《材料制备与加工 Materials Synthesis and Processing》课程教学资源（书籍文献）Advanced Ceramics Processing.pdf_大学文库

7 1 Introduction In this course on “Ceramic Processing and Microstructure” the main subject will be the fabrication of products from powders. Skills will be handed for understanding the processes, which play an important role in the fabrication of inorganic materials, especially ceramic materials. In this chapter first a “definition” of ceramics is given followed by some general aspects on ceramic processing. In general one can state that ceramics are inorganic (non-metallic) materials, mostly able to withstand elevated temperatures (in excess of 500°C). Ceramics are defined as inorganic compounds consisting of metals (e.g. silicon, aluminum, zirconium or titanium) and non-metals (oxygen, nitrogen or carbon) primarily held in ionic or covalent bonds. They are available as bulk materials or as a coating on other materials (metals, polymers or ceramics). One of the process steps in the fabrication of bulk ceramics (and sometimes also for ceramic coatings) is a firing or sintering step. Sintering means that it obtains a temperature treatment such that melting does not occur or on a limited scale only. A ceramic obtains its final characteristic properties after the sintering step. The resulting material has a certain microstructure; ceramics may be single-phase or mixed phase and there is generally porosity, ranging from almost 100% to near 0%. Ceramics find nowadays a widespread use in a broad range of applications. Generally ceramic materials are divided in two main groups: Traditional ceramics and advanced (also called engineering or technical) ceramics (see Figure 1-1). Examples are bricks, household porcelain, hightemperature furnace refractory materials, the insulating part of spark plugs, ferrite magnetic materials, BaTiO3 capacitor materials, etc. In this course we will focus on the processing of advanced ceramics. Advanced ceramics include ceramics for electrical, magnetic, optical, electronic and membrane (separation) applications (referred as to functional ceramics) and ceramics for structural applications (structural ceramics). However, it must be stated here that there is an analogy in processing of traditional and advanced ceramics. 1.1 The process – microstructure and homogeneity Up to about the first half of the 20th century most new products were seen as inventions rather than the planned outcome of research and development. Before the development of scientific insights of (ceramic) material processing, the properties of the product were often correlated with changes in a processing operation. The probability that adjustments, based on these empirical correlations, will produce significant advances is small, because the potential number of unsuccessful combinations of variables is always relatively large1 . The objectives of the science of ceramic processing are to identify (chemical and physical) characteristics of the material system and understand the effects of processing variables on the evolution of these characteristics. These characteristics or microstructural parameters determine the properties of the material. The engineering properties of (polycrystalline) ceramics are determined (controlled) by the microstructure which, in turn, depends on the process used for fabrication of the material. 1 “Principles of ceramic processing” ed. by James S. Reed, John Wiley & Sons (New York) 1995 Figure 1-1: Subdivision of ceramic materials in two main groups

1 Introduction 9 1.2 Fabrication of bulk ceramics The production process for ceramic materials is schematically given in Figure 1-5. The integral fabrication process is divided into four subsequent steps: 1. powder preparation, 2. compaction into a green shape, 3. sintering and 4. post treatment. Each step of this process has its specific influence on the microstructure and therefore control and knowledge of the whole process is important. Optimal properties require optimisation and control of each of these steps in a coherent way. Therefore, accurate attention should be paid to each processing step for obtaining (reproducibly) the desired, specific, property of the final material. The starting powder must have the desired chemical composition, particle size and morphology. Powder compacts are formed by bringing the powder particles close to each other. This processing step transforms the powder feed material into a green product with a specific size, shape, strength and microstructure. The formed compact should have sufficient strength for handling. Sintering this compact by heating it to high temperatures gives the product its final strength, microstructure and property. In order to obtain products that require close tolerances, post-treatments, like machining with more or less standard