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Erosion craters on Ti3SiC2 anode
Physics Letters A 378 (2014) 2417–2422
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Physics Letters A www.elsevier.com/locate/pla
Erosion craters on Ti3 SiC2 anode Peng Zhang, Tungwai Leo Ngai ∗ , Zhi Ding, Yuanyuan Li National Engineering Research Center of Near-net-shape Forming Technology for Metallic Materials, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 28 February 2014 Accepted 28 April 2014 Available online 23 June 2014 Communicated by F. Porcelli Keywords: Arc erosion Craters Ti3 SiC2 Decomposition
a b s t r a c t The erosion behavior of pure Ti3 SiC2 anode under vacuum discharge was investigated. By means of X-ray diffraction, energy dispersive spectroscopy and micro-Raman spectroscopy, the decomposition of Ti3 SiC2 into nonstoichiometric TiCx , amorphous carbon and other by-products was proved. The surface morphology was revealed by scanning electron microscope and 3D super depth digital microscope. Different kinds of craters with diameters varying from a few microns to a few hundred microns were observed on the anode surface after arcing. The smaller craters contain some TiCx , with a few tens of microns in diameter, are flower-like shaped with a protrusion pointing out from the center of the crater bottom. The larger craters are basically composed of TiCx , have diameters greater than one hundred microns but without the central protrusions, and are surrounded by collapse-fissures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ternary Ti3 SiC2 ceramic belongs to MAX phase family with a general formula of Mn+1 AXn (where M is an early transition metal, A is an A-group element, and X is either carbon or nitrogen). Its crystal structure can be described as two edge-shared CTi6 octahedron layers linked together by a two-dimensional closed packed Si layer [1,2]. The unique characters of this material are believed to have some relationship with its crystal structure. The chemical bonding in Ti3 SiC2 is an anisotropic metallic–covalent–ionic character with significant contributions from metallic and covalent bonds [2]. Intensive researches on the unusual properties of Ti3 SiC2 were initiated by Barsoum and colleagues [1,3–6]. It has merits of both metals and ceramics. Its electrical and thermal conductivities at the room-temperature are in the ranges of 4.3–4.5 × 106 ( m)−1 and 37–43 W/(m K), respectively [3,5,7], about 2–3 times of the value of titanium metal. Bulk Ti3 SiC2 material was found thermally stable under vacuum and argon atmosphere at temperatures as high as 1600 ◦ C for as long as 24 h [5]. Using samples obtained by reactive hot isostatic pressing (HIPing) of Ti, SiC and C powders, Gao and Miyamoto[8] obtained a flexural strength of 410 MPa and a fracture toughness of 11.2 MPa m1/2 , the former is in agreement with the value of 427 MPa that obtained by Li et al. [9]. Ti3 SiC2 exhibits excellent properties which include: high Young’s modulus, high-temperature plasticity, excellent chemical resistance, high-
*
Corresponding author. Tel.: +86 20 87112272; fax: +86 20 87112111. E-mail address: [email protected] (T.L. Ngai).
http://dx.doi.org/10.1016/j.physleta.2014.04.074 0375-9601/© 2014 Elsevier B.V. All rights reserved.
thermal shock resistance, high-temperature strength, good machinability with conventional tools, high damage tolerance, good oxidation resistance, etc. [3,5,7,9]. The key prerequisite for contact material is to conduct electricity and heat; to sustain the high-voltage breakdown; to keep away from welding and to bear severe arc erosion. In view of the distinguish properties of Ti3 SiC2 (for example, good electrical conductivity, excellent thermal conductivity and thermally stability), it is a potential candidate as an electric contact material [10]. However, there is no literature reported the arc erosion of Ti3 SiC2 . The purpose of this investigation is to analyze the erosion behavior of Ti3 SiC2 anode under vacuum discharge. 2. Experimental details Commercially available Ti (>99.9 wt% purity, −300 mesh), Si (>99.9 wt% purity, −300 mesh) and graphite (>99 wt% purity, −500 mesh) powders were mixed according to a molar ratio of 3Ti:1.2Si:1.8C. The powder mixtures were ball-mixed under Ar for 24 h at a rotating speed of 60 r/min. Zirconia balls with diameter of 6 mm and ball to powder mass ratio of 5:1 were used. After drying at 60 ◦ C for 24 hours under vacuum, the ball-mixed powder was pressed to form disc-shaped compacts in a steel mold by using a pressure of 500 MPa. The green compacts, with 20 mm in diameter and approximately 5 mm in thickness, were placed into lidded alumina crucible and pre-heated at 400 ◦ C for 2 h under vacuum. After that, high purity Ar (99.999%) was passed into the tube furnace and the furnace was raised to higher temperatures. When the furnace reached 750 ◦ C, a heating rate of 5 ◦ C/min was used to heat the samples up to 1500 ◦ C. Sintering was carried
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out at 1500 ◦ C for 2 h under Ar. Powder was obtained by drilling the as-sintered bulk materials and then ground in an agate mortar. Silicide impurity in the fabricated Ti3 SiC2 was removed by immersing the grounded powder into HF acid for about 1 hr and stirring from time to time, then draining the reacted acid and repeating the process by using fresh HF acid. Pure Ti3 SiC2 powder was obtained after rinsing the powder with deionized water to eliminate the residual acid. The purity of the rinsed powder was checked by a PANalytical X’Pert PRO X-ray diffractometer (XRD), with Cu Kα radiation, operating at 40 kV and 40 mA. The scanning rate used was 0.02◦ s−1 . The Pure Ti3 SiC2 powder was packed into a graphite mold with a diameter of 20 mm and sintered under vacuum (1 × 10−2 Pa) at 1500 ◦ C by using the spark plasma sintering technique (SPS) to produce pure Ti3 SiC2 bulk sample. The heating rate was fixed at 50 ◦ C/min; the soaking time was 5 min, while the applied pressure was kept constant at 50 MPa during sintering. The density of the SPS-sintered pure Ti3 SiC2 sample was measured by Archimedes method. After polishing, the sample was cleaned in acetone to remove surface grease and then rinsed with deionized water, followed by drying in a vacuum oven at 70 ◦ C for 15 min. The pure Ti3 SiC2 anode specimen was put in a sample holder, which was made of copper. The cathode was a polished pure molybdenum rod with a diameter of 5 mm. The gap between anode and cathode was 0.5 mm. The discharge experiment was carried out in a stainless steel vacuum chamber with a diameter of 30 cm and a height of 40 cm. The chamber was filled with argon and evacuated to 1 × 10−4 Pa by a mechanical pump and a sputter ion pump. The arcs were generated by a modified electric welder. A 10 kV DC voltage was applied across the cathode and anode. The applied current and arcing time were 20 A and 0.2 s, respectively. The current-time curve was recorded using a Tektronix TDS-210 digital memory oscilloscope. Microstructures and phase constituents of the cathode and anode sample were analyzed with a scanning electron microscope (JXA-8100) equipped with an energy dispersive spectrometer (EDS). Three-dimensional (3D) images of the anode craters were revealed by a 3D super depth digital microscope (KEYENCE, VHX-600E). Raman spectra were recorded with a micro-optical spectrometer system (LabRam Aramis Raman spectrometer). X-ray diffraction (XRD) with a Cu Kα source was used for phase identification. 