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Article

First Approach to ZrB2 Thin Films Alloyed with Silver Prepared by Magnetron Co-Sputtering

by
Tomáš Fiantok
1,*,
Martin Truchlý
1,
Viktor Šroba
2,
Tomáš Roch
2,
Vitalii Izai
1,
Marek Vidiš
1,
Marián Haršáni
3,
Leonid Satrapinskyy
2 and
Marián Mikula
2,4
1
Detached Workplace of Faculty of Mathematics, Physics and Informatics, Comenius University in Bratislava, Sadová 1148, 038 53 Turany, Slovakia
2
Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University in Bratislava, Mlynská Dolina F2, 842 48 Bratislava, Slovakia
3
Staton, s.r.o., Sadová 1148/9, 038 53 Turany, Slovakia
4
Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Dúbravská Cesta 9, 845 11 Bratislava, Slovakia
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(3), 663; https://doi.org/10.3390/coatings13030663
Submission received: 20 February 2023 / Revised: 17 March 2023 / Accepted: 20 March 2023 / Published: 22 March 2023
(This article belongs to the Section Thin Films)
Browse Figures
Versions Notes

Abstract

:
Hexagonal ZrB2 belongs to the group of ultra-high temperature ceramics representing an important class of materials with the potential to meet the high demands of today’s industry. However, this potential is limited by inherent brittleness and poor tribological properties. Here, the combination of density functional theory and experiment is used to investigate the effect of silver alloying on the mechanical and tribological properties of hexagonal ZrB2 thin films. Calculations indicate strong insolubility of Ag atoms in the ZrB2 metal sublattice and a significant effect on the mechanical properties, pointing out an improvement in ductility and tribological properties but at the cost of reduced hardness. The experiments confirmed the theoretical predictions of the strong insolubility of silver, where the magnetron-sputtered Zr1−xAgxB2+Δ films form a segregated nanostructure consisting of separated hexagonal ZrB2 and cubic Ag phases. With increased Ag content, values of Young’s modulus decrease from EZrB2.31 = 375 GPa to EZr0.26Ag0.74B0.89 = 154 GPa, followed by a decrease in hardness from HZrB2.31 = 30 GPa to a value of HZr0.26Ag0.74B0.89 = 4 GPa. The suppression of crack formation is also shown with the material flow around cube corner indents, indicating enhanced ductility. The improvement of tribological properties was also confirmed when the coefficient of friction (COF) was reduced from COFZrB2.31 ~0.9 to a value of COFZr0.26Ag0.74B0.89 ~0.25 for all counterpart materials—steel (100Cr6), Si3N4, and WC/Co.

