High temperature material on aerinautics engine

If you order your research paper from our custom writing service you will receive a perfectly written assignment on High temperature material on aerinautics engine. What we need from you is to provide us with your detailed paper instructions for our experienced writers to follow all of your specific writing requirements. Specify your order details, state the exact number of pages required and our custom writing professionals will deliver the best quality High temperature material on aerinautics engine paper right on time.

Our staff of freelance writers includes over 120 experts proficient in High temperature material on aerinautics engine, therefore you can rest assured that your assignment will be handled by only top rated specialists. Order your High temperature material on aerinautics engine paper at affordable prices!

High temperature

materials

for aero-engines

George FrenchCustom writing service can write essays on High temperature material on aerinautics engine

Philippe Grangier

Richard Halliday

Steven Farmer

Summary

1- Introduction

- Turbine composition and temperature

- Metals alloys

4- Ceramics

5-Thermal barrier coatings

6- Conclusion

7- Bibliography

Introduction

Since Sir Isaac Newton, in the 18th century, was the first to theorise that a rearward-channelled explosion could propel a machine forward at a great rate of speed, people have started to imagine that they could fly.

In 10, the Wright Brothers flew, The Flyer, with a 1 horse power gas powered engine.

But it was Frank Whittle, a British pilot, who designed the first turbo jet engine in 10. The first Whittle engine successfully flew in April, 17. This engine featured a multistage compressor, and a combustion chamber, a single stage turbine and a nozzle.

The first jet aeroplane to successfully use this type of engine was the German Heinkel He 178 invented by Hans Von Ohain. It was the worlds first turbojet powered flight.

Since this date, the engineers start to developed this kind of engine because they can offer more power to make the plane fly faster and Higher. Improvements in the performance of gas turbine have been intimately linked to the development of materials technologies for high-temperature components.

It¡¦s now over half a century since Franck Whittle and Otto Von Ohain demonstrated the practicality of aircraft powered by gas turbines. Engine performance and durability have been limited by the availability of suitable materials for the very high temperatures and high stresses endured by many of the components. Over the past 50-60 year, materials have been steadily developed so that peak metal temperatures of over 1100„aC¡K

Now the hot parts of the engine require materials which can operate at 1000„aC, the cooler parts at 600„aC. Furthermore, the environment is very harsh chemically and mechanical, with very large forces generated by the high rotational speeds and even the possibility of birds being sucked into the engine!

The maximum service temperature chart (on the bottom) is a useful way of identifying new possibilities for materials development. By drawing lines at 600¢XC and 1000¢XC it is possible to identify the materials classes which might be suitable in this case, namely metals and ceramics.

But the figure below shows that the remarkable improvements in aero-engine performance have come about because the materials designer has been able to provide the engineer with materials can be used at hotter temperatures. Higher engine temperatures are needed so that the engines can run more efficiently, while weight reductions require stiffer, stronger, and lighter materials. What will the future reserved to the engineers? The next generation of gas turbine will reach temperature of 1600„aC, so we need to foresee what the next generation of high temperature material will be.

At the present time titanium and nickel alloys are used for the low and high temperature parts, but some other solution have already been developed for allow highest temperature in aero-engine, and the future research are concerned with ceramics which appear to have the best high temperature properties.

That¡¦s why this report reviews some of the existing solutions with today¡¦s alloys, and try to explain how metal alloy can be use. It show also how we can use a combination of ceramics coatings and metal alloy to increase the operating temperature until the next development of ceramics.

Turbine composition and temperature

The basic mechanical arrangement of a gas turbine is relatively simple. It consists of only four parts

1. The compressor which is used to increase the pressure (and temperature) of the inlet air.

. One or a number of combustion chambers in which fuel is injected into the high-pressure air as a fine spray, and burned, thereby heating the air. The pressure remains (nearly) constant during combustion, but as the temperature rises, each kilogram of hot air needs to occupy a larger volume than it did when cold and therefore expands through the turbine.

. The turbine which converts some of this temperature rise to rotational energy. This energy is used to drive the compressor.

4. The exhaust nozzle which accelerates the air using the remainder of the energy added in the combustor, producing a high velocity jet exhaust.

