Appl. Phys. A DOI 10.1007/s00339-014-8544-7
Analysis of alumina-based titanium carbide composites by laser-induced breakdown spectroscopy Kaleem Ahmad • Walid Tawfik • Wazirzada A. Farooq • Jagdish P. Singh
Received: 14 November 2013 / Accepted: 28 May 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract In this work, alumina (Al2O3) containing different volume % of titanium carbide (TiC) ranging from 0 to 30 were consolidated by the novel spark plasma sintering. The spectroscopic analysis of the plasma generated by irradiation of laser Nd:YAG (k = 1,064 nm) on different concentrations of the composites in air atmospheric pressure was performed. The qualitative examination of the composites confirms the presence of aluminum, titanium, and carbon as major elements, while magnesium and sodium have been found as minor trace elements. Plasma parameters were estimated by assuming the LTE conditions for optically thin plasma. The electron density and temperature were evaluated by using the Stark broadening and intensity of selected aluminum emission lines, respectively. The addition of TiC to Al2O3 shows a linear behavior with plasma temperature corroborated by the calibration curve of Ti in the composites. The results suggest that calibration curve between plasma temperature and the composites can be used to estimate different concentrations of TiC in Al2O3 without analyzing
K. Ahmad (&) Sustainable Energy Technologies Center, King Saud University, Riyadh 11421, Saudi Arabia e-mail:
[email protected] W. Tawfik W. A. Farooq Department of Physics and Astronomy College of Science, King Saud University, P.O. Box 2455, Riyadh, Saudi Arabia W. Tawfik Department of Environmental Applications, National Institute of Laser Enhanced Sciences (NILES), Cairo University, Cairo, Egypt J. P. Singh Institute for Clean Energy Technology (ICET), Mississippi State University, Starkville, MS 39759, USA
the whole elements in the composites and thus opens up new applications of LIBS in ceramic industry.
1 Introduction Laser-induced breakdown spectroscopy (LIBS) has gained renewed interest recently due to its simplicity, high sensitivity, fast response, and applicability of this technique to a wide range of materials including polymers, nano-ceramics, semiconductors [1–5]. It provides a prompt real-time analysis of materials with high special resolution and depth profiling by obviating any lengthy sample preparation and being used in diverse fields such as nuclear, medical, aerospace, environment [1, 6–9]. LIBS has been demonstrated a proven and versatile technique for multi-elemental analysis of different materials such as ceramics [6, 10], nuclear materials [1], plants [11], semiconductors [5], and artifacts. In addition, its ability to analyze the samples in real time at remote distance, materials in hostile environment especially in the presence of high-temperature and high-radiation zone, and in situ make it promising for online monitoring of materials at different stages of production chain to improve the quality and product gain in the industry. In particular, LIBS has huge potential applications in ceramic industry for quality assurance and control of materials stoichiometry of different products. For example, the determination of impurities by the LIBS is of great importance for high-performance engineering materials as they are highly detrimental for proper functioning of engineering ceramics, and their presence may affect the mechanical and/or functional performance of the product [1]. Therefore, qualitative determination of impurities at different stages of production of ceramic-cutting tools is essential for quality assurance of the product.
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Recent advancement in high-performance advanced materials has introduced a number of ceramic composites for several engineering applications such as high-speed cutting tools, dental implants, and wear resistant parts. [12– 14]. Among them, alumina (Al2O3)-based titanium carbide (TiC) composites are getting more attention and are becoming promising candidate due to their excellent combination of structural and functional properties along with economy [15, 16]. They possess high-temperature structural stability, including retention of strength, superb hardness and chemical inertness, and widely used for tapehead materials, high-speed cutting of cast iron, super alloys, and hard steel [17]. In comparison with hard-metalcutting tools, Al2O3-based TiC-cutting tools remain intact and retain their mechanical stability up to 1,200 °C, while metal-based cutting tools degraded drastically above 800 °C. In this work, different Al2O3/TiC composites fabricated by the spark plasma sintering were characterized by the LIBS in detail for the first time. Elemental analysis was performed, and plasma parameters were estimated for potential applications in ceramic-cutting tool industry.