1. Introduction

Transition metal diborides (TMB2), compounds of boron, and transition metal (TM) from IV.—VI. groups (Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) [1] have received much attention in terms of their excellent properties such as extremely high melting temperature, chemical and thermal stability, high hardness, and high thermal and electrical conductivity. These exceptional characteristics predispose these materials to be used in extremely demanding space applications [2,3,4,5,6].
TMB2 thin films are most often synthesized by physical vapor deposition (PVD) techniques, most likely by magnetron sputtering in a non-reactive argon atmosphere. In this specific case, the deposition process is accompanied by two dominant effects: (i) Different angular distribution of light boron and heavy metal atoms [7]. (ii) The effect of resputtering of the growing film with reflected Ar neutrals. The two effects lead to significant changes in the stoichiometry, structure, and mechanical properties of the TMB2 films [8].
Overstoichiometry in TMB2 films is typical for light TM atoms (Ti, Zr). Such films consist of a nanocomposite structure composed of stoichiometric grains (nanofilaments) separated by an amorphous B-tissue phase [6,9]. On the other hand, the effect of resputtering, typical for heavy TM atoms (Hf, Ta, W, …), leads to the formation of understoichiometric films with grains containing vacancies in boron sublattice or the structure is completely amorphous [8].
The fundamental weakness of TMB2 films is their brittle nature (expressed by high values of Young’s modulus) [8,9]. In recent years, several approaches have been investigated to improve the ductility/toughness of ceramic thin films, e.g., structural modification of nanocomposites, vacancy engineering [8,10,11], strengthening through multilayer structures [12,13,14] or multicomponent alloying [9,15,16].
Recent research activities proved the positive influence of alloying on mechanical properties. Alloying of ZrB2 films with Ta is manifested by a hardness increase from 35 to 42 GPa and an increase in indentation toughness from 4 to 5.2 MPa.m−1/2 [16]. ZrB2 thin films alloyed with only 8 at.% of Al show a decrease in Young’s modulus of more than 20% compared with that of pure ZrB2.2. This indirectly indicates the improvement toward ductile behavior [9]. Similarly, Nedfors et al. [15] observed a decrease in Young’s modulus of approx. 18% from pure TiB2 after alloying of 8 at.% of Al.
Another weakness of TMB2 films is their relatively high coefficient of friction in humid air [17,18,19], which excludes these materials from a large number of tribological applications. Concerning the tribological properties, several authors report the coefficient of friction (COF) of bulk ZrB2 to be in the interval from 0.2 to 0.9 depending on the choice of measurement, load, surface morphology, chemical composition, environment, and material of the counterpart. Umeda et al. [17] pointed out the strong dependence of COF on environmental conditions. A typical value of the COF of sintered ZrB2 versus its ZrB2 counterpart is approximately 0.95 at humidity below 20%, but 0.4 at a humidity above 90%. Sonber et al. [20] investigated the behavior of sintered ZrB2 against a WC/Co ball at various loads and frequencies, at ambient conditions, and at 50% relative humidity. The COF was evaluated between 0.8 and 0.4 at different loads and sliding frequencies, where increasing the load and decreasing the frequency decreased the COF from 0.84 to 0.43. There are several studies [21,22] investigating the tribological properties of ZrB2-(B4C, ZrC, SiC) composites with a positive effect on COF reduction. Still, to our best knowledge, the impact of alloying ZrB2 with silver on the mechanical and tribological properties has not yet been investigated.
However, several studies [23,24] can be found on the investigation of the influence of Ag alloying on TM-Nitrides (TM—Zr, Ti, Mo, …), indicating improvement of tribological properties. Moreover, silver itself shows positive lubricating properties [25]. Thus, TMAgB2 represent a potential coating candidate with extraordinary mechanical and tribological properties on various type of substrates.
In this work, ZrB2 was chosen as a prospective hard coating to demonstrate the influence of silver alloying on the mechanical and tribological properties. Plain ZrB2 is a highly stable ceramic material with a melting temperature of 3246 °C and excellent mechanical properties due to strong covalent and ionic-covalent bonds between atoms [9,26]. Here, we combine theoretical predictions of phase stability and mechanical properties of α-Zr1−xAgxB2 using density functional theory (DFT) with experimental investigation of Zr1−xAgxB2 films prepared by magnetron sputtering. DFT calculations indicate a strong tendency towards phase separation of the α-Zr1−xAgxB2 solid solution. From the point of view of mechanical properties, increasing the concentration of silver in α-Zr1−xAgxB2 leads to more ductile behavior and improved lubrication properties, but it is accompanied by a decrease in hardness. The experimental results are in good agreement with the ab initio predictions and confirm the insolubility of silver in the ZrB2 phase. The effect of Ag alloying was manifested in the reduction of hardness, as the hardness value decreased from 30 GPa for Ag-free ZrB2 films to 4 GPa for the film with 39 at.% Ag. The coefficient of friction was reduced by adding silver from 0.95 for ZrB2 to 0.25 for the sample with 39 at.% Ag.

2. Materials and Methods

2.1. Calculation Methods

DFT calculations were carried out using QUANTUM ESPRESSO v. 6.4.1, employing projector-augmented wave pseudopotentials [27] and the Perdew–Burke–Ernzerhof parametrization [28,29] of the electronic exchange-correlation functionals. The α-Zr1−xAgxB2 solid solution, with x Є (0,1), was simulated using a 2 × 2 × 2 supercell (24 atoms). Overall, nine different compositions moving from stoichiometric ZrB2 to AgB2 (with a step in x of 0.125) were investigated, where Ag atoms randomly substituted Zr atoms. The influence of different choices of Ag substitution on results was also verified by comparison of total energies of various random configurations (for the same Ag content), resulting in only small differences up to 0.01 eV/at. Total energy and lattice parameters of all structures were determined by relaxing cell shapes, volumes, and atomic coordinates using a 9 × 9 × 9 k-point grid for Brillouin-zone sampling and a plane wave energy cutoff of 90 Ry, while imposing the convergence criteria of 10−7 Ry for energies and 10−5 Ry/bohr for forces. The chosen energy cutoffs and numbers of k-points ensured energy convergence within a few meV/atom. The stability of the α-Zr1−xAgxB2 in the form of a solid solution was investigated in terms of formation energy Ef [9] and mixing enthalpy ΔHmix [9].
Ab-initio mechanical properties were computed using the THERMO-PW driver as implemented in QUANTUM-ESPRESSO routines. α-Zr1−xAgxB2 solid solutions were investigated from the view of mechanical stability, where the elastic constants Cij must meet the necessary conditions of mechanical stability. From the elastic constants of stable systems, we calculated the bulk modulus (B) and shear modulus (G) by the Voight–Reuss–Hill approximation [30]. Furthermore, to discuss mechanical properties, Young’s modulus (E) and Poison’s ratio (ν) were also estimated [30]. Vickers hardness Hv was approximated from an empirical formula proposed by Chen et al. [31]. Moreover, the ductile/brittle behavior was investigated from the view of Cauchy’s pressures and Pugh’s indicator (G/B). Parameter μm = B/C44, known as the machinability index, assesses the suitability of materials for industrial applications, where large values of μm (values of order 10 and more) indicate excellent lubricant properties of materials [32].

