Metco 320 is a AlSi-hBN-polyester abradable, used in the high pressure compressorof commercial gas turbines. The material response to cyclic heating and cooling, and theresulting changes in microstructure, as well as their associated failure mechanisms wereinvestigated. It was found that the top surface layer of the abradable liner degrades overits lifetime. During thermal cycling hBN is removed from the material’s microstructure,which results in the degradation of the abradable and increased brittleness of the topsurface.Furthermore, material cracking and delamination behaviour during service was successfullyreproduced in the laboratory. The cracking and delamination observations made duringoverhaul, were replicated using cyclic water-quenching, but the spallation of abradablematerial did not occur. Investigations into material properties and their influenceupon the abradable failure mechanics revealed, that soft M320 matched the observationsmade during engine overhauls. It could also be established, that the plasma spray process,grit blasting, surface treatment after deposition and the transient of the substrateaffect the abradable’s performance and life-time, when heat cycled.Some service casings suffer from premature liner loss. These unscheduled overhaulsare costly and their number is desired to be reduced, if possible eliminated. In orderto control the material failures, the stresses introduced into the abradable seal duringmanufacturing need to be reduced, since this is one of main drivers for material crackingand delamination. Furthermore, it was established, that material at the top end ofthe hardness specification performed better in service. This is due to the fact, that moreAlSi metal matrix is present in the microstructure and the hBN loss does not affect thematerial integrity as much as in soft material. 2D and 3D modelling showed temperatureand strain profiles evolving during the quenching process. These show the areas of highstrain, which are consistent with the crack initiation areas observed during testing.It can be concluded, that M320 abradable is a very complex material system, which isinfluenced by several parameters. This research project highlighted, how sensitive thefailure modes are to changes in the material/substrate combination. Recommended isto increase the material hardness towards the upper end of the current specification (70HR15Y), reduce the stresses in the substrate and the abradable material by means ofannealing stages after grit blasting, and temperature control during plasma spraying. Furthermore,it would be beneficial to reduce the machining of the abradable’s surface afterdeposition, as well as carrying out further research into the failure modes of abradables.
展开▼
