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Verification of the flattening behavior of thermal-sprayed particles and free-falling droplets through controlling ambient pressure
Surface & Coatings Technology 205 (2011) 3816–3823
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Verification of the flattening behavior of thermal-sprayed particles and free-falling droplets through controlling ambient pressure K. Yang ⁎, Y. Ebisuno, K. Tanaka, M. Fukumoto, T. Yasui, M. Yamada Department of Mechanical Engineering, Toyohashi University of Technology, 1-1, Tempaku-cho, Toyohashi, Aichi 441–8580, Japan
a r t i c l e
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Article history: Received 7 November 2010 Accepted in revised form 22 January 2011 Available online 27 January 2011 Keywords: Thermal spraying Cu Ambient pressure Disk-shaped splat Splash splat Adsorption/desorption Heat transfer Wetting
a b s t r a c t Aiming at clarifying the role of ambient pressure on flattening behavior of molten particles on flat substrate surface, commercially available Cu powders were thermally sprayed onto mirror-polished AISI304 substrate using low-pressure plasma-spraying technique. The splat shapes on flat substrates underwent a transition from a distorted shape with splash to a disk-shaped one with a decrease of the ambient pressure, because there was no chemical modification and surface topography change took place, and the adsorption/desorption of the adsorbates and condensates on substrate surface played an important role on the splat flattening and solidification process. As a simulation of the splat formation process, the flattening behavior of millimetersized free-falling Cu droplet on the flat AISI304 substrate was investigated under designated ambient pressures. Better heat transfer and enhanced cooling rate were obtained under lower pressure in the deposition chamber due to the more intimate contact. Smaller contact angle of molten Cu droplet onto AISI304 substrate was maintained at lower pressure condition, which indicated that more favorable wetting can be obtained under such experiment condition. Consequently, the improved wetting generated by removing the adsorbates and condensates though reducing the ambient pressure might dominate the flattening behavior of the molten droplet on flat substrate surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Coating is a covering that is applied to the surface of an object, usually referred to as the substrate. In many cases coatings are applied to improve surface properties of the substrate, such as appearance, adhesion, wettability, corrosion resistance, wear resistance, and scratch resistance. Actually, this technique is widely used in many industrial applications over the last decade [1–3], for example, typically used as thermal barrier coating, TBC, in power plants. In other cases, in particular in printing processes and semiconductor device fabrication, the coating forms an essential part of the finished product. Thermal spraying is a process that can provide thick coatings over a large area at high deposition rate. However, the controllability or reliability of the process has not been established yet until today. As the individual splat is unit cell for the entire coating buildup, coating microstructure and corresponding properties, such as porosity and adhesion strength, depend strongly on the flattening nature of each splat [4]; therefore, it is necessary to study in detail the basic process of flattening behavior of the sprayed particles, not only from the point of view of scientific interest, but also from that of technical consequences as well.
⁎ Corresponding author. Tel.: +81 80 6914 5744; fax: +81 532 44 6690. E-mail address: [email protected] (K. Yang). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.01.049
In general, two typical types of splats, named disk-shaped splat and splash splat, can be obtained on the flat substrate surface under different conditions. Various investigations on splat formation have appeared in the literatures by theoretical [5,6], numerical [7–9] and experimental methods [10–14] in the past few decades. A transition phenomenon in a flattening behavior of the thermal-sprayed particle on the flat substrate surface was introduced by the authors [15–18], which reported that disk-shaped splat deposited instead of splash one when the spraying was carried out under low pressure in the deposition chamber for most of the feedstock materials. Sampath and Herman also reported that more contiguous Ni splats formed in a reduced pressure chamber than at atmospheric pressure [19]. However, we still cannot answer why or how does the disk-shaped splat appear, and what is the essential of the splat formation process. Therefore, there was a need for a detail study of this aspect. However, nearly all the previous studies observed the final splats without knowing the actual impact process, because thermal spraying is a complex and short-period process; hereby, it is difficult to clarify the flattening and solidification behavior of the thermal-sprayed particles directly with the prevailing technology. To overcome this difficulty, a free-falling experiment was carried out as a simulation of the thermal-spraying process, using both experiments and numerical simulation [20–28]. The experimental conditions were determined as equivalent Reynolds and Peclet numbers as practical thermalspraying process, despite the different impact velocity and diameter of the droplets. Weber numbers were also considered during the
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investigation. Moreover, wetting of the substrate surface by molten droplet played an important role in the droplet flattening because it affected not only the surface effects, but also the contact thermal resistance at the splat–substrate interface, which was an important parameter for the development of the coating structure. The degree of wetting was described by the contact angle (θ) [29], the angle at which the liquid–vapor interface met the solid–liquid interface. If the wetting was very favorable, the contact angle would be low, and the fluid would spread to cover a larger area of the surface. If the wetting was unfavorable, the fluid would form a compact droplet on the surface. In this study, the splats were collected under various ambient pressures. The flattening behavior of individual thermal-sprayed particles and millimeter-sized free-falling droplets onto flat substrate were investigated systematically. This article focused, in particular, on the heat transfer from molten droplet to substrate surface, and the wetting of molten droplet to flat substrate under designated experiment conditions.
2. Experimental procedures 2.1. Thermal-spraying and free-falling apparatus and raw materials
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Fig. 2. Micrograph of feedstock Cu powders.
with dimensions of 30 mm × 30 mm × 5 mm were used as substrates. The thermal history of the droplet on the substrate was measured during the flattening and solidification process. A hole with a diameter of 1 mm was drilled at the center of substrate, and J-type thermocouple with a diameter of 0.3 mm was inserted through this hole. A small amount of ceramic cement was forced into the hole, which also acted as an electrical insulator between the thermocouple and substrate. The thermo electromotive force was converted to digital signal and recorded by data logging system. The measuring method in detail can be referred to a previous article [30].