3. Results and discussion The SPS sintered Ti3 SiC2 samples are nearly fully dense, about 99% of the theoretical density. Fig. 1 shows the XRD patterns of the SPS as-sintered Ti3 SiC2 sample and the Ti3 SiC2 anode after 50 arcing. No impurity phase can be identified in the as-sintered sample. However, for the tested anode, weak TiC peaks can be founded. Figs. 2(a)–(d) show the worn surface morphology of the Ti3 SiC2 anode under different magnifications. As shown in Fig. 2(a), numerous craters can be seen on the anode surface. They are round shaped, with diameters varying from a few microns to a few hundred microns. Several flower-like craters, each with a protrusion pointing out from the center of the crater bottom, can be observed; their appearance are resemble to the cathode craters observed by Jüttner [11], as shown in Figs. 2(b) and (c). The major crater shown in Fig. 2(c) is about 25 μm in diameter and the central spheroidal protrusion is about 7 μm in diameter. Micro-craters with diameters of several microns can also be seen nearby; they have central protrusions as well. Fig. 2(d) shows a different kind of crater, which is much larger, has a dimension greater than one hundred microns but without a central protrusion. The crater is surrounded by collapse-fissures and a crack lying across the bottom.
Fig. 1. XRD patterns of the as-sintered pure Ti3 SiC2 and the Ti3 SiC2 anode after 50 arcings.
According to Miller [12,13], vacuum arc can exhibit five different anode discharge modes. Among them, two are low current modes, (which were named as diffuse arc modes by Miller). In these diffuse arc modes, basically the anode acted as a collector of particles emitted from the cathode and the anode erosion is negative. The third is medium current mode named as footpoint mode, which has a temperature above the melting point of the anode materials and the energy dissipations are mostly melting of the material and conduction into the matrix, thus, the anode erosion is relatively low to moderate. The fourth is high current mode named as anode spot mode, which has a temperature near the boiling point of the anode materials and the main energy loss is due to vaporization, while energy dissipation by radiation, melting and conduction contribute appreciable amounts, thus, the anode erosion in this case is severe, since the anode is a copious source of vapor and ions. The last is high current mode with narrow gap length named as intense current mode. In this mode anode spot is also present, but the severe anode erosion is accompanied by severe cathode erosion as well. As shown in Fig. 2(c) the appearance of the spheroidal protrusion and droplets indicate that liquid phase is involved in the arcing process, so we name it as the footpoint mode crater here. While neither spheroidal protrusion nor liquid droplets can be seen in the big crater that appeared in Fig. 2(d), the temperature rise of that particular area during arcing must be very high, so that the anode material was being vaporized or sputtered, therefore, we name the big crater as the anode spot mode crater. The type of anode discharge mode depends on the material, anode geometry, gap distance and arc current. In present study, both footpoint mode craters and anode spot craters can be found in a single tested anode sample. The energy dissipated through melting and conduction is relatively small compared to the energy inputted into the anode (crater) during arcing, but, with sufficient number of consecutive arcing, the heat loss due to conduction will lag far behind the power input, thus resulting in a significant increase in surface temperature. The temperature of the footpoint mode craters may gradually approach the boiling point, which makes transfer to the anode spot mode crater from footpoint mode crater possible. The large anode spot mode craters that found in this study may be originated from the clusters of the footpoint mode craters, as shown in Figs. 2(a) and (b). Another possible reason for the existence of those large craters is the presence of protrusions in the footpoint mode craters, since arc can be generated more easily from the pointed objects. EDS point analysis results on the chemical composition of the marked regions in Figs. 2(c) and (d) are listed in Table 1. As shown in Fig. 2(c), locations A and B are positioned at the protrusion and the rim of the footpoint mode crater, while location C is positioned on the flat surface of the tested anode. The chemical composition
P. Zhang et al. / Physics Letters A 378 (2014) 2417–2422
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Fig. 2. Surface morphology of the tested anode: (a) general view, (b) clusters of footpoint mode craters, (c) footpoint mode crater, (d) anode spot mode crater. Table 1 EDS results on the tested anode surface (refer to Fig. 2). Location Footpoint mode crater
Anode spot mode crater
A B C D E
Mo (at%)
Ti (at%)
Si (at%)
C (at%)
Ti:Si ratio
0.42
48.20 57.98 53.01 74.03 44.98
15.29 17.81 17.17 4.88 5.44
36.09 24.21 29.82 21.09 49.58
3.15 3.26 3.09 – –
4.33
at locations A, B and C revealed a Ti:Si:C atomic ratio of approximately 3:1:2, which indicated that these 3 locations are basically composed of Ti3 SiC2 . Locations A and C have been contaminated by Mo that came from the cathode, with location C having a higher Mo content of 4.33 at% compared to that of 0.42 at% at location A. Location B is at the rim of the crater, no Mo can be detected at that point. Although the EDS result did not show an exact Ti:Si:C atomic ratio of 3:1:2, but their Ti:Si atomic ratio is only slightly higher than 3:1, which implied Ti is in excess if we consider the original composition of the anode material is Ti3 SiC2 . Location C has a Ti:Si atomic ratio of 3.09, which indicated that location C contains the highest Ti3 SiC2 concentration and location B has the highest degree of decomposition among these analyzed data points. Consider the EDX technique cannot give accurate quantitative analyzing results on light elements such as C, it can be concluded that after arcing, some of the Ti3 SiC2 has been decomposed to form TiC as one of the major decomposition products. For the anode spot mode crater in Fig. 2(d), EDS point analysis results show that both locations D and E are composed of nonstoichiometric TiC (TiCx ), with about 5 at% of residual Si. These TiCx and Si are decomposition products of Ti3 SiC2 . It is interesting to note that after arcing, the Ti3 SiC2 that originally composed the anode spot mode crater decomposed into TiCx and Si, while the Ti3 SiC2 at the footpoint mode crater basically remained as Ti3 SiC2 .
