Nov_Dec_AMP_Digital

iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 3 7 iTSSe TSS 7 FEATURE ARTICLE E lectron backscatter diffraction (EBSD) is a characteri- zation technique that enables determination of crystal orientations, texture, boundary misorientations and de- formation behavior, and bonding mechanism in cold spray coatings. Although transmission electron microscopy (TEM) provides adequate crystallographic information, EBSD is a microstructural crystallographic characterization technique performed by means of a scanning electron microscope ( SEM) that is able to examine large areas of samples and provide sta- tistically significant data along with image quality. Especially in the case of cold spray coatings with a nonuniform deforma- tion, this technique is highly suitable for providing valuable information on the nature of bonding. Pattern quality maps obtained by the Kikuchi pattern qualities show local defect density and lattice strain. Sample preparation for EBSD in- volves metallography of the sample to yield a highly polished flat surface. Typical SEM accelerating voltages of 20 kV are used with step sizes of 50 to 500 nm to scan the coating with different grain sizes. Euler angle maps indicate locations with differing orientation by different colors (Fig. 1). Figure 1 (b) is a typical EBSD map showing the grain or crystallite size from aluminum feedstock powders to assess the initial crystallite size, taken from the work of Zou. In the EBSD map, each point is colored according to its crystal ori- entation. In this case, red corresponds to the [001] direction, blue to [111], and green to [101]. Characterization of the ther- momechanical behavior of cold spray coatings has been car- ried out extensively using EBSD. Electron backscatter diffraction, in conjunction with field-emission gun SEM, provides insights into large areas of samples to elucidate the particle bonding, porosity, and other qualities. As such, it provides statistically significant COLD SPRAY: ADVANCED CHARACTERIZATION METHODS—ELECTRON BACKSCATTER DIFFRACTION This article series explores the indispensable role of characterization in the development of cold spray coatings and illustrates some of the common processes used during coatings development. Dheepa Srinivasan, GE Power, GE India Technology Center, Bangalore Fig. 1 — (a) Scanning electron micrograph and (b) EBSDmap of feedstock aluminumpowder particles showing polycrystalline powder; (c) scanning electron micrograph of top surface of cold spray coating; (d) coating cross section; (e) EBSDmap of top surface; (f) grain size distribution.