An Introductory Overview 1.1 INTRODUCTION The subject of ceramics covers a wide range of materials. Recent attempts have been made to divide it into two parts: traditional ceramics and advanced ceramics. The use of the term advanced has, however, not received general acceptance and other forms including technical, special, fine, and engineering will also be encountered. Traditional ceramics bear a close relationship to those materials that have been developed since the earliest civilizations. They are pottery, structural clay products, and clay-based refractories, with which we may also group cements and concretes and glasses. Whereas traditional ceramics still represent a major part of the ceramics industry, the interest in recent years has focused on advanced ceramics, ceramics that with minor exceptions have been developed within the last 50 years or so. Advanced ceramics include ceramics for electrical, magnetic, electronic, and optical applications (sometimes referred to as functional ceramics) and ceramics for structural applications at ambient as well as at elevated temperatures (structural ceramics). Although the distinction between traditional and advanced ceramics may be referred to in this book occasionally for convenience, we do not wish to overemphasize it. There is much to be gained through continued interaction between the traditional and the advanced sectors. Chemically, with the exception of carbon, ceramics are nonmetallic, inorganic compounds. Examples are the silicates such as kaolinite [Al2Si2O5(OH)4] and mullite (Al6Si2O13), simple oxides such as alumina (Al2O3) and zirconia (ZrO2), complex oxides other than the silicates such as barium titanate (BaTiO3), and the superconducting material YBa2Cu3O6 (0    1). In addition, there are nonoxides including carbides such as silicon carbide (SiC) and boron carbide 1 2 Chapter 1 (B4C), nitrides such as silicon nitride (Si3N4) and boron nitride (BN), borides such titanium diboride (TiB2), silicides such as molybdenum disilicide (MoSi2) and halides such as lithium fluoride (LiF). There are also compounds based on nitride–oxide or oxynitride systems (e.g., ′-sialons with the general formula Si6-zAlzN8-zOz, where 0  z  4). Structurally, all materials are either crystalline or amorphous (also referred to as glassy). The difficulty and expense of growing single crystals means that, normally, crystalline ceramics (and metals) are actually polycrystalline—they are made up of a large number of small crystals, or grains, separated from one another by grain boundaries. In ceramics as well as in metals, we are concerned with two types of structure, both of which have a profound effect on properties. The first type of structure is at the atomic scale: the type of bonding and the crystal structure (for a crystalline ceramic) or the amorphous structure (if it is glassy). The second type of structure is at a larger scale: the microstructure, which refers to the nature, quantity, and distribution of the structural elements or phases in the ceramic (e.g., crystals, glass, and porosity). It is sometimes useful to distinguish between the intrinsic properties of a material and the properties that depend on the microstructure. The intrinsic properties are determined by the structure at the atomic scale and are properties that are not susceptible to significant change by modification of the microstructure, properties such as the melting point, elastic modulus, coefficient of thermal expansion, and whether the material is brittle, magnetic, ferroelectric, or semiconducting. In contrast, many of the properties critical to the engineering applications of materials are strongly dependent on the microstructure (e.g., mechanical strength, dielectric constant, and electrical conductivity). Intrinsically, ceramics usually have high melting points and are therefore generally described as refractory. They are also usually hard, brittle, and chemically inert. This chemical inertness is usually taken for granted, for example, in ceramic and glass tableware and in the bricks, mortar, and glass of our houses. However, when used at high temperatures, as in the chemical and metallurgical industries, this chemical inertness is severely tried. The electrical, magnetic, and dielectric behavior covers a wide range—for example, in the case of electrical behavior, from insulators to conductors. The applications of ceramics are many. Usually, for a given application one property may be of particular importance, but in fact, all relevant properties need to be considered. We are therefore usually interested in combinations of properties. For traditional ceramics and glasses, familiar applications include structural building materials (e.g., bricks and roofing tile), refractories for furnace linings, tableware