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Argon, Astatine, Bromine, Carbon, Chlorine, Fluorine, Helium, Hydrogen, Iodine, Krypton, Neon, Nitrogen, Oxygen, Phosphorus, Radon, Selenium, Sulphur and Xenon are the non-metal elements within the periodic table. Non-metals are either liquid or gas at room temperature; they have low electrical conductivity and have high melting and boiling points.
Teflon, nylon, carbon, rubber, Bakelite and a host of plastic materials are used in the gas turbine engine mainly as sealing and insulation materials. For example, nylon and Teflon are used to insulate and protect the shielded electricity wiring located on the outside of the engine. Teflon is also used on the J79 for the seal on the variable - stator-vane actuators. Carbon is used largely inside the engine in the form of carbon - rubbing oil seals. Some the these "face" carbon-rubbing seals must be flat to within two helium light bands. Rubber and rubberized fabric materials make up the sealing edge of the fire seal that divides the hot and cold sections of the engine when mounted in the nacelle. Synthetic rubber is used extensively throughout the engine in the form of O- ring or other shaped seals.
Role of Advanced Ceramics in Aerospace Industry
Aerospace manufacturers face extreme pressure to lower costs, while increasing performance and satisfying stringent safety standards. Producers in the commercial airline, defence and space exploration sectors continually seek new materials that are reliable and robust, and meet the needs of highly specialized applications.
Â Advanced ceramics, such as Alumina, Silicon Nitride and Aluminium Nitride are currently being used to manufacture critical aerospace components, because they have several advantageous physical properties. These inorganic, non-metallic materials retain dimensional stability through a range of high temperatures and exhibit very high mechanical strength. They also demonstrate excellent chemical resistance and stiffness-to-weight ratio, thereby providing manufacturers with the ability to design components that offer optimal performance in their intended application.
Important Properties of Ceramics
Oxidation/corrosion resistance is good compared to metals.
Creep resistance is also good.
Ceramic matrix composites (CMCs) are among advanced materials that have been identified as a key material system for improving the thrust-to-weight ratio of high-performance aircraft engines.
Requirements of aero engine
The requirements for aero-engines are high performance, light weight, low emission and noise, and low life cycle cost. It is necessary to increase the thrust-to-weight ratio (T/W) in aero-engines, that is, to increase performance it is necessary to increase turbine inlet temperature (TIT). Creep resistance is needed in all hot section components. High temperature materials are also required for the low-emissions combustor and for the noise suppressor nozzle.
Trends in aero-engine materials use
Though many monolithic ceramics materials exhibit intrinsic properties, the principal problem relative to their use in aero-engines has been their flaw sensitivity and brittle fracture modes. Continuous fibre CMCs are very interesting materials due to (i) their high temperature performance compared with super alloys and (ii) their higher fracture toughness compared with monolithic ceramics in aero-engines, in which structural integrity is most required.
Applications of ceramic metallic composites:
Blisk (bladed disk):
Blisk (rotating parts) design is driven strongly by the strength/density ratio differing with static components. Lightweight blisks permit additional weight to be removed, reducing shaft loads, bearing compartment loads, and others. This cascade of impacts can result in system benefits that are much greater than for the individual CMC applications.
The CFCC combustor model parts were also fabricated by slurry impregnation and a subsequent reaction sintering process. Their potential components are combustor liners, ducts, nozzle flaps, acoustic liners, turbine vanes, turbine blades, turbine disks, and so on. These include the development of the material system (thermal stability of SiC fibres, non-oxidizing interface and matrix), low cost manufacturing processes, the establishment of a design method and the development of non-destructive evaluation techniques.
A ceramic gas turbine where such advanced ceramics are used in high-temperature components such as turbine blades and nozzles makes it possible to increase the turbine inlet temperature (TIT) up to 1300 ~1400Â°C, resulting in high thermal efficiency.