The Raman spectra of Ti3 SiC2 single crystal showed four peaks at 224, 278, 625, and 673 cm−1 [14]. Amer et al. [15] reported six sharp peaks at 159, 228, 281, 312, 631, and 678 cm−1 for HIPing polycrystalline Ti3 SiC2 . Raman spectra obtained from the self-prepared polycrystalline pure Ti3 SiC2 and the tested anode are presented in Fig. 3. The sharp peaks at 218, 624, and 672 cm−1 and a broad peak at 280 cm−1 in the Ti3 SiC2 profile of Fig. 3(b) are in agreement with those observed by Mercier [14] and Amer [15]. The peaks observed at 159 and 312 cm−1 in HIPing samples [15] did not appear in our experiment and no second order Raman bands were detected. By means of micro-Raman spectroscopy, information about the chemical composition of the footpoint mode crater is revealed in Fig. 3(a). Profile A, B and C are the Raman spectra for the central protrusion, the rim and the flat area adjacent to the crater, respectively. In the 100–750 cm−1 region, profiles A and B display four broad peaks centered around 268, 365, 605 and 658 cm−1 , which are in agreement with the TiCx data reported by Amer et al. and Qi et al. [15,16]. There is no doubt that Ti3 SiC2 decomposed and TiCx formed in these locations. For profile C, however, peaks at 218, 624, and 672 cm−1 which associated with Ti3 SiC2 still existed while the broad peak at 280 cm−1 vanished, indicating the change in bonding character of Ti3 SiC2 . A broad and strong peak located at 906 cm−1 appears in profile C, and it appears in profile A also, but much weaker. Although the origin of this peak is not clear now, it
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Fig. 3. Raman spectra of self-prepared polycrystalline Ti3 SiC2 and four selected locations from (a) footpoint mode cater (profiles A–C), (b) anode spot mode cater (profile D).
is presumably related to the decomposition products of Ti3 SiC2 . In the 1000–2000 cm−1 region, broad peaks at 1337 and 1591 cm−1 indicate the presence of amorphous carbon [17]. The origin of the peaks located at 1457 and 1789 cm−1 cannot be identified yet. These Raman spectra results are in agreement with those obtained by EDS analysis. Base on the above analysis, it is reasonable to propose that the central protrusions were formed from the molten materials, and some of the Ti3 SiC2 in footpoint mode craters decomposed into TiCx , amorphous carbon and other unknown decomposition products; the flat areas adjacent to the crater are basically composed of Ti3 SiC2 , but contain amorphous carbon, Mo from the cathode and other unknown substances related to Mo or erupted materials from the crater. Profile D in Fig. 3(b) shows three broad peaks at 289, 360 and 605 cm−1 which correspond to the peaks of TiCx [15,16,18] and no carbon signal could be detected. The micro-Raman spectrum data confirmed the EDS result that nonstoichiometric TiC is the dominant phase found in the anode spot mode crater (Location D), indicating the complete decomposition of Ti3 SiC2 there. Furthermore, no spheroidal protrusion formed in the crater implied that liquid phase is not present at the bottom of the crater, therefore, it can be concluded that the formation temperature of the big crater is well above the melting point of the anode material; vaporization is the erosion mechanism there. The Raman peaks of Si, SiC and Ti–Si compounds cannot be found in all the 4 profiles. It should be mentioned that stoichiometric TiC has no Raman active vibrational mode and the Raman spectrum of TiCx is composed of broad peaks induced by carbon vacancies [19]. The stability of Ti3 SiC2 at elevated temperature has been studied by different authors [5,20,21]. Barsoum et al. [5] found that Ti3 SiC2 bulk samples are thermally stable at 1600 ◦ C under vacuum or argon atmosphere. Low et al. [20] reported that the phase stability and transition of Ti3 SiC2 at elevated temperature are strongly dependent on the oxygen partial pressure of the annealing atmosphere; Ti3 SiC2 decomposes slowly to nonstoichiometric TiCx and Ti5 Si3 Cx (Ti5 Si3 phase with C dissolution) at 1200 ◦ C but the decomposition becomes quite rapid at 1500 ◦ C under vacuum. Racault et al. [21] investigated the thermal stability of Ti3 SiC2
by annealing the samples at different temperatures under vacuum. Their result shows that Ti3 SiC2 is thermally stable up to about 1450 ◦ C in an alumina crucible. Above this temperature, Ti3 SiC2 undergoes decomposition according to the following reaction which exhibits a very slow kinetics.
Ti3 SiC2 (s) → 3TiC0.67 (s) + Si(g) Ti3 SiC2 has a laminar structure with Ti–C blocks separated by Si layers. Two types of Ti–C bonds are exhibited in a Ti3 SiC2 unit cell: the Ti(1)–C bonds adjacent to the Si layers and the Ti(2)–C bonds in the centre of the unit cell. The inter-atomic distance between Ti and Si is larger than those of Ti(1)–C and Ti(2)–C, and the Ti–Si bond is weaker than Ti–C bonds [1,2]. As a result, Si is the weakest bonded element in the Ti3 SiC2 structure and thus making the above decomposition equation reasonable. The protrusion with a spheroidal shape indicated that liquid phase was involved in the formation of footpoint mode craters. Comparing the melting points of all possible decomposition products of Ti3 SiC2 , the melting points of TiC, SiC and C are over 3000 ◦ C, while Si, Ti and the Ti–Si compounds have melting temperatures lower than 2000 ◦ C (with the exception of Ti5 Si3 , which has a melting point of 2130 ◦ C). The lowest temperature liquid phases exist at the two 1330 ◦ C Ti–Si eutectics, with one at 13.7 at% Si and the other at approximately 85 at% Si. It is believed that the Ti3 SiC2 anode under the footpoint mode discharge will partially decomposed into TiCx , with possibly Si, C and some form of Ti–Si phase. Due to the high energy of the footpoint mode discharge, the Si and/or Ti–Si phase will be melted first and eventually vaporized, leaving TiCx as the major decomposition product that found in footpoint mode craters. As pointed out earlier, footpoint mode discharge has a temperature higher than the melting point of the anode material, but not as high as those of the anode spot mode discharge, which has a temperature near or higher than the boiling point of the anode material. For anode spot mode crater, trace of liquid involvement can only be seen at the rim. As shown in Fig. 2(d), the center of anode spot mode crater is flat and basically composed of TiCx , which indicated that the decomposition of Ti3 SiC2 at that location is quite
P. Zhang et al. / Physics Letters A 378 (2014) 2417–2422
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Fig. 4. (a) Optical micrograph of a footpoint mode crater; (b) thee-dimensional topography of (a).
