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.

In the current work, a NiCrAlY and Fe-based alloy are HVOF-sprayed due to the combination of high coating density and customizable coating properties. The oxygen to fuel gas ratio was varied to modify coating defects in a targeted manner. The results demonstrate material dependent defect mechanisms. Further investigations regarded residual stresses, hardness, and electrical conductivity. In particular, the thermal diffusivity proved to be very promising. Moreover, the coatings were compared with previous work on arc-sprayed coatings of similar chemical composition regarding insulation capability.

A micro-plasma system was investigated for its capability in additive manufacturing (AM). Micro-plasma AM system has the advantage of lower cost and higher deposition rate over the laser-based AM systems, and generates leaner and cleaner weld deposit than other arc-based AM systems. However, the microplasma system is complex and involves a large number of process variables. In this study, the effects of two arc and wire feed modes on dimensional consistency and hardness were firstly examined. Subsequently, one set of the specimens was further subjected to oxidation tests and the results were compared to that from conventional wrought Inconel 718. It was found that all four processes could produce crack free samples without measurable distortion. Some surface discoloration was observed, ranging from light straw to a purple tint. After heat treatment, the hardness of the samples varies from 403 to 440 HV, with the transverse surface showing slightly lower hardness values. The oxidation tests at 900 °C yielded similar weight change for AM Inconel 718 and its counterpart wrought alloy; however, the rate constant for wrought alloy was slightly higher. Microstructural analysis with SEM and EDS revealed a dendritic structure in the AM Inconel 718 and the presence of Nb-rich compounds in the interdendritic region. The polycrystal grain structure was not delineated in AM material as that in wrought 718. With the increase of exposure time, the oxide layer continues to increase at a higher rate, along with a sublayer of Ni 3 Nb above the metal substrate. In addition, after 200 hours, the wrought alloy developed porous chromia, while AM material exhibited uneven oxide thickness. In consideration of all aspects of the evaluation carried out thus far, it is concluded that the AM material produced by micro-plasma process is equivalent to wrought material in mechanical properties and oxidation performance.

The formation of Nickel coatings on stainless steel substrates and YSZ (Yttria-Stabilized Zirconia) on NiCrAlY in the Atmospheric Plasma Spray (APS) process is investigated. Coating formation over a substrate with an arbitrary shape (an inclined step in this paper) is considered. The topography of the coatings, as well as their microstructure, e.g., porosity, average thickness, and average roughness, are evaluated. An algorithm, which is based on the Monte-Carlo stochastic model, is employed. The significant difference between the coating temperature and that of the substrate leads to the formation of residual thermal stresses. These stresses are analyzed using Object Oriented Finite-element software (OOF) developed by the National Institute of Standards and Technology (NIST). An image of the cross-section of the coating is imported into the code, which utilizes an adaptive meshing technique and Finite- Element Method to calculate residual thermal stresses. The maximum stress in the coatings occurs at the interface between the coating and the substrate. The coatings' topography and microstructure are compared with those of the experiments.

Self-fluxing alloys are an established thermal spray system in case of superimposed tribological and corrosive loads. A dense coating with high bonding strength can be formed by fusing. Such coating system represent the state of the art in valve technology. Diamond-like carbon (DLC) top coatings are used for friction-reduction. As an alternative approach, this study focuses on the possibility of incorporating solid lubricants in self-fluxing alloy coatings. This allows for higher local stress and failure tolerance as well as a reduced process chain. Molybdenum disulfide (MoS 2 ) was studied as solid lubricant in the self-fluxing alloy NiCrBSiFe. In this preliminary study, the optimization of the MoS 2 content with up to 10.0 wt% was performed in spark plasma sintered (SPS) bulk materials. The wear behavior under oscillating wear conditions was investigated. Besides the decrease in coefficient of friction (COF), the wear resistance was increased by incorporating MoS 2 . Furthermore, the distribution of the solid lubricants within the SPS bulk material and the influence of the production route were analyzed.

