QUALI ~ CASTELLl) 'J ISpA'''') ::'.•••" 96

THE CHARACTERIZATION OF PLASTICITY AND OBSERVATIONS ONAGING

William M. Carty

New York Stale Center for Advanced Technology and the Whiteware Research Center Dep artment of Ceramic Engineerin g and Scien ces New York State College of at Alfred University Alfred , NY 14802, U.S.A.

Abstract

The plasti city of a ceramic form ing bod y is critical to controlling the fabrication process. 1\ new technique has bee n developed to qu antitati vely measure plasticity using high pressure shear rh eom etry. The result s from the plasticity measurement s have also been correlated with plasti c forming processes. This dat a will be presented and observa tio ns on th e agin g of -based systems will also be discu ssed .

tntrodu ction

Plasti city is undoubtedly one of the most poorly underst ood properties of a clay body. It co u ld be argued that there are two types of plasticity : good and bad . If a material works within a specific process, the plasticity is perceived as good. If the mater ial does no t fun cti on . or creates defects in the produc t during the forming process. the pla sticity must be bad . Th e major problem lies in the fact that plastic masses are fricti on al so lids. placing them outside of the suspe nsion rheology co ncepts (for flu id materials). and thus closer to the range of ma terials ad dres sed by soil mech ani cs.

In eva luatio n of ma ter ial remaining in a extrusion mill. lead co ns isten tly to th e co ncl usion tha t the material failed in shea r under cons iderable pressure during th e forming process. Th e pressure for extrusion was provided by th e crew. and that material underwent shear-ty pe failure in order to extrude . This analys is provided an altern ative appro ach for the chrac terizatio n of plasticity. one which required th at the shear behavior be measured under pressure. In addition. becau se it appea red to be necessary fot the samples to be tested at eleva ted pressures. unconfined testing proced ures (i.e.. the Atterburg of Pferfferkorn tests. as well as torsion or mech ani cal tests such as those wich measu re the res istance to flow)

- 349 - QUALicav06 CASTEL L(l N (SPAIN) would not be applicab le. Other testing procedures, suc h as evalua ting the extrusion of samples. would be limited by whether or not the samples could be extruded - no extru sion, no data. A critical review of plasti cit y testing is currently awa iting publication. [11

In or der to better quantify the plasticity in ceramic particle systems, a new techniqu e has been developed using high pressure shear rheometry. This technique allo ws th e shear beh avior of samples to be mea sured as a fun ction of pressure, independent of the forming process. The range of operation of the material can be determied by evaluation g the test samples with the fabrication technique for whi ch the mat erial is int ended . One of the primary advantages of thi s technique is that it is independent ofthe fabri cation process, an d therefore does not require th e sample to actua lly be useable in order to be tested.

Concepts of the testing approach

Design of the high pressure shear rheom eter

The high pressure shear rheometer, referred to as the High Press ure An nular Shear Cell, or HPASC, is illustrated schematically in Figure 1. The instrument cons ists of an annular testing chamber (Figure 2a) a hydraulic loading a p p ara tus, and a va riab le rotati on sy s te m . Load Cell Measurements are tak en electronica ll y a n d c o llec te d v ia a n e lectron ic interfa ce in a sprea ds heet. The co n fining an d co mpaction pressure is ap plied via a hydraulic pr essure amplifier, in the form of a lever arm and weights, as pictured in Figure 2b. Because the instrument is still under eva luation, the testing procedure takes betw een one and two hours to complete.

T h e underlying princip le for the characterization of pla sticity using the HPASC stems from th e concept of a «Bingham-type» fluid , which is a Newtonian-like fluid with a yield stress (Figure 3). Sin ce plasti c masses are granular masses , (also referred to as fricti onal solids ) their behavior is highly pressure depen dent and thus th e Bingham yie ld stre ss will in cr ea se w it h increasing pressure, as illu strated in Figure 4. Rotation System From this perspecti ve, it is easy to recognize that the pressure dep enden ce of samples changes as Figure 1. Sc he matic ill ustrati on of the High ...... Pressure Ann ular Shear Cell. a function of water content, as illustrated in Figure 5. In th e wet case , water fills the pores (i.o., the system is saturated) and increasing pressure has little effect on the shear stre ngth of the mass. Since wat er is incompressible, an isostatic pressure case results and th ere is little change in the parti cle-particle int eracti ons. In the dry case , a sma ll in crease in pressure resu lts in a large increase in th e shear strength due to a correspon ding increase in the friction between the particles. Somewhere between these two extremes lies plasti c behavior. In addition , it is evident from Figure 5 that there is in finit e ran ge of possible behavio rs.

