Surface Engineering by Expanded Austenite
X-Ray Photoelectron Spectrscopy (XPS)
X-ray photoelectron spectroscopy (XPS) is a technique used for material surface analysis. It also has the capabilities of analysing the layers of material immediately adjacent to the surface when used in conjunction with sputtering. But this technique is only accurate up to a depth of approximately 12nm.
To explain the concept of XPS, further knowledge is required regarding:
Photoelectric Effect
During this investigation XPS was used to quantify the element composition on the surface and immediately below it to quantify how much carbon had been diffused into the matrix. XPS also has the ability to identify the chemical state of the elements. Due to time limitations opposed on using the machinery, only select samples could be investigated. Therefore it was decided that a one iron based sample and one K33 treated nickel based sample be used for the investigations.
The principal behind XPS involves irradiating a surface with an X-ray beam (in this case monochromatic K-α X-rays) whilst simultaneously measuring the kinetic energy and the electrons being emitted from the surface. The energy from the x-ray acts to ‘excite’ the electron, causing it become unbound from the atom, thus making the atom ionised. In order to return to its ground state, the atom emits a photon of a certain characteristic energy. This is known as the photoelectric effect. It is this energy which is measured by the XPS. All of which takes place within a vacuum. This process is carried out over a small surface area of 3mm x 3mm and an elemental composition quantification it calculated from this area.
By measuring this energy and knowing the energy of the x-ray beam (being 1486.6eV for the K-α beam) an energy balance can be formed:
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KE = Emitted Kinetic Energy
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hv = Energy of X-ray photons incident upon surface
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Eb = Binding energy of the electrons
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= Known work function dependent of the instrumentation
From this the binding energy of the electrons can be calculated. The binding energys are plotted as a spectrum of peaks showing intensity and energy, with each element having a known set of peaks at certain energy values. So by comparing the position and intensity of these emitted peaks to the known characteristics of elements, the composition of the material can be determined. An example of typical XPS spectrum is shown to the right of the text.
Binding Energy
The binding energy of the atom is not only dependant on the element, but also on which energy level the electron is released from. As can be seen from the adjacent image, electrons are arranged in a series of energy levels (shells) surrounding the nucleus within an atom, with the space between each level denoted by letters. These areas are known as orbitals.
The closer the electron is to the nucleus, the lower the binding energy of the electron. Electrons can be emitted from any of these energy levels, therefore this is the reason that spectrums of single elements can contain multiple peaks. For the purpose of determining the peaks, normally the peak with the highest intensity is examined. However this peak is not always easily identifiable due to the over-lapping of peaks from other elements. When this is the case, the smaller peaks should be examined. This over-lapping tends to occur due to the presence of Augur electrons.
Auger Effect
The Auger effect is one which makes the analysis of XPS spectrums more complicated. It is the phenomena by which an electron vacancy at a lower energy level (closer to the nucleus) is filled by an electron from a higher energy shell. The excess energy is either released as a photon, or transferred to another electron which is ejected from the atom. This electron is known as an auger electron. The process is illustrated in the figure below.
The kinetic energy of this emitted electron is picked up the XPS receiver, resulting in additional peaks in the spectrum. These peaks are often more undefined than the emission of standard electrons and therefore can cause difficulties in determining element compositions from the spectrum.
Sputtering
To gather further information regarding the depth profile of the elemental compositions, the process of sputtering is used. Sputtering is whereby particles are removed from a surface due to the bombardment by much higher energy particles. This creates a small crater in the surface of the material, without causing any changes or damage to the microstructure which allows an elemental composition depth profile to be obtained.
The sputtering was induced by a 40kV argon beam, producing an etch rate of 5.26nm/min.
To create the depth profile, the sample of the surface is subject to sputtering for a selected period of time, followed by XPS measurements. The element composition is noted and the process is repeated at a number of depths until a satisfactory depth has been reached.
For this investigation measurement were taken at depths of approximately 2.63nm, 5.26nm, 15.78nm, 26.3nm, 42.08nm, 57.86nm, 78.9nm, 105.2nm and 157.8nm
