Splat Morphology Research Articles

Comparison of solidity and fractal dimension of plasma sprayed splat with different spreading morphologies

The paper deals with the quantitative characterization of spreading morphologies of plasma sprayed Fe-based alloy droplets deposited on mirror polished steels with different preheated temperature. The plasma torch was utilized as heat producer. The influence of substrate temperature on the solidification mechanism of molten droplets was investigated. The image analysis method (IMA) was employed to identify single splat from the field emission scanning electron microscope (FE-SEM) morphology. The result shows that the substrate preheated temperature has a significant effect on the flattening behavior of molten droplets. With the increment of substrate temperature, the solidification mode of splat changes from flower-like and splashed splat to disk-like splat due to the modification of wettability and cooling velocity between molten droplet and substrate. Compared with area and perimeter, both fractal dimension (FD) and solidity could separately detect the solidification mode of splat to a certain extent, while the FD seems to be more excellent in characterizing irregular morphology of splat in contrast with solidity. However, the combination of FD and solidity is more efficient in classifying solidification mode of splat.

Microstructure formed by suspension plasma spraying: From YSZ splat to coating

In the current work, by means of suspension plasma spraying (SPS), yttria stabilized zirconia splats and coatings were deposited onto mirror-polished and rough substrates, respectively. The splats were characterized by scanning and transmission electron microscopes (TEM). The influence of substrate temperature on splat morphology was analyzed via comparing the splats deposited onto substrates held at different temperatures, namely room temperature (RT), 300°C, 500°C, 700°C and 900°C. As the substrates were heated from RT to 300°C, the feature of splats changed rapidly. However, when the substrate temperature further increased, the variation of morphology was found less conspicuous. Integrated splats were peeled off from the substrate using a “plating and etching” approach for TEM examination. The observation on morphological and crystallographic features of splats revealed that the cooling rate during impacting and spreading of SPS splats was lower than that of splats deposited by conventional atmospheric plasma spraying. Based on the characteristics of splats, a modified mechanism consisting changing of droplets trajectory and shadowing effect was proposed to explain the formation of the microstructure of as-sprayed SPS coating. In addition, the coatings were also examined by X-ray diffraction and nano-indenter to determine the phase composition and microhardness, respectively.

A numerical model for a hypoeutectic alloy droplet deposition with non-equilibrium solidification

A numerical model is developed to study the splat morphology and solidification characteristics of a molten hypoeutectic alloy droplet impinging and solidifying on a substrate. The study finds application in optimization and improvement of metal additive manufacturing processes such as solder jetting, microcasting, sputtering and 3D printing. The major mathematical and numerical challenges include solution of multiphase flow governing equations, interface tracking and modeling the non-equilibrium (rapid) solidification on a macroscopic domain. The free surface is tracked using a volume of fluid method with a piecewise linear interface construction while the mushy phase is modelled as a pseudo porous medium. An enthalpy formulation of the energy equation is coupled with the solute transport equation and the system is solved simultaneously for the temperature and concentration profiles until the eutectic point is reached; beyond which a special treatment is employed till complete solidification. Segregation models (with back diffusion) and eutectic phase diagram are incorporated in the solution procedure. The splat morphology, concentration profiles and microstructural properties are closely examined with emphasis on the convective effects and eutectic formation.

Cracking caused by cutting of plasma-sprayed hydroxyapatite coatings and its relation to the structural features of coatings deposited at different initial substrate temperatures

The present study estimated the cracking phenomenon in as-plasma-sprayed hydroxylapatite coatings (HACs) after they were being subjected to the severe cutting conditions in the direction perpendicular to the coating/substrate interface. In order to evaluate the effects of substrate preheating on the occurrence of micro-cracks, the HACs were deposited at different initial substrate temperatures (TS = 20, 100 and 200?C). The changes in phase composition and HA splat morphology with TS were observed and were correlated with the cracking occurrence. The results showed that severe cutting conditions introduced a localized cracking in the regions of HACs dominantly attributed to the brittle hydroxyl-deficient amorphous calcium phosphate (ACP) phase. This effect was particularly observable in the HACs deposited without preheating of substrate. On the other hand, the preheating of substrate reduced the presence of micro-cracks and caused insignificant changes in the average local phase composition. In HACs deposited with preheating of substrate, the HA splats (of which HACs are composed) were thinner and recrystallized HA regions seemed smaller in size and more evenly distributed. These results implied potentially important roles of the HA splat formation mechanism on the distribution of ACP and recrystallized HA regions in the as-plasma-sprayed HACs and the cracking resistance of HACs.

