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Surface modification of TiAlSiCN coatings to improve oxidation protection
Applied Surface Science 347 (2015) 713–718
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Surface modification of TiAlSiCN coatings to improve oxidation protection K.A. Kuptsov, Ph.V. Kiryukhantsev-Korneev, A.N. Sheveyko, D.V. Shtansky ∗ National University of Science and Technology “MISIS”, Leninsky Prospect 4, Moscow 119049, Russia
a r t i c l e
i n f o
Article history: Received 6 March 2015 Received in revised form 21 April 2015 Accepted 22 April 2015 Available online 30 April 2015 Keywords: Oxidation resistance TiAlSiCN coatings Magnetron sputtering Surface modification Oxide layers
a b s t r a c t Coatings with high thermal stability and oxidation resistance are highly anticipated for various hightemperature applications. In this work we compare three different approaches to increase the oxidation resistance of nanocomposite TiAlSiCN coatings with exceptionally high thermal stability: (i) deposition of a thin Al top-layer, (ii) Al ion implantation into their topmost surface, and (iii) deposition of a thin AlOx top-layer. The coatings were annealed in air at 1000, 1100, and 1200 ◦ C for 1 h and their oxidation was studied using scanning electron microscopy and glow discharge optical emission spectroscopy. The obtained results demonstrate that the deposition of a thin top-layer of amorphous AlOx increases the oxidation resistance of the TiAlSiCN coatings from 1000 to 1100 ◦ C. This decreases the gap between the high thermal stability (1300 ◦ C) and oxidation resistance of the TiAlSiCN coatings, which is particularly important for high-speed and dry cutting applications. In contrast, the deposition of either a thin Al top-layer or Al ion implantation resulted in a negative effect. The factors affecting the rapid oxidation of such coatings at 1000 and 1100 ◦ C are discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to their exceptional combination of properties, such as high hardness, wear resistance, thermal stability, and oxidation resistance, multicomponent nanostructured coatings are continuing to attract much attention for various high temperature applications. Superhard TiAlSiCN coatings with comb-like nanocomposite structure recently developed in our group demonstrated the highest thermal stability reported to date for nanocomposite coatings [1,2]. The main drawback of such coatings is, however, a relatively large difference in temperature between their thermal stability (1300 ◦ C) and oxidation resistance (1000 ◦ C), which limits their use at high temperatures. One possible way to enhance oxidation resistance of such coatings might be increasing the concentration of Al atoms at their topmost layer. The Al content of TiAlSiCN coatings with the best mechanical properties was reported to be relatively low (∼15 at.%) [3], but upon annealing at 1000 ◦ C it increases up to 30 at.% due to temperature-activated Al diffusion. It should be noted that a high Al content (above 54 at.%) is usually required to form a continuous Al2 O3 scale [4]. When Al content was high enough
∗ Corresponding author. Tel.: +7 499 236 6629; fax: +7 499 236 5298. E-mail address: [email protected] (D.V. Shtansky). http://dx.doi.org/10.1016/j.apsusc.2015.04.159 0169-4332/© 2015 Elsevier B.V. All rights reserved.
(about 60 at.%), a rapidly growing protective oxide scale, including rutile grains, was found to cover the whole sample surface [5]. It is well known that ternary and quaternary crystalline TiNbased coatings possess oxidation resistance well below 1000 ◦ C, for instance: 740 ◦ C (TiSiN) [6], 900 ◦ C (TiAlN) [7,8], 800–950 ◦ C (AlTiN) [9,10], 850 ◦ C (TiAlSiN) [11], and 800 ◦ C (TiAlCN) [12]. This limits the high-temperature applications of such coatings on high-speed cutting tools, turbine blades, special parts of super high-speed aircrafts, rockets, reusable launch vehicles, etc. [13]. In the present paper, three different approaches to increase the oxidation resistance of the nanocomposite TiAlSiCN coatings are considered: (i) deposition of a thin Al top-layer, (ii) Al ion implantation into the surface of TiAlSiCN coatings, and (iii) deposition of a thin AlOx top-layer. The high-temperature coating oxidation was studied using scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES).
2. Experimental The TiAlSiCN coatings, 2.1 m thick, were deposited by DC magnetron sputtering of a TiAlSiCN target in a gaseous mixture of Ar + 15% N2 . The target was manufactured by means of the selfpropagating high-temperature synthesis (SHS) method [14]. The composition of a powder mixture used for the target synthesis
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was as follows (at.%): Ti – 43.2, Al – 21.5, Si – 10.1, C – 12.2, N – 14.0. An Advanced Energy Pinnacle Plus+ unit was used as a magnetron power supply and the applied power was kept at 1100 W. During deposition, the total pressure was maintained at 0.1 Pa, the bias voltage was −50 V, and the substrate temperature was kept constant at about 350 ◦ C. Polycrystalline Al2 O3 was used as a substrate material. Prior to deposition, substrates were first ultrasonically cleaned in isopropyl alcohol for 5 min and then etched by Ar ions for 10 min using an additional ion source operating at 3 kV and 50 mA. To enhance coating adhesion to the substrate, high-energy ion bombardment using a MEVVA type ion implanter with an average Ti2+ ion energy of about 70 keV was applied for 5 min at the beginning of deposition stage. The accelerating voltage, ion current, and ion flux were kept constant at 30 kV, 10 mA, and 2 × 1014 ions cm−2 s−1 , respectively. The implantation of Al ions into the TiAlSiCN coatings (this type of coatings was hereafter denoted as Alii /TiAlSiCN) was realized using the same energetic regimes as for Ti implantation described above. To fabricate Al/TiAlSiCN coatings, a 160-nm-thick Al layer was sputterdeposited atop of TiAlSiCN coatings using pure Al as a target. AlOx /TiAlSiCN coatings were obtained by depositing a 250-nmthick AlOx layer on the TiAlSiCN surface using a pure Al2 O3 target. Both the Al and AlOx layers were deposited in an Ar atmosphere. To assess the oxidation resistance, the coatings were annealed in air at 1000, 1100, and 1200 ◦ C in a muffle furnace. The microstructure of coatings, both before and after annealing, was examined by a scanning electron microscope equipped with an EDS module. The elemental depth profiles for the as-deposited and oxidized coatings were obtained by glow discharge optical emission spectroscopy (GDOES) using a PROFILER-2 instrument. The XRD patterns of coatings after annealing at 1000 ◦ C were recorded on a D8 Advanced X-ray diffractometer (Bruker) with Cu K␣ radiation. Both normal and grazing incidence (3◦ ) geometries were used. The average surface roughness (Ra ) was determined by atomic force microscopy (AFM) using an NTEGRA Spectra unit.
