Thermal Barriers And High Temperatures Engineering Essay

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Thermal barrier coating have been around for about three decades now. They are used to protect the super alloy which makes up the turbine rotors from the high temperatures that are needed for the most efficient and powerful turbines.

The materials play an important role in the quality of the turbine rotor. The current material for this product is the yttria stabilized zirconia. Monolithic and composite ceramics are the potential new candidates in this industry.

We recommend the company to produce the turbine rotor using thermal barrier coating superalloy as the material and the solution precursor plasma spray as the processing route as the following reasons:

Our company will take lower risk using the proven yttria stabilized zirconia thermal barrier coating material.

Solution precursor plasma spray stands out from other methods in cost, durability, production rate.

Proceed no not

It is recommended Rennib do not proceed. The product that would be brought into production would not be particularly market leading and there is a lot of tough and very big companies already in this business. These companies have a lot of valuable experience in a product which has been found a fine art to master.

The chosen ceramic product is the jet turbine ceramic coated blades. These are used in many different applications for slightly different reasons and the type of ceramic and its processing route is also based upon its final application.

Jet Turbine blades can be split up into two main sectors:

Commercial planes

Military planes

Commercial planes can include cargo planes such as a mail plane, the other main type are passenger planes, taking people across the country or even the world.

Military planes mainly include fighter jets and then ordinance/stealth type planes.

The ceramic product on the jet turbine blades is actually now a coating and has been around for two to three decades now. These Thermal barrier coatings are typically 100-500 µm thick. [4] They are usually in a layer type layout, with a typical bond adhesive which will hold the coating on.

Turbine Blades have to withstand very high temperatures as well as a variety of large mechanical forces acting on them.

Temperatures within the turbine vary depending on service output and application. Most turbines have an initial gas temperature of 960-1100°C. Peak temperatures in military aircraft turbines is usually 1600+°C while commercial planes are up at around 1500+°C. The NI super alloy blades that are used have a maximal operating temperature of around 850-1100°C. It can be seen that the maximal operating temperature of the blades is less than the operating temperatures of the jet turbines. For this reason it becomes obvious why a ceramic thermal barrier coating is needed to be added to these blades to allow the design to work. [1]

The ceramic coating increases the engines overall durability and also increases the efficiency of the turbine. It does this by allowing an increase in the turbine inlet temperature and by also reducing the amount of cooling air that is needed by the hot components within the jet engine. This is all simply because the ceramic thermal barrier coating can withstand these higher temperatures and therefore protects the internals and blades themselves from this excessive heat. It was found that these coatings on the turbine would save a company/operator around 1-2% of fuel cost, which for large international businesses can see them saving over $10million per annum [2]

The manufacture of turbine blades has a high tooling cost, for this reason there is a huge incentive to predict what can become a good product rather than to find a better product from the building and then testing of it.

Instead of just a ceramic coating, the blade could become a solid ceramic. It has previously been looked into and may become a more popular idea again. The benefits of this would be much better high temperature operation and a longer durable life with these operating temperatures. However the problems with a solid ceramic blade are that ceramics in comparison to metal alloys very brittle. In the event of a foreign object hitting the blades at high speed the blades could easily be destroyed. There are also huge tensile forces due to the centrifugal force being created from the centre point of the turbine which the ceramic would have to withstand. Previously it has been looked into and through a design change the blades could possibly be made out of ceramic however it was thought of as very complicated and impractical at that time. But for those reasons in the future this may become another option. Other similar ideas involve making more of a hybrid blade with the outer part of the blade being ceramic and the inner core being a metal alloy, somewhat like the thermal barrier coatings being used today. [3]

A new processing route also shows as being potentially a very promising idea. Solution precursor plasma spray is this new process; it shows durability in testing of between 1.5-2.5 times better life than any current process. Manufacturing cost of this new process is also thought to be similar to other current methods and even lower than certain types such as EB-PVD. [5]

Section 3

Materials

The materials used in aircraft gas turbine rotors need to meet a wide range of property requirements due to their extremely harsh service condition, including high temperature, thermal stress, centrifugal stress, contact stress, high frequency and low frequency cyclic fatigue, static fatigue, creep, oxidation, and corrosion [6].

