MoSi2 Layer Research Articles

Microstructure and oxidation behaviors at 800°C and 1250°C of MoSi2/ReSi2/NbSi2 compound coating prepared by electrodeposition and then pack cementation

Abstract Mo Re duplex film has been electrodeposited from aqueous solution innovatively and then applied to fabricate protective silicide coating for the Nb Ti Si based alloy successively. The XRD, SEM and EDS results indicated the formation of MoSi2/ReSi2/NbSi2 compound coating after halide reactivated pack cementation (HAPC) treatment. Pesting oxidation of MoSi2 layer was suppressed and the Nb Ti Si based alloy was effectively protected at 800 °C with a mass gain of 0.48 mg cm−2 after oxidation for 100 h. At 1250 °C, continuous mass gain was observed with a mass gain of 2.6 mg cm−2 after exposure of 10 h. The synergistic combination of MoSi2 and NbSi2 layer enabled admirable protection of the substrate, in which MoSi2 layer worked as the major anti-oxidation coating and the NbSi2 layer could be a reservoir for Si. The contrast experiments indicate that ReSi2 layer could improve the integrity and adherence of MoSi2 layer, reduce the outward diffusion of alloying elements and oxidation of NbSi2 layer, and then improve oxidation resistance of the compound coating ultimately.

Deposition and oxidation behaviour of molybdenum disilicide coating on Nb based alloys substrate by combined AIP/HAPC processes

A MoSi2 coating was prepared on Nb-10Hf-1Ti alloys by a two-step process. The first step was arc ion plating (AIP) of the Mo layer, and the second step was halide-activated pack cementation (HAPC) of Si. The coating is composed of an outer layer of MoSi2 and an inner bond layer. After oxidation testing at 1200 °C for 100 h in a static air atmosphere, the mass gain of the coated specimen was 0.64 mg/cm2. The good oxidation resistance of the MoSi2 coating is attributed to the protective SiO2 scale generated by the oxidation process.

Stable Multilayer Reflective Coatings for λ(HeI) = 58.4 nm for the KORTES Solar Telescope

Multilayer aluminum-based structures reflecting at λ(HeI) = 58.4 nm were studied. Spectral and angular dependences of the reflection coefficient of these coatings were determined using a laboratory reflectometer and at the ELETTRA and BESSY II synchrotrons. The optical characteristics of multilayer Ru/Al and Mo/Al mirrors with a protective MoSi2 layer were found to be stable: the absolute change in the reflection coefficient within 18 months of storage in air was no larger than 1.5%. The peak near-normal reflection coefficient for Mo/Al mirrors at 58.4 nm was R = 26.1% with FWHM Δλ0.5 = 10 nm of the reflection curve.

Effect of hot-dip siliconizing time on phase composition and microstructure of Mo–MoSi2 high temperature structural materials

Mo–MoSi2 high temperature structural materials were obtained by hot-dip siliconizing method. The results show that the silicon melt and molybdenum substrate have good wettability, molten silicon reacts with molybdenum substrate to form a molybdenum silicide phase with a columnar structure, and a strong MoSi2 crystallographic preferred orientation (CPO) on the (110) crystal face is characterized. While, the Mo–MoSi2 high temperature structural materials consist mainly of silicon rich layer, MoSi2 layer, interface layer and molybdenum substrate. The coating surfaces contain a high silicon concentration (55–65 wt%) than the MoSi2 layer (30–35 wt%). Moreover, the thicknesses and the average grain sizes of siliconized coatings increase sharply with increasing siliconizing time. The coatings thicknesses are 20, 25, 30, and 50 µm, respectively. And the average grain sizes are 6.9, 9.3, 11.7, and 11.8 µm, respectively.

