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Segregation of Pt at clean surfaces of (Pt, Ni)3Al
Surface Science 601 (2007) 376–380 www.elsevier.com/locate/susc
Segregation of Pt at clean surfaces of (Pt, Ni)3Al F. Qin
b
a,c,*
, C. Jiang b,c, J.W. Anderegg c, C.J. Jenks c, B. Gleeson D.J. Sordelet c, P.A. Thiel a,b,c
b,c
,
a Department of Chemistry, Iowa State University, Ames, IA 50011, USA Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA c Ames Laboratory, Iowa State University, Ames, IA 50011, USA
Received 20 April 2006; accepted for publication 4 October 2006 Available online 24 October 2006
Abstract Experimental evidence for surface segregation of Pt at (1 1 1) surfaces of ternary (Pt, Ni)3Al alloys is presented, based upon Auger electron spectroscopy, low energy ion scattering, and angle-resolved X-ray photoelectron spectroscopy. Density functional calculations in the dilute limit confirm that Pt segregation is energetically favored. 2006 Elsevier B.V. All rights reserved. Keywords: Surface segregation; Density functional calculations; Auger electron spectroscopy; Nickel aluminide; X-ray photoelectron spectroscopy; Low energy ion scattering (LEIS); Low energy electron diffraction (LEED); Platinum
1. Introduction The binary alloy, Ni3Al, is used in numerous applications, such as turbine blades and cutting tools. It offers high hardness and wear resistance, low density, high melting point (1390 C), and good strength and corrosion resistance at high temperatures [1–3]. Recent work has shown that adding certain metals, notably Pt and Hf, can improve the oxidation resistance of this material significantly [4,5]. For instance, in high temperature oxidation, Pt improves the adhesion of the oxide scale and reduces the density of voids at the scale-metal interface. This naturally raises the question: What atomic-scale mechanisms are responsible for these beneficial effects of Pt? A basic starting point is to investigate the interaction of oxygen with the clean alloy surface. However, understanding the clean surface itself is necessary, and we take steps toward that end in this paper. Our primary objective is to * Corresponding author. Address: Department of Chemistry, Iowa State University, 220 Spedding Hall, Ames, IA 50011, USA. Tel.: +1 515 294 0905; fax: +1 515 294 4709. E-mail address: fl[email protected] (F. Qin).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.10.002
understand how Pt affects the composition and crystal structure at the surface, relative to the bulk. We address this issue with both theoretical work, i.e., density functional theory (DFT), and with experimental work, i.e., Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and low-energy electron diffraction (LEED). There have been many investigations of clean surfaces of the binary alloy, Ni3Al (which is also called the c 0 -phase), and these provide a strong background for the present work. It has been reported that the (1 1 1) and (1 1 0) faces are bulk-terminated, with no evidence of surface segregation [6–10]. The hexagonal (1 1 1) face contains 3/4 Ni and 1/4 Al, whereas the (1 1 0) surface preferentially exposes a termination that is 1/2 Ni and 1/2 Al. Thus, both surfaces preserve the bulk stoichiometry. They also preserve the bulk structure, except for interplanar relaxations, and for slight buckling of the first atomic layer, with Al atoms moving outward from the bulk [6,9,10]. The high-temperature Pt–Ni–Al phase diagram was first described by Jackson and Rairden [11], and was refined recently by Gleeson et al. [4,12]. The latter have shown that at 1150 C, Pt is highly soluble in Ni3Al, while the Al
F. Qin et al. / Surface Science 601 (2007) 376–380
content is invariant. This trend indicates that Pt substitutes for Ni in the bulk [4,12]. Such a conclusion is further supported by recent first-principles super cell calculations [13,14]. 2. Experimental description Large single crystals were grown by Lograsso and Wu of the Ames Laboratory’s Materials Preparation Center, using the Bridgman technique. Samples were oriented and sectioned from the ingot by electrical discharge machining (EDM). Surfaces were typically oriented to within ±0.25 of the desired orientation, then polished using standard metallographic techniques. The bulk compositions, based on electron probe micro-analysis (EPMA), are listed in Table 1. Three samples containing 10 at% Pt and three containing 20 at% Pt (hereafter referred to as 10%-Pt and 20%-Pt samples) were checked for spatial homogeneity, with 20 or more points per sample, using EPMA. The variations in atomic concentrations were small. For instance, one of the 20%-Pt samples showed standard deviations of 0.35 at% for Pt, 0.25 at% for Ni, and 0.09 at% for Al over the sample map, with no systematic trends in concentration vs. location. These numbers were typical. Thus, we regard the bulk samples as homogeneous. AES and LEED studies were performed in a single chamber that has been described previously [15], wherein the base pressure was 8 · 1011 Torr. Here, two (1 1 1) samples were characterized, a 10%-Pt and a 20%-Pt sample. Sample dimensions were 5 mm · 10 mm · 1.6 mm. They were spot-welded side by side onto a polycrystalline Ta plate, which was heated resistively. The sample temperature was monitored by a thermocouple (W/5%Re vs. W/ 26%Re) spot-welded on the back of the tantalum plate. After introduction from air, the samples were cleaned by repeated cycles of Ar+ sputtering followed by annealing at 1100 K, until the surface was judged clean by AES and LEED. For the clean surface, contaminants (mainly oxygen and carbon) were undetectable with AES. Between experiments, the samples were cleaned by 2–3 cycles of Ar+ sputtering at room temperature, followed by annealing at 1100 K for 60 min. The sputtering parameters were 1 keV and 1.0 lA to ground (without sample bias), and
377
