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A study on the effect of a dynamic contact force control for improving electrospark coating properties
Surface & Coatings Technology 204 (2010) 2613–2623
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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
A study on the effect of a dynamic contact force control for improving electrospark coating properties S. Frangini ⁎, A. Masci Dept. TER, ENEA CRE Casaccia, Via Anguillarese 301, I-00123 Rome, Italy
a r t i c l e
i n f o
Article history: Received 10 November 2009 Accepted in revised form 2 February 2010 Available online 10 February 2010 Keywords: Electrospark deposition (ESD) Aluminum-diffusion coating Stainless steel sheet Contact force Spark breakdown Streamer discharge
a b s t r a c t The electrospark deposition (ESD) technique was used to apply aluminum-diffusion coatings over a thin stainless steel sheet under low energy discharge conditions to minimize substrate damage. In order to produce uniform coating properties over relatively large areas, automatic deposition treatments were conducted in a multi-scan mode under a dynamic control of the contact force parameter, which was realized by means of a spring-loaded electrode contact. A continuous and uniform Al coating layer was obtained indicating that a dynamic contact force system may be effective in suppressing several coating defects, which are notoriously produced with conventional ESD processes. The benefits of the dynamic force control are mostly related to the concept of a mobile electrode contact, which creates a momentary gap expansion during the spark discharge. The expanded gap is believed to reduce the effect of surface roughness on spark reproducibility and at the same time to stabilize the streamer regime of the spark breakdown, which are both essential for more uniform ESD depositions and lower cathode erosion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Coating processes are an important technology in the modern materials industry. One of the major challenges facing this industry is to keep up with the ever-increasing requirements for energy savings and respect for the environment. In this context, the last decade has seen a growing interest for the electrospark deposition (ESD) as a low-pollution and cost-effective method for production of a broad variety of metallurgical and cermet coatings on high-precision components to be used in demanding applications [1–5]. In the ESD process the energy stored in a capacitor bank is discharged through a sequence of short sparks of low voltage/high current intensity. Small parts of the material are detached in form of molten droplets from the electrode tip (anode) during each spark and ejected onto the partially-melted substrate (cathode) where they create a thin alloying zone with the base material. The low heat transfer (typically, 10 W/cm2 [6]) associated to the mass transfer process results in an extremely rapid solidification of the inter-mixed zone producing fine-grained and diffusion-bonded coatings with a minimal transformed substrate zone. In spite of these interesting properties, serious shortcomings in process stability has so far prevented a wide application of ESD technique for large area treatments mostly due to irregular contact geometry in the discharge zone, which negatively affects the
⁎ Corresponding author. Tel.: + 39 06 30483138. E-mail address: [email protected] (S. Frangini). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.006
reproducibility of coating properties [7,8]. Irregular contact surfaces are caused by a change of electrode or discharge conditions (for instance, due to a change of electrode tip shape or coating roughness). Such irregular contacts may also generate harmful transient arcs, which are characterized by high energy transfer, spot localization, electrode overheating. All these effects are not desirable for an efficient and regular deposition resulting in coating discontinuity , high surface roughness, spatter layers and crater erosion. It is recognized that stabilization of the spark discharge alloying by reducing the effect of local surface topography would be of prime importance for extending the use of ESD to large area applications where coatings with high chemical homogeneity, thickness uniformity and 100% continuity are required. Scarce attention has been given so far in the literature to the influence of non-electrical process parameters on spark stability, although recent papers have found that coating properties can be heavily affected by factors such as magnetic fields [9] and discharge gap sizes [7] since they may determine the gas dynamics properties of the spark plasma and therefore also erosion and mass transfer conditions. With reference to the effect of gap size, Belik et al. have concluded that high coating properties cannot be obtained without a proper control of discharge gap, which was realized in their work by measuring the contact force exerted by the electrode tip on the substrate surface as the gap discharge is very difficult to measure directly [7]. As part of our development efforts to realize a stable ESD processing for large area applications, this paper describes the use of a spring-loaded electrode as a simple means to produce smooth coatings under force-controlled spark discharges, while providing
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sufficiently high material deposition. Deposition of aluminum onto a thin gauge stainless steel foil was used as an illustrative example of a coating problem facing several challenges such as burning-through, oxidation and distortion of the substrate. This non trivial example reflects technological problems inherent to aluminization of metallic bipolar plate components in the Molten Carbonate Fuel Cell (MCFC) technology [10,11]. 2. General statement of the problem In the ESD the process of material transfer by emission of molten droplets is the result of the electrical discharges taking place in micron gaps filled with a dielectric gas. These gas discharges are characterized by a strong deviation from the well known Paschen's law at gaps smaller than about 10 μm [12]. The two major contributing factors to this deviation are the ionization processes in the dielectric gas and the electric field inhomogeneities caused by the electrode roughness. For this reason, little attention has been paid so far to this subject and only recently the level of interest for micro-gap discharges has emerged due to their implications in the development of microelectronics, MEMS (Micro-Electric-Mechanical Systems) and nanotechnology devices [13–15]. In order to rationalize the effect of a spring-loaded contact on a micro-gap discharge behavior it appears useful to introduce the subject with a brief description of the various sources of instability in ESD discharge processes. 2.1. Physical interpretation of ESD process instabilities In general terms, an electrical discharge is defined as the phenomenon of electrical conduction through a dielectric material in a sufficiently strong applied electric field with a sudden release of energy usually accompanied by a flash of light. Partial or complete discharge phenomena may take place depending critically on the intensity of the electric field (see Fig. 1). As evident from Fig. 1, the spark discharge is not a stationary process but, in fully rigorous terms, only a transition mechanism characterized by a voltage activation peak, which unavoidably ends with a stationary thermal arc, if the short-circuit current is not interrupted. In an ESD process the spark is triggered by a contact discharge. The entire process can be ideally divided into three distinct phases. In the pre-breakdown phase a discharge current of several Amperes flows through two electrodes when they come into short-circuit at some contacting spots (see Fig. 2a). The intense Joule heating associated to the current passage creates some local gas ionization that triggers a low voltage spark discharge. The breakdown phase occurs when the
Fig. 2. Schematic illustration of the electrospark discharge phases: (a) short-circuit current passage (pre-breakdown phase); (b) spark discharge (breakdown phase); and (c) end of discharge and droplet mass transfer (post-breakdown phase).
Fig. 1. Ideal voltage–current characteristics of stationary electrical discharges in atmospheric-pressure gases.
contact is broken for the relative movement of one electrode with respect to the other creating a micro-gap in which high pressure and high temperature spark plasma is produced (Fig. 2b). At the end of the discharge phase when plasma pressure collapses, tiny molten pieces of anode material are ejected from the surface and attracted at high velocity on the work surface, mainly towards the high field zones (points , protrusions) where electrostatic forces prevail [16] (Fig. 2c). This can be defined as the post-breakdown phase. A schematic illustration of the various discharge phases is given in Fig. 3.
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Fig. 3. Schematic illustration showing the temporal succession of the various electrospark discharge phases.
