علم سرامیک

Ceramic Materials 1.1. Ceramics Defining what the term “ceramic” means is not simple, as there is no single definition on which everyone agrees; there are in fact various definitions depending on the point of view adopted. We can thus consider the points of view of a historian, a scientist (physicist, chemist, etc.), an engineer or a manufacturer. 1.1.1. Ceramics and terra cotta The concept of ceramics is historically related to the concept of terra cotta and pottery, from which the Greek term κεραμοσ derives. This vision refers to the soils, the crushed rocks, that is, the geological materials and it also highlights firing: ceramic art is the art of fire, even if the ceramist has his feet in clay… The potter chooses suitable soils, primarily clay soils, which in the wet state offer the plasticity required to model them in the desired form: cup, vase or statuette. Then the piece is dried and water loss makes it lose its plasticity, but rehydration would restore the clay’s initial properties. In fact, dried clay is not yet a ceramic, although it is utilized in the production of rudimentary bricks used in very dry countries – like Saharan Africa. It is in fact the firing that causes the irreversible Chapter written by Philippe BOCH and Jean-François BAUMARD. 4 Ceramic Materials physicochemical transformations resulting in a material that has lost its plasticity and is no longer capable of rehydration: ceramic. Terra cotta bricks or flower pots are examples of these products, whose visual appearance is not very different from dried clays, but whose mechanical resistance is much higher and, for which water- insensitivity constitutes an essential property. The identification between “ceramic” and “terra cotta” gathers together the basic concepts that we will continue to encounter throughout this book: powdery mineral raw materials [RIN 96], the shaping which is made possible by the plasticity of wet clay, the heat treatments which start by drying (reversible dehydration) and continue with firing (irreversible dehydration and permanent physicochemical modifications). We have not yet mentioned in the description a major characteristic that conditions the preparation techniques as well as the uses of ceramics: brittleness. The flower pot is hard (it can scratch a metal sheet) but is vulnerable to impact. This brittleness is a hydra with many heads, as it implies: i) lack of ductility and plasticity; ii) low toughness and therefore great sensitivity to the notch effect; iii) poor mechanical impact resistance, i.e. poor resilience; iv) vulnerability to differential expansions, therefore vulnerability to thermal shocks; v) mechanical tensile strength significantly lower than compression strength; vi) significant dispersion of the mechanical strengths in samples believed to be identical. But the hydra can be tamed and these drawbacks come with a set of qualities that make ceramics irreplaceable for innumerable applications. We must also point out that artists, historians or museologists often use the term “ceramic” as a noun rather than as an adjective: “a Greek ceramic…” meaning “a Greek ceramic vase…”. 1.1.2. Ceramics: physics, chemistry, science and materials engineering The definitions depend on the point of view adopted: – solid state physicists are particularly interested in the electronic structure of solids and their conduction properties [KIT 98]. It is therefore natural for them to classify solids – as they seldom speak about materials – in a ternary division: i) insulators ii) semiconductors and iii) conductors and superconductors. The term ceramic is rarely used, but when it is, it refers to oxides, whether they are insulators Ceramic Compounds: Ceramic Materials 5 or conductors, even superconductors like cuprates, which earned Bednorz and Müller the Nobel Prize [BED 86]; – solid state chemists attach particular importance to the nature of the bonding forces, by distinguishing three types of strong bonds (metallic bond, ionic bond and covalent bond) and various variants of weak bonds (like the Van der Waals bond) [JAF 88, WEL 84, WES 90]. Ceramics are therefore solids with essential bonds. However, solid state chemists usually reserve the term ceramics for polycrystalline materials, as opposed to crystals (monocrystalline) and glasses. Thus, a crystallized polycrystalline silica (SiO2) – for example, in the form of quartz – obtained by sintering (we will explain the meaning of this term a little further on) [GER 86] is regarded as a ceramic, but not the corresponding monocrystal (it is a crystal), or quartz glass – whose amorphous structure makes it unworthy of being qualified as a ceramic; – materials science adopts a ternary classification that distinguishes i) organic materials, ii) inorganic and metallic materials and iii) inorganic and non-metallic materials. The concept of ceramic compounds is extremely broad here, as it is synonymous with the third category of inorganic and non-metallic materials. Therefore, ceramic compounds include most minerals and rocks, i.e. almost the entire crust of our planet. It should be noted that this definition of ceramics shifts the