The amount of fuel added to the air will depend upon the temperature rise required. However, the maximum temperature is limited to within the range of 850 to 1700 ¢XC by the materials from which the turbine blades and nozzles are made. The air has already been heated to between 00 and 550 ¢XC by the work done in the compressor giving a temperature rise requirement of 650 to 1150 ¢XC from the combustion process

So the continuous flow of gas to which the turbine is exposed may enter the turbine at a temperature between 850 and 1700 ¢XC which is far above the melting point of current materials technology.

That¡¦s why, until the development of the high temperature material, engineers has developed tricks to cool the material used in their aero-engine. Because by cooling the material during its operation, they can increase the operating temperature of the turbine.

Materials scientists have worked hard to increase the operating temperature of todays alloys, and for reaching this aims they have made the following modification and use the following tricks

- turbine blades are grown as single crystals, because these are more resistant to creep (gradual changes in dimensions under stress and temperature);

- current nickel superalloys contain expensive alloying elements such as Hafnium and Rhenium in order to increase their high temperature performance;

- turbine blades have little networks of holes to air-cool the blade surface.

Metals alloys

In Aero Engines the development of highly engineered super alloys has become necessary.

Developments in advanced materials have contributed to the spectacular progress in thrust-to-weight ratio of the aero engine. And also this has enabled significant improvements in performance and reliability. Most modern jet engines contain at least 50% nickel based superalloys. This has been achieved mainly through the substitution of titanium and nickel alloys for steel. Aluminium has virtually disappeared from the aero engine, and the future projection illustrates the potential for composites of various types. the design of the aero engine requires a much wider range of materials than the airframe due to the large temperature range at which it must be run. The airframe is still mainly aluminium due to the lower requirements.

Aeropropulsion turbines will eventually use more of the light advanced high-temperature materials such as intermetallics, carbon matrix composites, and metal matrix composites. However, enhancements in coatings and cooling have extended the performance and value of nickel-based superalloys, so they are not yet out of the running.

In aero-engines, the blade of the high pressure turbine was for a long time the highest of the high technology in the aero gas turbine, and despite the complexity of the modern fan blade, the challenge it provides does not reduce. The ability to run at increasingly high gas temperatures has resulted from a combination of material improvements and the development of more sophisticated arrangements for internal and external cooling

1-Modern Alloys

A modern turbine blade alloy is complex in that it contains up to ten significant alloying elements, but its microstructure is very simple. The structure is analogous to an `Inca wall, which consisted of rectangular blocks of stone stacked in a regular array with narrow bands of cement to hold them together.

In the alloy case the `blocks are an intermetallic compound with the approximate composition Ni(Al,Ta), whereas the `cement is a nickel solid solution containing chromium, tungsten and rhenium. Magnesium alloys.

Magnesium alloy developments have traditionally been driven by aerospace industry requirements for lightweight materials to operate under increasingly demanding conditions such as high temperature ranges. Magnesium alloys have always been attractive to designers due to their low density, only two thirds that of aluminium. This has been a major factor in the widespread use of magnesium alloy castings and wrought products.

A further requirement in recent years has been for superior corrosion performance and dramatic improvements have been demonstrated for new magnesium alloys. Improvements in mechanical properties and corrosion resistance have led to greater interest in magnesium alloys for aerospace and speciality applications.

-Titanium alloys.

The high strength and low density of titanium and its alloys have from the first ensured a positive role for the metal in aero-engine applications. It is difficult to imagine how current levels of performance, engine power to weight ratios, strength, aircraft speed and range and other critical factors could be achieved without titanium.

Since the 150¡¦s, this temperature level has risen by about 00¢XC. Titanium alloys have progressively improved in temperature capability up to 60¢XC. Titanium alloys capable of operating at temperatures from sub zero to 600¢XC are used in engines for discs, blades, shafts and casings from the front fan to the last stage of the high pressure compressor, and at the rear end of the engine for lightly loaded fabrications such as plug and nozzle assemblies. This would allow most compressors to be designed completely in titanium. However, practice in the United States has been to switch at approximately 50¢XC to nickel alloys and incur a weight penalty.

-Beryllium.

