Abstract:
The development of aero-gas turbines is moving towards more efficient engines
with higher pressure ratios and increased Turbine Entry Temperatures. This leads to
increases in overall turbine blades temperatures which has resulted in the widescale
development of turbine blades with film cooling and Thermal Barrier Coatings (TBCs)
which reduce the metal temperature of the blade. The air used for film cooling is
directed around the blade by internal passages within the blade, current engines are
experiencing hot corrosion in areas of these internal passages, even with internal
aluminide coatings. The trend for more efficient engines means that corrosion of the
internal passages will become more common, coupled with the inability to inspect the
internal passages of turbine blades in service, results in a requirement for an improved
coating for the internal passages of turbine blades.
The aim of this study was to develop a coating which provides improved
corrosion and oxidation performance over a standard vapour aluminide on single crystal
CMSX-4 turbine blades material. The coating needs to be compatible with the Rolls-
Royce bond coat and the Rolls-Royce manufacturing strategy.
The study investigated a number of additions which could be used to
improve the performance of an aluminide coating. Silicon was selected as the optimum
addition on the basis of performance and ease of deposition. The work then assessed the
influence of various production parameters on the formation of a silicon-aluminide
coating. It was possible to control the level of silicon deposited in the coating.
Performance testing, using cyclic oxidation and salt recoat hot corrosion
tests, of various silicon aluminides developed in this programme demonstrated at least a
doubling in life compared with vapour aluminide coatings.
