Trans. Nonferrous Met. Soc. China 27(2017) 1063-1070

Structure and oxidation resistance of W_1-xAl_xN composite films

Xiao XIAO¹, Bei YAO^1,2

1. School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China;

2. Analysis Center, Tianjin University, Tianjin 300072, China

Received 25 March 2016; accepted 9 October 2016

Abstract: A series of W_1-xAl_xN films (01-xAl_xN films was investigated. The results show that WN film has a face-centered cubic structure. The preferred orientation changes from (111) to (200). The W_1-xAl_xN films consist of a mixture of face-centered cubic W(Al)N and hexagonal wurtzite structure AlN phases. The hardness of the W_1-xAl_xN films first increases and then decreases with the Al content increasing. The maximum hardness is 36 GPa, which is obtained at 32.4% Al (mole fraction). Compared with WN film, the W_1-xAl_xN composite films show much better oxidation resistance because of the formation of dense Al₂O₃ oxide layer on the surface.

Key words: W_1-xAl_xN film; microstructure; hardness; oxidation resistance; Al₂O₃ layer

1 Introduction

Hard thin films prepared by physical vapor deposition (PVD) are used to improve wear resistance of cutting tools undergoing tribological stress [1]. The first generation of PVD hard thin films (TiN, ZrN, CrN, etc.) has been widely used [2]. With the development of modern manufacturing, especially the appearance of dry and high speed machining, low friction coefficient, high hardness and low wear rate are demanded. Tungsten nitride thin film, as one of the first generation of thin films, has attracted growing attention due to its higher hardness, chemical stability, lower friction coefficient and better wear resistance at room temperature [3]. But it also shows poor oxidation resistance at high temperatures [4]. Since TiSiN composite films were prepared and studied [5], the researchers have observed that the poor oxidation resistance of binary films, at high temperatures, could be significantly improved with the incorporation of the elements which could improve the thermal stability [6-9], such as Al, Si and Y. Much work has been undertaken on the researches of ternary thin films (called them the second generation of hard thin films), such as CrAlN [10], TiSiN [11] and ZrAlN [12]. They showed improved properties like hardness as well as thermal stability and wear resistance at high temperature, due to the formation of an Al₂O₃ or SiO₂ oxide film on the surface. In this regard, the addition of aluminum into WN film is an effective way to improve its oxidation resistance. However, little attention has been paid to WAlN films.

The aim of this work was to prepare W_1-xAl_xN films and investigate the structure, mechanical properties and oxidation resistance. W_1-xAl_xN films with different Al contents were deposited by reactive magnetron sputtering. W₂N films were also produced for comparison purposes.

2 Experimental

W_1-xAl_xN films, (2.1±0.1) μm thick (deposition time: 2 h), were deposited by radio frequency magnetron reactive sputtering on Si (100) wafer with a base pressure of 6.0×10^-5 Pa. Water-cooled 75-mm-diameter W and Al targets with the same purity of 99.9%, were positioned at 78 mm from the substrate. The substrates were cleaned with successive rinses in ultrasonic baths of deionized water, alcohol and acetone and blown with dry air. High purity argon and nitrogen gases were introduced into the chamber from mass-flow controllers to reach a constant chamber pressure of 0.3 Pa. Just prior to initiating deposition, the targets were sputter cleaned for 10 min while the substrate was covered with a protective shutter. Then, AlN film was deposited for 15 min to increase the adhesion, and the target power of Al was 100 W, at Ar/N₂ volume ratio of 10:3. During deposition, the substrates were continuously rotated about the polar axis with 50 r/min in order to obtain optimal coating uniformity. The target power of W was 120 W, the target power of Al was set to be 0, 60, 90, 120 and 150 W, respectively, with at Ar/N₂ volume ratio of 10:10. In order to investigate the oxidation resistance of the films, the samples were annealed at different temperatures (500, 600, 700, 800 °C) for 2 h in ambient atmosphere using the annealing oven. The samples were naturally cooled down to room temperature in the furnace before moving out for other analysis.

