Table Of Content

1 Microstructural Targets for Ceramics Roger Morrell National Physical Laboratory, Teddington, Middlesex, U.K. List of Symbols and Abbreviations 2 1.1 Introduction 3 1.2 Controlled Porosity 4 1.2.1 Macroporous Bodies 4 1.2.2 Microporous Bodies 5 1.2.3 Nanoporous Bodies 6 1.3 Mechanical Strength at Room Temperature 6 1.3.1 Young's Modulus 7 1.3.2 Size of Flaw 7 1.4 Fracture Energy 9 1.5 Resistance to High-Temperature Deformation 13 1.6 Resistance to Thermal Shock 14 1.7 Hardness and Wear Resistance 14 1.7.1 Hardness 15 1.7.2 Sliding Wear Resistance 15 1.8 Thermal Conductivity 16 1.8.1 Enhanced Thermal Conductivity 16 1.8.2 Minimised Thermal Conductivity 17 1.9 Thermal Expansion 17 1.10 Optical Functions 19 1.10.1 Transparency and Translucency 19 1.10.2 Colour 19 1.10.3 Emissivity 19 1.10.4 Special Optical Functions 19 1.11 Specific Electrical Functions 20 1.12 Magnetic Functions 22 1.13 Resistance to Corrosion 22 1.14 Joinability 22 1.15 Concluding Notes 23 1.16 References 23 Materials Science and Technology Copyright © WILEY-VCH Verlag GmbH & Co KGaA. All rights reserved. 2 1 Microstructural Targets for Ceramics List of Symbols and Abbreviations A constant c size of crack E Young's modulus y toughness or work of fracture { a ultimate failure stress f YAG yttrium aluminium garnet 1.1 Introduction 1.1 Introduction - control of thermal conductivity, - control of thermal expansion, Commercial manufacture of ceramic - optical functions, products is only undertaken against a - specific electrical functions, number of targets, including: - specific magnetic functions, - properties of the material, - resistance to corrosion, - performance of the product, - size and shape of component, and re- representing the principal technical perfor- quired tolerances, mance criteria by which the suitability of - cost of the product. ceramics is judged for particular applications. The assumption is made that we are The first two of these are determined concerned with polycrystalline ceramics principally by the chemistry employed and produced from powders or other appropri- the microstructure achieved, while the lat- ate fabrication routes, not single crystals. ter two are questions of choice of plant, Clearly, chemistry is all-important in reliability of manufacture, and the overall defining the phases obtained within a cost-effectiveness of the process. Most ceramic, and hence the properties dis- ceramics, with the principal exceptions of played, but we will introduce chemistry tableware and decorative chinaware (al- only by way of examples to illustrate par- though some might dispute this exclusion), ticular types of product where optimisa- have technical functions for which particu- tion of microstructure has been achieved in lar sets of properties are required. To technical products. In this way we can dis- achieve these requires in turn, the selection cuss the principles uncluttered by excessive of chemistry, choice of raw materials, and and diverting practical detail. optimisation of processing towards an ide- al target microstructure. This opening Before dealing with each one of the chapter is devoted to a discussion of what above targets, it is necessary to look at the the target microstructure should be. To basic characteristics of ceramic micro- some extent, this is a very open-ended structures. When one takes an oxide or issue, and is difficult to generalise, but there non-oxide powder or powders, forms a are some guiding physical principles which shape from them and then consolidates the are usefully described as a starting point shape by a high-temperature process, the for the detailed formulation of materials. individual particles of the powder mass are encouraged thermodynamically (and, if In order to do this, this chapter consid- hot-pressing is applied, mechanically) to ers the targets from the point of view of the rearrange themselves and to join together following performance requirements or to form a solid body, usually by the move- desirable properties: ment of ions, and with or without the pres- - controlled porosity, ence of a liquid phase to assist the process. - mechanical strength at ambient temper- The final result is seldom the classical reg- ature, ular array of roughly equal-sized polyhe- - toughness, dral grains (Fig. 1-1), but is controlled by - resistance to deformation or creep at many factors, including: elevated temperatures, - thermal shock resistance, - the bulk chemical nature and overall - hardness, sliding wear or abrasion resis- composition of the powder(s), tance, - the surface chemistry of the powder(s), 1 Microstructural Targets for Ceramics results in the presence of residual porosity in the microstructure. Its complete elimination is limited by the ineffectiveness of sintering and grain growth processes, which are trying to reduce the total surface area of both exposed surface and grain boundaries. While complete elimination of pores is a desirable target for some mechanical properties, it is not for some oth- ers, such as filters, membranes, catalyst Figure 