC, andpressure slip casting, PSC), as well as the corre-sponding surface atomic ratios are summarized inTable 5. The analysis were performed indepen-dently on the bulk of green samples and on theexternal surface in contact with the moulds.In the case of slip casting, the presence of Ca2+

in the cast surface in contact with the plastermould was already detected in previous experi-ments.15 This contamination evidences a major

problem of slip casting as shaping technique, eitherin water or in nonaqueous solvents. However, thiscontamination is only detected in a thin layer of afew atoms and could be easily removed by simplemachining. The XPS spectrum of this external sur-face reveals that Si2p peak has a very low con-centration (6%) of oxidic particles (SiO2) with ahigh binding energy value (103.2 eV). The N/Siatomic ratio is abnormally low and the O/Si ratiois very high, which demonstrates that the contactwith the plaster mould not only produces calciumcontamination but also promotes a extremely highsurface oxidation. This is located as said before ina thin external layer and does not propagate to thebulk. On the other hand, the Al/Si and Y/Si atomicratios are much higher than those registered in thebulk of the samples, thus indicating that there is asigni®cative heterogeneity in the distribution of theoxidic sintering aids, whose concentrations increasemarkedly in the outer cast walls. This suggests thatthere is a migration or di�usion of these additivesduring forming, probably related to the particle¯ow during ®ltration process. On the other hand,the Al/Si and Y/Si ratios obtained for pressure castbodies in the external surface dupplicates thevalues registered in the bulk, suggesting also thatsome heterogeneity occurs, and those elementstend to migrate towards the external surface.Although this e�ect is very much reduced withrespect to slip cast bodies, it must be related notonly to interactions with the mould but also to theheterogeneity in the particle size.

Table 4. Green density of SN2 samples prepared by pressurecasting at ditterent pressures with as-received and with milled

yttria

Casting SN2 SN2-milledY2O3

pressure(MPa) �(g cmÿ3) �(% th) �(g cmÿ3) �(% th)

1.7 1.83�0.03 56.3 1.86�0.03 37.1

3.4 1.80�0.03 55.4 1.82�0.03 56.0

6.3 1.83�0.03 56.3 1.80�0.03 55.5

10.2 1.82�0.03 56.0 1.79�0.03 55.0

Fig. 8. Variation of the green body thickness with the appliedpressure for di�erent casting times of slip SN2.

Fig. 9. Pressure casting kinetics of SN2 slips prepared bymixing route B from previously attrition milled yttria, at dif-

ferent applied pressures

Table 5. Binding energy of core electrons and atomic ratiosdetermined by XPS for slip cast (SC) and pressure cast (PSC)

SN2 samples in the bulk and in the external surface

Sample N1s Si2p N/Si Al/Si Y/Si

Bulk-SC 397.6 (89)398.6 (11)

101.6 (65)102.3 (35)

1.34 0.020 0.026

Surface-SC 397.3 101.7 (94)103.2 (6)

0.929 0.299 0.094

Bulk-PSC 397.4 (86)398.4 (14)

101.5 (83)102.7 (17)

1.30 0.019 0.017

Surface-PSC 397.4 (90)398.5 (10)

101.6 (82)101.7 (18)

1.43 0.047 0.040

54 R. Moreno et al.

In order to clarify this point, XPS analysis werealso performed for slip cast green samples preparedusing the milled yttria. In this case, analysis weremade also on the bulk of the samples and in theexternal surface in contact with the mould. Thebending energy of the Nls and Si2p peaks and thecorresponding atomic ratios are shown in Table 6.The most important di�erences here are the Al/Siand Y/Si ratios, which are around 0.020 in the bulkof the samples and increases up to around 0.030 inthe external surface. This increased ratio is lowerthan in the case of pressure casting, whose atomicratios increase from around 0.018 in the bulk tomore than 0.040 in the surface, and very muchlower than in the slip casting performed with theas-received yttria.The caracterization of the green samples by XPS

