Thermal Sprayed Nanostructured La2Zr2O7 (LZ) /8YSZ ceramic thermal barrier coatings

 Now, aero engine thrust weight ratio and turbine inlet temperature is higher and higher. For example, the turbine inlet temperature of new type of foreign military aircraft engine has reached 1538~1871 degrees, and the design of 15~20 engine thrust weight ratio of turbine inlet temperature will reach 2077 degrees centigrade.

Thermal barrier coating (TBC) is widely used in aircraft engines, turbines and turbine blades, the protection of the high temperature alloy from high temperature oxidation and corrosion, heat insulation, to improve the engine inlet temperature and improve the effect of engine thrust weight ratio. The most common structural system of thermal barrier coating is made of alloy with ceramic layer and a working layer. The backing layer material is usually MCrAlY (M=Ni and/or Co), the working layer is mainly used 8% Y2O3 (mass fraction) stable ZrO2 (8YSZ) based ceramic layer.

However, a large number of literatures show that the 8YSZ coating of single ceramic layer can not be used for a long time under the conditions of high temperature above 1200. So, in order to meet the needs of the future advanced aviation engine for TBC more demanding, all kinds of new materials and new technology on TBC has been developed rapidly.

Double ceramic thermal barrier coatings were prepared by electron beam physical vapor deposition technology for Changchun of Applied Chemistry, Cao et al. The results show that: the double ceramic thermal barrier coatings can greatly improve the thermal cycle life. Ma also used electron beam physical vapor La2Ce2O7/YSZ double layer structure of ceramic thermal barrier coatings prepared by deposition process, effectively overcome the coating interfacial reaction, greatly improves the thermal cycle life of the coating. Guo design of GYbZ/YSZ double ceramic layer thermal barrier coatings, prepared GYbZ/YSZ coating composition similar to the stoichiometric phase by electron beam physical vapor deposition, the coating has good high temperature gas thermal shock resistance.

The literature has reported the thermophysical properties of zirconate ceramic block, seldom mention the thermal shock resistance of zirconate thermal barrier coating and high temperature oxidation resistance. For the first time in this paper by using nano powder granulation technology successfully control nano structure of lanthanum zirconate (La2Zr2O7) and 8YSZ powder feeding. Is prepared by using double ceramic thermal barrier coatings by plasma spraying technique was compared with single layer ceramic coating, exhibit better thermal shock resistance and high temperature oxidation resistance.

1 materials and methods

1.1 spray feed preparation and coating

The coating with GH4169 nickel based high temperature alloy (mass fraction of elements: 0.08%C, 17%Cr, 50%Ni, 1%Co, 2.8%Mo, 0.3%Al, 0.75%Ti, Fe, margin) processing size with 20 mm * 6 mm. Prior to spraying the ceramic layer, the transition layer of NiCoCrAlY alloy coating was first adopted. Parameters for spraying are shown in Table 1.

Table 1 thermal spraying process parameters

The quality of spraying on thermal spraying coatings with powder feeding structure plays a decisive role. For the first time in this paper by using nano powder granulation technology successfully control nano structure of lanthanum zirconate (La2Zr2O7) powder feeding (n-LZ) and nano 8YSZ powder feeding. As a comparison, the purchase of the traditional structure of 8YSZ powder feeding.

1.2 test methods and equipment

The bonding strength of the coating was measured by the method of the dual tension test. According to the HB5258-2000 of China aviation industry standard, the isothermal oxidation tests were carried out at 1000 and 1200 degrees Celsius, and the oxidation resistance of the coating was characterized by the oxidation of the coating samples. The thermal shock test of the coating is carried out with reference to the Japanese industrial standard JIS8666-1990. The microstructure and morphology of powder and coating were observed by S-570 scanning electron microscope produced by Japanese Hitachi company. The phase composition of the coating was analyzed by using D/max 2400 X - ray diffraction. Scanning speed is 5 degrees /min, the scanning range is 10 degrees ~90 degrees, the acceleration voltage is 40 kV, the current is 30 mA.

