Abstract:
Metco 320 is a AlSi-hBN-polyester abradable, used in the high pressure compressor
of commercial gas turbines. The material response to cyclic heating and cooling, and the
resulting changes in microstructure, as well as their associated failure mechanisms were
investigated. It was found that the top surface layer of the abradable liner degrades over
its 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 top
surface.
Furthermore, material cracking and delamination behaviour during service was successfully
reproduced in the laboratory. The cracking and delamination observations made during
overhaul, were replicated using cyclic water-quenching, but the spallation of abradable
material did not occur. Investigations into material properties and their influence
upon the abradable failure mechanics revealed, that soft M320 matched the observations
made during engine overhauls. It could also be established, that the plasma spray process,
grit blasting, surface treatment after deposition and the transient of the substrate
affect the abradable’s performance and life-time, when heat cycled.
Some service casings suffer from premature liner loss. These unscheduled overhauls
are costly and their number is desired to be reduced, if possible eliminated. In order
to control the material failures, the stresses introduced into the abradable seal during
manufacturing need to be reduced, since this is one of main drivers for material cracking
and delamination. Furthermore, it was established, that material at the top end of
the hardness specification performed better in service. This is due to the fact, that more
AlSi metal matrix is present in the microstructure and the hBN loss does not affect the
material integrity as much as in soft material. 2D and 3D modelling showed temperature
and strain profiles evolving during the quenching process. These show the areas of high
strain, 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 is
influenced by several parameters. This research project highlighted, how sensitive the
failure modes are to changes in the material/substrate combination. Recommended is
to increase the material hardness towards the upper end of the current specification (70
HR15Y), reduce the stresses in the substrate and the abradable material by means of
annealing stages after grit blasting, and temperature control during plasma spraying. Furthermore,
it would be beneficial to reduce the machining of the abradable’s surface after
deposition, as well as carrying out further research into the failure modes of abradables.