Preparation and annealing study of TaNx coatings on WC-Co substrates

Applied Surface Science 257 (2011) 6741–6749

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Preparation and annealing study of TaNx coatings on WC-Co substrates Yung-I Chen a,∗ , Bo-Lu Lin a , Yu-Chu Kuo a , Jen-Ching Huang b,c,d , Li-Chun Chang e,f , Yu-Ting Lin a a

Institute of Materials Engineering, National Taiwan Ocean University, Keelung, Taiwan Institute of Mechatronic Engineering, Tungnan University, New Taipei, Taiwan c Research Center for Micro/Nanotechnology, Tungnan University, New Taipei, Taiwan d Department of Mechanical Engineering, Tungnan University, New Taipei, Taiwan e Department of Materials Engineering, Mingchi University of Technology, New Taipei, Taiwan f Center for Thin Film Technologies and Applications, Mingchi University of Technology, New Taipei, Taiwan b

a r t i c l e

i n f o

Article history: Received 5 October 2010 Received in revised form 24 January 2011 Accepted 23 February 2011 Available online 2 March 2011 Keywords: TaN Diffusion barrier Oxidation Cemented carbide Glass molding

a b s t r a c t To prevent Co diffusion from cemented carbides at high temperatures, we fabricated TaNx coatings by reactive direct current (d.c.) magnetron sputtering onto 6 wt.% cobalt cemented carbide substrates, to form diffusion barrier layers. Varying the nitrogen flow ratio, N2 /(Ar + N2 ), from 0.05 to 0.4 during the sputtering process had a significant effect on coating structure and content. Deposition rate reduced as the nitrogen flow ratio increased. The effects of nitrogen flow ratio on the crystalline characteristics of the TaNx coatings were examined by X-ray diffraction. The TaNx coatings annealing conditions were 500, 600, 700, and 800 ◦ C for 4 h in air. We evaluated the performance of the diffusion barrier using both Auger electron spectroscopy depth-profiles and X-ray diffraction techniques. We also investigated oxidation resistance of the TaNx coatings annealed in air, and under a 50 ppm O2 –N2 atmosphere, to evaluate the fabricated layers effectiveness as a protective coating for glass molding dies. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Cemented carbides are composite materials comprised of individual tungsten carbide grains, imbedded in a ductile metal binder matrix of such as cobalt or nickel, and have been widely used in mechanical applications such as turning tools and mold material because of their high hardness, wear resistance, toughness, thermal shock resistance, thermal conductivity and low thermal expansion coefficient [1,2]. Hard coatings such as diamond layers have been used to improve the surface characteristics of cemented carbides [3,4]. However, it is not easy to deposit a diamond coating directly on WC-Co cemented carbide due to a catalytically mediated conversion of diamond to graphite. Various approaches for modifying the WC-Co substrate surface have been developed, such as (a) chemical etching of cobalt from the surface [3], (b) forming stable Co compounds as interfacial barriers [5,6], and (c) depositing a diffusion barrier layer, including Cr [7], Cr/CrN/Cr [4], TaN and NbN [8] layers on the WC-Co substrates. Recently, researchers used tungsten carbide as a glass-molding die, owing to its ability to withstand high temperature [9]. To avoid diffusion of Co and maintain surface optical properties at high temperature, attempts

∗ Corresponding author at: Institute of Materials Engineering, National Taiwan Ocean University, No. 2, Beining Rd., Keelung, Taiwan. E-mail address: [email protected] (Y.-I. Chen). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.115