Fig. 5. Schematic of the droplet formation model after Gray and Pharney [22].
complete, however, according to material balance, the decomposition products should include Si related phases. The absence of liquid phase involvement within the crater enables the authors to postulate that under the highly energetic anode spot mode discharge, all decomposition products with lower melting points are being evaporated immediately after decomposition. A number of reports had found that the temperature of the anode spot is above the boiling point of the anode material. For example, Boxman [22] measured the anode surface temperature of Ni electrodes and obtained an equilibrium temperature of about 2900 K. Anode spot temperatures between 2600 and 3300 ◦ C for Cu, and between 2000 and 2400 ◦ C for Al were observed by Grissom and Newton [23]. Cobine and Burger [24] estimated the anode spot temperature should be above the boiling temperature of the material. They predicted theoretical limit for tungsten, for example, is between 6700 and 8700 K and their. Predicted results have been confirmed by other authors [13] through direct measurements. Therefore, it is reasonable to conclude that the temperature of Ti3 SiC2 anode spot should exceed 2500 ◦ C, which is far above its decomposition temperature and close to the boiling points of Ti and Si. Due to the low current (20 A on average) used in this experiment, the arc erosion of the Mo cathode is dominated by the eruption of Mo. Experimental result showed that the erosion on the refractory Mo cathode is not serious at all and does not display any particular feature. EDS analysis showed that no Ti3 SiC2 or any of its decomposition product can be detected on the Mo cathode and only tiny amount of Mo can be detected on the Ti3 SiC2 anode surface. Therefore, we did not pay special attention on the Mo cathode. Fig. 4 shows the optical micrograph and 3D topography of a footpoint mode crater. As shown in Fig. 4(b), the depth of the crater measured from the bottom of the crater to the summit of the central protrusion is about 7 μm. The 3D image is consistent with the SEM observation that the crater has a distinct morphology of depressed crater surround by a rim with an elevated centre. The presence of central protrusion in the footpoint mode craters can be interpreted by the droplet formation model developed by Gray and Pharney [25]. During discharge process, sufficiently high arcing energy will melt the electrode material and create a crater, as shown in Fig. 5(a). A force introduced by ion bombardment
will act on the liquid pool at the crater bottom. The sudden extinction of the arc will cause an un-balanced recoil force directed outward from the electrode into the inter-electrode gap, as shown in Fig. 5(b). The surface tension force of the liquid will act against the recoil force, if the recoil force is larger than the surface tension force, the liquid pool will be displaced to the point of separation from the pool in the form of a molten droplet, as shown in Fig. 5(c). In case of the recoil force is less than the surface tension force; a central protrusion will be formed in the crater, as shown in Fig. 5(d). Similar description can be found in Jüttner’s work [11]. 4. Conclusion The erosion mechanism of Ti3 SiC2 anode is the decomposition of Ti3 SiC2 into TiCx as the major product and the evaporation and/or eruption of the decomposition products. Different kinds of craters with diameters varying from a few microns to a few hundred microns were observed on the anode surface. The smaller craters named as footpoint mode craters, with a few tens of microns in diameter, are flower-like shaped, each with a central protrusion pointing out from the crater bottom. Part of the Ti3 SiC2 in the footpoint mode crater decomposed into TiCx as the major product; amorphous C and Mo from the cathode can also be detected. The larger craters named as anode spot mode craters have diameters greater than one hundred microns but without the central protrusions and are surrounded by collapse-fissures. The decomposition of Ti3 SiC2 in the anode spot mode crater is quite complete; the crater is basically composed of TiCx . Acknowledgements The support for this research was from the National Natural Science Foundation of China (Grant No. 51074077). References [1] E.H. Kisi, J.A.A. Crossley, S. Myhra, M.W. Barsoum, J. Phys. Chem. Solids 59 (1998) 1437. [2] Y. Zhou, Z. Sun, J. Phys. Condens. Matter 12 (2000) 457. [3] M.W. Barsoum, T.E. Raghy, J. Am. Ceram. Soc. 79 (1996) 1953.
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[4] M.W. Barsoum, T.E. Raghy, Metall. Mater. Trans. A, Phys. Metall. Mater. Sci. 30 (1999) 363. [5] T.E. Raghy, M.W. Barsoum, J. Am. Ceram. Soc. 82 (1999) 2849. [6] M.W. Barsoum, Prog. Solid State Chem. 28 (2000) 201. [7] Z.M. Sun, H. Hashimoto, Z.F. Zhang, S.L. Yang, S. Tada, Mater. Trans. 47 (2006) 170. [8] N.F. Gao, Y. Miyamoto, D. Zhang, J. Mater. Sci. 34 (1999) 4385. [9] S. Li, J. Xie, J. Zhao, L. Zhang, Mater. Lett. 57 (2002) 119. [10] J.S. Liang, L.J. Wang, Adv. Mater. Res. 427 (2012) 185. [11] B. Jüttner, Beitr. Plasmaphys. 19 (1979) 25. [12] H.C. Miller, IEEE Trans. Dielectr. Electr. Insul. 4 (1997) 382. [13] R.L. Boxman, D.M. Sanders, P.J. Martin, Handbook of Vacuum Arc Science and Technology, Noyes, New Jersey, 1995, 308 pp. [14] F. Mercier, O. Chaix-Pluchery, T. Ouisse, D. Chaussende, Appl. Phys. Lett. 98 (2011) 081912.
