Design, manufacturing, and utilization of efficient heating systems for pipelines and closed-pressure equipment are necessary for cold regions to compensate for heat loss and prevent damages that are caused by freezing of the enclosed liquid. Given large-scale financial losses that stem from failure and bursting of the pipes, the development of novel, efficient, and affordable heaters, which can lead to improved efficiency, cost savings, and environmental benefits across various industries and applications, is of crucial importance. Heating systems have already been produced via different high-temperature thermal spraying techniques to achieve higher efficiency compared to conventional heating cables. In this study, tin, as the heating element, was deposited by using the cold spray process onto alumina coating that was fabricated by flame spraying (FS) to provide electrical insulation. Techno-economic assessment of fabrication and utilization of the coating-based heaters was conducted. It was found that cold-sprayed heater coatings exhibit improved performance compared to other thermally sprayed heater coatings and conventional heater cables. Further, their fabrication and utilization were more economically feasible. The results suggest that the new generations of coating-based heating systems may be competitive with conventional heat tracers that are widely used in industry.

In this work, we propose a novel visual navigation method to estimate the state of mobile and fixed cold-spray material deposition systems using a stereo-camera sensor installed in the workspace. Unlike other visual localization algorithms that exploit costly onboard sensors such as LiDARs or fully rely on distinct visual cues on the robot and grid markers in the environment, our method significantly reduces the cost and complexity of the sensory setup by utilizing a cost-effective remote stereo vision system. This allows for the localization of the target system regardless of its appearance or the environment, and enables scalability for tracking and operation of multiple mobile material deposition systems at the same time. To achieve this aim, deep neural networks, kinematic constraints, and learning-aided state observers are employed to detect and estimate the location and orientation of the deposition system. A physical model of the system with bounded uncertainty and fusion with a remote visual sensing module is proposed. This accounts for frames in which depth estimation accuracy is reduced due to perceptually degraded conditions in the cold spraying context. The algorithm is evaluated on a fixed and mobile setup that demonstrate the accuracy and reliability of the proposed method.

High entropy alloys (HEAs) are classified as a new class of advanced metallic materials that have received significant attention in recent years due to their stable microstructures and promising properties. In this study, three mechanically alloyed equiatomic HEA coatings – AlCoCrFeMo, AlCoCrFeMoW, and AlCoCrFeMoV – were fabricated on stainless steel substrates using flame spray manufacturing technique. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Vicker’s microhardness were utilized to characterize the fabricated HEA coatings. Furthermore, Joule heating experiments using a modified version of a two-probe test was used to measure the electrical resistivity of the HEA coatings. To prevent short-circuiting of the metallic coatings, a thin layer of alumina was deposited as a dielectric material prior to the deposition of HEA coatings. The microstructure of the HEA coatings showed the presence of multiple oxide regions along with solid-solution phases. The porosity levels were approximately 2 to 3% for all the HEA coatings. The HEA coatings had a thickness of approximately 130 to 140 μm, whereas the alumina layer was 120 to 160 μm thick. The electrical resistivity values were higher for all the HEA coatings compared to flame-sprayed Ni-20Cr and NiCrAlY coatings and AlCoCrFeNi HEA thin film, which may be attributed to the characteristics of HEAs, such as severe lattice distortion and solute segregations. The combined interaction of high hardness and increased electrical resistivity suggests that the flame-sprayed HEA coatings can be used as multifunctional wear-resistant materials for energy generation applications.

