J Therm Spray Tech https://doi.org/10.1007/s11666-018-0770-7
PEER REVIEWED
Thermally Sprayed Thin Copper Coatings by W-HVOF Satish Tailor1 • Ankur Modi1 • S. C. Modi1
Submitted: 9 April 2018 / in revised form: 4 October 2018 Ó ASM International 2018
Abstract Thermal spray technologies are widely employed to fabricate quality thick metallic and ceramic coatings for diverse applications. Today, all sectors of the industry demand better, faster and cheaper methods of production as it seems that manufacturing demands are ever increasing. However, if the coating thicknesses below 50 microns are demanded, as a result of economic or technological requirements, it constitutes a challenge for the established thermal spraying processes. So, in the present work, an attempt has been made to deposit a thin metallic coating below 40 microns by thermal spraying through wire feedstock materials rather using an expensive powder. For a broad spectrum of copper (Cu) applications, Cu is deposited on the low-carbon steel substrates using fast, easy and economical thermal spray process wire highvelocity oxy-fuel (W-HVOF): HIJET 9610Ò. As-sprayed coatings were analyzed using x-ray diffraction, scanning electron microscopy for phase, residual stresses and microstructural analysis, respectively. Roughness of coating surface, adhesion strength and porosity were also measured. Results show that the coating deposited through W-HVOF exhibited acceptable properties and provided with a direct economic advantage and time-saving process over existing thin coating techniques.
This article is an invited paper selected from presentations at the 2018 International Thermal Spray Conference, held May 7-10, 2018, in Orlando, FL, USA, and has been expanded from the original presentation. & Satish Tailor
[email protected] 1
Metallizing Equipment Company Pvt. Ltd., E-101, MIA Phase-II, Basni, Jodhpur, India
Keywords electrical conductivity microstructure porosity pull-off adhesion test residual stresses thin thermal spray coating wire-HVOF
Introduction Copper (Cu) has become the metal of choice for metallization replacing aluminum, owing to its high electrical and thermal conductivity, relatively higher melting temperature and correspondingly lower rate of diffusivity (Ref 1). These advantages have drawn significant research in the areas of pure Cu deposition. The thin Cu coating has a variety of applications such as decorative purpose, corrosion resistant, anti-microbial applications; copper is an excellent conductor, and copper plating can be used for EMI (electromagnetic interference) and RFI (radio frequency interference) shielding purposes (Ref 2-5). Copper electroplating on certain parts such as premium threaded connections, couplings, tubulars, risers, and alloy premium drill pipes which are used in oil and gas industry can be very helpful to reduce galling (Ref 6, 7). A thin coating of copper can be used to form the spinel coating on the interconnector plate (Ref 8, 9). Most of the current available thin film techniques can produce copper thin films but below the range of nano to a few microns (\ 2 lm) and at an expensive cost too. Ever-increasing changes regarding the needs of the thin film materials and devices are providing with new opportunities for the development of the new processes, materials and technologies. Basically, three categories of thin film deposition process lies: physical vapor deposition (PVD), chemical vapor deposition (CVD) and chemical methods. Thin films can be defined as ‘‘thin material layers ranging from fractions of a nanometer to several
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micrometers in thickness’’ (Ref 10). All standard procedures of thin metallic coatings are shown in Fig. 1. Thin film applications include very large scale production of electronic packaging, sensors, integrated circuits, optical film and devices and also protective and decorative coatings (Ref 10). The presence of enormous opportunities and rapidly changing needs for thin films and thin film devices are opening new frontiers for the development of new processes, materials and technologies. Still, there lie such areas of application in which thin film deposition techniques (Fig. 1) not only face technological restrictions regarding usable material but also economic disadvantages. Both PVD and more or less the CVD processes use high vacuum environment (Ref 11). Today, thin film technology itself is a separate branch of material science and has evolved into a set of techniques used to fabricate many products (Ref 12). Electrochemical deposition (electroplating) is widespread as one of the standard depositing methods for the application of thin metallic coatings up to 1 to 10 lm. It is well known that all thin film deposition techniques used to deposit a thin film, ranging between a few nanometers to several micrometers in thickness (not more than 1 or 2 lm) (Ref 13-18). Here, it can be said that existing thin film techniques have disadvantages and limitations to depositing a coating thickness above [ 10 lm in respect to technical and economic aspects. Economic aspects play a decisive role, particularly when there is requirement of coating thickness of [ 10 lm but less than \ 50 lm (compared to electroplating) such as a thin coating of 40 lm is enough to form the spinel coating on the interconnector plate in solid oxide fuel cell application (Ref 8, 9). A few more disadvantages, associated with existing thin film deposition techniques, are about job size Fig. 1 Thin film deposition techniques
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limitation and coating on a smaller area compared to the overall component. In such areas of applications, the process time of electroplating is often too long and a great deal of masking is necessary on smaller areas of large components, hence material loss due to masking. Below 50 lm (in the coating thickness ranging between 10 and 40 lm), there exists technological as well as an economical gap which even cannot be filled sensibly either by the existing thin coating techniques or the conventional thick coating technology such as thermal spraying. In general, thick coating technology doesn’t provide much control over the quality of coating yet it is comparatively cheaper to thin film technology. Thermally sprayed coatings are used extensively for producing a thick metallic as well as a ceramic coating on a metal or fiber substrate for a wide range of industrial applications (Ref 19, 20). Thermal spray process is different from all types of thin film deposition techniques. In it, the feed material is melted using energy from fuel combustion, electric arc or plasma. arc spray, flame spray, high-velocity oxy-fuel (HVOF), plasma spray (atmospheric plasma and vacuum plasma), cold spray and detonation gun are some of the most common thermal spray techniques for deposition a wide range of coatings (Ref 19, 20). A wide range of materials can be thermally sprayed for a variety of applications, ranging from automotive, oil & gas, gas turbine engines and electronics industry (Ref 20). Thermal spray process is well established in the area of forming coatings of thickness about greater than 50 microns: the so-called thick coatings. In thermal spray process, one of the economic disadvantages, i.e., unnecessarily thick coating, becomes apparent in the case of high-cost materials. It calls for the development of a coating deposition method providing a coating thickness
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less than 50 lm. In the row, a new processing technology, known as plasma spray-physical vapor deposition (PSPVD) has been developed in order to fulfill the gap between conventional APS and PVD techniques to deposit thin coatings (Ref 21, 22). With PVD, the coating material condenses directly on the substrate out of a vapor cloud. This enables us to produce coatings having characteristics which have not been achieved yet using any of the existing thermal spray processes. Plasma spray-PVD (PS-PVD) also known as very low pressure plasma spray (VLPPS) or low pressure plasma spray-thin film (LPPS-TF) is unique of LPPS where pressures are considerably lower, ranging from 0.5 to 10 Torr (Ref 23). But owing to higher operating temperature of the plasma, PS-PVD is considered to deposit only thin ceramic coatings for a few particular TBCs and SOFC applications, not used for deposition of thin metal coatings (Ref 21-25). For this purpose, in the present work, an attempt has been made to deposit a thin metallic coating less than 50 lm through a modified, fast, easy and economical thermal spray process. Due to wide range of copper (Cu) coating’s applications, Cu is deposited on the low-carbon steel substrates and tested for its properties.
