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TiCN composites
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Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive ␣- SiAlON/TiCN composites Hilmi Yurdakul a , Servet Turan b,∗ , Erhan Ayas b , Orkun Tunckan c , Alpagut Kara b a
Dumlupınar University, Faculty of Engineering, Department of Materials Science and Engineering, Evliya Celebi Campus, TR-43100 Kutahya, Turkey b Anadolu University, Faculty of Engineering, Department of Materials Science and Engineering, Iki Eylul Campus, TR-26480 Eskisehir, Turkey c Anadolu University, Faculty of Aeronautics and Astronautics, Department of Airframe and Powerplant Maintenance, Iki Eylul Campus, TR-26480 Eskisehir, Turkey Received 22 January 2014; received in revised form 20 March 2014; accepted 24 March 2014
Abstract Coating of ␣- SiAlON granules with TiCN powder led to a continuous 3D electrically conductive network formation. Here, the stability of TiCN grains following gas pressure sintering (GPS) and its effect on the electrical properties of an ␣- SiAlON/TiCN composite were analytically investigated by using transmission electron microscopy (TEM) based techniques. In situ formation of nano-sized SiC grains adjacent to TiCN and ␣- SiAlON grains were observed. These SiC grains may play a role on the high electrical conductivity of ␣- SiAlON/TiCN composite. Ti:C:N ratios from TiCN grains along the network showed that TiCN grains did not preserve their initial composition after sintering. Finally, Ti diffusion from TiCN grains into ␣- SiAlON and triple junction phases was observed. The incorporation of Ti into the SiAlON crystal lattices contributes to the high electrical conductivity of ␣- SiAlON/TiCN composite. © 2014 Elsevier Ltd. All rights reserved. Keywords: Composite; Electrical conductivity; SiAlON; TiCN; Transmission electron microscopy
1. Introduction In recent years, much effort has been devoted to broaden the application area of SiAlON-based composites with different approaches.1,2 Due to the finely adjustable mechanical, thermal and optical properties, these materials have been successfully employed so far in a wide range of applications such as cutting tools, ball bearings and light emitting diodes (LEDs).2–5 In particular interest, SiAlON-based composites can be utilized in electrical applications such as glow plugs and heaters to achieve the high electrical conductivity without deteriorating the mechanical properties.1 In addition, this feature gives the opportunity of producing complex shaped parts by employing electro-discharge machining (EDM) process, which excludes the time-consuming and costly secondary machining operations.6–8 In order to obtain electrical conductivity in such
∗
Corresponding author. Tel.: +90 222 335 0580x6350; fax: +90 222 323 9501. E-mail address: [email protected] (S. Turan).
systems, the addition of highly conductive secondary phases, e.g. TiN,9–12 SiC,13–15 TaN,16 MoSi2 ,17–19 Ti(C,N)20,21 and TiB2 ,22 has been extensively studied. Amongst these studies, TiN and Ti(C,N) have been found to be more effective additives in terms of their chemical compatibility and easy sinterability. With this particular approach, the conductive particles of at least 30 vol.% should be incorporated into Si3 N4 and/or SiAlON matrix to obtain desired electrical conductivity. However, addition of such a large amount of secondary phases leads to degradation in mechanical and thermal properties. This is due to covalent nature of these phases and undesirable reactions between constituents hindering the densification of designed when conventional sintering techniques like gas pressure and pressureless sintering have been performed. In previous study,23 the amount of conductive phase was decreased as low as to 5 vol.% through the segregated network concept. The electrical resistivity of insulator SiAlON matrix (∼1013 m) was found to be drastically decreased to 18 × 10−4 m values, when 5 vol.% nano TiCN particles were added. Reactions between the TiCN and Si3 N4 particles as
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well as liquid phase during the gas pressure sintering process were discussed by microstructural observations through scanning electron microscopy (SEM) and thermodynamic approach. However, due to high beam broadening in SEM, the precise data on the stability of TiCN grains after sintering and its effect on electrical conductivity in ␣- SiAlON/TiCN composites were missing. Therefore, advanced microstructural characterization techniques to further elucidate the aforementioned deficiencies were necessary. To the best of the authors’ knowledge, few data are available on the clarification of in situ phase formations and interactions between SiAlON and TiN/TiCN grains by transmission electron microscopy (TEM) based techniques. In this work, analytical TEM studies on a previously developed an electrically conductive ␣- SiAlON/TiCN composite were undertaken.
