May 2000 ECN-RX--00-003ADVANCED FORMING TECHNIQUES IN CERAMICS Z.S. Rak Revisions A 2 may 2000; Final version B Made by: Approved by: Issued by: ECN Energy Efficiency Materials Technology Z.S. Rak E.W. Schuring W.H. Tazelaar Verantwoording This paper will be presented on the Polish Ceramics 2000 conference, to be held in Spala from 29-31 of May 2000. Abstract Improvement in powder processing and forming technologies is the way to improve the reliability of advanced ceramics and lower their cost. Therefore a lot of effort is put into the development of low cost technologies for high volumes of complex-shaped components that will have the ability to form complex-shaped thin-walled parts. The near-net-shape forming of advanced ceramics are on the top of the interest. The following forming processes are actually in the stage of commercialisation or development: pressure slip casting, freeze casting, powder injection moulding, tape casting/lamination, rapid prototyping, and colloidal processing of powders. Especially the techniques based on the colloidal processing such as gelcasting, electrophoretic casting, hydrolysis assisted solidification, direct coagulation casting and similar are in a great interest due to an ability of reducing the critical defects size originated from manufacturing route. Specific manufacturing features, advantages, disadvantages and limitation of these novel processes are described in details in the paper. 2 ECN-RX--00-003 CONTENTS 1. INTRODUCTION 5 2. NEW FORMING TECHNIQUES 7 2.1 Aqueous injection moulding 7 2.2 Centrifugal slip casting 7 2.3 Direct coagulation casting 8 2.4 Electrophoretic casting 9 2.5 Gelcasting 10 2.6 Hydrolysis assisted solidification 11 2.7 Pressure slip casting 11 2.8 Temperature induced forming 12 2.9 Others 12 3. ADVANTAGES AND DISADVANTAGES OF NEW FORMING PROCESSES 13 4. CONCLUSIONS 15 REFERENCES 17 ECN-RX--00-003 3 4 ECN-RX--00-003 1. INTRODUCTION Ceramics materials are, in general, divided into conventional ceramics and advanced ceramics. Conventional ceramics are growing at a slow rate and some of their branches even show a decreasing tendency, i.e. refractory materials. In contradiction to the conventional ceramics the market for advanced ceramics is large and growing continuously [1, 2]. Advanced ceramics are generally divided into three categories - structural, electronic and coatings. Structural ceramics include applications such as industrial wear parts, bioceramics, cutting tools and engine components, and are the fastest growing segment. Electronic ceramics include dielectrics (capacitors, insulators and substrates), integrated circuit packages, piezoelectric ceramics, superconductors and magnets, and form the largest share of the advanced ceramics. The total market size of advanced ceramics was $6.5 B in 1997, of which 71% belongs to the electronic ceramics, 18% to structural ceramics and 11% to ceramic coatings. The highest average growth rate, 8.2% per year between 1998-2003, is predicted for structural ceramics [1]. The advanced electronic ceramics products are in general characterised by a simple shape, are produced in long series, and available conventional shaping techniques like slip casting, PIM, tape casting, extrusion, uniaxial pressing are sufficiently good for shaping process. On the other hand, their main feature is their functionality not mechanical strength, and even an average mechanical strength is sufficient for their final application. A quite opposite situation is in the field of advanced structural ceramics. The main parameters characterising these materials are the mechanical strength and reliability, the products are of complicated shape and therefore near net-shaping techniques are required for their production (i.e. components of engines, gas turbines and similar). Therefore all examples of applications, which are referred in this paper are for structural ceramic products. Critical factors in the commercialisation of advanced ceramics are the starting materials and the way they are shaped. Typical defects in ceramic components are often introduced by the powder itself, and/or in the forming and densification processes [3]. On the other hand there is a general requirement that all advanced ceramic materials should be characterised by a high mechanical strength. Therefore there are two lines of approach for improving the mechanical properties of ceramic materials at the present time [3, 4]. The first approach, according to Griffith's equation (1), favours reduction of the size of critical flaws, which may consist of residual pores, inhomogeneities, surface flaws or sintering induced cracks. Griffith’s equation: σ = Y (K1c √ ac) (1) where: ac – critical flaw size, σ – bending strength, K1c – fracture toughness, Y – shape factor. The