Experimental Materials Copper wire (99.9% pure) 3.17 mm in diameter was used as a feedstock material in the preparation of the coating samples. The coatings were deposited onto low-carbon steel substrates. Prior to spraying, substrates were cleaned with acetone for 5 min and also grit-blasted with alumina (Al2O3) in order to increase roughness of the surface, so that it could improve the adhesion strength of the coating to the substrate. Substrate roughness was found to be Ra * 2-2.5 lm. Thin Coating Deposition Cu coatings were deposited using a patented technology (Patent number 214843, Indian Patent application number 2529/DEL/1997)—W-HVOF thermal spray system (trade name: HIJET 9610Ò), designed and developed by R&D unit of the Metallizing Equipment Company Pvt. Ltd. Jodhpur, India. A schematic diagram of the gun and feeding of wire is shown in Fig. 2. HIJETÒ-9610 is a new generation high-velocity combustion wire spray system which provides supersonic spray velocities combined with the improved heating and melting of the wire particles. The W-HVOF process has wide range of opportunities where a porous free, very dense, high-
performance, wear resistance coating is the requirement also at economic cost. Unique design of the gun allows to coat thick as well as thin metallic coatings (\ 50 lm, in the thickness range of between 10 and 40 lm) and fulfills the gap and opens the scope for the users to use the metallic wires in HVOF rather than using expensive powders and thus makes it a promising solution to achieve better coating properties for various industrial applications. W-HVOF is cost-effective than existing thin film technologies, powder-HVOF and also plasma spraying. This modified HVOF thermal spray system uses wire as the feedstock which directly affects the cost of the process and makes it an inexpensive coating process as it is a well known fact that wires are cheaper to thermal spray grade powders and coating targets used in all thin film technologies. Similarly, the operating cost of above process is also very low in comparison with all thin film technologies, and it is also lower than most of the other available commercial thermal spray processes. Authors of this manuscript don’t want to compare this process with wire arc or twin-wire arc spraying process as the present work is demonstrating thin coating development with wire feedstock. However, a number of experiments have been performed, to develop a coating thickness ranging between 150 and 200 lm, using arc spray, flame spray and W-HVOF to calculate initial wire consumption and coating thicknesses. Experiments were performed using samples with dimensions of 250 9 200 9 4 mm. It was also observed that grinding allowance is 50% lower than arc spray and flame spray due to low Ra values in as-sprayed W-HVOF coating. In Table 1, there is a comparison between coatings thickness and wire consumption to these three processes. W-HVOF is economical in comparison with the PVD, EB-PVD and PS-PVD thin coating technologies, because the process cost of W-HVOF is very low and also there is no limitation of job size as well as coating can be deposited on the site. Thin coating on large size components by PVD, EB-PVD and PS-PVD is quite difficult due to size limits of the coating chambers. One more disadvantage associated with these processes is masking, i.e., if a small area is to be coated on a big component, masking must be applied on full job, and also there is loss of material as coating deposits on masking area too. No such limitations exist in thin coatings deposition of W-HVOF process. The W-HVOF system works on oxygen-propane and liquefied petroleum gas (LPG). Present experiments were carried out using oxygen-LPG as fuel. (Propane is not available in India.) The process parameters were already optimized in order to achieve a thin, dense microstructure, and high deposition efficiency. Forty experiments were performed using different spray parameters in order to optimize the processing parameters primarily air flow,
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J Therm Spray Tech Fig. 2 Schematic of the gun and wire feeding system [26]
Table 1 Comparison of initial costing of the coatings
Process
Wire consumption, gm
Arc spray
160
150
250 9 200 9 4 mm
200
200
250 9 200 9 4 mm
W-HVOF
132
170
250 9 200 9 4 mm
Values
Oxygen flow
230 slpm
LPG flow
55 slpm
Air flow
550-650 slpm
Spray distance
7 inches
Wire and size
Copper, 3.17 mm
Wire feed rate
160 cm/min
Slpm standard liter per minute
spray distance and wire feed rate. Optimized spray parameters are listed in Table 2. Samples were prepared for (1) metallographic evaluation, (2) bend test and (3) bond strength of the coatings.