Electrodeposition of Si in Molten CaCl2 Above 1300K

Metallic Si is produced by carbon reduction of SiO2 in an arc furnace above 2000K, and the energy consumption and CO2 exhaustion in the process are very large. The authors reported that TiO2 could dissolve in molten CaCl2 containing CaO above 1300K, and that Ti metal was electrodeposited in the bath. This result implies the possibility of electrolytic production of liquid metallic Si in molten CaCl2-CaO-SiO2 above its melting point, which can lead the innovative production process of metallic Si with lower energy consumption and lower CO2 exhaustion. To realize this process, the electrochemical behavior of Si in molten CaCl2-CaO-SiO2 at a very high temperature has to be studied in detail. In this study, the cathodic reaction has been investigated in molten CaCl2-CaSiO3 above 1300K, and the electrodesipotion of metallic Si has been attempted. Based on our previous studies on Ti electrolysis, calcium silicate was used instead of CaO and SiO2. Calcium chloride containing 0~10 mol% CaSiO3 in a Mo crucible was fused in an air-tight furnace, and electrolysis was carried out at 1373 ~ 1573 K under a pure Ar flow. The working electrode was mainly a Mo wire with an Al2O3 sheath, and a SiC rod was also used. Another Mo wire electrode was used as quasi-reference electrode, and its potential was calibrated with Mo dissolution potential in the bath. Cyclic voltammetry was performed to investigate the electrode reactions in the bath, and then potentio-static electrolysis was carried out to obtain electrodeposit. The electrode surfaces after electrolysis as well as the electrodeposit were analyzed by XRD and SEM-EDX. The cathodic current obviously enlarged with the CaSiO3 addition, and the CaSiO3 solubility seemed to increase with the bath temperature. A sharp cathodic current peak appeared around -1.1 V (vs. Mo dissolution potential) at the Mo electrode, while no cathodic current peak was observed at the SiC electrode. Electrodeposit as shown in Fig.1 was obtained at the Mo electrode, and metallic Si was detected in it. Although a MoSi2 layer was also formed on the surface by electrolysis as shown in Fig. 2, the amount of Si in the layer was limited considering the quantity of electricity during the electrolysis. Since CaSi2 was sometimes found in the deposit, the suitable electrolytic potential needs to be revealed. It was shown that metallic Si was obtained by electrolysis in molten CaCl2-CaSiO3 under the proper condition. However, the amount of metallic Si on the electrode could not be determined yet because of the powdery morphology of deposit. It should be desirable to obtain liquid metallic Si by electrolysis above its melting point. Figure 1

Oxidation Behavior of MoSi2-Coated TZM Alloys during Isothermal Exposure at High Temperatures

Coating properties and oxidation behaviors of Si pack cementation-coated TZM (Mo-0.5Ti-0.1Zr-0.02C) alloys were investigated in order to understand the stability of the coating layer at high temperatures up to 1350 °C in an ambient atmosphere. After the pack cementation coatings, MoSi2 and Mo5Si3 layers were formed. When MoSi2-coated TZM alloys were oxidized in air at high temperatures, the Si in the outer MoSi2 layer diffused and formed SiO2. Also, due to the diffusion of Si, the MoSi2 layer was transformed into a columnar shaped Mo5Si3 phase. During isothermal oxidation, the Mo5Si3 phase was formed both within the coated MoSi2 layer and between the MoSi2 and the substrate. The coating properties and the oxidation behavior of the Si pack-coated TZM alloys were discussed along with the identification of growth kinetics.

Effect of siliconizing temperature on microstructure and phase constitution of Mo–MoSi2 functionally graded materials

Mo–MoSi2 functionally graded materials were prepared by a liquid phase siliconizing method. The microstructure, phase constitution, cross-section elemental distribution, grains size, and coating thickness of these materials were investigated with scanning electron microscopy (SEM), back scattered electron (BSE), energy dispersive spectroscope (EDS), glow discharge spectrum (GDS) and X–ray diffraction (XRD). The results indicate that the Mo–MoSi2 functionally graded materials have a dense multi-layer structure, mainly composed of surface layer (Si–MoSi2 layer, 1–10 µm), intermediate layer (MoSi2 layer, 22–40 µm), transitional layer (Mo5Si3 and Mo3Si layer, 2–3 µm) and Mo substrate. Moreover, the silicon concentration, grains size, and coating thickness increase gradually with the increasing temperature. The surfaces silicon concentrations are about 68–75 wt%, the average grains sizes of MoSi2 columnar crystals are about 7.1–9.4 µm, and the coating thicknesses are about 28–35 µm.

