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Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering
Accepted Manuscript Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering Junhua Xu, Jian Chen, Lihua Yu PII:
S0042-207X(16)30169-5
DOI:
10.1016/j.vacuum.2016.05.030
Reference:
VAC 7039
To appear in:
Vacuum
Received Date: 29 March 2016 Revised Date:
30 May 2016
Accepted Date: 31 May 2016
Please cite this article as: Xu J, Chen J, Yu L, Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering, Vaccum (2016), doi: 10.1016/j.vacuum.2016.05.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering
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Junhua Xu, Jian Chen, Lihua Yu School of Materials Science and Engineering, Jiangsu University of Science and Technology, Mengxi Road 2, Zhenjiang, Jiangsu Province 212003, China
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E-mail address: Junhua Xu [email protected], Jian Chen [email protected], Lihua Yu [email protected]
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Corresponding author:Junhua Xu [email protected] Tel: 86 511 84411035,
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Mengxi Road 2, Zhenjiang, Jiangsu Province, 212003, China
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Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering
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Junhua Xu, Jian Chen, Lihua Yu School of Materials Science and Engineering, Jiangsu University of Science and Technology, Mengxi Road 2, Zhenjiang, Jiangsu Province 212003, China
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Abstract: A series of VSiN composite films with different Si content were deposited at room temperature by reactive magnetron sputtering. XPS, XRD, SEM, SAED,
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HRTEM, 3D Profilometer and Nano-indentation were employed to characterize the composition, microstructure, cross-sectional morphology and mechanical properties of the films. The results reveal that the c-VN phase and t-V5N phase coexist in films with different Si content and the c-VN is the major phase. The substitutional solid
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solution of (V,Si)N is formed when the Si content is less than 1.3 at.%. With a further increase of Si content, excess Si are aggregated in the grain boundary and amorphous Si3N4 is formed. It was found that minute traces of Si atoms can promote the growth
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of grain. A two-dimensional model was built in this study to reveal the existing forms of Si element and its influence on the microstructure and mechanical properties of
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VSiN films at different stages of the modelling process. Keywords: microstructure, solid solution, mechanical properties, model 1. Introduction
Over the last decades, transition-metal nitride (TMN) films have attracted widespread attention due to their excellent properties[1-3]. TMN films are successfully applied as wear-protection coatings for tools and mechanical components, decorative
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coatings, electrical contacts, and diffusion barriers in electronic devices[4]. Many binary, ternary and quaternary transition-metal nitride films have been investigated to
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meet the increasingly demanding industrial requirements[5-7]. Recently, vanadium nitrides (VN) have received attention because of their outstanding physical properties such as high hardness, low wear rates and good
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toughness[8-10]. Moreover, vanadium easily oxidizes at high temperature and forms Magneli-phase, which is one kind of high-temperature lubricious oxide phase with
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moveable shear planes[11]. Many studies attempt to add other third elements (for example, carbon, titanium, and silver) to further improve the properties of VN film[12-14]. Veprek reported that the microstructure of a Ti-Si-N nanocomposite film with the largest hardness is governed by a model, in which the nano-sized TiN grains
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are encapsulated by a thin amorphous Si3N4 monolayer approaching the percolation threshold[15]. Many studies show that Si can be added to improve certain properties (for example, the hardness, thermal stability and corrosion resistance) of NbN-based,
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TiN-based and WN-based films[16-18]. However, relatively few publication about VSiN system have been reported.
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Due to the high deposition rate, accurate control of composition, strong adhesion
and wide range of sputtering materials, reactive magnetron sputtering method is widely used in the preparation of thin films[6,9,18]. In this work, a series of VSiN
composite films with different Si content were deposited at room temperature by reactive magnetron sputtering. The influence of Si content on the microstructure and mechanical properties of VSiN films were discussed. Moreover, a new simple
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two-dimensional model to reveal the existing forms of Si and its influence on the microstructure and the mechanical properties of VSiN films at different stages was
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proposed. 2. Experimental procedures 2.1 Film depositions
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VSiN composite films were deposited on Si (100) wafers and 304 stainless steels after polishing by a radio frequency reactive (RF) magnetron sputtering system, which
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consists of two RF sputtering guns, each of them are mounted on water-cooled target holders. The distance between the substrate holder and the targets was 78 mm. More information about the RF reactive magnetron sputtering system is shown in Fig.1. The substrates were cleaned with successive rinsing in ultrasonic baths of deionized water,
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ethyl alcohol absolute (C2H6O≥99.7%) and acetone and blown dry with dry air. V target and Si target with the same purities of 99.9% were positioned at 78 mm from the substrate. Argon gas and nitrogen gas of very high purity (99.999%) was
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introduced. The base pressure was less than 6.0×10-4 Pa. Prior to deposition, the targets were cleaned by pre-sputtering for 10 minutes whiles the substrate was
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isolated from the plasma by a shutter. Between the substrate and films, a thin pure V transition layer was deposited to enhance the adhesion strength. More parameters for the deposition of VSiN films are listed in Table 1. 2.2 Characterization of the films The films deposited on 304 stainless steels were used to measure the composition
and chemical bonding states in order to avoid the Si signal coming from the substrate.
