A smart metal component with an integrated high temperature sensing capability is presented. The metallic structure in SS316 is manufactured by an additive layer process based on laser powder bed fusion (LPBF).  The sensor, a temperature sensitive in-fiber Fabry-Perot element, is encapsulated in a capillary and embedded into the structure during the LPBF build process. It is demonstrated that the sensor can measure the temperature inside the metal up to 1000°C with accuracy better than 10°C for extended periods far in excess of 300 h.

Laser-Powder Bed Fusion (LPBF) is rapidly becoming a highly versatile and important technology for the manufacture of complex shaped structures and components, often with highly intricate internal channels and designed-in voids for cooling and other functional capabilities [1,2]. In particular, power generation by gas turbines is an important technology where LPBF parts can drastically improve efficiency and operational lifetime. There are by now numerous other application areas and materials which benefit and gain functionalities from additive manufacturing technologies and the technology described here can be adapted and moved to other techniques although different temperature limits will apply depending on the material matrix used [3-5].
Integrating high temperature sensing capability into power generation components is increasingly important for real-time process condition monitoring and asset management. Such sensors must be capable of sustained high temperature operation and hence fused silica optical fibre sensors are an obvious choice; the high softening point of fused silica means that such sensors should be capable of operating at temperatures in excess of 1100°C, with a Fabry-Perot (F-P) design suitable for temperature measurement in this range[6,7]. Additive manufacturing involves building up structures layer by layer and it opens up the prospect of incorporating valuable internal features into structural components during their manufacture. A smart metal fabrication process has been developed which is based upon laser additive manufacturing (3D printing) to incorporate optical fiber sensors into metallic components during their construction [8,9]. The paper here describes the manufacture of LPBF manufactured coupons with integrated temperature sensors as substitute to the integration into actual turbine components for technology demonstration. These coupons can be manufactured in 10 minutes whereas a turbine part requires 2-3 hours processing time. 
Recent studies on the temperature characteristics of an embedded fiber Bragg grating (FBG) in a metallic structure show that due to the coefficient of thermal expansion (CTE) mismatch between silica and the host metal the differential strain at elevated temperatures can lead to delamination and fiber breakage [9-13] and limiting its application to ~450°C. Therefore, a different approach has been used in this paper in which a high temperature compatible F-P fiber sensors is encapsulated inside a fused silica capillary, in the metal component. In this way delamination or capillary breakage is less likely to affect the sensor because the sensor is sitting inside the region protected by the capillary. Although fiber embedment into LPBF components has been demonstrated by this research group [9] and F-P sensors of the described construction and type have been used in a number of projects[6], the overall combination of aspects described here and the extreme temperature environment of up to 1000°C is entirely novel.  
The F-P sensor is formed between two reflections along a length of fiber.  The first reflecting point is manufactured by producing an in-fiber reflective splice by splicing a fiber with a reflective layer of Chromium on its edit end onto another fiber. The reflectivity of the Chromium layer is selected to be in the order of 5 to 10% after the fibers have ben spliced.  This low reflectivity from the Cr layer provides a good intensity match with the 4% Fresnel reflection being returned from the second sensor interface, resulting in a high fringe visibility with low background. To achieve this post splicing reflectivity a slightly higher reflectivity of ~20% has to be applied to the end of the fiber prior to splicing since some of the Cr layer will be sputtered of during the splice. This second fiber is then cleaved at a distance of about 50-100 µm from the splice to form the optical cavity. The sensor is inserted into a fused silica capillary to prevent strain-induced damage, and a nickel jacket of thickness 140 m is deposited around the capillary to act as a thermal shield for the subsequent selective laser melting process to make the smart components.  Fabrication and testing of these devices is considered in detail below in this paper. It should be possible to use an FBG in a similar way; however, although recently developed FBGs such as regenerated gratings [14-17], chemical-composition gratings [18], and surface relief gratings [19] have been shown to be capable of measuring temperatures in excess of 1000°C, their long term stability at high temperature has not yet been proven for the power generation applications of interest here.

The sensor concept is shown schematically in [Figure 1] in which a thin reflective metal film is sandwiched between two single-mode fibres.

Figure 1: Schematic structure of the F-P cavity.  Light is guided in the core region, and the outer fibre diameter is typically 125 µm.

