The physical properties of any ceramic substance are a direct result of its crystalline structure and chemical composition.
Solid-state chemistry reveals the fundamental connection between microstructure and properties, such as localized density variations, grain size distribution, type of porosity, and second-phase content, which can all be correlated with ceramic properties such as mechanical strength σ by the Hall-Petch equation,
hardness,
toughness,
dielectric constant, and the
optical properties exhibited by
transparent materials.
Ceramography is the art and science of preparation, examination, and evaluation of ceramic microstructures. Evaluation and characterization of ceramic microstructures are often implemented on similar spatial scales to that used commonly in the emerging field of nanotechnology: from
nanometers to tens of micrometers (μm). This is typically somewhere between the minimum wavelength of visible light and the resolution limit of the naked eye. The microstructure includes most grains, secondary phases, grain boundaries, pores, micro-cracks, structural defects, and hardness micro indentions. Most bulk mechanical, optical, thermal, electrical, and magnetic properties are significantly affected by the observed microstructure. The fabrication method and process conditions are generally indicated by the microstructure. The root cause of many ceramic failures is evident in the cleaved and polished microstructure. Physical properties which constitute the field of
materials science and
engineering include the following:
Mechanical properties Mechanical properties are important in structural and building materials as well as textile fabrics. In modern
materials science, fracture mechanics is an important tool in improving the mechanical performance of materials and components. It applies the
physics of
stress and
strain, in particular the theories of
elasticity and
plasticity, to the microscopic
crystallographic defects found in real materials in order to predict the macroscopic mechanical failure of bodies.
Fractography is widely used with fracture mechanics to understand the causes of failures and also verify the theoretical
failure predictions with real-life failures. Ceramic materials are usually
ionic or
covalent bonded materials. A material held together by either type of bond will tend to
fracture before any
plastic deformation takes place, which results in poor
toughness and brittle behavior in these materials. Additionally, because these materials tend to be porous,
pores and other microscopic imperfections act as
stress concentrators, decreasing the toughness further, and reducing the
tensile strength. These combine to give
catastrophic failures, as opposed to the more ductile
failure modes of metals. These materials do show
plastic deformation. However, because of the rigid structure of crystalline material, there are very few available slip systems for
dislocations to move, and so they deform very slowly. To overcome the brittle behavior, ceramic material development has introduced the class of
ceramic matrix composite materials, in which ceramic fibers are embedded and with specific coatings form fiber bridges across any crack. This mechanism substantially increases the fracture toughness of such ceramics. Ceramic
disc brakes are an example of using a ceramic matrix composite material manufactured with a specific process. Scientists are working on developing ceramic materials that can withstand significant deformation without breaking. A first such material that can deform in room temperature was found in 2024.
Toughening mechanisms Many strategies are employed to improve the
toughness of ceramics to prevent fracture. This includes crack deflection, microcrack toughening, crack bridging, incorporation of ductile particles, and transformation toughening. Crack deflection is a toughening mechanism that involves deflecting cracks away from more rapid crack propagation paths, preventing catastrophic sudden failure. Cracks may be deflected using microstructures such as whiskers, as in the use of
silicon carbide whiskers to reinforce
molybdenum disilicide ceramic material detailed in a 1987 paper. Crack deflecting second phases may also take the form of
platelets, particles, or fibers. Microcrack toughening involves nucleation (creation) of microcracks near a macroscopic crack tip where the crack propagates, which lowers the stress experienced by the tip and therefore the urgency of crack propagation. To improve toughness, second phase particles can be incorporated into ceramic such that they are subject to microcracking, which relieves stress to prevent fracture. Crack bridging occurs when a strong discontinuous reinforcing phase applies a force behind the propagating tip of the crack that discourages further cracking. These second phase bridges essentially pin the crack to discourage its extension. Crack bridging can be used to improve toughness via the incorporation of second phase whiskers in the ceramic, as well as other shapes, to bridge cracks. Ductile particle ceramic matrix composites are composed of
ductile particles such as metals distributed in a ceramic matrix. These particles boost toughness by deforming plastically to absorb energy, and by bridging advancing cracks. To be most effective, the particles should be isolated from each other. The most studied iterations of these composites consist of an alumina matrix, and nickel, iron, molybdenum, copper, or silver metal particles. Transformation toughening occurs when a material undergoes stress-induced phase transformation. Some ceramics are capable of undergoing stress-induced martensitic transformation, which involves an energy barrier that must be overcome by absorbing energy. Martensitic transformations are
diffusionless shear transformations involving the transition between an "austenite" or "parent" phase that is stable at higher temperatures and a "martensitic" phase that is stable at lower temperatures. Because the transformation absorbs energy, stress-induced martensitic transformations can hinder crack progression and increases toughness. A key example of this phenomenon is
zirconia, whose martensitic transformation involves a crystal structure transformation from a
tetragonal crystal structure (the austenite phase) to a
monoclinic structure. The volume increase associated with transformation from tetragonal to monoclinic also relieves tensile stress at the crack, tip, further discouraging cracking and increasing toughness.
