Introduction

The role of advanced ceramics inengineering structures largelydepends on the possibility of reliablemass production of complex-shapedcomponents at acceptably low costs.Because of the near-net-shape pro-duction and the economic efficiencyin large series powder injectionmoulding (PIM) is the shaping tech-nique of choice for metal/ceramicparts of complex geometry [1]. Theprocess is capable of providing notonly shape complexity but also highprecision and high performanceproperties in the sintered parts [2].Tolerances of 0,3 % or better aregenerally claimed by PIM producers.The concept of the PIM technique isbased on mixing the ceramic pow-der with a liquid binder system (usu-ally a blend of molten polymers) tocreate a viscous feedstock, which isgranulated, cooled and fed into aninjection moulding machine. Themoulding mix is reheated to a highviscosity fluid state and is injectedunder high pressure into a die cavi-ty. The moulded part is removedafter the moulding mix has solidifiedin the die. The organic binder is thenremoved from the component attemperatures up to 500 °C, general-ly by some combination of wickingand thermal decomposition, solventdebinding, catalytic debinding orother. Finally, the component is sin-tered to obtain its final ceramicproperties [3]. Examples of compo-nents currently being producedcommercially include ceramic tur-bocharger and radial rotors, aluminathread guides, and rare earth mag-net pole pieces for hard disk drives.In the recent years following trendscould be observed in ceramic injec-tion moulding:(1) Development of multifunctional

ceramic components with a highdegree of complexity by two-component ceramic injectionmoulding (2C-CIM);

(2) Combination of contradictorymaterial properties in one ceram-ic part without additional joiningsteps; production of ceram-ic/ceramic- and ceramic/metal-compounds without additionaljoining by 2C-CIM and

(3) Reduction of processing costs,scrap-rate and tolerances such asan increase of accuracy.

First ideas and patents concerningco-injection moulding of two syn-thetic materials appeared 40 yearsago [4]. To the first applications oftwo-colour injection mouldingbelong buttons with abrasion-resis-tant symbols for computer key-boards or telephones [5]. Indeed,applications from the automotivebranch show that co-injectionmoulding is applied to a great vari-ety of automotive components. Bythis technique not only differentcolours but also thermal, mechani-cal, electrical and optical propertiesof plastics can be combined in oneprocessing step [5]. Since sintering isperformed at temperatures wherediffusion bonding is possible, a goalin PIM has been to form greenassemblies (by joining prior to sin-tering) and use the sintering step fordiffusion bonding [6].The ability to manufacture and sur-face engineer a component in a sin-gle process has attractive implica-tions, both technically and financial-ly, particularly when large numbersof complex shaped components areto be produced [7]. First researchresults have shown that ceramicmaterials can be combined in onepart by 2C-CIM [7-9]. Followingmodel systems have been produced:(1) a component with alumina skinand core, possessing a 0,5 µm parti-cle size skin layer surrounding a 1 µmparticulate core [7], (2) toughenedcomponents with a skin containing20 vol.-% partially stabilised zirconiasurrounding a 100 % alumina core[8], and (3) a ceramic heater consist-ing of different Al2O3/TiN-mixedceramics [9]. Moreover, the combi-nation of different ceramic materialsand ceramics with metals by two-component injection moulding isdescribed in [10]. Cai et al. [11] stud-ied the co-sintering of alumina andzirconia starting from tape-castgreen sheets. All studies on 2C-CIMhave disclosed that sintering ratecontrol is crucial to the success ofthis shaping method. Both compo-nents must sinter at similar rates andat similar positions in the sinteringtemperature profile to avoid delami-