High deposition rates as required for practical coating applications are usually obtained with long spark pulses (durationN50 μs) for their ability to promote abundant surface melting on the electrodes. The melting is the most important factor in the production of irregular craters on the electrode surface and of rough deposits on the work surface. The resulting irregular micro-gap space causes local electric field oscillations that, in cascade, generate large variability of the voltage activation peak and of the energy available for the discharge pulses, thereby being the cause of non uniform deposition. Unfortunately, the voltage breakdown instability is further exacerbated by the multi-spark nature of ESD processing. In fact, the rapid sequence of pulses may create the conditions for a progressive intensification of electric field with onset of discharges at lower voltages, thus enabling changes in material transfer conditions and risk of arc formation. The most important causes for such low voltage discharges are: 1. build-up of residual ionized gas in the interior of a micro-discharge gap after the initial discharges; 2. accumulation of debris (pulverization of embrittled white or recast layers) between electrode and work surface; and 3. heating up of the electrodes. Their main effect on the material transfer processes is to further promote preferential deposition areas potentially resulting in strongly uneven surface treatment. Another potential source of process instability is related to the nature of the dielectric gas as it heavily affects the mass transfer mechanisms and the surface coating topography. It is known that in air depositions the size of molten droplets increases resulting in highly irregular deposition. Although a precise mechanism behind this effect is not clear, this effect is generally ascribed to the fact that molecular gases like nitrogen and oxygen form a plasma of high thermal conductivity that promote globular/spatter mass transfer mechanisms [2]. Conversely, depositions carried out in argon gas produce the formation of very small droplets resulting in a fine spraying deposition and more even surfaces. In order to further improve ESD process stability in large area coating applications, a more precise understanding of the effect of argon on the mass transport mechanisms would be essential. Although not previously reported in literature, it is believed that the dielectric gas affects the mass transfer by controlling the mechanism of the electric spark including the development of avalanche or streamer plasma channels. Different mass transfer mechanisms can be envisaged as these breakdown processes interact differently with the electrodes in terms of energy loss mechanisms. At micron gap size dielectric breakdown can be explained through exponential multiplication of charge carriers (electron and ions) in
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successive electron collisions (Townsend avalanche model). At atmospheric pressure, the avalanche model is in satisfactory agreement with experiments only in micro-gaps such that the electrode distance is not much higher than the electron mean free path between collisions (typically, 2–7 um [17]). However, the breakdown is experimentally found to be too fast in larger gaps to be simply explained with repetitive electron avalanche mechanisms. This fact led to the streamer spark theory originally formulated independently by Loeb, Meek and Raether [18–20]. In this model, a Townsend-tostreamer transition occurs when the primary electron avalanche grows to a point (i.e., to a high charge density) to generate a critical space charge field comparable to the external applied field. From this point on, the primary electron avalanche proceeds forming multiple thin filaments of plasma (streamers). The streamer propagation is caused by secondary electron avalanches created near the streamer tip. When this process is accompanied by intense ultraviolet photoionization, the streamers grow preferentially in the cathode direction. Streamers of both polarities may be produced depending on applied voltage, local electric field characteristics and gap size, but many industrial applications (for instance, sterilization, ozone generation, gas cleaning) take advantage of positive streamers (cathode-directed) for their ability to initiate chemical reactions at low/moderate voltages [21]. In homogeneous fields, a transition from avalanche to streamer discharge is expected at some fraction of a millimeter gap [18,19]. However, the efficient photo-ionization behavior of an argon plasma can substantially sustain a stable streamer discharge regime [22] even at micron gaps. For instance, the critical gap spacing for a stable streamer regime in argon has been estimated at about 12 um, at 1 atm pressure [23]. It seems therefore possible to associate the fine spray mass transfer produced in argon gas with experimental conditions that favor a streamer discharge regime. In fact, the spread of the plasma channel in multiple filaments will enable the molten droplets to be sprayed over a wider coating area. Further, in a streamer the energy loss mechanisms are concentrated in the gas phase close to the anode ( the conducting plasma starts at the anode for the intense space charge potential therein) as opposed to a Townsend discharge where there is a predominance of energy loss processes on the cathode leading to higher energy densities in the cathode zone and formation of hot static spots [17]. Therefore, the higher anode thermal effects in a streamer discharge will result in the higher production of small sized molten droplets, which is in accordance with the needs for smooth
Fig. 4. Physical model of the micro-gap space under a spring-loaded contact: the interelectrode distance (h) during a discharge is the sum of a static (h1) and a dynamic (h2) component.
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and dense coating properties. If our assumptions are sound, high coating properties cannot be obtained for depositions carried out in air because the cathode experiences high energy loss mechanisms and thermal erosion for the effects of a prevalent avalanche discharge mechanism. One might speculate that any other factor that would be capable to extend the streamer discharge regime stability could also improve coating properties. The use of discharge gaps wider than in conventional ESD could be one of these improving factors.
2.2. The effect of a spring-loaded contact on spark discharge stability Typical gap spacings existing in conventional ESD processes may be estimated under the simplifying assumption that the location of the discharge terminal spots is a probabilistic phenomenon [24]. In that case, the irregular contact contour creates a multiplicity of possible inter-electrode discharge paths (h1) which increase with the cathode surface roughness (Ra), namely the average gap distance can be approximately expressed as: bh1N ≈ Ra. As coating roughness may
Fig. 5. The setup for the automatic ESD experiments: (a) general architecture of the coating process and (b) a schematic of the cantilever displacement measurement.
S. Frangini, A. Masci / Surface & Coatings Technology 204 (2010) 2613–2623 Table 1 Pulse parameters used for automatic Al coating onto stainless steel sheet substrates. Parameter
Minimum
Maximum
Voltage (V) Current (A) Pulse energy (mJ) Pulse duration (μs) Frequency (Hz)
14 18 16 62 55
18 21 27 72 90
typically change during a processing within the range 1–10 μm [3], this means that conventional depositions will take place in the prevalent avalanche plasma regime even under argon with a consequent high probability of non uniform coatings. According to literature, for gaps with h ≥ 10 Ra, the effect of surface roughness on spark voltage breakdown can be neglected [17]. For metallic ESD coatings this would mean that gap spacing discharges in the range 10–100 um could reduce such effects and at the same time stabilize the streamer discharge mechanism. According to Belik et al. [7], gap spacing may be indirectly controlled by measuring the contact force between the electrode tip and the work surface with a servomechanism system. As the authors used very short electrical pulses (1–2 μs) surface melting was practically absent in their experiments and only gaseous material transfer was occurring under prevalent electrostatic forces resulting in very smooth coating surfaces. Under those peculiar conditions, a servo-mechanism system allowed a satisfactory control of the contact force by simply monitoring the average voltage breakdown. However, a servo-mechanism is of little help in practical ESD applications where long pulse discharges are used (N 50 μs). As mentioned before, in such a context, the voltage breakdown largely oscillates due to the high surface roughness produced by the melting processes, thus impeding the use of this parameter for a reliable servo-mechanism control of the contact force. In this work a dynamic concept of contact force control was applied, which is based on the well known fact that during a short-circuit
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discharge a repulsion force generated by the spark may cause openings of the gap contacts and also an extended duration of the discharge phase. If the applied contact force is not higher than a few Newtons the gap openings may reach values up to several tens of microns [25]. This dynamic control system was realized with a spring-loaded contact resulting in a self-regulating contact force system capable to compensate the repulsive action of the spark by a momentary gap spacing expansion. The elastic force system promptly reacts to the spark force, thus the actual contact force during a discharge is the sum of a static and a dynamic component [7]. The momentary gap expansion is proportional to the dynamic force component as it gives a small yet reproducible negative contribution to the applied contact force (Fc). In this work, the static force component (i.e., the contact force) was applied by bending a cantilever spring. The contact force is given by Hooke's law as: Fc = kΔx, where Δx is the displacement from the equilibrium position and k is the Hooke constant. The repulsive spark force fsp can be expressed as follows: Fsp = cI, where I is the average discharge current and c is a proportional constant. With a proper spring-loaded design, the effects of natural mechanical oscillations can be minimized. With a such simplification, the average gap distance h is thus wider than in the conventional ESD case being the sum of two components, one static and one dynamic, namely: bhN = bh1N + bh2N, where h2 (the dynamic component) can be calculated on the basis of force analysis. The situation is depicted in the Fig. 4. During a spark discharge the opposing elastic force (Fel) acts on the moveable contact in a such way to generate the momentary deflection h2. At the new position equilibrium (maximum spring stroke) we have that Fel = Fc + kh2 = Fsp, from which h2 = cI/k−Δx. In other words, the h2 parameter is insensitive to contact gap geometry, being dependent only on discharge current, spring stiffness and contact force. In definitive, the greatest conceptual advantage of a dynamic contact force system lies in the possibility to obtain momentary gap expansion in a reproducible way.
Fig. 6. Microstructure of the Al coating surface following a manual ESD deposition experiment without a contact force control, at P = 16 mJ energy pulse and 55 Hz frequency: (a)–(c) secondary images , (b)–(d) backscattered images.
