Introduction
The TBC system is a typical double layer structure, which comprises a metal bond layer and a ceramic top layer. Adhesive layer is to protect the basic level of oxidation and corrosion and to improve the bonding strength between the ceramic layer and the base layer. Ceramic top layer compared to the metal body has a very low thermal conductivity, through the internal cold ceramic layer can achieve a large temperature difference (a few hundred K). Therefore, it can not only reduce the temperature of the metal substrate to improve the service life of the component, but also can improve the ignition temperature of the turbine engine to improve the working efficiency.
TBC started the industrialization development since 1850s the first military engine enamel coating. In 1860s, the first flame sprayed ceramic coating with a NiAl bond layer was applied to commercial aircraft engines. Over the next few decades, the thermal barrier coating material and spraying technology of continuous development. In 1880s, the rapid development of thermal barrier coatings. During these ten years, the yttrium oxide stabilized zirconia (YSZ) is considered as a special kind of ceramic top layer material because it is established as a standard in the past 30 years.
Two different methods have been established according to the different deposition processes. One is the electron beam physical vapor deposition (EB-PVD), the other is atmospheric plasma spraying (APS). The coating prepared by electron beam physical vapor deposition method is widely used in the high thermal mechanical loading of the aircraft engine. Compared with the electron beam physical vapor deposition method, atmospheric plasma spraying is proud of its extensive operation and economic feasibility, so now more TBC is used in this method. Typical static components, such as burner tanks and blade platforms are sprayed with APS.
In a fixed gas turbine, the blade of the gas turbine is used for spraying the thermal spraying method. Further improvement of gas turbine efficiency depends on the progress of combustion and cooling technology and the combination of higher turbine inlet temperature. This means that the standard material YSZ is bound to be close to its limit due to the sintering and phase transition at high temperatures.
Processed by EB-PVD and APS methods the YSZ contains a metastable T` phase. Long time at high temperature, it can be broken down into high yttrium oxide and low yttrium oxide phase. The latter in the cooling process will be transformed into a single crystalline phase and with a large volume increase, which will lead to the failure of TBC. The recognized upper limit temperature is 1200. In addition, due to the limited phase stability and the loss of the coating resulting in the tolerance of the coating, the high temperature performance of the coating can be reduced.
So, in the last few decades, people have done a lot of work to find better ceramic materials than YSZ. A lot of the literature contains the subject.
01 Pyrochlores
In order to more than 1300 DEG C under the condition of service, with pyrochlore structure TBC materials have better performance and more attractive than YSZ. Especially some thermal zirconate Pyrochlores lower rate makes this material more interesting. Again, they have very good thermal stability, which may be related to the location of the cations in the crystal. Extensive research, pyrochlore is rare earth zirconate (A2Zr2O7), where A can be any La, Gd, Sm, Nd, Eu and Yb in a or their mixture. Some of the hafnium (La2Hf2O7 and Gd2Hf2O7) and Ce (La2Ce2O7 and La2 (Zr0.7Ce0.3) 2O7)) as the base material is also the concern of the TBC material. In fact, cerium based oxides are often a defective fluorite structure, which makes it easier to exchange cations, which also explains why these materials have a high rate of sintering. The other elements in the La2Zr2O7 can increase the resistance of the sintering process.
In Pyrochlores, La2Zr2O7 (LZ) is one of the most promising materials for TBC application. Because it has more excellent features compared to YSZ, it has a good thermal stability, low thermal conductivity of 1.56 K W/m, sintering tendency is also very low. But it also has disadvantages, and its thermal expansion coefficient is low. The thermal expansion coefficient of YSZ is 10 - 11 x 10 - 6 K - 1 LZ, the thermal expansion coefficient is about 9 * 10 - 6 K - 1, the coefficient of mismatch will lead to higher thermal stress and thermal expansion. In this regard, Gd2Zr2O7 has more advantages, its thermal expansion coefficient is 1.1 x 10 - 6K - 1.
Because the substrate and the adhesive layer has a very high coefficient of thermal expansion (about 15 x 10 - 6 K - 1), due to the expansion of stress accumulation makes the crack can easily work in the process of TBC near the location of the bonding layer. This may be the reason why La2Zr2O7 and TBC are used separately as a top layer of ceramic material when Gd2Zr2O7's life is low.
In the double layer system (Figure 1) with a YSZ layer and a layer of material on the top by pyrochlores. The service life of the coating is significantly increased in the thermal cycling test.
In the double layer structure, YSZ which is close to the top of the bonding layer toughness, pyrochlore materials so that the coating has low sintering temperature and high temperature stability. The pyrochlore /YSZ bilayer systems exhibit excellent high-temperature performance than YSZ which is used to improve the thermal performance of gas turbine based on. Figure 2 is the use of NiCoCrAlY as the bonding layer, IN738 as the substrate and the use of different TBC systems to spray the experimental results of the burner. The YSZ system (about 2) of low altitude (about 12%) was significantly reduced at 1350 C under a given cycle (5 min heating, min cooldown). Due to the above reasons, the performance of the single layer TBC system is even worse. In the study by La2Zr2O7 J lich, powder by spray drying performance of double system made better and can improve the high temperature performance of TBC system over 100K. If the test before the test sample for commercial Gd2Zr2O7, the performance of the performance of a slight difference. On the other hand, the research used by different J Lich powder experiments show that it has a good cycle life (see Figure 2). It is obvious that the morphology and composition of the powder have important influence on the properties of the coating.