to use binderless tungsten carbides as die materials for glass molding met with brittle samples and high costs. Thus, the use of cobalt-cemented carbides with suitable diffusion barrier coatings is a promising option for cemented carbide applications. There are many reports of TaNx coatings use as diffusion barriers for Cu metallization [10–12]. Manaud et al. reported that a hexagonal TaN interlayer on WC-Co tools restricted Co-diffusion while depositing diamond coatings at 880 ◦ C, however, the TaN interlayer was carburized to cubic TaC during the deposition process [8]. In this study, we were concerned with the performance of TaNx coatings in the inhibition of Co-diffusion at different temperatures. A second aim was to evaluate the application of TaNx layers as protective hard coatings on the top surfaces of glass molding dies to prolong the lifetime of the die. Precious metal alloy coatings, such as platinum-alloys [13] or iridium-alloys [14], and carbon thin films [15] have found applications as protective coatings. Some nitride coatings such as TiAlN [16], CrWN [17], and TiAlN/ZrN [18] show promise as protective materials. However, the effects of thermal aging and oxidation of hard nitride coatings in the glass molding process are still uncertain. In a commercial die-transfer glass molding system, a continuous purging nitrogen gas-flow through the chamber, controls oxygen content in the molding atmosphere to 5–15 ppm, depending on flow rate, and purity of the nitrogen gas [19]. In this study, we fabricated TaNx coatings by reactive direct current magnetron sputtering. Annealing treatments in air and in a

6742

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

Table 1 Sputtering parameters of TaNx coatings. Sample

Argon flow rate (sccm)

Nitrogen flow rate (sccm)

N2 /(N2 + Ar) flow ratio

Pressure (Pa)

Power density (W/cm2 )

Target-substrate (mm)

Thickness (nm)

Deposition rate (nm/min)

TaN0.30 TaN0.62 TaN0.75 TaN1.00

19 18 16 12

1 2 4 8

0.05 0.10 0.20 0.40

0.4 0.4 0.4 0.4

7.4 7.4 7.4 7.4

100 100 100 100

460 520 500 480

23.0 18.6 15.6 12.0

50 ppm O2 –N2 atmosphere were carried out to evaluate resistance to both oxidation and to diffusion of Co.

PHI700, ULVAC-PHI, Japan) was utilized to evaluate the oxidation effect by recording chemical composition depth profiles. The nanostructure was examined by transmission electron microscopy (TEM, JEM-2010F, JEOL, Japan) under a 200 kV accelerating voltage.

2. Experimental details 6 wt.% cobalt cemented tungsten carbide, WC-Co, were used as the substrates. The substrates were ground and polished to 1 ␮m size diamond paste and then were ultrasonically cleaned in acetone for 10 min and ethanol for 15 min prior to deposition. A 99.95% pure Ta target with a diameter of 50.8 mm was adopted as source material. The target-to-substrate distance was kept at 100 mm for all sputtering runs. The chamber was evacuated down to 1.33 × 10−4 Pa followed by the inlet of plasma sources. Ta target was pre-sputtered in 99.99% pure Ar with a flow rate of 20 sccm resulting a working pressure of 4.0 × 10−1 Pa and a direct current power of 150 W for 10 min. Then, the reactive gas, 99.999% pure nitrogen, was introduced into the chamber. The total flow rate was sustained at 20 sccm and the flow rate ratios of N2 /(N2 + Ar) were 0.05, 0.1, 0.2 and 0.4, respectively, as Table 1 lists. The substrate holder was rotated with a speed of 5.0 rpm during sputtering. TaNx coatings were deposited onto the substrates with a direct current power of 150 W. The coating thickness was set around 500 nm. Two oxygen containing atmospheres were used for annealing. The annealing treatments in air were conducted in a furnace for 4 h at 500, 600, 700 and 800 ◦ C, respectively. The 50 ppm oxygen content atmosphere at 600 ◦ C was constructed by inletting nitrogen into a heated chamber and the oxygen content was controlled by adjusting nitrogen gas flow rate and recorded with a trace oxygen analyzer (Oxygen Measure Analyzer, COA-011, CNT, Taiwan). Chemical composition analysis was carried out with a field-emission electron probe microanalyzer (FE-EPMA, JXA-8500F, JEOL, Japan). A conventional X-ray diffractometer (XRD, X’Pert PRO MPD, PANalytical, Netherlands) with Cu K␣ radiation generated from a Cu anode operated at 45 kV and 40 mA was adopted to identify the phases of the coatings before and after annealing. The surface morphologies and the thickness values of TaNx coatings were determined by field emission scanning electron microscopy (FE-SEM, S4800, Hitachi, Japan). Auger electron spectroscopy (AES,