allows the prediction of the uniformity of theadditives distribution in the sintered material. Theother processing parameters, such as powder char-acterization, rheological behaviour, casting kinet-ics, etc., did not give this kind of information. Thispoint will be discussed below in the light of micro-structural observations in the sintered compacts.Summarizing the results of the surface analysis, it

can be stated that slip casting provides a sig-ni®cant contamination to the external walls of thebody. The major advantage of pressure casting isusually related to the increased casting rates facingan improved productivity. But a second and alsoimportant advantage is that it could be designedthe system in such a way that the cast is not con-tamined by the mould and then the impurities andthe local oxidation at the external surfaces arestrongly reduced. In addition, this promotes amore homogeneous distribution of the sintering aids.

3.4 Sintering experimentsThe spontaneous tendency to decomposition ser-iously di�cults the pressureless sintering of siliconnitride parts. Many di�erent parameters in¯uencethe characteristics of the ®red compact. Betweenthem, the sintering temperature and soaking time,the heating and cooling rates, the N2 ¯owingpressure in the furnace chamber, the possibility ofapplicating vacuum in the ®rst steps and the natureof the sintering bed, have been proven to be

determining parameters in the ®nal results. In thiswork, all these parameters were taken into accountby testing at di�erent conditions slip cast sampleswith composition SN2. According to these pre-vious studies the sintering temperature and timewere ®xed at 1750�C/1±2 hÿ1, with a heating rate of10±20�C minÿ1, using an overpressure of N2 of 20kPa and applying vacuum up to 750�C in order topromote O2 elimination.The most signi®cative di�erences were found in

relation with the sintering bed employed for thetests. Four sintering beds were considered: (1)Si3N4 powder; (2) Si3N4 powder after coating thesample with spray of BN; (3) a previously homo-genized mixture of Si3N4 with Al2O3 and Y2O3 inthe same proportions that in the sample to be sin-tered (SN2 composition); and (4) a bed composedby a mixture of SN3 composition, that is, with aconcentration of oxides higher that the sample SN2to be sintered. The ®nal densities and the weightloss obtained at the same conditions as a functionof the sintering bed are shown in Table 7. A gen-eral view of the sintered materials can be seen inthe micrographs of Fig. 10. When Si3N4 powder isused as sintering bed, the ®nal density is low andthe weight loss is large. In the correspondingmicrograph (Fig. 10) the presence of a porous layernear the edge sample is clearly observed. This is aconsequence of the volatilization of Si3N4. When

Table 6. Binding energy and atomic ratios determined by XPSfor slip cast (SC) SN2 samples prepared with milled Y2O3; in

the bulk and in the external surface

Sample zone N1s Si2p N/Si Al/Si Y/Si

Bulk 397.5 (90)398.6 (86)

101.6 (86)102.7 (14)

1.30 0.021 0.019

Surface 397.5 (92)398.5 (8)

101.6 (87)102.7 (13)

1.448 0.032 0.028

Table 7. Density of sintered compacts obtained by slip castingusing the di�erent mixing routes

Sample Mixingroute A

Mixingroute B

Mixingroute C

SN2 3.09�0.01 3.18�0.03 3.11�0.02SN2-milled Y2O3 3.08�0.02 3.24�0.02 3.26�0.02SN3 3.13�0.03 3.15�0.01 3.16�0.03

Fig. 10. General picture showing the in¯uence of the sinteringbed on the external surfaces of samples SN2 sintered at

1750�C/2 h.