A. Mannschatz, T. MoritzFraunhofer- IKTSDresdenwww.ikts.fraunhofer.de

cfi/Ber. DKG 86 (2009) No. 4 E 25

nations. The sintering behaviour canbe altered by lowering the powdercontent of one mix and hence itsgreen density, at the risk of a porouscomponent or by addition of a sec-ond non-sintering composite phase[7]. The variety and combination ofmaterials that can be used in themanufacture of metal or ceramiccomponents by powder co-injectionmoulding is more limited due to therequirement for compatible sinter-ing characteristics [7]. Dependingon the size of the contact area ofboth ceramic materials the feedstockcomponents can be injected simul-taneously or sequentially. Since September 2006 fourteenpartners from industry and RTD fromseven European countries have beendeveloping ceramic components forautomotive and railway applicationsby two-component ceramic injec-

Process Engineering

Challenges in Two-ComponentCeramic Injection Moulding

Fig. 1 Gear wheel in greenstage with sprue;perpendicular to thegates two weld linesform

100 1000 10000

10

100

1000

vis

cosity [

Pa s

]

shear rate [1/s]

140 C 150 C 160 C

Fig. 2 Rheological behav-iour of the aluminafeedstock at differ-ent temperatures

(1) ceramic glow plug, (2) ceramic gear wheel, (3) ceramic valve seat, and (4) ceramic braking pads for high

speed trains [12]. This article deals with one of the casestudy components of the CarCIMproject, the gear wheel, for showingexemplarily the requirements whichmust be met by producers of multicomponent ceramic parts for avoid-ing cracks, delaminations or distor-tion of the products. For that purpose the development of thistwo-component ceramic part begin-ning with the choice of the materialcombination is described. Forachieving a prototypical functionalceramic part a gear wheel had beenspecified by Robert Bosch GmbHwhich shall consist of an outerthough ring and an inner gear ringwith high hardness. Two materialcombinations, alumina toughenedzirconia (ATZ)/zirconia toughenedalumina (ZTA) and alumina/ZTA,had been taken into considerationand will be compared in this article.A possible application of this speci-fied component could be fluid orfuel pumps.

ExperimentalThe two material combinations havebeen realized with three ceramics. The powder CT3000SG (Almatis)was chosen for the alumina compo-nent. It was also used as basic mate-rial for the ZTA which was formed byaddition of 7 mass% nanostructuredzirconia (Evonik Degussa GmbH).The ATZ component consists ofPYT05.0-005H (Unitec Ceramics) zir-conia powder and 7 mass% finegrained alumina powder (Baikowski)(Tab. 1).The feedstocks were prepared on ashear roller (BSW 135-1000, Bel-laform) using the binder systemLicomont® EK583 (Clariant) plusadditives for enhanced wettingbehaviour. In order to ensure goodhomogenisation the feedstockspassed the shear roller three times.The raw materials of the particletoughened ceramics were mixed in-situ during feedstock preparation. For each of the material combina-tions one feedstock couple was

developed having a solids loading of56,5 vol-% and 55 vol-% forATZ/ZTA and ZTA/Al2O3, respective-ly. Flowability was tested by highpressure capillary rheometry (RH10,Malvern Instruments).Two-component injection mouldingwas performed on a two-compo-nent injection moulding machine(320S, Arburg). The two-compo-nent part consists of two concentricrings whereas the inner ring holdsthe gear teeth (fig.1). The tool workswith rotary plate technique (tooldesign: Fotec GmbH, tool construc-tion: Ernst Wittner GmbH). In a firststep the inner component is inject-ed, afterwards the tool opens andturns by 180°. Thereby the first com-ponent is transferred into the secondcavity and in the second step theouter component is injected. Toshorten flow paths the componentsare filled through two gates. Perpen-dicular to the gates the feedstockfronts meet and form two weld lines.Within this study three types of gearwheels were produced for which theouter and the inner component willbe marked with (o) and (i), respec-tively: (1) ATZ(o)/ZTA(i),

(2) ZTA(o)/Al2O3(i) and (3) Al2O3(o)/ZTA(i).