3. Results and discussion 3.1. As-deposited TaNx coatings As the flow rate ratio N2 /(N2 + Ar) was increased from 0.05, to 0.1, to 0.2, and to 0.4, the nitrogen content within the TaNx coatings, analyzed by FE-EPMA, increased from 0.229, to 0.380, to 0.426, to 0.492, respectively, as listed in Table 2. Since oxygen content in the TaNx coatings was only 0.6–1.3 at.%, weak oxidation due to residual oxygen in the vacuum chamber could be ignored. The TaNx coatings resulting from the variations in flow ratio listed above, are represented by TaN0.30 , TaN0.62 , TaN0.75 and TaN1.00 formulas, respectively. The deposition rate decreases from 23.0, to 18.6, to 15.6, to 12.0 nm/min as the N2 /(N2 + Ar) ratio increases from 0.05, to 0.1, to 0.2, to 0.4 (Table 1). Previous studies on TaN films have reported similar trends in the relationship of deposition rate and nitrogen content, either by d.c. [20,21] or radio frequency sputtering [22,23]. We attribute this phenomenon to the formation of TaN on the target at specific nitrogen pressures, the well-known target poisoning effect [21,24], resulting in the observed decrease in sputtering rate. Ta–N is a complex system with more than 11 reported equilibrium and metastable phases [25]. Shin et al. had proposed a growth phase map of TaNx layers prepared by reactive dc magnetron sputtering [25]. As the films were deposited at 100 ◦ C, the dominant phase varied from tetragonal ␤-Ta phase, prepared in pure Ar, to cubic TaN0.1 , to hexagonal Ta2 N, to cubic TaN, to bct TaNx , with increasing the nitrogen flow ratio in the discharge. Saha et al. reported the presence of a TaN0.1 phase when preparing Ta(N) coatings with low N2 /(N2 + Ar) ratios in the range 0.006–0.024 [26]. Both the works of Saha et al. and Shin et al. used oxidized Si substrates, and deposited a 500 nm thick coating by reactive magnetron sputtering. In this study, apart from substrate WC reflections, we

Table 2 Chemical compositions of as-deposited and annealed TaNx coatings on WC-Co substrates. Flow rate ratio N2 /(N2 + Ar)

Annealing temp. (◦ C)

Chemical composition (at.%) Ta

0.05

0.1

0.2

0.4

Atomic ratio

N

O

O/Ta

N/Ta

As-deposited 500 600 700 As-deposited 500 600 As-deposited 500 600

76.5 39.8 26.2 27.5 61.3 37.6 26.4 56.8 44.5 26.8

± ± ± ± ± ± ± ± ± ±

0.8 0.8 0.4 0.4 1.0 0.8 0.2 1.1 1.2 0.9

22.9 13.4 4.3 0 38.0 18.5 3.8 42.6 31.1 5.5

± ± ± ± ± ± ± ± ± ±

0.7 0.1 0.7 0 0.9 0.8 0.4 1.1 0.8 0.7

0.6 46.8 69.5 72.5 0.7 43.9 69.8 0.6 24.4 67.7

± ± ± ± ± ± ± ± ± ±

0.1 0.6 0.4 0.4 0.1 1.1 0.2 0.1 0.5 0.7

0.01 1.18 2.65 2.63 0.01 1.17 2.65 0.01 0.55 2.53

0.30 0.34 0.17 0 0.62 0.49 0.15 0.75 0.70 0.21

As-deposited 500 600 700

49.5 34.2 27.0 27.0

± ± ± ±

1.0 0.8 0.7 0.3

49.2 23.1 7.5 0.1

± ± ± ±

1.0 1.5 0.4 0.1

1.3 42.7 65.5 72.9

± ± ± ±

0.1 2.2 0.4 0.3

0.03 1.25 2.42 2.70

1.00 0.68 0.28 0

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

Fig. 1. XRD spectra of the as-deposited TaNx coatings grown on WC-Co substrates.