1-1. Typical single-phase, polycrystalline ce- supports, and thermal shock resistant and ramic structure comprising regular shaped polyhe- low thermal conductivity materials for a dral-shaped grains. variety of applications. However, the powder process allows us to control porosity by deliberately avoiding its removal in the - the particle morphology (surface area, consolidation process. particle size and shape), - the shaping process and the pressure 1.2.1 Macroporous Bodies used, - the thermal cycle employed to consoli- Packing of irregular similar-size parti- date the body, cles leaves a large volume fraction of pore space with irregular-shaped channels for and may be described by the following mi- permeation of fluids or for retention of air. crostructural features: This principle has been used for many - the chemistry of each phase, years for controlled filtration and for ther- - the size and shape of each phase, mal insulation. All that needs to be done to - the preferred orientation of each phase form a rigid ceramic filter body (e.g., Muil- (texture), wijk and Tholen, 1989) is to bond the large - the degree of elimination of porosity ini- particles together where they contact using tially present as the unfilled gaps in the a ceramic glue, which can be amorphous shaped body before firing (the "green" or crystalline in nature (Fig. 1-2 a). The shape). amount of bonding phase controls the strength of the material, but in small quan- This complex list of variables in real life tities does not affect the filter characteris- gives the ceramic manufacturer an almost tics. inexhaustible supply of options for making An interesting example is a ceramic dust products. It is the achievement of selected filter for power or incineration plant (Mor- microstructural features that allows ceram- rell et al., 1990). A coarsely porous body is ics to find their applications, and thus to used as the filter wall to give a low pressure create business. drop across the thickness of material, which obviously has to be adequately 1.2 Controlled Porosity strong to support itself mechanically under the service conditions. So that fine parti- The use of powder processing technolo- cles are trapped, a surface coating may be gy for the fabrication of ceramics usually applied which has finer pores. This coating 1.2 Controlled Porosity additional porosity is included by adding granular combustible or other fugitive organic material, such as sawdust, to the green body. Another example of a macroporous body is a metallurgical filter (Minjolle, 1990). The main ceramic material is designed to be dense, fine-grained and strong after consolidation, but the controlled pore or channel size is introduced typically by using a plastic-based foaming technique for green state shaping. The ceramic precursor powder is retained in the walls of the foam, especially in the network of thick ligaments between each void in the foam. On removal of the plastic and consolidation of the ceramic, a controlled channel size remains (Fig. l-2b). Channel sizes from a fraction of a millimetre, up to several millimetres, can be made in this way. By appropriate choice of foaming agent, both open cell and closed cell structures can be developed, the latter having potential val- Figure 1-2. Cross-sections of controlled open-porous ue for low thermal conductivity. ceramic structures for filters: (a) fabricated from ran- dom packing of large grains bonded by a small pro- portion of glass or ceramic bond (dark); (b) fabricated 1.2.2 Microporous Bodies by foaming a fine powder mix and sintering to give a dense skeletal structure. In contrast, microporous bodies are normally prepared simply by not allowing traps the first layer of dust, which then acts the consolidation process to proceed to as its own filter. greater than about 90% of the full theoret- The easiest way of controlling the pore ical density. Such products retain open size is by controlling the particle size of the pore channels through the structure, while precursor powder. A narrow size distribu- at higher densities, residual pores are no tion of large particles can be assembled longer interconnected. For electrical or with some fine particles which on firing act mechanical applications this would be un- as a bond to join the large particles togeth- desirable; penetration of water or dirt can er. The channel size left between the large represent limitations in service; but if particles is typically half the large particle porosity is controlled in size by the nature size. In the dust filter example, the pressure of the precursor powder, the product can drop across the filter body is controlled by be used to make particulate filters. the channel size, which is controlled by the Sometimes, porosity of this type is ad- starting large particle size of the ceramic ventitious, especially in conventional re- body, not the bonding phase. In the cases fractory products (Chesters, 1973). Large of insulating bricks and grinding wheels, grains and refractory bonding phases are 1 Microstructural Targets for