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Investigation on the oxidation behavior of Mo–Ru hard coatings
Surface & Coatings Technology 204 (2009) 860–864
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Investigation on the oxidation behavior of Mo–Ru hard coatings Yung-I Chen ⁎ Institute of Materials Engineering, National Taiwan Ocean University, No.2, Beining Rd., Keelung 202-24, Taiwan, ROC
a r t i c l e
i n f o
Available online 9 September 2009 Keywords: Mo–Ru Oxidation Microstructure Glass molding
a b s t r a c t Mo–Ru coatings are refractory metal alloy coatings that are used as protective coatings on the top surface of glass molding dies to prolong the lifetime of the die material. In a realistic molding environment, the atmosphere consists of steadily purged nitrogen and residual oxygen. The oxidation resistance of surface coatings under thermal cycling in mass production is a critical issue. In this study, the diffusion of oxygen into the Mo38.1Ru61.9 coatings annealed at 600 °C in an oxygen containing atmosphere was investigated. Three stages of oxidation behavior were verified. Solid solution strengthening and recrystallization due to the introduction of oxygen raised the surface hardness of the Mo38.1Ru61.9 coatings from 13 GPa to 19 GPa after 4 h annealing. The formation of oxide scale and Mo-depleted zone due to the preferential oxidation of Mo and high volatility of MoO3 were revealed after 8 h annealing. Cracks on the surface and Mo-depleted zone were observed after 20 h annealing. Chemical composition and structure variations and mechanical properties of the Mo38.1Ru61.9 coatings were intensively investigated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Glass molding technology has become common to manufacture high precision optical parts and components with aspherical surfaces [1–3]. Protective hard coatings have been widely used on the top surfaces of glass molding dies to prolong the lifetime of the die material. Precious metal alloy coatings, such as platinum-alloys [4] or iridium-alloys [5], and carbon thin films [6] have often been used in realistic mass production. Some nitrides such as TiAlN [7], CrWN [8], and TiAlN/ZrN composite coatings [9] have also been reported as possible protective materials. However, the effects of thermal aging and oxidation of hard coatings in the manufacture process have not been well investigated. Both effects result in the coarsening of the surface on the molding die, which reduces the quality of molded products. Recently, refractory metal alloy coatings of Mo–Ru have been fabricated by sputtering. Mo34.7Ru65.3 and Mo46.3Ru53.7 coatings, after annealing at 600 °C under vacuum, showed nanohardness, roughness and phase stability comparable to those of as-deposited ones [10]. However, in a typical molding atmosphere (~12 ppm oxygen and balanced nitrogen), the formation and volatility of MoO3 roughened the surface and limited the lifetime of the Mo56.7Ru43.3 coating after a 4 h annealing at 600 °C [11]. On the other hand, the Mo38.1Ru61.9 coating revealed a higher resistance to oxidation. In this study, oxidation treatments up to 20 h duration were conducted to evaluate the phase stability and oxidation resistance of the Mo38.1Ru61.9 coating.
The Mo–Ru alloy coatings were fabricated onto binderless tungsten carbide (WC, CW500, DIJET, Japan) substrate by RF magnetron sputtering with metal interlayers. The sputtering process was described in detail in a previous study [11]. The substrate holder was heated to 550 °C for all sputtering runs. The Mo38.1Ru61.9/Cr/Ti (O)/WC system was studied in this work. The thickness values of the Ti(O) and Cr interlayers and Mo38.1Ru61.9 coating were 0.10, 0.20 and 0.77 µm, respectively. In the process, oxygen was introduced to the titanium interlayer due to the target change, thus Ti(O) was suitable to denote the Ti interlayer, which was also described in the previous study [11]. The annealing treatments were performed in a glass molding machine (GMP-58-7Z, Toshiba Machine, Japan) at 600 °C. The chamber was purged with nitrogen in a steady flow and the residual oxygen content was monitored by an oxygen analyzer (COA011, CNT, Taiwan) with a zirconia sensor. The chemical composition of the coatings was evaluated with a field emission electron probe microanalyzer (FE-EPMA, JXA-8800 M, JEOL, Japan). Auger electron spectroscopy (AES, PHI700, ULVAC-PHI, Japan) was utilized to evaluate the oxidation effect by recording the chemical composition depth profiles. A conventional X-ray diffractometer (Rigaku Dmax-B, Tokyo, Japan) with Cu Kα radiation was adopted to identify the phases of the coatings. The surface morphologies of the coatings were evaluated by field emission scanning electron microscopy (FE-SEM, S4800, Hitachi, Japan). The microstructure was investigated by high resolution transmission electron microscopy (HRTEM, JEM-3000F, JEOL, Japan), in which the samples were prepared by focused ion beam technique (FEI Nova 200, U.S.A.). A Pt layer was deposited to protect the free surface in the sample preparation. The chemical states
