The Expanse Of Advanced Ceramics
When most of us picture a ceramic, we think of a coffee mug, a roof tile, perhaps a vase. These are all ceramics, but traditional ceramics are only a small portion of a much larger material family. Elsewhere in that family, advanced ceramics are used to protect spacecraft, scaffold the regrowth of bone, enable ultrasound scanning, and absorb radiation inside a nuclear reactor. This breadth and versatility is exactly what it’s all about.
Advanced ceramics are usually defined as non-metallic, inorganic materials that obtain their microstructure through the action of temperature.¹It is a deliberately broad definition, and it needs to be: there are theoretically billions of distinct combinations of elements that create a ceramic material, and the number of useful ones grows year on year as research pushes performance to meet engineering demand. Their functional properties range from thermomechanical and electrical to biological, optical, chemical, magnetic, and nuclear. One material family, finding applications in almost every branch of engineering.
The table below outlines some key examples of advanced ceramics and their engineering applications, as well as the necessary material properties.
| Material | Application | Property | Geometry |
|---|---|---|---|
| Silicon carbide (SiC) | Shell-and-tube heat exchangers, mechanical seal faces | High thermal conductivity, hardness, corrosion and thermal-shock resistance | Thin-walled tube bundles; flat, lapped seal faces |
| Silicon nitride (Si₃N₄) | Rolling-element bearings (machine-tool spindles, EV motors, aero mainshafts) | Low density, high fracture toughness, low friction, electrical insulation | Precision spheres and raceways, held to sub-micron tolerances |
| Alumina (Al₂O₃) | Power-electronics substrates; bioinert joint heads | Dielectric strength with useful thermal conductivity; hardness and bioinertness | Thin flat substrates; polished femoral-head spheres |
| Zirconia (ZrO₂, usually stabilised) | Dental and joint restorations; oxygen sensors and fuel-cell electrolytes; thermal-barrier coatings | Transformation toughening with biocompatibility; oxygen-ion conduction; low thermal conductivity | Machined crowns; thin sensor membranes; sprayed blade coatings |
| Boron carbide (B₄C) | Body and vehicle armour; reactor control rods | Extreme hardness at low density; high neutron-capture cross-section | Curved or flat plates on a polymer backing; rods and pellets |
| Lead zirconate titanate (Pb(Zr,Ti)O₃) | Ultrasound and sonar transducers, actuators, spark igniters | Piezoelectricity: a two-way conversion between mechanical and electrical energy | Discs, rings, and multilayer stacks |
| Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) | Bone-graft scaffolds; coatings on orthopaedic implants | Bioactivity and a close chemical likeness to bone mineral | Porous, interconnected scaffolds; thin coatings on metal |
So where do these properties come from? Fundamentally, a ceramic's material properties depend on the chemical elements within it, and how these elements bond together.²However, the ceramic processing workflow (shaping then sintering) also has effects on the properties and, at the component level, so does the geometry of the component. For a simplistic example, consider how the shape of a coffee cup affects its ability to perform its intended function; hold coffee.
The geometric requirements for the engineering applications are also given in the table above. The point is that a given property usually demands a particular form to express it. A material is only half-specified until you also say what shape it has to take. To describe an advanced ceramic component in full, then, you need both halves: the material and the geometry must be designed in parallel.
What’s more, ceramic chemistry is not necessarily fixed. Each row in the table above pairs a fixed composition with a fixed job, but the material properties can often be adjusted to meet the functional requirements. Broadly, there are two routes to doing so.
The first is doping: introducing a small amount of one element into the crystal lattice of another. Often the aim is to tune the end properties. Zirconia is a good example. On its own, it cracks as it cools, but add a small amount of yttria and its tough, high-temperature crystal phase becomes stable at room temperature.³ The same addition leaves the lattice with oxygen vacancies, which allow it to conduct oxygen ions. The result is a single material that serves both as a structural ceramic, in hip joints and cutting tools, and as the active element in oxygen sensors and fuel-cell electrolytes.
The second route is the composite: rather than dissolving an addition into the crystal lattice, two distinct ceramics are held together so the bulk behaviour reflects both. Zirconia-toughened alumina combines the hardness of alumina with the crack resistance of stabilised zirconia. Hydroxyapatite is usually used the same way; combined with tri-calcium phosphate for the combination of mechanical strength and bioactivity.
That is, the properties of functional ceramics are continually evolving as new ceramic materials and composites are developed. The co-evolution of flexible manufacturing processes is essential for rapid iteration in material formulation and functional component design. This will be driven by the demands of modern industry, including large-scale infrastructure projects, the digital transition, and sustainability initiatives. For example, imagine the wall of a nuclear fusion reactor made from a material that could not only dissipate the extreme heat of the fusion reaction but harness it directly using energy harvesting phenomena… Material properties of this kind are made possible by the expanse of advanced ceramics.
Science Corner
[1] This refers to the high-temperature sintering process, where adjacent ceramic particles are fused together to form a ceramic solid - more on this in The Complexity of Ceramic Processing.
[2] Defined as non-metallic, inorganic substances, ceramic materials (usually consisting of a metallic and a non-metallic element) form very strong interatomic bonds; a combination of covalent and ionic bonds. The balance of ionic or covalent bonds depends on the electronegativity ratio of the constituent elements. Elements with a large difference in electronegativity form ionic bonds whereas covalent bonds form in combinations of elements with similar electronegativities. These are said to have high covalent character.
[3] The mechanism in yttria-stabilised zirconia is a stress-induced phase transformation. The yttria holds the zirconia in its tetragonal phase, which is metastable at room temperature. When a crack begins to advance, the concentrated stress at its tip drives nearby grains into their monoclinic phase, which takes up slightly more volume; that local expansion compresses the crack tip and slows the crack propagation.