3. Results and discussion The plan-view and cross-sectional SEM images of the asdeposited TiAlSiCN, Alii /TiAlSiCN, Al/TiAlSiCN, and AlOx /TiAlSiCN coatings are shown in Fig. 1. The TiAlSiCN coating is seen to consist of two layers: (i) the top-layer with a dense, homogeneous, and fine columnar structure, and (ii) the underlayer, 200 nm thick, with a coarser columnar structure, which formed at the very beginning of deposition using ion implantation assisted magnetron sputtering (Fig. 1a). The Al ion implanted coating revealed single Al droplets on its surface (Fig. 1i, shown by arrow). This was because in the ion implanter an unfiltered arc evaporator was used as a metal ion source, and, therefore, some droplets reached the surface during ion implantation. In addition, Al ion implantation resulted in a more developed surface (Fig. 1j) compared with the non-modified TiAlSiCN coating. After Al ion implantation, the Ra value increased from 5.5 to 7.8 nm. The Al/TiAlSiCN coating with the thickness of a crystalline Al layer about 100 nm showed the highest surface roughness (Ra = 25.9 nm) due to Al grains nucleated and grown atop of the surface of the TiAlSiCN coating (Fig. 1e and f). In contrast, the amorphous AlOx layer was relatively smooth (Ra = 3.5 nm) and without any visible defects (Fig. 1m and n). The cross-sectional SEM images of the coatings heat-treated in air at 1000 ◦ C for 1 h are shown in Fig. 1 (columns 3 and 4). The coatings were only partly oxidized with oxidation depths increasing from 600 (TiAlSiCN) to 700 (AlOx /TiAlSiCN), to 880 nm (Al/TiAlSiCN) and finally to 2.7 m (Alii /TiAlSiCN). When comparing oxide scale of the TiAlSiCN and AlOx /TiAlSiCN samples, it should be noted that the thickness of the as-deposited AlOx layer was about
250 nm. Thus, in the case of AlOx /TiAlSiCN coating, the thickness of newly formed oxide scale was about 450 nm. GDOES depth profiles of the coatings annealed at 1000 ◦ C are presented in Fig. 2. The oxide scale formed on the surface of the TiAlSiCN coating consisted of three characteristic layers: TiOx , AlOx , and TiOx + SiOx (from top to bottom). The outermost layer, 0.3 m thick, did not form a dense film, merging as coarse and separate TiOx grains (Fig. 1d). The formation of a thin AlOx scale, 0.3 m thick, just beneath the TiOx layer was due to the enrichment of the near-surface layer with Al, which diffused to the coating surface during annealing [1]. While discussing the results, one should first highlight a positive role of Si. The diffusion coefficient of Al was shown to increase in the presence of Si [15]. In addition, the AlOx layer is followed by a mixed oxide layer containing nonprotective TiOx and SiOx phases (Fig. 2a). Unlike Al, Si did not diffuse to the coating’s surface, because Si ions are relatively immobile owing to the high-energy Si O bonding (465 kJ/mol) [16]. The SiOx phase, which was formed during oxidation, acted as an additional barrier against oxygen diffusion. Gong et al. [17] reported that the formation of SiO2 promotes Al2 O3 and suppresses TiO2 growth. Therefore, the high oxidation resistance of the TiAlSiCN coating can be explained by the synergetic effect of AlOx and SiOx phases, similar to that previously reported for TiAlSiN coatings [15,18]. The Gibb’s free energy (Go ) values for the formation of Al2 O3 , TiO2 and SiO2 are −954, −753, and −725 kJ/mol, respectively [19,20]. In addition, over a wide temperature range, Al has a very strong affinity to oxygen, and small aluminum atoms can easily diffuse through the grain boundaries in a cubic lattice [19]. Therefore, it is expected that a protective AlOx layer must be formed first, retarding the diffusion of oxygen inward and that of the coating elements outward. In order to check this hypothesis, the TiAlSiCN samples were annealed at 1000 ◦ C for 1, 6, 13, and 60 min, after which the elements depth distribution was analyzed by GDOES (Fig. 3a). It can be seen that the AlOx layer was detected immediately after annealing for as long as 1 min, whereas no TiOx scale was visible. After 6 min, TiOx layers were observed both atop and underneath the AlOx layer. These results indicate that despite the superior oxidation resistance of the TiAlSiCN coatings, their Al-rich oxide was not able to suppress completely the inward oxygen diffusion and outward Ti diffusion. The possible reason for this is the relatively low Al and Si contents in the TiAlSiCN coatings (14 and 6 at.%, respectively). The thickness of oxide scales was similar after annealing for 6 and 13 min (Fig. 3b). With further annealing, the thickness of the AlOx film remained relatively constant, whereas the thickness of TiO2 layers was observed to increase gradually. This agrees well with the previously published report on faster diffusion of Ti atoms toward the surface, compared to that of Al, which explains well the higher growth rate of TiOx compared to that of AlOx at temperatures between 900 and 1100 ◦ C [16]. Compared with the TiAlSiCN coating, two special features should be mentioned for the Al/TiAlSiCN coating: (i) the TiOx layer was thicker and (ii) its grains densely covered the whole surface of the annealed sample (Fig. 1g and h). This suggests that the AlOx scale did not act as a protective layer. On the contrary, it rather had a negative effect and decreased the oxidation resistance of the TiAlSiCN coating at 1000 ◦ C. This could be attributed to the fact that the surface of Al film had a high surface roughness giving rise to a porous AlOx scale. Another reason for the observed reduced oxidation resistance may be related to a relatively big difference in thermal expansion coefficients (CTEs) between Al and Al2 O3 (24.5 × 10−6 /K and 8.0 × 10−6 /K, respectively [21]). The tensile stress occurring during Al oxidation may lead to the formation of micro-cracks in the growing AlOx. This may result in two negative effects. On the one hand, Ti atoms can easily diffuse through the defects and micro-cracks in the AlOx layer toward the coating’s
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Fig. 1. Cross-sectional (1 and 3 vertical rows) and plan-view (2 and 4 vertical rows) SEM images of the coatings before and after annealing in air at 1000 ◦ C for 1 h. (a–d) TiAlSiCN, (e–h) Al/TiAlSiCN, (i–l) Alii /TiAlSiCN and (m–p) AlOx /TiAlSiCN.