other approach relies on attempts to increase the material's toughness, thus making it less susceptible to the effects of critical flaws. The first approach tries to improve monolithic ceramics via the application of new forming techniques, the second approach attempts to implement concepts aimed at increasing the toughness of ceramic materials. In the latter case higher fracture toughness values can be reached by utilising e.g. the martensitic phase transformation of tetragonal to monolithic ZrO2, layered structure, or reinforcement of the ceramic matrix by whiskers, fibre’s or platelets [4-6]. Shape forming techniques can be divided into traditional techniques such as dry pressing, powder injection moulding and extrusion, and wet colloidal forming techniques such as slip casting, tape casting, etc. The reliability of dry pressing ceramics is limited by defects due to agglomerates. This can be avoided by colloidal ECN-RX--00-003 5 forming, where ceramic powders can be deagglomerated completely [7, 8]. However most colloidal forming techniques are limited in the case of larger, complex parts with simultaneously thin and thick cross sections due to density gradients in the green body. The wet processing routes provide on the one hand the possibility of breaking down agglomerates but create on the other hand other problems like the differential sedimentation due to the particle size distribution in the starting powders. There are a few different approaches to overcome this problem such as the use of flocculated slurries or coagulated systems [6, 7]. The drawbacks of these methods are that flocculated and coagulated systems lead to green bodies with rather open structures and lower green densities, which need higher sintering temperatures to reach the theoretical density. Therefore, new techniques such as (hot) isostatic pressing, aqueous injection moulding [9-14], centrifugal slip casting [6, 14-18], direct coagulation casting [19, 21], electrophoretic forming [22-25], gelcasting [21, 26-31], hydrolysis assisted solidification [32, 33], pressure filtration [34-36], and temperature induced forming [37, 38] were developed in recent years. The pressure slip casting, electrophoretic forming and centrifugal slip casting were also included in this overview in spite of the fact that they are well-known techniques in the traditional ceramics sector. However they are still newcomers in the advanced ceramics world. 6 ECN-RX--00-003 2. NEW FORMING TECHNIQUES 2.1 Aqueous injection moulding Powder Injection Moulding (PIM) is a well-established forming technique for ceramics, particularly suited to the production of complex shapes. The PIM process is, in general, used for production of engineering ceramics, however quite recently the successful application of this technique was done for production of porcelain cups, even those with complex, unusual geometry’s [9]. In PIM, a mixture of thermoplastic polymers is used as a binder. A broad range of binders is commercially offered for this purpose. However the existing ceramic injection moulding processes require a large proportion of polymer binder which has to be removed before firing. Usually this process is quite slow taking from a few hours to a few days. Additionally, oxidation of polymers in the firing process produces dicarbon oxide (up to 3.5 times the weight of the polymer) and undesirable volatile organic by-products. An alternative to the traditional PIM process is low pressure injection moulding (LPIM) using water soluble polymers as binders which lower the temperature and pressure required to mould. A water- based system was invented by researchers from AlliedSignal Corporation (USA) and developed later by a number of European research centres in the Netherlands, UK, Italy and Spain [10-13]. Agar or agarose is used as a water-soluble binder which is set by a temperature change (gelling at 40-42oC during cooling). The aqueous slurry for the injection process consists, in general, of the following components: the ceramic powder, water as a solvent, agar/agarose as a binder, deflocculant and lubricant. The most important step is deagglomeration of the suspension before the injection step (usually by addition of deflocculant and ball milling). The addition of the binder, agar/agarose, also should be limited to 1-2 wt% to avoid an excessive growth in the viscosity of the prepared slurry. Therefore, in practice, the working suspension does not contain more than 50 vol.% of the ceramic powder. After injection of the low viscosity paste, the binder gels in the mould and provides the product with the strength needed for subsequent handling. The water can be removed easily by drying in a conventional oven or more uniformly by microwave drying. Aqueous Injection Moulding (AIM) technology is readily automisable, and preliminary technical cost estimates predict that industrial components can be fabricated within the required costs for commercial implementation [2]. 