Engineering - Royal Australian Navy

Naval Engineering Bulletin • June 2001
• Platinum-aluminide coatings - enhanced variant
of aluminide coating; inter-diffusion of Pt and Al creates
microstructural features that improve hot-corrosion
resistive properties of coating.
• Overlay coatings - superior ductility; wider range
of chemical compositions (can be tailored to produce
different protective oxide scales depending on service
environment).
Overlay coatings, despite their very high cost, are becoming
more widespread in their application to the most critically
stressed of hot-section components (i.e. HPT blades).
The most common form is MCrAlY (where M is usually cobalt,
nickel, or iron) - the chromium and aluminium provide
corrosion resistance, while yttrium improves oxide
scale adhesion.
In recognition of potential corrosion problems in the marine
application, GE uses two such overlay coatings (BC21
and BC23) on the LM2500 engine HPT blades, applied using
a plasma-coating process.
Coating technology is under continuous development, and
the focus of recent research has been on:
• quality control and non-destructive evaluation of
coated components post-production
• investigation of new coating application processes
(laser glazing, ion implantation)
• development of new coating materials (including ceramics).
Condition Monitoring and
Assessment
In the in-service environment, it is important to be able to
non-destructively assess the condition of hot-section components,
to provide a measure of remaining service life and
to take corrective action to prevent the progress of hot corrosion.
The costs associated with catastrophic internal failure
of gas turbine components are considerably high.
Engine Operating Parameter Monitoring
The most critical operating parameter for hot-corrosion is
the HPT inlet temperature. GE research has shown that
slight increases in HPT inlet temperature in the upper operating
range can have a dramatic effect on hot-corrosion
susceptibility. For the LM2500 engine, GE specifies hot-section
repair intervals based on the operating profile of the
engine (combined effects of inlet temperature value, and
cumulative operating time at that temperature). These
operating-profile-based hot-section repair intervals enable
maximum service life between overhauls, while still minimising
risk of catastrophic failure as a consequence of hot
corrosion.
Borescope Inspection and Limiting
Criteria
GE specifies a comprehensive borescope inspection regime
for all critical components of the LM2500 engine. Its technical
instructions give quite comprehensive guidance on the
limiting criteria for acceptability of defects, once observed.
In particular, no corrosion (or erosion) of the blade coating
is permissible, and the engine must be exchanged and overhauled
should such coating degradation be evident.
The changeout of a gas turbine engine is an expensive and
time-consuming evolution. Borescope inspection is an effective
means of detecting defects before the onset of catastrophic
failure, but is ultimately only as effective as the
naked eye in the initial detection process.
Other Non-destructive Evaluation
Techniques
The challenge in determining remaining life of hot-section
components is to be able to gain sufficient and accurate
data with minimal downtime, so as to maximise the ‘P-F
interval’ 5 . A number of techniques are the subject of current
research, such as optical thermography (identification
of hot-spots on stationary components, indicating localised
breakdown of coatings). Ultrasonic, eddy-current, and X-
ray methods are also applicable, and could be successfully
applied in the field (albeit, with the engine shutdown). As
described earlier, continued improvements in the use of NDE
in the component production phase also provides a higher
level of reliability in the field.
Conclusion and Future
Directions
The requirements for yet further improvements in gas turbine
engine performance remain as strong as they have
been since the 1940s. However, the HPT blades will always
remain the most highly stressed components in the engine,
and hence are still the focus of research and development.
Nickel-based superalloys are already being employed up
to the limit of their temperature capability, so no great improvements
in temperature of operation can be expected
without an entirely new direction in materials development.
5 ‘P-F interval’ is a concept used in Reliability Centred Maintenance and condition monitoring; it refers to the time period between detection of a potential
failure (‘P’) and the actual failure event (‘F’). For condition monitoring to be worthwhile, this interval must be longer than the time needed for predictive
maintenance action to avert the failure. Extending the P-F interval (i.e. by detecting the potential failure earlier) also affords flexibility in scheduling repair
action so as to minimise the operational effects of downtime.
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