![CVIpullout](https://upload.wikimedia.org/wikipedia/commons/e/e9/CVIpullout.jpg)
In
materials science, ceramic matrix composites (CMCs) are a subgroup of
composite material
A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...
s and a subgroup of
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s. They consist of ceramic
fiber
Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
s embedded in a ceramic
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** ''The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
. The fibers and the matrix both can consist of any ceramic material, whereby
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and
carbon fiber
Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
s can also be regarded as a ceramic material.
Introduction
The motivation to develop CMCs was to overcome the problems associated with the conventional technical ceramics like
alumina,
silicon carbide
Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal sin ...
,
aluminum nitride
Aluminium nitride ( Al N) is a solid nitride of aluminium. It has a high thermal conductivity of up to 321 W/(m·K) and is an electrical insulator. Its wurtzite phase (w-AlN) has a band gap of ~6 eV at room temperature and has a potent ...
,
silicon nitride
Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...
or
zirconia
Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant sta ...
– they
fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
easily under mechanical or thermo-mechanical loads because of cracks initiated by small defects or scratches. The crack resistance is very low, as in glass. To increase the crack resistance or
fracture toughness, particles (so-called
monocrystalline ''whiskers'' or ''platelets'') were embedded into the matrix. However, the improvement was limited, and the products have found application only in some ceramic cutting tools. So far only the integration of long multi-strand fibers has drastically increased the crack resistance,
elongation and
thermal shock
Thermal shock is a type of rapidly transient mechanical load.
By definition, it is a mechanical load caused by a rapid change of temperature of a certain point.
It can be also extended to the case of a thermal gradient, which makes different pa ...
resistance, and resulted in several new applications. The reinforcements used in ceramic matrix composites (CMC) serve to enhance the fracture toughness of the combined material system while still taking advantage of the inherent high strength and Young’s modulus of the ceramic matrix. The most common reinforcement embodiment is a continuous-length ceramic fiber, with an elastic modulus that is typically somewhat lower than the matrix. The functional role of this fiber is (1) to increase the CMC stress for the progress of micro-cracks through the matrix, thereby increasing the energy expended during crack propagation; and then (2) when thru-thickness cracks begin to form across the CMC at higher stress (proportional limit stress, PLS), to bridge these cracks without fracturing, thereby providing the CMC with a high ultimate tensile strength (UTS). In this way, ceramic fiber reinforcements not only increase the composite structure’s initial resistance to crack propagation but also allow the CMC to avoid abrupt brittle failure that is characteristic of monolithic ceramics. This behavior is distinct from the behavior of ceramic fibers in
polymer matrix composites (PMC) and
metal matrix composite
In materials science, a metal matrix composite (MMC) is a composite material with fibers or particles dispersed in a metallic matrix, such as copper, aluminum, or steel. The secondary phase is typically a ceramic (such as alumina or silicon carb ...
s (MMC), where the fibers typically fracture before the matrix due to the higher failure strain capabilities of these matrices.
Carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
(C), special
silicon carbide
Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal sin ...
(SiC),
alumina () and
mullite
Mullite or porcelainite is a rare silicate mineral formed during contact metamorphism of clay minerals. It can form two stoichiometric forms: 3 Al2 O32 SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge-balancing cations present. As a result, th ...
() fibers are most commonly used for CMCs. The matrix materials are usually the same, that is C, SiC, alumina and mullite. In certain ceramic systems, including SiC and
silicon nitride
Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...
, processes of
abnormal grain growth
Abnormal or discontinuous grain growth, also referred to as exaggerated or secondary recrystallisation grain growth, is a grain growth phenomenon through which certain energetically favorable grains ( crystallites) grow rapidly in a matrix of fin ...
may result in a microstructure exhibiting elongated large grains in a matrix of finer rounded grains.
AGG
Agg or AGG may refer to:
As an acronym:
* Anti-Grain Geometry, computer graphics rendering library
* Aesthetic group gymnastics, gymnastics In a group
* Abnormal grain growth, materials science phenomenon
* Art Gallery of Guelph
* AGG (programm ...
derived microstructures exhibit toughening due to crack bridging and crack deflection by the elongated grains, which can be considered as an in-situ produced fibre reinforcement. Recently
Ultra-high-temperature ceramics
Ultra-high-temperature ceramics (UHTCs) are a class of refractory ceramics that offer excellent stability at temperatures exceeding 2,000 °C being investigated as possible thermal protection system (TPS) materials, coatings for materials sub ...
(UHTCs) were investigated as ceramic matrix in a new class of CMC so-called
Ultra-high Temperature Ceramic Matrix Composites (UHTCMC) or Ultra-high Temperature Ceramic Composites (UHTCC).
Generally, CMC names include a combination of ''type of fiber/type of matrix''. For example, ''C/C'' stands for carbon-fiber-reinforced carbon (
carbon/carbon), or ''C/SiC'' for carbon-fiber-reinforced silicon carbide. Sometimes the manufacturing process is included, and a C/SiC composite manufactured with the liquid
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
infiltration (LPI) process (see below) is abbreviated as ''LPI-C/SiC''.
The important commercially available CMCs are C/C, C/SiC, SiC/SiC and . They differ from conventional ceramics in the following properties, presented in more detail below:
*
Elongation to rupture up to 1%
*Strongly increased
fracture toughness
*Extreme
thermal shock
Thermal shock is a type of rapidly transient mechanical load.
By definition, it is a mechanical load caused by a rapid change of temperature of a certain point.