Correlation of splat morphologies with porosity and residual stress in plasma-sprayed YSZ coatings

The Atmospheric Plasma Spray (APS) Process is a widely used technique for manufacturing ceramic coatings with unique properties for many applications, whereat the residual stresses within these coatings are an important factor affecting their performance. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so called splats. Understanding the mechanisms connecting these smallest structural elements with the overall coating properties enables the specific design of the coating process. In the present work, the correlation between coating porosity, Young's modulus, and residual stress with the particle melting was investigated. Therefore, single splats were collected on mirror-polished substrates and evaluated by identifying different types and their relative fractions under various spray conditions. Microscopic investigations were performed to obtain information on the spreading behavior of the different splat types to establish a relation to the coating properties. Furthermore, the particles were characterized by in-flight measurement of their temperature and the obtained results were evaluated in terms of their degree of melting. This was compared with the experimentally observed splat type amounts and finally correlated to the investigated coating properties.

Effect of Oxidation on the Bonding Formation of Plasma-Sprayed Stainless Steel Splats onto Stainless Steel Substrate

Stainless steel splats were deposited on 304 stainless substrates with different thicknesses of oxide layer to examine the effect of substrate oxidation on splat morphology and splat-substrate interface bonding by inert low-pressure plasma spraying. The cross sections of splats showing the splat-substrate interface were prepared by focus ion beam (FIB). The splat morphology and splat-substrate interface bonding state were characterized by scanning electron microscopy. The interface bonding was also examined by an electrolytic etching process. Results showed that with increasing oxide layer thickness and surface roughness, the morphology of splat changed from disk shape to splashed finger-like shape. The examination into the interface bonding by using FIB-prepared cross-sectional samples revealed that the splat interface bonding depended on the oxide roughness and composition. The interface bonding with a ratio of 44% was formed at the inner part of a splat on the pre-oxidized substrate when iron oxide presented on the surface, and the roughness of oxide scale was <5 nm. When the pre-oxidizing temperature exceeded 800 °C, the surface roughness increased to 14 nm and chromium oxide covered the pre-oxidized surface, resulting in no effective bonding forming at the whole interface. Thus, surface roughness and oxide composition have a significant influence on the splat interface bonding formation.

Impact of particle-laden drops: Particle distribution on the substrate

The splat morphology after the impact of suspension drops on hydrophilic (glass) and hydrophobic (polycarbonate) substrates was investigated. The suspensions were mixtures of water and spherical hydrophobic particles with diameter of 200μm or 500μm. The impact was studied by side, bottom and angled view images. At Reynolds and Weber numbers in the range 150⩽We⩽750 and 7100⩽Re⩽16,400, the particles distributed in a monolayer on the hydrophilic substrates. It was found that the 200μm particles self-arranged as rings or disks on the hydrophilic substrates. On hydrophobic substrates, many particles were at the air-water interface and 200μm formed a crown-like structure. The current study for impact of particle-laden drops shows that the morphology of splats depends on the substrate wettability, the particle size and impact velocity. We developed correlations for the inner and outer diameter of the particle distribution on the hydrophilic substrates, and for the crown height on hydrophobic substrates. The proposed correlations capture the character of the particle distributions after drop impact that depends on particle volume fraction, the wettability of both particles and the substrate, and the dimensionless numbers such as Reynolds and Weber.

Heat Flux Estimation of a Flame Thermal Spray Process Using a Thermally Thin Composite Calorimeter

Temperature measurements take on prime importance in the field of the thermal spray coating since the temperature variation greatly affects the formation of splat morphology and also the coating properties and qualities. The evaluation of the heat flux is therefore essential since temperature variation comes from the energy transfer and conduction of the thermal system. The aim of this study is to estimate the heat flux of a flame thermal spray by solving an inverse heat conduction problem. Firstly, the substrate material and geometry are well designed so that the Biot number is small enough to conform to the lumped capacitance conditions. A lumped capacitance model of a substrate with its coating subjected to a uniform echelon heat flux is evaluated by solving a heat balance equation in the Laplace domain. Then, a thermally thin calorimeter is designed and the experimental thermogram is obtained by embedding a thin-wire micro-thermocouple onto the front and rear faces of the substrate. The forced convective heat transfer coefficient as well as the net incident heat flux density brought to the substrate during the thermal spray process are estimated. The theoretical composite surface temperature is compared to the experimental recording, the result showing a good agreement.