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A such enlarged gap spacing would significantly improve the fine spray capability of the argon gas . In fact, an increased spark channel volume will lead to a more rapid cooling of the radial plasma jet, thus promoting a larger pressure drop that potentially would enhance the fine spray material removal process. Overall, at an optimal gap distance, which can be indirectly found by varying the intensity of the applied electrode force, the dynamic force control would combine a uniform material transfer process with a sufficiently high speed of droplets to impact on the work surface still in the molten state. This could contribute to improve both coating uniformity and density in accordance with the requirements for large area ESD applications.
position of the depositing area. Generally, keeping the workpiece heating below 6–7 °C was sufficient to avoid severe warping. For the purposes of this work, the spring-loaded contact was realized by placing the workpiece onto a rectangular cantilever made with a thin mild carbon steel foil. See the schematic illustration of ESD setup in Fig. 5a. The Hooke spring constant (k) of a rectangular cantilever can be determined if its length (L), width (w), thickness (t), Young modulus (E) are known using the following well known equation:
3. Experimental work
Placing E = 21.000 kg/mm2 (ca. 206 GPa); w = 100 mm; t =0.3 mm; L = 150 mm, we obtained a spring constant k of 4 gf/mm (0.04 N/mm). As the spring constant of a cantilever strongly depends on the measured position L, this value is valid only for measurements carried out at a 150 mm distance from the fixed end. As the coating area has a finite dimension of L = 10 mm, this generates an offset position that could require some correction. Approximate values of spring constant in other cantilever positions were estimated from the following formula [26]:
A commercial Al rod of 3 mm diameter was used as electrode material. Substrate material consisted of thin foils of SS310S austenitic stainless steel of 0.3 mm thickness. Rectangular 100 × 50mm strips were cut from the foils and used as workpiece onto which aluminum was deposited up to cover a 10 × 40 mm (4 cm2) area, unless otherwise specified. The foils were received in a mirror-like surface finish condition (BA standard finish) and rinsed with acetone before coating. Deposition experiments were made under argon gas shielding (gas flow = 50 ml/min) by using an Exair Cold Gun System Mod. 5015 for providing a sufficiently cold stream of argon capable to avoid oxidation of the work surface. Extreme care was also taken to avoid thermal stress distortions (warping) by probing the temperature in the coating area during deposition. A type K thermocouple was mounted in contact with the strip workpiece and placed close to the middle
Fig. 7. Effect of contact force on: (a) cathode and anode mass changes and (b) process efficiency (pulse frequency= 55 Hz; traverse speed = 5 mm/s, deposition time = 200 s).
k=
Ewt 3 : 4L3
k1 = k
L L−ΔL
ð1Þ
3
ð2Þ
where k1 is the spring constant at a distance ΔL from L. Locating the coating area center point at L = 150 mm, the ΔL offset is 5 mm with k1
Fig. 8. Effect of traverse speed on: (a) cathode and anode mass changes and (b) process efficiency (contact force = 0.12 N; pulse frequency = 55 Hz; deposition time (dt) can be calculated with the following formula , dt = 1000/ts, where ts is the traverse speed in mm/s).
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ranging between 4.40 and 3.60 gf/mm, namely the error in neglecting the position offset is only 10%. The contact force (Fc) exerted by the cantilever spring was calculated as follows: Fc = −kΔx
ð3Þ
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burn-through of the thin workpieces. Electrical parameters were therefore changed in a narrow window as described in Table 1. The kinetics of the mass transfer were followed by measuring both the cathode mass change (Δc) and anode mass change (Δa) with an analytical balance. These data served as a basis for process efficiency (ηESD) calculations as follows: Δc ·100: jΔaj
where Δx is the cantilever displacement (in mm) from the unloaded position (measured at workpiece center point). An automatic ESD coating system specifically devised to treat thin metallic foils was used in this work. The apparatus consists essentially of a commercial ESD pulse generator (SparkDepo™ machine, model 28–40 manufactured by TechnoCoat, Japan) electrically connected to the depositing electrode inserted in the mandrel of a vertical milling CNC machine (RM-MaxiMill 3D System by RonchiniMassimo S.A.S). The electrode was rotating at a fixed speed of 8.000 rpm while scanning the surface in a raster pattern mode (see Fig. 5a). Cantilever vertical displacements in the mm range were determined by using a laser-line generator and a CCD camera (Fig. 5b). The laser level was mounted in-plane with the unloaded cantilever. The laser beam is used to generate illumination spots on the lateral surface of electrode. The CCD camera aligned at 90 ° with respect to the laser sends the images of the illumination spots before and after cantilever loading to a computer for a precise determination of vertical displacement by video analysis. Resolutions were better than 0.2 mm. Coatings were applied under low energy intensive discharge conditions in order to avoid deformation, reverse side oxidation or
A few preliminary experiments with a hand-held ESD applicator were carried out to verify the effect of an Al deposition without electrode force control. The deposition was restricted to a small area of 0.5 cm2. Significant cracking by residual thermal stress and large spatter deposition was clearly visible on the coated surface (see Fig. 6). Backscattered electron images revealed also irregular aluminum composition of the coated surface. By EDX analysis it was found that the dark areas of the surface contain ca. 70 at.% as of Al dropping to about 25 at.% in the bright ones. The disuniform Al
Fig. 9. Effect of pulse frequency on: (a) cathode and anode mass changes and (b) process efficiency (pulse energy = 16 mJ; contact force = 0.12 N; traverse speed = 5 mm/s, deposition time = 200 s).
Fig. 10. Effect of the number of successive scans on: (a) cathode and anode mass changes and (b) process efficiency (pulse frequency = 55 Hz, contact force = 0.12 N, single scan deposition times = 200 s at 5 mm/s and 14 s at 70 mm/s).
ηESD =
ð4Þ
The morphology and chemical composition of the coatings were analyzed with a Scanning Electron Microscopy (SEM) Jeol model JSM5510 LV equipped with energy-dispersive X-ray (EDX) microanalysis. A portable contact profilometer model TR200 by TimeGroup Inc. was used to determine the average surface roughness of coated steels. 4. Results
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Fig. 11. Relationship between the number of deposition scans and coating continuity (contact force = 0.12 N; pulse frequency = 55 Hz, deposition times = 200 s at 5 mm/s and 14 s at 70 mm/s).
composition along with the spatter morphology of the coated surface could indicate that aluminum was transferred by a combination of contact and spray transfer mechanisms. Contact transfer (also known as short-circuit transfer) indicates a low energy material removal process in which the molten metal is being transferred to the substrate without spraying but simply bridging the electrode gap, thus leaving deposits with a highly spatter morphology [27]. Contact transfer may be a highly effective mechanism in aluminum because of its easy softening and melting at low currents. Because a contact bridge is being established when a pendant molten drop touches the
workpiece surface before its solidification, it is clear that the contact transfer of aluminum may be relevant in electrospark processes conducted with low energy pulses and in absence of a contact force control. In definitive, these preliminary trials confirmed the difficulty to obtain uniform aluminum deposits onto thin substrates without taking into consideration the contact force parameter. The effect of electrode force on both mass change rates and process efficiency is shown in Fig. 7a–b at two levels of pulse energy P, P1 = 16 mJ and P2 = 27 mJ. It can be seen that cathode weight gain is inversely proportional to the applied electrode force, whereas the anode erosion process is much less dependent on this parameter. At the P2 energy both cathode weight gain and process efficiency steeply decrease with increasing the contact force. Conversely, at P1 the deposition process appears less affected by the contact force showing a broad region of efficiencyN40% within the 0.12–0.4 N range. The results of Fig. 7b suggest that the severity of the cathode erosion increase noticeably along with substrate heating. All the following experiments were carried out at the optimal 0.12 N contact force. The effect of traverse speed and pulse frequency on the material transfer and substrate heating is shown in Figs. 8 and 9. A lower heating is observed at increasing traverse speed as a result of shorter processing times. Good efficiencies (N50%) with both minimal warping and thermal effects are obtainable at either P1 or P2 pulses by adjusting the traverse speed. Raising the frequency has the effect to rapidly increase the substrate heating even with the low pulse energy with a concomitant decrease of process efficiency. The variation of material transfer with the number of deposition scans is shown in Fig. 10 for the P1 and P2 pulse energy conditions with a low heating effect (below 5 °C). The cathode weight gain reaches a maximum after 4 scans at P2 and then slightly decreases, whereas a steady increase of cathode weight is observed with the number of scans when deposition is carried out with the P1 pulses. By knowing the density of coating material (ρ) and the coating area (S), the
Fig. 12. Microstructure of the Al coating surface following a multi-scan treatment under two different experimental conditions: (a)–(b) pulse energy = 16 mJ, traverse speed = 5 mm/s and 6 scans and (c)–(d) pulse energy = 27 mJ, traverse speed = 70 mm/s and 8 scans (contact force = 0.12 N, pulse frequency = 55 Hz).
