Cermet
Encyclopedia
A cermet is a composite material
composed of ceramic
(cer) and metal
lic (met) materials. A cermet is ideally designed to have the optimal properties of both a ceramic
, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder for an oxide
, boride
, or carbide
. Generally, the metallic elements used are nickel
, molybdenum
, and cobalt
. Depending on the physical structure of the material, cermets can also be metal matrix composite
s, but cermets are usually less than 20% metal by volume.
Cermets are used in the manufacture of resistor
s (especially potentiometer
s), capacitor
s, and other electronic
components which may experience high temperatures.
Cermets are being used instead of tungsten carbide in saws and other brazed tools due to their superior wear and corrosion properties. Titanium nitride (TiN
), Titanium carbonitride (TiCN), titanium carbide (TiC
) and similar can be brazed like tungsten carbide
if properly prepared however they require special handling during grinding.
More complex materials, known as Cermet 2 or Cermet II, are being utilized since they give considerably longer life in cutting tools while both brazing and grinding like tungsten carbide.
Some types of cermets are also being considered for use as spacecraft shielding as they resist the high velocity impacts of micrometeoroid
s and orbital debris much more effectively than more traditional spacecraft materials such as aluminum and other metals.
, the need to develop high temperature and high stress-resistant materials in the US became clear. During the war, German scientists developed oxide base cermets as substitutes for alloys. They saw a use for this for the high-temperature sections of new jet engines as well as high temperature turbine blades. Today ceramics are routinely implemented in the combuster part of jet engines because it provides a heat resistant chamber. Ceramic turbine blades have also been developed. These blades are lighter than steel and allow for greater acceleration of the blade assemblies.
The United States Air Force saw potential in the material technology and became one of the principal sponsors for various research programs in the US. Some of the first universities to research were Ohio State University
, University of Illinois
, and Rutgers University
.
The word cermet was actually coined by the United States Air Force
, the idea being that they are a combination of two materials, a metal
and a ceramic
. Basic physical properties of metals include ductility
, high strength, and high thermal conductivity
. Ceramics possess basic physical properties such as a high melting point
, chemical stability, and especially oxidation resistance
.
The first ceramic metal material developed used magnesium oxide
(MgO), Beryllium oxide
(BeO), and aluminum oxide (Al2O3) for the ceramic part. Emphasis on high stress rupture strengths was around 980C. Ohio State University was the first to develop Al2O3 based cermets with high stress rupture strengths around 1200C. Kennametal
, a metal-working and tool company based in Latrobe, PA, developed the first titanium carbide cermet with a 2800 psi and 100 hour stress-to-rupture strength at 980C. Jet engines operate at this temperature and further research was invested on using these materials for components.
Quality control in manufacturing these ceramic metal composites was hard to standardize. Production had to be kept to small batches and within these batches, the properties varied greatly. Failure of the material was usually a result of undetected flaws usually nucleated during processing.
The existing technology in the 1950s reached a limit for jet engines where little more could be improved. Subsequently, engine manufactures were reluctant to develop ceramic metal engines.
Interest was renewed in the 1960s when silicon nitride and silicon carbide were looked at more closely. Both materials possessed better thermal shock resistance, high strength, and moderate thermal conductivity.
Ceramic-to-metal mechanical seals have also been used. Traditionally they have been used in fuel cells and other devices that convert chemical, nuclear, or thermionic energy to electricity. The ceramic-to-metal seal is required to isolate the electrical sections of turbine-driven generators designed to operate in corrosive liquid-metal vapors.
Bioceramics play an extensive role in biomedical materials. The development of these materials and diversity of manufacturing techniques has broadened the applications that can be used in the human body. They can be in the form of thin layers on metallic implants, composites with a polymer component, or even just porous networks. These materials work well within the human body for several reasons. They are inert, and because they are resorbable and active, the materials can remain in the body unchanged. They can also dissolve and actively take part in physiological processes, for example, when hydroxylapatite
, a material chemically similar to bone structure, can integrate and help bone grow into it. Common materials used for bioceramics include alumina, zirconia, calcium phosphate, glass ceramics, and pyrolytic carbons.
