Background

Transition metal boride thin films are used for a wide variety of applications as they combine excellent mechanical properties with high thermal stability, oxidation, and corrosion resistance properties [1–3]. Directional covalent bonding of boron atoms and high electron concentrations introduced by transition metals are considered as two essential parameters for designing better mechanical properties materials [4, 5]. Considering the similar lattice constant with transition metal boride, NbB2 is endowed with excellent mechanical properties such as wear resistance and chemical inertness [6–8]. Compared to other boride, NbB2’s melting point is more than 3000 °C, appearing to be a promising high-temperature-resistant material. Due to these properties, NbB2 films have been considered as protective coatings for applications.

The incorporation of transition metal boride particulates improves the properties of ceramic matrix composites in terms of mechanical strength, abrasion, and wear of the composite of cutting tools in industrial applications [9–11]. Furthermore, cutting tools working in extreme conditions such as high temperature must possess thermostability and oxidation resistance. Different from boride, alumina is more applicable for its oxidation resistance, which becomes the new demand for cutting tools. The addition of borides to alumina is expected to result in a high mechanical strength and good oxidation resistance [12–14]. In our precious work, we found that the combination of Al2O3 and NbB2 can produce an improvement in films’ mechanical, thermal stability, and resistance properties [15]. However, the detailed influence of content and structure was not clear enough. Considering the hardness of Al2O3 monolithic film is much less than NbB2, we choose to control the content of Al2O3 to further research the Nb-B-Al-O nanocomposite films’ structure and properties.

In this paper, Nb-B-Al-O nanocomposite films were synthesized by multi-target magnetron co-sputtering NbB2 and Al2O3 targets. The power of Al2O3 were changed in order to explore their content’s effect on the microstructure and mechanical properties of Nb-B-Al-O films and to find an optimum composition that yield both high hardness and low friction coefficient. An important aim of this study is to investigate the change of Al2O3’s content distribution in the structure of nanocomposite films with XRD and TEM and correlate the mechanical properties to these structure changes. Finally, the effect on structure, orientation, mechanical, thermal stability, and oxidation resistance properties of nanocomposite films were discussed based on different power of Al2O3.

Methods

The nanocomposite Nb-B-Al-O films were synthesized on Si substrate by a computer controllable magnetron sputtering system. The NbB2 and Al2O3 (both 99.9 % purity) targets were respectively connected to DC-pulsed and RF source sputter guns, wh ich were fixed at both sides of the chamber. The base pressure of experiment was lower than 4.0 × 10−4 Pa. High purity argon (99.999 %) was used as the sputtering gas in order to deposit films. Si substrates were cleaned in an ultrasonic agitator in acetone and ethanol for 15 min and dried using compressed air before being placed into the chamber. Subsequently, the substrates were sputter-cleaned for 15 min at −600 V bias voltages and a pressure of 5.0 Pa in chamber prior to nanocomposite deposition. The deposition process was carried on at an Ar gas flow of 40 sccm and a work pressure of 0.5 Pa. We keep NbB2 target deposited at power of 120 W and different deposition powers of Al2O3 were changed from 40 to 160 W at room temperature with −100 V bias voltage. Furthermore, a little temperature test was performed to compare the thermostability in different content of Al2O3, and the detail process of substrate treatment was previously reported [15].

Scanning electron microscope (SEM, SU8010, Hitachi, Japan) was used to observe the fracture surface morphologies of the Nb-B-Al-O films. And, the surface topography was observed by a JEM-2100 electron microscope (TEM). Wide angle X-ray diffraction (XRD) was employed for the determination of the films’ structure and crystalline nature. An X-ray photoelectron spectrometer (XPS) was used to characterize the chemical composition and chemical bonds; the contaminated C1s (284.6 eV) was used as a reference for correcting charge shift. The XPS spectra were fitted by XPSPEAK software, and Shirley background was chosen for background calculation of the XPS spectra. The Ambios XP-2 surface profilometer was used to measure thickness and the residual stress of coatings, which was calculated by the Stoney equation [16]. Nano Indenter XP system measured hardness and modulus of the films. The properties were evaluated using the Oliver and Pharr’s analysis technique [17], this technique allowed to measure the contact stiffness as well as the load and displacement at any point on the loading curve. This system was also used to perform nanoscratch test.

