Applied Mechanics and Materials Vols. 284-287 (2013) pp 183-187 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.284-287.183
Oxidation Resistance and Corrosion Resistance of AlTiCrON Multilayered Coatings on AISI M2 Tool Steel Wei-Yu Ho1,a, Cheng-Hsun Hsu2,b, Zhong-Shen Yang1,a, Li-Wei Shen1,a, Woei-Yun Ho3,c 1
Department of Materials Science and Engineering, MingDao University, Taiwan 2
Department of Materials Engineering, Tatung University, Taiwan
3
Department of Applied Physics, National Ping Tung University of Education, Taiwan a
[email protected],
[email protected],
[email protected]
Keywords: Cathodic arc deposition; Multilayered coatings; Thermal stability; Corrosion resistance
Abstract. This study utilized the reaction of O2/N2 gases and Al0.67Ti0.33/Cr (99.9%) dual targets to synthesize (Ti,Al)ON/CrON multilayered coatings on AISI M2 steel by cathodic arc deposition system. Thermal stability and corrosion resistance of the multilayered coatings for aluminum die casting applications, along with coating structure were investigated. The results showed that the structure of multilayered coatings was a B1 NaCl type. The formation of oxide phases by introducing oxygen to react with Al, Cr, and Ti elements was confirmed by XPS. The thermal stability of oxygen-doped AlTiN/CrN coatings was higher than that of one without oxygen. After the immersion tests in Al-alloy melt, the oxygen-doped AlTiN/CrN coatings deposited at the O2/N2 ratio value of 0.3 had the best improvement on the corrosion resistance among all the coated specimens. Introduction It is known that hard coatings are commonly applied in cutting and forming tools for protection against oxidation and wear resistance. For example, (Ti,Al)N/CrN multilayered coatings were used to enhance wear resistance of tools [1-8]. The (Ti,Al)N/CrN multilayered coatings had the hardness of about 30-36 GPa, and a maximum hardness was obtained when the bi-layer thickness of the coatings ranged between 6 and 12 nm. Therefore, hardness of the (Ti,Al)N/CrN multilayered coatings depended on the bi-layer thickness and columnar grain size [5]. In addition, the addition of oxygen into the arc deposited (Ti,Al)N coatings leaded to the formation of layered structure comprising the phases of Al2O3, AlN, TiN, and Ti(O,N) [9]. Previous study also showed that Cr2O3/CrN double-layered coatings with 3-µm thickness possessed excellent corrosion resistance in aluminum melt [10]. To combine some of coating characteristics such as layered structure, high hardness, excellent thermal stability, and good oxidation resistance could offer the potential performance for the application of aluminum die casting. This was because die casting was a frequently manufacturing process to form the precision productions; the coatings deposited on the die have to overcome impediments during high speed forming. The impediments contained thermal shock, abrasive wear, adhesive wear, and corrosion of liquid aluminum. Thus, the study aims to synthesize O-doped (Ti,Al)N/CrN multilayered coatings by cathodic arc deposition (CAD) method, then to explore oxidation resistance of these coatings for industrial applications. Another type of (Ti,Al)N/CrN multilayered coatings without oxygen addition was also deposited for a comparison in this study. Experimental In this study, the substrates were made of AISI M2 high-speed tool steel (0.8-0.9 %C, 0.4 %Si, 0.4 %Mn, 3.8-4.5 %Cr, 4.5-5.5 %Mo, 5.5-6.7 %W, 1.6-2.2 %V, Fe balanced in wt.%), which were machined into circular φ 20 mm×5 mm in size. Prior to the CAD treatment, the substrates were