surface to form a TiOx scale. On the other hand, the Al (AlOx ) top layer retarded the oxidation of the upper layers of the TiAlSiCN coating, resulting in no additional barrier to prevent outward diffusion of Ti atoms. The Alii /TiAlSiCN coating demonstrated the worst oxidation resistance among all tested samples. The oxide scale was 4 times thicker compared with the TiAlSiCN coating (Figs. 1k and 2c). The Alii /TiAlSiCN coating also revealed layered oxide morphology. The outermost layer with a thickness of 1.1 m was assigned to TiOx . It can be clearly seen that TiOx facetted grains completely covered the sample surface (Fig. 1p). The TiOx layer was followed by AlOx and TiOx + SiOx layers with fine oxide grains. The thickness of the preserved, non-oxidized part of the coating was about
500 nm. High-energy ion implantation is well-known to cause atomic displacements, defect formation, and radiation-induced diffusion, which may facilitate the Ti ion diffusion to the surface and accelerate TiOx formation. The large rod-like TiOx grains formed on the sample surface did not prevent the inward diffusion of oxygen atoms, whereas defects induced by ion implantation facilitated oxygen transfer into the Alii /TiAlSiCN coating. This explains well the observed rapid oxidation of the sample. Note that the available literature data are contradictory, indicating either a positive [22] or negative [23] effect of metal ion implantation on the oxidation behavior. Improved oxidation resistance was achieved by doping with elements that form dense protective oxide layers [22], whereas a negative effect is associated with enhanced inward
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Fig. 2. GDOES depth profiles of the (a) TiAlSiCN, (b) Al/TiAlSiCN, (c) Alii /TiAlSiCN, and (d) AlOx /TiAlSiCN coatings after annealing in air at 1000 ◦ C for 1 h.
Fig. 3. GDOES depth profiles of the TiAlSiCN coatings after annealing in air at 1000 ◦ C and thickness of oxide scales versus annealing time. (1) Total oxide scale, (2) AlOx .
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Fig. 4. XRD patterns of (1) as-deposited TiAlSiCN coating and (2)–(6) those annealed at 1000 ◦ C for 1 h: (2) AlOx /TiAlSiCN (GIXRD mode), (3) TiAlSiCN, (4) Al/TiAlSiCN, (5) Alii /TiAlSiCN, (6) AlOx /TiAlSiCN.