problem to other definitions, including the exact meaning that must be given to the term metallic. Tungsten carbide (WC), for example, can be classified among ceramics, but it is an electronic conductor that physicists classify within the group of metallic solids; – materials engineering is an engineers’ science, and thus considers not only the chemical composition, structure and properties of solids, but also their method of preparation. It is therefore appropriate to split in two each of the three categories put forward by materials science and separate natural materials from synthetic materials. Organic materials are then differentiated into natural products (like wood) and synthetic products (like most polymers) – but under what can we classify plywood? Metallic materials distinguish the rare native metals (gold, sometimes copper) from all metals and alloys that are derived from industrial processes. Finally, inorganic and non-metallic materials, classify on one side minerals and on the other rocks and ceramics or, more exactly, the triplet: ceramics + glasses + hydraulic binders (cement and plaster are examples of hydraulic binders). All these definitions are useful and they are not mutually exclusive. It is advantageous, depending on the subject treated, to adopt a particular point of view – the vision of the physicist being undoubtedly preferable when we consider the electronic conduction properties of ceramics and the vision of the materials engineers being undoubtedly more relevant when we have industrial concerns. Now we will discuss the most widely used manufacturing techniques, as they help us to locate and define what we will mean by ceramics in this book. 6 Ceramic Materials 1.1.3. Powders; sintering We have said that brittleness is a predominant feature of ceramics – at least at normal temperatures, because at very high temperatures (typically above 1,000°C), a certain plasticity can be observed. Brittleness causes limitations in the uses of ceramics (it is advisable to avoid bulls in a china shop…), but it also induces restrictions in the production techniques applicable to ceramics. This opposition is most evident between metals and ceramics: – a metallic object is typically obtained in two stages: production of the material in the form of a semi-finished product (bars, sheets, wires, etc.), and then production of the object (body of a car or a bolt, etc.). The processing methods take advantage of the fact that most metals are sufficiently ductile and malleable to laminate them, to deform them plastically, to stretch them, etc. and that they are sufficiently soft to cut them, turn them, mill them, bore them, etc.; – the production of a ceramic object is not dissociated from the processing of the material: the potter does not cut his vase in an already consolidated ceramic preform! The firing of the wet clay rough shape solidifies the object in its final form while simultaneously allowing physicochemical reactions transforming the raw materials into ceramic phases. In most cases, the ceramist creates the object whilst working on the material that constitutes it. It is true that some metal parts are produced by foundry, a process where the processing of the material and preparation of the object are concomitant. But foundry is seldom applied to ceramics because of the high temperatures that would be required – as the melting points of ceramics are generally higher than those of common metals – and also because of its brittleness, which makes it necessary to avoid thermal shocks and differential expansions. All these limitations on the possible manufacturing processes explain why the basic technique for the preparation of ceramic parts is sintering, i.e. the transformation, using the mechanisms of atomic diffusion, of a powdery substance – a non-cohesive granular medium made up of loosely agglomerated particles, therefore without any marked mechanical properties – into a consolidated substance – a cohesive granular medium whose grains are strongly linked to one another, hence with strong mechanical properties of a solid and not the poor performance of a powder. We can give the example of a snowball, a non-cohesive material that we can throw at someone’s face without any risk of hurting that person, but which can be transformed into a block of ice, a cohesive material, which can hurt when thrown like a stone. The movements of matter (atomic diffusion) that allow sintering are activated thermally and it is only when the temperature is sufficiently high (typically 0.5 Tf to 0.8 Tf, where Tf is the melting point expressed in Kelvin) that sintering occurs at a usable speed. Snow is transformed easily into ice because, even in very cold areas, temperatures remain high in relation to melting, at 273 K. But for ceramics, where Tf generally exceeds Ceramic Compounds: Ceramic Materials 7 1,300 K and can exceed 2,500 K, sintering requires high temperature firing: that is why ceramics is an art of fire. We may note that sintering is the basic process for the preparation of ceramic parts, but it is also increasingly used for the manufacture of metallic objects (powder metallurgy). What