One important use of bioceramics is in hip replacement surgery. A hip joint essentially is a multiaxial ball and socket. The materials used for the replacement hip joints were usually metals such as titanium
with the hip socket usually lined with plastic. The multiaxial ball was tough metal ball but was eventually replaced with a longer lasting ceramic ball. This reduced the roughening associated with the metal wall against the plastic lining of the artificial hip socket. The use of ceramic implants extended the life of the hip replacement parts.
Cermets are also used in dentistry
as a material for fillings and prostheses.
es.
and British Army
have had extensive research in the development of cermets. These include the development of lightweight ceramic projectile proof armor for soldiers and also Chobham armor.
Cermets are also used in machining
on cutting tools.
A cermet of depleted fissiable material (e.g. uranium
, plutonium
) and sodalite
has been researched for its benefits in the storage of nuclear waste. Similar composites have also been researched for use as a fuel source.
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
composed of ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
(cer) and metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
lic (met) materials. A cermet is ideally designed to have the optimal properties of both a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder for an oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
, boride
Boride
In chemistry a boride is a chemical compound between boron and a less electronegative element, for example silicon boride . The borides are a very large group of compounds that are generally high melting and are not ionic in nature. Some borides exhibit very useful physical properties. The term...
, or carbide
Carbide
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
. Generally, the metallic elements used are nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, and cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
. Depending on the physical structure of the material, cermets can also be metal matrix composite
Metal matrix composite
A metal matrix composite is composite material with at least two constituent parts, one being a metal. The other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite...
s, but cermets are usually less than 20% metal by volume.
Cermets are used in the manufacture of resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s (especially potentiometer
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
s), capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s, and other electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
components which may experience high temperatures.
Cermets are being used instead of tungsten carbide in saws and other brazed tools due to their superior wear and corrosion properties. Titanium nitride (TiN
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
), Titanium carbonitride (TiCN), titanium carbide (TiC
Tic
A tic is a sudden, repetitive, nonrhythmic, stereotyped motor movement or vocalization involving discrete muscle groups. Tics can be invisible to the observer, such as abdominal tensing or toe crunching. Common motor and phonic tics are, respectively, eye blinking and throat clearing...
) and similar can be brazed like tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
if properly prepared however they require special handling during grinding.
More complex materials, known as Cermet 2 or Cermet II, are being utilized since they give considerably longer life in cutting tools while both brazing and grinding like tungsten carbide.
Some types of cermets are also being considered for use as spacecraft shielding as they resist the high velocity impacts of micrometeoroid
Micrometeoroid
A micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...
s and orbital debris much more effectively than more traditional spacecraft materials such as aluminum and other metals.
History
After World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, the need to develop high temperature and high stress-resistant materials in the US became clear. During the war, German scientists developed oxide base cermets as substitutes for alloys. They saw a use for this for the high-temperature sections of new jet engines as well as high temperature turbine blades. Today ceramics are routinely implemented in the combuster part of jet engines because it provides a heat resistant chamber. Ceramic turbine blades have also been developed. These blades are lighter than steel and allow for greater acceleration of the blade assemblies.
The United States Air Force saw potential in the material technology and became one of the principal sponsors for various research programs in the US. Some of the first universities to research were Ohio State University
Ohio State University
The Ohio State University, commonly referred to as Ohio State, is a public research university located in Columbus, Ohio. It was originally founded in 1870 as a land-grant university and is currently the third largest university campus in the United States...
, University of Illinois
University of Illinois at Urbana-Champaign
The University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...
, and Rutgers University
Rutgers University
Rutgers, The State University of New Jersey , is the largest institution for higher education in New Jersey, United States. It was originally chartered as Queen's College in 1766. It is the eighth-oldest college in the United States and one of the nine Colonial colleges founded before the American...
.
The word cermet was actually coined by the United States Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
, the idea being that they are a combination of two materials, a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
and a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
. Basic physical properties of metals include ductility
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
, high strength, and high thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
. Ceramics possess basic physical properties such as a high melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
, chemical stability, and especially oxidation resistance
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
.
The first ceramic metal material developed used magnesium oxide
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
(MgO), Beryllium oxide
Beryllium oxide
Beryllium oxide , also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and actually exceeds that of some metals. As an amorphous solid, beryllium...