and sanitaryware, electrical insulation (e.g., electrical porcelain and steatite), glass containers, and glasses for building and transportation vehicles. The applications for which advanced ceramics have been developed or proposed Ceramic Fabrication Processes 3 are already very diverse and this area is expected to continue to grow at a reasonable rate. Table 1.1 illustrates some of the applications for advanced ceramics (1). The important relationships between chemical composition, atomic structure, fabrication, microstructure, and properties of polycrystalline ceramics are illustrated in Fig. 1.1. The intrinsic properties must be considered at the time of materials selection. For example, the phenomenon of ferroelectricity originates in the perovskite crystal structure, of which BaTiO3 is a good example. For the production of a ferroelectric material, we may therefore wish to select BaTiO3. The role of the fabrication process, then, is to produce microstructures with the desired engineering properties. For example, the measured dielectric constant of the fabricated BaTiO3 will depend significantly on the microstructure (grain size, porosity, and presence of any secondary phases). Normally, the overall fabrication method can be divided into a few or several discrete steps, depending on the complexity of the method. Although there is no generally accepted terminology, we will refer to these discrete steps as processing steps. The fabrication of a ceramic body therefore involves a number of processing steps. In the next section, we examine, in general terms, some of the commonly used methods for the fabrication of ceramics. 1.2 CERAMIC FABRICATION PROCESSES Ceramics can be fabricated by a variety of methods, some of which have their origins in early civilization. Our normal objective is the production, from suitable starting materials, of a solid product with the desired shape such as a film, fiber, or monolith and with the desired microstructure. As a first attempt, we divide the main fabrication methods into three groups, depending on whether the starting materials involve a gaseous phase, a liquid phase, or a solid phase (Table 1.2). In the following sections, we examine briefly the main features of the processing steps involved in these methods and, from the point of view of ease of processing, their main advantages and disadvantages. 1.2.1 Gas-Phase Reactions By far the most important are vapor deposition methods, where the desired material is formed by chemical reaction between gaseous species. The reaction between a liquid and a gas is generally impractical but has been developed recently into an elegant technique, referred to as directed metal oxidation. Reaction between a gas and a solid, commonly referred to as reaction bonding (or reaction forming) has been used mainly for the production of Si3N4 but is now also being applied to the production of oxide ceramics. Reaction bonding (by a solid–liquid reaction) is also an important fabrication route for SiC. 4 Chapter 1 TABLE 1.1 Application of Advanced Ceramics Classified by Function Function Ceramic Application (Continued) Electric Magnetic Optical Insulation materials (Al2O3, BeO, MgO) Ferroelectric materials (BaTiO3, SrTiO3) Piezoelectric materials (PZT) Semiconductor materials (BaTiO3, SiC, ZnOBi2O3, V2O5 and other transition metal oxides) Ion-conducting materials (-Al2O3, ZrO2) Soft ferrite Hard ferrite Translucent alumina Translucent Mg-Al spinel, mullite, etc. Translucent Y2O3-ThO2 ceramics PLZT ceramics Integrated circuit substrate, package, wiring substrate, resistor substrate, electronics interconnection substrate Ceramic capacitor Vibrator, oscillator, filter, etc. Transducer, ultrasonic humidifier, piezolelectric spark generator, etc. NTC thermistor: temperature sensor, temperature compensation, etc. PTC thermistor: heater element, switch, temperature compensation, etc. CTR thermistor: heat sensor element Thick-film sensor: infrared sensor Varistor: noise elimination, surge current absorber, lightning arrestor, etc. Sintered CdS material: solar cell SiC heater: electric furnace heater, miniature heater, etc. Solid electrolyte for sodium battery ZrO2 ceramics: oxygen sensor, pH meter, fuel cells Magnetic recording head, temperature sensor, etc. Ferrite magnet, fractional horse power motors, etc. High-pressure sodium vapor lamp Lighting tube, special-purpose lamp, infrared transmission window materials Laser materials Light memory element, video display and storage system, light modulation element, light shutter, light valve Ceramic Fabrication Processes 5 TABLE 1.1 Continued Function Ceramic Application Source: Ref. 1. Chemical Thermal Mechanical Biological Nuclear Gas sensor (ZnO, Fe2O3, SnO2) Humidity sensor (MgCr2O4-TiO2) Catalyst carrier (cordierite) Organic