MoSi2 Coatings Research Articles

Research progress in modification of MoSi2 coatings on surface of refractory metals and their alloys: a review

Research progress in modification of MoSi2 coatings on surface of refractory metals and their alloys: a review

Temperature dependence mechanism of high-temperature oxidation of transition metal silicide MoSi2

Transition metal silicides represented by MoSi2 have excellent oxidation resistance and are widely used as high-temperature anti-oxidation coatings in hot end components of power equipment. However, the mechanism of temperature-dependent growth of MoSi2 oxidation products has not been revealed. Therefore, this study investigated the formation characteristics of oxide film and silicide-poor compound on MoSi2 at temperatures of 1000 °C–1550 °C through high-temperature oxidation experiments, combined with microscopic Raman spectroscopy, scanning electron microscope, and x-ray diffraction (XRD) characterizations. The result showed that MoSi2 underwent high-temperature selective oxidation reactions at 1000 °C–1200 °C, forming MoO2 and SiO2 oxide film on the substrate. As the oxidation temperature increased to 1550 °C, after 100 h of oxidation, along with the disappearance of MoO2 and the phase transformation of SiO2, a continuous Mo5Si3 layer with a thickness of approximately 47 μm was formed at the SiO2–MoSi2 interface. Thermodynamics and kinetic calculations further revealed the mechanism of temperature-dependent growth of oxidation products (MoO2 and Mo5Si3) during high-temperature oxidation process of MoSi2. As the temperature increased, the diffusion flux ratio of O and Si decreased, leading to a decrease in oxygen concentration at the interface and promoting the growth of the Mo5Si3 layer. Its thickness is an important indicator for evaluating the oxidation resistance of MoSi2 coatings during service. This study provides experimental and mechanistic insights into the temperature-dependent growth behavior of Mo5Si3 during the high-temperature oxidation of MoSi2 coating, and provides guidance for predicting the service life and improving the oxidation resistance of silicide coatings.

Oxidation behaviors of electrodeposited MoSi2 coating in a wide temperature range from 773 K to 1673 K

In this study, a MoSi2 coating was prepared on the surface of a molybdenum (Mo) substrate using the molten salt electrodeposition method, and the oxidation behavior of the coating over a wide temperature range from 773 K to 1673 K was studied in detail. The phase composition, surface and cross-sectional morphologies of the MoSi2 coatings after oxidation at various temperatures were characterized, and the evolution of the phase and microstructure were analyzed to elucidate the oxidation mechanism. The experimental results showed that the MoSi2 coating exhibited excellent oxidation resistance at lower temperatures (773 K) and did not experience the pesting phenomenon. When exposed to medium and high temperatures, a continuous and dense SiO2 layer was formed on the surface of the MoSi2 coating, which effectively prevented further oxidation inside the coating, thereby protecting the Mo substrate. Meanwhile, at higher temperatures, especially at 1473 K and 1673 K, the solid-phase diffusion of Si towards the substrate gradually transformed the MoSi2 into Mo5Si3 with a lower Si content. Notably, the thickness of the Mo5Si3 layer that was distributed between the original MoSi2 coating and the Mo substrate increased with the oxidation time and temperature. At 1673 K, a Mo3Si layer with a low Si content was formed beneath the Mo5Si3 layer. Thus, the diffusion of Si was an important factor restricting the service life of MoSi2 coating.

Experimental investigation and first-principles calculations of Nb and W alloying effects on the microstructure and properties of MoSi2 coatings fabricated via arc cladding

MoSi2 is one of the most promising refractory metal silicide materials, but its further use as a structural material is limited by its drawbacks such as poor room-temperature toughness and low high-temperature strength. The work performed a comprehensive investigation combining first-principles calculations and arc cladding experiments to explore the effects of Nb and W doping on the mechanical properties and electronic structure of MoSi2 coatings. The first-principles calculations revealed that Nb addition improved the B/G value and Poisson’s ratio of MoSi2, indicating enhanced ductility. W addition yields the opposite effect and led to a higher elastic modulus and improved hardness. Experimental results demonstrated that the arc-cladding MoSi2 coating mainly consisted of MoSi2 and Mo5Si3 phases with a dendritic microstructure. Upon doping with Nb and W, additional t-(Mo,Nb)Si2 and t-(Mo,W)Si2 phases were formed, which resulted in a denser and finer microstructure. Nb addition contributed to the solid-solution toughening of the coating, while W addition enhanced hardness but reduced toughness. Remarkably, the synergistic alloying of Nb and W significantly increased the hardness and fracture toughness of the coating by 30.7 and 70.7%, respectively, compared to pure MoSi2. The strengthening mechanism of the coating was attributed to solid-solution softening and fine-grain strengthening, while the crack extension mechanism involved the crack deflection and bridging. Furthermore, the coatings doped with 2% Nb and 4% W exhibited the lowest wear weight loss and superior wear resistance. The dominant wear mechanisms were oxidation wear and abrasive wear.