Effects of interfacial heat transfer, surface tension and contact angle on the formation of plasma-sprayed droplets through simulation study

Abstract A comprehensive numerical study of the effects of interfacial heat transfer, surface tension (1.8 N/m, 1.647 N/m, 1.35 N/m and temperature-dependent values) and contact angles (60°, 90° and 120°) on the droplet spreading behaviour in the formation of the plasma-sprayed splats was conducted. The evolution of splat morphologies with time was accurately captured using a volume of fluid (VOF) model in a 2-D computational domain. The results show that bubbles form at the time of impact and decrease the heat transfer efficiency at the contact points. During spreading, solidification occurs at the droplet edge before maximum spreading. The rapid growth of the underlying solidified layer induces fluid instabilities for the upper liquid layer, which triggers material jetting. The interfacial heat transfer is the key parameter influencing the droplet cooling process and the final splat morphology. Increasing the substrate preheating temperatures (from 27 °C to 300 °C) delays solidification but increases the potential of substrate melting. A low surface tension (1.35 N/m) readily promotes liquid projections, while the contact angle is of less importance in changing the surface morphologies.

Parametric Appraisal of Slurry-Sprayed Mullite Coatings for Coating Thickness

The slurry spray technique (SST) has been employed to deposit mullite-based coatings on ASTM 1018 low-carbon steel substrate for environmental barrier coating applications. A Taguchi L18 orthogonal array is adopted for optimization of the identified process variables and material parameters, namely stamping pressure, fly-ash content, and sintering additive, time, and temperature. The measured thickness of the produced coatings was chosen as the response characteristic for the present study. The optimum values of the process variables were predicted by employing analysis of variance based on raw data and the signal-to-noise (S/N) ratio. Based on analysis of the experimental results, the effect of each parameter level on the coating thickness is discussed. It is observed that the sintering temperature had a strong influence on the maximum coating thickness of the slurry-sprayed coating. The as-sprayed coatings fabricated using SST demonstrated splat morphology with continuous interface, suggesting good adherence to the substrate.

Splat Morphology and Influence of Feeding Rate During Reactive Plasma Spray of Aluminum Powder

Fabrication of aluminum nitride (AlN) coatings using conventional plasma spraying processes directly has been deemed impossible. It is attributed to the thermal decomposition of the AlN feedstock particles during spraying without a stable melting phase. Using the reactivity of the plasma (reactive plasma spraying: RPS) showed a promising consideration for in situ formation of AlN thermally sprayed coatings. Several AlN-based coatings were fabricated through the RPS of aluminum powders in the N2/H2 plasma. The focus of this study is in discussing the morphology of splat deposition during the nitriding of Al particles. Furthermore, the influence of the feeding rate during the RPS and nitriding of Al powders will be investigated. The nitride content, as well as the unreacted molten Al phase, strongly influences splat deposition and morphology during the RPS of Al. The collected splats can be divided into reacted, partially reacted, and unreacted splats. The reacted splats tend to show a disk or egg-shell shape. The partially reacted mainly had outside nitride shells and an unreacted molten Al part in the center. The unreacted splats tended to show a splash shape. The main controlling factor is the time of the droplet impact on the substrate during the reaction sequence. The particle size and spray distance showed significant effects on the splat formation due to their effect on the nitriding conversion and the melting behavior of the particles during RPS nitriding. The powder feeding rate was investigated through increasing the injection rate and by using a low carrier gas flow rate. Increasing the powder feeding rate significantly improved the coating thickness. However, it suppressed the nitriding conversion of the large Al particles. Thus, with increasing the amount of the powder in the plasma, the Al molten particles are easily aggregated and agglomerate together upon colliding on the substrate with an AlN shell on the surface. This prevents the N2 from having access to all of the aggregated particles. Therefore, the fabricated coatings using large Al particles consist of surface AlN layers and the central parts of AlN and Al composite layers. On the other hand, it was possible to fabricate about 500-μm-thick AlN coatings using fine Al particles of 15 μm and increasing the feeding rate. Using the fine particles improved the nitriding reaction due to the improvement of the surface area (the reaction area). Moreover, the nitriding process of the Al particles with increasing the feeding rate was also investigated.

Challenges Upon Reactive Plasma Spray Nitriding: Al Powders and Fabrication of AlN Coatings as a Case Study

Reactive plasma spraying (RPS) is a promising technology for the in situ formation of several ceramic coatings. The focus of this paper is to summarize the state of our current knowledge about the RPS process and using the nitriding of Al particles and the fabrication of aluminum nitride coatings, as a case study. The aspects and challenges in this process such as the influence of the plasma power, in-flight time, particle size, nitriding mechanism, splat morphology, in-flight particle diagnostics, N2 plasma gas, and the feeding rate on the RPS process are analyzed and discussed.