Oxidation behavior and microstructural evolution of a slurry sintered Si-Mo coating on Mo alloy at 1650 °C

A high-temperature oxidation-protective Si-Mo coating on a molybdenum alloy substrate was prepared by slurry sintering method. The structure, composition, phase distribution, and oxidation behavior of the coating were studied. The results show that the coating has a double-layer microstructure with an outer MoSi2 layer and an inner Mo5Si3 layer. After oxidation, the coating is composed of a SiO2 outer layer, a MoSi2 middle layer, and a Mo5Si3 inner layer. The mechanism of the phase distribution and the oxidation kinetic equation were also studied. The oxidation rate Kp determined over 12h at 1650°C, was found to be 0.061mg2cm−4h−1. The excellent oxidation protective ability of the coating at ultra-high temperatures is attributable to the formation of self-healing SiO2 on the coating surface after the oxidation of MoSi2.

Isothermal and cyclic oxidation resistance of pack siliconized Mo–Si–B alloy

Oxidation behaviour of MoSi2 coated Mo–9Si–8B–0.75Y (at.%) alloy has been investigated at three critical temperatures including 750, 900 and 1400°C in static air. Thermogravimetric analysis (TGA) data indicates a remarkable improvement in the oxidation resistance of the silicide coated alloy in both isothermal and cyclic oxidation tests. The cross-sectional scanning electron microscopy and energy dispersive spectroscopic analysis reveal the occurrence of internal oxidation particularly at the crack fronts formed in the outer MoSi2 layer during thermal cycling. The dominant oxidation mechanisms at 750–900°C and 1400°C are identified. Development of MoB inner layer further improves the oxidation resistance of the silicide coated alloy.

Interfacial characterization in ductile refractory metals reinforced MoSi2 based laminated composites

Efforts to develop MoSi2 for high temperature structural applications have been hampered by its extreme brittleness at temperatures below 1000°C. In the present work, toughening of MoSi2 at lower temperatures has been explored using the ductile refractory metals as reinforcements. Model tri-layer laminated composites were made using Mo, Ta and Nb sheets sandwiched in between two layers of MoSi2 based matrix. As these refractory metals are thermodynamically unstable with MoSi2, interfacial reaction layers were observed between MoSi2 and the refractory metal sheets. The main reaction product was identified to be as M5Si3 (M=Mo, Ta or Nb) consuming a part of the thickness of the ductile metal sheets. The Ta sheet reinforced laminated composite exhibited the minimum thickness of the reaction zone (~10μm) suggesting that the Ta has better chemical compatibility with MoSi2 as compared to Mo and Nb. Micro-hardness measurements made in the reaction zone revealed that the reaction products in all the three laminated composites were hard and brittle in nature. A quantitative idea of the thickness and micro-hardness values of the interfacial reaction layers is very useful to give some important guidelines to obtain the enhanced mechanical properties in MoSi2 based laminated composites.

Manufacturing of Composite Coatings by Atmospheric Plasma Spraying Using Different Feed-Stock Materials as YSZ and MoSi2

Yttria-stabilized zirconia (YSZ) is the state-of-the-art material for the top coat of thermal barrier coatings. To increase the efficiency and lifetime of gas turbines, the integration of MoSi2 as a healing material was proposed. A new method of manufacture was explored in order to enable the spraying of a homogeneous mixed layer of YSZ and MoSi2. As the chemical and physical properties of these powders are very different, they require contrasting process conditions. Due to the evaporation of Si from MoSi2 at spraying conditions suitable for YSZ, more moderate conditions and a shorter time of flight are required for depositing MoSi2. At the same time, the spraying conditions still need to be sufficient for melting the YSZ particles in order to produce a coating. To obtain a homogeneous mixture, both conditions can be matched using an injection system that allows powder injection at two different locations of the plasma jet. Two-color pyrometry during flight (DPV-2000, Tecnar) was used to monitor the actual particle temperature. By optimizing the injection point for the MoSi2, a mixed coating was obtained without decomposition of the MoSi2, which has been analyzed by means of XRD and SEM.