2.2 Centrifugal slip casting The Centrifugal Slip Casting (CSC) method combines the advantages of wet chemical powder preparation with a highly efficient and rather stress-free densification technology to produce massive and near-net-shape ceramic parts [6, 15-18]. It is well known that the wet processing route provides the possibility of breaking down agglomerates and other flaw sources. On the other hand, the drawback of wet chemical consolidation by centrifugal means might be the differential sedimentation due to the particle size distribution in the starting powders. One of the methods of avoiding mass segregation is to use slurries with a high solid content. Suspensions with repulsive, non-touching particle networks lead to bodies with high green densities which show good sintering behaviour and reach high final densities at much lower sintering temperatures. The advantage of this forming technique even in comparison to isostatically pressed bodies has been proven [26]. Centrifugally cast parts from optimised suspensions reach almost theoretical density at temperatures as low as 1300oC for TZP ceramic parts. Centrifugal cast sintered compacts show a finer and more homogenous microstructure than isostatically pressed TZP. After post-sintering annealing, fracture toughness could be obtained of up to 13 MPa.m1/2. Centrifugally cast materials also exhibit narrower size distributions than dry isostatically pressed materials. The main application is foreseen in the manufacturing of defect- free tubular membrane supports (15-18). The ceramic membranes are produced with a graded ECN-RX--00-003 7 pore-size distribution. Considering ultrafiltration membranes, up till now the filtration layer is formed on a support layer, which has larger pore size (1 to 6 μm) and is manufactured by slip casting or extrusion. Between the support layer and filtration layer, a discontinuous transition of material properties exists which, however, can be avoided by a gradient structure. To solve this problem, multimodal suspensions can be used to form a powder compact with a pore gradient if a particle size segregation can be achieved during the consolidation process. Oxide ceramics are in general used as materials for ultrafiltration membranes. In this process, a powder dispersed in a liquid with a stabilising agent, followed by axial rotation in a cylindrical mould for some time. The resulting cast is dried, released from the mould and sintered. If particles with a narrow distribution and low degree of agglomeration are used, a nearly random-close packed green compact can be formed. During centrifugical casting, the large particles form the large pores on the bottom of the sediment and the small particles form the small pores on the top layer of the sediment if the centrifugical acceleration is high enough. Ideally the pore-structure in each layer can be consolidated at the random packing structure of nearly monosized spheres. This pore- gradient structure is analogous to an asymmetric membrane, which is preferred in the filtration applications. The resistance of an asymmetric membrane to filtration mass transfer is determined by the thin top layer. The last layer for ultrafiltration layer can be manufactured also by the sol-gel technique. This technology lowers significantly a number of production steps in the membrane technology, results in improved strength of the support material and also lowers a number of incidental defects. The technology is still in the laboratory stage because instead of the perfection of the produced tubes higher prizes are serious limitation in mass-scale production. The second drawback of this technique is the limitation to only circular shapes [17]. 2.3 Direct coagulation casting Direct Coagulation Casting (DCC) is the solidification method for aqueous suspensions in a die, which has been recently presented by a group of scientists from ETH Zurich, SF [19, 20]. This process is based on the destabilisation of a suspension by internal chemical reactions leading to a decreased surface charge on the dispersed particles by a shift in pH, or by increasing the ionic strength in the suspension. In a polar liquid (water) solid particles become positive charged in the acidic region and negatively at higher pH values. If the particle surface charge is high and of the same sign, repulsive forces prevail over the Van der Waals attraction, and such a suspension is stable and its viscosity is low. If the repulsive forces are eliminated by shifting the pH toward the isoelectric point (IEP) or increasing the ionic strength, the