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A comparison between quantitative EELS and APFIM microanalysis of carbonitride grains in cermets
Ultramicroscopy 79 (1999) 273}281
A comparison between quantitative EELS and APFIM microanalysis of carbonitride grains in cermets J. Zackrisson *, W. Grogger, F. Hofer, H.-O. AndreH n Department of Experimental Physics, Chalmers University of Technology and Go( teborg University, SE-412 96 Go( teborg, Sweden Forschungsinstitut fu( r Elektronenmikroskopie, Technische Universita( t Graz, A-8010 Graz, Austria Received 12 September 1998; received in revised form 22 January 1999
Abstract Microstructural characterisation of cermet materials is often performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with energy dispersive X-ray analysis (EDX). Due to the poor detection e$ciency of light elements with EDX, carbon and nitrogen cannot be properly quanti"ed. Instead, atom-probe "eld-ion microscopy (APFIM) has been used to accurately determine the content of light elements. However, this is a time-consuming method, especially when taking into account the fact that several APFIM specimens have to be prepared in a controlled way to analyse all phases present in one TEM specimen. Therefore, it is of interest to evaluate whether transmission electron energy-loss spectroscopy (EELS) can be considered an alternative to APFIM for light element analysis. In this paper, we make a comparison of results from APFIM and quantitative EELS microanalysis of carbon and nitrogen in Ti(C, N) -based cermets. Our results show that the agreement between the two methods is good enough to permit most future analyses of light elements in these materials to be performed in the TEM using EELS. 1999 Elsevier Science B.V. All rights reserved. Keywords: TEM; Cermets; Hardmetals; Ti(C, N); Light elements
1. Introduction Cermets are Ti(C, N)-based cutting tool materials manufactured by sintering pressed raw material powders to a dense body. During the sintering process a liquid phase is formed enabling the powder particles to rearrange, dissolve and recrystallise. A cermet microstructure consists of hard carbonitride grains embedded in a tough binder phase, see Fig. 1. The carbonitride grains often have a
* Corresponding author. Tel.: #46-31-772-3325; fax: #4631-772-3224. E-mail address: [email protected] (J. Zackrisson)
core-rim structure [1,2], where the core is a remnant of the raw material and the rim is formed during sintering. The microstructure is determined by the raw material powder mixture as well as by the sintering conditions, and can be considered a link between the process and the properties. The distribution of carbon and nitrogen in the di!erent phases is of great interest to the understanding of cermet materials, since the solubility and di!usion of other elements during the sintering process can be controlled by these light elements. Hence, the total carbon and nitrogen content, and the N/(C#N) ratio are important parameters to control grain growth and phase composition, and thereby also the properties.
0304-3991/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 9 ) 0 0 0 5 4 - 6
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material. By optimising the parameters in#uencing the accuracy of quantitative EELS [3}7] so as to achieve a good agreement with the results obtained using atom-probe microanalysis, the potential of EELS for light element analysis in cermet materials is evaluated.
2. Experimental details 2.1. Material The experimental material was produced by sintering a raw material powder mixture of 64.5 Ti (C, N)-18.1 WC-17.4 Co (in wt%), resulting in two di!erent carbonitride phases, Ti (C, N) core and (Ti, W) (C, N) rim. Fig. 1. TEM micrograph of the cermet material used in this study. Hard carbonitride grains with a core-rim structure (Ti(C, N)-(Ti, W) (C, N)) are embedded in a tough metallic binder phase (Co-rich).
Quantitative microanalysis of light elements with su$cient accuracy and spatial resolution is often di$cult to achieve. So far, our group has used atom-probe "eld-ion microscopy (APFIM) for microanalysis of carbon and nitrogen in cermet materials. Energy dispersive X-ray analysis (EDX) in combination with transmission electron microscopy (TEM) provides too low detection e$ciency for light elements, and electron microprobe analysis (EMPA) lacks the required spatial resolution. However, the limited analysis volume of APFIM, together with its destructive nature, means that several APFIM specimens have to be prepared to analyse all phases present in one single TEM specimen. Hence, APFIM is a very time-consuming technique, and it would be desirable to be able to perform analysis of carbon and nitrogen, with good quantitative accuracy, in the TEM. Therefore, it is of interest to evaluate whether electron energy-loss spectroscopy (EELS) performed in the TEM can be considered an alternative to APFIM for light element analysis. In this paper, we compare results from quantitative EELS and APFIM microanalysis of the same carbonitride grains in a Ti(C, N)-WC-Co cermet
2.2. Specimen preparation Needle-shaped specimens for APFIM and EELS analysis were prepared by electropolishing rods with a cross section of 0.3 mm;0.3 mm in a solution of 5% H SO in methanol at !203C. Short (1}10 ms) 40 V pulses were used to achieve a specimen tip radius smaller than 50 nm and to back-polish the specimen until a carbonitride grain was present at the apex. All specimens were examined in the TEM during specimen preparation as well as before and after atom-probe analysis. One of the specimens used in this investigation is shown in Fig. 2.
Fig. 2. TEM micrograph of a needle-shaped specimen of the cermet material used in this study. This specimen geometry is required for APFIM so a special TEM specimen holder designed for needle-shaped specimens was used for EELS analysis.
