Self-organized columnar Zr 0.7 Ta 0.3 B 1.5 core/shell-nanostructure thin ﬁ lms

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While most hard ceramics are brittle [59], we have demonstrated that Zr 1−x Ta x B y thin films with x ≥ 0.2 are not only hard, but also tough [60].The alloys were grown in pure Ar by hybrid high-power impulse and dc magnetron (HiPIMS and DCMS) co-sputtering [61][62][63] in which a compound ZrB 2 target was continuously sputtered by DCMS, while a Ta target was operated in HiPIMS mode.A negative substrate bias was applied in synchronous with the metal-ion-rich portion of each HiPIMS pulse; at all other times, the substrate was electrically floating.Layers grown by pure DCMS were found to be overstoichiometric with composition ZrB 2.4 .The B-to-metal ratio y = B/(Zr + Ta) in Zr 1−x Ta x B y alloy films decreased continuously, while the Ta-cation ratio x = Ta/(Zr + Ta) increased, with increasing power applied to the HiPIMS Ta target.
A combination of x-ray diffraction, transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDX), electron energy-loss spectroscopy, and atom-probe tomography (APT) revealed that all films have the hexagonal AlB 2 crystal structure with a columnar nanostructure, in which the column boundaries of layers with x < 0.2 are B-rich, whereas those with x ≥ 0.  to 42 GPa, with a simultaneous increase of ~30% in nanoindentation toughness, from 4.0 to 5.2 MPa√m, compared to ZrB 2.4 [60].Here, we focus on the Zr 0.7 Ta 0.3 B 1.5 layers, which have both high hardness as well as the highest toughness, to determine their detailed atomic-scale nanostructure.

Experimental
The films, 1.6-μm thick, are deposited on 1.5 × 1.5 cm 2 Si(001) substrates at 475 °C in a CC800/9 CemeCon AG sputtering system with a base pressure of 3.8 × 10 −6 Torr (0.5 mPa) using hybrid Ta-HiPIMS/ ZrB 2 -DCMS co-sputtering.The ZrB 2 DCMS target is continuously sputtered at 5 kW, while the Ta magnetron is operated in HiPIMS mode, with 50-μs HiPIMS pulses, at an average power of 1800 W and a frequency of 300 Hz to provide a pulsed source of energetic Ta ions [60].A 100-V negative bias is applied to the substrate in synchronous with the metal-ion-rich portion of each HiPIMS pulse, as determined by timeresolved mass spectroscopy analyses at the substrate position [64,65], starting from t = 30 μs following pulse initiation (t = 0) to t = 130 μs.At all other times, the substrate is at a negative floating potential of 10 V.The target-to-substrate separation is 20 cm, yielding a deposition rate of ~10 Å/s (3.6 μm/h).Time-and energy-resolved mass spectroscopy measurements show that the Ta 2+ /(Ta + + Ta 2+ ) ratio incident at the substrate during each 100-μs synchronized substrate bias pulse is 0.052; additional details are given in reference 60.
Average film compositions are determined by time-of-flight elasticrecoil detection analyses in a tandem accelerator with a 36 MeV 127 I 8+ probe beam incident at 67.5°with respect to the sample surface normal; recoils are detected at 45°.High-resolution plan-view TEM analyses are carried out in a monochromated and aberration-corrected FEI Titan 3 60-300 electron microscope operated at 300 kV; high-angle annular dark-field (HAADF) images are acquired in scanning TEM (STEM) mode, using a 145-mm camera length, with the inner and outer acceptance HAADF-detector angles ranging from 56 to 200 mrad.EDX elemental maps are obtained using the SuperX and GIF Quantum ERS spectrometers embedded in the FEI instrument.Plan-view TEM specimens are prepared by mechanically polishing the samples from the substrate side, followed by Ar + ion milling at 5 keV, with a 6°incidence angle, during sample rotation in a Gatan precision ion miller.For the final stages of sample thinning, the ion energy is reduced to 2.5 keV.A change in contrast between the core and shell regions is also observed in the plan-view mass-sensitive (HAADF-STEM) image, Fig. 1(c), where the brighter shells correspond to enrichment in high-Z Ta.In addition, the enhanced brightness observed for some columns in Fig. 1(c), exemplified by an arrow, originates from well aligned columns that provide conditions for the electron channeling of the convergent beam.The contrast differences between cores and shells in all three images are attributed to differences in composition and/or structure.