The metal its self has a steel grey appearance. It has an extremely high melting point 10¢XC (54¢XF) and is the most lightweight except for magnesium of the common metals. It is nonmagnetic, has approximately 40% the electrical conductivity of copper and has a modulus of elasticity one third greater that of steel. It exhibits high permeability to X-rays. Beryllium powder can be hot pressed into blocks or billet form and can be thermo-mechanically processed to extrude billet and cross-rolled sheet. Beryllium parts are generally made by machining from blocks. This tends to leave behind a damaged surface layer, which is removed by etching for stressed applications.

Beryllium is used in applications where it is required for materials to be non sparking, non-magnetic and for components to be light weight, stiff and dimensionally stable. It can be used as an alloying element to produce beryllium-aluminium, beryllium-copper, and beryllium-nickel alloys.

4-Nickel.

Nickel based alloys dominate the high temperature area of aero engines mainly due to the increase of stable intermetallic super phases which strengthens the nickel matrix. Nickel based alloys are used for stationary components in aero engines. Modern jet engines contain at least 50% nickel based super alloys.

5-Super alloys.

Superalloys have always contained phases of this type, but in recent years the titanium in the original intermetallic has been replaced by tantalum. This change gave improved high temperature strength, and also improved oxidation resistance. However, the biggest change has occurred in the nickel, where high levels of tungsten and rhenium are present. These elements are very effective in solution strengthening.

6-Intermetallics.

Another material development project is the use of intermetallics. Compounds of nickel/aluminium and titanium/aluminium have been investigated with current emphasis on the latter. Most intermetallic compounds are brittle at room temperature. The first applications are therefore likely to be in small components such static and rotating compressor airfoils where the advantages over titanium include higher specific strength and stiffness as well as improved temperature and fire resistance.

The use of these materials could extend to more critical components. One possible application is as an alternative matrix to the titanium alloy in a metal matrix composite, although such an application will require alternative fibres, to minimise any thermal expansion mismatch, and novel processing technology.

7-The Future.

Eventually, operating temperatures up to about 800¢XC will be possible, and intermetallics could offer a very attractive weight saving of around 50% compared with nickel-based alloys. It is estimated that over the next twenty years a 00¢XC increase in turbine entry gas temperature will be required to meet the airlines demand for improved performance. Some of this increase will be made possible by the further adoption of thermal barrier coatings. These coatings are produced from ceramic pre-cursors and have the potential to contribute about 100¢XC through the protection they provide.

Ceramics

1-History

Since the mid-140s, researchers have investigated ceramic materials as a way to improve the performance of aero-engines, lengthen their life span, and reduce their fuel consumption substantially. Yet ceramics are just now approaching their first commercial use in turbines.

The problems involved have made progress slow. The material in modern turbines must survive temperatures of more than 1,000¢XC for thousands of hours; high thermal stresses caused by rapid temperature changes and large temperature gradients; high mechanical stresses; low and high frequency vibration loading; chemical reactions with adjacent components; oxidation; corrosion; and effects such as creep, stress rupture, and cyclic fatigue. Early ceramic materials were not able to withstand these conditions, and early turbine-component designs were not compatible with brittle materials.

A variety of oxides, borides, carbides, and cermets were evaluated in the 140s and 150s for potential use as turbine components. Some ceramics had favourable strength and oxidation resistance, but none survived the thermal shock conditions imposed by an engine. Some cermets could survive thermal shock and impact conditions but did not have adequate oxidation resistance and stress rupture life.

Interest was renewed in ceramics for turbines when new materials in the silicon nitride and silicon carbide families of ceramics were developed during the 160s. These materials had better thermal shock resistance, largely due to a combination of low thermal expansion, high strength, and moderate thermal conductivity. The first promising silicon nitride and silicon carbide materials were fabricated by reaction sintering. The silicon nitride was prepared by a reaction of a powder compact of silicon with nitrogen to form silicon nitride. This resulted in a reaction-bonded silicon nitride (RBSN) material. It was found that this material was strong (140MPa) up to a high temperature(1,400¢XC), but the material weakened over time when exposed at high temperature to an oxidising atmosphere. The silicon carbide was prepared by reacting a mixed powder compact of silicon carbide plus carbon with molten silicon to form an SiC-bonded silicon carbide, with any pores filled with silicon. Early reaction-sintered silicon carbide materials had strength that was similar to RBSN and superior oxidation resistance, making it more desirable.

