Preparation and high-temperature oxidation resistance of multilayer MoSi2/MoB coating by spent MoSi2-based materials

Spent MoSi₂ and MoB were used as raw materials to prepare multilayer MoSi₂/MoB coating on molybdenum by the two-step method of slurry deposition and spark plasma sintering. The results showed dense MoSi₂/MoB coating after sintering while penetrated cracks appeared in MoSi₂ coating due to coefficient of thermal expansion mismatch between the Mo substrate and coating. After the sintering of MoSi₂/MoB coatings, MoB and Mo₂B diffusion layers were formed between MoB transition layer and Mo substrate without defects, exhibiting good metallurgical bonding. The high-temperature oxidation behavior of coatings (1500°C) was also explored. After oxidation of 50 h at 1500°C, lowest mass gain (0.035 mg/cm²) was obtained for MoSi₂/MoB coating, and the oxide scale was dense and complete without voids, making the oxygen diffusion at elevated temperature inhibited. Compared with MoSi₂ coating under the same oxidation conditions, relatively thinner silica oxide scale was acquired by MoSi₂/MoB coating because of the reduction of cracks, and the multilayer coating exhibits better anti-oxidation properties at high temperature.     

Synthesis and Properties of MoSi2–MoB–SiC Ceramics

MoB and SiC particulate reinforced MoSi₂ matrix composites were synthesized in situ from Mo, Si, and B₄C powder mixtures by self‐propagating high‐temperature synthesis (SHS). The SHS MoSi₂–MoB–SiC products were vacuum hot‐pressed (HPed) at 1400°C for 90 min to fabricate high‐density (> 97.5% relative density) bulk composites. Microstructure refinement and improvements in the Vickers hardness and fracture toughness of the HPed composites were observed with increasing B₄C content in the reaction mixture. The HPed composite of composition MoSi₂–0.4MoB–0.1SiC exhibited grain size of 1–5 μm, Vickers hardness of 12.5 GPa, bending strength of 537 MPa, and fracture toughness of 3.8 MPa.m^1/2. These excellent mechanical properties indicate that MoB and SiC particulate reinforced MoSi₂ composites could be promising candidates for structural applications.     

Contact interaction and hot pressing of ZrB2-MoSi2 in CO/CO2 atmosphere

ZrB₂-15 vol% MoSi₂ ceramics were hot pressed in CO/CO₂ atmosphere in the 1700–1900^oС temperature range. During hot pressing, MoSi₂ decomposes into Mo and Si and the phase composition of the as-sintered ceramic results in ZrB₂, (Zr, Mo)B₂, SiC, SiO₂, and MoB. Contact melting between ZrB₂ and MoSi₂ was observed at 1800^oC, corresponding to the formation of (Zr, Mo)B₂. Ceramics obtained at1800–1850^oС had ∼ 500 МPа and 200 MPa strength at room at 1800^oC in vacuum, respectively. The thickness of the oxidized scales upon exposing the samples at 1600 ^oC for 120 min was 30–80 µm and depended on the amount of residual MoSi₂ and (Zr, Mo)B₂. The highest oxidation resistance was observed for the ceramic sintered at 1850 °C.     

Effect of MoSi2 addition on ablation behavior of ZrC coating fabricated by vacuum plasma spray

To improve the ablation resistance of ZrC coating, MoSi₂ incorporated ZrC composite coatings were fabricated by vacuum plasma spray. The ablation resistance of the composite coatings was evaluated using a plasma jet with a heat flux of 1.94?MW/m². The phase compositions and microstructures of the coatings before and after ablation were investigated, and the ablation mechanisms and effect of MoSi₂ were analyzed based on thermal dynamics and microstructure changes. Results showed that MoSi₂ addition could improve the ablation resistance of ZrC coating by means of decreasing the surface temperature and changing the microstructure of the oxidation layer. Si derived from the decomposition of MoSi₂, which occurred within coating, was beneficial to maintain the thickness and integrity of the SiO₂ layer and reduce the oxygen pressure beneath. The thickness of the SiO₂ layer was related to the formation rate (V_f) and the consumption rate (V_c) of SiO₂. The diffusion of Si was in favor of increasing the value of V_f. MoSi₂ could be one choice to improve the ablation resistance of the ZrC coating.     