The yield stress val ue s, when plotted as a function of pressure (illustrated in Figure 6a), reduce the data to a usab le form , and provide a quan titative characterization of plasticity. The slope (representing the pressure dependence, or sens itivity of the sample shear strength

· 350 · r.ASTELL 6~ (S PA I~ ) ~=~ QUAL I ~ 96 I!••

a} El'I.1BED \\'nrd.Picturn.6

b) Figure 2: laJ:\ photograph of the HI'ASC tes ting fixture w ith the tes ting chamber as il lustrated sr.humat ically in Figure 1. (II) Photographs of the three com ponents of HPASC instrument: (from left to rig ht) (j) the HPASC testing frame. Iii) data collection unit and computer terminal . and (i ii ) dead weight loading arm syste m with hydrau lic amplifier.

to the applied pressure) and the int ercept (or co hesion. representing a critical shear stress for zero shear rat e and zero pressure) are calc ulated from th e data and used as the two parameters of pla sti city. Th e intercept (on the y-ax is) is then pl ott ed as a fun ction of the s lope (on the x-axis) to create a two dimen sional surface (Figure Gh) which can he used to

rn rn Q) U'l9'\a "Tl ~ B INeYo1 oman Wit' a ylel::! st-ess] (j)

T 1. ! '1

Shear Rate

Figure 3. Schematic ill us trati on of Newtonian-Hke and Bingham Iluid behav iors.

· 351 - QUALI ~96 •• :. CASTELLO"l ISPAI"I)

en en PIS) Gl ~ iii P(4) ~ ., P(3) ..c:Gl

P( 1)

Shear Rate

Figure 4. Schematic illustration of the effect of pressure on a Bingham fluid. describe a broad range of plastic beh aviors. Using the process (such as extrusion) to screen the samples . the ran ge of plasti city necessary for th at process can then be defined . as represented by the region marked «extrudable range.» (Figure fib is only a cartoo n of the concept - this region mu st be defined and may not necessarily follow th is shape.)

To move around on the plasticity diagram requires changes in the slope and in tercept.

Dry

Plastic

Wet

Applied Pressure

Figure 5. Cartoon .depic ting the range of plasti c behaviors.

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Figure 7 illustrates that samples A and B have identical intercepts but different slopes. causing them to be located on a horizontal line in Figure 7b. Samples Band C have the sa me slope. but different in tercep ts. locating these samples on a verti cal line in th e plast icit y diagram (B b).

Intercept Exp e r im en ta l Approach Applied Pressure Testing procedure

Between 75 and 100 grams of sa mple are loaded into the test fixture. Th e load is first

A variety of samples were prepared using a mix-muller for mixing. Other samples have been prepared using a slurry approach meth od , followed by filter pressing. Som e samples have also been prepared via spray drying, but only th e da ta from th e mi x-mulling samples will be presented at thi s tim e.

Th e initial studies cons isted of the testing of a comme rcia lly availa ble (in the Un ited Stat es) kaoli n and ball clay (a pr imary clay cons isting of al most pure kao linite. and a secondary cla y with the major impu rity being qu artz, respectively). The clay samples were mixed with de-ionized water. a sample removed. and ad ditional water adde d, to provide a

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Number represerrts %water conlent based on /be dry I'reight of clay. ra ng e of sam ples with varying w at er 1oo0 ,--,------,- --,----r-- --,------, contents. Knowing that th e water conten t c -+- 13 (. 75" 0) can dramatically alter the properties of a -+-20 (=65vlo) clay-water mixture, this procedure allowed - '" ·25 (.60,101 the tren ds and contribution of water to a

~ clay sample to be evaluated without having a.~ to worry about achieving or maintaining ~ 500 exactly the same water conten ts. Samples o were dri ed pri or to testing to determine th e P exac t water content of the test specime ns . r _._!J. • .g_. ------_.-:J Samples for aging were prepared, .., marked , and sea led in air-tight containe rs. Multiple containe rs were used to eliminate J5----'-10----'-15--'-20-~ 25 --'---30 -35-'--40 va ri ations introduced by o pen ing the Applied Pressure (bar) sample containers to withdrawn samples.

Figure 8. Yield stress versus pressure for three ball clay­ To evaluate the contribution of non­ water mixtures. plastics (i.e., feldsp ar and quartz) to th e plasticity, a st andard batch w as a ls o prepared in an identical fashion usi ng th e mix-muller.

Results and Discussion

Figure 8 shows the yield stress versus applie d pressure dat a for samples of ball clay con taining three water contents. Th ese represent examples of wet, plastic, and dry, demonstrating that the concept appears to be valid and that th e data stro ngly reflects th e cartoon illustration depicted in Figure 5. It shou ld also be noted the extremely high degree r.;d.•. of linearity in the data, helping to keep the Number rep""'t' ..'er cool en! b.) evaluations objective. ,'" I