workshop tools, are carried out after the sintering step. Powder preparation In classical ceramics minerals, e.g. natural clays, are used as the raw material. These clays are in fact paste-like suspensions of silicate-based particles in water. With the development of ceramics and more specific, technical ceramics, in the past century requirements on composition, purity, particle morphology, size and degree of agglomeration increased enormously. Consequently the mineral-type raw materials were subjected to various refining steps such as particle and agglomerate reduction, purification and size classification. For ceramics with high added value the preparation of purely synthetic powders came into scope. Some examples of synthetic ceramic powder preparation are: • Recrystallization processes such as the Bayer process for the preparation of alumina powders • Precipitation is one of the wet chemical powder preparation methods. Here a precipitate is formed from a solution. A calcination process to form the final crystal phase often follows this process. • So-gel. With this technique particles are grown from a solution in a very controlled way in what one might call an “inorganic polymerisation process”. • Solid-state reactions in which the compound is prepared by reaction of two or more solid components. Preparation by solid-state reaction may have been preceded by a precipitation process in which for example hydroxides, nitrates, sulphates, acetates, citrates and carbonates are formed first and a final crystalline phase is formed by a calcinations (solid-state reaction) step. • Methods involving vapour as reactants. A process where all reactive molecules are in the gas phase is Chemical Vapour Deposition (CVD). Reaction bonding (or reaction forming) is a gasphase reaction were (porous) metal performs react with a gas (or sometimes a liquid) to produce chemical compounds like Reaction Bonded Aluminium Oxide (RBAO)2 , Si3N4 (RBSN)3 and SiC (RBSC) 2 N. Claussen, S. Wu and D. Holz, “Reaction bonding of aluminium oxide (RBAO) composites: processing, reaction mechanisms and properties”, J. Eur. Ceram. Soc., 14 (1994)97. Figure 1-5: Different processing steps in the fabrication of bulk ceramics

10 Advanced Ceramics Processing The purity and morphology of powders and the degree of agglomeration show a strong correlation with the method of preparation. For advanced, high quality ceramic components the powders are often prepared by controlled precipitation techniques. Powder preparation technology includes post-treatments to improve the particle morphology. The primary goal of these treatments is generally deagglomeration or deaggregation so that in the ideal case an assembly of “loose” primary particles is obtained with a narrow size distribution. Well-known examples of such treatments are wet attrition milling, vibration milling, dry yet milling and for lab scale applications ultrasonic deagglomeration treatments. Also controlled granulation by e.g. spray drying is a method to improve powder morphology (see chapter 6). Compaction Processing Table 1-1 gives a survey of the main processing techniques for the consolidation of a mass of fine particles to a shaped article, often referred as a green body. As can be seen from this table, the feed material for most of the processing techniques contains a mixture of powder and additives. Additives can be divided in solvents, deflocculants, binders, lubricants and plasticisers. It must be mentioned that all these additives must be evaporated or burned-out from the compact prior to the poreclosure step in the sintering process. Solvents are used to wet the powder particles and to provide fluidity. The extent of dispersion is influenced by deflocculants, which affect the electrostatic interactions or steric hindrance between particles. The main objective of deflocculants is the de-agglomeration of powders into uniform particles and the prevention of flocculation4 . Binders are added to provide enough adhesive strength, to increase the viscosity of the suspension or the paste and, thereby, influence the rheology of the system. In addition, binders are used to provide strength to the green body. Plasticisers are mostly used in combination with a binder to soften the binder in the dry state, thereby increasing the flexibility of the green compact. Lubricants are used in many compaction techniques to reduce the mutual friction between the particles and the friction between the particles and the die. For each of the processing techniques, which are discussed in this course, the use of additives will be addressed in combination with their purpose for that specific technique. After this green forming step the product must have sufficient strength for further handling. The most common compaction technique for ceramics