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Post-Test Characterization of a Chromium-Plated Copper Conductor
Post-Test Characterization of a Chromium-Plated Copper Conductor
Zachira Castro-Dettmer, Rachel Monfredo Gee, and Chadee Persad Institute for Advanced Technology, University of Texas at Austin, Austin, TX 78759 Chromium-plated copper rails previously tested as electromagnetic launcher rail conductors were studied. As-received specimens were characterized. To simulate the effect of liquid aluminum that is generated at the sliding interface, aluminum hot-dipping experiments were performed at high temperature. In addition, a further reliquification and spreading experiment to examine aluminum wetting on chromium was conducted. Numerous cracks and voids were found on the chromium layer of as-received specimens, and no molten aluminum–chromium interaction was found at the armature footprint because of the protection provided by the Cr2O3 scale. It was found that cracking due to the coefficient of thermal expansion mismatch upon heating allows oxide-free surfaces to interact with molten aluminum, and oxide formation on the surface of the aluminum deposit prevents the spreading of the deposit. © Elsevier Science Inc., 1999. All rights reserved.
INTRODUCTION
each of the coating materials studied was limited by poor fracture characteristics. Because chromium plating is successfully used in conventional gun barrels, this study was undertaken to assess the effects of using a hard chromium coating for the copper rails used as electromagnetic launcher rail conductors. Previous use of chromiumplated terminals in the circuits of current transformers has been reported [8]. Chromium is hard and corrosion resistant. The desirable properties of chromium are imparted to the surfaces of other materials by electrodeposition. This process is achieved by electrolysis: converting electrical energy to chemical energy by conducting it through electrodes in a primarily chromic acid solution to produce chromium metal. Two types of chromium plating can be the outcome of electrodeposition: hard chromium, and decorative chromium [9]. The thickness of hard chromium deposits varies from about 0.1 to 100mm, while decorative thickness ranges from 0.1 to 0.2mm. Hard chromium coatings are often used because of their low coefficient of fric-
Copper rail conductors have been widely used in electromagnetic launchers (EML) [1, 2]. Following launch experiments, softening of copper by grain growth has been observed [2]. Under extreme conditions, in which liquid aluminum is generated at the contact surface and the rail surface is radiated at power levels greater than 10MW/ cm2, the aluminum reacts with the copper to produce aluminides. Coatings offer protection to the copper substrate and inhibit formation of copper aluminides. Thin (,25mm thick) metal coatings and even thinner conductive ceramic coatings (,5mm) have been tested previously [3, 4]. These coatings are inadequate because they are too thin to protect the soft substrate conductor from mechanical damage, such as hypervelocity gouging [5, 6]. Thicker coatings of materials such as molybdenum and niobium have been investigated. Molybdenum has been evaluated for use as a coating for copper alloys in advanced EML studies [7]. The multitest performance of 251 MATERIALS CHARACTERIZATION 43:251–258 (1999) © Elsevier Science Inc., 1999. All rights reserved. 655 Avenue of the Americas, New York, NY 10010
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tion and good wear properties. For example, the wear resistance of automotive and aircraft components, drills, taps, dies, and gun barrels can be improved by hard chromium plating [10]. Previous studies have found that chromium coatings are brittle, and can only protect substrates if they do not crack [10]. Two major types of hard chromium plating are known: high contraction (HC), and low contraction (LC) [10]. In the railgun test environment, a solid aluminum alloy armature is accelerated by a Lorentz force between two rail conductors (Fig. 1). In this paper, several experiments and analyses were conducted to characterize chromium-plated copper rails previously tested as electromagnetic launcher rail conductors. An as-received specimen from the breech end of the rail and a specimen from the Al-rich footprint of the armature were characterized. To simulate the effect of liquid aluminum that is generated at the sliding interface, aluminum hot-dipping experiments were performed at a high temperature. In addition, a further reliquification and spreading experiment to examine aluminum wetting on chromium was conducted. In this experiment, a sample of the rail that had a preexisting aluminum deposit from the EML armature traverse was chosen.
EXPERIMENTAL PROCEDURE The posttest evaluation of the chromiumplated copper rail includes two types of
studies: (1) characterization of the as-received material using specimens A and B; and (2) the investigation of the interaction of molten aluminum with the chromium-plated copper rail using specimens C, D, E, and F. A summary of the investigations conducted is provided in Table 1. CHARACTERIZATION OF AS-RECEIVED MATERIAL The chromium-plated copper rail used in this investigation was 1.11m long 3 18.5mm wide 3 9.0mm thick and weighed 1732.5g. The rails had been used as electrodes in three EML experiments at the U.S. Army Research Laboratory [11]. The armature (sliding electrical contact) used for each shot was fabricated from 6061-T6 aluminum; the armature mass was 7g. A 12mm long sample was cut from the breech end of the rail, specimen A. At this location there is no interaction with the armature, and specimen A has only been subjected to pulsed Joule heating. A second cut, for specimen B, was made at the area of the rail rich in deposited aluminum for baseline characterization. This aluminumrich area, or armature footprint, is where the armature contacts the rails before it is propelled. Depending upon preload, some melting of the armature occurs prior to motion. This location corresponds to the portion of the conductor at which the armature has the longest residence. As the armature is propelled forward from the breech to the muzzle at increasing velocities, the duration of the local rail exposure is reduced. To prepare for metallographic analysis, Table 1 Summary of Characterizations and Experiments Conducted
FIG. 1. Sketch of the EM launcher and its essential features. Motion of the armature is from breech to muzzle.