Microstructure and oxidation behavior of Si–MoSi2 functionally graded coating on Mo substrate

The Si–MoSi2 functionally graded coating on Mo substrate consisting of a Si–MoSi2 layer (2.5µm), a MoSi2 layer (18µm) and a Mo–Mo5Si3–Mo3Si layer (2–4µm) was prepared by a liquid phase siliconizing method. The siliconized coating has a dense layered structure and no micro-cracks and holes. The Si element mainly enriches on the surface with the highest content of about 50wt%, and inhibits the formation of Mo5Si3 and volatile MoO3 and improves the high-temperature oxidation resistance of the coating. The mass gain of Si-MoSi2 coating is only 0.17wt% after oxidized at 1600℃ for 70h. The Si–MoSi2 functionally graded coating exhibits better high temperature oxidation resistance than pure MoSi2 coating.

Enhanced Oxidation Resistance of Mo–Si–B–Ti Alloys by Pack Cementation

As new high-temperature structural materials, Mo–Si–B alloys satisfy several requirements such as oxidation and creep resistance. Recently, novel Ti-rich Mo–Si–B alloys have shown an increased creep resistance compared to Ti-free alloys. However, due to the formation of a duplex SiO2–TiO2 oxide layer, which allows for fast ingress of oxygen, the oxidation resistance is poor. To improve the oxidation resistance, a borosilicate-based coating was applied to a Mo–12.5Si–8.5B–27.5Ti (in at.%) alloy. After co-deposition of Si and B by pack cementation at 1000 °C in Ar, a conditioning anneal at 1400 °C is used to develop an outer borosilicate layer followed by an inner MoSi2 and Mo5Si3 layer. During both isothermal and cyclic oxidation after an initial mass loss during the first hours of exposure, a steady state is reached for times up to 1000 h at temperatures ranging from 800 to 1200 °C, demonstrating a significantly enhanced oxidation resistance.

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.

Silicide Coating Fabricated by HAPC/SAPS Combination to Protect Niobium Alloy from Oxidation.

A combined silicide coating, including inner NbSi2 layer and outer MoSi2 layer, was fabricated through a two-step method. The NbSi2 was deposited on niobium alloy by halide activated pack cementation (HAPC) in the first step. Then, supersonic atmospheric plasma spray (SAPS) was applied to obtain the outer MoSi2 layer, forming a combined silicide coating. Results show that the combined coating possessed a compact structure. The phase constitution of the combined coating prepared by HAPC and SAPS was NbSi2 and MoSi2, respectively. The adhesion strength of the combined coating increased nearly two times than that for single sprayed coating, attributing to the rougher surface of the HAPC-bond layer whose roughness increased about three times than that of the grit-blast substrate. After exposure at 1200 °C in air, the mass increasing rate for single HAPC-silicide coating was 3.5 mg/cm(2) because of the pest oxidation of niobium alloy, whereas the combined coating displayed better oxidation resistance with a mass gain of only 1.2 mg/cm(2). Even more, the combined coating could significantly improve the antioxidation ability of niobium based alloy at 1500 °C. The good oxidation resistance of the combined silicide coating was attributed to the integrity of the combined coating and the continuous SiO2 protective scale provided by the oxidation of MoSi2.

Microstructural and Thermal properties of Plasma Sprayed YSZ Nano-Clusters Thermal Barrier Coatings

In the present study, a novel durable three layered thermal barrier coating (TBCs) were prepared using atmospheric plasma spray (APS) on Ni718 superalloy substrate consisting of the YSZ nano-clusters. In order to develop a functionally graded coating system, the non-transformable (t′) tetragonal YSZ nano-clusters (40 nm) were synthesized by a sol–gel process and characterized at the temperature 1200 °C for 100 h. NiCrAlY was used as bond coat. The developed coating system introduces a protective top layer of MoSi2 (top coat) for preventing diffusion of oxygen, oxidation of the bond coating, provides thermal insulation and protection against corrosion and high temperature erosion. Microstructural, thermal oxidation resistance, thermal shock and adhesion strength of TBCs were analyzed. Different properties of as-sprayed TBCs have no significant effect on thermal oxidation property. The TBCs have shown better thermal shock resistance but lower adhesion strength than the TBCs made of without MoSi2 layer.