The spraying work was carried out using low-pressure plasmaspraying (LPPS) as shown in Fig. 1, while keeping all the other conditions constant. AISI304 plates with dimensions of 20 mm × 20 mm × 6 mm were finally polished with 0.3 μm Al2O3 buff prior to spraying. Commercially available Cu powders with 75 μm or less in diameter as shown in Fig. 2 (Kojundo Chemical Lab. Co., Ltd., Japan.) were thermally sprayed onto the prepared substrates, operated at a current of 800 A and a voltage of 40 V, the ambient pressures ranged from 101.3 to 6.7 kPa. The spraying distance between the gun and the substrate was kept at 200 mm. The powder was injected at a feeding rate of 6 g/min. Argon and helium were used as operating gas with flow rate of 50 and 12 l/min, respectively. Spraying in LPPS was conducted under the designated pressures after once evacuated to the lowest pressure condition of the equipment. During deposition, the substrate surface was held vertically and the spray gun was held horizontally so that the direction of droplet stream was perpendicular to substrate surface. For the particle collection, both of the fixed steel slit and the moving graphite shutter were installed between the plasma torch and the substrate in order to collect the particles having the homogeneous thermal and velocity hysteresis. Particles were collected on the substrate by moving the shutter rapidly in one direction. The number of the particles deposited on the substrate in one passing of the shutter was around 100 or more. The free-falling experiments were conducted under the similar ambient pressures as thermal-spraying process. Commercially available Cu wire with a diameter of 2 mm (99.9% pure) was used as droplet material. It was melted by radio-frequency heating equipment prior to the falling. No.2000 waterproof paper polished AISI304 plates
The splat morphologies were observed using scanning electron microscope (SEM). (JSM-6390TY JEOL, Co., Ltd., Tokyo, Japan). First, the top surface morphologies of the splat collected under the designated conditions were observed. Following this, carbon tape was pressed of the sprayed region, then pulled off, and some splats were removed. The bottom surface of the splat collected on carbon tape was examined under SEM [15,31]. The cross section morphology at splat–substrate interface was investigated using a focused ion beam (FIB) microscope (Quanta 200 3D, Czech Republic). As the contact angle provided an inverse measure of wettability, the contact angle of Cu droplet was measured experimentally in this study, based on the standard sessile drop test. Mirror-polished AISI304 plates were used as the substrates. Spherical Cu with a diameter of 2 mm was used as droplet material. Self designed contact angle measurement equipment was used for the experiment (National Institute of Advanced Industrial Science and Technology: AIST). Fig. 3 indicates the schematic of the contact measurement apparatus. All the measurements were conducted in an argon atmosphere to prevent oxidation. The droplet was melted and profile photographs were captured by CCD camera with frame grabber rate of 30 fps. The substrate and droplet was maintained at 1373 K during the
Fig. 1. Schematic of low-pressure plasma-spraying (LPPS) equipment.
Fig. 3. Schematic of contact angle measurement apparatus.
2.2. Evaluation methods
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measurement, which is slightly higher than the melting point of Cu droplet; hence, no solidification occurred during the droplet spreading until the contact angle become stable. After that, the system was cooled and droplet solidification occurred. ImageJ imaging software (National Institutes of Health, Washington, DC, USA) was employed to calculate the contact angles.
3. Results and discussion 3.1. Characterization of thermal-sprayed splats The splat top surface morphologies of splats deposited under various ambient pressures are shown in Fig. 4. Most splats deposited at the atmospheric pressure performed a typical splash-like shape with a central splat surrounded by a ring of fragments (Fig. 4a), we named it as ring-shaped splash splat. By reducing the ambient pressure, most of the splat showed a uniform morphology with clear flow pattern and long projections along the periphery of the splat; however, the splash fingers always connected with the central solidification area (Fig. 4c). With the continuing reduction of ambient pressure, the splat pattern changed from the form with splashing to the one without splashing (Fig. 4e), only few short and smooth splash fingers around the central solidification area, which was defined as disk-shaped splat. Namely, the disk-shaped splat appeared only on the substrate located at low-pressure condition. Surface structure presented in Fig. 4 (d and f) showed the typical water-wave morphology from the high magnification images of the splash splat with radial fingers and disk-shaped splat. This phenomenon should due to the coexistence of liquid–solid, which might result in different viscosity. The difference between liquid drop and surrounding media kinematic viscosities and particle Reynolds number (Re) can result in convective movements [32,33] within the drop forming a spherical hill vortex. Furthermore, the droplet spread to cover a larger area of the surface due to the favorable wetting under lowered ambient pressure [15], resulting in the lateral fluid spreading upon the initial layer, which might enhance the formation of the water-wave morphology.
The bottom surface morphologies of Cu splats were observed as shown in Fig. 5. The splat with splashing obtained at atmospheric pressure is shown in Fig. 5 (a and b), and numerous pores were observed at the bottom surface of the splat. The pores should be generated by the evaporation of the adsorbates and condensates at the interface during the flattening and solidification process after the molten droplet impacted onto the substrate surface. Splash splat with radial fingers was maintained on the substrate located to the chamber with the ambient pressure lowered to 66.7 kPa as shown in Fig. 5(c and d), nano-pores still can be observed, either the amount or the diameter of the pores significantly decreased. With the continuing decrease of ambient pressure, almost no pore could be observed from the bottom surface view, and the solidification structure looks quite homogeneous as presented in Fig. 5 (e and f). In summary, both the area fraction and average size of the nano-pores at the bottom surface increase significantly with the increase of ambient pressure; hence, more intimate contact between the splat and substrate surface can be expected of the splat deposited at lowered ambient pressure. It is known that water and other substances can be adsorbed on clean solid surface, and the most common condensate is water from moisture. Desorption tends to occur when the substrate temperature rises and ambient pressure decreases. Actually, often molecules do form multi-layers, that is, some are adsorbed on already adsorbed molecules. BET isotherm [34] suggested that lack of a true chemical bond between adsorbed gas molecular and substrate except the first layer, so that the physical adsorption could be removed easily by reducing the ambient pressure. This must be the fundamental reason of the average size and area fraction of the nano-pores decreased along with the reduction of ambient pressure. The cross section morphologies of Cu splats deposited under various ambient pressures are shown in Fig. 6. Pores can be found at the interface for the splats deposited both high- and low-pressure conditions as indicated in mark 1 and 7. However, more pores exist inside the splat deposited under atmospheric pressure condition as marked with 2 and 3, and the pore size is remarkably larger than those exist at the interface between splat and substrate, because the amount of adsorbed gas molecules and their volume increased rapidly with the progress of flattening, which may be attributed to the temperature
Fig. 4. Top surface morphologies of Cu sprayed onto AISI304 substrate onto AISI304 substrate under various ambient pressures.
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Fig. 5. Bottom surface morphologies of Cu sprayed onto AISI304 substrate under various ambient pressures.
increased in this process. While the amount of adsorbates and condensates decreased significantly along with the decrease of ambient pressure, therefore, few pores can be observed inside the splat deposited under reduced chamber pressure. Meanwhile, as marked with 4, 5, 8 and 9, pores with large diameter paralleled to the substrate surface and thin thickness can be found at the periphery of the splats, in particular, deposited at high chamber pressure, because environmental gas could be trapped into the bottom of the splat from the part where the spreading droplet met the flat substrate before the solidification, in other words, due to the poor wetting of substrate by thermal-sprayed particle. According to the figure, the grain size became larger far away from the central solidification area, columnar structure grains could be found near the initial impinging zone, while the grain grew up paralleled to the substrate surface at the periphery of the splat as marked with 6 and 10. The contacting condition at central part was much better than that of the periphery of the splat, because the dynamic impact pressure was very large at the initial impact zone, but decreased rapidly with the flattening diameter [35]. Namely,
enhanced heat transfer might exist at the central zone, followed by faster cooling of molten droplet, resulting in the formation of columnar structure. 3.2. Characterization of free-falling droplets As a simulation of the splat formation process, millimeter-sized molten Cu droplets were deposited on AISI304 substrate surface by free-falling experiment under various ambient pressures. Droplet morphologies obtained under designated conditions are shown in Fig. 7. From the figure, the splat collected on the substrate under atmospheric pressure was a splash shape with sharp splash fingers; however, no ring-shaped splash fingers can be obtained, while they always connected with the central solidification area, whereas the splat morphology changed from splash shape to disk one with the decrease of ambient pressure. Actually, the shape of the splashing itself was different from the thermal-sprayed splat. The completely fragmented splat might be observed in thermal spraying, which cannot be found in the free-falling experiment. The Sommerfeld
Fig. 6. Cross section morphologies of Cu sprayed onto AISI304 substrate under various ambient pressures.