Specimen
Description
A B C D E F
As-received breech sample As-received footprint sample Hot dipped for 7200 s at 1023K Hot dipped for 1200 s at 1023K Hot dipped for 300 s at 1023K Heated for 900 s at 1023K in air
Chromium-Plated Copper Conductor
all samples were mounted in phenolic resin, machine ground using 120-, 320-, and 600-grit SiC paper, machine polished with 1mm alumina, and given a final polish with 0.5mm alumina. The chromium layer was etched swabbing the sample for 420 s with the following etchant: 7.5mL hydrochloric acid 1 3.75mL nitric acid 1 11.25mL glycerol [12]. Optical microscopy, scanning-electron microscopy, and energy-dispersive spectroscopy were used to characterize the chromium-plated copper rail samples. Vickers microhardness tests were conducted using a load of 100g on the copper and chromium regions of the rail specimens. The microhardness measurements were taken in three directions to evaluate the rail and its coating: longitudinal, transverse, and at a diagonal as sketched in Fig. 2. HOT-DIPPING EXPERIMENTS The hot-dipping experiments consisted of melting in a crucible, at 1023K, 6061 aluminum with a salt mixture (5mL NaCl 1 5mL KCl 1 1mL NaF), to preclude the formation of a surface oxide layer that would delay the interaction of the aluminum and the chromium. In previous work [13], this method was shown to exclude atmospheric gases and allow direct metal–metal interaction.
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Specimens C, D, and E of the Cr-plated Cu rail were dipped in the molten aluminum and salt mixture for a predetermined amount of time, (7200, 1200, and 300 s), and conducted in that order. The temperature chosen for this study was 1023K because a previous study [13] determined it to be an adequate temperature to melt all aluminum in the crucible and for the diffusion of aluminum to be significant. Specimens were air cooled following the dipping experiments. Hot-dipping experiments were conducted to characterize the behavior of molten aluminum on the chromium coating under static, controlled temperature conditions. DEPOSIT MELT-SPREADING EXPERIMENT A 12mm long section of the rail that was partly covered by an aluminum deposit from the armature, corroborated by energy-dispersive spectroscopy, was heated in a crucible from room temperature to 1023K, and was held at this temperature for 900 s. The sample was subsequently air cooled. The purpose of this experiment was to evaluate the effect of oxidation in the deposit on its subsequent melting and spreading characteristics.
RESULTS CHARACTERIZATION OF AS-RECEIVED MATERIAL
FIG. 2. The rounded corner of the rail illustrates the susceptibility of chromium corners to cracking, confirming the brittle nature of the chromium coating in specimen B. Microhardness measurement were taken in three directions: longitudinal, transverse, and at a diagonal, as indicated by the arrows.
During specimen preparation, specimen A was removed from the breech end of the rail for baseline characterization. As sawcutting proceeded, cracks along the chromium-plated surface immediately initiated, and propagated. Light optical microscopy of the cross-section shows a chromiumplated layer that is nominally 100mm thick. Both the Cu substrate and the Cr coating are rounded at the rail corners, as shown in Fig. 2. Because specimen A was obtained from the breech end, there was no contact with the aluminum armature. The chromium layer exhibited transcrystalline failure normal to the surface.
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Surface cracks, embedded cracks, and cracks from the Cr coating that propagated to the Cu substrate are present in the sample. Voids in the Cr coating along the interface with the Cu were also observed in specimen A of the rail (Fig. 3). Voids concentrated near the Cr–Cu interface extend no more than 28mm from the copper substrate. Figure 4 shows an etched transverse cross-section of the chromium layer in specimen A. The features observed in etched chromium deposits are dependent on the etching technique used [14]. The observed microstructure of the etched crosssection exhibits a fibrous columnar structure. The columnar grains are formed in the direction of current flow lines [14]. The measured intercolumnar spacing in specimen A is 14.14mm. The reported Knoop hardness value for the chromium coating before testing was HK 356, (approximately HV 350), under a 50g load (A. Zielinski, Army Research Laboratory, Private Communication, 1998). The hardness of the chromium layer in specimen A ranged from HV 384–459 and the copper substrate ranged from HV 81– 94. This data confirms that the chromium coating is considerably harder than the copper substrate. As part of the evaluation of the as-received material, a piece from the footprint of the rail was also examined, specimen B. Specimen B exhibited cracking and fracture damage to the chromium layer, but there
FIG. 3. Voids and crack found along the interface line in specimen A. This type of crack was prevalent in the chromium coating in all specimens.
Z. Castro-Dettmer et al.
FIG. 4. Etched transverse cross-section displaying the fibrous columnar microstructure of the chromium layer in specimen A.
