Effect of Ambient Pressure on Flattening Behavior of Thermal Sprayed Particles

Cu splats were thermally sprayed onto the mirror polished SUS304 substrate surface at various ambient pressures ranging from 6.66 to 101.33 kPa. The effect of ambient pressure on the flattening behavior of the particle was systematically investigated. It was observed that only around 10% or less of disk-shaped splats deposited at atmospheric pressure. The splat shape on the flat substrate had a transitional changing tendency from a splash splat to a disk one with a decrease of the ambient pressure. The tendency of splash splat change with increasing the ambient pressure agreed with the BET curve, which indicates that adsorption/desorption of the adsorbed gas/condensation plays an important role on the flattening behavior of thermal sprayed particles. Moreover, a part of substrates were polished to a mirror finish and heated to 573 K for 10 min, then elapsed to air atmosphere for different duration of up to 1 h. The fundamental static wetting behavior of the once heated substrate surface by a water droplet was investigated. The contact angle measurement results agreed well with the splat morphologies. No chemical change and surface topography change took place with the elapse time increasing. Hence, the occurrence of desorption caused by reducing the ambient pressure or by substrate preheating provided good wetting. Wetting of substrate surface by molten particles may dominate the flattening behavior of thermal sprayed particles.     

Effect of Substrate Temperature and Ambient Pressure on Heat Transfer at Interface Between Molten Droplet and Substrate Surface

Millimeter-sized molten Cu droplets were deposited on AISI304 substrate surface by free falling experiment. The roles of substrate temperature and ambient pressure on heat transfer at interface between molten droplet and substrate surface were systematically investigated. The splat characteristics were evaluated in detail. Temperature history of molten droplet was measured at splat-substrate interface. Cooling rate of the flattening droplet was calculated as well. Furthermore, the spreading behavior of molten droplet on substrate surface was captured by high speed camera. The heat transfer from splat to substrate was enhanced both by substrate heating and by ambient pressure reduction, which can be attributed to the good contact at splat bottom surface. The splats in free falling experiment showed similar changing tendency as thermal-sprayed particles. Consequently, substrate temperature and ambient pressure have an equivalent effect to contact condition at interface between droplet and substrate surface. Substrate heating and pressure reduction may enhance the wetting during splat flattening, and then affect the flattening and solidification behavior of the molten droplet.     

热喷涂镍粉在SUS304及A6063基体上的沉积行为

为阐明基体材料对单个颗粒沉积行为的影响,采用大气等离子喷涂技术将镍粉喷涂到镜面研磨后的不锈钢SUS304及铝合金A6063基体上,并通过扫描电子显微镜及聚焦离子束等技术观察在基体上收集的沉积物正面及截面形貌。结果表明:Ni在SUS304基体上呈明显的溅射沉积,单个沉积物的中心部位与周围溅射物相分离,沉积物底部及内部均可见明显气孔,溅射可能是由基体表面的吸附质在熔滴在基体表面铺展过程中脱附所导致。同时,Ni粉在A6063基体上呈现花朵状溅射,中心部位有明显的分层现象,这可能与基体材料的热导率、熔点以及基体表面吸附质的脱附等因素密切相关。     

Effect of interface wetting on flattening of freely fallen metal droplet onto flat substrate surface

A free-falling experiment was conducted as a simulation of a thermal spray process. The flattening behavior of the freely fallen metal droplet impinged onto a flat substrate surface was investigated in a fundamental way. The substrates were kept at various temperatures, and the substrates were coated with gold by physical vapor deposition (PVD) and were prepared in order to investigate the effect of wetting at the splat-substrate interface on the flattening behavior of the droplet. A falling atmosphere was created with atmospheric pressure of nitrogen to prevent the oxidation of the melted droplet. Experiments under low-pressure conditions also were conducted. The different types of splat morphology were recognized in experiments conducted under a nitrogen atmosphere with atmospheric pressure. The splat morphology on a substrate at room temperature was of the splash type, whereas that on a substrate at high temperature was of the disk type. The microstructure observed on a cross-section of the splat obtained on the substrate at room temperature was an isotropic coarse grain, whereas that on the substrate at high temperature was a fine columnar grain. The grain size changed transitionally with increasing substrate temperature. The temperature of the transition on the gold-coated substrate was higher than that on the naked substrate. The microstructure of the cross-section of the splat obtained under low pressure was finely columnar even on the substrate at room temperature. The results indicate that the metal droplet wets better under the low-pressure condition than under the atmospheric pressure nitrogen condition and that wetting has a significant role in the flattening of the droplet.     

