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Our company, Rennib Advanced Ceramics Ltd, has been dedicated to exploring advanced ceramic material. Among all those materials we have researched, thermal barrier coatings (TBCs) are the one of the most promising. Generally speaking, TBCs are kinds of ceramic coatings, which can enhance the high temperature performances for some components. Thermal barrier coatings, which are made from a wide range of materials, can be fabricated via different methods. Optimum materials and processing routes have been introduced in this report. In addition, according to the state of the current market, a reasonable assessment has been promoted in the final section.
Introduction of Thermal Barrier Coatings
What are thermal barrier coatings?
In recent years, researchers have become increasingly interested in thermal barrier coatings (TBCs) on turbine parts because the traditional gas turbine came up with many problems on its service temperature. TBCs usually refer to a ceramic coating, which is deposited on metal substrate, such as transition pieces, combustion lines, first-stage blade and vanes and other hot-path components of gas turbines .
Modern TBCs are evaluated by heat transfer through the coating, as well as protection against oxidation and corrosion. However, no single coating materials seem to meet the demands of multifunctional requirements. Consequently, a coating system has been invested . Generally speaking, two coatings are involved in this thermal barrier coatings system: the metallic bond coating and ceramic topcoating. The metallic bond coating has a good protective effect from oxidation and corrosion and increase the adhesive between the ceramic top coating and substrate. The ceramic topcoating can help the system decrease the temperature by its low thermal conductivity, thus employing an internal cooling [1, 3]. Figure 1. shows the typical structure of TBCs.
Figure1. Structure of TBCs
The concept of thermal barrier coatings was first put forward in the 1950s, when NASA Lewis Research Centre intended to improve the heat stability and wear resistance of gas turbine blade and rocket engines . In the 1960s, flame sprayed ceramic layers with Ni-Al bond coatings were used in commercial aerospace engines for the first time. In the early 80s, this exploration achieved a major breakthrough on the selection of coating material and process methods, which have laid a solid foundation for the application. The advanced ceramic coatings can reduce the temperature of the parts about 170K. During last 30 years, yttria stabilized zirconia (YSZ) in its metastable tetragonal-prime structure was regarded as a standard for advanced ceramic topcoating  . In the future, considerable efforts are being invested, therefore, in identifying new material with even better performance than the current YSZ.
Application for TBCs
Thermal barrier coatings are widely applied in heated components of aircraft, marine and land use gas turbine, civilian internal combustion engine, turbo, metallurgical industry and other devices with spray lances. It has broad prospects for application developments in recent years .
At first, TBCs was only used in the rocket engine of aircrafts, which put forward the development of aerospace by simply improving the durability under the high temperatures (Fig. 2). Later on, it has been popularized to civil use (Fig. 3). It was reported that almost all U.S. gas turbines are used by the TBCs, which is consumed 300t each year in the TBCs of zirconia materials and will reach 12% annual growth rate in the next decades, especially 25% growth rate in engine parts field. Moreover, since a great increase on inlet temperature, combustion chamber gas temperature and pressure, more and more TBCs will use in different field, which will stimulate the development of the TBCs and get us a good profit potential [1-3].
Figure 2. Aircraft that utilise the advances made in gas turbine technology. (Source: NASA Glenn research centre )
Figure 3. Thermal barrier coatings for gas-turbine engine applications. (Padture et al., Science, 2002
People have realized that the TBCs can not only increase the corrosion resistance under high temperature, but also decrease the fuel consumption, improve energy efficiency, as well as extend life span of the components or increase the firing temperature of the turbine, thereby improving its operation efficiency. That also means our project can get surpports from the government which may help us minimise the cost on manufacuture and product publicity.
The current state for thermal barrier coatings
The size of the current market and its projected future growth
As it is mentioned above, thermal barrier coatings (TBCs), which have certain favourable abilities of good chemical stability at high temperature, anti-corrosion resistance, and good thermal insulation, have been widely used since it was first put forward . They also have a good ability of protecting against oxidation, spalling and other associated heat induced effects. As a result, the main applications for TBCs are some heated components of aircrafts, gas turbines, and diesel generators . It was reported that recently thermal barrier coatings have been used in a majority of gas turbines in the USA, and in the next decade, the production of TBCs will have an increase of 25% in the components of engines . It is obvious that TBCs will have broad prospects for applications and potential value for developments in the future.
There are a large number of companies for manufacturing TBCs for various applications. Thermal barrier coatings are used to reduce the operating temperature of a metallic component which in turn leads to extended working life. In this field, compared with Asian markets, some western countries have more advanced products and widely applications for thermal barrier coatings. TBCs have been one of the most crucial techniques for the design and maintenance of aircraft engines. Some major international applications of TBCs are listed in Table 1 .