the beam was rastered over a total area of 10 mm · 6 mm. The ion beam was normal to the surface. Based on AES, the ion sputtering that was used in sample preparation caused preferential removal of Al and Ni. Annealing at 1100 K for periods on the order of an hour served to reverse these effects. Preferential sputtering of Al has also been observed on Ni3Al surfaces by several groups [7,10,16]. Angle-resolved XPS (ARXPS) and LEIS were performed in a separate chamber, wherein the base pressure was at or below 9 · 1010 Torr. The XPS-LEIS chamber, and sample handling arrangements, have been described elsewhere [17]. Sample cleaning procedures were similar to those used for AES. For the LEIS results reported here, the incident beam impinged on the surface with an angle of 40 with respect to the surface normal. The scattering angle was 135. In order to calculate relative atomic concentrations, the differential scattering cross-section for each element, given our scattering conditions, was calculated using the ZBL screened Coulomb potential [18]. The broad background arising from inelastic processes and subsurface scattering was removed prior to determining the peak areas. 3. Results 3.1. Experimental results In AES, compositions were obtained using the MNN transition of Pt at 1967 eV, the LMM transition of Ni at 848 eV, and the KLL transition of Al at 1396 eV. In order to calculate compositions, the AES data were corrected with matrix factors of 0.72 and 1.01 for the Al1396 and Pt1967 peaks, respectively. These matrix factors are based on Ni50Al50 and Ni50Pt50, and calculated using the method of Seah and Dench [19]. Table 1 shows compositions of the clean annealed surfaces, compared with bulk compositions determined from EPMA. For both the 10%-Pt and 20%-Pt samples, AES indicates that the surface contains more Pt and less Ni than the bulk. In the AES data, these two changes cancel, so that the amount of Al remains unchanged. XPS was not used to determine absolute surface compositions because several main peaks were too closely spaced.
Table 1 Experimental bulk and surface compositions of (Ni, Pt)3Al samples Sample name
Analysis method (bulk or surface sensitive)
Pt (at%)
Ni (at%)
Al (at%)
10%-Pt 10%-Pt 10%-Pt 20%-Pt 20%-Pt 20%-Pt
EPMA (bulk) AES (surface) LEIS (surface) EPMA (bulk) AES (surface) LEIS (surface)
9.7 ± 0.1 17.3 ± 1.3 17 ± 1.7 20.4 ± 0.1 26.6 ± 2.2 26.4 ± 0.6
65.3 ± 0.3 57.6 ± 1.5 50.2 ± 0.5 54.7 ± 0.2 47.6 ± 2.4 39.2 ± 0.5
25.0 ± 0.3 25.1 ± 1.1 32.8 ± 2.2 24.9 ± 0.2 25.8 ± 1.1 34.5 ± 0.1
Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1)
Standard deviations in bulk composition are based on EPMA analysis at different locations on the sample. Standard deviations in AES concentrations are based on different sample preparation-measurement cycles. AES data were measured with the sample at room temperature, while LEIS data were measured at 900 and 1100 K. Previous LEIS data showed no temperature-dependence for the binary alloy (Ni3Al) [16].
F. Qin et al. / Surface Science 601 (2007) 376–380
Concentration Ratio
(The Al2p and Pt4f peaks overlap completely, and are close to the Ni3p peak. Furthermore, the Al2s and Ni3s peaks are close enough to make background subtraction questionable). However, ARXPS could be used as a relative measure of surface vs. near-surface compositions, and the result is shown in Fig. 1. In these data, a higher value of the take-off angle corresponds to a lower contribution of the surface signal to the total signal. Panels (a) and (b) show that increasing the XPS take-off angle causes the Pt atomic composition to decrease for both 10%-Pt and 20%-Pt. This indicates that Pt is enriched at the surface. The Al concentration changes in parallel with the Pt, suggesting that there is surface enrichment of Al as well. At the same time, the Ni concentration increases with increasing angle, demonstrating depletion of surface Ni. The LEIS data, shown in Table 1, agree with the trends in the XPS data. Relative to bulk concentrations from EPMA, both Pt and Al are enriched at the clean, annealed surface, while Ni is depleted. However, the LEIS and ARXPS data together are inconsistent with the AES data, because AES shows no surface enrichment of Al (cf. Table 1). Part of this discrepancy is undoubtedly due to different surface sensitivities. AES probes deeper into the bulk than LEIS or ARXPS at grazing emission, and hence would be less sensitive to Al enrichment at the very top layers (although it is surprising that AES would be entirely insensitive to Al enrichment). However, this explanation does not explain the discrepancy entirely, because AES gives the same surface concentration of Pt as does LEIS (see
1.6
Pt
1.4 1.2 1
Al
0.8 0.6 0
Ni 20 40 60 Take-off Angle
80
Table 1). One expects AES to show a lower concentration of Pt than LEIS if Pt also segregates. The inconsistencies, both in Pt and Al surface concentration, indicate that LEIS or AES (or both) are in error. A source of error in LEIS could be ion neutralization factors [20]. However, the qualitative agreement between LEIS and ARXPS data is good, as noted above, and we know of no experimental errors that could invalidate our interpretation of the measured trends in ARXPS. (Sources of error are known for quantitative extraction of depth profiles from ARXPS, but these do not affect the qualitative interpretation of the data at the level used here [21,22].) Therefore it seems more likely that the matrix factors in AES are inaccurate. To obtain the values in the Table, we used matrix factors for Al and Pt that were calculated for Al50Ni50 and Pt50Ni50 binary alloys, respectively [19]. Error could arise from the deviation of Al and Pt from the stoichiometry used to predict the matrix factors, or from neglecting the third element in each case, because of its influence on Auger electron backscattering. A comparison of several approaches to calculate matrix factors shows that the method used to calculate the inelastic mean free path, k, is critical to the resulting matrix factor. For the case of (Pt, Ni)3Al ternary alloy, this makes calculations of the absolute values of the matrix factors difficult [23]. LEED patterns of the clean surfaces are shown in Fig. 2. The clean, well-annealed 10%-Pt and 20%-Pt (1 1 1) samples exhibit good LEED patterns. The surface lattice constants for the 10%-Pt and 20%-Pt samples are the same within
Concentration Ratio
378
1.6
Al
1.4 1.2 1
Pt
0.8
Ni
0.6
0
20 40 60 Take-off Angle
80
Fig. 1. XPS angle-resolved data for (1 1 1) samples of (Pt, Ni)3Al containing (a) 10%-Pt, and (b) 20%-Pt in the bulk. The y-axis is the ratio of elemental composition for a given take-off angle (with respect to the sample surface) normalized to the value at 70, which has the highest bulk contribution to the total signal.
