SYNTHESIS AND PROCESSING
2.02 Processing of Alumina and Corresponding Composites
2.02.4 Fabrication of Alumina Materials
One of the consequences of the high melting point of ceramics is that they are generally fabricated from powders and not from fused mixtures. The general processing schedule (Figure 2) includes the processing of the powders before the thermal treatment to reach a shaped powder form, usually named“green body”, and the thermal treatment. The word“sintering”is used to describe the phenomenon that occurs during the thermal treatment. Due to the high cost of machining hard materials such as alumina, near–net shape methods are being developed. Also, green bodies with sufficient strength as to be green machined are fabricated by optimization of the shaping methods. Green machining after isostatic-pressing is used routinely to fabricate insulators. The extreme geometrical requirements for hip and knee implants require intermediate andfinal machining andfinal polishing and two- or three-stage sintering.
When the powder compact is heated to a suitable temperature at which the mass transport mechanisms are operative, the porous compact shrinks as a result of the initial misfit of the particles. Apart from shrinkage, grain growth occurs during the process. In some cases, reaction between the constituents occurs, which might aid or impede shrinkage. This latter process is fundamental when the in situ formation of second phases is aimed. The packing structure of the green body determines the development of the microstructure during sintering. In order to get high-quality materials, the control of the shape-forming processes is of prime importance. In order to avoid defects in the green state, dispersal approaches have to be applied to the powders to bring them into the state that allows the use of the different shaping methods available.
The sintering of submicronic and high-purity (99.5%) aluminium oxide powders to near-full density (99%
of theoretical) was first reported in 1956 (Cahoon & Cristensen, 1956). Nowadays, for fabricating high- performing alumina materials, the new corundum powders with submicrometric particle sizes (average diametersz0.2–0.6mm) and high purity (>99.7%) commercially available since the late 1980s are used. Also, the use of new nanosized (<50 nm) powders to obtain materials with new properties is under investigation. In principle, the small size together with the increased particle curvature would increase the driving force for solid- state sintering, reducing the required temperature and consequently limiting the grain growth. However, fine
Powders Additives
Shaping
Debonding/Drying Green body Thermal treatment
Dense body
Machining Mixing
(Machining)
Figure 2 Basicflowchart for the fabrication of ceramic components. Shaping methods used for alumina ceramics are summarized inTable 3.
Processing of Alumina and Corresponding Composites 39
particles present larger tendency to agglomerate. Agglomeration hinders a locally homogeneous densification as explainedfirst by Lange (Lange, 1984) and confirmed later on for finer raw materials (Xue & Chen, 1990).
Therefore, the agglomerates in the green body evolve toward differential sintered areas constituted by large particles or particle assemblies surrounded by low-density zones. In order to get real profit of the sub- micrometric powders, the relationships between sintering temperature, sintered density, and grain size as a function of spatial homogeneity and interfaces have to be known and handled.
Most alumina-matrix composites are fabricated by powder processing and sintering methods of the same kind as those for single-phase alumina materials. However, each processing step presents specific problems due to the different characteristics that present the matrix and the reinforcing phase (size, shape, specific surface area, composition, density). First, it is necessary to prepare the homogeneous mixture of the matrix and the rein- forcing phase and consolidate it. Second, adequate thermal treatment has to be selected in order to reach a high-density matrix, even though the reinforcing phase can act as shrinkage inhibitor. And third, the interface between the matrix and the reinforcement has to be controlled to allow the action of the reinforcement mechanisms.
2.02.4.1 Powder Processing and Shaping
Shaping can be defined as the processing step in which a system constituted by isolated particles is transformed into a consolidated body with specific shape, size, and microstructure. The selection of one or another shaping procedure is based on the required characteristics of the sintered formdshape and sizedof the material microstructuredgrain size and shape, dispersion of secondary phases, and densitydon the number of pieces to be produced and its necessary reliability, and on environmental and economical aspects. The most expensive stage of the fabrication of pieces of hard materials such as alumina isfinal machining; thus, great effort is devoted to the development of shaping procedures to avoid machining.
The powder processing and shaping technology of alumina materials started on the basis of that of clay ceramics (Gitzen, 1970). Therefore, shaping methods were taken directly from those used for clays, which presented some problems due to the lack of plasticity of the available alumina powders. Most of the further developments of the traditional forming methodsdslip-casting, dry and semidry cold-pressing, isostatic- pressing, extrusion, etc.das well as new forming processes such as hot-pressing (HP) have been developed using alumina as the principal agent.