Organization structure and residual stress of the coating 2 2.1 feeding

Fig. 1 surface topography of 3 kinds of feed, it can be seen that the traditional 8YSZ feed showed polygonal and diamond shaped irregular morphology, and nanostructured 8YSZ powders and LZ feeding mainly spherical morphology, can see the surface of nanostructured LZ feedstock particles having a porous structure, thermal insulation coating of the structure to the following the preparation is more favorable.

                                    Fig. 1 surface topography of thermal spray feedstock

N-LZ powder and 8YSZ powder feeding feed prepared by double ceramic n-LZ/8YSZ nanostructured thermal barrier coating by plasma spraying. For comparative study, also prepared the traditional single micron structure ceramic 8YSZ thermal barrier coatings and nano structure ceramic 8YSZ thermal barrier coating (n-8YSZ coating).

2.2 the residual stress and microstructure of the coating

Figure 2 is a single layer ceramic nano 8YSZ coating and LZ/8YSZ coating residual double ceramic layer stress contrast can be seen, the residual ceramic coating of 8YSZ single radial stress, axial residual stress and shear stress were higher than that of double ceramic coating of LZ/8YSZ. Therefore, the selection of double ceramic layer LZ/8YSZ coating is very suitable. The preparation of 3 kinds of coating, that is, the traditional structure of 8YSZ coating, nano structure of the n-8YSZ coating and nano structure of the dual ceramic n-LZ/8YSZ coating and the substrate bonding strength were 22.3, 28.2 and 27.5 MPa.

      Figure 2 single ceramic coating of 8YSZ ceramic coating of LZ/8YSZ and residual stress comparison

Figure 3 shows the analysis of 3 kinds of feed and the corresponding coating XRD. The traditional structure of 8YSZ coating, the main component of thermal spraying for Y0.15Zr0.85O1.93, m-ZrO2, c-ZrO2. After spraying the main phase coating is divided into Y0.15Zr0.85O1.93, c-ZrO2. But for the n-8YSZ coating with nano structure, the main component of the spray feed into t phase, and the main phase in the coating after spraying for t 'phase, which has lower C and c/a values (a, C lattice constant). The n-LZ/8YSZ coating, spraying n-LZ feeding a single LZ phase, and after spraying, the phase structure of the surface layer n-LZ layer is LZ layer.

                          Figure 3 XRD pattern of thermal spraying coating and corresponding feeding

Cross section morphology of 3 kinds of thermal barrier coatings as shown in Figure 4, it can be seen from the chart, the 3 kinds of coating with layered structure, but the traditional 8YSZ coating structure is relatively more obvious. Compared with 4 (a) (b), we can find that the nanostructured n-8YSZ coating is more dense than the traditional 8YSZ coating. Figure 4 (c) is the two electron like double ceramic n-LZ/8YSZ thermal barrier coatings, figure 4 (d) is a type of double ceramic n-LZ/8YSZ thermal barrier coatings back scattered electrons, by elements of the line scan can be seen in n-LZ/8YSZ thermal barrier coatings with three layers, namely the substrate binding layer +n-8YSZ + n-LZ top. 3 layer in the total thickness of about 300 m, and the other two kinds of ceramic thermal barrier coating is.

                               Figure 4 different cross-section of thermal barrier coatings

High temperature properties of 3 coatings

3.1 heat insulation effect

Insulation temperature numerical test results as shown in Figure 5, the results show that the heat insulation effect of double ceramic coating with nano structure is better than other coatings, compared with the same thickness of single nano structure ceramic n-8YSZ thermal barrier coating, heat insulation effect was increased by approximately 35%, compared with the traditional structure of the same thickness of single micron ceramic 8YSZ thermal barrier coating, thermal insulation effect is improved by more than 70%. And the thermal insulation temperature is high, and the corresponding effective thermal conductivity is also low. Figure 6 shows the effective thermal conductivity of the 3 coatings at different temperatures, it can be found that the LZ/8YSZ coating has the lowest effective thermal conductivity, and the effective thermal conductivity of the conventional 8YSZ coating is the highest.

