Quaternary diborides—improving the oxidation resistance of TiB2 ± z coatings by disilicide alloying

To overcome the limited oxidation resistance of the emerging class of transition metal borides, we suggest within this study novel quaternary diborides, Ti-TM-Si-B2 ± z (TM = Ta, Mo), achieving the compromise between excellent oxidation resistance and requirements of hard coatings. Single-phase AlB2-type structured Ti-TM-Si-B2 ± z films (3–5 µm) are sputter-deposited from TiB2/TMSi2 targets. The Ti-Ta-Si-B2 ± z coatings exhibit 36 GPa in hardness, while maintaining strongly retarded oxidation kinetics till 1000°C. Ti-Mo-Si-B2 ± z coatings preserve a hardness up to 27 GPa, although outperforming all their counterparts by featuring outstanding oxidation resistance with 440 nm oxide scale thickness after 1 h at 1200°C. GRAPHICAL ABSTRACT IMPACT STATEMENT First report on quaternary Ti-TM-Si-B2 ± z coatings stabilized in hexagonal AlB2-prototype structures. These hard coating materials exhibit unprecedented oxidation resistance up to 1200°C due to the formation of Si-rich scales.

CONTACT Ahmed Bahr ahmed.bahr@tuwien.ac.atChristian Doppler Laboratory for Surface Engineering of high-performance Components, TU Wien, Vienna A-1060, Austria Supplemental data for this article can be accessed here.https://doi.org/10.1080/21663831.2023.2225554 The oxidation behavior for TiB 2 ± z bulk and thin film materials has been extensively studied [16,17,[19][20][21][22], and features specific morphological and kinetic-related aspects.According to Cai et al., monolithic bulk TiB 2 starts to oxidize at 400°C, whereas a rapid, anomalous oxidation sets in at around 500°C.The reported accelerated oxidation at 500°C is related to the formation of an outer mixed amorphous/crystalline B 2 O 3 /TiO 2 scale, and an inner unstable Ti-B-O layer [21].Between 650°C and 1000°C-also referred as the low-temperature regime-the scale formation changes to a laminated configuration with crystalline TiO 2 and a glassy amorphous B 2 O 3 providing a certain oxidation resistance [19][20][21][22].At higher temperatures, the accelerated kinetics predominates the oxidation process, forming volatile B 2 O 3 accompanied by non-protective porous TiO 2 scales [19,23,24].However, this oxidation sequence is reported to be different for TiB 2 ± z -based thin film materials.Huang et al. highlighted the formation of the B 2 O 3 (l) phase, evolving volatile at around 720°C, and hence results in a rapid oxidation of their chemical vapor deposited TiB 2 coating [22].In contrast, the oxidation of physical vapor deposited (PVD) TiB 2 ± z below 800°C in air did not exhibit the formation of B 2 O 3 (l), and the oxide scales have been reported to be Ti-rich [16][17][18].Thörnberg et al. indicated that the B concentration plays a prominent role on the oxidation kinetics of sputtered TiB 2 ± z thin films [16].Their sub-stoichiometric TiB 1.43 films exhibited lower oxidation rates compared to the Brich films, due to the absence of the rapidly oxidizing B-rich tissue phases.Nevertheless, all their investigated TiB 2 ± z films follow linear-rate laws with low oxidation resistance over 400°C [16].
Different routes have been attempted to enhance the oxidation resistance of TiB 2 ± z coatings mainly based on alloying with strong oxide formers such as Al [18,25,26] or Si [27,28]-which are prone to form protective oxide scales.Bakhit et al. reported an improvement in the oxidation resistance of sputtered Ti 0.68 Al 0.32 B 1. 35 with retarded kinetics at 800°C for 0.5 h, due to the formation of a dense Al-based oxide scale of 470 nm compared to a scale thickness of 1900nm obtained for their binary TiB 2.4 counterpart [18].Recently, Navidi et al. followed the Al-alloying strategy to deposit Al-rich but nearly stoichiometric (Ti 0.35 Al 0.65 )B 2 films revealing an outstanding oxidation resistance at 700°C by forming a thin Al-based oxide scale of only 39 ± 7 nm after 8 h [25].The high Al-content predominates the oxidation behavior, but on the expense of the mechanical properties with reported hardness between 9 and 24 GPa [25].The influence of Si-alloying on the oxidation resistance of several TMB 2 ± z based coatings was studied by Glechner et al. [27].For Ti-Si-B 2 ± z coatings, the Si addition provided high-temperature oxidation resistance with strongly retarded kinetics up to 1200°C, while the reported hardness drastically decreased upon high-Si content addition to a value around 16 GPa [27].Still, the above-mentioned alloying routes by Al and Si to form oxidation-resistant ternary diborides are limited by the deterioration of the coatings' mechanical properties.For bulk refractory diborides, a different strategy is followed to enhance the oxidation resistance.Through the addition of secondary Si-based phases (i.e.SiC or TMSi 2 ), which minorly influence the desired mechanical properties, highly protective glassy-like borosilicate scales can be formed [23,[29][30][31][32][33][34].A similar approach has been also applied for Zr-Mo-Si-B based-coatings to provide high-temperature oxidation resistance [35,36].Merging now these ideas to form quaternary, hexagonal structured diboride-based thin film materials by alloying TMSi 2 phases into binary TiB 2 ± z is a promising-yet relatively unexplored-strategy offering new possibilities to achieve the challenging compromise between good mechanical properties and highest oxidation resistance.