This forms one of the two reflecting surfaces that form the optical cavity. The other (labelled M2) is formed by the reflection at the end of the cavity fibre. The details of the sensor fabrication and its temperature response are given in [6]. A typical cavity reflection spectrum of the sensor is shown in [Figure 2]. In order to incorporate the developed sensor into engineering structures, and to avoid any strain transfer during temperature measurement, suitable strain-relief packaging is required. A fused silica capillary tube was found to be particularly suitable, as described in [6]
To manufacture the smart metallic coupon, a fused silica capillary of length ~150 mm (Z-FSS-150240, Postnova, ID and OD 150 and 220 ?m respectively) is sealed at one end using an electric arc. Approximately 60 mm of the polyimide jacket is subsequently removed from the sealed end by dipping it in hot (200°C) sulphuric acid.  A ~80 mm long metallic jacket is deposited on the capillary in a 2-stage process.  First a silver (Ag) layer is deposited using an electro-less deposition process [20,21]. A solution containing silver nitrate (Tollen’s reagent) and a reducing sugar (glucose) react to form Ag, which is deposited on the capillary. The Ag layer provides the anode for a subsequent nickel (Ni) electroplating process.  A Ni layer is applied to the Ag coated fibre by an electro plating process in a Ni-sulpha mate bath [8-10]. The thickness of the Ag and Ni layers of the jacket are ~5 µm and ~140 µm respectively.  The thermal shield provided by the metallic jacket around allows the capillary to be embedded into metallic structures.

Figure 2. Spectra recorded from the F-P cavity before embedding (black) and after embedding (red).

The capillary is then embedded into stainless steel components using laser powder bed fusion (LPBF); details of the customized LPBF set up used in the work are described in [8,9].The detailed approach used for capillary embedding is illustrated in [Figure 3 and 4]. The laser parameters used for the process are laser power 80 W, wavelength 1064 nm, and scan speed of 100 mm/sec, with the scan direction alternating between parallel and perpendicular to the capillary, and scan line separation of 50 µm.  Subsequent powder layers are ~50 µm thick. The laser parameters were selected to achieve melt pool reaching ~1 mm deep inside SS316 powder, to eliminate porosity and guaranteeing proper bonding between subsequent layers.  Initially a ‘U’-shaped groove is manufactured by LPBF process with dimension tailored to the diameter (~500 µm) of the metalized capillary, which is then placed in this groove, and the build process continued. To reduce the possibility of damage to the capillary, the laser scan strategy was optimized to minimize thermal loading, and ensure that the powder is melted in a controlled manner. Therefore, the first five layers immediately on top of the capillary are added with an increased scan line separation (100 µm) in the vicinity of the capillary to reduce the thermal load. The over layers on the sides of the capillary are deposited using the normal laser parameters given above. Also, for the first over layer on the capillary, the laser beam is scanned perpendicular to the direction of capillary in steps while clamping the capillary next to it to prevent its bending due to stresses induced by the process. Then alternate parallel and perpendicular scanning is carried out on a layer-by-layer basis to create a good weld which ensures good bonding of the capillary to the surrounding materials. After the capillary embedment, the standard process is resumed until the part is completed.

Figure 3: Schematic diagram showing the LPBF procedure for embedding the capillary.

Figure 4: Embedding process in three steps: Building the ‘U’-shaped groove with LPBF on SS 316 substrates (1), inserting Nickel coated capillary in the groove (2) and final encapsulation by continuing LPBF process (3).

The surface of the U-groove was examined using an Alicona IFM G4 (a microscope system suitable for 3D surface analysis), in order to analyse the surface roughness, and to check that the groove was free from any powder particles or other debris before placing the capillary in it. The measured 3D image of a typical U- groove, together with an example cross section are shown in [Figures 5(a) and 5(b)] respectively. The bottom surface of the U-Groove was measured to have a maximum peak to peak roughness (Rz) of 9 µm throughout the groove. In selected regions it is periodic with the period corresponding to the laser scan separation, and with an Rz of 4 µm. The edges of the LPBF part are always slightly higher than the interiors with a ΔZ between 40-100 µm, which could result in significant strain to the capillary at the ingress/egress point to the embedded structure [22]. It also prevents proper fitting of the capillary in to the groove. Therefore, the U- groove is designed to be 200 µm wider and deeper towards the end of the structure.  Also, in the test samples used in the experiments, a small amount of epoxy (Araldite precision) is applied at the capillary jacket and LPBF structure interface post fabrication to act as a temporary strain relief and to make the unit more robust during handling. Following installation of the sensor coupon at a test location or in a furnace, this temporary protection is no longer required, and the epoxy is burned off at the first thermal cycle. An image of typical 3D printed structure is given in [Figure 6]. The base plate or the build plate of the LPBF structure is removed later by milling or ideally by force free wire-based electro discharge machining for fragile components, which was not available to the researchers.

