The signal is then sent to a multichannel analyzer where the pulses are sorted by voltage. The energy, as determined from the voltage measurement, for each incident x-ray is sent to a computer for display and further data evaluation. The spectrum of x-ray energy versus counts is evaluated to determine the elemental composition of the sampled volume. Qualitative Analysis - The sample x-ray energy values from the EDS spectrum are compared with known characteristic x-ray energy values to determine the presence of an element in the sample.
Elements with atomic numbers ranging from that of beryllium to uranium can be detected. The minimum detection limits vary from approximately 0.
Quantitative Analysis - Quantitative results can be obtained from the relative x-ray counts at the characteristic energy levels for the sample constituents. Semi-quantitative results are readily available without standards by using mathematical corrections based on the analysis parameters and the sample composition.
The accuracy of standardless analysis depends on the sample composition. Greater accuracy is obtained using known standards with similar structure and composition to that of the unknown sample.
Elemental Mapping - Characteristic x-ray intensity is measured relative to lateral position on the sample. Among various coatings, amorphous diamond-like carbon DLC is considered by various authors as a good candidate for application on elastomer surfaces [5]. Such coatings have excellent tribological behaviour, i.
In general, it is employed in applications which demand a material with good mechanical properties and with a retained elastic nature [7]. Also it has a good resistance to degradation at elevated temperature, sunlight, in oxygen and, in particular, ozone [12]. Acrylonitrile-butadiene rubbers NBR and hydrogenated acrylonitrile-butadiene rubbers HNBR form another widely used family of elastomers. These elastomers are extensively employed in automotive industry, especially for lip seals, due to their moderate cost, excellent resistance to oils, fuels and greases, processability and very good resistance to swelling by aliphatic hydrocarbons [13].
Our study is focused on characterization of surface chemical composition of different elastomers subject to rubbing, surface modification and application of coatings. The main technique used for this study was X-ray photoelectron spectroscopy XPS , which is a very powerful technique for characterizing the chemical composition of very thin few nm surface layers.
XPS is particularly useful when analysing elastomers, as it provides information about the chemical environment of the elements, i. Thus, XPS is well suited for investigation the changes in binding energy of chemical elements situated within the first tens of nanometres of the material surface [4]. Elastomers are typically composed of carbon, hydrogen, oxygen and nitrogen. Their surface and bulk properties depend on the way these elements are combined rather than on the presence of other chemical elements.
XPS allows detection of new functional groups [4] and evaluation the variation in the amount of existing functional groups, e. C-O, as function of surface tailoring, ageing [1], or rubbing [14].
However, often it can be difficult to distinguish between different functional groups having similar binding energies. Therefore, in many cases some complementary techniques should be used to elucidate chemical features of elastomer surfaces.
One of these complementary techniques consists in measuring of contact angles CA of sessile drops of various liquids placed on the elastomer surface. This very simple method provides valuable information on the types of surface groups [15]. In particular, by using water, presence of polar groups, e.
C-O, can be determined. So, the degree of surface activation due to surface modification can be determined from measurements of surface hydrophobicity [16].
Then, more information on the surface chemistry and Surface Free Energy SFE can be obtained from measuring CA of various liquids with different characteristics. In the following sections we present some fundamental aspects of these techniques and case studies of elastomer surfaces.
XPS for characterization of elastomer surfaces 2. Introduction to the XPS technique XPS is an analytical technique that has its fundamental origin in the photoelectric effect, which was first explained by Einstein in [17]. A schematic drawing of typical XPS measurement device is shown in Figure 1a. The measurements are performed in ultrahigh vacuum UHV in order to control the surface cleanliness and to reduce the electron scattering on gas molecules.
To provide a beam of photons with given characteristics the device is equipped with an X-ray source focused on the sample surface. The photoelectrons emitted from the sample material at characteristic energies are analysed by a suitable electron analyser. The origin of the binding energies is related with Fermi level, Ef, whereas the kinetic energies are referenced to the vacuum level.