⁎ Tel.: +886 2 24622192x6404; fax: +886 2 24625324. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.08.047
Y.-I. Chen / Surface & Coatings Technology 204 (2009) 860–864
861
of the constitute elements were studied by X-ray photoelectron spectroscopy (XPS, PHI 1600, PHI, Japan). The surface morphologies and surface roughness of the coatings were evaluated by an atomic force microscope (AFM, DI-3100, Veeco, USA). The hardness of the Mo–Ru coatings was measured with a nanoindentation tester (Triboscope, Hysitron, USA). The nanoindenter was equipped with a Berkovich diamond probe tip. The applied load was set at 3000 to 4500 µN to exhibit an indentation depth of a 1/10 of the film thickness. The nanohardness and elastic modulus of each indent was calculated based on the Oliver and Pharr method [12]. 3. Results and discussion 3.1. Chemical composition variations As annealed at 600 °C under a molding atmosphere containing 12± 2 ppm oxygen, the Mo38.1Ru61.9 coatings were oxidized to different extents as listed in Table 1. Nitrogen was also considered as the possible incorporation of the coatings since the annealing was conducted in the nitrogen containing atmosphere. However, the contents of nitrogen were analyzed to be 0–0.3 at.%, and thus were ignored. Apparently, coatings annealed for longer times absorbed more oxygen atoms. The atomic ratio of O/(Mo + Ru) analyzed by FE-EPMA on the top surface increased from 0.16, to 0.29, to 0.39, to 0.45, to 0.41 as the annealing time was increased from 4, to 8, to 12, to 16, to 20 h. The slope of O/(Mo + Ru) vs. annealing time decreased as the time increased as shown in Fig. 1, which demonstrated that a diffusion controlled reaction should be investigated. Moreover, the volatility of MoO3 should be discussed, since its volatility at temperatures above 550 °C in 10.1 kPa of oxygen [13] or in air [14] is well-known. The variation of the Mo:Ru concentration ratio related to the absorbed oxygen content after annealing was neglected. The Auger depth profiles of the 16 and 20 h annealed Mo38.1Ru61.9 coatings are shown in Fig. 2a and b, respectively. 5 at.% of oxygen was observed at depths of 40, 110, 170 and 270 nm for the 4, 8, 12 and 16 h annealed coatings, respectively. The Mo/(Mo + Ru) ratios were in the range of 0.32–0.37 and 0.34–0.39 in most regions of the 16 and 20 h annealed coatings, respectively. Though oxygen diffused into the normal Mo–Ru regions, both Mo and Ru contents decreased, however the Mo/(Mo + Ru) ratios remained constant. Outside the normal Mo– Ru regions, a Mo-depleted zone was observed beneath the free surface except for the sample annealed for 4 h only.
Fig. 1. Annealing effect on the chemical composition of Mo38.1Ru61.9 coatings under a N2-12 ppm O2 atmosphere at 600 °C.
Mo0.31Ru0.69 (002), respectively. The Mo5Ru3 phase dominates in the annealed Mo38.1Ru61.9 coatings as seen in the de-convolution of the main peak as shown in Fig. 4 for the 8 h annealed sample. In a previous study, the XRD reflections of a Mo34.7Ru65.3 coating, after being annealed in vacuum at 600 °C for 4 h, did not reveal a peak shift [10]. It is known that magnetron sputtered coatings always exhibit defects and stress, which results in the formation of a metastable phase in the as-deposited state. The thermal effect from heat treatment was not
3.2. Structure variations The as-deposited Mo38.1Ru61.9 coating consisted of Mo5Ru3 and minor Ru phases. The Mo5Ru3 phase possesses a strong (202) peak and apparent (411), (422) and (433) reflections as seen in Fig. 3. The main peak, Mo5Ru3 (202) corresponding to the strong texture of columnar structure, shifted from 2θ = 41.16° to 40.90°, 40.84°, 40.84°, 40.76° and 40.82° for annealing times of 0 to 4, 8, 12, 16 and 20 h, respectively. Moreover, the shifted main peak can be split into two peaks with 2θ angles of 40.8° and 41.0°, which correspond to the Mo5Ru3 (202) and
Table 1 Annealing effect on the chemical composition of Mo38.1Ru61.9 coatings under a N2-12 ppm O2 atmosphere at 600 °C. Annealed time
Chemical composition (at.%)
Composition ratio
Hours
Mo
Ru
O
Mo:Ru
O/(Mo + Ru)
4 8 12 16 20
33.0 29.7 27.4 25.7 27.0
53.4 48.1 44.5 43.3 43.7
13.6 22.2 28.1 31.0 29.3
38.2:61.8 38.2:61.8 38.1:61.9 37.2:62.8 38.2:61.8
0.16 0.29 0.39 0.45 0.41
Fig. 2. AES depth profiles of (a) 16 h and (b) 20 h annealed Mo38.1Ru61.9 coatings.