attractive forces between particles will cause the suspension to flocculate or coagulate, and a stiff, wet solid is formed. Very well suited for this task are enzymes (to catalyse the synthesis or decomposition of molecules at ambient temperatures). Their reaction kinetics can easily be controlled by temperature and by the enzyme (catalyst) concentration. Time delayed decomposition of the substrates allows to change the pH or the ionic strength in suspensions. This may be used to precipitate powders from solutions, to trigger sol-gel transitions or to coagulate powder suspensions. The transitions from a well deagglomerated suspension to a viscoelastic body can be performed without disturbing the homogenous particle distribution from the outside. This results in very homogenous microstructured ceramic bodies, layers and micropatterns. Enzyme reactions were successfully applied to particle suspensions such as Al2O3, ZrO2, SiC or Si3N4 as well as to mixtures of powders with different isoelectric points in addition with specific adsorbing surfactants. Two enzymatic reactions for internal shifting of pH from the acidic to the alkaline in a controlled manner are hydrolysis of urea by urease (buffering at pH 9) or amides by amidase (buffering at pH 7) and glucose/glucoseoxidase, which enables an internal pH change from alkaline to acidic (pH 10 to 4) [19]. NH2CONH2 (urea) + H2O + urease (catalyst) → H2CO3 + 2NH3 + urease (2) pH 4 → pH 9 8 ECN-RX--00-003 Several other enzymatic reactions such as pectin, casein or protein hydrolysing enzymes are also useful for coagulating special ceramic suspensions. In case of particle suspensions with high solid loading the cast components do not undergo shrinkage during coagulation. This allows for production of complex shape bulk components, even with undercuts to near net shape in relatively short times (10 to 90 min.) It offers the possibility of low cost production of these complex shaped parts with extremely high strength and high reliability. The comparison of gelcasting and DCC forming techniques made by CERAM TEC AG, Germany proved the superiority of the DCC method [21]. Alumina ceramics produced by the DCC technique, sintered at 1400oC, were characterised by a density of 3.985 g/cm3, a mean bending strength of 623 MPa and a maximum flow size as low as a few micrometers. 2.4 Electrophoretic casting Eletcrophoresis has been known since 1850 but its application to the deposition of ceramic powders was first studied by Hamaker in 1940. Since that time, ElectroPhoretic Depositions (EPD) has been successfully used for traditional ceramics using aqueous suspensions [22]. The process received significant attention for production of advanced ceramics in the 1980's, generally using organic solvents, and in the 1990's, aqueous EPD has been used for processing technical ceramics. The EPD system works by applying to a slurry an external dc field that promotes the migration of particles (electrophoresis) and their subsequent deposition onto the opposite charged electrode. It is well known that fine ceramic powders form agglomerates causing inhomogeneities in ceramics. To disperse particles, at least one sufficiently strong repulsive force is necessary to overcome always-present attractive forces. There are three types of dispersion mechanisms - electrostatic (generation of a common surface charge on the particles), steric (adsorption of polymeric additives) and electrosteric stabilisations (absorbed ionic surfactants or polyelectrolytes). Water is the preferred solvent for suspension processing because it is economical, nontoxic, and ecologically beneficial. Aqueous suspensions can be stabilised electrostatically, that is, the forces can be changed from attractive to repulsive by adjusting the pH. It is possible to enhance the dispersability by incorporating polymeric additives. In the EPD process the particles must remain dispersed through the medium so they can move towards the electrode independently of each other. Particles can then deposit separately, without agglomerates, keeping the possibility of a structural reordering during packing. The suspended particles must have a high electrophoretic mobility. During deposition, the stability of conditions changes due to the distortion of the double layer that occurs when the particles migrate and to the increased concentration of particles and ions near the substrate. Packing begins when electrophoretic and electrostatic forces still dominate over the Van der Waals barrier, so that an attractive particles network starts to develop. The loss of stability leads to the formation of a deposit of particles. This is caused by their flocculation at near to contact distances, where attractive Van der Waals forces dominate. The charge transfer takes place only during the relaxation of the double layer. The colloidal parameters (zeta potential, viscosity and electrophoretic mobility) play an important role in EPD, while the electric ones (conductivity) determine the viability of the process. The kinetics of the EPD process follow a simple equation proposed by Hamaker (3), which relates the deposited mass per unit area (m) with the slurry properties (concentration, C, and electrophoretic mobility, μe), and the physical and electrical fixed conditions (electric field, E, and deposition time, t). m = C.