XPS Is a Successful Technique for Metal Surface Chemical Imaging
Surface analysis techniques are routinely applied to a broad range of materials, such as aluminum and stainless steel used in the manufacture of high technology products. Surface analysis studies aid in the examination of how one material will interact with another. The surface finish of a material also plays a critical role in properties such as chemical activity, adhesion, wettability, electrostatic behavior, corrosion resistance, bio-compatibility, and other factors which determine how the material will perform in various applications including medical implants, electronics, semiconductor devices, and magnetic media.
High resolution imaging tools are commonly used to identify surface contaminants that can influence a material’s performance. This information is used to pinpoint manufacturing or process failures that contribute to material surface impurity, take corrective action, and put processes in place for consistent operations and more reliable results.
While bulk analytical techniques may miss low levels of surface contamination, X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is ideally suited for surface analysis because it has a high degree of surface sensitivity.
The XPS technique identifies and quantifies the elements present on a surface and their chemical state. XPS works by irradiating the surface of a material with x-rays and detecting the electrons that are ejected from the surface. This is known as the photoelectric effect, discovered by Hertz in the 19th century, and explained by Einstein in a 1905 paper. The photoelectrons have a unique kinetic energy, which is related to the characteristic binding energy of the element, as well as the orbital and chemical environment of the atom. Because of the strong interaction of electrons with solid materials, only electrons generated near the surface, from less than 10 nm into the sample, can escape without losing too much energy. By measuring the kinetic energy using XPS, we can learn a great deal about a material’s surface.
A recent webinar demonstrated how XPS can be used to identify elemental and chemical composition of discolored areas on an aluminum assembly, along with a mapping experiment to look at the distribution of material across the surface. Four distinct areas were found using XPS, three of which could be seen optically. XPS allowed an image of these elements to be collected, and their distribution across the surface to be determined.
Aluminum, the second most plentiful metallic element on earth, is usually found in the oxide form known as bauxite. Noted for its longevity as a viable material and its structural integrity, it is the most widely used non-ferrous metal. Pure aluminum is used mostly as cladding material on aluminum alloys or as protecting coating for other metals.
Aluminum is actually very prone to corrosion. However, aluminum corrosion is aluminum oxide, a very hard material that actually protects it from further corrosion. Aluminum oxide corrosion also looks a lot more like aluminum (dull gray to powdery white in color), so it isn’t as easy to detect as rusted iron. Since XPS provides information on oxides and chemical states of materials, it is therefore highly useful in the analysis of aluminum surfaces and their interface to other materials and environments.
To see complete test data, including the survey spectra, distribution maps, methodology, and comments, see K-Alpha: A New Concept in XPS.
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