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.
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 GOODOLD, GOODNEW and BADNEW. The GOODOLD samples had already been subjected to constant tribological load throughout its service life, whereas the GOODNEW and BADNEW samples had not been worn before the analysis. The GOODOLD sample was analyzed to determine whether its friction stability behavior had been affected negatively because of aging under load.
The GOODNEW sample was found to exhibit good frictional behavior under tribological test. Conversely, the BADNEW 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 GOODOLD and GOODNEW 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 GOODOLD sample was found to be much higher than the BADNEW and GOODNEW samples. However, the total carbon content was very low on the surface of the GOODOLD 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 GOODOLD sample.
Peak fitting corroborates that a mixture of carbonate and C=O was present on the surface of the GOODNEW sample, whereas carbonate was almost absent in the case of the BADNEW sample.
Figure 5 presents the XPS imaging of the GOODOLD 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 GOODNEW sample, with a thickness slightly more than for the GOODOLD sample.
Figure 5. XPS imaging measurements of carbonate film thickness and surface composition
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 GOODOLD and GOODNEW.
Their XPS imaging revealed the presence of calcium carbonate tracks on their surfaces. The BADNEW 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 BADNEW 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 – Materials & Structural Analysis.
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