Splat Formation Research Articles

Crystallization dynamics of metallic droplet modified with nanoinoculators at its impact with substrate

A physical and mathematical model is developed, and numerical studies are carried out to investigate the processes of non-equilibrium crystallization of a metallic liquid droplet modified with refractory solid nano-inclusions at its impingement on a solid substrate. The conditions are considered, which are typical of thermal spraying of metal coatings, including the plasma spraying. An analysis of the splat formation depending on the concentration of refractory nanoparticles within the metallic droplet volume, their size and substrate temperature is given. The characteristics of splats computed using the non-equilibrium and equilibrium models of crystallization are compared under the same remaining conditions. The conditions of applicability of models are discussed.

Study of the Splat Microstructure and the Effects of Substrate Heating on the Splat Formation for Ni-Cr Particles Plasma Sprayed onto Stainless Steel Substrates

The plasma spraying process is still poorly understood in term of the processes by which the coating is built up, especially coating interactions with the substrate. This present study enhances this understanding by studying, through a range of electron microscopy techniques, single NiCr splats plasma sprayed onto stainless steel substrates, which were first exposed to different heat treatments. The microstructure of the splats, particularly the splat-substrate interface, was characterized, and the formation of the observed features is discussed. Evidence of localized substrate melting and inter-mixing with the splat material was found, showing metallurgical bonding. The structures observed were also correlated to the treatment of the substrate, demonstrating how such treatments can influence the properties of the fully deposited coating by modifying the splat formation process. Most notably, heating the substrate during spraying was found to significantly modify splat formation by reducing splashing and increasing the extent of substrate melting.

Study of the Splat Microstructure, Splat-Substrate Interface, and the Effects of Substrate Heating on the Splat Formation for Ni-Cr Particles Plasma Sprayed on to Aluminum Substrates

This study investigates the mechanisms of formation of plasma-sprayed coatings, through the study of splat morphology and the splat-substrate interface of NiCr single splats sprayed onto Al substrates, at both micro- and nano-scale levels, using a range of electron microscopy techniques. This study provides direct observation of extensive substrate melting, together with chemical inter-mixing with the splat and the formation of metastable interfacial phases (including both non-equilibrium phases and metallic glasses). In addition, voids and a range of oxide phases were observed. The mechanisms of formation of these features are discussed. Two aluminum alloys (with and without Mg additions) and three different sets of substrate surface conditions (polished, and heat treated prior to and during spraying) were used to investigate the variations in splat morphologies induced by the different preparation conditions. It was found that the heating of substrate during spraying significantly reduced splashing.

Effects of Substrate Roughness on Splat Formation for Ni-Cr Particles Plasma Sprayed onto Aluminum Substrates

Roughening of the substrate, for instance by grit-blasting or etching, is often used before plasma spraying in order to provide a high degree of roughness that promotes mechanical interlocking of the sprayed coating and consequently improved adhesion. This study investigates the morphology and microstructure of NiCr splats formed on such rough Al substrates, where roughness was generated by a number of methods including grinding and etching. Cross sections of the splats and the splat-substrate interface were examined using a range of electron microscopy techniques. Localized substrate melting and chemical mixing with the splat material was observed, forming very particular structures. The formation of various oxides phases and voids was also noted and found to increase, along with the degree of the substrate melting, with increasing substrate roughness. The structures observed were related to the spray conditions and substrate morphology.

Study of the Splat Formation for HVOF Sprayed NiCr on Stainless Steel Substrates and the Effects of Heating and Boiling Pre-Treatments

The HVOF process is a widely applied thermal spray technique used to form dense coatings with high bond strength. However, little is known about the mechanisms by which the coating forms and adheres to the substrate. The present study investigates the splat formation process by examining the morphology and microstructure of NiCr single splats sprayed on to stainless steel, using a range of electron microscopy techniques. Notable features include evidence of the deformation of the steel substrate by the impacting particle, the presence of porosity towards the centre of the splat, and under the rim due to the curling-up of the splat, and the identification of several oxide phases, including Cr2O3, FeO and NiO. From these observations, a description of the splat formation process is proposed. Effects of the substrate surface chemistry were studied by comparing the morphology of the splats on several substrates having undergone various pre-treatments.