It can be also extended to the case of a thermal gradient, which makes different pa ...
resistance
*Improved dynamical load capability
*
Anisotropic
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
properties following the orientation of fibers
Manufacture
The manufacturing processes usually consist of the following three steps:
#Lay-up and fixation of the fibers, shaped like the desired component
#Infiltration of the matrix material
#Final
machining
Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...
and, if required, further treatments like
coating
A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Pow ...
or impregnation of the intrinsic
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
.
The first and the last step are almost the same for all CMCs:
In step one, the fibers, often named rovings, are arranged and fixed using techniques used in fiber-reinforced plastic materials, such as lay-up of
fabric
Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...
s, filament winding,
braiding
A braid (also referred to as a plait) is a complex structure or pattern formed by interlacing two or more strands of flexible material such as textile yarns, wire, or hair.
The simplest and most common version is a flat, solid, three-strande ...
and
knot
A knot is an intentional complication in cordage which may be practical or decorative, or both. Practical knots are classified by function, including hitches, bends, loop knots, and splices: a ''hitch'' fastens a rope to another object; a ' ...
ting. The result of this procedure is called ''fiber-preform'' or simply ''preform''.
For the second step, five different procedures are used to fill the ceramic matrix in between the fibers of the preform:
#Deposition out of a gas mixture
#
Pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
of a pre-ceramic polymer
#
Chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
of elements
#
Sintering
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
at a relatively low temperature in the range
#
Electrophoretic
Electrophoresis, from Ancient Greek ἤλεκτρον (ḗlektron, "amber") and φόρησις (phórēsis, "the act of bearing"), is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric fi ...
deposition of a ceramic powder
Procedures one, two and three find applications with non-oxide CMCs, whereas the fourth one is used for oxide CMCs; combinations of these procedures are also practiced. The fifth procedure is not yet established in industrial processes. All procedures have sub-variations, which differ in technical details. All procedures yield a porous material.
The third and final step of
machining
Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...
–
grinding
Grind is the cross-sectional shape of a blade.
Grind, grinds, or grinding may also refer to:
Grinding action
* Grinding (abrasive cutting), a method of crafting
* Grinding (dance), suggestive club dancing
* Grinding (video gaming), repetitive and ...
,
drilling,
lapping
Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine.
Lapping often follows other subtractive processes with more aggressive material removal as a first step ...
or
milling – has to be done with diamond tools. CMCs can also be processed with a
water jet Water jet may refer to:
* A jet of water under pressure, like in an ornamental fountain
* Pump-jet, a marine propulsion mechanism for jetskis and other types of boats
* Water jet cutter, a tool for cutting and the machining of engineering material ...
,
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
, or
ultrasonic machining.
Ceramic fibers
![CMC3DStruktur](https://upload.wikimedia.org/wikipedia/commons/e/e8/CMC3DStruktur.JPG)
Ceramic fibers in CMCs can have a
polycrystalline
A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains.
Bacillite is a type of crystallite. It is rodlike with parallel longulites.
Stru ...
structure, as in conventional ceramics. They can also be
amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wi ...
or have
inhomogeneous
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
chemical composition
A chemical composition specifies the identity, arrangement, and ratio of the elements making up a compound.
Chemical formulas can be used to describe the relative amounts of elements present in a compound. For example, the chemical formula for ...
, which develops upon pyrolysis of organic
precursor
Precursor or Precursors may refer to:
* Precursor (religion), a forerunner, predecessor
** The Precursor, John the Baptist
Science and technology
* Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...
s. The high process temperatures required for making CMCs preclude the use of organic,
metallic
Metallic may be a reference to:
*Metal
* Metalloid, metal-like substance
*Metallic bonding, type of chemical bonding
* Metallicity, in astronomy the proportion of elements other than helium and hydrogen in an object
*Metallic color, a color that ...
or
glass fiber
Glass fiber ( or glass fibre) is a material consisting of numerous extremely fine fibers of glass.
Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the inventio ...
s. Only fibers stable at temperatures above can be used, such as fibers of alumina, mullite, SiC, zirconia or carbon. Amorphous SiC fibers have an elongation capability above 2% – much larger than in conventional ceramic materials (0.05 to 0.10%). The reason for this property of SiC fibers is that most of them contain additional elements like
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
,
titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
and/or
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
yielding a
tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
above 3 GPa. These enhanced elastic properties are required for various three-dimensional fiber arrangements (see example in figure) in
textile
Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...
fabrication, where a small bending radius is essential.
Manufacturing procedures
Matrix deposition from a gas phase
Chemical vapor deposition (CVD) is well suited for this purpose. In the presence of a fiber preform, CVD takes place in between the fibers and their individual filaments and therefore is called
chemical vapor infiltration Chemical vapour infiltration (CVI) is a ceramic engineering process whereby matrix material is infiltrated into fibrous preforms by the use of reactive gases at elevated temperature to form fiber-reinforced composites. The earliest use of CVI was th ...
(CVI). One example is the manufacture of C/C composites: a C-fiber preform is exposed to a mixture of
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
and a hydrocarbon gas (
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
,
propane
Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
, etc.) at a pressure of around or below 100 kPa and a temperature above 1000 °C. The gas decomposes depositing carbon on and between the fibers. Another example is the deposition of silicon carbide, which is usually conducted from a mixture of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
and methyl-
trichlorosilane
Trichlorosilane is an inorganic compound with the formula HCl3Si. It is a colourless, volatile liquid. Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to pr ...
(MTS, ; it is also common in
silicone production). Under defined condition this gas mixture deposits fine and crystalline silicon carbide on the hot surface within the preform.
