Oxidation Effects on the Critical Velocity of Pure Al Feedstock ...
Oxidation Effects on the Critical Velocity of Pure Al Feedstock Deposition in the Kinetic Spraying Process K. C. Kang, S. H. Yoon, Y. G. Ji, C. Lee Kinetic Spray Coating Laboratory (NRL), Division of Materials Science & Engineering, Hanyang University, Seoul, Korea
Abstract In the kinetic spraying process, the critical velocity is an important criterion which determines the deposition of a feedstock particle onto the substrate. It was experimentally and numerically proven that the critical velocity is determined by the physical properties and the state of materials such as initial temperature, size and the extent of oxidation. Compared to un-oxidized feedstock, oxidized feedstock required a greater kinetic energy of the in-flight particle to break away the oxide film during impact. The oxide film formed on the surface of particle and substrate is of a relatively higher brittleness and hardness than those of general metals. Because of its physical characteristics, the oxide significantly affected the deposition behavior and critical velocity. The effects of oxidation on the critical velocity and the deposition behavior of the feedstock were investigated and evaluated by individual particle impact tests in this study. The velocity of pure Al particles was measured for a wide range of process gas conditions.
The critical velocity is generally accepted as a minimum particle velocity for successful impact and occurrence of adiabatic shear instability. A number of studies have shown that the critical velocity is associated with the physical properties of the initial feedstock [1, 3] and substrate materials [4]. Different critical velocities have been reported for copper particles (500~640 m/s) by different investigators [5-7], and the reasons for the difference are still not well understood, or even ignored. Some authors attributed the discrepancies to the difference of feedstock features and gas conditions [8]. However, as mentioned by F. Gärtner [9] and Chang-Jiu Li [10], a significant effect of oxygen content in the feedstock on the critical velocity was found in the present paper. Thus, it is evident that the oxide film on the particle affects the impact behavior of the particle. In the present paper, the effect of oxygen content in the initial feedstock on the critical velocity was investigated experimentally by individual particle impact tests during kinetic spraying.
Introduction Experimental Procedures Kinetic spraying is a novel coating technology, which uses a low temperature supersonic gas jet to accelerate solid fine powders above the critical velocity at which the particles impact, deform plastically, and mechanically bond to the substrate to form a coating. Other than the conventional thermal spray technology, the deposition in kinetic spraying process is mainly dependent on the velocity (kinetic energy) of the impacting particles. Successful bonding requires localized deformation and adiabatic shear instabilities, which are induced by a kinetic energy conversion process of impacting particles. In general, the plastic deformation strain-rate of an impacting particle can be as high as 0.5×109 s-1 [1]. In the region of high strain-rate, localized adiabatic heating will lead to interfacial adiabatic shear instability, which benefits an increase of the bonding area at the particle-substrate and particle-particle interface. So far, some authors consider that the adiabatic shear instability might account for the bonding in kinetic spraying although the exact bonding mechanisms remain unknown [1, 2].
Feedstock Oxidation As shown in the Fig. 1, an as-received spherical Al powder with a mean particle size of 70 μm and a density of 2.70 g/cm3 was used. In addition, three other kinds of feedstock with different oxygen contents could be obtained by oxidation of the as-received Al powder at 150oC for 15 min in a furnace, oxidation at room temperature for 7 days in under atmospheric conditions, and by chemically etching with 10% hydrofluoric acid to remove the oxides on the particle, respectively. Spraying System In this study, a commercially available CGT kinetic spraying system with a MOC type WC nozzle was used. The equipment and the detailed coating process are described in the elsewhere [1, 11, 12]. Helium and nitrogen were used as the process and carrier gases, respectively. The process gas temperature was fixed at 300℃ to minimize any additional oxidation of the
feedstock. Gas pressures of 1.2, 1.6, 2.0 and 2.4 MPa were used, respectively. The flow quantity of carrier gas was set to 10% of the process gas. A standoff distance of 30 mm was used. The substrates of Al alloy were polished before deposition.
The flattening ratio decreased slightly with the increase of oxygen content in the feedstock.
