Thermal Barrier Coating Health
High performance modern engines, both aero and land-based, operate at ever increasing temperatures to take advantage of associated increases in thermodynamic efficiency. The engine operating temperatures are substantially higher than that sustainable by the existing base metal capability. The successful operation of engine components in the hot gas path, such as the stationary vanes and rotating blades, relies on both insulating ceramic thermal barrier coatings (TBCs) and hot part cooling.
Thermal barrier coating integrity is essential to the protection of hot parts. Both rotating and stationary parts need to be monitored. Prominent TBC failure modes that health monitoring must address include:
- TBC loss caused by erosion of TBCs or foreign object damage
- TBC spalation, debonding, or delamination caused by sintering when exposed to over temperatures or application defects
- Cracks cause by strain accumulation
Current Sensor Needs
Sensing methods are needed to monitor the thermal barrier coating integrity of both stationary and rotating parts in the high temperature turbine areas. Coverage is needed for all critical part surfaces. Temperature mapping of the surface is often used as an indication of thermal barrier coating health. Techniques currently applied to monitor TBC health include embedded sensors, radiation pyrometry or imaging, and thermographic phosphors.
Each of the methods under development has many technical and application challenges to meet test measurement and on-line TBC monitoring needs, including:
- Embedded sensors such as thermocouples are to measure critical properties at points on the surface. Limitations include the fact that they can create unacceptable stress concentrations on the part, only discrete points on the surface are measured, and life of the embedded sensor is limited.
- Radiation pyrometry has been used robustly as a nonntrusive method to monitor line of sight temperature indications. Short wavelength infrared (SWIR) pyrometers are a standard techniques used in testing. SWIR pyrometers can provide accurate surface temperature measurements for fully oxidized metal parts because their radiative properties provide high emittance, low reflectance, and no transmittance. Ceramics thermal barrier coatings tend to have low and variable emittance, significant reflectance, and some transmittance at short wavelengths, which will cause increased uncertainties in measurements of TBC coated parts.
- Long wavelength infrared (LWIR) pyrometry operates at wavelengths
around 10 microns where ceramics TBC are opaque and have high
emittance and low transmittance. The LIWR measurements, however,
have interferences from the radiance of combustion products
like H2O and CO2.
- IR imaging can be used to map the surface of hot coated parts. The IR mapping provides a visual image of the coating health and a qualitative temperature map. Imaging requires, however, line of sight access or access through optical fiber guides. IR imaging measurements have the same accuracy limitations as SWIR and LWIR pyrometry.
- Thermographic phosphors and other temperature indicating techniques have been used to measure temperature distributions as an indication of TBC health. Temperature indicating materials include temperature indicating paints and thermographic phosphors that use both thermal and laser excitation. The phosphors investigated are rare earth doped ceramics that can be either in separate layers or mixed with the TBC during preparation.
- The monitoring system should be capable of providing real-time data, and the sensors should have life capability in excess of normal outage times at operating conditions.
- The monitor will need to be an early indication of TBC coating condition changes.
- The monitor will need to monitor rotating and stationary parts globally so that deterioration of particular areas can be detected.
- The monitor will need to be sensitive to subtle changes in TBC conditions.
- Health monitoring resolution, accuracy, and temperature ranges requirements vary with engine type and specific test or monitoring objectives.
Strategic Advisory Board (SAB) Members Addressing This Need
Sporian Microsystems, Inc