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 low thermal conductivity in feedstock material and high plasma temperatures generally lead to inhomogeneous heating of particles in plasma spraying. Existing modeling methods can determine heat transfer within idealized spherical particles with homogenous morphology, but in many cases, particles have an agglomerated morphology, consisting of multiple smaller particles that are packed together. The reduced contact area between the individual smaller particles results in a drastic reduction of the effective thermal conductivity of the agglomerate. On the other hand, it enhances heat transfer from the plasma gas due to the increased particle surface area and penetration of the hot plasma into the agglomerate. Moreover, the momentum transfer from the plasma to the agglomerate differs from that of a homogenous spherical particle, which can significantly affect heating dynamics. This paper presents a novel particle modeling approach that accounts for all such phenomena. Differences in kinematics and heating dynamics of the agglomerates are analyzed with regard to their packing densities.

Coatings applied on steel molds used for casting aluminum parts have two main purposes: avoid mold metal reaction and control thermal transfer to obtain directional solidification. The coatings widely known to foundry operators are water-based sodium-silicate bonded ceramic suspensions; they are air sprayed and cured on mold surfaces and typically last for 100 casting cycles. Although thermal sprayed coatings have been shown to last more than 5000 casting cycles, they are not yet the preferred mold protection method. This study addresses the issue by developing a knowledge base of thermal transfer properties that can be achieved with air plasma sprayed magnesium zirconate powders. The properties are assessed on an instrumented mold using the inverse technique for different coating compositions.

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.

High temperature and fire-proof protection of metal structures issue involves formation of dense insulating coatings which possess low thermal diffusivity and capable to withstand high temperatures caused by open flame and other severe conditions. Such coating should be considered as sacrificial as it slowly decomposes during extreme high temperature impact. Such coatings intend significant extension of time required for heating and development of inelastic deformation in metal-based structures increasing service time in severe conditions. Several modifications of fire-proof coatings composed of organic binders were developed and investigated. Fireproof coatings were subjected to open flame test on an adapted burner rig. Open flame simulation with 1100°C was carried to estimate coating’s protection properties. Investigates coating showed reduction of temperature for 1000°C during 10 minutes. Such results achieved due to spumescent effect of coating’s coke layer.

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.

The objective of this work is the development of highly amorphous, iron based coatings for thermal barrier applications. Based on the results of previous work, a chemical composition of Fe 72 Si 4 B 20 Nb 4 was selected and modified in order to improve glass forming ability and corrosion resistance. Three metallic glass powder mixtures with different amounts of Cr and Fe were prepared, characterized, and deposited by air plasma and HVOF spraying. Different gas flow rates and standoff distances were used and particle temperatures and velocities were measured during spraying. The deposits were examined, tested, and compared and were found to have good potential for thermal barrier coating applications.

This study demonstrates a new surface treatment for thermal barrier coatings that combines ultrasonic vibration and laser remelting and assesses its effect on crack distribution, surface morphology, and grain refinement. YSZ coating samples were vibrated at 20 kHz at different power levels while being irradiated by a Nd:YAG pulsed laser operated at 5.2 J and 6 J. SEM examination revealed a uniform distribution of segmented network cracks in treated samples, which are shown to play an important role in relieving stress and increasing strain tolerance in topcoat layers, thus improving fracture toughness and thermal cycle life. Another important finding is that visible ribbon-like loops induced by variations in surface tension were eliminated as a result of improved surface convection facilitated by ultrasonic vibration. At a vibration power of 520 W, coating surfaces were uniform and flat, but at 1300 W, undulations and trough geometries were observed. The results of XRD analysis indicate that tetragonal to monoclinic phase transformations are prevented when ultrasonic vibration power is greater than 780 W.

This study investigates the effect of nozzle material on cold sprayed aluminum coatings produced using a downstream lateral injection system. It is shown through experimentation that nozzle material has a significant impact on deposition efficiency and particle velocity. It is proposed that the effects are related to complex interactions between particles and internal nozzle walls. The results obtained lead to the conclusion that nozzles with higher thermal diffusivity transfer more heat to particles when they make contact with internal surfaces, which increases deposition efficiency even though particle velocities are reduced.

In this study, 8YSZ and 24CeYSZ coatings were deposited on stainless steel by suspension plasma spraying. The suspensions were formulated using finely milled powder, water, and ethanol. Spraying parameters were modified by changing spray distance and torch scan speed and were the same for each material. Coating microstructure, phase composition, and porosity were assessed and thermal diffusivity was measured and used to calculate thermal conductivity.