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Fig. 3. XRD patterns of Mo–Ru/Cr/Ti(O)/WC coatings with different annealing times: (a) as-deposited, (b) 4 h, (c) 8 h, (d) 12 h, (e) 16 h and (f) 20 h. Inset is the XRD reflections from oxides for (b) 4 h, (e) 16 h, and (f) 20 h annealed coatings.
enough to induce lattice expansion as illustrated for the coatings annealed in vacuum, but the oxidation process caused it. As indicated in the inset of Fig. 3, the mixed XRD reflections of MoO2 and MoO3 phases are observed for the 16 and 20 h annealed coatings, but not for the 4 h annealed one. The Mo38.1Ru61.9 coating annealed for 4 h revealed the morphology, without oxide scales, similar to that of the as-deposited coating. Needle-like oxides were observed and covered partially the surfaces of the 8 to 16 h annealed samples. Fig. 5 shows the SEM images of the 16 and 20 h annealed coatings, huge oxides with cracks are observed on the latter one. It is recognized in the oxidation of Mo [15], that the oxide scale of MoO3 cracks due to thickening as oxidation proceeds, and subsequent oxygen may transport inward and forms lower-valence oxides. Fig. 6 shows the cross-sectional bright field TEM image of the 20 h annealed Mo38.1Ru61.9 coating. A distinct Mo oxide scale was observed on top of the Mo-depleted zone. The Mo-depleted zone expanded into the original columnar grains at a depth of 87 nm. The grain boundaries of the columnar grains in the Mo-depleted zone became the diffusion paths of oxygen ions. Cracks were observed in the transverse direction related to the grain boundaries.
Fig. 4. The estimated split of the main peak of the Mo38.1Ru61.9 coating after 8 h annealing at 600 °C under glass molding atmosphere.
The XPS spectra of O 1s, Mo 3d and Ru 3d core levels for the annealed Mo38.1Ru61.9 coating were evaluated. Beneath the free surface, oxygen was verified to be in the ionic state with the 1s core level of 530.71 ± 0.07 eV. It is interesting to note that Ru is retained in the metallic state in the oxidized alloy with the 3d5/2 core level of
Fig. 5. Surface morphologies of (a) 16 h and (b) 20 h annealed Mo38.1Ru61.9 coatings.
Y.-I. Chen / Surface & Coatings Technology 204 (2009) 860–864
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atoms tend to be oxidized easily. Beneath the free surface, the Mo 3d5/2 core level was 232.14 eV for Mo6+, 229.40 eV for Mo4+, and 228.04 eV for Mo0, which was comparable to the reported values [17,18] with a shift of 0.3–.5 eV. Fig. 7b shows the valence variation of Mo for the 20 h annealed coating. The outmost surface consisted only of Mo6+, the oxide scale region was dominated by Mo4+ and Mo6+, and the Modepleted zone was dominated by Mo0. Referring to the AES depth profiles, Fig. 2b, Mo diffused out and accumulated on the initial free surface to react with oxygen, which resulted in the formation of the oxide scale. 3.3. Surface quality and mechanical properties
Fig. 6. Cross-sectional TEM images of a Mo38.1Ru61.9 coating after 20 h annealing at 600 °C under glass molding atmosphere.
280.15 ± 0.08 eV. However, Mo changes its valence as shown in Fig. 7a. Similar results can be found in the 450 °C oxidation study of Ta–Ru thin films [16]. Ru remained metallic with a 3d5/2 core level of 280.17 eV due to the preferential oxidation of Ta in the Ta–Ru film. The Gibbs free energies of metal oxide formation for RuO2 and MoO2 at 600 °C are −154143 and − 371041 J/mol [11], respectively, the Mo
By AFM analysis, the surface roughness of Mo38.1Ru61.9 coatings annealed in the molding atmosphere for 4 h is lower than that of asdeposited coatings as listed in Table 2, which means that annealing, without forming an obvious Mo-depleted zone or oxide scale, could smooth the surface as the annealing temperature 600 °C was higher than the deposition temperature 550 °C. An increase of surface roughness accompanied the formation of a Mo-depleted zone after longer annealing treatment in the molding atmosphere. The AFM observations showed similar trends in two different scanned areas, 2 µm × 2 µm and 10 µm × 10 µm. The nanoindentation test was performed under 3000 µN load for as-deposited Mo38.1Ru61.9 coatings, 4000 µN load for 4, 8 and 12 h annealed ones, and 5000 µN load for 16 and 20 h annealed ones. To prevent the substrate effect, a suitable ratio of indentation depth to film thickness should be allowed to acquire the precise nanohardness and Young's modulus of films [19]. The indentation depth of the Berkovich tip is within the range of 73 to 88 nm as can be seen in Table 2. Since the Mo–Ru coatings exhibited a thickness of 0.77 µm, the indentation depth to film thickness was around 1/10. The hardness of as-deposited Mo38.1Ru61.9 coatings was 13.1 GPa. After being annealed in the molding atmosphere for 4 to 16 h, the hardness raised to 18–19 GPa. Accompanied by the introduction of oxygen, solid solution strengthening and recrystallization of Mo–Ru grains resulted in hardness enhancement. By the XRD analysis, the grain dimension calculated by Scherrer's formula [20] was 37 nm for the asdeposited sample. Using the Mo5Ru3 (202) Kα1 reflection of the annealed Mo38.1Ru61.9 coatings, the grain dimensions were around 25–31 nm. Since the crystalline feature of Mo38.1Ru61.9 coatings revealed a columnar grain structure, the calculated grain dimension should be relative to the cross-section dimension of the columnar grain. The grain dimension variation was small between the annealed samples, which implied the annealed coatings possessed similar hardness values. For the 20 h annealed Mo38.1Ru61.9 coating with 60 nm oxide scale, a hardness of 22 GPa was recorded. The Young's modulus data were sustained at the level around 210–240 GPa except for the 20 h annealed Mo38.1Ru61.9 coating, which exhibited an oxide scale and incorporated cracks, which revealed a distinct decrease of Young's modulus to 180 GPa only. 4. Conclusions
Fig. 7. (a) XPS spectra of Mo 3d core levels and (b) the valence variation of Mo in the depth distribution for the 20 h annealed Mo38.1Ru61.9 coating (sputter rate: 5.5 nm/min).
