Splat Formation Research Articles

Investigation of the Bonding Formation of a Plasma-Sprayed Cast Iron Splat on a Preheated Aluminum Substrate Using an Experimentally Based Numerical Simulation Method

In this study, the bonding formation of the cast iron splat deposited on a preheated aluminum substrate by atmospheric plasma spraying was investigated using a finite element method based on a heat transfer model built in a cylindrical coordinate system. To enhance the accuracy of calculations, thermal contact resistance (Rth) of the contact interface was incorporated into the calculation. A precise Rth value was obtained using a well-established relationship. The diameter of grains in the splat was determined by atomic force microscopy analysis. Furthermore, the physical parameters of the cast iron and the aluminum at different temperatures used in the simulation were calculated according to the composition of the materials. The calculations indicated that the interface temperature was below the substrate melting point during the entire observation period, confirming that no melting of the aluminum substrate had occurred. Thus, the formation of splat–substrate metallurgical bonding was attributed to the enhanced interface temperature and plastic deformation rather than the melting of the substrate surface.

Microstructural evolution and bonding of HVOF sprayed Ni particles on both mild and stainless-steel substrates

In the present study, Ni powder was sprayed onto both mild and stainless-steel substrates using high-velocity oxy-fuel (HVOF) deposition to comparatively analyse the effect of substrate material properties and surface condition on splat formation. A range of microscopy techniques including scanning electron microscopy (SEM), focused ion beam (FIB) microscopy, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) were employed to characterize both the splat morphologies, including their cross-sectional structure, as well as the nature of the splat-substrate interface. It was shown that the majority of the particles reached the substrate surface in a partially melted form owing to the high velocity typical in the HVOF process. Despite some splat splashing observed on the stainless-steel sample, the diffusion profiles, determined by STEM-EDX line scans, revealed evidence of elemental interdiffusion at the splat-substrate interface, suggesting metallurgical bonding in this sample. It was observed that splat morphologies, their frequency of occurrence and splat-substrate bond quality are all greatly affected by the surface condition of the substrate. These microstructural observations were correlated with the thermo-mechanical characteristics of the substrates to explain the mechanisms driving splat formation. Differences in the degree of plastic deformation of the substrates due to particle impact are also discussed.

Microstructural Characterization of HVOF-Sprayed Ni on Polished and Oxidized Stainless Steel Substrates

In the present study, microstructural analysis was carried out to investigate the effect of substrate surface chemistry on splat formation. Ni powder was sprayed onto stainless steel substrates exhibiting two different surface conditions (polished and oxidized) by high-velocity oxy-fuel (HVOF) thermal spray. X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical state of the surfaces which indicated the presence of the adsorbates on the oxidized substrate. Single splats of different morphologies from both samples were examined using a range of characterization techniques including scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy (EDS) interfaced to both SEM and TEM. This provided information on the particle solidification behavior and splat formation mechanism following impact on substrates with distinct surface conditions. The results showed the effect of oxidized surface adsorbates on splat morphology, pore formation and splat–substrate interactions. These microstructural findings, aided by theoretical models, revealed that a mixture of mechanical and metallurgical bonds exists between the splats and both substrates.

Effect of particle pre-oxidation on Ni and Ni20Cr splat formation during plasma spraying

During plasma spraying in air, molten metallic droplets can develop a surface oxide shell which is postulated to play a significant role on the subsequent impact and spreading on the substrate surface resulting in splat formation. In this work, oxide layers on Ni and Ni20Cr particles were formed through isothermal pre-oxidation prior to spraying and their influence on droplet spreading and splat-substrate interfacial interactions was investigated. The results showed that the presence of this oxide shell readily induced finger splashing, irrespective of the oxide shell thickness. In addition, small oxide nodules were frequently detected inside the splats. An oxygen-enriched interfacial interlayer was observed for the pre-oxidized-Ni splats. A mechanism is proposed to account for the inclusion of oxide fragments from the particle surface oxide shell, in the deposition of single plasma-sprayed Ni and Ni20Cr splats.