Results and Discussion

Figure 1a shows the elemental composition (at.%) energy-dispersive X-ray spectroscopy (EDX) from XPS of the Nb-B-Al-O nanocomposite films prepared at 120 W sputtering power of NbB2 and different sputtering power of Al2O3. With the increasing sputtering power of Al2O3, its content decreases. The total content of NbB2 is much higher than Al2O3’s. It is considered keeping the influence of NbB2’s mechanical properties is less affected by Al2O3. This EDX result from XPS is in agreement with the EDX from SEM, which is not shown here. Then, we kept the deposition time that is 2 h and measured the height of different power of Al2O3 by XP-2 surface profilometer (see from Fig. 1b). While increasing power of Al2O3, the deposition rate of film is also changed as it increases first before it decreases again. Considering the change trend of thickness, the film’s rate is not just determined by content of Al2O3 but also other factors changed in the nanocomposite films.

Fig. 1
figure 1

Nb and B elemental composition (at.%) EDX of Nb-B-Al-O nanocomposite films at different power of Al2O3 (a) and height of Nb-B-Al-O nanocomposite films at different power of Al2O3 (b)

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Figure 2 shows the XRD diffractograms from the nanocomposite films with different powers of Al2O3. The amorphous character of the Al2O3 and NbB2 monolithic film is also indicated in XRD pattern, which is not shown here. It was observed that all the films’ diffraction peaks can be assigned to monocrystalline NbB2 with (100) preferred orientation. At higher content of Al2O3, the monocrystalline NbB2 shows only a broad feature centered at about 34°, which has a weak crystallization, and the film’s grain size is small. The high content (40 and 60 W power) of Al2O3 is similar to each other without any clear crystalline peak of nanocomposite film in Fig. 2. With the decreasing of the Al2O3’s content, the peak of NbB2 (100) is became shaper, the whole film showed better crystalline (seen from 80 and 100 W images). TEM results also confirmed similar. When the power of Al2O3 reaches 120 W, the intensity peak of NbB2 (100) is continuing to increase to its maximum. But continuing to increase the power of Al2O3 to 140 W, the (100) peak is a little decreased. Furthermore, when the Al2O3’s power is higher than 120 W, there appears (441) crystal plane of cubic Al2O3. And, 140-W power film has more clear (441) peak, which is close to the Si substrate, and the strong substrate peak is unsymmetrical. It can be explained by that when the content of Al2O3 is decreased to some constant, it appears to crystalline.

Fig. 2
figure 2

XRD patterns of Nb-B-Al-O nanocomposite at different power of Al2O3

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The base peak of Al2O3 can be assigned to cubic (NaCl structure) with a lattice parameter of 7.95 Å. When we compared with Si substrate, we can find a clear peak of c-Al2O3’s (441), which is the main peak of cubic-crystallization Al2O3. When increasing the power of Al2O3, the nanocomposite film appears weak crystalline, and this result is in agreement with TEM images in Fig. 3. The light part represents Al2O3 and the dark part represents NbB2. The whole nanocomposite films show a typical nanocomposite microstructure with crystalline NbB2 grains embedded in a matrix of an amorphous Al2O3 phase from the low magnification image. When the content of Al2O3 is decreased to some extent, we can find the light part in the interface of Al2O3 and NbB2 appears crystallization from high magnification image. We can conclude that the decreasing Al2O3 content in nanocomposite films changes its structure from amorphous to weak crystalline in the interfaces of particle NbB2 and Al2O3. The film’s rate of deposition increased in low power of Al2O3 because less Al2O3 content can promote the film’s growth. We measured the Al2O3 and NbB2 monolithic films’ deposition rate and found that the rate of NbB2 is much higher than Al2O3. Then, the appearance of Al2O3’s crystalline plane in high power makes the films densification in the interface and its rate decreased. Besides that, the bombarding energy caused by increasing power of Al2O3 can become another important factor of deposition rate changes. A proper bombarding energy of lower power is beneficial in promoting the deposition of NbB2 and Al2O3. However, the high bombarding energy in higher power of Al2O3 tends to cause radiation damage [18]. And, more energetic ions and high-energy electrons will bombard newly formed film. Re-sputtering process of the deposited films will lead to the decrement of film thickness [19]. In order to eliminate the effect of the film’s thickness, we keep the thickness of nanocomposite Nb-B-Al-O films in different power of Al2O3 at 500–600 nm to explore its properties.