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heat-treated to obtain the hardness of about 63 HRC to imitate the real tool and mold in application [11,12]. A CAD system with dual arc sources was used to synthesize the experimental coatings. Two targets of 67%Al-33%Ti alloy and 99.9%Cr metal were vertically opposed and fixed on the chamber wall. The ratio values of O2 and N2 gas flow were set at four conditions of 0, 0.1, 0.2, and 0.3, respectively. These coatings were designed as a thin CrN interlayer as adhesive layer, and the (Ti,Al)ON/CrON multilayer as top layer. That is, the top layer was obtained to be a O-doped (Ti,Al)N/CrN type by addition of oxygen reactive gas. The bias voltage during deposition was kept at –150V. The multilayered coatings were obtained by rotating the substrate holder at a constant speed of 4 rpm. Details of the depositing parameters are listed in Table 1. Table 1 Parameters of the coating process in this study. Ar+ bombardment Substrate temperature (oC) Chamber pressure (Pa) Substrate bias (V) Target current (A) Distance between target and substrate (cm) CrN interlayer (only the use of Cr target) N2 flow rate (sccm) Depositing time (min) AlTiON/CrON top layer N2 flow rate (sccm) O2 flow rate (sccm) Depositing time (min) Holder rotation (rpm)
−1000 V bias for 10 min 220 0.3 −150 70 15 230 15 230 0, 23, 46, 69, respectively 35 4
Crystallographic characteristics of the as-deposited specimens were analysed by X-ray diffraction (XRD; model PAN analytical X’pert PRD (MRD)) with Cu Kα radiation. The Cu-Kα line at 0.15405 nm was used as the source for diffraction pattern analysis. X-ray photoemission spectroscopy (XPS) measurements were performed using a VG ESCALAB 250 electron spectrometer with a monochromatic Al Kα (1486.6eV) source. Scanning electron microscopy (SEM; model JOEL JSM-5600) was used to observe the fractured cross-section and surface morphology of the various coatings. Hardness of the films was measured using a nanoindenter (Hysitron Triboindenter) under an applied load of 5 mN. The penetrative depth of the indented tip was approximately 50 nm. Thermal analysis was tested in air using thermogravimetric analyzer (TGA) (Mettler Toledo TGA/SDAT851). The heating rate was set at 10 oC/min from room temperature to 1000 oC. Corrosion resistance evaluation of the coatings was performed by dipping tests for 5 hours in a molten A356 Al-alloy bath. After dipping, the tested samples were subsequently immersed in 1 M NaOH solution to remove the Al-alloy slag attached on the coated steel. And then the appearance of these samples was observed using an optical microscope (OM). Results and Discussion Microstructure and Hardness. Fig.1 shows SEM observation of the coated specimens deposited with the different ratio values of O2/N2 set at 0, 0.1, 0.2, and 0.3, respectively. According to the O2/N2 ratio values, the coated specimens were remarked as S0, S1, S2, and S3 in this study. For these SEM observations, it can be seen that the four coatings had similar columnar structures, and all of them had the averaged coating thickness of approximately 1.5 µm. Some coating droplets on their surface are also found. As we known, the cone-like droplets from the arc source are generally the major cause of coating defects during arc deposition process [13]. The correlation between roughness and droplets is also expected, that is, the more droplet amount results in the more coating roughness. XRD analysis was performed to investigate the microstructure of O-doped AlTiN/CrN coatings. For the AlTiN/CrN coatings (S0) in Fig.2, the XRD pattern shows the diffraction peaks of (111), (200), (220), and (311) preferred orientations, which are indexed as the cubic B1 NaCl structure. In S1 (O2/N2 ratio of 0.1) sample, the intensity of (111) orientation decreases whereas the (200) oriented peak of AlTiN/CrN raises. In S2 and S3 samples, as the oxygen flow increases the diffraction peak of (111) plane is disappeared, and the other planes of AlTiN/CrN also lessen their peaks. On the other hand, some specific peaks indexed as crystalline oxide of Cr2O3 and Al2O3 phases are observed. The aluminum oxide in the tendency to form an amorphous phase is more evident than the chromium oxide, as
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AlTiN/CrN(200)
30
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S0 (O2:N2 = 0) S1 (O2:N2=0.1) S2 (O2:N2=0.2)
50
60
CrO3(062)
AlTiN/CrN(311)
AlTiN/CrN(220)
S3 (O2:N2=0.3)
Al2O3(015)
Cr2O3(202)
Relative Intensity (a.u.)