J. Zackrisson et al. / Ultramicroscopy 79 (1999) 273}281
Thin foil TEM specimens were prepared by polishing and dimple grinding a 3 mm disc to a thickness of about 20 lm. Finally, the specimens were thinned to electron transparency by ion-milling in a Gatan DuoMill 600 using 4 kV Ar> ions with an incident angle of 123. 2.3. APFIM The atom-probe instrument used in this work is equipped with a Poschenrieder energy compensator which results in a high mass resolution. However, this increases detector pile-up which severely a!ects the detection of ions with a low massto-charge ratio such as carbon and nitrogen. Therefore, a spread in #ight path for identical ions evaporated at the same time is introduced by using a vertical line-focus on a tilted microchannel plate ion detector. A more detailed description of this instrument has been published previously [8]. To minimise e!ects of crystallography, "eld-ion microscopy was used to ensure that microanalysis was performed in high index directions only. All atom-probe spectra were statistically corrected for the detector dead-time. Spectra were evaluated and compositions calculated according to the suggestions given in Refs. [8}11]. Since atom-probe microanalysis is a destructive method, EELS has to be performed prior to APFIM to enable the same specimen region to be analysed. However, the contamination that occurs during EELS analysis is likely to increase the probability of specimen failure in the atom-probe. Therefore, a short APFIM analysis (about 3000 ions) was carried out before EELS as a precaution. After EELS analysis, the specimen was "nally analysed in the atom-probe for better statistics. 2.4. EELS Parallel EELS was performed in a 200 kV Philips CM200 SuperTwin Field Emission Gun TEM equipped with a Gatan Imaging Filter (GIF). In order to avoid crystallographic e!ects the specimen was tilted to high index directions prior to analysis. The convergence angle was always kept signi"cantly smaller than the collection angle so that convergence e!ects could be neglected. Since con-
275
tamination during EELS analysis makes carbon di$cult to quantify, di!erent TEM modes for acquisition were tried. The spectrometer entrance angle was 2 or 3 mm in diameter. The carbon K, nitrogen K and titanium L ionisation edges were used for quanti"cation. EELS spectra were evaluated using the Gatan EL/P software, both automatically by the program and by treating each ionisation edge individually. E!ects of plural scattering were removed according to the FourierRatio deconvolution method. To enable the same grain to be analysed with both techniques a special TEM specimen holder designed for needle-shaped specimens was used [12], since APFIM requires this specimen geometry. This TEM specimen holder allows the specimen to be tilted 903 around its axis, which made it possible to directly measure the specimen thickness and estimate the amount of contamination in the former beam direction. Quantitative EELS on thin-foil TEM specimens ascertained that the e!ect of the APFIM specimen geometry could be considered negligible for the experimental parameters we used.
3. Results 3.1. APFIM The compositions of a Ti(C, N) core and a (Ti, W) (C, N) rim as determined by atom-probe microanalysis are given in Table 1. Some atomic ratios are calculated from these results to allow direct comparison between APFIM and EELS (Tables 2 and 3). The APFIM spectra are shown in Fig. 3. The statistics are better for the Ti(C, N) core than for the (Ti, W) (C, N) rim as the number of ions detected and used for quanti"cation was signi"cantly larger in the analysis of the core. The substantially larger error limits for carbon and titanium than for the other elements are due to an overlap between the Ti> and the C> ions. 3.2. EELS The core and the rim previously analysed using APFIM (Table 1) were also subjected to EELS
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Table 1 Composition of a Ti(C, N) core and a (Ti, W)(C, N) rim as determined by APFIM (at%). The number of ions detected in these analyses were 51150 and 8380, respectively Element
Core
Rim
C N Ti O W
33.22$1.03 16.66$0.30 49.78$0.53 0.35$0.03 }
32.53$2.06 14.53$0.56 47.23$1.30 0.35$0.08 5.37$0.27
Table 2 APFIM and EELS results from the same Ti(C, N) carbonitride core. The EELS spectra have been quanti"ed using both an automatic and an optimised evaluation procedure Atomic ratio
APFIM
EELS automatic
EELS optimised
C/Ti N/Ti N/C
0.67$0.03 0.33$0.01 0.50$0.04
0.57$0.09 0.29$0.04 0.51$0.09
0.66$0.06 0.33$0.03 0.51$0.05
Table 3 APFIM and EELS results from the same (Ti, W) (C, N) carbonitride outer rim. The EELS spectra have been quanti"ed using both an automatic and an optimised evaluation procedure Atomic ratio
APFIM
EELS automatic
EELS optimised
C/Ti N/Ti N/C
0.69$0.06 0.31$0.02 0.45$0.05
0.70$0.11 0.32$0.05 0.46$0.10
0.68$0.06 0.31$0.03 0.46$0.04
analysis, see Fig. 4. The results achieved by using an automatic evaluation procedure are given as atomic ratios in Tables 2 and 3. To improve the accuracy, an evaluation method optimised to each individual ionisation edge was also performed on the same spectra (further discussed in Section 4.1). The results are given in Tables 2 and 3. Quanti"cation of tungsten using EELS is not accurate due to the low signal-to-noise ratio at high-energy losses (W M ionisation edge at 1809 eV). However,
Fig. 3. Atom-probe spectra of (a) Ti(C, N) core and (b) (Ti, W) (C, N) rim. The corresponding EELS spectra are shown in Fig. 4.