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X-ray photoelectron spectroscopy (XPS) was employed to characterize the composition and chemical bonding states of the films with Al Kα irradiation at a pass
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energy of 160 eV, which has a higher accuracy compared with EDX, especially for the measurement of light elements (for example, the nitrogen element involved in this
study). In order to reduce the pollution of lighter elements, the surfaces of VSiN films
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were clean-sputtered by Ar+ ion beam at a primary energy of 3 keV for 150s before
the XPS test. The XPS spectra were calibrated by the C 1s and the reasons are as
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follows: Carbon is widespread in the samples due to pollution from the air or the preparation process and the binding energy of carbon is 284.8 eV. Also, the peaks of Si3N4 and VN may be weak and overlap in some samples we studied, which are not conducive to calibrate. The films deposited on Si wafers were used to test XRD,
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HRTEM, SEM, microhardness and stress. The phases of VSiN films were explored by X-ray diffraction (XRD, Shimazu-600) with a Cu Kα source (wavelength =0.154056 nm), operated at 40 kV and 35 mA. The normal scanning speed and slow
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scanning speed were 4 °/min and 1 °/min, respectively. The average grain size D of
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VSiN films was calculated by Scherrer formula (Eq.(1)). D = Kγ / Bcosθ
(1)
where K is Scherrer constant (K=0.89), γ is the wavelength of X-ray (γ=0.154056 nm), B is the FWHM of XRD diffraction peak, θ is the diffraction angle. The cross-section of the films were taken by a field emission scanning electron microscope (FE-SEM, Merlin Compact-6170). The microstructure of the deposited films were analyzed by selected area electron diffraction (SAED) and high resolution transmission electron
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microscopy (HRTEM, field emission JEOL 2010F) operated at 200 kV. The hardness test was conducted using nano-indentation (CPX+NHT2+MST, CSM), which was
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equipped with a diamond Berkovich indenter tip (3-side pyramid). An automatic indentation mode was programmed to place indentations in a 3×3 array. The maximum load force is 3 mN; the loading rate and unloading rate were all 6 mN/min.
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As a general rule, the maximum penetration depth was always less than 10% of the
coating thickness to make sure that the hardness was not influenced by the Si (100)
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substrate. Based on surface curvature method, the average stresses σ in the VSiN composite films were determined by means of Stoney’s formula Eq.(2). σ = Es t s2 (1 / R − 1 / R0 ) /[6(1 − υ s )t f ]
Where
(2)
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Es is the elastic modulus of the substrate (E=170 GPa), υs is the Poisson’s ratio of the substrate (υs=0.3), ts is the thickness of the substrate, tf is the thickness of film, R is the curvature of film, R0 is the curvature of Si substrate. R and R0 were measured by
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Bruker 3D Profilometer. In measuring the curvature, the sliding length of the pointer was 10 mm and the sliding speed was 0.5 mm/s.
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3. Results and discussion
3.1 Microstructure analysis The chemical compositions of VSiN composite films with different Si target
powers are summarized in Fig.2. The Si content in the VSiN composite films increases from 0 at.% to 10.5 at.%, with a corresponding decrease of V content from 51.1 at.% to 40.9 at.% as the power of Si target increases from 0 W to 100 W.
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However, the N content in VSiN composite films remain almost constant at about 48 at.%, which is unaffected by Si target powers. This may be due to the high nitrogen
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flow rate (7sccm), which leads to the saturation of N content in the composite films. Fig.3 shows the XRD pattern of VSiN composite films deposited at room temperature with various Si content. For binary VN film, one strong peak and three
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weak peaks around 2θ=37.72°, 40.48°, 43.72° and 61.73° were detected, which corresponds to the (111) crystal plane of cubic VN (JCPDS#35-0768) and (101),
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(110) ,(200) crystal planes of tetragonal V5N (JCPDF#08-0380) using the software Jade. For each XRD pattern of VSiN composite films with different Si content, there is no obvious change diffraction peaks except for the intensities. The c-VN phase with (111) preferred orientation is the major phase in the films, coexisting with t-V5N
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phase. As is known to all, without taking into account the absorption of X-ray, the intensity of the diffraction peak is proportional to the relative content of the corresponding phase. In other words, the intensity of the diffraction peak can be used
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to roughly estimate the relative content of the phase. Although the diffraction peaks of V5N phase in Fig.3 look significant, in fact, the intensities of t-V5N diffraction peaks
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is below 2.5 per cent of the intensities of c-VN diffraction peaks, which suggest a very small amount of V5N phase. No diffraction peaks for crystalline silicon nitride or any metal silicide phases
were detected in any of the XRD patterns for the VSiN composite films, implying that Si may be present in an amorphous phase or may possibly exist as solute atoms in the c-VN lattice[19-20]. In order to obtain further information about these Si containing
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phases, as shown in Fig.4(a), the analysis of the shift of c-VN (111) diffraction peak with different Si content was carried out. The XRD pattern around c-VN (111) was
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scanned at a slow speed to make sure that the shift of the diffraction peak is not affected by scan speed. When the Si content is less than 1.3 at.%, it can be observed
that the diffraction peak of c-VN (111) crystal plane gradually shifted to a higher
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diffraction angle with increasing Si content. Fig.4(b) shows the lattice constant with different Si content. With Si content increasing from 0 at.% to 1.3 at.%, the lattice
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constant decreases from 0.4151 nm to 0.4135 nm.
The shift of diffraction peak and decrease of lattice constant may be caused by N content, stress or solid solution. As shown in Fig.2, the N content in VSiN composite films remains almost constant; this will not affect the position of diffraction peaks. As
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shown in Fig.5, the average stresses were determined by Stoney’s formula Eq.(2) based on surface curvature method[2,20]. The stresses in the films are always compressive stresses, which will lead to a decrease in the lattice constant. With Si
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content increasing from 0 at.% to 1.3 at.%, the stress decreases from 0.66 GPa to 0.05 GPa, which suggests that the influence of stress on the lattice constant is weak and
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disappears gradually. It can be inferred that, when the Si content increased from 0 at.% to 1.3 at.%, the shift of diffraction peak and the decrease in lattice constant was mainly
caused
by
solid
solution
of
silicon.
In
other
words,
Si atoms partially replace V atoms in the c-VN lattice and a substitutional solid solution of (V,Si)N is formed.