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thickness of the deposited layer (d) can be estimated from these cathode weight data with the following formula: d=
Δc ρS
ð5Þ
Thus, by way of example, assuming for Al ρ = 2.7 g/cm3, the coating thickness is estimated to be about 6 μm for the P1 discharge condition, after 10 scans. A multi-scan treatment was necessary for the formation of a continuous Al layer covering the entire substrate area. The degree of continuity was checked by SEM and image analysis of aluminum. It was found that coating continuity was achieved after a number of scans, which partly depends on the pulse energy and partly on the traverse speed as shown in Fig. 11. Excellent surface finish was also obtained (surface roughness around 1–2 μm). In comparison, manual deposition produced a much coarser surface roughness of between 4 and 6 μm (see Fig. 6). The corresponding surface and cross-section morphologies with the X-ray aluminum distribution map obtained after the multi-scan treatments are shown in Figs. 12–14. The deposits appear smooth, well adherent and with homogeneous distribution of aluminum inside the coating. The average coating thickness is about 10 μm being in close accordance with the above estimates from the weight change data. The absence of a clear-cut interface proves that a metallurgical bonding between aluminum and stainless steel was established even
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under the very low heat input conditions used in this work. The coating thickness is quite uniform with rare interface undulations. The narrow intermixing region between the substrate and the coating material and the low dilution of aluminum inside the coating (see Fig. 15) is a clear indication that a minimal melting of substrate was occurred during deposition. Accordingly, cross-section analysis of coatings shows a low level of defects such as porosity and microcracking, especially in the lower energy deposits. 5. Discussion In accordance with Belik et al. [7], the present large area deposit experiments have confirmed that a dynamic contact force control may substantially improve the uniformity of aluminum ESD coatings, representing thus a viable option to extend the fine mass transfer properties of the argon gas. The resulting momentary expansion of the micro-gap space when combined with the benefits of the argon gas is supposed to improve process stability in the following aspects: 1. by limiting the surface roughness effects on the spark reproducibility; 2. by performing deposition under a prevalent streamer discharge regime that has a marked effect in reducing cathode erosion; and 3. by promoting a more uniform spray deposition. The present experiments acquire a further significance if related to the challenge represented by aluminization of thin gauge steel substrates under low energy settings.
Fig. 13. Aluminum X-ray map and chemical analysis of the coating surface following the multi-scan treatments: (a) pulse energy = 16 mJ, traverse speed = 5 mm/s and 6 scans and (b) pulse energy = 27 mJ, traverse speed = 70 mm/s and 8 scans (contact force = 0.12 N, pulse frequency = 55 Hz).
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Fig. 14. Cross-sectional images of the Al coating following the multi-scan treatments: (a) pulse energy = 16 mJ, traverse speed = 5 mm/s and 6 scans and (b) pulse energy = 27 mJ, traverse speed = 70 mm/s and 8 scans (contact force = 0.12 N, pulse frequency = 55 Hz).
Firstly, in conventional ESD the mass transfer process is known to be very inefficient at energy pulses lower than 100 mJ as in these circumstances cathode erosion phenomena prevail over the deposited mass [3]. With reference to the results presented through Figs. 7–11 it appears that a high process efficiency can be still obtained with a combination of low energy parameters and low contact force. Raising the contact force (i.e. , to 0.8 N) had the effect of a rapid drop in efficiency as in conventional ESD treatments. As the rate of anode erosion did not change appreciably with the contact force, the
increasing cathode erosion with the contact force may be the sign of sparks occurring under prevalently growing avalanche discharges. Our results therefore indicate that a high process efficiency can be obtained even at low energy settings if a sufficiently low contact force is applied to ensure a stable streamer regime. Further, a low contact force increases the ohmic gap resistance, thereby lowering the probability of destructive arc discharges. Based on the assumption that the erosion of the alloying electrode is the result of a thermoelectrical action of the plasma, it should be stressed that the ESD
Fig. 15. EDX profile scan of Al, Cr, Fe, and Ni elements collected through a typical Al coating deposited in a multi-scan treatment.
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applicability onto thin-walled substrates is limited to the deposition of low melting-point materials like aluminum, where surface melting processes can take place even under extremely low energy discharge conditions. Conversely, ESD deposition of high melting metals or alloys onto thin substrates will introduce many additional difficulties as they would require high energy pulses. Secondly, the narrow intermixing region below coating observed in Fig. 15 undoubtedly indicates that a shallow molten pool was formed with a such combination of low energy pulse and low contact force, which allows to get sufficiently thick coatings in multi-scan deposition processes. The apparent absence of thickness limitation problems suggests that depositions made under a streamer discharge may reduce the tendency to fracture of ESD coatings [28,29]. It is known, in fact, that a too prolonged sequence of spark discharges may produce relatively deep molten pools on the substrate. The solidification of such molten pools forms what is called a surface recast layer. This layer may become very brittle due to thermal stress build-up and eventually begin to self-destruct and pulverize, which manifests as coating thickness limitations and reverse material transfer phenomena. It is believed therefore that the lower heat transfer to the cathode produced under a streamer discharge is a plausible explanation for the less brittle properties of Al coating. 6. Conclusions Aluminum was coated onto a thin gauge stainless steel sheet under argon gas shielding by an automatic electrospark deposition system which used a spring electrode contact for a dynamic control of the contact force during the process. The more regular deposition process allowed to obtain continuous and uniform aluminum coating properties over relatively large areas. The main results, which are strictly valid for Al coatings, can be summarized as follows: (1) Mass transfer processes taking place during a spark discharge are strongly affected by the contact force parameter, which is inversely proportional to the discharge gap size. Under a sufficiently low contact force it was found a net deposited mass even for low energy parameter depositions indicating that a force-controlled ESD can be also utilized for aluminization of thin gauge steel substrates under very low heat input. (2) The observed results are consistent with interpretations that take into considerations the effect of contact force on the mechanisms of spark discharges in micrometer gap sizes. Low contact forces (namely, large gap sizes) are believed to promote spark breakdown by a cathode-directed streamer mechanism,
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which operates for more uniform deposition and lower cathode erosion phenomena. (3) The increased performance stability promoted by the springloaded contact is also a direct consequence of spark discharges taking place at gap sizes larger than in conventional ESD processes, thus allowing to reduce the effect of surface roughness on the spark reproducibility. Acknowledgements The authors are indebted to the Italian Ministry of University and Research for the financial support provided under a FISR (Special Integrative Fund for Research) Programme. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
A.M. Paramonov, A.V. Koval, Surface Eng. Appl. Electrochem. 44 (2) (2008) 87. A. Agarwal, N.B. Dahotre, Surf. Coat. Technol. 106 (1998) 242. A. Lešnjak, J. Tušek, Sci. Technol. Weld. Join. 7 (6) (2002) 391. E.I. Zamulaeva, E.A. Levashov, A.E. Kudryashov, P.V. Vakaev, M.I. Petrzhik, Surf. Coat. Technol. 202 (2008) 3715. E.A. Levashov, P.V. Vakaev, E.I. Zamulaeva, A.E. Kudriashov, V.V. Kurbatkina, D.V. Shtansky, A.A. Voevodin, A. Sanz, Surf. Coat. Technol. 201 (2007) 6176. A. Agarwal, N.B. Dahotre, Mater. Charact. 42 (1999) 31. V.D. Belik, R.V. Litvin, M.S. Koval'chenko, Powder Metall. Met. Ceram. 45 (11–12) (2006) 593. US Patent No. 6,835,908 (issued Dec. 28, 2004). P.V. Peretyatku, Surface Eng. Appl. Electrochem. 43 (2) (2007) 87. S. Frangini, A. Masci, Surf. Coat. Technol. 184 (2004) 31. S. Frangini, S. Loreti, A. Masci, J. Fuel Cell Sci. Technol. 2 (1) (2005) 60. L.H. Germer, J. Appl. Phys. 30 (1959) 46. N. Sillon, R. Baptist, Sensor Actuat. B 83 (2002) 129. A. Chaitanya, K. Takahata, MEMS, Proceedings of the 21th IEEE International Conference on Micro-Electrical-Mechanical-Systems, Tucson, AZ, USA, January 13–17, 2008, 2008, p. 375. C. Wang, M. Madou, Biosens. Bioelectron. 20 (10) (2005) 2181. A.V. Ribalko, K. Korkmaz, O. Sahin, Surf. Coat. Technol. 202 (2008) 3591. [17 ] R. G. Longwitz, Thesis No. 2919, Lausanne (CH), EPFL (2004). J.M. Meek, Phys. Rev. 57 (1940) 722. L.B. Loeb, J.M. Meek, The Mechanism of the Electric Spark, Stanford University Press, Stanford, 1941. H. Raether, Zeits. f. Physik 107 (1937) 91. G. Dinelli, L. Civitano, M. Rea, IEEE Trans. Indus. Appl. 26 (1990) 535. S. Lachaud, J.F. Loiseau, Eur. Phys. J. Appl. Phys. 24 (2003) 67. M.S. Naidu, V. Kamaraju, High Voltage Engineering2nd ed., Mc Graw Hill, New York, 1999. M. Kunieda, B. Lauwers, K.P. Rajurkar, B.M. Schumacher, CIRP Ann. 54 (2) (2005) 64. D. Piccoz, P. Teste, R. Andlauer, T. Leblanc, J.P. Chabrerie, Eur. Phys. J. Appl. Phys. 13 (2001) 59. J.E. Sader, I. Larson, P. Mulvaney, L.R. White, Rev. Sci. Instrum. 66 (1995) 3789. M.A. Bejar, J. Mater. Process. Technol. 25 (1991) 185. A.V. Ribalko, O. Sahin, Surf. Coat. Technol. 201 (2006) 1724. C. Luo, S. Dong, X. Xiong, N. Zhou, Surf. Coat. Technol. 203 (2009) 3333.