AlTiN/CrN(111)
shown in Fig. 2. In addition, the peaks of titanium dioxide or oxynitride seem to be absent in all the coatings. The result mainly depends on the composition of Al0.67Ti0.33 and Cr targets to show a low content of crystalline TiO2 or amorphous TiOx in the coatings. Fig.3 presents the XPS result to indicate the existence of Cr2O3, Al2O3 and TiO2 phases in the thin films. For S0 sample in Fig.3 (a), the Al 2p XPS spectra was located at 74.4 eV to show AlN dominated in the AlTiN/CrN coatings. Moreover, the AlN phase transformed to Al2O3 phase (74.6 eV) as the oxygen flow increased. The Cr 2p XPS spectra are also presented in Fig.3 (b). Compared to S0 sample, the S2 and S3 samples had the shift of Cr 2p3/2 binding energy from 574.8 eV (CrN) to 576.5 eV (Cr2O3). The cause results from a change of electron binding state as the oxygen flow increased to activate the reactions of N, O and Cr. From Ti 2p XPS spectra (Fig. 3 (c)), the binding energy of Ti 2p shifted from 455.3 eV of TiN (S0, S1) to 458.7 eV of TiO2 (S2, S3) is seen. The above results supported the formation of oxides including Cr2O3, Al2O3, and TiO2. Thus, it is presumed that, in O-doped AlTiN/CrN films, the formation of TiO2 is feasible but the high Cr and Al content as well as the more O2 content may slow down this process, leading to the production of Cr2O3 and Al2O3.
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2 Theta
Fig. 2 XRD patterns of the TiAlN/CrN Fig. 1 SEM cross-sectional view of the coatings with the different O2/N2 flow ratios. coatings deposited with the different O2/N2 flow ratios. A comparison of hardness among these coatings after nano-indentation test is presented in Fig.4. The result showed that the three oxygen-doped AlTiN/CrN coatings (S1, S2, and S3) had a lower hardness than that of oxygen-free (S0). Moreover, the hardness of oxygen-doped AlTiN/CrN coatings decreased as the oxygen flow increased. The research presented that the growth of multiphase nanocrystalline-to-amorphous material is related to the lower hardness [9]. From the above results, it implies that either an amorphous or a nanocrystallized Cr2O3, Al2O3, and TiO2 phases in the thin film could decrease the hardness as compared to the highly crystallized AlTiN/CrN. (a)
(b)
Al2p
AlN(74.4eV)
CrN(574.8eV)
Relative Intensity (a.u.)
Cr2p3
(c)
TiN (455.3eV)
Cr 2 O 3 (576.5eV)
Al2O3(74.6eV)
Ti2p
TiO2(458.7eV) S0 (O2:N2 =0)
S0 (O 2 :N 2 =0)
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S1 (O 2 :N 2 =0.1)
S0 (O2:N2 = 0)
S2 (O 2 :N 2 =0.2)
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S2 (O2:N2=0.2)
S3 (O 2 :N 2 =0.3)
S2 (O2:N2=0.2)
S3 (O2:N2=0.3)
S3 (O2:N2=0.3) 65
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75 80 Binding Energy ( eV )
85
90
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580 585 590 Binding Energy ( eV )
595
450
455
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Binding Energy ( eV )
Fig. 3 XPS spectra of the TiAlN/CrN coatings with the different O2/N2 flow ratios. (a) Al 2p, (b) Cr 2p3, and (c) Ti 2p.
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1.6
S0 S1
30 Hardness (GPa)
Weight change ( % )
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S2
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S3
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100 200 300 400 Indentation lnto Surface (nm)
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S3 (O 2 :N 2 =0.3)
S3
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S0 (O 2 :N 2 =0)
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0.0 500
0
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400 600 800 o Temperature ( C)
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Fig. 5 Oxidation measurement of the various coatings by TGA in air.
Fig. 4 Hardness of the TiAlN/CrN coatings with the different O2/N2 flow ratios.
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(a) Corrosion Pits ( % )
(b)
16 14 12 10 8 6 4 2 0 0.0
Fig. 6 OM observation of the AlTiN/CrN coatings after the dipping tests of 5 hours in molten A356 Al alloy. (a)S0, and (b) S3.
0.1 0.2 O2/N2 Ratio
0.3
Fig. 7 Comparison of the fraction of corrosion pits among the coatings after the dipping tests of 5 hours.