diffusion of oxygen atoms along the defects resulting in a poor (porous and discontinuous) oxide scale. The AlOx /TiAlSiCN coating annealed at 1000 ◦ C showed an increased thickness of AlOx layer, which consisted of initial and newly formed parts (Fig. 1o). The XRD analysis of the top AlOx layer (not shown here) revealed its amorphous structure. Compared to crystalline structures, amorphous phases are typically expected to demonstrate better oxidation resistance as they have no fast paths for ion diffusion such as grain boundaries or atomic vacancies [24]. Note, however, that after annealing, the TiOx grains were still observed atop the AlOx /TiAlSiCN sample (Fig. 1o and p). This finding can be explained by the generation of micro-cracks during rapid heating, which was caused by the difference in CTEs between the AlOx layer and the TiAlSiCN coating. XRD patterns of the as-deposited TiAlSiCN coating and the TiAlSiCN, Al/TiAlSiCN, Alii /TiAlSiCN, and AlOx /TiAlSiCN coatings annealed at 1000 ◦ C for 1 h are presented in Fig. 4 and in general confirm the main findings described above. The as-deposited TiAlSiCN coating showed a strong (1 1 1) preferential orientation, while no other peaks from the cubic phase were observed. Using conventional XRD measurements, the main peaks observed in annealed samples were from the Al2 O3 and TiO2 phases. In the case of the TiAlSiCN, Al/TiAlSiCN, and AlOx /TiAlSiCN coatings, a broad asymmetrical peak was observed between 36 and 37.5 2 degrees, which can be interpreted as a superposition of the peaks from
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(1 0 0) planes of TiO2 and (1 1 1) planes of (Ti,Al)(C,N). Since the oxide scale on the surface of the Alii /TiAlSiCN coating was thick, the signals from the (1 1 1) planes of (Ti,Al)(C,N) were not visible. Note that conventional XRD does not permit to distinguish the signals from AlOx surface layer and Al2 O3 substrate. Therefore, to make XRD measurements surface sensitive, the AlOx /TiAlSiCN coating was also analyzed in the grazing-incidence small-angle scattering mode. In this mode, the peaks of Al2 O3 substrate completely disappeared while additional peaks, identified as those of AlOx scale on the coating’s surface, appeared at 35.9 and 44.2 2 degrees. During annealing at a higher temperature of 1100 ◦ C, all the coatings, except for the AlOx /TiAlSiCN, were completely oxidized. The coating morphology and elemental depth profiles of the asannealed AlOx /TiAlSiCN coating are shown in Fig. 5. The layered oxide structure was observed to be even more complex compared to that after annealing at 1000 ◦ C. The outermost layer consisting of individual TiOx grains was followed by a thin and dense AlOx , loose TiOx + AlOx , thick and dense AlOx , and finally a nonoxidized part of the initial AlOx /TiAlSiCN sample (Fig. 5a and b). Note that no mixed TiOx + SiOx layer was found this time. The total thickness of the oxide scale was estimated to be about 870 nm, which was only 170 nm thicker than that of the oxide formed at 1000 ◦ C for 1 h. It is also worth noting that after annealing at 1100 ◦ C, the outermost TiOx layer did not cover the coating surface completely (Fig. 5a (inset)), but the size of TiOx grains was larger compared with those observed after annealing at 1000 ◦ C (Fig. 1o and p). Although the AlOx /TiAlSiCN coatings annealed at 1000 and 1100 ◦ C revealed similar morphologies of dense AlOx layers with presumably amorphous structure, the question whether any amorphous-to-crystalline phase transformations occurred within this temperature range remains open. For instance, Zhang et al. [25] showed that amorphous Al oxide started to transform into ␣-Al2 O3 at 1050 ◦ C. The mixed TiOx + AlOx layer appeared to be formed due to simultaneous outward diffusion of Al and Ti atoms and inward oxygen diffusion through the defects and micro-cracks in the TiOx and AlOx layers. The formation of a mixed TiOx + AlOx layer just beneath the AlOx layer indicates that the AlOx film acted as a barrier for the outward diffusion of Ti atoms and promoted the formation of an additional oxide layer. During annealing at 1200 ◦ C, the AlOx /TiAlSiCN sample was completely oxidized, probably due to the decomposition of Al2 O3 crystallites [25]. It should also be mentioned that, among other factors, the oxidation of the coatings at such a high temperature could be controlled by the adhesive and cohesive failures of oxide layers
Fig. 5. (a) Cross-sectional and plan-view (inset) SEM images of the AlOx /TiAlSiCN coating after annealing at 1100 ◦ C for 1 h with (b) corresponding GDOES depth profiles.
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due to the difference in CTEs between Al2 O3 (8 × 10−6 /K) and TiO2 (10.5 × 10−6 /K [20]). 4. Conclusions To summarize, we investigated three different approaches to increase the oxidation resistance of TiAlSiCN coatings: (i) deposition of an Al top-layer, (ii) deposition of an AlOx top-layer, and (iii) Al ion implantation in the outmost TiAlSiCN layer. The as-prepared coatings were annealed in air at 1000, 1100, and 1200 ◦ C for 1 h and then evaluated using SEM, AFM, and GDOES. In order to analyze the oxidation kinetics, the TiAlSiCN coatings were annealed at 1000 ◦ C for 1, 6, 13, and 60 min, after which the elements depth distribution was analyzed by GDOES. The obtained results show that the deposition of a thin amorphous AlOx layer permitted to increase the oxidation resistance of the TiAlSiCN coating from 1000 to 1100 ◦ C. The oxidation process was shown to be accompanied by the diffusion of Al and Ti atoms to the surface and inward oxygen diffusion, which resulted in the formation of a layered structure. The thickness of the oxide scale was similar after heat treatments at 1000 and 1100 ◦ C, but the layered structures were different: TiOx and AlOx (1000 ◦ C) and TiOx , AlOx , TiOx + AlOx , and AlOx sublayers from the top to bottom of the AlOx /TiAlSiCN coating (1100 ◦ C). The deposition of amorphous AlOx layer led to a narrower temperature gap between the exceptionally high thermal stability (1300 ◦ C) and oxidation resistance of the TiAlSiCN coatings. This result is of great importance for high-speed and dry cutting applications because the operating temperatures on the cutting-tool edge during continuous cutting of hardened tool steel may exceed 1000 ◦ C [26]. In contrast, the formation of an Al rich top-layer, created via either depositing a metallic film or implanting Al ions, resulted in decreased oxidation resistance. Acknowledgements The authors gratefully acknowledge the financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (No. К2-2014-012) and the Russian Foundation for Basic Research (Agreement No. HK 13-03-12129\14). K.A.K. and D.V.S. also acknowledge a “Mega-Grant” award from the Ministry of Education and Science of the Russian Federation (No. 14.Y26.31.0005). References [1] D.V. Shtansky, K.A. Kuptsov, P.V. Kiryukhantsev-Korneev, A.N. Sheveyko, High thermal stability of TiAlSiCN coatings with comb like nanocomposite structure, Surf. Coat. Technol. 206 (2012) 4840–4849. [2] K.A. Kuptsov, P.V. Kiryukhantsev-Korneev, A.N. Sheveyko, D.V. Shtansky, Structural transformations in TiAlSiCN coatings in the temperature range 900–1600 ◦ C, Acta Mater. 83 (2015) 408–418.
