Ceramic engineering: Wikis

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The Materials science tetrahedron

Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties.[1]

Ceramic materials may have a crystalline or partly crystalline structure, with long-range order on atomic scale. Glass ceramics may have an amorphous or glassy structure, with limited or short-range atomic order. They are either formed from a molten mass that solidifies on cooling, formed and matured by the action of heat, or chemically synthesized at low temperatures using, for example the sol-gel technique.

The special character of ceramic materials gives rise to many applications in electrical engineering, materials engineering, chemical engineering and mechanical engineering. As ceramics are heat resistant, they can be used for many tasks that materials like metal and polymers are unsuitable for. Ceramic materials are used in a wide range of industries, including mining, aerospace, medicine, refinery, food and chemical industries, packaging science, electronics, industrial and transmission electricity, and guided lightwave transmission.

Contents

History

The word "ceramic" is derived from the Greek word κεραμικός (keramikos) meaning pottery. It is related to the older Indo-European language root "to burn",[2] "Ceramic" may be used as a noun in the singular to refer to a ceramic material or the product of ceramic manufacture, or as an adjective. The plural "ceramics" may be used to refer the making of things out of ceramic materials.

Ceramics engineering, like many sciences, evolved from a different discipline by today's standards. Materials engineering is grouped with ceramics engineering to this day. Universities with ceramics programs include a curriculum saturated with materials engineering classes.

Abraham Darby first used coke in 1709 in Shropshire, England, to improve the yield of a smelting process. Coke is now widely used to produce carbide ceramics. Potter Josiah Wedgwood opened the first modern ceramics factory in Stoke-on-Trent, England, in 1759. Austrian chemist Karl Bayer, working for the textile industry in Russia, developed a process to separate alumina from bauxite ore in 1888. The Bayer process is still used to purify alumina for the ceramic and aluminum industries. Brothers Pierre and Jacques Curie discovered piezoelectricity in Rochelle salt circa 1880. Piezoelectricity is one of the key properties of electroceramics. E.G. Acheson heated a mixture of coke and clay in 1893, and invented carborundum, or synthetic silicon carbide. Henri Moissan also synthesized SiC and tungsten carbide in his electric arc furnace in Paris about the same time as Acheson. Karl Schröter used liquid-phase sintering to bond or "cement" Moissan’s tungsten carbide particles with cobalt in 1923 in Germany. Cemented (metal-bonded) carbide edges greatly increase the durability of hardened steel cutting tools. W.H. Nernst developed cubic-stabilized zirconia in the 1920s in Berlin. This material is used as an oxygen sensor in exhaust systems. The main limitation on the use of ceramics in engineering is brittleness.

The first ceramic engineering course and department in the United States were established by Edward Orton, Jr., a professor of geology and mining engineering, at the Ohio State University in 1894. Orton and eight other refractory professionals founded the American Ceramic Society (ACerS) at the 1898 National Brick Manufacturers' Association convention in Pittsburgh. Orton was the first ACerS General Secretary, and his office at OSU served as the society headquarters in the beginning. Charles F. Binns established the New York State School of Clay-Working and Ceramics, now Alfred University, in 1900. Binns was the third ACerS president, and Orton the 32nd.[3] The Ceramic Society of Japan was founded in 1891 in Tokyo. The Deutsche Keramische Gesellschaft, the ceramic society of Germany, was founded in Berlin in 1919.

The military requirements of World War II (1939-1945) encouraged developments, which created a need for high-performance materials and helped speed the development of ceramic science and engineering. Throughout the 1960s and 1970s, new types of ceramics were developed in response to advances in atomic energy, electronics, communications, and space travel. The discovery of ceramic superconductors in 1986 has spurred intense research to develop superconducting ceramic parts for electronic devices, electric motors, and transportation equipment.

Current status

Ceramic bread knife
Ceramics used for inner plates of ballistic vests
Si3N4 ceramic bearing parts

Now a multi-billion dollar a year industry, ceramic engineering and research has established itself as an important field of science. Applications continue to expand as researchers develop new kinds of ceramics to serve different purposes.

  • Zirconium dioxide ceramics are used in the manufacture of knives. The blade of the ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.
  • Silicon nitride parts are used in ceramic ball bearings. Their higher hardness means that they are much less susceptible to wear and can offer more than triple lifetimes. They also deform less under load meaning they have less contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. The major drawback to using ceramics is a significantly higher cost. In many cases their electrically insulating properties may also be valuable in bearings.
  • In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, as shown by Carnot's theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is not feasible with current technology.
  • Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.
  • Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Most hydroxy apatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong, fully dense nano crystalline hydroxyapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic, but naturally occurring, bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.
  • High-tech ceramic is used in watchmaking for producing watch cases. The material is valued by watchmakers for its light weight, scratch-resistance, durability and smooth touch. IWC is one of the brands that initiated the use of ceramic in watchmaking. The case of the IWC 2007 Top Gun edition of the Pilot's Watch Double chronograph is crafted in high-tech black ceramic.[4]

Processing steps

The traditional ceramic process generally follows this sequence: Milling → Batching → Mixing → Forming → Drying → Firing → Assembly.