tungsten can be accurately quanti"ed using EDX in the TEM. Since this study is concentrated on absolute quanti"cation of carbon and nitrogen, tungsten has been excluded. Titanium is present in both carbonitride phases and used as a reference to be able to correlate EELS data to EDX data. Due to problems with contamination during EELS analysis, which severely complicates quanti"cation of carbon, the e!ect of acquiring spectra in di!erent TEM modes was investigated. It was found that by operating the TEM in image mode with a defocused electron beam instead of in diffraction mode with a focused beam, contamination could be almost completely avoided. Fig. 5 shows EELS spectra acquired in TEM image mode and TEM di!raction mode after di!erent times of illumination. Between acquisitions the specimen was
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4.1. EELS 4.1.1. Evaluation of cermet EELS spectra All EELS spectra were quanti"ed using both an automatic and an optimised evaluation procedure. In the automatic evaluation, the program treats all edges exactly the same, regardless of the spectrum appearance. By manually extrapolating the backgrounds and extracting the edge counts, it is possible to take, for example, edge overlap into account. Our results show that automatic evaluation of EELS spectra by the EL/P program works well for carbon and nitrogen but not always for titanium in cermet materials. This is mainly because the region where the background for the titanium L edge is to be extrapolated overlaps with the nitrogen K edge. Consequently, EELS spectra from cermet materials are preferably evaluated by treating each edge individually.
Fig. 4. EELS spectra of the same (a) a Ti(C, N) core and (b) a (Ti, W) (C, N) rim as in Fig. 3.
illuminated with a defocused beam, in TEM image mode as well as in TEM di!raction mode.
4.1.2. Background subtraction As with most microanalytical techniques, overlap between signals from di!erent elements present in the material is a problem di$cult to avoid also in EELS. In the case of cermet materials, the titanium L ionisation edge appears at an energy loss only 55 eV larger than that of the nitrogen K ionisation edge. This means that the structure of the nitrogen K ionisation edge is still present in the region most suitable to approximate the background for the titanium L ionisation edge. No other ionisation edges appear in the regions preceeding the carbon K edge nor the nitrogen K edge, making the background extrapolation of these edges straightforward.
4. Discussion Our results show that a good agreement between quantitative APFIM and EELS microanalytical results can be achieved. The evaluation procedure of EELS spectra optimised to each individual ionisation edge is superior to the automatic evaluation, especially for the atomic ratios involving titanium. For the N/C ratio, the results from the automatic evaluation are also satisfying. Below we discuss the use of EELS and APFIM for quantitative microanalysis of carbonitride grains in cermet materials in more detail.
4.1.3. Edge count extraction To include as many ionisation edge counts as possible in the integration window improves the statistics. Consequently, it is advisable to use as large an integration window as possible (+100 eV). However, this requires that the background approximation is good. If it is not, large errors are introduced when edge counts are extracted far away from the ionisation edge (i.e. far away from where the background extrapolation was made). In the case of cermet materials, the window widths for extracting nitrogen K and titanium
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Fig. 5. EELS spectra acquired in (a) TEM image mode with a defocused electron beam after 0, 10 and 20 min illumination, and (b) TEM di!raction mode with the beam focused on the specimen after being illuminated by a defocused beam for 0, 5 and 10 min.
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L edge counts were crucial. Since the titanium L edge lies close to the nitrogen K edge, an integration window larger than 45 eV for nitrogen K may also include titanium L edge counts. This also makes it di$cult to achieve a good background approximation for the titanium L edge, and an integration window extending further than 55 eV from the edge onset was found to introduce substantial errors. For the carbon K edge, integration windows between 50 and 100 eV gave the same result.
likely to cause drift problems. This makes accurate quanti"cation of these ionisation edges di$cult. Hence, in the TEM these heavier elements are preferably quanti"ed using EDX. In addition, the presence of molybdenum and niobium in carbonitride phases complicates the quanti"cation of carbon and nitrogen due to the overlap of molybdenum M and niobium M with the carbon K and nitrogen K ionisation edges.
4.1.4. Ionisation cross-sections The hydrogenic approximation [13,14] was used to compute the ionisation cross-sections for all edges used for quanti"cation. It has been suggested that the cross-section for the titanium L ionisa tion edge is better approximated with the Hartree}Slater model [15,16] together with an integration window starting after the white-lines [17]. In cermet materials, this approach introduced larger errors than using the hydrogenic appoximation with white-line corrections and an integration indow starting right at the edge onset. This is a consequence of the di$culty to achieve a good background approximation for the titanium L ionisation edge due to the overlap with the nitrogen K edge.
4.2.1. APFIM analysis of carbonitrides The Poschenrieder energy-compensator of the atom-probe instrument used in this investigation has a very high mass resolution. For microanalysis of carbonitride phases this is actually a disadvantage since ions in carbides, carbonitrides and nitrides tend to be "eld evaporated in pairs to a much higher extent than in metals. Hence, a signi"cant part of the most abundant ions is never detected due to the detector pile-up that arises at these events. In metals, it is possible to avoid detector pile-up by decreasing the "eld evaporation rate. In carbonitride phases, detector pile-up due to pair formation is a problem also at very low evaporation rates [8}11]. Therefore, a line-focus on a tilted detector is used to introduce a spread in #ight-time and thereby increase the detection e$ciency for these ion pairs. This worsens the mass resolution, but still increases the accuracy of quantitative APFIM microanalysis of the carbonitride phases in cermet materials. In addition, statistical corrections are made to take the remaining undetected ion pairs into account.
