MoSi2 Films Research Articles

Reliable and high-performance Ag-doped MoSi2 film with enhanced mechanical and tribological properties

The Mo-Ag-Si solid solution film was prepared using magnetron sputtering to enhance hardness and toughness while reducing friction and wear. Compared with the pure MoSi2 film, the Mo-Ag-Si solid solution film exhibited a 25.69% increase in hardness, a 5.17% increase in the H/E ratio, and a 33.33% boost in the H³/E² ratio. Tribological tests revealed that Ag atoms formed a self-lubricating AgMoxOy phase, reducing the friction coefficient to 0.09 and the wear rate to 3.89 × 10⁻⁷ mm³/N·m over 565m. First-principle calculations indicate that the Mo-Ag-Si solid solution film exhibited better mechanical and tribological properties than the pure MoSi2 film. These findings provide another approach for developing reliable MoSi2 films with high performance.

The effect of trace solute Au atoms on the phase structure, mechanical and self-lubricating properties of MoSi2 films

To overcome the high friction coefficient and low toughness bottleneck of MoSi2 used as protective films for moving parts, a novel strategy of doping and alloying through magnetron sputtering to build self-lubricating Mo-Si-Au solid solution film has been proposed. Herein, a series of MoSi2 films with different Au contents were prepared, and their phase structure, mechanical and tribological properties were investigated. The results indicate that when Au/Mo atomic ratio R = 0.10, Au atoms tend to replace Si atoms to form Mo-Si-Au solid solution film. Nevertheless, the Au atom would segregate to form a nanocomposite at R = 0.20. The nanoindentation test shows that the Mo-Si-Au solid solution film possesses the largest hardness, which is mainly attributed to the solid solution strengthening. The result of tribological testing exhibits that Au doping can significantly reduce the friction coefficient of MoSi2. More importantly, Mo-Si-Au solid solution film is realized with a lower coefficient of friction (∼0.08), which is ascribed to the formation of MoSiOx lubricating phase during the sliding induced solute Au atoms. Accordingly, the research would provide a considerable theoretical and technical foundation for the development of novel self-lubricating films with excellent mechanical properties.

Suppression of pesting failure in MoSi2 film by doping of Si

MoSi2 usually suffers pesting failure at 400– 700 °C due to severe oxidation of Mo, which has been a great hindrance for industrial applications. To address this issue, this work grew MoSi2(Si) thin films via magnetron co-sputtering of a Si single element target and a MoSi2 compound target on Si wafer substrates at room temperature, followed by annealing at 950 °C in an argon atmosphere. The as-annealed thin films were heated in a muffle furnace at 700 °C in air to experience different duration of oxidation. After that, the oxidation resistance was evaluated by examining the thickness, morphology, structure and composition of the as-heated thin films using field emission scanning electron microscopy and X-ray diffraction, respectively. In comparison with the MoSi2 without doping of Si, the MoSi2(Si) film is denser and exhibits a C11b crystalline structure with a smaller crystal size of 30 nm. The oxidation of Mo is effectively inhibited. There is no big bulge on the film surface after 1 h oxidation. But the oxidation of Si is severe, resulting in the continuous growth of grey SiO2 layer which help preventing the further oxidation of Mo. SiO2 islands are observed after oxidation for 9 h. The early pesting sites gradually become healed by the grown SiO2 layer during oxidation for 60 h. The mechanism for suppressing the pesting failure of MoSi2 by Si doping is discussed in details. A model is developed to explain the process.

Tailoring the strengthening-toughening behavior of the MoSi2 film by doping trace solute Au

