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Boron (B) is widely applied in microalloying of metals. As a typical light element, however, determination of boron in alloys with complex matrix spectra is still a challenge for laser-induced breakdown spectroscopy (LIBS) due to its weak line intensities in the UV-visible-NIR range and strong spectral interference from the matrix spectra. In this study, a wavelength-tunable laser was used to enhance the intensities of boron lines selectively. The intensities of B I 208.96 nm from boron plasmas were enhanced approximately 3 and 5.8 times while the wavelength-tunable laser was tuned to 249.68 and 249.77 nm, respectively. Utilizing the selective enhancement effect, accurate determinations of trace boron in nickel-based superalloys and steels were achieved by laser-induced breakdown spectroscopy assisted by laser-induced fluorescence (LIBS-LIF), with limits of detection (LoDs) of 0.9 and 0.5 ppm, respectively. The results demonstrated that LIBS-LIF can hopefully be used in boron determinations and has great potential for improving the ability of LIBS to determine light elements in alloys with a complex matrix.
© 2016 Optical Society of America
1. Introduction
As a typical light element, boron is widely applied in microalloying of metals. A minor amount of boron can improve the strength, endurance performance, and creep property of superalloys and steels; but an excessive amount of boron will decrease toughness, cause embrittlement, and produce hot shortness. For nickel alloy Inconel 690, boron higher than 0.003 wt. % can reduce its resistance to intergranular corrosion and caustic induced stress corrosion [1–3]. The standard boron analytical method (0.0005~0.20 wt.%) in both superalloys and steels is the methanol distillation curcumin photometric method (HB 5220.40-2008, GB/T 223.75-2008) which can determine boron down to 0.0005 wt.%. Although the chemical method has high sensitivity and accuracy, it requires complex procedures and is time consuming. Generally, specially trained operators are required and several hours are necessary even for one measurement. Therefore, a fast and accurate analytical method, such as laser-induced breakdown spectroscopy (LIBS), is urgently needed for boron analysis.
LIBS has proved to be a versatile analytical technique during the past decades [4, 5]. It has many advantages, including detection of nearly all elements in any physical form (liquid, solid, gas, etc.), rapid and simultaneous multielement determination, no or simple sample preparation, nearly nondestructive, capable of in situ and real-time analysis [6–8]. However, the determination of light elements including boron (B) in alloys with complex matrix spectra, such as steels, nickel alloys, and titanium alloys, is always a challenge for LIBS [9, 10]. Until now, determining B in alloys using LIBS has rarely been reported. Determination of other light elements, such as carbon (C), phosphorus (P), and sulfur (S), in steels using LIBS can be conducted only under vacuum ultraviolet (VUV) [9]. However, in the case of VUV, most parts of the LIBS system, including the sample, the light collection optics, the spectrometer, and the CCD detector, have to be placed in inert gas or vacuum environment. As a consequence, LIBS is no longer simple and fast and loses its advantages over traditional techniques, such as spark atomic emission spectroscopy (Spark-AES). Therefore, carrying out light element determinations in the ultraviolet-visible-near infrared (UV-visible-NIR) range is important for lots of applications of LIBS. However, just a few weak lines of light elements are available in the UV-visible-NIR range; and a strong spectral interference from the matrix spectra often cannot be ignored [10]. Therefore, it is important to find a way that is capable of enhancing the lines of light elements effectively.
Laser-induced breakdown spectroscopy assisted by laser-induced fluorescence (LIBS-LIF) is a potential solution for determining light elements in alloys with complex matrix spectra. LIBS-LIF uses a wavelength-tunable laser to irradiate the plasma of LIBS. The ground state atoms of the target element can be stimulated to an excited state by the wavelength-tunable laser tuned to one characteristic line of the element, resulting in a great increase in atom density in the corresponding excited state. Then transition lines from the excited state can be greatly enhanced. X. K. Shen et al. [11] and H. Kondo et al. [12] have demonstrated that LIBS-LIF is a successful solution for determining P in steels with limits of detection (LoDs) of 0.7 and 5.4 ppm, respectively.
In this study, a method of enhancing boron lines selectively by a wavelength-tunable laser was investigated. Utilizing the proposed method, determination of trace B in nickel-based superalloys and steels was carried out by LIBS-LIF.