Ball mill
  • Milling is the process by which materials are reduced from a large size to a smaller size. Milling may involve breaking up cemented material (in which case individual particles retain their shape) or pulverization (which involves grinding the particles themselves to a smaller size). Milling is generally done by mechanical means, including attrition (which is particle-to-particle collision that results in agglomerate break up or particle shearing), compression (which applies a forces that results in fracturing), and impact (which employs a milling medium or the particles themselves to cause fracturing). Attrition milling equipment includes the wet scrubber (also called the planetary mill or wet attrition mill), which has paddles in water creating vortexes in which the material collides and break up. Compression mills include the jaw crusher, roller crusher and cone crusher. Impact mills include the ball mill, which has media that tumble and fracture the material. Shaft impactors cause particle-to particle attrition and compression.
  • Batching is the process of weighing the oxides according to recipes, and preparing them for mixing and drying.
  • Mixing occurs after batching and is performed with various machines, such as dry mixing ribbon mixers (a type of cement mixer), Mueller mixers, and pug mills. Wet mixing generally involves the same equipment.
  • Forming is making the mixed material into shapes, ranging from toilet bowls to spark plug insulators. Forming can involve: 1) Extrusion, such as extruding "slugs" to make bricks 2) Pressing to make shaped parts. 3) Slip casting, as in making toilet bowls, wash basins and ornamentals like ceramic statues. Forming produces a "green" part, ready for drying. Green parts are soft, pliable, and over time will lose shape. Handling the green product will change its shape. For example, a green brick can be "squeezed", and after squeezing it will stay that way.
  • Drying is removing the water or binder from the formed material. Spray drying is widely used to prepare powder for pressing operations. Other dryers are tunnel dryers and periodic dryers. Controlled heat is applied in this two-stage process. First, heat removes water. This step needs careful control, as rapid heating causes cracks and surface defects. The dried part is smaller than the green part, and is brittle, necessitating careful handling, since a small impact will cause crumbling and breaking.
  • Firing is where the dried parts pass through a controlled heating process, and the oxides are chemically changed to cause sintering and bonding. The fired part will be smaller than the dried part.

Forming methods

Modern ceramic forming techniques include shaping by hand (or "throwing"), slipcasting, tape casting, injection molding, dry pressing, isostatic pressing, hot isostatic pressing (HIP) and others. Methods for forming ceramic powders into complex shapes are desirable in many areas of technology. Such methods are required for producing advanced, high-temperature structural parts such as heat engine components and turbines. Materials other than ceramics which are used in these processes may include: wood, metal, water, plaster and epoxy—most of which will be eliminated upon firing.

These forming techniques are well known for providing tools and other components with dimensional stability, surface quality, high (near theoretical) density and microstructural uniformity. The increasing use and diversity of specialty forms of ceramics adds to the diversity of process technologies to be used.

Thus, reinforcing fibers and filaments are mainly made by polymer, sol-gel, or CVD processes, but melt processing also has applicability. The most widely used specialty form is layered structures, with tape casting for electronic substrates and packages being preeminent. Photolithography is of increasing interest for precise patterning of conductors and other components for such packaging. Tape casting or forming processes are also of increasing interest for other applications, ranging from open structures such as fuel cells to ceramic composites.

The other major layer structure is coating, where melt spraying is very important, but chemical and physical vapor deposition and chemical (e.g., sol-gel and polymer pyrolysis) methods are all seeing increased use. Besides open structures from formed tape, extruded structures, such as honeycomb catalyst supports, and highly porous structures, including various foams, for example, reticulated foam, are of increasing use.

Densification of consolidated powder bodies continues to be achieved predominantly by (pressureless) sintering. However, the use of pressure sintering by hot pressing is increasing, especially for non-oxides and parts of simple shapes where higher quality (mainly microstructural homogeneity) is needed, and larger size or multiple parts per pressing can be an advantage.

The sintering process

See also: Sintering#Sintering_Mechanisms

Scanning electron micrographs (SEM) of room temperature Al2O3 fine powder compact formed using a) colloidal processing and b) slip casting techniques. *Note: Mean particle diameter = 0.6 microns.
Scanning electron micrographs (SEM) of Al2O3 fine powder compact formed using a) colloidal processing and b) slip casting techniques and sintered to 1200°C. *Note: Mean cluster size = 3 microns.
Scanning electron micrographs (SEM) of Al2O3 fine powder compact formed using a) colloidal processing and b) slip casting techniques and sintered to 1400°C. *Note: Mean grain size = 2 microns.
Scanning electron micrographs (SEM) of fully densified Al2O3 ceramic formed using a) colloidal processing and b) slip casting techniques and sintered to 1600°C. *Note: Mean grain size = 3 microns.

The principles of sintering-based methods are simple ("sinter" has roots in the English "cinder"). The firing is done at a temperature below the melting point of the ceramic. Once a roughly-held-together object called a "green body" is made, it is baked in a kiln, where atomic and molecular diffusion processes give rise to significant changes in the primary microstructural features. This includes the gradual elimination of porosity, which is typically accompanied by a net shrinkage and overall densification of the component. Thus, the pores in the object may close up, resulting in a denser product of significantly greater strength and fracture toughness.

Another major change in the body during the firing or sintering process will be the establishment of the polycrystalline nature of the solid. This change will introduce some form of grain size distribution, which will have a significant impact on the ultimate physical properties of the material. The grain sizes will either be associated with the initial particle size, or possibly the sizes of aggregates or particle clusters which arise during the initial stages of processing.