2.2. Experimental Materials and Methods

The Zr1−xAgxB2+Δ films were prepared by non-reactive direct current (DC) co-sputtering from ceramic ZrB2 (100 mm in diameter, 99.5 at.%, RHP Technology, Seibersdorf, Austria) and metal Ag (100 mm in diameter, 99.99 at.%, Kremnica Mint, Kremnica, Slovakia) targets in self-made deposition equipment. A chromium target (100 mm in diameter, 99.5 at.%, Testbourne, Basingstoke, UK) was used for the deposition of the 100 nm thick Cr buffer layer. After 5 min deposition of the Cr buffer layer, there was an additional 1 min where all three cathodes (ZrB2, Ag, Cr) deposited an interdiffusion layer to ensure a smoother transition from the buffer into the main layer. The mirror-polished c-cut sapphire (0001), silicon (001), and steel (K100, S600) plates were used as substrates. Silicon substrates were only used for chemical analysis, morphology, and x-ray diffraction measurements. Substrates were placed on the heated (400 °C) and non-biased (floating potential) holder located in the intersection of the three magnetrons—two magnetrons tilted at 45° with respect to substrates normal, one magnetron (Cr target) situated directly opposite the rotatable substrate holder. The distance of the substrates from the side magnetrons was set to approximately 15 cm and approximately 19 cm from the Cr target. Before deposition, the chamber was evacuated to the base pressure of 5 × 10−4 Pa and then filled with Ar gas to a total working pressure of 0.35 Pa. The magnetron power densities were kept at 10.2 W·cm−2 and 6.7 W·cm−2 for Cr and ZrB2, respectively. In the case of the Ag target, the magnetron power density varied in the range of 0–0.3 W·cm−2.
The morphology and thickness characterization of cross-sections of the Zr1−xAgxB2 samples was performed by scanning electron microscopy (SEM, Thermofisher Scientific Apreo 2, Waltham, MA, USA). The chemical composition of the films was measured by wave dispersive X-ray spectroscopy (SEM, Tescan Lyra—WDS, INCA Oxford Instruments, Abingdon, UK) calibrated with a mirror-polished ZrB2 standard. Phase analysis was carried out via X-ray diffraction (XRD) in the Bragg–Brentano (BB) and grazing incidence (GI) geometry using a PANalytical X’pert diffractometer (Malvern, UK) equipped with CuKα (λ = 0.15418 nm). Nanoindentation hardness H and Young’s modulus E were measured using a nanoindenter (Anton Paar NHT2, Graz, Austria) equipped with a Berkovich diamond tip. The applied load was varied from 2 to 10 mN to not exceed the indentation depth of 10% of the film thickness (~1.2–1.5 µm), thus avoiding substrate influence. H and E were determined via the Oliver and Phaar method [33]. The presented results are averages from a total of 16 measurements (4 × 4 patterns). Standard deviations for the measurements are presented in the form of error bars. In order to analyze enhanced ductility, a nanoindenter (Anton Paar NHT2) equipped with a diamond cube-corner tip was used. A total of 9 indentations were performed at each sample to minimize statistical error from any inhomogeneity for different loads from 20 mN to 100 mN to obtain similar indents for samples with different hardness.
The coefficient of friction (COF) was measured by a tribometer (Bruker UMT/2, Billerica, MA, USA) by the ball-on-disc method, and wear tracks were evaluated by SEM. The measurements were carried out against steel (100Cr6), Si3N4, and WC/Co (6 wt.% of cobalt) balls in ambient conditions with a relative humidity of 45%–50% and a load of 5 N and a total time of measurement 3600 s. For steel balls, the time of measurement was shortened to half due to their low resistance to wear. For wear-track analysis, the measurement time was set to 600 s due to the total wear of samples with a high amount of silver at 3600 s measurement.