Plasma generated by an SG-100 torch was applied to a spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) solid phase. The solids had a mean size of about 4.5 μm and were obtained by milling of commercial Metco 204 NS powder. The suspension was formulated with 20 wt% solid phase, 40 wt% water and 40 wt% ethanol. The plasma spray parameters were optimized with the electric power equal to 40 kW, working gases composition Ar 45 slpm and H 2 5 slpm, spray distance varying from 40 to 60 mm, and torch scan linear speed varying from 300 to 500 mm/s. Coatings with thicknesses ranging from 51 to 106 μm were sprayed onto stainless steel substrates. The porosity of the samples was found from the image analysis of metallographically prepared cross-sections of the samples to be in the range of 8 to 12%. Thermal diffusivity was measured with the use of the commercial NanoFlash system in the temperature range from room temperature to 523 K. The measurements were made with the use of the coatings sprayed on the substrate, and a 2-layer numerical model was developed to determine thermal diffusivity of the coatings. The diffusivity was in the range from 0.196 × 10 -6 to 0.352 × 10 -6 m 2 /s in room temperature depending on the spray parameters. The obtained data were then associated with the literature data of density and specific heat and experimental porosity to find thermal conductivity, which was in the range of 0.47 to 0.86 W/(mK) at room temperature, depending on the spray run.

Liquid injection plasma spraying is of growing interest for thermal spray applications like thermal barrier coatings and solid oxide fuel cells, since finely structured coatings offer improved properties over conventionally spray ones, for example lower thermal diffusivity and higher catalytic activity. One challenge is the optimization and understanding of the injection process. With a new high speed shadowgraphy setup, the injection and atomization of individual drops was observed and described in detail in this work which is, to our best knowledge, not reported before. A drop atomization cone model is derived from observations. A new modelling approach is developed which allows the prediction of the drop atomization cones by analytical calculations. The simulations are compared to measurements and deviations are explained by neglected effects which will be included in further developments of this model.

Cr 3 C 2 -NiCr coatings are commonly used to provide abrasion and erosion wear resistance on the surface of components, in particular for corrosive and atmospheric high-temperature environments. For these classical and new applications the knowledge of the thermophysical properties is highly important. In the present work the dependence of the heat conductivity on temperature of two HVOF-sprayed Cr 3 C 2 -25NiCr-coatings prepared by a liquid-fuelled HVOF-process from two different feedstock powders from room temperature up to 700 °C was determined. Thermal diffusivities, density functions, specific heat capacities and coefficient of thermal expansion (CTE) were measured in order to compute the heat conductivity for the coatings. All measurements were performed twice (as-sprayed and after a first thermal cycle) in order to take into account the structural and compositional changes. XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Heat conductivities (average of the two coatings) ranging from about 11 W/(mK) at 50°C up to about 20 W/(mK) at 700°C were determined. Differences between the two coatings were clearly detectable. The heat conductivity of the Cr 3 C 2 -NiCr coatings is significantly lower than determined previously for a WC-17%Co coating.

Many processes and systems require hot surfaces. These are usually heated using electrical elements located in their vicinity. However, this solution is subject to intrinsic limitations associated with heating element geometry and physical location. Thermally spraying electrical elements directly on surfaces can overcome these limitations by tailoring the geometry of the heating element to the application. Moreover, the element heat transfer is maximized by eliminating the air gap between the heater and the surface to be heated. This paper is aimed at modeling and characterizing resistive heaters sprayed on metallic substrates. Heaters were fabricated using a plasma-sprayed alumina dielectric insulator and a wire flame sprayed iron-based alloy resistive element. Samples were energized and kept at a constant temperature of 425°C for up to four months. SEM cross-section observations revealed the formation of cracks at very specific locations in the alumina layer after thermal use. Finite element modeling shows that these cracks originate from high local thermal stresses and can be predicted according to the considered geometry. The simulation model was refined using experimental parameters obtained by several techniques such as: emissivity and time-dependent temperature profile (infra-red camera), resistivity (four probe technique), thermal diffusivity (laser flash method) and mechanical properties (micro and nanoindentation). The influence of the alumina thickness and the substrate material on crack formation was evaluated.