The piezoresistivity of flame-sprayed NiCoCrAlTaY on an electrically insulated surface of a steel substrate was investigated through cyclic extension and compression cycles between 0 and 0.4 mm for 1000 cycles and uniaxial tensile test. The sprayed NiCoCrAlTaY was in grid form with grid thickness of 3 mm and grid length of 30 mm while the electrical insulation was fabricated by flame spraying alumina on the surface of the steel. During mechanical loading, instantaneous electrical resistance measurements were conducted to evaluate the corresponding relative resistance change. Images of the loaded samples were captured for strain calculations through Digital Image Correlation (DIC) technique. After consolidation of the pores within the coating, the behavior of the flame-sprayed NiCoCrAlTaY was consistent and linear within the cyclic compression and extension limits, with strain values of approximately -1000 με and +1700 με, respectively. The coating had a consistent and steady maximum relative resistance change of approximately 5% within both limits. The tensile test revealed that the coating has two gauge factors due to the bi-linearity of the plot of relative resistance change against strain. The progression of damage within the coating layers was analyzed from its piezoresistive response and through back-scattered scanning electron microscopy images. Based on the results, the nickel alloy showed high piezoresistive sensitivity for the duration of the loading cycles, with little or no damage to the coating layers. These results suggest that the flame-sprayed nickel alloy coating has great potential as a surface damage detection sensor.

High entropy alloys (HEAs) constitute a new class of advanced metallic alloys that exhibit exceptional properties due to their unique microstructural characteristics. HEAs contain multiple (five or more) elements in equimolar or nearly equimolar fractions compared to traditional alloy counterparts. Due to their potential benefits, HEAs can be fabricated with thermal spray manufacturing technologies to provide protective coatings for extreme environments. In this study, the AlCoCrFeMoW and AlCoCrFeMoV coatings were successfully developed using flame spraying. The effect of W and V on the HEA coatings were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness testing. Furthermore, performance of the coating under abrasive loading was investigated as per ASTM Standard G65. Microstructural studies showed different oxides with solid-solution phases for all the HEA coatings. Hardness results were higher for the AlCoCrFeMoV coatings followed by AlCoCrFeMoW and AlCoCrFeMo coatings. Lower wear rates were achieved for the AlCoCrFeMoV coatings compared to AlCoCrFeMoW and AlCoCrFeMo coatings. The evolution of multiple oxide phases and underlying microstructural features improved the resistance to abrasive damage for the AlCoCrFeMoV coatings compared to other HEA coatings. These results suggest that the flame-sprayed HEA coatings can be potential candidates for different tribological interfaces while concurrently opening new avenues for HEA coating utilization.

Boundary layers on surfaces will change from laminar to turbulent flow after a critical length. Due to the differing heat transfer coefficients of laminar and turbulent flow, the point of transition can be detected by heating the surface and measuring surface temperature by thermographic imaging. Locating the transition point is crucial for the aerodynamic optimization of components. In this study, fiber reinforced polymer composites (FRPCs) were chosen as the test substrate. Experiments were conducted using the flame spray process and NiCrAlY coatings. Multilayered coatings consisting of an aluminum bond coat, a layer of alumina as electrical insulation, and a heating layer of titania were fabricated by atmospheric plasma spraying. Free-flight tests were conducted with a functionalized winglet in order to assess the ability of thermally-sprayed heating elements to detect the location of transition of the flow regime. The results showed that the thermally-sprayed elements heat surfaces uniformly, with sufficient radiation losses for thermographic imaging. It was also shown that the change in temperature at the point of transition was readily observable using thermography.

In this study, the effect of the substrate roughness and thickness on the heat transfer coefficient of the impinging air jet upon a flat substrate was investigated. A low-pressure cold spraying unit was used to generate a compressed air jet that impinged on a flat substrate. A detailed mathematical model was developed and coupled with experimental data to determine the heat transfer coefficient and surface temperature of the substrate. It was found that increasing the roughness of the substrate enhanced the heat exchange between the impinging air jet and the substrate. As a result, higher surface temperatures on the rough substrate were measured. It was further found that the Nusselt number that was predicted by the model was independent of the thickness of the substrate. The results of the current study were aimed to cover the influential substrate parameters on surface temperature of the substrate that eventually can affect the final quality of the cold-sprayed coatings.

