OFF-LINE TEMPERATURE PROFILING UTILISING PHOSPHORESCENT THERMAL HISTORY PAINTS AND COATINGS. S. Karmakar Biswas

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1 Proceedings of ASME Turbo Expo 2014 GT2014 June 16-20, Düsseldorf, Germany GT OFF-LINE TEMPERATURE PROFILING UTILISING PHOSPHORESCENT THERMAL HISTORY PAINTS AND COATINGS J.P. Feist Sensor Coating Systems P.Y. Sollazzo Sensor Coating Systems S. Karmakar Biswas Sensor Coating Systems S. Berthier Sensor Coating Systems C. Pilgrim Sensor Coating Systems, Imperial College London, ABSTRACT Temperature profiling of components in gas turbines is of increasing importance as engineers drive to increase firing temperatures and optimise component s cooling requirements in order to increase efficiency and lower CO 2 emissions. However, on-line temperature measurements and, particularly, temperature profiling are difficult, sometimes impossible, to perform due to inaccessibility of the components. A desirable alternative would be to record the exposure temperature in such a way that it can be determined later, off-line. The commercially available Thermal Paints are toxic in nature and come with a range of technical disadvantages such as subjective readout and limited durability. This paper proposes a novel alternative measurement technique which the authors call Thermal History Paints and Thermal History Coatings. These can be particularly useful in the design process, but further could provide benefits in the maintenance area where hotspots which occurred during operation can be detected during maintenance intervals when the engine is at ambient temperature. This novel temperature profiling technique uses optical active ions in a ceramic host material. When these ions are excited by light they start to phosphoresce. The host material undergoes irreversible changes when exposed to elevated temperatures and since these changes are on the atomic level they influence the phosphorescent properties such as the life time decay of the phosphorescence. The changes in phosphorescence can be related to temperature through calibration such that in-situ analysis will return the temperature experienced by the coating. A major benefit of this technique is in the automated interpretation of the coatings. An electronic instrument is used to measure the phosphorescence signal eliminating the need for a specialist interpreter and thus increasing readout speed. This paper reviews results from temperature measurements made with a water based paint for the temperature range 100 C to 800 C in controlled conditions. Repeatability of the tests and errors will be discussed. Further, some measurements are carried out using an electronic hand-held interrogation device which can scan a component surface and provide a spatial resolution of below 3mm. The instrument enables mobile measurements outside of laboratory conditions. Further a robust Thermal History Coating is introduced demonstrating the capability of the coating to withstand long term exposures. The coating is based on Thermal Barrier Coating architecture with a high temperature bondcoat and deposited using an air plasma spray process to manufacture a reliable long lasting coating. Such a coating could be employed over the life of the component to provide critical temperature information at regular maintenance intervals for example indicating hot spots on engine parts. INTRODUCTION The knowledge of accurate temperature profiles on critical hot gas path components is of highest importance when designing novel more energy efficient components for new generation gas turbines, boilers, fuel cells or furnaces. Further, the ability to identify components which have seen excessive heat during operation is highly desirable as it enables improved maintenance procedures and reduces the number of unplanned outages. This paper introduces a novel technique based on the 1 Copyright 2014 by ASME

2 optical properties of luminescent ceramics which can operate in a wide range of temperatures, from room temperature to 1300 C or beyond. These materials can be used to detect past temperature exposure (temperature history temperature memory effect). After cooling down to ambient temperature the luminescence properties can be determined with an instrument such as a borescope, a camera or similar devices. The luminescence measurements are converted into temperature information using calibration data to enable the operator to read out the past temperature of the coating. The luminescent ceramic can be applied on a surface in two different ways. First, it can be embedded in a water based binder to form paint which is applied using a common air spray gun. Alternatively it can be applied using an industrial coating process called atmospheric plasma spraying (APS) which produces very robust and durable coatings for harsh environments. This new technique shows significant advantages over previous generations of temperature indicating systems (thermal crystals, thermocouples, thermal paints etc) in terms of toxicity, measurement