Characterizations All as-deposited coatings were tested and characterized in the R&D laboratory of MEC, India. Coated samples were cut and polished following a developed routine to analyze microstructures and other properties. Cross sections of the samples were examined under the scanning electron microscope (Carl ZEISS Evo18, UK) equipped with
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Sample dimensions
Flame spray
Table 2 Optimized spray parameters for the W-HVOF-sprayed Cu coatings Spray parameters
Coating thickness, lm
backscatter electron detector (BSC) and EDS analysis (Oxford Instruments, United Kingdom). X-ray diffraction (PANalytical Empyrean Series 2, Netherlands) was used to identify the phases present in the coating and to measure the residual stresses. The residual stresses in as-sprayed coating have been measured using Cu-Ka radiation in the XRD. The residual stresses in the thin Cu coating were measured by employing the sin2w method, and was reported that residual stresses of the coatings are compressive in nature. The copper radiation penetrates, approximately 10 microns, providing an investigation of the near-surface region of the coating. The residual stress measurement was performed in w geometry using sin2w for both positive and negative w tilt angles. With the sinw2 techniques, the strain-free lattice parameter is obtained from the w = 0 values. The anode settings were 40 kV and 40 mA. Porosity measurements were made with a standardized batch routine with the QSMIAS 4.0 Metallurgical Image Analysis System as per ASTM-E2109 method B. Specimens were prepared as per ‘‘E1920 Guide for Metallographic Preparation of Thermal Sprayed Coatings.’’ As copper is very soft material and pores may get closed due to short-time of polishing. Hence, sufficient time should be spent in polishing. The polished surface must reveal a clear distinction among inherent porosity, foreign matter, scratches and oxides. Coating deposition efficiency (DE) was calculated as per ISO 17836:2017. For DE calculation,
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these steps were followed—(1) initial substrate weight measurement in kg, (2) after coating substrate weight measurement in kg, (3) total powder deposition in grams by subtracting step 2 - step 1, (4) total powder consumption in grams, and (5) DE calculation in percentage = (step 3/step4) 9 100. The surface roughness of the as-sprayed coatings was measured using Surface Roughness Tester (Mitutoyo Model SJ-210, Japan). The adhesion strength of the coating was tested using a portable pull-off adhesion tester as per ASTM-D4541 (Elcometer). Bend tests were also conducted to check adhesion and crack characteristics of the thin Cu coatings. Electrical conductivity was also measured by Hall measurement.
Results and Discussion Optimization of Spray Parameter The advancement of the W-HVOF process lies in reaching of the temperature at melting point of the wire, and high degree of atomization takes place due to higher velocity, resulting in complete melting (Ref 26, 27). As such, no unmelted or semi-molten particles were found in the coatings. Further, to prove this statement, splats were collected and studied. By controlling the spraying parameters, mainly the air flow, spray distance and wire feed rate, many types of splats were obtained. During optimization, the air flow, spray distance and wire feed rates have been varied in the range of 300-700 slpm, 3-8 inches and 120-170 cm/min, respectively. At optimized spray parameters, droplet velocity and temperature were measured through SprayWatch system and found out to be 534 ± 12 m/s and 2210 ± 15 °C, respectively. These optimized spray parameters were also tested, at four different gun traverse speeds of 0.30, 0.40, 0.45 and 0.50 m/s, in order to investigate the coating microstructure and coating thickness. All coatings were deposited at room temperature 30 °C. It is discussed in detail in the next section. Coating Microstructures and Influence of Gun Traverse Speed To examine the influence of gun’s traverse speed (mounted on the robot) on coating microstructure, four different speeds of 0.30, 0.40, 0.45 and 0.50 m/s, respectively, were used to prepare thin Cu coatings. Figure 3 shows the cross sections of as-sprayed thin copper coatings at four different speeds. Very dense and micro-crack-free coating microstructures can be seen in the SEM images. It was observed that gun’s traverse speed has a strong influence on coating thickness as well as on the microstructure. At a