The ultimate microstructure (and thus the physical properties) of the final product will be limited by and subject to the form of the structural template or precursor which is created in the initial stages of chemical synthesis and physical forming. Ergo the importance of chemical powder and polymer processing as it pertains to the synthesis of industrial ceramics, glasses and glass-ceramics.

There are numerous possible refinements of the sintering process. Some of the most common involve pressing the green body to give the densification a head start and reduce the sintering time needed. Sometimes organic binders such as polyvinyl alcohol are added to hold the green body together; these burn out during the firing (at 200–350 °C). Sometimes organic lubricants are added during pressing to increase densification. It is common to combine these, and add binders and lubricants to a powder, then press. (The formulation of these organic chemical additives is an art in itself. This is particularly important in the manufacture of high performance ceramics such as those used by the billions for electronics, in capacitors, inductors, sensors, etc.

A slurry can be used in place of a powder, and then cast into a desired shape, dried and then sintered. Indeed, traditional pottery is done with this type of method, using a plastic mixture worked with the hands. If a mixture of different materials is used together in a ceramic, the sintering temperature is sometimes above the melting point of one minor component - a liquid phase sintering. This results in shorter sintering times compared to solid state sintering.

Strength of ceramics

A material's strength is dependent on its microstructure. The engineering processes to which a material is subjected can alter this microstructure. The variety of strengthening mechanisms that alter the strength of a material include the mechanism of grain boundary strengthening. Thus, although yield strength is maximized with decreasing grain size, ultimately, very small grain sizes make the material brittle. Considered in tandem with the fact that the yield strength is the parameter that predicts plastic deformation in the material, one can make informed decisions on how to increase the strength of a material depending on its microstructural properties and the desired end effect.

The relation between yield stress and grain size is described mathematically by the Hall-Petch equation which is

\sigma_y = \sigma_0 + {k_y \over \sqrt {d}}

where ky is the strengthening coefficient (a constant unique to each material), σo is a materials constant for the starting stress for dislocation movement (or the resistance of the lattice to dislocation motion), d is the grain diameter, and σy is the yield stress.

Theoretically, a material could be made infinitely strong if the grains are made infinitely small. This is, unfortunately, impossible because the lower limit of grain size is a single unit cell of the material. Even then, if the grains of a material are the size of a single unit cell, then the material is in fact amorphous, not crystalline, since there is no long range order, and dislocations can not be defined in an amorphous material. It has been observed experimentally that the microstructure with the highest yield strength is a grain size of about 10 nanometers, because grains smaller than this undergo another yielding mechanism, grain boundary sliding[5]. Producing engineering materials with this ideal grain size is difficult because of the limitations of initial particle sizes inherent to nanomaterials and nanotechnology.

Theory of chemical processing

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Microstructural uniformity

SEM micrograph of surface of colloidal solid. Structure and morphology consists of ordered domains with both interdomain and intradomain lattice defects.(Amorphous colloidal silica particles of average particle diameter 600 nm).
Manual highlighting reveals microstructural defects and domains in the above image

In the processing of fine ceramics, the irregular particle sizes and shapes in a typical powder often lead to non-uniform packing morphologies that result in packing density variations in the powder compact. Uncontrolled agglomeration of powders due to attractive van der Waals forces can also give rise to in microstructural inhomogeneities. [6][7]

Differential stresses that develop as a result of non-uniform drying shrinkage are directly related to the rate at which the solvent can be removed, and thus highly dependent upon the distribution of porosity. Such stresses have been associated with a plastic-to-brittle transition in consolidated bodies, [8] and can yield to crack propagation in the unfired body if not relieved.

In addition, any fluctuations in packing density in the compact as it is prepared for the kiln are often amplified during the sintering process, yielding inhomogeneous densification. [9][10] Some pores and other structural defects associated with density variations have been shown to play a detrimental role in the sintering process by growing and thus limiting end-point densities. [11] Differential stresses arising from inhomogeneous densification have also been shown to result in the propagation of internal cracks, thus becoming the strength-controlling flaws. [12]

It would therefore appear desirable to process a material in such a way that it is physically uniform with regard to the distribution of components and porosity, rather than using particle size distributions which will maximize the green density. The containment of a uniformly dispersed assembly of strongly interacting particles in suspension requires total control over particle-particle interactions. Monodisperse colloids provide this potential.[13]

Monodisperse powders of colloidal silica, for example, may therefore be stabilized sufficiently to ensure a high degree of order in the colloidal crystal or polycrystalline colloidal solid which results from aggregation. The degree of order appears to be limited by the time and space allowed for longer-range correlations to be established.[14][15]

Such defective polycrystalline colloidal structures would appear to be the basic elements of submicrometer colloidal materials science, and, therefore, provide the first step in developing a more rigorous understanding of the mechanisms involved in microstructural evolution in inorganic systems such as polycrystalline ceramics.

Self-assembly

An example of a supramolecular assembly.[16]

"Self-assembly" is the most common term in use in the modern scientific community to describe the spontaneous aggregation of particles (atoms, molecules, colloids, micelles, etc.) without the influence of any external forces. Large groups of such particles are known to assemble themselves into thermodynamically stable, structurally well-defined arrays, quite reminiscent of one of the 7 crystal systems found in metallurgy and mineralogy (e.g. face-centered cubic, body-centered cubic, etc.). The fundamental difference in equilibrium structure is in the spatial scale of the unit cell (or lattice parameter) in each particular case.