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
A study on the effect of a dynamic contact force control for improving electrospark coating properties S. Frangini ⁎, A. Masci Dept. TER, ENEA CRE Casaccia, Via Anguillarese 301, I-00123 Rome, Italy
a r t i c l e
i n f o
Article history: Received 10 November 2009 Accepted in revised form 2 February 2010 Available online 10 February 2010 Keywords: Electrospark deposition (ESD) Aluminum-diffusion coating Stainless steel sheet Contact force Spark breakdown Streamer discharge
a b s t r a c t The electrospark deposition (ESD) technique was used to apply aluminum-diffusion coatings over a thin stainless steel sheet under low energy discharge conditions to minimize substrate damage. In order to produce uniform coating properties over relatively large areas, automatic deposition treatments were conducted in a multi-scan mode under a dynamic control of the contact force parameter, which was realized by means of a spring-loaded electrode contact. A continuous and uniform Al coating layer was obtained indicating that a dynamic contact force system may be effective in suppressing several coating defects, which are notoriously produced with conventional ESD processes. The benefits of the dynamic force control are mostly related to the concept of a mobile electrode contact, which creates a momentary gap expansion during the spark discharge. The expanded gap is believed to reduce the effect of surface roughness on spark reproducibility and at the same time to stabilize the streamer regime of the spark breakdown, which are both essential for more uniform ESD depositions and lower cathode erosion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Coating processes are an important technology in the modern materials industry. One of the major challenges facing this industry is to keep up with the ever-increasing requirements for energy savings and respect for the environment. In this context, the last decade has seen a growing interest for the electrospark deposition (ESD) as a low-pollution and cost-effective method for production of a broad variety of metallurgical and cermet coatings on high-precision components to be used in demanding applications [1–5]. In the ESD process the energy stored in a capacitor bank is discharged through a sequence of short sparks of low voltage/high current intensity. Small parts of the material are detached in form of molten droplets from the electrode tip (anode) during each spark and ejected onto the partially-melted substrate (cathode) where they create a thin alloying zone with the base material. The low heat transfer (typically, 10 W/cm2 [6]) associated to the mass transfer process results in an extremely rapid solidification of the inter-mixed zone producing fine-grained and diffusion-bonded coatings with a minimal transformed substrate zone. In spite of these interesting properties, serious shortcomings in process stability has so far prevented a wide application of ESD technique for large area treatments mostly due to irregular contact geometry in the discharge zone, which negatively affects the
⁎ Corresponding author. Tel.: + 39 06 30483138. E-mail address: [email protected] (S. Frangini). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.006
reproducibility of coating properties [7,8]. Irregular contact surfaces are caused by a change of electrode or discharge conditions (for instance, due to a change of electrode tip shape or coating roughness). Such irregular contacts may also generate harmful transient arcs, which are characterized by high energy transfer, spot localization, electrode overheating. All these effects are not desirable for an efficient and regular deposition resulting in coating discontinuity , high surface roughness, spatter layers and crater erosion. It is recognized that stabilization of the spark discharge alloying by reducing the effect of local surface topography would be of prime importance for extending the use of ESD to large area applications where coatings with high chemical homogeneity, thickness uniformity and 100% continuity are required. Scarce attention has been given so far in the literature to the influence of non-electrical process parameters on spark stability, although recent papers have found that coating properties can be heavily affected by factors such as magnetic fields [9] and discharge gap sizes [7] since they may determine the gas dynamics properties of the spark plasma and therefore also erosion and mass transfer conditions. With reference to the effect of gap size, Belik et al. have concluded that high coating properties cannot be obtained without a proper control of discharge gap, which was realized in their work by measuring the contact force exerted by the electrode tip on the substrate surface as the gap discharge is very difficult to measure directly [7]. As part of our development efforts to realize a stable ESD processing for large area applications, this paper describes the use of a spring-loaded electrode as a simple means to produce smooth coatings under force-controlled spark discharges, while providing
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sufficiently high material deposition. Deposition of aluminum onto a thin gauge stainless steel foil was used as an illustrative example of a coating problem facing several challenges such as burning-through, oxidation and distortion of the substrate. This non trivial example reflects technological problems inherent to aluminization of metallic bipolar plate components in the Molten Carbonate Fuel Cell (MCFC) technology [10,11]. 2. General statement of the problem In the ESD the process of material transfer by emission of molten droplets is the result of the electrical discharges taking place in micron gaps filled with a dielectric gas. These gas discharges are characterized by a strong deviation from the well known Paschen's law at gaps smaller than about 10 μm [12]. The two major contributing factors to this deviation are the ionization processes in the dielectric gas and the electric field inhomogeneities caused by the electrode roughness. For this reason, little attention has been paid so far to this subject and only recently the level of interest for micro-gap discharges has emerged due to their implications in the development of microelectronics, MEMS (Micro-Electric-Mechanical Systems) and nanotechnology devices [13–15]. In order to rationalize the effect of a spring-loaded contact on a micro-gap discharge behavior it appears useful to introduce the subject with a brief description of the various sources of instability in ESD discharge processes. 2.1. Physical interpretation of ESD process instabilities In general terms, an electrical discharge is defined as the phenomenon of electrical conduction through a dielectric material in a sufficiently strong applied electric field with a sudden release of energy usually accompanied by a flash of light. Partial or complete discharge phenomena may take place depending critically on the intensity of the electric field (see Fig. 1). As evident from Fig. 1, the spark discharge is not a stationary process but, in fully rigorous terms, only a transition mechanism characterized by a voltage activation peak, which unavoidably ends with a stationary thermal arc, if the short-circuit current is not interrupted. In an ESD process the spark is triggered by a contact discharge. The entire process can be ideally divided into three distinct phases. In the pre-breakdown phase a discharge current of several Amperes flows through two electrodes when they come into short-circuit at some contacting spots (see Fig. 2a). The intense Joule heating associated to the current passage creates some local gas ionization that triggers a low voltage spark discharge. The breakdown phase occurs when the
Fig. 2. Schematic illustration of the electrospark discharge phases: (a) short-circuit current passage (pre-breakdown phase); (b) spark discharge (breakdown phase); and (c) end of discharge and droplet mass transfer (post-breakdown phase).
Fig. 1. Ideal voltage–current characteristics of stationary electrical discharges in atmospheric-pressure gases.
contact is broken for the relative movement of one electrode with respect to the other creating a micro-gap in which high pressure and high temperature spark plasma is produced (Fig. 2b). At the end of the discharge phase when plasma pressure collapses, tiny molten pieces of anode material are ejected from the surface and attracted at high velocity on the work surface, mainly towards the high field zones (points , protrusions) where electrostatic forces prevail [16] (Fig. 2c). This can be defined as the post-breakdown phase. A schematic illustration of the various discharge phases is given in Fig. 3.
