Microstructure of a High-Velocity Oxy-Fuel Thermal-Sprayed Nanostructured Coating Obtained from Milled Powder
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THERMAL spraying of milled nanostructured feedstock powders was proven to be an effective processing route for producing various metallic nanostructured coatings that often show enhancement in physical and/ or mechanical performances, compared with their conventional counterparts obtained using microcrystalline powders.[1–9] Among the various thermal spray techniques, the high-velocity oxy-fuel (HVOF) technique is considered to be the most efficient for depositing nanostructured metallic coatings, because of its combination of high flame velocity and low flame temperature that makes it easier to retain the nanostructure present in the feedstock powder. In a recent review article concerning nanostructured metallic coatings,[10] He and GANG JI, Research Fellow, and THIERRY GROSDIDIER, Professor, are with the Laboratoire d’Etude des Textures et Application aux Mate´riaux, UMR CNRS 7078, Universite´ Paul VerlaineMetz, Ile du Saulcy, 57045 Metz Cedex 01, France, and the Laboratoire d’Etudes et de Recherches sur les Mate´riaux, les Proce´de´s et les Surfaces, Universite´ de Technologie de Belfort-Montbe´liard, Site de Se´venans, 90010 Belfort Cedex, France. Contact e-mail: gang.ji@ brunel.ac.uk. GANG JI is also with the Laboratoire de Me´tallurgie Physique et Ge´nie des Mate´riaux, UMR CNRS 8517, Universite´ des Sciences et Technologies de Lille et Ecole Nationale Supe´rieure de Chimie de Lille, Cite´ Scientifique, 59655 Villeneuve d’Ascq Cedex, France, and the Brunel Centre for Advanced Solidification Technology, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom. JEAN-PAUL MORNIROLI, Professor, is with the Laboratoire de Me´tallurgie Physique et Ge´nie des Mate´riaux, UMR CNRS 8517, Universite´ des Sciences et Technologies de Lille et Ecole Nationale Supe´rieure de Chimie de Lille. Manuscript submitted October 1, 2006. Article published online August 29, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
Schoenung highlighted two major unresolved questions: (1) the origin of the nanocrystalline structure in nanostructured coatings and (2) their thermal stability. Microstructural investigations of nanostructured coatings have often revealed the presence of nanograins having a bimodal or a multiple size distribution. For example, in the analysis of Co-Cr coatings, Lau et al.[3] have found an average grain size of 21 nm together with some large grains above 80 nm. A detailed transmission electron microscopy (TEM) analysis of nanograins on cross-sectional thin foils of a nanostructured FeAl coating has recently shown that two types of nanograins are present in nanostructured metallic coatings. These are: (1) equiaxed (three-dimensional (3-D)) nanograins retained in unmelted powder particles and (2) columnar (two-dimensional (2-D)) ones that form by rapid solidification and extend along the thickness of fully melted splats.[11] In addition, it is also well established that equiaxed nanograins can be introduced in the coatings by crystallization, when both chemistry and undercooling dictate the retaining of an amor
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