μe .E . t (3) In general, the amount of deposited powder linearly increases with time when the concentration not varies with time and the electrical conditions imposed maintain without variation. The thickness of the deposited layer can vary from a few to a few hundred microns. To assure a constant electric field during EPD, the geometry of the electrophoretic cell must be constant. The process should be developed maintaining a constant current density. ECN-RX--00-003 9 The EPD is considered to be a powerful forming technique for three reasons; • it is extremely versatile, • it is a low cost process, • it is reliable. Problems that could be faced during the EPD process are: unwanted water electrolysis, galvanic reactions and water electro-osmosis. However the simplicity of the process and the capability of depositing metals, ceramics and polymer by electrophoresis opens great possibilities for application in different fields such as electroceramics (SOFC, multilayered capacitors, sensors, actuators, etc), structural ceramics (layered composites, long fibre CMC, functionally graded materials, laminated ceramic composite tubes, etc.) and protective coatings onto metals (EPD of sol-gel solutions or suspensions) [23-25]. This technique is already used commercially in traditional ceramics (electrophoretic spray coating of sanitary wares, enamel electrophoretic deposition, etc.). 2.5 Gelcasting Gelcasting (GC) is a new shaping process for advanced ceramic materials. Proof of concept was demonstrated by the scientists from Oak Ridge National Laboratory, USA in 1985 using non- aqueous systems. Development of an aqueous process using acrylamide as monomer was completed in 1988 [26]. However, concerns regarding health, safety and disposal of acrylamide caused industrial rejection of the process (acrylamide is a neurotoxin). Development of a low toxicity process was initiated to deal with the lack of acceptance, and it was fully demonstrated in 1990 [27]. In the gelcasting process, a small amount of organic monomer and cross linker is added to the ceramic aqueous slurry. The most useful systems are based on the monofunctional monomers methacrylamide (MAM), methoxy poly(ethylene glycol) monomethacrylate (MPEGMA), and n-vinyl pyrrolidone (NVP), the difunctional monomers methylene bisacrylamid (MBAM) and poly(ethylene glycol) dimethacrylate (PEG(1000) DMA), and the initiators ammonium persulfate/tetramerthyl ethylene diamine (APS-TEMED), azobis ([2-(2- imidazolin-2-yl)propane HCl (AZIP), and azobis (2-amidinopropane) HCl (AZAP). None of the monomers interact adversely with standard ceramic processing aids such as dispersants and defoamers. Solids loading as high as 55-60 vol.% were achieved in alumina slurries and 45-57 vol.% in silicon nitride suspensions using these systems [2]. Upon heating, the monomer polymerises, and the resulting gel (which is ca. 90% water) stiffens the ceramic powder slurry into the shape mould. Gellation times are as short as 15 min for turbine wheels. The gelled powder is then easily dried to remove the water, leaving a powder compact with an open pore network and 2-4 wt% dried gel. The gel is easily removed by a short pyrolysis cycle. The gelcasting process is 50-80% faster than the slip casting technique, generating a more uniform powder packing, having a much higher part strength after forming and after drying, and requiring simpler moulds (no need for dewatering surfaces). The gelcasting is more amenable for to automation than the pressure slip casting process of complex shapes. However densification is still an issue. Gelcasted parts typically shrink 23% during densification, resulting sometimes in distortion in the densified parts. Gelcasting starts to be used not only for manufacturing of complicated shaped dense products such as parts of turbines but also for manufacturing of porous ceramic objects [2, 27-31]. The new shaping technique is not limited to the USA but recently very broadly tested in other countries like Germany, Brazil, China, Spain, Poland, and others [21, 28-31]. The most advanced works in the field are already in the phase of commercialisation. AlliedSignal Ceramics Components (Torrance, CA, USA) working together with ORNL (Oak Ridge, TN, USA) has developed an automated gelcasting fabrication process of production ceramic turbine motors [2]. 