Three stages were verified for the diffusion of oxygen into the Mo– Ru matrix under a 12 ppm oxygen containing atmosphere at 600 °C. In the beginning, the oxygen atoms diffused into the Mo–Ru coatings at a low level and the diffusion out of Mo or Ru was ignored, which strengthened the surface hardness and became a valid protective coating for the die material with high surface quality. In further heat treatment, the preferential oxidation of Mo and high volatility of MoO3 resulted in the presence of a Mo-depleted zone and roughened the surface gradually, which limited the reliability and lifetime of the Mo–Ru protective coating. Finally, cracks occurred in the oxide scale and propagated in the Mo-depleted zone and the depth of the Modepleted zone spread into the deeper Mo–Ru coating.
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Table 2 Surface quality and mechanical properties of Mo38.1Ru61.9 coatings. Surface roughness
Mechanical properties
Annealed time
Scanned area
Young's modulus
Hardness
Hours
Ra (nm)
Rq (nm)
Ra (nm)
Rq (nm)
(GPa)
(GPa)
(nm)
– 4 8 12 16 20
3.4 ± 0.1 2.9 ± 0.2 3.9 ± 1.0 – 8.4 ± 3.2 17.6 ± 5.0
4.3 ± 0.2 3.7 ± 0.2 5.3 ± 1.8 – 11.9 ± 5.1 24.1 ± 7.9
4.3 ± 0.1 2.5 ± 0.2 3.5 ± 0.2 – 6.3 ± 0.4 24.8 ± 0.3
5.9 ± 0.2 3.8 ± 0.6 5.8 ± 1.1 – 9.0 ± 1.4 33.0 ± 0.3
230 ± 24 221 ± 2 244 ± 21 223 ± 8 207 ± 5 180 ± 17
13.1 ± 0.7 19.2 ± 1.7 18.2 ± 2.5 19.1 ± 1.8 18.2 ± 0.5 21.6 ± 2.9
73 ± 4 78 ± 3 77 ± 6 78 ± 4 88 ± 2 88 ± 6
2 µm × 2 µm
Indentation depth
10 µm × 10 µm
Acknowledgement The support of this work by the National Science Council, Taiwan, under Contract No. NSC-97-2218-E-019-002 is appreciated. Partial support from the National Taiwan Ocean University under NTOURD972-04-03-13-02 is also acknowledged. References [1] R.O. Maschmeyer, C.A. Andrysick, T.W. Geyer, H.E. Meissner, C.J. Parker, L.M. Sanford, Appl. Opt. 22 (1983) 2410. [2] R.O. Maschmeyer, R.M. Hujar, L.L. Carpenter, B.W. Nicholson, E.F. Vozenilek, Appl. Opt. 22 (1983) 2413. [3] A.Y. Yi, A. Jain, J. Am. Ceram. Soc. 88 (2005) 579. [4] H. Monji, M. Aoki, H. Torii, H. Okinaka, U.S. Patent No. 4629487, (1986). [5] K. Kuribayashi, M. Sakai, H. Monji, M. Aoki, H. Okinaka, H. Torii, U.S. Patent No. 4685948, (1987).