is true for ceramics can be transposed, partly, to glasses and hydraulic binders: these three types of materials are interrelated and we can say that all three bring into play ceramic compounds. But if we consider that a ceramic object is made of ceramic compounds processed by ceramic techniques, we can differentiate these materials by the order in which the three fundamental steps of the process take place: powders (P), forming of the object (F) and heat treatments of drying and sintering (HT): – P→F→HT: the manufacture of a ceramic component starts from a powdery medium (P), continues with its forming (F), and then ends with the heat treatments (HT). The consolidation of the material is done during sintering, therefore during the high temperature treatment; – P→HT→F: the manufacture of a glass component also starts with the powdery medium (P), but this is followed by heat treatments (HT), which must result in melting, whereas the consolidation occurs at the end of the process, at the time of the solidification of the magma on cooling, therefore at the same time as the forming of the object (F); – HT→P→F: the use of hydraulic binders starts with heat treatments (HT) – for example the firing, at about 1,450°C, of a mixture of limestone and clay for the preparation of cement clinker – then the reactive powders (P) thus produced are formed (F) and consolidated at normal temperatures, due to chemical reactions. This differentiation of ceramics, glasses and hydraulic binders based on the order of operations P/F/HT is rather simplistic, as it refers only to the most common cases. In fact, some ceramics are prepared by other sequences than the P→F→HT trio, some glasses are fashioned by sintering, and there are other exceptions, some of which will be discussed in detail in this book. However, the classification suggested is sufficiently relevant to be retained here, which implies that this volume on ceramics does not consider either glasses [ZAR 82] or hydraulic binders [BAR 96, TAY 97], these materials being studied in separate books. A logic emphasizing “the scientific aspects” rather than “the technical and industrial aspects” could have, on the other hand, justified combining the three categories of materials. The distinction made here between industrial ceramics and glasses lies in the method of preparation, not in the difference between crystalline solids and amorphous solids. In fact, some ceramics are mainly made up of vitreous (amorphous) phases, whereas vitroceramics are obtained by crystallizing a glass (devitrification). 8 Ceramic Materials 1.1.4. A few definitions In the reference work on ceramics [KIN 76], the authors place “the art and the science of ceramics in the production and use of objects formed of solids, whose essential components are inorganic and non-metallic materials”. This definition includes not only potteries, porcelains, refractory materials, terra cotta products, abrasives, sheet enamels, cements and glasses, but also magnetic non-metallic materials, ferroelectric materials, synthetic monocrystals and vitroceramics, not to mention “a variety of other products that did not exist a few years ago and the many others that do not yet exist…”. Kingery et al. stress that this definition largely exceeds terra cotta products consolidated by firing, to which the Greek term κεραμοσ refers, just as it exceeds the definitions given by most dictionaries – but our Petit Larousse is apt, defining the adjective “ceramic” as “relating to the manufacture of potteries and other terra cotta objects (including earthenware, stoneware, porcelain)” but also “ceramic material or ceramic: manufactured material that is neither a metal nor an organic material”. Kingery et al. adopt a scientific approach, which considers compounds rather than materials and thus includes glasses and cements among ceramics, but they do not make the jump to natural materials, since they mention only synthetic monocrystals, and yet the quartz that man has synthesized to cut out small piezoelectric resonators that form the heart of modern watches is a twin of the natural rock crystal. In the Dictionary of Ceramic Science and Engineering [OBA 84], we find a rather restrictive definition of ceramics: “Any inorganic and non-metallic product prepared by treatment at temperatures higher than 540°C (1,000°F) or used under conditions implying these temperatures, which includes metallic oxides and borides, carbides, nitrides and mixtures of these compounds”. We will see that kaolinite Al2(Si2O5)(OH)4 – which is the main mineral of clays, hence the name kaolin given to some clays rich in kaolinite – undergoes irreversible reactions that transform it into metakaolinite when it is fired above approximately 500°C, which could justify the temperature of 540°C selected in the definition used to make the distinction between dried earth and terra cotta. In the Concise Encyclopedia of Advanced Ceramic Materials [BRO 91], which presents the common viewpoint held in Europe, we can distinguish materials and processes: – ceramic materials are based on inorganic non-metallic compounds, primarily oxides, but also nitrides, carbides, silicides; they