Fan blades, Compressor blades, Combustion chamber, Turbine blades
more than 200 parts of engines, blade tip shroud
Combustion chamber, High-pressure turbine blades
Jet, Nozzle, Combustion chamber
Table 1. Major applications of TBCs
With the development of engines, it requires that turbine blades should have the ability of withstanding much higher temperature. Many corporations have made a lot of experiments to improve the characteristics of thermal barrier coatings. For example, the RR and NASA have succeeded in the development of high-temperature strain gauges by using plasma-sprayed thermal barrier coatings which will enhance the mechanical properties of high-temperature parts .
There are also many corporations making contributions to manufacturing varied thermal barrier coatings. For example, CAMCOAT , which is a European agent for tech-line coatings, has been applying and supplying their range of coatings since 1994. They have developed a wide range of TBCs to satisfy the requirements of customers. Several typical products of varied temperature capabilities are illustrated as follows:
Black Satin: layer thermal insulation Grey Ice: good corrosion resistance CBC2: water based cermet coating
coating, using on vehicles for steam temperature capacity is 1000° C, hard, durable surface, preventing
pipes, curing ovens, boilers and flues applied to automotive and marine crown damage
Figure 4. Typical TBCs of CAMCOAT 
For the development of the Rennib Advanced Ceramics (RAC) Ltd, we colleagues should have a good knowledge of the major international competitors of manufacturing thermal barrier coatings, which will bring us more improvements of our products and profits.
An outline of major international competitors:
General Electric Co. (GE)(US): a global infrastructure, finance and media company taking on the world's toughest challenges, offering the widest range of heavy duty gas turbines available
Rolls-Royce Co.(RR)(UK): a global business providing integrated power systems for use on land, at sea and in the air, a wide range of civil aerospace, defence aerospace, marine, energy, nuclear, etc.
Marcote U.K. Ltd : a specialist surface coatings application company offering a range of plasma coatings, thermal spray coatings.
POETON (UK) : world leaders in surface engineering; providing a wide choice of surface coatings; for TBCs, APTICOTE 800 widely used on aircraft engine parts.
Bodycote Metallurgical Coatings Ltd. (UK) : a specialist in the provision of plasma, thermal spraying and metal spray coating services, the TBCs designed to enable components to work within elevated or reduced temperatures.
AVIC CHENGDU ENGINE (Group) CO., LTD. (CN) : focusing on researching heat treatment, welding, chemical treatment, coatings and other special non-traditional machining process.
Current and new material used for TBCs
TBCs materials include , mullite, CaO/MgO+, YSZ, +YSZ, and zircon etc. . They are selected as TBCs materials for several reasons [1, 16]: (1) low thermal conductivity, (2) high melting point, (3) high temperature stability, (4) thermal expansion match with the metallic substrate, (5) good adherence to the metallic substrate, (6) With the extensive application of TBCs, (7) low sintering rate of the porous microstructure, (8) cost efficiently. However no single material listed above can achieve all of these properties. However, up to now, no single material has achieved all the properties listed above. More researches need to be made with material selection method. Dr. Cao wrote a paper in 2002, in which the comparison of different TBCs was first summarised. The table, TBCs material properties, he attached in the paper will help us to select proper material.
According to material selection methods, YSZ is one of a good current material choice. YSZ has a lower thermal conductivity and relative higher thermal expansion coefficient. Thus, YSZ has a good match with metal substrate as metals generally have high thermal expansion coefficient. Figure 5 shows the thermal Conductivity of traditional TBCs . In addition to low thermal conductivity, YSZ also have another benefit as a TBC material. It is one of the few refractory oxides that could be deposited as films using plasma spraying . However, it should have to sinter above 1473K and have phase transition in 1443K. Corrosion and oxygen-transparent can limit its application.
Fig 5. Thermal conductivity of current used material 
Mullite is another chemical component with low thermal conductivity. Besides, it also has low density, high thermal stability even in severe chemical environments. However, it crystalizes at 1023K - 1273K and unfavourable low thermal expansion coefficient.
Alumina is an important ceramic material because of its high corrosion resistance, high hardness and chemical inertness. The addition of Alumina into YSZ coating is supposed to be efficient on hardness without sacrificing the Young's modulus. However, phase change makes it not perfect. Except for, all the other aluminium oxides experience phase change.
The addition of certain can improve the thermal cycling life and limit the phase transition of YSZ. However, precipitation of will increase when the temperature is higher than 1373K.