Although the high hardness and wear resistance of transition metal silicides (TMSi) are widely used in protective films, overcoming the contradiction between hardness and toughness still faces huge challenges. Here the magnetron sputtering was used to introduce trace solute Au atoms into the MoSi2 to form a solid solution structure. X-ray diffraction and high-resolution transmission electron microscopy were used to characterize the microstructure of Mo-Si-Au film. In addition, hardness and toughness enhancement by solute Au was investigated the film through the combination of experimental and theoretical studies. The results show that the hardness and fracture toughness of Mo-Si-Au (Au/Mo atom ratio, R = 0.10) film is 15.4 GPa and 2.52 MPa m1/2, respectively. Compared with MoSi2, the hardness and toughness are increased by 41.3% and 68.0%, respectively. Density Function Theory calculations and electronic structure analyses further indicate that the increase in the hardness of the solid solution Mo-Si-Au film may be related to the increase in bulk modulus, C33, and C11. The increase in toughness is mainly due to the strengthening of the metal bonds in MoSi2 after doping a small amount of solute Au. The introduction of trace solute Au atoms in TMSi provides a new reliable scheme for overcoming the contradiction between hardness and toughness. As part of this research, a protective coating based on TMSi will be expected to be applied to aviation components.

Anodic Behavior of MoSi2 in Molten MgCl2-NaCl-CaCl2 Containing Oxide Ions

Mg and Li are smelted by electrolysis in molten chloride, and carbon-based material is used for the anode. Carbon-based anode reacts electrochemically with oxide ions dissolved in the bath, causing oxidative consumption. Therefore, the development of the so-called inert anode is desired. The authors proposed an inert anode by forming a stable passivation film on it and investigated the anodic behavior and the conditions to form a film on MoSi2 in molten LiCl-KCl containing Li2O. In this study, the behavior of MoSi2 anode and film formation on it have been investigated in molten MgCl2-NaCl-CaCl2.MgCl2-NaCl-CaCl2 electrolytic bath with about 1 mass% MgO was used at about 973K in an Ar atmosphere. A MoSi2 rod was used as working electrode. A Cu plate and Ag/AgCl were used as counter and reference electrode, respectively. Anodic behavior of MoSi2 was investigated by cyclic voltammetry and potentio-static electrolysis. The change in the MoSi2 surface with electrolysis was analyzed by XRD and SEM-EDX.Fig. 1 shows a cyclic voltammogram in the bath with MgO. Anodic current density increased at about 1.6 V (vs. Mg / Mg2+) and an anodic current peak appeared at about 1.8 V (vs. Mg / Mg2+). After the peak, the anodic current density sharply decreased. Since this anodic current peak was not seen without addition of MgO, it was indicated that the passivation phenomenon occurred in the bath with MgO.Fig. 2 shows the SEM image and element distributions of the cross-section of MoSi2 after potentio-static electrolysis for 4h at 3.1V (vs. Mg / Mg2+) in the bath with MgO. A passivating film of about 3 µm-thick was seen on the anode surface, and Si was found on it. Mg and O were concentrated in the passivation film, and SiO2 and MgSiO3 were detected by XRD. From the quantity of electricity during the potentio-static electrolysis, the thickness of the passivation film and the weight change, the current contributions for the passivation film formation, and for Mo dissolution were about 2% each, while about 96% of the electricity was shown to be spent by other reactions. Considering the electrolysis condition, 3.1V (vs. Mg/Mg2+), most of the current seemed to be used for the gas generation. It is concluded that MoSi2 is potential for the inert anode in molten chloride-containing oxide ions. Figure 1

Optical, Mechanical, and Thermal Properties of Free-Standing MoSi2Nx and ZrSi2Ny Nanocomposite Films

The optical, mechanical, and thermal properties of free-standing structures based on nitrided molybdenum and zirconium silicides have been studied. It has been shown that silicide nitriding considerably improves the thermal stability of films. While as-prepared amorphous MoSi2 and ZrSi2 films crystallize in the interval 330–370°C, nitriding of films has made it possible to increase their working temperatures in vacuum to 600–700°C (at least, for x ≥ 0.25, y ≥ 1.3). Tensile tests have shown that the ultimate strength of MoSi2Nx films (0 ≤ x ≤ 0.55) depends on nitrogen content only slightly. Comparison between the properties of MoSi2Nx and ZrSi2Ny films magnetron-sputtered at the same partial pressure of nitrogen has demonstrated that with transmission coefficients of films at a wavelength of 13.5 nm being close to each other, ZrSi2 films are more effective as protection coatings (they are less prone to oxidation and more degradation-resistant).