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Fig. 3. Schematic illustration showing the temporal succession of the various electrospark discharge phases.
High deposition rates as required for practical coating applications are usually obtained with long spark pulses (durationN50 μs) for their ability to promote abundant surface melting on the electrodes. The melting is the most important factor in the production of irregular craters on the electrode surface and of rough deposits on the work surface. The resulting irregular micro-gap space causes local electric field oscillations that, in cascade, generate large variability of the voltage activation peak and of the energy available for the discharge pulses, thereby being the cause of non uniform deposition. Unfortunately, the voltage breakdown instability is further exacerbated by the multi-spark nature of ESD processing. In fact, the rapid sequence of pulses may create the conditions for a progressive intensification of electric field with onset of discharges at lower voltages, thus enabling changes in material transfer conditions and risk of arc formation. The most important causes for such low voltage discharges are: 1. build-up of residual ionized gas in the interior of a micro-discharge gap after the initial discharges; 2. accumulation of debris (pulverization of embrittled white or recast layers) between electrode and work surface; and 3. heating up of the electrodes. Their main effect on the material transfer processes is to further promote preferential deposition areas potentially resulting in strongly uneven surface treatment. Another potential source of process instability is related to the nature of the dielectric gas as it heavily affects the mass transfer mechanisms and the surface coating topography. It is known that in air depositions the size of molten droplets increases resulting in highly irregular deposition. Although a precise mechanism behind this effect is not clear, this effect is generally ascribed to the fact that molecular gases like nitrogen and oxygen form a plasma of high thermal conductivity that promote globular/spatter mass transfer mechanisms [2]. Conversely, depositions carried out in argon gas produce the formation of very small droplets resulting in a fine spraying deposition and more even surfaces. In order to further improve ESD process stability in large area coating applications, a more precise understanding of the effect of argon on the mass transport mechanisms would be essential. Although not previously reported in literature, it is believed that the dielectric gas affects the mass transfer by controlling the mechanism of the electric spark including the development of avalanche or streamer plasma channels. Different mass transfer mechanisms can be envisaged as these breakdown processes interact differently with the electrodes in terms of energy loss mechanisms. At micron gap size dielectric breakdown can be explained through exponential multiplication of charge carriers (electron and ions) in
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successive electron collisions (Townsend avalanche model). At atmospheric pressure, the avalanche model is in satisfactory agreement with experiments only in micro-gaps such that the electrode distance is not much higher than the electron mean free path between collisions (typically, 2–7 um [17]). However, the breakdown is experimentally found to be too fast in larger gaps to be simply explained with repetitive electron avalanche mechanisms. This fact led to the streamer spark theory originally formulated independently by Loeb, Meek and Raether [18–20]. In this model, a Townsend-tostreamer transition occurs when the primary electron avalanche grows to a point (i.e., to a high charge density) to generate a critical space charge field comparable to the external applied field. From this point on, the primary electron avalanche proceeds forming multiple thin filaments of plasma (streamers). The streamer propagation is caused by secondary electron avalanches created near the streamer tip. When this process is accompanied by intense ultraviolet photoionization, the streamers grow preferentially in the cathode direction. Streamers of both polarities may be produced depending on applied voltage, local electric field characteristics and gap size, but many industrial applications (for instance, sterilization, ozone generation, gas cleaning) take advantage of positive streamers (cathode-directed) for their ability to initiate chemical reactions at low/moderate voltages [21]. In homogeneous fields, a transition from avalanche to streamer discharge is expected at some fraction of a millimeter gap [18,19]. However, the efficient photo-ionization behavior of an argon plasma can substantially sustain a stable streamer discharge regime [22] even at micron gaps. For instance, the critical gap spacing for a stable streamer regime in argon has been estimated at about 12 um, at 1 atm pressure [23]. It seems therefore possible to associate the fine spray mass transfer produced in argon gas with experimental conditions that favor a streamer discharge regime. In fact, the spread of the plasma channel in multiple filaments will enable the molten droplets to be sprayed over a wider coating area. Further, in a streamer the energy loss mechanisms are concentrated in the gas phase close to the anode ( the conducting plasma starts at the anode for the intense space charge potential therein) as opposed to a Townsend discharge where there is a predominance of energy loss processes on the cathode leading to higher energy densities in the cathode zone and formation of hot static spots [17]. Therefore, the higher anode thermal effects in a streamer discharge will result in the higher production of small sized molten droplets, which is in accordance with the needs for smooth
Fig. 4. Physical model of the micro-gap space under a spring-loaded contact: the interelectrode distance (h) during a discharge is the sum of a static (h1) and a dynamic (h2) component.
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and dense coating properties. If our assumptions are sound, high coating properties cannot be obtained for depositions carried out in air because the cathode experiences high energy loss mechanisms and thermal erosion for the effects of a prevalent avalanche discharge mechanism. One might speculate that any other factor that would be capable to extend the streamer discharge regime stability could also improve coating properties. The use of discharge gaps wider than in conventional ESD could be one of these improving factors.
2.2. The effect of a spring-loaded contact on spark discharge stability Typical gap spacings existing in conventional ESD processes may be estimated under the simplifying assumption that the location of the discharge terminal spots is a probabilistic phenomenon [24]. In that case, the irregular contact contour creates a multiplicity of possible inter-electrode discharge paths (h1) which increase with the cathode surface roughness (Ra), namely the average gap distance can be approximately expressed as: bh1N ≈ Ra. As coating roughness may
Fig. 5. The setup for the automatic ESD experiments: (a) general architecture of the coating process and (b) a schematic of the cantilever displacement measurement.
S. Frangini, A. Masci / Surface & Coatings Technology 204 (2010) 2613–2623 Table 1 Pulse parameters used for automatic Al coating onto stainless steel sheet substrates. Parameter
Minimum
Maximum
Voltage (V) Current (A) Pulse energy (mJ) Pulse duration (μs) Frequency (Hz)
14 18 16 62 55
18 21 27 72 90
typically change during a processing within the range 1–10 μm [3], this means that conventional depositions will take place in the prevalent avalanche plasma regime even under argon with a consequent high probability of non uniform coatings. According to literature, for gaps with h ≥ 10 Ra, the effect of surface roughness on spark voltage breakdown can be neglected [17]. For metallic ESD coatings this would mean that gap spacing discharges in the range 10–100 um could reduce such effects and at the same time stabilize the streamer discharge mechanism. According to Belik et al. [7], gap spacing may be indirectly controlled by measuring the contact force between the electrode tip and the work surface with a servomechanism system. As the authors used very short electrical pulses (1–2 μs) surface melting was practically absent in their experiments and only gaseous material transfer was occurring under prevalent electrostatic forces resulting in very smooth coating surfaces. Under those peculiar conditions, a servo-mechanism system allowed a satisfactory control of the contact force by simply monitoring the average voltage breakdown. However, a servo-mechanism is of little help in practical ESD applications where long pulse discharges are used (N 50 μs). As mentioned before, in such a context, the voltage breakdown largely oscillates due to the high surface roughness produced by the melting processes, thus impeding the use of this parameter for a reliable servo-mechanism control of the contact force. In this work a dynamic concept of contact force control was applied, which is based on the well known fact that during a short-circuit
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discharge a repulsion force generated by the spark may cause openings of the gap contacts and also an extended duration of the discharge phase. If the applied contact force is not higher than a few Newtons the gap openings may reach values up to several tens of microns [25]. This dynamic control system was realized with a spring-loaded contact resulting in a self-regulating contact force system capable to compensate the repulsive action of the spark by a momentary gap spacing expansion. The elastic force system promptly reacts to the spark force, thus the actual contact force during a discharge is the sum of a static and a dynamic component [7]. The momentary gap expansion is proportional to the dynamic force component as it gives a small yet reproducible negative contribution to the applied contact force (Fc). In this work, the static force component (i.e., the contact force) was applied by bending a cantilever spring. The contact force is given by Hooke's law as: Fc = kΔx, where Δx is the displacement from the equilibrium position and k is the Hooke constant. The repulsive spark force fsp can be expressed as follows: Fsp = cI, where I is the average discharge current and c is a proportional constant. With a proper spring-loaded design, the effects of natural mechanical oscillations can be minimized. With a such simplification, the average gap distance h is thus wider than in the conventional ESD case being the sum of two components, one static and one dynamic, namely: bhN = bh1N + bh2N, where h2 (the dynamic component) can be calculated on the basis of force analysis. The situation is depicted in the Fig. 4. During a spark discharge the opposing elastic force (Fel) acts on the moveable contact in a such way to generate the momentary deflection h2. At the new position equilibrium (maximum spring stroke) we have that Fel = Fc + kh2 = Fsp, from which h2 = cI/k−Δx. In other words, the h2 parameter is insensitive to contact gap geometry, being dependent only on discharge current, spring stiffness and contact force. In definitive, the greatest conceptual advantage of a dynamic contact force system lies in the possibility to obtain momentary gap expansion in a reproducible way.