Results and discussion
High-resolution plan-view HAADF-STEM images of Zr 0.7 Ta 0.3 B 1.5 films acquired along the [0001] zone axis, as well as off axis, are shown in Fig. 2. The plan-view zone-axis HAADF-STEM image, Fig. 2(a), for which the electron-channeling conditions are optimized, reveals that contrast within individual diboride crystalline cores is relatively homogeneous, while the shell regions are darker.We attribute the distinct contrast change to the presence of a disordered, primarily metal, shell phase surrounding each crystalline core.The slightly tilted plan-view image in Fig. 2(b), where the electron-channeling effects are strongly reduced and Z-contrast is dominant, reveals brighter shell regions.This indicates, in agreement with the above results, that the shell regions have a larger concentration of higher-mass Ta (m Ta = 180.9amu vs. m Zr = 91.2amu, and m B = 10.8 amu), which segregates toward boundaries with corresponding Zr-rich central core region.
A high-resolution plan-view HAADF-STEM image, Fig. 2(c), obtained along the [0001] zone axis, shows that the core regions are crystalline with a hexagonal structure, while the Ta-rich shells are disordered and similar to metallic glasses [66][67][68].This is shown more clearly in Fig. 2(d), a higher-resolution plan-view off-zone-axis image acquired from the same sample region as Fig. 2(b).Since the electronchanneling condition is minimized in this image, the variation in contrast is due to brighter Ta-rich compositional modulations.The average width, estimated from the zone-axis images, of the crystalline core regions is 80 ± 15 Å, while the disordered shell regions are 15 ± 8 Å wide.
The Ta-rich shell regions appear significantly wider in the off-zoneaxis images, Fig. 2(b) and (d), than in the zone-axis images, Fig. 2(a)  and (c).This is due to the projection of the tilted columnar structure with resulting overlay contrast from cores and shells.
Fig. 3 shows the high-resolution cross-sectional HAADF-STEM image of Zr 0.7 Ta 0.3 B 1.5 , which exhibits the shell structure between two adjacent cores, and confirms that the cores are crystalline, while the shells are disordered regions.In addition, it clearly reveals the collapse of diboride layered structure in the cores into a disordered shell.
In order to probe Zr and Ta elemental distributions in the Zr 0.7 Ta 0.3 B 1.5 core/shell nanostructures, plan-view EDX maps are acquired from the same sample region as the off-zone-axis image in Fig. 2(b).The Zr EDX map, shown in red in Fig. 4(a), reveals a relatively uniform distribution of Zr.The contrast variations in the Ta EDX map, Fig. 4(b), are locally much more pronounced.This is seen more clearly in the combined Zr and Ta EDX map in Fig. 4(c), which reveals that the amount of Ta in the disordered shell boundaries is higher than in the crystalline cores, consistent with previous APT results [60].
A schematic plan-view illustration of Zr 0.7 Ta 0.3 B 1.5 films grown using the hybrid Ta-HiPIMS/ZrB 2 -DCMS co-sputtering is shown in Fig. 5. ZrB 2 forms a solid solution with TaB 2 , both of which have the AlB 2 hexagonal crystal structure [24,69], in the central core regions.The ZrB 2 /TaB 2 lattice mismatch along the a direction is 2.2% (a ZrB2 = 3.169 Å and a TaB2 = 3.098 Å [70,71]), while the mismatch along the c direction is much larger, 8.6% (c ZrB2 = 3.530 Å and c TaB2 = 3.227 Å [70,71]), which provides lattice buckling and a driving force for phase separation during film deposition where adatom diffusivity is active and atomic layers can partly relax upwards (bulk diffusion quenched).The TM diborides are line compounds [23,56], for which TaB 2 has a much lower formation enthalpy (−2.16 eV/atom) [72,73] than ZrB 2 (−3.35 eV/atom) [72,73].Thus, the overall understoichiometry of the present Zr 0.7 Ta 0.3 B 1.5 alloys results in Ta segregation, during film growth, toward the shell regions in order to maintain the central core regions stoichiometric.This gives rise to a change in the cation fraction from Zr-rich central core regions to higher Ta concentrations in the shells, consistent with density-functional-theory calculations of Dahlqvist et al. [74] showing that TaB 2 is more tolerant than ZrB 2 to the formation of B vacancies.The understoichiometric Ta-rich/B-deficient shells are disordered due to the collapse of the hexagonal planes, confirmed in Fig. 3, as the B vacancy concentration becomes too high to sustain the AlB 2 structure in which TM atoms reside above hexagonal B interstices.
There is an increase of ~30% in the metal-atom volume density due to a transition from AlB 2 structure, in which the metal atoms are held in atop positions with respect to the neighboring metal atomic layer by B atoms arranged in hexagonal rings, to a disordered structure in which the metal atoms are more closely packed.The disordered shell has the structural characteristics of metallic-glass thin films, which have been shown to exhibit both high strength and toughness [66][67][68].

