稀有金属(英文版) 2015,34(10),717-724
收稿日期：1 September 2013
基金：financially supported by the Ministry of Industry and Information Technology of China(No.2012ZX04003011);the National Natural Science Foundation of China(No.51275323);
Structure and mechanical properties of Zr/TiAlN films prepared by plasma-enhanced magnetron sputtering
Guang Xian Hai-Bo Zhao Hong-Yuan Fan Hao Du
School of Manufacture Science and Engineering,Sichuan University
The Analysis and Testing Centre,Sichuan University
Abstract：
The purpose of this study was to investigate the effects of Zr interlayer on the structure and mechanical properties of TiAlN films, which were deposited on the M2 high-speed steel substrates by means of plasma-enhanced magnetron sputtering. The result shows that the crystal orientation of Zr/TiAlN films is similar to that of single-layered TiAlN films, but the difference is that AlN(111) of Zr/TiAlN films disappears completely. With respect to Zr interlayer, the texture coefficient of Zr/TiAlN films is approximately 1. Zr/TiAlN films exhibit a compact isometric structure, which is distinctly different from the columnar structure existing in the single-layered TiAlN films and Ti/TiAlN films. The hardness and H3/E*2 of Zr/TiAlN films are, respectively, enhanced to be 36.6 GPa and 0.147. With a few cracks emerging around the indention, the adhesion strength of TiAlN films is obviously advanced by adding Zr metal interlayer.
Keyword：
TiAlN films; Structure; Orientation; Hardness; Adhesion;
Author： Hai-Bo Zhao,e-mail:scdxzhaohaibo@sohu.com,zhaohaibocd@sohu.com;
Received： 1 September 2013
1 Introduction
Ti Al N films were successfully developed as a hard coating on cutting tools due to their excellent performance in hardness, abrasive resistance and thermal stability [1–4]. However, increasing requirements on high speed and dry cutting application make higher requests for the quality of hard films. To further improve the properties of Ti Al N films, the quaternary Ti Al N-based films such as Ti Al Si N, Ti Al Cr N, and Ti Al BN have been developed in recent years [5–7]. These films were proved to have higher hardness and better resistance to high temperature oxidation than Ti Al N films do. However, the brittleness of them increases and the adhesion strength between these films and substrates is still a prominent challenge. Another effective approach to enhance the properties of Ti Al N films is to deposit an intermediate interlayer between Ti Al N films and substrates. Vogli and co-workers [8, 9] already demonstrated that the Ti/Ti Al N films have a dense columnar structure and higher adhesion compared with non-interlayer films. In our previous study, it was also indicated that the Cr interlayer had a good effect on improving the mechanical properties and adhesion strength of Ti Al N films [10]. Therefore, one can expect that Zr metal interlayer has a favorable effect on Ti Al N films. Nevertheless, to our knowledge, few studies were about the effect of Zr interlayer on the structure and properties of Ti Al N films.
In the present study, the Zr/Ti Al N films were successfully deposited on M2 high-speed steel substrates by magnetron sputtering technique. In contrast with Zr/Ti Al N films, the single-layered Ti Al N films as well as Ti/Ti Al N films were also produced. The effects of Zr interlayer on the structure, morphology, hardness, and adhesion strength of outer Ti Al N films were investigated scientifically.
2 Experimental
2.1 Substrate materials and coating deposition
In this work, M2 high-speed steel (HSS) was used as substrates for the typical tool materials use. Zr/Ti Al N,Ti/Ti Al N, and Ti Al N films were deposited on HSS substrates using a plasma-enhanced magnetron sputtering technique, in which an ion plating evaporating method was combined with plasma-enhanced magnetron sputtering technique (Fig. 1) [10]. The ion plating evaporating method was designed to produce the inner interlayer and plasma-enhanced magnetron sputtering was to deposit the outer Ti Al N films. Some mechanical properties and parameters of Ti and Zr metal are summarized in Table 1. Two couples of Ti50Al50powder targets were symmetrically installed on the chamber wall and an evaporating crucible was located on the middle bottom of the chamber. In order to enhance ionization proportion, an ionizing source was placed on the right top of the vacuum chamber. During the process of deposition, a plasma area was produced among sputtering sources, evaporating source and ionizing source, and then the rotational substrate holders were located here.