Fig. 2. LEED patterns of (1 1 1) samples of (Pt,Ni)3Al containing (a) 10%-Pt and (b) 20%-Pt in the bulk.
F. Qin et al. / Surface Science 601 (2007) 376–380
±0.8%. The spots appear sharper for the 10%-Pt than for the 20%-Pt sample, indicating that the Pt causes increasing surface disorder. However, there is no superstructure as would be expected in the case of a surface reconstruction or ordered surface alloy. 3.2. Theoretical results In the present study, first-principles calculations based on density functional theory (DFT) were performed to obtain the surface segregation energy Eseg of Pt on the (1 1 1) surface of Ni3Al. We modeled the clean Ni3Al(1 1 1) surface using a periodically repeating seven-layer slab with a (2 · 2) ˚ . The surface unit cell separated by a vacuum region of 15 A Pt segregation energy can then be obtained as the total energy difference between a Pt atom occupying a Ni site in the top layer of the slab and a Ni site in the center layer of the slab (i.e., layer 4). The highly-efficient Vienna ab initio simulation package (VASP) [24,25] was employed to obtain the iterative solution of the self-consistent Kohn-Sham equations using projector augmented wave (PAW) pseudopotentials [26,27] and a plane wave basis set. For the exchange-correlation functional, we adopted the generalized gradient approximation (GGA) by Perdew and Wang (PW91) [28,29]. The semi-core 3p electrons of Ni were explicitly treated as valence. The plane wave cutoff energy was set at 459.9 eV. The k-point meshes for Brillouin zone sampling were constructed using the Monkhorst–Pack scheme [30] and a 12 · 12 · 1 k-point mesh was used for the (1 1 1) surface, which corresponds to 114 irreducible k-points in the Brillouin zone. We performed spin-polarized calculations to take into account the effects of the ferromagnetic nature of Ni. Atoms in the bottom layer of the slab were kept fixed at their bulk-like positions, whereas atoms in all other layers were fully relaxed to their equilibrium positions according to the quantum-mechanical Hellmann–Feynman forces using a conjugate-gradient scheme. After relaxations, the ˚. forces acting on the atoms were less than 0.02 eV/A Our calculations give the segregation energy of Pt on the (1 1 1) surface of Ni3Al to be the large negative value of EPt seg ¼ 0:45 eV/atom, which indicates that Pt has a strong tendency toward surface segregation. Such a conclusion is fully consistent with the present experimental results. 4. Discussion Based on all the data presented here, our main conclusion is that Pt segregates to the (1 1 1) surface of (Pt, Ni)3Al. This is also indicated by our DFT calculations. A recent study of two related alloys, 5%-Pt and 10%-Pt in NiAl, also showed segregation of Pt to the clean surfaces [31]. In order to understand the surface segregation of Pt, we can examine previous results for Ni-rich Pt–Ni binaries. These alloys can be considered models for the ternary alloys studied here, since Ni is the major constituent and Pt is a dilute solute in both cases. As reviewed by Varga
379
et al., experimental studies of single crystal PtxNi1x surfaces using LEED and LEIS have shown that for (1 1 1) and (1 0 0) surfaces, the topmost layer is enriched in Pt relative to bulk concentrations, at bulk concentrations of 10, 25, and 50 at% Pt. The second layer, however, is slightly Ptpoor [32–35]. By analogy with these data, Pt enrichment at (1 1 1) surfaces of the (Pt, Ni)3Al alloys is perhaps not surprising. In general, the driving force for surface segregation has been discussed in the literature in terms of many factors— often interrelated—such as surface energies, heats of solution, bond energies, surface relaxation and reconstruction, surface stress, atomic volumes, etc. Let us focus first on surface energies. If one compares only DFT-derived values to maximize consistency, the surface energies of Ni(1 1 1), Pt(1 1 1), and Ni3Al(1 1 1) are 2011 mJ/m2 [36], 2299 mJ/ m2 [36], and 1887 mJ/m2 [37], respectively. Thus for PtxNi1x binary alloys, the surface energies of the constituent elements, Ni and Pt, are close. In the absence of any other factors, one would