time, accuracy, ease of use and other aspects of their implementation. Based on the discussions with some users of Thermal Paints the authors speculate that this new technique could save 1.5m to 3m per engine development programme. In the current paper experimental results from some of the recently developed phosphor-based temperature indicating paints and coatings are reported. Also a very promising test showing direct comparison between a traditional commercial thermal paint and the new thermal history coating is provided using a joule furnace test on a butterfly shaped sample in the temperature range of 100ºC to 900ºC. REVIEW OF TEMPERATURE SENSORS The conventional temperature measurement techniques including pyrometry and thermocouples have fundamental application limitations in the hot section of gas turbine engines. Extraneous radiation from reflections or particles in the gas stream causes significant errors in pyrometry results [1-3]. Thermocouples require contact with the measured component and hence are intrusive and extremely difficult to practically apply, particularly on rotating components. Recent advances in phosphor thermometry have shown that accurate temperature measurements are possible on an operating gas turbine [4-6]. However, the requirement for optical access is not always feasible during the engine operation. Several techniques have been developed which do not require optical access to the engine during operation. These techniques determine the operating temperature of the component after service due to the thermal history effects. The most prominent of these techniques is known as temperature indicating paint or thermal paint. This paint is applied on the components of interest which are then installed and run in the engine. Thermally activated chemical reactions between the metal elements and other molecules in the paint cause colour changes, due to permanent changes in the reflection spectra [7]. The colour changes are then interpreted by a trained technician under controlled lighting in a laboratory and compared to calibration charts to determine the temperature of the exposure. Colour changes indicate isotherms across the component. The disadvantage with this technique is its subjective detection method, discrete measurements in contrast to continuous measurements, leaving gaps in the measurement range, and the limited durability of these paints. The longevity of the paints usually limits their application to dedicated short engine tests to avoid removal of the paint [8]. Most significantly some thermal paints rely on materials and pigments which are restricted under the EU REACH legislation [9], for example chromium, cobalt and lead. Although the paints themselves are exempt, the legislation will reduce the wholesale demand for these materials increasing their cost and eventually restricting supply. Other techniques have been developed for thermal history sensing which have greater durability and do not require restricted materials. These include metallurgical and crystal temperature sensors. Both rely on permanent changes to the material structure caused by temperature exposure. A range of different metallurgical sensors are available which rely on changes to hardness or magnetic susceptibility due to thermal exposure. The crystallographic sensors are irradiated with neutrons before testing which increases the spacing between atomic planes. Heating causes a reduction in the distortion due to the irradiation and this is measured by X-ray diffraction and compared to comprehensive calibration data. In both cases, however, the sensor material is inserted into drilled holes in the test component; hence it can only provide point measurements and, due to their destructive nature, can only be applied on development engines [10, 11]. The analysis of the thermally grown oxide by microscopy of sectioned samples is an established technique to infer the operating temperature. It has been used to develop statistical lifing models for the TBCs [12]. However, this method not only requires disassembly of the engine but also destruction of the components. In addition to drawbacks like their toxicity or destructive nature, the three previously discussed measurement techniques i.e., thermal paints, thermal crystals or metallographic characterization, are also extremely laborious and require substantial knowledge and care in preparation to achieve satisfactory results. This makes these techniques very time consuming and hence expensive. The luminescent thermal history technique, discussed in this paper, not only promises a faster and technologically smarter alternative, it will be more cost effective, too. The idea of luminescent thermal history sensors was first proposed by Feist et al [13]. Embedding optical active ions into oxide ceramics turns them into luminescence materials. During thermal exposure these ceramics can undergo permanent physical changes which can be quantified by analysis of the luminescence. Recent research has demonstrated that the luminescent thermal history technique can be applied using materials which are well established phosphors to provide temperature measurements over a large dynamic range, 600ºC 2 Copyright 2014 by ASME