By adopting the developed silicon nitride to the components including turbine blades, nozzles, combustor liners, and nose cones, TIT can be increased without cooling; this leads to high thermal efficiency of about 40 percent, as shown in Fig
Thermal efficiency curve
Silicon nitride has been recognized as one of the most promising ceramic materials for high-temperature structural components for nearly two decades, and high-temperature strength has been substantially improved, as shown in Fig. At high temperatures, the strength is degraded and the structural reliability is very often limited due to the softening of glassy phases, which are formed at grain boundaries as a result of processing with sintering additives. There are two regions in a delayed-fracture mechanism map of silicon nitride at the temperatures above 1200Â°C: slow crack growth failure and creep damage rupture is shown. The former is a fracture that occurs when a crack grows sub critically from a pre-existing flaw and reaches the critical size. This is predominant in the high-stress, short-term life region. The latter is due to the formation of a macro crack with the critical size by cavity nucleation and coalescence. This prevails in the low-stress, long-term life region. Generally, long-term durability for the practical service is estimated from the short-term data. The difference between these two fracture mechanisms is understood in terms of creep rate properties, creep life properties, micro structural changes, etc. The transition from the slow crack growth fracture to the creep damage rupture one occurs when the applied stress decreases below about 200 MPa.
Improved strength of ceramics at high
The creep curves of silicon nitride at high temperatures generally consist of three regimes: transient, steady-state, and accelerated creep regimes.
SiC-based ceramic matrix composites, consisting of carbon or SiC fibres embedded in a SiC matrix, are tough ceramics when the fibre/matrix bonding is properly optimized through the use of a thin inter-phase. They are fabricated according to different processing routes (chemical vapor infiltration, polymer impregnation/pyrolysis, liquid silicon infiltration or slurry impregnation/hot pressing) .SiC-matrix composites are highly tailorable materials in terms of fibre-type (carbon fibres of SiC-based fibres such as Si-C-O, SiC+C or quasi stoichiometric SiC reinforcements), inter-phase (pyrocarbon or hexagonal BN, as well as (PyC-SiC)n or (BN-SiC)n multilayered interphases), matrix (simple SiC or matrices with improved oxidation resistance, such as self-healing matrices) .
Silicon carbide generally does not contain glassy phases at grain boundaries, even when doped sintering additives such as alumina .Due to this rigid interface, the strength is not degraded at very high temperatures. Because of the good high-temperature mechanical properties as well as good corrosion resistance, silicon carbide is one of the most important candidate materials usable at high temperatures around 1400Â°C. In this section, creep and creep rupture behavior of silicon carbide doped with 5 wt percent alumina 1400Â°C is described. The TEM observation revealed that there is no glassy phase at the interfaces between two silicon carbide grains; even if any glassy phase is present, its thickness is in the order of atomic dimensions.
Then, the measured creep rate of this material at 1400Â°C, 200 MPa is as small as 6Ã-10-12/s. No cavity is formed during creep, though creep deformation should be controlled by grain boundary diffusion and creep failure is caused by slow crack growth from a pre-existing flaw. The crack grows sub critically along grain boundaries with diffusion process.
Elecro ceramic materials (piezoelectric and dielectric) are used in aerospace transducers and sensors such as accelerometers (for measurement of vibration), gyroscopes (for measurement of the acceleration and pitch of aircraft, missiles and satellites), and level sensors (such as fuel tanks).
One of the most successful commercial aircrafts in recent times, the Boeing 777, uses piezo ceramic material within the 60 ultrasonic fuel tank probes located on each aircraft. The ultrasonic transducers are installed at a variety of locations in each fuel tank. Similar ultrasonic fuel probes also used in fighter aircraft and other level sensing applications because of their ability to provide highly accurate readings, regardless of the orientation of the aircraft.
Seals and thermocouples:
Advanced ceramics are also ideally suited for aerospace applications that provide a physical interface between different components, due to their ability to withstand the high temperatures, vibration, and mechanical shock typical found in aircraft engines and other high - stress locations. Ceramics are commonly found in seals for gas turbine engines, fuel line assembly, and thermocouples. Where ceramic/metal assemblies are required, joining the two materials generally involves metalizing the ceramic surface and brazing the components together.
A structural composite is a material system consisting of two or more phases on a macroscopic scale, whose mechanical performance and properties are designed to be superior to those of the constituent materials acting independently. One of the phases is usually discontinuous, stiffer, and stronger and is called reinforcement, whereas the less stiff and weaker phase is continuous and is called matrix. Sometimes, because of chemical interactions or other processing effects, an additional phase, called inter phase, exists between the reinforcement and the matrix.
Phases of composite material
The phases of the composite system have different roles that depend on the type and application of the composite material. In the case of low to medium performance composite materials, the reinforcement, usually in the form of short fibers or particles, provides some stiffening but only local strengthening of the material. The matrix, on the other hand, is the main load bearing constituent governing the mechanical properties of the material. In the case of high performance structural composites, the usually continuous - fiber reinforcement is the backbone of the material that determines its stiffness and strength in the direction of the fibers. The matrix phase provides protection and support for the sensitive fibers and local stress transfer from one fiber to another. The inter phase, although small in size, can play an important role in controlling the failure mechanisms, fracture toughness, and overall stress- strain behaviour of the material.