Ice-templating for enhanced mechanical properties If a ceramic is subjected to substantial mechanical loading, it can undergo a process called
ice-templating, which allows some control of the
microstructure of the ceramic product and therefore some control of the mechanical properties. Ceramic engineers use this technique to tune the mechanical properties to their desired application. Specifically, the
strength is increased when this technique is employed. Ice templating allows the creation of macroscopic pores in a unidirectional arrangement. The applications of this oxide strengthening technique are important for
solid oxide fuel cells and
water filtration devices. To process a sample through ice templating, an aqueous
colloidal suspension is prepared to contain the dissolved ceramic powder evenly dispersed throughout the colloid, for example
yttria-stabilized zirconia (YSZ). The solution is then cooled from the bottom to the top on a platform that allows for unidirectional cooling. This forces ice crystals to grow in compliance with the unidirectional cooling, and these ice crystals force the dissolved YSZ particles to the solidification front of the solid-liquid interphase boundary, resulting in pure ice crystals lined up unidirectionally alongside concentrated pockets of colloidal particles. The sample is then heated and at the same the pressure is reduced enough to force the ice crystals to
sublime and the YSZ pockets begin to
anneal together to form macroscopically aligned ceramic microstructures. The sample is then further
sintered to complete the
evaporation of the residual water and the final consolidation of the ceramic microstructure. During ice-templating, a few variables can be controlled to influence the pore size and morphology of the microstructure. These important variables are the initial solids loading of the colloid, the cooling rate, the sintering temperature and duration, and the use of certain additives which can influence the microstructural morphology during the process. A good understanding of these parameters is essential to understanding the relationships between processing, microstructure, and mechanical properties of anisotropically porous materials.
Electrical properties Semiconductors Some ceramics are
semiconductors. Most of these are
transition metal oxides that are II-VI semiconductors, such as
zinc oxide. While there are prospects of mass-producing blue
light-emitting diodes (LED) from zinc oxide, ceramicists are most interested in the electrical properties that show
grain boundary effects. One of the most widely used of these is the varistor. These are devices that exhibit the property that resistance drops sharply at a certain
threshold voltage. Once the voltage across the device reaches the threshold, there is a
breakdown of the electrical structure in the vicinity of the grain boundaries, which results in its
electrical resistance dropping from several megohms down to a few hundred
ohms. The major advantage of these is that they can dissipate a lot of energy, and they self-reset; after the voltage across the device drops below the threshold, its resistance returns to being high. This makes them ideal for
surge-protection applications; as there is control over the threshold voltage and energy tolerance, they find use in all sorts of applications. The best demonstration of their ability can be found in
electrical substations, where they are employed to protect the infrastructure from
lightning strikes. They have rapid response, are low maintenance, and do not appreciably degrade from use, making them virtually ideal devices for this application. Semiconducting ceramics are also employed as
gas sensors. When various gases are passed over a polycrystalline ceramic, its electrical resistance changes. With tuning to the possible gas mixtures, very inexpensive devices can be produced.
Superconductivity demonstrated by levitating a magnet above a
cuprate superconductor, which is cooled by
liquid nitrogen Under some conditions, such as extremely low temperatures, some ceramics exhibit
high-temperature superconductivity (in superconductivity, "high temperature" means above 30 K). The reason for this is not understood, but there are two major families of superconducting ceramics.
Ferroelectricity and supersets Piezoelectricity, a link between electrical and mechanical response, is exhibited by a large number of ceramic materials, including the quartz used to
measure time in watches and other electronics. Such devices use both properties of piezoelectrics, using electricity to produce a mechanical motion (powering the device) and then using this mechanical motion to produce electricity (generating a signal). The unit of time measured is the natural interval required for electricity to be converted into mechanical energy and back again. The piezoelectric effect is generally stronger in materials that also exhibit
pyroelectricity, and all pyroelectric materials are also piezoelectric. These materials can be used to inter-convert between thermal, mechanical, or electrical energy; for instance, after synthesis in a furnace, a pyroelectric crystal allowed to cool under no applied stress generally builds up a static charge of thousands of volts. Such materials are used in
motion sensors, where the tiny rise in temperature from a warm body entering the room is enough to produce a measurable voltage in the crystal. In turn, pyroelectricity is seen most strongly in materials that also display the
ferroelectric effect, in which a stable electric dipole can be oriented or reversed by applying an electrostatic field. Pyroelectricity is also a necessary consequence of ferroelectricity. This can be used to store information in
ferroelectric capacitors, elements of
ferroelectric RAM. The most common such materials are
lead zirconate titanate and
barium titanate. Aside from the uses mentioned above, their strong piezoelectric response is exploited in the design of high-frequency
loudspeakers, transducers for
sonar, and actuators for
atomic force and
scanning tunneling microscopes.
