Ceramic Materials; Properties and Applications

Here you can find detailed information about ceramic materials. One can say that metals can be the most important material in engineering applications. But actually, there is another material group that has very unique properties and because of these properties, they have very common applications in engineering. We call this material group ceramics. In this article, we will explain the properties of ceramics, applications of ceramics, and classification of ceramics.

What is Ceramic? What are the Properties of Ceramics?

Ceramics are the type of material that includes metal and non-metal compounds in an inorganic manner. First of all, ceramics are very abundant on earth. The word ‘ceramic’ came from the Greek word ‘keramos’ which means pottery’s clay. The most important examples of ceramics are;

• They use alumina, or aluminum oxide (Al2O3), in applications ranging from abrasives to artificial bones.
• Silica, or silicon dioxide(SiO2), is the main ingredient in most glass products.
• Hydrous aluminum silicate (Al2Si2O5(OH)4), known as kaolinite, is the principal ingredient in most clay products.

If we take a look at the properties of general ceramic materials; High melting temperature that makes ceramic materials preferable in high-temperature applications. Ceramics has very good chemical stability. Good electrical and thermal insulation is also one of the most important properties of ceramics. Ceramics also have a high hardness and brittle nature.

What Are The Applications Of Ceramics?

• Construction products: Tiles, bricks, etc.
• Refractory: Environments that have high-temperature applications such as crucibles, molds, etc.
• Concrete is also ceramic material that we use in most buildings and other civil engineering applications.
• Whiteware products: pottery, stoneware, fine china, porcelain, other tableware, etc.
• Glass is also a type of ceramic.
• Fiberglasses.
• Aluminum oxide and silicon carbide are also very good abrasives.
• Cutting tool materials: cubic boron nitride(CBN), tungsten carbide, aluminum oxide.
• Ceramic insulation.

Basic Classification Of Ceramic Materials

• Traditional Ceramics: We use silicates as clay products such as bricks, pottery, etc.
• New Ceramics: Nonsilicate ceramic materials that developed recently. Has superior properties compared with traditional ceramics?
• Glasses: Non-crystalline ceramics.

Chemical and Atomic Properties of Ceramics

If we take a look at the atomic arrangements of ceramic materials, the prevailing atomic bonds in ceramics are ionic and covalent. These atomic bonds are much stronger if compare with metallic bonds. This is the main reason why ceramic materials’ hardness values are superior to metals. And we can explain the brittle nature of ceramics with this. The strong bond between ceramic atoms provides a high melting temperature.

As you know, metals conduct electricity with their ‘electron sea’ which means the free electrons of metals. There is no such a kind of phenomenon as this in ceramics. All the electrons in ceramics’ atomic structure are held strictly. Because of that, ceramics are electrically insulative materials.

Generally, ceramic materials have a crystalline structure like most metals. But, because of some reasons, the crystalline structure of ceramic materials is much more complex than metals. The main reason for it is, atom sizes in ceramics change drastically and ion charges also can be different. These factors lead to the very complex crystalline arrangement in ceramic materials.

Also, some ceramics can have an amorphous atomic structure. We call this structure of ceramics ‘glassy’ ceramics. The most known example of amorphous ceramics is glass. With the addition of other elements, we can change the colors of glass.

Ceramics can have single-crystalline and poly-crystalline structures. And like in metals, physical and mechanical properties can be affected by grain size. We can achieve higher strength and toughness with finer grains, for ceramics.

Ceramic Ores in the Nature

Generally, silicates, quartz, and other kinds of clays are the most common raw material for traditional ceramics. But clays are the most used raw material in nature to obtain traditional ceramics. Clays are the hydrous aluminum silicate, that we mix with water that we can shape plastically to desired shapes. Kaolinite(Al2Si2O5(OH)4) is the most common source of clay.

After shaping clay in plastic from that we mix it with water, a process called we apply ‘firing’ to obtain hard and brittle structures from clay. The firing temperature must be suitable for the clay that we process.

Silica(SiO2) is also the principal raw material for traditional ceramics. One of the most known products of Silica is glass. Silica is available in nature in the form of quartz. And we can find quartz from sandstone. Because of the abundancy of sandstone in nature, and the easiness of the process of sandstone to obtain Silica, silica is a very cheap material.

Feldspar is also a very important raw material for traditional ceramics. The structure of Feldspar is, aluminum silicate combined with calcium, barium, sodium, and potassium.

As you remember from the production of aluminum from its ore, we process the alumina from a mineral called bauxite. This bauxite material is the principal ore of aluminum production. Alumina is also a product of bauxite, which is also a very important raw material for traditional ceramics. We can use alumina ceramic as an abrasive in grinding wheels and as a refractory brick in furnaces.

Traditional Ceramic Applications

In industry, the refractoriness of some materials and parts is very important in such crucibles, furnaces, etc. For example at the casting of iron, temperatures high as 1600 celsius degrees. A refractory material has the property of high-temperature resistance, thermal insulation, and resistance to a chemical reaction. The most important traditional ceramic material that we use for refractory purposes is alumina. We use alumina with silica to obtain a very efficient refractory material. highest firing temperature to obtain finer products. Magnesium oxide and Calcium oxide materials are also very important materials for refractoriness. In general applications, the refractory lining has two layers. The first layer has a more porous structure, which increases thermal insulation properties.