This CVI procedure leaves a body with a porosity of about 10–15%, as access of reactants to the interior of the preform is increasingly blocked by deposition on the exterior.
Matrix forming via pyrolysis of C- and Si-containing polymers
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
polymers shrink during
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
, and upon
outgassing
Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which a ...
form carbon with an amorphous, glass-like structure, which by additional heat treatment can be changed to a more
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
-like structure. Other special polymers, known as
preceramic polymer The term preceramic polymer refers to one of various polymeric compounds, which through pyrolysis under appropriate conditions (generally in the absence of oxygen) are converted to ceramic compounds, having high thermal and chemical stability. Cer ...
s where some carbon atoms are replaced by silicon atoms, the so-called polycarbo
silane
Silane is an inorganic compound with chemical formula, . It is a colourless, pyrophoric, toxic gas with a sharp, repulsive smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental silicon. Sila ...
s, yield amorphous silicon carbide of more or less
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
composition. A large variety of such
silicon carbide
Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal sin ...
,
silicon oxycarbide Oxycarbide glass, also referred to as silicon oxycarbide, is a type of glass that contains oxygen and carbon in addition to silicon dioxide. It is created by substituting some oxygen atoms with carbon atoms. This glass may contain particles of amor ...
, silicon carbonitride and silicon oxynitride
precursors already exist and more
preceramic polymer The term preceramic polymer refers to one of various polymeric compounds, which through pyrolysis under appropriate conditions (generally in the absence of oxygen) are converted to ceramic compounds, having high thermal and chemical stability. Cer ...
s for the fabrication of
polymer derived ceramics Polymer derived ceramics (PDCs) are ceramic materials formed by the pyrolysis of preceramic polymers, usually under inert atmosphere.
The compositions of PDCs most commonly include silicon carbide (SiC), silicon oxycarbide (SiOxCy), silicon nitrid ...
are being developed.
[Wang X. et al]
Additive manufacturing of ceramics from preceramic polymers:
A versatile stereolithographic approach assisted by thiol-ene click chemistry, ''Additive Manufacturing 2019, volume 27 pages 80-90'' To manufacture a CMC material, the fiber preform is infiltrated with the chosen polymer. Subsequent
curing and pyrolysis yield a highly porous matrix, which is undesirable for most applications. Further cycles of polymer infiltration and pyrolysis are performed until the final and desired quality is achieved. Usually, five to eight cycles are necessary.
The process is called ''liquid polymer infiltration'' (LPI), or ''polymer infiltration and pyrolysis'' (PIP). Here also a porosity of about 15% is common due to the shrinking of the polymer. The porosity is reduced after every cycle.
Matrix forming via chemical reaction
With this method, one material located between the fibers reacts with a second material to form the ceramic matrix. Some conventional ceramics are also manufactured by
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
s. For example, reaction-bonded
silicon nitride
Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...
(RBSN) is produced through the reaction of silicon powder with nitrogen, and porous carbon reacts with silicon to form
reaction bonded silicon carbide, a silicon carbide which contains inclusions of a silicon phase. An example of CMC manufacture, which was introduced for the production of ceramic
brake discs, is the reaction of
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
with a porous preform of C/C. The process temperature is above , that is above the
melting point
The melting point (or, rarely, liquefaction point) of a substance 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 ...
of silicon, and the process conditions are controlled such that the carbon fibers of the C/C-preform almost completely retain their mechanical properties. This process is called ''liquid silicon infiltration'' (LSI). Sometimes, and because of its starting point with C/C, the material is abbreviated as ''C/C-SiC''. The material produced in this process has a very low porosity of about 3%.
Matrix forming via sintering
This process is used to manufacture oxide fiber/oxide matrix CMC materials. Since most ceramic fibers cannot withstand the normal
sintering
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
temperatures of above , special
precursor
Precursor or Precursors may refer to:
* Precursor (religion), a forerunner, predecessor
** The Precursor, John the Baptist
Science and technology
* Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...
liquids are used to infiltrate the preform of oxide fibers. These precursors allow sintering, that is ceramic-forming processes, at temperatures of 1000–1200 °C. They are, for example, based on mixtures of alumina powder with the liquids tetra-ethyl-ortho
silicate
In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
(as Si donor) and aluminium-
butyl
In organic chemistry, butyl is a four- carbon alkyl radical or substituent group with general chemical formula , derived from either of the two isomers (''n''-butane and isobutane) of butane.
The isomer ''n''-butane can connect in two ways, gi ...
ate (as Al donor), which yield a mullite matrix. Other techniques, such as
sol–gel process chemistry, are also used. CMCs obtained with this process usually have a high porosity of about 20%.
Matrix formed via electrophoresis
In the
electrophoretic
Electrophoresis, from Ancient Greek ἤλεκτρον (ḗlektron, "amber") and φόρησις (phórēsis, "the act of bearing"), is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric fi ...
process, electrically charged particles dispersed in a special liquid are transported through an
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
into the preform, which has the opposite electrical charge polarity. This process is under development, and is not yet used industrially. Some remaining porosity must be expected here, too.
Properties
Mechanical properties
Basic mechanism of mechanical properties
The high fracture toughness or crack resistance mentioned above is a result of the following mechanism: under load the ceramic matrix cracks, like any ceramic material, at an elongation of about 0.05%. In CMCs the embedded fibers bridge these cracks (see picture). This mechanism works only when the matrix can slide along the fibers, which means that there must be a weak bond between the fibers and matrix. A strong bond would require a very high elongation capability of the fiber bridging the crack and would result in a brittle fracture, as with conventional ceramics. The production of CMC material with high crack resistance requires a step to weaken this bond between the fibers and matrix. This is achieved by depositing a thin layer of pyrolytic carbon or boron nitride on the fibers, which weakens the bond at the fiber/matrix interface, leading to the
fiber pull-out at crack surfaces, as shown in the
SEM picture at the top of this article. In oxide-CMCs, the high porosity of the matrix is sufficient to establish a weak bond.