(BeO), and aluminum oxide (Al2O3) for the ceramic part. Emphasis on high stress rupture strengths was around 980C. Ohio State University was the first to develop Al2O3 based cermets with high stress rupture strengths around 1200C. Kennametal
Kennametal
Kennametal is a leading producer of metals founded in 1938 in the Pittsburgh suburb of Latrobe, Pennsylvania and still headquartered there. It delivers productivity to customers seeking peak performance in demanding environments by providing innovative custom and standard wear-resistant solutions...
, a metal-working and tool company based in Latrobe, PA, developed the first titanium carbide cermet with a 2800 psi and 100 hour stress-to-rupture strength at 980C. Jet engines operate at this temperature and further research was invested on using these materials for components.
Quality control in manufacturing these ceramic metal composites was hard to standardize. Production had to be kept to small batches and within these batches, the properties varied greatly. Failure of the material was usually a result of undetected flaws usually nucleated during processing.
The existing technology in the 1950s reached a limit for jet engines where little more could be improved. Subsequently, engine manufactures were reluctant to develop ceramic metal engines.
Interest was renewed in the 1960s when silicon nitride and silicon carbide were looked at more closely. Both materials possessed better thermal shock resistance, high strength, and moderate thermal conductivity.
Ceramic-to-metal joints and seals
Cermets were first used extensively in ceramic-to-metal joint applications. Construction of vacuum tubes was one of the first critical systems, with the electronics industry employing and developing such seals. German scientists recognized that vacuum tubes with improved performance and reliability could be produced by substituting ceramics for glass. Ceramic tubes can be outgassed at higher temperatures. Because of the high-temperature seal, ceramic tubes withstand higher temperatures than glass tubes. Ceramic tubes are also mechanically stronger and less sensitive to thermal shock than glass tubes. Today, cermet vacuum tube coatings have proved to be key to solar hot water systems.Ceramic-to-metal mechanical seals have also been used. Traditionally they have been used in fuel cells and other devices that convert chemical, nuclear, or thermionic energy to electricity. The ceramic-to-metal seal is required to isolate the electrical sections of turbine-driven generators designed to operate in corrosive liquid-metal vapors.
Bioceramics
Hydroxylapatite
Hydroxylapatite, also called hydroxyapatite , is a naturally occurring mineral form of calcium apatite with the formula Ca53, but is usually written Ca1062 to denote that the crystal unit cell comprises two entities. Hydroxylapatite is the hydroxyl endmember of the complex apatite group...
, a material chemically similar to bone structure, can integrate and help bone grow into it. Common materials used for bioceramics include alumina, zirconia, calcium phosphate, glass ceramics, and pyrolytic carbons.
One important use of bioceramics is in hip replacement surgery. A hip joint essentially is a multiaxial ball and socket. The materials used for the replacement hip joints were usually metals such as titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
with the hip socket usually lined with plastic. The multiaxial ball was tough metal ball but was eventually replaced with a longer lasting ceramic ball. This reduced the roughening associated with the metal wall against the plastic lining of the artificial hip socket. The use of ceramic implants extended the life of the hip replacement parts.
Cermets are also used in dentistry
Dentistry
Dentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders and conditions of the oral cavity, maxillofacial area and the adjacent and associated structures and their impact on the human body. Dentistry is widely considered...
as a material for fillings and prostheses.
Transportation
Ceramic parts have been used in conjunction with metal parts as friction materials for brakes and clutchClutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
es.
Other applications
The United States ArmyUnited States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...
and British Army
British Army
The British Army is the land warfare branch of Her Majesty's Armed Forces in the United Kingdom. It came into being with the unification of the Kingdom of England and Scotland into the Kingdom of Great Britain in 1707. The new British Army incorporated Regiments that had already existed in England...
have had extensive research in the development of cermets. These include the development of lightweight ceramic projectile proof armor for soldiers and also Chobham armor.
Cermets are also used in machining
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
on cutting tools.
A cermet of depleted fissiable material (e.g. uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
, plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
) and sodalite
Sodalite
Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent...
has been researched for its benefits in the storage of nuclear waste. Similar composites have also been researched for use as a fuel source.