Thus, self-assembly is emerging as a new strategy in chemical synthesis and nanotechnology. Molecular self-assembly has been observed in various biological systems and underlies the formation of a wide variety of complex biological structures. Thus, self-assembly is also emerging as a new strategy in chemical synthesis and nanotechnology. Molecular crystals, liquid crystals, colloids, micelles, emulsions, phase-separated polymers, thin films and self-assembled monolayers all represent examples of the types of highly ordered structures which are obtained using these techniques. The distinguishing feature of these methods is self-organization in the absence of any external forces.

In addition, the principal mechanical characteristics and structures of biological ceramics, polymer composites, elastomers, and cellular materials are being re-evaluated, with an emphasis on bioinspired materials and structures. Traditional approaches focus on design methods of biological materials using conventional synthetic materials. This includes an emerging class of mechanically superior biomaterials based on microstructural features and designs found in nature. The new horizons have been identified in the synthesis of bioinspired materials through processes that are characteristic of biological systems in nature. This includes the nanoscale self-assembly of the components and the development of hierarchical structures.[14][15][17]

Alternative methods

Melt processing

Several methods for making ceramics either use no powders at all, or do not directly make the ceramic from powder. The most extensive of these are the various melt processing methods, of which melt casting produces by far the largest volumes and individual sizes. Arc melting using graphite electrodes predominates, but some induction melting is used, directly coupling to the ceramic to be melted, usually after heating part of the mass by other means to about 1000 °C for sufficient coupling. (e.g. The growth of single crystals of cubic zirconia ZrO2) for the jewelry trade in a skull crucible).

In either case, the method is predominantly skull melting, in which the melt is contained within a layer of the same ceramic that is kept unmelted by a water-cooled shell. Non-oxide materials that melt can be processed by arc or electron beam melting. However, melting is predominantly applied to oxides, such as alumina (Al2O3) and zirconia (ZrO2). A solid ingot which is then crushed into grain for the manufacture of refractories, or into powder for plasma spraying. The advantage of melt-derived powders for plasma spraying lies in multi-constituent powders, since better compositional homogeneity is generally achieved.

Ceramic components cast from the melt, usually into graphite molds, typically have much (10-1000-fold) larger grains and substantial porosity (10-20%) compared to sintered ceramics. Grain sizes on the surface of such cast pieces will typically be finer than in the interior due to rapid surface chilling, especially in single-phase materials. However, even the surface grain size will typically be at least an order of magnitude or greater than typical sintered grain sizes.

While some porosity can arise due to extrinsic entrapped gases, as well as evolution of some dissolved gases during solidification, much of it arises intrinsically from the substantially smaller volume of the solidified material relative to its melt for oxide materials. The intrinsic porosity, as well as much of the extrinsic porosity, forms at the solid/liquid interface.

Directional solidification, as used in growing single crystals, can eliminate entrapment of any of the intrinsic porosity, and be used to give potentially beneficial eutectic structures. In order to minimize problems due to extrinsic porosity from trapped volatile species, most single crystals are grown from material that has previously been melted at least once, and commonly twice. Most ceramic castings are designed to result in a solidification front that in term results in the solidification porosity being located in regions where it is less detrimental to performance.

Melt processing is also one of the most widely used methods for making ceramic coatings, and can be done over sizable areas. This is most commonly done by feeding appropriate size powders, for example, spray dried agglomerates a few tens of micrometers in diameter, into a chemically (e.g., oxyacetylene) or electrically (e.g., plasma) derived flame. Such melt spray coatings, although used considerably in the past, have often suffered from variable quality and too much porosity. While some porosity is intrinsic in the process (and increases with subsequent heat treatment in materials such as Al2O3, that are quenched in unstable, lower density phases), denser coatings have been obtained in recent years by using chambers for spraying at reduced pressure for higher particle velocities. Chamber spraying also allows cleaner, more reliable surfaces to be obtained, for example, by sputtering, for better, more reliable adhesion. Computer control of spray parameters is leading to significant improvements in reproducibility.

Chemical vapor deposition

Another non-powder-base method is that of chemical vapor deposition (CVD). While used for making ceramic powders, this method is also used quite extensively for making ceramic coatings, as well as monolithic components. Typically, inorganic and related precursors, which are substantially less expensive, are utilized for bulk, as in structural bodies, with processing temperatures commonly in the 1000 - 1500 °C range. Organometallic precursors, typically much more expensive, can be used, for example, for coatings for electronics, with depositions at temperatures from a few to several hundred degrees Celsius. CVD has played a key role in the development of fiber optics by producing high-purity boules from which the fibers are drawn.

Under appropriate conditions, deposition rates as high as tens of micrometers per minute can be achieved for moderate process costs. CVD can produce some of the largest individual technical ceramic components. Another important advantage of CVD is that it can fairly readily produce a variety of quite important non-oxide materials, such as SiC, Si3N4, B4C, and BN, which otherwise require very high temperatures to produce and typically can only be produced from powder by using additives that may ultimately limit performance (especially at high temperatures). For example, CVD deposited SiC or Si3N4 have substantially higher creep resistance at elevated temperatures than do bodies made by densifying powders that require additives.