Properties under tensile and bending loads, crack resistance
![CMC SENB](https://upload.wikimedia.org/wikipedia/commons/0/08/CMC_SENB.jpg)
The influence and quality of the fiber interface can be evaluated through mechanical properties. Measurements of the crack resistance were performed with notched specimens (see figure) in so-called single-edge-notch-bend (SENB) tests. In
fracture mechanics, the measured data (force, geometry and crack surface) are normalized to yield the so-called
stress intensity factor
In fracture mechanics, the stress intensity factor () is used to predict the stress state ("stress intensity") near the tip of a crack or notch caused by a remote load or residual stresses. It is a theoretical construct usually applied to a h ...
(SIF), K
Ic. Because of the complex crack surface (see figure at the top of this article) the real crack surface area can not be determined for CMC materials. The measurements, therefore, use the initial notch as the crack surface, yielding the ''formal SIF'' shown in the figure. This requires identical geometry for comparing different samples. The area under these curves thus gives a relative indication of the energy required to drive the crack tip through the sample (force times path length gives energy). The maxima indicate the load level necessary to propagate the crack through the sample. Compared to the sample of conventional SiSiC ceramic, two observations can be made:
*All tested CMC materials need up to several orders of magnitude more energy to propagate the crack through the material.
*The force required for crack propagation varies between different types of CMCs.
In the table, CVI, LPI, and LSI denote the manufacturing process of the C/SiC-material. Data on the oxide CMC and SiSiC are taken from manufacturer datasheets. The tensile strength of SiSiC and were calculated from measurements of elongation to fracture and
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
, since generally only bending strength data are available for those ceramics. Averaged values are given in the table, and significant differences, even within one manufacturing route, are possible.
![CMCDehnungsverlauf](https://upload.wikimedia.org/wikipedia/commons/5/5d/CMCDehnungsverlauf.jpg)
Tensile tests of CMCs usually show nonlinear stress-strain curves, which look as if the material deforms plastically. It is called ''quasi-
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
'', because the effect is caused by the microcracks, which are formed and bridged with increasing load. Since the
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
of the load-carrying fibers is generally lower than that of the matrix, the slope of the curve decreases with increasing load.
Curves from bending tests look similar to those of the crack resistance measurements shown above.
The following features are essential in evaluating bending and tensile data of CMCs:
*CMC materials with a low matrix content (down to zero) have a high
tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
(close to the tensile strength of the fiber), but low
bending strength.
*CMC materials with a low fiber content (down to zero) have a high bending strength (close to the strength of the monolithic ceramic), but no elongation beyond 0.05% under tensile load.
The primary quality criterion for CMCs is crack resistance behavior or fracture toughness.
= High Temperature Creep Properties
=
Although CMCs are able to operate at very high temperatures,
creep deformation still occur around 1000 °C, in the range of certain high-temperature applications. Creep acts on either the matrix or fiber depending on the creep mismatch ratio (CMR) between the effective fiber strain rate and effective matrix strain rate. The component with the smaller strain rate bears the load and is susceptible to creep.
The three main creep stages are governed by the creep mismatch ratio. During primary creep,
internal stresses are transferred allowing the CMR to approach unity, as well as the secondary creep stage. The tertiary creep stage, where failure occurs, can be governed by fiber creep, where failure occurs due to fiber fracture, or matrix creep, which lead to matrix cracking. Usually, matrix creep strength is worse than the fiber, so the fiber bears the load. However, matrix cracking can still occur with weak fiber regions, resulting in
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
in oxidizing atmospheres, weakening the material. Increasing temperature, applied stress, and defect densities lead to greater creep deformation and earlier failure.
A
rule of mixtures
In materials science, a general rule of mixtures is a weighted mean used to predict various properties of a composite material . It provides a theoretical upper- and lower-bound on properties such as the elastic modulus, mass density, ultimate te ...
may be applied to find the strain rate of the composite given the strain rates of the constituents. For particulates, a simple sum of the product of the cross-sectional area fraction and creep response of each constituent can determine the composite’s total creep response. For fibers, a sum of the constituents’ creep response divided by the cross-sectional area fraction determines the total creep response.
Particulates:
Fibers:
where
is the creep response and
is the constituent cross sectional area fraction.
Other mechanical properties
In many CMC components the fibers are arranged as 2-dimensional (2D) stacked
plain
In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...
or
satin weave
A satin weave is a type of fabric weave that produces a characteristically glossy, smooth or lustrous material, typically with a glossy top surface and a dull back. It is one of three fundamental types of textile weaves alongside plain weave ...
fabric
Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...
s. Thus the resulting material is
anisotropic
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
or, more specifically,
orthotropic. A crack between the layers is not bridged by fibers. Therefore, the interlaminar
shear strength
In engineering, shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a materi ...
(ILS) and the strength perpendicular to the 2D fiber orientation are low for these materials.
Delamination
Delamination is a mode of failure where a material fractures into layers. A variety of materials including laminate composites and concrete can fail by delamination. Processing can create layers in materials such as steel formed by rolling a ...
can occur easily under certain mechanical loads. Three-dimensional fiber structures can improve this situation (see micrograph above).
