ICMCTF 2021 Virtual Conference Session TS2: New Horizons in Boron-Containing Coatings: Modeling, Synthesis and Applications

The coatings composed of transition metal (Me = Mo, W, Ta, Nb, …), boron, and carbon are promising candidates for next-generation hard protective coatings. The experimental studies are reporting a combination of high hardness and increased fracture resistance for Mo-B-C and W-B-C coatings, which are deposited using sputtering techniques at laboratory conditions. Such properties favor the use of Me-B-C coatings in the industry, where the demand for replacing the hard but brittle state-of-the-art ceramic coatings is significantly increasing.

For this study, the W-B-C coatings were sputter-deposited by non-reactive magnetron sputtering using an industrial system provided by the company SHM, Czech Republic. The system utilizes as a sputter source a cylindrical rotating segmented target composed of metal, boron carbide, and graphite segments. As an industrial standard, the planetary table capable of multi-axis rotation of substrates was used to simulate the batch coating of the tools. The depositions were carried out in both stationary regime and single-axis rotation regime to understand the differences between laboratory-like and industrial preparation of the coatings.

The properties of the W-B-C coatings were studied over a broad range of their chemical composition. Despite the coatings were mostly amorphous, they still exhibited high hardness (up to 30 GPa) and elastic modulus (up to 450 GPa). The coatings deposited in the rotation regime exhibited multilayered character due to different transport pathways of the light (B,C) and heavy (W) elements. Detailed analyses of their mechanical properties proved their superior fracture resistance compared to current ceramic based protective coatings. Low internal stress in the coatings together with good adhesion enabled to deposit sufficiently thick coatings on the test samples and tools used in forming applications. The application relevant testing was performed at the end-customers of SHM in the Czech Republic.

This research is conducted within the FV30262 project, which is funded through TRIO program of the Ministry of Industry and Trade with financial support from the state budget of the Czech Republic.

This work discusses and reviews the generally accepted assumptions commonly used in the XPS bonding analysis of different materials. These are in the presented case applied to amorphous W-B-C materials with different compositions. We focus ourselves on the chemical shift of 4f electronic states of W atoms. In the W-B-C systems, the formation of binary W–B and W–C bonds leads to a shift in core-level to lower and higher binding energies, respectively, of W 4f core electrons due to the differences in the Pauling electronegativity of W, B and C. For the theoretical verification of this assumption we generated three amorphous W-B-C atomic models of different compositions (W:B:C ratios), calculated the W 4f core electron binding energies from ab initio methods and compared them with the measured XPS profiles of deposited W-B-C with the same W:B:C ratios. The atomic models reproduced the behaviour of the deposited systems in terms of XPS profiles. In the next step, we used the models to analyse the relationships between the W 4f electron binding energies and the local atomic environments of W atoms. It was found that the relative W 4f peak positions depend on the B/C ratio in the system, as was expected and as is commonly assumed in the bonding analysis. However, the relative amount of W and its density fluctuations in the volume of the unit cell on the other hand, which is usually omitted in the bonding analysis, also led to a chemical shift of the W 4f peak. Therefore, several series of amorphous W-B-C coating were deposited in a way to obtain amorphous coatings with compositions allowing to separate the effect of B/C ratio and the amount of W on the W 4f peak positions. The experimental positions of the W 4f peaks confirmed the theoretical predictions and have shown that general simplified assumptions based on electronegativity of elements need to be taken with care for precise chemical analysis of the bonding in W-B-C coatings.

Multielement ceramic coatings are appropriate candidates for high-temperature applications due to their excellent behavior at temperatures exceeding 1000°C. When the excellent oxidation resistance is combined with the thermal stability of a high optical transparency, they can be applied for high-temperature passive protection of optical and optoelectronical devices. On the other hand, a combination of the high oxidation resistance and the thermal stability of the electrical conductivity makes them suitable for capacitive pressure, vibration or tip clearance sensors in a severe oxidation environments.

