For A723 steel large caliber gun barrel applications, high pressure (∼700MPa), rapid cyclic heating-cooling (∼1250°C), and aggressive propellant gases (CO, CO2, H2O, H2, N2, NH3, CH4, H2S etc) operational environment can cause severe wear and erosion damages to the bore, reducing the service life of the component. Refractory bore coatings can help protect the steel bore to extend service life. Cr electroplating process is currently being widely used in industry and in the military, but toxic wastes from the process are difficult and expensive to dispose. Plasma enhanced magnetron sputtering techniques are being developed for potential replacement of the Cr electroplating process. In this work, we compared A723 steel specimens coated with electroplated Cr and plasma enhanced magnetron sputtered Ta and Cr coatings. The specimens were subjected to analytical testing, including scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, metallography; and adhesion testing, including groove test, a pulsed laser heat test to simulate cyclic thermal environment of the bore, and actual firing test. Steel samples electroplated with HC (high contraction) Cr showed extensive pre-deposition cracks, it also showed cohesive failures under groove adhesion test. When subjected to cyclic high pressure and temperature operation, gas-metal interactions led the as-deposited cracks to grow wider and new cracks to develop, allowing hot pressurized propellant gases to penetrate the cracks and erode the steel substrate. Plasma enhanced sputtered Ta and Cr coatings on steel were harder and denser compared to conventional magnetron sputtered coatings. Under groove testing, they showed no cracks, no cohesive and no adhesive failures. When subjected to cyclic pulsed laser heating simulation at 1490°K, thick sputtered Ta showed no crack, no delamination, and no failures. A heat affected zone was observed in steel, which was caused by tempered to untempered martensite conversion. Our data also showed that Ta sputtered in Ar consisted of minimum interface meta-stable tetragonal Ta fingers, which converted to bcc Ta in the heat affected zone; Ta sputtered in Kr consisted of 100% stable bcc Ta under similar experimental conditions.
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