The
compressive strength
In mechanics, compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size (as opposed to tensile strength which withstands loads tending to elongate). In other words, compre ...
s shown in the table are lower than those of conventional ceramics, where values above 2000 MPa are common; this is a result of porosity.
![CMCLowCycleFatigueSiCSiC](https://upload.wikimedia.org/wikipedia/commons/f/f2/CMCLowCycleFatigueSiCSiC.jpg)
The composite structure allows high dynamical loads. In the so-called low-
cycle-fatigue (LCF) or high-cycle-fatigue (HCF) tests the material experiences cyclic loads under tensile and compressive (LCF) or only tensile (HCF) load. The higher the initial stress the shorter the lifetime and the smaller the number of cycles to rupture. With an initial load of 80% of the strength, a SiC/SiC sample survived about 8 million cycles (see figure).
The
Poisson's ratio
In materials science and solid mechanics, Poisson's ratio \nu ( nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Po ...
shows an anomaly when measured perpendicular to the plane of the fabric because interlaminar cracks increase the sample thickness.
Thermal and electrical properties
The thermal and electrical properties of the composite are a result of its constituents, namely fibers, matrix, and pores as well as their composition. The orientation of the fibers yields anisotropic data. Oxide CMCs are very good
electrical insulator
An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...
s, and because of their high porosity, their
thermal insulation
Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with s ...
is much better than that of conventional oxide ceramics.
The use of carbon fibers increases the
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
, provided the fibers contact each other and the voltage source. The silicon carbide matrix is a good thermal conductor. Electrically, it is a
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
, and its
resistance therefore decreases with increasing temperature. Compared to (poly)crystalline SiC, the amorphous SiC fibers are relatively poor conductors of heat and electricity.
Comments for the table: (p) and (v) refer to data parallel and vertical to fiber orientation of the 2D-fiber structure, respectively. LSI material has the highest
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
because of its low porosity – an advantage when using it for brake discs. These data are subject to scatter depending on details of the manufacturing processes.
Conventional ceramics are very sensitive to
thermal stress
In mechanics and thermodynamics, thermal stress is mechanical stress created by any change in temperature of a material. These stresses can lead to fracturing or plastic deformation depending on the other variables of heating, which include mater ...
because of their high Young's modulus and low elongation capability. Temperature differences and low
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
create locally different elongations, which together with the high Young's modulus generate high stress. This results in cracks, rupture, and brittle failure. In CMCs, the fibers bridge the cracks, and the components show no macroscopic damage, even if the matrix has cracked locally. The application of CMCs in brake disks demonstrates the effectiveness of ceramic composite materials under extreme thermal shock conditions.
Corrosion properties
Data on the
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
behaviour of CMCs are scarce except for
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
at temperatures above 1000 °C. These properties are determined by the constituents, namely the fibers and matrix. Ceramic materials, in general, are very stable to corrosion. The broad spectrum of manufacturing techniques with different sintering additives, mixtures, glass phases, and porosities are crucial for the results of corrosion tests. Less
impurities and exact
stoichiometry lead to less corrosion. Amorphous structures and non-ceramic chemicals frequently used as sintering aids are starting points of corrosive attack.
;Alumina
Pure alumina shows excellent corrosion resistance against most chemicals. Amorphous glass and silica
phases at the grain boundaries determine the speed of corrosion in concentrated
acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
s and
bases and result in
creep at high temperatures. These characteristics limit the use of alumina. For molten metals, alumina is used only with gold and platinum.
;Alumina fibers
These fibers demonstrate corrosion properties similar to alumina, but commercially available fibers are not very pure and therefore less resistant. Because of creep at temperatures above 1000 °C, there are only a few applications for oxide CMCs.
;Carbon
The most significant corrosion of carbon occurs in the presence of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
above about . It burns to form
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
and/or
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
. It also oxidizes in strong oxidizing agents like concentrated
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
. In molten metals, it dissolves and forms metal
carbide
In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece.
Interstitial / Metallic carbides
The carbides of the ...
s. Carbon fibers do not differ from carbon in their corrosion behavior.
;Silicon carbide
Pure silicon carbide is one of the most corrosion-resistant materials. Only strong bases, oxygen above about , and molten metals react with it to form carbides and
silicide
A silicide is a type of chemical compound that combines silicon and a (usually) more electropositive element.
Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides.
Similar to borides and carb ...
s. The reaction with oxygen forms and , whereby a surface layer of slows down subsequent oxidation (''
passive oxidation''). Temperatures above about and a low
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
of oxygen result in so-called ''active oxidation'', in which CO, and gaseous SiO are formed causing rapid loss of SiC. If the SiC matrix is produced other than by CVI, corrosion-resistance is not as good. This is a consequence of porosity in the amorphous LPI, and residual silicon in the LSI-matrix.
;Silicon carbide fibers
Silicon carbide fibers are produced via pyrolysis of organic polymers, and therefore their corrosion properties are similar to those of the silicon carbide found in LPI-matrices. These fibers are thus more sensitive to bases and oxidizing media than pure silicon carbide.
Applications
CMC materials overcome the major disadvantages of conventional technical ceramics, namely brittle failure and low fracture toughness, and limited thermal shock resistance. Therefore, their applications are in fields requiring reliability at high-temperatures (beyond the capability of metals) and resistance to corrosion and wear. These include:
*
Heat shield systems for
space vehicle
A space vehicle is the combination of a spacecraft and its launch vehicle which carries it into space. The earliest space vehicles were expendable launch systems, using a single or multistage rocket to carry a relatively small spacecraft in p ...
s, which are needed during the
re-entry
Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the entr ...
phase, where high temperatures,
thermal shock
Thermal shock is a type of rapidly transient mechanical load.