3 to 1000ºC and potentially 1400ºC, within ±15ºC error [14, 15]. These phosphor materials can also be combined with a binder to be applied as a non toxic paint. However, it is also possible to make these coatings without binders. Techniques such as APS and electron beam vapour deposition (EB-PVD) are routinely used to manufacture ceramic thermal barrier coatings (TBC) in gas turbines. Feist et al found that doping the TBCs with rare earth ions, enables them to act as a thermal history sensor [13, 16]. The TBCs are refractory coatings and typically consist of a bond coat and a ceramic top layer. Bond coats are made of metal alloy compositions and are oxidation resistant. In an APS coating, the feedstock ceramic powder is introduced to a plasma jet to cause rapid heating and melting. The molten particles impact on the surface of the relatively cool substrate where they rapidly cool and solidify. When a material is applied which has a high crystallisation temperature, the rapid cooling promotes a high fraction of amorphous content to be deposited. Yttrium aluminum garnet (YAG) has a crystallisation temperature of 850 C to 950 C [17] and has been shown to form the amorphous phase when applied by APS [18]. Furthermore, YAG doped with lanthanide ions (Ln) such as Eu, Dy, Ce, Tb etc provides bright phosphorescence, embodied in its wide application for solid-state lasers/ light-emitting diode and as an amorphous APS coating; it can potentially act as a robust thermal history sensor. These thermal history coatings and paints can provide twodimensional temperature profiles across the surface of a component. This paper will describe and discuss relevant aspects of this new technique, which shows temperature measurement capability from 100 C upwards. THEORETICAL REVIEW The irreversible changes in a luminescent thermal history sensor, when exposed to high temperatures can be reflected in the observed phosphorescence lifetime decay, the emission peak intensity or a shift in the emission lines of the spectrum. These parameters can be observed individually and in combination with a detailed calibration can provide accurate past temperature information. In a pure crystal every dopant ion is located on specific crystallographic locations in the lattice which are equivalent to each other. This results in narrow excitation bands and defined emission lines. In an amorphous state, however, the vicinity of each dopant ion is different hence the energy levels vary slightly within the material. The overall optical properties of the material are the sum of each individual dopant ion so the slight variation between each causes the excitation and emission bands to broaden. The greater number of defects in an amorphous host results in a higher probability of non-radiative transition (discussed below) of electrons from the excited state. This causes shorter luminescent decays in amorphous materials. The sensing material can be produced in an amorphous state, but when heat treated (or exposed to heat) it will start crystallizing. The crystallization process is not sudden but progresses while the temperature is being increased. By observing the luminescent properties of the material, the extent of the thermal exposure, in controlled conditions can be determined. The theoretical aspect of this physical process is described in detail as follows. When a phosphorescent material is excited, there is a transition of electrons from their ground states to excited states, followed by relaxation into lower energy levels [19, 20]. This relaxation can occur via two competitive processes: photon or phonon emission. While photon emission can normally be observed in the visible spectrum, phonon emission is nonradiative and releases energy as heat. The lifetime of the observed emission depends on the probability of occurrence of these two processes, according to [19]: In Eq. (1), τ is the lifetime decay, P R is the probability of radiative relaxation (photon) and P NR is the probability of nonradiative relaxation (phonon). The probability P R is temperature independent, while P NR increases with increasing temperature [19, 21]. This principle is used in on-line temperature detection methods, usually called phosphor thermometry and this is reported elsewhere [22-25]. The phosphorescence properties are used in a fundamentally different way for Thermal History Coatings and Paints. The presence of crystallographic defects and the random orientation of atoms in amorphous materials increase P NR. Crystallisation of the material decreases the number of defects and causes the dopant ions to occupy common sites. As a result P NR decreases and the measured lifetime decay, τ, increases. Therefore, irreversible changes to the structure of the material caused by thermal exposure can be observed in the phosphorescence. When the phosphor material is produced via the sol-gel route, as for the