[15] M. Amer, M.W. Barsoum, T. El-Raghy, I. Weiss, S. Leclair, et al., J. Appl. Phys. 84 (1998) 5817. [16] Q. Qi, W.Z. Zhang, L.Q. Shi, W.Y. Zhang, W. Zhang, B. Zhang, Thin Solid Films 520 (2012) 6882. [17] A.C. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 075414. [18] L. Zhang, Q. Qi, L.Q. Shi, D.J. O’Connor, B.V. King, E.H. Kisi, D.K. Venkatachalam, Appl. Surf. Sci. 258 (2012) 6281. [19] M.V. Klein, J.A. Holy, W.S. Williams, Phys. Rev. B 17 (1978) 1546. [20] I.M. Low, Z. Oo, K.E. Prince, J. Am. Ceram. Soc. 90 (2007) 2610. [21] C. Racault, F. Langlais, R. Naslain, J. Mater. Sci. 29 (1994) 3384. [22] R.L. Boxman, J. Appl. Phys. 46 (1975) 4701. [23] J.T. Grissom, J.C. Newton, J. Appl. Phys. 45 (1974) 2885. [24] J.D. Cobine, E.E. Burger, J. Appl. Phys. 26 (1955) 895. [25] E.W. Gray, J.R. Pharney, J. Appl. Phys. 45 (1974) 667.
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Erosion craters on Ti3 SiC2 anode Peng Zhang, Tungwai Leo Ngai ∗ , Zhi Ding, Yuanyuan Li National Engineering Research Center of Near-net-shape Forming Technology for Metallic Materials, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 28 February 2014 Accepted 28 April 2014 Available online 23 June 2014 Communicated by F. Porcelli Keywords: Arc erosion Craters Ti3 SiC2 Decomposition
a b s t r a c t The erosion behavior of pure Ti3 SiC2 anode under vacuum discharge was investigated. By means of X-ray diffraction, energy dispersive spectroscopy and micro-Raman spectroscopy, the decomposition of Ti3 SiC2 into nonstoichiometric TiCx , amorphous carbon and other by-products was proved. The surface morphology was revealed by scanning electron microscope and 3D super depth digital microscope. Different kinds of craters with diameters varying from a few microns to a few hundred microns were observed on the anode surface after arcing. The smaller craters contain some TiCx , with a few tens of microns in diameter, are flower-like shaped with a protrusion pointing out from the center of the crater bottom. The larger craters are basically composed of TiCx , have diameters greater than one hundred microns but without the central protrusions, and are surrounded by collapse-fissures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ternary Ti3 SiC2 ceramic belongs to MAX phase family with a general formula of Mn+1 AXn (where M is an early transition metal, A is an A-group element, and X is either carbon or nitrogen). Its crystal structure can be described as two edge-shared CTi6 octahedron layers linked together by a two-dimensional closed packed Si layer [1,2]. The unique characters of this material are believed to have some relationship with its crystal structure. The chemical bonding in Ti3 SiC2 is an anisotropic metallic–covalent–ionic character with significant contributions from metallic and covalent bonds [2]. Intensive researches on the unusual properties of Ti3 SiC2 were initiated by Barsoum and colleagues [1,3–6]. It has merits of both metals and ceramics. Its electrical and thermal conductivities at the room-temperature are in the ranges of 4.3–4.5 × 106 ( m)−1 and 37–43 W/(m K), respectively [3,5,7], about 2–3 times of the value of titanium metal. Bulk Ti3 SiC2 material was found thermally stable under vacuum and argon atmosphere at temperatures as high as 1600 ◦ C for as long as 24 h [5]. Using samples obtained by reactive hot isostatic pressing (HIPing) of Ti, SiC and C powders, Gao and Miyamoto[8] obtained a flexural strength of 410 MPa and a fracture toughness of 11.2 MPa m1/2 , the former is in agreement with the value of 427 MPa that obtained by Li et al. [9]. Ti3 SiC2 exhibits excellent properties which include: high Young’s modulus, high-temperature plasticity, excellent chemical resistance, high-
*
Corresponding author. Tel.: +86 20 87112272; fax: +86 20 87112111. E-mail address: [email protected] (T.L. Ngai).
http://dx.doi.org/10.1016/j.physleta.2014.04.074 0375-9601/© 2014 Elsevier B.V. All rights reserved.
