Microstructures, mechanical and tribological properties of VN films deposited by PLD technique

Abstract

Consistent stoichiometric FCC structured vanadium nitride (VN) films were fabricated by the pulsed laser deposition (PLD) technique at room temperature and 300 °C, for which the microstructures and mechanical and tribological properties were systematically investigated. The results indicated that the films deposited at 300 °C displayed a denser structure and possessed higher hardness and elastic modulus values than the films deposited at room temperature that exhibited a columnar structure. The wear behaviors of VN films were investigated at elevated temperature to 900 °C against an alumina ball under an ambient atmosphere. Due to the densified structure and excellent mechanical properties, the VN films deposited at 300 °C held lower friction coefficients over the investigated temperature range compared to those of the films deposited at room temperature and registered the lowest friction coefficient of about 0.21 at 900 °C. According to the XRD and Raman spectroscopy results, the oxidation behaviors of VN films at elevated temperatures formed a series of vanadium oxides, such as V₂O₅, V₃O₇, V₆O₁₁ and V₆O₁₃, which displayed easy crystallographic shear planes with reduced binding strength and influenced the tribological properties significantly. Moreover, liquid self-lubrication existed in the tribology process above 700 °C due to the melting of V₂O₅ that registered low melting points of 680 °C. Combining the vanadium oxides phase and lubrication phases with the lubricious oxide layer, as well as the liquid lubrication in the contact area, can cause decreased friction coefficients at higher temperatures.

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1. Introduction

Transition metal nitrides, i.e., titanium nitride (TiN) and vanadium nitride (VN), have attracted a great deal of attention over the past decades due to their excellent physical and chemical properties, including high melting point, metal-like conductivity, good chemical stability, and high hardness.^1–7 Moreover, TiN and VN have been widely used in various studies such as optical devices,^8,9 microelectronics,^10,11 catalysis,^12,13 energy storage devices such as batteries and supercapacitors,^14–16 and tribological applications.^17–19 In particular, VN in the form of thin films has been the focus of high-temperature shielding and lubrication applications over the past decade,^2,7,18–24 which not only enhanced the properties of common films, but also formed lubricating vanadium oxides on the wear track during friction processes at elevated temperatures, often referred to as Magnéli phases. As a matter of fact, a variety of vanadium oxides, spanning the oxygen-deficient homologous series with planar faults, show easy crystallographic shear planes with reduced binding strengths that can play a lubrication role during high-temperature friction^2,7,20,23. In addition, liquid self-lubrication often exists in the tribo-contact area because these formed oxide phases usually show low melting points, such as V₂O₅ with a melting point of 680 °C, and would be in the liquid form at a temperature above 700 °C.²⁵ Therefore, VN is supposed to be an effective candidate for high-temperature lubrication materials.

In recent years, several techniques have been utilized to deposit VN thin films, such as chemical vapor deposition,¹⁴ ion plating¹⁸ and magnetron sputtering,^{11,20,21,23,25} but VN films prepared by the pulsed laser deposition (PLD) technique are very rarely used in high-temperature lubrication. The PLD technique is more advantageous than the other methods due to its high deposition rate and good uniformity. Furthermore, the tribological properties and oxidation of VN films were characterized at elevated temperatures only to 700 °C in early literature,^18,20,21 while in present work, VN films deposited by the PLD technique, for which microstructures and mechanical properties were investigated in detail, tribological behavior was evaluated from room temperature to 900 °C in an ambient atmosphere. It is expected that the prepared VN films with satisfactory mechanical and tribological properties could be potentially applied to high-temperature lubrication.

2. Experimental details

2.1. Film preparation

VN films were deposited on a silicon wafer and Inconel 718 substrates by the PLD technique using a KrF excimer laser (λ = 248 nm, pulse duration = 25 ns, ComPexPro 205) at room temperature and 300 °C. The samples were denoted as VN_RT and VN₃₀₀, respectively. The individual VN target with a dimension of Φ 60 mm × 5 mm and 99.99% purity was used as a source material. The details of the deposition process have been reported elsewhere.²⁶ Briefly, a VN target was irradiated with 300 mJ of pulsed laser energy at 10 Hz in 36 000 pulses for each sample. The target was placed parallel to the substrate at a distance of approximately 50 mm. Prior to ablating the target, the deposition system was evacuated to 6.0 × 10⁻⁵ Pa and the working gas pressure was set at 0.3 Pa by N₂ flow of 90 ml min⁻¹.

