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.

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.

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.

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.

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.

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.

CaZrO 3 coatings were alternatively prepared by air plasma spray and flame spray processes. The microstructural characteristics and crystalline phases of the coatings were comparatively studied as a function of the spraying temperature achieved with each technique and the stand off distance. Image analyses were used to estimate their porosity. Thermal diffusivity was measured on free-standing thick coatings using the laser flash technique. Thermal conductivity was obtained from the experimental thermal diffusivity and density data. The hardness of the coatings was evaluated by Vickers indentation tests. Finally, different thermal treatments were carried out to evaluate the evolution of the crystalline phases and the properties of the coatings.

In this study, three different types of Al 2 O 3 coatings were prepared by atmospheric plasma spraying. Some physical properties of these coatings were measured. Microstructure and wear morphologies of coatings were investigated by scanning electron microscopy (SEM) and optical microscopy (OM). The tribological behaviour of various coatings against stainless steel using a block-on-ring configuration were investigated in terms of the microstructure, physical properties especially the thermal diffusivity of coatings and the evaluation of tribological heat. Results revealed that the wear resistance of coatings used in this work showed a high dependence on their thermal diffusivity rather than other properties such as porosity, microhardness and bond strength: the higher the thermal diffusivity of a ceramic coating, the better is its wear resistance.

Thermal barrier coatings were produced using both Ar and N 2 as the primary plasma gas. Various aspects of the process and the coatings were investigated. It was found that higher in-flight particle temperatures could be produced using N 2 , but particle velocities were lower. Deposition efficiencies could be increased by a factor of two by using N 2 as compared to Ar. Coatings having similar values of porosity, hardness, Young’s modulus and thermal diffusivity could be produced using the two primary gases. The coatings exhibited similar changes (increased hardness, stiffness and thermal diffusivity) when heat-treated at 1400°C. The results point to the potential advantage, in terms of reduced powder consumption and increased production rate, of using N 2 as compared to Ar as the primary plasma gas for TBC deposition.

Type of powder feedstock greatly affects properties of zirconia coatings since they interact differently in a plasma flame and hence influence microstructure development. In this study, ZrO 2 -8wt% Y 2 O 3 thermal barrier coatings (TBCs) have been produced by atmospheric plasma spraying fused and crushed (FC) and hollow sphere (HOSP) feedstock powders. The sprayed coatings contained segmentation cracks going through the coating thickness. High substrate temperature during spraying gave rise to increased segmentation crack density (Ds). The FC powder has the capability of providing coatings with high segmentation crack density compared with the HOSP one. At lower temperature, the HOSP coating was more porous than the FC coating sprayed at similar temperature and hence exhibited a much reduced thermal diffusivity. At high plasma temperature, the HOSP particles attained higher particle surface temperatures and velocities than the FC ones. The particles temperatures and velocities for the HOSP particles were influenced more significantly by spray condition than those for FC particles.

This study on ceramic thermal barrier coatings (TBCs) presents baseline thermal conductivity data on as-deposited 7-8 wt.% YSZ and a paired-cluster rare-earth oxide doped YSZ, prepared using air plasma spray (APS). The thermal diffusivity for each coating was measured up to 1100°C using the laser flash method, and from these values, the thermal conductivity was calculated. The maximum benefit for thermal conductivity reduction in TBCs with a (GdO 2 , Yb 2 O 3 )-doped YSZ composition was highest for APS dense, vertically macrocracked microstructures, whereas in the case of low density APS TBCs, the reduction in conductivity was found to be more strongly influenced by horizontally-oriented, sub-critical defects and porosity within the coating microstructure.

Influence of induction plasma gas composition on Ti coatings microstructure and composition has been studied. Plasma sprayed Ti powder and coatings were prepared by using induction plasma system. Spheroidization of irregular shaped particles was observed in the powder collected after spraying with Ar-air plasma. Microstructures of the coatings were analyzed by a high-resolution scanning electron microscope. Image analysis of the backscattered images of the cross-sectional view of the coatings was used to calculate coating porosities. X-ray diffraction analysis was also performed to identify secondary phases, which have been formed in the coatings during plasma spraying. Partially induced nanostructures were observed in fractured areas in the coatings. The nanostructures were preferentially formed on the surface of splats. X-ray diffraction pattern reveals that a secondary phase has been formed in the coatings during spraying with air vol. at 16 % and 20 % in plasma gas. Changes of the coating density and thermal diffusivity were studied with respect to the formation of secondary phases and induced nanostructures.

Alumina-zirconia composite coatings were fabricated by plasma spraying using both of mixture of commercial feedstocks and agglomerated feedstock from fine powders of about 100 nm. The coating from mixture had an ordinary lamella structure which consisted of alumina splats and zirconia splats, whereas that from fine powders showed the striped contrast in the splat. X-ray deflection profiles revealed that the latter contained gamma-alumina, monoclinic-zirconia and tetragonal-zirconia crystals, which sizes were under 10 nm. These facts mean the fine crystals dispersed inhomogeneously. The nano-composite coating had a higher Vickers hardness and lower thermal diffusivity than the coating from mixture. Moreover, it showed good thermal stability and its crystallite size kept under 50 nm even after the heat treatment at 1500°C for 100h.

Laser infrared photothermal radiometry (PTR) can be used to determine the thermophysical properties (thermal diffusivity and conductivity) and interfacial defects (i.e. disbonding) of various thermal sprayed coatings on carbon steel substrates. PTR experimental results are compared with a one-dimensional photothermal model that can take into account roughness affects and interfacial defects by considering a roughness equivalent-layer and an equivalent-thermal resistance, respectively. The foregoing thermophysical parameters of the thermal sprayed coatings are obtained when a multi-parameter optimization algorithm is used to fit the PTR experimental results. The potential of the PTR technique for in-situ monitoring of the coating process and the characterization of the thermal sprayed coatings will be discussed in this paper.

PTA (Plasma Transferred Arc) reclamation of aluminum alloys by hard materials with a much higher melting temperature is very difficult. This is due to the high thermal diffusivity of these al1oys. Below a critical heat flux φc nothing happens and over φc the substrate melts very rapidly contrarily to what is observed with steel substrates. That explains probably why PTA is mainly used for steel reclamation. Thus the knowledge of heat flux transferred to the anode is a critical point to develop PTA reclamation on aluminum alloys and this is the aim of this paper. An experimental set-up was built to study the heat transferred to three substrates made of different materials : cast iron for reference, aluminum alloy and copper for its high thermal conductivity. The plasma torch was a Castolin Eutectic gun and allowed to inject a sheath gas around the plasma column. The copper, aluminum alloy and cast iron substrates, easily interchangeable, were the top of a water-cooled calorimeter allowing to determine the variation of the received heat flux with the working parameters : arc current, stand off distance, plasma forming gas momentum, sheath gas composition and momentum. The determination of the arc electric field allowed to calculate the arc diameter which was compared first with pictures taken with a video camera and second, with wear traces left on the anode material. Several correlations have been established to characterize the arc voltage and the anode heat flux.

The influence of input spray parameters on the state of plasma-sprayed zirconia powder is studied. The particle temperature, velocity and diameter are measured using an integrated optical monitoring system. The monitoring system allows the investigation of the particles behavior in the spray jet. The collected information is correlated to coating characteristics such as deposition efficiency, microstructure and thermal diffusivity. Results show that, by monitoring the state of sprayed particles, a better understanding of the coating microstructure and properties can be achieved.