2. Material and methods The detailed production of the electrically conductive ␣- SiAlON/TiCN composites was given in a previous study.23 In that study the spray dried SiAlON-based granules (≤100 m) were coated with varying amounts of (up to 10 vol.%) nano size TiCN (≤150 nm) particles. All the composites together with a reference SiAlON sample were fully densified by gas pressure sintering (GPS) at 1990 ◦ C under 10 MPa N2 gas pressure. A continuous chain-type 3D microstructure to provide the electrical conductivity in the composites was successfully obtained. The investigated sample in this study was composed of 10 vol.% TiCN nano particles as a continuous network in ␣- SiAlON matrix, exhibiting high electrical conductivity. For analytical TEM investigations, an electron transparent specimen was prepared by conventional mechanical polishing and Ar-ion beam thinning (Leica Microsystems EM RES101). Afterwards, the sample was characterized by using a field emission TEM (Jeol 2100F), operating at 200 kV and equipped with a high angle annular dark field scanning transmission electron microscope (HAADF-STEM) detector (Fishione), annular dark field (ADF) detector, an energy filter (Gatan GIF Tridiem), parallel electron energy loss spectrometer (EELS), and an energy dispersive X-ray (EDX) spectrometer (Jeol JED-2300T). In STEM-based EDX, EELS and spectrum imaging (SI) analyses, an electron spot with 1–2 nm in diameter was used. Furthermore, a drift corrector was performed to avoid any possible drifts that may occur at nano-scale during the acquisition of STEM-EDX/EELS/SI analyses. During EDX analysis, STEM mode was preferred instead of TEM mode, because the spatial resolution of EDX in STEM is smaller (only a few nanometers).24 In STEM-based EELS analysis, the convergence and collection semi-angles were used as 9.2 and 15.7 mrad, respectively. The spectrometer energy dispersion was also chosen in 0.5 eV/channels. The backgrounds in EELS and energy filtering transmission electron microscope (EFTEM) 3-window elemental mapping analyses were subtracted according to power-law.25 For quantitative EELS analysis, the partial inelastic cross-sections were derived from Hartree-Slater model,25 and the calculations were based on the relative quantification method.25 Furthermore,
quantitative STEM-EDX analysis was carried out using standardless Cliff–Lorimer quantification method.26 3. Results and discussion In Fig. 1(a–b), bright field (BF) TEM images are shown from the different parts of the ␣- SiAlON/TiCN composite. Here, a well bonding and dense structure between the SiAlON and TiCN grains following the sintering was clearly observed. It was considered that this type of continuous TiCN network formation played a significant role on the attained high electrical conductivity.23 On the other hand, the presence of nano-sized intergranular and transgranular cracks as defect structures were detected around the TiCN grains (highlighted with white colored arrows in Fig. 1). These cracks were most likely generated during sintering due to the thermal expansion coefficient mismatch between the SiAlON and TiCN grains, which gives rise to thermal stresses.27 However, the visibility of these defects along the TiCN network was very limited. Spherical-shaped inclusions with approximately 50–100 nm in diameter were also observed in between and within the TiCN and SiAlON grains (indicated by black colored arrows in Fig. 1). It is suggested that these SiC inclusions are formed as a result of the reactions between the TiCN and Si3 N4 /SiO2 grains during sintering.20,28 In order to clarify the chemical composition of these inclusions, energy filtering TEM (EFTEM) 3-window elemental mapping analysis was performed along the different regions of TiCN network (Fig. 2). Based on the EFTEM elemental maps in Fig. 2(a–h), in situ formation of nano-sized SiC phase can be easily observed between the individual TiCN grains. This result revealed that SiC grains did not only occur in a certain region of TiCN segregated network structure, but also they formed throughout the network. Furthermore, considering Fig. 2(h) in detail (white colored arrows), nano-sized SiC grains were also found between and within SiAlON matrix grains. It is well known that electrical conductivity of SiC varies in terms of the dopant type and temperature for bulk materials. At room temperature, SiC shows insulating or semi-conducting behavior, but the electrical conductivity increases with the increasing temperature.29 The resistive behavior of trapped SiC grains may lead to decrease on the composite’s conductivity; however, as seen in the images, these particles are randomly and discontinuously formed and do not deteriorate the chain type TiCN particle interaction. Therefore, the composites have high electrical conductivity. More surprisingly, the large-size in situ SiC formations, almost as large as TiCN grains, were also detected between the TiCN and SiAlON grains (shown in circle in Fig. 2(g)). This gives important information about the intensive interactive diffusion evolved during the reaction of constituents. Fig. 3 shows the atomic number (Z)-contrast STEM image acquired from the TiCN network structure. Here, the TiCN and SiAlON grains can be seen as white and black contrast, respectively. This is due to the fact that the number of electrons scattered from the Ti (Z = 22) element in TiCN phase is much higher than that of Al (Z = 13) and Si (Z = 14) in SiAlON matrix. There was an evidence of liquid phase penetration in between
Please cite this article in press as: Yurdakul H, et al. Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive ␣- SiAlON/TiCN composites. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.03.024
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Fig. 1. (a and b) Bright-field (BF) TEM images acquired from the different regions of ␣- SiAlON/TiCN composite.
TiCN grains showing the densification mechanism of the composite (marked with red arrows in Fig. 3). Also, the drops of residual liquid phase on the TiCN grains were clearly visible. It is therefore believed that the penetration of considerable amount of liquid phase along the TiCN grains facilitates the reaction between TiCN and Si3 N4 /SiO2 particles to form in situ SiC. This finding is in agreement with a thermodynamic approach on the formation of in situ SiC phase, reported in a previous study.23 To reveal the stability of TiCN grains by using EELS analysis after sintering, STEM-SI data set was first collected from the electron transparent sample, shown in Fig. 4(a–b). Afterwards, qualitative EELS elemental maps were subtracted from the STEM-SI data set by using C-K (284 eV), N-K (401 eV) and Ti-L3,2 (456 eV) edges (Fig. 4(c–e)). Finally, a RGB
(RedGreenBlue) composite map by overlaying the individual EELS maps in Fig. 4(c–e) was constructed (Fig. 4(f)). Considering the EELS maps (Fig. 4(c–e)), the formation of nano-sized in situ SiC phase can be easily distinguished both between and within the TiCN and SiAlON grains. These results confirm the EFTEM observations given in Fig. 2(a–h). More surprisingly, by studying at the Ti-L3,2 EELS map in detail (Fig. 4(e)), Ti containing SiAlON grains can be recognized. Thus, it can be said that not only C diffuses from the TiCN grains during the sintering, but also Ti. Simple explanation of this phenomenon is that, the diffusion of Si4+ ions from the SiAlON lattice during the formation of nano-sized in situ SiC phase probably gives rise to an ion charge imbalance in Si(O,N)4 tetrahedrons. This charge imbalance can be compensated by the substitution of Ti4+ ions by Si4+ ions.30 This confirms the recent
Fig. 2. EFTEM-3 window elemental mapping results obtained from the same regions in Fig. 1(a and b); (a and b) C-K (284 eV) edge, (c and d) N-K (401 eV) edge, (e and f) Ti-L3,2 (456 eV) edge and (g and h) RGB (RedGreenBlue) composite maps. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Atomic number (Z) contrast scanning TEM (STEM) image.