Исследование оптических, механических и термических свойств свободновисящих пленок на основе нанокомпозитных материалов MoSi-=SUB=-2-=/SUB=-N-=SUB=-x-=/SUB=- и ZrSi-=SUB=-2-=/SUB=-N-=SUB=-y-=/SUB=-

The optical, mechanical, and thermal properties of freestanding films based on nitrided molybdenum and zirconium disilicides were investigated. It has been shown that nitriding of silicides leads to a significant increase in the thermal stability of the films. So, if crystallization of initially amorphous freestanding MoSi2 or ZrSi2 films is observed at temperatures of 330-370°C, the introduction of nitrogen into the film makes it possible to increase the temperature up to 600-700°С at which MoSi2Nx and ZrSi2Ny (at least for x ≥ 0.25, y ≥ 1.3) films can be used for many hours when heated under vacuum. The study of mechanical tensile strength showed that the ultimate strength is weakly dependent on the nitrogen content in MoSi2Nx films (0 ≤ x ≤ 0.55). Comparison of the properties of MoSi2Nx and ZrSi2Ny films obtained by the magnetron method at the same nitrogen partial pressure demonstrated that at similar values of the transmittance at 13.5 nm, nitrated ZrSi2 films are more effective as protective coatings (less susceptible to oxidation and more resistant to degradation at high temperatures).

Selective atomic layer deposition of MoSix on Si (0 0 1) in preference to silicon nitride and silicon oxide

Highly selective deposition of MoSix on Si in preference to SiO2 and SiNx was achieved via atomic layer deposition (ALD) using MoF6 and Si2H6 at 120 °C. The selectivity was enabled by the lack of chemical reactivity between the reactants and the SiO2 and SiNx substrates. In contrast, MoF6 nucleated in a self-limiting manner on H-terminated Si, and a following Si2H6 exposure reduced MoFx to Mo0 which is consistent with Mo-Si bond formation. X-Ray photoelectron spectroscopy (XPS) revealed that the 5 ALD cycles of MoF6 and Si2H6 selectively deposited a substoichiometric MoSi2 film on the Si substrate in contrast to previous results showing a nearly pure Mo deposition. Extra Si2H6 doses on the substoichiometric MoSi2 film incorporated more Si into the film without disturbing the inherent selectivity over SiO2 and SiNx. A depth-profiling study showed that the bulk of the film has Si/Mo = 1.7–1.9 with <10% F and O impurities. The data is consistent with higher pressure Si2H6 doses inducing silicide formation instead of metal deposition. To verify selectivity on the nanoscale, the selective deposition of MoSix was investigated on a patterned Si wafer containing three-dimensional (3D) nanoscale SiO2 and SiNx features. Cross-sectional transmission electron microscopy (TEM) showed that selective MoSix deposition was achieved on nanoscale 3D structures. AFM documented that there were less than 10 nuclei/µm2 on SiO2.; since SiO2 has ∼106/µm2 OH groups, this corresponds to an intrinsic selectivity of about 106:1 between the OH groups on SiO2 and Si-H groups on Si. This inherent substrate-dependent selectivity for silicide deposition allows the elimination of pre-positioning of passivants.

First-principles study of magnetic properties of ultra-thin MoSi2 films

The magnetic properties of ultra-thin tetragonal MoSi2 thin films were investigated by the first-principles method. Our results indicate that the Si terminated MoSi2 film is always metallic independent of its thickness and non-magnetic when its thickness is larger than three atomic layers. However, the three-atomic-layer MoSi2 film (1L MoSi2) exhibits magnetism with magnetic moments of ∼0.274 μB/atom for Mo atoms and ∼0.096 μB/atom for Si atoms. The system shows weaker magnetism with magnetic moments of ∼0.184 μB/atom for Mo and ∼0.079 μB/atom for Si after unilateral surface hydrogenation and becomes non-magnetic after bilateral hydrogenation. By comparing the ferromagnetic (FM) configuration with antiferromagnetic configurations, we found that the FM order is the ground state with the lowest energy. Furthermore, it is found that the magnetic properties of 1L MoSi2 can be tuned effectively by strain.