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Further details of the microstructure of the VSiN composite films at 1.3 at.% Si and 4.8 at.% Si are revealed by the cross-sectional HRTEM images and its
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corresponding SAED pattern as shown in Fig.6. As shown in Fig.6(a), for VSiN film at 1.3 at.% Si, only one set of lattice fringes with a lattice spacing of about 0.2385 nm
is detected, which corresponds to the c-VN (111), since the value of lattice spacing of
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c-VN (111) is 0.2389 nm. However, there is no lattice fringe corresponding with
t-V5N, whose diffraction rings appeared in SAED pattern. It may be due to the low
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content of t-V5N. It could be observed that the grain size of VSiN films at 1.3 at.% Si is about 30 nm and there is no amorphous phase seen in Fig.6(a). As shown in Fig.6(b), for VSiN film at 4.8 at.% Si, a nanocomposite microstructure consisting of crystal phases and amorphous phase are found. For the crystal phases, three sets of
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lattice fringes with a lattice spacing of about 0.2385 nm, 0.2088 nm and 0.1501 nm are observed, which are corresponding to the c-VN (111) and the t-V5N (110), (200), respectively. This is in agreement with the results obtained from its corresponding
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SAED pattern and XRD pattern. In order to further clarify further bonding state of the amorphous phase in the VSiN film at 4.8 at.%, Si 2p spectra with different Si content
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was carried out by XPS and the results are shown in Fig.7. No obvious peak is detected for VN film. A peak at 102.2 eV, which is in good agreement with Si3N4, is detected for VSiN composite film at 1.3 at.%[20-21]. When Si content increases to 4.8 at.%, the intensity of the peak at 102.2 eV increases, which indicates an increase in the amount of amorphous Si3N4 phase. As shown in Fig.6, we can also find that the
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grain size of VSiN film at 4.8 at.% Si is smaller compared with VSiN film at 1.3 at.% Si.
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To sum up, the results of XRD, HRTEM and SAED reveal that the c-VN phase and t-V5N phase coexist in VSiN composite films with different Si content and the c-VN is the major phase. When the Si content is less than 1.3 at.%, the V atoms in the
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c-VN lattice are replaced by smaller Si atoms and the substitutional solid solution of
(V,Si)N is formed. When the Si content is more than 1.3 at.%, excess Si are
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aggregated in grain boundary and amorphous Si3N4 phase is formed.
The grain sizes of VSiN composite films prepared at room temperature with different Si content are shown in Fig.8. The grain sizes are calculated using Scherrer’s equation, which is the Eq (1) as mentioned above. It was observed that, the grain size
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increases from about 20 nm at 0 at.% Si to 31 nm at 1.3 at.% Si. The coincidence is that, the formation of the substitutional solid solution of (V,Si)N is also within the same Si content range. However, with the further increase of Si content, the grain size
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decreases to 18 nm at 10.5 at.% Si. The calculation results of grain size are essentially in agreement with Fig.6. W. J. Meng et al[22] reported that the grain size of TiSiN
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coatings increases at the beginning and then decreases with the increase of Si content, which is consistent with our work. However, Hongjian Zhao et al[18] reported that the grain size of WSiN coatings with different Si content decreases gradually, which is different from what we found in this study. A few cross-sectional SEM micrographs of VSiN films are presented in Fig.9. As shown in Fig.9(a), for VN film, it exhibits a typical continuous columnar crystalline
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structure. The growth direction of the columnar crystals is perpendicular to the substrate/coating interface. The average width of columnar crystals in VN film is
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about 75 nm. As shown in Fig.9(b), for VSiN film at 1.3 at.% Si, nothing has changed compared with VN film except the width of columnar crystals, which increases to 140 nm. As shown in Fig.9(c), When the Si content increases to 4.8 at.%, the width of
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columnar crystals decreases to 60 nm. It can be observes from above that the degree
of crystallinity increases with Si content increasing from 0 at.% to 1.3 at.%, and then
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decreases with a further increase in Si content from 1.3 at.% to 10.5 at.%. However, Feng Huang et al[21] reported that the size of columnar crystals gradually decreases with the increase of Si content in VSiN system, which results is different from ours. It was found that minute traces of Si atoms can promote the growth of grain.
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3.2 Mechanical properties
The microhardness of VSiN composite films with different Si content are presented in Fig.10. With Si content increasing from 0 at.% to 1.3 at.%, the
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microhardness decreases from 19.4 GPa to 14.8 GPa. With Si content increasing from 1.3 at.% to 4.8 at.%, the microhardness increases from 14.8 GPa to 24.3 GPa. With a
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further increase in Si content to 10.5 at.%, the hardness decreases slightly to 23.4 GPa .
There are a lot of factors that affect the hardness of the composite films, for
example, grain size, residual stress, solid solution and grain boundary. Previous studies reported that the hardness of TMSiN composite films (for example, WSiN, TiSiN, NbSiN and CrSiN) will increase at the beginning and then decrease with
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increasing Si content[18,22-24]. The nc-TMN/a-Si3N4 composite structure plays an important role in the hardness enhancement process. Feng Huang et al[21] reported that
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the hardness of VSiN film prepared under some special parameters by magnetron sputtering can go beyond 50 GPa. However, in this work, the hardness of VSiN
composite films prepared at room temperature by reactive magnetron sputtering is
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different from what were mentioned above.
C.S. Sandu et al[23] proposed a three-step model for the film formation of the
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Nb-Si-N thin films. However, they did not discuss the influence of the binary film and defects on the growth of the composite films in detail. A simple four-stage model was proposed as shown in Fig.11 to reveal the existing form of Si elements and its influence on the microstructure, defects and mechanical properties of VSiN films.
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Before the analyses, there are three things to be explained. First and foremost, there was no differentiation between the V atoms and the N atoms in this model since the focus of our discussion is the existing forms of Si element. Secondly, the t-V5N phase
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was not considered, this is because diffraction intensity is too weak compared with the diffraction intensity of c-VN major phase. Lastly, there was a new growth process of
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the films at different stages. The defects are not possible at the same position. For purposes of clarity, the defects are drawn at the same position to show the influence of Si on the different stages of the thin film growth. Due to the low deposition temperature (RT) and the high N2 flow rate (7 sccm), the atoms have not enough energy to spread on the substrate sufficiently in the process of deposition of binary VN film. In other words, there must be a large number
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of defects generated in the crystal lattice, for example, vacancies, dislocations and sub-boundaries (as shown at stage 1).