2. Experimental setup
The experimental setup for LIBS and LIBS-LIF is schematically shown in Fig. 1. A Q-switched Nd:YAG laser (Quantel Laser, Brilliant 400) operating at 532 nm and 10 Hz was used to generate plasmas. The 532 nm laser beam was focused onto the target at the normal incidence by a plano-convex lens (f = 150 mm). The focal point was 8 mm below the target surface, and the ablation spot was about 0.8 mm in diameter. The target was mounted onto a motorized X-Y-Z translation stage.
A wavelength-tunable optical parametric oscillator (OPO) laser (OPOTEK Inc., VIBRANT HE 355, 5 ns, 10 Hz) was used for selective enhancement. The OPO laser was focused on the plasma horizontally by another plano-convex lens (f = 150 mm), with the optical axes at about 1 mm above the sample surface. The focal spot of the OPO laser was about 3 mm in diameter, which could fully cover the plasmas generated by the 532 nm laser. Light emitted from the target plasma was collected by a UV-NIR light collector and then coupled into a spectrometer through a UV-enhanced fiber. A Czerny-Turner spectrometer (Princeton Instruments, Isoplane SCT320) equipped with a grating of 3600 lines/mm and an intensified charge coupled device (ICCD) camera (Princeton Instruments, PIMAX3, 1024 × 256 pixels) was used for spectral analysis. The slit width used in experiments was 200 µm, and the corresponding spectral resolution was about 0.08 nm. The 532 nm laser, the OPO laser, and the ICCD camera were all triggered by a digit delay generator (Stanford Instrument, DG535) in experiments.
A pure boron sample (99.5%), purchased from Kurt. J. Lesker, was used in studying the selective enhancement of boron lines. Ten nickel-based superalloy samples, purchased from MBH Analytical Limited (28X06625, 28X07718, 28X7181, 28X7182, 28X7183, 28X7185, 28X7186), Brammer Standard Company, Inc. (BS 690A, BS H-6A) and Institute of Materials (IMR-11, boron content = 90 ppm), and eight steel samples, purchased from NIST (1264a, 1265a, 1761a, 1762a, 1763a, 1764a, 1765, 1767), were used in building quantitative models of boron. Every measurement consisted of 800 shots and was repeated 10 times unless specially specified.
3. Results and discussion
3.1 The selective enhancement of boron lines by a wavelength-tunable laser
A careful selection of the exciting line (the wavelength of the OPO laser) and the corresponding analytical line is very important to the successful selective enhancement of the target element. Some published works on LIBS-LIF are summarized in Table 1. As shown in Table 1, selection of a proper exciting line and the corresponding analytical line generally follows two rules: (1) the exciting line and the analytical line are both strong lines of the target element [13], and (2) the upper levels of the exciting line and the analytical line are the same or at least very similar. The main strong lines of atomic boron in the UV-visible-NIR ranges and the corresponding states provided by NIST [13, 14] are shown in Fig. 2.
Table 1. Reported works about LIBS-LIF.
As shown in Fig. 2, there are two potential ways of enhancing boron selectively: (1) B I 1166.32/1166.56 nm to be enhanced by laser of 208.89/208.96 nm; (2) B I 208.89/208.96 nm to be enhanced by laser of 1166.32/1166.56 nm. In the case of (1), most of the CCD/ICCD detectors used in LIBS can hardly respond to spectra longer than 1050 nm; and the OPO laser used in this study cannot be tuned to wavelengths shorter than 225 nm. Therefore, case (1) cannot be carried out in this study. In the case of (2), the OPO laser can be tuned to 1166.32/1166.56 nm and the CCD/ICCD can detect spectra of 208.89/208.96 nm; however, the expected selective enhancement effects of B I 208.89 and 208.96 nm were not observed in experiments. This phenomenon can be contributed to that the level of 40039.7 cm−1 which is the lower level of 1166.32/1166.56 nm is too high. If a Boltzmann distribution of the atoms population can be assumed and the plasma temperature is even on the order of 1 eV, the population of this level is just a few thousandth of the population of the ground state. As a consequence, just a very small portion of boron atoms can be pumped to 48611.9/48613.6 cm−1 by the 1166.32/1166.56 nm OPO laser and has nearly no impact on the enhancement of the analytical transition. Therefore, it is worth adding a third rule for the selection of a proper exciting line and the corresponding analytical line: the lower level of the excitation line is the ground state or a low energy state.