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Fig. 8. Bottom surface morphologies of free-falling droplet by controlling chamber pressure. Fig. 7. Dependence of free-falling droplet morphologies on ambient pressure.
number, K, has been often mentioned recently [36], which was described as 1=2
K = We
Re
2
1=4
connected by molten metal of the splat, the temperature in splat bottom part was recorded since droplet impacted onto substrate surface. The slope of this splat temperature curve, dT/dt, was defined
ð1Þ
We = ρdv = γ
ð2Þ
Re = ρdv = η
ð3Þ
Here, We and Re were the Weber and Reynolds number, and ρ: density, d: diameter, v: velocity, γ: viscosity, η: surface tension, respectively. According to the equations, equivalent Reynolds numbers were given by the thermal-spraying and free-falling droplets. Therefore, larger splashing number, K, can be obtained during thermal-spraying process, owing to the larger Weber number, which induced the more intensive splashing of thermal-sprayed particles, and the splash fingers can be deposited far away from the central solidification area. However, the splat shape of the free-falling droplet showed similar changing tendency as in the thermal-spraying process, despite the different impact velocity and diameter of the droplets. Therefore, investigation of the splat formation process of thermal-sprayed particle through observation on the flattening behavior of free-falling droplet is meaningful. The bottom surface morphologies of droplets obtained under various ambient pressures are shown in Fig. 8. It could be found that the number of pores gradually decreased with the decrease of ambient pressure. However, this pore decrease could be recognized only at central part of the splat and pores still existed at peripheral part even at 6.7 kPa (Fig. 8c). It can be estimated that central area of the splat was formed under a higher impact pressure impingement condition compared with other part. Li and Li [35] proposed that the impact pressure could be very high and concentrated at a small contacting area then spread quickly with droplet flattening. Meanwhile, pore basically inhibited the heat transfer, because contact area decreased due to the existence of pores at substrate–droplet interface. In order to understand the effect of ambient pressure on heat transfer at interface between molten droplet and substrate surface, splat temperature history was measured under the designated conditions as shown in Fig. 9. As thermocouple junction could be
Fig. 9. Dependence of droplet temperature history and cooling rate on ambient pressure.
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as the cooling rate of the splat. According to the figure, the temperature at substrate–droplet interface decreased faster when the substrate was located at lower ambient pressure environment, which indicated that better heat transfer could be obtained. From Fig. 9b, it was found that cooling rate in the splat increased with the decrease of ambient pressure, increasing from 4.4 × 103 K/s on the substrate under atmospheric pressure to 1.1 × 104 K/s on the substrate located at an ambient pressure of 6.7 kPa. It was quite noticeable that the ambient pressure affected the cooling rate after the droplet impacted onto the flat substrate. As the substrate material used in this study was stainless steel, and the substrates were stored for appropriate periods at room temperature in a dry condition, it was estimated that no significant oxidation change by reducing the ambient pressure. Therefore, no significant topography and surface roughness change took place, and only the adsorption/desorption of the adsorbates and condensates occurred by controlling the ambient pressure. Surely all the adsorbates and condensates might be evaporated once the molten droplet impacted onto the flat substrate surface; however, the more intimate contact between droplet and substrate could be obtained at reduced ambient pressure, owing to the less nano-pores formed at the bottom surface (Fig. 8c). Hence the heat transfer from the molten droplet to substrate surface was enhanced, followed by higher cooling rate of the droplet. 3.3. Dependence of wetting on ambient pressure Fig. 10 shows the contact angle of molten Cu droplet onto AISI304 substrate under various ambient pressures. The measurements were conducted after the droplet was completely melted and solidified on the substrate surface. According to the figure, the contact angle of droplets was maintained at 141° on the substrate under atmospheric pressure in an argon atmosphere, which meant that wetting of the substrate surface was unfavorable so the fluid would minimize contact with the surface and formed a compact liquid droplet; therefore, environmental gas could be trapped into the bottom of the droplet from the periphery part of this spreading droplet. Morks and his coworkers [37] also proposed that the spreading became slower under higher chamber pressure; accordingly, the flattening ratio of the final depositions became smaller for the thermal-sprayed splats. With the decrease of ambient pressure, the contact angle decreased significantly, which indicated that the wetting of substrate by molten droplet was enhanced along with the decrease of ambient pressure. Finally, a contact angle of 5° was maintained on the substrate located in the chamber with an ambient pressure of 5 Pa, wetting of the substrate surface was very favorable, and the fluid would spread over a large area of the substrate surface. Our previous studies [15,38,39] reported that good wetting might be generated by removing the adsorbates and condensates on the substrate surface through substrate preheating. As the desorption of
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adsorbed gas/condensation occurred by reducing the ambient pressure, the physical adsorbed water and other condensates, along with a significant fraction of adsorbed substances, were removed from the substrate surface rapidly, resulting in improvement of the wetting of substrate surface by molten droplet. As good wetting could be obtained at low-pressure condition, the solid layer grew by a significant amount during spreading, it would restrain the splat from spreading too far and becoming thin enough to rupture, the asdefined solidification parameter (Θ) [7,8] was kept in the favorable range. As a result, a disk-shaped splat would be produced. On the other hand, splashing of the molten droplet might occur. Actually, both the fraction of disk-shaped splat and wetting of flat substrate by molten droplet enhanced remarkably under the lowered ambient pressure [15,38]; in other words, the dependence of wetting on ambient pressure corresponded well to that of the splat shape. Aiming at clarifying the spreading process of molten Cu droplet onto flat AISI304 substrate in more detail, the dependence of contact angle on spreading time under various ambient pressures was recorded as shown in Fig. 11. The measurement started since the droplet was fully melted and spread onto the substrate surface, and the droplet temperature was kept at 1373 K during the measurement, which was slightly higher than its melting point. Here, it was assumed that this relation was applicable to dynamic wetting. According to the figure, it could clearly find that the molten droplet took several minutes to achieve the equilibrium state on the substrate surface, and the contact angle decreased with the decrease of ambient pressure. Also, the contact angle become stable faster at lower pressure than that of higher pressure condition, which indicated that the dynamic wetting was better at lowered ambient pressure. Even the flattening of the real thermal-sprayed particle onto the flat substrate was finished within several microseconds; however, it was believed that the flattening behavior became easier under the lower pressure condition because of the enhanced wetting. While the spreading speed was slowed down with the increase of ambient pressure, hereby, heat transfer from the molten droplet to the substrate surface was restrained, then affected the splat formation process. In summary, although there were not enough direct evidences to prove the sole domination at present, the wetting of substrate surface by molten droplet likely had a strongly affect on the splat formation. 3.4. Splat formation mechanisms through controlling ambient pressure When the spraying work was conducted at atmospheric pressure, low velocity and high particle temperature can be obtained due to the extended flying time [40–42]; in other words, the particle had low viscosity during the flattening process on the flat substrate surface. When the molten droplet impacted onto a flat substrate surface, the lateral flattening of the liquid fluid along the substrate surface took place, which was driven by the kinetic energy. The dynamic impact
Fig. 10. Final contact angle of molten Cu droplet onto AISI304 substrate under various ambient pressures.
