First, the application status of thermal barrier coatings
In order to make the engine get greater thrust to weight ratio, it is necessary to improve the engine turbine inlet temperature, the combustion chamber, gas turbine engine hot end components of the high temperature tolerance requirements increase. In the matrix alloy surface coating of thermal barrier coatings (Thermal Barrier Coating, TBC) is one of the ways to effectively enhance the ability to resist high temperature. Get the current in the turbine engine on the practical application of the thermal barrier coatings are double-layer structures: surface ceramic layer, mainly has the function of thermal insulation, also play a role of corrosion resistance, erosion and erosion; the inner layer is a metal bonding layer, mainly to improve between metal matrix and ceramic layer physical compatibility, enhanced high temperature resistant coating oxidation properties.
Aero engine thermal barrier coating
So far, the thermal barrier coating is the most widely used and most mature in yttrium oxide (mass fraction of 6% ~ 8%) of partially stabilized zirconia (YSZ) ceramic layer as a surface layer, TBCs system MCrAlY alloy layer for bonding layer. YSZ has a low thermal conductivity and a relatively high coefficient of thermal expansion, but it has the following problems in the use of the process:
(1) when the working temperature is higher than 1200 DEG C, along with the sintering time, the number of YSZ porosity and micro cracks gradually reduced, thus increase the coefficient of thermal conductivity, thermal insulation effect is reduced.
(2) in the high temperature environment, between the thermal barrier coating surface layer and the adhesive layer will generate thermally grown oxide with aluminum oxide (TGO), and the metal bonding layer will have a "poor aluminium belt", with the increase of thermal cycles, poor aluminum spinel oxide expanded, rich in Ni, Co formed in the TGO, so that TGO generated great stress, induce crack and causes the ceramic surface layer off.
(3) after entering the combustion chamber, the particulate matter in the air environment or on the air track forms a CMAS (CaO, MgO, Al2O3, SiO2, etc.) of the glass state sediment at high temperature. CMAS attached to the engine blades, the capillary force along the depth direction to YSZ coating pore permeability, then CMAS and YSZ coating reaction of Y2O3 in YSZ, accelerate the transformation, culminating in the interaction with thermochemical thermo mechanical, resulting in YSZ coating cracks.
(4) the differences of YSZ ceramic layer, metal bonding layer, the thermal expansion coefficient of TGO will cause the YSZ ceramic surface layer /TGO interface and TGO/ interface in metal bonding layer from the working temperature (thousand degrees Celsius) down to the misfit strain produced at room temperature in the process of forming thermal mismatch stress that will eventually lead to YSZ face off.
In order to improve the performance of YSZ coating, people have made a lot of exploration and research. Table 1 is a common problem affecting the service life of the YSZ coating and the improvement of the demand and the improvement of the service life.
1, improve the resistance to sintering
(1) to improve the purity of ceramic coating, reduce the content of SiO2 and Al2O3 impurities in YSZ coatings, the coating can significantly reduce the sintering rate, plane shrinkage tendency decreases, thereby reducing the thermal conductivity of the rate of increase, the coating showed some anti sintering.
(2) adding special chemical elements in the coating. For example, the Hf, Nd, Gd, Sm and other elements in the Lanthanum Series zirconium acid system (La2Zr2O7) coating can effectively improve the sintering resistance of the coating.
2, control the growth of TGO
Aero engine in high temperature service process, adhesive layer Al, Cr, Ni and other metal elements in contact with oxygen selective oxidation, the adhesive layer (BC) and the top ceramic layer (TC) formed on the surface of a layer of thermal grown oxide (TGO), causing the local expansion and tension on the coating of TC, when combined with the tension force more than TC will cause the crack, until the surface coating spalling.
(1) changes in the chemical composition of the adhesive layer. Some active elements (such as Y, Hf, Zr), which are suitably doped, reduce the growth rate of Al2O3 and inhibit the growth of TGO under the segregation of these elements;
(2) by cold spraying (CS), supersonic flame spraying (HVOF) process or pre depositing a layer of Al rich PVD thin layer, improve the coating structure, reduce the oxygen diffusion coefficient, thus slowing the growth rate of TGO.
3, improve the corrosion resistance of CMAS
The loss of the formation of CMAS engine blade will not only cause the phase change caused by ZrO2 yttrium melting unstable phase, coating stress increased CMAS deposition will accelerate the coating, erosion, thermal barrier coating service life greatly reduced. The study found that the coating can improve the corrosion resistance of CMAS coating from the following aspects:
(1) to change the chemical composition of the coating. In YSZ, Al, Ti, Si and other elements can be added to generate an oxygen based apatite phase, which can inhibit the internal corrosion of CMAS to the coating, reduce the infiltration properties of the interfacial layer, and enhance the anti CMAS performance of the coating.