10 ECN-RX--00-003 2.6 Hydrolysis assisted solidification The Hydrolysis Assisted Solidification (HAS) process was developed by the scientists of the Josef Stefan Institute in Ljubljana, Sl [32, 33]. The process is a sort of cross between hardening of cementitious products, DCC and gelling, providing some benefits of each. The process is based on thermally activated and accelerated hydrolysis of aluminium nitride (AlN) added to highly concentrated ceramic suspensions: AlN + H2O → Al(OH)3 + NH3 (4) During the hydrolysis of AlN, water is consumed. The solid loading will increase as water is bound into the metal hydroxide and ammonia is formed, which in turn may increase the pH of the suspension (the pH is shifted toward the isoelectric point of the alumina). Both mechanisms, i.e. water consumption and shift in pH, can be exploited for solidification of an aqueous suspension in a die. Furthermore, aluminium hydroxide, which is another reaction product obtained on AlN hydrolysis, gels on heating as well, which additionally assists in the solidification process and increases the strength of the moulded green body. The benefits of this process are excellent rheological properties, fast solidification, and high degree of density. The drawbacks include limited time and temperature stability, sensitive feedstock preparation, heat transfer during solidification and need of additional equipment to collect and neutralise ammonia. Additionally, the process is not suitable for all types of ceramics (introduces alumina). The technology is in an industrial scale-up phase. 2.7 Pressure slip casting Pressure Slip Casting (PSC) technology is well established in the table and sanitary ware industries but its use in technical ceramic fabrication has been limited (14, 34-36). Slip casting, with or without pressure, constituents an ideal combination of dewatering and shaping. Slip casting tableware using deflocculants in plaster moulds was been introduced on a commercial scale in Belgium as early as the 18th century [34]. Only little progress has been made up to the 1980s. The problem is that the casting slip contains about 30 wt% water. About half of it must be absorbed by the plaster mould in order to reduce the liquid slip to an acceptably firm body. This takes time, and it takes additional time to dry out the water stored in the plaster mould. Even nowadays this process only allows the production of two casts from one mould per working day. Using pressure to accelerate the dewatering process improves the productivity and economy of the process. The products with wall thickness in the range of 7-9 mm can be produced within 5-10 min. [36]. The main difference to slip casting is that the water is not removed by capillary suction (negative pressure in the plaster mould) but by pressurisation (positive slip pressure) using pressure up to 40 bar. Control of the filtration process is based on four parameters; the pressure differential on the body, the liquid-medium viscosity, the specific surface area of the slip's solids content and the body porosity (body formation is dependent on the permeability of the layer of body that has already formed from filtered material). However the heart of the system is the pressure casting mould. The pressure casting mould is made of a special-purpose type plastic with a defined pore structure. The advanced ceramic materials are produced mainly from submicronic ceramic powders such as alumina, zirconia or silicon nitride and therefore the main problem is the control and reduction of the pore size of the resin porous mould to allow filtration of the colloidal dispersion without clogging. The best particle packing is achieved by consolidating well-dispersed slurries, but the resulting body is dilettante in nature and tends to crack after drying. A lot of work was done to overcome these problems. Stable slips for pressure casting were developed, handleable green casts were produced and the problems of cracking during the drying process were solved [14, 36]. The use of pressure casting technology, however, has been retarded in its application to the manufacture of advanced ceramics components for several reasons. One reason may be the wide variety of materials which are used in structural ceramic applications and which demand specific development appropriate to the casting process. A second one may be the demand for components in quantities not sufficient to justify extensive development and investment. A third ECN-RX--00-003 11 one may be that existing conventional technologies are well established in this industry and that another shaping technology would be difficult to justify because of the capital investment required. 