TEM studies by Yurdakul and Turan,30,31 which demonstrated the incorporation of transition metals, i.e. Fe, Cr and Ti, into -SiAlON crystal structure. Therefore, herein the existence of Ti in ␣- SiAlON crystal structures is feasible. For the calculation of compositional variations on the TiCN segregated network, STEM-SI-EELS spot data from the
individual grains composing the network (points 1–15 in Fig. 4(b)) were used. The average composition of every individual TiCN grain in the network was summarized in Table 1. Here, the results showed that chemical compositions of individual TiCN grains existing in network differed from grain to grain because of the changeable diffusion of the Ti, C and N elements in the system. To strengthen the results, the atomic concentrations of Ti, C and N elements throughout an AB line were calculated based on the quantitative EELS elemental maps (Fig. 5(a–c)) and graphically shown in Fig. 5(d–f). By using these results, the chemical compositions of TiCN grains can be calculated at a specific distance of the AB line. For example, at 1 m, the stoichiometry of a TiCN grain was calculated to be TiC0.17 N0.49 (atomic %). Also note that the composition of any desirable point along the AB line can be computed from Fig. 5(d–f). Here, results extracted from the quantitative EELS elemental maps (Fig. 5(a–f)) are in well agreement with the average composition of the TiCN network, performed by EELS point analyses (Table 1). Here it should be mentioned that according to the supplier’s data, the starting C and N values in the TiCN particles were 0.7 and 0.3 (atomic %), respectively. Thus, it was concluded that the quantitative EELS analysis confirmed the diffusion of C atoms from TiCN particles to form the in situ SiC particles. For further characterization, nano-probed EELS analyses in STEM mode were also carried out from the in situ SiC, TiC and -SiAlON grains located close to network region (Fig. 6(a–c)).
Fig. 4. (a) Z-contrast STEM image indicating the square spectrum image area along with the selected spatial drift correction rectangular and last position of electron beam after acquisition was completed, (b) the view of spectrum imaging (SI) in STEM mode (Every pixel herein contains the chemical information, and the numbers on the TiCN phase correspond to where quantitative EELS analyses were obtained from), (c–e) qualitative EELS elemental maps of C-K (284 eV), N-K (401 eV) and Ti-L3,2 (456 eV) edges, respectively, and (f) RGB (RedGreenBlue) composite map. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Yurdakul H, et al. Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive ␣- SiAlON/TiCN composites. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.03.024
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Table 1 Atomic % quantitative EELS spot analysis results obtained from the points (1–15) shown in Fig. 4(b). Points
Elements (% Atomic) Ti
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Average
65.00 53.16 63.56 59.08 65.79 62.49 62.89 63.21 64.01 58.24 57.11 57.05 57.08 55.50 55.89 60.00
Composition (/Ti) C
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 4.01
6.25 10.05 6.78 10.30 3.04 10.03 9.69 10.26 9.83 10.26 13.97 13.86 13.26 13.91 13.71 10.35
N ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.014 0.027 0.014 0.025 0.007 0.023 0.022 0.023 0.022 0.027 0.035 0.034 0.033 0.035 0.035 3.18
Taking into account the analysis results, the observation of intensive C-K edge (Fig. 6(a)) confirms the formation of in situ SiC phase along the network. In addition, the C-K, N-K and Ti-L3,2 edges (Fig. 6(b)) reveal that the network structure consisted of TiCN grains. The EELS spectra in Fig. 6(a–b) were very similar to the spectra of SiC and TiCN phases acquired in the previous studies.32,33 Considering the EELS spectrum of -SiAlON grain in Fig. 6(c), the Ti-L3,2 edge can be clearly seen between
28.75 36.79 29.66 30.62 31.17 27.48 27.41 26.53 26.16 30.50 28.93 29.09 29.66 30.59 30.40 29.58
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.063 0.098 0.066 0.073 0.067 0.062 0.062 0.059 0.058 0.074 0.072 0.072 0.073 0.078 0.077 2.53
TiC0.10 N0.44 TiC0.19 N0.69 TiC0.11 N0.47 TiC0.17 N0.52 TiC0.05 N0.47 TiC0.16 N0.44 TiC0.15 N0.44 TiC0.16 N0.42 TiC0.15 N0.41 TiC0.19 N0.52 TiC0.24 N0.51 TiC0.24 N0.51 TiC0.23 N0.52 TiC0.25 N0.55 TiC0.25 N0.54 TiC0.17 N0.49
the N-K ve O-K edges, confirming the diffusion of Ti from TiCN to SiAlON grains. This result supports the visual observation of Ti-L3,2 EELS map in Fig. 4(e) and the earlier finding that Ti was detected in -SiAlON grain by EELS analysis.30 To determine the extent of Ti diffusion toward SiAlON matrix, the EDX spot analysis in STEM mode was performed. The EDX spectra of triple junction (TJ), ␣-SiAlON and SiAlON phases, acquired far away from the TiCN network, were
Fig. 5. (a–c) Atomic % quantitative EELS elemental maps of C-K (284 eV), N-K (401 eV) and Ti-L3,2 (456 eV) edges, respectively, and (d–f) % atomic concentration variation of C, N and Ti elements throughout a AB line labeled in (a–c) (red point corresponds to TiC0.17 N0.49 (atomic %) composition in a specific distance of the AB line at 1 m). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. (a–c) EELS spectra collected from the SiC, TiCN and -SiAlON grains, respectively, in STEM mode (the red point on the inset Z-contrast STEM images reveals where electron probe was focused during EELS analyses). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
given in Fig. 7(a–c). The quantification of the related phases was also summarized in Table 2. Considering the EDX spectra (Fig. 7(a–c)), the existence of Ti was shown in TJ, ␣-SiAlON and -SiAlON phases. Also, the amount of Ti in TJ, ␣-SiAlON and -SiAlON phases was calculated as 0.45, 0.15 and 0.24 (atomic %), respectively (Table 2). The results in Table 2 are the average
Fig. 7. (a–c) EDX spectra acquired from the triple junction (TJ), ␣-SiAlON and -SiAlON phases, respectively, in STEM mode (nano-probed beam positions during the EDX analyses were also shown as numbers on the inset Z-contrast STEM images).