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At stage 2, according to the results obtained from section 3.1, Si atoms replace the V atoms in the lattice and the substitutional solid solution of (V,Si)N is formed.
The formation of (V,Si)N will limit the formation of V vacancies. As results, the grain
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size becomes larger and the degree of crystallinity increases (as shown in Fig.6(a), Fig.8 and Fig.9(b)). At this stage, although the film is strengthened by the
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substitutional solid solution, the increase in grain size plays a very important role in the decrease of hardness. The larger the grain size, the fewer grain boundaries, the smaller the resistance to dislocation movement, the smaller the resistance of the material deformation, the lower the hardness. Moreover, previous study reported that [25]
. At this stage, the
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the compressive stress can enhance the microhardness of films
compressive stress in the film is gradually diminishing with an increasing of degree of crystallinity (as shown in Fig.5). The diminishing of the compressive stress is also
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an influential factor that cannot be ignored in the decrease of hardness [23]. With the further increase of Si content, the VN grains cannot accept the excess Si
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atoms due to the limit of solubility. The excess Si atoms gather in the grain boundaries and the Si3N4 phase is formed (as shown at stage 3). The formation of Si3N4 severely
inhibits the growth of grains, which resulted in a significant reduction in the size of the grains (as shown in Fig.6(b), Fig.8 and Fig.9(c)). The hardness enhancement at this stage is mainly attributed to the drop of the grain size[18]. The smaller grain size, the more grain boundaries, the greater the resistance to dislocation movement, the
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greater the resistance of the material deformation, the higher the hardness. Moreover, the existence of the proper amount of the amorphous phase of Si3N4 and the increase
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in compressive stress in films are all factors that cannot be ignored since it plays a role in enhancing hardness [26].
At stage 4, with the Si content increasing to 10.5 at.%, the film has a
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nanocomposite structure. The VN grains are coated with amorphous Si3N4 phase.
With the increase of amorphous Si3N4 phase, the grain size is further reduced. The
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decrease of hardness at this stage is mainly due to the increase of Si3N4 phase (about 22 GPa) and the compressive stress relaxation as shown in Fig.5[24]. 4. Conclusion
A series of VSiN composite films with different Si content were deposited at
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room temperature by reactive magnetron sputtering. The microstructure, growth behavior and mechanical properties have been investigated. The following conclusions were drawn:
The c-VN phase and t-V5N phase coexisted in films with different Si content and
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the c-VN is the major phase. The substitutional solid solution of (V,Si)N is
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formed when the Si content is less than 1.3 at.%. With an increase in Si content, excess Si are aggregated in grain boundary and amorphous Si3N4 is formed.
The grain size increases from about 20 nm at 0 at.% Si to 31 nm at 1.3 at.% Si, which indicates that the increase of the degree of crystallinity. However, with the further increase in Si content, the grain size decreases to 18 nm at 10.5 at.% Si due to the excess Si atoms gathering in the grain boundary and the Si3N4 phase
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formed.
With Si content increasing from 0 at.% to 1.3 at.%, the microhardness decreases
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from 19.4 GPa to 14.8 GPa. With Si content increasing from 1.3 at.% to 4.8 at.%, the microhardness increases from 14.8 GPa to 24.3 GPa. With Si content further increasing to 10.5 at.%, the hardness decreases marginally to 23.4 GPa. And a
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model for the film formation of VSiN ternary films is proposed. Acknowledgments
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Supported by the National Natural Science Foundation of China (613061302, 51574131), Nature Science Foundation of Jiangsu Province (BK2008240), Research Innovation Program for College Graduates of Jiangsu Province (CXZZ12_0717) and combined
study
of
Industry
Research
of
Jiangsu
Province
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(BY2013066-11).
University
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[26] C.W. Zou, H.J. Wang, M. Li, et al. Microstructure and mechanical properties of CrSiN
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nanocomposite coatings deposited by combined cathodic arc middle frequency magnetron
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sputtering, J. Alloys Compd. 485 (2009) 236-240.
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Table caption Table 1 The deposition parameters for VSiN composite films Table 1 Parameters
Base pressure
Less than 6.0×10-4 Pa
Total pressure
0.3 Pa
Ar flow rate
10 sccm
N2 flow rate Substrate holder rotation
7 sccm 3 r/min
V target power (RF)
200 W
Si target power (RF)
0 W, 20 W, 25 W, 30 W, 40 W, 60W, 100 W
Substrate bias voltage Deposition temperature
Room temperature
Deposition time
3h
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VSiN composite films
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Figure captions Fig.1 The schematic diagram of reactive magnetron sputtering system
target powers Fig.3 The XRD pattern of VSiN films with different Si content
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Fig.2 The chemical compositions of VSiN composite films as a function of the Si
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Fig.4 The shift of VN (111) diffraction peak (a) and lattice constant of c-VN (b) with different Si content
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Fig.5 The stress of VSiN composited films with different Si content
Fig.6 Cross-sectional HRTEM images and its corresponding SAED pattern of VSiN composite films at 1.3 at.% Si (a) and 4.8 at.% Si (b)
Fig.7 XPS spectra for VSiN composite films with different Si content
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Fig.8 The average grain sizes of VSiN composite films with different Si content Fig.9 The cross-sectional SEM micrographs: (a) VN film, (b) VSiN films at 1.3 at.% Si, (c) VSiN film at 4.8 at.% Si
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Fig.10 The microhardness of VSiN composite films with different Si content
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Fig.11 The model for the formation of VSiN composite films with different Si content Fig.1
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1. By using XRD, HRTEM and XPS, the microstructure of the VSiN films was studied in details.
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3. A new simple four-stage model was proposed.
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2. The microhardness and stress of VSiN films were investigated in details.