identified face centered cubic ␦-TaN [ICDD 49-1283], hexagonal ␧TaN [ICDD 39-1485], hexagonal ␥-Ta2 N [ICDD 26-0985] and body centered cubic TaN0.1 phases [ICDD 25-1278] in the X-ray diffraction (XRD) patterns of the as-deposited TaNx coatings, as shown in Fig. 1. The TaN0.30 coating, prepared under N2 /(N2 + Ar) = 0.05, exhibits overlapping reflections of cubic TaN (1 1 1), cubic TaN0.1 (1 1 0) and hexagonal Ta2 N (1 0 1). Also indicated in the inset of Fig. 1a is a broad peak (2 = 58–74◦ ), which implies the Ta–N compounds present in amorphous and nanocrystal states. Figs. 2 and 3 reveal the surface morphologies and cross sectional SEM images of the as-deposited TaNx coatings, respectively. The structural characteristic such as grain boundary is not clear in the TaN0.30 coating. Oxide particles were observed on the surface of TaN0.30 , TaN0.62

6743

and TaN0.75 coatings, accompanied with the examined oxygen contents. Similar morphology was reported by Chang et al. (Fig. 3a of Ref. [27]). The TaN0.62 coating, prepared under N2 /(N2 + Ar) = 0.10, presents a typical ␦-TaN structure, with apparent (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections (Fig. 1b). The integrated intensity ratio of I(1 1 1) :I(2 0 0) :I(2 2 0) :I(3 1 1) is calculated to be 100:71:39:27 when correlated to the standard ratio of 100:68:43:45 [ICDD 49-1283]. The lattice parameter is determined to be 0.437 nm, by correlation with the standard value of 0.434 nm. Though ␦-TaN is a metastable phase, it frequently appears during sputtering deposition [20–22,25,27]. In our sample, a ␥-Ta2 N (0 0 2) reflection was also present. Although the chemical composition ratio Ta:N for the TaN0.62 coating is 62:38, close to 2:1, the intensity of the ␥-Ta2 N (0 0 2) reflection is only 1/10 that of ␦-TaN (1 1 1). For transition metal nitride coatings, the ideal stoichiometry is generally not simple, and vacancy concentrations of up to 50 at.% can exist [2]. The cross sectional SEM image of the as-deposited TaN0.62 coating (Fig. 3b) reveals tightly packed fibrous grains [28], which do not extend through the coating thickness. Fig. 4 shows the bright field cross-sectional TEM image of the TaN0.62 coating. No cracks were observed in the coating or at the substrate/coating interface. Selected area diffraction pattern and magnified image of the middle region are also shown in Fig. 4, revealing a typical FCC TaN diffraction ring pattern and a polycrystalline structure with a grain size in nanometer scale. The TaNx coatings prepared under the higher N2 /(N2 + Ar) ratios of 0.2 and 0.4, produce hexagonal ␧-TaN and FCC ␦-TaN phase reflections in the XRD spectra, and are accompanied by a columnar structure (Fig. 3c and d) with polycrystalline surface morphology (Fig. 2c and d). The intensity ratios of ␧-TaN (1 1 0):␦-TaN (1 1 1) are 39:61 and 52:48 for the as-deposited TaN0.75 and TaN1.00 coatings, respectively. For the TaN0.75 coating prepared with N2 /(N2 + Ar) = 0.2, the integrated intensity ratio I(1 1 1) :I(2 0 0) :I(2 2 0) :I(3 1 1) of the ␦-TaN phase is 100:20:9:6, implies

Fig. 2. SEM images of the as-deposited (a) TaN0.30 , (b) TaN0.62 , (c) TaN0.75 and (d) TaN1.00 coatings.

6744

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

Fig. 3. Cross sectional SEM images of the as-deposited (a) TaN0.30 , (b) TaN0.62 , (c) TaN0.75 and (d) TaN1.00 coatings grown on WC-Co substrates.

a strong (1 1 1) texture. For the TaN1.00 coating prepared with N2 /(N2 + Ar) = 0.4, a strong (2 0 0) reflection of ␦-TaN is observed, with twice the intensity of the (1 1 1) reflection. Harper and Rodbell proposed 11 mechanisms for driving microstructure changes in thin films based on energy considerations [29]. Pelleg et al. have pointed out that the tendency towards a specific preferred orientation of TiN films is decided by a critical competition between the strain and surface energies [30]. However, this model did not apply to films with columnar grains. Manaud et al. claimed the texture of columnar TaN films resulting from the minimization of the surface energy and the deformation energy [8]. In this study, the (2 0 0) reflection of TaN1.00 coating possessed a wider width than that of the TaN0.75 coating, 0.039 and 0.033 in radian, respectively, which implied the grain size of the TaN1.00 coating was smaller and