4.1.5. Contamination The problem with contamination during EELS analysis could be avoided if performing analysis in TEM image mode with a defocused probe instead of in TEM di!raction mode with a focused probe. This generally worsens the spatial resolution somewhat. However, the resolution that can be achieved when operating the TEM in image mode during EELS analysis is still su$cient for quantitative microanalysis of cermet materials with a typical grain size of 0.5 lm. 4.1.6. Heavy elements Cermet materials often contain heavier elements such as tungsten, tantalum, molybdenum and niobium. The energy losses for these elements are high (+2000 eV) and the signal-to-noise ratio is often low. Consequently, core loss spectra from these elements require long acquisition times which are
4.2. APFIM
4.2.2. Overlap deconvolution The important overlaps to consider in cermet carbonitride phases are Ti> and C>, as well as Ti> and O>. The former is more important as the oxygen content is always very low, and seldom as interesting as the carbon content. The isotope distributions of both C> and O> consist of one main isotope, and the other isotopes have a very low abundance. This complicates accurate overlap deconvolution, since titanium is the only element with a proper isotope distribution and titanium has to be statistically corrected for the lost ion pairs
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before deconvolution. As a consequence, the error bars for titanium, carbon and oxygen are larger than for the other elements.
complicated and straightforward, mainly due to overlap problems.
4.2.3. Heavy elements The deliberately worsened mass resolution introduced by the tilted detector and the line-focus severely a!ects the quanti"cation of heavy elements if their isotope distributions overlap, as for tungsten and tantalum [18]. Hence, there is a con#ict in accurate quanti"cation of both heavy and light elements simultaneously in some cermet materials.
5. Conclusions
4.3. EELS vs APFIM We have shown that EELS and APFIM give the same quantitative results for carbonitride grains in cermet materials. Still, these methods are not completely interchangeable. Therefore, we would like to point out some advantages and drawbacks. A severe drawback with APFIM as compared to EELS is that APFIM is a destructive method. That is, the specimen is consumed during analysis which makes it impossible to perform analysis once more on the same specimen volume. In addition, APFIM analysis often causes specimen failure due to the high "eld applied to the specimen apex. Therefore, most APFIM specimens can be used only for one analysis. APFIM is a time-consuming method, especially from a specimen preparation point of view. Since most of our microstructural characterisation of cermets is already performed in the TEM it would be more time-e$cient for us to quantify the light element content using EELS instead of APFIM. Our results show that this is possible for a signi"cant part of our future needs. The easily achieved high-spatial resolution of APFIM is still di$cult for EELS to compete with. Hence, we still consider APFIM an alternative to EELS for example for the study of grain boundaries, phase boundaries, local compositional variations within grains and segregation. Both EELS and APFIM show problems with accurate quantitative microanalysis of heavy elements at the same time as achieving good results for light elements. None of the techniques have an evaluation and quanti"cation procedure that is un-
E The agreement between EELS and APFIM is good enough to permit most future light element analyses of these materials to be performed in the TEM using EELS. However, we still prefer APFIM for some speci"c problems that demand high-spatial resolution, such as quantitative microanalysis of grain or phase boundaries. E Contamination during EELS can to a large extent be avoided if the TEM is operated in image mode with a defocused electron beam instead of in di!raction mode with a focused probe. The disadvantage is worse spatial resolution, but for microanalysis of sub-micron grains the resolution that can be achieved with TEM image mode is su$cient. E In these materials, automatic evaluation of EELS spectra by the EL/P program works well for carbon and nitrogen but not always for titanium. This is mainly because the nitrogen K edge overlaps with the region where the background for the titanium L edge is to be extrapolated. Consequently, EELS spectra from cermet materials are preferably evaluated by treating each edge individually.
Acknowledgements Financial support from the Swedish Research Council for Engineering Science (TFR) is gratefully acknowledged. AB Sandvik Coromant is thanked for providing the materials.
References [1] P.O. Snell, Planseeberichte fuK r Pulvermetallurgie 22 (1974) 91. [2] A.H. Heuer, J.S. Sears, N.J. Zaluzec, in: Rhodes, E.A. Almond, C.A. Brookes, R. Warren (Eds.), Proceedings of International Conference on the Science of Hard Materials 2, Adam Hilger, Bristol and Boston, 1986, p. 321.
J. Zackrisson et al. / Ultramicroscopy 79 (1999) 273}281 [3] R.F. Egerton, Electron energy-loss spectrocopy in the transmission electron microscope, 2nd ed., Plenum Press, New York, 1996. [4] F. Hofer, Microsc. Microanal. Microstruct. 2 (1991) 215. [5] L. Reimer (Ed.), Energy-"ltering transmission electron microscopy, Springer, Berlin, 1995. [6] D.B. Williams, C.B. Carter, Transmission electron microscopy, Plenum Press, New York, 1996. [7] R.F. Egerton, Microsc. Microanal. Microstruct. 2 (1991) 203. [8] U. Rolander, H.-O. AndreH n, Appl. Surf. Sci. 76/77 (1994) 392. [9] U. Rolander, H.O. AndreH n, J. Phys. 49 (1988) 299.