was no evidence of an aluminum–chromium interaction. Figure 2 illustrates the susceptibility of chromium corners to cracking, confirming the brittle nature of the chromium plate. HOT-DIPPING EXPERIMENTS In specimen C (exposure time 7200 s), the molten aluminum aggressively attacked the copper substrate, which was also exposed to the molten aluminum. An Al–Cu– Cr alloy formed as a result of the extended hot-dipping time. Specimen D (exposure time 1200 s) displayed similar results as specimen C. For specimen E, the 5-mi (300 s) dipping was enough for the molten aluminum to seep into the cracks in the chromium coating and aggressively attack the copper substrate (Fig. 5). An aluminum–chromium interaction was detected at the crack edges in contact with the aluminum. In Fig. 5, the interaction region is the darker area on the crack edge. Energy-dispersive spectroscopy analysis suggests the intermetallic aAl9Cr4 [15] formed at the Al–Cr interface. Figure 6 shows a section of specimen E. Deformation is clearly visible resulting from heat-softened copper. The coefficient of thermal expansion (CTE) of copper is 16.5mm/m K, while the CTE of chromium is 6.2mm/m K [16]. Due to the CTE mismatch, the chromium coating cracked upon heating. The hardness of the chromium layer in
Chromium-Plated Copper Conductor
FIG. 5. In specimen E, molten aluminum seeped into a crack in the chromium coating and attacked the copper substrate. Some detachment of chromium is observed. The dark region along the crack edge is the Al–Cr interaction area.
specimen E ranged from HV 325–388, and the copper substrate ranged from HV 77–90. An effect of temperature on the chromium coating is a decrease in hardness. Electroplated chromium begins to decrease in hardness when it is exposed to temperatures above 478K [17]. Hardness decreases progressively with an increase in temperature, and it is recommended that chromium plating should not be used for wear resistance applications where temperature exceeds 693K [17].
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would spread and seep into the cracks of the chromium layer when subjected to further heating. Light optical microscopy showed no signs of the aluminum seeping into the cracks of the chromium layer. Also, there was no evidence of an aluminum– chromium interaction in the specimen. It was noted that this sample, after heating, had a minimum amount of cracks. This sample also contained a good example of a crack in the chromium coating that propagated beyond the Cr–Cu interface (Fig. 7). This type of crack could accelerate the attack of molten aluminum on the copper substrate. The hardness of the chromium layer in specimen F ranged from HV 264–337, and the copper substrate ranged from HV 52– 63. As expected, the hardness of both the chromium layer and the copper substrate decreased as a result of heat softening. Examination of the etched microstructure of the heated sample confirmed grain growth in the chromium layer, possibly the cause of the measured decrease in hardness.
DISCUSSION CHROMIUM PLATING APPLICATION
This test aimed to find if the aluminum-rich deposit that already existed on the rail
From 1940 to 1945, exhaustive work was done to develop materials suitable as liners for high-performance steel gun barrels [18]. One of the materials studied was chro-
FIG. 6. Due to the softening of the copper substrate, plastic flow occurred under load, and the chromium layer shifted, thus destroying the integrity of the protective coating in specimen D.
FIG. 7. The specimen used for the deposit-spreading experiment contained an example of a crack in the chromium coating that propagated into the chromium–copper interface.
DEPOSIT MELT-SPREADING EXPERIMENT
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mium, and it was found to be an excellent erosion-resistant material. Chromium has a high melting point, 2136K. An inexpensive electroplating process can apply the chromium coating, but it is brittle and develops cracks as a result of firing and engraving stresses. The early performance studies found that hot propellant gases in fired ammunition penetrate these cracks and attack the underlying steel substrate. In addition, if the coating was not thick enough to thermally insulate the substrate, the substrate deforms under the engraving stress, resulting in cracking of the chromium coating. Chromium coating has been successful in some conventional guns. The reason for this is electroplated chromium with microcracks and inclusions, resulting in a material with a modulus relatively close to that of steel. Highly contractile (HC) chromium is still the predominant coating used for erosion resistance in small-caliber guns. CHARACTERIZATION OF AS-RECEIVED MATERIAL Characterization of specimens A and B showed pervasive cracking of the chromium coating. Complete cracking of the chromium layer, as shown in Fig. 3, was prevalent. Cracks in the chromium plating are due to the relaxation of residual stresses caused by the presence of hydrides and hydrated oxides occluded during the electrodeposition process [18]. There was no evidence of an aluminum– chromium interaction at the armature footprint. The chromium oxide (Cr2O3) layer formed in the chromium surface must act as a barrier to molten aluminum attack. Cr2O3 has a melting point of 2708K, but as the oxide scale grows thicker it may eventually scale or spall off, leaving the metal unprotected [19]. Cracking of the chromium layer due to the CTE mismatch could also alter the protective Cr2O3 scale, eventually allowing an aluminum–chromium interaction. HOT-DIPPING EXPERIMENTS In the hot-dipping experiments, the reaction between the molten aluminum and the
copper substrate was favored over that between the molten aluminum and the chromium plating. The chromium layer is protected by chromium oxide, as previously discussed. It has been found that molten aluminum reduces copper oxide on contact [20]. Thus, the reaction of Al–Cu is preferred because there is no surface oxide protection. The aluminum–chromium interaction occurred at a crack surface in specimen E (exposure time 300 s). Cracking in the chromium coating due to the CTE mismatch allows the new oxide-free chromium surfaces to interact with the molten alumininum. These hot-dipping experiments showed the aggressive attack of molten aluminum on copper, but the interaction of the molten aluminum and the chromium layer, as shown by Barbier et al. [21], could not be isolated due to the exposed copper substrate. All EDS analysis of reaction products between aluminum and chromium revealed the presence of small amounts of copper, 0.95wt % or greater. Because of the proximity of the copper substrate, it was impossible to isolate the aluminum–chromium reactions from the effects of copper. DEPOSIT MELT-SPREADING EXPERIMENT The study of specimen F considered the question of melt spreading of deposits. Typically, railgun aluminum-rich deposits take the form of a thin (20mm thick) coating transferred from the armature contact surface to the cold rail. The morphology of lifted fragments, when studied by microscopy, shows a shiny side that was in contact with the rail and a dull, oxidized “air” side. This appearance is remarkably similar to that of thin metallic glass foils that are produced by planar flow casting of molten metal on a moving cold copper wheel [22]. Detailed characterization of these deposits has been reported elsewhere [5, 23]. The effect of oxide formation on the fluidity of molten aluminum has also been reported. It was found that the presence of alumina films and particles was a deciding
Chromium-Plated Copper Conductor
factor in lowering the fluidity of the melt [24]. As deposits are formed and rapidly quenched in an oxygen-containing atmosphere, the formation of aluminum oxide has been observed. These thin oxide-encased deposits are further oxidized in this experiment by heating in air, and even though the peak temperature is well above the melting point of this alloy, there is no evidence of melt flow that, upon resolidification, could wedge cracks open and facilitate multicycle crack propagation.