Flattening of droplets and formation of splats in thermal spraying: A review of recent work—Part 1

Properties of the coatings developed during thermal spraying are essentially determined by rapid solidification of splats formed as a result of impingement of the melted powder particles onto a substrate surface. The processes of flattening droplets and formation of splats in thermal spraying have been studied intensively during the last two decades. The last review on this topic was published at the end of 1994. Since then many papers have been dedicated to investigating splat formation, taking into account such important issues as roughness of the substrate surface, wetting phenomena, and splashing. This review, consisting of two parts, includes the main results obtained since 1994 and examines the influence of solidification of the lower part of the splat, substrate roughness, wetting at the substrate-coating interface, substrate deformation, oxidation, and splashing on the dynamics of flattening of droplets and the formation of splats. Flattening of composite powder particles, splat-substrate interaction, and development of splat-substrate adhesion and splat porosity are discussed. Part 1 of the review covers the following issues, which significantly influence the droplet flattening and splat formation: droplet solidification during flattening and roughness of the substrate surface, composite morphology of the powder particles, and oxidation processes. The results provide a better understanding of the thermal spray processes to increase their efficiency.     

Transient contact pressure during flattening of thermal spray droplet and its effect on splat formation

The flattening process of an isothermal droplet impinging on flat substrate in plasma spraying is studied numerically using “Marker-And-Cell” technique that enables the evolution of the droplet/substrate dynamic contact pressure. The distributions of the pressures upon substrate surface during flattening are calculated under different droplet conditions. The correlation of the distribution of the peak contact pressure along substrate surface with the observed splat morphology is examined. The results show that the transient contact pressure is initially high and concentrates at a small contacting area and then spreads quickly with droplet flattening. The maximum pressure is located at the front of the droplet at an early stage of deformation, which drives the fluid moving quickly along substrate and results in lateral flow. The contact pressure is mainly associated to droplet density and velocity. The peak pressure reduces monotonically with flattening and becomes negligible at the region where the flattening degree is larger than 2. The magnitude of the pressure resulting from evaporating gas by rapid heating of the adsorbed water on the substrate surface is comparable to that of the dynamic contact pressure at the region where the flattening degree ranges from 1.5 to 2. It is suggested that the reduced contact pressure at the late stage of spreading and disturbance by the evaporation-induced pressure resulting from rapid heating of the surface adsorbents by flattening droplet may contribute significantly to the splashing of flattening droplet and the formation of a reduced disk-like splat.     

Variation of splat shape with processing conditions in plasma sprayed alumina coatings

This paper is on various splat shapes obtained using three alumina based powders sprayed on various substrates. The parameters considered were substrate preheating temperature, nozzle diameter, and secondary and primary gas flow rates. The splat shape was found to be strongly dependent on spraying conditions. The substrate preheating temperature determined the degree of substrate wetting by the splat. A change in either nozzle diameter or primary gas flow rate brought about a change in the particle momentum and subsequently, a change in splat shape. The splat shape differed widely on an as - sprayed bond coat as compared to a polished one, owing to splat confinement by surface asperities. Sub-microscale surface roughness of polished substrate surfaces showed an increase with the preheating temperature and this in turn, resulted in better substrate wetting by the splats.     