Therefore, using an OPO laser with a wavelength of 249.68/249.77 nm became the only choice. In the experiment, boron plasmas were generated by a 532 nm laser with a pulse energy of 60 mJ; and then the 249.68/249.77 nm OPO laser was focused onto the plasma at 2 μs later. The spectra of boron plasmas with the 249.68/249.77 nm OPO laser and with LIBS only were recorded with a gate width of 8 ns, as shown in Fig. 3.
Fig. 3 Spectra of boron plasmas with the excitations of the 249.68/249.77 nm OPO laser and with LIBS only.
As shown in Fig. 3, even though there is a notable gap of about 0.969 eV between the excited state 40039.7 cm−1 and the excited states of 47857.1/47856.8 cm−1, the intensities of B I 208.96 nm were still enhanced for about 3 and 5.8 times when the OPO laser was tuned to 249.68 and 249.77 nm, respectively. Stimulated by the 249.68/249.77 nm OPO laser, an enormous amount of ground-state boron atoms in plasmas can be pumped to the excited state of 40039.7 cm−1. Because there are strong transitions whose transition probabilities are both 1.72E7 between the excited states of 48611.9/48613.6 cm−1 and of 40039.7 cm−1 [13, 14], boron atoms at the excited state of 40039.7 are easily transited to the excited states of 48611.9/48613.6 cm−1 by thermal transition. At last, the lines of B I 208.89/208.96 nm that transited from the excited states of 47857.1/47856.8 cm−1 will be enhanced.
As the lines of B I 208.89 and 208.96 nm were not resolved from each other, only B I 208.96 nm is mentioned in the following work. Furthermore, only the 249.77 nm OPO laser was used in further studies due to the better enhancement effect.
3.2 Influences of experimental parameters on the selective enhancement effect
Determining trace boron in nickel-based superalloys and steels by a 249.77 nm OPO laser was studied in detail. In order to simplify the process, only the nickel-based superalloy samples were used for optimizing experimental parameters. The signal-to-background ratio (SBR) of B I 208.96 nm was chosen for evaluating the selective enhancement effect, while the value of the continuum background was extracted from the averaged intensity of the blank area of 207.30 - 207.65 nm.
Influences of the 532 nm laser energies and the interpulse delays between two lasers on the SBR of B I 208.96 nm are shown in Fig. 4(a). Generally, the intensity of laser-induced fluorescence (B I 208.96 nm) is highly determined by the densities of boron atoms in the ground states (0.0, 15.3 cm-1), as shown in Fig. 2. A colder but higher density plasma will help to increase the densities of atoms in the ground states based on the Boltzmann distribution [23] and will thus increase the intensity of laser-induced fluorescence. Furthermore, a colder plasma will also be helpful to decrease the intensity of the continuum background. That is to say, a colder plasma obtained by a low laser fluency and a suitable gate delay can increase the SBR of laser-induced fluorescence. Thus, the maximum SBR of B I 208.96 nm was obtained with a laser energy of 10 mJ and an interpulse delay of 4 μs. It is worth mentioning that this phenomenon is quite different than that of conventional LIBS, in which the line (such as B I 249.77 nm) intensity is determined by the densities of atoms on the excited levels (such as 40039.7 cm−1); and a hotter plasma, which means a much higher laser energy, is helpful for increasing the intensities based on the Boltzmann distribution.
Fig. 4 Influences of the laser (532 nm) energies and the inter-pulse delays on the SBRs of B I 208.96 nm (a) and the OPO laser (249.77 nm) energies on the intensities and SBRs of B I 208.96 nm (b).
Influences of the OPO laser (249.77 nm) energies on the intensities and the SBRs of B I 208.96 nm are shown in Fig. 4(b). The intensities of B I 208.96 nm increased with the OPO laser energy until the energies of the OPO laser were higher than 0.5 mJ/pulse. After that, the intensities were saturated and kept nearly constant. The SBRs also increased with the OPO laser energies in a similar trend. However, the SBRs decreased quickly while the energies of the OPO laser were higher than about 1.9 mJ/pulse, which was different compared to that of the intensity. This is because the intensity of the continuum background increased when the OPO laser energies were higher than 1.9 mJ/pulse. Therefore, the optimal energies of the OPO laser were in the range of 0.5~1.9 mJ/pulse. While the energy of the OPO laser is in the optimal range, both the intensities and the SBRs of B I 208.96 nm can keep nearly constant even if the energies of the OPO laser fluctuate a little. This saturation effect of LIBS-LIF will probably be beneficial for improving the stability and reproducibility of quantitative analysis.