S0042-207X(16)30169-5
DOI:
10.1016/j.vacuum.2016.05.030
Reference:
VAC 7039
To appear in:
Vacuum
Received Date: 29 March 2016 Revised Date:
30 May 2016
Accepted Date: 31 May 2016
Please cite this article as: Xu J, Chen J, Yu L, Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering, Vaccum (2016), doi: 10.1016/j.vacuum.2016.05.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering
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Junhua Xu, Jian Chen, Lihua Yu School of Materials Science and Engineering, Jiangsu University of Science and Technology, Mengxi Road 2, Zhenjiang, Jiangsu Province 212003, China
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E-mail address: Junhua Xu [email protected], Jian Chen [email protected], Lihua Yu [email protected]
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Corresponding author:Junhua Xu [email protected] Tel: 86 511 84411035,
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Mengxi Road 2, Zhenjiang, Jiangsu Province, 212003, China
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Influence of Si content on the microstructure and mechanical properties of VSiN films deposited by reactive magnetron sputtering
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Junhua Xu, Jian Chen, Lihua Yu School of Materials Science and Engineering, Jiangsu University of Science and Technology, Mengxi Road 2, Zhenjiang, Jiangsu Province 212003, China
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Abstract: A series of VSiN composite films with different Si content were deposited at room temperature by reactive magnetron sputtering. XPS, XRD, SEM, SAED,
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HRTEM, 3D Profilometer and Nano-indentation were employed to characterize the composition, microstructure, cross-sectional morphology and mechanical properties of the films. The results reveal that the c-VN phase and t-V5N phase coexist in films with different Si content and the c-VN is the major phase. The substitutional solid
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solution of (V,Si)N is formed when the Si content is less than 1.3 at.%. With a further increase of Si content, excess Si are aggregated in the grain boundary and amorphous Si3N4 is formed. It was found that minute traces of Si atoms can promote the growth
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of grain. A two-dimensional model was built in this study to reveal the existing forms of Si element and its influence on the microstructure and mechanical properties of
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VSiN films at different stages of the modelling process. Keywords: microstructure, solid solution, mechanical properties, model 1. Introduction
Over the last decades, transition-metal nitride (TMN) films have attracted widespread attention due to their excellent properties[1-3]. TMN films are successfully applied as wear-protection coatings for tools and mechanical components, decorative
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coatings, electrical contacts, and diffusion barriers in electronic devices[4]. Many binary, ternary and quaternary transition-metal nitride films have been investigated to
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meet the increasingly demanding industrial requirements[5-7]. Recently, vanadium nitrides (VN) have received attention because of their outstanding physical properties such as high hardness, low wear rates and good
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toughness[8-10]. Moreover, vanadium easily oxidizes at high temperature and forms Magneli-phase, which is one kind of high-temperature lubricious oxide phase with
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moveable shear planes[11]. Many studies attempt to add other third elements (for example, carbon, titanium, and silver) to further improve the properties of VN film[12-14]. Veprek reported that the microstructure of a Ti-Si-N nanocomposite film with the largest hardness is governed by a model, in which the nano-sized TiN grains
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are encapsulated by a thin amorphous Si3N4 monolayer approaching the percolation threshold[15]. Many studies show that Si can be added to improve certain properties (for example, the hardness, thermal stability and corrosion resistance) of NbN-based,
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TiN-based and WN-based films[16-18]. However, relatively few publication about VSiN system have been reported.
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Due to the high deposition rate, accurate control of composition, strong adhesion
and wide range of sputtering materials, reactive magnetron sputtering method is widely used in the preparation of thin films[6,9,18]. In this work, a series of VSiN
composite films with different Si content were deposited at room temperature by reactive magnetron sputtering. The influence of Si content on the microstructure and mechanical properties of VSiN films were discussed. Moreover, a new simple
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two-dimensional model to reveal the existing forms of Si and its influence on the microstructure and the mechanical properties of VSiN films at different stages was
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proposed. 2. Experimental procedures 2.1 Film depositions
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VSiN composite films were deposited on Si (100) wafers and 304 stainless steels after polishing by a radio frequency reactive (RF) magnetron sputtering system, which
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consists of two RF sputtering guns, each of them are mounted on water-cooled target holders. The distance between the substrate holder and the targets was 78 mm. More information about the RF reactive magnetron sputtering system is shown in Fig.1. The substrates were cleaned with successive rinsing in ultrasonic baths of deionized water,
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ethyl alcohol absolute (C2H6O≥99.7%) and acetone and blown dry with dry air. V target and Si target with the same purities of 99.9% were positioned at 78 mm from the substrate. Argon gas and nitrogen gas of very high purity (99.999%) was
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introduced. The base pressure was less than 6.0×10-4 Pa. Prior to deposition, the targets were cleaned by pre-sputtering for 10 minutes whiles the substrate was
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isolated from the plasma by a shutter. Between the substrate and films, a thin pure V transition layer was deposited to enhance the adhesion strength. More parameters for the deposition of VSiN films are listed in Table 1. 2.2 Characterization of the films The films deposited on 304 stainless steels were used to measure the composition
and chemical bonding states in order to avoid the Si signal coming from the substrate.