As you know from basic history knowledge, pottery has a very long history, especially in China. General pottery products that produced from traditional ceramic materials; earthenware, stoneware, and china. Earthenware is the most porous one and is generally glazed. Glazing is a special coating applied. Stoneware is less porous and obtained with higher firing applications. Porcelain is like china and obtained with the firing of clay, silica, and feldspar. Porcelain has also very important applications in engineering such as electrical insulation. The structure and properties of china are also like porcelain.

Other Important Applications

Brick and tile applications are one of the major applications of traditional ceramic materials. Techniques that we use in traditional brick and tile production date back to very old times. Because of this reason, the materials that we use in brick and tile applications are called traditional ceramic materials. Some special applications are building brick, clay pipe, unglazed roof tiles, and drain tiles. In general, we produce these brick and tile applications by molding and firing operations.

One other application of traditional ceramics is abrasives. Because of the high hardness values of some ceramic materials, we can use traditional ceramics in abrasive processes Silicon carbide is a very important traditional ceramic material that we use in abrasive applications. And this material has a very high hardness value(2200-2600 HV). Grinding wheels is one of the most important abrasive applications in the metal industry. But we produce grinding wheels from Al2O3 because it gives better results. These abrasive traditional materials bond to grinding wheels with shellac, and special resins.

As you know, most aspects of our civilization depend upon these applications. Abrasive processes are a very important process to obtain the required surface parameters of metal parts. Bricks and tiles are very important parts of out buildings. 

A Special Applications of Ceramics: Glass Ceramics

In the history of human civilization, they produce various types of materials to obtain different kinds of properties for different applications. The expected properties of materials can change according to the application type. For example, lightweight applications are very important in the aerospace industry, and we produce these materials according to these necessities. 

For example, they produce glass materials because of their opaque nature and their capability to withstand static loads. 

From glass, we produce there are various kinds of materials also such as glass ceramics for much more superior properties. In this article, we elaborate on the glass-ceramic material’s properties and we use them. 

What is Glass Ceramic?

As you know from the basic material science and engineering courses, glass has an amorphous microstructure which gives its opaque characteristic of it. Also, glass has a very brittle nature. 

As its name implies, we produce glass ceramics from glass materials. With the application of the crystallization process, the amorphous microstructure of the glass turns into the crystalline microstructure. But this crystallization is not occurring 100%. In general, the crystallization rate of glass ceramics is from 55% to 90% according to the expectations from that material. We call the non-crystallized percentage of glass ceramics residual glass.

This crystallization of glass-ceramic materials adds extra superior properties to glass materials. 

Properties of Glass-Ceramic Materials

With the addition of the crystalline phase to glass-ceramic materials, we obtain these kinds of superior properties;  

• In material design, designing the micro and nanostructures can be a very important factor for different loading cases. We can produce glass-ceramics through conventional glass shaping processes, and we can design the micro and nanostructures of glass ceramics. So we can obtain different kinds of desired material properties in glass-ceramic materials. 
• Nearly zero thermal expansion coefficient makes the glass-ceramic materials proper for high-temperature applications. Higher thermal expansion coefficients can lead to fractures because of the structural limitations in most high-temperature applications. 
• High fracture toughness is another important factor of glass-ceramic materials, compared with glass. Because of this property, we can use glass-ceramic materials in structural applications where the mechanical strength of normal glass is not sufficient enough. 
• Improved impact strength is another important feature of glass ceramics, compared with normal glass again. 

How We Produce the Glass Ceramics?

We produce glass-ceramic materials from glass, it is true. But we do not use every type of glass material to produce glass ceramics. To obtain glass-ceramic crystallization, we must make the proper glass selection. For example, we are not using the conventional window glass type for glass-ceramic production. Because window glasses have a very stable amorphous state that is very hard to transform them into a crystalline phase. Also, there are other kinds of glasses that very high rate of crystallization in a rapid way. In this case, we can obtain unintended microstructures. But, it is very important to obtain desired microstructures from glass ceramics. 

Before of crystallization process of primary glass material, we produce glass with conventional methods. After that, we give the intended shape to glass. After the solidification of the net shape, we heat the glass up to glass-transition temperature to start the crystallization process. 

During this heat treatment process, we add special kinds of agents to make the crystallization process much easier. These special agents, noble metals, and fluorides make the nucleation for crystallization easy. 

We apply different kinds of heat treatment methods to produce glass-ceramic materials. But apart from the heat treatment procedures, powder metallurgy is another different method to produce glass-ceramic materials. Because of the very high sintering temperatures and high costs of ceramic powder production, we do not prefer the powder metallurgy production technique in glass-ceramic production. 

Where We Use the Glass Ceramics?