By definition, it is a mechanical load caused by a rapid change of temperature of a certain point.
It can be also extended to the case of a thermal gradient, which makes different pa ...
conditions and heavy vibration loads take place.
*Components for high-temperature
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
s such as
combustion chamber
A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process.
Intern ...
s,
stator vanes,
exhaust mixer
In aviation, exhaust mixer is a feature of many turbofan engines, where the bypass (cold/slow) air is mixed with the core (hot/fast) exhaust gases, before exhausting to atmospheric pressure through a common (mixed flow) propelling nozzle.
Benef ...
s and
s.
*Components for
burners
Burning Man is an event focused on community, art, self-expression, and self-reliance held annually in the western United States. The name of the event comes from its culminating ceremony: the symbolic burning of a large wooden effigy, referre ...
,
flame holder
A flame holder is a component of a jet engine designed to help maintain continual combustion. In a scramjet engine the residence time of the fuel is very low and complete penetration of the fuel into the flow will not occur. To avoid these conditio ...
s, and hot gas ducts, where the use of oxide CMCs has found its way.
*
Brake disks and brake system components, which experience extreme thermal shock (greater than throwing a glowing part of any material into water).
*Components for
slide bearings under heavy loads requiring high corrosion and wear resistance.
In addition to the foregoing, CMCs can be used in applications which employ conventional ceramics or in which metal components have limited
lifetimes due to corrosion or high temperatures.
Developments for applications in space
During the re-entry phase of space vehicles, the heat shield system is exposed to temperatures above for a few minutes. Only ceramic materials can survive such conditions without significant damage, and among ceramics, only CMCs can adequately handle thermal shocks. The development of CMC-based heat shield systems promises the following advantages:
*Reduced weight
*Higher load-carrying capacity of the system
*Reusability for several re-entries
*Better steering during the re-entry phase with CMC flap systems
![CMC-X38-Bodyflaps](https://upload.wikimedia.org/wikipedia/commons/6/64/CMC-X38-Bodyflaps.jpg)
In these applications, the high temperatures preclude the use of oxide fiber CMCs, because under the expected loads the creep would be too high. Amorphous silicon carbide fibers lose their strength due to re-
crystallization
Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...
at temperatures above . Therefore, carbon fibers in a silicon carbide matrix (C/SiC) are used in development programs for these applications. The European program HERMES of
ESA
, owners =
, headquarters = Paris, Île-de-France, France
, coordinates =
, spaceport = Guiana Space Centre
, seal = File:ESA emblem seal.png
, seal_size = 130px
, image = Views in the Main Control Room (120 ...
, started in the 1980s and for financial reasons abandoned in 1992, has produced first results. Several follow-up programs focused on the development, manufacture, and qualification of nose cap, leading edges and steering flaps for the
NASA X-38
The X-38 was an experimental re-entry vehicle designed by NASA to research a possible emergency crew return vehicle (CRV) for the International Space Station (ISS). The 1995–2002 program also developed concepts for a crew return vehicle design ...
space vehicle.
This development program has qualified the use of C/SiC bolts and nuts and the bearing system of the flaps. The latter were ground-tested at the DLR in Stuttgart, Germany, under expected conditions of the re-entry phase: , 4
tonne
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s load, oxygen partial pressure similar to re-entry conditions, and simultaneous bearing movements of four cycles per second. A total of five re-entry phases were simulated. Design and manufacture of the two steering flaps and its bearings, screws and nuts was performed by
MT Aerospace in Augsburg, Germany based on the
CVI-process for the production of carbon fiber reinforced silicon carbide (see manufacturing procedures above).
Furthermore, oxidation protection systems were developed and qualified to prevent burnout of the carbon fibers. After mounting of the flaps, mechanical ground tests were performed successfully by NASA in Houston, Texas, US. The next test – a real re-entry of the unmanned vehicle X-38 – was canceled for financial reasons. One of the
Space Shuttle
The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...
s would have brought the vehicle into orbit, from where it would have returned to the Earth.
These qualifications were promising for only this application. The high-temperature load lasts only around 20 minutes per re-entry, and for reusability, only about 30 cycles would be sufficient. For industrial applications in a hot gas environment, though, several hundred cycles of thermal loads and up to many thousands of hours of lifetime are required.
The
Intermediate eXperimental Vehicle
The Intermediate eXperimental Vehicle (IXV) is a European Space Agency (ESA) experimental suborbital re-entry vehicle. It was developed to serve as a prototype lifting body orbital return vehicle to validate the ESA's work in the field of reus ...
(IXV), a project initiated by
ESA
, owners =
, headquarters = Paris, Île-de-France, France
, coordinates =
, spaceport = Guiana Space Centre
, seal = File:ESA emblem seal.png
, seal_size = 130px
, image = Views in the Main Control Room (120 ...
in 2009, is Europe's first lifting body reentry vehicle. Developed by
Thales Alenia Space
Thales Alenia Space () is a Franco-Italian aerospace manufacturer. A joint venture between the French technology corporation Thales Group (67%) and Italian defense conglomerate Leonardo (33%), the company is the largest satellite manufactur ...