The composition of the samples was determined by an electron probe micro analyzer (EPMA). The crystal structures of as-deposited and annealed W_1-xAl_xN films were investigated by X-ray diffraction (XRD) with a Cu K_α source, operated at 40 kV and 40 mA. The glancing incidence angle was 1° and the scanning speed was 4 (°)/min range of 30°-65°. The chemical bonds of the films were characterized by X-ray photoelectron spectroscopy (XPS) with Al K_α irradiation at a pass energy of 160 eV after removing the surface contaminants on the films by sputtering with Ar⁺ ion beam at a primary energy of 3 keV for 3 min. The spectra were calibrated by the C 1s line with a binding energy of 284.5 eV, and then corrected for the linear emission background and decomposed into peaks with Gaussian-Lorentzian line shapes by a non-linear least-square fitting method. The surface morphologies before and after heat treatment were observed by scanning electron microscope (Nanosem 430). The microstructure was analyzed by a high resolution transmission electron microscope (HRTEM, field emission JEOL 2010F) operated at 200 kV. Hardness tests were conducted with a commercial nano- indentation hardness tester (Hysitro, Ti Premier), which equipped with a diamond Berkovich indenter tip (3-side pyramid). To ensure that the hardness was not influenced by the substrate, the maximum penetration depth of 100 nm which was always less than 10% of the coating thickness (2.0 μm) was used.

3 Results and discussion

3.1 Composition and microstructure

The composition of the W_1-xAl_xN films as a function of Al target powers was determined by EPMA. The results are listed in Table 1. It is found that the Al content increases from 0 to 38.6% (mole fraction) with increasing the Al target power from 0 to 150 W, however, the W content appears to decrease gradually. The hardness values of the W_1-xAl_xN films are also listed in Table 1. As shown in Table 1, the hardness gradually increases from 28 GPa for pure WN film to 36 GPa for the film containing 32.4% Al. As the Al content increases to 38.6%, the hardness decreases to 35 GPa.

Table 1 Chemical composition, hardness and average grain size of W_1-xAl_xN composite films

The XRD patterns of W_1-xAl_xN films with different Al contents are presented in Fig. 1. As shown, WN film has a δ-NaCl FCC-structure with the (111) and (200) preferred orientation (PDF card No. 25-1257), which is corresponding to WN phase. W_1-xAl_xN films share a similar structure regardless of Al content. With the increase of Al content, the (111) diffraction peak of the W_1-xAl_xN films becomes weaker and the (200) diffraction peak shifts toward higher diffraction angles, indicating a decrease in the lattice parameter.

Fig. 1 XRD patterns of W_1-xAl_xN composite films with different Al contents

According to the Bragg equation [13], the lattice parameters (a) of W_1-xAl_xN films with different Al contents were calculated. The results are shown in Fig. 2. As shown in Fig. 2, the incorporation of Al results in a decrease of the lattice parameter. This is probably due to the fact that the W atoms (atomic radius: 0.139 nm) are substituted by Al atoms (atomic radius: 0.125 nm) during the preparation of W_1-xAl_xN films. With the increase of Al content, more W atoms are substituted. This results in a further decrease of lattice parameters of W_1-xAl_xN. No XRD signals corresponding to AlN phase are found.

Fig. 2 Lattice constants of W_1-xAl_xN composite films with different Al contents

In order to study the bonding state of W_1-xAl_xN films, XPS analysis was carried out. The XPS analysis results are shown in Fig. 3. As shown in Fig. 3, two peaks are found at 73.0 and 74.6 eV, which are ascribed to W(Al)—N and Al—N bonds, respectively [14,15]. With the increase of Al content, the peak intensity of W(Al)—N decreases coupling with the increase of the peak intensity of Al—N. This suggests that the amount of AlN increases and the amount of W(Al)N decreases.

Fig. 3 XPS analysis of Al 2p photoelectron spectra of W_1-xAl_xN films

The HRTEM images of the WN and W_0.78Al_0.22N films are shown in Fig. 4. As shown in Fig. 4(a), for WN film, two lattice fringes are observed and the lattice spacing values are 0.238 and 0.206 nm, respectively. For WN film, the values of (111) and (200) lattice spacing are 0.24 and 0.208 nm in JCPDS data. This suggests that the calculated values are nearly in agreement with the theoretical values. As shown in Fig. 4(b), for W_0.78Al_0.22N film, two lattice fringes are also observed and the lattice spacing values are 0.236 and 0.210 nm, respectively. According to the reported studies [14-16], these values of the lattice fringes are nearly consistent with the values of HCP-AlN (101) and FCC-W₂N (200) in JCPDS data. According to the above analysis, the films consist of a mixture of FCC-W(Al)N and HCP-AlN phases.

Fig. 4 HRTEM images of different films

The crystalline sizes of W_1-xAl_xN composite films were calculated utilizing Scherre’s equation [13]. The results are also listed in Table 1. As listed, the average grain size of WN is 16.8 nm. As the Al content increases, the grain size first decreases gradually and then increases. A minimum value of 12.9 nm is obtained as the Al content is 32.4%.