3. Results and Discussion

3.1. Calculation Results

In this study, ab initio DFT calculations were implemented to investigate the stability and elastic properties of hexagonal α-Zr1−xAgxB2. With the increase in x (Ag content), the formation energy Ef (Figure 1a) gradually increases, indicating that the system is converting into a less structurally stable one. The Zr0.375Ag0.625B2 already has a positive value of Ef, which suggests strong structural instability. The values of mixing enthalpies ΔHmix (Figure 1b) calculated with respect to ZrB2 and AgB2 are positive in the whole concentration range. Moreover, the values only slightly differ with increasing temperature, pointing to strong insolubility even in the state at 0 K with only a minor contribution from increasing temperature. Similar behavior/tendencies were theoretically predicted and experimentally observed on the similar TiAlB2 and ZrAlB2 systems [9,15]. In the ZrAlB2 system, a more moderate trend in values of Ef and ΔHmix led to the formation of a dual structure. As-dep samples with 8 at.% of alloying metal contained a decomposed solid solution. Therefore, a similar structure is expected in Zr1−xAgxB2+Δ even at such low Ag content. Systems with an amount of x > 0.625 do not meet the necessary conditions [34] for mechanical stability; therefore, they will not be further considered. Structural analysis reveals an almost linear increase of the c lattice parameter, as was expected. Due to the presence of strong covalent B-B bonds in (000z) planes, the c parameter has a tendency to expand more. The lattice parameter a is slightly decreased upon silver alloying.
From the view of theoretically obtained elastic properties, all of the investigated polycrystalline moduli—Young’s moduli (E), shear moduli (G), and bulk moduli (B)—linearly decrease with an increasing amount of Ag from EZrB2 = 494 GPa to EZr0.375Ag0.625B2 = 210 GPa (decrease of 58%), GZrB2 = 216 GPa to GZr0.375Ag0.625B2 = 81 GPa (decrease of 63%), and BZrB2 = 230 GPa to BZr0.375Ag0.625B2 = 183 GPa (decrease of 21%). Moreover, with increasing Ag amount, hardness also drastically decreases from HZrB2 = 40 GPa to HZr0.375Ag0.625B2 = 7 GPa (decrease of 83%). Ductile/brittle behavior is non-directly investigated from the values of Pugh’s ratio G/B, Poisson’s ratio, and Cauchy’s pressures of C13-C44 (0001) and C12-C66 (10-10). Therefore, from Table 1, a tendency toward ductile behavior with increasing Ag amount can be seen. Specifically, G/B increases from G/BZrB2 = 0.94 to G/BZr0.375Ag0.625B2 = 0.44 (crossing the critical empirical value of 0.57 between brittle and ductile materials). As Poisson’s ratio increases from νZrB2 = 0.14 to νZr0.375Ag0.625B2 = 0.3 (crossing the critical empirical value of 0.26 between brittle and ductile materials), Cauchy’s pressures increase from C12-C66ZrB2 = −113 GPa to C12-C66Zr0.375Ag0.625B2 = 75 GPa (10-10) and from C13-C44ZrB2 = −113 GPa −184 to C13-C44Zr0.375Ag0.625B2 = −60 GPa (positive values indicate ductile behavior). Tribological behavior was investigated via analysis of machinability index parameter μm = B/C44. As can be clearly seen from Table 1, μm increases from μmZrB2 = 0.98 to μmZr0.375Ag0.625B2 = 6.34, indicating very promising development of tribological properties upon Ag alloying, where from the experimental point of view, significant reduction of coefficient of friction can be expected.

3.2. Experimental Results

3.2.1. Chemical Composition Analysis

The ab-initio results (presented in Section 3.1) indicate the possible formation of metastable ZrAgB2 solid solutions at Ag concentrations below 21 at.%. In addition, increasing the silver content in ZrAgB2 continuously decreases the hardness, which practically means that only films with an Ag content of less than 15 at.% maintaining relatively high hardness values are usable in a demanding abrasive environment. On the contrary, higher Ag concentrations are promising from the point of view of tribological properties, and therefore our experimental investigation also includes these compositions.
Quantitative elemental WDS analysis of the reference ZrB2+Δ film revealed a slightly overstoichiometric B/Zr ratio of 2.3. In the following steps, we prepared Zr1−xAgxB2+Δ films by co-deposition from ZrB2 and Ag targets. Due to the very high deposition rate of silver, each subtle increase in the power density on the Ag target led to a significant increase in the Ag content in the Zr1−xAgxB2+Δ films. At the same time, the boron content also decreased, which can be seen in the gradual reduction of the B/(Zr + Ag) ratio from 2.07 to 1.61. However, a relatively high jump of B/(Zr + Ag) to a value of 0.89 can be seen in the sample prepared at the highest power density of 2.4 W/cm2. This fact can be attributed to the relatively high voltage at the Ag target (470 V). Then, the reflected Ar neutrals can cause resputtering of the growing films [8]. Because the sum of elemental composition is 100 at.%, with increasing Ag content, Zr and B content is reduced relative to Ag. The proportion of impurities (C, O) incorporated in the films is less than 3 ± 1 at.% which is a relatively high but common value for ZrB2 thin films due to the high affinities of Zr and B to oxygen [35,36,37]. Because our base pressure is below 5 × 10−4 Pa, we assume that contaminants originate from the target material. With increasing power density on the Ag target, deposition time was optimized to obtain a similar thickness of all samples of ~1.2–1.5 µm. The authors of the manuscript are aware of the sensitivity of WDS analysis to light elements that bond in various coordination and WDS accuracy compared with different analytical methods [38]. In the case of WDS analysis, the B/metals ratio is affected by systematic error. However, every measurement is equally laden with error, thus, we assume that trends are correct and relative values within the deposition series are comparable. The uncertainty shown in Table 2 refers to the typical standard deviation of five measurements across different sites of the sample, not the uncertainty due to the precision and systematic error of WDS analysis.