                                Figure 5 temperature thermal insulation coatings at different temperatures

                                 Fig. 6 effective thermal conductivity of coating at different temperatures

3.2 thermal shock resistance

Thermal shock test was carried out at 1000 C and 1200 C at two temperatures. The macroscopic crack on the surface of the coating is observed when the coating is removed from the water. If the coating surface is relatively coarse or macro crack of 20% of the total area peeling, peeling, cracking phenomenon, as coatings have failed, to terminate the test. Fig. 3 thermal cycling life of the 7 coatings at different temperatures. Can be seen at 1000 degrees and 1200 degrees, double ceramic n-LZ/8YSZ showed superior performance of single ceramic coating of 8YSZ, thermal shock, especially at 1200 degrees, the thermal shock resistance of double ceramic type n-LZ/8YSZ is more than two times of nano structured n-8YSZ coatings, while at the same time, at the same temperature, the thermal cycle life single layer ceramic nano structure is higher than the traditional single layer ceramic thermal structure.

                        The thermal cycle life of Figure 7 coatings at different heat shock temperatures

Figure 8 XRD spectra of 3 kinds of coating thermal shock failure under different temperatures, from figure 8 (a) can be seen, the traditional structure of 8YSZ thermal barrier coatings at 1000 DEG C under the condition of thermal shock, the surface is mainly composed of Zr0.92Y0.08O1.96 and t-ZrO2 phase, and 1200 degrees in the heat shock condition, generation division Y0.15Zr0.85O1.93, c-ZrO2, also contain Cr2Ni3, AlNi3, Al0.9Ni4.22, Al4CrNi15. From figure 8 (b) shows that nanostructured n-8YSZ thermal barrier coating in 1000 degrees of thermal shock conditions, the surface composition is mainly Y0.15Zr0.85O1.93 and c-ZrO2, and 1200 degrees in the heat shock condition, generation phase still is Y0.15Zr0.85O1.96 and c-ZrO2. From figure 8 (c) shows that the double ceramic n-LZ/8YSZ thermal barrier coating at 1000 DEG C under the condition of thermal shock, composed of surface of almost all La2Zr2O7 phase, and 1200 degrees in the heat shock condition, generating phase containing La2Zr2O7 phase, and Zr0.92Y0.08O1.96 and c-ZrO2.

                           Figure 8 coating on the XRD chromatogram of different temperature after failure

Figure 9 shows the fracture morphology of 3 kinds of coatings with different thermal shock temperature after failure, FIG. 9 (a) (d) can be seen, the traditional structure of 8YSZ coating has a layered structure obviously, its failure in 1000 C is mainly layer internal fracture and spalling failure, 1200 degrees in thermal shock conditions is the separation between the interface layer between the failure, and was accompanied by a larger droplet sheet drawing effect. From Figure 9 (b) (E) can be seen that the nanostructured n-8YSZ coating at 1000 DEG C under the condition of thermal shock is mainly due to the internal stress of the coating had a longitudinal crack, and under the condition of 1200 DEG C, is mainly due to the failure of longitudinal cracks along the direction of the rapid expansion of the horizontal spraying and coating interface.

             The fracture morphology of Figure 9 coating at different temperatures after the earthquake

The n-LZ/8YSZ double ceramic coating, thermal shock at 1000 DEG C under the condition of failure is mainly separating large droplet layers between the particles, and 1200 degrees in the heat shock condition, the LZ layer had internal micro cracks, crack propagation resistance along a relatively small place, such as the rapid expansion of interface layer to the LZ layer and 8YSZ layer interface to failure (Fig. 9 (c) (f)).

3.3 high temperature oxidation behavior

Figure 10 shows the function relationship between the 3 kinds of coatings at different oxidation temperature and oxidation weight increase with time, it can be seen from the chart, the oxidation process can be divided into 3 stages, namely fast oxidation stage, stable oxidation stage and failure stage. In the fast oxidation stage, the oxidation weight obviously, can be seen at 1000 DEG C, 3 kinds of coating rapid oxidation phase in 25~50 h, and at 1200 DEG C, 3 kinds of coating rapid oxidation phase in 20~25 H. Under the conditions of the temperature, the n-LZ/8YSZ coating has the lowest oxidation weight gain and the lowest oxidation rate. 1000 degrees of oxidation, the traditional structure of 8YSZ coating after 225 h after a large area of spalling, nanostructured n-8YSZ coating failure occurred only in 300 h, and n-LZ/8YSZ in 400 h after oxidation is no obvious change in weight. In 1200 c oxidation, the traditional structure of 8YSZ coating in 175 h after a large area of spalling, nanostructured n-8YSZ coating failure occurred only in 225 h, and n-LZ/8YSZ in 400 h after oxidation is still not obvious change in weight. Therefore, it can be seen that n-LZ/8YSZ has very excellent high temperature oxidation resistance, oxidation of 1200 degrees, the static high temperature oxidation life of at least 400 h or more.