expect that Pt would not segregate to the surface of Ni(1 1 1) [38] or Ni3Al(1 1 1). However, experimentally, Pt has been shown to segregate to the surface of Ni [35] and, as we have shown, to the surface of Ni3Al(1 1 1). Tre´glia and Legrand [39], using a mean-field approach, tested the relative importance of various segregation driving forces including chemical bonding, and stress due to different atomic sizes, in surface segregation in Pt–Ni. They found almost no surface segregation when only chemical bonding effects were allowed, but strong Pt segregation in the first layer when only atomic size effects were operative. The atomic volume of Pt is over 35% larger than Ni, based on lattice constants [40]. The Treglia and Legrand results combined with a comparison of surface energies above, suggest that Pt surface segregation in Pt–Ni alloys is driven mainly by size differences. In the ternary case, one would expect that chemical bonding alone might have an effect on segregation. However, since both Pt and Al tend to form alloys with Ni, there is no readily apparent driving force leading to preferential segregation of one element over the others. Additional factors must play a role. The atomic volume of Al and Pt are similar and both are substantially larger than Ni. Thus, Ni is not expected to segregate to the surface based on atomic volume differences. Given though that Pt substitutes for Ni in the bulk alloy [4,12] the larger Pt segregating to the surface would serve to reduce strain. Thus, we postulate that relative size of Pt and Ni is mainly responsible for the surface segregation of Pt in the (Pt, Ni)3Al(1 1 1) ternary as well. The LEIS and ARXPS data also suggest that Al segregates at the surface of the (Pt, Ni)3Al(1 1 1) ternary. This is somewhat surprising, since several studies have indicated that the (1 1 1) surface of the related binary, Ni3Al, is bulkterminated [7,8,10]. The surface energy of Al(1 1 1) is calculated as 1199 mJ/m2 [36], which is much lower than the values for Ni(1 1 1), Pt(1 1 1), and Ni3Al(1 1 1). Furthermore, Al is slightly larger than Pt. Hence, both surface
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F. Qin et al. / Surface Science 601 (2007) 376–380
energies and size favor surface segregation of this element. This is apparently counterbalanced by the fact that the Al– Al bond is considerably weaker than the Ni–Al bond [41], which disfavors disruption of the binary lattice by surface segregation. It is possible that Pt changes the balance of these forces and promotes Al segregation. If so, this may partially explain why Pt facilitates oxidation of Al at the expense of Ni—it increases the amount of Al available, relative to Ni, in the initial stages of surface oxidation. 5. Conclusions Platinum segregates to the surface of clean (Pt, Ni)3Al(1 1 1), based upon experimental data for samples in which the bulk content is 10 at% and 20 at% Pt. Density functional calculations confirm that it is energetically favorable for Pt to migrate to the surface, in the dilute limit. By comparing surface energies of the pure elements and of Ni3Al, and also by drawing an analogy with results for PtNi alloys, we postulate that size effects are mainly responsible for this phenomenon. There is also some surface segregation of Al. Acknowledgement This work was supported by the Director, Ames Laboratory, US Department of Energy, Contract No. W-405Eng-82. References [1] Y. Lu, W. Chen, R. Eadie, Intermetallics 12 (2004) 1299. [2] V.K. Sikka, S.C. Devvi, S. Viswanathan, R.W. Swindeman, M.L. Santella, Intermetallics 8 (2000) 1329. [3] N.S. Stoloff, C.T. Liu, S.C. Devvi, Intermetallics 8 (2000) 1313. [4] B. Gleeson, W. Wang, S. Hayashi, D. Sordelet, Mater. Sci. Forum 461–464 (2004) 213. [5] C. Leyens, B.A. Pint, I.G. Wright, Surf. Coat. Technol. 133–134 (2000) 15. [6] G.F. Cotterill, H. Niehus, D.J. O’Connor, Surf. Rev. Lett. 3 (1996) 1355. [7] S.G. Addepalli, B. Ekstrom, N.P. Magtoto, J.-S. Lin, J.A. Kelber, Surf. Sci. 442 (1999) 385. [8] C. Becker, J. Kandler, H. Raaf, R. Linker, T. Pelster, M. Draeger, M. Tanemura, K. Wandelt, J. Vac. Sci. Technol. A 16 (1998) 1000.