Microstructure and high-temperature oxidation behaviour of MoSi2 protective coatings on Mo-based combustion chamber in rocket engines

This paper presents a detailed characterization of the microstructure and high-temperature oxidation behaviour of MoSi2 coatings obtained by pack cementation methods on a Mo-based substrate. The author focused on the challenges of applying Mo-based combustion chambers in rocket engines. Due to these assumptions, the high-temperature oxidation resistance of MoSi2 coatings was analyzed in the temperature range of 1200–1700 °C. A particular area of interest was the surface condition of MoSi2 coatings after oxidation tests, characterized by their roughness. The presented studies showed very beneficial behaviour of oxidized MoSi2 coatings from the point of view of both protective coating's morphology and durability under these conditions and the surface roughness responsible for the gas flow stability in actual working conditions. The identified microstructural phenomena related to MoSi2-based coating formation, its thermal stability and oxidation interaction were described from the thermodynamic point of view. Those analyses revealed that thermodynamically the most stable silicide compound during the first stage of coating formation is Mo5Si3, not MoSi2. It suggests that the interaction between the Si-rich gas atmosphere and the Mo5Si3 layer determines the growth progress of silicide coating. The necessary condition for further growth is an excess of active silicon sources in the reactive atmosphere in the pack cementation process.

Effect of Reaction Temperature on the Molten Salt Electrodeposition of Silicide Coating on Molybdenum Substrate and its Oxidation Behavior

In this study, silicide coatings were successfully prepared on Mo substrates using NaCl-KCl-NaF molten salt as the reaction medium and K2SiF6 as the silicon source. The electrochemical reduction mechanism of Si(IV) was studied by cyclic voltammetry and chronoamperometry technology. The effect of reaction temperature on the phase composition, microstructure, and growth kinetics of the electrodeposited coatings were investigated in detail. The long-term oxidation behaviors of Mo and MoSi2-coated Mo samples at 873 K were also compared. The results revealed that the electrodeposition of silicide coatings was closely related to the reaction temperature. Dense and coherent single-layer MoSi2 coatings could be obtained at 1023 K and 1073 K, while double-layer coatings with inner MoSi2 layer and outer Si layer could be obtained at 1123 K. Under the same electrodeposition time, the overall thickness of the silicide coatings increased with the reaction temperature. Moreover, long-term oxidation tests at 873 K proved that MoSi2 coatings could effectively prevent the Mo substrate from oxidation. The mass gain of the MoSi2-coated Mo sample was only 0.46 wt% after 500 h of oxidation, which could be attributed to the formation of the continuous SiO2 protective layer.

High-temperature oxidation resistance of spent MoSi2 modified ZrB2–SiC–MoSi2 coatings prepared by spark plasma sintering

The spent MoSi2 modified ZrB2–SiC–MoSi2 coatings were prepared on carbon matrixes by spark plasma sintering. A continuous metallurgical bonding was formed at the interface between the coating and matrix, and no obvious defects such as pores and cracks were observed inside. The effects of spent MoSi2 content and trace doping in the spent powder on the oxidation behavior of the coatings in air at 1700 °C were investigated. During the active oxidation stage, the spent MoSi2 promoted the densification of the coating and enhanced the structural oxygen barrier properties. With the increase of service time, during the inert oxidation stage, doping an appropriate amount of spent MoSi2 helped to increase the fluidity of the rich-SiO2 protective layer so that the Zr oxides fully dispersed in the generated Zr–B–Si–O–Al multiphase glass layer, which could impede the penetration of oxygen and enhance the oxidation protection efficiency. However, excessive spent MoSi2 exacerbated the volatilization of gas by-products, forming pores and cracks in the glass layer and rising the oxidation loss. When the content of spent MoSi2 was 20 vol%, the glass layer is dense and uniform, with few defects and the best oxygen resistance property. Moreover, compared with commercial powders, spent MoSi2 contained Al2O3 and SiO2. Al2O3 had an excellent modification effect, while SiO2 glass can promote liquid phase sintering and seal the defects in the coatings. By adding spent MoSi2, the modified ZrB2–SiC–MoSi2 composite coatings could inhibit the formation of defects and improve the dynamic stability of the coatings effectively.

