Microstructural Refinement within a Cold-Sprayed Copper Particle
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INTRODUCTION
COLD spray has gained considerable interest in recent years as a method for rapid production of coatings and free-standing components.[1] A unique feature of the process is that deposit buildup occurs by virtue of shock-induced, solid-state deformation of microparticles during supersonic impact. In-flight particle heating is low due to the significant cooling of the jet stream as it expands through the cold spray nozzle. Finite element modeling has shown that upon impact, some material within each particle undergoes high levels of strain, but this deformation is unevenly distributed over the particle volume.[2] For example, it was demonstrated that in the 900 m/s impact of a 10-lm spherical copper particle onto a flat copper surface, the strain at a point on the interface 5 ns after the beginning of impact was 300 pct, but moving along a radial path toward the center of the particle, this dropped to ~83 pct at 1 lm from the interface, and to ~13 pct at 5 lm.[2] Furthermore, since the total deformation phase of an individual cold-sprayed particle lasts for an extremely short time (3 mm) deposits were also made by increasing the feed rate and number of passes of the gun to around 15 to 16. The thermal history of a deposit was monitored during spray using an Agema Thermovision 570 infrared camera (AGEMA Infrared Systems, Danderyd, Sweden is now part of FLIR Systems, Portland, OR). The camera used a 320 9 240 pixel uncooled microbolometer with a spectral range of 7.5 to 13 lm and measurement accuracy of ±2 pct. It was equipped with a 24 9 18 deg lens. Temperature readings were captured at a rate of 1 per every 1.4 seconds. The camera was held ~1 m from the sample at an oblique angle to avoid reflections from the nozzle coinciding with the deposit zone. Images were analyzed using the Irwin Research 2.01 software package (AGEMA Infrared Systems, Danderyd, Sweden is now part of FLIR Systems, Portland, OR). The thermal emissivity of the copper powder had been previously determined by heating a small amount on a hot plate to ~200 C and adjusting the emissivity setting on the camera to match the output temperature with a thermocouple reading of the powder temperature. The emissivity was 0.45, which was identical to that of a copper cold spray coating, measured by the same technique. The internal microstructures of both an unsprayed particle and a cold-sprayed particle adhered to the Cu surface were revealed by cross sectioning using the Ga+ beam in an xT Nova NanoLab 200 dual-focused ion beam/secondary electron microscope (FIB/SEM) (FEI Company, Hillsboro, OR). The unsprayed powder was mounted in bakelite prior to FIB/SEM. The FIB milling technique has been described in several review articles.[13,14] Grain contrast is particularly good in polycrystalline copper due to preferential etching of certain crystallographic directions by the ion beam.[15] Secondary electron imaging could be performed using the primary electron beam (conventional SEM mode) or the Ga+ beam. Foils for transmission electron microscopy (TEM)
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