thermal shock resistance, high-temperature strength, good machinability with conventional tools, high damage tolerance, good oxidation resistance, etc. [3,5,7,9]. The key prerequisite for contact material is to conduct electricity and heat; to sustain the high-voltage breakdown; to keep away from welding and to bear severe arc erosion. In view of the distinguish properties of Ti3 SiC2 (for example, good electrical conductivity, excellent thermal conductivity and thermally stability), it is a potential candidate as an electric contact material [10]. However, there is no literature reported the arc erosion of Ti3 SiC2 . The purpose of this investigation is to analyze the erosion behavior of Ti3 SiC2 anode under vacuum discharge. 2. Experimental details Commercially available Ti (>99.9 wt% purity, −300 mesh), Si (>99.9 wt% purity, −300 mesh) and graphite (>99 wt% purity, −500 mesh) powders were mixed according to a molar ratio of 3Ti:1.2Si:1.8C. The powder mixtures were ball-mixed under Ar for 24 h at a rotating speed of 60 r/min. Zirconia balls with diameter of 6 mm and ball to powder mass ratio of 5:1 were used. After drying at 60 ◦ C for 24 hours under vacuum, the ball-mixed powder was pressed to form disc-shaped compacts in a steel mold by using a pressure of 500 MPa. The green compacts, with 20 mm in diameter and approximately 5 mm in thickness, were placed into lidded alumina crucible and pre-heated at 400 ◦ C for 2 h under vacuum. After that, high purity Ar (99.999%) was passed into the tube furnace and the furnace was raised to higher temperatures. When the furnace reached 750 ◦ C, a heating rate of 5 ◦ C/min was used to heat the samples up to 1500 ◦ C. Sintering was carried
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out at 1500 ◦ C for 2 h under Ar. Powder was obtained by drilling the as-sintered bulk materials and then ground in an agate mortar. Silicide impurity in the fabricated Ti3 SiC2 was removed by immersing the grounded powder into HF acid for about 1 hr and stirring from time to time, then draining the reacted acid and repeating the process by using fresh HF acid. Pure Ti3 SiC2 powder was obtained after rinsing the powder with deionized water to eliminate the residual acid. The purity of the rinsed powder was checked by a PANalytical X’Pert PRO X-ray diffractometer (XRD), with Cu Kα radiation, operating at 40 kV and 40 mA. The scanning rate used was 0.02◦ s−1 . The Pure Ti3 SiC2 powder was packed into a graphite mold with a diameter of 20 mm and sintered under vacuum (1 × 10−2 Pa) at 1500 ◦ C by using the spark plasma sintering technique (SPS) to produce pure Ti3 SiC2 bulk sample. The heating rate was fixed at 50 ◦ C/min; the soaking time was 5 min, while the applied pressure was kept constant at 50 MPa during sintering. The density of the SPS-sintered pure Ti3 SiC2 sample was measured by Archimedes method. After polishing, the sample was cleaned in acetone to remove surface grease and then rinsed with deionized water, followed by drying in a vacuum oven at 70 ◦ C for 15 min. The pure Ti3 SiC2 anode specimen was put in a sample holder, which was made of copper. The cathode was a polished pure molybdenum rod with a diameter of 5 mm. The gap between anode and cathode was 0.5 mm. The discharge experiment was carried out in a stainless steel vacuum chamber with a diameter of 30 cm and a height of 40 cm. The chamber was filled with argon and evacuated to 1 × 10−4 Pa by a mechanical pump and a sputter ion pump. The arcs were generated by a modified electric welder. A 10 kV DC voltage was applied across the cathode and anode. The applied current and arcing time were 20 A and 0.2 s, respectively. The current-time curve was recorded using a Tektronix TDS-210 digital memory oscilloscope. Microstructures and phase constituents of the cathode and anode sample were analyzed with a scanning electron microscope (JXA-8100) equipped with an energy dispersive spectrometer (EDS). Three-dimensional (3D) images of the anode craters were revealed by a 3D super depth digital microscope (KEYENCE, VHX-600E). Raman spectra were recorded with a micro-optical spectrometer system (LabRam Aramis Raman spectrometer). X-ray diffraction (XRD) with a Cu Kα source was used for phase identification. 3. Results and discussion The SPS sintered Ti3 SiC2 samples are nearly fully dense, about 99% of the theoretical density. Fig. 1 shows the XRD patterns of the SPS as-sintered Ti3 SiC2 sample and the Ti3 SiC2 anode after 50 arcing. No impurity phase can be identified in the as-sintered sample. However, for the tested anode, weak TiC peaks can be founded. Figs. 2(a)–(d) show the worn surface morphology of the Ti3 SiC2 anode under different magnifications. As shown in Fig. 2(a), numerous craters can be seen on the anode surface. They are round shaped, with diameters varying from a few microns to a few hundred microns. Several flower-like craters, each with a protrusion pointing out from the center of the crater bottom, can be observed; their appearance are resemble to the cathode craters observed by Jüttner [11], as shown in Figs. 2(b) and (c). The major crater shown in Fig. 2(c) is about 25 μm in diameter and the central spheroidal protrusion is about 7 μm in diameter. Micro-craters with diameters of several microns can also be seen nearby; they have central protrusions as well. Fig. 2(d) shows a different kind of crater, which is much larger, has a dimension greater than one hundred microns but without a central protrusion. The crater is surrounded by collapse-fissures and a crack lying across the bottom.
Fig. 1. XRD patterns of the as-sintered pure Ti3 SiC2 and the Ti3 SiC2 anode after 50 arcings.
According to Miller [12,13], vacuum arc can exhibit five different anode discharge modes. Among them, two are low current modes, (which were named as diffuse arc modes by Miller). In these diffuse arc modes, basically the anode acted as a collector of particles emitted from the cathode and the anode erosion is negative. The third is medium current mode named as footpoint mode, which has a temperature above the melting point of the anode materials and the energy dissipations are mostly melting of the material and conduction into the matrix, thus, the anode erosion is relatively low to moderate. The fourth is high current mode named as anode spot mode, which has a temperature near the boiling point of the anode materials and the main energy loss is due to vaporization, while energy dissipation by radiation, melting and conduction contribute appreciable amounts, thus, the anode erosion in this case is severe, since the anode is a copious source of vapor and ions. The last is high current mode with narrow gap length named as intense current mode. In this mode anode spot is also present, but the severe anode erosion is accompanied by severe cathode erosion as well. As shown in Fig. 2(c) the appearance of the spheroidal protrusion and droplets indicate that liquid phase is involved in the arcing process, so we name it as the footpoint mode crater here. While neither spheroidal protrusion nor liquid droplets can be seen in the big crater that appeared in Fig. 2(d), the temperature rise of that particular area during arcing must be very high, so that the anode material was being vaporized or sputtered, therefore, we name the big crater as the anode spot mode crater. The type of anode discharge mode depends on the material, anode geometry, gap distance and arc current. In present study, both footpoint mode craters and anode spot craters can be found in a single tested anode sample. The energy dissipated through melting and conduction is relatively small compared to the energy inputted into the anode (crater) during arcing, but, with sufficient number of consecutive arcing, the heat loss due to conduction will lag far behind the power input, thus resulting in a significant increase in surface temperature. The temperature of the footpoint mode craters may gradually approach the boiling point, which makes transfer to the anode spot mode crater from footpoint mode crater possible. The large anode spot mode craters that found in this study may be originated from the clusters of the footpoint mode craters, as shown in Figs. 2(a) and (b). Another possible reason for the existence of those large craters is the presence of protrusions in the footpoint mode craters, since arc can be generated more easily from the pointed objects. EDS point analysis results on the chemical composition of the marked regions in Figs. 2(c) and (d) are listed in Table 1. As shown in Fig. 2(c), locations A and B are positioned at the protrusion and the rim of the footpoint mode crater, while location C is positioned on the flat surface of the tested anode. The chemical composition