Thermal Analysis. The thermal analysis was conducted using TGA to compare the mass change of the various coatings in this study, as shown in Fig. 5. No drastic weight changes were observed throughout oxidation experiments for all the coatings before 800 oC. The oxidation of the coated samples occurred gradually over time. After heating up to 800oC, the oxidation curves for all the samples increased rapidly. Especially, S0 sample had a sharp increase of the weight gain as compared to the other oxygen-doped coatings. The results revealed the oxidation resistance of the studied coatings is at least above 800oC which is higher than the melting point of A356 aluminum alloy. Therefore, it is worthily suggested that the thermal stability of the studied coatings is available for the aluminum die casting process. Moreover, the oxidation temperature of the O-doped AlTiN/CrN coatings is visible higher than that of oxygen-free one. The cause of the better thermal stability was due to the presence of the higher oxygen content in AlTiN/CrN samples. Literature presented that the oxidation mechanism for the AlTiN films was established by outward diffusion of Al from the coating to form Al-rich oxide at the topmost surface and inward diffusion of O to form Ti-rich oxide at the interface to TiAlN [14]. Similarly, the higher oxygen content causes the formation of the more quantities of Al2O3, TiO2, and Cr2O3 in the coatings, which plays a decisive role in reducing oxidation rates for AlTiN/CrN coatings. Corrosion Resistance in Al melt. After dipping in the Al-alloy melt of 620 oC for 5 hours, the coated steels were attacked to cause discernable pits in local surface, i.e. hemispherical corrosion pits were formed on the coated steels. There was no thermal crack to be observed in the coatings. Fig. 6 illustrates the damaged appearance of the coated samples after the dipping test. It is observed that the
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O-doped AlTiN/CrN coatings with the higher O2/N2 ratio had the less pits. Further, the fraction of corrosion pits on the surface of these coated samples is compared to evaluate the corrosion resistance, as presented in Fig. 7. The result is noticed that no obvious pitting was observed on S3 sample after dipping test of 5 hours. In contrast, the pitted area was discernable for the S0 and S1 samples. Hereby, M2 steel coated with AlTiN/CrN films via the set of O2/N2 ratio at 0.3 could more effectively retard pitting corrosion than the other coatings in this study. Conclusions The O-doped AlTiN/CrN coatings were smoothly synthesized on M2 steel by cathodic arc deposition method with the different O2/N2 ratios. With the increase of O2/N2 ratio, the coatings trended to the production of Cr2O3 and Al2O3. After thermal tests to know, oxidation resistance of the O-doped AlTiN/CrN coatings is higher than that of the one without oxygen. After dipping tests in Al-alloy melt, the AlTiN/CrN coated with O2/N2 ratio of 0.3 had the best improvement on corrosion resistance as compared the other coatings. Acknowledgments The authors express their sincere thanks for the financial support of the National Science Council (Taiwan, ROC) under Contract No. NSC 100-2221-E-451-002. References [1] Wadsworth, I. J. Smith, L.A. Donohue, W. D. Münz, Surf. Coat. Technol., 94-95 (1997), 315. [2] W.M. Rainforth, A.J. Leonard, C. Perrin, A. Bedolla-Jacuinde, Y. Wang, H. Jones, Q. Luo, Tribology International 35 (2002), 731. [3] S. PalDey, S.C. Deevi, Mater. Sci. Eng., A342 (2003), 58. [4] Y. Y. Chang, S. J. Yang, D. Y. Wang, Surf. Coat. Technol., 201 (2006), 4209. [5] C. L. Chang, J. Y. Jao, W. Y. Ho, D. Y. Wang, Vacuum 81 (2007), 604. [6] C. Ducros, F. Sanchette, Surf. Coat. Technol., 201 (2006), 1045. [7] M. Panjan, S. Šturm, P. Panjan, M. Čekada, Surf. Coat. Technol., 202 (2007), 815. [8] V.K. William Grips, H. C. Barshilia, V. Ezhil, S. Kalavati, K.S. Rajam, Thin Solid Films 514 (2006), 204. [9] J. Sjölén, L. Karlsson, S. Braun, R. Murdey, A. Hörling, L. Hultman, Surf. Coat. Technol., 201 (2007) 6392. [10] Wei-Yu Ho, Dung-Hau Huang, Li-Tian Huang, Cheng-Hsun Hsu, Da-Yung Wang, Surf. Coat. Technol., 177 –178 (2004) 172. [11] W.F. Smith, Structure and Properties of Engineering Alloys, McGraw-Hill, 2nd ed., New York, 1993, p.425. [12] C.H. Hsu, Y.D. Chen, Thin Solid Films 517 (2009) 1655. [13] M. Čekada, P. Panjan, D. Kek-Merl, M. Panjan, G. Kapun, Vacuum 82 (2008), p-252. [14] S.G. Harris, E.D. Doyle, Y.-C. Wong, P.R. Munroe, J.M. Cairney, J.M. Long, Surf. Coat. Technol., 183 (2004) 283–294.