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X-ray photoelectron spectroscopy (XPS) was employed to characterize the composition and chemical bonding states of the films with Al Kα irradiation at a pass
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energy of 160 eV, which has a higher accuracy compared with EDX, especially for the measurement of light elements (for example, the nitrogen element involved in this
study). In order to reduce the pollution of lighter elements, the surfaces of VSiN films
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were clean-sputtered by Ar+ ion beam at a primary energy of 3 keV for 150s before
the XPS test. The XPS spectra were calibrated by the C 1s and the reasons are as
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follows: Carbon is widespread in the samples due to pollution from the air or the preparation process and the binding energy of carbon is 284.8 eV. Also, the peaks of Si3N4 and VN may be weak and overlap in some samples we studied, which are not conducive to calibrate. The films deposited on Si wafers were used to test XRD,
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HRTEM, SEM, microhardness and stress. The phases of VSiN films were explored by X-ray diffraction (XRD, Shimazu-600) with a Cu Kα source (wavelength =0.154056 nm), operated at 40 kV and 35 mA. The normal scanning speed and slow
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scanning speed were 4 °/min and 1 °/min, respectively. The average grain size D of
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VSiN films was calculated by Scherrer formula (Eq.(1)). D = Kγ / Bcosθ
(1)
where K is Scherrer constant (K=0.89), γ is the wavelength of X-ray (γ=0.154056 nm), B is the FWHM of XRD diffraction peak, θ is the diffraction angle. The cross-section of the films were taken by a field emission scanning electron microscope (FE-SEM, Merlin Compact-6170). The microstructure of the deposited films were analyzed by selected area electron diffraction (SAED) and high resolution transmission electron
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microscopy (HRTEM, field emission JEOL 2010F) operated at 200 kV. The hardness test was conducted using nano-indentation (CPX+NHT2+MST, CSM), which was
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equipped with a diamond Berkovich indenter tip (3-side pyramid). An automatic indentation mode was programmed to place indentations in a 3×3 array. The maximum load force is 3 mN; the loading rate and unloading rate were all 6 mN/min.
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As a general rule, the maximum penetration depth was always less than 10% of the
coating thickness to make sure that the hardness was not influenced by the Si (100)
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substrate. Based on surface curvature method, the average stresses σ in the VSiN composite films were determined by means of Stoney’s formula Eq.(2). σ = Es t s2 (1 / R − 1 / R0 ) /[6(1 − υ s )t f ]
Where
(2)
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Es is the elastic modulus of the substrate (E=170 GPa), υs is the Poisson’s ratio of the substrate (υs=0.3), ts is the thickness of the substrate, tf is the thickness of film, R is the curvature of film, R0 is the curvature of Si substrate. R and R0 were measured by
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Bruker 3D Profilometer. In measuring the curvature, the sliding length of the pointer was 10 mm and the sliding speed was 0.5 mm/s.
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3. Results and discussion
3.1 Microstructure analysis The chemical compositions of VSiN composite films with different Si target
powers are summarized in Fig.2. The Si content in the VSiN composite films increases from 0 at.% to 10.5 at.%, with a corresponding decrease of V content from 51.1 at.% to 40.9 at.% as the power of Si target increases from 0 W to 100 W.
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However, the N content in VSiN composite films remain almost constant at about 48 at.%, which is unaffected by Si target powers. This may be due to the high nitrogen
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flow rate (7sccm), which leads to the saturation of N content in the composite films. Fig.3 shows the XRD pattern of VSiN composite films deposited at room temperature with various Si content. For binary VN film, one strong peak and three
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weak peaks around 2θ=37.72°, 40.48°, 43.72° and 61.73° were detected, which corresponds to the (111) crystal plane of cubic VN (JCPDS#35-0768) and (101),
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(110) ,(200) crystal planes of tetragonal V5N (JCPDF#08-0380) using the software Jade. For each XRD pattern of VSiN composite films with different Si content, there is no obvious change diffraction peaks except for the intensities. The c-VN phase with (111) preferred orientation is the major phase in the films, coexisting with t-V5N
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phase. As is known to all, without taking into account the absorption of X-ray, the intensity of the diffraction peak is proportional to the relative content of the corresponding phase. In other words, the intensity of the diffraction peak can be used
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to roughly estimate the relative content of the phase. Although the diffraction peaks of V5N phase in Fig.3 look significant, in fact, the intensities of t-V5N diffraction peaks
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is below 2.5 per cent of the intensities of c-VN diffraction peaks, which suggest a very small amount of V5N phase. No diffraction peaks for crystalline silicon nitride or any metal silicide phases
were detected in any of the XRD patterns for the VSiN composite films, implying that Si may be present in an amorphous phase or may possibly exist as solute atoms in the c-VN lattice[19-20]. In order to obtain further information about these Si containing
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phases, as shown in Fig.4(a), the analysis of the shift of c-VN (111) diffraction peak with different Si content was carried out. The XRD pattern around c-VN (111) was
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scanned at a slow speed to make sure that the shift of the diffraction peak is not affected by scan speed. When the Si content is less than 1.3 at.%, it can be observed
that the diffraction peak of c-VN (111) crystal plane gradually shifted to a higher
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diffraction angle with increasing Si content. Fig.4(b) shows the lattice constant with different Si content. With Si content increasing from 0 at.% to 1.3 at.%, the lattice
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constant decreases from 0.4151 nm to 0.4135 nm.
The shift of diffraction peak and decrease of lattice constant may be caused by N content, stress or solid solution. As shown in Fig.2, the N content in VSiN composite films remains almost constant; this will not affect the position of diffraction peaks. As
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shown in Fig.5, the average stresses were determined by Stoney’s formula Eq.(2) based on surface curvature method[2,20]. The stresses in the films are always compressive stresses, which will lead to a decrease in the lattice constant. With Si
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content increasing from 0 at.% to 1.3 at.%, the stress decreases from 0.66 GPa to 0.05 GPa, which suggests that the influence of stress on the lattice constant is weak and
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disappears gradually. It can be inferred that, when the Si content increased from 0 at.% to 1.3 at.%, the shift of diffraction peak and the decrease in lattice constant was mainly
caused
by
solid
solution
of
silicon.
In
other
words,
Si atoms partially replace V atoms in the c-VN lattice and a substitutional solid solution of (V,Si)N is formed.
