Temperature sensors are critical components in many industrial and research applications, particularly in harsh environments where high temperatures, corrosion and mechanical stress are prevalent. In this paper, we investigate the use of plasma spray technique as a versatile and simple method to print thermocouples and Resistance Temperature Detectors (RTDs) on metallic and ceramic substrates. The thermocouples based on NiCr-NiAl coatings were directly printed using thick metallic masks, while the RTD’s were structured using laser ablation. The manufacturing methods and the preliminary characterization of these temperature sensors are presented and discussed.

A new method for fabricating microsensors which can provide accurate real-time temperature monitoring of thermal barrier coatings on gas turbine engines was developed. A high temperature K-type thermocouple sensor for hostile environments was deposited using a coaxial pulsed laser cladding process with optimized process parameters giving minimal intrusive features to the substrate and afterwards embedded in typical ceramic layers. The dimensions of the cladded thermocouple were about one hundred microns in thickness and width. The thermal and electrical response of the cladded thermocouple was tested before and after embedding over temperatures ranging from ambient up to approximately 500 °C in a furnace with flowing argon as protective gas. The results were compared to that of a commercial standard K-type thermocouple, which indicate that laser cladding is a promising technology for manufacturing microsensors for in-situ monitoring in harsh operation environments.

A substrate surface thermocouple was developed for thermal spraying. The substrate used for the study is a porous 430 stainless steel disk, though the thermocouple concept can be applied with other materials. Type N thermocouple wires are cemented in holes through the substrate, and then a copper coating is deposited across the surface to electrically connect the wire tips to complete the thermocouple circuit. The copper also promotes temperature equalization between the wire tips and the surrounding substrate surface to increase accuracy. Using finite element analysis (FEA), it was determined that the optimum thickness of the copper layer is 38 µm. With this thickness, the thermocouple should be able to measure peak-to-peak surface temperature swings due to a passing plasma jet within +/-3% when the copper thickness is uniform and all physical properties of the coating and substrate system are well-known. However, a number of assumptions were used for the FEA, so a detailed uncertainty analysis was performed. This analysis found that the expected accuracy window of the thermocouple is +19%/-10% as implemented for measuring surface temperature swings. For measuring average temperatures, the thermocouple is very accurate, because large heat fluxes into the substrate occur only when the plasma torch is directly in front of the substrate. Experimental measurements of surface temperatures with the optimized thermocouple are presented.

A study of thermal fluxes transferred during the HEATCOOL process is proposed. The concept of this process, specially designed to enhance the residual stresses relaxation, consists in the use of a consecutive three-step procedure during the coating elaboration (heating / spraying / cooling). The present study focuses on thermal exchanges occurring during the heating step. For this, the elaborated experimental equipment incorporates a series of ten holes aligned equidistantly with 5 mm separation. A burning gas mixture (premixed acetylene and oxygen) is injected through these holes and the burning gas jets impinge and heat the substrate. The stand-off distance between the heating device and the substrate may be adjusted between 30 and 90 millimeters. Concerning thermal fluxes transferred using this experimental device, a front work piece incorporating several thermocouples was used to perform heat flux measurements. In a first step, the case of a single hole was considered. Since this method is not able to provide the thermal flux directly, the corresponding thermal fluxes were deduced using an inverse heat conduction problem method that was specially developed. Results obtained using this inverse problem method based on experimental measurements are then compared with numerical predictions obtained using a computational fluid dynamic model representing the system. For this part, the PHOENICS software was used to perform the corresponding computations.

The aim of the present study concerns the effect of the chamber pressure on the structure of a plasma jet. Conventional vacuum plasma spray equipments are currently designed to operate at chamber pressures higher than 10 mbar. Nevertheless, recently, the exploration of lower pressure level conditions seems to have become a new challenge. In the present study, pressure levels as low as 0.5 mbar have been achieved and tested. It is shown that a decrease in the chamber pressure provides a longer and enlarged plasma jet whose length may be longer than one meter for the lowest pressure levels considered. CFD modeling results and photographs performed under vacuum conditions are proposed for the case a standard conical nozzle. Some additional characterizations (using thermocouples and emission spectroscopy) were also conducted but the results are currently being analyzed.

The integration of thermocouples into thermal spray deposits and especially into vacuum thermal spray coatings could provide temperature monitoring between the substrate and the coating or between two different coatings during the spray process and later during post treatments and service life. Thermocouples of 251µm in diameter were made using Chromel and Alumel wires. Electrical insulation was obtained using a ceramic cement. Astroloy and Copper coatings were successfully sprayed over these sensors and the temperature given by an embedded thermocouple was compared to the response of an infrared pyrometer during the spraying process.

A new instrument has been developed for measuring stresses due to particle quenching, temperature gradients during spraying, temperature fluctuations during coating formation and expansion mismatch between coating and substrate upon cooling. It records in situ and continuously the curvature of a substrate during spraying and upon cooling after spraying with a contacting displacement sensor. The substrate is fixed onto a pair of knife edges by springs. The knife edges are disposed on a water-cooled rotating cylindrical substrate holder and the substrate (2*15*100 mm 3 ) is parallel to the holder axis. The torch is moved back and forth parallel to the holder axis and the substrate temperature is recorded by a thermocouple spot welded to it. Examples of results with alumina coatings on steel substrates are presented.