3.2.2. Structure and Morphology Analysis

Cross-sectional SEM images of the ZrB2.31 film sputtered from a single ZrB2 target show featureless fracture morphology. In our case, low ion energy (due to floating potential) did not result in clear columnar morphology, typical for crystalline ZrB2 [9,26,39]. SEM fracture images of co-sputtered Zr1−xAgxB2+Δ containing 2 and 8 at.% Ag indicate the presence of columnar morphology (Figure 2). As can be seen, columnar growth is suppressed within the samples with 20 and 39 at.% of Ag. The EDS map of Zr0.65Ag0.35B1.67 film reveals chemically separated Zr-rich and Ag-rich regions, suggesting structure segregation and the formation of a dual structure. Unfortunately, such an investigation was not carried out in samples with 2 and 8 at.% due to the insufficient signal from the low number of Ag atoms.
Figure 3a shows a series of XRD ϴ/2ϴ diffraction patterns of as-deposited ZrB2+Δ and Zr1−xAgxB2+Δ films with different silver contents. The X-ray pattern of the reference as-dep ZrB2.31 film shows a broad reflection centered near the table value of 2ϴ~42° for the (101) reflection of the hexagonal (P6/mmm) α-ZrB2 phase. This broadening indicates a very fine nanocrystalline nature of the structure. The dominant cubic (Im-3m) Cr reflection was also recognized at 2ϴ~44.1°, belonging to the buffer layer. In the case of Ag-alloyed films, XRD patterns also show a peak at 2ϴ = 25°, belonging to the reflection (001) from the α-ZrB2 phase. With increasing silver content in the films, the intensity of the dominant cubic reflection (Fm-3m) belonging to silver at 2ϴ~44.3° increases. However, this reflection is very close to the cubic (110) Cr reflection, so in the case of lower silver contents, it is not possible to evaluate with certainty the solubility of Ag in the ZrB2 grains or its segregation from BB. In the case of Zr0.65Ag0.35B1.67 (Ag~12.6 at.%), the reflection located at 2ϴ~38.1° belonging to silver (111) begins to dominate. The coexistence of ZrB2 and Ag phases is in good agreement with SEM fractures according to EDS analysis (Figure 2). To suppress dominant Cr (110) buffer layer reflection, the GIXRD method was used. As can be seen from Figure 3b, the reflection located at 2ϴ~44.1° is still present after the suppression of the signal from the Cr buffer layer in the sample with 2 at.% of Ag (Zr0.93Ag0.07B2.07), which suggests phase separation even at such low Ag content. The presence of Ag (111) reflection is also recognizable in the sample with 8 at.% of Ag. However, complete suppression of the signal from the buffer layer is not possible. Due to the overlay of Cr–Ag reflections, a wider range GIXRD of Zr0.93Ag0.07B2.07 is provided in Figure 3c, where the increased signal in the position of Ag (111) reflection is noticeable.