New materials for TBCs
In seeking potential new materials for TBCs, other refractory materials can be our viable candidates. However, material selection can be very complex, since there are thousands of crystal structures made from numerous compounds. The structures of zirconia can be a clue to search a proper compound. Using "atomistic simulations" and "crystal chemistry", the researchers find fluorite oxides, pyrochlore oxides and nanocrystalline materials . David R. Clarke and Simon R. Phillpot summarized in their paper that materials with low thermal conductivity have certain attributes: high average atomic weight, loose bonding, and highly disordered and distorted structures .
Fluorite oxides include . Reduction of thermal conductivity has been reported for the composition in which co-dipped zirconia and hafnia, as well as replacing with .
Pyrochlore oxides usually have a structure of . Among them is one of the most promising TBCs material with a lower thermal conductivity and higher temperature stability up to 2000 . The disadvantage is a lower thermal expansion coefficient. A double layer based on pyrochlore and YSZ has been reported to improve the overall thermal properties.
Nanocrystalline materials are considered as TBCs materials because of their long-term stability at evaluate temperature . While, they also turn to be coarse rapidly which worsen the situation. The potential of nanocrystalline to be TBCs has not been evaluated scientifically which can be a particularly interesting research.
Heat barrier coatings can be formed from a solid, a liquid, and a gas or plasma state of materials . Various processing routes can be used to fabricate either thin or thick films of coatings. They are shown in appendix II . This section focuses on the synthesis routes and the characteristics about some of these coating processing methods.
Chemical vapour deposition (CVD)
If the growth of a coating can be allowed without the necessity of interactions between substrates and deposits, this category of materials can be obtained by CVD. CVD is dependent on interactions between heated substrate surfaces and gaseous phases. The general operation temperature ranges from 500â„ƒ to 1100â„ƒ and can be conducted by using open loop systems or closed loop systems. However, the open loop system is most widely used.
Interdiffusion is an important criterion to identify the specific type of coatings to be fabricated. If interdiffusin exists, layers that are produced will be separated from the substrate. If not, layers will have a strong interfacial bonding with substrates. Other parameters of processing, such as temperature, pressure, gas flow rate, apparatus' structures, etc., can lead to deposits with various characteristics . Two principles are involved in CVD. One is pyrolysis, which means to decompose some chemical and the other is the reactions between gaseous reactants . Various coatings with complex shapes can be produced by CVD. It also provides high deposition rates and good-quality deposits. However, high temperatures and low pressures involved can be hard to achieve. The heating of substrates also brings a lot of difficulties. Sometimes accurate geometry of reactors needs to be guaranteed in order to produce complex items .
Physical vapour deposition (PVD)
Generally, PVD is performed to deposit a wide range of metals, alloys, compounds, their mixtures or some organic materials onto the substrate in vapour state under vacuum conditions. This process does not involve any chemical reaction. As it takes place in a vacuum chamber, gases involved should be properly chosen such as N2 . The deposits produced by PVD presents as a distinctive part from the substrate.
Evaporation, sputtering and ion plating are three major techniques in PVD. Evaporation has higher rates than sputtering. However, sputtering provides a good control of the composition of alloys and the rates of depositions. Good adhesion can be achieved by the means of ion plating . Compared with CVD, PVD has some advantages over it. The steps of the processing can be controlled by PVD, which cannot be obtained via CVD. Lower temperatures are involved in PVD. The final products produced by PVD do not need further heat treatments . PVD can deposit a wide range of materials. Both thin and thick films can be fabricated. Various techniques of PVD can be chosen to produce deposits. However, the equipment used in PVD is quite expensive which inhibits the usage of PVD .
Pack process can fall into two categories: pack cementation and vacuum pack process.
Those coatings which need an interaction between layers and substrates require pack cementation instead . In this case, chemical vapour reaction happens when the substrate surrounded by powders of the master alloys and additives is placed into a semi-permeable box [19, 20]. During the procedure, the temperature of the container, powder and substrate remains the same and only a slight H2 flow is allowed around the container .
This method usually uses Al and Cr to deposit onto some metal-based alloys. And the substrates should remain clean and out of impurities such as oxides. During pack cementation, reaction occurs firstly within vapour of metals or alloys. Then vapour deposits on the substrate. Finally, deposit begins to grow and interdiffusion takes between substrate and deposit . Thin, brittle and hard coatings with high uniformity and close tolerance can be achieved by this process. Moreover, pack cementation is a very cost-effective process. However, the size of substrate is critical and high- temperature substrates are required.