Fig. 6. Microstructure of the Al coating surface following a manual ESD deposition experiment without a contact force control, at P = 16 mJ energy pulse and 55 Hz frequency: (a)–(c) secondary images , (b)–(d) backscattered images.
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A such enlarged gap spacing would significantly improve the fine spray capability of the argon gas . In fact, an increased spark channel volume will lead to a more rapid cooling of the radial plasma jet, thus promoting a larger pressure drop that potentially would enhance the fine spray material removal process. Overall, at an optimal gap distance, which can be indirectly found by varying the intensity of the applied electrode force, the dynamic force control would combine a uniform material transfer process with a sufficiently high speed of droplets to impact on the work surface still in the molten state. This could contribute to improve both coating uniformity and density in accordance with the requirements for large area ESD applications.
position of the depositing area. Generally, keeping the workpiece heating below 6–7 °C was sufficient to avoid severe warping. For the purposes of this work, the spring-loaded contact was realized by placing the workpiece onto a rectangular cantilever made with a thin mild carbon steel foil. See the schematic illustration of ESD setup in Fig. 5a. The Hooke spring constant (k) of a rectangular cantilever can be determined if its length (L), width (w), thickness (t), Young modulus (E) are known using the following well known equation:
3. Experimental work
Placing E = 21.000 kg/mm2 (ca. 206 GPa); w = 100 mm; t =0.3 mm; L = 150 mm, we obtained a spring constant k of 4 gf/mm (0.04 N/mm). As the spring constant of a cantilever strongly depends on the measured position L, this value is valid only for measurements carried out at a 150 mm distance from the fixed end. As the coating area has a finite dimension of L = 10 mm, this generates an offset position that could require some correction. Approximate values of spring constant in other cantilever positions were estimated from the following formula [26]:
A commercial Al rod of 3 mm diameter was used as electrode material. Substrate material consisted of thin foils of SS310S austenitic stainless steel of 0.3 mm thickness. Rectangular 100 × 50mm strips were cut from the foils and used as workpiece onto which aluminum was deposited up to cover a 10 × 40 mm (4 cm2) area, unless otherwise specified. The foils were received in a mirror-like surface finish condition (BA standard finish) and rinsed with acetone before coating. Deposition experiments were made under argon gas shielding (gas flow = 50 ml/min) by using an Exair Cold Gun System Mod. 5015 for providing a sufficiently cold stream of argon capable to avoid oxidation of the work surface. Extreme care was also taken to avoid thermal stress distortions (warping) by probing the temperature in the coating area during deposition. A type K thermocouple was mounted in contact with the strip workpiece and placed close to the middle
Fig. 7. Effect of contact force on: (a) cathode and anode mass changes and (b) process efficiency (pulse frequency= 55 Hz; traverse speed = 5 mm/s, deposition time = 200 s).
k=
Ewt 3 : 4L3
k1 = k
L L−ΔL
ð1Þ
3
ð2Þ
where k1 is the spring constant at a distance ΔL from L. Locating the coating area center point at L = 150 mm, the ΔL offset is 5 mm with k1
Fig. 8. Effect of traverse speed on: (a) cathode and anode mass changes and (b) process efficiency (contact force = 0.12 N; pulse frequency = 55 Hz; deposition time (dt) can be calculated with the following formula , dt = 1000/ts, where ts is the traverse speed in mm/s).
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ranging between 4.40 and 3.60 gf/mm, namely the error in neglecting the position offset is only 10%. The contact force (Fc) exerted by the cantilever spring was calculated as follows: Fc = −kΔx
ð3Þ
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burn-through of the thin workpieces. Electrical parameters were therefore changed in a narrow window as described in Table 1. The kinetics of the mass transfer were followed by measuring both the cathode mass change (Δc) and anode mass change (Δa) with an analytical balance. These data served as a basis for process efficiency (ηESD) calculations as follows: Δc ·100: jΔaj
where Δx is the cantilever displacement (in mm) from the unloaded position (measured at workpiece center point). An automatic ESD coating system specifically devised to treat thin metallic foils was used in this work. The apparatus consists essentially of a commercial ESD pulse generator (SparkDepo™ machine, model 28–40 manufactured by TechnoCoat, Japan) electrically connected to the depositing electrode inserted in the mandrel of a vertical milling CNC machine (RM-MaxiMill 3D System by RonchiniMassimo S.A.S). The electrode was rotating at a fixed speed of 8.000 rpm while scanning the surface in a raster pattern mode (see Fig. 5a). Cantilever vertical displacements in the mm range were determined by using a laser-line generator and a CCD camera (Fig. 5b). The laser level was mounted in-plane with the unloaded cantilever. The laser beam is used to generate illumination spots on the lateral surface of electrode. The CCD camera aligned at 90 ° with respect to the laser sends the images of the illumination spots before and after cantilever loading to a computer for a precise determination of vertical displacement by video analysis. Resolutions were better than 0.2 mm. Coatings were applied under low energy intensive discharge conditions in order to avoid deformation, reverse side oxidation or
A few preliminary experiments with a hand-held ESD applicator were carried out to verify the effect of an Al deposition without electrode force control. The deposition was restricted to a small area of 0.5 cm2. Significant cracking by residual thermal stress and large spatter deposition was clearly visible on the coated surface (see Fig. 6). Backscattered electron images revealed also irregular aluminum composition of the coated surface. By EDX analysis it was found that the dark areas of the surface contain ca. 70 at.% as of Al dropping to about 25 at.% in the bright ones. The disuniform Al
Fig. 9. Effect of pulse frequency on: (a) cathode and anode mass changes and (b) process efficiency (pulse energy = 16 mJ; contact force = 0.12 N; traverse speed = 5 mm/s, deposition time = 200 s).
Fig. 10. Effect of the number of successive scans on: (a) cathode and anode mass changes and (b) process efficiency (pulse frequency = 55 Hz, contact force = 0.12 N, single scan deposition times = 200 s at 5 mm/s and 14 s at 70 mm/s).
ηESD =
ð4Þ
The morphology and chemical composition of the coatings were analyzed with a Scanning Electron Microscopy (SEM) Jeol model JSM5510 LV equipped with energy-dispersive X-ray (EDX) microanalysis. A portable contact profilometer model TR200 by TimeGroup Inc. was used to determine the average surface roughness of coated steels. 4. Results
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Fig. 11. Relationship between the number of deposition scans and coating continuity (contact force = 0.12 N; pulse frequency = 55 Hz, deposition times = 200 s at 5 mm/s and 14 s at 70 mm/s).