Thermal stability of plasma-sprayed La2Ce2O7/YSZ composite coating

A composite coating composed of La2Ce2O2 (LCO) and yttria-stabilized zirconia (YSZ) in a weight ratio of 1:1 was deposited by the plasma spraying using a blended YSZ and LCO powders, and the stability of the LCO/YSZ interface exposed to a high temperature was investigated. The LCO/YSZ deposits were exposed at 1300°C for different durations. The microstructure evolution at the LCO/YSZ interface was investigated by quasi-in-situ scanning electron microscopy assisted by X-ray energy-dispersive spectrum analyses and X-ray diffraction measurements. At an exposure temperature of 1300°C, the grain morphology of LCO splats in contact with YSZ splats changed from columnar grains to quasi-axial grains with interface healing, and some grains tended to disappear during the thermal exposure. The results indicate that the phases in LCO–YSZ composite coating are not stable at 1300°C. The element La in the LCO splat diffused towards the adjacent YSZ splat during the exposure, generating the reaction product layers composed of La2Zr2O7 between the LCO and YSZ splats. After exposed for 200h, the composite coating consisted of a mixture of mainly La2Zr2O7 and CeO2 and a minor amount of YSZ, accounting for the unusual decrease in the thermal conductivity at the late stage of exposure.

Substrate temperature dependence of splat morphology for plasma-sprayed cast iron on aluminum surface

Abstract In the plasma-sprayed coatings, the morphology of the splats plays an important role in optimizing the microstructure and performance of the coating. Especially, the splashing of impinging droplets during deposition weakens the splat–substrate/intersplat bonding and increases the porosity of the coating. Consequently, the integration of the coating is deteriorated. In the present study, cast iron particles were plasma-sprayed on the surface of polished aluminum substrate to form a single splat. During spraying, the surface of aluminum substrate was preheated in a temperature range from 25 °C to 320 °C. The impact of the substrate preheating temperature on the morphology of the splats was studied using a field emission scanning electron microscope. Results showed that the substrate temperature had significant effects on the morphology of splats. At room temperature, the splats mainly exhibited a splash type with network or radial lines on the splat periphery. While, the splashed splats deposited onto a high temperature substrate showed a star shape on the splat periphery. When the substrate was preheated to 130 °C, the mean percentage of the splashed splats decreased to a minimum value of 18.4% and the disk-like splats prevailed. With the increase of the substrate temperature from 130 °C to 290 °C, the mean percentage of the splashed splats increased monotonically to 78.3%. When the substrate temperature reached to 320 °C, the mean percentage of the splashed splats slightly reduced to 76.6%.

Effect of the Bond Coating Surface Morphology on Ceramic Splat Construction

In this work, yttria-stabilized zirconia splats deposited onto different bond coatings were characterized. The influence of both substrate topography and temperature on splat morphologies was investigated. The substrate surface was described using a method characterized by multi-level roughness. On non-heated substrate, the peak-valley structure in scale of micrometer contributed to the formation of crater-like holes on splats. The amount of such holes was reduced with the increase of substrate temperature. It was also observed that on micro-smooth surface, island-like fragments in the center of splat would warp and their edges could be detached from the underlying substrate, especially when the substrate was heated. The obtained results reveal that the influence of substrate pre-heating on the splat formation is realized via thinning the air film near substrate surface.

Automated image inspection system to quantify thermal spray splat morphology

Nowadays, coating materials such as ceramic, metal or alloys, are widely used in the industry to improve properties of the base materials. There is growing interest in knowing the parameters governing the promotion of high quality coatings, mainly adhesion strength. It would be really interesting using Automated Measurement Tools based on Image Processing procedures in order to obtain an image inspection system, owing to the fact that these techniques are contrasted tools in other inspection and quality control systems. In this work alumina splats were plasma-sprayed onto an aluminium surface under atmospheric conditions. Top view images of these splats were obtained by a scanning electron microscope and processed by Image Processing Techniques. As the microstructure of the splats forming a coating is a very important factor which determines the adhesion strength of the coated piece, this system was developed to characterise these splats in terms of their area, perimeter, circularity, eccentricity and solidity. As a result, our system can be used as the initial stage of an automated inspection system, which could be helpful to a human expert, to take a decision whether the spraying splats, and hence the spraying parameters, are the appropriated ones. In a later stage, our interest will be to substitute this human expert for an expert system based on the knowledge theory, which would be able to decide about the quality of these splats without human intervention. In this work, an automated image inspection system to assist human experts to quantify thermal spray splat morphology is shown. The automatic system was able to classify the coating by means of a decision tree, foretelling the suitability of each type of splats in a coating, and thus the spraying process parameters utilised.

