Hot Contents: Ceramic fiber reinforced ceramic

Ceramic matrix composite

Ceramic matrix composites (CMCs) are a group of materials within composite materials or within technical ceramics. They consist of ceramic fibers embedded in a ceramic matrix, thus forming a ceramic fiber reinforced ceramic (CFRC) material. The matrix and fibers can consist of any ceramic material; for example, carbon fibers are also considered a ceramic material.

Introduction

The motivation to develop CMCs was to overcome the problems associated with the conventional technical ceramics like alumina, silicon carbide, aluminium nitride, silicon nitride or zirconia – they fracture easily under mechanical or thermo-mechanical loads because of cracks initiated by very small defects or scratches. The crack resistance is – like in glass – very small. To increase the crack resistance or fracture toughness, particles, so-called mono-crystalline 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 fibers has drastically increased the crack resistance, elongation and thermal shock resistance, and resulted in several new applications.

Carbon (C), special silicon carbide (SiC), alumina (Al₂O₃) and mullite (Al₂O₃–SiO₂) fibers are the ones most commonly used for CMCs. The matrix materials are basically the same ones: C, SiC, alumina and mullite.

Generally the 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 manufactured with the liquid polymer infiltration (LPI) process (see below) is abbreviated as LPI-C/SiC.

The important commercially available CMCs are C/C, C/SiC, SiC/SiC and Al₂O₃/Al₂O₃. They differ from conventional ceramics in the following properties, presented in more detail below:

Manufacture

The manufacturing processes usually consist of the following three steps:

Lay-up and fixation of the fibers, shaped as the desired component
Introduction of the matrix material
Final machining and, if required, further treatments like coating or generation of porosity.

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 fabrics, filament winding, braiding and knotting. The result of this procedure is called fiber-preform or simply preform. The third and final step of machining has to be done with diamond tools: grinding, drilling, lapping or milling. The special properties of CMCs also allow cutting them with a water jet or a laser.

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 of a pre-ceramic polymer
Chemical reaction of elements
Sintering at low temperatures (1000 to 1200 °C)
Electrophoretic deposition of a ceramic powder

The fifth procedure is not yet established in industrial processes. All procedures have sub-variations, which differ in technical details. Combinations of these procedures are also possible and applied. Procedures one, two and three are applied for non-oxide CMCs, whereas the fourth one is used for oxide CMCs. All procedures yield a porous material.

Ceramic fibers

Ceramic fibers in CMCs can have polycrystalline structure, like in conventional ceramics. They can also be amorphous or have inhomogeneous chemical composition, which develops upon pyrolysis of organic precursors. The high CMC process temperatures are incompatible with organic, metallic or glass fibers. Only fibers stable at temperatures above 1000 °C can be used, such as fibers of alumina, mullite, crystalline SiC, zirconia, carbon or amorphous SiC. SiC fibers have an elongation capability beyond 2%, much larger than in conventional ceramic materials (0.05 to 0.10%). The reason for this quality of SiC fibers is that most of them contain additional elements like oxygen, titanium and/or aluminium yielding a tensile strength beyond 3000 MPa. These properties are used and needed to allow textile fabrication like weaving and an arrangement of three-dimensional structures (see figure), where a small bending radius is essential.

Manufacturing procedures

Matrix deposition from a gas phase

Chemical vapor deposition (CVD) is usually used for this purpose. In presence of a fiber preform, CVD takes place in between the fibers and their filaments and therefore is called chemical vapor infiltration (CVI). One example is the manufacture of C/C: a C-fiber preform is exposed to a mixture of argon and a hydrocarbon gas (methane, propane, etc.) at low pressures (around or below 100 kPa) and high temperatures (above 1000 °C). The gas decomposes and deposits carbon on and in between the fibers. Another example is the deposition of silicon carbide. Generally a mixture of hydrogen and methyl-trichlorosilane (MTS, CH₃SiCl₃) is used, which is also common for silicone production. Under defined condition this gas mixture deposits fine and crystalline silicon carbide on a hot surface. This CVI procedure leaves a body with a porosity of about 10–15%, because after a certain time the surface of the preform is covered tightly.