composition along with the spatter morphology of the coated surface could indicate that aluminum was transferred by a combination of contact and spray transfer mechanisms. Contact transfer (also known as short-circuit transfer) indicates a low energy material removal process in which the molten metal is being transferred to the substrate without spraying but simply bridging the electrode gap, thus leaving deposits with a highly spatter morphology [27]. Contact transfer may be a highly effective mechanism in aluminum because of its easy softening and melting at low currents. Because a contact bridge is being established when a pendant molten drop touches the
workpiece surface before its solidification, it is clear that the contact transfer of aluminum may be relevant in electrospark processes conducted with low energy pulses and in absence of a contact force control. In definitive, these preliminary trials confirmed the difficulty to obtain uniform aluminum deposits onto thin substrates without taking into consideration the contact force parameter. The effect of electrode force on both mass change rates and process efficiency is shown in Fig. 7a–b at two levels of pulse energy P, P1 = 16 mJ and P2 = 27 mJ. It can be seen that cathode weight gain is inversely proportional to the applied electrode force, whereas the anode erosion process is much less dependent on this parameter. At the P2 energy both cathode weight gain and process efficiency steeply decrease with increasing the contact force. Conversely, at P1 the deposition process appears less affected by the contact force showing a broad region of efficiencyN40% within the 0.12–0.4 N range. The results of Fig. 7b suggest that the severity of the cathode erosion increase noticeably along with substrate heating. All the following experiments were carried out at the optimal 0.12 N contact force. The effect of traverse speed and pulse frequency on the material transfer and substrate heating is shown in Figs. 8 and 9. A lower heating is observed at increasing traverse speed as a result of shorter processing times. Good efficiencies (N50%) with both minimal warping and thermal effects are obtainable at either P1 or P2 pulses by adjusting the traverse speed. Raising the frequency has the effect to rapidly increase the substrate heating even with the low pulse energy with a concomitant decrease of process efficiency. The variation of material transfer with the number of deposition scans is shown in Fig. 10 for the P1 and P2 pulse energy conditions with a low heating effect (below 5 °C). The cathode weight gain reaches a maximum after 4 scans at P2 and then slightly decreases, whereas a steady increase of cathode weight is observed with the number of scans when deposition is carried out with the P1 pulses. By knowing the density of coating material (ρ) and the coating area (S), the
Fig. 12. Microstructure of the Al coating surface following a multi-scan treatment under two different experimental conditions: (a)–(b) pulse energy = 16 mJ, traverse speed = 5 mm/s and 6 scans and (c)–(d) pulse energy = 27 mJ, traverse speed = 70 mm/s and 8 scans (contact force = 0.12 N, pulse frequency = 55 Hz).
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thickness of the deposited layer (d) can be estimated from these cathode weight data with the following formula: d=
Δc ρS
ð5Þ
Thus, by way of example, assuming for Al ρ = 2.7 g/cm3, the coating thickness is estimated to be about 6 μm for the P1 discharge condition, after 10 scans. A multi-scan treatment was necessary for the formation of a continuous Al layer covering the entire substrate area. The degree of continuity was checked by SEM and image analysis of aluminum. It was found that coating continuity was achieved after a number of scans, which partly depends on the pulse energy and partly on the traverse speed as shown in Fig. 11. Excellent surface finish was also obtained (surface roughness around 1–2 μm). In comparison, manual deposition produced a much coarser surface roughness of between 4 and 6 μm (see Fig. 6). The corresponding surface and cross-section morphologies with the X-ray aluminum distribution map obtained after the multi-scan treatments are shown in Figs. 12–14. The deposits appear smooth, well adherent and with homogeneous distribution of aluminum inside the coating. The average coating thickness is about 10 μm being in close accordance with the above estimates from the weight change data. The absence of a clear-cut interface proves that a metallurgical bonding between aluminum and stainless steel was established even
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under the very low heat input conditions used in this work. The coating thickness is quite uniform with rare interface undulations. The narrow intermixing region between the substrate and the coating material and the low dilution of aluminum inside the coating (see Fig. 15) is a clear indication that a minimal melting of substrate was occurred during deposition. Accordingly, cross-section analysis of coatings shows a low level of defects such as porosity and microcracking, especially in the lower energy deposits. 5. Discussion In accordance with Belik et al. [7], the present large area deposit experiments have confirmed that a dynamic contact force control may substantially improve the uniformity of aluminum ESD coatings, representing thus a viable option to extend the fine mass transfer properties of the argon gas. The resulting momentary expansion of the micro-gap space when combined with the benefits of the argon gas is supposed to improve process stability in the following aspects: 1. by limiting the surface roughness effects on the spark reproducibility; 2. by performing deposition under a prevalent streamer discharge regime that has a marked effect in reducing cathode erosion; and 3. by promoting a more uniform spray deposition. The present experiments acquire a further significance if related to the challenge represented by aluminization of thin gauge steel substrates under low energy settings.
Fig. 13. Aluminum X-ray map and chemical analysis of the coating surface following the multi-scan treatments: (a) pulse energy = 16 mJ, traverse speed = 5 mm/s and 6 scans and (b) pulse energy = 27 mJ, traverse speed = 70 mm/s and 8 scans (contact force = 0.12 N, pulse frequency = 55 Hz).
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Fig. 14. Cross-sectional images of the Al coating following the multi-scan treatments: (a) pulse energy = 16 mJ, traverse speed = 5 mm/s and 6 scans and (b) pulse energy = 27 mJ, traverse speed = 70 mm/s and 8 scans (contact force = 0.12 N, pulse frequency = 55 Hz).
Firstly, in conventional ESD the mass transfer process is known to be very inefficient at energy pulses lower than 100 mJ as in these circumstances cathode erosion phenomena prevail over the deposited mass [3]. With reference to the results presented through Figs. 7–11 it appears that a high process efficiency can be still obtained with a combination of low energy parameters and low contact force. Raising the contact force (i.e. , to 0.8 N) had the effect of a rapid drop in efficiency as in conventional ESD treatments. As the rate of anode erosion did not change appreciably with the contact force, the
increasing cathode erosion with the contact force may be the sign of sparks occurring under prevalently growing avalanche discharges. Our results therefore indicate that a high process efficiency can be obtained even at low energy settings if a sufficiently low contact force is applied to ensure a stable streamer regime. Further, a low contact force increases the ohmic gap resistance, thereby lowering the probability of destructive arc discharges. Based on the assumption that the erosion of the alloying electrode is the result of a thermoelectrical action of the plasma, it should be stressed that the ESD
Fig. 15. EDX profile scan of Al, Cr, Fe, and Ni elements collected through a typical Al coating deposited in a multi-scan treatment.
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applicability onto thin-walled substrates is limited to the deposition of low melting-point materials like aluminum, where surface melting processes can take place even under extremely low energy discharge conditions. Conversely, ESD deposition of high melting metals or alloys onto thin substrates will introduce many additional difficulties as they would require high energy pulses. Secondly, the narrow intermixing region below coating observed in Fig. 15 undoubtedly indicates that a shallow molten pool was formed with a such combination of low energy pulse and low contact force, which allows to get sufficiently thick coatings in multi-scan deposition processes. The apparent absence of thickness limitation problems suggests that depositions made under a streamer discharge may reduce the tendency to fracture of ESD coatings [28,29]. It is known, in fact, that a too prolonged sequence of spark discharges may produce relatively deep molten pools on the substrate. The solidification of such molten pools forms what is called a surface recast layer. This layer may become very brittle due to thermal stress build-up and eventually begin to self-destruct and pulverize, which manifests as coating thickness limitations and reverse material transfer phenomena. It is believed therefore that the lower heat transfer to the cathode produced under a streamer discharge is a plausible explanation for the less brittle properties of Al coating. 6. Conclusions Aluminum was coated onto a thin gauge stainless steel sheet under argon gas shielding by an automatic electrospark deposition system which used a spring electrode contact for a dynamic control of the contact force during the process. The more regular deposition process allowed to obtain continuous and uniform aluminum coating properties over relatively large areas. The main results, which are strictly valid for Al coatings, can be summarized as follows: (1) Mass transfer processes taking place during a spark discharge are strongly affected by the contact force parameter, which is inversely proportional to the discharge gap size. Under a sufficiently low contact force it was found a net deposited mass even for low energy parameter depositions indicating that a force-controlled ESD can be also utilized for aluminization of thin gauge steel substrates under very low heat input. (2) The observed results are consistent with interpretations that take into considerations the effect of contact force on the mechanisms of spark discharges in micrometer gap sizes. Low contact forces (namely, large gap sizes) are believed to promote spark breakdown by a cathode-directed streamer mechanism,
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which operates for more uniform deposition and lower cathode erosion phenomena. (3) The increased performance stability promoted by the springloaded contact is also a direct consequence of spark discharges taking place at gap sizes larger than in conventional ESD processes, thus allowing to reduce the effect of surface roughness on the spark reproducibility. Acknowledgements The authors are indebted to the Italian Ministry of University and Research for the financial support provided under a FISR (Special Integrative Fund for Research) Programme. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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