Influence of Content of Al2O3 on Structure and Properties of Nanocomposite Nb-B-Al-O films.

Nb-B-Al-O nanocomposite films with different power of Al2O3 were successfully deposited on the Si substrate via multi-target magnetron co-sputtering method. The influences of Al2O3's content on structure and properties of obtained nanocomposite films through controlling Al2O3's power were investigated. Increasing the power of Al2O3 can influence the bombarding energy and cause the momentum transfer of NbB2. This can lead to the decreasing content of Al2O3. Furthermore, the whole films showed monocrystalline NbB2's (100) phase, and Al2O3 shaded from amorphous to weak cubic-crystalline when decreasing content of Al2O3. This structure and content changes were proof by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). When NbB2 grains were far from each other in lower power of Al2O3, the whole films showed a typical nanocomposite microstructure with crystalline NbB2 grains embedded in a matrix of an amorphous Al2O3 phase. Continuing increasing the power of Al2O3, the less content of Al2O3 tended to cause crystalline of cubic-Al2O3 between the close distances of different crystalline NbB2 grains. The appearance of cubic-crystallization Al2O3 can help to raise the nanocomposite films' mechanical properties to some extent. The maximum hardness and elastic modulus were up to 21.60 and 332.78 GPa, which were higher than the NbB2 and amorphous Al2O3 monolithic films. Furthermore, this structure change made the chemistry bond of O atom change from the existence of O-Nb, O-B, and O-Al bonds to single O-Al bond and increased the specific value of Al and O. It also influenced the hardness in higher temperature, which made the hardness variation of different Al2O3 content reduced. These results revealed that it can enhance the films' oxidation resistance properties and keep the mechanical properties at high temperature. The study highlighted the importance of controlling the Al2O3's content to prepare well-defined films with high mechanical properties and thermal stability.


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][2][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, NbB 2 is endowed with excellent mechanical properties such as wear resistance and chemical inertness [6][7][8]. Compared to other boride, NbB 2 's melting point is more than 3000°C, appearing to be a promising hightemperature-resistant material. Due to these properties, NbB 2 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][10][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][13][14]. In our precious work, we found that the combination of Al 2 O 3 and NbB 2 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 Al 2 O 3 monolithic film is much less than NbB 2 , we choose to control the content of Al 2 O 3 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 NbB 2 and Al 2 O 3 targets. The power of Al 2 O 3 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 Al 2 O 3 '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 Al 2 O 3 .