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pressure influenced zone must be increased because favorable wetting could be obtained at low-pressure condition. Therefore, the impact pressure would be enough to force liquid into surface crevices without splashing, finally, formed as disk-shaped splat on the flat substrate surface. Briefly speaking, as the flattening and solidification of the individual thermal-sprayed particle was the fundamental process for the coating fabrication process, coating properties depend strongly on the flattening nature of each splat. Therefore, the investigation of the flattening behavior of the sprayed particles on the substrate surface is significantly meaningful for the practical usage of the thermal-spraying process. By selecting the optimum operating conditions in thermal spraying, we can control the coating microstructure and the corresponding properties well. Fig. 11. Dependence of contact angle on spreading time under various ambient pressures.
4. Conclusions pressure towards the substrate surface would be generated to keep the fluid flowing along the substrate surface. This impact pressure could be very high and concentrated at a small contacting area and then spread quickly with droplet flattening. The maximum pressure was located at the front of the droplet at an early stage of deformation, which drove the fluid moving quickly along substrate and resulted in lateral flow [35]. Heating of molten droplet to the substrate occurred because the heat flow from the droplet to the substrate occurred simultaneously. The intensively rapid heating of the substrate surface would cause desorption of adsorbates and condensates to form gas or to evolve into gas phase. Certain amount of environmental gas molecules would be trapped into the bottom surface of the splat from the periphery of the spreading particle, due to the poor wetting of substrate by the molten particle. With the progress of flattening, more gas was accumulated at the interface between the fluid and the substrate. This adsorbed gas and trapped gas induced pressure tended to detach the fluid from the substrate surface, and such pressure might depend on the gas amount accumulated and evolution rate. Accordingly, the gas-induced pressure would be increased with the flattening process, while the dynamic impact pressure spread out and dissipated quickly with droplet flattening, and the radius increased with the progress of flattening and solidification. Moreover, the cooling rate was low due to the weakened heat transfer and poor wetting, which resulted in the rapid flattening with low viscosity. When the gas-induced pressure became larger than the impact pressure at the last stage of spreading, the flowing fluid would be pushed up or thrown away in jetting by inertial of flowing fluid [43]; in other words, deposited as splash splat on the flat substrate surface. With the decrease of ambient pressure, higher velocity and lower temperature of the in-flight particle prior to impact onto the substrate could be obtained [40–42], which resulted in higher dynamic impact pressure and viscosity during the flattening process. When the molten droplet impacted onto a flat substrate surface, the lateral flattening of the liquid fluid along the substrate surface took place, which was driven by the kinetic energy. Meanwhile, most of the adsorbates and condensates on substrate surface were removed by reducing the ambient pressure, and more intimate contact and enhanced heat transfer between the molten droplet and the clean surface could be obtained. A smaller contact angle at the interface where the molten particle met the substrate surface could be gotten, owing to the favorable wetting. Therefore, less environmental gas would be trapped into the bottom surface of the splat, and the spreading on the surface became easier. Higher dynamic impact pressure towards the substrate surface would be generated to keep the faster fluid flowing along the substrate surface. Less gas was accumulated and evaporated at the interface for lower ambient pressure. Accordingly, the adsorbed gas and trapped gas induced pressure should be lower than that achieved at higher pressure condition, while the dynamic impact pressure should be higher. The diameter of the impact
Commercially available Cu powders were thermally sprayed onto AISI304 substrates under various ambient pressures, and the splat formation process of the individual Cu particle had been investigated systematically. The spreading behavior of millimeter-sized molten Cu droplet and contact angle under designated ambient pressures was evaluated as well. The results obtained in this study are summarized as follows: (1) The splat shape of the thermal-sprayed particles changed from a splash splat to a disk-shaped one with the decrease of ambient pressure. As there was no chemical modification and surface morphology change took place, this transition was likely attributed to the adsorption/desorption of the adsorbates and condensates on substrate surface through controlling ambient pressure. (2) The shape of the splashing itself was different from the thermal-sprayed splat; however, the splat of the free-falling droplet showed similar changing tendency as in the thermalspraying experiment, despite the different impact velocity and diameter of the droplets. Heat transfer from the molten droplet to substrate surface was enhanced, followed by improved cooling rate when the experiment was conducted at low pressure in the chamber. (3) Favorable wetting was maintained under low-pressure condition, which corresponded well with the splat shape. The favorable wetting might generated by removing the adsorbates through reducing the ambient pressure. In other words, wetting of substrate surface by molten droplet may dominate the splat formation process. Acknowledgments The authors would like to acknowledge the assistance provided by Dr. Shimojima and Dr. Matsumoto in National Institute of Advanced Industrial Science and Technology (AIST) for the contact angle measurement. The authors’ thanks also go to Mr. S. Yoshida and D. Mano for their assistance and discussions in the experiments. The research was supported both by the Grant-in-Aid for Scientific Research of the Ministry of Education, Science, Culture and Sports in Japan, and by a special research fund in Toyohashi University of Technology. References [1] M.L. Thorpe, Adv. Mater. Process 143 (5) (1993) 50. [2] F. Kassabji, G. Jacq, J.P. Durand, in: C. Coddet (Ed.), Proceedings of the International Thermal Spraying Conference 1998, Pub. ASM International, Materials Park, OH, USA, 1998, p. 1677. [3] M. Ducos, J.P. Durand, in: C.C. Berndt, K.A. Khor, E.F. Lugscheider (Eds.), Proceedings of the International Thermal Spraying Conference 2001, Pub. ASM International, Materials Park, OH, USA, 2001, p. 1267.