3.2.3. Mechanical and Tribological Properties

According to the results presented in Figure 4, the hardness values of alloyed Zr1−xAgxB2+Δ films are significantly decreased compared with those of very hard ZrB2.31 due to the reduction of cohesive strength at the interfaces between grains. This trend is in good agreement with DFT predictions. We can see a decrease in hardness from a value of HZrB2.31 = 30 ± 2 GPa to HZr0.26Ag0.74B0.89 = 4.2 ± 0.9 GPa (decrease of 86%). The lower hardness of ZrB2.31, compared with that of other studies and DFT calculations [9,26], can be explained by the absence of crystalline structure (mainly the presence of (001) texture, typically responsible for high hardness) [8,10] and the overstoichiometry of our samples. The lowered values of Young’s modulus non-directly indicate improvement in ductility, again correlating to ab-initio calculations, where the experimental value of Young’s modulus decreased from EZrB2.31 = 375 ± 13 GPa to EZr0.26Ag0.74B0.89 = 154 ± 18 GPa (59% decrease).
Cube-corner indents of the reference ZrB2+Δ and Zr1−xAgxB2+Δ films are shown in Figure 5. In pure ZrB2+Δ, the presence of clear radial cracks can be observed. With increasing Ag content, a reduction in crack length, circular corrugation, and accumulation of material that can be attributed to increased toughness can be seen.
Results of ball-on-disc tribotests against different types of ball-sliding or counter material can be seen in Figure 6. Due to similar behavior, regardless of the choice of substrate material, tribotest results on the S600 substrate are not shown to preserve the clarity of the figures. As can be clearly seen, Ag-alloying positively impacts COF decrease compared with all materials. High saturated values of COF ~0.9 for pure ZrB2 were measured against steel and Si3N4. However, a lower COF ~0.8 was observed against WC/Co. As we assumed, the lower COF of ZrB2-WC/Co tribocontact can be explained by higher flash temperatures during measurement. The higher hardness of the WC/Co counterpart compared with ceramic Si3N4 may lead to the formation of the lubricating B2O3 which can occur even at room temperature [20]. During the tribo test, heat generation occurs. The generated heat is sufficient to start the oxidation of the ZrB2, when ZrB2 is reacting with 5/2O2 into ZrO2 and B2O3. The oxygen required for the reaction is available from the surrounding air [20]. This assumption is supported by the WDS analysis in Figure 6d which indicates a higher concentration of oxygen within the wear track worn by WC/Co compared with other counterparts used. The uncertainty shown in Figure 6d refers to the standard deviation of five measurements from different sites on the wear track. Alloying with 8 at.% of Ag leads to a decrease in COF in the case of Si3N4 to a value of 0.65 and in the case of WC/Co to a value of 0.45. The sample with 8 at.% of Ag against steel shows still relatively high COF with a value of 0.75. Such a value can be explained by the lower temperatures during measurement against the steel counterpart with the lower hardness value compared with those of Si3N4 and WC/Co. The lowering of COF, in general, can be attributed to the lubricating properties of silver itself. Still, due to the small amount of silver, no visible change was seen in SEM wear track analysis.
Further increase of Ag amount to 20 at.% and 39 at.% resulted in a significant decrease of COF against all counterpart materials to the lowest value of COF ~0.25. As we assumed, lubricating properties and better thermal conductivity of Ag resulted in lower flash temperatures during the tribotest, insufficient for B2O3 formation, which is supported by lower oxygen concentration in the wear tracks (Figure 6d). However, phase analysis of oxides by the X-ray diffraction methods was not feasible. Due to the low volume of wear track compared with irradiated volume, diffraction peaks belonging to B- and Zr- oxides were absent in the diffraction pattern. The hump within the values of COF for the sample with 20 at.% can be explained through the accumulation of material during measurement. The change from tribo-chemical reaction to abrasive wear mechanism due to increased Ag concentration can be seen in Figure 7, where clear abrasive grooves are shown in the details of Zr0.26Ag0.74B0.84 wear tracks. Due to the low hardness values of Zr0.47Ag0.53B1.61 and Zr0.26Ag0.74B0.89, an increase in COF can be seen in all cases caused by total wear out of the coatings. Therefore, shorter measurements of 600 s had to be conducted for correct wear track SEM analysis.

4. Conclusions

In the present study, the DFT calculations were employed to investigate the effect of silver alloying on the structural and chemical stability and mechanical properties of the hexagonal Zr1−xAgxB2 system. The calculations point out the strong insolubility of silver in Zr1−xAgxB2 over the whole concentration range. Increasing silver content leads to a decrease in hardness, Young’s, bulk, and shear moduli. The improvement of ductility was also indirectly supported by an increase in Cauchy pressures and Poisson’s ratio and a decrease in Pugh’s ratio. Calculations also suggest improvements in tribological properties where the machinability index µm was increased upon silver alloying.
Motivated by the computational results, we experimentally prepared Zr1−xAgxB2±Δ with different silver contents using magnetron co-sputtering. For Ag concentrations above 12 at.%, analysis suggests the formation of a dual structure with the columnar character of segregated hexagonal α-ZrB2 and cubic Ag structure. Further structural analysis confirmed the formation of a dual structure in the sample with a silver content of 2 at.%.
As was predicted by the ab-initio calculations, Ag alloying led to a decrease in hardness values from HZrB2.31 = 30 GPa to HZr0.26Ag0.74B0.89 = 4.2 GPa, followed by a decrease of EZrB2.31 = 375 GPa to EZr0.26Ag0.74B0.89 = 154 GPa. Improvement in toughness and ductile behavior was experimentally confirmed by cube-corner indentations, where the crack formation was suppressed with increasing Ag content.
Further motivated by the calculations, tribological properties via the ball-on-disc method were investigated. Based on the counterpart material used, the coefficient of friction decreased with an increasing amount of silver, with the most significant improvement being a reduction from 0.95 for ZrB2 to 0.25 for the sample with 39 at.% Ag. Further analysis indicates a change in wear from tribo-oxidative to abrasive, indicating positive lubricant properties upon silver alloying.