Matrix forming via pyrolysis of C- and Si-containing polymers

Hydrocarbon polymers shrink during pyrolysis, and upon outgassing form carbon with amorphous, glass-like structure, which by heat treatment can be changed to a more graphite-like structure. Other special polymers, where some carbon atoms are replaced by silicon atoms, the so-called polycarbosilanes, yield amorphous silicon carbide of more or less stoichiometric composition. A large variety of such SiC-, SiNC-, or SiBNC-producing precursors exist and are being developed. To manufacture 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 established. 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

Here some material is already arranged between the fibers and then reacts with another material to form the ceramic matrix. Some conventional ceramics are manufactured by chemical reactions. For example, reaction-bonded silicon nitride (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 for CMC manufacture, which has been introduced in the production of ceramic brake discs, is the reaction of silicon with a porous preform of C/C. The process temperature is above 1414 °C, that is above the melting point of silicon, and the process conditions are controlled so that the carbon fibers of the C/C-preform are mostly untouched. This process is called liquid silicon infiltration (LSI). Sometimes, and because of its starting point with C/C, it 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 of the ceramic fibers can not withstand the normal sintering temperatures of above 1600 °C, special precursor 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-orthosilicate (as Si donor) and aluminium-butylate (as Al donor), yielding a mullite matrix. Other techniques, such as sol-gel chemistry, are also used. CMCs obtained with this process usually have a high porosity of about 20%.

Matrix formed via electrophoresis

In the electrophoretic process, electrically loaded particles dispersed in a special liquid are transported by direct current voltage into the preform, which has the opposite electrical load. This process is under development, and is not yet integrated to industrial processes. Some remaining porosity must be expected here, too.

Properties

Mechanical properties

Basic mechanism of mechanical properties

The mentioned high fracture toughness or crack resistance is reached by 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 are bridging these cracks (see picture). This mechanism works only when the matrix can slide along the fibers, which means that there must be a week bond between the fibers and matrix. A strong bond would require an infinite elongation capability of the fiber bridging the crack and yield brittle fracture like conventional ceramic. 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, the so-called fiber/matrix interphase (sometimes "interface"), leading to the so-called fiber pull-out-picture of crack surfaces shown at the top of this article. In oxide-CMC the high porosity of the matrix is sufficient to establish the weak bond.

Properties under tensile and bending loads, crack resistance

The influence and quality of the fiber interphase can be evaluated through mechanical properties. Measurements of the crack resistance have been performed with notched specimen (see figure) in so-called single-edge-notch-bend (SENB) tests. In fracture mechanics the measured data (force, geometry and crack surface) are standardized and after some mathematics yield the so-called stress intensity factor (SIF), K_Ic. Because of the complex crack surface (see figure at the top of this article) the real crack surface 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; they require identical geometry for comparing different samples. The area under those curves corresponds to the energy required to drive the crack tip through the sample (force times path length gives energy). The maximum indicates the load level necessary to propagate the crack through the sample. Compared to the sample of conventional SiSiC ceramic two observations can be made:

In the table, CVI, LPI, and LSI denote the manufacturing process of the C/SiC-material. Data of the oxide CMC and SSiC are taken from manufacturer data sheets. Tensile strength of SSiC and Al₂O₃ were calculated results of elongation to fracture and Young's modulus, 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.

Tensile tests of CMCs usually show nonlinear stress-strain curves, which look as if the material deforms plastically. It is called quasi-plastic, because the effect is caused by the microcracks, which are formed and bridged with increasing load. Since the Young's modulus 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.

The following features are essential in evaluating bending and tensile data of CMCs:

The primary quality criterion for CMCs is the crack resistance behavior or fracture toughness.