CONCLUSIONS As-received multispecimen characterization and two types of thermochemical experiments were conducted using sections of a chromium-plated copper bar previously used as an electromagnetic launcher rail conductor. An as-received specimen from the breech end of the rail and a specimen from the Alrich footprint of the armature were characterized. Numerous cracks and voids were found on the chromium layer of both specimens. Voids were concentrated near the Cr–Cu interface extending no more than 28mm from the copper substrate. Existent cracks propagated and new cracks developed easily in the brittle chromium layer. No molten aluminum–chromium interaction was found at the armature footprint because of the protection provided by the Cr2O3 scale. Hot-dipping experiments were conducted at high temperature (1023K). In these experiments, the molten aluminum attacked the exposed copper substrate by seeping into the cracks in the chromium coating. A molten aluminum–chromium interaction was found at one crack. Cracking due to the CTE mismatch upon heating allows oxide-free surfaces to interact with molten aluminum. The reaction between molten aluminum and copper was favored to that of molten aluminum and chromium because chromium oxide acts as a barrier to molten aluminum attack while copper oxide is easily reduced on contact by molten aluminum.
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Further heating of a rail sample that had some aluminum deposit from the armature was performed. The aluminum deposit did not spread or seep into the existing cracks. Oxide formation on the surface of the deposit prevented the spreading of the aluminum deposit, and there was no aluminum remelting. Thus, spreading and seeping of aluminum into the cracks could only occur with subsequent armature deposits and in the presence of high pressure between the armature and the rail. No molten aluminum–chromium interaction was found in this particular experiment. From the results of this investigation it has been concluded that the use of this type of electroplated chromium, as a coating for copper rail conductors, is inadequate. The presence of cracks in the chromium coating, the decrease in hardness of both the coating and the substrate upon heating, the cracking and deformation of the chromium coating due to the CTE mismatch, and the attack of molten aluminum on the copper substrate through cracks in the chromium layer are all unfavorable factors in extending the life of rail conductors.
References 1. A. J. Bedford: Rail damage and armature parameters for different railgun rail materials. IEEE Transact. Magnet. 20:352–355 (1984). 2. C. Persad and S. Raghunathan: Post-test characterization of the hardness and microstructure of copper rails from a 9-MJ electromagnetic launcher. IEEE Transact. Magnet. 31:740–774 (1995). 3. G. R. Colombo, M. Otooni, M. P. Evangelisti, N. Colon, and E. Chu: Application of coatings for electromagnetic gun technology. IEEE Transact. Magnet. 31:704–708 (1995). 4. D. S. Wright: Wear behavior of copper and molybdenum conductor pairs in simple railgun electromagnetic launchers. MS Thesis, University of Texas at Austin, Austin, TX (1994). 5. C. Persad, C. J. Lund, Z. Eliezer, D. R. Peterson, J. Hahne, and R. Zowarka: Wear of conductors in railguns: Metallurgical aspects. IEEE Transact. Magnet. 25:433–437 (1989). 6. C. Persad, G. Prabhu, G. White, A. Yeoh, and Z. Eliezer: Characterization of hypervelocity gouge craters in rail conductors. IEEE Transact. Magnet. 33:401–405 (1997).
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258 7. A. Yeoh: Evaluation of a Mo-on-Cu Explosively Bonded Bimetallic Strip for Use as an Electromagnetic Launcher Rail Conductor. Part 1: Static Armature Experiments. Report No. IAT.R 0146, Institute for Advanced Technology, Austin, TX (1997).
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8. P. J. Betts: Nickel and chromium-plated terminals in current transformer circuits. IEEE Proc. A: Sci. Measure. Technol. 132:178–180 (1985).
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9. J. Segerson: The two faces of chromium. Hydraul. Pneumat. 42:14 (1989). 10. R. Weil and K. Sheppard: Electroplated coatings. ASM Handbook, vol. 18. ASM International, Materials Park, Ohio, pp. 834–839 (1992). 11. C. Le and A. Zielinski: Preliminary Findings on Magnetic Shielding Effectiveness for Electromagnetic (EM) Railgun Applications. Proc. 6th European Symposium on Electromagnetic Launch Technology, The Hague, The Netherlands (May 1997).
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12. G. Petzow: Metallographic Etching. American Society for Metals, Metals Park, OH (1978).
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13. Z. Castro: Interaction at high temperature of liquid metal aluminum 7075 with H13 tool steel. MS Thesis, University of Texas at Austin, TX (1997).