2.2. Film characterization

X-ray diffraction (Philips, Cu Kα radiation, λ = 0.15 nm) was conducted to investigate the phase composition and identify possible oxide phases formed after tribological tests by comparing to standard ICSD patterns (89/54378) with the Jade 6.0 software. Measurements were carried out at a potential of 40 kV and current of 40 mA; the scanning range of 2θ was from 15° to 80° at the grazing incidence angle of 1° and the scan step size was fixed at 0.06° s⁻¹. The crystalline size of the films was calculated by the Scherrer equation and the Williamson–Hall plot method.²⁷ Microstructure morphologies of the films were characterized by a SU-8020 scanning electron microscope. The micro-hardness and elastic modulus of the films were evaluated using an in situ nanomechanical testing system (TI950, Hysitron TriboIndenter, USA) with a cube-corner diamond tip and set to run five indents on each sample.

The tribological behavior of the VN films was conducted by a ball-on-disk tribometer (UMT-3, Bruker) at elevated temperatures from room temperature up to 900 °C against an alumina ball (10 mm in diameter) in an ambient atmosphere (relative humidity 40 ± 5%). The tests were run at a rotating velocity of 200 rpm with the radius of 3 mm under the normal load of 10 N in 10 min for every sample. Tribochemical reactions and wear products during the tests were monitored by Raman spectroscopy (Horiba Raman microscope, He–Ne laser, wavelength of 532 nm).

3. Results and discussion

3.1. Microstructures

Fig. 1 shows the grazing incidence X-ray diffraction (GIXRD) patterns of VN films deposited at room temperature and 300 °C, indicating that the consistent stoichiometric VN films exhibit a polycrystalline FCC VN structure.^20,21 In the case of the VN_RT film, there is no obvious difference in the peak intensity but it still has a light texture in the [200] direction because it presents the highest intensity. As the substrate temperature increases, the VN₃₀₀ film is strongly oriented in [200] orientation, suggesting that it retains the largest volume fraction in the film. The preferred orientation observed in the films as a function of the deposition temperatures have been explained in detail in our previous work²⁶ where TiN films showed a strong [200] orientation at higher temperatures and was explained by considering the competition between surface energy and epitaxy. The lattice parameter, grain size and micro-strain were calculated on the basis of the patterns in Fig. 1 and tabulated in Table 1. There is no significant change in the lattice parameters of the deposited films at different substrate temperatures. VN films possessing a small grain size, in the range of 8–10 nm, can be due to the formation of highly textured grains. Texture-controlled grain growth in addition to higher nucleation kinetics during the deposition may be explained as similar to the reported work by Jayaganthan²⁸ where the formation of nanograins in TiN films was influenced by factors such as ion energy, ion flux, trace impurities, and textures. In addition, all peaks from the VN₃₀₀ film shifted to lower diffraction angles than in the VN_RT film, which may be due to the influence of enhanced surface energy and mobility of adatoms at a higher substrate temperature and residual compressive stresses,^20,29 resulting in higher micro-strain of the VN₃₀₀ film.

Fig. 1 GIXRD patterns of the VN_RT and VN₃₀₀ films.

Table 1 Structural parameters of the VN_RT and VN₃₀₀ films

Samples	Lattice parameter/nm	Grain size/nm	Micro-strain/%
VNRT	0.4113	8.2(0.2)	0.907(0.11)
VN300	0.4116	9.7(0.3)	0.549(0.08)

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The surface and section morphology of the VN_RT and VN₃₀₀ films is shown in Fig. 2. It can be clearly seen that the surface of the VN_RT film is much coarser and looser than that of the VN₃₀₀ film. There are some large cauliflower-like particles of about 400–600 nm in diameter on the VN_RT film surface. On the contrary, the VN₃₀₀ film surface is very flat and compact and had nearly no large particles on the surface except for some ball-like particles with a diameter of 100 nm. There is also an evident difference in section morphology between the VN_RT and the VN₃₀₀ films; the former, with a thickness of 1.2 μm, displayed the typical columnar structure, but the latter, with a thickness of 1.4 μm, showed a densified structure. The growth rate and the crystallization parameters are enhanced at higher deposition temperatures compared to room temperature due to the PLD films growth depending on the mobility of adatoms and their diffusion. At room temperature, the adatoms, with high temperature and energy, arrive on the pre-deposit surface and the film grows along the opposite direction of heat transfer, and this results in a typical columnar structure. At higher substrate temperatures for the VN₃₀₀ film, the high-energy-mobility adatoms move along random directions on the pre-deposit surface, meet and aggregate together, and result in enhancing the crystallization and formation of a dense microstructure. This phenomenon is similar to the dense structure of TiN films fabricated by unbalanced D.C. magnetron sputtering³⁰ and also agree well with early investigations that the VN¹⁰ and TiN²⁶ films obtained at higher temperature were thicker than the films deposited at room temperature.