2.8 Temperature induced forming In this process the sol-gel processing was combined with the concept of solidification by pH control, which in this case brought about the thermal decomposition of formamide to ammonia. The new process was developed by scientists from the Max-Planck-Institute für Metallforschung (Stuttgart, Germany) and is called Temperature Induced Forming (TIF) [37]. In this process the boemite sol is added to the well-dispersed alumina powder, both components are ball-milled for a better dipersion and 1.5wt% formamide (methamamide –HCONH2) is added. The suspension after de-aerating under vacuum is poured into an impermeable mould. The mould is sealed and placed in an oven at 60oC for 12 hours. The process utilises the effect of increased temperature to change the surface charge of suspended particles thus creating a weakly attractive network. Simultaneously, the solubility of the particles increases and drying induces precipitation to create an inorganic binder phase, which provides green strength. The thermal decomposition of formamide to ammonia is responsible for the shift of the pH to the isoelectric point and for the transformation of a fluid to a rigid state (solidification of the slurry) [37, 38]. HCONH2 → NH3 + CO (5) pH 4 (60oC) → pH 9 The technology is still in the laboratory development stage. 2.9 Others The above overview of new forming techniques for advanced structural ceramics is not complete. There are some other forming techniques which have been neglected in this overview due to a lower potential for implementation in industrial practice such as freeze casting [39], adiabatic casting [40], AIM using methyl cellulose binders gelling during heating [41], etc. 12 ECN-RX--00-003 3. ADVANTAGES AND DISADVANTAGES OF NEW FORMING PROCESSES Most of the advantages, disadvantages and process limitations which could be labelled as specific characteristics of colloidal processing technologies arise from the solidification principle exploited while all the others are inherited from the parent technologies, i.e pressure slip casting from pressure casting, etc. In Table 1 a provisional list of major benefits, drawbacks and specific limitations of the discussed variants of the wet processing route are presented. Table 1 Benefits and drawbacks of selected wet processing routes Processing Advantages Disadvantages Commer- Remarks method cialisation Aqueous Simple, fast High viscosity, Size Not yet Solidification by injection limited, Heat transfer gelling, moulding in process Environmentally friendly Centrifugal slip One step process Additional equipment Not yet Solidification by casting for filters needed centrifugal forces, Recommended for tube shape objects Direct Excellent Expensive additives, Yes Solidification by coagulation rheological Narrow pH windows, coagulation, casting properties, No Gaseous products Excellent mechanical size and wall formation, Poor green properties of final thickness strength products limitations, high green density Electrophoretic Suitable for Sensitive to current Not yet Solidification by casting functionally parameters electrophoresis, Used graded in conventional composites and ceramics coatings Gelcasting Widely Limited time stability, Yes Solidification by acceptable, No Low solidification gelling, Used for size and wall rate, Low green dense and porous thickness density ceramics limitations Hydrolysis Simple, Fast, No Alumina Yes Solidification by assisted size and wall contamination, hydrolysis, Not solidification thickness Gaseous product suitable for all types limitations, High formation, Poor green of ceramics green density strength Pressure slip Fast Drying, Additional Not yet Solidification by casting equipment needed pressure gradient, Used in conventional ceramics Temperature Simple Heat transfer, Gaseous Not yet Solidification by induced forming product formation coagulation, Newness ECN-RX--00-003 13 Environmental aspects of all the presented colloidal processes using water as a liquid vehicle to replace organic solvents and binders was not discussed in the paper but these are obvious. According to the author a final successful application of a specific technology to production practice will be decided, however, by an ability for automation because if the market for advanced ceramics will be developed continuously, then it will be necessary to go to automation. 14 ECN-RX--00-003 4. CONCLUSIONS The following conclusions can be drawn: • The ceramic components manufactured by the reviewed wet forming techniques are characterised by a lower number of smaller processing defects than in the case of ceramic components manufactured by pressing, extrusion or PIM. 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