Fig. 3
figure 3

XRD patterns of Nb-B-Al-O nanocomposite film at 140 W power of Al2O3 (a), TEM images taken at low (b) and high (c) magnification of 140 W power of Al2O3

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Figure 4a shows the variations of hardness and elastic modulus as different power of Al2O3 in the 120 W power of NbB2, together with hardness and elastic modulus of NbB2 and Al2O3 monolithic films. With increasing the power of Al2O3 to 140 W, the hardness of the nanocomposite films increases first before it decreases again. The elastic modulus of the samples also followed the similar trend as that of hardness. In order to make sure the accuracy results of hardness, we also measured the 110 and 130 W power of Al2O3; the trend of hardness change is still remaining stability. The maximum hardness and elastic modulus are up to 21.60 and 332.78 GPa, which are higher than the NbB2 and Al2O3 monolithic films and keep constant in 110 to 130 W power of Al2O3. Figure 4b shows the critical fracture load of Nb-B-Al-O films with power of Al2O3. We can, through the critical fracture load Lc, characterize the adhesion strength of the film or the film’s fracture resistance. Further, other factors such as inherent internal stress, hardness, and plastic recovery can also influence the film’s fracture resistance. Just like the trend of hardness, the Lc of nanocomposite films is increased with the increasing power of Al2O3 and then remains constant.

Fig. 4
figure 4

Hardness and elastic modulus (a), critical load (b) of the Nb-B-Al-O nanocomposite films at different power of Al2O3

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From earlier research [20, 21], nanocomposite films also show hardness enhancement compared to monolithic films. Our results showed that the interfaces in Al2O3 and NbB2 play a leading role to its mechanical properties. When increasing the power of Al2O3, the content of Al2O3 is decreased because of higher bombarding energy’s radiation damage. The decreasing Al2O3 makes the distance of NbB2 grains close to each other, and the size of crystallization NbB2 grains are increased (see from Fig. 5). The films showed a typical nanocomposite microstructure with crystalline NbB2 grains embedded in a matrix of an amorphous Al2O3 phase in higher content of Al2O3. Continuing increasing the power of Al2O3, there appear crystalline of cubic-Al2O3 between the close distances of different crystalline NbB2 grains in some area. The appearance of cubic-crystallization Al2O3 can help to raise the nanocomposite films’ mechanical properties to some extent. The interfaces of NbB2 and Al2O3 act as barriers to the motion of dislocations glide in nanocomposite film when explain the mechanisms of promoted hardness [22]. Furthermore, the dislocation blocking due to coherency strains for different nanocrystalline grains also makes a contribution to hardness enhancement [23].

Fig. 5
figure 5

TEM images taken at high magnification of 60 W (a), 140 W (b) power of Al2O3, and its model representation (c, d)

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The compressive stress of the nanocomposite films determined by XP-2 profiler is influenced by the power of Al2O3 as is shown in Fig. 6. Residual stress is generated during the coating growth process. High residual stress (σ) is the main reason for film delamination and plastic deformation. Therefore, the reduced residual stress in films is a key factor for these films to explore more applications. All the films’ residual stress is less than 1 GPa. It is due to introduction of Al2O3 into crystalline phase NbB2 that helps to relax the stress. The higher content of Al2O3 has composed by amorphous Al2O3 and small size of crystalline NbB2 grains. The momentum transferred in electron collisions is less efficient to induce mass transport, as occurs the radiation enhanced diffusion by electrons is not enough to induce film densification, as can be verified by TEM results in Fig. 5, suggesting that this less dense structure permits the relaxation of the film with lower residual stress [24]. The change of structure in the lower content of Al2O3 makes the films’ residual stress increased because of the crystallization c-Al2O3 in the interface of large size crystalline NbB2 grains.