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Fig. 2. Surface morphology of the tested anode: (a) general view, (b) clusters of footpoint mode craters, (c) footpoint mode crater, (d) anode spot mode crater. Table 1 EDS results on the tested anode surface (refer to Fig. 2). Location Footpoint mode crater
Anode spot mode crater
A B C D E
Mo (at%)
Ti (at%)
Si (at%)
C (at%)
Ti:Si ratio
0.42
48.20 57.98 53.01 74.03 44.98
15.29 17.81 17.17 4.88 5.44
36.09 24.21 29.82 21.09 49.58
3.15 3.26 3.09 – –
4.33
at locations A, B and C revealed a Ti:Si:C atomic ratio of approximately 3:1:2, which indicated that these 3 locations are basically composed of Ti3 SiC2 . Locations A and C have been contaminated by Mo that came from the cathode, with location C having a higher Mo content of 4.33 at% compared to that of 0.42 at% at location A. Location B is at the rim of the crater, no Mo can be detected at that point. Although the EDS result did not show an exact Ti:Si:C atomic ratio of 3:1:2, but their Ti:Si atomic ratio is only slightly higher than 3:1, which implied Ti is in excess if we consider the original composition of the anode material is Ti3 SiC2 . Location C has a Ti:Si atomic ratio of 3.09, which indicated that location C contains the highest Ti3 SiC2 concentration and location B has the highest degree of decomposition among these analyzed data points. Consider the EDX technique cannot give accurate quantitative analyzing results on light elements such as C, it can be concluded that after arcing, some of the Ti3 SiC2 has been decomposed to form TiC as one of the major decomposition products. For the anode spot mode crater in Fig. 2(d), EDS point analysis results show that both locations D and E are composed of nonstoichiometric TiC (TiCx ), with about 5 at% of residual Si. These TiCx and Si are decomposition products of Ti3 SiC2 . It is interesting to note that after arcing, the Ti3 SiC2 that originally composed the anode spot mode crater decomposed into TiCx and Si, while the Ti3 SiC2 at the footpoint mode crater basically remained as Ti3 SiC2 .
The Raman spectra of Ti3 SiC2 single crystal showed four peaks at 224, 278, 625, and 673 cm−1 [14]. Amer et al. [15] reported six sharp peaks at 159, 228, 281, 312, 631, and 678 cm−1 for HIPing polycrystalline Ti3 SiC2 . Raman spectra obtained from the self-prepared polycrystalline pure Ti3 SiC2 and the tested anode are presented in Fig. 3. The sharp peaks at 218, 624, and 672 cm−1 and a broad peak at 280 cm−1 in the Ti3 SiC2 profile of Fig. 3(b) are in agreement with those observed by Mercier [14] and Amer [15]. The peaks observed at 159 and 312 cm−1 in HIPing samples [15] did not appear in our experiment and no second order Raman bands were detected. By means of micro-Raman spectroscopy, information about the chemical composition of the footpoint mode crater is revealed in Fig. 3(a). Profile A, B and C are the Raman spectra for the central protrusion, the rim and the flat area adjacent to the crater, respectively. In the 100–750 cm−1 region, profiles A and B display four broad peaks centered around 268, 365, 605 and 658 cm−1 , which are in agreement with the TiCx data reported by Amer et al. and Qi et al. [15,16]. There is no doubt that Ti3 SiC2 decomposed and TiCx formed in these locations. For profile C, however, peaks at 218, 624, and 672 cm−1 which associated with Ti3 SiC2 still existed while the broad peak at 280 cm−1 vanished, indicating the change in bonding character of Ti3 SiC2 . A broad and strong peak located at 906 cm−1 appears in profile C, and it appears in profile A also, but much weaker. Although the origin of this peak is not clear now, it
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Fig. 3. Raman spectra of self-prepared polycrystalline Ti3 SiC2 and four selected locations from (a) footpoint mode cater (profiles A–C), (b) anode spot mode cater (profile D).
is presumably related to the decomposition products of Ti3 SiC2 . In the 1000–2000 cm−1 region, broad peaks at 1337 and 1591 cm−1 indicate the presence of amorphous carbon [17]. The origin of the peaks located at 1457 and 1789 cm−1 cannot be identified yet. These Raman spectra results are in agreement with those obtained by EDS analysis. Base on the above analysis, it is reasonable to propose that the central protrusions were formed from the molten materials, and some of the Ti3 SiC2 in footpoint mode craters decomposed into TiCx , amorphous carbon and other unknown decomposition products; the flat areas adjacent to the crater are basically composed of Ti3 SiC2 , but contain amorphous carbon, Mo from the cathode and other unknown substances related to Mo or erupted materials from the crater. Profile D in Fig. 3(b) shows three broad peaks at 289, 360 and 605 cm−1 which correspond to the peaks of TiCx [15,16,18] and no carbon signal could be detected. The micro-Raman spectrum data confirmed the EDS result that nonstoichiometric TiC is the dominant phase found in the anode spot mode crater (Location D), indicating the complete decomposition of Ti3 SiC2 there. Furthermore, no spheroidal protrusion formed in the crater implied that liquid phase is not present at the bottom of the crater, therefore, it can be concluded that the formation temperature of the big crater is well above the melting point of the anode material; vaporization is the erosion mechanism there. The Raman peaks of Si, SiC and Ti–Si compounds cannot be found in all the 4 profiles. It should be mentioned that stoichiometric TiC has no Raman active vibrational mode and the Raman spectrum of TiCx is composed of broad peaks induced by carbon vacancies [19]. The stability of Ti3 SiC2 at elevated temperature has been studied by different authors [5,20,21]. Barsoum et al. [5] found that Ti3 SiC2 bulk samples are thermally stable at 1600 ◦ C under vacuum or argon atmosphere. Low et al. [20] reported that the phase stability and transition of Ti3 SiC2 at elevated temperature are strongly dependent on the oxygen partial pressure of the annealing atmosphere; Ti3 SiC2 decomposes slowly to nonstoichiometric TiCx and Ti5 Si3 Cx (Ti5 Si3 phase with C dissolution) at 1200 ◦ C but the decomposition becomes quite rapid at 1500 ◦ C under vacuum. Racault et al. [21] investigated the thermal stability of Ti3 SiC2
by annealing the samples at different temperatures under vacuum. Their result shows that Ti3 SiC2 is thermally stable up to about 1450 ◦ C in an alumina crucible. Above this temperature, Ti3 SiC2 undergoes decomposition according to the following reaction which exhibits a very slow kinetics.
