Effect of Nano-Y2O3 Content on Microstructure and Mechanical Properties of Fe18Cr Films Fabricated by RF Magnetron Sputtering

In this work, FeCr-based films with different Y2O3 contents were fabricated using radio frequency (RF) magnetron sputtering. The effects of Y2O3 content on their microstructure and mechanical properties were investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductive coupled plasma emission spectrometer (ICP) and a nanoindenter. It was found that the Y2O3-doped FeCr films exhibited a nanocomposite structure with nanosized Y2O3 particles uniformly distributed into a FeCr matrix. With the increase of Y2O3 content from 0 to 1.97 wt.%, the average grain size of the FeCr films decreased from 12.65 nm to 7.34 nm, demonstrating a grain refining effect of Y2O3. Furthermore, the hardness of the Y2O3-doped FeCr films showed an increasing trend with Y2O3 concentration, owing to the synergetic effect of dispersion strengthening and grain refinement strengthening. This work provides a beneficial guidance on the development and research of composite materials of nanocrystalline metal with a rare earth oxide dispersion phase.


Introduction
The discovery and application of nuclear energy are regarded as two of the most important scientific and technological achievements of the twentieth century. As a kind of clean and sustainable energy, nuclear energy can extensively replace conventional fossil energy [1,2]. So far, the studies on nuclear energy systems have made great progress, but material issue is still one of the key challenges that restrict the development of nuclear energy [3][4][5][6]. In particular, for future advanced nuclear reactors, such as fast reactors [7], thermonuclear fusion reactors (TFR) [8] and accelerator driven sub-critical systems (ADS) [9], due to their severely harsh service environment which includes a high temperature, strong irradiation and strong corrosion, materials with superior service performance and reliability are urgently needed.
FeCr-based steels have been widely considered as one of the most important structure materials for nuclear reactors because of their outstanding mechanical properties. However, in the long-term nuclear service process, FeCr-based structural alloys, such as 304 and 316 stainless steel, have to bear high-dose irradiation damages, which accordingly induce Nanomaterials 2021, 11, 1754 2 of 8 irradiation hardening [5], porosity evolution and swelling [10], high temperature helium embitterment (HTHE) [11], etc. Therefore, how to increase the irradiation resistance of FeCrbased alloys is still a key problem to be solved until now. Oxide dispersion-strengthened (ODS) steels are a great development in nuclear energy structure materials, owing to their excellent irradiation resistance property [12,13]. S. Ukai et al. found that nanoparticles could act as sinks for the trapping of helium atoms and point defects, thus retarding radiation-induced material degradation [14]. Moreover, FeCr alloys with the addition of oxide particles have been shown to have high tensile, creep and fatigue strength, have thermal stability through the promotion of radiation-induced defect recombination, trap He atoms and impede dislocation climb and glide [15]. In particular, the addition of a small amount of Y 2 O 3 nanoparticles (0.1~0.5 wt.%) in the alloy matrix can not only improve its mechanical properties, but also annihilate the defects effectively caused by irradiation [16][17][18][19]. Moreover, the grain refinement of structural materials is also treated as one of the effective methods of optimizing the strength and irradiation resistance properties, owing to the high density of interface, especially in nanocrystalline materials, as reported in nanocrystalline Au (grain size~23 nm) [20] and TiNi alloys (grain size 23-31 nm) [21]. This is because the high-density interface in nanocrystalline materials can effectively absorb and annihilate the irradiation defects, which shows a better anti-irradiation performance than that of coarse grain materials. Meanwhile, an increase in the strength of nanostructured materials has been achieved over the last several decades based on extrapolations of the grain size dependence of mechanical properties of conventional materials [22]. The general enhancement in the materials' strength is dependent on the grain size, which is often described by the empirical Hall-Petch relationship [23].
Magnetron sputtering is known as one of the most useful methods of fabricating nanocrystalline materials [24][25][26][27], in which grain size, density and film thickness can be well controlled through adjusting the sputtering parameters, such as substrate temperature, sputtering power, sputtering time and sputtering pressure. It is known that thin films and bulks materials have different application fields. The thin films cannot be used as the construction materials of nuclear reactors. However, the FeCr-based thin films can be adopted as a platform to model the nanocrystalline metal materials with an extremely high volume fraction of grain boundaries, and to investigate the mechanism of Y 2 O 3 -induced strengthening. Therefore, in this work, the magnetron sputtering ODS-FeCr films were prepared and researched in order to provide a beneficial guidance for the research and development of fission reactor structural materials in the future.