of several EDX analyses. These findings clearly showed that Ti from TiCN grains diffuses toward the SiAlON matrix via liquid phase formed during sintering. Therefore, as a consequence, the observed Ti diffusion may play a significant role on the high electrical conductivity of the investigated ␣- SiAlON/TiCN
Table 2 Atomic % quantitative STEM-EDX spot analysis results acquired from the triple junction (TJ), ␣-SiAlON and -SiAlON phases shown in Fig. 7(a–c). Phase
TJ ␣ 
Elements (atomic %) Y
Sm
Ca
Ti
Si
Al
O
N
18.23 ± 0.02 1.36 ± 0.22 –
1.06 ± 0.14 0.02 ± 8.79 –
0.33 ± 0.53 0.06 ± 2.69 –
0.45 ± 0.34 0.15 ± 0.96 0.24 ± 0.69
48.16 ± 0.00 51.01 ± 0.00 52.51 ± 0.00
6.79 ± 0.03 4.32 ± 0.04 2.52 ± 0.09
9.50 ± 0.03 6.67 ± 0.03 9.63 ± 0.03
15.48 ± 0.04 36.41 ± 0.01 35.10 ± 0.02
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composite. It is because when Ti was even slightly incorporated into different host crystal structures, their electrical properties were enhanced.34,35 Furthermore, it is known that the incorporation of Ti into glass structures led to a positive effect on the evolution of electrical property.36 4. Conclusions In this paper, an electrically conductive bulk ␣- SiAlON/TiCN composite was examined by using different TEM-based analytical techniques. In addition, the stability of TiCN grains following sintering and its effect on the electrical properties of ␣- SiAlON/TiCN sample were performed at nanoscale. Based on the TEM results, a well bonding and dense contact structure between the SiAlON and TiCN grains was clearly observed. This type of continuous TiCN network formation primarily played a significant role on the high electrical conductivity. Liquid phase penetration to the TiCN particle network was also observed. Thus, a fully dense composite was obtained. In situ formation of nano-sized SiC phase between the TiCN grains was detected. The SiC grains also found to be encapsulated within the ␣- SiAlON matrix. Nano-sized in situ SiC grains might generate a positive effect on the electrical property of bulk ␣- SiAlON/TiCN composite with the increasing temperature. In EELS analyses, it was shown that the TiCN grains did not preserve their starting composition after sintering. This compositional change reveals a clear evidence of in situ formed nano-size SiC grains. Finally, Ti diffusion from TiCN grains toward ␣- SiAlON matrix and triple junction phases was detected. The presence of Ti in the SiAlON crystal lattice is believed to play an important role on the evolution of high electrical conductivity attained from ␣- SiAlON/TiCN composite. References 1. Izhevskiy VA, Genova LA, Bressiani JC, Aldinger F. Progress in SiAlON ceramics. J Eur Ceram Soc 2000;20:2275–95. 2. Riley F. Silicon nitride and related materials. J Am Ceram Soc 2000;83(2):245–65. 3. Mandal H, Kara F, Turan S, Kara A. Novel SiAlON ceramics for cutting tool applications. Key Eng Mat 2003;237:193–202. 4. Bernd B, Sebastian B, Kilian F. SiAlON based ceramic cutting tools. J Eur Ceram Soc 2008;28(5):989–94. 5. Xie RJ, Mitomo M, Uheda K, Xu FF, Akimune Y. Preparation and luminescence spectra of calcium- and rare-earth (R=Eu, Tb, and Pr)-co doped SiAlON ceramics. J Am Ceram Soc 2002;85:1229–34. 6. Martin C, Cales B, Viver P, Mathieu P. Electrical discharge machinable ceramic composites. Mater Sci Eng A 1989;109:351–6. 7. Lok YK, Lee TC. Processing of advanced ceramics using the wire cut EDM process. J Mat Process Tech 1997;63:839–43. 8. Liu CC, Huang JL. Micro-electrode discharge machining of TiN/Si3 N4 composites. Brit Ceram Trans 2000;99(4):149–52. 9. Bellosi A, Guicciardi S, Tampieri A. Development and characterization of electro conductive Si3 N4 –TiN composites. J Eur Ceram Soc 1992;9(2):83–93. 10. Boskovic S, Sigulinski F, Zivkovic L. Liquid phase sintering and properties of Si3 N4 –TiN composites. J Mat Synt and Proces 1999;7(2):119–26. 11. Zivkovic L, Nikolic Z, Boskovic S. Electrical properties percolation concentration in Si3N4–TiN based composites. Euro Ceram 2002;VII(Pt 1–3):1489–92.