catalysts Electrodes (titanates, sulfides, borides) ZrO2, TiO2 Cutting tools (Al2O3, TiC, TiN, others) Wear-resistant materials (Al2O3, ZrO2) Heat-resistant materials (SiC, Al2O3, Si3N4, others) Alumina ceramics implantation, hydroxyapatite, bioglass UO2, UO2-PuO2 C, SiC, B4C SiC, Al2O3, C, B4C Gas leakage alarm, automatic ventilation alarm; hydrocarbon, fluorocarbon detectors, etc. Cooking control element in microwave oven, etc. Catalyst carrier for emission control Enzyme carrier, zeolites Electrowinning aluminum, photochemical processes, chlorine production Infrared radiator Ceramic tool, sintered CBN; cermet tool, artificial diamond; nitride tool Mechanical seal, ceramic liner, bearings, thread guide, pressure sensors Ceramic engine, turbine blade, heat exchangers, welding burner nozzle, high frequency combustion crucibles) Artificial tooth root, bone and joint. Nuclear fuels Cladding materials Shielding materials FIGURE 1.1 The important relationships in ceramic fabrication. 6 Chapter 1 TABLE 1.2 Common Ceramic Fabrication Methods Starting materials Method Product Gases Gas–liquid Gas–solid Liquid–solid Liquids Solids (powders) Chemical vapor deposition Directed metal oxidation Reaction bonding Reaction bonding Sol–gel process Polymer pyrolysis Melt casting Sintering of powders Films, monoliths Monoliths Monoliths Monoliths Films, fibers Fibers, films Monoliths Monoliths, films 1.2.1.1 Chemical Vapor Deposition Chemical vapor deposition (CVD) is a process by which reactive molecules in the gas phase are transported to a surface at which they chemically react and form a solid film. It is a well-established technique that can be used to deposit all classes of materials, including metals, ceramics, and semiconductors, for a variety of applications. Large areas can be coated and the process is amenable to mass production. Thick films or even monolithic bodies can also be produced by basically prolonging the deposition process so that the desired thickness is achieved. Table 1.3 shows some of the important reactions used for the fabrication of ceramics together with the temperature range of the reactions and the applications of the fabricated articles. There are several excellent texts on CVD and related processes covering the fundamental physics and chemistry, equipment, applications, and reaction chemistry for most materials (2); fundamental aspects of thermodynamics, kinetics, and transport phenomena (3,4); deposition of thin films (5); microelectronic applications (6,7); and common deposition strategies for Si3N4, SiC and other materials (8). The apparatus used for CVD depends on the reaction being used, the reaction temperature, and the configuration of the substrate. Figure 1.2 shows examples of reactors for the deposition of films on substrates such as Si wafers (9). The general objective for any design is to provide uniform exposure of the substrate to the reactant gases. CVD has a number of process variables that must be manipulated to produce a deposit with the desired properties. These variables include the flow rate of the reactant gases, the nature and flow rate of any carrier gases, the pressure in the reaction vessel, and the temperature of the substrate. Substrate heating is required in CVD reactors because the films are produced preferably by endothermic reactions. The temperature of the substrate influences the deposition rate and is the main factor controlling the structure of the Ceramic Fabrication Processes 7 TABLE 1.3 Some Important CVD Reactions for the Fabrication of Ceramics Reaction Temperature (°C) Application 2CxHy → 2xC  yH2 CH3Cl3Si → SiC  3HCl W(CO)6 → WC  CO2  4CO TiCl4  O2 → TiO2  2Cl2 SiCl4  2CO2  2H2 → SiO2  4HCl  2CO SiCl4  2H2O → SiO2  4HCl SiCl4  2H2 → Si  4HCl TiCl4  2BH3 → TiB2  4HCl  H2 SiH4  CH4 → SiC  4H2 3SiH4  4NH3 → Si3N4  12H2 3HSiCl3  4NH3 → Si3N4  9HCl  3H2 BCl3  NH3 → BN  3HCl 900–2400 1000–1300 400–800 900–1200 800–1000 500–1000 500–800 1000–1300 1000–1400 800–1500 800–1100 700–1000 Pyrolytic carbon and graphite Composites Coatings Films for electronic devices Films for electronic devices, optical fibers Films for electronic devices, optical fibers Films for electronic devices Monoliths, composites Coatings Films for semiconductor devices Composites Monoliths deposit. In general, high temperatures will yield crystalline deposits while low temperatures result in amorphous materials. Between these two extremes a polycrystalline deposit will be formed. The pressure in the reaction vessel influences the concentration of the reactant gases, the diffusion of reactants toward the substrate, and the diffusion of the products away from the surface. The higher diffusivity at lower pressure leads to the formation of films with better uniformity, so that most CVD reactors are