Abradable seal systems play a crucial role in ensuring the efficient functioning of the compressor and turbine stages in gas turbines. These systems involve a sacrificial abradable lining which is cut by a blade, either bare or abrasive tipped. The interaction between these components during engine break-in defines the blade tip clearance, a critical factor for engine efficiency. Tight tip clearances are essential to prevent increased fuel consumption and reduce component lifespan. In the challenging conditions of the high-pressure turbine, where extreme temperatures prevail, the material selection for abradable seal systems becomes crucial.
Magnesia Alumina Spinel (MAS) ceramic material, known for its stability at high temperatures, is employed as a sacrificial lining in the High Pressure Turbine (HPT) and due to the hardness of ceramics, abrasive tipped blades are required for their cut. The current abrasive solutions, electroplated c-BN tipped blades, have reported durability limitations in service due to the matrix (NiCoCrAlY) weakening at high temperatures and alternative Directed Energy Deposition (DED) c-BN tipped blade variants with higher thermally stable matrices (Inconel 792 and NiAlTa) have been proposed to overcome these limitations.
The absence of a standardized testing method for evaluating blade performance at elevated temperatures has prompted the development of a High-Temperature Incursion Rig (HTR). This rig aims to assess novel Directed Energy Deposition (DED) solutions in comparison to the current electroplated baseline, simulating representative gas turbine conditions. Alongside rig development, new methodologies have been implemented to evaluate the performance and wear of both blade architectures.
Incursion tests conducted at both room and high temperatures have unveiled specific wear mechanisms observed in electroplated and DED c-BN tipped blades during the incursion process. Although these wear mechanisms vary between the two blade architectures, they are united by a common triggering factor: heat accumulation. The primary cause of this heat accumulation is the frictional heat generated from the contact, with blade morphology and incursion rate playing critical roles. Additionally, external heat sources can contribute to the rise in blade temperature, thereby exerting additional influence on the resulting wear mechanisms.
In a direct comparison between TBT429 and DELTA abrasive coatings on MAS abradable linings at high temperature, the electroplated abrasive coating (TBT429) demonstrated superior performance. Although DELTA features a more thermally stable matrix material (Inconel 792), the abrasive coating design where both grits and matrix are in constant contact with the abradable surface results in less effective interaction than the electroplated coating, in which only the grits lead the cutting process. This interaction disadvantage in DELTA coatings diminishes the matrix strength gains achieved by using the high-yield-strength Inconel 792 superalloy, which, despite its benefits, does not fully compensate for the inherent limitations observed above 650 °C.