By measuring the electron kinetic energies and knowing the spectrometer work function, it is possible to determine the binding energies of various inner levels or core electrons , as well as those of the outer or valence electrons involved in chemical bonding. A typical photoemission PE spectrum, i. PE yield vs. The spectrum consists of a series of peaks on a background signal which generally increases at low kinetic energy due to secondary electrons, i. In summary, the XPS spectra consist of peaks at discrete kinetic energies corresponding to atomic core levels CLs and Auger transitions.
Note that each element has a unique elemental spectrum. Quantitative information can be derived from the peaks areas, whereas chemical states can often be identified from the exact positions of the peaks and separations between them.
The presence of chemical bonding causes binding energy shifts, which can be used to infer the chemical nature such as atomic oxidation state from the sample surface. Here, we limit ourselves to study elastomer samples. A complete description of XPS technique can be found in specialized literature [20, 21]. Advantages and shortcomings of XPS technique for characterization of elastomer surfaces.
Operating conditions, measurements and semi-quantitative analysis The standard XPS measurements are carried out under vacuum conditions by retarding- fields techniques. The X-ray lines from these sources are narrow less than 0. Initially, a survey scan or wide energy range scan, typically from to 0 eV, should be obtained in order to identify the elements present on the surface.
As each element emits electrons at characteristic energies, it is possible to identify all the elements present in the sample surface, except hydrogen and helium which are not detectable by this technique. Elastomers usually contain a small number of elements, of which the most common are C, O, N, F and Cl. Other elements like sulphur and zinc can be detected in small quantities.
Sulphur is a typical curing agent, whereas zinc is usually employed as a curing activator [22]. In most of the cases, these elements will not be taken into account as they have no real influence on the surfaces properties. Normally, the elements are uniformly distributed in the bulk; however, under certain circumstances surface segregation may take place.
It should be stressed that XPS is a semi-quantitative technique. In order to quantify the amount of each element the integrated area of a particular peak should be divided by the corresponding relative sensitivity factor. The denominator corresponds to the atomic fraction of other elements in the sample. Assuming a homogeneous distribution of elements, a strong line for each element in the spectrum should be analyzed.
In case the requirement of homogeneity is not fulfilled, the assumption of homogeneity can be used as a starting point for further calculations. Reference published data on elemental sensitivity factors could be used for determination of S, although the type of instrument and analysis conditions should be considered.
With this technique it is also possible to identify chemical states of a given element by measuring the high resolution or core level peaks. In the first one, ion sputtering is used to remove surface layers. Sputtering and XPS can be applied consecutively or simultaneously. In the non-destructive mode, depth profiling is obtained by varying the detection angle of the emitted electrons. More detailed information about both methods can be found elsewhere [20].
Another important problem in XPS analysis is related with sample degradation due to X-ray radiation. In fact, this degradation comes from the secondary electrons emitted during the X-ray exposure [23, 24]. In most of the cases this degradation is slow enough as compared with time required for XPS analysis, thus the changes in composition due to X-ray can be neglected. Notwithstanding, this problem should be considered when analysing chemically unstable materials.
In our work, no sample degradation due to X-ray radiation has been observed for all studied elastomers. To avoid X- ray damage on the samples low X-ray power of W was used. The core level narrow spectra were recorded using pass energy of 15 eV. The binding energy, EB, scale was calibrated with respect to the C 1s core level peak at eV. The surface area subject to XPS analysis was around 5. When modified samples were analyzed, the surface area subject to XPS analysis was smaller than the treated area, thus the contribution from untreated surfaces was negligible.
The shape of C 1s core level peak HR C1s measured with high energy resolution was analysed using peak fitting in order to identify functional groups.
Depending on the chemical environment of the carbon atoms, important chemical shift of C 1s peak can be observed. Decomposition of the experimental peak in components allows identification of the contribution from each component. For the analysis of HR C1s, the spectrum recorded from the untreated sample was used as a reference. The only remaining free parameter in the fit procedure was the area of the peaks. By doing so, new carbon species derived from the treatment processes could be identified.