Due to the lack of plasticity of advanced ceramic powders, organic additives (binders, plasticizers, and lubricants) are usually added to provide plasticity during forming and green strength of the formed product.
Starting materials for forming are powder, additives, and solvents, and the three main material states for forming are granules, masses, and slurries. InTable 3, the main ceramic cold-forming (shaping) methods used to fabricate alumina parts that are discussed in which follows are summarized. In general, they are classified on the basis of the degree of moisture in the starting material.
Table 3 Shaping methods used to fabricate alumina materials classified by the degree of moisture (those which are not specifically discussed in the text is indicated in italic.)
Dry Plastic (semi-dry and wet) Direct slurry shaping
Cold pressing:
Uniaxial Biaxial Isostatic
Hot pressing, hot isostatic pressing
Extrusion Injection moulding Others: pressure moulding
Filtration:
Slip casting Pressure casting Vacuum casting Deposition/evaporation:
Tape casting Centrifugal casting
Others: electrophoretic deposition, screen printing, dip coating Polymerization/gelation:
Gel-casting
Others: freeze casting, rapid prototyping, floculation/coagulation
40 Processing of Alumina and Corresponding Composites
The powder processing before consolidation is of prime importance to avoid defects in the sintered material.
Major heterogeneities in the powders are soft and hard agglomerates, organic and inorganic inclusions, and large grains. Moreover, more than one phase have to be homogeneously mixed when considering composites.
Mixing difficulty is increased when particles of different shape are considered as occurs in the alumina–SiCw composites used for cutting tools. Even when the desired homogeneity is reached during mixing, mass segre- gation during shaping can lead to heterogeneities in the green body in multiphase ceramics. Most heteroge- neities can be eliminated by manipulating and controlling interparticle forces as practiced in colloid science (Lange, 1989, 2001). Repulsive interparticle forces are used to break apart weak agglomerates, fractionate inclusions greater than a given size, and mix different fractionated powders. Moreover, colloidal technology can be combined with milling procedures. Once fractioned and mixed, the interparticle forces can be made attractive to form a network that prevents mass segregation.
2.02.4.1.1 Cold-Pressing Methods
Pressing technologies involve the compaction of the powder by applying either a rigid (axial-pressing) or flexible (isostatic-pressing) pressure. For specific shapes such as tubes or insulators, part of the mold used for isostatic-pressing can be rigid. Fundamentals of pressing are discussed by Hofmann et al. (Hoffman, Scharrer, Czerch, Fruhauf, & Burck, 1962). These authors identified the Coulomb force between the moisture ions adsorbed on the surface of the ceramic particles as the main cause for the cohesion in the green body.
As the saturation degree is very low (0.01–0.5), the required applied pressures are high (z50–200 MPa).
The degree of moisture in the starting material may change from less than 7% (dry-pressing) to 15–20%
(semidry and wet). Axial-pressing can involve the application of pressure on one direction (uniaxial) or two directions (biaxial). Wall friction during axial-pressing is the major source of density variation in the pressed body (Allen, 1961). When isostatic-pressing is used, pressure is applied equally through the whole mass and the density gradients characteristics of uniaxial-pressing are avoided. However, axial-pressing is the most used at industrial level when it is possible to reach the required shape due to the easiness of automation and the high production speed. High-purity (98.5%) alumina dense (>95%) parts such as tap seals with smaller height than diameter are routinely fabricated by uniaxial-pressing. Isostatic-pressing is used for shapes that cannot be obtained by uniaxial-pressing or for added value products that require high density and isotropic green bodies. Molds for axial-pressing are usually made of steel and for isostatic-pressing they are made of elastomeres, silicone, and polyurethanes. The latter have better properties but are more difficult to synthesize.