[9] D. Sondericker, F. Jona, P.M. Marcus, Phys. Rev. B 34 (1986) 6775. [10] D. Sondericker, F. Jona, P.M. Marcus, Phys. Rev. B 34 (1986) 6770. [11] M.R. Jackson, J.R. Rairden, Metall. Trans. A 8 (1977) 1697. [12] S. Hayashi, S.I. Ford, D.J. Young, D.J. Sordelet, M.F. Besser, B. Gleeson, Acta Mater. 53 (2005) 3319. [13] C. Jiang, M.F. Besser, D.J. Sordelet, B. Gleeson, Acta Mater. 53 (2005) 2101. [14] C.Y. Geng, C.Y. Wang, T. Yu, Acta Mater. 52 (2004) 5427. [15] S.-L. Chang, W.B. Chin, C.-M. Zhang, C.J. Jenks, P.A. Thiel, Surf. Sci. 337 (1995) 135. [16] Y.G. Shen, D.J. O’Connor, R.J. MacDonald, Surf. Interf. Anal. 17 (1991) 903. [17] S.-L. Chang, J.W. Anderegg, P.A. Thiel, J. Non-Cryst. Solids 195 (1996) 95. [18] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [19] M.P. Seah, in: D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, Wiley, New York, 1990, p. 212. [20] C.J. Jenks, A.R. Ross, T.A. Lograsso, J.A. Whaley, R. Bastasz, Surf. Sci. 521 (2002) 34. [21] W.S.M. Werner, I.S. Tilinin, J. Vac. Sci. Technol. A 12 (1994) 2337. [22] W.S.M. Werner, G.C. Smith, A.K. Livesey, Surf. Interf. Anal. 21 (1994) 38. [23] R.-S. Li, C.-F. Li, Surf. Sci. 231 (1990) 433. [24] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [25] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15. [26] P.E. Blo¨chl, Phys. Rev. B 50 (1994) 17953. [27] G. Kresse, J. Joubert, Phys. Rev. B 59 (1999) 1758. [28] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. [29] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [30] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1972) 5188. [31] P.Y. Hou, K.F. McCarty, Scripta Mater. 54 (2006) 937. [32] R. Baudoing, Y. Gauthier, M. Lundberg, J. Rundgren, J. Phys. C 19 (1986) 2825. [33] P. Weigand, W. Hofer, P. Varga, Surf. Sci. 287/288 (1993) 350. [34] Y. Gauthier, Y. Joly, R. Baudoing, J. Rundgren, Phys. Rev. B 31 (1985) 6216. [35] P. Varga, M. Schmid, W. Hofer, Surf. Rev. Lett. 3 (1996) 1831. [36] L. Vitos, A.V. Ruban, H.L. Skriver, J. Kolla´r, Surf. Sci. 411 (1998) 186. [37] V.M. Kuznetsov, R.I. Kadyrov, G.E. Rudenskii, J. Mater. Sci. Technol. 14 (1998) 320. [38] A.R. Miedema, Z. Metallk. 69 (1978) 455. [39] G. Tre´glia, B. Legrand, Phys. Rev. B 35 (1987) 4338. [40] CRC Handbook of Chemistry and Physics, Boca Raton, FL, CRC Press LLC, 2001. [41] I. Al-Lehyani, M. Widom, Y. Wang, N. Moghadam, G.M. Stocks, J.A. Moriarty, Phys. Rev. B 64 (2001) 75109.
Segregation of Pt at clean surfaces of (Pt, Ni)3Al F. Qin
b
a,c,*
, C. Jiang b,c, J.W. Anderegg c, C.J. Jenks c, B. Gleeson D.J. Sordelet c, P.A. Thiel a,b,c
b,c
,
a Department of Chemistry, Iowa State University, Ames, IA 50011, USA Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA c Ames Laboratory, Iowa State University, Ames, IA 50011, USA
Received 20 April 2006; accepted for publication 4 October 2006 Available online 24 October 2006
Abstract Experimental evidence for surface segregation of Pt at (1 1 1) surfaces of ternary (Pt, Ni)3Al alloys is presented, based upon Auger electron spectroscopy, low energy ion scattering, and angle-resolved X-ray photoelectron spectroscopy. Density functional calculations in the dilute limit confirm that Pt segregation is energetically favored. 2006 Elsevier B.V. All rights reserved. Keywords: Surface segregation; Density functional calculations; Auger electron spectroscopy; Nickel aluminide; X-ray photoelectron spectroscopy; Low energy ion scattering (LEIS); Low energy electron diffraction (LEED); Platinum
1. Introduction The binary alloy, Ni3Al, is used in numerous applications, such as turbine blades and cutting tools. It offers high hardness and wear resistance, low density, high melting point (1390 C), and good strength and corrosion resistance at high temperatures [1–3]. Recent work has shown that adding certain metals, notably Pt and Hf, can improve the oxidation resistance of this material significantly [4,5]. For instance, in high temperature oxidation, Pt improves the adhesion of the oxide scale and reduces the density of voids at the scale-metal interface. This naturally raises the question: What atomic-scale mechanisms are responsible for these beneficial effects of Pt? A basic starting point is to investigate the interaction of oxygen with the clean alloy surface. However, understanding the clean surface itself is necessary, and we take steps toward that end in this paper. Our primary objective is to * Corresponding author. Address: Department of Chemistry, Iowa State University, 220 Spedding Hall, Ames, IA 50011, USA. Tel.: +1 515 294 0905; fax: +1 515 294 4709. E-mail address: fl[email protected] (F. Qin).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.10.002
understand how Pt affects the composition and crystal structure at the surface, relative to the bulk. We address this issue with both theoretical work, i.e., density functional theory (DFT), and with experimental work, i.e., Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and low-energy electron diffraction (LEED). There have been many investigations of clean surfaces of the binary alloy, Ni3Al (which is also called the c 0 -phase), and these provide a strong background for the present work. It has been reported that the (1 1 1) and (1 1 0) faces are bulk-terminated, with no evidence of surface segregation [6–10]. The hexagonal (1 1 1) face contains 3/4 Ni and 1/4 Al, whereas the (1 1 0) surface preferentially exposes a termination that is 1/2 Ni and 1/2 Al. Thus, both surfaces preserve the bulk stoichiometry. They also preserve the bulk structure, except for interplanar relaxations, and for slight buckling of the first atomic layer, with Al atoms moving outward from the bulk [6,9,10]. The high-temperature Pt–Ni–Al phase diagram was first described by Jackson and Rairden [11], and was refined recently by Gleeson et al. [4,12]. The latter have shown that at 1150 C, Pt is highly soluble in Ni3Al, while the Al