Fig. 4. Cross sectional TEM image of the as-deposited TaN0.62 coating grown on WC-Co substrates.

the grain boundaries area was larger. Shin et al. reported a similar texture variation as increasing the N2 ratio from 0.150 to 0.250 [25]. 3.2. TaNx coatings annealed in air Annealing in air introduced oxygen into all the TaNx coatings, at temperature in excess of 500 ◦ C, for 4 h. The variations in chemical composition are listed in Table 2. The Gibbs free energies of compound formation for TaN, Ta2 N and Ta2 O5 at 500 ◦ C are −187,662, −100,866 and −850,804 J/(mol Ta atoms), respectively [31]. Thus, TaN and Ta2 N oxidizes to Ta2 O5 during high temperature annealing in air. When annealed at 500 ◦ C, the N/Ta ratios of TaNx coat-

Fig. 5. XRD spectra of the 500 ◦ C annealed TaNx coatings.

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

6745

Fig. 6. SEM images of the 500 ◦ C annealed TaNx coatings.

ings decrease, with the exception of the sample prepared under N2 /(N2 + Ar) = 0.05. Fig. 5 shows XRD patterns of the 500 ◦ C annealed TaNx coatings. For the TaN0.30 coating, the ␥-Ta2 N phase forms after 500 ◦ C annealing, accompanied with an increase in the N/Ta ratio

from 0.30 to 0.34. Figs. 6 and 7 show the surface and cross sectional SEM images of the 500 ◦ C annealed TaNx coatings. The grain size of the 500 ◦ C annealed TaN0.30 coating grows to over 500 nm (Fig. 7a), accompanied with a narrow width of Ta2 N reflections

Fig. 7. Cross sectional SEM images of the 500 ◦ C annealed TaNx coatings.

6746

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

Fig. 8. AES depth profiles of (a) TaN0.62 and (b) TaN1.00 coatings annealed at 500 ◦ C in air for 4 h.

Fig. 9. XRD spectra of the 500–800 ◦ C annealed TaN1.00 coatings.

(Fig. 5). The surface of the 500 ◦ C annealed TaN0.30 coating presents cavities with a diameter less than 100 nm on the surface (Fig. 6a). For the annealed TaN0.62 , TaN0.75 and TaN1.00 coatings, ␦-TaN and ␧-TaN phases are maintained. Fig. 8a and b show the AES depth profiles of the 500 ◦ C annealed TaN0.62 and TaN1.00 coatings on WCCo substrate. The TaNx coating and WC-Co substrate interfaces are

still well defined. The outward diffusion of Co at 500 ◦ C is inhibited by the TaNx coatings. Following annealing in air, oxygen diffused into the TaNx coatings; 5 at.% of oxygen is observed at depths of 53 and 104 nm for the TaN0.62 and TaN1.00 coatings, respectively. The TaN0.62 and TaN1.00 coatings exhibit coarse fibrous grain and columnar microstructures as shown in Fig. 7b and d, respectively, thus the TaN1.00 coating provides fast diffusion paths for oxygen through inter-columns voids [32] and forms a thicker oxide layer. The N/Ta atomic ratios in the interior of the TaN0.62 and TaN1.00 coatings, as determined by AES, are 0.48 ± 0.01 and 0.73 ± 0.09, respectively, while the N/Ta ratios in the oxidized portions are 0.30 ± 0.03 and 0.22 ± 0.03, respectively. Thus, nitrogen was replaced by oxygen during the annealing process. Since the in-diffusion of oxygen was limited, only a tiny Ta2 O5 (0 0 1) reflection is observed following 500 ◦ C annealing in air as shown in Fig. 5. The TaNx coatings, with various chemical compositions annealed above 600 ◦ C produce similar XRD spectra. Fig. 9 shows the XRD patterns of the annealed TaN1.00 coatings. Orthorhombic L-Ta2 O5 [ICDD 25-0922] presents a preferred orientation of (0 0 1) after 600 ◦ C annealing. The O/Ta ratio increases and the N/Ta ratio decreases for annealing temperatures of 600 or 700 ◦ C for each TaNx coatings as listed in Table 2. During four hour annealing of TaN1.00 at 600 ◦ C, oxygen replaces nitrogen throughout the coating, except at the original interface as shown in Fig. 10a. An inter-diffusion zone, with residual nitrogen and tungsten out-diffusion characteristics, forms between the oxidized coating and the substrate. For comparison, a TaN1.00 coating was annealed at 600 ◦ C for 4 h in a vacuum chamber at 3 Pa. No W-diffusion is observed in the AES

Fig. 10. AES depth profiles of TaN1.00 coatings annealed at 600 ◦ C for 4 h in (a) air and (b) vacuum at 3 Pa.