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Surface modification of TiAlSiCN coatings to improve oxidation protection K.A. Kuptsov, Ph.V. Kiryukhantsev-Korneev, A.N. Sheveyko, D.V. Shtansky ∗ National University of Science and Technology “MISIS”, Leninsky Prospect 4, Moscow 119049, Russia
a r t i c l e
i n f o
Article history: Received 6 March 2015 Received in revised form 21 April 2015 Accepted 22 April 2015 Available online 30 April 2015 Keywords: Oxidation resistance TiAlSiCN coatings Magnetron sputtering Surface modification Oxide layers
a b s t r a c t Coatings with high thermal stability and oxidation resistance are highly anticipated for various hightemperature applications. In this work we compare three different approaches to increase the oxidation resistance of nanocomposite TiAlSiCN coatings with exceptionally high thermal stability: (i) deposition of a thin Al top-layer, (ii) Al ion implantation into their topmost surface, and (iii) deposition of a thin AlOx top-layer. The coatings were annealed in air at 1000, 1100, and 1200 ◦ C for 1 h and their oxidation was studied using scanning electron microscopy and glow discharge optical emission spectroscopy. The obtained results demonstrate that the deposition of a thin top-layer of amorphous AlOx increases the oxidation resistance of the TiAlSiCN coatings from 1000 to 1100 ◦ C. This decreases the gap between the high thermal stability (1300 ◦ C) and oxidation resistance of the TiAlSiCN coatings, which is particularly important for high-speed and dry cutting applications. In contrast, the deposition of either a thin Al top-layer or Al ion implantation resulted in a negative effect. The factors affecting the rapid oxidation of such coatings at 1000 and 1100 ◦ C are discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to their exceptional combination of properties, such as high hardness, wear resistance, thermal stability, and oxidation resistance, multicomponent nanostructured coatings are continuing to attract much attention for various high temperature applications. Superhard TiAlSiCN coatings with comb-like nanocomposite structure recently developed in our group demonstrated the highest thermal stability reported to date for nanocomposite coatings [1,2]. The main drawback of such coatings is, however, a relatively large difference in temperature between their thermal stability (1300 ◦ C) and oxidation resistance (1000 ◦ C), which limits their use at high temperatures. One possible way to enhance oxidation resistance of such coatings might be increasing the concentration of Al atoms at their topmost layer. The Al content of TiAlSiCN coatings with the best mechanical properties was reported to be relatively low (∼15 at.%) [3], but upon annealing at 1000 ◦ C it increases up to 30 at.% due to temperature-activated Al diffusion. It should be noted that a high Al content (above 54 at.%) is usually required to form a continuous Al2 O3 scale [4]. When Al content was high enough
∗ Corresponding author. Tel.: +7 499 236 6629; fax: +7 499 236 5298. E-mail address: [email protected] (D.V. Shtansky). http://dx.doi.org/10.1016/j.apsusc.2015.04.159 0169-4332/© 2015 Elsevier B.V. All rights reserved.
(about 60 at.%), a rapidly growing protective oxide scale, including rutile grains, was found to cover the whole sample surface [5]. It is well known that ternary and quaternary crystalline TiNbased coatings possess oxidation resistance well below 1000 ◦ C, for instance: 740 ◦ C (TiSiN) [6], 900 ◦ C (TiAlN) [7,8], 800–950 ◦ C (AlTiN) [9,10], 850 ◦ C (TiAlSiN) [11], and 800 ◦ C (TiAlCN) [12]. This limits the high-temperature applications of such coatings on high-speed cutting tools, turbine blades, special parts of super high-speed aircrafts, rockets, reusable launch vehicles, etc. [13]. In the present paper, three different approaches to increase the oxidation resistance of the nanocomposite TiAlSiCN coatings are considered: (i) deposition of a thin Al top-layer, (ii) Al ion implantation into the surface of TiAlSiCN coatings, and (iii) deposition of a thin AlOx top-layer. The high-temperature coating oxidation was studied using scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES).
2. Experimental The TiAlSiCN coatings, 2.1 m thick, were deposited by DC magnetron sputtering of a TiAlSiCN target in a gaseous mixture of Ar + 15% N2 . The target was manufactured by means of the selfpropagating high-temperature synthesis (SHS) method [14]. The composition of a powder mixture used for the target synthesis
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was as follows (at.%): Ti – 43.2, Al – 21.5, Si – 10.1, C – 12.2, N – 14.0. An Advanced Energy Pinnacle Plus+ unit was used as a magnetron power supply and the applied power was kept at 1100 W. During deposition, the total pressure was maintained at 0.1 Pa, the bias voltage was −50 V, and the substrate temperature was kept constant at about 350 ◦ C. Polycrystalline Al2 O3 was used as a substrate material. Prior to deposition, substrates were first ultrasonically cleaned in isopropyl alcohol for 5 min and then etched by Ar ions for 10 min using an additional ion source operating at 3 kV and 50 mA. To enhance coating adhesion to the substrate, high-energy ion bombardment using a MEVVA type ion implanter with an average Ti2+ ion energy of about 70 keV was applied for 5 min at the beginning of deposition stage. The accelerating voltage, ion current, and ion flux were kept constant at 30 kV, 10 mA, and 2 × 1014 ions cm−2 s−1 , respectively. The implantation of Al ions into the TiAlSiCN coatings (this type of coatings was hereafter denoted as Alii /TiAlSiCN) was realized using the same energetic regimes as for Ti implantation described above. To fabricate Al/TiAlSiCN coatings, a 160-nm-thick Al layer was sputterdeposited atop of TiAlSiCN coatings using pure Al as a target. AlOx /TiAlSiCN coatings were obtained by depositing a 250-nmthick AlOx layer on the TiAlSiCN surface using a pure Al2 O3 target. Both the Al and AlOx layers were deposited in an Ar atmosphere. To assess the oxidation resistance, the coatings were annealed in air at 1000, 1100, and 1200 ◦ C in a muffle furnace. The microstructure of coatings, both before and after annealing, was examined by a scanning electron microscope equipped with an EDS module. The elemental depth profiles for the as-deposited and oxidized coatings were obtained by glow discharge optical emission spectroscopy (GDOES) using a PROFILER-2 instrument. The XRD patterns of coatings after annealing at 1000 ◦ C were recorded on a D8 Advanced X-ray diffractometer (Bruker) with Cu K␣ radiation. Both normal and grazing incidence (3◦ ) geometries were used. The average surface roughness (Ra ) was determined by atomic force microscopy (AFM) using an NTEGRA Spectra unit.