Apart from these fundamental while complex requisites, the demand of higher thermal efficiency raised even more rigorous requirements for the turbine engine components, since the most essential way to increase engine efficiency is to raise turbine inlet temperature (TIT) due to the Cornet equation [6][7][8][9]. Consequently, an increase of operational temperature in hot section of the engine including turbine rotors is desirable and inevitable [7][8][9].

To fulfil these long existing demands, materials used in aircraft turbine rotor were updated rapidly since its first practical application in the mid 20th century [9]. From wrought alloys to cast alloys with cooling channel; from equiaxed cast to single crystal cast with thermal barrier coating, each of these key modification mentioned above raised the operating temperature for about 50oC, which then induced a similar amount of increase in TIT and consequently the engine efficiency [6].

The following part of this section will focus on the current materials as well as some other potential new candidates for turbine blade production.

Current materials

As is shown in Figure 1, the current materials used in turbine rotor production is a state of the art four layers system, consisting of the superalloy substrate, the bond coat, the thermally grown oxides (TGO) and the top ceramic coat [6].

A high durability is obtained from this four-layer system by the lowered thermal stress and the toughened ceramic layer. The gradual shift of thermal expansion coefficient as well as thermal conductivity in this multilayer system greatly reduces the thermal stress, a detrimental weak point of any laminate system, between the superalloy substrate and the ceramic thermal barrier coat (TBC) [9]. In addition, the deliberately made microcracks on the top coat provide enough spaces for the expansion of the relatively brittle ceramic layer, which increases the strain tolerance and inhibits the brittle fracture induced by the thermal expansion during operation. Therefore, the durability of this materials system is high enough to withstand thousands of take-off/landing cycles and up to 30000 hours of operation [6].

A high operational temperature is also derived from this materials system. As a key thermal barrier layer, the top coat of this multilayer system is typically produced using yttia stabilized zirconia (YSZ). Stabilized by the 7-8 wt% yttia, the most desirable tetragonal phase zirconia provides the ideal properties for TBC applications, including extremely low thermal conductivity, high thermal-expansion coefficient, low density, good hardness, high ambient and hot corrosion resistance[8][9]. Together with bond coat and TGO, this TBC system could effectively reduce the superalloy surface temperature by 100 oC to 300oC and enable the operating temperature to surpass the melting temperature of its superalloy substrates [6][9][10].

The properties of both prolonged durability and higher operating temperature without fundamental change of the matrix superalloys make YSZ TBC turbine blade the current best choice for aircraft turbine engines.

Figure the multilayer structure of the thermal barrier coating [4]

New candidates

Due to the potential significant increase of thermal efficiency, the ceramic materials which possess excellent strength and durability at high temperature were reckoned as possible materials for aircraft turbine engines.

Silicon carbide/ silicon nitride monolithic ceramics

Roode and Bose argued that, one type of the ceramic materials designated for the application in aircraft turbine engines was the silicon based monolithic ceramics [7][10]. Compared with the traditional and current materials, both silicon nitride and silicon carbide possess superior strength and durability at high temperature, which shows the potential of increasing the TIT, reducing the air-cooling system and raising the efficiency of the engine by using this type of materials [10]. However, the poor impact resistance of the monolithic ceramics (KIc= 5-9 MPa m0.5) compared with the current superalloys (KIc= 35-65 MPa m0.5) is the major problem for their application on the turbine rotor [10]. Besides, the environmental degradation resistance, especially to the water vapour, of the monolithic ceramics is also relatively low compared with the conventional superalloys [7].

Ceramic matrix composites

According to Roode, Bose and Richerson, another type of the ceramic candidates is the SiC/SiC ceramic matrix composites (CMC), which have a threefold impact toughness (KIc= ~20 MPa m0.5) than those of silicon based monolithic materials [6][7][8]. This type of material has already been used in some of the stationary turbine engine parts including nozzle, shrouds and liners [5]. However, the strength of this CMC is slightly insufficient for the rotating components, which makes it difficult to be applied on the turbine rotor. The environmental degradation problem of the CMC similar to the monolithic ceramics has been successfully overcome recently via the environmental barrier coatings (EBC) [7][10].