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

6747

Fig. 11. (a) Cross-sectional TEM image of the TaN1.00 coating annealed at 600 ◦ C in air for 4 h, and selected area diffraction patterns of (b) region A and (c) region B in (a).

depth profiles (Fig. 10b). Due to the formation of the inter-diffusion zone, ␦-TaN, WO3 [ICDD 20-1324], and CoWO4 [ICDD 15-0867] are present in the XRD pattern as shown in Fig. 9. A WN phase [ICDD 251256] is also a possible product, but cannot be distinguished from the WC substrate phase [ICDD 51-0939]. Fig. 11a shows a crosssectional bright field TEM image of the 600 ◦ C annealed TaN1.00 coating on WC-Co substrate. Fig. 10a shows correlation to the AES depth profiles, and Fig. 11a indicates an inter-diffusion zone with a width of 70 nm. Fig. 11b and c shows selected area diffraction patterns of regions (A) and (B) from Fig. 11a. Region (A) includes the oxidized coating, exhibiting Ta2 O5 and ␦-TaN diffraction spots, while region (B) consists of the inter-diffusion zone and WC-Co substrate, revealing the ␦-TaN and WN, or WC diffraction spots. Samples annealed at temperatures above 700 ◦ C, show no detectable nitrogen content in the oxidized TaNx coatings, and the O/Ta atomic ratio is close to, or greater than, 2.5, as listed in Table 2. Over-stoichiometric TaOx films, with x as high as 2.8 [33], or 3 [34], were previously reported. The TaN phases disappear after annealing at 700 ◦ C, but Ta2 O5 , WO3 and CoWO4 phases are still detectable. Fig. 12 shows the cross-sectional SEM images of a

1000 nm oxide scale and a 170 ␮m oxide scale for the 600 ◦ C and 700 ◦ C annealed TaN1.00 coatings, respectively. The oxide is dense for the 600 ◦ C annealed coating but porous for the 700 ◦ C annealed one due to the generation of CO and CO2 during oxidation of WC [35]. When annealed at 800 ◦ C, an oxide scale of around 3 mm in thickness forms. XRD diffraction patterns reveal this scale to be orthorhombic CoWO4 and WO3 phases. Basu and Sarin reported that the oxidation rate of WC-Co increases rapidly with temperature increase above 600 ◦ C, and at atmospheric oxygen content [35]. The non-protective nature of the oxide allows rapid in-diffusion of oxygen to the oxide/sample interface. In this study, the asdeposited TaNx coatings were only 500 nm in thickness and do not inhibit the in-diffusion of oxygen. The Ta2 O5 content is negligible in relation to the 3 mm oxide scale and is not detectable by XRD for the 800 ◦ C annealed coatings. 3.3. TaNx coatings annealed in a 50 ppm O2 –N2 atmosphere Glass molding technology has become ubiquitous in the manufacture of high precision optical elements with aspheric surfaces.

Fig. 12. Cross-sectional SEM images of the (a) 600 ◦ C and (b) 700 ◦ C annealed TaN1.00 coatings.

6748

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

annealed TaN1.00 coatings is stable at 0.73 ± 0.03, the same as that recorded for TaN1.00 samples annealed at 500 ◦ C in air. It appears that the TaN1.00 coating is an effective protective coating for glass molding dies at 600 ◦ C under an atmosphere with low oxygen content. However, Ta2 O5 is a soft oxide, with a hardness of 5 GPa, and Young’s modulus of 120 GPa [33], the feasibility of the surface oxidized TaNx coatings require further evaluation under pressing loads and longer annealing times. 4. Conclusions

Fig. 13. XRD spectra of the TaN0.62 and TaN1.00 coatings annealed in a 50 ppm O2 –N2 atmosphere at 600 ◦ C.

