Oxidation behavior of TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films deposited by multi-arc ion plating

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Available online at www.amse.org.cn Acta Metall. Sin.(Engl. Lett.)Vol.23 No.

1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.23 No.6 pp December 2010 Oxidation behavior of TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films deposited by multi-arc ion plating Shilu ZHAO and Jun ZHANG School of Mechanism Engineering, Shenyang University, Shenyang , China Changsheng LIU Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang , China Manuscript received 10 December 2009; in revised form 22 February 2010 The two Ti-Al-Zr targets and one pure Cr target were used to prepare the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films on high speed steel (W18Cr4V) substrates by multi-arc ion plating technique. Short-term isothermal (at 600 C, 700 C, 800 C and 900 C for 4 h) and long-term cyclic (at 700 C and 800 C for 100 h) high temperature oxidation behavior of the gradient films were studied. Then the oxide scales formed on the film specimens were characterized by scanning electron microscopy (SEM), energy dispersion X-ray spectroscopy (EDS) and X-ray diffraction (XRD). It was showed that, under short-term isothermal condition, the high temperature oxidation resistance of the gradient film was excellent up to 800 C and an oxide scale comprising TiO 2 was observed. On the other hand, under long-term cyclic high temperature condition, the oxidation resistance of the gradient film was excellent at about 700 C. KEY WORDS Multi-arc ion plating; TiAlZrCr/(Ti, Al, Zr, Cr)N gradient film; High speed steel; High temperature oxidation resistance 1 Introduction The multi-arc ion plating technique for the deposition of hard films is well known for the high ionization in the plasma and allows the deposition of dense films [1,2]. Industrial applications of the hard films deposited by multi-arc ion plating technique are increasing rapidly due to their superior mechanical properties and excellent high temperature oxidation resistance [3,4]. TiN hard films were widely used as protective films in cutting tools and molds. The TiN films, despite the excellent mechanical properties, could not be used at elevated temperatures in the oxidative atmosphere due to their poor thermal stabilities. They could oxidize easily above 600 C [5]. A potential approach to further improve high temperature oxidation resistance was the addition of small concentrations of alloying elements [6]. The additions such as Al, Zr and Cr elements frequently exhibited the improved mechanical properties of the nitride films, and also improved the high temperature oxidation resistance [3,7 12]. It was known that the (Ti, Al)N films showed the elevated Corresponding author. Lecturer, PhD; Tel: address: zhaoshilu@sina.com (Shilu ZHAO)

2 474 high oxidation resistance, which had been used widely under high-speed dry machining conditions [13]. Moreover, the addition of Zr or Cr element to form the (Ti, Al, Zr)N or (Ti, Al, Cr)N film was believed to have high red hardness and high oxidation resistance [14]. Therefore, the simultaneous additions of Zr and Cr elements to form the (Ti, Al, Zr, Cr)N multi-component films were expected to combine the potential advantages of the (Ti, Al, Zr)N and (Ti, Al, Cr)N films. Up to now, the various hard films have been developed from the mono-component nitride films to the multi-component nitride films, and then to the multi-layered and gradient films for improving properties [3,15,16]. As for the gradient films, the thermal stress resulted from the thermal expansion coefficient difference between the film and the substrate could decrease significantly. Consequently, the adhesive strength between the (Ti, Al, Zr, Cr)N film and the W18Cr4V substrate might increase obviously due to the (Ti, Al, Zr, Cr)N gradient film with the TiAlZrCr alloy interlayer. In our previous studies [17], it showed that the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films had the superior mechanical properties, for which the micro-hardness of the gradient films was up to 3500HV 0.01, and the adhesive strength between the film and the substrate deposited at all the bias voltages was excellent, in terms of critical load which was about 190 N 200 N. The purpose of this present study is to investigate the high temperature oxidation behavior of the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient film on the W18Cr4V substrate by multi-arc ion plating technique, using the Ti-Al-Zr target and pure Cr target. 2 Experimental A MAD-4B multi-arc ion plating equipment was used to deposit the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films along with the (Ti, Al, Zr, Cr)N films for comparison [18]. The schematic structure of the deposition system is illustrated in Fig.1. Two Ti63-Al32-Zr5 (at. pct) alloy targets and one pure Cr target were manufactured by a powder metallurgical process and used for the deposition. The high speed steel (W18Cr4V) was adopted as the substrates. They were first cut into dimensions of φ30 mm 6 mm. The surfaces were then grounded down to 1000 grit SiC paper and polished to an average peak-tovalley roughness R a of about 0.1 µm. Finally, the specimens were degreased ultrasonically in acetone, cleaned by ethanol and dried in air. The protective gas (Ar) and reactive gas (N 2 ) were introduced around the target to accelerate the reaction of the plasma. The elemental gradient distributions of the films were made as functions of nitrogen pressure varied from ( ) 10 1 Pa to ( ) 10 1 Pa and arc current of targets Fig.1 Schematic structure of MAD-4B multiarc ion plating equipment: (1 and 2. Ti-Al-Zr target; 3. Cr target; 4 and 5. gas inlet; 6. vacuum system; 7. specimens; 8. bias voltage supply).