The challenges of CVD are mainly control of microstructure and residual stresses. CVD commonly results in substantial grain sizes (from a few tens to a few hundreds of micrometers), development of growth cones (colonies of similarly oriented grains) usually growing at a much higher rate perpendicular to the growth surface rather than parallel with it, or both. Large grains typically result in less optimal mechanical properties. Other disadvantages include rough surfaces that require more machining and may limit the surface finish quality.

Residual stresses can be a very serious problem for CVD, as for any deposition process. Some stresses arise due to differential expansion between the material being deposited and the substrate- usually graphite for producing freestanding bodies, since it is easily removed, is relatively modest in cost, and allows a fair range of accommodation of thermal expansion. However, the major sources of residual stresses are apparently variations in stoichiometry and resultant lattice strains, since some stresses can actually substantially distort or destroy a component at deposition temperature, where there is little or no strain differential between the component and the surface onto which it is being deposited.

Control of both residual stress and microstructure is a common reason for CVD being conducted at relatively modest deposition rates, that is, reduced temperatures, pressures, and flow rates. However, the issue appears in part to be one of controlling appropriate nucleation, which should be addressable more fundamentally by chemical and physical means. There appears to be no intrinsic reason why one cannot utilize the cost advantages of high deposition rates while achieving acceptable microstructures and residual stresses. This will likely be in an area for important research and development.

Physical vapor deposition

Various methods of physical vapor deposition (PVD) are also used. These include evaporation (e.g., electron beam), sputtering, and reaction process (e.g., reactive sputtering). Since the deposition rates are quite low, such processes are restricted to thin coatings.

Coatings for wear applications, especially for many cutting and related tools-for example, with TiN by reactive deposition-are now widely done on an industrial scale. An important opportunity presented by PVD, as well as CVD and probably some other deposition processes, is to learn to control preferred orientations in coatings. If one can obtain coatings with proper preferred orientation of some anisotropic ceramics, much better matching of ceramic coating and substrate thermal expansions can be achieved.

A number of ceramics can also be deposited by electrochemical means which, in principle, could be used for producing monolithic components as well as coatings as with CVD. However, there has been only limited investigation of this technique for coating purposes. The vehicle for deposition is usually a molten salt, which in itself presents a substantial challenge of temperature and corrosion, as does the problem of small pockets of the bath being incorporated in the coating, limiting coating quality. Much more development of the process is needed, ranging from basic chemistry to control of factors influencing film microstructure.

Polymer pyrolysis

One of the newest nonpowder-based methods of preparing ceramics or ceramic coatings is polymer pyrolysis. Here, one obtains a ceramic by the pyrolysis of an appropriate metal organic polymer, in direct analogy with the fabrication of glassy carbon or graphite fibers by pyrolysis of appropriate organic polymers. While this method is applicable to some oxides, it is predominantly for non-oxides. The preparation of SiC from Si-C-based polymers and Si3N4 from Si-N-based polymers has been demonstrated and progress has been made toward obtaining B4C and BN-based ceramics from appropriate polymeric precursors.

The term "based" is used to denote the fact that one typically does not obtain exact stoichiometry of the desired ceramic or the precursor polymer. Instead, an excess of one constituent or a mixture of products is generally obtained. It is common to obtain mixtures of Si3N4 and SiC, or to produce Si3N4 with excess Si (which can be converted to Si3N4 by pyrolyzing in a nitrogen producing atmosphere, such as N, or NH3).

The weight yield of the resultant ceramic from the polymer must typically be in the range of SO-80% for a practical process. However, for some systems yields as high as 90% have been calculated theoretically and approached very closely experimentally with the appropriate precursor, one that results in essentially exclusive loss of hydrogen in the forming of the final ceramic product. Substantial shrinkage and/or cracking, as well as porosity, are the typical mechanisms by which the substantial differences in densities between the starting polymer and the resultant ceramic composition are accommodated.

One basic limitation of polymer pyrolysis is that it cannot produce fully dense ceramic materials unless at least one dimension is small, on the order of micrometers. Typical resultant porosities of bulk bodies from pyrolysis are in the range of 20-40%, at least for pieces a few mm thick. Thicker pieces may have more porosity, and thus the achievable sizes are limited. Nevertheless, glassy carbon parts at least 30 cm long have been produced. The process is potentially most widely applicable for fabrication of coatings or fibers, for which it is already in commercial production, as well as composites.

Sol-gel processing is also of interest for composites. Melt processing is used extensively for producing oxide fibers. Industrially, this includes fibers for fiber optics, reinforcement (fiber-glass composites), and insulation fibers for products ranging from home to high-temperature furnace applications.

Insulation fibers are typically formed by blowing molten streams, while optical and reinforcement fibers are drawn. Previously, only quite viscous melts, such as silicate, could be used, otherwise surface tensions would result in droplet formation. However, the development of inviscid melt spinning has relaxed this limitation. This is accomplished by extruding the molten fiber into a chamber where the hot fiber acts as a substrate for CVD (commonly of graphite, from CH4, for example) such that sufficiently rapid CVD coating onto the fiber prevents its breakup into droplets. New glass compositions for fibers are of interest, including halides for low-loss IR fibers, and oxynitride or oxycarbide compositions for higher stiffness.