An example of the analysis of HR C1s is presented in Figure 2 where some spectra of untreated and modified elastomers are compared.
The contribution from these groups varied depending on the surface treatment. However, identification of chemical groups can be difficult when different species produce similar chemical shifts see Table 1 for EB of main carbon bonds identified in the present study.
Detailed analysis of XPS spectra is presented in section 3. Figure 2. Components employed for the analysis of the C 1s core levels 2. Complementary techniques for the interpretation of the results of XPS As we mentioned in the introduction section, surface and bulk properties of elastomers depend on the way the main constituents C, H, O, N, etc. Therefore, in some cases and depending on the light source employed for XPS analysis, it is difficult to distinguish between the presence of different functional groups, as occurs for HR C1s with C-O and C-N groups.
Complementary information on surface chemistry of elastomers can be obtained from spectroscopy of inelastic scattering of light, e. Measurements of SFE of elastomer using sessile drop method is another very simple but powerful method which can provide valuable information on the type of the surface groups. Wenzel [33] proposed to introduce a roughness factor, r, which is the real contact area divided by the geometrical, or projected, area.
In [34] it was demonstrated that CA behaviour is determined by interactions of the liquid and the solid at the three-phase contact line alone and that the interfacial area within the perimeter is irrelevant. Figure 3. For non-metallic solid surfaces, in addition to apolar LW interactions, electron acceptor — electron donor interactions, or Lewis acid-base AB interactions may often occur.
Unlike LW interactions, polar interactions are essentially asymmetrical. As this equation is underdetermined, the components of surface tension for the solid can be found by measuring CAs using at least three different liquids with known and different components of surface tension.
To overcome this problem, CA measurements should be performed with more than three liquids. These will constitute an overdetermined system of linear equations which can be solved by least-square method. In order to reduce the measurement error, each measurement of the CA should be repeated several times.
The resulting set of simultaneous equations is the following: 0. In case some of the parameters are not statistically significant, it can be zero set and removed from b. Then, the calculation should be repeated using modified matrix A.
By doing so, the standard error of the parameters of solid can be reduced. Matrix method is also very useful for the analysis of surface tension variation in time, e. In this case, CA measurements are performed at different periods of time using a set of several liquids as described above.
This constitutes a set of simultaneous equations at the selected points of time. The values of the components of surface tension for these liquids are listed in Table 2.
After corresponding modification of A and b, solution of 20 can be found by the matrix method described above. Case studies 3. Understanding of the chemical mechanisms of elastomer degradation is a key for designing advanced elastomers with higher resistance to oxidative degradation.
Surface chemical composition of the samples determined from these spectra as a function of ageing duration is shown in Table 3. The high carbon content in all samples arises from the contribution from the backbone structure of the elastomer. Oxygen, nitrogen, silicon and zinc are generally attributed to curing agents, amine-based accelerators and additives [1, 14].
In addition, for both temperatures there was certain increase in nitrogen and silicon concentrations for the days ageing. As XPS is a superficial analysis technique, the variation of these elements present in small amounts on the surface could be related to diffusion processes during ageing and segregation of impurities on the surface. The broad carbon peak in the range of EB from eV to eV can be attributed to different carbon- based surface functional groups.
This effect was similar to the evolution of the oxygen content registered in the wide energy range scan Table 3. Related titles. Carousel Previous Carousel Next. X-ray spectrometry and X-ray microtomography techniques for soil.
Jump to Page. Search inside document. Spectroscopy -X-ray spectroscopy is a technique of using characteristic X-rays to identify chemical elements. How does x-ray spectroscopy differ from X-ray diffraction for crystal structure analysis?
Fluorescent yield of chemical elements Fig: Fluorescent yield variation with atomic number and with the K, L, and M characteristic lines. Wavelength Dispersive Spectra A WDS spectrum is presented as a diagram in which the characteristic X-ray lines are dispersed in a range of the X-ray wavelengths Figure : WDS spectra of a nickel-based superalloy: a spectrum obtained with a LiF analyzing crystal and b spectrum obtained with a TAP crystal. Chad Smith.
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