Electrochemical Formation of MoSi2 Film on Ni-Based Superalloy

The growth of MoSi2 films by the electrochemical siliciding of Mo was investigated and MoSi2 film formation was attempted on a Ni-based superalloy (CMSX-4) via a two-step coating process. An oxidation test of the MoSi2 film was also performed. A thick MoSi2 film was obtained at a temperature below 1073 K. At a relatively high current density of 100 mA·cm−2, the supply of Si to the Mo surface was too fast and pure Si was deposited outward instead of alloying. The growth rate of MoSi2 at 50 mA·cm−2 was 57 μm·h−1, abnormally fast compared to that by normal diffusion. This indicated that Si diffused through the grain boundaries, rather than through grain bodies. A dense and homogeneous MoSi2 film, approximately 100 μm thick, was successfully formed on CMSX-4. The boundaries in MoSi2/Mo and Mo/CMSX-4 were coherent. After oxidation testing, a multilayer comprising SiO2, MoSi2, Mo5Si3, Mo, an intermediate layer, and CMSX-4 was formed. The SiO2 film was significantly thick at 10–30 μm compared to literature values. To obtain a thin SiO2 film, a MoSi2 layer with fewer defects might be required.

Electrochemical Siliciding of Nickel and Molybdenum in Molten Fluoride

Electrochemical siliciding of molybdenum in a molten fluoride bath was attempted with the aim of producing an oxidation-resistant MoSi2 film. Prior to this study, the electrochemical siliciding of nickel was investigated. As a result, a nickel silicide film composed of Ni3Si2 and NiSi was obtained. At a high current density of over 50 mA⋅cm−2, irregular needle-like crystals were observed and the preferential growth phase was Ni3Si2, which was consistent with the results of a previous study on diffusion coefficient determination using a diffusion couple. On the other hand, at low current density of 10 mA⋅cm−2, a nickel silicide film with a smooth surface and smooth interface was formed, which was not observed previously. In the electrochemical siliciding of molybdenum, a dense MoSi2 film was formed along with the deposition of pure silicon. The thickness of the MoSi2 film did not change significantly even when the current density was varied, which indicated that the film growth rate was determined by the diffusion rate of silicon in MoSi2. The film growth rate was significantly higher than that estimated from the diffusion calculation. This may be because not only body diffusion but also fast diffusion, such as grain boundary diffusion, occurred.

Infrared absorption of thin films MoSi2/SiNx micro-bridge

In this paper, we report the infrared absorption and opto-electrical characteristics of multilayered thin films of MoSi2/SiNx with a micro-bridge structure. The thin films of MoSi2 deposited by radio frequency magnetron sputtering exhibit a relatively smooth surface (RMS roughness &amp;lt;1 nm, ∼0.98 Å/s), on which their square resistance is linearly increased from 340 to 550 Ω/sq. as the thickness decreases from 32 to 16 nm. Fourier transform infrared spectroscopy analysis indicates that the thin film MoSi2 with a thickness of ∼24 nm (450 Ω/sq.) presents the maximum infrared absorption on nearly dielectric thin film SiNx. Optical simulation verifies that the resistivity of the nearly dielectric thin film SiNx has an important influence on the infrared absorption of the thin film MoSi2. This work provides the physical understanding regarding the building of micro-bridges with the high infrared absorption.

First-principles study of electronic properties of MoSi2 thin films

Electronic properties of tetragonal MoSi2 thin films are studied by the first-principles method. The results show that the MoSi2 film is always metallic, and its density of states and electronic structure are gradually close to their bulk counterpart as the film thickness increases. We further show that the three-atomic-layer film with the lowest energy is magnetic and has a magnetic moment of 0.33 B for its unit cell, and the film becomes non-magnetic when its thickness is more than three atomic layers. Moreover, we investigate the electronic properties of the three-atomic-layer MoSi2 films under unilateral and bilateral hydrogenation and find that the film with unilateral hydrogenation is magnetic and has a magnetic moment of 0.26 B, while the film with bilateral hydrogenation is non-magnetic. The spin polarizations for the films without hydrogenation and unilateral hydrogenation are 30% and 33%, respectively. These results suggest that three-atomic-layer MoSi2 film is metallic or magnetic when it is under suspension or grown on substrate, indicating its potential applications in nanoscale electronic and spintronic devices.