Other mechanical properties

In many CMC components the fibers are arranged as 2-dimensional (2D) stacked plain or satin weave fabrics. Thus the resulting material is anisotropic or, more specifically, orthotropic. A crack between the layers is not bridged by fibers. Therefore, the interlaminar shear strength (ILS) is low for this materials as well as the strength perpendicular to the 2D fiber orientation. Delamination can occur easily under certain mechanical loads. Three-dimensional fiber structures can improve this situation (see micrograph above).

In the table, because of the porosity, the compressive strength is lower than that of conventional ceramics, where values above 2000 MPa are common. Because of porosity and lack of fiber bridges the through-thickness strength is very low.

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 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 insulators, and because of their high porosity their thermal insulation is much better than that of conventional oxide ceramics.

The use of carbon fibers increases the electrical conductivity, provided the fibers contact each other and the voltage source. Silicon carbide matrix is a good thermal conductor. Electrically, it is a semiconductor, and its resistance decreases with increasing temperature. Compared with (poly)crystalline SiC the amorphous SiC fibers are relatively poor conductors of heat and electricity.

Comments for the table: (p) and (v) refer to the data for the parallel and vertical orientation to the 2D-fiber structure, respectively. LSI material has the highest thermal conductivity 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 because of their high Young's modulus and low elongation capability. Temperature differences and low thermal conductivity 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 are bridging 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 behavior of CMCs are scarce except for oxidation 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 the chemicals, which are often used for easier sintering and do not belong to ceramics because of their chemical composition, are starting points of corrosive attack.

Alumina

Pure alumina shows excellent corrosion resistivity against most chemicals. Amorphous glass and silica phases at the grain boundaries decide the speed of corrosion in concentrated acids and bases. Creep at high temperatures and loads is limiting the use of alumina. In liquid metals alumina is used only in gold and platinum.

Alumina fibers

These fibers behave 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 few applications for oxide CMCs.

Carbon

The most important corrosion stage of carbon occurs beyond about 500 °C in presence of oxygen; it burns into carbon dioxide and/or carbon monoxide. It also oxidizes in strong oxidizing agents like concentrated nitric acid. In liquid metals it dissolves and forms metal carbides. 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 800 °C, and liquid metals react with it; carbides and silicides are formed with the metals. The reaction with oxygen forms SiO₂ and CO₂, where SiO₂ gives a surface layer, which slows down the oxidation process (passive oxidation). At temperatures above about 1600 °C and in a low partial pressure of oxygen the so-called active oxidation starts. Lack of oxygen leads to the formation of SiO and CO gases, and SiC evaporates very quickly. If the SiC matrix is not produced via CVI, corrosion-resistance is not as good. The amorphous LPI-matrix and the leftover silicon in the LSI-matrix are the reason for this.

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 produced for LPI-matrices: the fibers are more sensitive to bases and oxidizing media than pure silicon carbide.

Applications

CMC materials lack the major disadvantages of conventional technical ceramics, namely brittle failure and small toughness as well as limited thermal shock resistance. Therefore, their applications are in the fields requiring reliability at high-temperature conditions (beyond the capability of metals) and resistance to corrosion and wear. They include:

Beyond that all fields are of interest in which conventional ceramics are applied 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 1500 °C for a few minutes. Only ceramic materials are able to 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:

In this application the temperatures do not allow the use of oxide CMCs, because under the expected loads the creep is too high. Amorphous silicon carbide fibers lose their strength due to re-crystallization at temperatures above 1250 °C. Therefore carbon fibers in a silicon carbide matrix (C/SiC) are used in development programs for these applications. The European program HERMES of ESA, 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 space vehicle X-38.

This development program has qualified the use of C/SiC screws and nuts, and the bearing system of the flaps. The latter have been ground-tested at the DLR in Stuttgart, Germany, under expected conditions of the re-entry phase: 1600 °C, 4 tonnes load, oxygen partial pressure according to re-entry conditions, bearing movements with four cycles per second. A total of five re-entry phases has been simulated. Furthermore, oxidation protection systems have been developed and qualified to prevent burn-out of the carbon fibers. After mounting of the flaps, mechanical ground tests were performed successfully by NASA in Houston, Texas, US. For financial reasons, the next test – a real re-entry of the unmanned vehicle X-38 – was cancelled. One of the space shuttles 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 hot gas environment, though, several hundred cycles of thermal loads and up to many thousands hours of lifetime are required.