Low-pressure cold spray has been used as an innovative method to deposit metal matrix composite (MMC) coatings: boron carbide-nickel (B4C-Ni) and tungsten carbide-cobalt-nickel (WC-Co-Ni) composites. The coatings were studied using scanning electron microscopy, X-ray diffraction with Rietveld refinement, and acoustic emission-coupled four-point flexural test. Indentation fracture toughness tests were performed on the WC-Co-Ni coatings, only. The results showed that the composites had reinforcing particle volume fractions of 45.8 ± 0.3 vol.% and 22.7 ± 0.1 vol.% for the WC-Co-Ni and B4C-Ni MMC coatings, respectively. Flexural tests were used to evaluate the fracture strain of the composites. In these tests, the WC-Co-Ni composite failed by brittle facture at approximately 0.5% nominal strain. The B4C-Ni composite showed flexural behaviour similar to that of an unreinforced Ni matrix. These results suggest that there was insufficient B4C within the coating to affect significantly the ductile failure mode of Ni matrix. Post bending fracture analysis showed the presence of straight, continuous cracks on the WC-Co-Ni surface and the indentation fracture toughness of WC-Co-Ni was found to be 1.2 ± 0.2 MPa·m0.5. Discontinuous, random cracks were observed on the B4C-Ni surface. The quantification of these properties is essential in evaluating the performance of the low-pressure cold sprayings to determine their potential applications.

In this study, a mathematical model based on 2D heat conduction was developed to determine the temperature distribution within different substrate materials during cold gas dynamic spraying. Heat transfer between the hot gas and substrate was estimated theoretically and experimentally and the results were compared with those obtained from numerical studies. The heat transfer coefficient was found to be dependent on the distance from the stagnation point of the impinging air jet. It was also concluded that at higher air jet temperatures, the Nusselt number of the spreading air film near the stagnation point could be affected by external heat exchange with colder ambient air.

This study evaluates the possibility of depositing hard B 4 C and TiC reinforcing particles in a Ni matrix using low-pressure cold spraying. It also investigates the effect of particle velocity and kinetic energy on deposition efficiency, microstructure, hardness, and wear resistance. B 4 C and TiC powders were blended at 50, 75, and 92 wt% carbide content with Ni powder comprising the remainder of the mixture. The impact velocity of sprayed carbide particles was calculated using a mathematical model based on the thermodynamics of compressible fluid flow through a converging-diverging nozzle. The model showed that the kinetic energy of TiC particles prior to impact was three times smaller than that of B 4 C, resulting in a higher carbide content (18 wt% compared to 8 wt%) due to reduced fracture and rebound of the TiC particles. Although the hardness values of both coatings are within the range of cold-sprayed WC-Co-Ni, wear rates were found to be high.

A low-cost, low-pressure (less than 1 MPa) cold spray unit was used to deposit tungsten carbide (WC)-based metal matrix composite (MMC) coatings on low carbon steel substrates. The coatings were then friction-stir processed (FSP) by using a flat cylindrical tool. Scanning electron microscopy (SEM), image analysis, micro-hardness testing, and ASTM Standard G65 dry abrasion wear testing were conducted to study the influence of FSP on the coating properties and its wear rate. It was found that porosity increased following FSP on the coating due to insufficient flow of the metal matrix material (nickel). The hardness of the WC-based MMC coating decreased after FSP as a result of increase in porosity and possible decarburization of the WC caused by the heat of the FSP. The SEM images taken from the cross sections of the FSPed coatings confirmed the effectiveness of FSP in distributing the WC particles within the matrix to produce a coating with uniform distribution of WC particles in the matrix. As a result, the abrasion wear resistance of the coatings after FSP increased compared to that of the as-sprayed coatings. This suggested that FSP can be considered as a method to improve the wear properties of MMC coatings.