Compared with the current TBC superalloy turbine blades, ceramic substitutes possess several advantages including higher thermal resistance, lower weight and simpler structure without internal cooling vents. Although several disadvantages hinders the application of the ceramics, they are still of great potential as new candidates for the aircraft turbine rotor productions.

Section 5 - current manufacturing

Rolls Royce manufacture of turbine blades

Engine materials are chosen for their ability to withstand the environment they are required to operate in. In this circumstance strength at temperature and corrosion resistance are major properties. These sorts of materials cause a manufacturing challenge. [12]

HP turbine components work at high gas temperature in relation to the melting point and will need to be designed and made to perform and survive these conditions. Turbine blades have complex internal cooling systems as well as ceramic and intermetallic coatings for heat resistance. [12]

Turbine blades will either have single crystal or directionally solidified structures to maximise their strength. [12]

Casting is the only way to manufacture these structures.

Casting

In engine manufacture two types of casting are use, structural casting and 'hot end' components. Both use investment casting technology where a highly accurate, hollow, ceramic mould or shell is created and filled with molten metal to create a part. [12]

To extend the life span of turbine blades they are cast so that there are no grain boundaries, this is formed by making the blade out of single crystal. Another way is to have the crystals orientated in a certain manner; this is formed by directional solidifying. The moulds used for these two methods differ from usual casting as the mould is open at both ends. [12]

Metal is entered into the mould cavities via a ceramic filter. [12]

Machining

To achieve precision fits some form of machining then has to be undertaken on all components. High speed, multi axis, computer controlled machine tools, using ceramic and intermetallic cutter materials and high pressure coolants, has resulted in chip machining being used. Chip machining is used to remove metal around casing bosses and to machine holes. [12]

Surface finish

Surface finish affects the aerodynamics of an aerofoil. Most aerofoils undergo treatments such as barrelling, vibro polishing, shot peening, or vapour blasting, this creates a uniform smooth surface. [12]

Application of the ceramic powder onto the turbine blade

There are many different ways to apply ceramics to turbine. Some of these include oxyfeul gas powder spraying, plasma spraying in air, vacuum plasma spraying, detonation coating and high velocity oxyfuel process. [13]

Plasma spraying in air

A plasma arc spray torch contains a tubular copper anode; in this is a tungsten cathode. These electrodes are water cooled and are surrounded by an insulating body; this holds them in correct relation to each other and serves as an arc chamber. Inside the torch a high current arc is generated, a gas is then injected into the arc chamber, heated, then passed through a constriction in the anode bore, here it is converted to high temperature plasma. Powdered surfacing material is then added to the plasma jet; the surfacing material is then heated and accelerated onto the substrate. [13]

Injecting the spray axially into the centre of the plasma jet is said to be the best way, giving the best results, with high deposition rates and efficiencies. [13]

Robots can be used to hold the torch in position allowing the spray of complex geometries. [13]

Vacuum plasma spraying

Vacuum plasma spraying or low pressure plasma spraying involves spraying in a chamber initially evacuated to a pressure of 10-² mbar; this is then backfilled with argon to 40 mbar. [13]

By excluding oxygen from the process neither the spray particles nor the substrate surface become oxidised during spraying, deposits are dense and well bonded. The plasma jet is longer than in air and the work piece reaches a higher temperature. [13]

Recommendation

Materials

As is discussed above, a choice could be made between the current YSZ TBC superalloys and the cutting edge ceramics in turbine blade production. A brief evaluation between these two types of materials will be conducted in the following paragraphs.

For the monolithic and composite ceramics, the risk is extremely high. The potential increase of TIT and therefore engine efficiency by using ceramic materials is distinctive. However, the disadvantages are also obvious. First, large amount of investment is demanded in research and development section to overcome the several major defects and find out the possible manufacturing routes. Second, a long period is needed before profits can be made from this product. Optimistic estimations were given from different sources that the production of ceramic turbine blade could be realized on aircraft turbine engines in more than 10 years [7][10]. In addition, the academic interest in this area was greatly reduced during the past several years, which demonstrated the difficulties for the technological breakthrough, after some major research programmes funded by government and big companies ending with limited results [8][10]. Therefore, the ceramics are more like materials for the next generation products rather than the current one.