Deposition and cavitation erosion behavior of multimodal WC-10Co4Cr coatings sprayed by HVOF

Cavitation erosion is one of the major failure modes for engineering components operating in the marine environment. In this study, WC-10Co4Cr coatings with multi-scale WC particles were prepared on 304 stainless steel substrates by two high velocity oxygen fuel (HVOF) systems differentiated by the types of fuel. The splat formation process of the multimodal cermet particles was investigated, as well as coatings' porosity, microhardness, fracture toughness and electrochemical performance. In addition, the cavitation erosion resistance of the coatings was evaluated by ultrasonic vibratory apparatus in 3.5 wt% NaCl solution. The results show that the splats sprayed by high velocity oxygen liquid fuel (HVOLF) had a more disk-like shape, leading to much lower porosity and superior mechanical properties in the corresponding coating. The low porosity also significantly hindered the formation of cavitation sources and reduced the cavitation rate. More importantly, the synergistic effect of corrosion on cavitation erosion made the coating much more vulnerable in NaCl solution than in fresh water.

Splat formation mechanism of droplet-filled cold-textured groove during plasma spraying

In this study, a numerical simulation and experiments were performed to analyze the formation process of splat in the laser texture groove of filling molybdenum droplets during atmospheric plasma spraying. Texture grooves were extracted using a 3D surface topography instrument, and the heat transfer and phase transition processes of droplet hydrodynamics were analyzed. A 3D numerical model of the droplet-filling texture groove was developed using a commercial software. The droplet diameter, impact velocity, and temperature were set to 50 μm, 110 m/s, and 2917 °C, respectively, to fill the cold-textured grooves. The cross-sectional morphology and element distribution of the filled coating were characterized using scanning electron microscopy and energy dispersive spectroscopy, respectively. Numerical results are consistent with those of the experiment. This study reveals the splat formation mechanism of droplet impact, spreading, and solidification in textured grooves and the grooves’ wall temperature. Results show that the fastest spreading velocity of the droplet contact groove is 4.63 times that of the initial impact velocity. The groove wall has a maximum melting depth of 2.6 μm. In the process of droplet spreading, the fluid layer gradually solidifies, and the transient spreading is accompanied by solidification.

Protective Plasma Sprayed Coating forThermo-Sensitive Substrates

Plasma spray is one of the surface treatment techniques that consist on the deposition of a thin coating onto a targeted substrate. Coating is built up by successive accumulation of layered splats resulting from impact and solidification of molten particles into thin ‘‘splats’’ onto the substrate. The process of droplet impact, spreading and solidification is then a crucial process in coating formation. This technique may be also used for thermo-sensitive materials such as wood by applying a metallic coating for protective or decorative purposes. However, when applying a ceramic coating which provides a high protection against hot temperatures like fire, wood may be damaged because of the high temperature at which the ceramic molten particles arrive at the substrate. In this paper, a numerical simulation based on the Finite Elements Method is carried out in order to simulate the process of the first splat formation onto a wood substrate under traditional plasma spraying conditions. The computations are carried out on a fixed eulerian structured mesh using the level set method to track the interface between the molten particle and surrounding gas. The effects of operating conditions as well as the droplet characteristics that allow applying ceramic coating onto a wood substrate without any damage to this thermo-sensitive material are investigated.

An Investigation on Splat and Flattening Behavior of Thermally Sprayed Copper on A Rough Surface: A New Approach

All thermal spray coatings are finally deposited on a rough and active grit-blasted surface of the job. But, available literatures are reporting splat and flattening behavior on a polished surface. There is a gap in thermal spraying to understand actual solidification on a rough surface. Therefore, in the present work an attempt has been made to study of splat formation of thermally sprayed copper onto grit-blasted rough surface. An optimization study is done to collect rounded/semi-rounded disk like splats to set spray parameters. Optimized parameters were also tested at four different gun traverse speeds to fabricate thin Cu coatings (30-50µm).