3. Results and discussion The plan-view and cross-sectional SEM images of the asdeposited TiAlSiCN, Alii /TiAlSiCN, Al/TiAlSiCN, and AlOx /TiAlSiCN coatings are shown in Fig. 1. The TiAlSiCN coating is seen to consist of two layers: (i) the top-layer with a dense, homogeneous, and fine columnar structure, and (ii) the underlayer, 200 nm thick, with a coarser columnar structure, which formed at the very beginning of deposition using ion implantation assisted magnetron sputtering (Fig. 1a). The Al ion implanted coating revealed single Al droplets on its surface (Fig. 1i, shown by arrow). This was because in the ion implanter an unfiltered arc evaporator was used as a metal ion source, and, therefore, some droplets reached the surface during ion implantation. In addition, Al ion implantation resulted in a more developed surface (Fig. 1j) compared with the non-modified TiAlSiCN coating. After Al ion implantation, the Ra value increased from 5.5 to 7.8 nm. The Al/TiAlSiCN coating with the thickness of a crystalline Al layer about 100 nm showed the highest surface roughness (Ra = 25.9 nm) due to Al grains nucleated and grown atop of the surface of the TiAlSiCN coating (Fig. 1e and f). In contrast, the amorphous AlOx layer was relatively smooth (Ra = 3.5 nm) and without any visible defects (Fig. 1m and n). The cross-sectional SEM images of the coatings heat-treated in air at 1000 ◦ C for 1 h are shown in Fig. 1 (columns 3 and 4). The coatings were only partly oxidized with oxidation depths increasing from 600 (TiAlSiCN) to 700 (AlOx /TiAlSiCN), to 880 nm (Al/TiAlSiCN) and finally to 2.7 m (Alii /TiAlSiCN). When comparing oxide scale of the TiAlSiCN and AlOx /TiAlSiCN samples, it should be noted that the thickness of the as-deposited AlOx layer was about
250 nm. Thus, in the case of AlOx /TiAlSiCN coating, the thickness of newly formed oxide scale was about 450 nm. GDOES depth profiles of the coatings annealed at 1000 ◦ C are presented in Fig. 2. The oxide scale formed on the surface of the TiAlSiCN coating consisted of three characteristic layers: TiOx , AlOx , and TiOx + SiOx (from top to bottom). The outermost layer, 0.3 m thick, did not form a dense film, merging as coarse and separate TiOx grains (Fig. 1d). The formation of a thin AlOx scale, 0.3 m thick, just beneath the TiOx layer was due to the enrichment of the near-surface layer with Al, which diffused to the coating surface during annealing [1]. While discussing the results, one should first highlight a positive role of Si. The diffusion coefficient of Al was shown to increase in the presence of Si [15]. In addition, the AlOx layer is followed by a mixed oxide layer containing nonprotective TiOx and SiOx phases (Fig. 2a). Unlike Al, Si did not diffuse to the coating’s surface, because Si ions are relatively immobile owing to the high-energy Si O bonding (465 kJ/mol) [16]. The SiOx phase, which was formed during oxidation, acted as an additional barrier against oxygen diffusion. Gong et al. [17] reported that the formation of SiO2 promotes Al2 O3 and suppresses TiO2 growth. Therefore, the high oxidation resistance of the TiAlSiCN coating can be explained by the synergetic effect of AlOx and SiOx phases, similar to that previously reported for TiAlSiN coatings [15,18]. The Gibb’s free energy (Go ) values for the formation of Al2 O3 , TiO2 and SiO2 are −954, −753, and −725 kJ/mol, respectively [19,20]. In addition, over a wide temperature range, Al has a very strong affinity to oxygen, and small aluminum atoms can easily diffuse through the grain boundaries in a cubic lattice [19]. Therefore, it is expected that a protective AlOx layer must be formed first, retarding the diffusion of oxygen inward and that of the coating elements outward. In order to check this hypothesis, the TiAlSiCN samples were annealed at 1000 ◦ C for 1, 6, 13, and 60 min, after which the elements depth distribution was analyzed by GDOES (Fig. 3a). It can be seen that the AlOx layer was detected immediately after annealing for as long as 1 min, whereas no TiOx scale was visible. After 6 min, TiOx layers were observed both atop and underneath the AlOx layer. These results indicate that despite the superior oxidation resistance of the TiAlSiCN coatings, their Al-rich oxide was not able to suppress completely the inward oxygen diffusion and outward Ti diffusion. The possible reason for this is the relatively low Al and Si contents in the TiAlSiCN coatings (14 and 6 at.%, respectively). The thickness of oxide scales was similar after annealing for 6 and 13 min (Fig. 3b). With further annealing, the thickness of the AlOx film remained relatively constant, whereas the thickness of TiO2 layers was observed to increase gradually. This agrees well with the previously published report on faster diffusion of Ti atoms toward the surface, compared to that of Al, which explains well the higher growth rate of TiOx compared to that of AlOx at temperatures between 900 and 1100 ◦ C [16]. Compared with the TiAlSiCN coating, two special features should be mentioned for the Al/TiAlSiCN coating: (i) the TiOx layer was thicker and (ii) its grains densely covered the whole surface of the annealed sample (Fig. 1g and h). This suggests that the AlOx scale did not act as a protective layer. On the contrary, it rather had a negative effect and decreased the oxidation resistance of the TiAlSiCN coating at 1000 ◦ C. This could be attributed to the fact that the surface of Al film had a high surface roughness giving rise to a porous AlOx scale. Another reason for the observed reduced oxidation resistance may be related to a relatively big difference in thermal expansion coefficients (CTEs) between Al and Al2 O3 (24.5 × 10−6 /K and 8.0 × 10−6 /K, respectively [21]). The tensile stress occurring during Al oxidation may lead to the formation of micro-cracks in the growing AlOx. This may result in two negative effects. On the one hand, Ti atoms can easily diffuse through the defects and micro-cracks in the AlOx layer toward the coating’s
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Fig. 1. Cross-sectional (1 and 3 vertical rows) and plan-view (2 and 4 vertical rows) SEM images of the coatings before and after annealing in air at 1000 ◦ C for 1 h. (a–d) TiAlSiCN, (e–h) Al/TiAlSiCN, (i–l) Alii /TiAlSiCN and (m–p) AlOx /TiAlSiCN.