Experimental
Composite targets composed of Fe 18 Cr-based steel holder and Y 2 O 3 slices were designed ( Figure 1a). By changing the number of Y 2 O 3 slices, the Y 2 O 3 contents could be adjusted in the deposited film. Si single crystal wafers with (111) orientation were used as substrates. Y 2 O 3 -doped FeCr films were fabricated by radio frequency (RF) magnetron sputtering technique. The rotating target was used in this deposition experiment to guarantee the homogeneous distribution of chemical elements in the films. There was an annular magnetic field around the target and the electrons moved in a cycloid-like manner on the surface of the target. Before sputtering, the vacuum chamber was evacuated to 8.0 × 10 −4 Pa by a molecular pump, and then the working argon (Ar) gas was filled into the chamber and the working pressure was kept at about 1.0 Pa. The sputtering time was 4 h, the sputtering power was controlled at 70 W and the substrate temperature was fixed at 200 • C. the as-deposited films were operated by a transmission electron microscopy (TEM, Tecnai TF20 TMP, Gatan Company USA) with an accelerating voltage of 200 kV. Cross-sectional TEM samples were prepared by a method combining the methods of mechanical thinning with Ar ion milling.
Microhardness tests were carried out on surface of the films by the nanoindentation (Nano-indenter G200, Agilent, USA) with a strain rate of 0.05 s −1 . The average hardness values were calculated from 15 separate indents with the depth range of 200-500 nm.

Results and Discussion
In order to investigate the effects of Y2O3 addition on the microstructure and mechanical properties of FeCr films, Y2O3/FeCr composite targets with 0, 1, 2 and 4 pieces of Y2O3 slices were used during sputtering (Figure1a), and the resultant films were designated as FeCr, Y2O3-FeCr, 2Y2O3-FeCr, 4Y2O3-FeCr, respectively, herein. The concentration of Y2O3 in the FeCr films was analyzed by the inductive coupled plasma emission spectrometer (ICP), as shown in Table 1. It can be seen that as the number of Y2O3 slices is 0, 1, 2 and 4, the Y2O3 content is 0, 0.12, 0.23 and 1.97 wt.%, respectively. The abnormal increase of Y contents from 0.23 to 1.97 as the Y2O3 slice number increase from 2 to 4 was difficult to understand in terms of the sputtering area of the Y2O3 slice, but was tentatively attributed to the change in glow discharge mode induced by the Y2O3. The crystalline structure of the films was characterized using a grazing incident X-ray diffractometer (GIXRD, PANalytical Company, Netherlands, Philips X'pert PRO, Cu Kα radiation, wavelength~0.15418 nm) with a step size of 0.06 • and grazing angle of 1.5 • . The surface and cross-section morphology of the films were observed by field emission scanning electron microscopy (FESEM, Sirion 200FEG, HITACHI, Japan, accelerating voltage 5 keV) with equipment of a secondary electron detector. Observations of Y 2 O 3 in the as-deposited films were operated by a transmission electron microscopy (TEM, Tecnai TF20 TMP, Gatan Company USA) with an accelerating voltage of 200 kV. Cross-sectional TEM samples were prepared by a method combining the methods of mechanical thinning with Ar ion milling.
Microhardness tests were carried out on surface of the films by the nanoindentation (Nano-indenter G200, Agilent, USA) with a strain rate of 0.05 s −1 . The average hardness values were calculated from 15 separate indents with the depth range of 200-500 nm.