Ceramics needed to ensure minimal dimensional 1.2.3 Nanoporous Bodies change on consolidation, and dimensional If controlled porosity at submicrometre stability and retention of strength at very levels is mandatory, a different strategy is high temperatures. These large grains tend required. Very small pores tend to be elim- to restrict the elimination of the pore space inated very rapidly in the powder consoli- in the shaped green body because they are dation process. It is then usually necessary not as sinter-active as very fine particles to consider specialized methods of obtain- with large surface area, and as a result, ing the required pore structure, and to most bulk refractories have open porosity. counter the thermodynamically driven ten- However, this is of considerable advantage dency for their removal. This usually calls in controlling thermal stress damage resis- for processing tricks to generate appropri- tance. The presence of pores reduces the ate structures at low temperatures where elastic stiffness of the material, so less ener- pore elimination rates are much reduced. gy than in a fully dense body is available For example, the process for forming sili- from thermal strains to cause cracking con nitride by the nitridation of silicon (the damage. In addition, pores may tend to reaction bonding process) can be used to pin cracks, and such enhanced resistance make very finely porous bodies by careful to damage is much sought after by refrac- control of the starting silicon powder size tory engineers. and homogeneity. Other routes can be A useful attribute of many oxide-based used to make thin membranes with con- ceramics is low thermal conductivity. trolled pore size, such as by applying a very However, the thermal conductivity of air fine powder surface layer to a pre-sintered in the absence of convection is even lower, coarsely porous body (Van Praag et al., and thus the trapping of air inside a low 1989), or by using chemical vapour deposi- conductivity solid is the chief route for ob- tion partially to seal coarser open porosity, taining improvements in thermal insula- or by anodising aluminium to form partic- tion (Budnikov, 1964). However, there is a ular structures in aluminium oxide. limit to how low the density of a solid body On an even finer scale, members of the formed from powder particles can be be- zeolite family have crystal structures with fore the strength is insufficient for practi- defined "atomic" tunnels in them, which cal use. Lower densities are better achieved allow the passage of small gas molecules, by using either hollow spheres (bubble alu- but not large ones (Breck and Anderson, mina, glassy cenospheres from fly ash) or, 1981). Zeolites can be formed into ceramic much more commonly, fine fibres, which bodies, or can be used as a coating on a are now very widely used for ambient and porous substrate. The chemical composi- high-temperature thermal insulations. For tion is used to control the structure type, near-ambient temperature use, even better and hence the tunnel size. properties can be achieved from so-called "aerogels" (Gowda and Harrison, 1987; Fricke and Caps, 1988; Fricke and 1.3 Mechanical Strength Emmerling, 1992), which are basically very at Room Temperature low density skeletal forms of silica and other oxides, which need encapsulating The mechanical strength of ceramic within rigid boundaries in order to retain products is controlled by microstructural them in place. defects which, if not already in the form of 1.3 Mechanical Strength at Room Temperature small cracks produced during microstruc- 1.3.2 Size of Flaw tural development or by external damage, form weak points or regions from which a Flaws or defects in ceramics take many crack can propagate when subjected to a forms, from large scale voids or delamina- sufficiently high stress level. The optimisa- tions several millimetres in length to tion of strength in any microstructure thus micrometre sized intentional features of requires the minimisation of the size and the microstructures. The larger the flaw (or number of such defects or flaws by atten- its effective zone of influence) the lower tion to processing conditions. This target will be the strength. Thus in generating a applies irrespective of any additional ma- microstructure, strength can only be opti- terial modifications designed to enhance mised by removing as many flaws as possi- strength and/or toughness further. ble, but especially the largest ones. Very The weakest link concept of brittle frac- small flaws can be considered as having an ture in ceramics (Davidge, 1979) means insignificant role in determining strength that the ultimate failure stress, c, is deter- while large flaws remain. Table 1-1 shows f mined by the size of crack c and the tough- origins of and the typical scale of various ness or work of fracture, y of the material types of defects or flaws. i5 a"t the- inmstant of failure: The size of defect that can be tolerated in a ceramic product depends on the pur- (1-1) pose of the product and the strength level