The present work focuses on systematic investigation of high-temperature behavior of two amorphous Hf-B-Si-C-N coatings with different electrical and optical properties. The coatings were prepared by reactive pulsed dc magnetron co-sputtering of a B₄C-15%Hf-20%Si target in two Ar+N₂ mixtures (15% and 20% N₂ fractions). Particular attention is paid to thermogravimetric analysis and differential scanning calorimetry of the coatings in air and Ar, and to the evolution of the film structure, microstructure and elemental composition with increasing annealing temperature from 1100°C to 1700°C.

The coating prepared at the 15% N₂ fraction has an elemental composition of Hf₇B₂₃Si₂₂C₆N₄₀ and is electrically conductive, while the coating prepared at the 20% N₂ fraction has an elemental composition of Hf₇B₂₃Si₁₇C₄N₄₅ and is optically transparent. Both coatings are sufficiently hard (≈ 20 GPa) and exhibit a superior oxidation resistance up to 1600°C due to the formation of a nanocomposite diffusion barrier surface layer consisting of HfO₂ nanocrystallites surrounded by a borosilicate amorphous matrix. A small difference in the composition results, however, in a different thermal stability of the amorphous structure of the coatings above 1400°C. While the Hf₇B₂₃Si₁₇C₄N₄₅ coating underneath the oxide layer retains its amorphous structure, the Hf₇B₂₃Si₂₂C₆N₄₀ coating crystallizes into different phases (Si₃N₄, B(C)N, HfCN, HfB₂) with a unique self-organized structures [1,2].

References:

[1] “Superior high-temperature oxidation resistance of magnetron sputtered Hf–B–Si–C–N film”, P. Zeman, S. Zuzjakova, P. Mares, R. Cerstvy, M. Zhang, J. Jiang, E.I. Meletis, J. Vlcek, Ceram. Int. 42, 4853 (2016).

[2] “Microstructure evolution in amorphous Hf–B–Si–C–N high temperature resistant coatings after annealing to 1500 °C in air”, Y. Shen, J. C. Jiang, P. Zeman, V. Simova, J. Vlcek, E. I. Meletis, Sci. Rep. 9, 3603 (2019).

The nickel-based superalloys are designed to meet industrial requirements to withstand higher temperatures during the long-term service. They have wide applications in the aerospace, marine, nuclear, and petrochemical industries due to their excellent oxidation and corrosion resistance. However, because their high nickel content, these alloys are susceptible to abrasive, erosive and adhesive wear damage. Boriding process is considered as an efficient method for improving the tribological properties of nickel-based superalloys. These superalloys were successfully borided by using different methods without impairing corrosion and oxidation resistance. Among the boriding processes, the powder-pack boriding is a highly effective and low-cost method to produce hard nickel borides with outstanding properties.

From a kinetic viewpoint, the growth kinetics modeling is very substantial to select the optimum values of boride layers’ thicknesses matching the practical use of nickel-based superalloys in terms of wear conditions.

The Heat Balance Integral Method (HBIM) is an alternative to investigate the boriding growth kinetics of nickel-based superalloys. This approach, originally applied for analyzing thermal problems, is based on numerical resolution of a system formed by differential algebraic equations (DAE) under certain assumptions.

In the present work, new results about the growth kinetics of nickel boride layers on an Inconel 718 superalloy was estimated considering a diffusion model based on the HBIM. The boron diffusion coefficients in nickel boride layers were estimated in the temperature range of 1123 - 1223 K at different exposure times. In addition, the layers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and Berkovich indentation techniques to determine the phases formed on the surface, the microstructure and the chemical composition across the depth of the layer-substrate, and the distribution of the hardness, respectively.