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Verification of the flattening behavior of thermal-sprayed particles and free-falling droplets through controlling ambient pressure K. Yang ⁎, Y. Ebisuno, K. Tanaka, M. Fukumoto, T. Yasui, M. Yamada Department of Mechanical Engineering, Toyohashi University of Technology, 1-1, Tempaku-cho, Toyohashi, Aichi 441–8580, Japan
a r t i c l e
i n f o
Article history: Received 7 November 2010 Accepted in revised form 22 January 2011 Available online 27 January 2011 Keywords: Thermal spraying Cu Ambient pressure Disk-shaped splat Splash splat Adsorption/desorption Heat transfer Wetting
a b s t r a c t Aiming at clarifying the role of ambient pressure on flattening behavior of molten particles on flat substrate surface, commercially available Cu powders were thermally sprayed onto mirror-polished AISI304 substrate using low-pressure plasma-spraying technique. The splat shapes on flat substrates underwent a transition from a distorted shape with splash to a disk-shaped one with a decrease of the ambient pressure, because there was no chemical modification and surface topography change took place, and the adsorption/desorption of the adsorbates and condensates on substrate surface played an important role on the splat flattening and solidification process. As a simulation of the splat formation process, the flattening behavior of millimetersized free-falling Cu droplet on the flat AISI304 substrate was investigated under designated ambient pressures. Better heat transfer and enhanced cooling rate were obtained under lower pressure in the deposition chamber due to the more intimate contact. Smaller contact angle of molten Cu droplet onto AISI304 substrate was maintained at lower pressure condition, which indicated that more favorable wetting can be obtained under such experiment condition. Consequently, the improved wetting generated by removing the adsorbates and condensates though reducing the ambient pressure might dominate the flattening behavior of the molten droplet on flat substrate surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Coating is a covering that is applied to the surface of an object, usually referred to as the substrate. In many cases coatings are applied to improve surface properties of the substrate, such as appearance, adhesion, wettability, corrosion resistance, wear resistance, and scratch resistance. Actually, this technique is widely used in many industrial applications over the last decade [1–3], for example, typically used as thermal barrier coating, TBC, in power plants. In other cases, in particular in printing processes and semiconductor device fabrication, the coating forms an essential part of the finished product. Thermal spraying is a process that can provide thick coatings over a large area at high deposition rate. However, the controllability or reliability of the process has not been established yet until today. As the individual splat is unit cell for the entire coating buildup, coating microstructure and corresponding properties, such as porosity and adhesion strength, depend strongly on the flattening nature of each splat [4]; therefore, it is necessary to study in detail the basic process of flattening behavior of the sprayed particles, not only from the point of view of scientific interest, but also from that of technical consequences as well.
⁎ Corresponding author. Tel.: +81 80 6914 5744; fax: +81 532 44 6690. E-mail address: [email protected] (K. Yang). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.01.049
In general, two typical types of splats, named disk-shaped splat and splash splat, can be obtained on the flat substrate surface under different conditions. Various investigations on splat formation have appeared in the literatures by theoretical [5,6], numerical [7–9] and experimental methods [10–14] in the past few decades. A transition phenomenon in a flattening behavior of the thermal-sprayed particle on the flat substrate surface was introduced by the authors [15–18], which reported that disk-shaped splat deposited instead of splash one when the spraying was carried out under low pressure in the deposition chamber for most of the feedstock materials. Sampath and Herman also reported that more contiguous Ni splats formed in a reduced pressure chamber than at atmospheric pressure [19]. However, we still cannot answer why or how does the disk-shaped splat appear, and what is the essential of the splat formation process. Therefore, there was a need for a detail study of this aspect. However, nearly all the previous studies observed the final splats without knowing the actual impact process, because thermal spraying is a complex and short-period process; hereby, it is difficult to clarify the flattening and solidification behavior of the thermal-sprayed particles directly with the prevailing technology. To overcome this difficulty, a free-falling experiment was carried out as a simulation of the thermal-spraying process, using both experiments and numerical simulation [20–28]. The experimental conditions were determined as equivalent Reynolds and Peclet numbers as practical thermalspraying process, despite the different impact velocity and diameter of the droplets. Weber numbers were also considered during the
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investigation. Moreover, wetting of the substrate surface by molten droplet played an important role in the droplet flattening because it affected not only the surface effects, but also the contact thermal resistance at the splat–substrate interface, which was an important parameter for the development of the coating structure. The degree of wetting was described by the contact angle (θ) [29], the angle at which the liquid–vapor interface met the solid–liquid interface. If the wetting was very favorable, the contact angle would be low, and the fluid would spread to cover a larger area of the surface. If the wetting was unfavorable, the fluid would form a compact droplet on the surface. In this study, the splats were collected under various ambient pressures. The flattening behavior of individual thermal-sprayed particles and millimeter-sized free-falling droplets onto flat substrate were investigated systematically. This article focused, in particular, on the heat transfer from molten droplet to substrate surface, and the wetting of molten droplet to flat substrate under designated experiment conditions.
2. Experimental procedures 2.1. Thermal-spraying and free-falling apparatus and raw materials
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Fig. 2. Micrograph of feedstock Cu powders.
with dimensions of 30 mm × 30 mm × 5 mm were used as substrates. The thermal history of the droplet on the substrate was measured during the flattening and solidification process. A hole with a diameter of 1 mm was drilled at the center of substrate, and J-type thermocouple with a diameter of 0.3 mm was inserted through this hole. A small amount of ceramic cement was forced into the hole, which also acted as an electrical insulator between the thermocouple and substrate. The thermo electromotive force was converted to digital signal and recorded by data logging system. The measuring method in detail can be referred to a previous article [30].