Green parts were inspected by X-raycomputed tomography (CT Com-pact, Fraunhofer Development Cen-tre X-Ray Technique Fürth and Pro-con X-ray) to detect injectionmoulding defects and characterizethe interface. The binder system contains a watersoluble component which allowstwo-stage debinding. Prior to ther-mal treatment, pores are created byextraction debinding that supportthe transport of reaction products inthe later thermal decomposition. Intwo-component parts swelling ofbinder molecules might damage theinterface of the green composites.Therefore the samples were onlysubjected to thermal debinding inair up to 450 °C. The debindered samples were sin-tered at 1620 °C for 4 h. Linearshrinkage of the individual feed-stocks was determined using one-component injection moulded barshaped samples (3,5 x 7 x 70 mm³)in order to avoid any interactionwith a second component. Sinteringbehaviour and thermal expansionwas studied by dilatometry ondebindered injection moulded sam-ples at temperatures up to 1550 °C.Roundness of green and sinteredbodies was tested by 3D-coordinatemeasuring technique.

Fig. 3 Computer tomo-graphic image of agreen two-compo-nent gear wheel

E 26 cfi/Ber. DKG 86 (2009) No. 4

tion ceramic moulding (2C-CIM) inthe frame of the European STREPproject “CarCIM”. The project willresult in four 2C-CIM prototypeparts integrated in systems for func-tional testing and verification such asfor techno-economical assessmentof the complete processing chainwhich will be developed in four par-allel case studies:

Process Engineering

0 200 400 600 800 1000 1200 1400 1600

-12

-10

-8

-6

-4

-2

0

2

Al2O3 ZTA ATZ

shrinkage [

%]

temperature [ C]

0 200 400 600 800 1000 1200 1400 1600

-13,0

-12,8

-12,6

-12,4

-12,2

-12,0

-11,8

-11,6

-11,4

-11,2

co

ntr

actio

n [

%]

temperature [ C]

ATZ

ZTA

Al2O

3

Fig. 4 Sintering curves of alumina, ATZ and ZTA measured ondebindered injection moulded samples

Fig. 5 Thermal contraction during cooling

Powder D50 [ m] BET [m /g]

PYT05.0-005H (UnitecCeramics) 1,0 4,3

CT3000SG (Almatis) 0,8 6,5

n-ZrO2 (Evonik-Degussa) 0,02 43,3

Al2O3 (Baikowski) 0,14 19,9

Tab.1Powder properties

Results and Discussion

In 2C-CIM the two materials arejoined in green stage and the finalcomposite is formed during co-sin-tering by diffusion bonding at hightemperatures. This means that thepartners have to be processed simul-taneously and sintering behaviour,thermal expansion as well as finalshrinkage have to be considered formaterial choice.Final shrinkage is defined by the par-ticle packing density of the greenbody and the achieved sinteringdensity. In powder injection mould-ing, the design of the feedstockoffers the opportunity to affect thegreen density by modifying thepowder-binder-ratio. In terms offlowability and debinding behaviourthe optimal binder content is deter-mined by the powder properties likeparticle size and surface since thepowder particle surface has to becoated completely. While lacking ofbinder decreases injectability excessbinder leads to powder binder sepa-rations. Therefore, owing to the dif-fering powder particle sizes for eachfeedstock couple the binder andadditive composition was adaptedindividually to result in equal solidsloadings. High pressure capillaryrheometry showed that all devel-oped feedstocks were suited forinjection moulding since their vis-cosities were well below 1000 Pas[13]. Because of the shear thinningbehaviour the viscosity decreaseswith increasing shear rate, and athigher temperatures the viscositydecreases due to improved flowabil-ity of the binder components (Fig. 2). The injection mouldedgreen gear wheels were flawless forall feedstock combinations. Non-destructive testing by X-raycomputed tomography showed thatno injection moulding defects likebubbles appeared and that the firstcomponent was not damaged byinjecting the second feedstock (Fig.3). The interface could be estab-lished defect-free without stress-induced cracks. In two-componentinjection moulding, stresses mightarise between the two componentsif the feedstocks undergo differingthermal contraction while coolingfrom processing to room tempera-ture or if the temperature differencebeween first and second componentis too high in the moment of joining.This effect can be limited by usingthe same binder system.By adapting the solids loadingsshrinkage adjustment was accom-

