Wear-resistant amorphous and nanocomposite steel coatings
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INTRODUCTION
RECENTLY, a significant effort has been directed at developing nanoscale structures in coatings produced by thermal deposition techniques. Two approaches have been commonly used; the first approach is to start with heavily deformed, mechanically processed micron-sized powder that has a nanoscale structure,[1–7] and the second approach relies on directly spraying either nanosized powders[8] or nanopowder precursors.[9,10,11] Both approaches rely on maintaining the existing nanoscale structure during spraying and subsequent impact on the substrate. In the spraying of mechanically processed micron-sized powders with nanoscale structures, complete melting of the powder must be avoided,[5,7] and the elevated-temperature exposure of the particles must be sufficiently short so that grain growth and coarsening do not occur to a significant extent,[6] or the desired nanostructure will be lost. The second approach, the direct spraying of nanophase powder, has its own set of limitations associated with the process. The primary issue is the introduction of nanosized powders into the high-velocity thermal spray jet and their impingement on the substrate. Nanosized powders tend to agglomerate, resulting in plugged particle feed lines, and the extremely small particles do not readily penetrate the jet and, hence, are not exposed to the temperatures required for melting. The other issue is that nanosized powders readily follow the gas streamlines and do not impinge on the substrate to be coated,[9] which makes it difficult to form a coating with high deposition efficiency. This problem can be circumvented by agglomeration of nanosized particles with a binder, especially in cases where the binder is a lower-melting-point material such as Co, used D.J. BRANAGAN, Staff Scientist, W.D. SWANK, Advisory Scientist, D.C. HAGGARD, Principal Technical Specialist, and J.R. FINCKE, Science Fellow, are with the Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID 83415-2218. Manuscript submitted April 26, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
in the Co-WC cermet system.[9] Unfortunately, this approach is useful for only a limited number of material systems. Another methodology that has been used is to introduce the material as a liquid or gaseous precursor, which reacts inflight to form nanosized particles.[10,11,12] This approach is promising; however, it suffers from low deposition efficiency, as the in-flight synthesized powder tends to follow the gas-flow streamlines when performed at atmospheric pressure. This limitation can be overcome by operating at extremely high velocity and reduced pressure.[12] Under these conditions, the aerodynamic drag force on the particle is reduced and the rate of impact on the substrate is greatly increased. The disadvantage is the increased cost and complication of operating at reduced pressure in a controlled-atmosphere environment. The deposition of coatings from liquid or gaseous precursors has been demonstrated for a limited number of oxide and carbide system
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