Various components of the boeing 757 aircraft made of composite materials
Composite materials are made by combining two or more materials to give a unique combination of properties. Many common materials are indeed "composites," including wood, concrete, and metals alloys. However, fiber-reinforced composite materials differ from these common materials in that the constituent materials of the composite (eg the two or more phases) are macroscopically distinguishable and eventually mechanically separable. In other words, the constituent materials work together but remain essentially in their original bulk form (apart from the thin hybrid interface between the phases). The main component of a composite is the matrix material. The reinforcement (qv) can be fibers, particulates, or whiskers. The fibers can be continuous, long, or short. In advanced composites, the fibers (ie the reinforcing phase) are present as unidirectional strands or woven fabric and provide strength and stiffness to the composite. The matrix acts as a load transfer medium assuring rigidity and protects the fibers and the whole composite from environmental attack. Short chopped fibers and mat are used in nonstructural polymer matrix composites. In these cases the fibers provide comparatively less strength and stiffness to the composite.
Fiberglas is the most common composite material, and consists of glass fibres embedded in a resin matrix. Thermoplastics are a relatively new material that is replacing thermoses as the matrix material for composites. They hold much promise for aviation applications. One of their big advantages is that they are easy to produce. They are also more durable and tougher than thermo sets, particularly for light impacts, such as when a wrench dropped on a wing accidentally. The wrench could easily crack a thermo set material but would bounce off a thermoplastic composite material.
Types of composite materials:
Polymer-matrix composites (PMCs)Â
Lightest type of composite materials and applications of PMCs in aircraft propulsion systems, such as General Electric`s F-404 engine, have resulted in substantial reductions in both engine weight and manufacturing costs. Commercially available state-of-the-art high-temperature PMCs, such as graphite fibre/PMR-15 and graphite fibre/PMR-11-55, are capable of withstanding thousands of hours of use at temperatures between 290 and 345Â°C).
Inter metallic-matrix composites.
The initial phase of the IMC program involves investigating available fiber compositions (SiC and Al2O3) in aluminides of iron, titanium, nickel, and niobium. These aluminides are Ti3Al and FeAl for applications to 1000Â°C and NiAl and Nb-alloy/aluminides for higher temperature applications.
The new class of materials - ceramic matrix composites (CMCs) - is concerned with a ceramic matrix reinforced by ceramic fibres, whiskers or particles. The matrix is made of either a monolithic ceramic (SiC, Al2O3, Si3N4,) or a glass-ceramic. The first ones are prepared from ceramic routes (melting or chemical vapour infiltration -CVI-, polymer infiltration -PIP- processes) and the second ones result from the glass route which is easier to produce and needs a lower temperature.
In CMCs fabricated by CVI, the design of the fibre/matrix interfacial zone is based on precoated fibres where a weak interface (a pyrocarbon interphase for example) is deposited on the fibre prior to the matrix. To improve the oxidation resistance, the use of multilayer interphases for example (PyC-SiC)n has been developed . A self healing process of the ceramic matrix can also be achieved by addition of boron to pyrocarbon because oxidation of boron gives rise to a low melting glass healing the micro cracks.
These fiber-reinforced ceramics (FRCs) have lower densities, better oxidation resistance, and potential to operate at significantly higher temperatures than super alloys. Compared to monolithic ceramics, CMCs present higher toughness and tolerance to the presence of cracks, which implies a non-catastrophic mode of failure. Creep resistance is one of the main requirements for these materials because potential applications of CMCs, for example as parts of gas turbines for aircrafts, require maintaining the material properties over long periods of time (thousands of hours) at high temperature.
Glass ceramic composite based material:
Glass and glass-ceramic matrices are silicates which exhibit thermal expansion coefficients close to those of the SiC fibres (3-5 10âˆ’6Â·Kâˆ’1). A key point from a mechanical point of view is the presence of a thin layer of carbon, often found textured. The carbon-rich layer is relatively weak and consequently increases the fracture toughness of the composite. It allows crack deflection along the fibre/matrix interface and load transfer to occur from matrix to fibre. The nature of the interfacial zone, its thickness and its kinetics of growth depend on many parameters such as the glass composition, the hot-pressing condition.