Effect of substrate oxidation on spreading of plasma-sprayed nickel on stainless steel

Plasma-sprayed, molten nickel particles (∼ 60 μm diameter) were photographed during impact on oxidized 304L stainless steel surfaces that were maintained at either room temperature or at 350 °C. Steel coupons were oxidized by heating them at different temperatures. A fast charge-coupled device (CCD) camera captured time-integrated images of the spreading splat. A two-color pyrometer collected thermal radiation from particles and recorded the evolution of their temperature after impact. Molten nickel particles impacting on oxidized steel at room temperature fragmented significantly, while heating the surfaces produced splats with disk-like morphologies. Impact on steel that was highly oxidized induced the formation of finger-like splash projections at the splat periphery. Thermal contact resistance between splats and non-heated oxidized steel was calculated from splat cooling rates and found to decrease as the degree of oxidation increased. On heated, oxidized steel thermal contact resistance was much lower and did not change significantly with the degree of oxidation. It was concluded that thermal contact resistance was largely influenced by adsorbates on the steel surface that evaporated when the surface was heated or oxidized.     

Examination of substrate surface melting-induced splashing during splat formation in plasma spraying

Impacting of a molten droplet with a melting point much higher than the substrate results in melting of the substrate around the impact area. Melting of the substrate surface to a certain depth alters the flow direction of the droplet. The significant change of fluid flow direction leads to the detaching of fluid from the substrate. Consequently, splashing occurs during the droplet-spreading process. In the current study, molybdenum (Mo) splats were formed on a stainless steel substrate under different plasma-spraying conditions. For comparison, Mo splats were also deposited on an Mo surface. The substrate surface was polished prior to deposition. The powders used had a narrow particle size distribution. The results show that the morphology of splats depends significantly on the thermal interaction between the molten particle and the substrate. The splat observed was only a central part of an ideal disk-like complete splat. The typical pattern of Mo splats was of the split type, presenting a small split structure on the surface of the stainless steel substrate. With Mo particles, the preheating of a steel substrate has no effect on splat morphology. On the other hand, a disk-like Mo splat with a reduced diameter of a dimple-like structure at the central area of the splat was formed on Mo substrates, and splashing can be suppressed through substrate preheating. Based on the experimental results, a surface melting-induced splashing model was proposed to explain the formation mechanism of the Mo splat on a steel surface. The influence of droplet condition on splat formation is discussed. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Effect of oxidation on droplet flattening and splat-substrate interaction in thermal spraying

The processes of oxidation that occur during particle inflight motion and during splat solidification in an oxygen-rich atmosphere were considered for the thermal spray process. The effect of oxidation on droplet flattening, splat-substrate mechanical and thermal interaction, splat morphology, and development of coating porosity and adhesion was studied. The influence of wetting and oxygen dissolution on flattening and splat-substrate adhesion was also investigated. The results from theoretical treatment agree with experimental observations.     

Influence of surface character change of substrate due to heating on flattening behavior of thermal sprayed particles

The authors have confirmed that in the thermal spraying of practical powder materials, the splat shape changes with increasing substrate temperature to a circular disk shape from a fringe shape with splashing at a critical substrate temperature,T _t. The increase of the substrate temperature may accompany a kind of essential change on the substrate surface, because the effect is maintained until the substrate is cooled down to room temperature. However, the nature of the substrate surface change due to the heating has not been clearly understood yet. In this study, AISI 304 stainless steel was used as a substrate material, and the substrate was heated in an air at mosphere or laser treated as a pretreatment. Substrate surface topography in nanometer scale was analyzed precisely by atomic force microscope (AFM). The relationship between surface topography in nanometer scale and splat morphology was discussed. Moreover, to evaluate the effect of chemical composition of the substrate surface, gold was coated onto the substrate surface by physical vapor deposition (PVD) after the heat treatment. The effect of adsorbate/condensate on the substrate surface on the flattening behavior of thermal sprayed particles was also verified. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Knowledge concerning splat formation: An invited review

This paper summarizes our knowledge at the beginning of 2003 about splat formation. First, the analytical and numerical models related to the impact and flattening of single particles on smooth or rough substrates with different tilting are recalled. Then, the different diagnostic methods, including imaging, are briefly described. The last part of the paper is devoted to the results and their discussion. Studies are related to the effect of various parameters on particle flattening. They include the characteristics of particles prior to impact: normal impact velocity, temperature, molten state, oxidation state, etc.; the parameters related to the substrate: tilting angle, roughness, oxide layer composition, thickness and crystallinity, desorption of adsorbates and condensates, wetting properties between impacting particle and substrate, etc.; and, finally, the parameters related to the heat exchange between the flattening particle and the substrate. They depend on previous parameters and control the propagation of the solidification front within the flattening particle, eventually modifying its liquid flow. It is obvious from this review that, if our understanding of the involved phenomena has been drastically improved during the last years, many points have still to be clarified. This is of primary importance because all the coating properties are linked to the particle flattening, splat formation, and layering.     