Sensors in Spray Processes

This paper presents what is our actual knowledge about sensors, used in the harsh environment of spray booths, to improve the reproducibility and reliability of coatings sprayed with hot or cold gases. First are described, with their limitations and precisions, the different sensors following the in-flight hot particle parameters (trajectories, temperatures, velocities, sizes, and shapes). A few comments are also made about techniques, still under developments in laboratories, to improve our understanding of coating formation such as plasma jet temperature measurements in non-symmetrical conditions, hot gases heat flux, particles flattening and splats formation, particles evaporation. Then are described the illumination techniques by laser flash of either cold particles (those injected in hot gases, or in cold spray gun) or liquid injected into hot gases (suspensions or solutions). The possibilities they open to determine the flux and velocities of cold particles or visualize liquid penetration in the core of hot gases are discussed. Afterwards are presented sensors to follow, when spraying hot particles, substrate and coating temperature evolution, and the stress development within coatings during the spray process as well as the coating thickness. The different uses of these sensors are then described with successively: (i) Measurements limited to particle trajectories, velocities, temperatures, and sizes in different spray conditions: plasma (including transient conditions due to arc root fluctuations in d.c. plasma jets), HVOF, wire arc, cold spray. Afterwards are discussed how such sensor data can be used to achieve a better understanding of the different spray processes, compare experiments to calculations and improve the reproducibility and reliability of the spray conditions. (ii) Coatings monitoring through in-flight measurements coupled with those devoted to coatings formation. This is achieved by either maintaining at their set point both in-flight and certain spray parameters (spray pattern, coating temperature…), or defining a good working area through factorial design, or using artificial intelligence based on artificial neural network (ANN) to predict particle in-flight characteristics and coating structural attributes from the knowledge of processing parameters.

Microstructural Study of Splat Formation for HVOF Sprayed NiCr on Pre-Treated Aluminum Substrates

In this study, the mechanisms of splat formation and the influence of substrate surface conditions were studied for NiCr splats HVOF sprayed onto Al. The specimens, having undergone various substrate pre-treatments, were observed at high resolution using various electron microscopy techniques with emphasis on the splat microstructure and the splat-substrate interface. From the observations made here, a model for splat formation is proposed, whilst the effects of surface chemistry on this process were investigated. It was found that that due to the deformation of the substrate on impact of the partially molten particle, effective contact may be achieved between the substrate and the splat. However, in many cases, much of the initial projected particle did not adhere to the substrate after impact. The presence of oxide or hydroxide layers on the surface of the substrate was influential on the adhesion of the NiCr splat

Study of the splat formation for plasma sprayed NiCr on aluminum substrate as a function of substrate condition

Abstract The study of splat formation, splat-substrate interactions and the influence of the substrate surface conditions on splat behavior is a key step in understanding and improving the plasma spray process. In the present work, NiCr particles were plasma sprayed onto aluminum substrates to form single splats, and their microstructure, along with the splat-substrate interface, was studied using a range of microscopy techniques (SEM, FIB, TEM and AFM). From the observations made, a model summarizing the splat formation process has been proposed. Substrates with various surface conditions, induced by heating and/or boiling treatment prior to spraying, were used. It was observed that on the non-boiled specimens, the contact between splat and substrate was poor, possibly due to the desorption of adsorbates/condensates present on the substrate surface, and no splats were formed on the boiled specimens, due to the presence of a thick hydroxide layer.

Comparison of In-Flight Particle Properties, Splat Formation, and Coating Microstructure for Regular and Nano-YSZ Powders

The relationship between atmospheric pressure plasma spray parameters and in-flight particle characteristics was determined. The morphologies of individual splats and the coating microstructure were studied for different stand-off distances and arc currents. Coating cross-sectional analysis showed that the total porosity of the coating increased with decreasing arc current, and increasing stand-off distance. Two different materials were used: the regular (r-YSZ) feed stock and the nano size (n-YSZ) agglomerated powder. The results illustrate that the r-YSZ coating has higher total porosity at higher arc currents than n-YSZ coating. The splat flattening degree and circularity was examined at different substrate temperatures for both powders. The results indicate that the flattening degree increased at high temperatures for the two materials, but the values for n-YSZ were higher than those for the r-YSZ. This study showed the operating regimes in which the use of n-YSZ yields improved coating properties.

The Solution Precursor Plasma Spray Coatings: Influence of Solvent Type

The influence of solvent type on splat formation and coating microstructure using the Solution Precursor Plasma Spray process was studied. Droplet with a high surface tension and high boiling point solvent experiences incomplete solvent evaporation process in the plasma jet leading to a porous coating. Droplet created from a low surface tension and low boiling point solvent undergoes rapid solvent evaporation, solute precipitation, pyrolysis, melting process in the plasma jet and forms splat upon impacting on the substrate; the build-up of splats results in a dense coating.

