Analysis of Nitride Layers on Steel using a Low Energy X-Ray Spectrometer (LEXS)

The EDAX Low Energy X-Ray Spectrometer (LEXS) is a wavelength dispersive spectrometry (WDS) spectrometer, which consists of X-ray optics capable of executing parallel beam operation. The high collection optics offer high efficiency for testing light elements, particularly O B, N and C, with a complete range of operation covering from 80 eV to 2.4 keV. In the following article, the method of using the LEXS to detect light elements found in nitride layers on steel is discussed.

Application

To understand the composition and distribution of the nitrides in steel, highly accurate X-ray data collection is needed for X-ray maps, quantification, and line scans.

Spectrometer Conditions

The LEXS WDS spectrometer was chosen for this application. As shown in Figure 1, a boron nitride standard was measured together with some pure elements to facilitate quantitative analysis of the unidentified nitride elements to be calculated. All of the samples were analyzed at a beam current between 20 to 25 nA and an accelerating voltage of 10 kV.

A tungsten filament scanning electron microscope (SEM) along with a Keithley Pico ammeter was used to track the beam current. Energy dispersive spectroscopy (EDS) spectra were derived from the samples, including the WDS data.

The concurrent acquisition of WDS and EDS data allows the application of EDS data for certain higher atomic number samples, but this did not happen in this case. The two detectors had a takeoff angle of 35°. The instrument’s working distance was 17 mm. This distance is precise to the instrument, and can be relatively smaller if the SEM chamber allows.

WDS (top) and EDS (bottom) spectra of BN. The WDS spectra shows the peaks of B K and N K.

Figure 1. WDS (top) and EDS (bottom) spectra of BN. The WDS spectra shows the peaks of B K and N K.

Nitride Measurements

The nitride layer on steel was measured and a nitrogen X-ray map was derived from the substrate and the layer with the aid of the WDS system. Figure 2 illustrates the nitrogen K X-ray map and the SEM image (BSE image).

BSE image and N K X-ray map of the layers.

Figure 2. BSE image and N K X-ray map of the layers.

The top layer is an Al coat, the bottom layer is the substrate, and the bright middle layer is the nitride layer. The image is 128 x 100 pixels with 200 msec/pixel dwell time. The layer displays varied levels of N, possibly from different phases inside the layer. X-ray data was obtained from a spot on the lower and middle portions of the nitride layer. Figures 3 and 4 show the collected spectra.

WDS (top) and EDS (bottom) spectra of the spot analysis, lower part of the nitride layer.

Figure 3. WDS (top) and EDS (bottom) spectra of the spot analysis, lower part of the nitride layer.

WDS (top) and EDS (bottom) spectra of the spot analysis, middle part of the nitride layer.

Figure 4. WDS (top) and EDS (bottom) spectra of the spot analysis, middle part of the nitride layer.

The two spectra illustrated in Figure 4 were measured using the boron nitride standard. The results in Tables 1 and 2 show the higher concentration of nitrogen in the brighter part of the nitride layer, and the lower concentration of nitrogen in the darker part of the layer.

Table 1. Lower spot analysis.

Element Wt % At % K-Ratio Z A F
N K 7.81 25.63 0.0502 1.205 0.5331 1.0014
Fe L 90.32 74.37 0.8311 0.9799 0.939 1
Total 98.133 100        

Table 2. Middle spot analysis.

Element Wt % At % K-Ratio Z A F
N K 9.32 28.97 0.0609 1.201 0.5435 1.0014
Fe L 91.14 71.03 0.8324 0.9767 0.9351 1
Total 100.467 100        

LEXS

The LEXS has been optimized to handle low energy microanalysis. It utilizes innovative high collection optics (HCO), which are able to capture the maximum count rates available, offering fast X-ray analysis at the highest resolutions possible.

The LEXS is exclusively designed for low energy X-ray microanalysis, offering resolutions of below 20 eV and high peak-to-background ratios for X-ray energies less than 2.5 keV. It is suitable for resolving such overlaps as N Ka from Ti La or Si Ka from W or Ta Ma, with count rates sufficiently high that testing speed is not a limitation for this system.

Conclusion

The LEXS, fitted with high-performance optics, is perfectly suited to test light elements from Be to S. The system can produce a number of data sets, allowing material characterization researchers to completely understand the chemical and composition trends throughout the layers, such as the nitride layer tested.

The high performance of the LEXS system for nitrogen is highly useful for calculating concentration levels inside the layers. An X-ray map of the entire target area and line scan profiles show the nitride intensities throughout the numerous layers.

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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