[10] [11] [12] [13] [14]
[15] [16] [17] [18]
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U. Rolander, H.-O. AndreH n, J. Phys. 50 (1989) 371. U. Rolander, H.-O. AndreH n, J. Phys. 50 (1989) 529. B. Loberg, H. NordeH n, Arkiv Fysik 39 (1969) 383. R.F. Egerton, Ultramicroscopy 4 (1979) 169. R.F. Egerton, in: G.W. Bailey (Ed.), Proceedings of the 39th Ann. EMSA meeting, Claitor's Publishing, Baton Rouge, Lousiana, 1981, p. 198. R.D. Leapman, P. Rez, D.F. Mayers, J. Chem. Phys. 72 (1980) 1232. P. Rez, Ultramicroscopy 28 (1989) 16. C.C. Ahn, P. Rez, Ultramicroscopy 17 (1985) 105. J. Zackrisson, M. Thuvander, P. Lindahl, H.-O. AndreH n, Appl. Surf. Sci. 94/95 (1996) 351.
A comparison between quantitative EELS and APFIM microanalysis of carbonitride grains in cermets J. Zackrisson *, W. Grogger, F. Hofer, H.-O. AndreH n Department of Experimental Physics, Chalmers University of Technology and Go( teborg University, SE-412 96 Go( teborg, Sweden Forschungsinstitut fu( r Elektronenmikroskopie, Technische Universita( t Graz, A-8010 Graz, Austria Received 12 September 1998; received in revised form 22 January 1999
Abstract Microstructural characterisation of cermet materials is often performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with energy dispersive X-ray analysis (EDX). Due to the poor detection e$ciency of light elements with EDX, carbon and nitrogen cannot be properly quanti"ed. Instead, atom-probe "eld-ion microscopy (APFIM) has been used to accurately determine the content of light elements. However, this is a time-consuming method, especially when taking into account the fact that several APFIM specimens have to be prepared in a controlled way to analyse all phases present in one TEM specimen. Therefore, it is of interest to evaluate whether transmission electron energy-loss spectroscopy (EELS) can be considered an alternative to APFIM for light element analysis. In this paper, we make a comparison of results from APFIM and quantitative EELS microanalysis of carbon and nitrogen in Ti(C, N) -based cermets. Our results show that the agreement between the two methods is good enough to permit most future analyses of light elements in these materials to be performed in the TEM using EELS. 1999 Elsevier Science B.V. All rights reserved. Keywords: TEM; Cermets; Hardmetals; Ti(C, N); Light elements
1. Introduction Cermets are Ti(C, N)-based cutting tool materials manufactured by sintering pressed raw material powders to a dense body. During the sintering process a liquid phase is formed enabling the powder particles to rearrange, dissolve and recrystallise. A cermet microstructure consists of hard carbonitride grains embedded in a tough binder phase, see Fig. 1. The carbonitride grains often have a
* Corresponding author. Tel.: #46-31-772-3325; fax: #4631-772-3224. E-mail address: [email protected] (J. Zackrisson)
core-rim structure [1,2], where the core is a remnant of the raw material and the rim is formed during sintering. The microstructure is determined by the raw material powder mixture as well as by the sintering conditions, and can be considered a link between the process and the properties. The distribution of carbon and nitrogen in the di!erent phases is of great interest to the understanding of cermet materials, since the solubility and di!usion of other elements during the sintering process can be controlled by these light elements. Hence, the total carbon and nitrogen content, and the N/(C#N) ratio are important parameters to control grain growth and phase composition, and thereby also the properties.
0304-3991/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 9 ) 0 0 0 5 4 - 6
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material. By optimising the parameters in#uencing the accuracy of quantitative EELS [3}7] so as to achieve a good agreement with the results obtained using atom-probe microanalysis, the potential of EELS for light element analysis in cermet materials is evaluated.
2. Experimental details 2.1. Material The experimental material was produced by sintering a raw material powder mixture of 64.5 Ti (C, N)-18.1 WC-17.4 Co (in wt%), resulting in two di!erent carbonitride phases, Ti (C, N) core and (Ti, W) (C, N) rim. Fig. 1. TEM micrograph of the cermet material used in this study. Hard carbonitride grains with a core-rim structure (Ti(C, N)-(Ti, W) (C, N)) are embedded in a tough metallic binder phase (Co-rich).