Methods
The nanocomposite Nb-B-Al-O films were synthesized on Si substrate by a computer controllable magnetron sputtering system. The NbB 2 and Al 2 O 3 (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 NbB 2 target deposited at power of 120 W and different deposition powers of Al 2 O 3 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 Al 2 O 3 , 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 NbB 2 and different sputtering power of Al 2 O 3 . With the increasing sputtering power of Al 2 O 3 , its content decreases. The total content of NbB 2 is much higher than Al 2 O 3 's. It is considered keeping the influence of NbB 2 's mechanical properties is less affected by Al 2 O 3 . 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 Al 2 O 3 by XP-2 surface profilometer (see from Fig. 1b). While increasing power of Al 2 O 3 , 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 Al 2 O 3 but also other factors changed in the nanocomposite films. Figure 2 shows the XRD diffractograms from the nanocomposite films with different powers of Al 2 O 3 . The amorphous character of the Al 2 O 3 and NbB 2 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 NbB 2 with (100) preferred orientation. At higher content of Al 2 O 3 , the monocrystalline NbB 2 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 Al 2 O 3 is similar to each other without any clear crystalline peak of nanocomposite film in Fig. 2. With the decreasing of the Al 2 O 3 's content, the peak of NbB 2 (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 Al 2 O 3 reaches 120 W, the intensity peak of NbB 2 (100) is continuing to increase to its maximum. But continuing to increase the power of Al 2 O 3 to 140 W, the (100) peak is a little decreased. Furthermore, when the Al 2 O 3 's power is higher than 120 W, there appears (441) crystal plane of cubic Al 2 O 3 . 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 Al 2 O 3 is decreased to some constant, it appears to crystalline. The base peak of Al 2 O 3 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-Al 2 O 3 's (441), which is the main peak of cubic-crystallization Al 2 O 3 . When increasing the power of Al 2 O 3 , the nanocomposite film appears weak crystalline, and this result is in agreement with TEM images in Fig. 3. The light part represents Al 2 O 3 and the dark part represents NbB 2 . The whole nanocomposite films show a typical nanocomposite microstructure with crystalline NbB 2 grains embedded in a matrix of an amorphous Al 2 O 3 phase from the low magnification image. When the content of Al 2 O 3 is decreased to some extent, we can find the light part in the interface of Al 2 O 3 and NbB 2 appears crystallization from high magnification image. We can conclude that the decreasing Al 2 O 3 content in nanocomposite films changes its structure from amorphous to weak crystalline in the interfaces of particle NbB 2 and Al 2 O 3 . The film's rate of deposition increased in low power of Al 2 O 3 because less Al 2 O 3 content can promote the film's growth. We measured the Al 2 O 3 and NbB 2 monolithic films' deposition rate and found that the rate of NbB 2 is much higher than Al 2 O 3 . Then, the appearance of Al 2 O 3 '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 Al 2 O 3 can become another important factor of deposition rate changes. A proper bombarding energy of lower power is beneficial in promoting the deposition of NbB 2 and Al 2 O 3 . However, the high bombarding energy in higher power of Al 2 O 3 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 Al 2 O 3 at 500-600 nm to explore its properties. Figure 4a shows the variations of hardness and elastic modulus as different power of Al 2 O 3 in the 120 W power of NbB 2 , together with hardness and elastic modulus of NbB 2 and Al 2 O 3 monolithic films. With increasing the power of Al 2 O 3 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 Al 2 O 3 ; 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 NbB 2 and Al 2 O 3 monolithic films and keep constant in 110 to 130 W power of Al 2 O 3 . Figure 4b shows the critical fracture load of Nb-B-Al-O films with power of Al 2 O 3 . 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 Al 2 O 3 and then remains constant.
From earlier research [20,21], nanocomposite films also show hardness enhancement compared to monolithic films. Our results showed that the interfaces in Al 2 O 3 and NbB 2 play a leading role to its mechanical properties. When increasing the power of Al 2 O 3 , the content of Al 2 O 3 is decreased because of higher bombarding energy's radiation damage. The decreasing Al 2 O 3 makes the distance of NbB 2 grains close to each other, and the size of crystallization NbB 2 grains are increased (see from Fig. 5). The films showed a typical nanocomposite microstructure with crystalline NbB 2 grains embedded in a matrix of an amorphous Al 2 O 3 phase in higher content of Al 2 O 3 . Continuing increasing the power of Al 2 O 3 , there appear crystalline of cubic-Al 2 O 3 between the close distances of different crystalline NbB 2 grains in some area. The appearance of cubiccrystallization Al 2 O 3 can help to raise the nanocomposite films' mechanical properties to some extent. The interfaces of NbB 2 and Al 2 O 3 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]. The compressive stress of the nanocomposite films determined by XP-2 profiler is influenced by the power of Al 2 O 3 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 Al 2 O 3 into crystalline phase NbB 2 that helps to relax the stress. The higher content of Al 2 O 3 has composed by amorphous Al 2 O 3 and small size of crystalline NbB 2 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 Al 2 O 3 makes the films' residual stress increased because of the crystallization c-Al 2 O 3 in the interface of large size crystalline NbB 2 grains.
From the analysis of different power of Al 2 O 3 's structure, we choose two typical contents of Al 2 O 3 (60 and 120 W) as example to explore its oxidation resistance properties. XPS O 1s spectra from different power of  Figure 8 shows the nanocomposite films' XPS whole energy spectra of different power of Al 2 O 3 . The atomic concentration specific value of B and Nb is basically remaining stable. On the contrary, as the power of  In order to explore the films' thermostability properties, we test the hardness from room temperature to 300°C with different power of Al 2 O 3 . 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 Al 2 O 3 '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 Al 2 O 3 contents is reduced. The less content of Al 2 O 3 can make this film remains the good mechanical properties and also a good thermostability.

Conclusions
Nb-B-Al-O nanocomposite films were deposited on Si substrate by magnetron sputtering. The effect of Al 2 O 3 's content on structure and properties were investigated. Decreasing the content of Al 2 O 3 through increasing the power can appear cubic-crystallization of Al 2 O 3 between large sizes of NbB 2 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 Al 2 O 3 . The change of interface structure between Al 2 O 3 and NbB 2 and theory of bombarding energy plays an important role in its properties. Because of crystallization of Al 2 O 3 '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 NbB 2 can produce a positive effect on properties. Nb-B-Al-O films appear to be a promising nanocomposite system suitable for engineering applications.