Results and Discussion
In order to investigate the effects of Y 2 O 3 addition on the microstructure and mechanical properties of FeCr films, Y 2 O 3 /FeCr composite targets with 0, 1, 2 and 4 pieces of Y 2 O 3 slices were used during sputtering (Figure 1a), and the resultant films were designated as FeCr, Y 2 O 3 -FeCr, 2Y 2 O 3 -FeCr, 4Y 2 O 3 -FeCr, respectively, herein. The concentration of Y 2 O 3 in the FeCr films was analyzed by the inductive coupled plasma emission spectrometer (ICP), as shown in Table 1. It can be seen that as the number of Y 2 O 3 slices is 0, 1, 2 and 4, the Y 2 O 3 content is 0, 0.12, 0.23 and 1.97 wt.%, respectively. The abnormal increase of Y contents from 0.23 to 1.97 as the Y 2 O 3 slice number increase from 2 to 4 was difficult to understand in terms of the sputtering area of the Y 2 O 3 slice, but was tentatively attributed to the change in glow discharge mode induced by the Y 2 O 3 .   Table 1. The reason is that under the same sputtering conditions, the sputtering yield of Y 2 O 3 ceramic is lower than that of the FeCr metal in the composite target. Figure 2a shows the XRD patterns of FeCr-based films with four kinds of Y 2 O 3 contents. All films exhibit the typical cubic ferrite phase structure, but the Y 2 O 3 -related diffraction peaks were not observed owing to their extremely low concentrations below the detecting limit of the XRD instrument. Based on the Scherrer equation D = kλ/Bcosθ, where D is the coherent domain size, k is constant, B is the full width at half maximum (FWHW) and θ is the diffraction angle, the coherent domain size was roughly evaluated as about 7~12 nm (shown in Table 1) in terms of the FWHM of the (110) main diffraction peaks. It is worth pointing out that the coherent domain size D has a gradually decreasing tendency with an increase in Y 2 O 3 content, which implies the grain refinement effect of Y 2 O 3 . This demonstrates that magnetron sputtering is a powerful technique for the preparation of ODS-nanocrystalline metal thin films.
incorporated with 1, 2 and 4 pieces of Y2O3 slices, respectively.  Figure 1b,c show the surface and cross-section morphology of pure FeCr films and 2Y2O3-FeCr films, respectively. The large columnar crystal consisting of small columnar nanocrystals was observed in both the pure and 2Y2O3-FeCr films. For the 2Y2O3-FeCr film (Figure 1c), the number of grains per unit area was denser compared with that of the pure FeCr film (Figure 1b). In addition, the film thickness of the FeCr, Y2O3-FeCr, 2Y2O3-FeCr and 4Y2O3-FeCr films were 2.5, 1.5, 1.2, 1.0 μm, respectively, showing a decreasing trend with the increasing number of Y2O3 slice as shown in Table 1. The reason is that under the same sputtering conditions, the sputtering yield of Y2O3 ceramic is lower than that of the FeCr metal in the composite target. Figure 2a shows the XRD patterns of FeCr-based films with four kinds of Y2O3 contents. All films exhibit the typical cubic ferrite phase structure, but the Y2O3-related diffraction peaks were not observed owing to their extremely low concentrations below the detecting limit of the XRD instrument. Based on the Scherrer equation D = kλ/Bcosθ, where D is the coherent domain size, k is constant, B is the full width at half maximum (FWHW) and θ is the diffraction angle, the coherent domain size was roughly evaluated as about 7~12 nm (shown in Table 1) in terms of the FWHM of the (110) main diffraction peaks. It is worth pointing out that the coherent domain size D has a gradually decreasing tendency with an increase in Y2O3 content, which implies the grain refinement effect of Y2O3. This demonstrates that magnetron sputtering is a powerful technique for the preparation of ODS-nanocrystalline metal thin films.