In situ fabrication and properties of 0.4MoB-0.1SiC-xMoSi2 composites by self-propagating synthesis and hot-press sintering

Mo, Si and B₄C powders were used to fabricate 0.4MoB-0.1SiC-xMoSi₂ composites by self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The effects of MoSi₂ content (x = 1, 0.75, 0.5 and 0.25) on phase composition, microstructure and properties of the composites were investigated. The results showed that the 0.4MoB-0.1SiC-xMoSi₂ composite exhibited Vickers hardness of 10.7–15.2 GPa, bending strength of 337–827 MPa and fracture toughness of 4.9–7.0 MPa m^1/2. The fracture toughness increased with the increasing volume fraction of MoB and SiC particles which were promoted by the toughening mechanisms, such as crack bridging, cracks deflection and crack branching. Moreover, the electrical resistivity showed an increasing trend with decreasing volume fraction of MoSi₂.     

Recycling MoSi2 heating elements for preparing oxidation resistant multilayered coatings

Spent MoSi₂ heating elements were used to fabricate MoSi₂, ZrB₂/MoSi₂ and MoSi₂-ZrB₂/MoSi₂ oxidation resistant coatings via a two-step process of slurry painting and spark plasma sintering. Cracks appeared in simple MoSi₂ and ZrB₂/MoSi₂ coatings, whilst MoSi₂-ZrB₂/MoSi₂ coating was crack-free as CTE mismatch was reduced. Mo₅Si₃ and MoB diffusion layers formed under MoSi₂-ZrB₂ transition layer in MoSi₂-ZrB₂/MoSi₂ coating, revealing good metallurgical combination between the substrate and coating. On one side the silica layers covering MoSi₂ and ZrB₂/MoSi₂ coatings displayed diffused cracks after oxidation at 1773 K for 100 h, on the other side it was dense and continuous for MoSi₂-ZrB₂/MoSi₂ coating, showing low oxidation rate constant at 1773 K. Furthermore, MoSi₂-ZrB₂ transition layer in MoSi₂-ZrB₂/MoSi₂ coating possessed good short-term oxidation protective properties at 1873 and 1973 K since the formation of ZrSiO₄ improved the stability of Si-O-Zr composite oxide scale and diminished the oxygen permeability, therefore demonstrating good high-temperature oxidation resistance.     

High temperature oxidation behavior of MoSi2–Al2O3 composite coating on TZM alloy

A novel MoSi₂–Al₂O₃ composite coating was prepared on Mo-based TZM alloy by slurry sintering method. The oxidation behavior of the coating was evaluated at 1600 °C in static air. Microstructure and phase composition of the as-prepared and oxidized coatings were characterized, and the antioxidant mechanism of the coating at high temperature was discussed. A three-layer structure was observed in the as-prepared coating, consisting of a ～2 μm thick Mo₅Si₃ diffusion layer, a ～65 μm thick MoSi₂ inner layer and a ～36 μm thick outer layer of mixture of MoSi₂ and Al₂O₃. After oxidation at 1600 °C for 5 h, all MoSi₂ phases were completely converted to intermediate silicide Mo₅Si₃ by solid-state diffusion, and the formed Mo₅Si₃ phase would be transformed into Mo₃Si phase with further extending the oxidation time. Furthermore, a dense oxide layer of SiO₂-mullite was formed on the specimen surface, which can effectively protect the material to further oxidation. The MoSi₂–Al₂O₃ coating could protect the substrate effectively at 1600 °C for 20 h without failure. The enhanced oxidation resistance of MoSi₂–Al₂O₃ coating is due to the formation of multi-layer structure containing a SiO₂-mullite composite oxide outer layer with high thermal stability and low oxygen permeability.     

