THE WEAR MECHANISMS OCCURRING IN ABRADABLE SEALS OF GAS-TURBINES

Metallographic investigations of worn abradable coatings have been carried out to identify and characterize the main wear mechanisms occurring in turbomachine seals. The following mechanisms have been found: cutting, smearing, adhesive transfer, crushing, melting and tribo-oxidation. Three other mechanisms occur without blade-seal interaction: erosion, corrosion and high-temperature oxidation. Using the data from these investigations as a reference, rub tests have been conducted on a specially designed high-speed-high-temperature rig. The many correlations obtained between test values and wear mechanisms allow the generation of 'wear mechanism maps' for coatings of the AlSi-plastic family. Mechanism orientated model tests are needed to understand the correlations between structural parameters and coating behaviour during blade-coating interactions. Wear mechanism maps will help modelling coating systems to be able to withstand severe service conditions. Avoiding the onset of adhesive or melting wear and simultaneously favouring cutting wear produces better abradable seals which will help improve both the operational safety and the economy of modern turbomachines.