The influence of flame spraying parameters on coating microstructure and electrical conductivity of aluminum- 12silicon coatings deposited on polyurethane substrates was studied. In order to evaluate the effect of the spray parameters on temperature distribution and its corresponding effect on coating characteristics, an analytical model based on a Green’s function approach was employed. It was found that the addition of air to the flame decreased the temperature within the substrate. Dynamic mechanical analysis (DMA) of the PU substrate revealed that the PU softened as the temperature increased. Therefore, by increasing the pressure of the air injected into the flame spray torch from 35 kPa to 69 kPa, the particles impacted a stiffer substrate. This led to increased deformation of the particles into splats upon impact, improved interlocking, and the overall coating had lower porosity and lower electrical resistance. The results obtained indicated that coating properties are sensitive to both thermal spraying parameters and temperature distribution within the substrate when depositing on elastomeric materials. The effect of torch stand-off distance on coating properties was also evaluated. It was found that higher air pressure can cool the substrate and, therefore, allow for a decrease of the stand-off distance. As a result of shorter stand-off distances, a coating with lower porosity and electrical resistance was deposited.

The temperature distribution of glass fiber-reinforced epoxy flat plates coated with a thin oxy-acetylene flame-sprayed aluminum-12silicon coating was determined experimentally. The composite plates were fabricated by filament winding. Following winding, but prior to and during curing, garnet sand was uniformly distributed on the glass fiber-reinforced epoxy plate surface. The sand roughened the surface such that there was adhesion of the aluminum-12silicon particles to the surface. A resistive heating wire was attached to the coated surface. Thermocouples were attached to the composite and coating surfaces to measure transient and spatial surface temperature distributions. The spatial temperature of the coating and polymer surfaces decayed uniformly throughout the coating-composite ensemble from the heating wire. It was also observed that the coating served to increase the surface temperature of the coating-polymer system compared to uncoated samples. This was attributed to the large thermal conductivity of the metal coating and the low thickness of the samples.

In this study, the temperature distribution of the surfaces of several substrates under an impinging gas jet from a cold spray nozzle was determined. A low-pressure cold-gas dynamic spraying unit was used to generate a jet of hot compressed nitrogen that impinged upon flat substrates. Computer codes based on a finite differences method were used to solve a simplified 2-D temperature distribution equation for the substrate to produce non-dimensional relationships between the surface temperature and the radius of the impinging fluid jet, the substrate thickness, and the heating time. It was found that a single profile of the transient non-dimensional maximum surface temperature could be used to estimate the dimensional maximum surface temperature, regardless of the value of the compressed gas temperature. It was found further that as the thermal conductance of the substrate increased, the maximum surface temperature of the substrate beneath the gas jet decreased. The close agreement of the numerical results with the experimental results suggests that the non-dimensionalized results may be applied to a wide range of conditions and materials.

Cold-gas dynamic spraying (“cold-spraying”) at low pressure (1034kPa/150 psig) was used to fabricate WC-Ni-Cu metal matrix composite (MMC) coatings. Tungsten carbide (WC)- based powder was mechanically blended with nickel (Ni) and copper (Cu) powder at various compositions. X-ray diffraction (XRD), scanning electron microscopy (SEM), and Vickers micro-hardness testing were conducted on the cold-sprayed coatings. Image analysis was used to determine the WC content in the coatings. XRD profiles showed that no decarburization or oxidation of the WC reinforcing particles occurred in any of the coatings. The WC content in the coatings increased as the WC content in the powder increased, but did not increase further beyond 96 wt. % WC content in the powder blend. The results from Vickers micro-hardness testing confirmed that the coatings with the highest amount of WC had the highest hardness value. The coatings fabricated with a powder composition of 96 wt. % WC + 2 wt. % Ni + 2 wt. % Cu yielded a hardness of 385 ± 73 HV 0.3 /10 (n = 50). These results suggest that it is possible to use cold-spraying at low pressure to fabricate WC-based MMC coatings with improved hardness.

This study assesses the photocatalytic properties of HVOF-sprayed nanostructured TiO 2 coatings, particularly their bactericidal effect. The surfaces of the coatings were lightly polished before being exposed to bacterial solutions of known concentration. The solution was dispensed on the coating and irradiated with white light in 30-minute intervals up to 120 minutes. On polished HVOF-sprayed TiO 2 coatings, 24% of the bacteria were killed after 120 minutes of exposure. On stainless steel controls, the percentage of bacterial cells killed was approximately 6% for most of the exposure times studied.