N-LZ/8YSZ coating has excellent oxidation resistance, mainly because the LZ is not transmitted to oxygen at high temperatures, the oxygen in the air can not be through 8YSZ theory only through the pores and cracks parallel to the direction of the coating by spraying, but the thickness of pore and crack must be used throughout the LZ layer, it's more oxygen easy to pass. But in fact, it is very difficult to have such defects, so the oxidation rate of LZ/8YSZ coating is much lower than that of the single layer 8YSZ coating.
Nanostructured 8YSZ coatings have higher resistance to high temperature oxidation than the micron structure n-8YSZ coatings. The main reason is that the nanostructured n-8YSZ coatings have a much smaller pore, and the oxygen is not easy to pass through. Even in the stable oxidation stage will be due to the rapid oxidation phase coating on the formation of continuous and dense thermally grown oxide (TGO), thereby preventing further by oxygen, the oxidation process in stable oxidation stage.
Thermal protective effect of 3.4 LZ layer on 8YSZ layer
The advantage of dual ceramic coatings is the thermal protection of the top layer La2Zr2O7 on the underlying nanocrystalline 8YSZ. Because La2Zr2O7 has low thermal conductivity, the top layer with a certain thickness to 8YSZ nanocrystalline layer below a layer of heat plays a protective role, thereby inhibiting the growth of nanocrystalline 8YSZ layer below a layer of grain in a certain extent.
Figure 11 (a) (b) is a nanostructured n-8YSZ and n-LZ/8YSZ coatings at 1050 8C TEM morphology after 25 h, it can be seen from the chart, the nanostructured n-8YSZ coating after 1050 calcined at 25 h after the grain has been raised, and the protection of LZ n-LZ/8YSZ coating of 8YSZ grains without obviously growing up. Figure 11 (c) (d) is a nanostructured n-8YSZ and n-LZ/8YSZ coatings at 1200 8C TEM morphology after 45 h, it can be seen from the chart, improve the calcination temperature and prolonging the time after n-8YSZ single coating grain ceramic layer grew more obvious, and n-LZ/8YSZ coating LZ protection in grain layer 8YSZ the coating also grew up, but the grain tend to grow up without n-8YSZ coating single ceramic layer grew significantly. There are many factors that affect the grain growth, the initial grain size, the heating rate, the cooling rate and the over heat will affect the size of the grain size.
                                                

                           The morphology of TEM coating in Figure 11 after calcination

In fact, the actual use of coating in the engine, turbine, turbine blades are usually fully coated type, then coated with a layer of LZ coating on the outside, will be on the inside or the bottom of the nanostructured n-8YSZ coating to thermal protection, control or inhibit the excessive grain growth, prolong the service life of thermal barrier coatings. In addition, if the LZ layer in the actual application process occurred spalling failure can, through the re manufacturing means to repair coating, save resources to prolong life.

4 conclusions

(1) the use of nano structure control technology for the preparation of Plasma Sprayed Nanostructured n-8YSZ powders and n-LZ feedstock, and can be prepared by a new type of Thermal Sprayed Nanostructured Ceramic Coatings of n-LZ/8YSZ type double.

(2) under the same test temperature, the double ceramic n-LZ/8YSZ coating has the best thermal insulation effect and thermal shock resistance, the nano structure n-8YSZ coating layer, the traditional 8YSZ coating is the worst.

(3) at 1000 degrees and 1200 degrees temperature under the condition of double ceramic n-LZ/8YSZ coating with high temperature oxidation resistance of n-LZ is the highest, because of its low thermal conductivity, the top layer with a certain thickness to the thermal protection effect on the n-8YSZ nanocrystal layer.