Key Factors for Dense Copper Coating by HVOF Spraying

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Table 1: Spray conditions. Unit Condition A Condition B Condition C Condition D Fuel flow rate dm 3·min -1 0.25 0.28 0.32 0.38 Oxygen flow rate dm 3·min -1 1050 968 920 838 Combustion pressure MPa 0.69 0.69 0.69 0.69 Fuel/oxygen ratio* - 0.46 0.56 0.66 0.87 Barrel length mm 203 Powder feed rate g·min -1 75 Torch velocity mm·s -1 700 Spray distance mm 380 Powder feed gas - Nitrogen(N 2 ) Film thickness µm 200 *1.0 corresponds to stoichiometric mixture ratio. Experimental Method Spraying and coating Coatings were prepared with the HVOF thermal spray equipment (JP5000, TAFA Co., Concord, NH, US) and the flame was made from kerosene and oxygen. The copper powder (FUKUDA METAL FOIL & POWDER Co., Kyoto, JPN) was used as the feedstock and sieved from 63 to 75 µmin size. The substrate was JIS SS400 low carbon steel and its surface was blasted by alumina grit and degreased by ultrasonic cleaning in acetone. Primary spray conditions are listed in Table 1. The combustion pressure in this table was the maximum value settable under the condition that all the spray particles could exist as the unmelted ones upon impingement to the substrate. The molten state was estimated by capturing spray particles with a gel target, as described below. The specified combustion pressure was expected to fix the flight velocity of spray particles, based on the assumption that the combustion pressure determines the flight velocity. On the other hand, the oxygen/fuel ratio was expected to change mainly the temperature of the spray particles. Characterization of sprayed particle We measured in-flight velocity of sprayed particles by the flight thermal sprayed particle analyzer (TECNAR Co., DPV-2000, St-Hubert, Qc, Canada). Its principle and mechanism was described in detail elsewhere [4]. This equipment is usually used in order to calculate the surface temperature of in-flight sprayed particles from their synchrotron radiation. In addition, the in-flight velocity is also determined by dividing the interval (160 µm) of two slits by the time between two radiation signals detected when one particle passes through the slits. This method based on the thermal emission is called the Non-Plasma configuration and abbreviated as NP below. Copper, however, is highly reflective material, on the contrary poorly emissive, because emissivity = 1 – reflectivity. Therefore, it was difficult for NP to obtain detectable numbers of thermally radiated particles, especially in comparatively low temperature of the spray particles. Under such a “cold” condition, the additional system of Cold Particle Sensor (abbreviated as CPS below) was effective. This system enabled us to acquire signals from copper particles on the basis of reflection of laser, which was irradiated to the in-flight particles. Our research group reported that the molten fraction of HVOF sprayed particles could be evaluated by capturing the sprayed particles softly with an agar gel as the target material and by separating melted and unmelted particles at different depths [4]. Gel targets were passed once horizontally by the spray gun and captured sprayed copper particles. As shown later, cross sectional views of the sprayed gels showed clearly that the spray particles were captured by the gel target at shallow and deep positions separately. Such views could be seen in thin films with thickness of approximately 50 µm, made by slicing the gels manually with a knife along the depth direction. When the molten fraction was determined, the gels were shaved with a cutter from the sprayed surface forward to the depth direction and the shavings contained melted or unmelted particles. Such shavings were put into a different test tube and heated after addition of distilled water. When heated, copper particles were precipitated at the bottom of test tube by removing supernatants containing agar. The precipitated copper was dissolved by adding 5 ml of concentrated HNO 3 solution (60.0 62.0wt%). After the total volume of such solutions was adjusted to 100 ml by adding 0.5 mol.dm-3 HCl solution, the concentration of the copper solutions was determined by inductively coupled plasma (ICP) atomic emission spectrometry using an analyzer (SPS 3000, EKO Instruments Inc. Tokyo JPN). The molten fraction of the sprayed particles was calculated from the ratio of copper amount in the shallow and deep parts of each agar gel capturing copper particles. Deposited particles (splats) were observed by the optical microscope (Olympus, BX60M, Tokyo, JPN) and by laser microscope (Lasertec Co. 1LM21, Kanagawa, JPN). Splats were formed on AISI 304 stainless steel (SUS304) with a mirror-polished surface. In order to obtain the splats, the spray gun passed the targets of once horizontally during spraying. 756