surface to form a TiOx scale. On the other hand, the Al (AlOx ) top layer retarded the oxidation of the upper layers of the TiAlSiCN coating, resulting in no additional barrier to prevent outward diffusion of Ti atoms. The Alii /TiAlSiCN coating demonstrated the worst oxidation resistance among all tested samples. The oxide scale was 4 times thicker compared with the TiAlSiCN coating (Figs. 1k and 2c). The Alii /TiAlSiCN coating also revealed layered oxide morphology. The outermost layer with a thickness of 1.1 m was assigned to TiOx . It can be clearly seen that TiOx facetted grains completely covered the sample surface (Fig. 1p). The TiOx layer was followed by AlOx and TiOx + SiOx layers with fine oxide grains. The thickness of the preserved, non-oxidized part of the coating was about
500 nm. High-energy ion implantation is well-known to cause atomic displacements, defect formation, and radiation-induced diffusion, which may facilitate the Ti ion diffusion to the surface and accelerate TiOx formation. The large rod-like TiOx grains formed on the sample surface did not prevent the inward diffusion of oxygen atoms, whereas defects induced by ion implantation facilitated oxygen transfer into the Alii /TiAlSiCN coating. This explains well the observed rapid oxidation of the sample. Note that the available literature data are contradictory, indicating either a positive [22] or negative [23] effect of metal ion implantation on the oxidation behavior. Improved oxidation resistance was achieved by doping with elements that form dense protective oxide layers [22], whereas a negative effect is associated with enhanced inward
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Fig. 2. GDOES depth profiles of the (a) TiAlSiCN, (b) Al/TiAlSiCN, (c) Alii /TiAlSiCN, and (d) AlOx /TiAlSiCN coatings after annealing in air at 1000 ◦ C for 1 h.
Fig. 3. GDOES depth profiles of the TiAlSiCN coatings after annealing in air at 1000 ◦ C and thickness of oxide scales versus annealing time. (1) Total oxide scale, (2) AlOx .
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Fig. 4. XRD patterns of (1) as-deposited TiAlSiCN coating and (2)–(6) those annealed at 1000 ◦ C for 1 h: (2) AlOx /TiAlSiCN (GIXRD mode), (3) TiAlSiCN, (4) Al/TiAlSiCN, (5) Alii /TiAlSiCN, (6) AlOx /TiAlSiCN.
diffusion of oxygen atoms along the defects resulting in a poor (porous and discontinuous) oxide scale. The AlOx /TiAlSiCN coating annealed at 1000 ◦ C showed an increased thickness of AlOx layer, which consisted of initial and newly formed parts (Fig. 1o). The XRD analysis of the top AlOx layer (not shown here) revealed its amorphous structure. Compared to crystalline structures, amorphous phases are typically expected to demonstrate better oxidation resistance as they have no fast paths for ion diffusion such as grain boundaries or atomic vacancies [24]. Note, however, that after annealing, the TiOx grains were still observed atop the AlOx /TiAlSiCN sample (Fig. 1o and p). This finding can be explained by the generation of micro-cracks during rapid heating, which was caused by the difference in CTEs between the AlOx layer and the TiAlSiCN coating. XRD patterns of the as-deposited TiAlSiCN coating and the TiAlSiCN, Al/TiAlSiCN, Alii /TiAlSiCN, and AlOx /TiAlSiCN coatings annealed at 1000 ◦ C for 1 h are presented in Fig. 4 and in general confirm the main findings described above. The as-deposited TiAlSiCN coating showed a strong (1 1 1) preferential orientation, while no other peaks from the cubic phase were observed. Using conventional XRD measurements, the main peaks observed in annealed samples were from the Al2 O3 and TiO2 phases. In the case of the TiAlSiCN, Al/TiAlSiCN, and AlOx /TiAlSiCN coatings, a broad asymmetrical peak was observed between 36 and 37.5 2 degrees, which can be interpreted as a superposition of the peaks from
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(1 0 0) planes of TiO2 and (1 1 1) planes of (Ti,Al)(C,N). Since the oxide scale on the surface of the Alii /TiAlSiCN coating was thick, the signals from the (1 1 1) planes of (Ti,Al)(C,N) were not visible. Note that conventional XRD does not permit to distinguish the signals from AlOx surface layer and Al2 O3 substrate. Therefore, to make XRD measurements surface sensitive, the AlOx /TiAlSiCN coating was also analyzed in the grazing-incidence small-angle scattering mode. In this mode, the peaks of Al2 O3 substrate completely disappeared while additional peaks, identified as those of AlOx scale on the coating’s surface, appeared at 35.9 and 44.2 2 degrees. During annealing at a higher temperature of 1100 ◦ C, all the coatings, except for the AlOx /TiAlSiCN, were completely oxidized. The coating morphology and elemental depth profiles of the asannealed AlOx /TiAlSiCN coating are shown in Fig. 5. The layered oxide