Fig.1 Schematic diagram of depositing system
Table 1 Compilation of metals used as interlayer and some of their properties 下载原图
Table 1 Compilation of metals used as interlayer and some of their properties
Before being put in the vacuum chamber, the HSS substrates experienced mechanical polishing and abrasive blasting, followed by ultrasonic cleaning in acetone and alcohol baths for 20 min. After being dried, the substrates were immediately placed on the work holder. When the coating chamber evaluated to 5.0910-3Pa, it began to heat the specimen and the duration was 60 min. And then they were cleaned again by ion bombardment at a DC biasvoltage of -200 V and pulse bias voltage of -800 V (the duty ratio is 80 %) in Ar atmosphere of 0.18 Pa for 25 min. Subsequently, the Ti or Zr metal interlayer was deposited for 10 min (this step was left out for depositing the single Ti Al N films only). At last, the Ti Al N films were deposited from Ti50Al50targets at the working pressure of 0.28 Pa. The typical process parameters of depositing interlayer and outer Ti Al N films are listed in Table 2.
Table 2 Typical process parameters of deposition 下载原图
Table 2 Typical process parameters of deposition
2.2 Sample characterizations
The composition of films was measured by energy dispersive X-ray (EDX) analysis, which was as an affiliated apparatus installed on S4800 scanning electron microscopy (SEM, Hitachi, Japan). The bonding structure of films was characterized by X-ray photoelectron spectroscopy (XPS, V. G. Microtech) system with a monochromatic Al Ka X-ray beam (energy = 1486.5 e V and power = 150 W). The C 1s peak with a binding energy of 285.0 e V was used to make correction for the charge shift. The phases and textures of the films were investigated using X-ray diffractometer (XRD, Philips X’ Pert PRO SUPER, Holland) with a Cu Ka radiation (k = 0.154056 nm). The analyzed range of diffraction angle 2h was between 30° and 85°. The coating morphology was examined using S4800 SEM (Hitachi, Japan) at 10.0 k V voltage. Hardness and Yong’s modulus of films were measured using a nano-indentation technique with a trigonal shaped Berkovich diamond indenter (TB15192-12-8-20). Six indents were measured to obtain the average hardness and elastic modulus values. Rockwell HRC indentations were carried out to evaluate the adhesion of the films, with tip radius of 200 lm. And the morphology of the indentations was detected by OLYMPUS GX51 optical microscope.
3 Results and discussion
3.1 Chemical composition and chemical structure
The relative composition of as-deposited Ti Al N, Ti/Ti Al N, and Zr/Ti Al N films as detected by EDX is presented inTable 3. Result indicates that the content of Al is greater than that of Ti, especially in Zr/Ti Al N films, which is deviated from the original proportion of Ti50Al50targets. This phenomenon might be owing to the greater saturated vapor pressure of Al atom than Ti atom. The content of N is more than 50 at% in these three films and thus the N/(Ti ? Al) ratio reaches about 1.3. In addition, a bit of O is detected in all the three samples. This may be due to the oxygen adsorption of films in air environment after deposition. Another possible source of oxygen in films is likely to be the residual gas in depositing chamber because of the low base pressure [11].