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14. J. K. Dennis and T. E. Such: Nickel and Chromium Plating, Newnes-Butterworth, London, pp. 49, 55 (1972).
can Society for Metals, Metals Park, Ohio, pp. 170– 187 (1982). I. Ahmad: The problem of gun barrel erosion: an overview. In Gun Propulsion Technology, L. Stiefel, ed., American Institute of Aeronautics and Astronautics, Washington, DC, p. 336 (1988). S. Bradford: Fundamentals of corrosion in gasses. In ASM Handbook (vol. 13), ASM International, Materials Park, Ohio, pp. 64–65 (1987); I. G. Wright: High-temperature corrosion. In ASM Handbook (vol. 13), ASM International, Materials Park, Ohio, pp. 97–98 (1987). C. Persad, A. Yeoh, G. Prabhu, G. White, and Z. Eliezer: On the nature of the armature–rail interface: Liquid metal effects. IEEE Transact. Magnet. 33:140–145 (1997). F. Barbier, D. Manuelli, and K. Bouche: Characterization of aluminide coatings formed on 1.4914 and 316L steels by hot-dipping in molten aluminum. Scripta Mater. 36:425–431 (1997). Metallic Glasses: Production, Properties and Applications, T. R. Anantharaman, ed., Trans Tech Publications, Switzerland (1984). G. B. Prabhu, D. L. Bourell, and C. Persad: Characterization of Nanocrystalline 7075 Aluminum Alloy Produced by Rapid Solidification. On Processing and Properties of Nanocrystalline Materials, Proceedings of the TMS Materials Week ‘95 Symposium, Cleveland, OH (Oct 29–Nov 2, 1995), pp. 233–244 (1996). L. Liu and F. H. Samuel: Assessment of melt cleanliness in A356.2 aluminum casting alloy using the porous disc filtration apparatus technique. Part II: Inclusion analysis. J. Mater. Sci. 32:5927–5944 (1997).
15. Alloy phase diagram. In ASM Handbook (vol. 3), H. Baker, ed., ASM International, Materials Park, Ohio, pp. 2–43 (1992).
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16. ASM Handbook (vol. 2), ASM International Handbook Committee, ed., ASM International, Metals Park, Ohio, pp. 1108–1111 (1990). 17. H. Chessin and E. H. Fernald, Jr: Hard chromium plating. In Metals Handbook (9th ed.; vol. 5), Ameri-
Received March 1999; accepted May 1999.
Zachira Castro-Dettmer, Rachel Monfredo Gee, and Chadee Persad Institute for Advanced Technology, University of Texas at Austin, Austin, TX 78759 Chromium-plated copper rails previously tested as electromagnetic launcher rail conductors were studied. As-received specimens were characterized. To simulate the effect of liquid aluminum that is generated at the sliding interface, aluminum hot-dipping experiments were performed at high temperature. In addition, a further reliquification and spreading experiment to examine aluminum wetting on chromium was conducted. Numerous cracks and voids were found on the chromium layer of as-received specimens, and no molten aluminum–chromium interaction was found at the armature footprint because of the protection provided by the Cr2O3 scale. It was found that cracking due to the coefficient of thermal expansion mismatch upon heating allows oxide-free surfaces to interact with molten aluminum, and oxide formation on the surface of the aluminum deposit prevents the spreading of the deposit. © Elsevier Science Inc., 1999. All rights reserved.
INTRODUCTION
each of the coating materials studied was limited by poor fracture characteristics. Because chromium plating is successfully used in conventional gun barrels, this study was undertaken to assess the effects of using a hard chromium coating for the copper rails used as electromagnetic launcher rail conductors. Previous use of chromiumplated terminals in the circuits of current transformers has been reported [8]. Chromium is hard and corrosion resistant. The desirable properties of chromium are imparted to the surfaces of other materials by electrodeposition. This process is achieved by electrolysis: converting electrical energy to chemical energy by conducting it through electrodes in a primarily chromic acid solution to produce chromium metal. Two types of chromium plating can be the outcome of electrodeposition: hard chromium, and decorative chromium [9]. The thickness of hard chromium deposits varies from about 0.1 to 100mm, while decorative thickness ranges from 0.1 to 0.2mm. Hard chromium coatings are often used because of their low coefficient of fric-
Copper rail conductors have been widely used in electromagnetic launchers (EML) [1, 2]. Following launch experiments, softening of copper by grain growth has been observed [2]. Under extreme conditions, in which liquid aluminum is generated at the contact surface and the rail surface is radiated at power levels greater than 10MW/ cm2, the aluminum reacts with the copper to produce aluminides. Coatings offer protection to the copper substrate and inhibit formation of copper aluminides. Thin (,25mm thick) metal coatings and even thinner conductive ceramic coatings (,5mm) have been tested previously [3, 4]. These coatings are inadequate because they are too thin to protect the soft substrate conductor from mechanical damage, such as hypervelocity gouging [5, 6]. Thicker coatings of materials such as molybdenum and niobium have been investigated. Molybdenum has been evaluated for use as a coating for copper alloys in advanced EML studies [7]. The multitest performance of 251 MATERIALS CHARACTERIZATION 43:251–258 (1999) © Elsevier Science Inc., 1999. All rights reserved. 655 Avenue of the Americas, New York, NY 10010
1044-5803/99/$–see front matter PII S1044-5803(99)00011-X
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tion and good wear properties. For example, the wear resistance of automotive and aircraft components, drills, taps, dies, and gun barrels can be improved by hard chromium plating [10]. Previous studies have found that chromium coatings are brittle, and can only protect substrates if they do not crack [10]. Two major types of hard chromium plating are known: high contraction (HC), and low contraction (LC) [10]. In the railgun test environment, a solid aluminum alloy armature is accelerated by a Lorentz force between two rail conductors (Fig. 1). In this paper, several experiments and analyses were conducted to characterize chromium-plated copper rails previously tested as electromagnetic launcher rail conductors. An as-received specimen from the breech end of the rail and a specimen from the Al-rich footprint of the armature were characterized. To simulate the effect of liquid aluminum that is generated at the sliding interface, aluminum hot-dipping experiments were performed at a high temperature. In addition, a further reliquification and spreading experiment to examine aluminum wetting on chromium was conducted. In this experiment, a sample of the rail that had a preexisting aluminum deposit from the EML armature traverse was chosen.