Ceramic composites

The Porsche Carrera GT's carbon-ceramic (silicon carbide) composite disc brake

Substantial interest has arisen in recent years in fabricating ceramic composites. While there is considerable interest in composites with one or more non-ceramic constituents, the greatest attention is on composites in which all constituents are ceramic. These typically comprise two ceramic constituents: a continuous matrix, and a dispersed phase of ceramic particles, whiskers, or short (chopped) or continuous ceramic fibers. The challenge, as in wet chemical processing, is to obtain a uniform or homogeneous distribution of the dispersed particle or fiber phase.

Consider first the processing of particulate composites. The particulate phase of greatest interest is tetragonal zirconia because of the toughening that can be achieved from the phase transformation from the metastable tetragonal to the monoclinic crystalline phase, aka transformation toughening. There is also substantial interest in dispersion of hard, non-oxide phases such as SiC, TiB, TiC, boron, carbon and especially oxide matrices like alumina and mullite. There is also interest too incorporating other ceramic particulates, especially those of highly anisotropic thermal expansion. Examples include Al2O3, TiO2, graphite, and boron nitride.

In processing particulate composites, the issue is not only homogeneity of the size and spatial distribution of the dispersed and matrix phases, but also control of the matrix grain size. However, there is some built-in self-control due to inhibition of matrix grain growth by the dispersed phase. Particulate composites, though generally offer increased resistance to damage, failure, or both, are still quite sensitive to inhomogeneities of composition as well as other processing defects such as pores. Thus they need good processing to be effective.

Particulate composites have been made on a commercial basis by simply mixing powders of the two constituents. Although this approach is inherently limited in the homogeneity that can be achieved, it is the most readily adaptable for existing ceramic production technology. However, other approaches are of interest.

From the technological standpoint, a particularly desirable approach to fabricating particulate composites is to coat the matrix or its precursor onto fine particles of the dispersed phase with good control of the starting dispersed particle size and the resultant matrix coating thickness. One should in principle be able to achieve the ultimate in homogeneity of distribution and thereby optimize composite performance. This can also have other ramifications, such as allowing more useful composite performance to be achieved in a body having porosity, which might be desired for other factors, such as limiting thermal conductivity.

Much attention is now being focused on controlling bonding between fiber and matrix in ceramic fiber composites. Dramatic benefits have been demonstrated by coating fibers with an appropriate material such as BN. This has been most extensively done by using CVD. However, sputtering has also been used with certain boron-containing fibers. Reaction in a reactive nitrogen atmosphere (e.g. ammonia) has also been utilized.

There are also some opportunities to utilize melt processing for fabrication of ceramic, particulate, whisker and short-fiber, and continuous-fiber composites. Clearly, both particulate and whisker composites are conceivable by solid-state precipitation after solidification of the melt. This can also be obtained in some cases by sintering, as for precipitation-toughened, partially stabilized zirconia. Similarly, it is known that one can directionally solidify ceramic eutectic mixtures and hence obtain uniaxially aligned fiber composites. Such composite processing has typically been limited to very simple shapes and thus suffers from serious economic problems due to high machining costs.

There are possibilities of combining this technique with methods of shaping single crystals in the growth process to produce useful aligned fiber composites directly from the melt. Note also that use of a eutectic mixture in principle gives a more homogeneous and potentially more useful distribution of ZrO than would normal solid-state processing.

Clearly, there are possibilities of using melt casting for many of these approaches. Potentially even more desirable is using melt-derived particles. In this method, quenching is done in a solid solution or in a fine eutectic structure, in which the particles are then processed by more typical ceramic powder processing methods into a useful body. There have also been preliminary attempts to use melt spraying as a means of forming composites by introducing the dispersed particulate, whisker, or fiber phase in conjunction with the melt spraying process.

Besides many process improvements, the first of two major needs for fiber composites is lower fiber costs. The second major need is fiber compositions or coatings, or composite processing, to reduce degradation that results from high-temperature composite exposure under oxidizing conditions.

Applications

Radial rotor made from Si3N4 for a gas turbine engine
Silicon nitride thruster. Left: Mounted in test stand. Right: Being tested with H2/O2 propellants

The products of technical ceramics include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones.

Its products are often made from materials other than clay, chosen for their particular physical properties. These may be classified as follows:

Ceramics can be used in many technological industries. One application are the ceramic tiles on NASA's Space Shuttle, used to protect it and the future supersonic space planes from the searing heat of reentry into the Earth's atmosphere. They are also used widely in electronics and optics. In addition to the applications listed here, ceramics are also used as a coating in various engineering cases. An example would be a ceramic bearing coating over a titanium frame used for an airplane. Recently the field has come to include the studies of single crystals or glass fibers, in addition to traditional Polycrystalline materials, and the applications of these have been overlapping and changing rapidly.

Aerospace

  • Engines; Shielding a hot running airplane engine from damaging other components.
  • Airframes; Used as a high-stress, high-temp and lightweight bearing and structural component.
  • Missile nose-cones; Shielding the missile internals from heat.
  • Space Shuttle tiles
  • Space-debris ballistic shields -- Ceramic fiber woven shields offer better protection to hypervelocity (~7 km/s) particles than aluminum shields of equal weight.[18]
  • Rocket Nozzles; Withstands and focuses the exhaust of the rocket booster.