slower speed of 0.30 m/s, coating thickness was found to be 44 lm, while at a slightly higher speed of 0.40 m/s coating thickness was found to be 37 lm. It was observed that at a slower traverse speed, deposition efficiency (DE) was high and it decreases with increasing traverse speed. At traverse speed of 0.45 m/s, coating thickness was found to be in the range of 25 to 35 lm, as speed and DE both are low in this case. But at traverse speed of 0.50 m/s, coating deposition was not good, a decent coating thickness was not achieved. DE, at four different gun traverse speeds of 0.30, 0.40, 0.45 and 0.50 m/s, was found to be 75 ± 5%, 70 ± 5% 60 ± 7% and 50 ± 8%, respectively. Further it should be noted that to form a thin coating, all coatings were deposited having just two regular passes. Interval between two neighboring parallel passing was 4 inches. In the W-HVOF process, the feedstock is in wire form and when melting takes place comparatively big molten droplets are formed. Due to slightly higher traverse speed of 0.50 m/s combined with higher particle velocity of 534 m/ s, the random dispersion of splats takes place which do not cover all substrate surfaces after two regular passes. Figure 4 represents a schematic of splat deposition at different traverse speeds. Here it can be stated that by varying gun traverse speed, the thin coating can be fabricated ranging between 10 and 50 lm. The surface roughness of first three as-sprayed coatings (at traverse speeds 0.30, 0.40, 0.45 m/ s) was found to be in range of Ra * 3 to 4 lm, while at traverse speed of 0.50 m/s it was 10 lm. Gun traverse speed plays a significant role on the microstructure as well as on the mechanical properties of coatings. The porosity in the coatings (deposited on four different traverse speeds of 0.30, 0.40, 0.45 and 0.50 m/s) was found to be in the range of 0.5 to 1.5% with the error percentage of ± 0.5%. The lowest porosity of 0.5% was found at a gun traverse speed of 0.30 m/s and maximum 1.5% at a gun traverse speed of 0.50 m/s. In Fig. 6(d), it can be seen that thickness is not uniform at the gun traverse speed of 0.50 m/s and it is highly porous in comparison with rest of the three coatings, so this is relatively a weak coating. In the case of the gun traverse speeds of 0.40 and 0.45 m/s, only DE is different, while coating properties are almost same. The next section discusses mechanical properties of the thin coatings. Effect of Surface Roughness of the Substrate Few coating samples were prepared on substrates (nonblasted) which were cleaned particularly with emery paper. This was done in order to investigate the effect of substrate roughness on the final coating roughness. It is a wellknown fact that blasting increases the coating adhesion strength of the substrate but if a thin coating is deposited on blasted surface, surface roughness of the as-sprayed
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J Therm Spray Tech Fig. 3 Cross sections of assprayed thin copper coatings at four different traverse speeds (a) 0.30 m/s, (b) 0.40 m/s, (c) 0.45 m/s and (d) 0.50 m/s
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J Therm Spray Tech Fig. 4 Schematic representation of splat deposition in W-HVOF at four different traverse speeds
Random dispersion of splats
Fig. 5 Copper thin coating on the without blasting substrate
coating will directly depend on the surface roughness of substrate. But when a smooth surface with a low surface roughness is a requirement instead of strong bonding strength, also normal adhesion is acceptable; such coatings can be deposited by W-HVOF. Figure 5 shows the cross section of the thin Cu coating deposited on non-blasting substrate at optimized parameters as shown in Table 2 at traverse speed of 0.45 m/s. Substrate roughness was found to be Ra * 1 lm. A good microstructure and low porosity (\ 1%) can be observed in the coating as shown in Fig. 5. Top surface morphology of as-sprayed coatings can be seen elsewhere (Ref 26). The surface roughness of as-sprayed coating was found to be less than Ra * 2 lm. In general, during coating deposition the coating roughness decreases with the very fine particle sizes. In W-HVOF, low splash fraction, higher direction angle of the overspray due to high impact, and particle velocity are the key factors of low surface roughness.
XRD Analysis and Oxide Content in the Coating The XRD pattern of thin copper coating is shown in Fig. 6. All peaks are of pure copper, and no oxide phases were detected. However, in thermal spraying oxides are formed. In EDS analysis of thin Cu coating (Fig. 7), it was found that oxygen content is so low that its peak does not appear in the results. It means high particle velocity (534 m/s) in the W-HVOF prevents oxide formation during the spraying. In the W-HVOF flame reduces naturally, it shows that supplied oxygen gas was getting fully consumed building a proper flame. Further, it can be attributed to the fact that lack of oxygen in the reducing flame and shorter dwelling time of particles, within the flame, reduces in-flight oxidative processes. Controlling of oxide formation in the thermal spraying process is the advantage of this W-HVOF system. Oxidation in the coating alters its electrical properties.