Fig. 6
figure 6

Residual stresses of Nb-B-Al-O nanocomposite films at different power of Al2O3

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From the analysis of different power of Al2O3’s structure, we choose two typical contents of Al2O3 (60 and 120 W) as example to explore its oxidation resistance properties. XPS O1s spectra from different power of Al2O3 are presented in Fig. 7. The spectrum for low power of Al2O3 in Fig. 7a shows three features: the strongest peak at 531.7 eV assigned to O-Al bond and other peak at 530.7 and 533.0 eV assigned O-Nb and O-B chemistry bonds, respectively. The exits of O-Nb and O-B chemical bonds are due to the combination of NbB2 and Al2O3. Besides that, the film’s oxidation in air condition can also lead to this chemistry bonding. Since the content of Al2O3 is pretty small in the nanocomposite films (from Fig. 1), the chemistry bonding of O-Nb and O-B is also too little to find from XRD results. As the content of Al2O3 is decreased in the nanocomposite films, a slight shift is seen in the O1s spectra, from about 531.2 eV for the low power of Al2O3 to about 531.7 eV for the high power of Al2O3(b). And, there only exits O-Al chemical bond, which means Al only exists in the form of Al2O3. This shift can be explained that by decreasing the content of Al2O3, it can change the oxidation resistance properties of nanocomposite film. When the content of Al2O3 is higher, the film shows a nanocomposite microstructure with amorphous Al2O3 embedded in weak NbB2 nanocrystalline phase. The weak crystallization of NbB2 means the B atoms and Nb atoms are easy to form chemistry bonds with O atoms. With the decreasing of Al2O3, the NbB2 shows good crystalline with large size and there appears to cubic-Al2O3 crystallization, so the O atoms are only formed by the Al-O bond. This is in agreement with the results from XRD and TEM.

Fig. 7
figure 7

Nb-B-Al-O Nanocomposite films’ high-resolution XPS spectra of O1s in 60 W power (a) and 120 W power (b) of Al2O3

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Figure 8 shows the nanocomposite films’ XPS whole energy spectra of different power of Al2O3. The atomic concentration specific value of B and Nb is basically remaining stable. On the contrary, as the power of Al2O3 rises to 120 W, the atomic concentration specific value of Al and O is increased from 10.56 % in 40 W to 69.62 % in 120 W, and the main existing form in 120 W is Al-O bond. This can also be explained by the change of crystalline in NbB2 and Al2O3 phase and is confirmed with the results showed in XPS O1s spectra. A general observation is that decreasing the content of Al2O3 can enhance the films’ oxidation resistance properties.

Fig. 8
figure 8

Nb-B-Al-O nanocomposite films’ XPS whole energy spectra of different power of Al2O3

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In order to explore the films’ thermostability properties, we test the hardness from room temperature to 300 °C with different power of Al2O3. With the increasing of temperature, the change trend of hardness is just the same. So we choose 100 °C to make a comparison. From Fig. 9, we can see that the influence of Al2O3’s content is pretty clear on the hardness of nanocomposite films at room temperature. But when the temperature is increased, the hardness change variation of different Al2O3 contents is reduced. The less content of Al2O3 can make this film remains the good mechanical properties and also a good thermostability.

Fig. 9
figure 9

Hardness and elastic modulus with room temperature and 100 °C deposition temperature of Nb-B-Al-O nanocomposite films at different power of Al2O3

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Conclusions

Nb-B-Al-O nanocomposite films were deposited on Si substrate by magnetron sputtering. The effect of Al2O3’s content on structure and properties were investigated. Decreasing the content of Al2O3 through increasing the power can appear cubic-crystallization of Al2O3 between large sizes of NbB2 grains. This structure change can enhance the mechanical properties and oxidation resistance properties of nanocomposite films and keep thermal stability. The maximum hardness and elastic modulus were up to 21.60 and 332.78 GPa at higher power of Al2O3. The change of interface structure between Al2O3 and NbB2 and theory of bombarding energy plays an important role in its properties. Because of crystallization of Al2O3’s less content, the mechanical properties can keep better oxidation resistance and stability at high temperature. Our results showed that the combined aluminum oxide and NbB2 can produce a positive effect on properties. Nb-B-Al-O films appear to be a promising nanocomposite system suitable for engineering applications.