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A.J. Melmed and T.A. Lusby Materials Science and Engineering Department, Johns Hopkins University, Baltimore, Maryland 21218-2689

R. Schweinfest and M. Ru¨hle Max-Planck-Institute fu¨r Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany

E.J. Lavernia Department of Chemical Engineering and Materials Science, University of California, Irvine, Irvine, California 92697-2575 (Received 22 January 2001; accepted 12 February 2002)

Gas-atomized Ni powders were mechanically milled under liquid nitrogen and subsequently sprayed using a high-velocity oxygen-fuel thermal spraying system. The resultant coatings were evaluated using various analytical methods, including scanning electron microscopy (SEM), transmission electron microscopy, and atom probe field ion microscopy (APFIM). The results indicated that the majority of the grains in nanocrystalline Ni coating were equiaxed with an average grain size of 50 ± 23 nm. SEM elemental dot mapping analysis suggested that oxide particles were distributed along the prior droplet boundaries and interpass boundaries in the coatings. The APFIM analysis indicated that the distribution of oxygen was very inhomogeneous in the nanocrystalline coating samples studied. Mostly small NiO precipitates with a size range of 1–7 nm were found throughout the grains, and the average size of these precipitates was approximately 4 nm. However, large oxide precipitates with a size up to 55 nm were also observed. In addition, increased amounts of oxygen were found at the grain boundaries and at structural defects. I. INTRODUCTION

Significant advancements have been made in the synthesis of nanocrystalline powders with controllable chemical homogeneity. Several methods have been scaled up for commercial use.1,2 As a result, the application of nanocrystalline materials has diversified into various fields. For instance, thermal spraying of nanoscale materials has generated coatings with properties reportedly superior to those of the micron-sized counterparts.3–11 In a recent study, nanocrystalline Ni powders prepared by mechanical milling in methanol for 10 h were sprayed by high-velocity oxygen-fuel (HVOF) spraying to produce nanocrystalline coatings.12 In this study, the coating exhibited a 20% increase in microhardness despite a higher volume fraction of porosity as compared to the coatings produced using conventional inert-gas-atomized powders.12 Amongst the various techniques available, cryomilling offers the advantage of changing the deformation kinetics of the powders by minimizing heat generation and favoring fracturing over welding of the particles.13 The technique was first employed by Luton et al. in the mechanical alloying of J. Mater. Res., Vol. 17, No. 5, May 2002

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aluminum and dilute aluminum alloys.14 The alloyed powders synthesized by this technique were strengthened by aluminum oxynitride particles (2–10 nm in diameter with a mean spacing of 50–100 nm).14 Whittenberger et al. employed cryomilling to disperse Y2O3 in prealloy