F. Qin et al. / Surface Science 601 (2007) 376–380
content is invariant. This trend indicates that Pt substitutes for Ni in the bulk [4,12]. Such a conclusion is further supported by recent first-principles super cell calculations [13,14]. 2. Experimental description Large single crystals were grown by Lograsso and Wu of the Ames Laboratory’s Materials Preparation Center, using the Bridgman technique. Samples were oriented and sectioned from the ingot by electrical discharge machining (EDM). Surfaces were typically oriented to within ±0.25 of the desired orientation, then polished using standard metallographic techniques. The bulk compositions, based on electron probe micro-analysis (EPMA), are listed in Table 1. Three samples containing 10 at% Pt and three containing 20 at% Pt (hereafter referred to as 10%-Pt and 20%-Pt samples) were checked for spatial homogeneity, with 20 or more points per sample, using EPMA. The variations in atomic concentrations were small. For instance, one of the 20%-Pt samples showed standard deviations of 0.35 at% for Pt, 0.25 at% for Ni, and 0.09 at% for Al over the sample map, with no systematic trends in concentration vs. location. These numbers were typical. Thus, we regard the bulk samples as homogeneous. AES and LEED studies were performed in a single chamber that has been described previously [15], wherein the base pressure was 8 · 1011 Torr. Here, two (1 1 1) samples were characterized, a 10%-Pt and a 20%-Pt sample. Sample dimensions were 5 mm · 10 mm · 1.6 mm. They were spot-welded side by side onto a polycrystalline Ta plate, which was heated resistively. The sample temperature was monitored by a thermocouple (W/5%Re vs. W/ 26%Re) spot-welded on the back of the tantalum plate. After introduction from air, the samples were cleaned by repeated cycles of Ar+ sputtering followed by annealing at 1100 K, until the surface was judged clean by AES and LEED. For the clean surface, contaminants (mainly oxygen and carbon) were undetectable with AES. Between experiments, the samples were cleaned by 2–3 cycles of Ar+ sputtering at room temperature, followed by annealing at 1100 K for 60 min. The sputtering parameters were 1 keV and 1.0 lA to ground (without sample bias), and
377
the beam was rastered over a total area of 10 mm · 6 mm. The ion beam was normal to the surface. Based on AES, the ion sputtering that was used in sample preparation caused preferential removal of Al and Ni. Annealing at 1100 K for periods on the order of an hour served to reverse these effects. Preferential sputtering of Al has also been observed on Ni3Al surfaces by several groups [7,10,16]. Angle-resolved XPS (ARXPS) and LEIS were performed in a separate chamber, wherein the base pressure was at or below 9 · 1010 Torr. The XPS-LEIS chamber, and sample handling arrangements, have been described elsewhere [17]. Sample cleaning procedures were similar to those used for AES. For the LEIS results reported here, the incident beam impinged on the surface with an angle of 40 with respect to the surface normal. The scattering angle was 135. In order to calculate relative atomic concentrations, the differential scattering cross-section for each element, given our scattering conditions, was calculated using the ZBL screened Coulomb potential [18]. The broad background arising from inelastic processes and subsurface scattering was removed prior to determining the peak areas. 3. Results 3.1. Experimental results In AES, compositions were obtained using the MNN transition of Pt at 1967 eV, the LMM transition of Ni at 848 eV, and the KLL transition of Al at 1396 eV. In order to calculate compositions, the AES data were corrected with matrix factors of 0.72 and 1.01 for the Al1396 and Pt1967 peaks, respectively. These matrix factors are based on Ni50Al50 and Ni50Pt50, and calculated using the method of Seah and Dench [19]. Table 1 shows compositions of the clean annealed surfaces, compared with bulk compositions determined from EPMA. For both the 10%-Pt and 20%-Pt samples, AES indicates that the surface contains more Pt and less Ni than the bulk. In the AES data, these two changes cancel, so that the amount of Al remains unchanged. XPS was not used to determine absolute surface compositions because several main peaks were too closely spaced.
Table 1 Experimental bulk and surface compositions of (Ni, Pt)3Al samples Sample name
Analysis method (bulk or surface sensitive)
Pt (at%)
Ni (at%)
Al (at%)
10%-Pt 10%-Pt 10%-Pt 20%-Pt 20%-Pt 20%-Pt
EPMA (bulk) AES (surface) LEIS (surface) EPMA (bulk) AES (surface) LEIS (surface)
9.7 ± 0.1 17.3 ± 1.3 17 ± 1.7 20.4 ± 0.1 26.6 ± 2.2 26.4 ± 0.6
65.3 ± 0.3 57.6 ± 1.5 50.2 ± 0.5 54.7 ± 0.2 47.6 ± 2.4 39.2 ± 0.5
25.0 ± 0.3 25.1 ± 1.1 32.8 ± 2.2 24.9 ± 0.2 25.8 ± 1.1 34.5 ± 0.1
Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1) Ni3Al(1 1 1)
Standard deviations in bulk composition are based on EPMA analysis at different locations on the sample. Standard deviations in AES concentrations are based on different sample preparation-measurement cycles. AES data were measured with the sample at room temperature, while LEIS data were measured at 900 and 1100 K. Previous LEIS data showed no temperature-dependence for the binary alloy (Ni3Al) [16].