required for that purpose. For materials where E is Young's modulus and A is a where only intermediate to low strength is constant determined by the geometry of required, such as in refractories, often the crack in relation to the direction of strength tends to be controlled by the ef- stressing. In order to maximise the fectiveness of bonding between large parti- strength, it is desirable to maximise E and cles, with factors such as particle size and y whilst minimising c. Let us consider total porosity (0-40%) playing a lesser i9 each of these contributory factors. role. Unintentional defects, such as foreign contaminants, which play a major role in strength control in strong materials, can often be safely ignored, especially in 1.3.1 Young's Modulus coarse-grained products. The Young's modulus of ceramics is de- Where medium levels of strength are re- termined by two factors, the chemistry of quired (typically 100-400 MPa), it is es- the crystalline and amorphous phases, and sential to remove the larger defects, but the level of total amount of porosity. In small ones can usually be tolerated. For order to maximise E, it is necessary to se- example, in high-alumina ceramics with lect crystalline phases with inherently high flexural strengths of typically 300 MPa, modulus and, particularly, to reduce strength and elastic modulus can be en- porosity levels as far as possible. The for- hanced by removing as much of the total mer of these options may clearly be re- porosity as possible, but usually a few per- stricted by other performance attributes cent remains. Strength tends then to be required of the material, but the latter is a controlled by the grain size rather than by desirable target in the majority of circum- pores less than about 10 jim across, unless stances. they form groups (e.g., resulting from a 8 1 Microstructural Targets for Ceramics Table 1-1. Flaws - their size ranges and origins. Type of flaw or defect Typical size range Origin Delaminations 1-100 mm Problems in pressing or removal of binder; drying shrinkage Foreign organic matter 0.1-5 mm Contamination in powder handling Foreign inorganic matter 0.01-1 mm Contamination in powder handling Green machining surface faults 0.01-0.05 mm Binder not strong enough to withstand machining Aggregates causing voided regions 0.03-1 mm Powder binder mix too hard, aggregates not crushed by pressing Individual pores 0.001-1 mm Inhomogeneity of compacted powder; lack of completeness of sintering Large grains Microstructure Selection and processing of raw materials; scale dependent chemical formulation; fired too hot Machining damage 0.001 -0.5 mm Grinding with too coarse a diamond grit or too great a depth of cut Grain size 0.0005-1 mm Chemistry and physics of powder; firing conditions porous seam or aggregate). It may not be strength should not be ruined by inade- worthwhile trying to remove a small quate attention to machining procedures amount of fine-scale porosity. which generate surface flaws. In fact, the For ultra-high strength (typically latter may be the fundamental limitation in > 400 MPa), the defect or flaw size must any ceramic product, either in shaping to be reduced as far as possible by the appro- the final size and tolerances, in handling, priate attention to selection of raw materi- or in service. als and their processing conditions. It is The mechanical integrity, or for that imperative to minimise the risk of contam- matter many other functional attributes, ination by adopting clean processing con- of advanced types of ceramics when pro- ditions such that no foreign bodies enter cessed to avoid extraneous defects is thus the batch. Even mill-ball debris has to be essentially determined by a combination of avoided, and consideration should be giv- pore size and grain size. To obtain uni- en to using the fired product as the wear- form, consistent performance, a uniform resistant surfaces in processing equipment. grain size with a minimum of porosity If binders are added, they should not form (preferably with pores rather smaller than hard agglomerates which do not crush on the grains) is the practical target in many shaping the green body, or which do not circumstances. Even then, the grains them- shrink as much as surrounding material on selves will act as the fundamental limita- firing. Chemistry must be controlled to tion. A grain boundary can act as a flaw, achieve close-to-theoretical density with and the following are examples: minimum residual porosity whilst main- 1. In some cases, the flaw will be present taining control on the maximum grain size. as a crack in unstressed material, simply as Finally, the achievement of high intrinsic a result of differences in thermal expansion 1.4 Fracture Energy coefficients between contacting grains 3. Surface machining by abrasives re- which do not have the same crystallo- moves material by a ploughing and/or graphic orientation, or between grains or chipping process that requires the forma- grain boundary phases of different chemis- tion of microcracks. These can run into the