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J Therm Spray Tech Fig. 6 XRD pattern of assprayed thin Cu coating at 0.40 m/s speed
Fig. 7 EDS analysis of as-sprayed thin Cu coating
Residual Stress Analysis Residual stresses are defined at two stages of the process— first at the deposition, where the sprayed molten particles strike the substrate and finally get quenched to the room temperature of the underlying material and second at cooling down after thermal spraying using compressed air. These types of coatings generally exhibit the state of residual stress at the top surface that changes from tensile to compressive as the temperature increases due to difference in a thermal mismatch and the way of the cooling method employed after thermal spray coating (Ref 27). Previous studies have reported that electroplated Cu thin coatings have tensile stresses in nature (Ref 28-30). The residual stresses have been identified to be tensile in nature and are found to be in the range of 60-90 MPa. Bend Test Bend test was conducted as per ASTM E290-14 to check delamination characteristics of the thin Cu coating, and it was considered as a test to check the adhesion strength of the thin film. Gun traverse speed plays a significant role in
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Fig. 8 Bend test of as-sprayed thin Cu coating
adhesion strength of the coatings. At gun traverse speed of 0.50 m/s, coating microstructure was not good, uniform and was also highly porous. After the bend test, coating got separated from the substrate. But at the gun traverse speeds of 0.40 and 0.45 m/s, the coating exhibits acceptable and good adhesion strength. Both the coatings showed almost the same characteristics in the bend test. However, at a gun traverse speed of 0.30 m/s, coating exhibited low adhesion strength in comparison with the case of the gun traverse speeds of 0.40 and 0.45 m/s, respectively. Figure 8 shows the bend strip of the thin Cu coating deposited at a gun traverse speed of 0.45 m/s, before (a) and after (b) the test. As per standard, generated cracks are minor and acceptable. The coating deposition on non-blasting surface is also showing good bonding characteristics still it was not better to coating deposition on blasted surfaces.
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microstructure was not good at a gun traverse speed of 0.50 m/s, so the test was not performed for this sample. However, as-sprayed thin Cu coatings show good electric conductivity but slightly less than thick bulk Cu wire which was used as feedstock in the present work. The electric conductivity of the Cu wire was found to be as 2.90E?07 Siemens/cm.
Conclusions
Fig. 9 Pull-off adhesion testing, (a) as-sprayed thin Cu coating, (b) dolly specimens were directly adhesive bonded on a coated substrate (c) sample after the test
Pull-Off Adhesion Test The adhesion strength of the coating was tested using a portable PosiTest pull-off adhesion tester as per ASTMD4541, D7234, ISO 4624. The specimens were directly adhesive bonded onto the Cu-coated substrate. Figure 9(a), (c), respectively, shows thin Cu coating before and after the adhesive pull-off strength test. The test was conducted on each as-sprayed sample (coated at four different gun traverse speeds of 0.30, 0.40, 0.45 and 0.50 m/s) at three different locations. The adhesion strength of the four different thin Cu coatings was found to be 15, 19, 20, and 10 Mpa, respectively, which is acceptable at the initial stage of the development of thermal-sprayed thin coating. However, the adhesion strength of the thin Cu coating deposition on without blasting substrate was found to be in the range of 10-11 Mpa. Substrate roughness conditions, with blasting and without blasting, were found to be Ra * 2-2.5 lm and Ra * 1 lm, respectively. Efforts are still ongoing in the MEC R&D laboratory with continuous experiments to achieve adhesion strength in the range of 25-30 MPa. Electrical Conductivity Test Hall measurement setup (at the Materials Research Centre, Malaviya National Institute of Technology, Jaipur, India) was used to measure the electrical conductivity of assprayed thin Cu coating. Readings were collected three times. In the present work, as-sprayed thin Cu coatings at three different gun traverse speeds of 0.30, 0.40 and 0.45 m/s exhibited a good electrical conductivity of 2.0E?07 Siemens/cm, 2.20E?07 Siemens/cm, 2.21E? 07 Siemens/cm, respectively, at room temperature. Coating
Thin metallic copper coatings were fabricated successfully by means of thermal spraying in order to provide an economical, fast and easy coating process. Unique design of the W-HVOF gun allows to coat thick ([ 50 to 2 mm) as well as thin metallic coatings (\ 50 lm, in the thickness ranging between 10 and 40 lm) and fulfills an existing gap and opens the scope for the users to use the metallic wires in HVOF rather than using expensive powders. As-sprayed Cu thin coating exhibits good properties such as uniform deposition, low porosity (1-2%), good adhesion strength (20 MPa), low Ra value (\ 3 to 4 lm) and good conductivity of 2.21E?07 Siemens/cm. This makes it a promising solution for many industrial applications.
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