Seal coating, also called abradable coating, was mainly applied in fan, compressor and turbine of aircraft engine to protect rotating component and meanwhile reduce the clearance and improve the efficiency. Yet due to the harsh working conditions such as super high sliding speed and environment temperature (more than 100 m/s and 1000°C) and intermittent contact form, the tribological behavior between seal coating and counterpart blade have not been symmetrically studied and the wear mechanism was not fully understood. The selection and design of the seal coating were lack of the guidance of the theory and preparation of the coating was mainly done by imitation of foreign brand. Because of the reasons mentioned above, the seal coating damaged the blade or adhered to the blade, which could bring disastrous consequences to the engine during service.
Thermal Control Elements for Spacecraft and Materials Tribology Research Group in Specialized Materials and Devices Division, Institute of Metal Research, Chinese Academy of Sciences, is the pioneer in the experimental equipment development and tribology study on the seal coating and the counterpart blade. The first high-speed-and-high-temperature rubbing tester for simulating the working condition of seal coating and blade was built by this group in China. So far, the tester has been upgraded twice and lots of research works have been completed.
Recently the adhesion mechanism of Al-hBN coating to TC4 blade was investigated by Prof. DUAN Deli, Dr. GAO Siyang, Dr. XUE Weihai, et al. The adhesion was found to be closely related to working condition. The combination of low incursion per strike and high rubbing speed resulted in the most serious adhesion. By comparing the adhesive layer on the blade tip with the original coating, the fierce friction-induced heat under high sliding speed was found to be the key factor that made the metal phase Al melted and transferred to the blade tip during blade rubbing process.
Tribological behavior of CuAl-NiCg coating and GH4169 blade was further studied and shear lip phenomenon occurred in real service condition of the aircraft engine have been reproduced. The volume of shear lip was the maximum and the blade wear degree was the highest under the condition with high sliding speed and low incursion rate. The mechanism model on forming shear lip was established by characterization analysis on metallographic structure evolution, considering the friction heat conduction and distribution between the coating and the blade.
Based on the summary of wear mechanisms of different sealing couples, the effect of the friction heat on the tribological behaviors of the seal coatings and blades was identified particularly. Therefore, the one-dimensional heat conduction model was established to discover the important influence of thermal properties of sealing couples on the temperature raising rate of contact area, which brought about different tribological behavior of the coating adhering to the blade or damaging the blade. Those findings were published in Wear recently.
In recent years, the work on seal coatings have been funded by National Natural Science Foundation of China, APTD and Innovation Funds from Aviation Industry Corporation of China, National Key Technologies Program of China, and Projects on equipment development, repairing and purchasing from Chinese Academic of Sciences.
Schematic of the formation mechanism of the shear lip and the transfer mix layer (Image by IMR)