EXPERIMENTAL PROCEDURE The posttest evaluation of the chromiumplated copper rail includes two types of
studies: (1) characterization of the as-received material using specimens A and B; and (2) the investigation of the interaction of molten aluminum with the chromium-plated copper rail using specimens C, D, E, and F. A summary of the investigations conducted is provided in Table 1. CHARACTERIZATION OF AS-RECEIVED MATERIAL The chromium-plated copper rail used in this investigation was 1.11m long 3 18.5mm wide 3 9.0mm thick and weighed 1732.5g. The rails had been used as electrodes in three EML experiments at the U.S. Army Research Laboratory [11]. The armature (sliding electrical contact) used for each shot was fabricated from 6061-T6 aluminum; the armature mass was 7g. A 12mm long sample was cut from the breech end of the rail, specimen A. At this location there is no interaction with the armature, and specimen A has only been subjected to pulsed Joule heating. A second cut, for specimen B, was made at the area of the rail rich in deposited aluminum for baseline characterization. This aluminumrich area, or armature footprint, is where the armature contacts the rails before it is propelled. Depending upon preload, some melting of the armature occurs prior to motion. This location corresponds to the portion of the conductor at which the armature has the longest residence. As the armature is propelled forward from the breech to the muzzle at increasing velocities, the duration of the local rail exposure is reduced. To prepare for metallographic analysis, Table 1 Summary of Characterizations and Experiments Conducted
FIG. 1. Sketch of the EM launcher and its essential features. Motion of the armature is from breech to muzzle.
Specimen
Description
A B C D E F
As-received breech sample As-received footprint sample Hot dipped for 7200 s at 1023K Hot dipped for 1200 s at 1023K Hot dipped for 300 s at 1023K Heated for 900 s at 1023K in air
Chromium-Plated Copper Conductor
all samples were mounted in phenolic resin, machine ground using 120-, 320-, and 600-grit SiC paper, machine polished with 1mm alumina, and given a final polish with 0.5mm alumina. The chromium layer was etched swabbing the sample for 420 s with the following etchant: 7.5mL hydrochloric acid 1 3.75mL nitric acid 1 11.25mL glycerol [12]. Optical microscopy, scanning-electron microscopy, and energy-dispersive spectroscopy were used to characterize the chromium-plated copper rail samples. Vickers microhardness tests were conducted using a load of 100g on the copper and chromium regions of the rail specimens. The microhardness measurements were taken in three directions to evaluate the rail and its coating: longitudinal, transverse, and at a diagonal as sketched in Fig. 2. HOT-DIPPING EXPERIMENTS The hot-dipping experiments consisted of melting in a crucible, at 1023K, 6061 aluminum with a salt mixture (5mL NaCl 1 5mL KCl 1 1mL NaF), to preclude the formation of a surface oxide layer that would delay the interaction of the aluminum and the chromium. In previous work [13], this method was shown to exclude atmospheric gases and allow direct metal–metal interaction.
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Specimens C, D, and E of the Cr-plated Cu rail were dipped in the molten aluminum and salt mixture for a predetermined amount of time, (7200, 1200, and 300 s), and conducted in that order. The temperature chosen for this study was 1023K because a previous study [13] determined it to be an adequate temperature to melt all aluminum in the crucible and for the diffusion of aluminum to be significant. Specimens were air cooled following the dipping experiments. Hot-dipping experiments were conducted to characterize the behavior of molten aluminum on the chromium coating under static, controlled temperature conditions. DEPOSIT MELT-SPREADING EXPERIMENT A 12mm long section of the rail that was partly covered by an aluminum deposit from the armature, corroborated by energy-dispersive spectroscopy, was heated in a crucible from room temperature to 1023K, and was held at this temperature for 900 s. The sample was subsequently air cooled. The purpose of this experiment was to evaluate the effect of oxidation in the deposit on its subsequent melting and spreading characteristics.
RESULTS CHARACTERIZATION OF AS-RECEIVED MATERIAL
FIG. 2. The rounded corner of the rail illustrates the susceptibility of chromium corners to cracking, confirming the brittle nature of the chromium coating in specimen B. Microhardness measurement were taken in three directions: longitudinal, transverse, and at a diagonal, as indicated by the arrows.
During specimen preparation, specimen A was removed from the breech end of the rail for baseline characterization. As sawcutting proceeded, cracks along the chromium-plated surface immediately initiated, and propagated. Light optical microscopy of the cross-section shows a chromiumplated layer that is nominally 100mm thick. Both the Cu substrate and the Cr coating are rounded at the rail corners, as shown in Fig. 2. Because specimen A was obtained from the breech end, there was no contact with the aluminum armature. The chromium layer exhibited transcrystalline failure normal to the surface.