Deposition characteristics of CeO2-Gd2O3 co-stabilized zirconia (CGZ) coating prepared by solution precursor plasma spray

A 16 mol% CeO2-4 mol% Gd2O3 co-stabilized zirconia (CGZ) ceramic top coat for thermal barrier coating (TBC) was obtained by solution precursor plasma spray (SPPS). This work describes the influence of the spray parameters, such as spray distance, atomizing pressure, and solution flow rate on microstructures of the CGZ coating. Single scan experiments were performed to better understand the formation and microstructure of splat. Scanning electron microscopy (SEM) show the atomizing pressure and spray distance have a strong influence on splat and characteristics of coating. The atomization pressure of 1.3 bar and spray distance of 35 mm are conducive to obtain larger sized splat and dense coating. The melting state of the coating became worse with increasing solution flow rate. Precursor decomposition is an exothermic reaction, which occurs mainly in the temperature range of 200–600 °C, analysed by thermogravimetry (TG)/differential scanning calorimetry (DSC). The phase compositions of the coatings were investigated by X-ray diffraction (XRD) and Raman spectra, which confirmed that the coatings were composed of t and c-ZrO2, with minor m-ZrO2 under certain processing parameters.

The Surface Chemistry of the Mild Steel and Glass Substrates for Thermal Spray

The surface chemical condition of substrates plays a key role in single splats formation. Single splats are the foundation of thermal spray coatings. In this study, the surface chemistry of the mild steel and glass substrates were investigated. The result indicated that the 100 °C heat treatment released the absorbents of mild steel substrates. The adsorbed water disappears from the surface of the mild steel sample after one hour of heat treatment at 100 °C; there is no adsorbed water on the surface of the glass sample.

Computer-aided detection of the optimal spraying modes of coatings based on intermetallic titanium-aluminum alloys

The computational experiments results on simulation of a layered structure formation and the prediction of characteristics (porosity, roughness, adhesion) of metallic coatings based on intermetallic titanium-aluminum alloys (α2-Ti3Al, γ-TiAl and TiAl3) are given. The coatings under conditions typical of atmospheric plasma spraying (APS) and for high-velocity spraying (HVOF, D-Gun Spraying, etc.) by computer-aided design have been prepared. The ranges of the “key physical parameters (KPPs)” values of the sprayed particles, which ensure the stable formation of spreading and solidified melt particles (splats) on the sprayed surface for each of the three types of intermetallic titanium-aluminum coatings, have been detected. Taking into account the stable splat formation conditions, as well as the minimum of porosity (and roughness) and the maximum adhesion strength of the simulated coatings, the optimal spraying modes of coatings based on intermetallic titanium-aluminum alloys (Ti3Al, TiAl and TiAl3) were determined.

Plasma-sprayed nickel splats on chromium substrates: The role of substrate preheating and thermal conductivity

The effect of chromium substrate preheating on the spreading behavior of plasma-sprayed nickel splats was characterized. The interfacial features along the splat-substrate interface were analyzed by focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In addition, the transient droplet spreading process was modelled in a simulation study. The results showed that the presence of a large fraction of surface moisture on the substrates induced splat fragmentation. However, splat fragmentation was completely constrained on chromium preheated to a low temperature (373 K) where a certain amount of surface moisture was still present. The high thermal conductivity of chromium substrates increased droplet solidification rates, decreasing the interaction time between the spreading droplet and the vaporized gas layer. Accordingly, splat fragmentation was suppressed and disk-shaped splats were formed. Fast solidification not only refined the final splat microstructure, but also promoted the formation of finger-splashed disk splats. In addition, the extent of elemental diffusion across the splat-substrate interface was decreased due to the low interfacial temperature.

Prediction of the functional characteristics of plasma titanium aluminide coatings by computer simulation

The results of stochastic simulation (computer-aided design) of a layered structure formation and functional characteristics (porosity, roughness, adhesion) of metal coatings based on titanium aluminide powders (γ-phase TiAl) under conditions typical of atmospheric plasma spraying are given. Taking into account the stable formation of a metal splats and titanium aluminide coatings, as well as minimum porosity and maximum adhesive strength of coatings, the optimal plasma spraying mode has been obtained: the particle velocity Up = 250 m/s, the particle temperature Tp ≤ 2050 K, the particle diameter Dp = (30 ± 10) μm.