Thermal History Paints, the product contains bonds with residual precursors such as OH, H 2 O, NO 3 and CH 2. The residuals have high phonon energies relative to the lattice phonons. The high energy phonons allow efficient quenching of the phosphorescence, i.e. a high P NR value. Release of these residuals by evaporation during subsequent heat treatment augment the rise in decay time due to structural changes, hence increase the rate the lifetime decay changes with temperature. A steeper gradient for the lifetime decay versus temperature improves the resolution of the temperature measurements. When a phosphor is illuminated by a short laser pulse, the subsequent phosphorescence decays exponentially following the equation below [19]: (2) In Eq. (2), I 0 is the initial intensity, τ is the lifetime decay and t is time. Each dopant ion is in thermal equilibrium with the surrounding ions and has its own decay time. The observed luminescence decay τ is the sum of many individual emitting ions and hence a deviation from a single exponential behaviour (Eq. 2) is often observed. However, a Levenberg-Marquardt fit (1) 3 Copyright 2014 by ASME

4 Intensity I (Volts) of equation Eq. (2) can be applied to each recorded signal. Figure 1 shows a typical signal showing a lifetime decay curve after the end of the applied laser pulse. The dashed line represents the single exponential fit for this signal. The signal represents a single (not averaged) shot and shows very little noise. The fitting routine provides the life time decay, τ, and this is plotted as a function of the heat treatment temperature. An example of a lifetime versus temperature graph is given in Figure 6 and Figure 8 for two different materials Laser Operating Single pulse decay curve Single exponential fit using Eq(2) Mobile hand-held instrumentation The mobile hand held system for the lifetime decay measurement operates on the same principle as the laboratory instrumentation shown in Figure 2. In order to improve compactness and mobility the Q-switched Nd:YAG laser was replaced with a smaller diode pumped solid state laser (DPSS), radiating at 532nm. The PMT and the oscilloscope in the bench top system were replaced by a smaller detector and an acquisition card. The laser light and the phosphorescence light are delivered and collected respectively with the same optical fibre probe. An external laptop controls the hand-held device and makes the entire set-up portable. 0.6 Laser Not Operating 0.4 Decay curve Time(ms) Figure 1. A typical single pulse decay curve measured with the hand held instrumentation (see next section). The constant region at the beginning of the curve signifies the time when the laser is operating. The red dashed line shows the best fit to the decay curve with Eq(2). EXPERIMENTAL ASPECTS Bench top instrumentation The experimental arrangement for the laboratory based bench top system is illustrated in Figure 2. The sample is excited by laser light generated by a Quantel Brilliant B laser (QBB). The QBB is composed of a Q-switch laser head which generates pulsed laser light at 1064nm, up to 600mJ. The fundamental output is frequency doubled or tripled to generate 532nm (<400mJ) and 355nm (<300mJ) light respectively. The pulses are 4ns to 6ns long and generated at a rate of 10Hz. 90% of the energy of the laser pulses is used to excite the sample while the remaining 10% is used to monitor the power of the laser pulses with a power meter (Gentec). The 90 to10% energy (beam) split is done with UV grade fuse silica (LiNOS) plate positioned on the path of the laser beam. The phosphorescence signal is collected by an optical probe, OPETS, manufactured and designed by SCS [6, 26], which couples collimated light into a fibre bundle. The fibre bundle routes the light to a photomultiplier tube (PMT: Hamamatsu). The phosphorescence light is filtered by an optical filter centred on the peak of the phosphorescence emission spectrum. The PMT signal is digitalised by an oscilloscope (Tektronix DPO3034) and transferred to a computer through USB interface. The computer runs a customised LabVIEW program which fits the signal with a Levenberg-Marquardt algorithm to extract the lifetime decay. Figure 2. A typical laboratory bench top set up for lifetime decay measurements. (a) (b) Figure 3. (a) Photograph showing a measurement being taken with the laser pen of the hand held instrumentation. The small diameter of the fiber provides high spatial resolution temperature profiling across the sample surface. (b) Photograph showing a user measuring past temperatures on the inner liner of a combustion chamber of a Rolls-Royce jet engine. Paint based coatings Thermal history paints are composed of two materials, a phosphor powder and a high temperature binder. The europium (Eu) doped phosphor powder was synthesized in its amorphous state using a standard sol-gel preparation route [15, 27]. To monitor the quality of the phosphor product, it is characterised 4 Copyright 2014 by ASME

