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What are ceramics?

Industrial ceramic, Ceramics are broadly defined as inorganic, nonmetallic materials that exhibit such useful properties as high strength and hardness, high melting temperatures, chemical inertness, and low thermal and electrical conductivity but that also display brittleness and sensitivity to flaws. As practical materials, they have a history almost as old as the human race. Traditional ceramic products, made from common, naturally occurring minerals such as clay and sand, have long been the object of the potter, the brickmaker, and the glazier. Modern advanced ceramics, on the other hand, are often produced under exacting conditions in the laboratory and call into play the skills of the chemist, the physicist, and the engineer. Containing a variety of ingredients and manipulated by a variety of processing techniques, ceramics are made into a wide range of industrial products, from common floor tile to nuclear fuel pellets. Yet all these disparate products owe their utility to a set of properties that are universally recognized as ceramic-like, and these properties in turn owe their existence to chemical bonds and atomic structures that are peculiar to the material. The composition, structure, and properties of industrial ceramic, their processing into both traditional and advanced materials, and the products made from those materials are the subject of many articles on particular traditional or advanced ceramic products, such as whitewares, abrasives, conductive ceramics, and bioceramics. For a more comprehensive understanding of the subject, however, the reader is advised to begin with the central article, on the composition, structure, and properties of ceramic materials.

A ceramic is an inorganic non-metallic solid made up of either metal or non-metal compounds that have been shaped and then hardened by heating to high temperatures. In general, they are hard, corrosion-resistant and brittle.

'Ceramic' comes from the Greek word meaning ‘pottery’. The clay-based domestic wares, art objects and building products are familiar to us all, but pottery is just one part of the ceramic world.

Nowadays the term ‘ceramic’ has a more expansive meaning and includes materials like glass, advanced ceramics and some cement systems as well.

Traditional ceramics – pottery

Pottery is one of the oldest human technologies. Fragments of clay pottery found recently in Hunan Province in China have been carbon dated to 17,500–18,300 years old.

The major types of pottery are described as earthenware, stoneware and porcelain.

Earthenware is used extensively for pottery tableware and decorative objects. It is one of the oldest materials used in pottery.

The clay is fired at relatively low temperatures (1,000–1,150°C), producing a slightly porous, coarse product. To overcome its porosity, the fired object is covered with finely ground glass powder suspended in water (glaze) and is then fired a second time. Faience, Delft and majolica are examples of earthenware.

Stoneware clay is fired at a high temperature (about 1,200°C) until made glass-like (vitrified). Because stoneware is non-porous, glaze is applied only for decoration. It is a sturdy, chip-resistant and durable material suitable for use in the kitchen for cooking, baking, storing liquids and as serving dishes.

Porcelain is a very hard, translucent white ceramic. The earliest forms of porcelain originated in China around 1600BC, and by 600AD, Chinese porcelain was a prized commodity with Arabian traders. Because porcelain was associated with China and often used to make plates, cups, vases and other works of fine art, it often goes by the name of ‘fine china'.

To make porcelain, small amounts of glass, granite and feldspar minerals are ground up with fine white kaolin clay. Water is then added to the resulting fine white powder so that it can be kneaded and worked into shape. This is fired in a kiln to between 1,200–1,450°C. Decorative glazes are then applied followed by further firing.

Bone china – which is easier to make, harder to chip and stronger than porcelain – is made by adding ash from cattle bones to clay, feldspar minerals and fine silica sand.

Industrial ceramic is typically crystalline or partly crystalline in structure. They are made of inorganic, non-metallic matter. Early ceramics consisted mainly of clay and clay-mixtures, as used to make pottery. The natural mineral deposits of readily available clay and sand, combined to reach the right consistency when mixed with various liquids, are ideal for creating moldable material useful for traditional ceramics. This traditional ceramics mixture is used by potters and bricklayers around the world, in part because it is so readily available, easy to mix, and inexpensive.

Current developments have enabled ceramics to be used in technological applications far more complex than their traditional ceramics predecessors. Using precise ingredients, measurements, and procedures, modern advanced ceramic machining often calls upon the skills of physicists, chemists, and multiple engineers. They are used to create products as simple as a floor tile, or as complicated and intricate as a nuclear fuel pellet.

Modern advanced ceramics relies on high-quality ingredients, not just sand and clay, to create ceramics that exhibit properties needed to withstand extreme hazardous environmental conditions. At the same time, these same ceramics must be made with exacting precision to allow for flaws to be evidently visible.

Most traditional ceramics are known for their hardness, brittleness, and strength. In the past, traditional ceramics have been used as electric insulators since porcelain is resistant to the flow of electricity. Modern industrial ceramic can be made to be as tough and as conductive as the hardest metals. These ceramics are created with such precision, that their very cellular structure is controlled, manipulated, and created. Such highly conductive ceramics are often used in superconductors and many types of superior mechanical devices. This makes these heat conductive ceramics a highly sought after commodity.