Deposition Behavior of Nanostructured WC–23Co Particles in Cold Spraying Process

The nanostructured WC–23Co particles were deposited on three kinds of substrates in different accelerating gas temperature by using cold spraying process. The morphologies of nanostructured WC–23Co splats attached on the polished surface of different substrates were characterized via scanning electron microscopy. It is revealed that the splats are partially embedded in Q235 and stainless steel with a comparative integrated contour similar to that of the original particles. There exist some revers around the splats and some corrugations or ripples on the Q235 and stainless steel substrates. In contrast, the splats on the surface of WC–12Co substrate present clearly a flat shape with central hump surrounded by a ringy band. By comparison of the morphologies of splats deposited on three kinds of substrates with the accelerating gas temperature of 850 ± 10°C and 750 ± 10°C, it can be found that the deformation of nanostructured WC–23Co particles is enhanced by the increment of substrate hardness and gas temperature.

Effect of Substrates Temperatures on the Spreading Behavior of Plasma-Sprayed Ni and Ni-20 wt.% Cr Splats

In the formation of plasma-sprayed splats, the spreading behavior of molten droplets is essential for forming desirable lamella with good adhesion to the substrate. To understand the effect of the active element chromium on droplet spreading, pure Ni and Ni-20 wt.% (Ni20Cr) alloyed powders were plasma sprayed on mirror-polished 304 stainless steel substrates heated to different temperatures (below 200 °C). The substrate heating resulted in very little change in the surface roughness. However, there was a measureable change in the surface chemistry of the outermost few nanometers, which became increasingly enriched in Fe at higher temperatures. The splat morphologies were characterized and the transition temperatures were estimated. The results show that the transition from splashed to disk splats was not solely dependent on the temperature of the substrate. In some cases, some splashing still occurred to a measureable extent even at relatively high substrate temperatures, even above temperatures at which adsorbates (moisture) were totally removed from the surface. The splashing behavior could be correlated to a combination of the change in the surface chemistry of the substrate and the presence of active elements in the coating materials.

Morphology and Size Evolution of Interlamellar Two-Dimensional Pores in Plasma-Sprayed La2Zr2O7 Coatings During Thermal Exposure at 1300 °C

La2Zr2O7 (LZO) is widely expected to be one of the promising thermal barrier coating materials for application in high-temperature conditions (1200 °C). However, high-temperature exposure causes sintering which heals interlamellar two-dimensional (2D) pores and intrasplat pores. This sintering effect increases the stiffness and thermal conductivity of thermal barrier coatings, consequently reducing their durability. In this study, to reveal the possible critical opening of 2D pores above which they are free from sintering, LZO coating and splat were deposited by atmospheric plasma spraying and were exposed to 1300 °C for different durations. Thereafter, the evolution of the parameters of residual 2D pores in the coating and the surface morphology of LZO splat were characterized. It was found that there is a critical opening width for 2D pores above which grain bridging does not occur across the gaps. Accordingly, pores with an opening larger than this critical width are free from sintering across the 2D pores despite surface roughening of splats, whereas pores with an opening less than the critical width sinter rapidly at the early stage of thermal exposure through the formation of grain bridges.

Effect of suspension characteristics on in-flight particle properties and coating microstructures achieved by suspension plasma spray

This paper focuses on the influence of suspension properties on the manufacturing of coatings by suspension plasma spraying (SPS). For this purpose, alumina suspensions were formulated with two different liquid phases: water and ethanol. Suspensions were atomized with a twin-fluid nozzle and injected in an atmospheric plasma jet. Suspension injection was optimized thanks to shadowgraphy observations and drop size distribution measurements performed by laser diffraction. In-flight particle velocities were evaluated by particle image velocimetry. In addition, splats were collected on glass substrates, with the same conditions as the ones used during the spray process. Scanning electron microscopy (SEM) and profilometry analyses were then performed to observe the splat morphology and thus to get information on plasma / suspension interactions, such as particle agglomeration. Finally, coatings were manufactured, characterized by SEM and compared to each other.