Formation Dynamics of Ni-Graphite Abradable Coatings

In turbomachinery, clearances are left beyond the free ends of the blades to compensate for thermal expansions, vibrations, among others. To increase the equipment efficiency and reduce fuel consumption, a tight sealing can be achieved by rubbing the rotating component against a softer stationary abradable counterpart. To meet the delicate compromise between sufficient abradability and sufficient mechanical resistance (to withstand the harsh operational conditions), abradable coatings are typically composed of composite materials such as AlSi-Polyester and Ni-Graphite. In this study, a fundamental understanding on the formation dynamics of composite abradable coatings is pursued. Detailed study of Ni-graphite particle transformations in flight, splat quantification, and microstructure phase quantification was performed for different air plasma spray processing parameters. Microstructure results were correlated with elastic modulus and residual stresses. In flight particle analysis revealed that the Ni-cladded graphite particles are susceptible to shearing and separation in flight, being the Ni attached to the substrate, but not the graphite. This effect increased with the processing energy, resulting in a proportional increase in the fraction of Ni in the coating. Coatings processed with intermediate particle velocities had higher elastic modulus and more compressive residual stresses indicating a better balance between dwell time and processing energy.