Biomedical

A titanium hip prosthesis, with a ceramic head and polyethylene acetabular cup.

Electronics

Optical

Biomaterials

The DNA structure at left (schematic shown) will self-assemble into the structure visualized by atomic force microscopy at right.[19]

Silicification is quite common in the biological world and occurs in bacteria, single-celled organisms, plants, and animals (invertebrates and vertebrates). Crystalline minerals formed in such environment often show exceptional physical properties (e.g. strength, hardness, fracture toughness) and tend to form hierarchical structures that exhibit microstructural order over a range of length or spatial scales. The minerals are crystallized from an environment that is undersaturated with respect to silicon, and under conditions of neutral pH and low temperature (0-40 °C). Formation of the mineral may occur either within or outside of the cell wall of an organism, and specific biochemical reactions for mineral deposition exist that include lipids, proteins and carbohydrates. The significance of the cellular machinery cannot be overemphasized, and it is with advances in experimental techniques in cellular biology and the capacity to mimic the biological environment that significant progress is currently being reported.

Most natural (or biological) materials are complex composites whose mechanical properties are often outstanding, considering the weak constituents from which they are assembled. These complex structures, which have risen from hundreds of million years of evolution, are inspiring the design of novel materials with exceptional physical properties for high performance in adverse conditions. Their defining characteristics such as hierarchy, multifunctionality, and the capacity for self-healing, are currently being investigated.[20]

The basic building blocks begin with the 20 amino acids and proceed to polypeptides, polysaccharides, and polypeptides–saccharides. These, in turn, compose the basic proteins, which are the primary constituents of the ‘soft tissues’ common to most biominerals. With well over 1000 proteins possible, current research emphasizes the use of collagen, chitin, keratin, and elastin. The ‘hard’ phases are often strengthened by crystalline minerals, which nucleate and grow in a biomediated environment that determines the size, shape and distribution of individual crystals. The most important mineral phases have been identified as hydroxyapatite, silica, and aragonite. Using the classification of Wegst and Ashby, the principal mechanical characteristics and structures of biological ceramics, polymer composites, elastomers, and cellular materials have been presented. Selected systems in each class are being investigated with emphasis on the relationship between their microstructure over a range of length scales and their mechanical response.

Thus, the crystallization of inorganic materials in nature generally occurs at ambient temperature and pressure. Yet the vital organisms through which these minerals form are capable of consistently producing extremely precise and complex structures. Understanding the processes in which living organisms control the growth of crystalline minerals such as silica could lead to significant advances in the field materials science, and open the door to novel synthesis techniques for nanoscale composite materials, or nanocomposites.

The iridescent nacre inside a Nautilus shell.

High-resolution SEM observations were performed of the microstructure of the mother-of-pearl (or nacre) portion of the abalone shell. Those shells exhibit the highest mechanical strength and fracture toughness of any non-metallic substance known. The nacre from the shell of the abalone has become one of the more intensively studied biological structures in materials science. Clearly visible in these images are the neatly stacked (or ordered) mineral tiles separated by thin organic sheets along with a macrostructure of larger periodic growth bands which collectively form what scientists are currently referring to as a hierarchical composite structure. (The term hierarchy simply implies that there are a range of structural features which exist over a wide range of length scales).[21]

Future developments reside in the synthesis of bio-inspired materials through processing methods and strategies that are characteristic of biological systems. These involve nanoscale self-assembly of the components and the development of hierarchical structures.[15][17][22][14]