The corrosion susceptibilities of the borided CoCrMo alloy exposed to the diffusion annealing process (condition I) and the borided CoCrMo alloy (condition II) immersed during 7 days in Hank's solution were evaluated. Firstly, the CoCrMo alloy was exposed to the powder-pack boriding process (PPBP) during 5 h at 1273 K, to obtain a biphasic layer (CoB-Co₂B) with a total thickness around 30 µm. Then, the diffusion annealing process (DAP) was performed in the borided CoCrMo alloy during 2h at 1273 K in an inert argon atmosphere to obtain a 30 µm of monophasic Co₂B layer. The potentiodynamic polarization technique was used to evaluate the corrosion resistance (R_p) using the linear polarization technique (LPR) and the corrosion current density (j_corr) from the Tafel extrapolation technique. In addition, the corrosion resistance estimated from conditions I and II were compared with the CoCrMo alloy (reference material). The results showed, in the 7^th day of immersion in the Hank’s solution, that the condition I (~357 kΩ cm²) exhibited corrosion values ~24 times higher than those obtained by the condition II (~15 kΩ cm²) reaching values close to the reference material (~385 kΩ cm²). The corrosion behavior of the borided CoCrMo alloy exposed to the diffusion annealing process was attributed to: i) the presence of the monophasic Co₂B layer on the surface of the borided CoCrMo alloy, ii) the redistribution of Cr and Mo after the process, which increased the content of these elements at the surface of the borided alloy and iii) the inhibition of the B₂S₃ and CrPO₄ species, which has a detrimental effect on the corrosion behavior at the surface of borided CoCrMo alloy. Finally, the effect of the diffusion annealing process (condition I) reduced the corrosion susceptibility of the cobalt boride layer around 7% in comparison to the value of 96 % estimated for the borided CoCrMo alloy (condition II).

The conventional powder-pack boriding process (PPBP) is a surface hardening treatment, which results in the formation of boride layers to enhance the mechanical and chemical properties of the materials. Particularly, in the PPBP, the growth kinetics of the boride layers depends basically on the boriding temperature, the exposure time, and the content of the powder mixture (B₄C, SiC and KBF₄), using temperatures for the process in the range of 1123 to 1223 K with long exposure times (4 to 12 h).

Facing this problem, several studies have developed an alternative process to reduce exposure times in the formation of boride layers at the surface of various steels and alloys, using a direct current field in the powder-pack boriding process (PBDCF). During the process, the current flux between the electrodes follows the direction of the conventional current (from positive to negative), however, the migration of the boron atoms, on the surface exposed to the cathode, is opposite to the normal direction of diffusion, consequently, the growth of the boride layer is slower than that exposed to the anode.

Based on the aforementioned, this study develops a novel treatment called pulsed-DC powder-pack boriding (PDCPB). This treatment employs a DC power supply coupled to an electronic programable control device (EPCD) to generate timed cyclic polarity changes on the flux of ionized boron atoms. The effect of the pulsed DC current field provides a homogeneous boron diffusion during the PDCPB, which promotes the growth of similar boride layer thicknesses on both surfaces (anode and cathode) exposed to the direct current field. The PDCPB treatment was performed on the AISI 316 L steel and Inconel 718 alloy at temperatures of 1123 - 1223 K with different exposure times for each temperature. The powder mixture consisted of B₄C, SiC and KBF₄, supplying a current input of 5 A and polarity inversion cycles of 10 s.

Ternary transition metal diborides M¹_1-xM²_xB₂ represent a promising class of materials for hard wear-resistant coatings [1]. By using co-deposition from two binary stoichiometric targets, it is possible to prepare a ternary system in a wide x range. However, the varying boron-to-metal ratio often embodies obstacles/difficulties in the evaluation of the effect of x on coating properties. The mechanisms that determine the resulting boron-to-metal ratio are dependent on the target material and may significantly alter the resulting coatings’ properties [2].

We use magnetron co-sputtering from TiB₂ and TaB₂ stoichiometric targets to prepare AlB₂-prototype ternary Ti_1-xTa_xB_2±Δ solid solution, with x in the range from 0 to 1. Using this technique, the boron-to-metal ratio (B/Me) varies with the actual Ti and Ta content. The boron-to‑tantalum ratio can be increased by decreasing the TaB₂ target voltage, which has a considerable effect on the coating structure. Coatings with B/Me > 2 reveal highly textured nanocolumnar structure, while the coatings with B/Me < 2 tend to be nanocrystalline (without any preferred crystallite orientation) or amorphous. All the deposited coatings have a hardness higher than 32 GPa. The under-stoichiometric (B/Me < 2) coatings show material pile-up around the cube-corner indent edges, an indication for plastic flow and increased ductility.