try or crystallography (Davidge, 1982) bulk of the material to a depth controlled (Fig. 1-3). This can be shown to occur by the forces applied and by the toughness. when the elastic energy associated with the It is thought that one factor limiting depth thermal expansion mismatch exceeds that of damage may be the grain size, provided required to fracture the boundary. Exam- that this is greater than about 5 jim. There ples include coarse-grained alumina, alu- have been a number of demonstrations in minium titanate, magnesium dititanate the literature on dense, single-phase ce- (Kuszyk and Bradt, 1973), and beta-eu- ramics in which the flexural strength of cryptite (Li O A1 O -2SiO ). The effect machined test-bars is proportional to c~1/2 2 2 3 2 can be minimised by reducing grain size, as predicted by the Griffith equation (1-1) although this may not always be possible above (e.g., Davidge, 1979). This suggests whilst retaining other desirable properties that damage is limited in depth by the (e.g., low expansion coefficient in alumini- grain boundaries that the microcracks um titanate). meet. Below about 5 jim grain size, dam- 2. Phase changes in the ceramic during age seems more likely to jump several cooling after firing can in some circum- grains, and the c~1/2 relationship tends to stances produce microcracks between disappear. grains. A classic example is the quartz In summary, the first microstructural transition seen in conventional siliceous target for strength should be the reduction porcelains (Kingery et al., 1976), but it in size and amount of all types of defect, also occurs with unstabilized zirconia used including pores, large grains, foreign bod- as a reinforcing agent in many different ies and machining damage. The second types of advanced technical ceramic. Opti- should then be the control of phase con- misation of strength clearly requires such tent, grain shape, and the use of any rein- phase changes to be controlled or sup- forcement that can conveniently and pressed. simultaneously enhance the toughness. This is discussed below. 1.4 Fracture Energy The fracture energy of ceramics is close- ly related to the phase composition, and to some extent the grain structure (Davidge, 1973, Evans, 1988). Glasses, having fea- tureless microstructures, show very little resistance to the propagation of small cracks, and thus possess low toughness. Substantially crystalline microstructures Figure 1-3. As Fig. 1-1, but with microcracks on cer- which tend to fracture across the grains tain boundaries caused by thermal expansion mismatch. (transgranularly) (Fig. 1-4 a) also show 10 1 Microstructural Targets for Ceramics in its path. In ceramic/metal systems, such as hardmetals or cermets, a metallic phase bonding ceramic particles together is used to ensure that fracture involves plastic deformation of the metal phase, which ab- sorbs considerable amounts of energy. In fact, ligaments of metal can often be seen across the fracture (Fig. 1-5). However, in an all-ceramic, i.e., all-brittle system, we (a) cannot employ plastically deforming ligaments. Enhancement of toughness requires absorption of energy by increasing the de- viation of the crack from a planar condi- tion. We already have some roughness if the microstructure naturally cracks inter- granularly, but we should seek to encour- age either further deviations of the fracture surface, or the generation of more damage over a wider zone than just the immediate crack plane, or we can make it more difficult for the crack to open by ensuring brit- (b) tle ligaments remain uncracked or wedged across the fracture (Fig. 1-6), or by grow- Figure 1-4. Brittle fracture through the structure shown in Fig. 1-1: (a) transgranular fracture (strong ing elongated grains. Reinforcement using grain boundaries, weak grains); (b) intergranular frac- strong whiskers (Becher etal., 1986) is ture (weak grain boundaries, strong grains). probably the most effective. This concept is shown schematically in Fig. 1-7. low toughness compared with those which There has been much research in recent tend to fail along grain boundaries (inter- years into such toughening mechanisms, granularly) (Fig. l-4b), because in the for- with a better understanding being devel- mer case, grain boundaries offer little if any resistance to the propagating crack while in the latter case, the propagating crack meets added resistance as segments of it are diverted in fresh directions when they reach the end of one grain facet. This is a simple view of the typical situation in a polycrystalline material. In practice, the microstructure may be manipulated both in chemistry and in physical arrangement of phases to maximise the toughness (Clarke, 1992), undoubtedly topics of con- Figure 1-5. Toughening the microstructure shown in tinuing research for many years to come. Figure 1-1 by ductile metal particles which bridge the To impede the development of a small crack faces. This effect results in rising crack resis- flaw into a crack, obstacles must be placed tance with increasing crack length.