SiC/BN dual-coated Nicalon-fibre-reinforced glass-ceramic matrix composites
The embrittlement of the glass-ceramic matrix which appears in oxidizing environment can be either the consequence of oxygen diffusion to the fibre/matrix interface via matrix microcracks reacting with the carbon layer or a "pipeline diffusion" from cut ends of fibres exposed to air at the composite surface. To maintain an oxidative stability at high temperatures, one approach consists of using fibre coatings applied to the fibres prior to composite processing. Such coatings should have two key functions: a mechanical fuse to allow crack deflection and load transfer from matrix to fibre and an improvement of the oxidation resistance.
The mechanical properties of the composites were evaluated by three point bending and tensile testing at both room temperature and high temperatures (up to 1573 K) in air. The composite strength was excellent up to 1473 K, the ultimate strength at 1473 K and the elastic modulus were found respectively to be 565 MPa and 69 GPa. Mechanical properties decrease significantly at 1573 K because of matrix softening. A degradation of the mechanical properties occurs after annealing in air for 500 h at 1473 K. A nanoscale silica/carbon sublayer was formed at the BN/SiC Nicalon fibre interface during long-term exposure to oxygen at high temperature. This sublayer appearing between 1373 K and 1473 K was supposed to be responsible for the decrease in the fibre/matrix bonding strength at high temperature. But good interfacial properties are maintained at 1373 K for long-term exposures.
Bending creep behaviour of sic / BN coated fibre /BMAS composites:
Below 1408 K, the constant creep rates were extremely low (~10âˆ’9Â·sâˆ’1) and at 1473 K constant creep rates were an order magnitude higher. The 0/90â-¦ fibre-reinforced composites exhibited long creepstrain recovery. From the microstructure investigations, it was concluded that the dual SiC/BN coating provides an effective barrier to reaction and diffusion. Moreover the BN coating allows debonding to occur with an extensive fibre pull-out of the fibres.
Cyclic creep and recovery behaviour of NextelTM720/alumina ceramic composite at 1200 â-¦C
Two primary mechanisms responsible for strain recovery process in fibre -reinforced ceramics: intrinsic strain recovery of the constituents upon decrease in stress and mechanical driving force arising from the residual stress state that develops in a composite upon unloading. This residual stress state develops as a result of unequal creep rates and different elastic constants generally exhibited by the fibres and matrix. The tensile creep-recovery behaviour of the N720/A composite was investigated at 1200 â-¦C in air and in steam. The creep strain recovery was quantified using strain recovery ratios. In air the composite exhibits considerable creep strain recovery with creep strain recovery ratios reaching 90%. The extent of primary creep is significantly reduced with each creep-recovery cycle. The reduction in the duration of primary creep is attributed to the strain recovery and the associated changes in the residual stress state of the composite that occur during unloading. For a given creep stress, the overall creep rate is much lower in a cyclic creep-recovery test than in a sustained creep test. Furthermore, total creep strain accumulated during cyclic creep-recovery is significantly less than the creep strain accumulated during an equivalent length of time in a sustained creep test. Change in the primary creep behaviour is behind the reduction in creep strain accumulated during cyclic creep-recovery. Strain recovery leads to a significant improvement in creep lifetime. For a given creep stress, creep lifetimes obtained in cyclic creep-recovery tests significantly exceed those obtained in sustained creep testes. Life predictions that do not account for strain recovery may notably underestimate service life of the component. Presence of steam has a profound effect on the cyclic creep recovery behaviour of N720/A composite at 1200 â-¦C. In steam the composite exhibits much less strain recovery than in air, with creep-strain recovery ratios reaching only 34%. The primary creep behaviour remains relatively stable after the first cycle. Consequently, the creep rate in a cyclic creep-recovery test is close to that produced in a sustained creep test. Because in steam the strain recovery is minimal, creep strains accumulated in cyclic creep recover tests are close to those accumulated in sustained creep tests. Likewise creep lifetimes produced in cyclic creep-recovery tests are similar to those produced in sustained creep tests.
Maintainability, Serviceability and durability
Composites can operate in hostile environments for long periods of time. They have long fatigue lives and are easily maintainable and required. However, they suffer from sensitivity to hygro thermal environments. Service - induced damage growth may be internal, requiring sophisticated, non destructive techniques for its detection and monitoring. Sometimes it is necessary to apply protective coatings against erosion, surface damage, and lightning strike.
One of the important advantages of composites is reduction in acquisition and /or life cycle costs. This is effected through weight savings, lower tooling costs, reduced number of parts and fewer assembly operations.