A computational approach for predicting microstructure and mechanical properties of plasma sprayed ceramic coatings from powder to bulk

In this study, a multi-pronged computational approach is developed to predict the effect of spray parameters on aluminum oxide splat formation and mechanical properties of the coatings. Effect of plasma power and substrate preheat temperature on the splat formation and coating's elastic modulus are studied. The splat morphology is investigated using computational fluid dynamics approach. Simulated splat morphologies show a good agreement with the experimentally obtained splats. Three-dimensional coating structure is constructed using the stochastic approach with different simulated splat morphologies, such as the disk, fragmented and fingered shapes, and their volume fractions. Microstructure-based Finite Element Analysis is used to compute the elastic modulus of the coating, which is found to be higher (~12%) than the experimentally obtained value. A correction factor (fis) is introduced, which takes into account inter-splat cracks, interface bonding, and other effects like curling of splats and splat sliding. After the correction factor, the computed elastic modulus for simulated coating was comparable to the experimental values. This study shows that the proposed computational approach can predict the mechanical properties of the coating and is promising for developing plasma-sprayed coatings with predictable properties. This methodology can be extended to other materials systems resulting in a desktop manufacturing approach of plasma sprayed coatings with a reduced number of experiments.

Impact interaction of in-flight high-energy molten volcanic ash droplets with jet engines

The turbine technology incorporated in jet engines is inherently vulnerable to attack by environmental silicate debris. Amongst the various kinds of such debris, volcanic ash is a particular threat as its glass softens to a liquid at temperatures of 500–800 °C, far below jet engine operating temperatures of ∼1500 °C. As a result, ingested re-molten droplets impact and form splats on the protective thermal barrier coatings (TBCs). Investigation of the damage to jet engines ensuing from this process has, to date been restricted to forensic observations after critical encounters. Here, we employ a thermal spray technology to recreate the ‘in-situ’ generation of molten volcanic ash droplets and observe their morphological evolution and interaction with TBCs. The mechanism of splat formation is found to depend both on substrate topography and on in-flight droplet characteristics, whereby splat circularity increases with surface roughness and with the product of the Weber and Reynolds numbers. The experiments reveal that the molten ash droplet adhesion rate is dictated by droplet temperature and viscosity, ash concentration and substrate roughness. A new dimensionless number, S, is developed to quantify the molten ash droplet adhesion rate to both substrate topography and in-flight droplet characteristics. These findings provide a greatly improved basis for the quantification of the hazard potential of volcanic ash to jet engines and should be incorporated into protocols for operational aviation response during volcanic crises.

Flattening behavior of micro- and nano-sized yttria-stabilized zirconia particles plasma-sprayed on smooth preheated (610 K) nickel substrate: part I

The work presented in this paper is devoted to the impact on Ni substrates of molten oxide particles (yttria-stabilized zirconia (YSZ)) either in the micrometer-sized range (between 20 and 100 µm) or in the submicrometer range (between 0.3 and 1.1 µm). YSZ was chosen because no oxidation occurs when plasma-spraying it in air. Micrometer-sized particles were injected into a hot plasma stream obtained with a DC plasma torch (Oerlikon Metco F4 torch) working with a mixture of Ar and H2 (Ar 33 slm and H2 10 slm). Correlatively an ethanol suspension of submicrometer-sized particles was injected into an Ar–He (Ar 40 slm and He 20 slm) DC plasma jet. Upon impact on a smooth Ni substrate preheated in air at 610 K both sizes of particles flatten and form splats. To achieve a much better understanding of splat formation, measurements of the velocity and temperature of an in-flight single particle just prior to its impact and at its impact were developed. Unfortunately, with submicrometer-sized particles, in-flight measurements of a single particle become impossible and only mean values resulting from a few particles are measured, making interpretation trickier. This study focused on the flattening on smooth substrates, presented as Part I, and will be followed by another study, to be presented as Part II, concerning splat formation on rough substrates adapted to the size of sprayed particles and then splat layering. Of course, the rough surfaces will also be preheated as the smooth ones at 610 K.

