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 | J U L Y / A U G U S T 2 0 2 1 5 3 iTSSe TSS COLD SPRAY: ADVANCED CHARACTERIZATION METHODS—RAMAN SPECTROSCOPY, OXYGEN ANALYSIS, AND SURFACE ROUGHNESS MEASUREMENT 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 coating development. Dheepa Srinivasan Raman spectroscopy is a structural characterization tool used to evaluate the arrangement of bonding in a material. An intense monochromatic light beam is applied to the sample, resulting in the electrical field of the incident radiation distorting the electron clouds that make up the chemical bonds in the sample, which have a particular energy. On reversal of the field, the wave passes, the distorted electron clouds relax, and the stored energy is reradiated. Most of the stored energy is reradiated at the same frequency as the incident exciting light; this phenomenon is called Rayleigh scattering. However, a small portion is transferred to the sample itself, exciting vibrational modes that appear as weak side bands in the spectrum at frequencies less than the incident beam. These are the Raman lines characteristic of the material. Kliemann et al. carried out micro-Raman spectroscopy to detect phase transitions in ceramic TiO2 coatings. Figure 1 shows the intensities obtained from a cold spray coating of TiO2 starting with an anatase powder. The presence of both rutile and anatase forms of TiO2 in the starting powder is clearly established, the two forms being distinguished by their individual patterns in the Raman spectrum. However, the 100% anatase structure is evident in the cold spray coating, regardless of gas temperature. OXYGEN ANALYSIS One of the key features that distinguishes cold spray from thermal spray is the lack of oxidation during the spray process. However, if the starting powders contain oxygen, then it becomes difficult to avoid subsequent oxidation in the coating. It is therefore important to characterize the oxygen content in the coating—starting with the powders—prior to any spraying. A LECO analyzer is specifically designed to estimate very minute (5 to 10 ppm) traces of oxygen, nitrogen, and hydrogen in samples. It is imperative to characterize for oxygen levels, especially withmaterials and alloys prone to oxygen pickup such as titaniumalloys. Because titaniumcan dissolve up to 13wt% oxygen, it is important to know howmuch oxygen exists in the coating in order to interpret the mechanical properties as well as bonding characteristics with the substrate. Figure 2 shows an example of the measured oxygen levels in both warm and cold spray coatings, using the inert gas fusionmethod. Other practitioners have detected the oxygen using high-resolution energy-filtered transmission electron microscopy and have found that most of the oxygen exists near the 8 FEATURE Fig. 1 — Raman shift of TiO2 coating sprayed using nitrogen at 4 MPa (580 psi) compared with Raman spectra of rutile and anatase. Fig. 2 — LECO analysis of oxygen and nitrogen at different cold spray gas temperatures in titanium and Ti-64. JTST HIGHLIGHTS FEATURE 7
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