Advanced aero-engine technology is one of the important symbols of a country’s scientific and technological level, industrial base and comprehensive national strength. In order to improve the performance of aeroengine, higher requirements should be put forward for turbine inlet gas temperature. Generally speaking, the temperature at the front inlet of a first-stage turbine with a thrust-to-weight ratio of 10 is about 1,950 K, and the temperature at the front inlet of a first-stage turbine with a thrust-to-weight ratio of 12 to 15 is more than 2,100 K. When the thrust-to-weight ratio increases to 15 to 20, the inlet temperature will reach more than 2350 K [1]. Therefore, in the process of developing aero-engines with higher thrust-to-weight ratio and higher thermal efficiency, increasing turbine inlet temperature has become a problem that researchers have to solve.
At present, the working temperature of some key engine hot end components, such as turbine blades (including guide blades and working blades) and nickel-based superalloy, commonly used as matrix material, is about 1100 ℃ [2], far lower than the turbine inlet temperature of advanced aero-engines. Therefore, in order to improve the service temperature of engine turbine blades, the following three methods can be adopted: first, to develop more high-temperature resistant matrix materials; The second is to reduce the surface temperature of the matrix by air film cooling technology; Third, the thermal barrier coating with low thermal conductivity is prepared on the substrate surface [3]. These three types of technologies are also known as the three key technologies of advanced aeroengine turbine blades
Gadolinium zirconate (Gd2Zr2O7) is a rare earth zirconate that has applications in thermal barrier coatings, nuclear waste cured substrates, and solid oxide battery electrolytes. In 2004, Vassen et al. [6] first reported the application of rare earth zirconate in thermal barrier coatings. After that, it was found that among many ceramic materials whose thermal conductivity was lower than YSZ, rare earth zirconate had the lowest thermal conductivity [18], while gadolinium zirconate in A2B2O7 rare earth zirconate had the lowest thermal conductivity and the highest thermal expansion coefficient. Due to its excellent thermal insulation performance and high temperature stability, the research reports on gadolinium zirconate in the field of thermal barrier coatings have been continuous in recent years. Driven by people’s strong demand for new thermal barrier coatings, the research on gadolinium zirconate materials is in the ascendant
Ordered and disordered transformation of gadolinium zirconate materials
Gadolinium zirconate at low temperature can be regarded as an ordered defective fluorite structure. With the increase of temperature, the disorder of the pyrochlorite structure increases. After reaching a certain transition temperature, the crystal structure begins to change from order to disorder, and finally forms a disordered defective fluorite structure.
The transition temperature of A2B2O7 type rare earth zirconate is related to the ionic radius ratio between rare earth cation and zirconium ion. With the increase of rare earth cation radius, the transition temperature increases gradually. According to the literature reports, under the standard atmospheric pressure, the condition of rare earth zirconate forming stable pyrochlorite structure is 1.46≤(A3+)/(Zr4+)≤1.78; When (A3+)/(Zr4+)<1.46, defective fluorite structure is formed. Through computer simulation, Rushton et al. predicted the order and disorder transition temperature of various rare earth zirconate materials from the perspective of cluster formation energy. With the increase of the cationic radius of rare earth, the disorder energy and temperature required for the order and disorder transition gradually increased, which was consistent with the above change law.
Thermal properties of gadolinium zirconate materials
Gadolinium zirconate shows good thermal properties at high temperature. In fact, there are some differences in the thermal conductivity and thermal expansion coefficient of gadolinium zirconate in different reports, mainly due to the preparation process, test conditions, and material density. However, gadolinium zirconate has lower thermal conductivity than YSZ and good phase stability at high temperatures no matter what experimental scheme is chosen. This excellent thermal insulation performance depends on the crystal structure of gadolinium zirconate. There is an oxygen vacancy in each molecular unit. The high concentration of oxygen vacancy enhances phonon scattering and reduces the mean free energy of phonons, thus reducing the thermal conductivity of gadolinium zirconate.
Gadolinium zirconate thermal barrier coating
Although gadolinium zirconate material has advantages such as low thermal conductivity, high thermal expansion coefficient, no phase transformation at high temperature, and good corrosion resistance, it also has defects that cannot be ignored, namely, gadolinium zirconate material has lower fracture toughness and lower thermal expansion coefficient than traditional YSZ material [49]. As a result, the single layer of gadolinium zirconate coating has the defects of difficult forming, easy peeling at high temperature and low thermal cycle life. Therefore, it is necessary to improve the fracture toughness and comprehensive properties of gadolinium zirconate coating by doping modification or structural design in practical applications. At the same time, in order to obtain high performance thermal barrier coating, in addition to the material and structure, we can also try to use advanced coating preparation technology.