2 Experimental procedure The steps involved in processing, fabrication and spectroscopy analysis of the composites are schematically shown in Fig. 1. In brief, Al2O3 and TiC powders in purity around 99.99 % were obtained from commercial Japanese source. After weighing in the appropriate proportions, seven compositions ranging from 0, 5, 10, 15, 20, 25, and 30 volume (vol) % in Al2O3 were prepared. For uniform mixing, the composite powders were ball-milled in ethanol for 24 h and then dried by the rotary vaporizer. Subsequently, the dried composite powders were spark plasma sintered (Dr. Sinter 1050, Sumitomo Coal Mining Co., Japan) at 1,400 °C at a pressure of 40 MPa in a graphite die of inner diameter 12.5 mm. The relative densities of all the composites were above *95 %. After grinding the surface of the composite pellets, the LIBS was carried out on spectrolaser 7000 (Photon Machines Inc.). Spectrolaser 7000 as shown schematically in Fig. 1 is an integrated system consists of excitation laser, optical spectrographs, and CCD detectors. It is fully controlled through software (Laser Analysis Technologies Pty. Ltd, Australia). It has a high-power mini Nd:YAG laser with a wavelength 1,064 nm that yields 7 ns pulse duration with a frequency of 15 Hz. The laser energy can be adjusted from 05 to 300 mJ depending on the sample. The detection system consists of seven Czerny–Turner spectrometers with corresponding CCD detectors that cover the spectral range from 188 to 990 nm. The seven spectrometers cover the
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Fig. 1 Schematic of powder processing for Al2O3/TiC composites and LIBS experimental setup
whole range as 188–260, 260–315, 315–360, 360–500, 500–625, 625–820, and 820–990 nm. The system has the ability to focus on the fresh region of the sample through a short focal length to each successive laser pulses in order to minimize the matrix effect. Each CCD detector has its own fast A/D circuitry to record the corresponding spectral ranges from the respective spectrograph simultaneously. The PC-controlled interactive software allows adjusting system parameters such as delay time, laser power, and number of shots. The laser energy was optimized in this experiment to avoid the oxygen influence from the air. This was achieved by optimizing the laser energy using 1,064 nm laser of 7 ns pulse duration focused with f/7.6 lens. We found that using 50 mJ laser produces a beam waist w0 * 50 lm FWHM at the surface of the sample (measured with CDD detector built inside the system to follow up during laser firing).
Analysis of alumina-based titanium carbide composites
This in turn gives laser intensity of &9.091 9 1010 W/ cm2, which is much lower than the breakdown threshold of 1,064 nm in air (*1 9 1011 W/cm2) [18–20]. In addition, we did not detect any significant emission of atomic lines of nitrogen in LIBS spectrum of our samples. These indicate that environmental influences have been fully avoided due to optimized experimental conditions. In the present study, optimized laser energy 50 mJ has been used with a delay time of 1 ls (gate delay between the laser firing and the CCD detector). The LIBS measurements were taken
with average of ten shots for each point having an average of three points for each sample.
3 Results and discussion 3.1 Identification of main elements Figure 2 shows the entire spectra obtained from the emission of spectral lines from the 30 vol% of Al2O3/TiC
Fig. 2 Full spectrum for 30 vol% Al2O3/TiC composite in six different windows ranging from 240 to 820 nm wavelengths
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K. Ahmad et al. Table 1 Selected spectral atomic lines of major elements detected in the alumina/TiC composites
Table 2 Aluminum atomic lines used for calculation of plasma temperatures
Elements
Charge state
Emission lines (nm)
Species
Wavelength (nm)
Aki (s-1)
Al I
226.91
C
I
247.85
Al I
Al
I
256.79, 308.21, 309.27, 394.40, 396.15,
Al I
Ti
I
334.18, 365.34, 399.86, 453.32, 498.17, 499.12
composite in the wavelength range from 240 to 820 nm. Each wavelength of the peaks in the spectrum were analyzed thoroughly, compared and finally identified with the help of US National Institute of Standards and Technology (NIST) database available online [21]. The major identified elements and their corresponding wavelengths are shown in Fig. 2. Aluminum, titanium, and carbon were identified as major elements by the comparing the strong lines with the standard database. Figure 2a shows the strongest emission lines of aluminum and carbon elements, while several titanium lines are present in almost each window of the spectrum. The major identified elements and their corresponding emission line wavelengths are shown in Table 1. In addition, few minor elements such as sodium and magnesium were identified as trace elements to be discussed in Sect. 3.4. 3.2 Plasma temperature The plasma parameters were determined by assuming the Local Thermodynamic Equilibrium (LTE). The plasma generated by irradiation of laser on the Al2O3/TiC composites has been characterized by the electron temperature and number density. Generally, LIBS plasma does not exhibit the LTE condition at the initial stages of its expansion [22]. However, LIBS was performed with a delay time of 1 ls, which is adequate for thermalization of plasma to occur [23]. The population density of atomic and ionic states in LTE conditions are generally described by the Boltzmann distribution. Therefore, the temperature was estimated using the Boltzmann plot method with aluminum lines for the seven samples as shown in Table 2. The selected lines are isolated and do not have interference with other elements. The aluminum spectral lines wavelength, upper level energies, statistical weights, and transition probabilities were taken from the Refs. [21, 24]. It is assumed that the re-absorption effect of plasma emission in optically thin plasma is negligible; therefore, the emitted spectral lines’ intensity is a measure of the population of the corresponding energy level of that particular element in the plasma. The emitted spectral lines’ intensity has been used for measuring the plasma temperature by using the following equation [24, 25].