Quantitative microanalysis of light elements with su$cient accuracy and spatial resolution is often di$cult to achieve. So far, our group has used atom-probe "eld-ion microscopy (APFIM) for microanalysis of carbon and nitrogen in cermet materials. Energy dispersive X-ray analysis (EDX) in combination with transmission electron microscopy (TEM) provides too low detection e$ciency for light elements, and electron microprobe analysis (EMPA) lacks the required spatial resolution. However, the limited analysis volume of APFIM, together with its destructive nature, means that several APFIM specimens have to be prepared to analyse all phases present in one single TEM specimen. Hence, APFIM is a very time-consuming technique, and it would be desirable to be able to perform analysis of carbon and nitrogen, with good quantitative accuracy, in the TEM. Therefore, it is of interest to evaluate whether electron energy-loss spectroscopy (EELS) performed in the TEM can be considered an alternative to APFIM for light element analysis. In this paper, we compare results from quantitative EELS and APFIM microanalysis of the same carbonitride grains in a Ti(C, N)-WC-Co cermet
2.2. Specimen preparation Needle-shaped specimens for APFIM and EELS analysis were prepared by electropolishing rods with a cross section of 0.3 mm;0.3 mm in a solution of 5% H SO in methanol at !203C. Short (1}10 ms) 40 V pulses were used to achieve a specimen tip radius smaller than 50 nm and to back-polish the specimen until a carbonitride grain was present at the apex. All specimens were examined in the TEM during specimen preparation as well as before and after atom-probe analysis. One of the specimens used in this investigation is shown in Fig. 2.
Fig. 2. TEM micrograph of a needle-shaped specimen of the cermet material used in this study. This specimen geometry is required for APFIM so a special TEM specimen holder designed for needle-shaped specimens was used for EELS analysis.
J. Zackrisson et al. / Ultramicroscopy 79 (1999) 273}281
Thin foil TEM specimens were prepared by polishing and dimple grinding a 3 mm disc to a thickness of about 20 lm. Finally, the specimens were thinned to electron transparency by ion-milling in a Gatan DuoMill 600 using 4 kV Ar> ions with an incident angle of 123. 2.3. APFIM The atom-probe instrument used in this work is equipped with a Poschenrieder energy compensator which results in a high mass resolution. However, this increases detector pile-up which severely a!ects the detection of ions with a low massto-charge ratio such as carbon and nitrogen. Therefore, a spread in #ight path for identical ions evaporated at the same time is introduced by using a vertical line-focus on a tilted microchannel plate ion detector. A more detailed description of this instrument has been published previously [8]. To minimise e!ects of crystallography, "eld-ion microscopy was used to ensure that microanalysis was performed in high index directions only. All atom-probe spectra were statistically corrected for the detector dead-time. Spectra were evaluated and compositions calculated according to the suggestions given in Refs. [8}11]. Since atom-probe microanalysis is a destructive method, EELS has to be performed prior to APFIM to enable the same specimen region to be analysed. However, the contamination that occurs during EELS analysis is likely to increase the probability of specimen failure in the atom-probe. Therefore, a short APFIM analysis (about 3000 ions) was carried out before EELS as a precaution. After EELS analysis, the specimen was "nally analysed in the atom-probe for better statistics. 2.4. EELS Parallel EELS was performed in a 200 kV Philips CM200 SuperTwin Field Emission Gun TEM equipped with a Gatan Imaging Filter (GIF). In order to avoid crystallographic e!ects the specimen was tilted to high index directions prior to analysis. The convergence angle was always kept signi"cantly smaller than the collection angle so that convergence e!ects could be neglected. Since con-
275
tamination during EELS analysis makes carbon di$cult to quantify, di!erent TEM modes for acquisition were tried. The spectrometer entrance angle was 2 or 3 mm in diameter. The carbon K, nitrogen K and titanium L ionisation edges were used for quanti"cation. EELS spectra were evaluated using the Gatan EL/P software, both automatically by the program and by treating each ionisation edge individually. E!ects of plural scattering were removed according to the FourierRatio deconvolution method. To enable the same grain to be analysed with both techniques a special TEM specimen holder designed for needle-shaped specimens was used [12], since APFIM requires this specimen geometry. This TEM specimen holder allows the specimen to be tilted 903 around its axis, which made it possible to directly measure the specimen thickness and estimate the amount of contamination in the former beam direction. Quantitative EELS on thin-foil TEM specimens ascertained that the e!ect of the APFIM specimen geometry could be considered negligible for the experimental parameters we used.
3. Results 3.1. APFIM The compositions of a Ti(C, N) core and a (Ti, W) (C, N) rim as determined by atom-probe microanalysis are given in Table 1. Some atomic ratios are calculated from these results to allow direct comparison between APFIM and EELS (Tables 2 and 3). The APFIM spectra are shown in Fig. 3. The statistics are better for the Ti(C, N) core than for the (Ti, W) (C, N) rim as the number of ions detected and used for quanti"cation was signi"cantly larger in the analysis of the core. The substantially larger error limits for carbon and titanium than for the other elements are due to an overlap between the Ti> and the C> ions. 3.2. EELS The core and the rim previously analysed using APFIM (Table 1) were also subjected to EELS
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Table 1 Composition of a Ti(C, N) core and a (Ti, W)(C, N) rim as determined by APFIM (at%). The number of ions detected in these analyses were 51150 and 8380, respectively Element
Core
Rim
C N Ti O W
33.22$1.03 16.66$0.30 49.78$0.53 0.35$0.03 }
32.53$2.06 14.53$0.56 47.23$1.30 0.35$0.08 5.37$0.27
Table 2 APFIM and EELS results from the same Ti(C, N) carbonitride core. The EELS spectra have been quanti"ed using both an automatic and an optimised evaluation procedure Atomic ratio
APFIM
EELS automatic
EELS optimised
C/Ti N/Ti N/C
0.67$0.03 0.33$0.01 0.50$0.04
0.57$0.09 0.29$0.04 0.51$0.09
0.66$0.06 0.33$0.03 0.51$0.05
Table 3 APFIM and EELS results from the same (Ti, W) (C, N) carbonitride outer rim. The EELS spectra have been quanti"ed using both an automatic and an optimised evaluation procedure Atomic ratio
APFIM
EELS automatic
EELS optimised
C/Ti N/Ti N/C
0.69$0.06 0.31$0.02 0.45$0.05
0.70$0.11 0.32$0.05 0.46$0.10
0.68$0.06 0.31$0.03 0.46$0.04
analysis, see Fig. 4. The results achieved by using an automatic evaluation procedure are given as atomic ratios in Tables 2 and 3. To improve the accuracy, an evaluation method optimised to each individual ionisation edge was also performed on the same spectra (further discussed in Section 4.1). The results are given in Tables 2 and 3. Quanti"cation of tungsten using EELS is not accurate due to the low signal-to-noise ratio at high-energy losses (W M ionisation edge at 1809 eV). However,
Fig. 3. Atom-probe spectra of (a) Ti(C, N) core and (b) (Ti, W) (C, N) rim. The corresponding EELS spectra are shown in Fig. 4.