, the IXV is scheduled to make its first flight in 2014 on the fourth
Vega
Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, a ...
mission (VV04) over the Gulf of Guinea. More than 40 European companies contributed to its construction. The thermal protection system for the underside of the vehicle, comprising the nose, leading edges and lower surface of the wing, were designed and made by
Herakles
Heracles ( ; grc-gre, Ἡρακλῆς, , glory/fame of Hera), born Alcaeus (, ''Alkaios'') or Alcides (, ''Alkeidēs''), was a divine hero in Greek mythology, the son of Zeus and Alcmene, and the foster son of Amphitryon.By his adoptive ...
using a ceramic matrix composite (CMC), carbon/silicon-carbide (C/SiC), in this case based on the liquid silicon infilration (LSI) process (see manufacturing procedures above). These components should have been functioned as the vehicle's heat shield during its atmospheric reentry.
The
European Commission
The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body o ...
funded a research project, C3HARME, under the NMP-19-2015 call of
(H2020) in 2016 for the design, development, production, and testing of a new class of
Ultra-high-temperature ceramic matrix composites (UHTCMC) reinforced with silicon carbide fibers and
carbon fibers suitable for applications in severe aerospace environments such as propulsion and
Thermal protection systems (TPSs).
Developments for gas turbine components
The use of CMCs in gas turbines permit higher turbine inlet temperatures, which improves engine efficiency. Because of the complex shape of stator vanes and turbine blades, the development was first focused on the combustion chamber. In the US, a combustor made of SiC/SiC with a special SiC fiber of enhanced high-temperature stability was successfully tested for 15,000 hours. SiC oxidation was substantially reduced by the use of an oxidation protection coating consisting of several layers of oxides.
The engine collaboration between
General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
and
Rolls-Royce
Rolls-Royce (always hyphenated) may refer to:
* Rolls-Royce Limited, a British manufacturer of cars and later aero engines, founded in 1906, now defunct
Automobiles
* Rolls-Royce Motor Cars, the current car manufacturing company incorporated in ...
studied the use of CMC stator vanes in the hot section of the F136
turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
engine, an engine which failed to beat the Pratt and Whitney F-135 for use in the
Joint Strike Fighter
Joint Strike Fighter (JSF) is a development and acquisition program intended to replace a wide range of existing fighter, strike, and ground attack aircraft for the United States, the United Kingdom, Italy, Canada, Australia, the Netherlands ...
. The engine joint venture,
CFM International
CFM International is a 50/50 Franco-American joint venture between GE Aviation and Safran Aircraft Engines (formerly known as Snecma). It was formed to build and support the CFM56 series of turbofan engines. The company is the world’s leading s ...
is using CMCs to manufacture the high-temperature turbine shrouds. General Electric is using CMCs in combustor liners, nozzles, and the high-temperature turbine shroud for its upcoming GE9X engine.
CMC parts are also being studied for stationary applications in both the cold and hot sections of the engines since stresses imposed on rotating parts would require further development effort. Generally, development continues of CMCs for use in turbines to reduce technical issues and cost reduction.
After in investment and 20 years of
research and development
Research and development (R&D or R+D), known in Europe as research and technological development (RTD), is the set of innovative activities undertaken by corporations or governments in developing new services or products, and improving existi ...
, by 2020
GE Aviation
GE Aviation, a subsidiary of General Electric, is headquartered in Evendale, Ohio, outside Cincinnati. GE Aviation is among the top aircraft engine suppliers, and offers engines for the majority of commercial aircraft. GE Aviation is part of the ...
aims to produce per year up to of CMC
prepreg
Pre-preg is a composite material made from "pre-impregnated" fibers and a ''partially cured'' polymer matrix, such as epoxy or phenolic resin, or even thermoplastic mixed with liquid rubbers or resins. The fibers often take the form of a weave ...
and 10 t of
silicon carbide
Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal sin ...
fiber.
Chemical vapor deposition can apply
coating
A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Pow ...
s on a laid-able fiber tape in large quantities and GE managed to Vapor Infiltration, infiltrate and casting, cast parts with very high silicon densities, higher than 90% for High-cycle fatigue, cyclic fatigue environments, thanks to thermal processing.
Environmental Barrier Coatings (EBCs) to Protect Gas Turbine Components
Environmental Barrier Coatings (EBCs) provide a barrier to the CMCs to reduce the amount of oxygen and other corrosive substances from diffusing through the surface of CMC components.
''Design Requirements for EBCs:''
* Relative coefficient match with CMC component to reduce probability of cracking
* Low volatility to minimize stead-induced corrosion/recession
* Resistant to molten ingested particulate
* High temperature capability
* Phase stability at high temperatures
* Chemical Compatibility with the CMC and additional layers
* High Hardness and toughness to protect against Foreign Object Damage (FOD) and erosion
Typically when coating with an EBC a bond coat is required to support good adhesion to the CMC component. NASA has developed a slurry based EBC which starts with a mullite-based coating before being layered with an additional 2-3 layers. In order for EBCs to actively protect the CMC surface, sintering aids must be added to the slurry coat to create a dense coating that will block the penetration of oxygen, gaseous, and molten deposits from the engine. Sintering creates a densified coating and enhances bonding and performance of the coating.
Currently, research is being done to combat common failure modes such as delamination, erosion, and cracking caused by steam or molten deposits. Delamination and cracking due to molten deposits are typically caused by the reaction with the EBC creating an unexpected microstructure leading to CTE mismatch and low toughness in that phase. Steam degradation is caused by the volatilization of the thermally grown oxide layer between the EBC and the ceramic. The steam produced from this leads to a rapid recession of SiC, i.e. degradation of the EBC. The success of EBCs are imperative to the overall success of CMC components in the gas flow of the turbine in jet engines.