Author Contributions

Conceptualization, T.F. and M.T.; validation, T.F. and M.T.; methodology T.F. and M.M.; investigation, T.F., M.T., V.Š., T.R., V.I., M.V., M.H. and L.S.; resources, M.M.; data curation, T.F.; writing—original draft preparation, T.F. and M.M.; writing—review and editing, T.F., M.T., T.R., V.I., M.V., M.H. and M.M.; visualization, T.F. and M.M.; supervision, M.T. and M.M.; project administration, M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by an Operational Program Integrated Infrastructure (Project No. ITMS 313011AUH4).

Data Availability Statement

The data presented in this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Energy of formation (Ef) and (b) mixing enthalpy (ΔHmix) of α-Zr1−xAgxB2 solid solution as a function of Ag content (x), where the red circle and blue triangle illustrate the impact of the temperature of 773 K and 1273 K on ΔHmix, respectively. (c) Relative values of the hexagonal lattice parameters (a, c) with volume (V) and density (ρ) as a function of Ag content (x) compared with reference ZrB2.
Figure 1. (a) Energy of formation (Ef) and (b) mixing enthalpy (ΔHmix) of α-Zr1−xAgxB2 solid solution as a function of Ag content (x), where the red circle and blue triangle illustrate the impact of the temperature of 773 K and 1273 K on ΔHmix, respectively. (c) Relative values of the hexagonal lattice parameters (a, c) with volume (V) and density (ρ) as a function of Ag content (x) compared with reference ZrB2.
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Figure 2. (a) Cross-sectional micrographs of ZrB2+Δ and Zr1−xAgxB2+Δ films and (b) Zr0.65Ag0.35B1.67 (13 at.% Ag) cross-sectional micrograph with corresponding EDS map of elemental distribution.
Figure 2. (a) Cross-sectional micrographs of ZrB2+Δ and Zr1−xAgxB2+Δ films and (b) Zr0.65Ag0.35B1.67 (13 at.% Ag) cross-sectional micrograph with corresponding EDS map of elemental distribution.
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Figure 3. (a) Bragg–Brentano XRD patterns of reference ZrB2+Δ and Zr1−xAgxB2+Δ films, (b) grazing incidence XRD detail on selected reflections indicating presence of segregated Ag phase even at 2 at.% of Ag content, and (c) grazing incidence XRD detail Zr0.93Ag0.07B2.07 pointing out at the presence of (111) Ag reflection.
Figure 3. (a) Bragg–Brentano XRD patterns of reference ZrB2+Δ and Zr1−xAgxB2+Δ films, (b) grazing incidence XRD detail on selected reflections indicating presence of segregated Ag phase even at 2 at.% of Ag content, and (c) grazing incidence XRD detail Zr0.93Ag0.07B2.07 pointing out at the presence of (111) Ag reflection.
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Figure 4. Experimental hardness (H) and Young’s moduli (E) of the reference ZrB2+Δ and Zr1−xAgxB2+Δ films plotted as a function of the power density on Ag target and Ag content.
Figure 4. Experimental hardness (H) and Young’s moduli (E) of the reference ZrB2+Δ and Zr1−xAgxB2+Δ films plotted as a function of the power density on Ag target and Ag content.
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Figure 5. Top view on the cube-corner nanoindentations of the reference ZrB2+Δ and Zr1−xAgxB2+Δ films.
Figure 5. Top view on the cube-corner nanoindentations of the reference ZrB2+Δ and Zr1−xAgxB2+Δ films.
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Figure 6. Results of dry sliding ball-on-disc tribological test for the reference ZrB2+Δ and Zr1−xAgxB2+Δ films on K100 substrate against different ball materials including (a) Si3N4, (b) steel, (c) WC/Co and (d) analysis of oxygen content in the wear track areas, where Ag content refers to content in films according to Table 2.
Figure 6. Results of dry sliding ball-on-disc tribological test for the reference ZrB2+Δ and Zr1−xAgxB2+Δ films on K100 substrate against different ball materials including (a) Si3N4, (b) steel, (c) WC/Co and (d) analysis of oxygen content in the wear track areas, where Ag content refers to content in films according to Table 2.
