Air plasma spraying is a variation of thermal spraying that is used, among others, for the production of thermal barrier and wear resistant coatings. High plasma temperatures enable the processing of ceramic powder particles which have a high melting point and cannot be processed otherwise. Due to their low heat conductance, the ceramic particles are not necessarily fully melted during their flight in the free jet and prior to the impact on the substrate surface. Experimental particle temperature measurements by means of particle diagnostics systems deliver merely the surface temperature of the particles while the melting degree of the ceramic particles remains unknown. Therefore, the temperature field within spherical Al 2 O 3 particles is numerically investigated for a commonly used particle size distribution by considering different particle sizes. The model includes a two-way coupled particle-laden free jet model and takes the latent heat of melting and evaporation into account. The effect of the particles size as well as the stand-off distance on the melting degrees of the particles in the given powder size distribution is determined.

Plasma Facing Materials (PFMs) suffer from very high heat load including quasi-stationary high heat load during normal operation and transient events with extremely high heat load during normal plasma operation and off-normal events. In this paper, W/Cu functional gradient coating was applied on CuCrZr substrate (250mm × 120mm × 30mm) with compositionally gradient W/Cu as bond coat (0.4-0.6 mm) and 1.5 mm thickness W coating as top coat via VPS for continuous deposition duration of 5 h. VPS-W/CuCrZr mockup with built-in cooling channel was prepared for evaluating the transient vertical displacement and plasma disruption events applied by high energy electron beam. The formation of cracks and surface melting of VPS W/Cu mockup were investigated under the two transient high heat loads (HHL). The coatings were able to absorb about 2 MJ/m2 in HHL without significant damage.

The homogeneity of thermal spray plumes is mostly dependent on the type of feedstock used. Powdery feedstocks, for example, promote homogeneity. If in-flight particles are atomized from a melting bath, however, as in twin wire arc spraying (TWAS), the spray jet is less homogeneous due to the fact that particles are generated by the impingement of an airflow on the melting tips of electrically conducting wires. This work aims to contribute to the understanding of the initiation of such particles in the TWAS process. To that end, cored wires filled with W-rich particles were sprayed, then the process was halted and the wire tips were examined to analyze how the filling powder interacts with the melted part of the velum. 3D tomograms show that the resolidified melt bath is interspersed with spherical and irregular-shaped W-rich particles. The irregular shape implies a partial melting of the W-rich particles.

Owing to the arc ignition in twin wire arc spraying (TWAS) the wire tips are heated in three different zones. The outer part of the wire tips (contact zone) is heated directly by the arc ignition (zone I). The wires material in this zone becomes fully liquefied. The heat propagation phenomenon raises the temperature of the area immediately adjacent (towards the fed wires) generating a doughy material (zone II). Next to the doughy zone (towards the fed wires) the transferred heat softens the wire material causing a permanent deformation (zone III). The deformation is due to the exerted aerodynamic forces of the atomization gas pressure. A high speed imaging system was used to observe the melting behavior, metal break up, and particle formation under different operating conditions. The liquidus metal in zone I is directly atomized in the form of smaller droplets. Their size is a function of the specific properties of the molten metal and the exerting aerodynamic forces. The doughy area (zone II) is the origin of the extruded metal sheets at the anode and cathode side. The extruded metal sheets in case of cored wires are shorter than the ones observed by solid wires. The extruded metal sheets support the re-ignition of the arc and therefore enhance the process stability in twin wire arc spraying. In this study the effects of adjustable parameters and powder filling on melting behavior, particle formation and process instability were revealed and a comparison between solid and cored wires was made. The findings can improve the accuracy of TWAS process modeling and enhance the atomization of metal droplets through the adoption of specific nozzle geometry modifications.

In this study, porous molybdenum (Mo) materials were prepared by flame spraying semi-molten particles to low velocity levels. The influence of spray particle state, including particle velocity and melting degree, on microstructure and porosity was investigated to understand the formation mechanism of the pore structure and connection between particles. The results showed that Mo sprayed particles at low velocities (<20 m/s) and limited semi-molten state can be generated by flame spraying. The annealed Mo deposits with the porosity ranged from 39% to 61% were deposited. High porosity in the deposit was achieved through the shielding effect of deposited particles, bonded by the settled melt in the particle/particle interface. Moreover, the porosity generally decreased with the increase of melting degree of spray particles prior to impact.

It is usually difficult to deposit effectively spray particles with a much limited melting in thermal spraying. In the present study, flame spraying was employed to produce yttria-stabilized zirconia (YSZ) particles at a limited melting. The melting degree of YSZ particles was controlled by flame combustion intensity and flying distance of spray particles within gas flame. The velocity of spray particles was quantitatively measured. The effects of spray distance and acetylene flow rate on the particle velocity, and deposition behavior were examined. The surface morphology of deposited particles was characterized by scanning electron microscopy. The bonding between deposited YSZ particles and YSZ substrate was examined from cross section. The spray particles in different melting states were obtained by changing flame spray parameters. The deposition experiment revealed that the YSZ particles at different melting states from only surface layer melted state, semi-melted state to substantially melted state can be successfully deposited on YSZ substrate by controlling substrate temperature. As a result, YSZ surfaces with different morphologies of particles from a near spherical shape, hat-on-frustum shape and hat-shape with different neck sizes, to a pancake shape and well spread disk shape have been created.