powders is the relative cheap pressing technique. Sintering Sintering is the crucial technological step for obtaining the final ceramic material with its desired properties. It can generally be defined as a high-temperature process of matter redistribution stimulat- 3 A.J. Moulsen, “Reaction-bonded silicon nitride: its formation and properties”, J. Mater. Sci., 14(1979)1017 4 The word deflocculant is derived from the process of converting a flocked system or a batch of powder into a system of well-dispersed particles. In the same way the word dispersant is used, derived from the process of dispersing powders in a liquid. A stabiliser is added to maintain a system in a well-dispersed state. If these agents are described in chemical terms one speaks of surfactants in case of small interface-active molecules while larger molecules are simply indicated as polymers. In addition it is well-known that simple ionic compounds such as HNO3 can have a significant colloidal activity too through surface charging effects. Table 1-1; Feed materials and shapes of the green compact for the common forming methods. Forming method Feed material Shape of green product dry or semi dry processing uniaxial pressing isostatic pressing powder or free-flowing granules powder or fragile granules Small simple shapes Larger, more complex shapes Suspension/colloidal processing slip casting, colloidal filtration tape casting suspension with low concentration of additives suspension with relatively high binder content Thin complex shapes Thin sheets Paste processing extrusion injection moulding high viscous mixture of powder and binder mixture of powder and thermoplastic binders Uniform cross section Small complex shapes

1 Introduction 11 ed by the free energy, associated with the large free surface energy of fine particles in powder compacts. Decrease of the internal free surface during sintering leads, as a rule, to densification and strengthening of the compact. Generally no liquid phase (solid state sintering) or just a limited amount of liquid phase (liquid phase sintering) is present during sintering of technical (advanced) ceramics. So the driving force for sintering is the reduction of the interface energy. If no second phases are present equation (1.1) holds: ∆G = ∆Gs + ∆Gb = s∆As + b∆Ab γ γ (1.1) Where ∆G is the change in Gibbs energy of the compact, γs and γb the surface energy, and ∆As and ∆Ab the change in surface area of the solidvapour and the solid-solid (grain boundary) interface, respectively. Sintering occurs if ∆G < 0. Early in the process the decrease in Gs is the most important factor. Later in the process ∆Gs is smaller and accordingly the increase in ∆Gb must slow down to keep ∆G negative. This can be realised by grain growth. Solid state sintering is commonly divided into three, not clearly distinguishable, stages (see also Figure 1-6): • Initial stage; the powder particles grow together and necks are formed at the interfaces. Some particle rearrangement may occur as well. Porosity is in the range of 40-60%. A large influence of the particle size on the rate is observed. • Intermediate stage; the pores and particles form an intersecting network. The density increases to 85 or 90% where pore channels break up and form discrete pores. Consequently, the porosity type changes from open to closed, and grain growth occurs. • Final stage; discrete pores are present, which can only be removed by further grain growth. 1.3 Phase diagrams in ceramics In materials science phase diagrams are important tools. A phase diagram is a graphic representation of the phases as present in equilibrium state of materials with a certain composition and as function of temperature. In this paragraph the zirconia-yttria (ZrO2 –Y2O3) system will be treated as an example. Also in other parts of this course zirconia-yttria systems will be used as examples for the different steps in the ceramic fabrication process. At ambient pressure (undoped, pure) ZrO2 can occur in three crystallographic distinct polymorphs, depending on temperature: cubic, tetragonal and monoclinic. At room temperature, pure zirconia has a monoclinic (m) structure that is stable up to about 1170ºC, at which temperature it transforms into the tetragonal (t) structure. Upon further heating a transformation to the cubic phase takes place at 2370ºC and this phase is stable up to the melting point of about 2680ºC. By addition of stabilising oxides like MgO, CaO, Y2O3 and CeO2 the cubic phase can be stabilised to room temperature. In Figure 1-7 a phase diagram is given for the zirconia rich part of the ZrO2-Y2O3 system. Figure 1-6: Schematic representation of the microstructure development during the several stages of sintering: (A) “green” compact, (B) neck formation, (C) pore channels, (D) closed pores and grain growth