''Overall benefits of EBCs:''
* Extends the life of CMC components allowing for overall cost savings in jet engine production
* Improves Oxidation Resistance of CMC components
* Provides greater oxidation resistance to CMC components exposed to gaseous compounds from the jet engine
Application of oxide CMC in burner and hot gas ducts
Oxygen-containing gas at temperatures above is rather corrosive for metal and silicon carbide components. Such components, which are not exposed to high mechanical stress, can be made of oxide CMCs, which can withstand temperatures up to . The gallery below shows the
flame holder
A flame holder is a component of a jet engine designed to help maintain continual combustion. In a scramjet engine the residence time of the fuel is very low and complete penetration of the fuel into the flow will not occur. To avoid these conditio ...
of a crispbread bakery as tested after for 15,000 hours, which subsequently operated for a total of more than 20,000 hours.
Flaps and ventilators circulating hot, oxygen-containing gases can be fabricated in the same shape as their metal equivalents. The lifetime for these oxide CMC components is several times longer than for metals, which often deform. A further example is an oxide CMC lifting gate for a sintering furnace, which has survived more than 260,000 opening cycles.
Application in brake disk
Carbon/carbon (C/C) materials have found their way into the disc brakes of sports car, racing cars and airplanes, and C/SiC brake disks manufactured by the LSI process were qualified and are commercially available for sports cars. The advantages of these C/SiC disks are:
*Very little wear, resulting in lifetime use for a car with a normal driving load of , is forecast by manufacturers.
*No brake fade, fading is experienced, even under high load.
*No surface humidity effect on the friction coefficient shows up, as in C/C brake disks.
*The corrosion resistance, for example to the road salt, is much better than for metal disks.
*The disk mass is only 40% of a metal disk. This translates into less unsprung and rotating mass.
The weight reduction improves shock absorber response, road-holding comfort, agility, fuel economy, and thus driving comfort.
The SiC-matrix of LSI has a very low porosity, which protects the carbon fibers quite well. Brake disks do not experience temperatures above for more than a few hours in their lifetime. Oxidation is therefore not a problem in this application. The reduction of manufacturing costs will decide the success of this application for middle-class cars.
Application in slide bearings
![CMCGleitlager](https://upload.wikimedia.org/wikipedia/commons/e/e1/CMCGleitlager.jpg)
Conventional SiC, or sometimes the less expensive SiSiC, have been used successfully for more than 25 years in plain bearing, slide or journal bearings of pumps. The pumped liquid itself provides the lubricant for the bearing. Very good corrosion resistance against practically all kinds of media, and very low wear and low friction coefficients are the basis of this success. These bearings consist of a static bearing, shrink-fitted in its metallic environment, and a rotating shaft sleeve, mounted on the shaft. Under compressive stress, the ceramic static bearing has a low risk of failure, but a SiC shaft sleeve does not have this situation and must, therefore, have a large wall thickness and/or be specially designed. In large pumps with shafts in diameter, the risk of failure is higher due to the changing requirements on the pump performance – for example, load changes during operation. The introduction of SiC/SiC as a shaft sleeve material has proven to be very successful. Test rig experiments showed an almost triple specific load capability of the bearing system with a shaft sleeve made of SiC/SiC, sintered SiC as static bearing, and water at as lubricant.
[K. Gaffal, A.-K. Usbeck, W. Prechtl: ''Neue Werkstoffe ermöglichen innovative Pumpenkonzepte für die Speisewasserförderung in Kesselanlagen''. VDI-Berichte Nr. 1331, VDI-Verlag, Düsseldorf, 1997, p. 275] The specific load capacity of a bearing is usually given in Watt, W/mm
2 and calculated as a product of the load (MPa), the surface speed of the bearing (m/s) and friction coefficient; it is equal to the power loss of the bearing system due to friction.
This slide bearing concept, namely SiC/SiC shaft sleeve and SiC bearing, has been used since 1994 in applications such as in the boiler feedwater pumps of power stations,
which pump several thousand cubic meters of hot water to a level of , and in tubular casing pumps for water supply, water works or seawater desalination plants, pumping up to to a level of around .
This bearing system has been tested in pumps for liquid oxygen, for example in oxygen turbopumps for thrust engines of space rockets, with the following results. SiC and SiC/SiC are compatible with liquid oxygen. In an auto-making fire, ignition test according to the French standard NF 28-763, no auto-ignition was observed with powdered SiC/SiC in 20 bar pure oxygen at temperatures up to . Tests have shown that the friction coefficient is half, and wear one-fiftieth of standard metals used in this environment. A hydrostatic bearing system (see picture) has survived several hours at a speed up to 10,000 revolutions per minute, various loads, and 50 cycles of start/stop transients without any significant traces of wear.
Other applications and developments
*Thrust control flaps for military jet engines
*Components for Nuclear fusion, fusion and Nuclear fission, fission reactors
*Friction systems for various applications
*Nuclear applications
*heat treatment, high temperature, soldering fixtures
[J. Demmel, U. Nägele (ed.): "CFC. The perfect material for new heat treatment fixtures". European Carbon Conference 1998. Science and technology of carbon. Vol.2. Strasbourg. pp. 741-742]
References
Further reading
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* {{cite book , title=Ceramic Matrix Composites: Materials, Modeling and Technology , publisher=Wiley , location=Hoboken , editor1-first=N. P. , editor1-last=Bansal , editor2-first=J. , editor2-last=Lamon , name-list-style=amp , date=2015 , isbn=978-1-118-23116-6
Ceramic materials
Composite materials