There are various kinds of application areas of glass-ceramic materials because of their superior properties; 

• Heat exchangers: We use glass-ceramic materials in structural parts of heat exchangers because of their high resistance to high temperatures and ability to preserve mechanical properties at high temperatures. 
• Vitroseramic hobs: In Vitro ceramic hobs, some heaters are heated by electricity resistance. Because of the heat production, we must use a material that needs high-temperature resistance. In this case, glass-ceramic materials are ideal for this application both for high-temperature resistance and the visual appearance of devices. 
• High-precision optical instruments: We are expecting very good optical properties from the material that we use in optical instruments. One may think that we need to use glass in optical instruments. It can be correct but in very high precisions, we must consider the thermal expansion of the glass. Considering all these cases, glass-ceramic materials can be very good materials for high-precision optical instruments. 
• Solar panels: This is another common application of glass ceramics because of the very good thermal and mechanical properties of glass ceramics. 

Oxide Ceramics

Ceramic technique and ceramic technology are very important branches. Like other technologies and techniques, ceramic techniques and technology have also been very important innovations in themselves. One of these innovations in ceramic technique and technology is oxide ceramics. In this article, we will explain oxide ceramics in a very basic term.

The difference between new ceramics from traditional ceramics is compounds and process steps. But compounds in new ceramics can be inside some traditional ceramics also. The main difference between new ceramics and traditional ceramics is control over material properties through compounds in new ceramics, with more recent processes and techniques. We can assume the oxide ceramics are also new.

The most important material in the oxide ceramic family is alumina. As we explained in another article, we produce alumina with bauxite synthetically by the arc furnace method. Control over particle size and blending with other kinds of elements through controlling the production process of alumina gives much better products than its historical counterparts. With very good properties of hot hardness, very good corrosion resistance, and low thermal conductivity, new alumina finds a very important place in various engineering applications such as; alloying ingredients in the glass, abrasives(grinding wheel grit), bioceramics(artificial bones and teeth), electrical insulators, electronic components, spark plug barrels, refractory brick, cutting

Ceramic Nitride Materials and Applications

The general ceramic nitride materials that are used in engineering applications are; titanium nitride(TiN), boron nitride(BN), and silicon nitride(Si3N4). These nitride materials have very specific properties and applications.

• Titanium Nitride: It is the only nitride material that has electrical conductivity. It has very good wear resistance and high hardness properties. And has a very low friction coefficient with ferrous metals. Because of these properties, titanium nitride is used in tool making.
• Silicon Nitride: It is a very suitable ceramic nitride material for high-temperature applications. Because silicon nitride oxidizes at 1200 celsius degrees and decomposes at around 1600 celsius degrees. Because of that resistance to high temperature, and high creep resistance, Silicon Nitride has used ar rocketry, crucibles and other high-temperature applications.
• Boron Nitride: Boron nitride has various molecular structures such as hexagonal as carbon and cubic as diamond. Also, boron nitride has comparable properties with diamond for example in terms of hardness. Cubic boron nitride(CBN) which is a very important material in toolmaking, is produced by heating hexagonal boron nitride to obtain cubic molecular structure. CBN has extreme hardness. In some areas of machining, even CBN is not comparable with diamond. CBN is generally used in steel machining and diamond is used in non-steel machining operations.

Ceramic Carbides

Ceramic carbide materials are very important materials that have specific properties to use in specific engineering applications. There are a bunch of ceramic carbide materials available. In this article, we will explain these ceramic carbide materials and their compositions, and production processes.

Ceramic carbide materials that are generally used in most engineering applications, tungsten carbide(WC), chromium carbide(Cr3C2), titanium carbide (TiC), tantalum carbide(TaC), and silicon carbide(SiC). Silicon carbide is the most historical ceramic carbide material, because of that, this ceramic carbide material can be classified as traditional ceramic. Silicon carbide has various kinds of applications such as an additive in steelmaking, resistance heating elements, etc. Chromium carbide is also very important in such applications as chemical stability and oxidation resistance.

Except for SiC, all the other ceramic carbides must have a metallic binder such as nickel or cobalt, etc. This gives a very solid structure to ceramic carbide materials. These materials in which carbides and metals are combined are called cemented carbides. In the market, these cemented carbide materials also known as ‘cermets’ means ‘ceramic metals’.

Titanium carbide, tantalum carbide, and tungsten carbide materials have very good hardness and wear resistance. Because of that, these materials are generally used in toolmaking for machining operations. Tungsten carbide is the most historical one, and it is used for long historical centuries. The production of tungsten carbide includes carburizing of tungsten powders that are obtained from tungsten ores which have a chemical composition of FeMnWO4 called wolframite, and CaWO4 called scheelite. Chromium carbide is produced by carburizing chromium oxide which has a chemical composition of Cr3C2. Titanium carbide is also produced from rutile(TiO4) and ilmenite(FeTiO3). And tantalum carbide is made by carburizing either pure tantalum powders or tantalum pentoxide(Ta2O5).

Conclusion

As you see above, ceramics are a very important class of materials. So, their applications are very common in engineering. There are different kinds of applications and types of ceramic materials that we need to know.

Also, there are important advanced uses of these materials. As in cutting tool technology, the use of ceramic applications is very common. And also, we are using them as refractory materials.

These are the general important points about ceramic materials.

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