Influence of hBN on the abradable performance of AlSi-based coatings: Bridging the gap between tribological assessment and abradability testing

Abstract

Abradable coatings have been proven to significantly improve the performance of turbomachinery by serving as a protective barrier between rotating blades and stationary casing. However, there is still a pressing need to develop more efficient abradable materials that can withstand the growing energy requirements and prolong the service life of gas turbines, which continues to pose a significant challenge to the aerospace industry. In the present study, two thermally sprayed aluminum‑silicon-based (AlSi) coatings were investigated, the first containing 40 wt% polyester (referred to as AlSi-Poly), and the second containing 6 wt% hexagonal boron nitride (hBN) and 20 wt% polyester (referred to as AlSi-hBN-Poly) as filler material. The microstructure, HR15Y hardness, and surface roughness were systematically examined in the fabricated coatings. Tribological studies were conducted using ball-on-flat and ball-on-disk testing conditions against titanium counterballs. In addition, a cost-effective abradable test rig has been developed to assess the abradability performance of both coatings under closer simulated aircraft turbine conditions. The microstructural examination of the coatings revealed that the filler materials were uniformly distributed throughout the coating, with the hBN particles embedded within the matrix. The average hardness of the AlSi-hBN-Poly coating was higher than that of the AlSi-Poly coating, which is consistent with the area fraction of filler materials present in each coating. The tribological assessment of the coatings revealed similar frictional coefficients, no apparent wear on the counterballs and higher wear depth and width for AlSi-hBN-Poly coating. This was attributed to significant third-body abrasion and higher transfer during testing in the case of the AlSi-hBN-Poly coating. Similarly, the abradability evaluation showed no blade wear against both coatings and slightly different transfer film formation. However, the AlSi-Poly coating presented a smoother rubbing wear track than the AlSi-hBN-Poly, which showed deeper rubbing grooves. These findings provide valuable insights into the tribological behaviour of these coatings and can aid in the development of more effective wear-resistant coatings.