[1] B. Alling, et al. Scientific Reports 5 (2015)

[2] B. Grančič, et al. Surface & Coatings Technology 367 (2019) 341–348

Authors acknowledge funding from the Slovak Research and Development Agency [APVV-17-0320], VEGA 1/0381/19 and Operational Program Research and Development [project ITMS 26210120010]

DC magnetron sputtering (DCMS) is a widely used technique for the deposition of decorative, protective, and wear-resistive coatings. However, high power impulse magnetron sputtering (HiPIMS), due to the pulsed nature and higher peak current/power, is able to provide a much higher degree of sputtered atom ionization. This allows for a better control of the material flux, and a pathway for tuning of thin-film stoichiometry and microstructure. The physical vapor deposition synthesis of metal diborides (MB₂) is an expanding area of research and is primarily carried out by sputtering from a compound target. For both HiPIMS and DCMS, the material flux from the target goes through three primary stages – sputter ejection, plasma transport, and film deposition. The target composition is rarely preserved in the resulting film stoichiometry. In the present work, we have performed a systematic study of HiPIMS and DCMS plasmas generated from circular 5 cm diameter ZrB₂, TaB₂, NbB₂, HfB₂, TiB₂, VB₂, W₂B₅, and CrB₂ targets at Ar pressures of 3 and 20 mTorr. For all depositions, the average power is kept constant at 50 W, with HiPIMS being operated at a repetition rate of 100 Hz and a pulse length of 100 µs. Plasma characterization, including plasma chemistry and time- and charge-resolved ion energy distributions, is performed with a spectrometer-to-target distance of 15 cm. Corresponding films were deposited using the same geometry, for compositional characterization and a comparison to the plasma and target composition. The obtained results are discussed as a function of the material properties of the metal diboride targets, and in view of the differences in mass of the metal component. The study is aimed at developing an understanding of the synthesis of stoichiometric MB₂ using sputtering techniques.

Future tasks in many different fields of academia and industry are directed towards environmental sustainability, asking also for new advance in the field of protective coating materials. Especially, transition metal diboride based thin films exhibit a great potential to be applied in various applications because of their extreme refractory character as well as interesting electrical properties. Latest studies on various diborides emphasized their strong and weak points being on the one hand high hardness and phase stability in a wide range but on the other limited fracture and oxidation resistance. Alloying concepts involving transition metals (TM) such as W, Ta, or Zr forming ternary diborides (TMI_1-xTMII_xB_2-z) suggest to be a proper solution to overcome these restriction [1-3].

Therefore, within this study we provide a detailed insight on the thermomechanical properties and oxidation resistance of ternary W_1-xTa_xB_2-z thin films prepared by DC magnetron sputtering utilizing diboride based target materials. Based on theoretical investigations using density functional theory, we classified the two binary systems α-WB_2-z and α-TaB_2-z as highly ductile materials according to semi-empirical criteria e.g. Pettifor and Pugh. On behalf of structural investigations, we could confirm that single phased structured α-W_1-xTa_xB_2-z thin films are formed up to Ta contents of x=0.26. The addition of Ta also clearly enhances the phase stability in inert atmosphere shifting the decomposition of the metastable α- into the thermodynamically more stable ω-structure from 800 to 1400 °C for x=0.26. During oxidation treatments up to 800 °C the high Ta alloyed coatings outperform its counterparts, forming dense and stable oxide scale also after long exposure times (1000 min). In addition, these films were investigated by nanoindentation and in-situ micromechanical bending tests to evaluate the mechanical properties. The fracture toughness only slightly decreases with increasing Ta content from 3.7 to 3.0 MPam^1/2. In summary, the study highlights the potential of Ta alloyed WB_2-z coatings and confirms the strategy to form ternary or multinary diborides to expand the limitations in specific material properties.