Oxidation behavior and interfacial microstructure evolution of MoSi2/MoB coatings on Mo1 substrate at 600 and 1400 °C

Boron was incorporated into the MoSi2 coating via BSi co-cementation and a two-step process, i.e., boronizing followed by siliconizing. Compositional and microstructural changes arising from B-doping remarkably improved the performance of the MoSi2 coating. At 600 °C, compositional change caused by B-doping led to the formation of borosilicate scale with lower viscosity relative to vitreous SiO2, which considerably enhanced the oxidation resistance. However, the pores that formed within the middle and lower part of the B-doping MoSi2 coatings during preparation had a negative effect on the development of a continuous borosilicate scale. At 1400 °C, the interfacial reaction between MoB and Mo5Si3 led to the formation of a continuous Mo5SiB2 interlayer, which delayed the inward diffusion of Si within the coating. Accordingly, the growth rate of Mo5Si3 was roughly halved.

Comparative study on high-temperature oxidation behavior of MoSi2 and MoSi2/SiC-Mo2C composite coatings on Mo substrates at 1300 °C

In this study, MoSi2/SiC-Mo2C composite coatings and single-layer MoSi2 coatings are prepared on pure molybdenum substrates using the pack cementation method. The oxidation behavior of the coatings at 1300 °C is investigated. The calculated thermal stress of the MoSi2/SiC-Mo2C composite coatings is 366.76 MPa, whereas that of the single-layer MoSi2 coating is 1775.98 MPa. After oxidation for 30 h, the masses of the MoSi2/SiC-Mo2C composite coatings and MoSi2 single-layer coating increased by 0.61 and 0.38 mg/cm2, respectively. The average service life of the MoSi2/SiC-Mo2C composite coatings is 56.67 h, whereas that of the MoSi2 single-layer coating is only 46.67 h. The excellent performance of the MoSi2/SiC-Mo2C composite coatings can be attributed to the formation of a SiC-depleted zone, which reduces the generation of cracks within the coating. In addition, the Mo2C transition layers can effectively delay the mutual diffusion between the MoSi2 coating and substrate.

Ablation behavior and mechanisms of ZrC-SiC-MoSi2 coated C/C-SiC-ZrC ceramic matrix composites under oxyacetylene torch

To further improve the anti-ablation property of C/C composites in different ablation environments, ZrC−SiC−MoSi2 coatings containing 34 wt.% and 60 wt.% Zr were prepared on C/C−SiC−ZrC ceramic matrix composites via slurry painting (SP) method. And the ablation behavior of the coated composites was investigated in oxyacetylene torch environment with three kinds of fluxes of oxygen and acetylene. The results show that with the increase of Zr content, the particle sizes of ZrC and SiC in the coatings are significantly reduced, with the particles distributing more homogeneously. The linear ablation rates of the coatings with 60 wt.% Zr increase with the increase of oxygen and acetylene fluxes, while for the coatings with 34 wt.% Zr, the linear ablation rates increase at the beginning and then decrease. The ablation mechanisms of the coatings under different conditions were discussed as well. With the increase of oxygen and acetylene fluxes, main ablation mechanisms change from oxidation to oxidation plus evaporation and then to the combination of oxidation, evaporation and denudation.

Corrosion Behavior of MoSi<sub>2</sub> Coated Hastelloy X Utilized in Iodine -Sulfur Cycle for Hydrogen Production Application

Molybdenum di silicide (MoSi2) has lower mismatch of coefficient of thermal expansion (CTE) with Hastelloy X as compare to silicon carbide. So, MoSi2 coatings can be a potential candidate to protect Hastelloy X against high temperature sulfuric acid. Herein, MoSi2 thin films were deposited via electron beam physical vapor deposition (EBPVD) method. Corrosion behavior of MoSi2 coated HX and bare HX was analyzed. Corrosion tests of bare Hastelloy X and coated samples was performed at different temperatures (60°C and 120°C) in 98% sulfuric acid using the weight-loss method. The morphology of pre and post corrosion test was also analyzed by using optical microscopy. Bare Hastelloy X showed a slight weight gain at 60°C and a weight loss at 120°C. The MoSi2 coated Hastelloy X showed slight weight gain at 60°C and a weight loss at 120 °C. The weight gain is the result of oxides formation whereas, weight loss may be attributed to corrosion due to H+ ions. These findings were also accompanied by optical microscopy as corroded and very rough patches were evident for bare HX, whereas, for MoSi2 coated samples, the coating was still intact.