Since the plasma sprayed coatings always present a limited interlamellar bonding, it is difficult for a plasma sprayed coating to be applied in corrosion environment without any post-spray treatment. In this study, a NiCr powder alloyed with boron was employed to fabricate fully dense corrosionresistant coating by plasma spraying through in-situ deoxidation effect of boron. As reported previously, plasma sprayed Ni 20 Cr 4 B coating presents fully dense microstructure with few isolated pores. Due to the oxide-free state of the inflight particles by the deoxidation effect of boron, the splats were effectively bonded upon impact so that the inter-splat boundaries were indiscernible. A long-term immersion corrosion test in NaCl solution was conducted for 80 days to confirm that the plasma sprayed Ni 20 Cr 4 B coating presents the superior resistance against the corrosion, which was comparable to the flame spray-fused NiCrBSi coating. Furthermore, the cross-sectional microstructure of the Ni 20 Cr 4 B coated Al alloy samples after 80 days immersion revealed that the plasma sprayed Ni 20 Cr 4 B coating was dense enough to completely block the penetration of corrosive substance in such an aqueous corrosion environment.

Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution suspension plasma sprayed YSZ TBC coating was applied on the spike contour of an additively manufactured aerospike engine demonstrator. The engine was designed for a hydrogen peroxide / kerosene 6 kN thrust at 2.0 MPa chamber pressure and was manufactured from nickel-based superalloy Inconel 718 powder using the laser powder bed fusion process (LPBF). Due to the novelty of the application of suspension sprayed YSZ thermal protection coatings on additively manufactured Inconel 718 components, extensive tests were necessary to characterize the interaction between the coating and the component.

In the current work, a typical NiCrAlY alloy and, moreover, amorphous Fe-based alloys are arc-sprayed for the desired application in cryogenic environments. Nitrogen is used as process gas, while the stand-off distance and number of passes were varied. The results demonstrate coatings with low, but varying porosity and oxide content and mostly high electrical conductivity. Especially the amorphous Fe-based coatings reveal homogeneous coating structures and promising properties. Further investigations regarded the deposition efficiency, tensile adhesive strength, hardness, durability under cryogenic conditions and the thermal diffusivity.

In the field of additive manufacturing, the demand for Extreme High-Speed Laser Material Deposition (EHLA) is increasing due to its unique process characteristics, economic efficiency as well as its great resource efficiency. The process is currently mostly used for surface functionalization through coating, by means of corrosion and wear protection. Thereby, almost all materials can be processed and nearly all material combinations can be created. The layers produced are dense and metallurgical bonded, and furthermore the surface roughness produced is low, so that only 20-100 μm has to be removed to produce a finished surface. However, it can also be used for the generation of 3D geometries. The greatest cost factor in the production is the coating material. With increasing requirements, for example in wear protection, cost-intensive special alloys or materials must be used. An opportunity to increase the areas of application in the field of wear resistance as well as increasing material efficiency is offered by combining EHLA with the innovative post-processing methods of hammering, solid as well as smooth rolling. Using these processes, the surface roughness can be reduced to a value of Rz 1-3 μm on the one hand and the surface hardness can be increased on the other hand. The hammering and solid rolling processes differ in their depth of impact. In the case of hammering, the impact depth can be a few millimeters and in the case of solid rolling only a few tenths of a millimeter. So far, the influence of hammering or solid rolling of additive manufactured volumes or surfaces has not been investigated. In the context of this study, the influence of hammering and solid rolling on a volume produced with EHLA is investigated. For this purpose, an EHLA produced volume of IN718 is built up and the influence of hammering as well as solid rolling on the surface roughness and hardness is analyzed.