F. Qin et al. / Surface Science 601 (2007) 376–380
Concentration Ratio
(The Al2p and Pt4f peaks overlap completely, and are close to the Ni3p peak. Furthermore, the Al2s and Ni3s peaks are close enough to make background subtraction questionable). However, ARXPS could be used as a relative measure of surface vs. near-surface compositions, and the result is shown in Fig. 1. In these data, a higher value of the take-off angle corresponds to a lower contribution of the surface signal to the total signal. Panels (a) and (b) show that increasing the XPS take-off angle causes the Pt atomic composition to decrease for both 10%-Pt and 20%-Pt. This indicates that Pt is enriched at the surface. The Al concentration changes in parallel with the Pt, suggesting that there is surface enrichment of Al as well. At the same time, the Ni concentration increases with increasing angle, demonstrating depletion of surface Ni. The LEIS data, shown in Table 1, agree with the trends in the XPS data. Relative to bulk concentrations from EPMA, both Pt and Al are enriched at the clean, annealed surface, while Ni is depleted. However, the LEIS and ARXPS data together are inconsistent with the AES data, because AES shows no surface enrichment of Al (cf. Table 1). Part of this discrepancy is undoubtedly due to different surface sensitivities. AES probes deeper into the bulk than LEIS or ARXPS at grazing emission, and hence would be less sensitive to Al enrichment at the very top layers (although it is surprising that AES would be entirely insensitive to Al enrichment). However, this explanation does not explain the discrepancy entirely, because AES gives the same surface concentration of Pt as does LEIS (see
1.6
Pt
1.4 1.2 1
Al
0.8 0.6 0
Ni 20 40 60 Take-off Angle
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Table 1). One expects AES to show a lower concentration of Pt than LEIS if Pt also segregates. The inconsistencies, both in Pt and Al surface concentration, indicate that LEIS or AES (or both) are in error. A source of error in LEIS could be ion neutralization factors [20]. However, the qualitative agreement between LEIS and ARXPS data is good, as noted above, and we know of no experimental errors that could invalidate our interpretation of the measured trends in ARXPS. (Sources of error are known for quantitative extraction of depth profiles from ARXPS, but these do not affect the qualitative interpretation of the data at the level used here [21,22].) Therefore it seems more likely that the matrix factors in AES are inaccurate. To obtain the values in the Table, we used matrix factors for Al and Pt that were calculated for Al50Ni50 and Pt50Ni50 binary alloys, respectively [19]. Error could arise from the deviation of Al and Pt from the stoichiometry used to predict the matrix factors, or from neglecting the third element in each case, because of its influence on Auger electron backscattering. A comparison of several approaches to calculate matrix factors shows that the method used to calculate the inelastic mean free path, k, is critical to the resulting matrix factor. For the case of (Pt, Ni)3Al ternary alloy, this makes calculations of the absolute values of the matrix factors difficult [23]. LEED patterns of the clean surfaces are shown in Fig. 2. The clean, well-annealed 10%-Pt and 20%-Pt (1 1 1) samples exhibit good LEED patterns. The surface lattice constants for the 10%-Pt and 20%-Pt samples are the same within
Concentration Ratio
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1.6
Al
1.4 1.2 1
Pt
0.8
Ni
0.6
0
20 40 60 Take-off Angle
80
Fig. 1. XPS angle-resolved data for (1 1 1) samples of (Pt, Ni)3Al containing (a) 10%-Pt, and (b) 20%-Pt in the bulk. The y-axis is the ratio of elemental composition for a given take-off angle (with respect to the sample surface) normalized to the value at 70, which has the highest bulk contribution to the total signal.
Fig. 2. LEED patterns of (1 1 1) samples of (Pt,Ni)3Al containing (a) 10%-Pt and (b) 20%-Pt in the bulk.
F. Qin et al. / Surface Science 601 (2007) 376–380
±0.8%. The spots appear sharper for the 10%-Pt than for the 20%-Pt sample, indicating that the Pt causes increasing surface disorder. However, there is no superstructure as would be expected in the case of a surface reconstruction or ordered surface alloy. 3.2. Theoretical results In the present study, first-principles calculations based on density functional theory (DFT) were performed to obtain the surface segregation energy Eseg of Pt on the (1 1 1) surface of Ni3Al. We modeled the clean Ni3Al(1 1 1) surface using a periodically repeating seven-layer slab with a (2 · 2) ˚ . The surface unit cell separated by a vacuum region of 15 A Pt segregation energy can then be obtained as the total energy difference between a Pt atom occupying a Ni site in the top layer of the slab and a Ni site in the center layer of the slab (i.e., layer 4). The highly-efficient Vienna ab initio simulation package (VASP) [24,25] was employed to obtain the iterative solution of the self-consistent Kohn-Sham equations using projector augmented wave (PAW) pseudopotentials [26,27] and a plane wave basis set. For the exchange-correlation functional, we adopted the generalized gradient approximation (GGA) by Perdew and Wang (PW91) [28,29]. The semi-core 3p electrons of Ni were explicitly treated as valence. The plane wave cutoff energy was set at 459.9 eV. The k-point meshes for Brillouin zone sampling were constructed using the Monkhorst–Pack scheme [30] and a 12 · 12 · 1 k-point mesh was used for the (1 1 1) surface, which corresponds to 114 irreducible k-points in the Brillouin zone. We performed spin-polarized calculations to take into account the effects of the ferromagnetic nature of Ni. Atoms in the bottom layer of the slab were kept fixed at their bulk-like positions, whereas atoms in all other layers were fully relaxed to their equilibrium positions according to the quantum-mechanical Hellmann–Feynman forces using a conjugate-gradient scheme. After relaxations, the ˚. forces acting on the atoms were less than 0.02 eV/A Our calculations give the segregation energy of Pt on the (1 1 1) surface of Ni3Al to be the large negative value of EPt seg ¼ 0:45 eV/atom, which indicates that Pt has a strong tendency toward surface segregation. Such a conclusion is fully consistent with the present experimental results. 4. Discussion Based on all the data presented here, our main conclusion is that Pt segregates to the (1 1 1) surface of (Pt, Ni)3Al. This is also indicated by our DFT calculations. A recent study of two related alloys, 5%-Pt and 10%-Pt in NiAl, also showed segregation of Pt to the clean surfaces [31]. In order to understand the surface segregation of Pt, we can examine previous results for Ni-rich Pt–Ni binaries. These alloys can be considered models for the ternary alloys studied here, since Ni is the major constituent and Pt is a dilute solute in both cases. As reviewed by Varga