3.3 Quantitative analysis of trace boron in nickel-based superalloys
The quantitative analysis of trace boron in nickel-based superalloys was carried out under optimized conditions, in which the energy of 532 nm laser, the interpulse delay, and the energy of the 249.77 nm OPO laser were 10 mJ, 4 μs, and 1.2 mJ, respectively. The calibration curve of LIBS-LIF for determining boron in nickel-based superalloys is shown in Fig. 5(a). The intensities of B I 208.96 nm were normalized by the averaged intensities of the background spectra in 207.2~207.3 nm. The horizontal error bars of data points in Fig. 5(a) identify the uncertainties of boron content from the certification files of the samples, and the vertical error bars indicate the uncertainties of intensities measured by LIBS-LIF. The corresponding spectra of the nickel-based superalloys by LIBS-LIF are shown in Fig. 5 (b).
As shown in Fig. 5(a), the intensities of B I 208.96 nm by LIBS-LIF exhibit a great linear relationship with the contents of boron; and the R2 factor of calibration is 0.987 in the range of 3~114 ppm. In contrast, the R2 factors of calibration curves acquired by LIBS only with the best condition (pulse energy = 60 mJ, gate delay = 8 μs, gate width = 1 μs) were 0.12, 0.55 and 0.72 for B I 208.96 nm, B I 249.68 nm and B I 249.77 nm, respectively. The RMSECV of boron measurement is only 6.5 ppm, which shows a satisfactory accuracy for practical applications. The LoDs of boron in nickel-based superalloys calculated by the 3σ criterion [24] is 0.9 ppm, which shows a very high sensitivity. This high sensitivity is contributed to the improved SBR of B I 208.96 nm by LIBS-LIF. For LIBS-LIF, the net SBR of B I 208.96 nm (boron content = 107 ppm, acquired by divide the intensity and the intercept of the calibration curve) was as much as 9.8. However, the net SBR of B I 208.96 nm by LIBS only was only nearly 1, and the net SBR of B I 249.77 nm by LIBS was only about 2. Therefore, a quantitative determination of trace boron in nickel-based superalloys was achieved by LIBS-LIF.
3.4 Quantitative analysis of trace boron in steels
The quantitative analysis of trace boron in steels was also carried out under the same conditions as used for nickel-based superalloys. The calibration curve for boron in steels is shown in Fig. 6(a), and the corresponding spectra is shown in Fig. 6(b). The intensities of B I 208.96 nm were normalized by the averaged intensities of matrix spectra of 208.53~208.65 nm. The calibration curve in Fig. 6(a) also shows a great linear relationship between the line intensities and the contents of boron. The R2 factor, the RMSECV, and the LoD of boron analysis were 0.966, 5.3 ppm, and 0.5 ppm, respectively. Therefore, trace boron in steels can also be determined by LIBS-LIF.
The results above have shown that LIBS-LIF is capable of determining trace boron in alloys with complex matrix spectra under the UV-visible-NIR circumstance. Not only having a high sensitivity and accuracy, LIBS-LIF also retains the advantages of LIBS, including no sample preparation, nearly nondestructive, and fast analysis, etc. Therefore, LIBS-LIF can be used in boron determination in practice. The results have also shown that LIBS-LIF has great potential for improving the sensitivity and accuracy of light element determinations.
4. Conclusions
The selective enhancement of boron lines using a wavelength-tunable laser was investigated. With the stimulations of 249.68 and 249.77 nm OPO lasers, the line intensities of B I 208.96 nm were selectively enhanced 3 and 5.8 times, respectively. Successful quantitative analysis of trace boron in nickel-based superalloys and steels by LIBS-LIF was carried out with the 249.77 nm OPO laser. The R2 factor, RMSECV and LoD of boron determination were 0.987, 6.5 ppm, and 0.9 ppm, respectively, for nickel-based superalloys and were 0.966, 5.3 ppm, and 0.5 ppm, respectively, for steels. The results have shown that LIBS-LIF can be used in boron detection in practice, and it has also great potential in improving the ability of LIBS analysis in detecting light elements in alloys with complex matrix spectra.
Acknowledgments
This research was financially supported by the National Special Fund for the Development of Major Research Equipment and Instruments (No. 2011YQ160017) of China, and the National Natural Science Foundation of China (No. 61575073, 51429501 & 61378031).
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