must contain at least 30% of crystallized phases in volume; they exhibit a fragile behavior, with a stress-strain curve that obeys Hooke’s law of linear elasticity; Ceramic Compounds: Ceramic Materials 9 – ceramic processes bring sintering primarily into play, at temperatures higher than 800°C. We will now pause the study on the definitions of the word “ceramic”, which was necessary to bring to light some of the keywords that we will find throughout this book (non-metallic inorganic compounds, mineral powders, heat treatments and sintering, brittleness, etc.) and also to justify the differences that arise from the variety of possible points of view. If we propose our own definition, it could be the following: ceramic materials are synthetic materials, mainly composed of ionocovalent inorganic phases, not fully amorphous, and generally consolidated by the sintering at high temperatures of a powdery “compact” formed into the shape of the desired object, the starting powders being frequently prepared from crushed rocks. We do not think it necessary to define a threshold for the sintering temperature; it is important, with the development of composites, to suggest that besides inorganic phases, minority organic phases can occur; the existence of interesting ceramic materials with electronic conduction means that the reference to the iono-covalent character of the bonds does not exclude a partially metallic nature; lastly, it is necessary to insist on the closeness between the world of ceramics and that of minerals and rocks. If we were to expand this idea, we could affirm that “ceramics are synthetic rocks”. 1.2. Ceramic compounds Defining a ceramic compound as a non-metallic inorganic compound is a paradoxical choice, because it supposes that the concept of metal is sufficiently unambiguous to serve as basis for clarifying its antonym. However, confusions are frequent here, and are aggravated by the fact that ceramic compounds are metallic- non-metallic compounds, which we shall now study in detail. 1.2.1. Chemistry of ceramics Metallic elements form a majority among the elements of the periodic table. We know that these elements are located on the left of this table, and are therefore electropositive elements – which tend to lose electrons to yield positively charged cations. Non-metallic elements, sometimes denoted by their old name metalloids, are located on the right of the table: they are electronegative and tend to capture electrons. The ionic bond is illustrated by the attractions that develop between a metal, sodium, and a non-metal, chlorine, to create sodium chloride NaCl – also written as Na+Cl-. 10 Ceramic Materials Oxygen, a non-metal, is the most abundant element in the 45 km-thick Earth’s crust (approximately 47% in mass) [EMS 95]; then comes an element at the border between metals and non-metals, silicon (≈ 28%), and then metallic elements, particularly aluminum (≈ 8%) and iron (≈ 4%). Aluminum and iron are the most widespread metals – if we reason in terms of elements, but in a world rich in oxygen, almost all metals tend to oxidize, so that only noble metals remain in the native state, more stable than their oxides: we can find in the soil a few gold nuggets and a little native copper, but never blocks of aluminum or iron; in other words, we do not find in an isolated state materials whose constitutive elements are so abundant. Let us remind ourselves that we left the Stone Age only a few thousand years ago to reach the Bronze Age and then the Iron Age; that it was only in 1824 that Berzelius isolated silicon and in 1825 that Oersted isolated aluminum. In short, it is obvious that the reduction towards the constitutive metal of very stable metallic oxides – their enthalpies of formation are several hundred kilojoules per mole – is a difficult process. The essentially iono-covalent, non-metallic compounds that constitute ceramics are compounds formed between metals and non-metals. The opposition – the word is not too strong – between a metallic material and a metallic oxide can be illustrated by the comparison between a metal, aluminum, and its oxide, Al2O3 (alumina in English, where oxides are named by adding the ending “a” to the name of the metal: aluminum/alumina, silicon/silica, magnesium/magnesia, uranium/urania, etc.): – aluminum melts at low temperatures (660°C); it is an excellent conductor of electricity and heat, opaque to visible light, soft and very ductile, its modulus of elasticity is low (one-third that of steel) and it is vulnerable to the aggressions of multiple chemical reactants; – alumina melts at high temperatures (2,050°C); it is one of best known electrical insulators and a poor heat conductor; it is transparent to visible light, very hard but brittle, its modulus of elasticity is high (double that of steel) and it resists most aggressions of chemical reactant: in particular, it is perfectly stable in an oxidizing medium, as it “is already oxidized”. This comparison brings to light the fact that metals and metallic oxides exhibit different, even opposite, characteristics. Regarded as compounds formed between metals and non-metals, ceramics can therefore be classified with reference to the non-metal that is involved in the bond. The abundance of oxygen is such that the largest share belongs to oxides, while within these oxides, the abundance of silicon privileges silicates – which combine oxygen and silicon. Besides oxides, we find carbides and nitrides (the non-metals being respectively carbon and nitrogen), as well as borides, silicides (not to be Ceramic Compounds: Ceramic Materials 11 confused with silicates), even halides (chlorides, fluorides, iodides, etc.), sulphides, etc. However, another separation distinguishes silicate ceramics and non-silicate ceramics [CER 99]. 