In order to improve the flowing of the fine powders during pressing, they are usually granulated before compaction. Otherwise, the specific volume of nonagglomerated particles in or beyond the micrometer range is so large that on compaction, the decrease in volume is excessive, causing extensive elastic relaxation (and associated cracking) on removing the load. The granules are most often produced by colloidal dispersion techniques followed by spray drying ceramic powder slurries containing additions of organic materials to act as binders, usually polymers, and have to fulfill several requirements in order to avoid defects in the sintered body (see, e.g.,Eckhard & Nebelung, 2011; Lange & Metcalf, 1983; Nakamura, Tanaka, Kato, & Uematsu, 2009). They have to be large (>50mm) and strong enough for handling but soft enough to be destroyed by compaction to avoid relicts of the granules in the green body that will evolve to defects by differential sintering. In addition, they have to uniformly deform tofill interagglomerate void space during pressing (Frey & Halloran, 1984; Lin &
Lin, 2008). It is necessary to avoid the movement of the binders toward the surface of the slurry droplets, in the stream of hot air during the spray drying process, because it would produce binder segregation at the surface of the granules after the liquid evaporates. In order to reduce segregation, binders that bond strongly on the powder surface should be used. In the case of alumina powders, strong chemical bonding has been proved with binders containing functional groups of the polyacrylic type (Tanaka, Pin, & Uematsu, 2006).
For optimized powder conditioning before isostatic-pressing and sintering schedules, materials with den- sities higher than 98% of theoretical and grain sizes of about 1–5mm can be fabricated from undoped alumina powders of about 0.5mm average particle size.
2.02.4.1.2 Extrusion and Injection Molding
Higher moisture contents increase the plasticity of the powder, and plastic-forming methods such as extrusion, pressure molding, or injection molding can be used. Pressure molding is mainly used for tableware. In these cases, the starting material deforms at relatively low pressures (1–20 MPa) due to the high saturation degree (>0.9).
Processing of Alumina and Corresponding Composites 41
Extrusion is adequate when long pieces of constant transversal section, such as tubes, cylinders, or tiles, are required. However, it is scarcely used for advanced materials because the noncompressible powders used require additives, which make difficult the dimensional control and require special treatments to eliminate. Extrusion is mainly used in clay systems that confer plasticity to the shaped material.
Injection moulding is a special case because the solid is saturated by a fused polymer that solidifies due to the temperature change in the mould after injection. Due to the high viscosity (103 Pa$s) of the mixtures, which are prepared by colloidal techniques, it is necessary to apply high pressures (10–150 MPa) at temperatures of about 120–200C. The shaped piece has to be subjected to a treatment to eliminate organics, usually done by a slow thermal treatment, which increases the price. An attractive characteristic of this method, broadly used to fabricate textile guides, is that a wide spectrum of near–net-shaped complicated shapes can be fabricated, limiting machining in the sintered state. Due to the high viscosity of the mixtures and the high organic content (20–50 vol%), this procedure usually leads to defect formation, so it is not generally used for advanced ceramics. However, in order to take profit of its advantages, developments are being done to incorporate it into their standard fabrication methods (Mannschatz, Höhn, & Moritz, 2010). In fact, some companies are already offering high-purity sintered alumina (99.8) pieces green shaped using injection moulding (e.g.,http://www.
adamoutechnicalceramics.com/news/8/57/Adamou-featured-in-Powder-Injection-Moulding-International).
By using low–molecular weight thermoplastic polymers, it is possible to decrease the applied pressure (down to z0.3–0.3 MPa) significantly. This relatively new process is called “low pressure injection moulding” (Mangels, 1994; Novak, Vidovic, Sajko, & Kosmac, 1997; Rak & Hamburg, 1995). The low viscosity required for this method limits the acceptable solid contents of the slurries (max 65 vol%), which leads to a significant shrinkage during sintering and the possibility of low-density defects (Mangels, 1994).
2.02.4.1.3 Direct Slurry Consolidation Methods
Colloidal technology is used to produce the slurries used for spray drying and injection molding. However, a major issue for exploiting the advantages of the colloidal approach is the direct shaping of green bodies from the slurry without addition of the large amounts of forming additives. Direct slurry consolidation includes a variety of procedures in which the applied pressures are very low (forfiltration,P<1 MPa) or zero. Main problem of the direct slurry consolidation methods is that homogenization and pouring introduces air into the slurry, forming“bubbles”, whose complete elimination is difficult. Therefore, relatively large spherical pores might be formed in the green compact, which cannot be eliminated by solid-state sintering, as shown inFigure 3, where the fracture surfaces of green (Figure 3(a)) and sintered (Figure 3(b)) bodies show the evolution of one of these bubbles after sintering.