plished. According to the higherbinder content the shrinkage ofZTA/Al2O3 was higher (ZTA: 16,8 %,Al2O3: 16,3 %) than for ATZ/ZTA(ATZ: 15,7 %, ZTA: 15,4 %).Besides the final shrinkage sinteringkinetics play an important role.Dilatometric measurements revealthat the used materials start to den-sify at differing temperatures (Fig.4). By addition of zirconia the sintertemperature of the alumina is shiftedtowards higher temperatures sincethe ZrO2 particles hinder graingrowth in ZTA. The onset tempera-ture moves from 1150 °C to 1280 °Cand the maximum shrinkage differ-ence is 3,9 % in Al2O3/ZTA. For thecombination ATZ/ZTA the onsettemperatures vary by 80 K and themaximum shrinkage difference of3.0 % is slightly lower. The shrinkagemismatch can cause stressesbetween the two materials which isespecially critical for the less sinteractive component, since it is still ininitial sintered or even debinderedstate and only weak particle interac-tions contribute to the strength.After sintering the parts cool downto room temperature and shrinkaccording to their coefficient of ther-mal expansion (CTE). The CTEs ofalumina and ZTA were comparablewith 9,35*10-6 K-1 and 9,48*10-6 K-1,respectively (Fig. 5). In contrast, ATZhas a CTE of 12,05*10-6 K-1 and con-tracts to a higher degree.Despite of the differing shrinkageintervals defect-free gear wheelswith intact interfaces could beobtained for the combinationAl2O3/ZTA. However, for ATZ/ZTAthe inner ring was destroyed at theweld line (Fig. 6). In parts whose out-er components sinter at lower tem-peratures (ATZ(o)/ZTA(i) andAl2O3(o)/ZTA(i)) the inner ring issubjected to compression stresses. Ifthose stresses exceed the limitingstrength the inner component isdamaged at critical sites like inATZ(o)/ZTA(i) at the weld lines. Inthis case additional forces are

Fig. 6 Sintered gearwheels for thematerial combina-tionsa) ATZ(o)/ZTA(i), b) ZTA(o)/Al2O3(i),c) Al2O3(o)/ZTA(i)

cfi/Ber. DKG 86 (2009) No. 4 E 27

applied during cooling because ofthe greater CTE of the outer ring. InAl2O3(o)/ZTA(i) the comparableCTEs allow synchronal contractionand the samples remain defect-free.In the couple ZTA(o)/Al2O3(i) thereverse forces act because the innerring sinters earlier and applies ten-sion forces onto the interface. TheSEM image in fig. 7 shows the bot-tom of a gear tooth and emphasizeshow well the interface was estab-lished already in green state. Whilethe alumina starts to sinter cracks

Process Engineering

Fig. 7 SEM image of bottom at gear tooth with alumina as innercomponent

Fig. 8 SEM image of bottom at gear tooth with ZTA as inner com-ponent

suming and expensive assembling orjoining steps. However, two-compo-nent ceramic injection moulding is avery challenging shaping method.For each processing step of the tech-nological chain both ceramic mate-rials have to be adjusted to each oth-er. Following principle requirementsshould be taken into account for thedevelopment of two-componentceramic parts:1. The powders chosen for two-

component injection mouldingmust be sinterable to full densityat comparable temperatures andunder the same gaseous atmos-phere.

2. For avoiding critical stresses dur-ing sintering the powders shallhave a similar sintering behaviour,i.e. the onset of shrinkage shall fallin a narrow temperature range forboth powders and the shrinkingrate shall be comparable.

3. For ensuring a precise adjustmentin total shrinkage the volume con-tent of solid in the feedstocksmust be the same.

4. Since differences in the thermalexpansion coefficents of the feed-stocks may cause cracks, distor-tion or delamination of the com-pounds already in the green statethe same binder system or bindersystems with comparable thermalexpansion behaviour must beused for feedstock preparation.