3.2 Oxidation resistance

Figure 5 shows the XRD patterns of W_1-xAl_xN films after annealing at different temperatures for 2 h. As shown in Fig. 5(a), after oxidation at 500 °C, WO₃ diffraction peaks are only detected on the surface of WN film. This suggests that the films containing Al content are not oxidized after oxidation at 500 °C for 2 h. As shown in Fig. 5(b), after oxidation at 600 °C, the intensities of WO₃ diffraction peaks detected on the surface of WN film become stronger, while no evident oxide peaks are detected in W_1-xAl_xN films. This indicates that the oxidation degree of WN film increases but the W_1-xAl_xN films still maintain a good oxidation resistance. Figure 5(c) shows that, when the annealing temperature increases up to 700 °C, some WO₃ peaks could be observed on the surface of the W_0.876Al_0.124N film. However, no diffraction peaks corresponding to oxides are detected on the surface of W_1-xAl_xN films (x>12.4%), indicating that the W_1-xAl_xN films with more than 12.4% Al still keep a good oxidation resistance. Figure 5(d) illustrates that the W_0.614Al_0.386N composite film can maintain good oxidation resistance after annealing at 800 °C, because no WO₃ peaks are detected on the surface of W_0.614Al_0.386N film. According to Fig. 5, the oxidation resistance of WN film is improved by adding the aluminum element.

Fig. 5 XRD patterns of W_1-xAl_xN composite films at different annealing temperatures

Figure 6 illustrates the W 4f and Al 2p spectra of the W_0.676Al_0.324N film after oxidation at 700 °C for 2 h. As shown in Fig. 6(a), two contributions are found at 34 and 36 eV, which are ascribed to W(Al)—N and W—O bonds, respectively [17]. This can be assigned to WAlN and WO₃, respectively. Similarly, the Al 2p spectrum can be deconvolved into two subpeaks, centered at about 73.0 and 75.3 eV, which can be assigned to WAlN and Al₂O₃ [17,18], respectively. The XPS results demonstrate that the surface of W_0.676Al_0.324N film is only partially oxidized.

The SEM images of W_0.676Al_0.324N films oxidized at different temperatures are presented in Fig. 7. As shown in Fig. 7, the oxidized surface of films still maintains dense pebble-like structure with uniform grains at temperatures below 600 °C. As the oxidation temperature increases to 700 °C, some micropores and upheavals can be observed to distribute among the grain boundaries. This could speed up the diffusion of oxygen to promote the oxidation of films. As the oxidation temperature further increases to 800 °C, some cracks are found on the surface, which makes it easy to promote the inward diffusion of oxygen. According to Ref. [19], volume expansion generated by the oxide reaction and phase transformation should result in the formation of microcracks. The bulk densities of WAlN, WO₃ and Al₂O₃ are 16.2, 7.16 and 3.99 g/cm³, respectively. The bulk densities of the films decrease after oxidation. This would result in an enormous volume expansion. At 700 °C, the difference of the bulk densities resulted from the formation of WO₃ and Al₂O₃ is just enough to produce some pores and upheavals. At 800 °C, the difference of the bulk densities resulted from the formation of WO₃ and Al₂O₃ could produce cracks.

Fig. 6 XPS spectra of W_0.676Al_0.324N film after oxidation at 700 °C for 2 h

Fig. 7 SEM images of W_0.676Al_0.324N film oxidized at different temperatures

Figure 8 illustrates the changing curve of Al content on the surface of W_0.676Al_0.324N film oxidized at different temperatures. As shown in Fig. 8, with the temperature increasing, the relative content of Al on the oxidized surface increases gradually. During the high-temperature oxidation, it is interesting to note that Al₂O₃ is formed more easily than WO₃, because the Gibbs free energies for Al₂O₃ and WO₃ are -1456.1 and -764 kJ/mol, respectively. For this reason, the Al atoms would diffuse to the film surface to bond with O atoms. Thus, the relative content of Al on the film surface increases slightly.