Conclusions
Zr 0.7 Ta 0.3 B 1.5 alloy films, grown at 475 °C by hybrid high-power impulse (HiPIMS) and dc magnetron (DCMS) co-sputtering, in which a ZrB 2 target is continuously sputtered by DCMS and a Ta target is operated in HiPIMS mode, have a highly oriented [0001] fiber texture.They are both hard and ductile as reported in reference 60.Here, a combination of high-resolution TEM, HAADF-STEM, and EDX analyses is used to determine the atomic-scale nanostructure of Zr 0.7 Ta 0.3 B 1.5 alloys, which is responsible for their excellent mechanical properties.The columns, with average diameters of 95 ± 17 Å, are core/shell nanostructures in which the 80 ± 15-Å cores are crystalline Zr-rich Bstoichiometric Zr 1−x Ta x B 2 .The shell structure between adjacent cores is a narrow dense, disordered region which is Ta-rich and highly Bdeficient.The cores are formed under intense ion mixing via preferential Ta segregation, due to the lower formation enthalpy of TaB 2 than ZrB 2 (which are both line compounds), in response to the chemical driving force to form a stoichiometric compound.Such a self-organized core/shell nanostructure combines the benefits of crystalline diboride nanocolumns, providing the high hardness, with the dense metallicglass-like shells, which gives rise to enhanced toughness.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to

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Typical plan-view bright-field (BF) TEM, dark-field (DF) TEM, and HAADF-STEM images of Zr 0.7 Ta 0.3 B 1.5 films are shown in Fig. 1.The inset in Fig. 1(a) presents a cross-sectional BF-TEM image, with the corresponding selected-area electron diffraction (SAED) pattern, showing that the films are highly oriented with a [0001] fiber texture and consist of nanocolumns which extend in the growth direction.The plan-view images are acquired normal to the [0001] fiber texture axis.The plan-view BF-TEM image, Fig. 1(a), reveals that the nanocolumns have an average size of 95 ± 17 Å, with a contrast difference between lighter column cores and darker shell regions.The plan-view DF-TEM image in Fig. 1(b) exhibits dark cores surrounded by bright shells.Individual columns that are perfectly aligned with their [0001] zone axis, normal to the beam, appear dark in the BF-TEM image, while they are bright in the DF-TEM image, as indicated by arrows in Fig. 1(a) and (b).In both images, the difference in intensities between columns originates from differences in beam scattering and absorption conditions between individual columns.The plan-view SAED pattern in the Fig. 1(b) inset is characterized by a dominant [1010] reflection, which is in agreement with the strong [0001] fiber texture of the film in the growth direction.

Fig. 1 .
Fig. 1.Plan-view (a) bright-field TEM image, with cross-sectional bright-field TEM image and the corresponding SAED pattern shown in the inset; (b) dark-field TEM image together with the corresponding SAED pattern; and (c) HAADF-STEM image of Zr 0.7 Ta 0.3 B 1.5 films.

Fig. 5 .
Fig. 5. Schematic plan-view illustration of TM cation distributions in Zr 0.7 Ta 0.3 B 1.5 films with self-organized crystalline Zr-rich ZrTaB 2 cores surrounded by disordered Ta-rich/B-deficient shells.Zr atoms are shown in red, while Ta atoms are green (B atoms are not shown).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)