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Figure 7. Wear tracks of ZrB2.31 sample from the view of SEM worn against different counterpart materials: (a) WC/Co, (b) Si3N4, (c) Steel. Details of these wear tracks ((a1) WC/Co, (b1) Si3N4, and (c1) steel) and wear tracks of Zr0.26Ag0.74B0.89 from the view of SEM against different counterpart material ((d) WC/Co, (e) Si3N4, and (f) steel), with corresponding wear track details ((d1) WC/Co, (e1) Si3N4, and (f1) steel).
Figure 7. Wear tracks of ZrB2.31 sample from the view of SEM worn against different counterpart materials: (a) WC/Co, (b) Si3N4, (c) Steel. Details of these wear tracks ((a1) WC/Co, (b1) Si3N4, and (c1) steel) and wear tracks of Zr0.26Ag0.74B0.89 from the view of SEM against different counterpart material ((d) WC/Co, (e) Si3N4, and (f) steel), with corresponding wear track details ((d1) WC/Co, (e1) Si3N4, and (f1) steel).
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Table
Table 1. Computed elastic properties of ZrB2 and Zr1−xAgxB2 solid solutions with respect to Ag content (x). Specifically, E, G, B, ν, and H denote the elastic modulus, shear modulus, bulk modulus, Poisson’s ratio, and Vickers hardness, respectively, while B/C44 is the machinability index, C12-C66 and C13-C44 are directional Cauchy pressures in the hexagonal (0001) and (10-10) directions and G/B denotes Pugh’s ratio.
Table 1. Computed elastic properties of ZrB2 and Zr1−xAgxB2 solid solutions with respect to Ag content (x). Specifically, E, G, B, ν, and H denote the elastic modulus, shear modulus, bulk modulus, Poisson’s ratio, and Vickers hardness, respectively, while B/C44 is the machinability index, C12-C66 and C13-C44 are directional Cauchy pressures in the hexagonal (0001) and (10-10) directions and G/B denotes Pugh’s ratio.
SystemE (GPa)G (GPa)B (GPa)νH (GPa)µmC12-C66 (GPa)C13-C44 (GPa)G/B
ZrB24942162300.14400.98−184−1130.94
Zr0.875Ag0.125B24561962260.16341.19−156−780.87
Zr0.75Ag0.25B23871622140.20251.59−129−120.76
Zr0.625Ag0.375B23211302030.24172.18−114290.64
Zr0.5Ag0.5B22831121970.26133.11−90430.57
Zr0.375Ag0.625B2210811830.3076.35−60750.44
Zr0.25Ag0.75B2179681760.3254.39−33460.39
Zr0.125Ag0.875B286331590.32−0.6−130410780.21
AgB2−9−31470.58-−3329772−0.02
Table
Table 2. WDS quantitative elemental analysis of ZrB2+Δ and Zr1−xAgxB2+Δ films with deposition current (I), voltage (U), and power density on the Ag target.
Table 2. WDS quantitative elemental analysis of ZrB2+Δ and Zr1−xAgxB2+Δ films with deposition current (I), voltage (U), and power density on the Ag target.
I (A)U (V)P (W/cm2)Zr (at.%)Ag (at.%)B (at.%)B/(Zr + Ag)
00029.6 ± 1068.4 ± 12.31
0.053380.2229.5 ± 12.2 ± 165.8 ± 12.07
0.073500.3127.4 ± 18.0 ± 161.4 ± 11.74
0.13740.4823.9 ± 112.6 ± 161.0 ± 11.67
0.24151.0617.5 ± 119.8 ± 159.9 ± 11.61
0.44702.3913.9 ± 139.0 ± 147.2 ± 10.89
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Fiantok, T.; Truchlý, M.; Šroba, V.; Roch, T.; Izai, V.; Vidiš, M.; Haršáni, M.; Satrapinskyy, L.; Mikula, M. First Approach to ZrB2 Thin Films Alloyed with Silver Prepared by Magnetron Co-Sputtering. Coatings 2023, 13, 663. https://doi.org/10.3390/coatings13030663

AMA Style

Fiantok T, Truchlý M, Šroba V, Roch T, Izai V, Vidiš M, Haršáni M, Satrapinskyy L, Mikula M. First Approach to ZrB2 Thin Films Alloyed with Silver Prepared by Magnetron Co-Sputtering. Coatings. 2023; 13(3):663. https://doi.org/10.3390/coatings13030663

Chicago/Turabian Style

Fiantok, Tomáš, Martin Truchlý, Viktor Šroba, Tomáš Roch, Vitalii Izai, Marek Vidiš, Marián Haršáni, Leonid Satrapinskyy, and Marián Mikula. 2023. "First Approach to ZrB2 Thin Films Alloyed with Silver Prepared by Magnetron Co-Sputtering" Coatings 13, no. 3: 663. https://doi.org/10.3390/coatings13030663

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