Using XPS to Evaluate the Elemental and Chemical Composition of Wear Resistant Coatings

The role of additives is crucial in lubricating oils as it creates friction-modifying layers on metal surfaces. Wear resistant layers are deposited on steel surfaces using overbased calcium sulfonate additives.

Using zinc dialkyldiphosphonate (ZDDP), a protective, glassy phosphonate coating is formed on surfaces that are subjected to tribological load. The distribution of the layers created on surfaces under tribological load is inhomogeneous across the sample.

Figure 1 presents optical images of worn surfaces, showing darker and brighter tracks (50–100µm wide) due to non-uniform processes while performing tribological testing.

Figure 1. Optical images taken by the instrument of the analyzed surfaces

The surface area of the samples analyzed in in tribological testing may be much larger than the size of a typical, superior quality X-ray spot (30–400µm).

Therefore, for the characterization of the wear tracks, it is necessary to have an experimental protocol comprising both micro and macro analyses.

The various chemical bonding states within narrow interesting features can be detected and differentiated using small spot X-ray analysis, whereas their distribution across the surface can be explored using the large area mapping.

Instrumentation

Thermo Scientific K-Alpha (Figure 2) has superior imaging capability, chemical selectivity and surface sensitivity, which make them an ideal analytical instrument to analyze the elemental and chemical composition of wear resistant coatings.

The K-Alpha is designed to perform highly sensitive direct analysis even for small spot analysis of anti-wear coatings that have magnetizable substrates like steel or are electrically insulating.

This article describes the application of this integrated XPS tool to correlate the elemental and chemical composition of wear resistant coatings with their behavior.

Figure 2. The Thermo Scientific K-Alpha

Experimental and Results

This experiment analyzed three steel samples for friction stability, namely GOOD_OLD, GOOD_NEW and BAD_NEW. The GOOD_OLD samples had already been subjected to constant tribological load throughout its service life, whereas the GOOD_NEW and BAD_NEW samples had not been worn before the analysis. The GOOD_OLD sample was analyzed to determine whether its friction stability behavior had been affected negatively because of aging under load.

The GOOD_NEW sample was found to exhibit good frictional behavior under tribological test. Conversely, the BAD_NEW sample exhibited abnormal friction properties. In this experiment, these different tribological characteristics of the samples were correlated with their surface composition and chemistry using the K-Alpha XPS.

The elements present on each sample surface were identified and quantified by acquiring survey spectra over wide areas on each sample (Figure 3). The samples had very small amount of zinc after tribological testing (Table 1) albeit the presence of a high level of ZDDP additive in the oil.

Conversely, the level of calcium was high in the GOOD_OLD and GOOD_NEW samples. The overbased detergents that had been added in various ratios along with the ZDDP into the oil were the source of calcium.

Figure 3. Elemental identification of the elements on the steel surfaces

Table 1. Elemental surface quantification of samples

The level of calcium on the surface of the GOOD_OLD sample was found to be much higher than the BAD_NEW and GOOD_NEW samples. However, the total carbon content was very low on the surface of the GOOD_OLD sample. Hence, the carbon chemistry on each sample was thoroughly analyzed.

As shown in Figure 4, high-energy resolution carbon spectra were acquired to analyze the chemical bonding states of carbon on each of the sample surface. Four chemical states were revealed, namely organically derived C-C, C-O and C=O, and inorganic carbonate.

Figure 4. Chemometric analysis of carbon

The data obtained was fitted with a series of synthetic Lorentzian-Gaussian peakshapes to perform a thorough analysis of the carbon chemistry. Consequently, the chemical states on the sample surfaces were fully quantified. The carbonate and organic carbon content was much higher on the surface of the GOOD_OLD sample.

Peak fitting corroborates that a mixture of carbonate and C=O was present on the surface of the GOOD_NEW sample, whereas carbonate was almost absent in the case of the BAD_NEW sample.

Figure 5 presents the XPS imaging of the GOOD_OLD sample, showing well-defined wear tracks with alternating regions of low and high carbonate. These areas were in correlation with the high and low calcium concentrations, revealing the formation of the calcium carbonate in the tracks.

The carbonate film thickness at each point in the map was simultaneously measured with the composition. The thickness of the film in the carbonate tracks can be as high as 87Å. Thick calcium carbonate tracks were observed in the XPS imaging of the GOOD_NEW sample, with a thickness slightly more than for the GOOD_OLD sample.

Figure 5. XPS imaging measurements of carbonate film thickness and surface composition

Conclusion

The results clearly demonstrate the comprehensive characterization of three coated steel samples using the Thermo Scientific K-Alpha. Good friction stability properties were observed for two of the samples, labeled GOOD_OLD and GOOD_NEW.

Their XPS imaging revealed the presence of calcium carbonate tracks on their surfaces. The BAD_NEW showed poorer friction stability properties and virtually no calcium carbonate was observed on its surface.

From the K-Alpha XPS analysis, it can be concluded that the appropriate ratio of ZDDP to calcium detergent will lead to the generation of calcium carbonate during tribological load, and this carbonate imparts good friction stability properties to the surfaces.

The absence of calcium carbonate on the surface of the BAD_NEW sample indicates that the ratio of ZDDP to detergent in the oil formulation is inappropriate.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).

For more information on this source, please visit Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).