Vacuum pack process
Vacuum pack only needs metals, alloys or intermetallic powders without applications of inert fillers. The heat transfer during a vacuum pack is much better than other conventional pack processes . However, in general, the heat transfer in pack processes is poor. Defects inherent in the coatings severely affect the performance of the components .
Products fabricated by spraying techniques have compositional flexibility, but are difficult to achieve homogeneity. Five kinds of spraying processes are available nowadays. They are "liquid-metal spraying, wire explosion spraying, flame spraying, detonation spraying and plasma spraying ." Plasma spraying is the most advanced and economical one by using the mixture of coating materials powder and an inert gas in the form of plasma. It leads to the products with high porosity and poor interfacial bonding .
Cladding includes several methods such as "explosive impact and magnetic impact bonding, or hot isostatic pressing or cladding, or mechanical bonding such as extrusion ." Rolling and extrusion are the most economical one. The most common materials used are steels with various compositions and can be employed effectively. Both methods have the limitations of all the normal metallurgies .
Compared with slurry techniques, electro-deposition requires lower temperature. It can be regarded as a derivation of the traditional metal working with aqueous electrolytes with an additional step that powders or fibres are added to the electrolytes. The electrolytes are always nickel based, cobalt based or the mixture of nickel and cobalt based due to high temperature. A wide range of materials can be employed in this case. Compared with PVD, it is simpler to conduct .
Hot dipping is a very traditional way to produce coatings. It can be used under high temperature to produce coatings with various thickness and uniformity onto substrates made from ferrous alloys, zinc, aluminium and some alloys of these metals . The advantages and disadvantages are listed in appendix 2 to help make the optimum way to produce heat barrier coatings.
However, some newly developed methods are also promising such as slurry, so-gel, laser treatment and rapid solidification processes .
Recommended materials and processing route
CVD is favoured among those techniques because it is economical, easy to performance and able to produce relatively high-quality products. Among the two methods, reactions between vapour reactants are preferred as the processing of polymer precursors is expensive.
As extremely high temperatures are the main drawbacks of CVD process, processing temperature needs to be controlled as low as possible. Ceramic coatings can be nitrides, oxides, silicides, borides or carbides. The processing temperatures of these materials are listed below in figure 6. As the temperature is the main concern and materials need to be available, SiO2 is the best choice.
Fig. 6 compound formed by CVD
Reasoned arguments as to whether RAC Ltd. should proceed or not
Intellectual property considerations
Intellectual property refers to a number of distinct types of intellectual innovations of the mind for which property rights are recognized and the corresponding fields of law . Therefore, if our company makes a profit with the aid of others' intellectual property of manufacturing thermal barrier coatings, we will pay a large amount of remuneration. A large number of patents concerning TBCs have been invented. For example, a patent about optimized high temperature thermal barrier was invented by Doesburg J.C., Xie L.D., Schimid R., Gold M. in 2011. United technologies corp. made an invention of thermal barrier coatings with low thermal conductivity in 2010. It was about a TBC comprising from 0.5 to 1.0 mol% of at least one oxide or mixtures. As a result, if we only make efforts to manufacture TBCs instead of the process of inventing, our profits will be sharply decreased. Consequently, we should set up a researching group to make contributions to inventing intellectual creations to cut down the cost.
Means to gain market share
Besides a researching group, some competitive marketing measures, which will affect customer satisfaction, should also be taken into considerations. Several measures can be taken as follows.
Inventing creative innovations and improving techniques of products
Setting a reasonable and acceptable price
Establishing an outstanding group for after-sale service
The expectations, quality and price should affect customer satisfaction, and in turn customer satisfaction should affect profitability . It forms a circle that customer satisfaction will bring more profits which can be dedicated in the research of innovative TBCs. the creations will satisfy more customers who can are more in need of TBCs. Therefore, a group which can cross all-sided will attain most profits and gain the largest market share.
The size and probable future growth of the market
With the development of advanced technology, a large majority of scientists have devoted themselves on the research of aerospace, aircrafts and jet engines. It is definitely true that the requirements of thermal barrier coatings will have a bright future. Therefore, developing a TBC with low conductivity, high thermal expansion coefficient is the first crucial task for scientists, which will accelerate the growth of the market for TBCs. A number of different TBC materials have been investigated in the past. The pyrochlore as well as the defect cluster materials accords with requirements most . "However, further development might reveal certain advantages also of the other materials with respect to thermal cyclic performance and thermal stability ". All in all, developing creative innovations of thermal barrier coatings to satisfy the requirements is the best measure to enlarge the size and enhance the growth of the market.
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