Thermal sprayed NiCrBSi wear resistant coating is widely used in industry life increasing of worn parts for protective coating of the metallic surfaces. The coating is based on the mechanical anchoring of the splats onto the substrate surface and thermal integration among the splats in the coating is not possible during the spraying process due to the operation temperature, which is lower than the melting temperature of the coating material. This, in turn, results in the formation of non-homogeneous structures and voids in the coating. One of the techniques to avoid such situations is to integrate the splats through the controlled melting. This can be achieved through the laser melting process. In the present study, laser melting of thermal sprayed coating is modeled to determine the melt layer thickness. A lump parameter analysis is introduced in the model study and simulations are carried out in relation to the actual laser melting conditions. The melt layer predicted is compared with the experimental measurements. The micro structural analyses prior and post laser melting process are carried out using SEM and XRD, and the mechanical properties such as hardness, were also investigated. It is found that the predictions of the melt layer are in good agreement with experimental results.

Thermally sprayed cermet powder coatings as well as bulk cermet materials sintered of carbide/metal powder blends are widely used in applications with severe abrasive wear conditions. A cost-saving alternative can be provided by using iron-based melt-atomised hard alloy powder feedstocks. Among them, commercial alloys containing high amounts of vanadium and carbon obtain outstanding wear resistance due to their high volume fraction of finely dispersed, hard vanadium carbides. However, their performance is still exceeded by cemented carbides. A further improvement of the wear properties of hard alloys basically can be attained by increasing their carbide content, concurrently considering the limitations of the melting and atomisation process regarding the melting temperature. A possible solution can be provided by alloying the basic system Fe-V-C with an additional strong carbide former like niobium. Subject of this work is the comparing investigation of the technologically important melting equilibria in the systems Fe-V-C and Fe-V-C-Nb.

Plasma spraying allows melting totally or partially micrometer sized particles, which flatten in about one is onto the substrate to build the coating by layering resulting solidified splats. The coating adhesion is essential and depends mainly on the behaviour of first lamellae in contact with the substrate. But in the plasma spray process about 108 particles/sec impact onto the substrate, and thus it is difficult to understand the role of the different spray parameters onto the coating quality. In order to get a better understanding of phenomena involved, it is necessary to study a single lamella formation. The experimental set-up is composed of a fast (50ns) two-colour pyrometer and an imaging system, comprising two fast (1 to 10 µs) CCD cameras triggered by the velocity signal of the particle in flight prior to its impact. This work is focused on alumina particles flattening onto stainless steel (304L) substrates preheated at different temperatures during different times.

An adapted HVOF system has been computationally investigated in order to test the effects of injecting a cooling gas on both the gas phase dynamics and particle behaviour through the system. An existing liquid-fuelled HVOF thermal spray gun is modified by introducing a centrally located mixing chamber. The gas phase model incorporates liquid fuel droplets which heat, evaporate and then exothermically combust within the combustion chamber producing a realistic compressible, supersonic, turbulent jet. The trajectory of each discrete phase powder particle is tracked using the Lagrangian approach, with the inclusion of heating, melting and solidification through each particle. The results obtained give an insight to the complex interrelations present between the gas and particle phases, and demonstrates the usefulness of this modelling approach in aiding the development of thermal spray devices.

Asymmetric melting behaviour of the electrodes is a process related feature of the twin wire arc spraying (TWAS) technique since the heating of the negative connected wire is different from that of the positive connected wire. Due to these differences in melting behaviour a tracking of particle velocity and temperature for both electrodes individually is very important. Particle velocity and temperature have been recorded from anode and cathode by positioning the tracking device on each side of the spraying gun. To draw the whole picture of the spraying jet particles have been tracked also from the top side of the spray gun. The goal of this study is to have an experimental data set-up for model building and simulation of depositing process in TWAS. Corresponding measuring devices have been employed to investigate the TWAS process by spraying of massive and cored wires.

A numerical method and software were developed to predict non-stationary conjugated conductive heat transfer, melting and possible evaporation of materials under high energy fluxes impinging onto solid body surface (plasma jet, arc spot, laser or electron beam), and also subsequent cooling and solidification of the melted substrate layer. In the numerical procedure, the finite-element method was employed. The processes of interest can have different characteristic time and spatial scales, which in addition can suffer drastic changes at heat flux densities q ?[108 ;1014 ] W/m2. An advanced procedure was developed to enable dynamic adaptive triangulation of domains involved in the current numerical solution and characterizing the different phase states (liquid or solid) of the materials. This procedure, belonging to the class of the frontal algorithms, allows one to break a solution domain into triangles based only on the domain boundaries. The model applications of the developed simulation software are illustrated.

The melting behavior of in-flight particle and its impact on splat morphology are studied. A group parameter, “melting index”, has been derived to correlate the melting status of inflight particles with particle size, velocity, and temperature which can be measured experimentally. Numerical simulations have been used to determine the unknown parameters in the melting index. The effect of particle size on its melting behavior has been investigated.