Study of the effects of surface chemistry on splat formation for plasma sprayed NiCr onto stainless steel substrates

The plasma spray process involves the melting and spraying of a fine powder onto a substrate using a plasma jet. However, understanding of the processes of splat formation and bonding with the substrate is limited because of the difficulties in performing high resolution studies of the splat–substrate interface. In this study, a nickel–chromium powder was sprayed onto mirror polished stainless steel substrates that have undergone various thermal and boiling pre-treatments to modify their surface chemistry. Detailed microstructural characterization of the splats and their interfaces with the substrate was performed using several electron microscopy techniques. Splats' shapes, the nature of the splat–substrate interface and occurrence of substrate melting and chemical intermixing, along with the presence of various oxide phases (Ni, Fe and Cr oxides) were observed and discussed in relation to the splat formation process. Such features were found to vary depending on the surface chemistry of the substrate.

The Role of Substrate Surface Chemistry on Splat Formation During Plasma Spray Deposition by Experiments and Simulations

NiCr single splats were plasma-sprayed on aluminum and stainless steel substrates, which were modified by immersion in boiling water, to grow specific types of oxide/oxyhydroxide on the surface. It was observed that there was no splat formation on aluminum substrate. In contrast, a significant number of splats were formed on stainless steel substrate. The differences in splat formation on aluminum and stainless steel surfaces corresponded to the variations of thickness and proportions of the oxide/oxyhydroxide layer on the surfaces. A three-dimensional numerical model was developed to simulate the impact of a droplet onto the substrate. The simulation illustrated good agreement with experimental observations. The effect of the oxide layer on the splat morphology was also examined. It was suggested that the splat morphology was more strongly influenced by water release from the dehydration of oxyhydroxide to oxide rather than by simple presence of the oxide layer on the substrate surface.

Wear and Corrosion Behavior of HVOF-Sprayed WC-CoCr Coatings on Al Alloys

WC-CoCr coatings were HVOF-sprayed onto an AA6082T6 substrate. Thickness values between 50 and 150 μm were produced by stepwise increase of the number of torch scans. This increase made the coatings not only thicker but also denser. This was due both to peening effects and by modifications to the splat formation mechanism, investigated by focused ion beam technique. Thanks to such densification, the hardness, the wear and impact resistance, and the corrosion protectiveness of the layers increased with the number of torch scans. The largest improvement occurred from 2 to 3 torch scans. These coatings were also compared to anodized films: cermets had superior wear and impact resistance but offered less corrosion protection.

Effect of Substrate Concave Pattern on Splat Formation of Yttria-Stabilized Zirconia in Atmospheric Plasma Spraying

Splat morphology of yttria-stabilized zirconia (YSZ) on a microconcave-patterned substrate was investigated by both numerical and experimental approaches under a dc-rf hybrid-plasma spray condition. The spreading behavior of molten droplets on a microdimple pattern was numerically simulated in a three-dimensional form. For comparison, impact of a YSZ droplet onto a microdimple pattern of a quartz glass substrate was studied in situ utilizing thermal emissions from the droplet. Concave aspects of a substrate surface play an important role in fingering/splashing of a spreading droplet as well as convex patterns. The main mechanism that causes splashing is likely due to the slipping of a spreading droplet at the edge of concave patterns. The viscosity decrease of the spreading droplet enhances the droplet splash.

Mathematical Modeling and Numerical Simulation of Splat Cooling in Plasma Spray Coatings

Particle deformation and cooling significantly affect the characteristics of thermally sprayed coatings, such as the adhesion and cohesion strength between a splat and a substrate and between splats, as well as the internal stresses of deposits. It is essential to understand these processes for the successful industrial application of thermal spray technology. However, to date, the microstructure of the boundary of a splat and the substrate has not been clarified, although much research has been conducted on splat formation and the cooling process. We have developed a microstructure model of the boundary between the splat and the substrate, based on splat morphology obtained from experiments. In the model, it is assumed that gaps, or voids, and contact areas are arranged on the splat boundary with the substrate in an orderly fashion. The model includes phase changes and heat resistance simulating the function of the microstructure during splat cooling. Assumptions in the model are that ambient gas trapped in the gaps, or voids, transfers heat only by conduction and not by convection or radiation. The results of the simulation indicated that the extent of gaps, or voids, significantly affects the rate of decrease of the average temperature of the splat surface, as well as the temperature distribution inside the splat.