(2) to change the coating structure. Burn into Y2Zr2O7 CMAS pyrochlore structure is much less than the general structure of YSZ. The inner layer of YSZ + rare earth zirconate Pyrochlores layer (Ln2Zr2O7) "and" YSZ +Sm2Zr2O7 "and" YSZ +Gd2Zr2O7 "double layer thermal barrier coatings, the pyrochlore outer layer can reduce CMAS infiltration, so the thermal barrier coating anti CMAS corrosion has been greatly improved.
4, improved YSZ surface layer strain tolerance
Using EB-PVD technology, plasma physical vapor deposition (PS-PVD) technique, suspension plasma spraying (SPS) can be prepared "YSZ ceramic surface layer of columnar structure, through the column longitudinal cracking release ceramic surface layer /TGO interface on the thermal mismatch stress to thermal barrier coating can withstand higher the misfit strain, so as to improve the YSZ ceramic layer strain tolerance extension
1, improve the resistance to sintering
(1) to improve the purity of ceramic coating, reduce the content of SiO2 and Al2O3 impurities in YSZ coatings, the coating can significantly reduce the sintering rate, plane shrinkage tendency decreases, thereby reducing the thermal conductivity of the rate of increase, the coating showed some anti sintering.
(2) adding special chemical elements in the coating. For example, the Hf, Nd, Gd, Sm and other elements in the Lanthanum Series zirconium acid system (La2Zr2O7) coating can effectively improve the sintering resistance of the coating.
2, control the growth of TGO
Aero engine in high temperature service process, adhesive layer Al, Cr, Ni and other metal elements in contact with oxygen selective oxidation, the adhesive layer (BC) and the top ceramic layer (TC) formed on the surface of a layer of thermal grown oxide (TGO), causing the local expansion and tension on the coating of TC, when combined with the tension force more than TC will cause the crack, until the surface coating spalling.
(1) changes in the chemical composition of the adhesive layer. Some active elements (such as Y, Hf, Zr), which are suitably doped, reduce the growth rate of Al2O3 and inhibit the growth of TGO under the segregation of these elements;
(2) by cold spraying (CS), supersonic flame spraying (HVOF) process or pre depositing a layer of Al rich PVD thin layer, improve the coating structure, reduce the oxygen diffusion coefficient, thus slowing the growth rate of TGO.
3, improve the corrosion resistance of CMAS
The loss of the formation of CMAS engine blade will not only cause the phase change caused by ZrO2 yttrium melting unstable phase, coating stress increased CMAS deposition will accelerate the coating, erosion, thermal barrier coating service life greatly reduced. The study found that the coating can improve the corrosion resistance of CMAS coating from the following aspects:
(1) to change the chemical composition of the coating. In YSZ, Al, Ti, Si and other elements can be added to generate an oxygen based apatite phase, which can inhibit the internal corrosion of CMAS to the coating, reduce the infiltration properties of the interfacial layer, and enhance the anti CMAS performance of the coating.
(2) to change the coating structure. Burn into Y2Zr2O7 CMAS pyrochlore structure is much less than the general structure of YSZ. The inner layer of YSZ + rare earth zirconate Pyrochlores layer (Ln2Zr2O7) "and" YSZ +Sm2Zr2O7 "and" YSZ +Gd2Zr2O7 "double layer thermal barrier coatings, the pyrochlore outer layer can reduce CMAS infiltration, so the thermal barrier coating anti CMAS corrosion has been greatly improved.
4, improved YSZ surface layer strain tolerance
Using EB-PVD technology, plasma physical vapor deposition (PS-PVD) technique, suspension plasma spraying (SPS) can be prepared "YSZ ceramic surface layer of columnar structure, through the column longitudinal cracking release ceramic surface layer /TGO interface on the thermal mismatch stress to thermal barrier coating can withstand higher the misfit strain, so as to improve the YSZ ceramic layer strain tolerance extension
3, improve the corrosion resistance of CMAS
The loss of the formation of CMAS engine blade will not only cause the phase change caused by ZrO2 yttrium melting unstable phase, coating stress increased CMAS deposition will accelerate the coating, erosion, thermal barrier coating service life greatly reduced. The study found that the coating can improve the corrosion resistance of CMAS coating from the following aspects:
(1) to change the chemical composition of the coating. In YSZ, Al, Ti, Si and other elements can be added to generate an oxygen based apatite phase, which can inhibit the internal corrosion of CMAS to the coating, reduce the infiltration properties of the interfacial layer, and enhance the anti CMAS performance of the coating.
(2) to change the coating structure. Burn into Y2Zr2O7 CMAS pyrochlore structure is much less than the general structure of YSZ. The inner layer of YSZ + rare earth zirconate Pyrochlores layer (Ln2Zr2O7) "and" YSZ +Sm2Zr2O7 "and" YSZ +Gd2Zr2O7 "double layer thermal barrier coatings, the pyrochlore outer layer can reduce CMAS infiltration, so the thermal barrier coating anti CMAS corrosion has been greatly improved.
