Timothy J. Bencic

Depth-penetrating temperature measurements of thermal barrier coatings incorporating thermographic phosphors

Thermographic phosphors have been previously demonstrated to provide effective non-contact, emissivity-independent surface temperature measurements. Due to the translucent nature of thermal barrier coatings (TBCs), thermographic-phosphor-based temperature measurements can be extended beyond the surface to provide depth-selective temperature measurements by incorporating the thermographic phosphor layer at the depth where the temperature measurement is desired. In this paper, thermographic phosphor (Y2O3:Eu) fluorescence decay time measurements are demonstrated to provide through-the-coating-thickness temperature readings up to 1100 °C with the phosphor layer residing beneath a 100-µm-thick TBC (plasmasprayed 8 wt.% yttria-stabilized zirconia). With an appropriately chosen excitation wavelength and detection configuration, it is shown that sufficient phosphor emission is generated to provide effective temperature measurements, despite the attenuation of both the excitation and emission intensities by the overlying TBC. This depth-penetrating temperature measurement capability should prove particularly useful for TBC diagnostics where a large thermal gradient is typically present across the TBC thickness. The fluorescence decay from the Y2O3:Eu layer exhibited both an initial short-term exponential rise and a longer-term exponential decay. The rise time constant was demonstrated to provide better temperature indication below 500 °C while the decay time constant was a better indicator at higher temperatures.

Monitoring delamination of plasma-sprayed thermal barrier coatings by reflectance-enhanced luminescence

Plasma-sprayed thermal barrier coatings (TBCs) present a challenge for optical diagnostic methods to monitor TBC delamination, because the strong scattering exhibited by plasma-sprayed TBCs severely attenuates light transmitted through the TBC. This paper presents a new approach that indicates delamination in plasma-sprayed TBCs by utilizing a luminescent sublayer that produces significantly greater luminescence intensity from delaminated regions of the TBC. Freestanding coatings were produced with either a Eu-doped or Er-doped yttria-stabilized zirconia (YSZ) luminescent layer below a plasma-sprayed undoped YSZ layer. A NiCr backing layer was added to represent an attached substrate in some sections. For specimens with a Eu-doped YSZ luminescent sublayer, luminescence intensity maps showed excellent contrast between unbacked and NiCr-backed sections. Discernable contrast between unbacked and NiCr-backed sections was not observed for specimens with a Er-doped YSZ luminescent sublayer, because luminescence from Er impurities in the undoped YSZ layer overwhelmed luminescence originating from the Er-doped YSZ sublayer.

Advances in High Temperature Phosphor Thermometry for Aerospace Applications

Phosphor thermometry has been used for many years for non-contact temperature measurements in hostile environments. Aerospace systems are particularly prone to adverse high temperature environments, including large blackbody background, vibration, rotation, fire/flame, pressure, or noise. These environments often restrict the use of more common thermocouples or infrared thermometric techniques. Temperature measurements inside jet turbines, rocket engines, or similar devices are especially amenable to fluorescence techniques. Often the phosphor powders are suspended in binders and applied like paint or applied as high temperature sprays. Thin coatings will quickly assume the same temperature as the surface to which they are applied. The temperature dependence of phosphors is a function of the base matrix atoms and a small quantity of added activator or “dopant” ions. Often for high temperature applications, the selected materials are refractory and include rare earth ions. Phosphors like Y3Al5O12 (YAG) doped with Eu, Dy, or Tm, Y2O3 doped with Eu, or similar rare earth compounds, will survive high temperatures and can be configured to emit light that changes rapidly in lifetime and intensity. Recently, a YAG:Cr phosphor paint emitted fluorescence during short duration tests in a high Mach number hydrogen flame at 2,200 °C. One of the biggest challenges is to locate a binder material that can withstand tremendous variations in temperature in an adverse aerospace environment. This presentation will give research results applicable to the use of phosphors for aerospace thermometry. Emphasis will be placed on the selection of phosphor and binder combinations that can withstand high temperatures.

Smart coatings for health monitoring and nondestructive evaluation

Luminescent coatings applications have been increased dramatically over the last decade as imaging capacities have advanced. These coatings have been used to monitor surface temperature and air pressure (oxygen sensing) in testing facilities around the world. Through the commercial suppliers of these coatings, custom assembled hardware systems and especially data reduction and analysis software, the use of smart luminescent coatings are starting to find their way in to inspection monitoring and nondestructive evaluation testing. The use of a temperature sensitive paint for example, can be a potential replacement for infrared imaging where IR techniques are limited due to access, reflections and complex geometries. Detection of the luminescent signal can use simple intensity ratio methods with synchronized pulsing systems to capture frequency responses in imaging applications. Time or frequency methods allow signals to be detected in the presence of high background noise that allow measurements that were previously unobtainable. This paper describes general luminescent sensors, detection methods and examples of coatings that are applied over test examples or embedded in materials to measure or monitor the health of a specimen.

Phosphor Thermometry in an Operating Turbine Engine

It is well known that phosphor thermometry may be used in environments characterized by high brightness from blackbody background or combustion flames. The present work describes measurements performed inside an operating turbine engine. For this application, two thermographic phosphors were chosen to cover the expected temperature range. The first, Yttrium Oxide doped with Europium (Y2O3:Eu) has received much attention as a temperature-indicating material for higher temperature ranges. The second, Yttrium Vandidate doped with Thulium (YVO4:Tm) is relatively new, and was chosen for several reasons. First, it exhibits a bright emission line at 475 nm, which is much bluer than Y2O3:Eu line at 611 nm. Blackbody emission will have less effect on the detection of blue emission as opposed to the red. Second, YVO4:Tm also exhibits temperature dependence from at 100 C to above 900 C in the range not covered by Y2O3:Eu. This testing proved that temperature measurements are possible in this environment, given proper selection of the emission wavelength and temperature range.

Progress in Measuring Water Impingement Characteristics on Aircraft Surfaces

Validation of trajectory computer codes, for icing analysis, requires experimental water droplet impingement data for a wide range of aircraft geometries as well as flow and icing conditions. This paper provides a summary of experimental water droplet impingement research starting with the early efforts of NACA in the 1950s. Industry requirements for additional impingement data are discussed based on results from a recent industry survey. A new research program aimed at expanding and modernizing the existing water droplet impingement data is outlined. Improved experimental and data reduction methods for obtaining water impingement data are presented. A discussion on sources of error in the experimental and data reduction methods is provided. Relative humidity is shown to have a significant effect on the repeatability of the experimental data. Recent wind tunnel test results for an MS(1)-0317 airfoil and a three element McDonnell Douglas high lift system are compared with computational results obtained with the LEWICE code. In most cases, the correlation between the analytical and experimental impingement efficiency distributions is good. The experimental and analysis results include impingement characteristics for median volumetric diameters of 11.5, 21 and 92 microns. Test repeatability for the experimental results presented ranged from 4% to 10% with respect to the average data. This represents a significant improvement compared to previous experimental results.