During the molding process, the molding dies need to endure repeated thermal cycles between room temperature and the molding temperature, of around 600 ◦ C, under high pressing loads, in an atmosphere consisting nitrogen and oxygen atmosphere, with oxygen present in ppm concentration. When annealed in a 50 ppm O2 –N2 atmosphere at 600 ◦ C for 4 h, the XRD spectra of the TaN0.62 and TaN1.00 coatings show WC, L-Ta2 O5 and ␦-TaN phase reflections, as shown in Fig. 13. The AES depth profiles of the annealed TaN0.62 and TaN1.00 coatings reveal in-diffusion of oxygen with 5 at.% O at depths of 56 and 12 nm, respectively, as shown in Fig. 14. The outward diffusion of Co at 600 ◦ C in a 50 ppm O2 –N2 atmosphere is inhibited by the TaNx coatings. As for the TaN0.62 sample annealed in air at 500 ◦ C, the oxidized and un-oxidized parts present different N/Ta ratios of 0.30 ± 0.03 and 0.48 ± 0.01, respectively. These ratios imply that nitrogen is replaced by oxygen during annealing in air. However, for the 50 ppm O2 –N2 atmosphere and 600 ◦ C annealing of the TaN0.62 coating, the N/Ta ratios for the oxidized and non-oxidized parts are 0.55 ± 0.04 and 0.49 ± 0.02, respectively, which suggests that nitrogen and oxygen diffused into the sub-stoichiometric ␦-TaN structure. Since the TaN0.62 coating is further from stoichiometry than the TaN1.00 coating is, the oxygen content for TaN0.62 is higher after annealing. The N/Ta ratios in the

We reported the preparation of crystalline TaNx coatings deposited on WC-Co substrates using reactive d.c. magnetron sputtering without heating the substrates. The nitrogen flow ratio, N2 /(Ar + N2 ), applied during the sputtering process, significantly affected the coating structure and content. The as-deposited TaN0.30 coating revealed a mixture of nanocrystal and amorphous states, and transferred to Ta2 N phase after 500 ◦ C annealing in air, accompanied with the increase of nitrogen content in the coating. The as-deposited TaN0.62 coating presented a fibrous grain FCC TaN structure, while the as-deposited TaN0.75 and TaN1.00 coatings revealed a columnar feature, composed of mixed FCC TaN and hexagonal TaN phases. During the annealing process in air, oxygen diffused into the TaNx coatings and partially substituted nitrogen. The columnar structure, present in the TaN0.75 and TaN1.00 samples provided fast channels for oxygen diffusion above 500 ◦ C, limiting the suitability of TaNx coating applications at high temperatures in air. No cracks were observed in the coating or at the substrate/coating interface for the as-deposited and the 500 and 600 ◦ C annealed TaNx coatings, but porous oxide layer revealed for the 700 ◦ C annealed coatings. Both the TaN0.62 and TaN1.00 coatings served as diffusion barriers for outward diffusion of Co during annealing at 500 ◦ C in air and 600 ◦ C in a 50 ppm O2 –N2 atmosphere for 4 h. When annealed in a glass molding, 50 ppm O2 –N2 atmosphere, the in-diffusion of oxygen is restricted to shallow depths within the TaNx layer, thus, the TaN1.00 coating is an appropriate protective coating under such physical conditions. Acknowledgements The support of this work from the National Science Council, Taiwan, under Contract No. NSC-99-2221-E-019-009 is appreciated. Partial support from the National Taiwan Ocean University under NTOU-RD981-04-03-13-01 is also acknowledged.

Fig. 14. AES depth profiles of (a) TaN0.62 and (b) TaN1.00 coatings annealed in a 50 ppm O2 –N2 atmosphere at 600 ◦ C for 4 h.