Developments for gas turbine components

The use of CMCs in gas turbines would allow increasing the turbine inlet temperature and thus the turbine efficiency. Because of the complexity of the shape of stator vanes and turbine blades the development has focused on the combustion chamber first. 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 slowed down heavily by the use of an oxidation protection coating of several layers of oxides. The engine partnership between General Electric and Rolls-Royce is studying the use of CMC stator vanes in the hot section of the F136 turbofan engine presently used in the Joint Strike Fighter. The engine partnership CFM International is also considering the use of CMC parts to reduce weight in its Leap-X demonstrator engine program, which is aimed at providing next-generation turbine engines for narrow-body airliners. CMC parts are also studied for stationary applications inside both the cold and hot sections of the engines, since stresses imposed on rotating parts would require further development efforts. Generally, a successful application in turbines still needs a lot of technical and economical work for all high-temperature components to verify the efficiency gain. Furthermore, cost reduction for fibers, manufacturing processes and protective coatings is essential.

Application of oxide CMC in burner and hot gas ducts

Oxygen-containing gas heated to temperatures of about 1000 °C is rather corrosive for metal and silicon carbide components. Several of such components are not exposed to high mechanical stress and thus can be made of oxide CMCs, which can withstand temperatures up to 1200 °C. In the gallery below a flame holder of a crisp bread bakery is shown after testing for 15,000 hours, and it operated for more than 20,000 hours in total.

Flaps and ventilators circulating hot, oxygen-containing gases can be realized in the same shape as metal components. The lifetime is several times longer for these oxide CMC components 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 as brake disk

Carbon/carbon (C/C) materials have found their way in brake disks of racing cars and airplanes, and C/SiC brake disks manufactured by the LSI process were qualified and are commercially available for luxury vehicles. The advantages of these C/SiC disks are:

The SiC-matrix of LSI has a very low porosity and protects the carbon fibers quite well, and brake disks do not heat up above 500 °C for more than a few hours in their lifetime. Therefore, oxidation is not essential in this application. The reduction of manufacturing costs will decide the success of this application for middle-class cars.

Application in slide bearings

Conventional SiC or sometimes the less expensive SiSiC has been successfully applied for more than 25 years in slide or journal bearings of pumps. The pumped liquid itself is used as the lubricant of the bearing. Very good corrosion resistance against practically all kinds of media, very low wear and low friction coefficients are the cause 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 therefore is used with high wall thickness and/or mounted with special designs. In big pumps with shafts 100–350 mm in diameter the risk of failure is higher due to the changing requirements on the pump performance – for example, load changes under 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 the shaft sleeve made of SiC/SiC, sintered SiC as static bearing, and water of 80 °C as lubricant. The specific load capacity of a bearing is usually given in W/mm and calculated as a product of the load (MPa), surface speed of the bearing (m/s) and friction coefficient; it is equal to the power loss of the bearing system due to friction.

In boiler feedwater pumps of power stations, which are pumping several thousand cubic meters of hot water to a level of 2000 m, and in tubular casing pumps for water works or sea water desalination plants (pumping up to 40,000 m to a level of around 20 m) this slide bearing concept, namely SiC/SiC shaft sleeve and SiC bearing, is used since 1994. A picture of such shaft sleeves is shown above.

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-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 525 °C. Tests have yielded a twice lower friction coefficient and 50 times lower wear compared to the standard metals used in this environment. A hydrostatic bearing system (see picture) has survived several hours at a speed up to 10,000 cycles per minute, various loads and 50 cycles of start/stop transients without any significant traces of wear.

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2011/06/16

Ceramic fiber reinforced ceramic