Although a lot of pyrochlore materials compared to the perovskite structure materials, using the method of thermal spraying is easier to handle, but there are still some problems. One of them is the loss of composition in the process of spraying. Due to the loss of La2Zr2O7 in La2O3, the impurity phase of the unstable ZrO2. This is not conducive to the performance of the coating.
Although the high temperature performance of the double layer system is very promising, it is necessary to improve the performance of the system. Chen proposed a hierarchical YSZ/La2Zr2O7 structure that can improve the cycle life of the furnace slightly. On the other hand, the results of a gradient test of Vassen R. show that the performance of the double layer system is better than that of the hierarchical structure.
These two relative results may be due to a reduction in the average stress in the gradient test, the average stress in the coating at room temperature, and the storage of elastic energy when compared to the isothermal test.
By baking powder (50% La2Zr2O7, 50% YSZ) on the chemical stability of mixed YSZ and La2Zr2O7 double system, no reaction in the calcination temperature below 1250 DEG C, which means that the La2Zr2O7 and YSZ by chemical is suitable to manufacture double TBC system.
On the other hand, the reaction temperature and the increase of pyrochlore aluminum. Therefore, in the process of long-term use, the adhesive layer is formed on the oxide (al, the thermally grown oxide layer (TGO)) and Pyrochlores reaction between is expected to. However, this problem is avoided due to the double layer structure.
Some specific reactions with corrosion may also be beneficial in some cases. Recently, the so-called CMAS (Ca Mg Al Si) attack has attracted much attention. Firstly, this kind of damage mechanism can be observed at high temperature, and the formation of CMAS in is deposited on the TBC. At high surface temperatures they begin to be liquefied and penetrate into the coating. During the cooling process, they solidify and reduce the strain tolerance of the coating. Like some Gd2Zr2O7 Pyrochlores can react to crystallization and Portland, CMAS penetration will stop very early. Therefore, compared to the YSZ, some Pyrochlores can better against CMAS.
02 TBC defect group
In this new TBC materials, zirconium doped rare earth cations to different. This mixed form inclusion aggregation like ZrO2 - Y2O3 - Nd2O3 (Gd2O3, Sm2O3) - Yb2O3 (Sc2O3) system, can reduce the thermal conductivity of about 20 to 40%. For 5.5 Y2O3 mol% - 2.25 Gd2O3 mol% - 2.25 Yb2O3 the thermal conductivity of mol% is decreased from 2.3 to 2.6 W/m/K ZrO2 - 4.5 Y2O3DF mol% to 1.6 - 1.9 W/m/K.. For 8.5 Y2O3 mol% - 0.75 Gd2O3 mol% - 0.75 Yb2O3 the thermal conductivity of mol% is 1.8 and 2.1 W/m/K. In addition, the parameters can improve the thermal stability of the coating. Compared with the traditional YSZ, the thermal defects of cluster TBC rate increase with time significantly reduced (for example: 1315 degrees, the traditional YSZ thermal conductivity is 2.9 * 10 - 7, the defect cluster TBC thermal conductivity 2.7 * 10 - 6 W/m Ks). It is proved that it can improve the thermal stability of the coating. Compared with the conventional YSZ, the thermal cycling performance of the conventional is improved or similar to that of the conventional. The use of zirconium or hafnium oxide. The deficiencies of the model cluster method to make up to 1650 DEG C high temperature resistant ability possible.
For higher doping level, the cubic phase is stable. Compared to the conventional 7 - 8 wt.%, we can observe that the thermal cycling performance is decreased due to the decrease of the toughness of the zirconia stabilized zirconia. Discussion with Pyrochlores similar, can significantly improve the performance of double layer structure. At 1135 degrees, 45 minutes /15 minutes of heating / cooling cycle, the thermal cycle of the coating from 300-400 to 500-800 times.
03 six -aluminum lanthanate
Six aluminum lanthanate magnetoplumbite structure is often used in laser technology, catalyst and magnetic field. Due to its high melting point, high thermal expansion coefficient, low thermal conductivity, excellent sintering resistance and long time up to 1800 DEG C structure stability and other characteristics of this kind of material has its advantages in the application of thermal barrier coatings. Its composition is (La, Nd) MAl11O19, where M can be Mn Zn Cr Sm Mg. Experiments show that the addition of Li to its advantage. One of the most interesting is (La, Nd) MAl11O19, according to its thermo physical properties and the problems that have emerged in the APS have been extensively studied. Due to the rapid quenching from the molten state, the atmospheric plasma sprayed coatings are partially amorphous. According to the initial heat treatment recrystallization occurs at 800 C to 1200 C, which is accompanied by a substantial reduction in the volume of the coating.