Y.-I. Chen et al. / Applied Surface Science 257 (2011) 6741–6749

References [1] G.S. Upadhyaya, Mater. Des. 22 (2001) 483. [2] L.E. Thod, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [3] R. Haubner, B. Lux, Int. J. Refract. Met. Hard Mater. 14 (1996) 111. [4] Z. Xu, L. Lev, M. Lukitsch, A. Kumar, Diamond Relat. Mater. 16 (2007) 461. [5] T. Kolber, R. Arno Koepf, H. Haubner, Hutter, Int. J. Refract. Met. Hard Mater. 17 (1999) 445. [6] W. Tang, S. Wang, F. Lu, Diamond Relat. Mater. 9 (2000) 1744. [7] C.C. Chou, J.W. Lee, Y.I. Chen, Surf. Coat. Technol. 203 (2008) 704. [8] J.P. Manaud, A. Poulon, S. Gomez, Y. Le Petitcorps, Surf. Coat. Technol. 202 (2007) 222. [9] M. Katsuki, Proc. SPIE 6149 (2006), 61490 M. [10] M. Stavrev, C. Wenzel, A. Möller, K. Drescher, Appl. Surf. Sci. 91 (1995) 257. [11] K.H. Min, K.C. Chun, K.B. Kim, J. Vac. Sci. Technol. B 14 (1996) 3263. [12] A.E. Kaloyeros, E. Eisenbraun, Annu. Rev. Mater. Sci. 30 (2000) 363. [13] H. Monji, M. Aoki, H. Torii, H. Okinaka, U.S. Patent No. 4,629,487 (December 16, 1986). [14] K. Kuribayashi, M. Sakai, H. Monji, M. Aoki, H. Okinaka, H. Torii, U.S. Patent No. 4,685,948 (August 11, 1987). [15] S. Hirota, Y. Oogami, K. Hashimoto, U.S. Patent No. 6,776,007 (August 17, 2004). [16] G. Kleer, W. Doell, Surf. Coat. Technol. 94–95 (1997) 647. [17] C.H. Lin, J.G. Duh, B.S. Yau, Surf. Coat. Technol. 201 (2006) 1316. [18] M. Hock, E. Schäffer, W. Döll, G. Kleer, Surf. Coat. Technol. 163–164 (2003) 689.

6749

[19] Y.I. Chen, L.C. Chang, J.W. Lee, C.H. Lin, Thin Solid Films 518 (2009) 194. [20] B. Mehrotra, J. Stimmell, J. Vac. Sci. Technol. B 5 (1987) 1736. [21] .L. Gladczuk, A. Patel, J.D. Demaree, M. Sosnowski, Thin Solid Films 476 (2005) 295. [22] W.H. Lee, J.C. Lin, C. Lee, Mater. Chem. Phys. 68 (2001) 266. [23] X. Sun, E. Kolawa, J.S. Chen, J.S. Reid, M.-A. Nicolet, Thin Solid Films 236 (1993) 347. [24] M. Stavrev, D. Fischer, C. Wenzel, K. Drescher, N. Mattern, Thin Solid Films 307 (1997) 79. [25] C.S. Shin, Y.W. Kim, D. Gall, J.E. Greene, I. Petrov, Thin Solid Films 402 (2002) 172. [26] R. Saha, J.A. Barnard, J. Cryst. Growth 174 (1997) 495. [27] C.C. Chang, J.S. Jeng, J.S. Chen, Thin Solid Films 413 (2002) 46. [28] J.A. Thornton, J. Vac. Sci. Technol. 11 (1974) 666. [29] J.M.E. Harper, K.P. Rodbell, J. Vac. Sci. Technol. B 15 (1997) 763. [30] J. Pelleg, L.Z. Zevin, S. Lungo, N. Croitoru, Thin Solid Films 197 (1991) 117. [31] I. Barin, Thermochemical Data of Pure Substances, 3rd edn., VCH, New York, 1995, pp. 1602, 1603, 1606. [32] N. Fréty, F. Bernard, J. Nazon, J. Sarradin, J.C. Tedenac, J. Phase Equilib. Diffus. 27 (2006) 590. [33] O. Banakh, P.-A. Steinmann, L. Dumitrescu-Buforn, Thin Solid Films 513 (2006) 136. [34] Z. Geretovszky, T. Szörényi, J.-P. Stoquert, I.W. Boyd, Thin Solid Films 453–454 (2004) 245. [35] S.N. Basu, V.K. Sarin, Mater. Sci. Eng. A 209 (1996) 206.