Table 3 Composition of films determined using EDX analysis 下载原图
Table 3 Composition of films determined using EDX analysis
In order to know the chemical state of each component in outer Ti Al N film, the high-resolution XPS core-level spectra of Zr/Ti Al N films is shown in Fig. 2. As shown in Fig. 2a, the peak associated with Ti 2p consists of four peaks center at 456.0, 457.8, 461.3, and 463.3 e V. The peaks centered at 456.0 and 461.3 e V originate from Ti 2p3/2and Ti 2p1/2electrons in titanium oxynitride [12, 13]. The peaks centered at 457.8 and 463.3 e V correspond to Ti 2p3/2and Ti 2p1/2electrons, respectively, in Ti O2[14, 15]. In Fig. 2b, the Al 2p spectrum shows a characteristic peak at a binding energy of 73.6 e V, which corresponds to Al N [16]. In Fig. 2c, the N 1s spectrum of Zr/Ti Al N films reveals the presence of two peaks at 396.0 and 399.4 e V. The high intensity peak at 396.0 e V corresponds to nitrogen in Ti N or Al N, as the binding energy of Ti N and Al N were reported to be very similar [17]. The low intensity peak centered at 399.4 e V may be attributed to the presence of impurities in the films such as N2[16]. In Fig. 2d, the peak associated with O 1s consists of three peaks centered at 530.4, 531.7 and 533.0 e V. The peak centered at 530.4 e V corresponds to Ti O2[14]. The peak centered at 531.7 e V corresponds to oxygen in titanium oxynitride [18]. The peak located at 533.0 e V corresponds to absorbed oxygen species [18, 19].
3.2 Crystal structure
XRD patterns of Ti Al N, Ti/Ti Al N, and Zr/Ti Al N films on M2 substrate in Ar ? N2atmosphere are shown in Fig. 3.In these three samples, two fcc B1 (Na Cl type) phases are detected with diffraction peaks at positions close to those of Ti N phase (JCPDS 65-0414) and Al N phase (JCPDS 46-1200) in respective. It is obvious that all the Ti N peaks are moved to the increase of 2h direction, which is due to that the lattice constant of Ti N is reduced by the smaller Al atoms dissolved in Ti N lattice and substituted Ti atoms [20, 21]. Inversely, all the Al N peaks are shifted to the lower 2h direction owing to Al atoms partly replaced by Ti atoms. As seen from Fig. 3, the single-layered Ti Al N film is Ti N (200) strongly predominated while Ti N (111) is merely the secondary peak for Ti N phase. Besides, the Al N (111) next to Ti N (111) is also obviously detected in Ti Al N film while the rest peaks of Al N phase are very weak. As to Ti/Ti Al N films, the Ti N (200) peak is completely vanished and thus the preferred orientation changes to Ti N (111). This might be owing to the similar atom packing between (111) in Ti N and (0001) in Ti, which results in kinetically favorable conditions in forming Ti N (111) on the pre-deposited Ti interlayer [22]. Also, the Al N (111) totally disappears, which probably means that the Ti Al N (111) phase is produced between Ti N (111) and Al N (111). With respect to Zr/Ti Al N films, however, the orientation of Ti N crystal is not obviously changed but the intensity of Ti N (111) increases in comparison with Ti Al N films. And the Al N (111) also disappears in Zr/Ti Al N films.
Fig.2 XPS core-level spectra of Zr/Ti Al N films: a Ti 2p, b Al 2p, c N 1s, and d O 1s
In order to quantize the changes of preferred orientation, the texture coefficient (TC) of films was calculated from their respective XRD peaks using the following formula [23].
where Ihklrepresents the measured intensity of (hkl) plane, Iohklis the standard intensity of (hkl) plane in PDF card, and n is the number of existing crystal faces. Withoutconsideration of Ti N (220), Ti N (311), and Ti N (222), the texture coefficient of Ti N (111) and Ti N (200) in films is shown in Fig. 4. It can be clearly found that the TC values of Ti/Ti Al N films deviate greatly from 1 since Ti N (200) completely disappears. For Ti Al N films, the TC value of (111) and (200) plane is 0.82 and 1.18, respectivelv, indicating the film is slightly (200) oriented. However, using Zr metal as interlayer, both the TC value of (111) and (200) of Zr/Ti Al N films are approximately 1. This result indicates that, compared with the ideal Ti N crystal (JCPDS 65-0414), the Zr/Ti Al N films do not undergo any preferred orientation in the process of growth. In conclusion, it is clear that the observed changes in textures are influenced by the interlayer materials.