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Surface cracks, embedded cracks, and cracks from the Cr coating that propagated to the Cu substrate are present in the sample. Voids in the Cr coating along the interface with the Cu were also observed in specimen A of the rail (Fig. 3). Voids concentrated near the Cr–Cu interface extend no more than 28mm from the copper substrate. Figure 4 shows an etched transverse cross-section of the chromium layer in specimen A. The features observed in etched chromium deposits are dependent on the etching technique used [14]. The observed microstructure of the etched crosssection exhibits a fibrous columnar structure. The columnar grains are formed in the direction of current flow lines [14]. The measured intercolumnar spacing in specimen A is 14.14mm. The reported Knoop hardness value for the chromium coating before testing was HK 356, (approximately HV 350), under a 50g load (A. Zielinski, Army Research Laboratory, Private Communication, 1998). The hardness of the chromium layer in specimen A ranged from HV 384–459 and the copper substrate ranged from HV 81– 94. This data confirms that the chromium coating is considerably harder than the copper substrate. As part of the evaluation of the as-received material, a piece from the footprint of the rail was also examined, specimen B. Specimen B exhibited cracking and fracture damage to the chromium layer, but there
FIG. 3. Voids and crack found along the interface line in specimen A. This type of crack was prevalent in the chromium coating in all specimens.
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FIG. 4. Etched transverse cross-section displaying the fibrous columnar microstructure of the chromium layer in specimen A.
was no evidence of an aluminum–chromium interaction. Figure 2 illustrates the susceptibility of chromium corners to cracking, confirming the brittle nature of the chromium plate. HOT-DIPPING EXPERIMENTS In specimen C (exposure time 7200 s), the molten aluminum aggressively attacked the copper substrate, which was also exposed to the molten aluminum. An Al–Cu– Cr alloy formed as a result of the extended hot-dipping time. Specimen D (exposure time 1200 s) displayed similar results as specimen C. For specimen E, the 5-mi (300 s) dipping was enough for the molten aluminum to seep into the cracks in the chromium coating and aggressively attack the copper substrate (Fig. 5). An aluminum–chromium interaction was detected at the crack edges in contact with the aluminum. In Fig. 5, the interaction region is the darker area on the crack edge. Energy-dispersive spectroscopy analysis suggests the intermetallic aAl9Cr4 [15] formed at the Al–Cr interface. Figure 6 shows a section of specimen E. Deformation is clearly visible resulting from heat-softened copper. The coefficient of thermal expansion (CTE) of copper is 16.5mm/m K, while the CTE of chromium is 6.2mm/m K [16]. Due to the CTE mismatch, the chromium coating cracked upon heating. The hardness of the chromium layer in
Chromium-Plated Copper Conductor
FIG. 5. In specimen E, molten aluminum seeped into a crack in the chromium coating and attacked the copper substrate. Some detachment of chromium is observed. The dark region along the crack edge is the Al–Cr interaction area.
specimen E ranged from HV 325–388, and the copper substrate ranged from HV 77–90. An effect of temperature on the chromium coating is a decrease in hardness. Electroplated chromium begins to decrease in hardness when it is exposed to temperatures above 478K [17]. Hardness decreases progressively with an increase in temperature, and it is recommended that chromium plating should not be used for wear resistance applications where temperature exceeds 693K [17].
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would spread and seep into the cracks of the chromium layer when subjected to further heating. Light optical microscopy showed no signs of the aluminum seeping into the cracks of the chromium layer. Also, there was no evidence of an aluminum– chromium interaction in the specimen. It was noted that this sample, after heating, had a minimum amount of cracks. This sample also contained a good example of a crack in the chromium coating that propagated beyond the Cr–Cu interface (Fig. 7). This type of crack could accelerate the attack of molten aluminum on the copper substrate. The hardness of the chromium layer in specimen F ranged from HV 264–337, and the copper substrate ranged from HV 52– 63. As expected, the hardness of both the chromium layer and the copper substrate decreased as a result of heat softening. Examination of the etched microstructure of the heated sample confirmed grain growth in the chromium layer, possibly the cause of the measured decrease in hardness.
DISCUSSION CHROMIUM PLATING APPLICATION
This test aimed to find if the aluminum-rich deposit that already existed on the rail
From 1940 to 1945, exhaustive work was done to develop materials suitable as liners for high-performance steel gun barrels [18]. One of the materials studied was chro-
FIG. 6. Due to the softening of the copper substrate, plastic flow occurred under load, and the chromium layer shifted, thus destroying the integrity of the protective coating in specimen D.
FIG. 7. The specimen used for the deposit-spreading experiment contained an example of a crack in the chromium coating that propagated into the chromium–copper interface.
