Synthesis and nanoindentation study of high-velocity oxygen fuel thermal-sprayed nanocrystalline and near-nanocrystallin
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HIGH-VELOCITY oxygen fuel (HVOF) spraying, characterized by a high particle velocity and relatively low temperatures (⬃2000 K) compared to other plasma-based thermal-spray techniques, has been successfully used as a means of producing nanocrystalline coatings.[1,2] The extremely brief exposure of the precursor nanocrystalline particles to the high temperatures of the HVOF process, and the intrinsic thermal stability of the powders, are important factors that help to preserve the nanocrystalline structure in most of the particles deposited on the substrate. Thermal spraying of nanoscale materials has generated coatings with properties reportedly superior to those of micron-sized counterparts.[3–8] In a recent study, the effect of grain-size reduction on the wear resistance of electrodeposited nanocrystaline pure nickel coatings was investigated quantitatively by the Taber abrasive-wear test, a standard test often applied in industrial testing. The nanocrystalline coating (grain size of 13 nm) was approximately 100 pct more resistant to abrasive wear than the conventional polycrystalline (grain size of 90 m).[9] Depth-sensing nanoindentation techniques have been widely used for probing the mechanical response of materials L. AJDELSZTAJN, Graduate Student, K. CHUNG, Postdoctoral Student, and E.J. LAVERNIA, Professor, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. Contact e-mail: [email protected] J. LEE, formerly Postdoctoral Student, Department of Chemical and Biochemical Engineering and Materials Science, University of California, is with Samsung, Suwon, South Korea. F.L. BASTIAN, Professor, is with the Department of Metallurgy and Materials Engineering, Federal University of Rio de Janeiro, Rio de Janiero, Casixa Postal 68505, Brazil. Manuscript submitted June 11, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
such as thin films, nanomaterials, and ultrafine structures on a submicrometer scale.[10–13] A limited number of nanoindentation studies on thermal-spray coatings have been reported.[14] To date, nanoindentation has not been a popular method of determining mechanical properties of thermalspray coatings since, being relatively thick (⬎30 m), there does not seem to be any benefit for a method which relies on a shallow penetration depth (submicrometer scale) of indentation. However, in order to clearly understand the mechanical behavior of thermal-spray coatings, the submicrometer-scale mechanical properties of individual splats need to be evaluated, since splats are the very basic structural unit of thermal-spray deposits.[15] This is particularly important for the investigation of nanocrystalline coatings where, despite the fact that the basic lamellar structure is similar to that of conventional coatings, the intrinsic properties and the microstructure are quite different. In view of the previous discussion, the objective of the present study is to investigate the microstructure and properties of nanocrystallin
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