Fig.3 XRD patterns of deposited Ti Al N, Ti/Ti Al N, and Zr/Ti Al N films
Compared with Ti Al N films, it is observed that the full width at half maximum (FWHM) of Ti N (111) peak in Ti/ Ti Al N films becomes broader, which indicates that the crystalline grains of the films are refined by adding Ti metal interlayer between outer Ti Al N film and substrate. However, the Ti N (111) peak in Zr/Ti Al N films becomes very sharp and the FWHM is relatively narrow. As a result, the grains of Zr/Ti Al N films are greater than those of Ti Al N films and Ti/Ti Al N films. To obtain the size of crystalline grains, the Scherrer formula as given in Eq. (2) is adopted to estimate the grains of films [24]. The calculated results are shown in Fig. 5.
where λ is the Cu Ka wavelength (λ = 0.154056 nm), h is the Bragg angle of the diffraction peak, and FWHM is the full width half maximum of the (hkl) peak. As shown in Fig. 5, the Ti/Ti Al N films have the smaller grain size at 10.4 nm according to Ti N (111). Nevertheless, the Zr/ Ti Al N films in contrast with single-layered Ti Al N film possess the greater grain size at 14.7 nm.
Fig.4 Texture coefficient of Ti Al N, Ti/Ti Al N, and Zr/Ti Al N films
Fig.5 Crystalline size of Ti Al N, Ti/Ti Al N and Zr/Ti Al N films
3.3 Cross-sectional morphology
The cross-sectional SEM images of Ti Al N, Ti/Ti Al N, and Zr/ Ti Al N films are presented in Fig. 6. Figure 6a displays an unconspicuous columnar structure of Ti Al N films, with the common orientation perpendicular to the interface of the films and the substrate. The columnar width does not change significantly along with the direction of growth. Figure 6b reveals the morphology of the Ti/Ti Al N films, whose upper Ti Al N layer has a typical columnar structure. This might be attributed to the heavy Ti N (111) preferred orientation in crystals. From Fig. 6c it can be found that the Zr/Ti Al N films display a compact isometric crystal structure, which results from the balanced crystal structure without preferred orientation. This indicates that the Zr metal interlayer is in favor of growing isometric crystal structure in outer Ti Al N films. In addition, the thickness of upper Ti Al N films in these specimens is nearly 2.5 lm, which suggests that the growth rate is not influenced by interlayer.
3.4 Hardness, Yong's modulus, and H3/E*2
The hardness and Yong’s modulus of films were determined by depth-sensing indentation tests. In order to avoidthe effect of the substrate, the indentation depth was controlled within 10 % of the coating’s thickness. Six indentation tests were taken in each sample to minimize the accidental error. The average values are calculated and listed in Fig. 7. It is suggested that the H3/E*2(or H/E) ratio can be used to evaluate the ability of the films to resist plastic deformation and cracking, where H refers to the hardness, E refers to the Young’s modulus, and E* is the effective Young’s modulus. And E* = E/(1 - v2) in which m is the Poisson’s rate [25, 26].
Fig.7 Hardness and Yong’s modulus of Ti Al N, Ti/Ti Al N, and Zr/ Ti Al N films
In Fig. 7, it can be seen that the maximum hardness (36.6 GPa) and Young’s modulus (541.9 GPa) are obtained in Zr/Ti Al N films. Meanwhile, the hardness and Young’s modulus of the Ti Al N films are, respectively, 35.1 and 530.4 GPa. However, the hardness of the Ti/Ti Al N films decreases to 33.9 GPa. It is well known that the materials with high hardness are always exhibiting high elastic modulus. However, H3/E*2ratio makes a difference. It is noticeable that H3/E*2of Zr/Ti Al N films is greater than that of Ti Al N and Ti/Ti Al N films, as shown in Fig. 8.
The reason for the largely increased hardness of Zr/ Ti Al N films is connected to the compact isometric crystal structure and the greater grain size of Zr/Ti Al N films. Keunecke et al. [27] studied the changes in hardness ofmodified Ti Al N films and the result suggested that the changes in morphology were accompanied by different indentation hardness values. When the growth structure changed to dense and featureless morphology, the films reached the maximum hardness. As is well known, the grain size has a significant influence on the hardness and other mechanical properties of both bulk materials and films, which follows by the classical Hall–Petch effect [28, 29]. But as to nano-crystalline films, the hardening mechanism actually was proved to be an an inverse Hall–Petch effect [30]. For Zr/Ti Al N films, the grain size of (111) peak reaches to 14.7 nm, which is greater than that of the corresponding peak of Ti Al N films and Ti/Ti Al N films. In conclusion, the difference of mechanical properties like that of hardness in outer Ti Al N films results from the combined effect of microstructure, morphology, and grain size [27, 30].