5 using x-ray diffraction and Fourier transform infrared spectroscopy. To form a smooth paint, the phosphor powder needs to be homogenously mixed with a binder. Important requirements are, first, that the binder allows a paint to adhere to the surfaces of interest and can survive at the high temperatures likely to be encountered and measured. Secondly, the binder must not react with the active ingredient or negatively affect its sensing capability. There is a range of commercially available non-toxic silicate and silica based binders specifically designed for the preparation of paints intended to survive at high temperatures. A typical water based alkali silicate binder was found to be most compatible with the phosphors in terms of high temperature durability and surface adherence. A SRi Pro compliant smart / spot repair gravity feed spray gun was used for applying the thermal history paint to the stainless steel substrates. Spraying paint requires careful preparations and some experience in order to produce a smooth and uniform surface. First, the substrate surface was roughened by grit blasting followed by cleaning with Iso-propanol. After air drying and curing, 17 pieces of paint samples were heat-treated in a tube furnace at temperatures between 100ºC and 900ºC in a steady state for 30 minutes. The samples were quenched to room temperature and cooled under ambient conditions. In order to generate a calibration curve, the lifetime decay values of the samples heated at different temperature were measured using the mobile hand held instrumentation. The calibration curve is shown in Figure 6(b) where the lifetime decay of the luminescence is plotted as a function of the heat treatment temperature. APS coatings (a) YAG:Ln on titanium substrates The YAG:Ln APS coatings were applied on flat, square titanium tiles of dimensions 20x20x1mm at Forschungszentrum Jülich GmbH. A standard bond coat was first applied by vacuum plasma spraying. The samples were heat treated at various temperatures from 300 C to 1000 C. The samples were left to dwell in a steady state in a tube furnace for 45 minutes. The approximate heat up and cool down times were 5 and 3 minutes respectively. A Nd:YAG laser (QBB) was used to excite the phosphorescence. The phosphorescence was collected using the OPETS indicated in Figure 2. The probe has a uniform observation volume and eliminates the requirement for precise alignment. A more detailed description is given elsewhere [4, 26]. APS coatings (b) Butterfly samples To compare the performance of the robust thermal history coatings with thermocouples and thermal paints, a typical butterfly shaped substrate (stainless steel,1.5mm thick) as shown in Figure 4 was coated with YAG:Ln, thermal history coating on one side and a commercial thermal paint (TMC- MC350-8) on the other side. The YAG:Ln coating was applied using an APS method as discussed in the preceding section, while the thermal paint was applied using an air-spray process. Additionally eight thermocouples were welded on the thermal paint side at locations described in Table 1. The butterfly sample was then heated by electrical current flow (13Amps) in a Joule furnace. The corresponding experimental arrangement is shown in Figure 5. Thermocouple T3, located at the thinnest part of the butterfly sample, was used by the Joule furnace controller to maintain the centre of the sample at 900 C for 30 minutes. Because of its specific geometry the butterfly shaped sample, has a sharp resistance gradient between its centre and its coating ends (L and R, shown above). When a high current is passed through the sample, a corresponding temperature gradient is formed, with the highest temperature (900 C) being attained at the centre of the sample. The steady state thermocouple temperature for an individual location was calculated by averaging the temperature for 30 minutes of the experiment. Table 1 Location of the thermocouples on the butterfly sample in mm. The distances are measured starting with zero, from the vertical dashed line at the left of T1 (Coating end L ). Thermocouple T1 T2 T3 T4 T5 T6 T7 T8 Distance (mm) Centre of the sample Coating end L T1 T2 T3 T4 T5 T6 T7 T8 Coating end R Figure 4. Sketch of the butterfly sample. One side of the sample was painted with a commercial thermal paint while the other side was coated with APS YAG:Ln thermal history coating. T1 to T8 represent the locations of the 8 thermocouples welded to the sample on the thermal paint side. The distance measurements indicated in the figure are in mm. The calibration procedure for the thermal paint was performed by the thermal paint manufacturer, using a set of calibration samples. Each of these samples was heated in a box furnace for 30 minutes in 20 C steps from 290 C to 1170 C. A calibration chart was generated from the changing colours of the heat treated samples and was used to identify the temperature on the thermal paint side of the butterfly samples. The measured lifetime decay values of the luminescence thermal history coating were translated into temperatures using a pre-recorded calibration curve. The calibration curve was obtained earlier by heating five APS coated (YAG:Ln) samples 5 Copyright 2014 by ASME