Figure 9 sho ws the variation between 1 plasticity obtained from kaolin, ball clay, a mi xture of two parts ball clay to one part kaolin, and to the standard triaxial batch. This plot demonstrates severa l interesting results. First, it is clear th at differen ces between seco ndar y clays and primary clays are easily dis cernible and can eve n be distinguished from a mixture of the two cla ys . Second, the wa ter con ten t necessary to reach a maximum cohesi on is highest for the secondary cla y, lowest for the 10 15 20 25 30 35 40 primary clay, falls in between the two for the Applied Pressure Dependence mixture. This value should represent th e saturation of the body with water, and may reflect the water of plasticity necessary to Figure 9. Plasticity of ball cla y ( O ~1 #4 J . kao lin {EPK}. a 2:1 provide plastici ty to the material. In ad dition, mixture (ball clay.kaolin). and standard po rcelain batch (23 % quartz. 33 % fe ldspar, an d 44 % ba ll clay. kaolin the wat er co n tent required for maximum mixtu re) showing the de pende nce of plasticity on water cohesion should correspond to the specific content (deno ted by the values on the plot nex t to data sur face areas for the clays and as illustrated points).

- 3S4- CASTELLON (SPA IN) dit QUAlICQJu 96

8a ll Ctay @22%De ionized Water (d.w.b. of clay) in Table I, the ba ll clay has a higher specific ~ Internal Friction -e-Cohesiveness sur face area co mpared to the kaolin. It 11 ~,~~~~~~~~ -r-' ~'--'-' 110 ~ sho uld also be noted that the pressure dependence decreases with water content . ~ ~ as is predicted by th e overall conc ept and ] that as the satura tion poin t is exceeded , the j 10 · - -- - J:.--- 90 g pressu re d epende n c e b e come s more se ns itive to wa ter content. ~ I Ul... o ­CD (/J :J When evaluating th e co ntribution of 9 •~------, 70 ~ th e n on- plas tics , it is evi d e n t tha t , ~ j ~ significantly higher cohesion stresses are obtained at lower water co n te n ts than JL....~~~ "--,--~--,---"-----,--,---,~150 ~ require d by a clay-water mi xture by itself, Two factors are at work: (i) th e specific o 5 10 15 w surface area of the mi xture is signi ficantly AgingTime(Day) reduced by th e substitution of th e coa rser particl e size non-p lasti c fracti on s; and (ii) the particle size distribution of the bod y Figure 10. Plastici ty as a function of tim e for mixture of ball­ h as been dramatically a ltered by the clay and 22% de-io nized \vater (on a dry we ight bas is). introducti on of th e non-plastics. With regards to the secon d p oin t, the introducti on of larger particle size material significantly improves th e packing behavior of the body, resulting in increased particle-pa rticle contacts, and grea ter cohesion stres ses, The reduction in surface area resu lts in less water being necessary before the bod y saturates.

Figures 10 and 11 dep ict the change in plasticity of a clay-water mixture and of the standard compos itio n, respectively, as a funct ion of tim e. Both of thes e sam ples were prepared w it hou t the use of any d is pers an ts or su r factan ts a n d -&-Internal Friction -+-Cohesiveness represent the flocculated sta te of the o samples in de-ionized water. It is o evi d e n t from th e data that th e ~ 3.5 Q properties of the sam ple s cha nge VI dr amatica lly with time and that the o • 60 most subs tan tial change occurs in the III 3 first day or severa l hours. In add itio n, a. 37% Sal/clay 1__ 1 ~ o I 16%Kaolin it is apparent that th e non-plastic 22% a d d it io ns a ccele ra te th e aging (J) 2.5 25%Silica I ~ process, requ iring sign ifica ntly less 22% Deionized water 40 (d. w.b. ofsolids) ~ tim e to reach a stea dy-state condition I 1 2 '------than that required for the clay mixture by itself. o, Summary and Conclusions 24 48 72 96 120 Agi ngTime (HR) A new technique for evaluating plasticity has been develo ped which is i nde p e nde n t of t h e forming Figure 11. Plasticity as a function of time the st andard porcela in batch (23% qu artz. 33% feld spa r. and 44% ball cla v:kaolin mi xture). (Note: S lope = internal fri c tio n : a n d Cohe-s ian streng th = cohesive ne ss).

- 3SS . CASTEL L6~ (S PAl~ ) process. This technique, using high pres sure shear rheometry, allows plasticity to be quantitatively evaluated and characterized. From the results presen ted , it was possible to demonstrate the differences between a primary clay and a secondary clay, to distinguish between mixtures of the two, and to identify the water content necessary for the highest cohesion. In addition , the role of the non-plastic additions (feldspar and quartz) in the plasticity of a clay body to be measured, showing that significantly lower water contents are required to obtain high cohesive stresses. Finally, the impact of time on the plasticity of a clay-based system has been, for the first time, quantitatively determ ined.

References 1. Nowak, P, and Carty, w. , «A Critical Review of Plasticity Charac teriza tion, » Proceedings of the Science Technology and Commercializa tion of Powder Synthesis and Shape Forming Processes Symposium , #SXIX, Cincinnati, OH, 1994, American Ceramic Society, Westerville, OH, (awaiting publicati on).

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