Focused Ion Beam Study of Ni5Al Single Splat Microstructure
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0983-LL04-04-KK04-04
Focused Ion Beam Study of Ni5Al Single Splat Microstructure Yuhong Wu1, Meng Qu1, Lucille Gianuzzi2, Sanjay Sampath1, and Andrew Gouldstone1 1 Materials Science and Engineering, SUNY Stony Brook, 130 Heavy Engineering, SUNY Stony Brook, Stony Brook, NY, 11794-2275 2 FEI Corp, HIllsboro, OR, 97124 ABSTRACT
Thermally sprayed (TS) coatings are widely used for surface engineering across a range of industries, including aerospace, infrastructure and biomedical. TS materials are formed via the successive impingement, rapid quenching and build-up of molten powder particles on a substrate. The impacted ‘splats’ are thus the fundamental microstructural constituents of the coatings, and their intrinsic properties, as well as intersplat bonding and morphology, dictate coating behavior. Beyond the obvious practical considerations, from a scientific standpoint, splats represent a fascinating template for study, due to the highly non-equilibrium processing conditions (rapid deceleration from sub-sonic velocities, million-degree/sec cooling rates). In the literature, many studies of isolated splats on substrates have been carried out, but these have focused on overall morphology (disc-shape vs fragmented). Direct observations of microstructure, in particular cross-section, are limited in the specimen preparation stage due to splat size (tens of microns in diameter, 1-2 microns in thickness). However, Focused Ion Beam (FIB) techniques have allowed this problem to be addressed in a robust manner; in this paper we will discuss such approaches to observe Ni5Al splats on stainless steel substrates. Cross-sections through the splat and the substrate were created by recourse to ion milling and the ion beam itself provided good channeling contrast for grain imaging. The typical splat microstructure with submicron Ni(Al) columnar grains, a chill zone at the bottom and a lift off area is observed in high detail. In addition, an amorphous aluminum oxide top layer of 100-200 nm is partially present on top of the Ni(Al) columnar grains. At the splat/substrate interface, defects such as micro- and nano-scale pores were characterized for the first time and will be discussed. These observations provide insights into splat and interface formation during the deposition process and may drastically improve our current understanding of Ni5Al splat properties.
INTRODUCTION
Thermal spray (TS) has been extensively used to deposit coatings in various engineering applications for protection against high temperature, wear, oxidation and corrosion [1]. In addition, TS-derived materials and coatings are now being explored for functional thick-film sensors [2]. The tremendous industrial interest in passive coatings, as well as the emerging interest in active structures necessitates a fundamental understanding of the process-structureperformance of these materials. TS coatings are formed by accelerating molten droplets of metals or ceramics in a hot plasma flame toward a substrate, where they impinge and build up. Typical feedstock powd
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