During the thermal cycling process in MCrAlY-YSZ thermal barrier coating system, stresses are produced at bondcoat (BC)-topcoat (TC) interface due to the mismatch of the thermal expansion coefficients of the two coating layers. The stresses at the interface are not a single value and can be affected by the coatings’ microstructure. In this paper, finite element (FE) modeling method was used to study the behavior of the stress distribution at the coatings’ interface. The influence of the pore structure in the ceramic TC and the micro bulge structure at the metal BC surface was investigated. The results showed that both structures can change the stress distribution. The pores played a “stone-in-river” role, which trapped higher stress around them and simultaneously reduced the size of the macro stress zones in TC. The micro bulges at the TC/BC interface also trapped high stresses which could cause more interaction between TC cracks and BC roughness.

The use of suspensions in the thermal spraying process, makes it possible to apply sufficiently thin (<30 μm), metallic coatings made of nickel-chromium alloy 2.4869 (NiCr8020). High velocity oxy-fuel suspension flame spraying (HVSFS) is used to manufacture these thin metallic coatings in order to be able to effectively use them as electric panel heaters. Area heating capacities of 25 W cm -2 are possible with them and heating rates of 15 K s -1 even outperform many ceramic heating elements. In addition, it provides a flexible way to apply the heating coatings directly to the components to be heated. The use of fine powders in the micron and sub-micron ranges allows a more precise adjustment of the coating thickness, compared to conventional thermal spraying techniques, even in the thickness range below 10 μm. Therefore, an adaption to customer needs is possible regarding the electric panel heater characteristics, like electric resistance, applied voltages and current range and heating rates.

Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution, the influence of heat treatment and surface conditioning of the additively manufactured Inconel 718 substrates on the thermocycling performance of suspension sprayed YSZ coatings was investigated. The different surface conditions included as-built, sandblasted and milled substrate surfaces with and without heat treatment. YSZ coatings were applied using suspension plasma spraying (SPS) with commercial available suspensions. Thermal cycling tests (FCT) at 1100°C, 1300 °C, and 1500 °C were applied to coating systems until failure occurred. The microstructures of the samples were characterized before and after thermal cycling. The performance of the coatings was mainly influenced by the coating morphology and FCT test conditions and less by the state of the AM substrates. Columnar-like YSZ SPS sprayed coatings on AM Inconel 718 substrates seemed to be a promising candidate for rocket engine applications.

Different surface protection technologies were investigated in a waste-wood fired fluidized bed boiler. This biomass fuel environment is more aggressive than those firing virgin wood due to the elevated presence of sodium, potassium, lead, and zinc, leading to the deposit of alkali metal chlorides in conjunction with ash on boiler tube surfaces. As laboratory tests are seldom representative of the complex firing, chemistry, temperature, and local heat flux encountered in actual operating conditions, five different commercial, near commercial, and development coatings were applied to a 1 m length of plain carbon steel tubing used in the furnace walls. The coatings were fully characterized and measured prior to installation and after exposure. Iron and nickel-based weld overlays, two high velocity thermal spray coatings, and a laser-clad nanosteel coating were tested. After exposure, the tube was extracted from the boiler and corrosion scales and material losses were evaluated in comparison to unprotected tube material.

In this study, NiCr alloy coatings were deposited by arc spraying using different combinations and mixtures of pressurizing gases and other process modifications. Coating properties were examined mainly by SEM, EDS, and conductivity measurements. The results show significantly reduced oxygen contents and improved coating morphologies compared to reference coatings produced using current plasma processes. Improved microstructure is shown to have a positive effect on surface quality and specific resistivity, making it possible to texture arc-sprayed coatings just as successfully as the plasma-sprayed reference layers. Moreover, the temperature coefficients and resistivities of arc-sprayed NiCr were found to be superior to those of conventionally manufactured coatings.