t v e lig h I n -F 1 - a . u y / s i t t e n Characterization of coating The crystal structure of coatings were characterized by the X-ray diffraction measurement (Rigaku Co., RINT 2000, Tokyo, JPN). The cross section of the coated specimens were examined by the optical microscope . The cross section was prepared by embedding the coated specimen into the epoxy resin, part of which was removed by abrading and polishing treatments. Through-porosity of the coatings was evaluated by the quantitative analysis with the ICP Spectrometry of dissolved ion from coated specimens immersed in the acid solution [5]. In combination of electrochemically noble copper and less noble steel, the dissolved Fe ion came from the steel substrate through the connecting pores of copper coating after formation by corrosion reaction of the substrate prior to the coating, i.e. by galvanic corrosion. Accordingly, the dissolution amount and rate of Fe ion corresponds to the through-porosity of coating. The experimental procedure of this method was described as follows. A SS400 specimen coated by HVOF spraying of copper was cut into square pieces with one side of 2.5cm. A SUS304 wire was connected to the back surface of the substrate plate by spot welding. The sprayed area of 2cm 2 left exposed and the rest of the specimen surface was insulated with the silicon resin. This specimen was immersed in 0.5 mol·dm-3 HCl solution at 300K for 3 days. During immersion, 5 ml of solution was sampled at a predetermined time and the amount of dissolved ion was determined by ICP spectrometry. Simultaneously, corrosion potential of the coatings in the solution was measured by using the corrosion monitor (Riken Denshi, Model CT-5, Tokyo, JPN). Results and Discussion velocity on the fuel/oxygen ratio. Original data of the velocity were obtained at each ratio by two measurement methods of NP and CPS, and their number and weight average were represented in the figure. The NP method has larger values in average velocity than the CPS method. This is explained by the deduction that smaller and lighter particles are heated up to their thermal radiation temperature in shorter time and accelerated up to higher velocity, compared to larger and heavier particles. This deduction can be confirmed by the fact that the number average velocity of NP is higher than its weight average, i.e. larger and heavier particles are estimated to be slower. As for the CPS method, the number and weight average values were almost identical at each ratio and were approximately 490 m·s -1 at all ratios. From this result, the in-flight velocity was constant even at the various ratios of fuel to oxygen under the constant combustion pressure in this paper. Cleanliness of coating Figure 2 compares the XRD patterns of copper coatings with that of copper powder as the feedstock. All the peaks of the coatings prepared under conditions A and D were in accordance with those of the feedstock powder in 2θ position and were attributed to those of copper. In general, the temperature of spray particles increases with the fuel/oxygen ratio up to 1.0 in the HVOF process because the flame temperature increases as the fraction of oxygen gas acting as a cooling medium decreases. However, no additional peaks could be observed even under the Cond. D although this condition was expected to cause oxidation of the coating to the highest degree. Therefore, the XRD results showed that the coatings prepared under all the conditions in this paper were not oxidized. In-flight velocity of sprayed particles Figure 1 shows dependence of the spray particles’ in-flight (a ) (1 1 1) (2 0 0) (2 2 0) (3 1 1) (2 2 2) 700 e d y a r p s f s m / article p y o 600 . (b ) 500 lo c it 400 300 (c ) (111) (2 0 0) (111) (2 2 0) (3 1 1) (222) (2 0 0) (2 2 0) (3 1 1) (2 2 2) I n 200 0.4 0.5 0.6 0.7 0.8 0.9 Ratio(=Rf/o) Figure 1: Relationship between in-flight velocity of sprayed particles and fuel/oxygen ratio: ●: number average (CPS), ■: weight average (CPS), ○: number average (NP), and □: weight average (NP). θ / d e g .(C u K α ) 2 Figure 2: XRD patterns of (a) supply powder of Cu, and HVOF sprayed coatings of Cu prepared under (b) Cond A and (c) Cond D. 2 0 4 0 6 0 8 0 1 0 0 757
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Key Factors for Dense Copper Coating by HVOF Spraying

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