Influence of SiO2 glass phase (SGP) content on the oxidation and diffusion behavior of MoSi2 coatings on Nb-based alloys

MoSi2 coatings with different SiO2 glass phase (SGP) content were prepared on Nb-based alloy by hot press sintering. Then, the effect of SGP content on the growth behavior of the diffusion and oxidation layers was studied. The results showed that the desired coating could withstand 150 h of static oxidation at 1550 ℃ without failure. The coating with lower SGP content possessed thinner diffusion and oxidation layers after oxidation, resulting in better oxidation resistance. This improvement was due to low SGP content, which reduced the MoSi2/SGP or SiO2/oxide interfaces and restrained the diffusion of silicon and oxygen.

Fabrication of MoSi2 coatings on molybdenum and its high-temperature anti-oxidation properties

Industrial spent MoSi2-based materials were used to fabricate oxidation-resistant coatings on molybdenum via slurry painting in air. The microstructure, phase constituent and high-temperature oxidation behaviors of the coatings at 1500 °C were explored. The results show that the bonding layer is generated in the coatings after sintering, which strengthens the metallurgical combination between the coating and the substrate because of the formation of diffusion layers. Rare cracks appear in the coating using pure MoSi2 (PM coating) while the coating using spent MoSi2 (SM coating) is free of cracks due to decreased thermal expansion mismatch. After oxidation, the oxide scale of PM coating possesses large-sized pores while a relatively dense oxide scale is acquired by SM coating. Compared with PM coating, thinner glassy oxide scale with lower mass gain is obtained by SM coating, exhibiting better anti-oxidation properties at 1500 °C.

Self-diffusion mechanisms based defect complexes in MoSi2

MoSi2 is widely concerned due to excellent electrical conductivity, oxidation resistance as a typical transition metal silicide. The high-temperature diffusion behavior is one of the important factors for the degradation of MoSi2 coatings. However, the diffusion mechanism in MoSi2 is still unclear. Prior theoretical work mostly focused on defect formation energy, but these are not consistent with the self-diffusion experiments because the migration behaviors were not considered. Therefore, the purpose of this work was to investigate the microscopic diffusion mechanisms of Mo and Si atoms in MoSi2 using density functional theory and the CI-NEB method. We confirmed that the temperature-dependent vibrational contribution has a significant impact on the defect formation free energy. The isolated point defects in MoSi2 will tend to aggregate to form defect complexes, which participate in the atomic diffusion as mediators. The defect migration behaviors of atoms for vacancy mediated, vacancy complex mediated, and antisite assisted jumps were obtained based on electronic structures analysis. The results show that Si diffusion is mediated by intrasublattice jumps of the nearest neighbor Si vacancies. Moreover, the destroyed covalent Mo–Si bonds by Si vacancies and the non-directional weak metal bonds formed by the Mo antisites and Mo atoms could improve the mobility of the Mo atom which results in the low migration barrier. The agreement between our calculations and the reported experimental results indicates that the dominant diffusion mechanism for Mo atoms is mediated by vacancy complex mediated jumps and antisite assisted jumps. It is concluded that the Si vacancy-based defect complexes are likely the diffusion mediators for Mo atom self-diffusion in MoSi2. This work provides a deeper insight into the connection between the atomic mechanism and the macroscopic behavior for the diffusion in the MoSi2, and establishes the basis for further optimizing high-temperature coating materials.

Influence of Ta2O5 on the micromorphology and high-temperature oxidation resistance of MoSi2-based composite coating for protecting niobium

Spent MoSi2-based materials were recycled as coating raw-materials, and MoSi2 and MoSi2-Ta2O5 coatings were fabricated to protect niobium using Spark Plasma Sintering (SPS). The micromorphology and anti-oxidation properties of the coatings at 1773 K were researched. Penetrating cracks were detected in the MoSi2 coating surface while no cracks appeared in MoSi2-Ta2O5 composite coating since the thermal expansion mismatch of the substrate and coating was reduced by adding Ta2O5. (Mo,Nb)5Si3 diffusion layer formed in MoSi2-Ta2O5 composite coatings, showing good metallurgical combination between substrate and coating. After oxidation at 1773 K for 20 h, crack-free Ta-Si-O layer with low oxygen permeability was formed on the composite coating. Compared with the oxidized MoSi2 coating, lower mass gain is gained by MoSi2-Ta2O5 composite coating under same conditions, presenting improved oxygen-barrier properties at high temperature.

