About Ceramics

Overall introduction to ceramics

Here we explain the definition of ceramics, what ceramics consist of, material properties, design tips, areas of use and a general description of how ceramics are manufactured.

ceramic ball bearings of silicon nitride Si3N4 and zirconium dioxide ZrO2

What is Ceramics?

According to the definition, ceramics are inorganic, non-metallic solid materials, which during manufacture are heat treated at temperatures above 600°C. The latter is to leave out naturally occurring stones. Ceramic materials are divided in traditional ceramics (porcelain, ceramic ware, pottery, refractories, enamel and glass) and in technical ceramics (for instance pure aluminium oxide and silicon nitride). While traditional ceramics are well known to most of us, technical ceramics are not as apparent at first glance. Technical ceramics are historically a young phenomenon and have only developed and become widespread in recent decades. These ceramics have exceptional material-specific properties that are used in applications where other materials do not perform as well or do not work at all.

Technical ceramics are divided into two subgroups – engineering ceramics och functional ceramics. Engineering ceramics, also known as structural ceramics, are used mainly due to their excellent mechanical properties, often in combination with other useful physical properties such as chemical inertness and electrical insulation. For the functional ceramics, it is mainly inherent physical properties (such as conductivity, dielectricity, piezoelectricity, ferroelectricity, ferromagnetism and permeability) that are of interest for certain applications. The mechanical properties in this case come second. Traditional ceramics can be included in the group of technical ceramics but are made from traditional clay-based ceramic materials.

Ceramic materials may have a fully or partially crystalline structure or in some cases be completely amorphous, such as ordinary glass. An example of a common technical ceramic that is crystalline is alumina. This ceramic is made up of tightly connected micrometer-sized crystals of the elements aluminum and oxygen.

Technical ceramics – typical material properties

What distinguishes ceramics from other materials are their characteristic properties such as hardness, compressive strength, stiffness (high modulus of elasticity), high temperature resistance, chemical stability and corrosion and abrasion resistance. Ceramic materials usually have good electrical and thermal insulating ability and a low coefficient of thermal expansion.

At the same time, ceramics are more brittle than other materials. The brittleness limits the plastic deformation in the material, which means that the crack sensitivity is greater. For special needs in special applications, there are several ways to increase the toughness of the material, for example by particle and fiber reinforcement.

Material data for some common ceramics can be found in Table of material properties for construction ceramics.

Design right with ceramic materials

As mentioned, ceramics are hard and brittle. In order to obtain high strength and reduce the risk of fracture or chipping, a number of design rules should be followed. A couple of primary such rules are to simplify the geometry as far as possible and to avoid sharp edges. Since the compressive strength is considerably higher than both the bending and tensile strength, it is better to subject the components to pressure and adapt the application accordingly. Critical stress concentrations should also be avoided in the materials.

Something that also affects the design is which ceramic material has been chosen for a particular application, as the properties of the materials vary a lot. Silicon nitride, for example, has higher fracture toughness than alumina, which makes it more resistant to cracking. For this reason, silicon nitride can often be used in applications where the design is more complex or where the material requirements are tougher. The disadvantage is that silicon nitride is more expensive to produce than alumina and that it begins to oxidize at temperatures above 1200 ° C.

Technical ceramics – applications

Today, technical ceramics (also known as structural ceramics, advanced ceramics or industrial ceramics) constitute essential and often indispensable components in a wide range of technical applications. As previously mentioned, these ceramics are divided into engineering ceramics and functional ceramics, which also include electroceramics. Some examples of applications for construction ceramics are nozzles for both spraying and welding, valves, ceramic ball bearings, cutting edges including indexable inserts, turbine rotors in gas turbines, thermal barrier layers and other coatings, ballistic protection, plates in bulletproof vests, biocompatible components in medical technology gas and catalyst and combustion catalysts. Functional ceramics are used, for example, as magnets for electric motors, sensors in sensors, actuators in mechanics and vibrators, circuit board substrates, electrolyte in fuel cells (SOFC), varistors for surge protection and dielectric components for microwave applications.

Traditional ceramics have a history that stretches far back in time and is still market-dominant. This group includes everyday porcelain that we come in daily contact with, glaze to protect and seal ceramics, glass for a variety of applications, stoneware, earthenware, sanitary ware for bathrooms and electrical porcelain for, for example, low and high voltage insulators. This group also includes refractory materials for use at temperatures above 1500 ° C such as fired chamotte bricks and refractory masses for oven lining. The traditional ceramics also include technical ceramics, as this group of ceramics consists of the same ceramic materials as some of the traditional ceramics, see the paragraph below. The difference is that technical ceramics are used in technical applications.