There are three general categories of ceramics: oxides, non-oxides, and composites. This article breaks down those categories and looks at the different types of ceramics.

Types of Oxide Ceramics

The introduction of oxide fibers in a ceramic mixture can help the final component withstand oxidation, and provide added strength and reinforcement. Although they are available in a range of compositions and can be formed through different processes, all oxide fibers are formed first, usually through a chemical process, and then heated to finalize the ceramic. Several common methods include polymer pyrolysis, a chemical deposition process that occurs at high heat, and sol-gel, wherein chemical solution deposition takes place through spinning fibers from a liquid.

Ceramic oxide fibers often include combinations of zirconium dioxide, aluminum trioxide, and titanium dioxide. Silica, phosphorous, and Boria are typically required in large quantities, as they are glass-forming oxides.

Alumina Ceramics

Alumina ceramics tend to offer high chemical resistance, increased strength, and high temperature resistance. They can be made using a process similar to sol-gel, and then fired over high-heat. The end product has a rough surface but a strong polycrystalline structure. The surface can be smoothed using a silica coating, which further enhances the strength of the component. Alumina-zirconia ceramic fiber offers better retention of mechanical properties when exposed to extreme heat, and is typically more useful in composites that must withstand continual exposure to higher temperatures. Alumina-silica ceramic fiber features similar properties as alumina-zirconia.

Beryllium Oxide Ceramics

Beryllium oxide ceramics have good thermal conductivity, high insulation, low dielectric constant, low medium loss, and good process adaptability. These ceramics are sometimes used as a component in glass. Glass containing beryllium oxide, which can pass through x-rays, is used to make X-ray tubes that can be used for structural analysis and medically to treat skin diseases. Beryllium oxide ceramics are also used for high-power microwave packaging and high-frequency transistor packaging because of their stability and insulation properties.

Zirconia Ceramics

Zirconia ceramics have a low thermal conductivity with excellent thermal insulation, and very high resistance to crack propagation. As such, they’re often used for protective coatings and as tools for wire forming. They are used in dentistry applications such as dental prostheses, and for other medical devices. Zirconia ceramics are less brittle than other ceramics, so they’re commonly used for ceramic knives.

Types of Non-Oxide Ceramics

Because oxide ceramics are not always well-suited to use in extreme environments or as a replacement in applications required to bear significant loads, ceramic non-oxides respond to this need. Silicon nitride and silicon carbide, two commonly used ceramic non-oxide fibers, offer high heat resistance. They do not degrade until temperatures pass 2400 degrees Celsius.

Additionally, non-oxide ceramics offer incredibly high corrosion resistance, hardness, and oxidation resistance. Fiber manufacturing techniques involve spinning and heat treating the final fiber to cure it, as is done in pyrolysis and sintering.

Silicon Nitride Ceramics

Silicon nitride ceramics have a very low density, a high fracture toughness, good flexural strength, and excellent thermal shock resistance. Silicon nitride can be machined in several states: green, biscuit, or fully dense. Silicon nitride ceramics are used for rotating bearing balls and rollers, cutting tools, moving engine parts, turbine blades, and weld positioners.

Silicon Carbide Ceramics

Silicon carbide ceramics are much lighter and harder than other ceramics and are resistant to acids and lyes. Pressureless sintering techniques make it possible to manufacture dense compacts of silicon carbide, making it a widespread structural material. These ceramics are lightweight because silicon carbide consists primarily of lightweight elements. They also have low thermal expansion and high conductivity and are exceptionally chemically stable. They are used for fixed and moving turbine components, suction box covers, seals, bearings, ball valve parts, and heat exchangers.

Types of Composite Ceramics

A composite material is composed of two or more constituent materials with significantly different physical or chemical properties. These materials combine to produce a material with characteristics different from the individual components. The components remain distinct within the finished structure, differentiating composites from mixtures and solid solutions. Composite ceramics have ceramic fibers embedded in a ceramic matrix. The matrix and the fibers can be made up of any ceramic material.

Fiber-Reinforced Ceramics

Fiber-reinforced ceramics, or ceramic matrix-fiber composites, have increased toughness and strength. The ceramic fibers can have a polycrystalline structure, as in conventional ceramics. They can also be amorphous or have inhomogeneous chemical composition. The high process temperatures required for making the composite ceramics prevent the use of organic, metallic, or glass fibers. Only fibers stable at temperatures above 1000 °C can be used, such as fibers of alumina, mullite, SiC, zirconia, or carbon. Fiber-reinforced ceramics don’t have the major disadvantages of conventional ceramics, namely brittleness and low fracture toughness, and limited thermal shock resistance. Consequently, their applications are in fields requiring reliability at high-temperatures and resistance to corrosion and wear. These applications include heat shield systems for space vehicles, components for high-temperature gas turbines, components for burners, flame holders, and hot gas ducts, brake system components, which experience extreme thermal shock, and components for slide bearings under heavy loads requiring high corrosion and wear resistance.

Summary

This article presented an understanding of the different kinds of ceramics and ceramic machining. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.

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