Role of Cr additions in enhancing wear and oxidation resistance of (Mo1−xCrx)Si2 nanocrystalline films

In order to explore the effects of Cr alloying on the wear and oxidation resistance of MoSi2 films, four kinds of (Mo1−xCrx)Si2 (x=0, 0.05, 0.10 and 0.15) nanocrystalline films of differing Cr contents were fabricated onto Ti6Al4V substrates by a double cathode glow discharge apparatus. The microstructure features of the as-deposited films were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The hardness and elastic modulus of the (Mo1−xCrx)Si2 nanocrystalline films were evaluated by nano-indentation. The dry sliding wear properties of the as-deposited films sliding against GCr15 rolling bearing steel were studied using a ball-on-disc type tribometer at room temperature. The results showed that the friction coefficients and specific wear rates of the (Mo1−xCrx)Si2 nanocrystalline films decreased with increasing Cr content in the films. The oxidation behavior of the (Mo1−xCrx)Si2 nanocrystalline films was investigated at 500°C under cyclic oxidation conditions in air for exposure time of 100h. The oxidation kinetics of the films was found to obey a subparabolic behavior, and three Cr-containing MoSi2 films showed a much lower weight gain and the oxidation rate constant as compared to the monolithic MoSi2 film during a 100h exposure. Surface morphologies, phase constituents and electrochemical behavior of the oxide scales formed on the (Mo1−xCrx)Si2 nanocrystalline films were characterized using FESEM, XRD and electrochemical-impedance spectroscopy (EIS). The electrochemical results revealed that the structure of the oxide scales changed from a homogeneous and dense structure to a duplex structure consisting of a porous outer layer and a denser inner layer. The overall resistance of the oxide scales depended strongly on both the oxidation time and Cr content in the films.

Enhanced corrosion-resistance in nanocrystalline MoSi2 films enabled by Cr additions

Nanocrystalline C40-structured (Mo1−xCrx)Si2 films were engineered onto Ti6Al4V substrates by a double glow discharge plasma technique. The electrochemical behavior of the newly developed films in a 3.5wt.% NaCl solution was characterized by electrochemical techniques including open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy. To gain a deeper understanding of the role of Cr alloying in corrosion resistance, both cohesive energy and Mulliken population of the interatomic bonds in C40 (Mo1−xCrx)Si2 were calculated based on the first-principles density-functional theory.

A critical role for Al in regulating the corrosion resistance of nanocrystalline Mo(Si1−xAlx)2films

Novel nanocrystalline Mo(Si1−xAlx)2 films, with differing Al contents were synthesized by double cathode glow discharge. The films exhibited a compact columnar microstructure having a pronounced (111) preferred orientation. The corrosion behaviour of these films were characterized by using various electrochemical techniques including open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution. The corrosion resistance of the films increased with increasing Al content in the as-synthesized films. The composition and chemical state of the passive layers formed on the films were investigated by X-ray photoelectron spectroscopy (XPS). It was demonstrated that the passive layer formed on the binary MoSi2 film was highly enriched in SiO2 with minor amounts of MoO42−, MoO2 and SiOx. With the increase of Al content in the films, Al2O3 was generated and incorporated into the passive layers, enhancing the corrosion resistance of the films by inhibiting the dissolution of Mo. Built upon the experimental results, the first-principles density-functional theory was applied to calculate the inter-atomic bonding strength in Mo(Si1−xAlx)2 and elucidate the role of Al in controlling the corrosion resistance of the films. The new findings lay a solid basis for the development and application of MoSi2 based corrosion-resistant films.

Sputter Deposition of ${\rm MoSi}_{2}$ Film as a Barrier for Nb-Based Josephson Junction

Recently, sputter-deposited MoSi2 films have been successfully used as a barrier material in Nb/MoSi2/Nb Josephson junctions. In this report, we present our study on the deposition conditions of MoSi2 film for electronics application. We investigated the film stress and sheet resistance as a function of deposition conditions. We also studied the micro-structural change in the film according to the deposition conditions using electron microscopes. We suggest that the thermal strain need to be considered in order to get a reliable junction.