References

[1] C. Fuger, V. Moraes, R. Hahn, H. Bolvardi, P. Polcik, H. Riedl, P.H. Mayrhofer, MRS Com. (2019) 1–6.

[2] V. Moraes, C. Fuger, V. Paneta, D. Primetzhofer, P. Polcik, H. Bolvardi, M. Arndt, H. Riedl, P.H. Mayrhofer, Scr. Mater. 155 (2018) 5–10.

[3] B. Bakhit, D.L.J. Engberg, J. Lu, J. Rosen, H. Högberg, L. Hultman, I. Petrov, J.E. Greene, G. Greczynski, J. Vac. Sci. Technol. A. 37 (2019) 031506.

Physical vapor deposited transition metal borides are an emerging class of materials. Their inherent promising properties range from ultra-low compressibility, highest thermal stability to chemical inertness, allowing an application as protective coating in quite harsh environments. Our recent ab initio calculations [1] suggest an attractive combination of high stiffness and appropriate high ductility for α-structured WB_2-z (space group 191, AlB₂-prototype, P6/mmm). This leads to an interesting combination of high hardness while maintaining a sufficient fracture toughness. The stabilization of the α-structure over the intrinsically favored ω-structure (space group 194, W₂B₅-prototype, P6₃/mmc) is based on omnipresent growth defects (e.g. various types of 0-dimensional vacancies) in the PVD process. However, next to the stabilized phases (hence prevalent bonding nature) also the morphology, especially column size and grain boundary interior, has a huge impact on the mechanical response.

Therefore, within this study we deposited various WB_2-z coatings using different deposition techniques as well as parameters to modify the crystallite size and grain boundary constitution but also predominating phases. Subsequently, the mechanical properties of these coatings in the as deposited and annealed state have been analyzed by means of nanoindentation, microcantilever bending tests, and micropillar compression testing. Depending on the coating morphology, which varies from nanocrystalline (amorphous) to crystalline fibrous grown structures, the hardness, indentation modulus, and fracture toughness obtains a strong variation. Hardness and indentation modulus can be varied between 31±2 to 42±2 GPa and 440±30 and 560±30 GPa, respectively. The most significant variation though was found in the fracture toughness of these coatings: We calculated values within the framework given by Matoy et al. [2] between 2.5 and 4.7 MPa√m. Furthermore, we critically evaluate the comparability of distinct micromechanical testing techniques assessing the fracture behavior also with respect to the residual stress state.

[1] V. Moraes, H. Riedl, C. Fuger, P. Polcik, H. Bolvardi, D. Holec, P.H. Mayrhofer, Sci. Rep. (2018).

[2] K. Matoy, H. Schönherr, T. Detzel, T. Schöberl, R. Pippan, C. Motz, G. Dehm, Thin Solid Films 518 (2009) 247–256.

Ti_1-xAl_xB₂ metastable ceramic alloys were predicted in [1] to exhibit a tendency for isostructural decomposition despite the fact that binary TiB₂ and AlB₂ belong to the same hexagonal space group and have very similar lattice parameters. Due to the reported high hardness of TiB₂ and the prospect of age-hardening through isostructural clustering, these alloys could be good candidates for hard protective coatings on industrial cutting tools. In this work, we present theoretical predictions regarding phase stability and temperature dependence of structural parameters, based on first-principles density functional theory. The phase diagram for this ternary system is derived and shows an isostructural miscibility gap that closes only at temperatures beyond the melting temperatures of TiB₂ and AlB₂.