Slip-casting and tape-casting are the two conventional slurry consolidation methods. During slip-casting, the slurry is poured into a permeable mould, which allows the fabrication of complex-shaped pieces. High-density (>98% of theoretical) alumina materials can be fabricated from high-purity (>99.9%), undoped powders of about 0.5mm average grain size, with grain sizes dependent on the sintering temperature (e.g.,z3–5mm for temperatures ranging between 1450 and 1550C;Bueno et al., 2008; Sánchez Herencia, Moreno, & Baudín, 2000). The homogeneous dispersion of 5 vol% zirconia stabilized with 3 mol% Y2O3(YTZP) allows keeping the grain size smaller thanz2mm (Ruiz-Hervías, Bruno, Gurauskis, Sánchez-Herencia, & Baudín, 2006). The main limitation derives from the low kinetics of thefiltering process that limits the allowable thicknesses of the shaped pieces. There have been different techniques proposed to accelerate the process such as performing thefiltration under vacuum or microwaves, centrifugation, or application of a pressure (<1 MPa) that has to be high (>5 MPa) when fine-grained powders such as those used for advanced aluminas are considered (Yu & Lange, 2010; Zhang & Lange, 2006). From an industrial point of view, slip-casting is used for the fabrication of thin-walled complex pieces in short series such as a wide spectrum of laboratory ware. To avoid excessive shrinkage duringfluid removal and densification, slurries containing the highest possible fraction of particles are needed, which requires the careful manipulation of particle interaction to maintain the stability of the slurry (Cesarano, Aksay, & Bleier, 1988; Lewis, 2000; Sigmund, Bell, & Bergstrom, 2000).
Tape-casting (Shanefield & Mistier, 1974; Shanefield, Morzenti, & Mistier, 1974; Williams, 1976) is generally used when substrates with high microstructural and thickness control and good surfacefinish are needed. This process involves the deposition of the slurry on a substrate where consolidation occurs due to the evaporation of the liquid medium. The thickness of the deposited tape is controlled by a blade. The obtained tape has to be flexible to be manipulated, so special additives, binders, and plasticizers, which have to be eliminated by the thermal treatment, have to be added (Moreno, 1992a, 1992b). Tape-casting is limited to the fabrication of relatively thin (z25–400mm) tapes. Thicker tapes and bulk pieces can be fabricated by stacking cast tapes and 42 Processing of Alumina and Corresponding Composites
sintering (Gurauskis, Sánchez-Herencia, & Baudín, 2006; Lambrinou et al., 2007), with microstructural char- acteristics similar to those of bulk pieces obtained by slip-casting (Ruiz-Hervías et al., 2006). Relatively porous (z90–92% of theoretical density) alumina (96% purity) substrates designed to sustain thermal gradients are fabricated by tape-casting.
Centrifugal-casting is a particle consolidation process in which particles in a colloidal suspension settle under centrifugal acceleration (Chang, Lange, Pearson, & Pollinger, 1994; Huisman, Graule, & Gauckler, 1995). When compared with other colloidal-forming methods, it has the advantage of producing massive ceramic objects combined with a reduced risk of stress gradients in the formed part because stresses during the centrifugal forming process act on every volume element. During sedimentation, the particles move in the direction of gravity, whereas the liquidflows in the opposite direction. This means that the total volume of liquid in the dispersion does not have to be transported through the sedimented powder, leaving defects such asfiltration channels as in the case of slip-casting and pressurefiltration. The drawback of this technology, however, is that particle size separation might occur due to differential settling during the consolidation stage.
In order to avoid this problem, consolidation of the system in theflocculated state or in the coagulated state or the use of highly concentrated slurries was proposed. Centrifugal-casting is limited for relatively thin-walled shapes.
A lot of effort is being put in the study of processes that transform a high solid content (>50 vol%)flowable slurry into a firm body without liquid phase removal. This solid body retains the structure of the stable suspension. This transformation can be done by manipulation of the electrical characteristics of the particles (the electrical double layer) or by changing the pH or the additive concentration, as occurs in the process of direct coagulation (Prabhakaran, Raghunath, & Melkeri, 2008). Another possibility is the addition of chemical species that polymerize under controlled conditions. The most usually investigated methods are the addition of catalysts that produce the polymerization of monomers added to the suspension (gel-casting).
(a)
(b)
Figure 3 Characteristic defects formed in bodies shaped directly from the slurries. Scanning electron microscopy of fracture surfaces. (a) Spherical pore in the green body formed as a result of an air bubble in the slurry. (b) Spherical pore in the sintered material formed as a consequence of the evolution during sintering of a spherical pore in the green body.
Processing of Alumina and Corresponding Composites 43