Materials and methods
All the ternary and quaternary Ti-TM-Si-B 2 ± z coating materials have been deposited in a laboratory-scale magnetron sputtering system using 3-inch sized target materials from Plansee Composite Materials GmbH.Each of the seven targets was solely DC-sputtered at a target current of 0.5 A in pure argon atmosphere (working pressure of 0.4 Pa).Additionally, a binary TiB 2.57 coating was deposited from a TiB 2 target at a pressure of 0.56 Pa.The coatings were grown onto sapphire and single-crystalline Si substrates (10 N 11-oriented, 10 × 10 × 0.53 mm 3 and 100-oriented, 20 × 7 × 0.38 mm 3 ) as well as poly-crystalline Al 2 O 3 (20 × 7 × 0.38 mm 3 ).The obtained film thicknesses of the quaternary Ti-TM-Si-B 2 ± z coatings were in the range between 3.2 and 4.9 μm.
The chemical composition of the coatings was determined by ion beam analysis techniques using Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA) and Rutherford Backscattering Spectrometry (RBS) at the 5 MV Pelletron Tandem accelerator laboratory at Uppsala University [37].For ToF-ERDA, 127 I 8+ projectiles with a primary energy of 36 MeV were employed with an incident angle of 67.5 o with respect to the surface normal and a recoil detection angle of 45 o .RBS was carried out using 3 MeV 4 He + ions and a detection angle of 170°.The analysis of the ToF-ERDA experimental data was performed using the Potku software [38], while the RBS data were analyzed using the SIMNRA software [39].The total systematic and statistical uncertainties were estimated to be 5-8% of the deduced value for the major constituents.
The oxidation behavior of the coatings was investigated using DTA/TG system (Netzsch STA 449 F1).The dynamic measurements up to 1400°C were done at a heating rate of 10°C/min, under flowing synthetic air (50 ml/min) and helium (20 ml/min).The employed samples for these measurements were the coated polycrystalline Al 2 O 3 .Further isothermal annealing measurements were done in ambient air using conventional furnace at 800°C and 1200°C.Moreover, the mechanical properties of the coatings were investigated using an ultra-micro indentation (UMIS) system equipped with Berkovich diamond tip.For each sample, 31 surface indents were done in a load-controlled mode with indentation loads varied between 3 and 45 mN and consequently evaluated based on the Oliver and Pharr method [40].The Poisson's ratios were taken from [41].
The structure of the as-deposited coatings was investigated by X-ray diffraction (XRD) in Bragg-Brentano configuration using a Panalytical Xpert Pro MPD system equipped with Cu-K α radiation source (λ = 1.54 Å).Furthermore, the morphology for selected oxidized samples was investigated using transmission electron microscopy (TEM FEI TECNAI F20) combined with a selected area electron diffraction analysis.Additionally, electron energy-loss spectroscopy (EELS) mappings were performed to determine the elemental chemical composition.