operated in the pressure range of 1–15 kPa. The reactant gases, also referred to as precursor molecules, are chosen to react and produce a specific film. Properties necessary for a good precursor include thermal stability at its vaporization temperature and sufficient vapor pressure (at least 125 Pa) at a reasonable temperature (300C) for effective gas phase delivery to the growth surface. In addition, the molecules must be obtainable at high purity and must not undergo parasitic or side reactions which would lead to contamination or degradation of the film (10). Examples of the classes of precursor molecules (e.g., hydrides, halides, carbonyls, hydrocarbons, and organometallics) and the types of chemical reaction (pyrolysis, oxidation/hydrolysis, reduction, carbidization/nitridation, and disproportionation) are summarized in Table 1.3. CVD technology has been attracting much interest recently for the production of diamond films or coatings (11). Diamond has several attractive properties 8 Chapter 1 FIGURE 1.2 Typical reactors used in chemical vapor deposition:(a) pancake reactor; (b) barrel reactor; (c) horizontal reactor; (d) low-pressure (LPCVD) reactor. (From Ref. 9.) but, in the past, high pressures and high temperatures have been required to produce synthetic diamond. In contrast, a plasma-assisted CVD process allows the production of diamond films at relatively low temperatures and low pressures (Fig. 1.3). The deposition process is complex and is not understood clearly at present. The basic reaction involves the pyrolysis of a carbon-containing precursor such as methane: CH4(g)→C(diamond)+ 2H2(g) (1.1) The typical process consists of the reactant gas at less than atmospheric pressure and containing 95% H2. The gas is activated by passing it through a plasma or past a heated filament (at 2000C) before deposition on a substrate at 800–1000C. Ceramic Fabrication Processes 9 FIGURE 1.3 Schematic diagram of microwave-plasma-assisted chemical vapor deposition (MPACVD) diamond growth system. (From Ref. 11.) CVD technology has also been attracting significant interest as a fabrication route for ceramic composites (12). For fiber-reinforced ceramics, one approach that has shown considerable promise is chemical vapor infiltration (CVI). The fibers, preformed into the shape and dimensions of the finished body, are placed into the reactant gases and held at the desired temperature so that the deposited material is formed in the interstices between the fibers. Significant effort has been devoted to SiC matrix composites reinforced with SiC or C fibers. The SiC matrix is typically deposited from methyltrichlosilane, CH3Cl3Si, at temperatures of 1200C and pressures of 3 kPa. The process is slow and a serious problem is the tendency for most of the reaction to occur near the surface of the fiber preform, leading to density gradients and the sealing off of the interior. A promising route involves the exploitation of forced flow of the reacting gas into the preform using pressure and temperature gradients (Fig. 1.4). Matrices with reasonably high density (typically 10% porosity) have been produced. The CVI route has an inherent advantage over conventional ceramic powder processing routes that commonly require higher temperatures and high pressures for fabrication: mechanical and chemical degradation of the composite during fabrication is not severe. Composites containing as high as 45 vol% of fibers have been fabricated with an open porosity of 10%. The measured fracture toughness (13) remained 10 Chapter 1 unchanged at 30 MPam1/2 up to 1400C, which is considerably better than unreinforced SiC with a fracture toughness of 3 MPam1/2. Table 1.3 indicates that the reaction temperatures for the CVD fabrication of most of the highly refractory ceramics listed are rather low. Therefore, CVD methods provide a distinct advantage of fairly low fabrication temperatures for ceramics and composites with high melting points that are difficult to fabricate by other methods or require very high fabrication temperatures. The low reaction temperatures also increase the range of materials that can be coated by CVD, especially for the highly refractory coatings. However, a major disadvantage is that the material deposition rate by CVD is very slow, typically in the range of 1–100 m/h. The production of monolithic bodies can therefore be very time consuming and expensive. Another problem that is normally encountered in the fabrication of monolithic bodies by CVD is the development of a microstructure consisting of fairly large, columnar grains which leads to fairly low intergranular strength. These difficulties limit CVD methods primarily to the formation of thin films and coatings. FIGURE 1.4 Schematic diagram of chemical vapor infiltration process exp