Figure 5: (a) Surface profile image of a typical U- groove measured using 3D surface profiling microscope (Alicona IFM G4).  (Note: The white patches are unresolved data from the microscope processing and are not related to any specific sample artefacts). (b) Typical cross section of the U-groove measured using Alicona IFM G4.

After the embedding process the other end of the capillary is cleaved using a fibre cleaving tool (FK11, PK Technology Ltd). The cleaved end is heated using electric arc to slightly melt the sharp edges in order to prevent them from damaging the sensor fibre. A 125 µm diameter F-P sensor with a cavity length ~75 µm and with a fibre jacket removed length of 150 mm is inserted into the capillary. The capillary-fibre interface is partially sealed by collapsing the capillary onto the outside of the sensor fibre. This does not form a hermetic seal but prevents the fibre inside the capillary from being dislocated laterally. The capillary has a very smooth internal surface which allows very long lengths of fibre (i.e., 1 m and above) being inserted without difficulties and damage to the sensor fibre. Depending on the temperature environment at the end of the capillary an additional protective fibre sleeve can be applied to further protect the transition region, where the hot- melt adhesive inner tube (Ethylene Vinyl Acetate) of the sleeve bonds to both the fibre/capillary and the heat shrinkable outer tube (Polyolefin) of it to encapsulate the capillary-fibre interface and provides vibration damping and an environmental seal, protecting the fibre/capillary from damage and contaminants. The outer heat shrinkable tubing provides an instant shrink-force and drives the adhesive liner into all areas of the capillary-fibre interface. A stainless-steel strength member provides additional rigidity to prevent misalignment, micro bending or breakage of the capillary-fibre interface. A schematic diagram of the F-P sensor inside metalized capillary is shown in [Figure 7]. The spectrum of the sensor before embedding and after embedding are the same [Figure 2] implying no transmission loss introduced during the embedding process. This configuration of the sensor is robust and hence suitable for test in harsh or high temperature environments.

Figure 6: Image of a 3D printed smart metallic structure on a 50×20×1.2 mm base plate.

Figure 7: Schematic of an LPBF built SS316 coupon with embedded F-P sensor suitable for high temperature applications (NB not to scale).

In order to test the embedded sensor for measurement of temperature they were placed it together with a reference thermocouple (N type) into a tube furnace (Carbolite). The spectra were recorded using a swept wavelength interrogator (sm125-500, Micron Optics).  The recorded data (reflected light intensity values over a wavelength range of 1510-1590 nm with wavelength resolution step size of 5 pm) were collected and processed using Lab VIEW software [6]. A typical interferogram recorded from a sensor in the wavelength space is shown in [Figure 2]. The key information lies in the periodicity of the fringes which changes with the optical length of the cavity formed between M1 and M2. This information is extracted from the data by a FFT algorithm within the Lab View application after converting the data to frequency space. The great strength of this type of analysis is its robustness and insensitivity to intensity fluctuations in the light source and the optical interconnections.  A single sweep of the fibre optic interrogator used takes 1 second, and much faster systems are now available.

Thermal cycling of the embedded sensor was carried out with an initial cycle from ~30°C to 200°C and back to ~30°C. After each cycle the upper temperature of the next cycle was increased by 100°C until it reached 900°C following which the step increment was reduced to 50°C.  The heating and cooling rates were set to 3°C/minute and 2°C/minute respectively. The sensor was kept at each upper temperature for approximately 20 minutes before the cooling process started to ascertain whether any drift occurs at these elevated temperatures and to ensure that a thermal equilibrium had been reached. The temperature calibration curve obtained during the cycling process is given in [Figure 8]. The calculated repeatability of the temperature cycling up to 950°C is within ± 5°C (see Figure 8 inset). Above this temperature the sensor shows some drift which is still being explored.  After annealing, for further cycling the sensor repeatability is within the above accuracy.  The measured temperature sensitivity of the sensor at 400°C is ~7.8 m rad/°C which corresponds to a temperature resolution achievable of 3°C for the sensor system with a measurement speed of 0.5 Hz. The thermal mass of the actual sensor fibre is absolutely miniscule at 2.7?10-6g and the overall mass at the sensor end of the embedded capillary is around 100?10-6g.  Heat losses into the fibre optic down-lead and radioactive losses are also very small, hence the nominally response speed of the sensor is extremely fast, in the order of 100 °C/s and is easily capable of following any fast temperature excursion of the SS coupon in which it is embedded.