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Further details of the microstructure of the VSiN composite films at 1.3 at.% Si and 4.8 at.% Si are revealed by the cross-sectional HRTEM images and its
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corresponding SAED pattern as shown in Fig.6. As shown in Fig.6(a), for VSiN film at 1.3 at.% Si, only one set of lattice fringes with a lattice spacing of about 0.2385 nm
is detected, which corresponds to the c-VN (111), since the value of lattice spacing of
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c-VN (111) is 0.2389 nm. However, there is no lattice fringe corresponding with
t-V5N, whose diffraction rings appeared in SAED pattern. It may be due to the low
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content of t-V5N. It could be observed that the grain size of VSiN films at 1.3 at.% Si is about 30 nm and there is no amorphous phase seen in Fig.6(a). As shown in Fig.6(b), for VSiN film at 4.8 at.% Si, a nanocomposite microstructure consisting of crystal phases and amorphous phase are found. For the crystal phases, three sets of
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lattice fringes with a lattice spacing of about 0.2385 nm, 0.2088 nm and 0.1501 nm are observed, which are corresponding to the c-VN (111) and the t-V5N (110), (200), respectively. This is in agreement with the results obtained from its corresponding
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SAED pattern and XRD pattern. In order to further clarify further bonding state of the amorphous phase in the VSiN film at 4.8 at.%, Si 2p spectra with different Si content
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was carried out by XPS and the results are shown in Fig.7. No obvious peak is detected for VN film. A peak at 102.2 eV, which is in good agreement with Si3N4, is detected for VSiN composite film at 1.3 at.%[20-21]. When Si content increases to 4.8 at.%, the intensity of the peak at 102.2 eV increases, which indicates an increase in the amount of amorphous Si3N4 phase. As shown in Fig.6, we can also find that the
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grain size of VSiN film at 4.8 at.% Si is smaller compared with VSiN film at 1.3 at.% Si.
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To sum up, the results of XRD, HRTEM and SAED reveal that the c-VN phase and t-V5N phase coexist in VSiN composite films with different Si content and the c-VN is the major phase. When the Si content is less than 1.3 at.%, the V atoms in the
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c-VN lattice are replaced by smaller Si atoms and the substitutional solid solution of
(V,Si)N is formed. When the Si content is more than 1.3 at.%, excess Si are
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aggregated in grain boundary and amorphous Si3N4 phase is formed.
The grain sizes of VSiN composite films prepared at room temperature with different Si content are shown in Fig.8. The grain sizes are calculated using Scherrer’s equation, which is the Eq (1) as mentioned above. It was observed that, the grain size
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increases from about 20 nm at 0 at.% Si to 31 nm at 1.3 at.% Si. The coincidence is that, the formation of the substitutional solid solution of (V,Si)N is also within the same Si content range. However, with the further increase of Si content, the grain size
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decreases to 18 nm at 10.5 at.% Si. The calculation results of grain size are essentially in agreement with Fig.6. W. J. Meng et al[22] reported that the grain size of TiSiN
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coatings increases at the beginning and then decreases with the increase of Si content, which is consistent with our work. However, Hongjian Zhao et al[18] reported that the grain size of WSiN coatings with different Si content decreases gradually, which is different from what we found in this study. A few cross-sectional SEM micrographs of VSiN films are presented in Fig.9. As shown in Fig.9(a), for VN film, it exhibits a typical continuous columnar crystalline
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structure. The growth direction of the columnar crystals is perpendicular to the substrate/coating interface. The average width of columnar crystals in VN film is
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about 75 nm. As shown in Fig.9(b), for VSiN film at 1.3 at.% Si, nothing has changed compared with VN film except the width of columnar crystals, which increases to 140 nm. As shown in Fig.9(c), When the Si content increases to 4.8 at.%, the width of
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columnar crystals decreases to 60 nm. It can be observes from above that the degree
of crystallinity increases with Si content increasing from 0 at.% to 1.3 at.%, and then
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decreases with a further increase in Si content from 1.3 at.% to 10.5 at.%. However, Feng Huang et al[21] reported that the size of columnar crystals gradually decreases with the increase of Si content in VSiN system, which results is different from ours. It was found that minute traces of Si atoms can promote the growth of grain.
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3.2 Mechanical properties
The microhardness of VSiN composite films with different Si content are presented in Fig.10. With Si content increasing from 0 at.% to 1.3 at.%, the
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microhardness decreases from 19.4 GPa to 14.8 GPa. With Si content increasing from 1.3 at.% to 4.8 at.%, the microhardness increases from 14.8 GPa to 24.3 GPa. With a
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further increase in Si content to 10.5 at.%, the hardness decreases slightly to 23.4 GPa .
There are a lot of factors that affect the hardness of the composite films, for
example, grain size, residual stress, solid solution and grain boundary. Previous studies reported that the hardness of TMSiN composite films (for example, WSiN, TiSiN, NbSiN and CrSiN) will increase at the beginning and then decrease with
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increasing Si content[18,22-24]. The nc-TMN/a-Si3N4 composite structure plays an important role in the hardness enhancement process. Feng Huang et al[21] reported that
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the hardness of VSiN film prepared under some special parameters by magnetron sputtering can go beyond 50 GPa. However, in this work, the hardness of VSiN
composite films prepared at room temperature by reactive magnetron sputtering is
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different from what were mentioned above.
C.S. Sandu et al[23] proposed a three-step model for the film formation of the
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Nb-Si-N thin films. However, they did not discuss the influence of the binary film and defects on the growth of the composite films in detail. A simple four-stage model was proposed as shown in Fig.11 to reveal the existing form of Si elements and its influence on the microstructure, defects and mechanical properties of VSiN films.
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Before the analyses, there are three things to be explained. First and foremost, there was no differentiation between the V atoms and the N atoms in this model since the focus of our discussion is the existing forms of Si element. Secondly, the t-V5N phase
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was not considered, this is because diffraction intensity is too weak compared with the diffraction intensity of c-VN major phase. Lastly, there was a new growth process of
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the films at different stages. The defects are not possible at the same position. For purposes of clarity, the defects are drawn at the same position to show the influence of Si on the different stages of the thin film growth. Due to the low deposition temperature (RT) and the high N2 flow rate (7 sccm), the atoms have not enough energy to spread on the substrate sufficiently in the process of deposition of binary VN film. In other words, there must be a large number
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of defects generated in the crystal lattice, for example, vacancies, dislocations and sub-boundaries (as shown at stage 1).