The spraying work was carried out using low-pressure plasmaspraying (LPPS) as shown in Fig. 1, while keeping all the other conditions constant. AISI304 plates with dimensions of 20 mm × 20 mm × 6 mm were finally polished with 0.3 μm Al2O3 buff prior to spraying. Commercially available Cu powders with 75 μm or less in diameter as shown in Fig. 2 (Kojundo Chemical Lab. Co., Ltd., Japan.) were thermally sprayed onto the prepared substrates, operated at a current of 800 A and a voltage of 40 V, the ambient pressures ranged from 101.3 to 6.7 kPa. The spraying distance between the gun and the substrate was kept at 200 mm. The powder was injected at a feeding rate of 6 g/min. Argon and helium were used as operating gas with flow rate of 50 and 12 l/min, respectively. Spraying in LPPS was conducted under the designated pressures after once evacuated to the lowest pressure condition of the equipment. During deposition, the substrate surface was held vertically and the spray gun was held horizontally so that the direction of droplet stream was perpendicular to substrate surface. For the particle collection, both of the fixed steel slit and the moving graphite shutter were installed between the plasma torch and the substrate in order to collect the particles having the homogeneous thermal and velocity hysteresis. Particles were collected on the substrate by moving the shutter rapidly in one direction. The number of the particles deposited on the substrate in one passing of the shutter was around 100 or more. The free-falling experiments were conducted under the similar ambient pressures as thermal-spraying process. Commercially available Cu wire with a diameter of 2 mm (99.9% pure) was used as droplet material. It was melted by radio-frequency heating equipment prior to the falling. No.2000 waterproof paper polished AISI304 plates
The splat morphologies were observed using scanning electron microscope (SEM). (JSM-6390TY JEOL, Co., Ltd., Tokyo, Japan). First, the top surface morphologies of the splat collected under the designated conditions were observed. Following this, carbon tape was pressed of the sprayed region, then pulled off, and some splats were removed. The bottom surface of the splat collected on carbon tape was examined under SEM [15,31]. The cross section morphology at splat–substrate interface was investigated using a focused ion beam (FIB) microscope (Quanta 200 3D, Czech Republic). As the contact angle provided an inverse measure of wettability, the contact angle of Cu droplet was measured experimentally in this study, based on the standard sessile drop test. Mirror-polished AISI304 plates were used as the substrates. Spherical Cu with a diameter of 2 mm was used as droplet material. Self designed contact angle measurement equipment was used for the experiment (National Institute of Advanced Industrial Science and Technology: AIST). Fig. 3 indicates the schematic of the contact measurement apparatus. All the measurements were conducted in an argon atmosphere to prevent oxidation. The droplet was melted and profile photographs were captured by CCD camera with frame grabber rate of 30 fps. The substrate and droplet was maintained at 1373 K during the
Fig. 1. Schematic of low-pressure plasma-spraying (LPPS) equipment.
Fig. 3. Schematic of contact angle measurement apparatus.
2.2. Evaluation methods
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measurement, which is slightly higher than the melting point of Cu droplet; hence, no solidification occurred during the droplet spreading until the contact angle become stable. After that, the system was cooled and droplet solidification occurred. ImageJ imaging software (National Institutes of Health, Washington, DC, USA) was employed to calculate the contact angles.
3. Results and discussion 3.1. Characterization of thermal-sprayed splats The splat top surface morphologies of splats deposited under various ambient pressures are shown in Fig. 4. Most splats deposited at the atmospheric pressure performed a typical splash-like shape with a central splat surrounded by a ring of fragments (Fig. 4a), we named it as ring-shaped splash splat. By reducing the ambient pressure, most of the splat showed a uniform morphology with clear flow pattern and long projections along the periphery of the splat; however, the splash fingers always connected with the central solidification area (Fig. 4c). With the continuing reduction of ambient pressure, the splat pattern changed from the form with splashing to the one without splashing (Fig. 4e), only few short and smooth splash fingers around the central solidification area, which was defined as disk-shaped splat. Namely, the disk-shaped splat appeared only on the substrate located at low-pressure condition. Surface structure presented in Fig. 4 (d and f) showed the typical water-wave morphology from the high magnification images of the splash splat with radial fingers and disk-shaped splat. This phenomenon should due to the coexistence of liquid–solid, which might result in different viscosity. The difference between liquid drop and surrounding media kinematic viscosities and particle Reynolds number (Re) can result in convective movements [32,33] within the drop forming a spherical hill vortex. Furthermore, the droplet spread to cover a larger area of the surface due to the favorable wetting under lowered ambient pressure [15], resulting in the lateral fluid spreading upon the initial layer, which might enhance the formation of the water-wave morphology.
The bottom surface morphologies of Cu splats were observed as shown in Fig. 5. The splat with splashing obtained at atmospheric pressure is shown in Fig. 5 (a and b), and numerous pores were observed at the bottom surface of the splat. The pores should be generated by the evaporation of the adsorbates and condensates at the interface during the flattening and solidification process after the molten droplet impacted onto the substrate surface. Splash splat with radial fingers was maintained on the substrate located to the chamber with the ambient pressure lowered to 66.7 kPa as shown in Fig. 5(c and d), nano-pores still can be observed, either the amount or the diameter of the pores significantly decreased. With the continuing decrease of ambient pressure, almost no pore could be observed from the bottom surface view, and the solidification structure looks quite homogeneous as presented in Fig. 5 (e and f). In summary, both the area fraction and average size of the nano-pores at the bottom surface increase significantly with the increase of ambient pressure; hence, more intimate contact between the splat and substrate surface can be expected of the splat deposited at lowered ambient pressure. It is known that water and other substances can be adsorbed on clean solid surface, and the most common condensate is water from moisture. Desorption tends to occur when the substrate temperature rises and ambient pressure decreases. Actually, often molecules do form multi-layers, that is, some are adsorbed on already adsorbed molecules. BET isotherm [34] suggested that lack of a true chemical bond between adsorbed gas molecular and substrate except the first layer, so that the physical adsorption could be removed easily by reducing the ambient pressure. This must be the fundamental reason of the average size and area fraction of the nano-pores decreased along with the reduction of ambient pressure. The cross section morphologies of Cu splats deposited under various ambient pressures are shown in Fig. 6. Pores can be found at the interface for the splats deposited both high- and low-pressure conditions as indicated in mark 1 and 7. However, more pores exist inside the splat deposited under atmospheric pressure condition as marked with 2 and 3, and the pore size is remarkably larger than those exist at the interface between splat and substrate, because the amount of adsorbed gas molecules and their volume increased rapidly with the progress of flattening, which may be attributed to the temperature
Fig. 4. Top surface morphologies of Cu sprayed onto AISI304 substrate onto AISI304 substrate under various ambient pressures.
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Fig. 5. Bottom surface morphologies of Cu sprayed onto AISI304 substrate under various ambient pressures.
increased in this process. While the amount of adsorbates and condensates decreased significantly along with the decrease of ambient pressure, therefore, few pores can be observed inside the splat deposited under reduced chamber pressure. Meanwhile, as marked with 4, 5, 8 and 9, pores with large diameter paralleled to the substrate surface and thin thickness can be found at the periphery of the splats, in particular, deposited at high chamber pressure, because environmental gas could be trapped into the bottom of the splat from the part where the spreading droplet met the flat substrate before the solidification, in other words, due to the poor wetting of substrate by thermal-sprayed particle. According to the figure, the grain size became larger far away from the central solidification area, columnar structure grains could be found near the initial impinging zone, while the grain grew up paralleled to the substrate surface at the periphery of the splat as marked with 6 and 10. The contacting condition at central part was much better than that of the periphery of the splat, because the dynamic impact pressure was very large at the initial impact zone, but decreased rapidly with the flattening diameter [35]. Namely,
enhanced heat transfer might exist at the central zone, followed by faster cooling of molten droplet, resulting in the formation of columnar structure. 3.2. Characterization of free-falling droplets As a simulation of the splat formation process, millimeter-sized molten Cu droplets were deposited on AISI304 substrate surface by free-falling experiment under various ambient pressures. Droplet morphologies obtained under designated conditions are shown in Fig. 7. From the figure, the splat collected on the substrate under atmospheric pressure was a splash shape with sharp splash fingers; however, no ring-shaped splash fingers can be obtained, while they always connected with the central solidification area, whereas the splat morphology changed from splash shape to disk one with the decrease of ambient pressure. Actually, the shape of the splashing itself was different from the thermal-sprayed splat. The completely fragmented splat might be observed in thermal spraying, which cannot be found in the free-falling experiment. The Sommerfeld
Fig. 6. Cross section morphologies of Cu sprayed onto AISI304 substrate under various ambient pressures.