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et al., experimental studies of single crystal PtxNi1x surfaces using LEED and LEIS have shown that for (1 1 1) and (1 0 0) surfaces, the topmost layer is enriched in Pt relative to bulk concentrations, at bulk concentrations of 10, 25, and 50 at% Pt. The second layer, however, is slightly Ptpoor [32–35]. By analogy with these data, Pt enrichment at (1 1 1) surfaces of the (Pt, Ni)3Al alloys is perhaps not surprising. In general, the driving force for surface segregation has been discussed in the literature in terms of many factors— often interrelated—such as surface energies, heats of solution, bond energies, surface relaxation and reconstruction, surface stress, atomic volumes, etc. Let us focus first on surface energies. If one compares only DFT-derived values to maximize consistency, the surface energies of Ni(1 1 1), Pt(1 1 1), and Ni3Al(1 1 1) are 2011 mJ/m2 [36], 2299 mJ/ m2 [36], and 1887 mJ/m2 [37], respectively. Thus for PtxNi1x binary alloys, the surface energies of the constituent elements, Ni and Pt, are close. In the absence of any other factors, one would expect that Pt would not segregate to the surface of Ni(1 1 1) [38] or Ni3Al(1 1 1). However, experimentally, Pt has been shown to segregate to the surface of Ni [35] and, as we have shown, to the surface of Ni3Al(1 1 1). Tre´glia and Legrand [39], using a mean-field approach, tested the relative importance of various segregation driving forces including chemical bonding, and stress due to different atomic sizes, in surface segregation in Pt–Ni. They found almost no surface segregation when only chemical bonding effects were allowed, but strong Pt segregation in the first layer when only atomic size effects were operative. The atomic volume of Pt is over 35% larger than Ni, based on lattice constants [40]. The Treglia and Legrand results combined with a comparison of surface energies above, suggest that Pt surface segregation in Pt–Ni alloys is driven mainly by size differences. In the ternary case, one would expect that chemical bonding alone might have an effect on segregation. However, since both Pt and Al tend to form alloys with Ni, there is no readily apparent driving force leading to preferential segregation of one element over the others. Additional factors must play a role. The atomic volume of Al and Pt are similar and both are substantially larger than Ni. Thus, Ni is not expected to segregate to the surface based on atomic volume differences. Given though that Pt substitutes for Ni in the bulk alloy [4,12] the larger Pt segregating to the surface would serve to reduce strain. Thus, we postulate that relative size of Pt and Ni is mainly responsible for the surface segregation of Pt in the (Pt, Ni)3Al(1 1 1) ternary as well. The LEIS and ARXPS data also suggest that Al segregates at the surface of the (Pt, Ni)3Al(1 1 1) ternary. This is somewhat surprising, since several studies have indicated that the (1 1 1) surface of the related binary, Ni3Al, is bulkterminated [7,8,10]. The surface energy of Al(1 1 1) is calculated as 1199 mJ/m2 [36], which is much lower than the values for Ni(1 1 1), Pt(1 1 1), and Ni3Al(1 1 1). Furthermore, Al is slightly larger than Pt. Hence, both surface
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energies and size favor surface segregation of this element. This is apparently counterbalanced by the fact that the Al– Al bond is considerably weaker than the Ni–Al bond [41], which disfavors disruption of the binary lattice by surface segregation. It is possible that Pt changes the balance of these forces and promotes Al segregation. If so, this may partially explain why Pt facilitates oxidation of Al at the expense of Ni—it increases the amount of Al available, relative to Ni, in the initial stages of surface oxidation. 5. Conclusions Platinum segregates to the surface of clean (Pt, Ni)3Al(1 1 1), based upon experimental data for samples in which the bulk content is 10 at% and 20 at% Pt. Density functional calculations confirm that it is energetically favorable for Pt to migrate to the surface, in the dilute limit. By comparing surface energies of the pure elements and of Ni3Al, and also by drawing an analogy with results for PtNi alloys, we postulate that size effects are mainly responsible for this phenomenon. There is also some surface segregation of Al. Acknowledgement This work was supported by the Director, Ames Laboratory, US Department of Energy, Contract No. W-405Eng-82. References [1] Y. Lu, W. Chen, R. Eadie, Intermetallics 12 (2004) 1299. [2] V.K. Sikka, S.C. Devvi, S. Viswanathan, R.W. Swindeman, M.L. Santella, Intermetallics 8 (2000) 1329. [3] N.S. Stoloff, C.T. Liu, S.C. Devvi, Intermetallics 8 (2000) 1313. [4] B. Gleeson, W. Wang, S. Hayashi, D. Sordelet, Mater. Sci. Forum 461–464 (2004) 213. [5] C. Leyens, B.A. Pint, I.G. Wright, Surf. Coat. Technol. 133–134 (2000) 15. [6] G.F. Cotterill, H. Niehus, D.J. O’Connor, Surf. Rev. Lett. 3 (1996) 1355. [7] S.G. Addepalli, B. Ekstrom, N.P. Magtoto, J.-S. Lin, J.A. Kelber, Surf. Sci. 442 (1999) 385. [8] C. Becker, J. Kandler, H. Raaf, R. Linker, T. Pelster, M. Draeger, M. Tanemura, K. Wandelt, J. Vac. Sci. Technol. A 16 (1998) 1000.
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