6 Numbers Normalized lifetime decay( / 100 o C ) at pre-defined temperatures (from 300ºC to 900ºC) for 30 minutes in a single zone tube furnace. To assure high temperature accuracy additional thermocouples were attached to each individual sample during the heat treatment process. The temperature readings of these calibration samples were recorded with a data logger (Omega) connected to a computer. Current controller (13 Amps, 240V) T3 to furnace for temperature control Wire for electrical connection (a) ( C) Operator 1 Operator 2 Operator 3 Eu Based Paint 1.2 Thermocouples Data logger Figure 5. Diagram representing the experimental arrangement for the Joule heating procedure. APPLICATIONS & RESULTS Paint coating applications Figure 6 (a) shows photograph of a selection of individual thermal history paint samples after being heat treated at various temperatures. The samples get darker in appearance as a result of high temperature exposure. This is due to the changes in the binder and this resulted in some signal degradation. It was possible to measure the lifetime decay of these samples at room temperature for heat treatment temperature ranging from 100 C to 900 C. Each point in the calibration plot (Figure 6 (b)) represents the mean value extracted from 30 repeated lifetime decay measurements. The maximum observed standard deviation of these 30 measurements was less than 0.3% of the mean lifetime decay. The error bars in Figure 6(b) represent the standard deviation of these 30 measurements, multiplied by a factor of 5 for visibility reasons. The lifetime decay data has been normalized, with respect to the decay time for 100 C sample, to visualize the dynamic range of more than 75% in the decay time as the heat treatment temperature increased. This large increase in the calibration parameter i.e., the decay time as a function of heat treatment temperature implies higher sensitivity of the thermal history paint. The life time decay showed a monotonic variation within the temperature range 100ºC to 800ºC. Consequently, the paint can be used for temperature measurements in this regime. To test the accuracy of the thermal history paint, lifetime decay measurements for the calibration curve were repeated by three different operators at separate times and locations on the samples. Figure 6(b) shows an excellent agreement between these three separate measurements, indicating high reliability of the paints as well as the mobile hand held instrumentation Heat treatment temperature ( o C) Figure 6. (a) Photograph of the thermal history paint samples heat treated at different temperatures. (b) A normalized calibration curve for a typical Eu based phosphor paint. The three different symbol types represent measurements done by separate operator at separate times and locations. The error bars were multiplied by a factor of 5 for clearer visibility. Validation of the calibration curve shown in Figure 6 was done by measuring temperature on several independent samples painted with the Eu based thermal history paint. Figure 7 shows statistical distribution of 30 single measurements done on a typical paint sample heat treated at 650 C for 30 minutes indicating high precision and accuracy of this paint single measurements Exposure Temeperature 650 o C Temperature error = +/- 7.6 o C (Std. Dev) Temperature Sensed ( o C) Figure 7. Statistical distribution of the 30 single temperature measurements done on a typical Eu based thermal history paint exposed to 650 C for 30 minutes. The solid curve represents a Gaussian fit to the distribution data. APS coating applications YAG:Ln on titanium The change in decay time with temperature were measured for the three APS coated YAG:Ln samples. The results from the lifetime decay measurements for all these three samples are 6 Copyright 2014 by ASME

7 Normalized Lifetime decay ( untreated ) shown in Figure 8. One sample was heat treated and measured incrementally between 300 C and 780 C (square symbols in Figure 8). A second sample was heat treated solely to 795 C (circles). A third was heat treated and measured incrementally from 780 C to 950 C (triangles). The data has been normalized, with respect to the decay time for the untreated sample to visualize relative change with heat treatment. As the heat treatment temperature is increased, there is a gradual and monotonic increase in the lifetime decay until approximately 800 C where it rapidly increases at 900 C (~250% with respect to the untreated sample). After 900 C the decay time appears to be constant, this is considered to be because the material is fully crystalline [13, 17]. The monotonic change in decay time between 300 C and 900 C indicates that APS YAG:Ln coatings could be used for temperature measurements in this range. change contours are visible. The black crescent shaped lines indicating the visible colour changes and thus reflect isotherms on the component. The temperature estimations for the isotherms were made using the