Materials
To confirm the formation of the Y2O3, the XPS (X-ray photoelectron spectroscopy) analysis was conducted. As shown in Figure 2b, a specific electron binding energy was found to coincide with the energy value of Y3d5/2 electrons in molecular Y2O3. This result indicates that the Y elements exist in the form of Y2O3 in fabricated films, which is in line with the expectation of realizing oxide dispersion strengthening in FeCr based films by radio frequency magnetron sputtering (RFMS) methods.   To confirm the formation of the Y 2 O 3 , the XPS (X-ray photoelectron spectroscopy) analysis was conducted. As shown in Figure 2b, a specific electron binding energy was found to coincide with the energy value of Y 3d5/2 electrons in molecular Y 2 O 3 . This result indicates that the Y elements exist in the form of Y 2 O 3 in fabricated films, which is in line with the expectation of realizing oxide dispersion strengthening in FeCr based films by radio frequency magnetron sputtering (RFMS) methods.    A nanoindenter technique was used to explore the microhardness of these ODS-FeCr-based films. Figure 5a displays the hardness vs. displacement curves of the FeCrbased films with different Y2O3 contents. The indentation depth h was controlled in the   A nanoindenter technique was used to explore the microhardness of these ODS-FeCr-based films. Figure 5a displays the hardness vs. displacement curves of the FeCrbased films with different Y2O3 contents. The indentation depth h was controlled in the A nanoindenter technique was used to explore the microhardness of these ODS-FeCr-based films. Figure 5a displays the hardness vs. displacement curves of the FeCr-Nanomaterials 2021, 11, 1754 6 of 8 based films with different Y 2 O 3 contents. The indentation depth h was controlled in the range of 0 to 500 nm in order to exclude the influence of the Si substrate. The effective nanoindentation hardness of the ODS-FeCr-based films deposited under different Y 2 O 3 concentrations was calculated by averaging the relatively stable platform of hardnessdepth profiles located in 100-500 nm, as shown in Figure 5b. It can be seen that the film hardness distinctly increases with the increase of the Y 2 O 3 concentration, and the hardness value increases from 10.4 GPa to 16.5 GPa, exhibiting the obvious nano-oxide dispersion strengthened effect. Song et al. [27] raised an Orowan model demonstrating that the moving dislocation lines can be markedly hindered by the oxide particle. In the present Y 2 O 3 -doped FeCr films, the uniformly distributed nano Y 2 O 3 particles act as an obstacle for the dislocation motion during deformation, which is beneficial for improving the mechanical property. Moreover, grain refining is another way to increase the hardness of hard films. Wang et al. demonstrated that the hardness (H) of a metallic material increases with a decreasing grain size d, according to the Hall-Petch relation H = H 0 + kd −1/2 (k is constant) [28,29]. Grain refinement increased the number of grain boundaries, which improved the obstacle to material deformation and dispersed the stress concentration. Based on the analysis above, the extremely high hardness of Y 2 O 3 -doped FeCr films is attributed to the synergetic strengthen effect of dispersion strengthening and grain refinement strengthening.  [28,29]. Grain refinement increased the number of grain boundaries, which improved the obstacle to material deformation and dispersed the stress concentration.
Based on the analysis above, the extremely high hardness of Y2O3-doped FeCr films is attributed to the synergetic strengthen effect of dispersion strengthening and grain refinement strengthening.

Conclusions
Y2O3-dispersed FeCr films were grown through a RF magnetron sputtering technique and the effects of Y2O3 addition on their microstructures and hardness were thoroughly investigated. The main results can be concluded as follows: (1) The coherent domain size of ODS-FeCr-based film is about 7~12 nm, and it decreases with an increase in Y2O3 content. (2) The addition of Y2O3 can obviously enhance the hardness of FeCr-based films, resulting in an extremely high value of 16.5 GPa.
(3) The strengthening mechanism was from the nano-Y2O3 particle dispersion strengthening as well as the grain refinement strengthening.