Molybdenum powder has been plasma sprayed on stainless steel, brass and aluminum substrates. The substrate melting phenomenon is observed and investigated by means of scanning electron microscopy (SEM) and scanning white light interferometery (SWLI). It is found that the flower-shape splat morphology is typical for molybdenum on all three substrate materials when the substrate is at room temperature. Notable substrate melting is manifested through the energy dispersion analysis of X-ray (EDAX) map and Robinson back-scattered image of cross-sections of splats. It has been shown that the substrate material plays an important role in substrate melting phenomenon. The lift angle of the petals of splats and the maximum crater depth have been characterized and compared. Both of these increase in the sequence, from stainless steel, brass to aluminum. A ‘volume of fluid’ (VOF) based model coupled with rapid solidification has been used to simulate splat deformation, solidification, substrate melting and resolidification. The numerical & analytical results agree quite well with the experimental data. A substrate melting mechanism is proposed based on the time scales of the droplet solidification and substrate melting to explain the formation of flower like splat morphologies.

This study examines the melting characteristics of wire feedstock used in arc and flame spray processes and how they relate to coating quality. A high-speed CCD camera reveals important details in how different types of wire melt away during spraying. Solid and tube cored wires, for example, melt off in a much more continuous manner than grooved wires during high-velocity combustion spraying. During arc spraying, however, no significant differences are observed. The paper also analyzes various coating microstructures and explains how they correlate with the melt off behavior of different wire designs. Paper includes a German-language abstract.

In the present study, hydroxyapatite coatings were deposited on Ti-6Al-4V alloy substrate by high velocity oxy-fuel (HVOF) spray technique. The as sprayed HA powders and coatings were analyzed with the aim to reveal the melting state of HA powders and its influence on coating properties. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were employed for the characterization of the starting powders and as-sprayed coatings. Differential scanning calorimetry (DSC) was performed to determine the recrystallization temperature of the amorphous phase in HVOF HA coating. Results show that different melting state of HA powders can be achieved through altering HA powder size and/or spray parameters. XRD result reveals that the as sprayed HA coating made from large powders with size of ~50 µm is composed of crystalline HA and very small amount of a-tricalcium phosphate (TCP). While the coatings deposited using fine powders around 30 μm demonstrated a lot of amorphous phase besides crystalline HA and small amount of a-TCP. The recrystallization temperature of the amorphous phase in HA coating is ~720°C. The adhesive strength of the HVOF sprayed HA coatings is ~31MPa and is largely dependent on the melting state of HA powders. This suggests that the fully melted state of the feedstock can result in the formation of amorphous phase, and simultaneously decrease the adhesive strength. It also suggests that the melted fraction of the powders is the most critical factor influencing the adhesive strength and phase composition of HVOF HA coatings. The partial melting state of HA powders is beneficial in terms of adhesive strength and crystallinity.

A numerical finite difference model has been developed to treat the transfer of heat and momentum between a gas environment and a particle injected into it. The model is based on an explicit solution scheme for the thermal field and explicit treatment of the momentum exchange. The latent heat associated with phase changes is simulated via a post-iterative heat accumulation scheme. Particle-gas heat transfer is represented by a heat transfer coefficient, which is a function of relative gas velocity. The validity of the model is confirmed via comparisons between predicted behaviour and previously-published experimental data for thermal histories and particle trajectories. Comparisons are also presented with predictions from previously-developed models. Results are then presented for the behaviour of hollow zirconia particles, with particular attention being paid to in-flight melting characteristics. It is shown that there is an optimum combination of particle size and wall thickness for the promotion of efficient melting, for a given gas flow and temperature field.

By means of Schlieren photography, enthalpy probe, mass spectrometry and the particle measuring system DPV 2000 the influence of the internal and external anode nozzle and torch geometry, on plasma jet quality for atmospheric plasma spraying was investigated. It turned out that there is a strong geometrical effect of the inner contour and that with a proper expansion of the hot core of the plasma jet a considerable improvement of the melting and deposition quality can be obtained. Also the outer torch contour is of influence on the spray process because it controls the formation and the intensity of turbulence and the interaction of the plasma jet with its surrounding and hence the cold gas entrainment.

This paper presents the experimental validation of a curvilinear novel nozzle design. Their effect on the speed and temperature of the particles in flight, on the porosity and adhesive force of the coatings is measured and compared with conventional conical nozzle devices. The nozzle testing is performed using a Miller SG100 dc plasma torch. It is observed that the curvilinear nozzle produced denser and more uniform coatings with lower porosity and higher adhesive force. This could be achieved by increasing the flight temperature of the particles and ensuring more complete melting of the particles. The velocity profile of the particles on the substrate remained unchanged. Paper includes a German-language abstract.

Dispersion welding can be used both for repairing worn machine parts and for atomization on surfaces with novel components. Knowledge of the temperature changes during melting and the shifting process of the scattered layer are important for the exact technological design, because the expansion and stress processes can be calculated from this data. In this paper, the expansion calculations are presented, which were determined on the basis of the measurement data during induction melting. Paper includes a German-language abstract.