tungsten can be accurately quanti"ed using EDX in the TEM. Since this study is concentrated on absolute quanti"cation of carbon and nitrogen, tungsten has been excluded. Titanium is present in both carbonitride phases and used as a reference to be able to correlate EELS data to EDX data. Due to problems with contamination during EELS analysis, which severely complicates quanti"cation of carbon, the e!ect of acquiring spectra in di!erent TEM modes was investigated. It was found that by operating the TEM in image mode with a defocused electron beam instead of in diffraction mode with a focused beam, contamination could be almost completely avoided. Fig. 5 shows EELS spectra acquired in TEM image mode and TEM di!raction mode after di!erent times of illumination. Between acquisitions the specimen was
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4.1. EELS 4.1.1. Evaluation of cermet EELS spectra All EELS spectra were quanti"ed using both an automatic and an optimised evaluation procedure. In the automatic evaluation, the program treats all edges exactly the same, regardless of the spectrum appearance. By manually extrapolating the backgrounds and extracting the edge counts, it is possible to take, for example, edge overlap into account. Our results show that automatic evaluation of EELS spectra by the EL/P program works well for carbon and nitrogen but not always for titanium in cermet materials. This is mainly because the region where the background for the titanium L edge is to be extrapolated overlaps with the nitrogen K edge. Consequently, EELS spectra from cermet materials are preferably evaluated by treating each edge individually.
Fig. 4. EELS spectra of the same (a) a Ti(C, N) core and (b) a (Ti, W) (C, N) rim as in Fig. 3.
illuminated with a defocused beam, in TEM image mode as well as in TEM di!raction mode.
4.1.2. Background subtraction As with most microanalytical techniques, overlap between signals from di!erent elements present in the material is a problem di$cult to avoid also in EELS. In the case of cermet materials, the titanium L ionisation edge appears at an energy loss only 55 eV larger than that of the nitrogen K ionisation edge. This means that the structure of the nitrogen K ionisation edge is still present in the region most suitable to approximate the background for the titanium L ionisation edge. No other ionisation edges appear in the regions preceeding the carbon K edge nor the nitrogen K edge, making the background extrapolation of these edges straightforward.
4. Discussion Our results show that a good agreement between quantitative APFIM and EELS microanalytical results can be achieved. The evaluation procedure of EELS spectra optimised to each individual ionisation edge is superior to the automatic evaluation, especially for the atomic ratios involving titanium. For the N/C ratio, the results from the automatic evaluation are also satisfying. Below we discuss the use of EELS and APFIM for quantitative microanalysis of carbonitride grains in cermet materials in more detail.
4.1.3. Edge count extraction To include as many ionisation edge counts as possible in the integration window improves the statistics. Consequently, it is advisable to use as large an integration window as possible (+100 eV). However, this requires that the background approximation is good. If it is not, large errors are introduced when edge counts are extracted far away from the ionisation edge (i.e. far away from where the background extrapolation was made). In the case of cermet materials, the window widths for extracting nitrogen K and titanium
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Fig. 5. EELS spectra acquired in (a) TEM image mode with a defocused electron beam after 0, 10 and 20 min illumination, and (b) TEM di!raction mode with the beam focused on the specimen after being illuminated by a defocused beam for 0, 5 and 10 min.
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L edge counts were crucial. Since the titanium L edge lies close to the nitrogen K edge, an integration window larger than 45 eV for nitrogen K may also include titanium L edge counts. This also makes it di$cult to achieve a good background approximation for the titanium L edge, and an integration window extending further than 55 eV from the edge onset was found to introduce substantial errors. For the carbon K edge, integration windows between 50 and 100 eV gave the same result.
likely to cause drift problems. This makes accurate quanti"cation of these ionisation edges di$cult. Hence, in the TEM these heavier elements are preferably quanti"ed using EDX. In addition, the presence of molybdenum and niobium in carbonitride phases complicates the quanti"cation of carbon and nitrogen due to the overlap of molybdenum M and niobium M with the carbon K and nitrogen K ionisation edges.