A lot of research on the thermal physical properties of six parameter description of LnMgAl11O19 aluminate impurities (Ln=La, Gd, Sm, Yb) for thermal expansion only related to La, but the thermal conductivity can be reduced by a total.
The volume shrinkage due to the crystallization of amorphous part six aluminate. This is related to the gradual phase transition, including the second phase formed in the Al - Mg - La oxide system. The formation of six aluminate phase at 1500 deg. Below 1400 degrees Celsius can be observed in such a typical perovskite like LaAlO3.
Similar results also appeared in the simple LaAl11O18 six aluminate, due to the dynamic effect, no six pure aluminate phase found in 1650 DEG C. We believe that low Young's modulus and high fracture toughness are the reasons for the longer thermal cycling life of these coatings. This is mainly due to the random arrangement of the six lanthanum aluminate films, which is the cause of the equilibrium porosity and reduces the thermal conductivity of the ceramic. This lamellar morphology depends on the origin and composition of the samples. High transverse and vertical ratio of the chip to make it more fracture toughness.
The recrystallization phenomenon is considered the disadvantages of six aluminate coatings by plasma spraying deposition of the main. People have been looking for ways to replace it. Unfortunately, the requirement of temperature and the volatilization of Mg in the electron beam physical vapor deposition during the sol gel or impregnation technique will bring some difficulties to the precipitation process,
However, can the growth of cutting crack network of plasma sprayed coating six aluminate which increases the strain tolerance, coating and produce a thermal shock resistance in TBC application which is advantageous (see Figure 3). This may be due to a stress release mechanism in the interlocking network, which is similar to a mechanism found in the flexible sand and gravel. When it is used as the top layer of a double layer TBC system, it has a very good life in the combustion chamber test of a thermal gradient of up to 1350 degrees centigrade. Recently, the use of the AL and six Al - acid - La - graded composites was proposed to improve the ductility and fracture toughness of TBC.
04 perovskite
The crystal structure of this kind of ABO3 is the positive eight plane structure of crystal angle atom sharing, which is rigid. In solid solution, it can hold a variety of ions, including the mass of the ions. Most of this material at high temperature is very stable, which makes them become a concern in the development of TBC application in material candidates.
A zirconium acid salt
Candidate materials for early TBC applications are BaZrO3. Although its melting temperature is as high as 2600 degrees Celsius, its thermal stability and chemical stability are very poor, resulting in thermal cycling test when the surface temperature of 1200 degrees, the failure of the coating premature. In contrast to SrZrO3, either as a separate ceramic top layer or in a double layer structure in the YSZ layer, when the surface temperature above 1250 degrees Celsius, it has a better performance of the thermal cycle. At about 730 degrees Celsius, it changes from the angle of the slope to the pseudo tetragonal crystal, which we do not want to see the phase transformation. By doping Gd or Yb can inhibit the phase transformation but also can improve the thermal physical properties of the coating at high temperature. On the other hand, CaZrO3 is considered to be the latest material for this class of materials in TBC applications. Although its melting point is lower than YSZ, its thermal conductivity is very low, only 2W/M and K.
B complex form
In addition to the high melting point, another notable feature of the complex perovskite is the order effect of the B cation, which can regulate the properties of the material. In atmospheric plasma spraying, the deposited coating is often shown to be disordered cubic phase, which can be converted to an ordered one when heat treatment is higher than 1250. Similar sequence effects can be observed in O3 (Al1/4Mg1/2Ta1/4) La. This material shows a more promising coating performance because of the presence of a vertical crack network in the plasma sprayed coating. The microstructure of the coating using this material as a double layer structure is shown in figure 4.
Although the overall characteristics of the perovskite is very good, but its toughness is not as good as YSZ. In the process of atmospheric plasma spraying, due to the different oxide vapor pressure lead to the composition of zirconium acid salts of non - zirconium oxide composition and the complexity of the perovskite in the first evaporation of Magnesium Oxide. This effect will lead to the deposition of the non stoichiometric phase which is not conducive to the coating performance. Recent studies have indicated that by optimizing the parameters of plasma spraying, the time of particle in the plasma plume can be reduced to minimize the effect of plasma spraying parameters.
However, plasma spraying BaLa2Ti3O10 did not observe the phenomenon of volatile components. This kind of perovskite is advantageous to reduce the thermal conductivity, because there is a weak bonding surface between the different planes of the layered oxide. In 1200 C. its thermal cycling performance of 7YSZ is better than in the past, this is because the segmentation of crack in coating on atmospheric plasma spraying, does not exist because the deposition of non stoichiometric impurity phases and the disadvantages caused by.
However, further experiments are required to demonstrate whether such materials with a rather low melting point are suitable for high temperature applications of more than 1200.