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Ek (cm-1)
gk
7.90 9 107
44,168.85
6
236.71
7
7.20 9 10
42,233.74
4
237.31
8.60 9 107
42,237.78
6
Al I
256.79
2.30 9 107
38,929.41
4
Al I
257.54
2.80 9 107
38,933.97
6
Al I
308.22
6.30 9 107
32,435.45
4
7
Al I Al I
309.27 396.152
7.40 9 10 9.80 9 107
32,436.79 25,347.76
6 2
Al I
394.40
4.93 9 107
25,347.76
2
ln
Ik Ek CF ¼ þ ln Aki gk UðTÞ kT
ð1Þ
where k is the wavelength of emission line, Aki is the transition probability, Ek is the excited level energy, T is the temperature in LTE, k is the Boltzmann constant, U(T) is the partition function, C is the species concentration, F is the experimental factor, and gk is the statistical weight for the upper level. By plotting the left hand side of Eq. (1) against the excited energy level Ek, the plasma temperature has been calculated from the slope of the straight lines as shown in the Fig. 3. The graph for plasma excitation temperature versus different vol% of TiC in Al2O3 for seven samples has been plotted in Fig. 4a. The data were linearly fit by the straight line and show a linear relationship with the increase in TiC composition in Al2O3. There are three major detected elements in the composites that include aluminum, carbon, and titanium having the ionization potentials 5.98, 11.26, and 6.82 eV, respectively [21]. It is well documented that the spectral line intensities from laser-ablated material have strong correlation with its constituent components and their values depend on multiple factors such as porosity, grain size, melting point, and mechanical strain present in the material [26]. Therefore, the physical properties of materials such as hardness strongly affect the mass of ablated materials through laser surface interaction [27]. The result suggests that as we increase the TiC composition in aluminum oxide, the same effect is observed in the laser target interaction volume, and the concentration of aluminum oxide decreases, while the concentration of TiC increases. TiC has superior physical properties in comparison with Al2O3. Therefore, with the addition of TiC in Al2O3, improved structural properties have been observed in several studies [28, 29]. The addition of TiC concentration increases the hardness of the composites that leads to smaller quantity of ablated materials for the fixed value of laser energy, consequently this will in turn increase the plasma temperature [30]. It indicates that the plasma
Analysis of alumina-based titanium carbide composites
Fig. 3 Typical Boltzmann plots determined from the emission lines of aluminum observed in the samples of Al2O3/TiC composites for pristine Al2O3 and 15 vol% of TiC composites
temperature increases due to improvement in the physical properties of the composite with the addition of TiC, and this behavior is consistent with the previous investigations [25, 31, 32]. Recently, it has been reported that there is almost a linear correlation between properties especially hardness of solid and plasma temperature that also corroborates our results [10, 30]. The incorporation of TiC in Al2O3 not only expected to improve its structural and electrical properties but also to improve its thermal conductivity and thermal shock resistance, which are required to enhance its robustness for harsh environment applications such as cutting tools [33]. To further support our discussion, the line intensity of Ti I at wavelength 399.86 nm is plotted with the TiC concentrations of all samples in the composite (Fig. 4b) [34]. The Ti intensity increases as the vol% of TiC increases in the laser target interaction volume of the composite. The resulting graph shows a linear behavior similar to calibration curve of plasma temperature (Fig. 4a) and in turn corroborates the plasma temperature calibration curve due to improvement in physical properties caused by the addition of TiC. Furthermore, in TiC/Al2O3 composites, the TiC and Al2O3 phases remain intact during densification, i.e., no
Fig. 4 a Calibration plot for plasma temperature versus different vol% of TiC in the Al2O3 composites b calibration curve for Ti line intensity versus TiC concentrations in the composites
reaction exists between them. It is likely that due to nonhomogenous distribution of TiC phase in Al2O3 at the laser ablation area may result in lower plasma temperature for 30 % composite than 25 % TiC/Al2O3 composite and similar variation in Ti intensity as shown in Fig. 4. 3.3 Electron density Generally, electron density is determined by calculating the broadening of suitable emission line of the laser plasma spectrum [35]. The famous line broadening mechanisms in plasma includes pressure broadening, Doppler broadening, Stark broadening [35, 36]. It has been reported that the selfabsorption phenomenon is caused by the line broadening and spectral shift of emission lines [37]. Therefore, in this work, special attention was given to observe the line splitting and spectral shift that are good evidence of selfabsorption. However, no sign of self-absorption has been