Improvement of heating characteristics of molybdenum silicide thin film electric heaters

AbstractMolybdenum silicide (MoSi2) has an electrical conductivity as high as that of a metal, and greater chemical stability than that of, for example, SiC, in various atmospheres. Therefore, many kinds of MoSi2 bulk‐type heaters are used in practical operations up to 1800°C, which is higher than the temperature of SiC heaters. However, MoSi2 is fragile at room temperature and has low creep resistance at high temperature. The purpose of this study is to fabricate heaters using thin films of MoSi2 deposited on alumina substrates and crucibles by RF magnetron sputtering and to evaluate their characteristics. MoSi2 thin film was deposited on the outside of an alumina crucible without heating the substrate and then Pt wire was attached using a Pt paste with sintering in a vacuum. This MoSi2 thin film heater showed almost linear resistance–temperature (R–T) characteristics and a uniform heating state. It also showed good controllability of voltage and stability in the power–T characteristics for operations up to 1000°C. However, at a heating temperature of 1300°C, the heating area of MoSi2 thin film decreased because of the reaction between Pt and MoSi2 in the case of long‐term heating. Thus, Mo thin film was deposited as a buffer layer between Pt and MoSi2 thin film to prevent such a reaction. This thin film heater showed good linear R–T characteristics up to 1200°C. However, the temperature coefficient of resistance changed with repeated heating operation as a result of the diffusion of Mo atoms into MoSi2. Thus, a thin film heater was fabricated with Mo3Si, having a higher Mo content than MoSi2. This heater showed a low degree of diffusion of Mo or Pt atoms into the thin film and had excellent practical characteristics up to 1000°C. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(2): 11–19, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20806

ケイ化モリブデン薄膜高温発熱体の発熱特性の改善

Molybdenum silicide (MoSi2) has an electrical conductivity as high as that of a metal, and greater chemical stability than that of, for example, SiC, in various atmospheres. Therefore, many kinds of MoSi2 bulk-type heaters are used in practical operations up to 1800°C, which is higher than the temperature of SiC heaters. However, MoSi2 is fragile at room temperature and has low creep resistance at high temperature. The purpose of this study is to fabricate heaters using thin films of MoSi2 deposited on alumina substrates and crucibles by RF magnetron sputtering and to evaluate their characteristics. MoSi2 thin film was deposited on the outside of an alumina crucible without heating the substrate and then Pt wire was attached using a Pt paste with sintering in a vacuum. This MoSi2 thin film heater showed almost linear resistance–temperature (R–T) characteristics and a uniform heating state. It also showed good controllability of voltage and stability in the power–T characteristics for operations up to 1000°C. However, at a heating temperature of 1300°C, the heating area of MoSi2 thin film decreased because of the reaction between Pt and MoSi2 in the case of long-term heating. Thus, Mo thin film was deposited as a buffer layer between Pt and MoSi2 thin film to prevent such a reaction. This thin film heater showed good linear R–T characteristics up to 1200°C. However, the temperature coefficient of resistance changed with repeated heating operation as a result of the diffusion of Mo atoms into MoSi2. Thus, a thin film heater was fabricated with Mo3Si, having a higher Mo content than MoSi2. This heater showed a low degree of diffusion of Mo or Pt atoms into the thin film and had excellent practical characteristics up to 1000°C. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(2): 11–19, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20806

Fabrication and Application of Thin MoSi2 Film Electric Heaters

Molybdenum silicide (MoSi2) has high electrical conductivity as much as metal and high chemical stability than SiC in various atmospheres. Therefore, many kinds of MoSi2 bulk-type heaters are practically used for operation up to 1800°C, which is higher than that of SiC heaters. However, MoSi2 is fragile at room temperature and has low creep resistance at high-temperatures. The purpose of this study is to fabricate heaters using thin films of MoSi2, which are deposited on alumina plate-type substrates or crucible-type substrates by RF magnetron sputtering and to evaluate their characteristics. MoSi2 thin films were deposited on the outside of the alumina crucible, then Pt wires were attached using Pt paste by sintering in vacuum. This MoSi2 thin film heaters showed linear R-T characteristics and uniform heating state. They also showed good controllability of voltage and stability of power-T characteristics to operate up to 1000°C.