Fig.6 Cross-sectional SEM images of films: a Ti Al N, b Ti/Ti Al N, and c Zr/Ti Al N
Fig.8 H3/E*2of Ti Al N, Ti/Ti Al N and Zr/Ti Al N films
3.5 Rockwell adhesion test
The adhesion of Ti Al N, Ti/Ti Al N, and Zr/Ti Al N films to M2 substrate was assessed by Rockwell test, and the results are shown in Fig. 9. The indentations were made at 60 kg load, and they were studied with optics microscope (OM). Six classes of indentation characters (HF1–HF6) were recommended and recorded in Ref. [31]. Class HF 1 shows a small amount of cracks in coating around the indentation and class HF 2 displays higher density of cracks. Small parts of detachment of films are occurred in class HF 3 and the detachment increases orderly in class HF 4 and HF 5. The last class HF 6 is characterized by a large area of delamination of the coating from the substrate. In Fig. 9a, it is obvious that a large area of spallation from the substrate occurs in the indention of Ti Al N films. It can be also found that a great amount of cracks exist in the remaining films, like the referred class of HF5. In Fig. 8b, it can be seen that the indent in Ti/Ti Al N films presents the decrease spalling and cracking, which still implies that the bad adhesion of Ti/Ti Al N films to substrate like HF4 class. However, the Zr/Ti Al N films do not show delamination but only a few cracks, as shown in Fig. 9c. The indent of Zr/ Ti Al N films resembles the referred class of HF2, which indicates that the good adhesion strength of films to substrate.
Gerth et al. [31] studied the adhesion strength of PVD Ti N films prepared with various adhesive interlayers on high-speed steel substrate. The interlayer materials contained W, Mo, Nb, Cr, Ti, Ag, and Al. The result showed that the Ti N films with Mo, Nb, and Ti interlayer had the good adhesion on high-speed steels substrate. This phenomenon was analyzed and attributed to the fact that the mechanical properties of interlayer materials are similar to those of films and substrate. In the present study, the structure and mechanical properties of Zr metal are very close to those of Ti metal in that Zr and Ti elements locate at the same main group in Periodic Table of Chemical Elements. However, the good adhesion of Zr/Ti Al N films on M2 substrate happens through adding Zr interlayer, whose effect is better than that of adding Ti interlayer. This phenomenon might suggest that the adhesion strength is not only related to the mechanical properties of interlayer materials but also related to the texture and morphology structure of films interlayer affected.
Fig.9 OM images of indentations made with 60 kg load in films: a Ti Al N, b Ti/Ti Al N, and c Zr/Ti Al N
4 Conclusion
The effects of Zr interlayer on the structure and properties of magnetron-sputtered Ti Al N films were investigated. The preferred orientation of outer Ti Al N films is obviously influenced by adding interlayer. With respect to Ti interlayer, the texture of Ti/Ti Al N films is changed to Ti N (111) from Ti N (200) predominated in single-layered Ti Al N films. In case of Zr interlayer, the texture coefficient of Zr/ Ti Al N films is approximately 1. As a result, the Zr/Ti Al N films exhibit a compact isometric structure, which is distinctly different from the columnar structure existing in the single-layered Ti Al N films and Ti/Ti Al N films. Comparing with single-layered Ti Al N films, the hardness and H3/E*2of Zr/Ti Al N films are, respectively, enhanced to be 36.6 GPa and 0.147. However, the hardness and H3/E*2somewhat decrease in Ti/Ti Al N films. The adhesion strength of Ti Al N films is obviously advanced by adding Zr metal interlayer. Both the single-layered Ti Al N films and the Ti/Ti Al N duplex films cause a certain area of spallation from substrates, whereas the samples with Zr interlayer show only a few cracks.