On-Line Measurement of Plasma-Sprayed Ni-Particles during Impact on a Ti-Surface: Influence of Surface Oxidation

The objective of this study was to analyze the impact of plasma-sprayed Ni5%Al particles on polished and grit-blasted Ti6Al4V samples under oxidized and nonoxidized conditions. For this purpose, measurements of thermal radiation and velocity of individual plasma-sprayed particles were carried out. From the thermal radiation at impact, splat diameter during flattening and temperature evolution during cooling were evaluated. Characteristic parameters related to the quality of contact between the splat and the substrate were retrieved. The flattening speed was introduced to characterize wetting, while the cooling rate was used to characterize solidification. The idea was to get a signature of particle impact for a given surface roughness and oxidation state by identifying parameters which strongly affect the splat behavior. Sieved Ni5%Al powder in a narrow range (+65 −75 μm) was sprayed on four sets of titanium alloy surfaces, consisting of polished and grit-blasted samples, one set had a nonoxidized surface and the other one was oxidized in an oven at 600 °C for two hours. Resulting splats after impact were characterized by scanning electron microscopy, the splats on oxidized surface showed pores in their core and detached fingers at the periphery. The cooling rate and flattening degree significantly increased on the oxidized smooth surface compared to the nonoxidized one. This trend was not found in grit-blasted surfaces, which implies that impact phenomena are different on grit-blasted surfaces than on smooth surfaces thus further work is needed.     

基底对等离子喷涂Mo层片形成的影响

运用扫描电镜和台阶仪研究了在不同状态下的1Cr18Ni9Ti不锈钢和纯Cu基底上大气等离子喷涂Mo层片的形成机制。结果表明:室温光滑基底上沉积的Mo层片均呈不规则飞溅花瓣状,随预热温度升高,形成的层片逐渐变得较完整;熔滴撞击铺展过程中在基底上可能会形成凹坑;基底导热率越高,层片铺展度越小;粗糙基底表面阻碍熔滴铺展,促进熔体飞溅。理论分析表明,"花瓣状"层片的形成是熔体表面张力、熔体与基底接触热阻的不均匀性和基底凹坑相互作用的结果;基底凹坑的形成主要由基底熔化所控制。     

Wettability of pure Ti by molten pure Mg droplets

The wetting behavior of molten pure Mg droplets on pure Ti substrate, a crucial phenomenon in the design of Mg matrix composites reinforced with Ti particles, was investigated by the sessile drop method. The contact angle was measured in high-purity argon (99.999%) at 1073 K. In particular, the effects of two important parameters on the contact angle were evaluated: Mg evaporation during the wetting test; and surface oxide film of the substrate. The calculation method to estimate the modified contact angle involved taking the morphological changes of the droplet outline due to the evaporation into consideration. By changing the thickness of the surface oxide films on the Ti substrate, it was possible to examine the wettability and the chemical reactions at the interface between the solidified Mg drop and the substrate were investigated by scanning electron microscopy–energy dispersive X-ray spectrometry analysis. At the initial wetting stage, a large contact angle with 95–110° was obtained, which depended on the reduction of TiO₂ surface films by Mg droplets. When the molten Mg contacts an area of pure Ti after reduction, the contact angle suddenly decreased. The equilibrium value at the stable state strongly depended on the surface roughness of the Ti plate.     