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of LTE condition along low temperature plasma also confirms the optically thin plasma and consequently supports the nonexistence of self-absorption under current conditions [39]. Stark broadening has been found to be the dominant broadening effect as broadening increases with the increase in energy level [40]. Therefore, Stark broadening has been used to calculate the electron density, while other mechanisms such as Vander Walls broadening, resonance broadening, etc. may be neglected [35]. Since the instrumental line broadening exhibits a Lorentzian shape function [41, 42], therefore Stark broadening line width DkFWHM can be obtained by subtracting the instrumental line broadening Dkinstrument from the Dkobserved as reported earlier [41, 42]. The predetermined value for instrument broadening is &0.05 nm in the present study. DkFWHM ¼ Dkobserved Dkinstrument
ð2Þ
The following expression has been used for non H-like electron density estimation using the FWHM of the line [24]. Ne ð
DkFWHM Þ 1016 2w
ð3Þ
where w is the Stark broadening parameter. The aluminum line 266.03 has been selected for electron density estimation. Figure 5a–c shows typical aluminum 266.03 lines with sufficient resolution to measure the full width at half maximum (k1/2) for selected 0, 15, and 30 vol% of Al2O3/TiC composites, respectively. All the data points in three samples were simulated with the Lorentzian fitting function in Origin software to measure the full width at half maximum. The values of k1/2 for the pristine Al2O3 5, 10, 15, 20, 25 and 30 vol% of TiC in the composites have been determined as 0.074, 0.067, 0.088, 0.127, 0.135, 0.159, and 0.162 nm, respectively. The value increases with the addition of TiC in Al2O3. By substituting the value of k1/2 and the value of Stark broadening parameter w (4.2900 9 10-04) [24] in Eq. (3) of the corresponding composite, the electron densities have been estimated as shown in Table 3. The plasma parameters increase with the addition of TiC concentration in Al2O3. This behavior is well in agreement with the previous reported results [25]. Fig. 5 Effect of TiC addition on the Stark broadening of aluminum lines 266.03 nm used for calculation of electron density for pristine Al2O3, 15 and 30 vol% of Al2O3/TiC composites
observed. In addition, resonance lines have been avoided in determining the electron density in order to decrease the contribution of self-absorption [38]. Furthermore, validation
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3.4 Trace elements Figure 6 shows some of the impurities identified in Al2O3/ TiC composites by the LIBS. The presence of these impurities as shown in Table 4 may have adverse effect on the mechanical and functional properties of the composites. Figure 6a, b shows the famously identified impurity lines of Na and Mg, respectively.
Analysis of alumina-based titanium carbide composites Table 3 The electron density determined from the aluminum spectral lines in the seven samples Composites
Electron density (cm-3)
Pristine alumina
2.77 9 1017 ± 7.3 %
5 % Alumina/TiC composite
1.98 9 1017 ± 9.1 % 17
Table 4 Spectral atomic lines of the trace elements detected in the 30 vol% alumina/TiC composite
Elements
Charge state
Emission line (nm)
Na
I
588.99, 589.59
Mg
I
285.21
10 % Alumina/TiC composite
4.43 9 10
15 % Alumina/TiC composite
8.97 9 1017 ± 10.1 %
20 % Alumina/TiC composite
9.91 9 1017 ± 8.9 %
25 % Alumina/TiC composite
18
± 11.1 %
of LTE condition because this criteria is a necessary condition, but not sufficient for LTE [45].
18
± 13.8 %
Ne 1:6 1012 DE3 T 1=2
30 % Alumina/TiC composite
1.27 9 10 1.31 9 10
± 11.5 %
ð4Þ
where T is the plasma temperature and DE is the largest energy transition [46]. In the present study, DE = 4.34 eV for Mg at 285.212 nm for the composites have been used. By inserting the values of the observed plasma temperatures for all the samples, the resulting electron density values from Eq. (4) vary in the range from 9.72 9 1015 to 1.03 9 1016 cm-3, which are well below the electron densities values estimated for the composites. This shows that McWhirter criteria has been satisfied and almost validate the LTE assumption [22].
4 Conclusions LIBS has been performed to analyze the Al2O3/TiC composites ranging from 0, 5, 10, 15, 20, 25, and 30 vol% of TiC in Al2O3. The qualitative analysis of the spectrum for 30 vol% of Al2O3/TiC composites confirmed the presence of aluminum, titanium, and carbon as major elements, while magnesium and sodium as minor trace elements. The estimated values of electron density and plasma temperature increase with the increase in TiC contents in the composites. This behavior was further supported by drawing the calibration curve for Ti I (399.86 nm) line intensity versus different concentrations of TiC in the composite. These results suggest that LIBS can be employed in ceramic industry for estimating the composition just by calculating the plasma temperature. Acknowledgments The authors acknowledge financial support from the Research Center, College of Engineering King Saud University.
Fig. 6 Trace elements identified in 30 vol% Al2O3/TiC composite
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