3 475 varied from 50 A to 70 A. A DC power supply connected to substrate holder was used at 150 V bias voltage. The detailed operating parameters of the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films are listed in Table 1. For comparison, the (Ti, Al, Zr, Cr)N films were also deposited under the same conditions, and the operating parameters were the same as the last deposition process parameters at a N 2 pressure of ( ) 10 1 Pa and a arc current of TiAlZr target of 70 A [18]. Meanwhile, the W18Cr4V substrate specimens were also used for comparative study. Table 1 Operating parameters of the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films Introdu- Gas Bias Arc current Arc current Substrate Deposition Stage ced gas pressure voltage of TiAlZr of Cr temper- time 10 1 Pa V target/a target/a ature/ C min Ion bombardment Ar TiAlZrCr interlayer deposition Ar (Ti, Al, Zr, Cr)N Incremental Incremental gradient N 2 change from 150 change from film deposition to to 70 Note: The base pressure was Pa. Following the deposition process, the short-term isothermal oxidation tests at 600 C, 700 C, 800 C and 900 C for 4 h in static air were carried out in a muffle furnace (SG2-5-12). Moreover, the long-term cyclic oxidation tests at 700 C and 800 C for up to 100 h were conducted. The specimens were kept in the furnace for 4 h and then taken out to cool at room temperature for mass measurements, which was defined as one cycle, and the duration of the whole oxidation process was 25 cycles. An electronic balance (FA2104) was used to measure the mass gain change (the sensitivity was 0.1 mg). A scanning electron microscope (SEM: Shimadzu SSX 505) was used to observe the surface morphologies of the as-deposited films and the oxide scales after oxidation. A field emission scanning electron microscope (SEM: JOEL JSM-7001F) was used to characterize the cross-section morphology and measure the film thickness. The compositions of the as-deposited films and the oxide scales were determined by energy disperse spectroscopy (EDS) equipped with SEM. The crystal structures of the as-deposited films and the oxide scales were analyzed by X-ray diffraction (XRD: X Pert Pro MPD) with CuK α radiation. 3 Results and Discussion 3.1 Characterization of the as-deposited films The surface morphology of the as-deposited gradient film is shown in Fig.2a. The gradient film was isotropy and dense, but appearing many macro-particles (or micro-drops) on the film surface, which is the major disadvantage of the multi-arc ion process. Pores were also observed on the film surface due to the more intense ion bombardment energy that could lead to the back-sputter. As examined by SEM, the cross-section morphologies revealed the typical and fully dense columnar structure characterized by Zone T of Thornton s microstructure zone model (Fig.2b). There were no grain boundary porosity or other defects observed at the interface between the film and the substrate, and the film thickness was approximately 1 µm.

476 Fig.2 SEM images of the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient film: (b) cross-section morphology. (a) surface morphology; The average composition of the gradient film was Ti27.8-Al11-Zr1.3-Cr11.