sintering was performed with samples coated byBN higher densities are obtained but the weightloss is also high. In the microstructure of this sam-ple [Fig. 10(b)] the porous layer near the surface isalso clearly observed. This could be related to thedi�erent shrinkage behaviour allowing the compactsurface to be directly exposed to the Si3N4 bedduring cooling, and the same aspect is seen in thepicture. When the bed has the same composition ofthe sample the density increases and the weight lossis reduced because the atmosphere around thesample is the same that into the sample, thusreducing di�usion and volatilization phenomena.In this case a more homogeneous microstructure isobtained without the porous external layer[Fig. 10(c)]. In the case of a bed of SN3 composition[Fig. 10(d)] the picture is similar but the density isreduced. The problem in this case is the stronginteraction between the sample and the bed.Independently of the sintering bed, the presence

of bright precipitates near the external surfaces isobserved in all cases. This is probably originated asa consequence of local changes in the liquid phasecomposition during sintering. This e�ect is notonly observed in slip cast samples but also in pres-sure cast ones, as can be observed in Fig. 11, where

the microstructures of bulk and edge areas ofpressure cast SN2 samples are shown. Conse-quently it cannot be related to contaminationpromoted by plaster moulds, When milled yttria isemployed a more homogeneous microstructure isobtained and these precipitates are still present butat lower concentrations. Figure 12 shows themicrostructure of the edge of a sintered SN2 slipcast specimen prepared with milled yttria. Theseresults are in good agreement with the previouslyreported surface analysis, where the heterogeneousdistribution of the sintering aids was detectedbefore sintering.The ®nal densities of the sintered slip cast com-

pacts obtained for SN2 and SN3 compositionswith the as-received sintering aids and for SN2prepared with milled yttria are shown in Table 7.The values are in line with all previous processingsteps. Mixing procedure B gave lower viscositiesand lower casting rates and hence, the ®nal den-sities are higher. The ®nal value is enhanced whenyttria is previously milled, as expected from theXPS and microstructural analysis. In the case ofpressure casting the ®nal densities are lower than inslip casting, as shown in Table 8, but no signi®cantdi�erences in density were found for the di�erent

Fig. 11. Microstructure of (A) the centre and (B) the edge of apressure cast, sintered SN2 sample (using the as-received yttria).

Fig. 12. Optical micrograph of the edge of a slip cast SN2sample prepared with milled yttria.

Table 8. Density, shrinkage and weight loss of sintered com-pacts obtained by pressure casting using di�erent pressures

Sample Pressure(MPa)

Density(g cmÿ3)

Linearshrinkage

(%)

Weightloss(%)

SN2 1.7 3.14�0.02 Ð Ð3.4 3.18�0.01 14.19 2.106.3 3.17�0.01 16.43 2.2110.2 3.16�0.02 18.20 1.84

SN2-milled Y2O3 1.7 3.11�0.03 18.79 3.393.4 3.10�0.02 18.94 3.386.3 3.12�0.01 19.70 3.7310.2 3.13�0.02 21.52 3.38

56 R. Moreno et al.

applied pressures. On the other hand, and oppo-sitely to that found for slip casting, the use of milledyttria produces a decrease in the sintered densities.Another problem of silicon nitride materials is

their preparation for SEM observations. In thiswork chemical etching was performed at 275�Cusing an alkaline mixture. The best grains de®nitionwas achieved after 25±30 min etching. Figure 13shows the microstructures obtained by SEM onsintered and etched surfaces for slip cast [Fig. 13(a)]and pressure cast [Fig. 13(b)] SN2 compacts. Someagglomerates can be detected, specially in the caseof pressure casting. The homogeneity is improvedusing milled yttria, as can be observed in theSEM micrographs of Fig. 14, corresponding toslip cast [Fig. 14(a)] and pressure cast [Fig. 14(b)]specimens.After chemical etching liquid phases located at

grain boundaries are eliminated. The silicon nitridecrystals present a bimodal microstructure with amajor presence of rod-like grains, typical of �-Si3N4,

Fig. 13. SEM micrographs of (a) slip cast and (b) pressure castSN2 samples sintered at 1750�C/2 h.

Fig. 15. SEM micrographs of (a) slip cast and (b) pressure castSN2 samples sintered at 1750�C/2 h, after plasma etching.