Effect of substrate temperature on the microstructure and interface bonding formation of plasma sprayed Ni20Cr splat

Ni20Cr splats were sprayed onto polished substrates at different preheating temperatures in an Ar atmosphere by low pressure plasma spray to reveal the dominating factor driving the bonding formation of the splats. The splat morphology, microstructure and splat-substrate interface were characterized by SEM, EBSD and TEM. The interface for examination of typical splats was prepared by focus ion beam (FIB). Disk-like splats were obtained on AISI 304 stainless steel substrate preheated to temperatures of 100 °C (cooling down from 350 °C) and 350 °C, whereas few short projections in the periphery were observed on the splat deposited at 550 °C. For all splats, three typical distinct zones with different microstructure features were observed on the splat surface by SEM and EBSD, including central coarse grain zone, transitional medium grain zone and peripheral fine grain zone. The interface bonding formation is closely related to the microstructure distribution. TEM observations demonstrated that the bonded interface only formed in the central coarse grain zone which provides a facile approach to access the bonding of plasma sprayed splat. The present work also reveals that increasing the substrate temperature cannot significantly improve the interface bonding due to the existence of oxide layer. The mechanism dominating the formation of those microstructures was also discussed.

On growth of suspension plasma-sprayed coatings deposited by high-enthalpy plasma torch

Introduction of suspension plasma spraying (SPS) brought novel coating microstructures unachievable by other thermal spraying techniques. Both columnar and dense layers are of particular interest for various applications. Deeper understanding of the coating buildup mechanisms is essential for tailoring of the coatings microstructure and properties. This paper contributes to elucidation of the buildup mechanisms with focus on high enthalpy plasma torches, in particular the hybrid water/argon-stabilized plasma torch WSP-H 500. The main differences between the formation of finely structured dense coatings and columnar coatings were studied for alumina and yttria-stabilized zirconia (YSZ) by comprehensive evaluation of individual splat formation, gradual coating growth, and geometrical characteristics of the deposits.It was clearly demonstrated that the coating microstructure is generally controlled by morphology of individual deposited splats which is influenced namely by formulation of the suspension and mean trajectory of the fragmented suspension droplets in the plasma jet, both influencing size, flattening ratio and thermal history of deposited splats.Larger droplets having proper thermal treatment provided miniature lamellae similar to conventional plasma spraying, eventually leading to the development of low porosity (dense) structures. With decreasing splat size and increasing distance from the plasma jet axis, coatings tended to develop more porous columnar structures. After characteristic number of deposition cycles, lateral growth of the columnar structures became saturated, i.e. the equivalent diameter of the columnar structures became stabilized at about 75 μm, when observed from the free-surface. Material deposited from the plasma fringes was only loosely bonded and could be easily blown away from the surface which explains absence of interpass porosity in the deposited coatings.

Analysis of the Near-Wall Flow in a Turbine Cascade by Splat Visualization.

Turbines are essential components of jet planes and power plants. Therefore, their efficiency and service life are of central engineering interest. In the case of jet planes or thermal power plants, the heating of the turbines due to the hot gas flow is critical. Besides effective cooling, it is a major goal of engineers to minimize heat transfer between gas flow and turbine by design. Since it is known that splat events have a substantial impact on the heat transfer between flow and immersed surfaces, we adapt a splat detection and visualization method to a turbine cascade simulation in this case study. Because splat events are small phenomena, we use a direct numerical simulation resolving the turbulence in the flow as the base of our analysis. The outcome shows promising insights into splat formation and its relation to vortex structures. This may lead to better turbine design in the future.

Understanding the formation of plasma-sprayed Ni20Cr splats through interface observation

In the formation of plasma-sprayed metallic splats, substrate preheating plays a critical role in determining the splat morphology as well as the splat-substrate and the splat-splat interfacial interactions. In the present work, the splat formation mechanism of regular-shaped Ni20Cr splats on stainless steel substrates preheated at 423 K, 573 K and 673 K was studied. Focused ion beam (FIB) microscopy and transmission electron microscope (TEM) were used to characterize the splat solidification microstructure, pore formation, splat delamination, and the elemental diffusion along the splat-substrate interface. The substrate surface roughness was consistent up to 573 K preheating temperature, while the great degree of surface oxide formation increased the roughness at 673 K. For disk splats, good interfacial contact with a long distance of elemental diffusion was observed near the center. With increased substrate preheating temperatures, the degree of curling-up of the splat edge decreased. At high substrate preheating temperatures (673 K) where splats slowly solidified, Cr segregated to the top surface forming a thick layer of NiCr2O4 spinel. During FIB milling, a dense layer consisting of Fe and Cr was formed at the delaminated splat edge due to substrate material redeposition.