Major efforts have been conducted world-wide since the early 170s to improve the high-temperature properties of silicon nitride. Some have focused on finding a composition with a higher-temperature intergranular glass phase; others have focused on compositions that can be heat-treated to crystallise the grain-boundary phase and avoid the glass phase. Only recently have the properties been adequate to consider long-life applications.

Ceramic turbine components are fabricated starting with powders of the raw materials. The quality of the final part depends on the quality of the starting powder and on each step in the fabrication process. Early powders were coarse and contained impurities, and they were not widely available until the mid-170s. Around that time, researchers demonstrated that silicon nitride and silicon carbide could be densified by pressureless sintering if the starting powder was of very small particle size. Powder synthesis techniques were refined during the 180s, making powders with a smaller particle size and relatively high purity available.

Research during the 10s has been concerned with improving the properties of sintered materials to minimise flaw size and refining the microstructure to increase fracture toughness. Higher fracture toughness means a larger critical flaw size for a given stress. Whereas the early materials had a critical flaw size around 150 microns for a 00MPa stress, the improved materials can withstand flaws several times larger. Fracture toughness is extremely important in aero-engine materials due to the high rotational speeds involved, and also to withstand bird strikes etc.

-Present

At the moment, toughness means that ceramics are still not used on a large scale in aero-engines although it has the best high temperature properties

PropertyMetalsCeramics

Toughness (bird strikes)GoodVery Poor

Corrosion/oxidation resistanceFairGood

FormingGood (forging)Fair (sintering)

JoiningGoodDifficult

Creep resitanceFairGood

CostHighHigh

-Future

Over the next 0 years, demand for improved performance means that gas entry temperatures will have to rise by 00„aC.

Looking at this table shows that the strength and especially the maximum service temperatures of ceramics means that some form of ceramic must be considered which will overcome the toughness limitation. One method is the use of ceramics as Thermal Barrier Coatings.

Thermal barrier coatings

So far ceramic components have not found application within aero engines because no one has yet been able to develop a method for increasing the fracture toughness of ceramics to a level which would allow them to survive application in the hottest parts of such engines. Although metals have a significantly higher fracture toughness, they are also less resistant to high temperatures, which have the effect of accelerating corrosion and creep. The turbine blades, which experience high centrifugal forces due to the high rotation speeds, are at the greatest risk of creep. Indeed it is known that the creep life of turbine blades is halved for every 10-15„aC increase in operating temperature.

A common method for combining the best qualities of both types of material is to use a thermal barrier coating (TBC). TBC¡¦s are ceramic coatings which are applied to the surface of the metal components to insulate them from the high surrounding temperatures. The coatings are applied to parts such as burner cans, flame holders or turbine vane segments in order to protect the substrate material from too high operating temperatures or too severe thermal shocks.

The most widely used TBC is yttria stabilised zirconia (ZrO-8%YO). The main advantages of using yttria stabilised zirconia as a TBC material lies in its low thermal conductivity, a thermal expansion coefficient close to that of the substrate material, and a

good resistance to thermal shocks.

It is important that the thermal expansion coefficient is similar to that of the substrate metal because otherwise a significant thermal strain would be imposed as the coating expands by a greater or lesser amount than the substrate. If this were to happen then the repeated stress caused by this thermal strain would cause the coating to fail in a relatively short time span.

The lifetime of the coating is decided both by its stability at the high operating temperatures and by its adhesion to the component under cyclic thermal loading. Even though the coating is chosen so that it has a similar thermal expansion coefficient to the substrate material, relatively minor thermal strains can be accentuated by the high temperatures and the regular cycles experienced by the engines. The regular rapid temperature changes from atmospheric temperature when shut down, to operating temperature can still result in fatigue failure of the bond between the coating and the substrate.

It is for this reason that typical coatings consist of a bonding layer between the component and the thermal barrier coating. The bond coat enhances the adhesion of the component with the overlying coating and will usually also be required to act as a barrier to prevent oxidation of the metal component underneath. This is because the TBC on it¡¦s own is relatively porous and, although provides excellent thermal insulation, it provides minimal protection against corrosion. Obviously any corrosion of the surface of the substrate would result in the bond between the coating and the substrate becoming weaker, potentially to the point where it fails.