Bubble formation in NiCr splat on aluminum substrate during plasma spray: Surface chemistry effect

NiCr single splats were plasma-sprayed on aluminum substrates which were modified by thermal treatments over a range of specific temperatures. It was observed that the splat morphologies changed from highly fingered splats to disk splats with increasing thermal treatment temperature. In addition, the density of the bubble pores which formed at the underside of NiCr splats significantly reduced with an increase of substrate temperature. X-ray photoelectron spectroscopy was used to examine the variation of surface chemistry with thermal treatment. The release of water from the dehydration of an oxyhydroxide layer, which was found to form on the substrate surface, was strongly correlated with the splat morphology and bubble formation at the splat underside. It is proposed that the water desorption from the substrate surface causes turbulence in the spreading process of the splat and the bubble formation at the splat underside, which leads to the splashing of the splat. Preheating or heating the substrate during spraying can partly remove the water prior to spraying, thereby greatly improving the contact between the splats and the substrate which enhanced the formation of a disk splat and significantly reduced the bubble formation at the splat underside.

Modeling and Simulation of Microstructure Formation for Porosity Prediction in Thermal Barrier Coatings Under Air Plasma Spraying Condition

Effective physical and mechanical properties of thermal barrier coatings are strongly dependent on the coating microstructure. The main objective of this study is the coating porosity prediction during the coating formation by simulation. For this purpose, two simulation approaches are presented. The first model takes into account physical impact, deformation, and overlying of powder particles on the solid substrate. Therefore, computational fluid dynamics and the volume of fluid method for this model were used. In the second approach, a faster and therefore more efficient model was developed, hence it was strongly simplified to simulate the formation of coatings and their microstructure. The splat formation was handled by calculating the flattening degree as a function of the Reynolds number. The disc-shaped particles were discretized by cuboids. The neighboring cuboids are moveable against each other at their contact areas. The displacement of those depends on material properties and the Reynolds number as well. Both approaches for predicting the microstructure were mutually compared.

Microstructure and microhardness characterization of a Fe-based coating deposited by high-velocity oxy-fuel thermal spraying

Based on the merits of high-velocity oxy-fuel (HVOF) thermal spraying, hardness and cavitation erosion resistance of steel were improved with a Fe–Cr–Si–B (5.8 wt.% B) alloy coating applied to stainless steel (1Cr18Ni9Ti) substrate by HVOF thermal spraying. The microstructures and microhardness of the coating were investigated. It was found that the morphologies of the coating were of well-flattened splats (containing some un-melted borides), semi-molten particles, featureless structures and a small amount of pores. The average Vickers microhardness of the coating reached 9.9 GPa, about four times that of the substrate (2.8 GPa). From the substrate to coating, the indentation marks or the corresponding microhardness values changed gradually. The higher microhardness values of the substrate near the coating can be attributed to work hardening resulting from the impact of the melted or semi-molten particles and grit sand on the substrate. The microhardness of the well-flattened splats (10.8 GPa) is higher than that of the semi-molten particles (about 6.2 GPa) and the featureless regions (6.4 GPa), and the microhardness of the featureless regions is higher than that of the semi-melted particles. This suggests that the fully molten state of the feedstock can result in the formation of featureless structures and well-flattened splats (containing some un-melted borides) present high microhardness.

Formation of Solid Splats During Thermal Spray Deposition

This paper reviews the findings of recent research on the formation of solid splats by the impact of thermal spray particles on solid substrates. It discusses methods of describing the substrate, by characterizing both chemical (oxide layers) and physical (surface topography, adsorbed and condensed contaminants) aspects. Recent experiments done to observe impact of thermal spray particle are surveyed and techniques used to photograph particle impact and measure cooling rates described. The use of numerical modeling to simulate impact and deformation of impacting particles is appraised. Two different break-up mechanisms are identified: solidification around the edges of splats; and perforations in the interior of thin liquid films created by droplet spreading without solidification. These two modes can be reproduced in numerical models by varying the value of thermal contact resistance between the splat and substrate. A simple criterion to predict the final splat shape is presented.

Plasma processing and deposition of powdered metal oxides consisting of hollow spherical particles

Features of the plasma processing of agglomerated metal oxide powders and the deposition of coatings of these powders, which consist of hollow spherical particles are considered in a particular case of powdered zirconium dioxide. It is shown that the formation of zirconia splats (melt drops spread over and solidified on a substrate surface) during the collision of hollow drops with the substrate has a more stable character as compared to the case of continuous (dense) particles of the same mass. Advantages of using hollow powders in the thermal spraying of coatings are discussed.