4, improve the YSZ surface layer strain tolerance
Using EB-PVD technology, plasma physical vapor deposition (PS-PVD) technique, suspension plasma spraying (SPS) can be prepared "YSZ ceramic surface layer of columnar structure, through the column longitudinal cracking release ceramic surface layer /TGO interface on the thermal mismatch stress to thermal barrier coating can withstand higher the misfit strain, so as to improve the YSZ ceramic layer strain tolerance, prolong the coating thermal cycle life. This method has the advantages of simple process, low cost, but the longitudinal crack is generated by surface centralized heating mode, the density is not high and the form can not be controlled, so the cycle life of the coating is not stable, so that the coating application has been limited.
Sol gel Florence by Professor Philippe and Professor Toulouse Albi of the French University College (Sol-Gel) method in YSZ layer deposited on the bottom of the metal bonding, the occurrence of cracks by high temperature heat treatment, and then through the APS technique for filling cracks, strengthen the original structure, form a mesh partition structure of YSZ ceramic layer. The research results show that the vertical partition structure with thermal barrier coating can reach the thermal cycle life is similar to that of EB-PVD thermal barrier coatings.
Second, the new development direction of thermal barrier coatings
The search for new materials to meet the higher engine outlet temperature is an important direction of the development and preparation of thermal barrier coatings. In order to obtain the ideal thermal barrier coatings, must have more low conductive top ceramic layer rate, developed a new type of low thermal conductivity thermal barrier coatings and ceramic matrix composite thermal barrier coatings has become a research hotspot.
1, a new low thermal conductivity thermal barrier coatings
To increase the thermal resistance of thermal barrier coatings from reducing the thermal conductivity of the heat transfer in thermal barrier coatings are mainly electronic conduction, thermal radiation and phonon conduction in three ways, thus reducing the phonon mean free path, phonon velocity or density of the material, can effectively reduce the heat transfer of the phonon. The inherent defects of the material itself and the doping can significantly improve the scattering of the external defects, thereby reducing the thermal conductivity of the material.
(1) to change the chemical composition of the coating
The use of Y2O3 doped ZrO2, various lanthanide elements (including La, Gd, Er, Nd, Dy and Yb) alone or Co doped ZrO2, transition metal elements (such as Ni, Nb and Ta) doped and Hf doped ZrO2, induced by lattice strain, vibration scattering and phonon anharmonic lattice increases, thereby reducing the thermal conductivity of materials.
(2) improved coating crystal structure
Pyrochlore structure system (A2B2X7) by virtue of its low thermal conductivity rate in recent years has become a hot research model of thermal barrier coatings. For example, rare earth zirconate Ln2Zr2O7 (Ln = rare earth) pyrochlore structure ceramic coatings (Gd2Zr2O7, Sm2Zr2O7) compared to the YSZ, has better thermal properties and good mechanical properties. When the structure is similar to the doping of Lanthanum Series and zirconium salts, the solid solution can be formed, and the thermal conductivity of the material can also be reduced. And the thermal conductivity of La2Hf2O7 is lower than that of La2Zr2O7. In addition, increasing the porosity of the coating also helps to reduce the thermal conductivity of the coating.
2, ceramic matrix composite coatings
High temperature metallic materials (such as nickel, cobalt or iron based alloy) was used for the production of turbine engine blades, but they are usually served in more than the melting point temperature, so the heat stability has been a great challenge. The ceramic matrix composites (CMC) can withstand 1250 DEG C high temperature, a new CMC composite is thus able to withstand higher temperature has become the main candidate for next generation aero engine high temperature parts, thereby improving the service temperature of engine hot end components, protect the engine power and conversion efficiency.
The CMC material is different from the metal material, the hot end member in the work does not need to carry on the air cooling, and also can improve the durability of the parts, so as to greatly improve the engine's thrust and work rate. In 2013, the Rolls Royce SiC/SiC composite manufacturing engine high pressure turbine blade test results showed that the leaf quality can be reduced by about 50%. However, the oxidation of CMC at high temperature limits its application in aircraft. Based on SiC- fiber /SiC- matrix CMC material as an example, in the high temperature oxidation under the condition of high pressure turbine, will form a layer of protective layer of SiO2 CMC to prevent further oxidation, but also SiO2 layer and water vapor reaction of hydroxide, which resulted in the erosion of SiC matrix in CMC. The preparation of a layer of environmental thermal barrier coating on the substrate of CMC (EBC) is the key to solve this problem.
Schematic diagram of ER7 multilayer coating structure
EBC is usually composed of three parts, such as the bonding layer, the transition layer and the top layer. The adhesive layer is composed of Si elements, the main role is to ensure the good combination of EBC and CMC matrix; transition layer consists of barium strontium aluminum silicate (BSAS) and mullite mixture, mainly from the high temperature oxidation resistance and inhibition and water vapor reaction; top layer composed of BSAS, mainly to the high temperature corrosion resistance and resistance foreign object impact.