1.2.2. Silicate ceramics and non-silicate ceramics The abundance of oxygen, silicon and aluminum implies that the Earth’s crust consists predominantly (more than 97%) of silicate minerals, particularly aluminosilicates. Granite, for example, is a rock made up of quartz (the usual crystallized form of silica SiO2), feldspars (for example potassic feldspar KAlSi3O8) and various micas-aluminosilicates containing iron, magnesium or sodium; clay is a rock that always contains a high proportion of the mineral kaolinite Al2(Si2O5)(OH)4, etc. We can mention here a practice of ceramists that can lead to confusion: they write chemical formulas of oxide compounds as if they were elementary mixtures of oxides and not compounds: KAlSi3O8 is written as K2O-Al2O3-6SiO2 and Al2(Si2O5)(OH)4 is written as Al2O3-2SiO2-2H2O. This does not obviously mean that potassic feldspar is an agglomerate of the oxide of potassium, aluminum and silicon, and that kaolinite is a mixture of alumina, silica and water. A common error is therefore to think that the high proportion of aluminum in the soil, which is the reason behind the frequent occurrence of “alumina” in the writing of the constitution of minerals, makes this alumina a common mineral: this is not the case, as can be testified by the fortunate but rare owners of the main gem whose chemical composition is Al2O3: sapphire! Since the Earth’s crust is primarily made up of silicates, we can understand why all ceramics that come close, by near or by far, to terra cotta are silicate materials. Silicate ceramics form, in tonnage, the majority of the world of ceramics. They are often described as traditional ceramics, a term which we do not endorse because it may be understood as opposed to progress and technical improvements – whereas many silicate ceramics are sophisticated materials – but which is justified by history: it was only at the end of the 19th century that non-silicate ceramics came to the scene, with specific uses that explain their other name, technical ceramics. Our choice here is to use “silicate ceramics” and “non-silicate ceramics”, rather than opposing tradition and advanced technology. We may note that almost all industrial glasses and cements are also silicate compounds. 12 Ceramic Materials 1.3. Silicate ceramics If we consider industrial products, and forget for the moment glasses and cements, the world of silicate ceramics (see Chapter 4) consists of the usual products: – terra cotta products, the most important of these being bricks and tiles, sometimes known as red products because of the color that they owe to the iron oxides they contain; – ceramic tiles; – sanitary ceramics; – tableware: colored, opaque and waterproof sandstone, often colored, opaque and porous earthenware, but generally covered with a waterproofing enamel – sometimes well, sometimes badly when it is cracked, but the esthetic quality is then enhanced – white, translucent and waterproof porcelain; white, opaque and waterproof vitreous objects; – technical ceramics, for example porcelains for electric insulation or porcelains for resistance to chemical attacks; – bioceramics (dental prostheses, etc.); – certain refractory materials, which serve as protection or insulation elements within devices that must function at high temperatures (lining of chimney hearths, etc.); – enamel sheets, to cover and protect steel sheets that are part of our refrigerators and washing machines. These enamel sheets are too often forgotten in works dealing with ceramics and glasses – but as these enamels are primarily amorphous and prepared by melting, it is better to classify them among glasses than among ceramics; – silica products – but it is common practice to classify them among technical ceramics, therefore paradoxically, to exclude them from silicate ceramics. Though non-exhaustive, this list shows the variety of the products involved. As regards the composition on the one hand, and the raw materials brought into play on the other, a silicate ceramics can be located primarily in a ternary diagram: – by adopting the conventional writing in the form of “mixtures” of elementary oxides, the composition of most silicate ceramics is in the pseudo-ternary diagram SiO2-Al2O3-MxOy, where MxOy is an oxide like Fe2O3, K2O, Na2O, MgO, TiO2, etc. The natural abundance of the element iron explains, as we have seen, the red, brown and yellow coloring of a number of silicate ceramics, where the degree of oxidation of iron (Fe3+: ferric-iron or Fe2+: ferrous