Modeling of Rapid Solidification with Undercooling Effect During Droplet Flattening on a Substrate in Coating Formation

The coating deposit on the substrate in thermal spray coating process develops by solidification of individual molten particle which impacts, flattens and solidifies on the surface of the substrate. Droplet flattening and solidification typically involves rapid cooling. In this paper, a model for non-equilibrium rapid solidification of a molten droplet spreading onto a substrate is presented. Transient flow during droplet impact and its subsequent spreading is considered using the volume of fluid surface tracking method which was fully coupled with the rapid solidification model. The rapid solidification model includes undercooling, nucleation, interface tracking, non-equilibrium solidification kinetics and combined heat transfer and fluid flow as required to treat a non-stagnant splat formed from droplet flattening. The model is validated with the literature results on stagnant splats. Subsequently, using the model the characteristics of the rapidly solidifying interface for non-stagnant splat, such as interface velocity and interface temperature, are described and the effect of undercooling and interfacial heat transfer coefficient are highlighted. In contrast to the stagnant splat, the non-stagnant splat considered in this study displays interesting features in the rapidly solidifying interface. These are attributed to droplet thinning and droplet recoiling that occur during the droplet spreading process.     

Microstructural Characteristics and Heat Transfer of Flattened Millimeter-size Nickel Droplet

The splat is the fundamental unit of thermally sprayed coatings,which has been one of the hottest issues for decades.In order to study the splat formation,an experiment was designed and conducted,in which a millimeter-size nickel metal droplet fell freely and impacted on aluminum and stainless steel substrate.The microstructural characteristics of the splat and the heat conduction and solidification processes during the flattening process have been studied numerically and experimentally.The effect of the droplet temperature,impact velocity as well as the substrate temperature was investigated.The phenomenon of substrate melting was observed after the spreading of nickel droplet,which became more pronounced when the initial substrate temperature increased.Increasing the impact velocity of droplet resulted in a decrease in the interfacial temperature between droplet and substrate.     

Simultaneous independent measurement of splat diameter and cooling time during impact on a substrate of plasma-sprayed molybdenum particles

In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed two-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique, the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma-sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 μs after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 μs.     

Finite element analysis of effect of substrate surface roughness on liquid droplet impact and flattening process

A computational program using the finite element method has been developed to simulate the impact and flattening of a metal droplet impacting onto a solid surface with different surface roughness occurring in the plasma thermal spray. The model is based on Navier-Stokes equations combining with friction conditions on the substrate surface to simulate the effect of substrate surface roughness on the flattening process of the droplet. In this study, a moving free surface model based on the Lagrangian method with an automatic adaptive remeshing technique has been developed to handle the large deformation of droplets and to ensure the computational accuracy of the numerical results. The numerical results show that the substrate surface roughness has a significant influence on the spreading velocity, flattening ratio, flattening time, splat size, and shape. The spreading process of a droplet is governed not only by the inertia and viscous forces, but also by the frictional resistance of the substrate surface.     

Numerical simulation of impingement of molten Ti,Ni, and W droplets on a flat substrate

Thermal spraying has been widely applied to process thin protective coatings on preshaped parts and to manufacture metallic preforms of a variety of geometries. The quality of materials produced by thermal spraying depends critically on the impact conditions of the droplets. In the present study, the deformation behavior and interaction of molten droplets impinging onto a flat substrate during thermal spraying have been numerically simulated. The calculated results reveal that a droplet spreads uniformly in the radial direction during impingement and eventually forms a thin splat with final diameter and thickness up to 11.3 times and down to 0.02 times the impact diameter, respectively. The final splat diameter increases rapidly with increasing impact velocity and melt density or decreasing melt viscosity. For the processing conditions of interest, the final splat diameter and the spreading time may be approximated by correlations: d_s/do = l.04Re^0.2 and t_s/(do/uo) = 0.62Re^0.2, where ds/do is the dimensionless splat diameter; ts/(do/uo) is the dimensionless spreading time; and Re is the Reynolds number. A fully liquid droplet impinging onto a flat solid substrate leads to good contact between the splat and the substrate. Multiple fully liquid droplets striking simultaneously onto other flattening, fully liquid splats cause ejection and rebounding of the liquid, as well as formation of voids within the liquid.