5. The thermal expansion coeffi-cients of the sintered ceramicmaterials play also a very impor-tant role as shown in this article,because differences in this proper-ty can result in stresses in the two-component part during coolingafter sintering or during applica-tion of the part under cyclic heat-ing and cooling conditions.

If stresses between both compo-nents cannot be excluded totally,they should be taken into considera-tion already in the design of theinjection moulded parts. For the

experiments we carried out with thethree ceramic materials the mostcritical aspect for the two-compo-nent parts has been the difference inthe thermal expansion coefficientbetween the materials ZTA and ATZ.

AcknowledgementThe CarCIM project is funded by theEuropean Commission within the 6th

Frame Programme (TST5-CT-2006-031462).

Literature[1] Z. Y. Liu: Micro-powder injection

moulding. Journal of Materials Pro-cessing Technology 127 (2002) pp.165-168.

[2] Z. S. Rak: New trends in powder injec-tion moulding. cfi/Ber. DKG 75 (1998)[9] 19-26.

[3] R. M. German: Technological barriersand opportunities in powder injectionmoulding. Am. Ceram. Soc. Bull. 73(1994) [5] 37-41.

[4] B. Rief: Mehr Farbe und Funktion.Kunststoffe 6 (2003), 20-26.

[5] G. Steinbichler: Multifunktionalität ineinem Arbeitsschritt. Kunststoffe –Synthetics 42 (1995) [9] 44-52.

[6] J. L. Johnson et al.: Design guidelinesfor processing bi-material compo-nents via powder-injection molding.JOM Oct. (2003), 30-34

[7] J. R. Alcock et al. : Surface engineeringby co-injection moulding. Surface andCoatings Technology 105 (1998), pp.65-71.

[8] S. Hanson: Surface engineering bypowder coinjection moulding. SurfaceEngineering 15 (1999) [2] 159-162.

[9] G. Finnah et al.: Drei Sonderverfahrenin einem – 2K-Mikro-Pulver-spritzgießen. Kunststoffe (2005) 1,pp. 58-61.

[10] DE 196 52 223 C2[11] P. Z. Cai et al.: J. Am. Ceram. Soc. 80

(1997) 1929[12] T. Moritz: Two-component CIM parts

fort he automotive and railway sec-tors. Powder Injection MouldingInternational 2 (2008) 4, pp. 38-39

[13] B. C. Mutsuddy, R. G. Ford: CeramicInjection Molding, Chapman & Hall(1995)

Fig. 9 Outer diameter ofsintered samplewith a mean diameter of 22,9mm determined by 3D-coordinate measuring tech-nique; deviationsfrom roundness aredisplayed enlarged

form instead of delaminationsbetween the two components. Forthe combination Al2O3(o)/ZTA(i) thestress conditions prevent crackgrowth (Fig. 8).The diameter of defect-free sampleswas measured by 3D-coordinatemeasuring technique around thewhole circumference. Only smalldeviations from roundness werefound which are displayed enlargedin Fig. 9. The slightly elliptic shapecan be referred to the location of thegates at the part. While the smallestdiameter was found at the gates, thelargest dimension was at the weldlines. This could already be observedat green samples with the diameterof 27,09 ± 0,02 mm. During sinter-ing this effect is enhanced by possi-ble marginal lower packing densitiesat the weld lines. For ZTA(o)/Al2O3(i)and Al2O3(o)/ZTA(i) the diameterwas 22,83 ± 0,09 mm and 22,98 ±0,07 mm, respectively.

ConclusionTwo-component ceramic injectionmoulding is a very promising shap-ing technique for large-scale pro-duction of ceramic parts with novelfunctionalities and complex geome-tries. The advantage of this technol-ogy can be seen in the fact that twodifferent ceramics can be combinedwithout any additional time-con-

Process Engineering