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 | J U L Y / A U G U S T 2 0 2 1 5 4 iTSSe TSS iTSSe TSS well as the specific process parameters. Surface roughness is considered an indirect indicator of coating density and coating process integrity and is often used as a characterization tool during process optimization. Figures 3c and 3d show the variation of coating roughness with deposition efficiency and microhardness, respectively, for a cold spray titanium coating. Another consideration, especially in applications that involve direct use of cold spray coatingswithout anymachining, is that coating roughness could play an important role in determining the reliability of both coating life and fatigue properties. ~iTSSe For more information: This article series is adapted from Chapter 5, Cold Spray—Advanced Characterization authored by Dheepa Srinivasan in “High Pressure Cold Spray—Principles and Applications,” edited by Charles M. Kay and J. Karthikeyan (ASM, 2016). Complete references are included in this volume. The author may be reached at dheepasrinivasan6@gmail. com. 8 interface of the substrate/particle or particle/particle boundaries. Inert gas fusion is also used to determine the oxygen content in the coatings. Typically, the oxygen content of the cold spray coating is largely determined by the oxygen content of the original powder. SURFACE ROUGHNESS The first step in any coating process is characterization of the surface. Roughness (Ra) is the average of the deviation from the average height of a surface profile. This can be measured using a simple handheld surface profilometer (a mechanical profiler with a spring-loaded stylus) or by more sophisticated methods such as interferometry (using an optical profiler), atomic force microscopy, or scanning tunneling microscopy. Simple to use, a profilometer run on a cold spray coating is an indirect indicator of the extent of deformation and bonding of the powder particles in the coating. In many instances, nitrogen coatings have a higher roughness than helium sprayed ones, as shown in Fig. 3b. Coating surface roughness is a combination of many factors, such as powder size distribution and morphology, as 9 FEATURE Fig. 3 — Root mean square surface roughness (Ra) measurements: (a, b) Optical micrographs showing variation in surface roughness from a helium-sprayed (4 MPa, or 580 psi, and 600°C, or 1110°F) versus nitrogen-sprayed (5 MPa, or 725 psi, and 800°C, or 1470°F) IN625 coating on AISI steel. Insets in the micrographs indicate the average surface roughness measurement; (c) roughness versus deposition efficiency (DE) of a cold spray titanium coating; and (d) microhardness versus roughness for a titanium coating. (a) (b) (c) (d)
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