Cross-section
50㎛
Figure 1: Feedstock: pure Al powder morphology. Figure 2: Mounting of the SprayWatch system. Individual particle impact tests were carried out to evaluate the particle impact behavior and critical velocity. A low feed rate (
Abstract In the kinetic spraying process, the critical velocity is an important criterion which determines the deposition of a feedstock particle onto the substrate. It was experimentally and numerically proven that the critical velocity is determined by the physical properties and the state of materials such as initial temperature, size and the extent of oxidation. Compared to un-oxidized feedstock, oxidized feedstock required a greater kinetic energy of the in-flight particle to break away the oxide film during impact. The oxide film formed on the surface of particle and substrate is of a relatively higher brittleness and hardness than those of general metals. Because of its physical characteristics, the oxide significantly affected the deposition behavior and critical velocity. The effects of oxidation on the critical velocity and the deposition behavior of the feedstock were investigated and evaluated by individual particle impact tests in this study. The velocity of pure Al particles was measured for a wide range of process gas conditions.
The critical velocity is generally accepted as a minimum particle velocity for successful impact and occurrence of adiabatic shear instability. A number of studies have shown that the critical velocity is associated with the physical properties of the initial feedstock [1, 3] and substrate materials [4]. Different critical velocities have been reported for copper particles (500~640 m/s) by different investigators [5-7], and the reasons for the difference are still not well understood, or even ignored. Some authors attributed the discrepancies to the difference of feedstock features and gas conditions [8]. However, as mentioned by F. Gärtner [9] and Chang-Jiu Li [10], a significant effect of oxygen content in the feedstock on the critical velocity was found in the present paper. Thus, it is evident that the oxide film on the particle affects the impact behavior of the particle. In the present paper, the effect of oxygen content in the initial feedstock on the critical velocity was investigated experimentally by individual particle impact tests during kinetic spraying.
Introduction Experimental Procedures Kinetic spraying is a novel coating technology, which uses a low temperature supersonic gas jet to accelerate solid fine powders above the critical velocity at which the particles impact, deform plastically, and mechanically bond to the substrate to form a coating. Other than the conventional thermal spray technology, the deposition in kinetic spraying process is mainly dependent on the velocity (kinetic energy) of the impacting particles. Successful bonding requires localized deformation and adiabatic shear instabilities, which are induced by a kinetic energy conversion process of impacting particles. In general, the plastic deformation strain-rate of an impacting particle can be as high as 0.5×109 s-1 [1]. In the region of high strain-rate, localized adiabatic heating will lead to interfacial adiabatic shear instability, which benefits an increase of the bonding area at the particle-substrate and particle-particle interface. So far, some authors consider that the adiabatic shear instability might account for the bonding in kinetic spraying although the exact bonding mechanisms remain unknown [1, 2].
Feedstock Oxidation As shown in the Fig. 1, an as-received spherical Al powder with a mean particle size of 70 μm and a density of 2.70 g/cm3 was used. In addition, three other kinds of feedstock with different oxygen contents could be obtained by oxidation of the as-received Al powder at 150oC for 15 min in a furnace, oxidation at room temperature for 7 days in under atmospheric conditions, and by chemically etching with 10% hydrofluoric acid to remove the oxides on the particle, respectively. Spraying System In this study, a commercially available CGT kinetic spraying system with a MOC type WC nozzle was used. The equipment and the detailed coating process are described in the elsewhere [1, 11, 12]. Helium and nitrogen were used as the process and carrier gases, respectively. The process gas temperature was fixed at 300℃ to minimize any additional oxidation of the
feedstock. Gas pressures of 1.2, 1.6, 2.0 and 2.4 MPa were used, respectively. The flow quantity of carrier gas was set to 10% of the process gas. A standoff distance of 30 mm was used. The substrates of Al alloy were polished before deposition.
The flattening ratio decreased slightly with the increase of oxygen content in the feedstock.
Cross-section
50㎛
Figure 1: Feedstock: pure Al powder morphology. Figure 2: Mounting of the SprayWatch system. Individual particle impact tests were carried out to evaluate the particle impact behavior and critical velocity. A low feed rate (