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Fig. 8. Bottom surface morphologies of free-falling droplet by controlling chamber pressure. Fig. 7. Dependence of free-falling droplet morphologies on ambient pressure.
number, K, has been often mentioned recently [36], which was described as 1=2
K = We
Re
2
1=4
connected by molten metal of the splat, the temperature in splat bottom part was recorded since droplet impacted onto substrate surface. The slope of this splat temperature curve, dT/dt, was defined
ð1Þ
We = ρdv = γ
ð2Þ
Re = ρdv = η
ð3Þ
Here, We and Re were the Weber and Reynolds number, and ρ: density, d: diameter, v: velocity, γ: viscosity, η: surface tension, respectively. According to the equations, equivalent Reynolds numbers were given by the thermal-spraying and free-falling droplets. Therefore, larger splashing number, K, can be obtained during thermal-spraying process, owing to the larger Weber number, which induced the more intensive splashing of thermal-sprayed particles, and the splash fingers can be deposited far away from the central solidification area. However, the splat shape of the free-falling droplet showed similar changing tendency as in the thermal-spraying process, despite the different impact velocity and diameter of the droplets. Therefore, investigation of the splat formation process of thermal-sprayed particle through observation on the flattening behavior of free-falling droplet is meaningful. The bottom surface morphologies of droplets obtained under various ambient pressures are shown in Fig. 8. It could be found that the number of pores gradually decreased with the decrease of ambient pressure. However, this pore decrease could be recognized only at central part of the splat and pores still existed at peripheral part even at 6.7 kPa (Fig. 8c). It can be estimated that central area of the splat was formed under a higher impact pressure impingement condition compared with other part. Li and Li [35] proposed that the impact pressure could be very high and concentrated at a small contacting area then spread quickly with droplet flattening. Meanwhile, pore basically inhibited the heat transfer, because contact area decreased due to the existence of pores at substrate–droplet interface. In order to understand the effect of ambient pressure on heat transfer at interface between molten droplet and substrate surface, splat temperature history was measured under the designated conditions as shown in Fig. 9. As thermocouple junction could be
Fig. 9. Dependence of droplet temperature history and cooling rate on ambient pressure.
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as the cooling rate of the splat. According to the figure, the temperature at substrate–droplet interface decreased faster when the substrate was located at lower ambient pressure environment, which indicated that better heat transfer could be obtained. From Fig. 9b, it was found that cooling rate in the splat increased with the decrease of ambient pressure, increasing from 4.4 × 103 K/s on the substrate under atmospheric pressure to 1.1 × 104 K/s on the substrate located at an ambient pressure of 6.7 kPa. It was quite noticeable that the ambient pressure affected the cooling rate after the droplet impacted onto the flat substrate. As the substrate material used in this study was stainless steel, and the substrates were stored for appropriate periods at room temperature in a dry condition, it was estimated that no significant oxidation change by reducing the ambient pressure. Therefore, no significant topography and surface roughness change took place, and only the adsorption/desorption of the adsorbates and condensates occurred by controlling the ambient pressure. Surely all the adsorbates and condensates might be evaporated once the molten droplet impacted onto the flat substrate surface; however, the more intimate contact between droplet and substrate could be obtained at reduced ambient pressure, owing to the less nano-pores formed at the bottom surface (Fig. 8c). Hence the heat transfer from the molten droplet to substrate surface was enhanced, followed by higher cooling rate of the droplet. 3.3. Dependence of wetting on ambient pressure Fig. 10 shows the contact angle of molten Cu droplet onto AISI304 substrate under various ambient pressures. The measurements were conducted after the droplet was completely melted and solidified on the substrate surface. According to the figure, the contact angle of droplets was maintained at 141° on the substrate under atmospheric pressure in an argon atmosphere, which meant that wetting of the substrate surface was unfavorable so the fluid would minimize contact with the surface and formed a compact liquid droplet; therefore, environmental gas could be trapped into the bottom of the droplet from the periphery part of this spreading droplet. Morks and his coworkers [37] also proposed that the spreading became slower under higher chamber pressure; accordingly, the flattening ratio of the final depositions became smaller for the thermal-sprayed splats. With the decrease of ambient pressure, the contact angle decreased significantly, which indicated that the wetting of substrate by molten droplet was enhanced along with the decrease of ambient pressure. Finally, a contact angle of 5° was maintained on the substrate located in the chamber with an ambient pressure of 5 Pa, wetting of the substrate surface was very favorable, and the fluid would spread over a large area of the substrate surface. Our previous studies [15,38,39] reported that good wetting might be generated by removing the adsorbates and condensates on the substrate surface through substrate preheating. As the desorption of
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adsorbed gas/condensation occurred by reducing the ambient pressure, the physical adsorbed water and other condensates, along with a significant fraction of adsorbed substances, were removed from the substrate surface rapidly, resulting in improvement of the wetting of substrate surface by molten droplet. As good wetting could be obtained at low-pressure condition, the solid layer grew by a significant amount during spreading, it would restrain the splat from spreading too far and becoming thin enough to rupture, the asdefined solidification parameter (Θ) [7,8] was kept in the favorable range. As a result, a disk-shaped splat would be produced. On the other hand, splashing of the molten droplet might occur. Actually, both the fraction of disk-shaped splat and wetting of flat substrate by molten droplet enhanced remarkably under the lowered ambient pressure [15,38]; in other words, the dependence of wetting on ambient pressure corresponded well to that of the splat shape. Aiming at clarifying the spreading process of molten Cu droplet onto flat AISI304 substrate in more detail, the dependence of contact angle on spreading time under various ambient pressures was recorded as shown in Fig. 11. The measurement started since the droplet was fully melted and spread onto the substrate surface, and the droplet temperature was kept at 1373 K during the measurement, which was slightly higher than its melting point. Here, it was assumed that this relation was applicable to dynamic wetting. According to the figure, it could clearly find that the molten droplet took several minutes to achieve the equilibrium state on the substrate surface, and the contact angle decreased with the decrease of ambient pressure. Also, the contact angle become stable faster at lower pressure than that of higher pressure condition, which indicated that the dynamic wetting was better at lowered ambient pressure. Even the flattening of the real thermal-sprayed particle onto the flat substrate was finished within several microseconds; however, it was believed that the flattening behavior became easier under the lower pressure condition because of the enhanced wetting. While the spreading speed was slowed down with the increase of ambient pressure, hereby, heat transfer from the molten droplet to the substrate surface was restrained, then affected the splat formation process. In summary, although there were not enough direct evidences to prove the sole domination at present, the wetting of substrate surface by molten droplet likely had a strongly affect on the splat formation. 3.4. Splat formation mechanisms through controlling ambient pressure When the spraying work was conducted at atmospheric pressure, low velocity and high particle temperature can be obtained due to the extended flying time [40–42]; in other words, the particle had low viscosity during the flattening process on the flat substrate surface. When the molten droplet impacted onto a flat substrate surface, the lateral flattening of the liquid fluid along the substrate surface took place, which was driven by the kinetic energy. The dynamic impact
Fig. 10. Final contact angle of molten Cu droplet onto AISI304 substrate under various ambient pressures.