On the contrary, the production of TBC superalloy turbine rotors has much lower ventures. As a mature while still modern technology, the YSZ Zirconia TBC superalloy provides the currently best properties for the turbine rotors. Besides, the processing methods have already been highly developed, which means the production can start without too much difficulties when the equipments are bought and installed. The main problem of the TBC material is the inevitable competition with several world's leading gas turbine suppliers that have already been in this industry for many years. It will be a difficult task to distinguish our product from these competitors.

Although both types of the materials have pros and cons, YSZ TBC superalloy is recommended as the candidate material mainly due to its low risk and shorter rewarding period.

Processing routes

Since YSZ TBC superalloy is chosen as the candidate material, the choice of the processing routes are mainly among the APS, EB-PVD and SPPS.

For the APS processing route, the lowest thermal conductivity can be gained together with the lowest cost during production. In addition, it can be produced in a fast rate. The major disadvantage for this production route is the relatively low durability, especially in high thrust turbine engines [9].

For the EB-PVD method, high thermal and mechanical properties can be obtained, which promises a much better durability compared with APS. The main defects of this method are its very slow production rate together with the high cost. This method is the current major technology for the production of high thrust turbine engine blade TBC [8][9].

For the novel SPPS route, a very high durability which is 2.5 times of APS and 1.5 times of EB-PVD is acquired due to its very fine microstructures and vertical microcracks on the top ceramic coat [11]. In addition, the cost, which is similar to APS, is much lower than that of EB-PVD. This method also possesses a high production rate. The major disadvantage of this method is that there is very limited experience relating to this method. Therefore a higher risk is predicable compared with the other two proven and mature techniques.

Despite the relatively higher risks, the SPPS stands out from other methods due to its better durability, lower cost and higher production rate. By choosing this processing route, our company may have the core competitiveness to distinguish from other competitors.

Conclusion

In conclusion, the TBC superalloy is recommended as the candidate material and the SPPS is recommended as the processing route of the turbine rotors.

Based on all findings it is thought that Rennib Advanced Ceramics should not continue and produce this product. The company would not be able to bring anything new to the current market that would make them stand out from the rest of the competition. A lot of research, time, money and testing has already been done by the biggest market holders in this field and this puts them at a huge advantage. It has been found that thermal barrier coatings are very hard to get exactly correct and to work in the way they are designed. Failures of the coatings do happen especially in companies who destructively test their products, the problem is the variety of reasons that the product can fail. Diffusion, oxidation, phase transformation, elastic deformation, plastic deformation, creep deformation, thermal expansion, thermal conduction, radiation, fracture, fatigue, and sintering [4] are all reasons that can cause failure. For this reason as earlier mentioned it would be very hard for Rennib to be able to catch up and attain as much knowledge as possible; even then the product that will be produced will not be something that can be seen as to stand out from the other market leaders.

If Rennib ever did make this product any change they may want to eventually make to the process would also be very difficult. There are a huge number of patents that have been filed making it very hard to produce a certain variation in the design as well as meaning a lot of research time looking through possible gaps in patents that allow a new design.

The time thermal barrier coatings have been used is around 30+ years now, this has allowed the companies who started off from the beginning to now be very good at what they do. Anything they have done that would be considered as new and innovative will already have a patent.

Finally, the market future growth predictions. The only major positive which can be seen for Rennib is that the market may open itself up. It has been predicted that around 17,300 new passenger and freight planes will be needed between 2004 and 2023. The market is expected to grow by about 3 times. With air freight growing at a predicted 5.9% per annum. This could allow an opening for Rennib however caution must be taken, thew other leading companies are probably aware of this and may accommodate. It must also be remembered that the product Rennib would be making would not be hugely advanced from other companies.[14]

In conclusion however it is recommended for Rennib not to proceed because:

TBC's are very fine arts, other companies have spent years perfecting them the best they can

Any possible market leading designs which could be made in the future have been patented so it will be difficult to create designs that can be seen as better than competitors

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