Selection of spraying method for preparing zirconia (ZrO2) thermal barrier coating

       Due to the high melting point (2760℃) and low thermal conductivity (1.0~2.0W/(m·K)) of ZrO2 ceramic materials, in addition to arc spraying, cold air power spraying, high-speed flame spraying, oxygen-acetylene flame remelting and The intermediate frequency induction remelting technology and the plasma spray welding process cannot be used to prepare the ZrO2 ceramic coating, other thermal spraying processes can be used for the preparation.



However, with the improvement of coating performance requirements, the preparation of thermal barrier coatings is basically dominated by plasma spraying. However, in practical applications, or limited by conditions, or in order to reduce costs and ensure performance, different thermal spraying process methods are often used. Depending on whether a single equipment or multiple equipments are used to prepare the thermal barrier coating, the preparation process of the thermal barrier coating can be divided into a single preparation process and a composite preparation process.



A single preparation process refers to a process in which the bonding bottom layer of the thermal barrier coating and the ZrO2 ceramic surface layer are prepared by the same spraying method, including: atmospheric plasma spraying, low-pressure plasma spraying, vacuum plasma spraying, explosive spraying, high-speed Plasma spraying process, etc.



The composite preparation process refers to the process in which the bonding bottom layer of the thermal barrier coating and the ZrO2 ceramic surface layer are prepared by different spraying methods, including:



1) Vacuum + atmospheric plasma composite spraying process, the bonding bottom layer of thermal barrier coating is prepared by vacuum plasma spraying process, and the ZrO2 ceramic surface layer is prepared by atmospheric plasma spraying process.



2) High-speed flame + atmospheric plasma composite spraying process, the bonding bottom layer of thermal barrier coating adopts high-speed flame spraying, and the ZrO2 surface layer adopts atmospheric plasma spraying.



3) High-speed flame + high-speed plasma composite spraying process, the bonding bottom layer adopts high-speed flame spraying, and the ZrO2 surface layer adopts high-speed plasma spraying, etc.



In order to solve the problems of high porosity and crack-induced oxidation resistance and reduced coating life of plasma sprayed TBC, extensive researches have been carried out at home and abroad for laser preparation of TBC in two different fields: laser surface remelting and laser cladding. Applied exploratory research. There are two methods for preparing TBC by laser method, namely, the laser primary cladding method and the laser secondary cladding method. The preparation of TBC by the laser cladding method belongs to a new field, and there are only research reports in the past ten years, mainly including the preset method and the powder feeding method.



The presetting method is to pre-set the partially stabilized YPSZ and Ni-based composite powder on the substrate, and then use CO2 laser cladding to obtain a composite coating with a layered structure. The surface is a dense ZrO2 ceramic layer, and the bottom is Ni-based. In the alloy transition layer, the upper part of the ZrO2 ceramic layer is equiaxed crystal, and the middle and lower part is columnar crystal, which is mainly composed of t' phase. The powder feeding method is to use a powder feeding device to send the partially stabilized YPSZ and alloy composite powder into the laser irradiation area, and use the laser to clad it on the substrate to obtain an automatically layered ceramic layer area, all of which are columnar crystal structures. And basically consists of t phase. The laser secondary cladding method refers to the process of first spraying the ZrO2 ceramic layer on the surface of the substrate and then performing laser cladding treatment on it. This process can obtain a ceramic with a smooth, continuous, dense surface, without cracks and pores and other defects. The cladding layer avoids the crack problem that cannot be solved by the powder feeding laser cladding process. The structure of the ceramic cladding layer is columnar crystal, and its growth direction is perpendicular to the substrate. The laser secondary cladding method provides a feasible way for the preparation of high-performance and low-cost TBC, but it is still in the preliminary research stage, and the high-temperature performance test is blank. The effect of high temperature performance on the coating life needs to be further studied.



In recent years, the preparation of TBCs with high thermal shock resistance by electron beam physical vapor deposition (EB-PVD) has attracted much attention. The research on EB-PVD thermal barrier coating began in the 1970s, and Pratt & Whitney made a breakthrough in the 1980s. Subsequently, the technology has also been successfully applied in Germany and other countries. The EB-PVD thermal barrier coating is formed by heating and vaporizing the ceramic source with high-energy electron beams to form ceramic vapor, which is then deposited on the substrate in atomic units. There is metallurgical bonding between the body and the matrix, and the stability is very good. In a high temperature environment, the columns can be separated to relieve the thermal stress caused by the difference in the thermal expansion coefficient, thereby greatly improving the thermal fatigue resistance of the coating. The research shows that an Al2O3 region is found between the ZrO2 ceramic and the bonding bottom layer, and the existence of this region is beneficial to improve the oxidation resistance of the thermal barrier coating. In addition, the thermal barrier coating prepared by EB-PVD has a smooth surface and can reproduce the original bottom layer roughness without reprocessing, which can reduce the gas resistance and improve the service life of the coating, and the process parameters are easier to control than plasma spraying, but, The EB-PVD process also has shortcomings such as low thermal insulation capability of the coating, uncontrollable coating thickness, complex surface cleaning, complex and expensive equipment, relatively low deposition rate, and cumbersome process flow, all of which are in great need of research and improvement.



In view of the big difference between TBC prepared by plasma spraying method and TBC prepared by EB-PVD method, some people refer to TBC prepared by plasma spraying method as the first generation TBC, and TBC prepared by EB-PVD method as the first generation TBC. Second generation TBC.