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Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive ␣- SiAlON/TiCN composites Hilmi Yurdakul a , Servet Turan b,∗ , Erhan Ayas b , Orkun Tunckan c , Alpagut Kara b a
Dumlupınar University, Faculty of Engineering, Department of Materials Science and Engineering, Evliya Celebi Campus, TR-43100 Kutahya, Turkey b Anadolu University, Faculty of Engineering, Department of Materials Science and Engineering, Iki Eylul Campus, TR-26480 Eskisehir, Turkey c Anadolu University, Faculty of Aeronautics and Astronautics, Department of Airframe and Powerplant Maintenance, Iki Eylul Campus, TR-26480 Eskisehir, Turkey Received 22 January 2014; received in revised form 20 March 2014; accepted 24 March 2014
Abstract Coating of ␣- SiAlON granules with TiCN powder led to a continuous 3D electrically conductive network formation. Here, the stability of TiCN grains following gas pressure sintering (GPS) and its effect on the electrical properties of an ␣- SiAlON/TiCN composite were analytically investigated by using transmission electron microscopy (TEM) based techniques. In situ formation of nano-sized SiC grains adjacent to TiCN and ␣- SiAlON grains were observed. These SiC grains may play a role on the high electrical conductivity of ␣- SiAlON/TiCN composite. Ti:C:N ratios from TiCN grains along the network showed that TiCN grains did not preserve their initial composition after sintering. Finally, Ti diffusion from TiCN grains into ␣- SiAlON and triple junction phases was observed. The incorporation of Ti into the SiAlON crystal lattices contributes to the high electrical conductivity of ␣- SiAlON/TiCN composite. © 2014 Elsevier Ltd. All rights reserved. Keywords: Composite; Electrical conductivity; SiAlON; TiCN; Transmission electron microscopy
1. Introduction In recent years, much effort has been devoted to broaden the application area of SiAlON-based composites with different approaches.1,2 Due to the finely adjustable mechanical, thermal and optical properties, these materials have been successfully employed so far in a wide range of applications such as cutting tools, ball bearings and light emitting diodes (LEDs).2–5 In particular interest, SiAlON-based composites can be utilized in electrical applications such as glow plugs and heaters to achieve the high electrical conductivity without deteriorating the mechanical properties.1 In addition, this feature gives the opportunity of producing complex shaped parts by employing electro-discharge machining (EDM) process, which excludes the time-consuming and costly secondary machining operations.6–8 In order to obtain electrical conductivity in such
∗
Corresponding author. Tel.: +90 222 335 0580x6350; fax: +90 222 323 9501. E-mail address: [email protected] (S. Turan).
systems, the addition of highly conductive secondary phases, e.g. TiN,9–12 SiC,13–15 TaN,16 MoSi2 ,17–19 Ti(C,N)20,21 and TiB2 ,22 has been extensively studied. Amongst these studies, TiN and Ti(C,N) have been found to be more effective additives in terms of their chemical compatibility and easy sinterability. With this particular approach, the conductive particles of at least 30 vol.% should be incorporated into Si3 N4 and/or SiAlON matrix to obtain desired electrical conductivity. However, addition of such a large amount of secondary phases leads to degradation in mechanical and thermal properties. This is due to covalent nature of these phases and undesirable reactions between constituents hindering the densification of designed when conventional sintering techniques like gas pressure and pressureless sintering have been performed. In previous study,23 the amount of conductive phase was decreased as low as to 5 vol.% through the segregated network concept. The electrical resistivity of insulator SiAlON matrix (∼1013 m) was found to be drastically decreased to 18 × 10−4 m values, when 5 vol.% nano TiCN particles were added. Reactions between the TiCN and Si3 N4 particles as
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well as liquid phase during the gas pressure sintering process were discussed by microstructural observations through scanning electron microscopy (SEM) and thermodynamic approach. However, due to high beam broadening in SEM, the precise data on the stability of TiCN grains after sintering and its effect on electrical conductivity in ␣- SiAlON/TiCN composites were missing. Therefore, advanced microstructural characterization techniques to further elucidate the aforementioned deficiencies were necessary. To the best of the authors’ knowledge, few data are available on the clarification of in situ phase formations and interactions between SiAlON and TiN/TiCN grains by transmission electron microscopy (TEM) based techniques. In this work, analytical TEM studies on a previously developed an electrically conductive ␣- SiAlON/TiCN composite were undertaken.