Results and discussion
In Table 1 the chemical compositions evaluated by Timeof-Flight Elastic Recoil Detection Analysis (ToF-ERDA) and Rutherford Backscattering Spectrometry (RBS) are summarized for all grown films.The binary TiB 2.57 coating shows boron super-stoichiometry, while the B/TM ratio tends to decrease in all the alloyed coatings by increasing the Si content.The lowest total metal content (Ti + TM) of 25 ± 1 at.% was evaluated for the ternary Ti-Si-B 2 ± z coatings, while the Ti-Ta-Si-B 2 ± z coatings exhibit nearly 34 ± 1 at.% metal content for both compositions, followed by 30 ± 1 at.% for Ti-Mo-Si-B 2 ± z coatings.The oxygen content in all grown films is below 2.6 at.%.
Figure 1 presents the X-ray diffractograms of the as-deposited Ti-TM-Si-B 2 ± z coatings in comparison with the binary TiB 2.57 .Only peaks corresponding to the hexagonal-TiB 2 phase (SG 191)-in addition to the Al 2 O 3 substrate-can be indexed.Apart from the amorphous Ti 0.20 Mo 0.11 Si 0.26 B 0.43 , all alloyed coatings exhibit a single-phased hexagonal structure with broad 001 peaks as the preferred orientation.The high Si content in Ti 0.20 Mo 0.11 Si 0.26 B 0.43 leads to the amorphous character with diminished peaks.In contrast, the higher Tacontaining Ti 0.28 Ta 0.07 Si 0.12 B 0.53 shows an increase in the predominant 001 peak intensity accompanied by a shift towards lower 2θ values, suggesting the dissolution of Ta in the hexagonal phase.Moreover, the calculated c/a ratios are in the range between 1.03 and 1.05.The quaternary systems show slightly lower c/a values compared to the binary TiB 2.57 (see Table 1), indicating the substitution of Ti by the dissolved Ta or Mo in the hexagonal lattice.
Moreover, the mechanical properties (surface hardness and Young's modulus) of the coatings are summarized in Table 1.The Ta-alloyed coatings maintained relatively high hardness values with an observed hardening effect by increasing the Ta-Si content from 32.8 to 36 GPa for Ti 0.31 Ta 0.04 Si 0.06 B 0.59 and Ti 0.28 Ta 0.07 Si 0.12 B 0.53 , respectively.The increase in hardness is related to solid solution hardening with Ta addition, which was also reported for other Ta-alloyed borides, i.e.ZrTaB 2 ± z [42] and WTaB 2 ± z [43].In contrast, the Ti-Si and Mo-Si alloying routes lead to decreased hardness with increasing the alloying content.Generally, the alloying of TiB 2 ± z with Si was emphasized to result in material softening [27,28].Grančič et al. reported hardness values between 14 and 24 GPa for their amorphous Ti-Si-B 2 ± z films [28].The here reported hardness exceeds those values, even at higher Si-contents.This difference is related to the formation of single-phase structured coatings and a predominant 001 orientation-being the preferred one for the anisotropic hardness of hexagonal diborides [8].
Figure 2 summarizes the mass change during dynamic oxidation of Ti-TM-Si-B 2 ± z coatings as a function of the annealing temperature up to 1400°C.The onset oxidation temperature for the un-alloyed TiB 2.57 is observed to be around 490°C.Above this temperature, the coating exhibits a mass increase with accelerated oxidation   MoSi 2 ) at around 700°C (also confirmed by XRD analysis, see in supplementary Figure S1), followed by preferential oxidation of Si to form protective Si-based oxide scales, which inhibit oxygen inward-diffusion.MoSi 2 is known as an effective oxidation resistant phase due to the capability to form protective SiO 2 scales, especially at high temperatures.In contrast, TaSi 2 is reported to exhibit a proper oxidation resistance only up to 800°C, due to competing Ta-based oxides [44,45].