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
S. Hirota, Y. Oogami, K. Hashimoto, U. S. Patent No. 6776007, (2004). G. Kleer, W. Doell, Surf. Coat. Technol. 94–95 (1997) 647. C.H. Lin, J.G. Duh, B.S. Yau, Surf. Coat. Technol. 201 (2006) 1316. M. Hock, E. Schäffer, W. Döll, G. Kleer, Surf. Coat. Technol. 163–164 (2003) 689. Y.I. Chen, K.T. Liu, F.B. Wu, J.G. Duh, Thin Solid Films 515 (2006) 2207. Y.I. Chen, L.C. Chang, J.W. Lee, C.H. Lin, Thin Solid Films 518 (2009) 194. M.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. E.A. Gulbransen, K.F. Andrew, F.A. Brassart, J. Electrochem. Soc. 110 (1963) 952. N. Solak, F. Ustel, M. Urgen, S. Aydin, A.F. Cakir, Surf. Coat. Technol. 174–175 (2003) 713. Z. Li, Y. He, W. Gao, Oxid. Met. 53 (2000) 577. D.S. Wuu, C.C. Chan, R.H. Horng, J. Vac. Sci. Technol., A 17 (6) (1999) 3327. C.V. Ramana, V.V. Atuchin, V.G. Kesler, V.A. Kochubey, L.D. Pokrovsky, V. Shutthanandan, U. Becker, R.C. Ewing, Appl. Surf. Sci. 253 (2007) 5368. J.G. Choi, L.T. Thompson, Appl. Surf. Sci. 93 (1996) 143. R. Saha, W.D. Nix, Acta Mater. 50 (2002) 23. B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd Ed. Prentice Hall, USA, 2001.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Investigation on the oxidation behavior of Mo–Ru hard coatings Yung-I Chen ⁎ Institute of Materials Engineering, National Taiwan Ocean University, No.2, Beining Rd., Keelung 202-24, Taiwan, ROC
a r t i c l e
i n f o
Available online 9 September 2009 Keywords: Mo–Ru Oxidation Microstructure Glass molding
a b s t r a c t Mo–Ru coatings are refractory metal alloy coatings that are used as protective coatings on the top surface of glass molding dies to prolong the lifetime of the die material. In a realistic molding environment, the atmosphere consists of steadily purged nitrogen and residual oxygen. The oxidation resistance of surface coatings under thermal cycling in mass production is a critical issue. In this study, the diffusion of oxygen into the Mo38.1Ru61.9 coatings annealed at 600 °C in an oxygen containing atmosphere was investigated. Three stages of oxidation behavior were verified. Solid solution strengthening and recrystallization due to the introduction of oxygen raised the surface hardness of the Mo38.1Ru61.9 coatings from 13 GPa to 19 GPa after 4 h annealing. The formation of oxide scale and Mo-depleted zone due to the preferential oxidation of Mo and high volatility of MoO3 were revealed after 8 h annealing. Cracks on the surface and Mo-depleted zone were observed after 20 h annealing. Chemical composition and structure variations and mechanical properties of the Mo38.1Ru61.9 coatings were intensively investigated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Glass molding technology has become common to manufacture high precision optical parts and components with aspherical surfaces [1–3]. Protective hard coatings have been widely used on the top surfaces of glass molding dies to prolong the lifetime of the die material. Precious metal alloy coatings, such as platinum-alloys [4] or iridium-alloys [5], and carbon thin films [6] have often been used in realistic mass production. Some nitrides such as TiAlN [7], CrWN [8], and TiAlN/ZrN composite coatings [9] have also been reported as possible protective materials. However, the effects of thermal aging and oxidation of hard coatings in the manufacture process have not been well investigated. Both effects result in the coarsening of the surface on the molding die, which reduces the quality of molded products. Recently, refractory metal alloy coatings of Mo–Ru have been fabricated by sputtering. Mo34.7Ru65.3 and Mo46.3Ru53.7 coatings, after annealing at 600 °C under vacuum, showed nanohardness, roughness and phase stability comparable to those of as-deposited ones [10]. However, in a typical molding atmosphere (~12 ppm oxygen and balanced nitrogen), the formation and volatility of MoO3 roughened the surface and limited the lifetime of the Mo56.7Ru43.3 coating after a 4 h annealing at 600 °C [11]. On the other hand, the Mo38.1Ru61.9 coating revealed a higher resistance to oxidation. In this study, oxidation treatments up to 20 h duration were conducted to evaluate the phase stability and oxidation resistance of the Mo38.1Ru61.9 coating.