Out of the constituent binaries, TiB₂ is a well-studied line compound that has received more focus than AlB₂. In this work, initial phonon calculations of stoichiometric AlB₂ indicates that the thermal expansion trend does not follow experimental measurements as closely as TiB₂. Furthermore, it is known in the literature that there is an inherent metal deficiency present in AlB₂. Therefore, special attention is put on AlB₂ to further our fundamental understanding of the compound. We investigate configurational thermodynamics of metal vacancies, their origin in the electronic structure and their interplay with lattice vibrations. Through the use of cluster expansion, effective cluster interactions are obtained and used in Monte Carlo simulations in order to study vacancy order-disorder transition temperature and to derive a phase diagram. For a subset of vacancy structures, phonon vibrational contributions in the quasiharmonic approximation are calculated to obtain volume expansion for metal deficient AlB₂, and are compared with experimental measurements performed by our partners.

[1] B. Alling et al. “A theoretical investigation of mixing thermodynamics, age-hardening potential, and electronic structure of ternary M¹_1-xM²_xB₂ alloys with AlB₂ type structure". Scientific Reports 5 2015, 09888.

Transition metal diborides (TMB₂) are very interesting due to their high temperature chemical stability and excellent mechanical properties. Growth of diboride films by physical vapor deposition (PVD) methods, such as magnetron sputtering from stoichiometric compounds, is accompanied by several interesting aspects. Different angular distribution of sputtered elements plays an important role leading to the formation of under/overstoichiometric nanocomposite films. The best-known superhard overstoichiometric TiB_2+x film consists of oriented α-TiB₂ hexagonal filaments embedded in the boron-rich tissue phase. In addition, thermodynamically non-equilibrium PVD processes lead in Nb-, Mo-, WB₂ to the synthesis of metastable structures by incorporating point defects such as vacancies. Understoichiometric MoB_1.6 and NbB_1.7 films form despite boron deficiency vacancy-containing α-nanofilaments surrounded by a boron matrix. In the case of slightly understoichiometric α-WB_2-x films, boron vacancies promote α-type of the structure exhibiting more ductile behavior in comparison with brittle character of stoichiometric ω-WB₂. Tantalum is probably the most interesting metal of the abovementioned transition metals of VB and VIB group due to its superior oxidation resistance and mechanical properties. Grančič et al. [1] sputtered TaB_2-x films from a stoichiometric compound. They drew attention to the significant impact of reflected Ar neutrals from heavy Ta atoms in the target toward the growing film. This led to a large re-sputtering of boron and formation of a significantly understoichiometric amorphous TaB_1.2 film. Only the maximum decrease in acceleration voltage reduced the energy of Ar neutrals resulting in the nanocrystalline TaB_1.7 film, but without the typical nanocolumnar character as in the other diborides. Here, we grow under/overstoichiometric TaB_2±x films using High Target Utilisation Sputtering (HiTUS) technology where it is possible to independently change the kinetic energy of the argon particles accelerated towards the target (target voltage) while maintaining the same amount (target current). We used SRIM (Stopping and Range of Ions in Matter) simulation of sputtering processes and experiments supported by density functional theory (DFT) calculations to investigate the effect of reflected Ar neutrals on the chemical composition, structure and mechanical properties of tantalum diboride films.

Authors acknowledge funding from the Slovak Research and Development Agency [APVV-17-0320], VEGA 1/0381/19 and Operational Program Research and Development [project ITMS 26210120010].

In the last couple of years, borides or diborides in particular, have drawn great attention in the development of protective coatings with exceptional properties. Their high melting points and excellent hardness are promising attributes to outperform nitride-based materials in various fields of applications.

Transition metal diborides tend to crystallize in two related but slightly different hexagonal structures. Whereas early TMB₂s have their stable configuration in the AlB₂-prototype (α-type, SG-191), late TMB₂s energetically prefer the W₂B_5-x prototype (ω-type, SG-194) showing a difference in the crystallographic ordering. Recent investigations on WB₂ have shown that in the case of thin films, the material crystallizes in the α-type, though its energetically preferred structure is the ω-type. Moreover, the thin films show a rather boron deficient chemical composition compared to measurements of bulk materials. On the contrary, HfB₂ clearly crystallizes in the α-type (bulk as well as thin films) with a tendency to an increased boron content.