Development of MoSi2 coating on Nb-1Zr-0.1C alloy

MoSi2 coating has been developed on Nb-1Zr-0.1C alloy surface by firstly depositing Mo using magnetron sputtering technique and subsequent Si deposition by a chemical vapor deposition process namely pack siliconizing. About 2 μm thick interdiffusion zone is formed after annealing the Mo-coated Nb-alloy at 1500°C. The detailed characterization studies using XRF, SEM, EDS and EBSD of the pack siliconized coated alloy revealed the formation of an outer MoSi2 layer and an inner NbSi2 layer. About 25 μm thick MoSi2 layer comprising of fine grained microstructure is formed at 1100°C.

Development of Protective Coating of MoSi2 over TZM Alloy Substrate by Slurry Coating Technique

TZM alloy possesses high strength at elevated temperature but due to high volatility of its oxide, MoO3 the alloy undergoes catastrophic oxidation in air above 700°C. In the present study, development of high temperature oxidation resistant MoSi2 coating over TZM alloy was investigated by slurry coating technique. As-coated TZM alloy by Si powder was exposed to thermal treatment at 1200°C to develop MoSi2 coating. Characterization studies like surface, cross section morphologies, phase identification, evaluation of hardness, adhesive strength, oxidation resistance etc. were carried out to determine the quality of coatings. A double layered coating structure was obtained with thick outer surface layer of MoSi2(thickness ∼ 60 μm) and thin interfacial layer of Mo5Si3 (thickness ∼ 5 μm). The coating showed good adhesion strength (∼ 25 MPa) and stable oxidation resistance with negligible mass gain at 1000°C in air.

Development of a new TBC system for more efficient gas turbine engine application

To protect the superalloy blades in industrial gas turbine engines from failure Atmospheric plasma-sprayed YSZ (yttria-stabilized zirconia) thermal barrier coatings (TBCs) are widely used. The spalling of YSZ coating is the main failure of TBCs. This failure usually occurs within the YSZ coating near the YSZ/bond coat interface. In the present feasibility study, a novel durable multilayer TBC was fabricated using atmospheric plasma spray (APS) on Ni718 superalloy substrate consisting of a NiCrAlY-20%YSZ- MoSi2 top coat. The developed system introduces a protective top layer of MoSi2 for preventing diffusion of oxygen, oxidation of the bond coating, provides thermal insulation and protection against corrosion and high temperature erosion. The results of hot gas flow exposure test (H2+O2) to the triple layered coating, a kinetic heating study, showed that a decreased temperature 400 °C for heating of 145 second achieved without any visible damage or delamination in such drastic combustion condition exposure even after 27 cycles of such repeated exposure.

Effect of filler on the oxidation protective ability of MoSi2 coating for Mo substrate by halide activated pack cementation

To protect molybdenum against oxidation, MoSi2 coatings were prepared by halide activated pack cementation, and the effect of filler cases, Al2O3, SiO2 and SiC, on the microstructure and oxidation resistance of MoSi2 coating was studied. The deposited coatings using Al2O3 and SiO2 filler presented a double-layer microstructure of the outer MoSi2 and the inner Mo5Si3 layer. Whereas, the coating deposited in SiC-filler case displayed only one MoSi2 layer. Lower inert ability of SiC filler in the pack and the possible reaction with molybdenum are responsible for the mono-layer structure. The coating prepared with SiO2 filler presented rough morphologies and more residual SiO2 fillers embedded on the coating surface than the Al2O3 or SiC filler cases. Attributed to the residual embedded fillers, all three kinds of deposited coatings exhibited perfect anti-oxidation property at 773K and could effectively protect molybdenum substrate against pest oxidation. As for high temperature oxidation of 1473K in air, the coating corresponding to SiO2 filler displayed the best oxidation protective ability with only 0.05% mass change after 110h oxidation. The protective SiO2 layer formed on the surface of MoSi2 coating at high temperature is responsible for the good oxidation resistance of the coating.