The Mo–Ru alloy coatings were fabricated onto binderless tungsten carbide (WC, CW500, DIJET, Japan) substrate by RF magnetron sputtering with metal interlayers. The sputtering process was described in detail in a previous study [11]. The substrate holder was heated to 550 °C for all sputtering runs. The Mo38.1Ru61.9/Cr/Ti (O)/WC system was studied in this work. The thickness values of the Ti(O) and Cr interlayers and Mo38.1Ru61.9 coating were 0.10, 0.20 and 0.77 µm, respectively. In the process, oxygen was introduced to the titanium interlayer due to the target change, thus Ti(O) was suitable to denote the Ti interlayer, which was also described in the previous study [11]. The annealing treatments were performed in a glass molding machine (GMP-58-7Z, Toshiba Machine, Japan) at 600 °C. The chamber was purged with nitrogen in a steady flow and the residual oxygen content was monitored by an oxygen analyzer (COA011, CNT, Taiwan) with a zirconia sensor. The chemical composition of the coatings was evaluated with a field emission electron probe microanalyzer (FE-EPMA, JXA-8800 M, JEOL, Japan). Auger electron spectroscopy (AES, PHI700, ULVAC-PHI, Japan) was utilized to evaluate the oxidation effect by recording the chemical composition depth profiles. A conventional X-ray diffractometer (Rigaku Dmax-B, Tokyo, Japan) with Cu Kα radiation was adopted to identify the phases of the coatings. The surface morphologies of the coatings were evaluated by field emission scanning electron microscopy (FE-SEM, S4800, Hitachi, Japan). The microstructure was investigated by high resolution transmission electron microscopy (HRTEM, JEM-3000F, JEOL, Japan), in which the samples were prepared by focused ion beam technique (FEI Nova 200, U.S.A.). A Pt layer was deposited to protect the free surface in the sample preparation. The chemical states
⁎ Tel.: +886 2 24622192x6404; fax: +886 2 24625324. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.08.047
Y.-I. Chen / Surface & Coatings Technology 204 (2009) 860–864
861
of the constitute elements were studied by X-ray photoelectron spectroscopy (XPS, PHI 1600, PHI, Japan). The surface morphologies and surface roughness of the coatings were evaluated by an atomic force microscope (AFM, DI-3100, Veeco, USA). The hardness of the Mo–Ru coatings was measured with a nanoindentation tester (Triboscope, Hysitron, USA). The nanoindenter was equipped with a Berkovich diamond probe tip. The applied load was set at 3000 to 4500 µN to exhibit an indentation depth of a 1/10 of the film thickness. The nanohardness and elastic modulus of each indent was calculated based on the Oliver and Pharr method [12]. 3. Results and discussion 3.1. Chemical composition variations As annealed at 600 °C under a molding atmosphere containing 12± 2 ppm oxygen, the Mo38.1Ru61.9 coatings were oxidized to different extents as listed in Table 1. Nitrogen was also considered as the possible incorporation of the coatings since the annealing was conducted in the nitrogen containing atmosphere. However, the contents of nitrogen were analyzed to be 0–0.3 at.%, and thus were ignored. Apparently, coatings annealed for longer times absorbed more oxygen atoms. The atomic ratio of O/(Mo + Ru) analyzed by FE-EPMA on the top surface increased from 0.16, to 0.29, to 0.39, to 0.45, to 0.41 as the annealing time was increased from 4, to 8, to 12, to 16, to 20 h. The slope of O/(Mo + Ru) vs. annealing time decreased as the time increased as shown in Fig. 1, which demonstrated that a diffusion controlled reaction should be investigated. Moreover, the volatility of MoO3 should be discussed, since its volatility at temperatures above 550 °C in 10.1 kPa of oxygen [13] or in air [14] is well-known. The variation of the Mo:Ru concentration ratio related to the absorbed oxygen content after annealing was neglected. The Auger depth profiles of the 16 and 20 h annealed Mo38.1Ru61.9 coatings are shown in Fig. 2a and b, respectively. 5 at.% of oxygen was observed at depths of 40, 110, 170 and 270 nm for the 4, 8, 12 and 16 h annealed coatings, respectively. The Mo/(Mo + Ru) ratios were in the range of 0.32–0.37 and 0.34–0.39 in most regions of the 16 and 20 h annealed coatings, respectively. Though oxygen diffused into the normal Mo–Ru regions, both Mo and Ru contents decreased, however the Mo/(Mo + Ru) ratios remained constant. Outside the normal Mo– Ru regions, a Mo-depleted zone was observed beneath the free surface except for the sample annealed for 4 h only.
Fig. 1. Annealing effect on the chemical composition of Mo38.1Ru61.9 coatings under a N2-12 ppm O2 atmosphere at 600 °C.
Mo0.31Ru0.69 (002), respectively. The Mo5Ru3 phase dominates in the annealed Mo38.1Ru61.9 coatings as seen in the de-convolution of the main peak as shown in Fig. 4 for the 8 h annealed sample. In a previous study, the XRD reflections of a Mo34.7Ru65.3 coating, after being annealed in vacuum at 600 °C for 4 h, did not reveal a peak shift [10]. It is known that magnetron sputtered coatings always exhibit defects and stress, which results in the formation of a metastable phase in the as-deposited state. The thermal effect from heat treatment was not
3.2. Structure variations The as-deposited Mo38.1Ru61.9 coating consisted of Mo5Ru3 and minor Ru phases. The Mo5Ru3 phase possesses a strong (202) peak and apparent (411), (422) and (433) reflections as seen in Fig. 3. The main peak, Mo5Ru3 (202) corresponding to the strong texture of columnar structure, shifted from 2θ = 41.16° to 40.90°, 40.84°, 40.84°, 40.76° and 40.82° for annealing times of 0 to 4, 8, 12, 16 and 20 h, respectively. Moreover, the shifted main peak can be split into two peaks with 2θ angles of 40.8° and 41.0°, which correspond to the Mo5Ru3 (202) and
Table 1 Annealing effect on the chemical composition of Mo38.1Ru61.9 coatings under a N2-12 ppm O2 atmosphere at 600 °C. Annealed time
Chemical composition (at.%)
Composition ratio
Hours
Mo
Ru
O
Mo:Ru
O/(Mo + Ru)
4 8 12 16 20
33.0 29.7 27.4 25.7 27.0
53.4 48.1 44.5 43.3 43.7
13.6 22.2 28.1 31.0 29.3
38.2:61.8 38.2:61.8 38.1:61.9 37.2:62.8 38.2:61.8
0.16 0.29 0.39 0.45 0.41
Fig. 2. AES depth profiles of (a) 16 h and (b) 20 h annealed Mo38.1Ru61.9 coatings.