To gain a more detailed understanding on the oxide scale formation process, the morphology of selected oxidized samples was investigated using TEM analysis.Figure 3 presents the cross-sectional TEM analysis for Ti 0.28 Ta 0.07 Si 0.12 B 0.53 after 1 h oxidation in ambient air at 800°C.The bright-field image shows an oxide scale of 535 nm on top of an unoxidized intact coating featuring columnar morphology (see Figure 3(a)).In more detail, the formed oxide scale is composed of two layers with a distinct interface: an outer dense glassy amorphous layer with a thickness of 140 nm, and an inner layer composed of mixed equiaxed and columnar crystallites (see Figure 3(b)).The inner scale exhibits a relatively dense morphology with small, globular crystallites near to the coating-oxide interface, while more columnar structures with larger grains predominate the upper interface.The corresponding EELS maps (see Figure 3(d) and the respective elements) clearly reveal that the outer oxide scale is Si-rich with small amounts of boron, while the inner crystalline scale mainly consists of Ti and Ta-based oxides (Ta is also confirmed by EDX, not shown) with no boron detected.The formation of an outer dense Sirich borosilicate scale is the key to the excellent oxidation resistance with retarded kinetics.Furthermore, the SAED pattern presented in Figure 3(c) reveals an initiation of phase separation processes between the TiB 2 -based matrix and TaSi 2 after annealing at 800°C. Figure 4 depicts the TEM analysis for the airannealed Ti 0.23 Mo 0.07 Si 0.16 B 0.54 after 1 h at 1200°C.The unoxidized coating exhibits globular morphology with clear indications for recrystallization processes as bulk diffusion was already activated at 1200°C-evidence for reaching about 0.4 of the melting temperature [46]-(see Figure 4(a)).Moreover, the BF-image clearly shows a dense oxide scale of 440 nm which is amorphous according to the SAED analysis (see Figures 4(b,c 1 )).This coating experienced a separation of the MoSi 2 phase as indicated in SAED image (Figure 4(c 2 )) after the annealing at 1200°C.The EELS maps in Figure 4(d) reveal that the oxide scale is based only on Si with no competing boron.However, boron-rich pockets can be observed at the coating-oxide interface due to the formation of MoB phase according to reaction (1).The formation of MoB was reported by Silvestroni et al. for ZrB 2 /MoSi 2 bulk system at 1200°C [30].Here, the seperated MoSi 2 phase beneficially acts as an active reservoir for selective oxidation of Si and the formation of the highly dense and protective Si-based scale, while concomitantly suppressing the detrimental volatile B 2 O 3 phase (see equation 1).
To present the best compromise between mechanical properties and oxidation resistance, we correlate in highlighted an outstanding oxidation resistance up to 1100°C in synthetic air for the ternary Ti 0.13 Si 0.41 B 0.46 , while the high Si content results also in reduced hardness of 16 GPa [27].The here described TiB 2 ± z coatings with alloyed TMSi 2 secondary phases exhibit delayed oxidation kinetics compared to their Al-alloyed counterparts, but a higher hardness with respect to the Sialloyed literature data.This data underlines the need for further alloying concepts, i.e. quaternary diborides.The Ti 0.28 Ta 0.07 Si 0.12 B 0.53 coating exhibits delayed oxidation kinetics obtaining a scale thickness of 550 nm after 1 h at 800°C, while featuring a relatively high hardness of 36 GPa in the as-deposited state.Moreover, the Ti-Mo-Si-B  S2).
binary and ternary counterparts.The Ti-Ta-Si-B 2 ± z coatings maintained high hardness up to 36 GPa due to solid solution hardening effect of Ta.In addition, the Ti 0.28 Ta 0.07 Si 0.12 B 0.53 coating exhibited strongly retarded oxidation kinetics at 800°C owing to the formation of an oxide scale with an outer protective glassy Si-rich borosilicate phase.Furthermore, the Ti-Mo-Si-B 2 ± z alloyed coatings preserved an outstanding oxidation resistance up to 1200°C, which is attributed to the selective oxidation of Si and the formation of highly stable and protective Si-based oxide scales, inhibiting oxygen inward diffusion, while suppressing the formation of the detrimental volatile B 2 O 3 phase.The alloying strategy by incorporating TMSi 2 phases into TiB 2 ± z provides a wide playground to stabilize single-phase quaternary Ti-TM-Si-B 2 ± z coatings, featuring remarkable oxidation resistance and good mechanical stability.Nevertheless, the B stoichiometry, as well as high-temperature phase separation processes, need to be considered for further improvements of these novel quaternary diborides.