4 476 Fig.2 SEM images of the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient film: (b) cross-section morphology. (a) surface morphology; The average composition of the gradient film was Ti27.8-Al11-Zr1.3-Cr11.4-N48.5 (at. pct) confirmed by EDS analysis on the film surface. The atomic ratio of N/(Ti+Al+Zr+Cr) was close to 1:1, which was quite similar to the stoichiometrical ones. Consequently, the gradient film surface could be described in the form of (Ti, Al, Zr, Cr)N. The atomic ratio of (Al+Zr+Cr)/(Ti+Al+Zr+Cr) was 0.46, at which the mechanical properties of the film were excellent [17]. The EDS line scans of the film cross-section showed that the relative intensity of Cr element increased throughout the film and no N element presented in the interlayer. Meanwhile, the relative intensity of Ti, Al, Zr and N elements decreased. The elemental gradient distributions of the film varied as the functions of nitrogen pressure and arc current of targets. The (Ti, Al, Zr, Cr)N gradient film with the TiAlZrCr alloy interlayer was confirmed, and the interlayer thickness was about 0.5 µm [17]. The gradient film were of the TiN (B1-NaCl) type face-centered cubic structure, based on one main diffraction peak of (220) plane and other diffraction peaks of (111), (200), (311) and (222) planes by X-ray diffraction patterns [17]. 3.2 Short-term oxidation behavior The short-term high temperature oxidation tests at 600 C, 700 C, 800 C and 900 C for 4 h were conducted, and the tint change of all the specimens was studied as described in Table 2. The tint of TiAlZrCr/(Ti, Al, Zr, Cr)N film specimens changed more slowly than that of W18Cr4V substrates and their (Ti, Al, Zr, Cr)N film specimens. Consequently, the oxidation rate of gradient films was decreased significantly at the different oxidation temperatures. The TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films on W18Cr4V substrates had much higher oxidation resistance than W18Cr4V substrates and their (Ti, Al, Zr, Cr)N films due to the high Al and Cr concentrations and crystal structure of their oxide scales. The oxidation mass gain curves of the W18Cr4V substrates and their (Ti, Al, Zr, Cr)N and TiAlZrCr/(Ti, Al, Zr, Cr)N films for 4 h at 600 C 900 C are shown in Fig.3. After oxidation at 600 C in air, there was almost no mass gain for all the bare and film specimens. At 700 C, there was also no visual mass gain for the (Ti, Al, Zr, Cr)N and TiAlZrCr/(Ti, Al, Zr, Cr)N film specimens, whereas mass gain occurred for the W18Cr4V substrate specimens. As the oxidation temperature reached 800 C, no evident mass gain occurred for the TiAlZrCr/(Ti, Al, Zr, Cr)N film specimens, while (Ti, Al, Zr, Cr)N film specimens showed certain mass gain. On the other hand, the W18Cr4V substrate specimens exhibited very high oxidation rate. When the oxidation temperature was about

5 477 Table 2 Tint change of W18Cr4V substrates and their (Ti, Al, Zr, Cr)N and TiAlZrCr/(Ti, Al, Zr, Cr)N films after oxidation for 4 h Oxidation W18Cr4V (Ti, Al, Zr, Cr)N TiAlZrCr/(Ti, Al, Zr, Cr)N temperature/ C substrates films gradient films 600 Light gray and lusterless Purplish blue and bright Yellow-red and bright 700 Gray and lusterless Purplish blue and bright Purplish blue and bright 800 Dark gray and lusterless Purplish gray and Purplish blue decreased brightness and bright 900 Dark gray and lusterless Gray and lusterless Purplish green and lusterless 900 C, the mass gain increased sharply for all the specimens, and the oxidation resistance of the TiAlZrCr/(Ti, Al, Zr, Cr)N film specimens improved to certain extent. As presented in Fig.3, it was revealed that the oxidation resistance of TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films on W18Cr4V substrates increased remarkably, which was excellent at 800 C in air. The surface morphologies of the oxide scales formed on the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient film specimens after isothermal oxidation for 4 h at 700 C 900 C are presented in Fig.4. It can be seen that the size of the micro-drops increased with increasing oxidation temperatures at 700 C Mass gain / mg mm W18Cr4V (Ti, Al, Zr, Cr) N Ti, Al, Zr, Cr/(Ti, Al, Zr, Cr) N Oxidation temperature / o C Fig.3 Oxidation mass gain curves of W18- Cr4V substrates and their (Ti, Al, Zr, Cr)N and TiAlZrCr/(Ti, Al, Zr, Cr)N films after 4 h oxidation at 600 C 900 C. and 800 C (Fig.4a and b). When the temperature increased to 900 C, the size and amount of the oxide grains increased rapidly. The oxidation morphology showed flocculent and acicular (Fig.4c), and the barbotage and crack occurred due to the thermal stress (Fig.4d). Meanwhile, the compositions of coarse oxides were confirmed to be the substrate elements of Fe and C in addition to the oxide scale elements of Ti, Al, Zr, Cr, N and O by EDS analysis. Fig.5 shows XRD patterns of the oxide scales formed on the TiAlZrCr/(Ti, Al, Zr, Cr)N film specimens after isothermal oxidation for 4 h at 700 C 900 C. Apart from TiN, α-fe and Fe 3 W 3 C, the peak corresponding to TiO 2 was also detected at 700 C and 800 C during the oxidation process. After plated with TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films, the TiO 2 oxide scales formed. Although Al and Cr elements had higher affinity for oxygen, the concentrations of Al and Cr in the films were not high enough to form the continuous Al 2 O 3 and Cr 2 O 3 oxides. The poor oxidation resistance of the W18Cr4V substrates was attributed to the formation of non-protective oxides comprising TiO 2 in oxidative environment. When the temperature was about 900 C, no peaks corresponding to TiN appeared in the XRD patterns of gradient film specimens. Therefore, the appearance of Fe 2 O 3 in Fig.5 demonstrated that the specimens had been fully oxidized during the oxidation process. Thus, the oxidation resistance of TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films on W18Cr4V substrates was excellent up to 800 C in the short-term isothermal oxidation.