Effect of Temperature on the Microwave-Absorbing Properties of an Al2O3–MoSi2 Coating Mixed with Copper

To obtain a high temperature-resistant microwave absorbing coating, an Al2O3–MoSi2/Cu composite coating was prepared by atmospheric plasma spraying. Compared with a normal temperature environment, there are a few reports on Cu as an absorbent for high temperature microwave absorbing coatings. Therefore, in this regard, wave absorbing property can be improved by using a Cu absorbent. The microstructure of a Al2O3–MoSi2/Cu coating was observed, and the dielectric properties of the composite coating in the high-temperature environment of the X-band were tested. The experimental results show that with the increase in temperature, Cu transforms Cu2O in the high-temperature environment and improves the coating’s wave absorption performance with MoSi2. In addition, a 1.4 mm-thick coating showed best microwave absorbing performance at 700 °C. The reflection loss was −19.09 dB, and the effective microwave absorbing bandwidth was 2.83 GHz (Reflection Loss < −10 dB). It was found that the Al2O3–MoSi2/Cu composite coating has good wave-absorbing performance in a 700 °C high-temperature environment.

Formation of MoSi2 and Si/MoSi2 coatings on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy by hot dip silicon-plating method

MoSi2 and Si–MoSi2 coatings were deposited on TZM alloy by using a hot dip silicon-plating method. The composite coatings mainly consist of MoSi2 and Si with a small amount of C/SiO2/ZrSi2. The composite coatings have fine MoSi2 grain size and higher surface silicon concentration. The diffusion layer mainly consists of MoSi2 layer and Mo5Si3C layer under the deposition time is 10 min. The diffusion layer is divided into three layers when the deposition time ranges from 15 to 25 min, which consists of Si–MoSi2 layer, MoSi2 layer and Mo5Si3 interface layer. The gradient structure can reduce the stress mutation between coating and the substrate, and further reduce the possibility of crack propagation. SEM and CLSM results show that a large amount of MoSi2 grains are covered by a silicon layer when the deposition time is longer than 10 min, which results in a very low coating surface roughness. And the lowest values of Sa and Sq are 0.292 and 0.391 μm, respectively. Meanwhile, about 65.375%–79.898% of the coating surfaces are covered by silicon when the deposition time is 20–25 min.

Preparation and 1500 °C oxidation behavior of crack-free bentonite doped MoSi2 protective coating on molybdenum

Spark Plasma Sintering technique was applied to fabricate MoSi2 and MoSi2-bentonite coatings on molybdenum. The phase composition, morphologies and anti-oxidation properties of the coatings at 1500 °C were investigated. Penetrated cracks generated in MoSi2 coating after SPS while the composite coating was crack-free because of the reduction of CTE mismatch. Cracks existed in the silica layer of MoSi2 coating after oxidation while the silica layer of bentonite-containing coating showed dense and crack-free morphologies. Compared with MoSi2, the composite coating possessed thinner Mo5Si3 diffusion layer and silica layer, exhibiting less loss of substrate and coating with improved oxidation resistance.

Influence of Bentonite Content on the Oxidation Behavior of Mosi2 Coating Prepared by Hot Pressing Sintering on Nb-Based Alloy

Influence of Bentonite Content on the Oxidation Behavior of Mosi2 Coating Prepared by Hot Pressing Sintering on Nb-Based Alloy

Microstructural Characterization and Anti‐Oxidation Properties of Molybdenum Disilicide Coating on Niobium by Spent MoSi2‐Based Materials

MoSi2 coatings are prepared using spent and commercial MoSi2 powders as raw materials to protect niobium (Nb) substrate by spark plasma sintering (SPS) technique, and the microstructure and oxidation resistance of the coatings at 1500 °C are investigated. Results showed that obvious pores are found in MoSi2 coating using commercial MoSi2 (MC coating) after SPS while MoSi2 coating using spent MoSi2 (MS coating) is dense with low porosity. (Mo,Nb)5Si3 diffusion layer is generated between MS coating and the substrate with no gap or cracks, showing excellent metallurgical bonding. After long‐term oxidation at elevated temperature, pores and exposed Mo5Si3 can be observed in the oxide scale of MC coating, while dense and complete oxide scale is obtained by MS coating. Lowest mass gain (2.78 mg cm−2) is acquired by MS coating after oxidation at 1500 °C for 40 h because SiO2 oxide scale is completely formed on the dense and bentonite‐containing coating, blocking the diffusion of oxygen at high temperature. Compared with MC coating, no further oxidation occurs inside the MS coating or substrate, exhibiting better high‐temperature oxidation resistance.