Figure 2 .
Figure 2. Thermogravimetric (TG) curves of mass change during dynamic oxidation of (a) Ti-Si-B 2 ± z , (b) Ti-Ta-Si-B 2 ± z , and (c) Ti-Mo-Si-B 2 ± z coatings in synthetic air under heating rate of 10°C/min.The TG curve for the binary coating TiB 2.57 is indicated by a dashed line in (a), (b) and (c).

Figure 3 .
Figure 3. TEM analysis of Ti 0.28 Ta 0.07 Si 0.12 B 0.53 coating oxidized in ambient air at 800°C for 1 h.(a) BF image of the whole coating with the substrate at the bottom and oxide scale on top.(b) magnified area of the oxide scale with coating interface.(c) SAED image for the area indicated in (a).(d) STEM image and corresponding EELS maps for the area illustrated in (b).

Figure 4 .
Figure 4. TEM analysis of Ti 0.23 Mo 0.07 Si 0.16 B 0.54 coating oxidized in ambient air at 1200°C for 1 h.(a) BF image of the whole coating with the substrate at the bottom and oxide scale on top.(b) magnified area for oxide scale with coating interface.(c 1 ) and (c 2 ) SAED patterns for areas indicated in (a).(d) elemental EELS maps for the area illustrated in (b).

Figure 5
in an uncommon way the as-deposited hardness to the oxidation resistance for selected Ti-(TM)-Si-B 2 ± z coatings in relation to literature data.Please note, that the oxidation temperature and the given time refer to an oxidation treatment in air, where a stable and adherent scale was formed.The unalloyed TiB 2 ± z film exhibits high hardness of up to 43 GPa with the lowest onset oxidation temperature of 400°C as reported by Thörnberg et al.[16]-see red star in Figure5.Increasing the alloying content, in detail Al, the Ti 0.9 Al 0.1 B 1.3 provides an improvement in the oxidation resistance up to 600°C for 10 h, while maintaining a high hardness of 45 GPa[26].Navidi et al. achieved higher oxidation resistance for their stoichiometric Ti 0.35 Al 0.65 B 2.0 at 700°C, while the high Al-content leads to reduced hardness value of 19 GPa[25].In comparison, Bakhit et al. employed a lower Al-content to maintain high hardness of 39 GPa for Ti 0.68 Al 0.32 B 1.35 leading also to delayed oxidation kinetics at 800°C for 0.5 h[18].On the other hand, Grančič et al. reported at 800°C higher scale thickness of 1.2 μm for their alloyed Ti-Si-B 2 ± z with 20 at.% Si obtaining only a hardness of 14 GPa[28].Recently, Glechner et al.

Table 1 .
Chemical composition, crystallographic parameters (c/a ratio), and mechanical properties (H and E) for all grown Ti-(TM)-Si-B 2 ± z coating materials.
2 ± z alloyed coatings show moderate hardness up to 27 GPa, but outperforming all the reported coatings concerning the high-temperature oxidation resistance up to 1200°C due to the formation of protective Si-based scales.The Ti 0.20 Mo 0.11 Si 0.26 B 0.43 coating preserved superior oxidation resistance at 1200°C by obtaining thin Si-based scale of 335 nm after 10 h at 1200°C (see in supplementary Figure