Not the ceramics of the kitchen
Most people think of ceramics as plates, tiles and bathroom fixtures. Those are traditional ceramics, made mainly from natural clays and fired at moderate temperatures. Advanced ceramics, also called engineering or technical ceramics, are a different class. They are made from carefully purified, finely controlled powders of compounds such as aluminum oxide, silicon carbide, silicon nitride and zirconium oxide, and they are formed and densified to meet a specific performance target.
The chemistry is simpler than the processing. What sets these materials apart is the control: particle size, purity, shaping and firing are managed closely so that the finished part has predictable properties.
What makes them different from metals and polymers
The atoms in ceramics are held together by strong ionic or covalent bonds. This one fact explains most of their typical behavior.
- Hardness. Strong bonds resist the sliding of atomic planes, so ceramics are generally very hard and resist wear.
- Heat resistance. Many hold their strength at temperatures where most metals soften or oxidize badly. Some are discussed further in the article on high-temperature materials.
- Chemical stability. Many ceramics resist corrosion and attack by acids or molten materials, though not all resist every chemical.
- Electrical behavior. Most are electrical insulators, though some are semiconductors and a few are used for their conductivity.
- Stiffness and low density. Many are stiff for their weight, and some are lighter than steel.
The same bonding is responsible for the main drawback. Because atomic planes cannot slide to absorb stress, ceramics cannot yield the way metals do. A tiny flaw can concentrate stress and start a crack that runs through the part. In everyday terms, ceramics are hard but brittle. The difference between hardness and the ability to resist cracking is explored in fracture toughness, and designers of ceramic parts spend much of their effort managing it.
Alumina
Aluminum oxide is the most widely used advanced ceramic. It is relatively affordable, hard, electrically insulating and resistant to wear and many chemicals. Typical uses include electrical insulators, wear liners, seals, substrates for electronic circuits and parts that handle abrasive materials. Purity and grain size vary between grades, and those differences change the properties. A fuller discussion appears in the article on alumina ceramics.
Silicon carbide
Silicon carbide is very hard, stiff and conducts heat better than most ceramics, with good resistance to oxidation at high temperature. It is used in kiln furniture, heat exchanger components, mechanical seals, pump parts and as an abrasive. Its place at the hard end of the spectrum is covered in the article on silicon carbide.
Other important members
- Zirconia is notable because it can be made much tougher than most ceramics. A mechanism called transformation toughening absorbs energy near a crack tip and slows its growth, which is described in zirconia toughening.
- Silicon nitride combines good strength, thermal shock resistance and low density, and is used in bearing balls and some engine components, as detailed in silicon nitride.
- Aluminum nitride is an electrical insulator that conducts heat well, which makes it useful as a base for electronics that need to shed heat.
- Boron carbide is extremely hard and relatively light, used in armor and abrasive-blast nozzles.
How the parts are made
Ceramics generally cannot be melted and cast like metals, because their melting points are very high. Instead, fine powder is shaped by pressing, molding or similar methods into a “green” body, then heated until the particles bond and the part densifies. This step, called sintering, is described in the sintering process. Shrinkage during firing is substantial and must be planned for, so achieving tight tolerances often requires grinding the fired part with hard abrasives, commonly diamond.
Porosity left after firing weakens a part, so much of the process development aims at reducing flaws and pores.
Typical applications
Wear and flow handling. Nozzles, liners, seals and pump components in abrasive or corrosive service take advantage of hardness and chemical stability. Similar reasoning applies to ceramic bearings, which can run at high speed and tolerate harsh conditions.
Electronics. Insulating substrates, packages and heat-conducting bases are common uses, since ceramics combine electrical insulation with dimensional stability.
High-temperature equipment. Furnace components, heat exchangers, burner parts and kiln supports use ceramics where metals would creep or oxidize.
Cutting and machining. Ceramic cutting inserts are used for some high-speed machining, and ceramic particles form many abrasives.
Medical and dental. Some ceramics are used for joint replacement components and dental restorations because they are stable inside the body. Suitability depends heavily on the specific material and design.
Aerospace and energy. Ceramic coatings and ceramic-based composites protect hot components, topics treated in thermal barrier coatings and ceramic matrix composites.
Limits to keep in mind
Brittleness is the main limit. Designers avoid sharp corners and impact loading, and they try to keep ceramic parts in compression where possible, since ceramics tend to tolerate compression better than tension. Joining ceramics to metals can be difficult because they expand by different amounts when heated. Manufacturing cost is generally higher than for common metals, so ceramics tend to be chosen where their particular combination of properties justifies the expense.
Advanced ceramics are therefore best viewed as specialists. They are not replacements for metals in general. They are the answer when heat, wear, corrosion or electrical insulation is the controlling problem, and the design can respect their brittleness.