Ti3 SiC2 (s) → 3TiC0.67 (s) + Si(g) Ti3 SiC2 has a laminar structure with Ti–C blocks separated by Si layers. Two types of Ti–C bonds are exhibited in a Ti3 SiC2 unit cell: the Ti(1)–C bonds adjacent to the Si layers and the Ti(2)–C bonds in the centre of the unit cell. The inter-atomic distance between Ti and Si is larger than those of Ti(1)–C and Ti(2)–C, and the Ti–Si bond is weaker than Ti–C bonds [1,2]. As a result, Si is the weakest bonded element in the Ti3 SiC2 structure and thus making the above decomposition equation reasonable. The protrusion with a spheroidal shape indicated that liquid phase was involved in the formation of footpoint mode craters. Comparing the melting points of all possible decomposition products of Ti3 SiC2 , the melting points of TiC, SiC and C are over 3000 ◦ C, while Si, Ti and the Ti–Si compounds have melting temperatures lower than 2000 ◦ C (with the exception of Ti5 Si3 , which has a melting point of 2130 ◦ C). The lowest temperature liquid phases exist at the two 1330 ◦ C Ti–Si eutectics, with one at 13.7 at% Si and the other at approximately 85 at% Si. It is believed that the Ti3 SiC2 anode under the footpoint mode discharge will partially decomposed into TiCx , with possibly Si, C and some form of Ti–Si phase. Due to the high energy of the footpoint mode discharge, the Si and/or Ti–Si phase will be melted first and eventually vaporized, leaving TiCx as the major decomposition product that found in footpoint mode craters. As pointed out earlier, footpoint mode discharge has a temperature higher than the melting point of the anode material, but not as high as those of the anode spot mode discharge, which has a temperature near or higher than the boiling point of the anode material. For anode spot mode crater, trace of liquid involvement can only be seen at the rim. As shown in Fig. 2(d), the center of anode spot mode crater is flat and basically composed of TiCx , which indicated that the decomposition of Ti3 SiC2 at that location is quite
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Fig. 4. (a) Optical micrograph of a footpoint mode crater; (b) thee-dimensional topography of (a).
Fig. 5. Schematic of the droplet formation model after Gray and Pharney [22].
complete, however, according to material balance, the decomposition products should include Si related phases. The absence of liquid phase involvement within the crater enables the authors to postulate that under the highly energetic anode spot mode discharge, all decomposition products with lower melting points are being evaporated immediately after decomposition. A number of reports had found that the temperature of the anode spot is above the boiling point of the anode material. For example, Boxman [22] measured the anode surface temperature of Ni electrodes and obtained an equilibrium temperature of about 2900 K. Anode spot temperatures between 2600 and 3300 ◦ C for Cu, and between 2000 and 2400 ◦ C for Al were observed by Grissom and Newton [23]. Cobine and Burger [24] estimated the anode spot temperature should be above the boiling temperature of the material. They predicted theoretical limit for tungsten, for example, is between 6700 and 8700 K and their. Predicted results have been confirmed by other authors [13] through direct measurements. Therefore, it is reasonable to conclude that the temperature of Ti3 SiC2 anode spot should exceed 2500 ◦ C, which is far above its decomposition temperature and close to the boiling points of Ti and Si. Due to the low current (20 A on average) used in this experiment, the arc erosion of the Mo cathode is dominated by the eruption of Mo. Experimental result showed that the erosion on the refractory Mo cathode is not serious at all and does not display any particular feature. EDS analysis showed that no Ti3 SiC2 or any of its decomposition product can be detected on the Mo cathode and only tiny amount of Mo can be detected on the Ti3 SiC2 anode surface. Therefore, we did not pay special attention on the Mo cathode. Fig. 4 shows the optical micrograph and 3D topography of a footpoint mode crater. As shown in Fig. 4(b), the depth of the crater measured from the bottom of the crater to the summit of the central protrusion is about 7 μm. The 3D image is consistent with the SEM observation that the crater has a distinct morphology of depressed crater surround by a rim with an elevated centre. The presence of central protrusion in the footpoint mode craters can be interpreted by the droplet formation model developed by Gray and Pharney [25]. During discharge process, sufficiently high arcing energy will melt the electrode material and create a crater, as shown in Fig. 5(a). A force introduced by ion bombardment
will act on the liquid pool at the crater bottom. The sudden extinction of the arc will cause an un-balanced recoil force directed outward from the electrode into the inter-electrode gap, as shown in Fig. 5(b). The surface tension force of the liquid will act against the recoil force, if the recoil force is larger than the surface tension force, the liquid pool will be displaced to the point of separation from the pool in the form of a molten droplet, as shown in Fig. 5(c). In case of the recoil force is less than the surface tension force; a central protrusion will be formed in the crater, as shown in Fig. 5(d). Similar description can be found in Jüttner’s work [11]. 4. Conclusion The erosion mechanism of Ti3 SiC2 anode is the decomposition of Ti3 SiC2 into TiCx as the major product and the evaporation and/or eruption of the decomposition products. Different kinds of craters with diameters varying from a few microns to a few hundred microns were observed on the anode surface. The smaller craters named as footpoint mode craters, with a few tens of microns in diameter, are flower-like shaped, each with a central protrusion pointing out from the crater bottom. Part of the Ti3 SiC2 in the footpoint mode crater decomposed into TiCx as the major product; amorphous C and Mo from the cathode can also be detected. The larger craters named as anode spot mode craters have diameters greater than one hundred microns but without the central protrusions and are surrounded by collapse-fissures. The decomposition of Ti3 SiC2 in the anode spot mode crater is quite complete; the crater is basically composed of TiCx . Acknowledgements The support for this research was from the National Natural Science Foundation of China (Grant No. 51074077). References [1] E.H. Kisi, J.A.A. Crossley, S. Myhra, M.W. Barsoum, J. Phys. Chem. Solids 59 (1998) 1437. [2] Y. Zhou, Z. Sun, J. Phys. Condens. Matter 12 (2000) 457. [3] M.W. Barsoum, T.E. Raghy, J. Am. Ceram. Soc. 79 (1996) 1953.
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