specific calibration chart provided by the manufacturer. The obtained temperature values are shown at the bottom of figure 9. Since the thermal paint read outs are only possible at the colour change boundaries, it was not possible to make a continuous read out along the sample leaving gaps between the isotherms of up to 100 C. This is less beneficial in real environments where temperature changes can occur on short distances and are of the range of 100 C to 200 C eg cooled turbine blades Incremental heating from 330 o C Single stage heating at 795 o C Incremental heating from 780 o C Untreated Heat treatment temperature ( o C) Figure 8. The change in lifetime decay for YAG:Ln samples at different heat treatment temperatures when exposed for 45 minutes. Three different heat treatment routines were conducted on three different samples (see legend). A significant observation is the similarity in the results for the three samples, heat treated in different steps. The results show that prolonged exposure at lower temperatures has no effect on the decay time. The decay times of the two samples which are heat treated at 780 C are within 0.1%. One of these samples was heat treated for an accumulated time of 7.5 hours before being heated at 780 C, where as the other sample was heated directly at 780 C for 30 minutes. This indicates that the coatings record the maximum temperature of heat treatment and the duration of exposure at a lower temperature is less significant for the temperature measurement. This promises that the coating could be applied for testing under cycling conditions and will sense the maximum temperature of operation. This is particularly useful since the maximum temperature of operation is typically of most importance. APS coating application - Butterfly test results Figure 9 shows a picture of the thermal paint side of the butterfly sample after the Joule heating process. Several colour Figure 9. Estimation of the temperature profile on the butterfly sample from the thermal paint. The black curves are guide to eyes, indicating the location of colour change interface. The black spots on the sample show the locations of the welded thermocouples. Figure 10 shows the temperature results obtained along the horizontal axis of the butterfly sample (dashed line in Figure 9), by the three different techniques; thermocouple, thermal paint, and thermal history coating. The x axis represents the distance from the coating at point L (for left), as shown in Figure 9. The data obtained during the heating process from the thermocouple T5 was unstable and noisy and hence was removed. The lifetime decay measurements on the YAG:Ln thermal history coating side were obtained after the heating process at room temperature. Using the hand held device point measurements were carried out every 5mm along the dashed centre line in Figure 9. Most significant is that the temperature sensed by the thermal paint and the thermal history coating agreed well with each other. As expected, both the techniques sensed the highest temperature, approximately 900 C around the narrower part of the butterfly sample. The lowest temperatures were measured towards the wider end of the butterfly sample (point L left and R right). The data from the luminescent thermal history coating exhibited small humps at distances 60, 100 and 120 mm from the point L. This is due to the rough nature of the calibration data available at the time which only had six data points and intermediate values were interpolated as shown in Figure 11. The decay time is normalized and plotted versus the 7 Copyright 2014 by ASME

8 Temperature ( o C) Normalized lifetime decay ( Untreated ) heat treatment temperatures. Each data point in the figure represents the mean of 30 repeated lifetime decay measurements. The maximum observed standard deviation of these 30 measurements was less than 0.2% of the mean lifetime decay. The error bars in Figure 10 represent the standard deviation of these 30 measurements, multiplied by a factor of 2 for better visibility in the graph. Surprisingly most of the thermocouples T1, T2, T4, T6 and T7 detected lower temperatures when compared to the thermal paint and the thermal history paint. The authors speculate that this is probably due to a current leak between the Joule furnace and the battery powered thermocouple data logger. The electric ground potential of the battery is different from the ground potential of the Joule furnace causing this current leak. Consequently this might lead to differences in the potential differential of the thermocouple, hence incorrect temperature readings. However, the temperature reading of T3, used by the Joule furnace controller, is correct, because it shares the same electric ground potential than the Joule furnace. This is confirmed by both temperature readings from thermal paint and thermal history coating as they indicate similar values. While the precision of the thermocouples was still high the accuracy was extremely bad due to the systematic error introduced by the current leakage. Unfortunately, as