November AMP_Digital

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 9 4 2 iTSSe TSS iTSSe TSS FEATURE COLD SPRAY: ADVANCED CHARACTERIZATION METHODS—X-RAY PHOTOELECTRON SPECTROSCOPY, X-RAY FLUORESCENCE, AND AUGER ELECTRON SPECTROSCOPY 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 X -ray photoelectron spectroscopy (XPS) is a technique used to determine the elemental and chemical state on the surface of the coating. In XPS, soft x-rays bombard a sample material, causing electrons to be ejected. Based on the kinetic energy of the ejected photoelectrons, the chemical state of the sample can be determined, and the relative con- centration of the elements can be discerned from the relative photoelectron intensities. XPS typically probes to a depth of two to 20 atomic layers and, depending on the material, can be used to probe depths of 5 to 50 Å. This technique serves as a valuable tool in cold spray characterization to determine the oxidation or nitridation state of the coatings. In general, a higher inlet gas temperature and therefore a higher substrate temperature is found to be beneficial for cold-sprayed coatings. However, when trying to achieve these conditions in practice, excessive oxidation and nitridation of the resultant coatings can occur. XPS is used to accurately determine the temperature effect on the coatings. Figure 1 illustrates this application for a commercially pure titanium coating, showing the effect of substrate temperature. The in- fluence of particle temperature can also be determined by XPS when an external gas heater is used to heat the particles. XPS is also a useful technique for determining the surface contami- nation in cold-sprayed coatings. X-ray fluorescence (XRF) is a nondestructive method used for elemental analysis of materials. This technique is used for cold- sprayed feedstock powders to provide accurate estimates of the chemistry, especially the oxygen and nitrogen levels, because coat- ing chemistry is known to affect mechanical properties. With this method, an x-ray source is used to irradiate the specimen and cause the elements in the coating pow- ders to emit or fluoresce their char- acteristic x-rays. A detector is used to detect the emitted x-ray peaks for quantitative estimation of the elements present. All elements except hydrogen, helium, and lith- ium can be detected using XRF. It is possible to determine accurate val- ues of elemental information using this method. The impurity levels and the moisture/oxygen content in the powders can lead to significant- Fig. 1 — Comparison of surface characteristics of helium-sprayed versus nitrogen-sprayed titanium coatings for oxidation and nitridation characteristics. 8