Characterization and Comparison of Magnetron Sputtered and Electroplated Gun Bore Coatings

[+] Author and Article Information
Christopher P. Mulligan

 U.S. Army Armament Research, Development and Engineering Center, Benét Laboratories, Bldg 115, Watervliet, NY 12189mulligan@pica.army.mil

Stephen B. Smith, Gregory N. Vigilante

 US Army Armament Research, Development and Engineering Center, Watervliet, NY 12189

J. Pressure Vessel Technol 128(2), 240-245 (Dec 21, 2005) (6 pages) doi:10.1115/1.2172963 History: Received December 01, 2005; Revised December 21, 2005

The demands to increase range, rate of fire, and muzzle velocity have resulted in increased wear and erosion problems in gun tubes. To increase the service life of gun tubes, a number of bore-coating systems are being considered for replacement of the current electroplated high-contractile chromium coating. Two such coating systems are cylindrical magnetron sputtered (CMS) Cr coatings and CMS TaCr bilayer coatings. Cylindrical magnetron sputtering is a high-rate vacuum deposition process that has been applied to 120mm tubes. Characterization studies of the electroplated and CMS coatings were completed to determine the applicability of these coating/substrate systems for gun bore protection. Each coating system is subjected to a series of tests, including adhesion, microhardness, compositional analysis, and vented erosion-simulation testing (VES). VES testing is completed via a laboratory combustion chamber that reproduces the transient thermal and chemical environments of tank cannon firing on small chord sections of 120mm coated gun tubes. In addition to the aforementioned characterization tests, metallography, scanning electron microscopy, and energy dispersive spectroscopy are conducted on each specimen before and after VES testing to evaluate the thermal stability of the coating and the severity of the thermal damage imposed. The mechanisms of damage are investigated, including void formation and micropit growth, oxidation and erosion, and thermomechanical cracking. In addition, methods to further increase resistance to thermal damage are discussed to increase the service life of future gun tube systems.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

(a) Surface appearance of an HC–Cr–plated gun bore after 750 rounds and (b) the surface appearance after 150 benign rounds and 39 experimental higher energy rounds

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Figure 2

Schematic of the CMS process to apply Cr, Ta, and Ta∕Cr bilayer coatings

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Figure 3

Thickness profile of sputtered Ta over a 650mm segment of a 120mm tube section

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Figure 4

Metallographic image of an as-deposited electroplated HC–Cr coating

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Figure 5

Metallographic image of an as-deposited sputtered Cr coating

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Figure 6

Metallographic image of Ta sputtered directly on steel and (inset) a high-magnification image of the interface exhibiting a thin dispersed β Ta layer

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Figure 7

Metallographic image of a Ta∕Cr bi-layer coating sputtered on steel. The Cr interlayer is represented by the thin (∼10μm) white layer.

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Figure 8

Plan view images of an (a) HC–Cr sample, (b) poorly adhered Ta sample, and (c) a well-adhered Ta sample subjected to groove adhesion testing

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Figure 9

Schematic portrayal of the vented erosion simulator (VES)

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Figure 10

Illustration of the damage observed in gun firing and that exhibited in the VES

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Figure 11

Illustration of the type of failure exhibited in tantalum on steel fired in the VES

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Figure 12

SEM surface analysis of (a) HC–Cr, (b) sputtered Cr, (c) Ta∕Cr bilayer coatings after VES firing, and (d) SEM image of the resolidified Ta oxide

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Figure 13

Metallographic cross sections of (a) etched HC–Cr after 100 VES firings, (b) sputtered Cr after 50 VES firings, (c) Ta over Cr with adequate adhesion after 100 VES firings, and (d) Ta over Cr with inadequate adhesion after 50 VES firings

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Figure 14

(a) Interfacial images of a delaminated chip taken from a VES sample and (b) the mating steel surface of the delaminated chip

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Figure 15

Typical microhardness profile on a metallographic cross section. Sample illustrated is an electroplated Cr coating fired 100cycles in the VES

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Figure 16

Microhardness profiles of sputtered Cr and Ta (SP–Cr and SP–Ta, respectively) and electroplated HC–Cr prior to (a) and after (b) VES firing



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