See also

References

  1. ^ Chambers Science and Technology Dictionary, Chambers, 1992
  2. ^ A. R. von Hippel (1954). "Ceramics". Dielectric Materials and Applications. Technology Press (M.I.T.) and John Wiley & Sons. ISBN 1580531237.  
  3. ^ The American Ceramic Society: 100 Years, American Ceramic Society, 1998, p 169-173, ISBN 1-888903-04-X.
  4. ^ Ceramic in Watchmaking
  5. ^ Schuh, Christopher; Nieh, T.G. (2003). "Hardness and Abrasion Resistance of Nanocrystalline Nickel Alloys Near the Hall-Petch Breakdown Regime". Mat. Res. Soc. Symp. Proc. 740.  
  6. ^ Onoda, G.Y., Jr. and Hench, L.L. Eds. (1979). Ceramic Processing Before Firing (Wiley & Sons, New York).  
  7. ^ Aksay, I.A., Lange, F.F., Davis, B.I. (1983). "Uniformity of Al2O3-ZrO2 Composites by Colloidal Filtration". J. Am. Ceram. Soc. 66: C-190. doi:10.1111/j.1151-2916.1983.tb10550.x.  
  8. ^ Franks, G.V. and Lange, F.F. (1996). "Plastic-to-Brittle Transition of Saturated, Alumina Powder Compacts". J. Am. Ceram. Soc. 79: 3161. doi:10.1111/j.1151-2916.1996.tb08091.x.  
  9. ^ Evans, A.G. and Davidge, R.W. (1969). "Strength and fracture of fully dense polycrystalline magnesium oxide". Phil. Mag. 20: 373. doi:10.1080/14786436908228708.  
  10. ^ Evans, A.G. and Davidge, R.W. (1970). "Strength and fracture of fully dense polycrystalline magnesium oxide". J. Mat. Sci. 5: 314.  
  11. ^ Lange, F.F. and Metcalf, M. (1983). "Processing-Related Fracture Origins in A12O3/ZrO2 Composites II: Agglomerate Motion and Crack-like Internal Surfaces Caused by Differential Sintering". J. Am. Ceram. Soc. 66: 398. doi:10.1111/j.1151-2916.1983.tb10069.x.  
  12. ^ Evans, A.G. (1987). "Considerations of Inhomogeneity Effects in Sintering". J. Am. Ceram. Soc. 65: 497. doi:10.1111/j.1151-2916.1982.tb10340.x.  
  13. ^ Mangels, J.A. and Messing, G.L., Eds. (1984). "Microstructural Control Through Colloidal Consolidation". Advances in Ceramics: Forming of Ceramics 9: 94.  
  14. ^ a b c Whitesides, G.M., et al. (1991). "Molecular Self-Assembly and Nanochemistry: A Chemical Strategy for the Synthesis of Nanostructures". Science 254: 1312. doi:10.1126/science.1962191.  
  15. ^ a b c Dubbs D. M, Aksay I.A. (2000). "Self-Assembled Ceramics". Ann. Rev. Phys. Chem. 51: 601. doi:10.1146/annurev.physchem.51.1.601.  
  16. ^ Dalgarno, S. J. (2005). "Fluorescent Guest Molecules Report Ordered Inner Phase of Host Capsules in Solution". Science 309: 2037. doi:10.1126/science.1116579.  
  17. ^ a b Ariga, Katsuhiko; Hill, Jonathan P; Lee, Michael V; Vinu, Ajayan; Charvet, Richard; Acharya, Somobrata (2008). "Challenges and breakthroughs in recent research on self-assembly" (free download). Sci. Technol. Adv. Mater. 9 (1): 014109 (96 pages). doi:10.1088/1468-6996/9/1/014109.  
  18. ^ Ceramic Fabric Offers Space Age Protection, 1994 Hypervelocity Impact Symposium
  19. ^ M. Strong (2004). "Protein Nanomachines". PLoS Biol. 2 (3): e73-e74. doi:10.1371/journal.pbio.0020073.  
  20. ^ Perry, C.C. (2003). "Silicification: The Processes by Which Organisms Capture and Mineralize Silica". Rev. Miner. Geochem. 54: 291. doi:10.2113/0540291.  
  21. ^ Meyers, M.A., Chen (2008). "Biological Materials: Structure and Mechanical Properties". Prog. Mat. Sci. 53: 1–206. doi:10.1016/j.pmatsci.2007.05.002.  
  22. ^ Heuer, A.H., et al. (1992). "Innovative Materials Processing Strategies: A Biomimetic Approach". Science 255: 1098. doi:10.1126/science.1546311.  

Further reading

  • Hench, L.L., and West. J.K., The Sol-Gel Process, Chem. Rev., Vol.90, p. 33 (1990)
  • Matijevic, E., Monodispersed Colloids: Art and Science, Langmuir, Vol.2, p. 12 (1986)
  • Engineered Materials Handbook, Volume 4: Ceramics and Glasses, ASM International, 1991, ISBN 0-87170-282-7.
  • M.W. Barsoum, Fundamentals of Ceramics, McGraw-Hill Co., Inc., 1997, ISBN 978-0070055216.
  • W.D. Callister, Jr., Materials Science and Engineering: An Introduction, 7th Ed., John Wiley & Sons, Inc., 2006, ISBN 978-0471736967 .
  • W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics, John Wiley & Sons, Inc., 1976, ISBN 0-471-47860-1.
  • M.N. Rahaman, Ceramic Processing and Sintering, 2nd Ed., Marcel Dekker Inc., 2003, ISBN 0-8247-0988-8.
  • J.S. Reed, Introduction to the Principles of Ceramic Processing, John Wiley & Sons, Inc., 1988, ISBN 0-471-84554-X.
  • D.W. Richerson, Modern Ceramic Engineering, 2nd Ed., Marcel Dekker Inc., 1992, ISBN 0-8247-8634-3.
  • W.F. Smith, Principles of Materials Science and Engineering, 3rd Ed., McGraw-Hill, Inc., 1996, ISBN 978-0070592414.
  • Wachtman, John B. (1996). Mechanical Properties of Ceramics. New York: Wiley-Interscience, John Wiley & Son's. ISBN 0-471-13316-7.  
  • L.H. VanVlack, Physical Ceramics for Engineers, Addison-Wesley Publishing Co., Inc., 1964, ISBN 0201080680.
  • Ceramic Processing Before Firing, Onoda, G.Y., Jr. and Hench, L.L. Eds., (Wiley & Sons, New York, 1979)
  • Colloidal Dispersions, Russel, W.B., et al., Eds., Cambridge Univ. Press (1989)
  • Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing by C. Jeffrey Brinker and George W. Scherer, Academic Press (1990)
  • Sol-Gel Optics: Processing and Applications, Lisa Klein, Springer Verlag (1994)
  • Zarzycki, J., Glasses and the Vitreous State, (Cambridge University Press, 1991)

External links


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