Positive thermal coefficient Temperature increases can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials, mostly mixtures of
heavy metal titanates. The critical transition temperature can be adjusted over a wide range by variations in chemistry. In such materials, current will pass through the material until
joule heating brings it to the transition temperature, at which point the circuit will be broken and current flow will cease. Such ceramics are used as self-controlled heating elements in, for example, the rear-window defrost circuits of automobiles. At the transition temperature, the material's
dielectric response becomes theoretically infinite. While a lack of temperature control would rule out any practical use of the material near its critical temperature, the dielectric effect remains exceptionally strong even at much higher temperatures. Titanates with critical temperatures far below room temperature have become synonymous with "ceramic" in the context of ceramic capacitors for just this reason.
Optical properties with
synthetic sapphire output window
Optically transparent materials focus on the response of a material to incoming light waves of a range of wavelengths.
Frequency selective optical filters can be utilized to alter or enhance the brightness and contrast of a digital image. Guided lightwave transmission via frequency selective
waveguides involves the emerging field of fiber
optics and the ability of certain glassy compositions as a
transmission medium for a range of frequencies simultaneously (
multi-mode optical fiber) with little or no
interference between competing
wavelengths or frequencies. This
resonant mode of
energy and
data transmission via electromagnetic (light)
wave propagation, though low powered, is virtually lossless. Optical waveguides are used as components in
Integrated optical circuits (e.g.
light-emitting diodes, LEDs) or as the transmission medium in local and long haul
optical communication systems. Also of value to the emerging materials scientist is the sensitivity of materials to radiation in the thermal
infrared (IR) portion of the
electromagnetic spectrum. This heat-seeking ability is responsible for such diverse optical phenomena as
night-vision and IR
luminescence. Thus, there is an increasing need in the
military sector for high-strength, robust materials which have the capability to transmit
light (
electromagnetic waves) in the
visible (0.4 – 0.7 micrometers) and mid-
infrared (1 – 5 micrometers) regions of the spectrum. These materials are needed for applications requiring
transparent armor, including next-generation high-speed
missiles and pods, as well as protection against improvised explosive devices (IED). In the 1960s, scientists at
General Electric (GE) discovered that under the right manufacturing conditions, some ceramics, especially
aluminium oxide (alumina), could be made
translucent. These translucent materials were transparent enough to be used for containing the electrical
plasma generated in high-
pressure sodium street lamps. During the past two decades, additional types of transparent ceramics have been developed for applications such as nose cones for
heat-seeking missiles,
windows for fighter
aircraft, and
scintillation counters for computed
tomography scanners. Other ceramic materials, generally requiring greater purity in their make-up than those above, include forms of several chemical compounds, including: •
Barium titanate, often mixed with
strontium titanate), displays
ferroelectricity, meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in
electromechanical transducers, ceramic
capacitors, and
data storage elements.
Grain boundary conditions can create
PTC effects in
heating elements. •
Sialon (silicon aluminium oxynitride) has high strength, resistance to thermal shock, chemical and wear resistance, and low density. These ceramics are used in non-ferrous molten metal handling, weld pins, and the chemical industry. •
Silicon carbide (SiC) is used as a
susceptor in microwave furnaces, a commonly used abrasive, and as a
refractory material. •
Silicon nitride (Si3
N4) is used as an
abrasive powder. •
Steatite (magnesium silicates) is used as an
electrical insulator. •
Titanium carbide is used in space shuttle re-entry shields and scratchproof watches. •
Uranium oxide (
UO2) is used as
fuel in
nuclear reactors. •
Yttrium barium copper oxide (Y
Ba2
Cu3
O7−x) is a
high-temperature superconductor. •
Zinc oxide (
ZnO) is a
semiconductor used in the construction of
varistors. •
Zirconium dioxide (zirconia), which in pure form undergoes many
phase changes between room temperature and practical
sintering temperatures, can be chemically "stabilized" in several different forms. Its high oxygen
ion conductivity recommends it for use in
fuel cells and automotive
oxygen sensors. In another variant,
metastable structures can impart
transformation toughening for mechanical applications; most
ceramic knife blades are made of this material. Partially stabilised zirconia (PSZ) is much less brittle than other ceramics and is used for metal forming tools, valves and liners, abrasive slurries, kitchen knives and bearings subject to severe abrasion. ==Products==