2. Material and methods The detailed production of the electrically conductive ␣- SiAlON/TiCN composites was given in a previous study.23 In that study the spray dried SiAlON-based granules (≤100 m) were coated with varying amounts of (up to 10 vol.%) nano size TiCN (≤150 nm) particles. All the composites together with a reference SiAlON sample were fully densified by gas pressure sintering (GPS) at 1990 ◦ C under 10 MPa N2 gas pressure. A continuous chain-type 3D microstructure to provide the electrical conductivity in the composites was successfully obtained. The investigated sample in this study was composed of 10 vol.% TiCN nano particles as a continuous network in ␣- SiAlON matrix, exhibiting high electrical conductivity. For analytical TEM investigations, an electron transparent specimen was prepared by conventional mechanical polishing and Ar-ion beam thinning (Leica Microsystems EM RES101). Afterwards, the sample was characterized by using a field emission TEM (Jeol 2100F), operating at 200 kV and equipped with a high angle annular dark field scanning transmission electron microscope (HAADF-STEM) detector (Fishione), annular dark field (ADF) detector, an energy filter (Gatan GIF Tridiem), parallel electron energy loss spectrometer (EELS), and an energy dispersive X-ray (EDX) spectrometer (Jeol JED-2300T). In STEM-based EDX, EELS and spectrum imaging (SI) analyses, an electron spot with 1–2 nm in diameter was used. Furthermore, a drift corrector was performed to avoid any possible drifts that may occur at nano-scale during the acquisition of STEM-EDX/EELS/SI analyses. During EDX analysis, STEM mode was preferred instead of TEM mode, because the spatial resolution of EDX in STEM is smaller (only a few nanometers).24 In STEM-based EELS analysis, the convergence and collection semi-angles were used as 9.2 and 15.7 mrad, respectively. The spectrometer energy dispersion was also chosen in 0.5 eV/channels. The backgrounds in EELS and energy filtering transmission electron microscope (EFTEM) 3-window elemental mapping analyses were subtracted according to power-law.25 For quantitative EELS analysis, the partial inelastic cross-sections were derived from Hartree-Slater model,25 and the calculations were based on the relative quantification method.25 Furthermore,
quantitative STEM-EDX analysis was carried out using standardless Cliff–Lorimer quantification method.26 3. Results and discussion In Fig. 1(a–b), bright field (BF) TEM images are shown from the different parts of the ␣- SiAlON/TiCN composite. Here, a well bonding and dense structure between the SiAlON and TiCN grains following the sintering was clearly observed. It was considered that this type of continuous TiCN network formation played a significant role on the attained high electrical conductivity.23 On the other hand, the presence of nano-sized intergranular and transgranular cracks as defect structures were detected around the TiCN grains (highlighted with white colored arrows in Fig. 1). These cracks were most likely generated during sintering due to the thermal expansion coefficient mismatch between the SiAlON and TiCN grains, which gives rise to thermal stresses.27 However, the visibility of these defects along the TiCN network was very limited. Spherical-shaped inclusions with approximately 50–100 nm in diameter were also observed in between and within the TiCN and SiAlON grains (indicated by black colored arrows in Fig. 1). It is suggested that these SiC inclusions are formed as a result of the reactions between the TiCN and Si3 N4 /SiO2 grains during sintering.20,28 In order to clarify the chemical composition of these inclusions, energy filtering TEM (EFTEM) 3-window elemental mapping analysis was performed along the different regions of TiCN network (Fig. 2). Based on the EFTEM elemental maps in Fig. 2(a–h), in situ formation of nano-sized SiC phase can be easily observed between the individual TiCN grains. This result revealed that SiC grains did not only occur in a certain region of TiCN segregated network structure, but also they formed throughout the network. Furthermore, considering Fig. 2(h) in detail (white colored arrows), nano-sized SiC grains were also found between and within SiAlON matrix grains. It is well known that electrical conductivity of SiC varies in terms of the dopant type and temperature for bulk materials. At room temperature, SiC shows insulating or semi-conducting behavior, but the electrical conductivity increases with the increasing temperature.29 The resistive behavior of trapped SiC grains may lead to decrease on the composite’s conductivity; however, as seen in the images, these particles are randomly and discontinuously formed and do not deteriorate the chain type TiCN particle interaction. Therefore, the composites have high electrical conductivity. More surprisingly, the large-size in situ SiC formations, almost as large as TiCN grains, were also detected between the TiCN and SiAlON grains (shown in circle in Fig. 2(g)). This gives important information about the intensive interactive diffusion evolved during the reaction of constituents. Fig. 3 shows the atomic number (Z)-contrast STEM image acquired from the TiCN network structure. Here, the TiCN and SiAlON grains can be seen as white and black contrast, respectively. This is due to the fact that the number of electrons scattered from the Ti (Z = 22) element in TiCN phase is much higher than that of Al (Z = 13) and Si (Z = 14) in SiAlON matrix. There was an evidence of liquid phase penetration in between
Please cite this article in press as: Yurdakul H, et al. Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive ␣- SiAlON/TiCN composites. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.03.024
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Fig. 1. (a and b) Bright-field (BF) TEM images acquired from the different regions of ␣- SiAlON/TiCN composite.
TiCN grains showing the densification mechanism of the composite (marked with red arrows in Fig. 3). Also, the drops of residual liquid phase on the TiCN grains were clearly visible. It is therefore believed that the penetration of considerable amount of liquid phase along the TiCN grains facilitates the reaction between TiCN and Si3 N4 /SiO2 particles to form in situ SiC. This finding is in agreement with a thermodynamic approach on the formation of in situ SiC phase, reported in a previous study.23 To reveal the stability of TiCN grains by using EELS analysis after sintering, STEM-SI data set was first collected from the electron transparent sample, shown in Fig. 4(a–b). Afterwards, qualitative EELS elemental maps were subtracted from the STEM-SI data set by using C-K (284 eV), N-K (401 eV) and Ti-L3,2 (456 eV) edges (Fig. 4(c–e)). Finally, a RGB
(RedGreenBlue) composite map by overlaying the individual EELS maps in Fig. 4(c–e) was constructed (Fig. 4(f)). Considering the EELS maps (Fig. 4(c–e)), the formation of nano-sized in situ SiC phase can be easily distinguished both between and within the TiCN and SiAlON grains. These results confirm the EFTEM observations given in Fig. 2(a–h). More surprisingly, by studying at the Ti-L3,2 EELS map in detail (Fig. 4(e)), Ti containing SiAlON grains can be recognized. Thus, it can be said that not only C diffuses from the TiCN grains during the sintering, but also Ti. Simple explanation of this phenomenon is that, the diffusion of Si4+ ions from the SiAlON lattice during the formation of nano-sized in situ SiC phase probably gives rise to an ion charge imbalance in Si(O,N)4 tetrahedrons. This charge imbalance can be compensated by the substitution of Ti4+ ions by Si4+ ions.30 This confirms the recent
Fig. 2. EFTEM-3 window elemental mapping results obtained from the same regions in Fig. 1(a and b); (a and b) C-K (284 eV) edge, (c and d) N-K (401 eV) edge, (e and f) Ti-L3,2 (456 eV) edge and (g and h) RGB (RedGreenBlue) composite maps. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Atomic number (Z) contrast scanning TEM (STEM) image.