478 Fig.4 Surface morphologies of the oxide scales formed on the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films after oxidation for 4 h at 700 C (a), 800 C (b) and 900 C (c, d). 3.

6 478 Fig.4 Surface morphologies of the oxide scales formed on the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films after oxidation for 4 h at 700 C (a), 800 C (b) and 900 C (c, d). 3.3 Long-term oxidation behavior Based on the results of the short-term isothermal oxidation behavior, the cyclic oxidation kinetics curves of the bare and film specimens for 100 h at 700 C and 800 C are depicted in Fig.6. It could be seen that the mass gain of the (Ti, Al, Zr, Cr)N film specimens was lower than that of the bare specimens, and the oxidative rate of the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films was remarkably decreased due to their compositions of the elemental gradient distributions of the (Ti, Al, Zr, Cr)N film with the TiAlZrCr alloy interlayer. No obvious spallation of the oxide Intensity / a.u. 900 o C 800 o C 700 o C TiO 2 Fe 2 O 3 Fe 3 W 3 C -Fe TiN / deg. Fig.5 XRD patterns of the oxide scales formed on the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films after oxidation for 4 h at C. scales or the gradient films took place during the cyclic oxidation of the gradient film specimens, which indicated that the adhesive strength between the film and the substrate or the oxide scale and the film was very high. After plated with TiAlZrCr/(Ti, Al, Zr, Cr)N gradient film, the mass gain greatly reduced at 700 C, and the oxidation kinetics curve was smooth after about 10 h. Accelerated mass gain was observed on the TiAlZrCr/(Ti,

479 Mass gain / mg mm -2 0.12 0.10 0.08 0.06 0.04 0.02 0.00 (a) W18Cr4V (Ti, Al, Zr, Cr) N 0 20 40 60 80 100 Oxidation time / h 0.021 (b) (Ti, Al, Zr, Cr) N 0.018 TiAlZrCr/(Ti, Al, Zr, Cr) N 0.015 0.

7 479 Mass gain / mg mm (a) W18Cr4V (Ti, Al, Zr, Cr) N Oxidation time / h (b) (Ti, Al, Zr, Cr) N TiAlZrCr/(Ti, Al, Zr, Cr) N Oxidation time / h 0.40 (c) 0.35 W18Cr4V 0.30 (Ti, Al, Zr, Cr)N 0.25 TiAlZrCr/(Ti, Al, Zr, Cr)N Oxidation time / h Fig.6 Oxidation kinetics curves of W18Cr4V substrates and their (Ti, Al, Zr, Cr)N and TiAlZrCr/(Ti, Al, Zr, Cr)N films at 700 C (a, b) and 800 C (c). Fig.7 Surface morphologies of the oxide scales formed on the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films after cyclic oxidation for 100 h at 700 C (a) and 800 C (b, c). Al, Zr, Cr)N film specimens at 800 C, which was much lower than that on the bare and (Ti, Al, Zr, Cr)N film specimens. The surface morphologies of the oxide scales formed on the gradient film specimens after long-term cyclic oxidation were similar to those after short-term isothermal oxidation, as seen in Fig.7. It showed that the size of the micro-drops increased significantly compared with those in short-term oxidation at 700 C (Fig.7a). When the oxidation temperature reached 800 C, the oxidation mor- Intensity / a.u. 800 o C 700 o C TiO 2 Fe 2 O 3 TiN -Fe Fe 3 W 3 C / deg. Fig.8 XRD patterns of the oxide scales formed on the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films after cyclic oxidation for 100 h at 700 C and 800 C. phology was fully acicular (Fig.7b), and the small barbotage began to appear (Fig.7c). The XRD patterns of the oxide scales formed on the gradient film specimens after oxidation for 100 h at 700 C and 800 C are shown in Fig.8. At 700 C, the peak corresponding to TiO 2 was detected, and the single oxide scale appearance of Fe 2 O 3 demonstrated that the specimens had been entirely oxidized at 800 C. The TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films were nearly failure. Consequently, the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films on W18Cr4V substrates had excellent oxidation resistance, which was also excellent at 700 C in the long-term cyclic oxidation. 4 Conclusions The TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films (with the single TiN type FCC struc-