Fig. 2 SEM images of VN films: (a, b) VN_RT and (c, d) VN₃₀₀.

3.2. Mechanical properties

The results of nano-indentation measurements on VN films deposited at different temperatures are presented in Fig. 3. It is obvious that the hardness and elastic modulus of VN_RT films are lower than that of VN₃₀₀ films that hold a maximum hardness and elastic modulus value of 21.5 GPa and 256.8 GPa, respectively. To exclude the influence of substrates, nano-indentation experiments were performed on controlled contact depths of 60, 120 and 200 nm. Take the VN₃₀₀ film for example, inset in Fig. 3, there is a small difference among the results, especially in the controlled contact depth of 200 nm where the five load–displacement curves are basically in coincidence. It is worth mentioning that the mechanical properties of the VN₃₀₀ films are uniform. The results of VN films from PLD are higher than those from magnetron sputtering¹¹ where the hardness and elastic modulus are 11 GPa and 187 GPa, respectively. The mechanical properties of the films have a direct corresponding relationship to the microstructures. Numerous studies have reported³¹ that the deformation mechanisms are different at the different grain sizes of the materials. When the grain size was in the range of 1–20 nm, grain-boundary sliding became important and grain-boundary effects had been identified to dominate the deformation process. As analyzed by microstructure results, both of the prepared VN films exhibited nanograins in the size of 8–10 nm. When an external load was exerted on the films, sliding occurred at every grain boundary and dislocation density increased due to the great number of grain boundaries; thus, it could resist the external load. However, the columnar structure, such as in the VN_RT film, contains more intergranular cracking during indentation tests than that of the VN₃₀₀ films that displayed a dense and flat structure.^32,33 Consequently, a denser microstructure, smaller grain size and higher residual compressive stress may be important contributions to the higher hardness and elastic modulus of the VN₃₀₀ films.^34,35

Fig. 3 The hardness and elastic modulus of VN_RT and VN₃₀₀ films. Inset: load–displacement curves of VN₃₀₀ films (a 60 nm, b 120 nm, c 200 nm).

The stationary friction coefficients, as a function of temperature, for the VN_RT and VN₃₀₀ films are shown in Fig. 4. Obviously, the VN₃₀₀ film shows lower friction coefficients over the whole test range compared with VN_RT films. At lower temperatures, both the VN films displayed differences in the friction coefficients. However, the films exhibit decreased friction coefficients at higher temperatures, and both drop to a similar level at higher temperatures around 700 °C (VN_RT-0.26, VN₃₀₀-0.25). This may be due to the continuous formation of easily sheared vanadium oxide phases and the appearance of a lubricious layer in the tribo-contact area.^21,25,34 According to previous statements,^2,7,20,23 a variety of vanadium oxides can form on the worn surface and display low shear strengths due to its crystallographic structure, which can play a significant lubrication role in the contact area during high-temperature friction. Additionally, liquid self-lubrication often exists in the contact area due to melting of V₂O₅ (with melting point ∼ 680 °C) above 700 °C, which could contribute to the similar friction coefficients at 700 °C for both films. Therefore, vanadium oxides and the protective lubricious oxide layer formed in the wear track during tests, as well as liquid lubrication in the contact area, can be responsible for the decreased friction coefficients of VN films at higher temperatures. Furthermore, increased hardness is also reported to lead to better wear resistance,³⁵ the denser microstructure combined with the excellent mechanical properties of the VN₃₀₀ film may be responsible for the better tribological properties compared to the VN_RT film. In addition, it was found that the friction coefficients were slightly lower than that of VN films (μ ∼ 0.45–0.25) prepared by reactive unbalanced magnetron sputtering from early studies,^20,23 where the wear tests were performed from room temperature to 700 °C against a ceramic alumina ball (6 mm in diameter) at a load of 5 N in humid air.

Fig. 4 Temperature dependence of the friction coefficient for VN_RT and VN₃₀₀ films.

In order to illustrate the tribology process of the films at different temperatures, the friction coefficient curves corresponding to test time for VN₃₀₀ films are shown in Fig. 5 and are mainly steady during the friction tests. After a very short running-in period, the friction coefficient slightly decreases and reaches a steady condition. In combination with the homogeneous dense microstructure and excellent mechanical properties, this could be reasonably attributed to the steady friction process of the VN₃₀₀ films. It might also be concluded that the films deposited by the PLD technique at 300 °C were greatly uniform.

Fig. 5 The friction vs. rotating time curves of the VN₃₀₀ film at different temperatures.