TEM studies by Yurdakul and Turan,30,31 which demonstrated the incorporation of transition metals, i.e. Fe, Cr and Ti, into -SiAlON crystal structure. Therefore, herein the existence of Ti in ␣- SiAlON crystal structures is feasible. For the calculation of compositional variations on the TiCN segregated network, STEM-SI-EELS spot data from the
individual grains composing the network (points 1–15 in Fig. 4(b)) were used. The average composition of every individual TiCN grain in the network was summarized in Table 1. Here, the results showed that chemical compositions of individual TiCN grains existing in network differed from grain to grain because of the changeable diffusion of the Ti, C and N elements in the system. To strengthen the results, the atomic concentrations of Ti, C and N elements throughout an AB line were calculated based on the quantitative EELS elemental maps (Fig. 5(a–c)) and graphically shown in Fig. 5(d–f). By using these results, the chemical compositions of TiCN grains can be calculated at a specific distance of the AB line. For example, at 1 m, the stoichiometry of a TiCN grain was calculated to be TiC0.17 N0.49 (atomic %). Also note that the composition of any desirable point along the AB line can be computed from Fig. 5(d–f). Here, results extracted from the quantitative EELS elemental maps (Fig. 5(a–f)) are in well agreement with the average composition of the TiCN network, performed by EELS point analyses (Table 1). Here it should be mentioned that according to the supplier’s data, the starting C and N values in the TiCN particles were 0.7 and 0.3 (atomic %), respectively. Thus, it was concluded that the quantitative EELS analysis confirmed the diffusion of C atoms from TiCN particles to form the in situ SiC particles. For further characterization, nano-probed EELS analyses in STEM mode were also carried out from the in situ SiC, TiC and -SiAlON grains located close to network region (Fig. 6(a–c)).
Fig. 4. (a) Z-contrast STEM image indicating the square spectrum image area along with the selected spatial drift correction rectangular and last position of electron beam after acquisition was completed, (b) the view of spectrum imaging (SI) in STEM mode (Every pixel herein contains the chemical information, and the numbers on the TiCN phase correspond to where quantitative EELS analyses were obtained from), (c–e) qualitative EELS elemental maps of C-K (284 eV), N-K (401 eV) and Ti-L3,2 (456 eV) edges, respectively, and (f) RGB (RedGreenBlue) composite map. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Atomic % quantitative EELS spot analysis results obtained from the points (1–15) shown in Fig. 4(b). Points
Elements (% Atomic) Ti
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Average
65.00 53.16 63.56 59.08 65.79 62.49 62.89 63.21 64.01 58.24 57.11 57.05 57.08 55.50 55.89 60.00
Composition (/Ti) C
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 4.01
6.25 10.05 6.78 10.30 3.04 10.03 9.69 10.26 9.83 10.26 13.97 13.86 13.26 13.91 13.71 10.35
N ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.014 0.027 0.014 0.025 0.007 0.023 0.022 0.023 0.022 0.027 0.035 0.034 0.033 0.035 0.035 3.18
Taking into account the analysis results, the observation of intensive C-K edge (Fig. 6(a)) confirms the formation of in situ SiC phase along the network. In addition, the C-K, N-K and Ti-L3,2 edges (Fig. 6(b)) reveal that the network structure consisted of TiCN grains. The EELS spectra in Fig. 6(a–b) were very similar to the spectra of SiC and TiCN phases acquired in the previous studies.32,33 Considering the EELS spectrum of -SiAlON grain in Fig. 6(c), the Ti-L3,2 edge can be clearly seen between
28.75 36.79 29.66 30.62 31.17 27.48 27.41 26.53 26.16 30.50 28.93 29.09 29.66 30.59 30.40 29.58
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.063 0.098 0.066 0.073 0.067 0.062 0.062 0.059 0.058 0.074 0.072 0.072 0.073 0.078 0.077 2.53
TiC0.10 N0.44 TiC0.19 N0.69 TiC0.11 N0.47 TiC0.17 N0.52 TiC0.05 N0.47 TiC0.16 N0.44 TiC0.15 N0.44 TiC0.16 N0.42 TiC0.15 N0.41 TiC0.19 N0.52 TiC0.24 N0.51 TiC0.24 N0.51 TiC0.23 N0.52 TiC0.25 N0.55 TiC0.25 N0.54 TiC0.17 N0.49
the N-K ve O-K edges, confirming the diffusion of Ti from TiCN to SiAlON grains. This result supports the visual observation of Ti-L3,2 EELS map in Fig. 4(e) and the earlier finding that Ti was detected in -SiAlON grain by EELS analysis.30 To determine the extent of Ti diffusion toward SiAlON matrix, the EDX spot analysis in STEM mode was performed. The EDX spectra of triple junction (TJ), ␣-SiAlON and SiAlON phases, acquired far away from the TiCN network, were
Fig. 5. (a–c) Atomic % quantitative EELS elemental maps of C-K (284 eV), N-K (401 eV) and Ti-L3,2 (456 eV) edges, respectively, and (d–f) % atomic concentration variation of C, N and Ti elements throughout a AB line labeled in (a–c) (red point corresponds to TiC0.17 N0.49 (atomic %) composition in a specific distance of the AB line at 1 m). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Yurdakul H, et al. Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive ␣- SiAlON/TiCN composites. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.03.024
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Fig. 6. (a–c) EELS spectra collected from the SiC, TiCN and -SiAlON grains, respectively, in STEM mode (the red point on the inset Z-contrast STEM images reveals where electron probe was focused during EELS analyses). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
given in Fig. 7(a–c). The quantification of the related phases was also summarized in Table 2. Considering the EDX spectra (Fig. 7(a–c)), the existence of Ti was shown in TJ, ␣-SiAlON and -SiAlON phases. Also, the amount of Ti in TJ, ␣-SiAlON and -SiAlON phases was calculated as 0.45, 0.15 and 0.24 (atomic %), respectively (Table 2). The results in Table 2 are the average
Fig. 7. (a–c) EDX spectra acquired from the triple junction (TJ), ␣-SiAlON and -SiAlON phases, respectively, in STEM mode (nano-probed beam positions during the EDX analyses were also shown as numbers on the inset Z-contrast STEM images).
