Study on the design, preparation and properties of abradable coating

Yueguang Yu

Beijing General Research Institute of Mining &Metallurgy,
Building 23, Zone 18, Headquarters Base, 188 South Fourth Ring West Road, Beijing

The abradable seal coating is one of the most important coatings in contemporary turbines and also the coating with the largest usage amount. Its main function is to control the clearance between the rotor and the stator, which is of great significance for improving the operation efficiency, extending the service life, and enhancing the safety. Research shows that for every 0.01-inch reduction in the clearance between the rotor and the stator achieved through the abradable coating, the operation efficiency can be increased by approximately 1%, and the fuel consumption can be reduced by 1.5%.
However, this coating has a complex composition and special service requirements. The prepared coatings often suffer from serious problems such as peeling off, chipping, or blade wear during use, leading to failure. In this study, a high-temperature and high-speed abradable testing machine developed was used to conduct research on the abradability and service mechanism of the coating, and two types of abrasion mechanisms, namely weak connection peeling and plastic deformation, were clarified.
Based on the abrasion mechanism, a refined design and preparation of the particle structure of the composite powder materials used for coating preparation were carried out. Composite powder materials such as NiCrAl/NiCg with thin-shell nickel-coated graphite as the abradable component were prepared. In response to the problem of easy burning loss of the composite powder during the thermal spraying process, through the study of the burning loss law, the stable control of the abradable components of the coating was achieved, and thus, uniformly and stably structured coatings such as AlSi/AHB coating, CuAl/NiCg coating, and NiCrAl/NiCg coating were prepared. The coatings have a moderate hardness (with an HR15Y value ranging from 58.4 to 62.5), high bonding strength (≥7MPa), and excellent abradability and thermal shock resistance.
Through a full-chain research including mechanism research, material design, material preparation, coating preparation, and application, a series of various high-performance abradable coatings were developed, with the service temperature ranging from low to high, including AlSi/AHB, CuAl/NiCg, NiCrAl/NiCg, NiCrFeAl/BN, MCrAlY/AHB, and porous YSZ, etc. The series of abradable coatings have been widely applied in gas turbines and steam turbines.