thermocouples are an online technology this inaccuracy was not being noticed until later when the analysis of the thermal paints and the thermal history coatings was obtained. The standard deviation of the thermal history coating is typically below 5 C and can be as low as 0.05 C in the temperature range between 450 C and 900 C. This is based on 5 repeated measurements of 120 life time decay measurements per data point. The measurement duration is typically 15 seconds for one reading. A standard deviation for the thermal paint can t be obtained as this is a manual readout Thermocouple Thermal History Coating Thermal Paint Distance (mm) Figure 10. Temperatures measured on the butterfly sample by the thermocouples, the thermal paint and the thermal history coating. The significance of the test is that continuous readings were obtained from the thermal history coating across the surface while the thermal paint only provided discrete values. This means that with an improved calibration curve of multiple data points a more accurate temperature profiling can easily be achieved using the thermal history coating. Consequently, a thermal history coating can be used ideally in situations where temperature gradients are experienced over small distances e.g. cooling hole arrays on engine components. Also a full temperature profile can be obtained by scanning the surface of a component as there are no gaps in the dynamic temperature range in contrast to thermal paints. The luminescent thermal history coatings in combination with the hand held readout device promises the user a fast alternative to measure the past temperatures continuously with a spatial resolution of less than 3mm and at any coated location Untreated Heat treatment Temperature ( o C) Figure 11. The calibration curve for the APS YAG:Ln coating. The error bars were multiplied by a factor of 2 for improved visibility. SUMMARY AND CONCLUSION The concept of thermal history paint and a thermal history coating was introduced allowing past temperatures to be detected. Basics of the measurement technique and the physics were briefly explained and compared with other common techniques. A typical laboratory set-up for life time decay measurements was described and turned into a portable readout device. The thermal history paint application has shown excellent repeatability in the temperature range 100 C to 800 C. The standard deviation of individual data points was typically below 0.3%. This demonstrates a high degree of robustness for both the thermal history paint and the associated instrumentation. A more durable coating approach was tested using an industrial air plasma spray process to deposit a lanthanide doped YAG material on a butterfly sample. The butterfly sample was used in a Joule furnace to generate temperature variations across the surface. Temperature profiles were successfully obtained from attached thermocouples, the commercial thermal paint and from the novel thermal history coating. Most of the thermocouples returned inaccurate results during operation due to the specific wiring of the Joule furnace. However, thermal paint and the thermal history coating have shown a high degree of agreement in the temperature range 8 Copyright 2014 by ASME

9 between 400 C and 900 C. Temperature precision of the thermal history coating was estimated to be typically below 5 C, but showed values down to 0.05 C. Also potential measurement uncertainties caused by past accumulated temperature exposures proved to be very small around 0.1% for this material. This shows the capability of the material to register peak temperatures accurately even when previously exposed for several hours at lower temperature regimes. This ideally is suited for warranty and maintenance applications when overheating of components can occur. Eventually, both thermal history paint and coating using non-toxic materials in contrast to currently available thermal paints and provide a serious technical alternative. The paper has successfully demonstrated the feasibility and applicability of a novel temperature detection method based on phosphorescent materials. This new method provides quantitatively reliable measurements with high accuracy over wide temperature ranges. The results indicate that these thermal history paints and thermal history coatings hold great potential for applications from jet engines and power generation to the electronics industry and automotive sectors in areas of research and development, maintenance and non-destructive testing. ACKNOWLEDGMENTS Parts of the work described herein were kindly supported by the Office of Naval Research and the Technology Strategy Board, UK. REFERENCES [1] Douglas, J., Smith, C. A., and Taylor, S. J. R, "An integrated approach to the application of high bandwidth optical pyrometry to turbine blade surface temperature mapping," Proc. Instrumentation in Aerospace Simulation Facilities, ICIASF th International Congress on, pp. 4/1-4/6. [2] Feist, J. P., and Heyes, A. 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