Figure 8: Temperature response of the embedded sensor over 10 temperature cycles.  The upper temperature range on each cycle was increased from 200°C to 1000°C as indicated in the graph key.  A typical F-P spectrum is presented in Figure 2.

Figure 9: Calculated sensitivity of F-P sensors as a function of cavity length.
Sensors with different cavity lengths were manufactured and tested, and results are as expected, as shown in [Figure 9], demonstrating that the embedded sensor is well isolated from any external strain influences. The increase in sensitivity with temperature is due to the nonlinearity of the thermo-optic coefficient of SiO2, as reported in earlier work [23].

Figure 10: Stability of the embedded sensor at 1000°C.  Temperature drift given in right side of the plot is also from the sensor.

Detailed studies of the long-term stability of the F-P sensors are reported in [6,24] demonstrating long term stability of ~10°C over 300 hours.  The long-term stability of the embedded sensor at 1000°C is given in Fig. 10.  The drift of the embedded sensor at 1000°C is <10°C for a period of ~10 hours.
The cross-sectional image of the embedded sensor coupon is taken after the temperature cycling experiments is shown in [Figure 11].  The coupon was above 1000°C for ~50 hours.  It is clear from the picture that silica capillary is not damaged due to the CTE mismatch of the host metal.  Cracks in the Ni jacket is visible in [Figure 11] which is believed to be due to the outgassing of the sulphate at high temperatures from the phosphor-sulphate content trapped in it from the Ni-Sulpha mate bath during its deposition.  Apart from poor bonding underside of the capillary no cracks are seen in the LPBF built SS316 structure. The density of the LPBF structure excluding the sensor part is estimated to be >99%.  From the point of view of structural integrity, the embedded capillary and sensor fibre are a foreign inclusion, and if considered as an element of zero strength in a stress analysis then should not necessarily pose a structural problem.  Hence, considering the overall cross section of the fibre and capillary and any remaining void as a non-load-bearing part, the occurrence of small gaps is acceptable.  

Figure 11: Cross section of coated capillary embedded in metal coupon.
Compared to previously reported[9-13]metal embedded fibre optic temperature sensors, the smart metal demonstrated in this paper have an upper temperature range of operation nearly twice as high and thus highlights the potential of incorporating embedded intelligence into the components of power generation or jet engine industry exposed to very high operational temperatures for their process optimization.

The long-term stability of such high temperature in-fibre Fabry-Perot sensors have been demonstrated at temperatures up to near 1000°C work [24]. Above this temperature the main limiting factor on the stability of the sensor becomes dopant diffusion and verification of the glass which modify the optical properties of the cavity.

In conclusion, a robust method to strain isolate and embed an in-fibre F-P cavity based high temperature sensor in to LPBF build metallic components is described. The sensor consists of an optical fibre end and a reflective splice to form the optical cavity. Silica capillary encapsulation isolates the sensor from any external mechanical influences. Additive manufacturing via LPBF is used to embed the sensors into metallic structures to permit in-situ temperature monitoring at highly elevated temperatures. The metallic jacket provides thermal shield and protects the capillary or sensor during the LPBF process.  The repeatability and then the accuracy of the embedded sensor are found to be better than ± 5°C. The long term (10 h) stability of the sensor at 1000°C is ~10°C. The results reported in this paper demonstrate the feasibility to reliably monitor temperature up to 1000°C inside 3D printed metallic components and the technology is not limited to LPBF processes but could equally be employed in similar 3D build processes such as e-beam fusion or blown powder fusion.

J. Mathew was the main researcher carrying out the experimental study and data analysis and drafted the main elements of this manuscript.  D. Polyzos operated the high temperature furnaces and provided analysis of the thermal cycling experiments and RRJ Maier conceived the main research topic and coordinated the research.

All authors contributed to the manuscript and J Mathew or RRJ Maier are the corresponding authors.

The work was supported by the European Union’s Seventh Framework Program for research, technological development and demonstration project under Grant 310279 (“OXIGEN”). The authors thank Alicona for additional support.

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Corresponding Author: Jinesh Mathew, Department of Physics, GITAM School of Science, India. E-mail: jmathew@gitam.edu.

Copyright: © 2022 All copyrights are reserved by Jinesh Mathew, published by Coalesce Research Group. This This work is licensed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.