This study investigates the microstructure and efficiency of coating-based heating elements produced by deposition of various powders, including aluminum oxide (Al 2 O 3 ), alumina-titania (Al 2 O 3 -TiO 2 ), nickel-chromium (NiCr), and copper, using flame spraying, suspension plasma spraying, high-velocity oxyfuel (HVOF) spraying, and cold spraying techniques. The main goals are to assess the dielectric strength of flame and plasma sprayed alumina, compare the electrical resistivity of HVOF and flame sprayed NiCr, and obtain coating cross-sectional images to shed light on the challenges and potential of different heating element designs. The Al 2 O 3 layer produced by suspension plasma spraying appeared to be more reliable due to its cauliflower-like structure, corundum content, and hygroscopic properties. Resistivity was found to be higher in the flame sprayed NiCr than in the HVOF deposit mainly due to discontinuities and imperfections such as cracks, pores, and oxygen content. The micrographs taken from sample cross-sections show penetration of flame-sprayed NiCr into the flame-sprayed Al 2 O 3 and Al 2 O 3 -TiO 2 layers, which decreases the effective thickness of the dielectric. However, interlocking between NiCr and Al 2 O 3 -TiO 2 coatings can be beneficial when cohesion is a concern.

The economic feasibility of using thermal-sprayed heat generating coatings for temperature control in steel pipes was investigated. A data-intensive model was developed to compare fabrication, installation, operation, and maintenance expenditures with those of conventional heating cables. The multi-layered coating consists of flame-sprayed Al 2 O 3 and NiCr layers and cold-sprayed copper. Scalability factors were incorporated in the model to estimate the total projected costs for fabricating the coatings as opposed to installing heat tracing. Although material costs for the coating and heat tracing were approximately the same, the cost of fabrication for the coating was higher due mainly to labor expenses. However, the coating-based system was found to be more energy efficient than heat tracing due to the good adhesion and reduced thermal contact resistance between the heating elements and pipe.

This study investigates the effect of incorporating different reinforcing particles on the microstructure, electrical resistance, and heating efficiency of flame-sprayed nickel-based coatings. Feedstock powders were prepared by adding Al 2 O 3 , TiO 2 , and WC particles to NiCrAlY powder, and the various combinations were applied to alumina-coated carbon steel substrates. A number of Joule heating experiments were conducted by creating voltage differences across the coatings and measuring temperature changes due to induced electron flow and associated resistive heating. It was found that the electrical properties of the ceramic particles have a major effect on heat generation and that there is considerable room for improvement.

This study demonstrates a method of plasma spraying in which the plasma is maintained in a laminar (rather than turbulent) state, achieving a much greater jet length with less ambient air engulfment. In the experiments, NiCr coatings were produced at spraying distances between 250 mm and 500 mm, showing that specific structures can be realized by changing stand-off distance. Structures with high porosity, for example, are generated at relatively short distances; dense structures, on the other hand, are obtained at longer stand-off distances that allow feedstock powder to reach a fully melted state. XRD analysis shows that the spraying process does not change the chemical composition of the material, and EDS results indicate that chemical and metallurgical bonding are achieved.

In this study, NiCrBSi powders with a size range of 30-50 μm were deposited on mild steel substrates by self-fusing atmospheric plasma spraying. Particle temperatures exceeded 2400 °C and the deposits were remarkably dense with low oxygen content. Based on the results, a novel strategy is proposed to directly deposit dense, oxide-free coatings by plasma spraying without the need of post-spray fusing processes.

Sand blasting and high-velocity thermal spray processes can produce residual stresses in superalloy substrates that can significantly influence microstructure development. To investigate this effect, single-crystal superalloy substrates were sand blasted using different levels of force (zero, light, and heavy) and then coated with a MCrAlY layer by HVOF spraying. Cross-sectional analysis of an as-sprayed sample revealed a subsurface depletion zone with a composition rich in Mo nano precipitates. Cross-sectional examinations after vacuum heat treating and at various points during oxidation testing showed that elemental interdiffusion occurred between the coating and substrate and that sand blasting intensity has a major influence on the depth of the interdiffusion zones.