February/March_AMP

FEATURE ARTICLE

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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 | F E B R U A R Y / M A R C H

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BE imaging reveals atomic number contrast to illustrate coat-

ing phases. Spray angle and deposition efficiency effects are

analyzed using SEM microstructures of etched samples. For

example, nozzle erosion sometimes occurs during cold spray,

especially for hard materials such as MCrAlY or Inconel pow-

ders. The erodedmaterial could end up embedded in the coat-

ing and distinguished as an artifact.

SEM is used extensively to characterize coating chem-

istry via EDS. Figures 3 (a) to (d) show SEM micrographs of

IN625 feedstock powders and a cold spray coating, along with

the corresponding energy-dispersive x-ray analysis pattern

from the powder, and coating chemistry. Elemental mapping

is often done on powders prior to spray in order to evaluate

the level of segregation in the chemistry, as shown in Fig. 4

(a) to (d) on two types of MCrAlY powders. Coating porosity

characterization provides feedback to achieve optimum thick-

ness for a dense coating (Fig. 5) such as titanium. Coating po-

rosity evolution with different feedstock powders and powder

surface topography is also shown in Fig. 5.

Coating thickness and interface contour can be char-

acterized in a manner that makes SEM an indispensable and

versatile tool for evaluating coating integrity in the as-sprayed

condition, as well as in developing an understanding of bond-

ing to the substrate. Substrate hardness helps achieve good

coating deposition. Figures 6 (a) and (b) show the interface of a

Fig. 3 —

SEMs show: (a) IN625 powder morphology and (b) as-

sprayed coating, and (c) energy-dispersive x-ray analysis pattern

and (d) listing of the elements from the spectrum in the coating.

Fig. 4 —

Energy-dispersive x-ray analysis mapping along with

SEMs taken from an MCrAlY cold spray coating illustrate elemental

segregation in the powders: (a, b) Powder type A and (c, d) Powder

type B.

Fig. 5 —

SEMs show powder morphology and cross-sectional

structure from two different feedstocks of WC-Co powder.

Fig. 6 —

SEMs from a NiCr cold spray coating (using helium gas)

on (a) AISI 4130 steel, (b) aluminum substrate, and (c) coating-sub-

strate interface after heat treatment, revealing an interdiffusion

zone; (d) composition across the interface taken by EDS.