The main limitations of yttria stabilised zirconia are that at high temperatures the material properties begin to change. Densification due to sintering and phase transformation to more stable phases involving volume expansions are obviously undesirable phenomena that can be seen at temperatures above 150 ¢XC. This temperature is therefore often used as the maximum design temperature when working with this material.

Because of these limitations, alternatives to YSZ are being sought as temperatures are expected to continue to rise as higher efficiency engines having higher power to weight ratios are being designed. The main demand for these higher performance engines predictably comes from the military, because of a strong trend in the development of military engines to decrease the amount of cooling media used for cooling of the components in the hot sections of the engine, which will increase the thermal load on turbine components. Calculations indicate coating surface temperatures as high as 1400 ¢XC in future applications. This will most possibly require development of new coating materials with high temperature stability. This means that a suitable material will undergo no severe phase transformations and a have a high resistance to sintering, but still have a low thermal conductivity coefficient and a high resistance to thermal shock failure.

One possible way to increase the operating temperature of the TBC is simply to use a thicker coating. Current TBC¡¦s are around 0.-0.4mm thick. However, standard production procedures of thick TBC¡¦s (1 mm) result in coatings with an insufficient thermal shock life. The main problem with thick thermal barrier coatings is that the higher temperature gradient across the material results in higher internal stresses which in turn lead to higher stresses at the interface between the coating and the bonding layer between the substrate and the coating.

This means that over time there is a greater risk that a thick TBC will fail due to the fatigue caused at the bond interface by the thermal cycling associated with aero engines.

Until now, studies with thick TBC¡¦s have been with YSZ which still has an operating limit of 150„aC because it is unstable at higher temperatures and the strain tolerance of the coating can be lost rapidly if too high coating surface temperatures are allowed.

Because of this temperature ceiling of YSZ, there is a continual need to try to find a better material for the job. Certain rare earth zirconates such as GdZrO7 and SmZrO7, have been shown to have lower thermal conductivity than YSZ, but the fact that YSZ is stable at high temperatures when in contact with alumina, which is the oxide formed on the surface of all present bond coat alloys at high temperatures, and the fact that YSZ has a similar thermal expansion coefficient to the nickel based substrate material, will make it hard to replace as a TBC.

Conclusion

The temperature in the aero engine is still increasing and materials scientists have to develop more and more suitable materials. By looking at the development of aero engine, we see that the next generation of aero engine will work at 1600„aC. Ceramics are not available because of their resistance properties, but by combination of the thermal barrier coatings and the metals alloys which have been developed since a long time, we can imagine that scientists have already find the solution for this next generation of engine.

But the progress in aero engine science will not stop, and the working temperature of aero engine will keep increase. But the research on the ceramics will go on because they seem to have many qualities required for work at high temperature, and they are already use in ceramic coatings. But they can maybe offer more if we can use them alone. Ceramics, the way ahead? The future will tell us¡K

Bibliography

Materials world,vol.4,16, ¡¨advanced materials mean advanced engine¡¨

Stewart Miller

¡§The contribution of advanced high-temperature materials to the future of aero-engine¡¨

Article from Proc Instn Mech Engrs Vol 15 Part L

M R Winstone, A Partridge, J W Brooks,

¡§High temperature structural materials¡¨

Chapman and Hall, 16

¡§Advantages/disadvantages of various TBC systems as perceived by engine manufacturer¡¨

Article from thermal barrier coatings, nato agard report 8, 18

P Morrell , D S Rickerbery

¡§Engineering approaches to high temperature design¡¨

B Wilshire and D.R J Owen

Please note that this sample paper on High temperature material on aerinautics engine is for your review only. In order to eliminate any of the plagiarism issues, it is highly recommended that you do not use it for you own writing purposes. In case you experience difficulties with writing a well structured and accurately composed paper on High temperature material on aerinautics engine, we are here to assist you. Your persuasive essay on High temperature material on aerinautics engine will be written from scratch, so you do not have to worry about its originality.

Order your authentic assignment and you will be amazed at how easy it is to complete a quality custom paper within the shortest time possible!