DEPOSIT MELT-SPREADING EXPERIMENT
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mium, and it was found to be an excellent erosion-resistant material. Chromium has a high melting point, 2136K. An inexpensive electroplating process can apply the chromium coating, but it is brittle and develops cracks as a result of firing and engraving stresses. The early performance studies found that hot propellant gases in fired ammunition penetrate these cracks and attack the underlying steel substrate. In addition, if the coating was not thick enough to thermally insulate the substrate, the substrate deforms under the engraving stress, resulting in cracking of the chromium coating. Chromium coating has been successful in some conventional guns. The reason for this is electroplated chromium with microcracks and inclusions, resulting in a material with a modulus relatively close to that of steel. Highly contractile (HC) chromium is still the predominant coating used for erosion resistance in small-caliber guns. CHARACTERIZATION OF AS-RECEIVED MATERIAL Characterization of specimens A and B showed pervasive cracking of the chromium coating. Complete cracking of the chromium layer, as shown in Fig. 3, was prevalent. Cracks in the chromium plating are due to the relaxation of residual stresses caused by the presence of hydrides and hydrated oxides occluded during the electrodeposition process [18]. There was no evidence of an aluminum– chromium interaction at the armature footprint. The chromium oxide (Cr2O3) layer formed in the chromium surface must act as a barrier to molten aluminum attack. Cr2O3 has a melting point of 2708K, but as the oxide scale grows thicker it may eventually scale or spall off, leaving the metal unprotected [19]. Cracking of the chromium layer due to the CTE mismatch could also alter the protective Cr2O3 scale, eventually allowing an aluminum–chromium interaction. HOT-DIPPING EXPERIMENTS In the hot-dipping experiments, the reaction between the molten aluminum and the
copper substrate was favored over that between the molten aluminum and the chromium plating. The chromium layer is protected by chromium oxide, as previously discussed. It has been found that molten aluminum reduces copper oxide on contact [20]. Thus, the reaction of Al–Cu is preferred because there is no surface oxide protection. The aluminum–chromium interaction occurred at a crack surface in specimen E (exposure time 300 s). Cracking in the chromium coating due to the CTE mismatch allows the new oxide-free chromium surfaces to interact with the molten alumininum. These hot-dipping experiments showed the aggressive attack of molten aluminum on copper, but the interaction of the molten aluminum and the chromium layer, as shown by Barbier et al. [21], could not be isolated due to the exposed copper substrate. All EDS analysis of reaction products between aluminum and chromium revealed the presence of small amounts of copper, 0.95wt % or greater. Because of the proximity of the copper substrate, it was impossible to isolate the aluminum–chromium reactions from the effects of copper. DEPOSIT MELT-SPREADING EXPERIMENT The study of specimen F considered the question of melt spreading of deposits. Typically, railgun aluminum-rich deposits take the form of a thin (20mm thick) coating transferred from the armature contact surface to the cold rail. The morphology of lifted fragments, when studied by microscopy, shows a shiny side that was in contact with the rail and a dull, oxidized “air” side. This appearance is remarkably similar to that of thin metallic glass foils that are produced by planar flow casting of molten metal on a moving cold copper wheel [22]. Detailed characterization of these deposits has been reported elsewhere [5, 23]. The effect of oxide formation on the fluidity of molten aluminum has also been reported. It was found that the presence of alumina films and particles was a deciding
Chromium-Plated Copper Conductor
factor in lowering the fluidity of the melt [24]. As deposits are formed and rapidly quenched in an oxygen-containing atmosphere, the formation of aluminum oxide has been observed. These thin oxide-encased deposits are further oxidized in this experiment by heating in air, and even though the peak temperature is well above the melting point of this alloy, there is no evidence of melt flow that, upon resolidification, could wedge cracks open and facilitate multicycle crack propagation.
CONCLUSIONS As-received multispecimen characterization and two types of thermochemical experiments were conducted using sections of a chromium-plated copper bar previously used as an electromagnetic launcher rail conductor. An as-received specimen from the breech end of the rail and a specimen from the Alrich footprint of the armature were characterized. Numerous cracks and voids were found on the chromium layer of both specimens. Voids were concentrated near the Cr–Cu interface extending no more than 28mm from the copper substrate. Existent cracks propagated and new cracks developed easily in the brittle chromium layer. No molten aluminum–chromium interaction was found at the armature footprint because of the protection provided by the Cr2O3 scale. Hot-dipping experiments were conducted at high temperature (1023K). In these experiments, the molten aluminum attacked the exposed copper substrate by seeping into the cracks in the chromium coating. A molten aluminum–chromium interaction was found at one crack. Cracking due to the CTE mismatch upon heating allows oxide-free surfaces to interact with molten aluminum. The reaction between molten aluminum and copper was favored to that of molten aluminum and chromium because chromium oxide acts as a barrier to molten aluminum attack while copper oxide is easily reduced on contact by molten aluminum.
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Further heating of a rail sample that had some aluminum deposit from the armature was performed. The aluminum deposit did not spread or seep into the existing cracks. Oxide formation on the surface of the deposit prevented the spreading of the aluminum deposit, and there was no aluminum remelting. Thus, spreading and seeping of aluminum into the cracks could only occur with subsequent armature deposits and in the presence of high pressure between the armature and the rail. No molten aluminum–chromium interaction was found in this particular experiment. From the results of this investigation it has been concluded that the use of this type of electroplated chromium, as a coating for copper rail conductors, is inadequate. The presence of cracks in the chromium coating, the decrease in hardness of both the coating and the substrate upon heating, the cracking and deformation of the chromium coating due to the CTE mismatch, and the attack of molten aluminum on the copper substrate through cracks in the chromium layer are all unfavorable factors in extending the life of rail conductors.
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258 7. A. Yeoh: Evaluation of a Mo-on-Cu Explosively Bonded Bimetallic Strip for Use as an Electromagnetic Launcher Rail Conductor. Part 1: Static Armature Experiments. Report No. IAT.R 0146, Institute for Advanced Technology, Austin, TX (1997).
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Received March 1999; accepted May 1999.