In order to explain the phenomenon that the friction coefficients decreased with increasing temperature, the films after tribological testing were characterized using GIXRD for identification of oxide phases. Take the VN₃₀₀ film in Fig. 6 as an example. It can be apparently seen that an FCC VN structure is stable from room temperature to 300 °C, and no significant oxidation can be observed until test temperatures rise to 500 °C. High quantities of new peaks in the XRD pattern, indicating generation of various vanadium oxides on the film surface at 500 °C, can be attributed to be V₂O₅, VO₂, V₃O₇, V₆O₁₁ and V₆O₁₃. As the test temperatures rise to 700 and 900 °C, more vanadium oxides can be identified as V₃O₇, V₃O₅, V₄O₇, V₆O₁₁ and V₆O₁₃. Furthermore, since only a very weak signal of the VN peaks can be observed at higher temperatures, this suggests that a massive oxidation took place on the film surface. For many materials, there is a transformation temperature after which continuous oxidation occurs and an oxide layer is established, which provides good protection against wear.^2,23,36,37 For VN, oxidation starts to take place above the temperature of 400 °C, the formed vanadium oxides also undergo a reversible change in the crystal structure during the wear test at its transformation temperature,^38,39 leading to phase transformations such as V₂O₅ that dominate the XRD patterns of the oxidized VN films surface at 500 °C, but disappears at higher temperatures with the appearance of new V₄O₇ and V₃O₅ phases in the XRD patterns. On the basis of other reports,^20,21,23 the vanadium oxides can be assigned to the lubricious Magnéli oxide series V_nO_2n−1 and V_nO_3n−1 with n = 1, 2, 3, which exhibited easy crystallographic shear planes with reduced binding strength, and played a significant lubrication role in the contact area during high-temperature friction thus resulting in a decreased friction coefficient with increasing temperature.

Fig. 6 GIXRD patterns of the VN₃₀₀ film after tribological tests at different temperatures.

For further investigation of oxidation products, the wear track of VN₃₀₀ films after testing at different temperatures was analyzed by Raman spectroscopy, as shown in Fig. 7. Raman investigations indicate that the patterns obtained at room temperature and 300 °C show the standard curves of stoichiometric VN, which agree well with the reports in ref. 20 and 23. As the test temperature increases to 500 °C, significant peaks suggesting ongoing oxidation reactions during the tribological test (identified as V₂O₅, V₃O₇, V₆O₁₁ and V₆O₁₃) mainly agree with the abovementioned XRD results. In the case of the wear track produced at 700 and 900 °C, apart from vanadium oxides that the XRD investigations detected, a high number of new phases corresponding to AlVO₄ can be observed, which exist in the range between 900 and 1000 cm⁻¹. The generation of AlVO₄ is assumed to be an interaction between the formed V₂O₅ and the alumina ball counterpart during the high-temperature tribological tests.²⁰ Combining the lubrication action of vanadium oxides and the AlVO₄ phase are supposed to be responsible for the low friction coefficients for both types of films. Furthermore, the AlVO₄ peaks dominate in both Raman patterns but were not found by the XRD investigation. This may result from the fact that Raman spectroscopy investigated the tiny area of the wear track but XRD investigations were conducted on a larger area consisting of both wear track and film surface.

Fig. 7 Raman patterns of the wear track on the VN₃₀₀ film after tribological test at different temperatures.

4. Conclusions

The aim of this study was to investigate the microstructures, mechanical and tribological properties of VN films fabricated by the PLD technique. Results from these studies revealed that.

(1) The VN film deposited at 300 °C displayed a densified structure and possessed higher hardness and elastic modulus values than that of the film deposited at room temperature, which registered a looser columnar structure.

(2) The VN films exhibited decreased friction coefficients at high temperatures. The film deposited at 300 °C held lower friction coefficients over the investigated temperature range compared to the films deposited at room temperature due to its densified structure and excellent mechanical properties and registered the lowest friction coefficient of about 0.21 at 900 °C.

(3) The oxidation behavior of VN films at elevated temperatures formed a series of vanadium oxides that influenced the tribological properties. In combination with lubricious oxidation products (vanadium oxides) and a lubricious oxide layer, as well as liquid lubrication in the contact area, all these reasons could be responsible for the decreased friction coefficients at high temperatures.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51471181), National Defense Science and Technology Innovation Foundation of Chinese Academy of Sciences (Grant No. CXJJ-14-M39), the Young Science and Technology Foundation of Gansu Province (Grant No. 1506RJYA058), and the Project of Gansu Province Longyuan young Creative Talents (Grant No. 2015GS06462) in China for providing the financial support.

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