Fig. 8 Al content on surface of W_0.676Al_0.324N film oxidized at different temperatures

The SEM images of surface and fracture morphologies of WN, W_0.876Al_0.124N and W_0.676Al_0.324N films, after oxidation at 700 °C for 2 h, are shown in Fig. 9. Some cracks are observed on the oxidized surface of Al-free WN film. This is because the larger difference in the bulk densities of WAlN and WO₃ according to the above analysis. Numbers of micropores are found on the oxidized surface of W_0.876Al_0.124N film, because of the difference in the bulk densities of WAlN, WO₃ and Al₂O₃. Some upheavals are found on the surface of W_0.676Al_0.324N film. This is also attributed to the difference in the bulk densities of WAlN, WO₃ and Al₂O₃. All the films exhibit dense and adherent oxide scale (marked by red lines), which share a columnar structure. The formation of oxide scale with thickness of ~0.9 μm is found for Al-free WN film. With further increasing Al content to 12.4%, the thickness of the oxide scale decreases to ~0.6 μm. As the Al content reaches 32.4%, the thickness of the oxide scale further decreases to ~0.3 μm.

Fig. 9 SEM images of surface and fracture morphologies of different films oxidized at 700 °C for 2 h

An analytical TEM study was performed to investigate the structural insight into the oxide scale of the W_1-xAl_xN films. Figure 10 shows the fracture image of the W_0.676Al_0.324N film after oxidation at 700 °C for 2 h and the corresponding HRTEM images of zones A, B and C. As shown in zone A, a lattice fringe with a lattice spacing of about 0.21 nm is observed. This value is nearly consistent with that shown in JCPDS data, where the value of (402) lattice spacing for α-Al₂O₃ is 0.209 nm. In other words, this lattice fringe is corresponding to the α-Al₂O₃ (402). Similarly, WO₃ is observed in zone B and only W₂N exists in zone C. Thus, the oxide scale consists of an Al₂O₃ outer layer (zone A) and a WO₃ inner layer (zone B).

Fig. 10 Fracture image of W_0.676Al_0.324N film oxidized at 700 °C for 2 h and corresponding HRTEM images

It can be learned from the above analyses that the addition of aluminum can improve the oxidation resistance of WN films. This can be considered as follows. Firstly, the added Al atoms replace the situation of W atoms to form FCC-W(Al)N structure, which results in the metallic bonding (W—N) transferring to covalent bonds (Al—N). The increase in bonding energy could improve the thermal stability and inhibit the decomposition of FCC-W(Al)N. Secondly, the W⁶⁺ and Al³⁺ combine with the oxygen by diffusing outward the film surface to form the mixture oxide scale of Al₂O₃ and WO₃. These oxide layers with dense structure can act as an effective diffusion barrier to slow down the inward diffusion of oxygen at high temperatures and increase the oxidation resistance of W_1-xAl_xN films.

4 Conclusions

1) WAlN films consist of a mixture of face-centered cubic W(Al)N and hexagonal wurtzite structure AlN phases. With the increase of Al content, the lattice parameters decrease gradually, while the grain size and hardness decrease first and then increase.

2) The surface morphologies have obvious change for the films oxidized at different temperatures due to the difference of the bulk densities of WAlN, WO₃ and Al₂O₃. For W_0.676Al_0.324N film, the oxidized surface still maintains dense pebble-like structure with uniform grains at temperatures below 600 °C; some micropores and upheavals distributing among the grain boundaries can be observed at 700 °C; some cracks are found on the surface at 800 °C.

3) The oxidation resistance of WN film is improved by adding the aluminum element. All the films exhibit dense and adherent oxide scale, which share a columnar structure. The oxide scale consists of an Al₂O₃ outer layer and a WO₃ inner layer.

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W_1-xAl_xN复合膜的显微组织和抗氧化性能

肖 晓¹，姚 琲^1,2

1. 天津大学 材料科学与工程学院，天津 300072；2. 天津大学 分析中心，天津 300072

摘  要：采用反应磁控溅射仪制备一系列不同铝含量(0~38.6%，摩尔分数)的W_1-xAl_xN复合膜。利用EPMA、XRD、XPS、纳米压痕仪、SEM及HRTEM等对膜组成、显微组织、力学及抗氧化性能进行表征，研究Al含量对WAlN膜显微组织和抗氧化性能的影响。结果表明，WN膜为面心立方结构，W_1-xAl_xN膜由面心立方W(Al)N和六方纤锌矿结构AlN组成，具有很强的(111)和(200)取向。其硬度随Al含量增加先增大后减小。当Al含量为32.4%(摩尔分数)时硬度达到最大值，为36 GPa。与WN相比，W_1-xAl_xN膜表现出更优的抗氧化性能，这主要归因于表层形成的致密Al₂O₃层。

关键词：W_1-xAl_xN膜；显微组织；硬度；抗氧化性能；Al₂O₃层

(Edited by Wei-ping CHEN)

Corresponding author: Bei YAO; Tel: +86-13389072752; E-mail: pyaotiju@163.com

DOI: 10.1016/S1003-6326(17)60124-9