4.1.4. Ionisation cross-sections The hydrogenic approximation [13,14] was used to compute the ionisation cross-sections for all edges used for quanti"cation. It has been suggested that the cross-section for the titanium L ionisa tion edge is better approximated with the Hartree}Slater model [15,16] together with an integration window starting after the white-lines [17]. In cermet materials, this approach introduced larger errors than using the hydrogenic appoximation with white-line corrections and an integration indow starting right at the edge onset. This is a consequence of the di$culty to achieve a good background approximation for the titanium L ionisation edge due to the overlap with the nitrogen K edge.
4.2.1. APFIM analysis of carbonitrides The Poschenrieder energy-compensator of the atom-probe instrument used in this investigation has a very high mass resolution. For microanalysis of carbonitride phases this is actually a disadvantage since ions in carbides, carbonitrides and nitrides tend to be "eld evaporated in pairs to a much higher extent than in metals. Hence, a signi"cant part of the most abundant ions is never detected due to the detector pile-up that arises at these events. In metals, it is possible to avoid detector pile-up by decreasing the "eld evaporation rate. In carbonitride phases, detector pile-up due to pair formation is a problem also at very low evaporation rates [8}11]. Therefore, a line-focus on a tilted detector is used to introduce a spread in #ight-time and thereby increase the detection e$ciency for these ion pairs. This worsens the mass resolution, but still increases the accuracy of quantitative APFIM microanalysis of the carbonitride phases in cermet materials. In addition, statistical corrections are made to take the remaining undetected ion pairs into account.
4.1.5. Contamination The problem with contamination during EELS analysis could be avoided if performing analysis in TEM image mode with a defocused probe instead of in TEM di!raction mode with a focused probe. This generally worsens the spatial resolution somewhat. However, the resolution that can be achieved when operating the TEM in image mode during EELS analysis is still su$cient for quantitative microanalysis of cermet materials with a typical grain size of 0.5 lm. 4.1.6. Heavy elements Cermet materials often contain heavier elements such as tungsten, tantalum, molybdenum and niobium. The energy losses for these elements are high (+2000 eV) and the signal-to-noise ratio is often low. Consequently, core loss spectra from these elements require long acquisition times which are
4.2. APFIM
4.2.2. Overlap deconvolution The important overlaps to consider in cermet carbonitride phases are Ti> and C>, as well as Ti> and O>. The former is more important as the oxygen content is always very low, and seldom as interesting as the carbon content. The isotope distributions of both C> and O> consist of one main isotope, and the other isotopes have a very low abundance. This complicates accurate overlap deconvolution, since titanium is the only element with a proper isotope distribution and titanium has to be statistically corrected for the lost ion pairs
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before deconvolution. As a consequence, the error bars for titanium, carbon and oxygen are larger than for the other elements.
complicated and straightforward, mainly due to overlap problems.
4.2.3. Heavy elements The deliberately worsened mass resolution introduced by the tilted detector and the line-focus severely a!ects the quanti"cation of heavy elements if their isotope distributions overlap, as for tungsten and tantalum [18]. Hence, there is a con#ict in accurate quanti"cation of both heavy and light elements simultaneously in some cermet materials.
5. Conclusions
4.3. EELS vs APFIM We have shown that EELS and APFIM give the same quantitative results for carbonitride grains in cermet materials. Still, these methods are not completely interchangeable. Therefore, we would like to point out some advantages and drawbacks. A severe drawback with APFIM as compared to EELS is that APFIM is a destructive method. That is, the specimen is consumed during analysis which makes it impossible to perform analysis once more on the same specimen volume. In addition, APFIM analysis often causes specimen failure due to the high "eld applied to the specimen apex. Therefore, most APFIM specimens can be used only for one analysis. APFIM is a time-consuming method, especially from a specimen preparation point of view. Since most of our microstructural characterisation of cermets is already performed in the TEM it would be more time-e$cient for us to quantify the light element content using EELS instead of APFIM. Our results show that this is possible for a signi"cant part of our future needs. The easily achieved high-spatial resolution of APFIM is still di$cult for EELS to compete with. Hence, we still consider APFIM an alternative to EELS for example for the study of grain boundaries, phase boundaries, local compositional variations within grains and segregation. Both EELS and APFIM show problems with accurate quantitative microanalysis of heavy elements at the same time as achieving good results for light elements. None of the techniques have an evaluation and quanti"cation procedure that is un-
E The agreement between EELS and APFIM is good enough to permit most future light element analyses of these materials to be performed in the TEM using EELS. However, we still prefer APFIM for some speci"c problems that demand high-spatial resolution, such as quantitative microanalysis of grain or phase boundaries. E Contamination during EELS can to a large extent be avoided if the TEM is operated in image mode with a defocused electron beam instead of in di!raction mode with a focused probe. The disadvantage is worse spatial resolution, but for microanalysis of sub-micron grains the resolution that can be achieved with TEM image mode is su$cient. E In these materials, automatic evaluation of EELS spectra by the EL/P program works well for carbon and nitrogen but not always for titanium. This is mainly because the nitrogen K edge overlaps with the region where the background for the titanium L edge is to be extrapolated. Consequently, EELS spectra from cermet materials are preferably evaluated by treating each edge individually.
Acknowledgements Financial support from the Swedish Research Council for Engineering Science (TFR) is gratefully acknowledged. AB Sandvik Coromant is thanked for providing the materials.
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