Fig. 14. SEM micrographs of (a) slip cast and (b) pressure castSN2 samples sintered at 1750�C/2 h, obtained using the milled

yttria powder.


as it was identi®ed by X-ray di�raction. The inter-granular phases can be better visualized inFig. 15(a) and (b), where the microstructure of slipcast and pressure cast samples treated by plasmaetching with CF4 are observed.In order to evaluate the properties of the

obtained sintered materials, room temperatureYoung's modulus and toughness were determined,as summarized in Tables 9 and 10. These propertieswere determined by microindentation tests for anapplied load of 50 kg. The ratio between thecrack length (c) and the indenter diameter (a)was c/a>2.5, as desired for indentation tests.Taking into account that the studied compositionswere selected for adjusting the processing para-meters the obtained values are high enough andcan be compared with the obtained by other authorsby pressure sintering. On the other hand, nosigni®cative di�erences are found between the dif-ferent forming processes, and between the di�erentapplied pressures in pressure casting. However,there are di�erences as demonstrated by surface andmicroscopical analysis. These di�erences are notdetected in the E and H room temperature values,but probably these properties are a�ected at highertemperatures by processing conditions.

4 Summary

In this series of works all the processing stepsfor the obtention of pressureless sintered siliconnitride parts by colloidal ®ltration techniques havebeen studied, relating any processing step withprevious and next processing steps. As demon-strated along the paper the best rheological prop-

erties are obtained for prolongated low energeticalmixing of 65wt% solid loadings at pH=l1.5 in aball mill (24 h), which produce lower castingrates and subsequently, higher green densities (near60% th). Surface analysis by XPS and micro-structural observations con®rmed all the proces-sing parameters. Slip casting moulds produce asigni®cant Ca contamination and promotes a notice-able surface oxidation in a thin external layer. Theuse of ®ne grained sintering aids tend to reduce thise�ect but it does not completely disappear. On theother hand, pressure casting kinetics is 20 timesfaster than slip casting one, then allowing theprocess scale-up to industrial productions. Thedecrease in the green density can be neglected whenconsidering the technological improvements ofthe technique for a continuous production. Fur-thermore, the use of pressure cast avoids the con-tamination derived from the plaster moulds andthe subsequent surface oxidation degree.Finally, the controlled slip processing studied in

this work allows the obtention of pressureless sin-tered silicon nitride with relative densities higherthan 96% th, and good values of hardness andtoughness are measured, very near to those repor-ted for pressure sintered parts.Obviously, the starting compositions were not

selected facing the improvement of any particularproperty, but the controlled sequency of processingsteps stated along the paper allow us to design andprepare any composition of this system searchingthe enhancement of a determined property.

Acknowledgements

This work has been supported by CICYT (Spain)under contract no. MAT97-0676, and a bilateralproject Spain±Italy (H194-066) The authors thanktoo Professor J. L. G. Fierro (Instituto de CataÂ lisisy PetroleoquõÂmica, CSIC, Madrid) for XPS mea-surements and discussions and Professor M. Reece(Queen Mary College, London) for plasma etching.S. M. C. acknowledges RHAE-CPq (Brazil) for theconcession of a grant.

References

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Table 10. Hardness and toughness of sintered pressure castcompacts

Composition SN2 SN2-milledY2O3

Pressure(MPa)

Hv

(GPa)KIC

(MPam1/2)Hv

(GPa)KIC

(MPam1/2)

1.7 12.4�0.3 6.2�0.2 12.4�0.2 5.9�0.33.4 13.6�0.5 6.4�0.l 13.0�0.7 6.3�0.15.4 12.9�0.5 6.3�0.1 Ð Ð6.3 Ð Ð 13.0�0.2 6.7�0.110.2 13.3�0.4 5.8�0.4 13.9�0.7 6.2�0.1

Table 9. Hardness and toughness of sintered slip cast compacts

Composition Hv (GPa) KIc(MPam1/2)

SN2 12.8�0.3 6.4�0.1SN2-milled Y2O3 12.2�0.3 6.2�0.1SN3 12.6�0.3 6.2�0.3SN3-milled Y2O3 14.1�0.3 6.6�0.1

58 R. Moreno et al.

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colloidal filtration of silicon nitride aqueous slips ... · non-oxide ceramics and, in particular,...

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