8 480 ture) were deposited successfully on the high speed steel (W18Cr4V) substrates by multiarc ion plating technique. The average composition of the gradient film surface was Ti27.8- Al11-Zr1.3-Cr11.4-N48.5 (at. pct). Compared with the W18Cr4V substrates and their (Ti, Al, Zr, Cr)N films, the TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films greatly improved the high oxidation resistance, and the TiO 2 oxide scales on them were formed. The oxidation resistance of TiAlZrCr/(Ti, Al, Zr, Cr)N gradient films on W18Cr4V substrates was excellent up to 800 C in the short-term (4 h) isothermal oxidation and to 700 C in the long-term (100 h) cyclic oxidation. Acknowledgements This work was financially supported by Program for Liaoning Excellent Talents in University (No.RC-05-05) and Program for Changjiang Scholars and Innovative Research Team in University (No.IRT0713). REFERENCES [1] H.G. Prengel, A.T. Santhanam, R.M. Penich, P.C. Jindal and K.H. Wendt, Surf Coat Technol (1997) 597. [2] J. Vetter, Surf Coat Technol (1995) 719. [3] S. Paldey and S.C. Deevi, Mater Sci Eng A342 (2003) 58. [4] S.G. Harris, E.D. Doyle, A.C. Vlasveld, J. Audy and D. Quick, Wear 254 (2003) 723. [5] D. McIntyre, J.E. Greene, G. Hakansson, J.E. Sundgren and W.D. Münz, J Appl Phys 67 (1990) [6] W.D. Münz, Werkstoffe Korros 41 (1990) 753. [7] G.S. Fox-Rabinovich, K. Yamomoto, S.C. Veldhuis, A.I. Kovalev and G.K. Dosbaeva, Surf Coat Technol 200 (2005) [8] Kenji Yamamoto, Susumu Kujime and Kazuki Takahara, Surf Coat Technol 200 (2005) [9] L. Rebouta, F. Vaz, M. Andritschky and M.F. Da Silva, Surf Coat Technol (1995) 70. [10] L.A. Donohue, I.J. Smith, W.D. Münz, I. Petrov and J.E. Greene, Surf Coat Technol (1997) 226. [11] K. Yamamoto, T. Sato, K. Takahara and K. Hanaguri, Surf Coat Technol (2003) 620. [12] R. Cremer and D. Neuschütz, Acta Metall Sin (Engl Lett) 15 (2002) 6. [13] A. Mitsuo, S. Uchida, N. Nihira and M. Iwaki, Surf Coat Technol (1998) 98. [14] Q. Luo, W.M. Rainforth, W.D. Münz, Surf Coat Technol (2001) 430. [15] S.L. Zhao, Y. Li, J. Zhang, C. Wang and C.S. Liu, Heat Treat Met 33 (2008) 99. [16] M.P. Le Blanc, Z.F. Zhou, Y.G. Shen, Y.W. Mai and K.Y. Li, Acta Metall Sin (Engl Lett) 18 (2005) 242. [17] S.L. Zhao, J. Zhang and C.S. Liu, Vacuum Technol Surf Eng (Proceedings of the 9 th Vacuum Metallurgy and Surface Engineering Conference, 2009) p.83. [18] S.L. Zhao, J.Zhang and C.S. Liu, J Chin Soc Corros Prot 29 (2009) 296.

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