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At stage 2, according to the results obtained from section 3.1, Si atoms replace the V atoms in the lattice and the substitutional solid solution of (V,Si)N is formed.
The formation of (V,Si)N will limit the formation of V vacancies. As results, the grain
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size becomes larger and the degree of crystallinity increases (as shown in Fig.6(a), Fig.8 and Fig.9(b)). At this stage, although the film is strengthened by the
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substitutional solid solution, the increase in grain size plays a very important role in the decrease of hardness. The larger the grain size, the fewer grain boundaries, the smaller the resistance to dislocation movement, the smaller the resistance of the material deformation, the lower the hardness. Moreover, previous study reported that [25]
. At this stage, the
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the compressive stress can enhance the microhardness of films
compressive stress in the film is gradually diminishing with an increasing of degree of crystallinity (as shown in Fig.5). The diminishing of the compressive stress is also
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an influential factor that cannot be ignored in the decrease of hardness [23]. With the further increase of Si content, the VN grains cannot accept the excess Si
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atoms due to the limit of solubility. The excess Si atoms gather in the grain boundaries and the Si3N4 phase is formed (as shown at stage 3). The formation of Si3N4 severely
inhibits the growth of grains, which resulted in a significant reduction in the size of the grains (as shown in Fig.6(b), Fig.8 and Fig.9(c)). The hardness enhancement at this stage is mainly attributed to the drop of the grain size[18]. The smaller grain size, the more grain boundaries, the greater the resistance to dislocation movement, the
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greater the resistance of the material deformation, the higher the hardness. Moreover, the existence of the proper amount of the amorphous phase of Si3N4 and the increase
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in compressive stress in films are all factors that cannot be ignored since it plays a role in enhancing hardness [26].
At stage 4, with the Si content increasing to 10.5 at.%, the film has a
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nanocomposite structure. The VN grains are coated with amorphous Si3N4 phase.
With the increase of amorphous Si3N4 phase, the grain size is further reduced. The
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decrease of hardness at this stage is mainly due to the increase of Si3N4 phase (about 22 GPa) and the compressive stress relaxation as shown in Fig.5[24]. 4. Conclusion
A series of VSiN composite films with different Si content were deposited at
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room temperature by reactive magnetron sputtering. The microstructure, growth behavior and mechanical properties have been investigated. The following conclusions were drawn:
The c-VN phase and t-V5N phase coexisted in films with different Si content and
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the c-VN is the major phase. The substitutional solid solution of (V,Si)N is
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formed when the Si content is less than 1.3 at.%. With an increase in Si content, excess Si are aggregated in grain boundary and amorphous Si3N4 is formed.
The grain size increases from about 20 nm at 0 at.% Si to 31 nm at 1.3 at.% Si, which indicates that the increase of the degree of crystallinity. However, with the further increase in Si content, the grain size decreases to 18 nm at 10.5 at.% Si due to the excess Si atoms gathering in the grain boundary and the Si3N4 phase
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formed.
With Si content increasing from 0 at.% to 1.3 at.%, the microhardness decreases
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from 19.4 GPa to 14.8 GPa. With Si content increasing from 1.3 at.% to 4.8 at.%, the microhardness increases from 14.8 GPa to 24.3 GPa. With Si content further increasing to 10.5 at.%, the hardness decreases marginally to 23.4 GPa. And a
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model for the film formation of VSiN ternary films is proposed. Acknowledgments
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Supported by the National Natural Science Foundation of China (613061302, 51574131), Nature Science Foundation of Jiangsu Province (BK2008240), Research Innovation Program for College Graduates of Jiangsu Province (CXZZ12_0717) and combined
study
of
Industry
Research
of
Jiangsu
Province
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(BY2013066-11).
University
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Ceramics International. 40 (2014) 8621-8630. [20] Junhua Xu, Hongbo Ju, Lihua Yu. Influence of silicon content on the microstructure, mechanical and tribological properties of magnetron sputtered Ti-Mo-Si-N films, Vacuum. 110 (2014) 47-53.
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Table caption Table 1 The deposition parameters for VSiN composite films Table 1 Parameters
Base pressure
Less than 6.0×10-4 Pa
Total pressure
0.3 Pa
Ar flow rate
10 sccm
N2 flow rate Substrate holder rotation
7 sccm 3 r/min
V target power (RF)
200 W
Si target power (RF)
0 W, 20 W, 25 W, 30 W, 40 W, 60W, 100 W
Substrate bias voltage Deposition temperature
Room temperature
Deposition time
3h
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VSiN composite films
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Figure captions Fig.1 The schematic diagram of reactive magnetron sputtering system
target powers Fig.3 The XRD pattern of VSiN films with different Si content
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Fig.2 The chemical compositions of VSiN composite films as a function of the Si
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Fig.4 The shift of VN (111) diffraction peak (a) and lattice constant of c-VN (b) with different Si content
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Fig.5 The stress of VSiN composited films with different Si content
Fig.6 Cross-sectional HRTEM images and its corresponding SAED pattern of VSiN composite films at 1.3 at.% Si (a) and 4.8 at.% Si (b)
Fig.7 XPS spectra for VSiN composite films with different Si content
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Fig.8 The average grain sizes of VSiN composite films with different Si content Fig.9 The cross-sectional SEM micrographs: (a) VN film, (b) VSiN films at 1.3 at.% Si, (c) VSiN film at 4.8 at.% Si
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Fig.10 The microhardness of VSiN composite films with different Si content
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Fig.11 The model for the formation of VSiN composite films with different Si content Fig.1
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Fig.7
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Fig.10
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1. By using XRD, HRTEM and XPS, the microstructure of the VSiN films was studied in details.
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3. A new simple four-stage model was proposed.
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2. The microhardness and stress of VSiN films were investigated in details.