What do technical and traditional ceramic materials consist of?

Technical ceramics consist either of refined and purified natural raw materials or of synthetically produced compounds based on oxides, nitrides, carbides and borides. The largest group consists of the oxides and some common such construction ceramics are alumina (Al2O3), zirconia (ZrO2), silica (SiO2) and mullite (Al6Si2O13). Of these, alumina is the dominant ceramic in the market for construction and other advanced applications. Some common nitrides are silicon nitride (Si3N4), sialon (Si-Al-O-N), aluminum nitride (AlN) and boron nitride (BN) and a couple of common carbides are silicon carbide (SiC) and boron carbide (B4C). Both silicon nitride and silicon carbide are included in the group of high-performance ceramics and are often used as construction materials thanks to excellent mechanical properties. The functional ceramics are available in a large number of variants, most of which are oxide-based. An example of such an oxide is barium titanate (BaTiO3). This material has a high dielectric constant and is therefore used, among other things, as an insulating intermediate layer in capacitors with high capacitance.

Unlike the technical ceramics, the traditional ceramics are made from naturally occurring raw materials such as clay and various quartz compounds. No actual processing normally takes place before the production of the various ceramic materials. The traditional ceramics therefore have a more complex composition of chemical compounds and consist of a mixture of amorphous and often crystalline oxides. For example, the basic materials in porcelain production are feldspar (NaAlSi3O5), kaolin clay (Al2 (OH) 4Si2O5) and quartz (crystalline silica, SiO2). The starting materials in the manufacture of ordinary glass are soda (Na2CO3), lime (CaCO3, Ca (OH) 2, CaO) and sand (SiO2, SixOy) which on heating form a melt of Na2O, CaO and SiO2. During cooling, an amorphous structure of these compounds is maintained, which is transparent in the visible light region.

Unique properties of each ceramic material

Ceramic materials have many unique and useful properties. It is often one or a couple of distinctive properties of each ceramic that make that particular material very important in a particular application. An example is aluminum titanate (Al2TiO5) which has a coefficient of thermal expansion of only 1 ppm / ° C up to 1000 ° C, which results in the material having very good resistance to thermal shock. The material is therefore used, among other things, for exhaust ducts in internal combustion engines as a thermally insulating housing. The ceramic remains intact despite repeated and extreme temperature changes in the engine.

Another example of a special material is aluminum nitride (AlN). Compared with most other ceramics, this material has very good thermal conductivity, approx. 160 W / m-K, and at the same time good electrical insulation, which means that it is currently used as a heat-conducting substrate in the electronics industry. In the past, beryllium oxide (BeO) was used, which has even higher conductivity, but due to its toxicity, this material has been replaced by aluminum nitride.

Manufacture of ceramic components

Compared to other materials, components of ceramic materials are relatively cumbersome to manufacture. This is because the production is based on fine-grained powder that needs to be prepared in several steps. First, the required raw powders for a certain ceramic must be weighed in and mixed with solvents, usually water, and various process aids, such as dispersants. The resulting slurry is mixed and ground in a ball mill to homogenize and reduce the particle size. The continued preparation depends on the method chosen to form components.

The choice of forming method is governed by the geometry of the components. The molding is divided into dry and wet molding. The most common dry forming method for simpler geometries is uniaxial pressing, which is a fast and cost-effective manufacturing process. In order to be able to press components, granules need to be produced. Granules consist of spherical clusters of ceramic particles and are obtained by spray drying. Some typical wet forming techniques are slurry casting, tape casting, extrusion and injection molding. For wet forming, various organic binders and stabilizers are added to the preparation to achieve a good forming result. In the case of tape casting, for example, adhesive is a necessary additive for the shaped thin layers to stick together.

After molding, the components are dried, processed and fired (sintered) at a high temperature, resulting in solid and durable products. These products are usually finished either to meet the dimensional requirements or just to get an appealing finish. Due to the hardness of the materials, special equipment for processing and polishing is normally required.

The above describes the basic principles of making ceramics. Much development is taking place in the area to reduce costs and adapt production for the specific end product. For example, it is important to shape components in such a way that post-sintering post-processing is minimized. In addition to optimizing the process, a simplification of the design of the product and a modification of the application can reduce the total cost significantly.

ceramic screws of zirconia ZrO2 and alumina Al2O3

More information about technical ceramics and technical ceramics

This was an overall introduction to the ceramics area. If you want further information about specific technical ceramic materials, manufacturing methods or application possibilities, you are welcome to contact us.