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pressure influenced zone must be increased because favorable wetting could be obtained at low-pressure condition. Therefore, the impact pressure would be enough to force liquid into surface crevices without splashing, finally, formed as disk-shaped splat on the flat substrate surface. Briefly speaking, as the flattening and solidification of the individual thermal-sprayed particle was the fundamental process for the coating fabrication process, coating properties depend strongly on the flattening nature of each splat. Therefore, the investigation of the flattening behavior of the sprayed particles on the substrate surface is significantly meaningful for the practical usage of the thermal-spraying process. By selecting the optimum operating conditions in thermal spraying, we can control the coating microstructure and the corresponding properties well. Fig. 11. Dependence of contact angle on spreading time under various ambient pressures.
4. Conclusions pressure towards the substrate surface would be generated to keep the fluid flowing along the substrate surface. This impact pressure could be very high and concentrated at a small contacting area and then spread quickly with droplet flattening. The maximum pressure was located at the front of the droplet at an early stage of deformation, which drove the fluid moving quickly along substrate and resulted in lateral flow [35]. Heating of molten droplet to the substrate occurred because the heat flow from the droplet to the substrate occurred simultaneously. The intensively rapid heating of the substrate surface would cause desorption of adsorbates and condensates to form gas or to evolve into gas phase. Certain amount of environmental gas molecules would be trapped into the bottom surface of the splat from the periphery of the spreading particle, due to the poor wetting of substrate by the molten particle. With the progress of flattening, more gas was accumulated at the interface between the fluid and the substrate. This adsorbed gas and trapped gas induced pressure tended to detach the fluid from the substrate surface, and such pressure might depend on the gas amount accumulated and evolution rate. Accordingly, the gas-induced pressure would be increased with the flattening process, while the dynamic impact pressure spread out and dissipated quickly with droplet flattening, and the radius increased with the progress of flattening and solidification. Moreover, the cooling rate was low due to the weakened heat transfer and poor wetting, which resulted in the rapid flattening with low viscosity. When the gas-induced pressure became larger than the impact pressure at the last stage of spreading, the flowing fluid would be pushed up or thrown away in jetting by inertial of flowing fluid [43]; in other words, deposited as splash splat on the flat substrate surface. With the decrease of ambient pressure, higher velocity and lower temperature of the in-flight particle prior to impact onto the substrate could be obtained [40–42], which resulted in higher dynamic impact pressure and viscosity during the flattening process. When the molten droplet impacted onto a flat substrate surface, the lateral flattening of the liquid fluid along the substrate surface took place, which was driven by the kinetic energy. Meanwhile, most of the adsorbates and condensates on substrate surface were removed by reducing the ambient pressure, and more intimate contact and enhanced heat transfer between the molten droplet and the clean surface could be obtained. A smaller contact angle at the interface where the molten particle met the substrate surface could be gotten, owing to the favorable wetting. Therefore, less environmental gas would be trapped into the bottom surface of the splat, and the spreading on the surface became easier. Higher dynamic impact pressure towards the substrate surface would be generated to keep the faster fluid flowing along the substrate surface. Less gas was accumulated and evaporated at the interface for lower ambient pressure. Accordingly, the adsorbed gas and trapped gas induced pressure should be lower than that achieved at higher pressure condition, while the dynamic impact pressure should be higher. The diameter of the impact
Commercially available Cu powders were thermally sprayed onto AISI304 substrates under various ambient pressures, and the splat formation process of the individual Cu particle had been investigated systematically. The spreading behavior of millimeter-sized molten Cu droplet and contact angle under designated ambient pressures was evaluated as well. The results obtained in this study are summarized as follows: (1) The splat shape of the thermal-sprayed particles changed from a splash splat to a disk-shaped one with the decrease of ambient pressure. As there was no chemical modification and surface morphology change took place, this transition was likely attributed to the adsorption/desorption of the adsorbates and condensates on substrate surface through controlling ambient pressure. (2) The shape of the splashing itself was different from the thermal-sprayed splat; however, the splat of the free-falling droplet showed similar changing tendency as in the thermalspraying experiment, despite the different impact velocity and diameter of the droplets. Heat transfer from the molten droplet to substrate surface was enhanced, followed by improved cooling rate when the experiment was conducted at low pressure in the chamber. (3) Favorable wetting was maintained under low-pressure condition, which corresponded well with the splat shape. The favorable wetting might generated by removing the adsorbates through reducing the ambient pressure. In other words, wetting of substrate surface by molten droplet may dominate the splat formation process. Acknowledgments The authors would like to acknowledge the assistance provided by Dr. Shimojima and Dr. Matsumoto in National Institute of Advanced Industrial Science and Technology (AIST) for the contact angle measurement. The authors’ thanks also go to Mr. S. Yoshida and D. Mano for their assistance and discussions in the experiments. The research was supported both by the Grant-in-Aid for Scientific Research of the Ministry of Education, Science, Culture and Sports in Japan, and by a special research fund in Toyohashi University of Technology. References [1] M.L. Thorpe, Adv. Mater. Process 143 (5) (1993) 50. [2] F. Kassabji, G. Jacq, J.P. Durand, in: C. Coddet (Ed.), Proceedings of the International Thermal Spraying Conference 1998, Pub. ASM International, Materials Park, OH, USA, 1998, p. 1677. [3] M. Ducos, J.P. Durand, in: C.C. Berndt, K.A. Khor, E.F. Lugscheider (Eds.), Proceedings of the International Thermal Spraying Conference 2001, Pub. ASM International, Materials Park, OH, USA, 2001, p. 1267.
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