Heat exchangers play a vital role in ongoing efforts to conserve energy. Plate-type heat exchangers typically consist of two flat separated flow paths in which heat transfer enhancing matrices are inserted. The combined effects of small irregular hydraulic diameters along with elevated heat transfer areas results in highly-efficient heat transfer to the external fluid. This allows for very versatile and compact heat exchanger designs. Typical plate-type heat exchanger fabrication methods such as brazing are labour intensive and limit post-processing operations like welding. In this paper, a novel micro-heat exchanger fabrication method using recently patented technologies is presented. The approach uses thermal spray processes such as Pulsed Gas Dynamic Spraying (PGDS) as an alternative to brazing for the production of a pressure barrier and integration of flow headers. Mesh wafer surfaces sealed using PGDS

The development of new hardmetal coating applications such as fatigue-loaded parts, structural components and tools for metal forming is connected with improvement of their performance and reliability. For modelling purposes the knowledge of thermophysical, mechanical and other material data is required. However, this information is still missing today. In the present work the thermophysical data of a WC-17Co coating sprayed with a liquid-fuelled HVOF-process from a commercial agglomerated and sintered feedstock powder from room temperature up to 700 °C was determined as an example. The dependence of the heat conductivity on temperature was obtained through measurement of the coefficient of thermal expansion, the specific heat capacity and the thermal diffusivity. Heat conductivities ranging from 29.2 W/(mK) at 50°C to 35.4 W/(mK) at 700 °C were determined. All measurements were performed twice (as-sprayed and after the first thermal cycle) in order to take into account the structural and compositional changes. Extensive XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Bulk samples obtained by spark plasma sintering from the feedstock powder were studied for comparison.

Thermal spray coating processes have been employed in the current study to deposit well-adhered, dense skins on the surfaces of open-cell nickel foams. Using foam with 10 and 40 PPI (pores per inch) pore sizes, square channels were made with a height of 20mm and having a length of 250mm. In a unique process that prevents the deposited skin from penetrating the foam substrate via a paste comprised of a thermoset resin and powder particles, a dense stainless steel skin with an average thickness of 400 μm is applied to the exterior of the foam sample. The result is a channel that consists of a Ni foam core and a stainless steel skin wall that can be used as a compact heat-exchanger by directing the coolant flow through the foam. To study the feasibility of the metallic foam heat-exchangers, hydraulic and heat-transfer characteristics were investigated experimentally. The local wall and fluid temperature distribution and the pressure drop along the length of the heat exchanger were measured for heat-flux of 1540.35 – 9627.38 W/m 2 . Experiments were conducted using air as the coolant and varying flow velocity from 10 – 80 L/min. For non-Darcy flow with inertia effects in the porous media, the Dupuit and Forchheimer modification is employed with the experimental results to determine foam characteristics such as permeability (K), Ergun coefficient (CE) and the friction factor (f). To measure the heat-transfer performance of the metal foam filled channels, a length average Nusselt number is derived based on the local wall and fluid temperatures. Heat transfer was shown to have nearly doubled compared to that of a channel without a foam core.

Cold and detonation spraying methods are based on the interaction of high-velocity particles with substrate. High quality coatings from various powder materials can be deposited. In both processes, the substrate experiences insignificant thermal effect. Thermally sensitive powder can be sprayed with no oxidation and decomposition. The initial powder microstructure and even nanostructure can be preserved under properly selected spraying conditions. This study is based on a comparative analysis of the mechanical, electrical, and heat transfer properties of a series of coatings deposited by cold and detonation spraying technologies. The coatings are produced from copper and aluminum powders using a commercial Cold Spray equipment CGT-4000 and an original computer-controlled detonation spraying (CCDS) installation developed by the authors. The coating microhardness, density, electrical and heat conductivity, adhesion, cohesion, etc. are measured and compared. Particular advantages and drawbacks of both spraying methods are discussed.

Analysis of melting mode of core wire at Arc Spraying helped to explain a possibility of incomplete fusion of a charge of a core wire. Influence of physicochemical parameters of feedstock and atomization modes was established. Analysis results helped to develop a metastable austenite type Core Wire for Arc Spraying of good wear resistance. Applications results are shown.

This paper presents a new approach for computer aided generation of optimized trajectories in thermal spray systems. Through a combination of CFD and FEM modeling, the influence of torch trajectory and speed profile on heat and mass transfer during deposition is assessed along with its effect on residual stress and coating properties. Coating experiments with WC-Co as the spray material were performed on real components in order to validate the developed programming and simulation tools.