of several EDX analyses. These findings clearly showed that Ti from TiCN grains diffuses toward the SiAlON matrix via liquid phase formed during sintering. Therefore, as a consequence, the observed Ti diffusion may play a significant role on the high electrical conductivity of the investigated ␣- SiAlON/TiCN
Table 2 Atomic % quantitative STEM-EDX spot analysis results acquired from the triple junction (TJ), ␣-SiAlON and -SiAlON phases shown in Fig. 7(a–c). Phase
TJ ␣ 
Elements (atomic %) Y
Sm
Ca
Ti
Si
Al
O
N
18.23 ± 0.02 1.36 ± 0.22 –
1.06 ± 0.14 0.02 ± 8.79 –
0.33 ± 0.53 0.06 ± 2.69 –
0.45 ± 0.34 0.15 ± 0.96 0.24 ± 0.69
48.16 ± 0.00 51.01 ± 0.00 52.51 ± 0.00
6.79 ± 0.03 4.32 ± 0.04 2.52 ± 0.09
9.50 ± 0.03 6.67 ± 0.03 9.63 ± 0.03
15.48 ± 0.04 36.41 ± 0.01 35.10 ± 0.02
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composite. It is because when Ti was even slightly incorporated into different host crystal structures, their electrical properties were enhanced.34,35 Furthermore, it is known that the incorporation of Ti into glass structures led to a positive effect on the evolution of electrical property.36 4. Conclusions In this paper, an electrically conductive bulk ␣- SiAlON/TiCN composite was examined by using different TEM-based analytical techniques. In addition, the stability of TiCN grains following sintering and its effect on the electrical properties of ␣- SiAlON/TiCN sample were performed at nanoscale. Based on the TEM results, a well bonding and dense contact structure between the SiAlON and TiCN grains was clearly observed. This type of continuous TiCN network formation primarily played a significant role on the high electrical conductivity. Liquid phase penetration to the TiCN particle network was also observed. Thus, a fully dense composite was obtained. In situ formation of nano-sized SiC phase between the TiCN grains was detected. The SiC grains also found to be encapsulated within the ␣- SiAlON matrix. Nano-sized in situ SiC grains might generate a positive effect on the electrical property of bulk ␣- SiAlON/TiCN composite with the increasing temperature. In EELS analyses, it was shown that the TiCN grains did not preserve their starting composition after sintering. This compositional change reveals a clear evidence of in situ formed nano-size SiC grains. Finally, Ti diffusion from TiCN grains toward ␣- SiAlON matrix and triple junction phases was detected. The presence of Ti in the SiAlON crystal lattice is believed to play an important role on the evolution of high electrical conductivity attained from ␣- SiAlON/TiCN composite. References 1. Izhevskiy VA, Genova LA, Bressiani JC, Aldinger F. Progress in SiAlON ceramics. J Eur Ceram Soc 2000;20:2275–95. 2. Riley F. Silicon nitride and related materials. J Am Ceram Soc 2000;83(2):245–65. 3. Mandal H, Kara F, Turan S, Kara A. Novel SiAlON ceramics for cutting tool applications. Key Eng Mat 2003;237:193–202. 4. Bernd B, Sebastian B, Kilian F. SiAlON based ceramic cutting tools. J Eur Ceram Soc 2008;28(5):989–94. 5. Xie RJ, Mitomo M, Uheda K, Xu FF, Akimune Y. Preparation and luminescence spectra of calcium- and rare-earth (R=Eu, Tb, and Pr)-co doped SiAlON ceramics. J Am Ceram Soc 2002;85:1229–34. 6. Martin C, Cales B, Viver P, Mathieu P. Electrical discharge machinable ceramic composites. Mater Sci Eng A 1989;109:351–6. 7. Lok YK, Lee TC. Processing of advanced ceramics using the wire cut EDM process. J Mat Process Tech 1997;63:839–43. 8. Liu CC, Huang JL. Micro-electrode discharge machining of TiN/Si3 N4 composites. Brit Ceram Trans 2000;99(4):149–52. 9. Bellosi A, Guicciardi S, Tampieri A. Development and characterization of electro conductive Si3 N4 –TiN composites. J Eur Ceram Soc 1992;9(2):83–93. 10. Boskovic S, Sigulinski F, Zivkovic L. Liquid phase sintering and properties of Si3 N4 –TiN composites. J Mat Synt and Proces 1999;7(2):119–26. 11. Zivkovic L, Nikolic Z, Boskovic S. Electrical properties percolation concentration in Si3N4–TiN based composites. Euro Ceram 2002;VII(Pt 1–3):1489–92.
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Please cite this article in press as: Yurdakul H, et al. Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive ␣- SiAlON/TiCN composites. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.03.024