structure was observed to be even more complex compared to that after annealing at 1000 ◦ C. The outermost layer consisting of individual TiOx grains was followed by a thin and dense AlOx , loose TiOx + AlOx , thick and dense AlOx , and finally a nonoxidized part of the initial AlOx /TiAlSiCN sample (Fig. 5a and b). Note that no mixed TiOx + SiOx layer was found this time. The total thickness of the oxide scale was estimated to be about 870 nm, which was only 170 nm thicker than that of the oxide formed at 1000 ◦ C for 1 h. It is also worth noting that after annealing at 1100 ◦ C, the outermost TiOx layer did not cover the coating surface completely (Fig. 5a (inset)), but the size of TiOx grains was larger compared with those observed after annealing at 1000 ◦ C (Fig. 1o and p). Although the AlOx /TiAlSiCN coatings annealed at 1000 and 1100 ◦ C revealed similar morphologies of dense AlOx layers with presumably amorphous structure, the question whether any amorphous-to-crystalline phase transformations occurred within this temperature range remains open. For instance, Zhang et al. [25] showed that amorphous Al oxide started to transform into ␣-Al2 O3 at 1050 ◦ C. The mixed TiOx + AlOx layer appeared to be formed due to simultaneous outward diffusion of Al and Ti atoms and inward oxygen diffusion through the defects and micro-cracks in the TiOx and AlOx layers. The formation of a mixed TiOx + AlOx layer just beneath the AlOx layer indicates that the AlOx film acted as a barrier for the outward diffusion of Ti atoms and promoted the formation of an additional oxide layer. During annealing at 1200 ◦ C, the AlOx /TiAlSiCN sample was completely oxidized, probably due to the decomposition of Al2 O3 crystallites [25]. It should also be mentioned that, among other factors, the oxidation of the coatings at such a high temperature could be controlled by the adhesive and cohesive failures of oxide layers
Fig. 5. (a) Cross-sectional and plan-view (inset) SEM images of the AlOx /TiAlSiCN coating after annealing at 1100 ◦ C for 1 h with (b) corresponding GDOES depth profiles.
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due to the difference in CTEs between Al2 O3 (8 × 10−6 /K) and TiO2 (10.5 × 10−6 /K [20]). 4. Conclusions To summarize, we investigated three different approaches to increase the oxidation resistance of TiAlSiCN coatings: (i) deposition of an Al top-layer, (ii) deposition of an AlOx top-layer, and (iii) Al ion implantation in the outmost TiAlSiCN layer. The as-prepared coatings were annealed in air at 1000, 1100, and 1200 ◦ C for 1 h and then evaluated using SEM, AFM, and GDOES. In order to analyze the oxidation kinetics, the TiAlSiCN coatings were annealed at 1000 ◦ C for 1, 6, 13, and 60 min, after which the elements depth distribution was analyzed by GDOES. The obtained results show that the deposition of a thin amorphous AlOx layer permitted to increase the oxidation resistance of the TiAlSiCN coating from 1000 to 1100 ◦ C. The oxidation process was shown to be accompanied by the diffusion of Al and Ti atoms to the surface and inward oxygen diffusion, which resulted in the formation of a layered structure. The thickness of the oxide scale was similar after heat treatments at 1000 and 1100 ◦ C, but the layered structures were different: TiOx and AlOx (1000 ◦ C) and TiOx , AlOx , TiOx + AlOx , and AlOx sublayers from the top to bottom of the AlOx /TiAlSiCN coating (1100 ◦ C). The deposition of amorphous AlOx layer led to a narrower temperature gap between the exceptionally high thermal stability (1300 ◦ C) and oxidation resistance of the TiAlSiCN coatings. This result is of great importance for high-speed and dry cutting applications because the operating temperatures on the cutting-tool edge during continuous cutting of hardened tool steel may exceed 1000 ◦ C [26]. In contrast, the formation of an Al rich top-layer, created via either depositing a metallic film or implanting Al ions, resulted in decreased oxidation resistance. Acknowledgements The authors gratefully acknowledge the financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (No. К2-2014-012) and the Russian Foundation for Basic Research (Agreement No. HK 13-03-12129\14). K.A.K. and D.V.S. also acknowledge a “Mega-Grant” award from the Ministry of Education and Science of the Russian Federation (No. 14.Y26.31.0005). References [1] D.V. Shtansky, K.A. Kuptsov, P.V. Kiryukhantsev-Korneev, A.N. Sheveyko, High thermal stability of TiAlSiCN coatings with comb like nanocomposite structure, Surf. Coat. Technol. 206 (2012) 4840–4849. [2] K.A. Kuptsov, P.V. Kiryukhantsev-Korneev, A.N. Sheveyko, D.V. Shtansky, Structural transformations in TiAlSiCN coatings in the temperature range 900–1600 ◦ C, Acta Mater. 83 (2015) 408–418.
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