In this study, we investigate the theoretical and experimental possible influences on stoichiometry, microstructure and the preferred structure type of various transition metal diborides. Therefore, we used Density Functional Theory calculations to study phase stabilities and the impact of point defects in combination with experimental variations on binary and ternary diboride systems such as HfB₂, CrB₂, V(W)B₂, or W(Ta)B₂.

A group of thin film metallic glasses have been reported to exhibit properties different from conventional crystalline metal films, though their bulk forms are already well-known for properties such as high strength because of their amorphous structure. Boron is one of the widely-used constituents for forming metallic glasses. Amorphous FeB-based alloys are excellent examples for this case, which have been developed for widely use as the energy efficient transformer core material. In this presentation, I will give a review of unique properties of this type of coating, followed by the first-ever metallic glass nanotube (MGNT) arrays on Si by a simple lithography and sputter deposition process for very large-scale integration. Like biological nanostructured surfaces, MGNTs show some surprising water repelling and attracting properties. Nanotubes are 500-750 nm tall and 500-750 nm in diameter, with wall thicknesses of ~103 nm. Two examples are presented based on modifications of this scheme. First, the MGNT array is prepared on a heating device on Si wafers and, with an applied electric voltage to the heating device underneath, the array surface is functioned as biomimetic artificial suckers for thermally adhesion response in biological systems. Second, after modification of biotin, the MGNT array acts as a waveguiding layer for an optical sensor. The MGNT sensor waveguide could readily detect the streptavidin. The detection limit of the MGNT arrays for streptavidin is estimated to be 25 nM. Thus, the MGNT arrays may be used as a versatile platform for high-sensitive label-free optical biosensing.

We report on comprehensive analysis of TiB_x thin films by x-ray photoelectron spectroscopy. Films were grown by both direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) from a compound TiB₂ target in Ar discharge. By varyig the deposition parameters, the film composition could be tuned over the wide range 1.34 ≤ B/Ti ≤ 3.0, as determined by elastic recoil detection analysis (ERDA) and Rutherford backscattering spectrometry (RBS). By comparing spectra over this wide range of compositions, we can draw new conclusions about how to interpret XPS spectra of TiB_x. By careful spectra deconvolution, the signals from Ti-Ti and B-B bonds can be resolved from those corresponding to stoichiometric TiB₂. The intensities of the off-stoichiometric signals can be directly related to the B/Ti ratio of the films. Furthermore, we demonstrate a way to obtain consistent and quantum-mechanical accurate peak deconvolution of the whole Ti 2p envelop, including plasmons, for both oxidized and sputter-cleaned samples. Due to preferential suttering of Ti over B, the film B/Ti ratio is best determined without sputter etching of the sample surface. This allows compositional determination, assuming extensive levels of oxygen has not penetrated below the sample surface. Fully dense films can be accurately quantified for at least a year after deposition, while under-dense samples do not give reliable data if the O/Ti ratio on the un-sputtered surface is ≥ 3.5. Titanium sub-oxides detected after sputter etching is also indicative of too much oxygen penetrating the sample, and quantification by XPS should not be trusted.

Refractory high-entropy alloys (HEAs) show outstanding mechanical and thermal properties than traditional alloys. Recently, the application of HEA thin films as functional and protective coatings has been widely investigated. Following our previous studies on VNbMoTaW based refractory HEA thin films, the mechanical properties of the thin films were further investigated by micropillar compression and nanoindentation methods in this study. A series of VNbMoTaWCrB thin films were fabricated by a pulse direct current magnetron sputtering method. The hardness and elastic modulus of thin films were evaluated by nanoindentation. The yield strength of each HEA micropillar under uniaxial compression was analyzed. The elastic and plastic deformation behaviors and the fracture toughness of these HEA thin films were studied. The effect of Cr and B concentrations on the hardness, elastic modulus, and fracture toughness of VNbMoTaWCrB HEA thin films were also discussed in this work.