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Y.-I. Chen / Surface & Coatings Technology 204 (2009) 860–864
Fig. 3. XRD patterns of Mo–Ru/Cr/Ti(O)/WC coatings with different annealing times: (a) as-deposited, (b) 4 h, (c) 8 h, (d) 12 h, (e) 16 h and (f) 20 h. Inset is the XRD reflections from oxides for (b) 4 h, (e) 16 h, and (f) 20 h annealed coatings.
enough to induce lattice expansion as illustrated for the coatings annealed in vacuum, but the oxidation process caused it. As indicated in the inset of Fig. 3, the mixed XRD reflections of MoO2 and MoO3 phases are observed for the 16 and 20 h annealed coatings, but not for the 4 h annealed one. The Mo38.1Ru61.9 coating annealed for 4 h revealed the morphology, without oxide scales, similar to that of the as-deposited coating. Needle-like oxides were observed and covered partially the surfaces of the 8 to 16 h annealed samples. Fig. 5 shows the SEM images of the 16 and 20 h annealed coatings, huge oxides with cracks are observed on the latter one. It is recognized in the oxidation of Mo [15], that the oxide scale of MoO3 cracks due to thickening as oxidation proceeds, and subsequent oxygen may transport inward and forms lower-valence oxides. Fig. 6 shows the cross-sectional bright field TEM image of the 20 h annealed Mo38.1Ru61.9 coating. A distinct Mo oxide scale was observed on top of the Mo-depleted zone. The Mo-depleted zone expanded into the original columnar grains at a depth of 87 nm. The grain boundaries of the columnar grains in the Mo-depleted zone became the diffusion paths of oxygen ions. Cracks were observed in the transverse direction related to the grain boundaries.
Fig. 4. The estimated split of the main peak of the Mo38.1Ru61.9 coating after 8 h annealing at 600 °C under glass molding atmosphere.
The XPS spectra of O 1s, Mo 3d and Ru 3d core levels for the annealed Mo38.1Ru61.9 coating were evaluated. Beneath the free surface, oxygen was verified to be in the ionic state with the 1s core level of 530.71 ± 0.07 eV. It is interesting to note that Ru is retained in the metallic state in the oxidized alloy with the 3d5/2 core level of
Fig. 5. Surface morphologies of (a) 16 h and (b) 20 h annealed Mo38.1Ru61.9 coatings.
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atoms tend to be oxidized easily. Beneath the free surface, the Mo 3d5/2 core level was 232.14 eV for Mo6+, 229.40 eV for Mo4+, and 228.04 eV for Mo0, which was comparable to the reported values [17,18] with a shift of 0.3–.5 eV. Fig. 7b shows the valence variation of Mo for the 20 h annealed coating. The outmost surface consisted only of Mo6+, the oxide scale region was dominated by Mo4+ and Mo6+, and the Modepleted zone was dominated by Mo0. Referring to the AES depth profiles, Fig. 2b, Mo diffused out and accumulated on the initial free surface to react with oxygen, which resulted in the formation of the oxide scale. 3.3. Surface quality and mechanical properties
Fig. 6. Cross-sectional TEM images of a Mo38.1Ru61.9 coating after 20 h annealing at 600 °C under glass molding atmosphere.