Plasma-sprayed, molten molybdenum particles (~55 µm diameter) were photographed during impact on grit-blasted glass surfaces that were maintained at either room temperature or at 350°C. Droplets approaching the surface were sensed using a photodetector and after a known delay, a fast charge-coupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat from behind the glass. A rapid two-color pyrometer was used to collect the thermal radiation from the spreading droplets to follow the evolution of their temperature and calculate the splat cooling rates. It was found that as the surface roughness increased, the maximum spread diameters of the molten molybdenum droplets decreased, while the splat cooling rates increased. Impact on non-heated and heated roughened glass with similar roughness values produced splats with approximately the same maximum spread diameters, skewed morphologies, and cooling rates. On smooth glass, the splat morphologies were circular, with larger maximum spread diameters and smaller cooling rates on non-heated smooth glass. An established model was used to estimate the splat-substrate thermal contact resistances. On highly roughened glass, the thermal contact resistance decreased as the glass roughness increased, suggesting that splat-substrate contact was improved as the molten metal penetrated the spaces between the large asperities.

Plasma-sprayed, molten nickel particles (60 µm diameter) were photographed during impact on oxidized 304L stainless steel surfaces that were maintained at room temperature or at 350oC. The steel samples were oxidized at different temperatures. Droplets approaching the surface were sensed using a photo detector and after a known delay, a fast charge-coupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat from the substrate front surface. A two-color pyrometer was used to collect the thermal radiation from the particles to follow the evolution of their temperature after impact. Molten nickel particles impacting on oxidized steel at room temperature fragmented significantly, while heating the surfaces produced splats with disk-like morphologies. Impact on steel that was highly oxidized induced the formation of finger-like splash projections at the splat periphery. The splat cooling rate and thermal contact resistance between the splat and non-heated oxidized steel varied significantly as the degree of oxidation increased; heating the oxidized steel greatly reduced the variations. It was suggested that the large variations in splat cooling rates and thermal contact resistances on the non-heated oxidized steel was due primarily to the presence of adsorbates on the steel surface.

In this work we present the numerical simulation results for the molten nickel and zirconia (YZS) droplets impact on different micro-scale patterned surfaces of silicon. The numerical simulation clearly showed the effect of surface roughness and the solidification on the shape of the final splat, as well as the pore creation beneath the material. The simulations were performed using a computational fluid dynamic software, Simulent Drop, The code uses a three-dimensional finite difference algorithm solving full Navier Stokes Equation with heat transfer and phase change. Volume of fluid (VOF) tracking algorithm is used to track the droplet free surface. Thermal contact resistance at the droplet– substrate interface is also included in the model. Specific attention is paid to the simulation of droplet impact under plasma spraying conditions. The droplet sizes ranged from 15 to 60 microns with the initial velocities of 70-250 m/s. The substrate surface was patterned by a regular array of cubes spaced at 1 µm and 5 µm from each other. The peak to valley height of each cube was between 1 to 3 µm. Different splat morphologies will be compared with those obtained from the experimental results under the same impact and surface conditions.

Plasma-sprayed, molten molybdenum particles (~40 µm diameter) were photographed during impact (with velocity ~110 m/s) on Inconel surfaces that were preheated or maintained at room temperature or 400oC. A droplet approaching the surface was sensed using a photodetector and after a known delay, a fast CCD camera was triggered to capture images of the spreading splat from the substrate front surface. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting on surfaces at room temperature disintegrated and splashed, after achieving a maximum diameter larger than 400 µm. Impact on preheated and heated Inconel produced splats with maximum diameters between 200 µm and 300 µm and with less splashing. The cooling rate of splats on the preheated Inconel was larger than that of splats on non-heated Inconel, suggesting that the splat-substrate contact was improved.