Studies of single crystal artificial superlattices (SL) have been instrumental to understand hardening mechanisms at the nano-scale in transition metal (TM) nitride and carbide multilayers. The lack of such fundamental studies for TM diborides is the motivation for this study of CrB_x/TiB_y (0001) SLs, grown epitaxially onto Al₂O₃ (0001) wafer substrates by direct current magnetron sputtering from compound diboride targets. Growth conditions for obtaining well-defined SLs with good interface quality are found at a sputter gas pressure of 4 mTorr Ar and a substrate temperature of 600 °C. 1-µm-thick SL films deposited with modulation periods Λ between 1 and 10 nm, and Λ = 6 nm SLs with TiB_y-to-Λ layer thickness ratios Γ ranging from 0.2-0.8 are characterized by means of HRTEM, STEM, EDX, EELS, XRD, ToF-ERDA, RBS, and XPS. It is found that SLs with Λ = 6 nm and Γ in the range of 0.3-0.4 exhibit the highest structural quality. The effects of Γ and the stoichiometries (B/TM ratio) of the CrB_x and TiB_y layers on the distribution of B in the SL structures are highlighted. We show that by increasing the relative thickness of TiB_y, the crystalline quality of SLs starts to deteriorate due to B segregation in over-stoichiometric TiB_y layers, resulting in narrow epitaxial SL columnar growth with structurally-distorted boron-rich boundaries. An increase in relative thickness of under-stoichiometric CrB_x on the other hand, enhances the SL quality and hinders formation of boron-rich boundaries. Detailed structural analyses, mechanical properties with hardness, and stress as well as possible solutions for further improved SL definition will be presented.

The European Spallation Source (ESS), currently being built in Sweden, will be capable of delivering neutron beams more than an order of magnitude brighter than existing neutron sources, which will enable faster neutron scattering experiments on new materials using new techniques. For best utilization of the brightness, novel thin film based neutron-optical components, such as neutron mirrors and area detectors, are being developed. The Thin Film Physics division at Linköping University aims at developing novel thin film concepts enabling future neutron technologies.

Together with ESS, affordable ¹⁰B -based thin film neutron converter coatings have been developed that allows for eliminating the present unsustainable use of the rare isotope ³He in state-of-the-art neutron detectors. The primary material choice is magnetron sputter deposited B₄C, enriched in the neutron absorbing isotope ¹⁰B. We have shown that ¹⁰B₄C thin films can be grown by magnetron sputter deposition with properties that fulfill the requirements of mechanical and chemical stability as well as being radiation hard, while maintaining a high volume density of ¹⁰B. For example1µm thick films with residual stress as low as 0.09 GPa can be grown by DC magnetron sputtering and by employing HiPIMS it is possible to lower the substrate temperature to ~100°C which is useful for sensitive substrates. This ¹⁰B₄C thin film technology is proven superior to ³He-based detectors and is chosen for most of the foreseen detectors for scattering, reflectometry, as well as time-of-flight inelastic spectroscopy at the ESS.

Novel solid-state neutron detector designs, requiring neutron converter filling of narrow high-aspect ratio wells and trenches in semiconductor surfaces, require conformal coverage CVD methods. We have recently shown that conformal coatings of BC_x in wells with aspect ratios as high as 10 000:1 are possible using triethylborane, B(C₂H₅)₃ as precursor. This allows for future ¹⁰B-based solid state neutron detectors with sensor pixels in the range of 10x10 µm² (rather than the present ~cm²).

Nanometric metallic multilayers act as neutron mirrors and are key to guiding neutrons from source to experiment. By combining incorporation of ¹¹B in the multilayers with temporal ion-assistance modulation during sputter deposition, we obtain a significant reduction of interface width which, in turn, leads to significantly improved neutron reflectivities. For example in Ni/Ti:¹¹B multilayer mirrors reflectivity increased by 43% using this technique, implying up to 10x higher neutron flux throughput and significantly increased spectral range of future neutron guides.