280.15 ± 0.08 eV. However, Mo changes its valence as shown in Fig. 7a. Similar results can be found in the 450 °C oxidation study of Ta–Ru thin films [16]. Ru remained metallic with a 3d5/2 core level of 280.17 eV due to the preferential oxidation of Ta in the Ta–Ru film. The Gibbs free energies of metal oxide formation for RuO2 and MoO2 at 600 °C are −154143 and − 371041 J/mol [11], respectively, the Mo
By AFM analysis, the surface roughness of Mo38.1Ru61.9 coatings annealed in the molding atmosphere for 4 h is lower than that of asdeposited coatings as listed in Table 2, which means that annealing, without forming an obvious Mo-depleted zone or oxide scale, could smooth the surface as the annealing temperature 600 °C was higher than the deposition temperature 550 °C. An increase of surface roughness accompanied the formation of a Mo-depleted zone after longer annealing treatment in the molding atmosphere. The AFM observations showed similar trends in two different scanned areas, 2 µm × 2 µm and 10 µm × 10 µm. The nanoindentation test was performed under 3000 µN load for as-deposited Mo38.1Ru61.9 coatings, 4000 µN load for 4, 8 and 12 h annealed ones, and 5000 µN load for 16 and 20 h annealed ones. To prevent the substrate effect, a suitable ratio of indentation depth to film thickness should be allowed to acquire the precise nanohardness and Young's modulus of films [19]. The indentation depth of the Berkovich tip is within the range of 73 to 88 nm as can be seen in Table 2. Since the Mo–Ru coatings exhibited a thickness of 0.77 µm, the indentation depth to film thickness was around 1/10. The hardness of as-deposited Mo38.1Ru61.9 coatings was 13.1 GPa. After being annealed in the molding atmosphere for 4 to 16 h, the hardness raised to 18–19 GPa. Accompanied by the introduction of oxygen, solid solution strengthening and recrystallization of Mo–Ru grains resulted in hardness enhancement. By the XRD analysis, the grain dimension calculated by Scherrer's formula [20] was 37 nm for the asdeposited sample. Using the Mo5Ru3 (202) Kα1 reflection of the annealed Mo38.1Ru61.9 coatings, the grain dimensions were around 25–31 nm. Since the crystalline feature of Mo38.1Ru61.9 coatings revealed a columnar grain structure, the calculated grain dimension should be relative to the cross-section dimension of the columnar grain. The grain dimension variation was small between the annealed samples, which implied the annealed coatings possessed similar hardness values. For the 20 h annealed Mo38.1Ru61.9 coating with 60 nm oxide scale, a hardness of 22 GPa was recorded. The Young's modulus data were sustained at the level around 210–240 GPa except for the 20 h annealed Mo38.1Ru61.9 coating, which exhibited an oxide scale and incorporated cracks, which revealed a distinct decrease of Young's modulus to 180 GPa only. 4. Conclusions
Fig. 7. (a) XPS spectra of Mo 3d core levels and (b) the valence variation of Mo in the depth distribution for the 20 h annealed Mo38.1Ru61.9 coating (sputter rate: 5.5 nm/min).
Three stages were verified for the diffusion of oxygen into the Mo– Ru matrix under a 12 ppm oxygen containing atmosphere at 600 °C. In the beginning, the oxygen atoms diffused into the Mo–Ru coatings at a low level and the diffusion out of Mo or Ru was ignored, which strengthened the surface hardness and became a valid protective coating for the die material with high surface quality. In further heat treatment, the preferential oxidation of Mo and high volatility of MoO3 resulted in the presence of a Mo-depleted zone and roughened the surface gradually, which limited the reliability and lifetime of the Mo–Ru protective coating. Finally, cracks occurred in the oxide scale and propagated in the Mo-depleted zone and the depth of the Modepleted zone spread into the deeper Mo–Ru coating.
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Table 2 Surface quality and mechanical properties of Mo38.1Ru61.9 coatings. Surface roughness
Mechanical properties
Annealed time
Scanned area
Young's modulus
Hardness
Hours
Ra (nm)
Rq (nm)
Ra (nm)
Rq (nm)
(GPa)
(GPa)
(nm)
– 4 8 12 16 20
3.4 ± 0.1 2.9 ± 0.2 3.9 ± 1.0 – 8.4 ± 3.2 17.6 ± 5.0
4.3 ± 0.2 3.7 ± 0.2 5.3 ± 1.8 – 11.9 ± 5.1 24.1 ± 7.9
4.3 ± 0.1 2.5 ± 0.2 3.5 ± 0.2 – 6.3 ± 0.4 24.8 ± 0.3
5.9 ± 0.2 3.8 ± 0.6 5.8 ± 1.1 – 9.0 ± 1.4 33.0 ± 0.3
230 ± 24 221 ± 2 244 ± 21 223 ± 8 207 ± 5 180 ± 17
13.1 ± 0.7 19.2 ± 1.7 18.2 ± 2.5 19.1 ± 1.8 18.2 ± 0.5 21.6 ± 2.9
73 ± 4 78 ± 3 77 ± 6 78 ± 4 88 ± 2 88 ± 6
2 µm × 2 µm
Indentation depth
10 µm × 10 µm
Acknowledgement The support of this work by the National Science Council, Taiwan, under Contract No. NSC-97-2218-E-019-002 is appreciated. Partial support from the National Taiwan Ocean University under NTOURD972-04-03-13-02 is also acknowledged. References [1] R.O. Maschmeyer, C.A. Andrysick, T.W. Geyer, H.E. Meissner, C.J. Parker, L.M. Sanford, Appl. Opt. 22 (1983) 2410. [2] R.O. Maschmeyer, R.M. Hujar, L.L. Carpenter, B.W. Nicholson, E.F. Vozenilek, Appl. Opt. 22 (1983) 2413. [3] A.Y. Yi, A. Jain, J. Am. Ceram. Soc. 88 (2005) 579. [4] H. Monji, M. Aoki, H. Torii, H. Okinaka, U.S. Patent No. 4629487, (1986). [5] K. Kuribayashi, M. Sakai, H. Monji, M. Aoki, H. Okinaka, H. Torii, U.S. Patent No. 4685948, (1987).
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