Preparation,properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi₂-based materials and HfO₂ were recycled as raw materials to fabricate MoSi₂-HfO₂ composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi₂ coating while no cracks can be found in the HfO₂-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)₅Si₃ diffusion layer was found in the HfO₂-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO₂ layer on MoSi₂ coating while the HfO₂-containing composite coating possessed crack-free oxide scale. HfSiO₄ with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi₂ coating, Nb coated MoSi₂-HfO₂ has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.     

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.     

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.     

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.     

Evolution behavior of rare-earth yttria modified silicide oxidation-resistant coating at 1700 oC

Yttria-modified silicide (MoSi₂-Y₂O₃) oxidation-resistant coatings with multiple Y₂O₃ contents were prepared using supersonic atmospheric plasma spraying, and the oxidation resistance was investigated at 1700 °C with static atmosphere. Experimental results indicated that the sprayed MoSi₂-Y₂O₃ coating possessed good compactness, which adhered well to the SiC transition layer. Meanwhile, the Y₂O₃ addition greatly enhanced the bonding strength of the coating, and extended its service life at 1700 °C. Wherein the MoSi₂-20 wt.%Y₂O₃ coating exhibited the highest adhesive strength and best oxidation resistance with the lowest mass loss among all the coatings. In practice, the Y₂O₃ changed the microstructures of formed oxide glass scale during oxidation, and then modified the oxidation resistance of the coating. The action mechanism of Y₂O₃ on the oxidation behavior of MoSi₂-Y₂O₃ coating was analyzed.     

Influence of different coating structures on the oxidation resistance of MoSi2 coatings

Two different structures of MoSi₂ coatings were prepared on Niobium based alloys by using a two step process. The as-deposited type(a) MoSi₂ coating structure consists of a MoSi₂ layer on the surface and a NbSi₂ layer underneath, while the type(b) MoSi₂ coating consists of an outer MoSi₂ layer and an inner unsiliconized Mo layer. The oxidation behaviors of the two different types MoSi₂ coatings were examined at 1200 °C for 100 h in air, and the mass gains of type(a) and type(b) MoSi₂ coated specimens were 0.64 mg/cm² and 0.59 mg/cm² respectively. The excellent oxidation resistance of both type(a) and type(b) MoSi₂ coated samples at 1200 °C was due to the formation of a dense and continuous SiO₂ scale during oxidation. As the CTE mismatch between the outer MoSi₂ coating and the inner layer, cracks distributed within both type(a) and type(b) MoSi₂ coating structures.     

A MoSi2-SiOC-Si3N4/SiC anti-oxidation coating for C/C composites prepared at relatively low temperature

In this study, SiC coating for C/C composites was prepared by pack cementation method at 1773 K, and MoSi₂-SiOC-Si₃N₄ as an outer coating was successfully fabricated on the SiC coated samples by slurry method at 1273 K. The microstructure and phase composition of the coatings were analyzed. Results showed that a porous β-SiC inner coating and a crack-free MoSi₂-SiOC-Si₃N₄ coating are formed. Effect of Si₃N₄ content on the oxidation resistance of the coated C/C composites at 1773 K in air was also investigated. The weight loss curves revealed that introducing the appropriate proportion of Si₃N₄ could improve the oxidation resistance of coating. The MoSi₂-SiOC/SiC coated C/C sample had an accelerated weight loss after oxidation in air for 20 h. However, the coating containing 45% Si₃N₄ could protect C/C composition from oxidation for 100 h with a minute weight loss of 0.63%.     

Protective properties of SiO2 with SiO2 and Al2O3 nanoparticles sol-gel coatings deposited on FeCrAl alloys

Five layer SiO₂ coatings containing SiO₂ or Al₂O₃ nanopowders were deposited on FeCrAl alloy support by sol-gel method. Studies of protective properties of the coatings were carried out during high temperature cyclic oxidation. Changes in surface topography, structure and chemical composition of the surface layer of FeCrAl alloy were investigated. It has been shown that the type of nanofillers present in the SiO₂ coating (about 2.5?wt%) affects morphology of Al₂O₃ growing scale and determines the heat resistance of FeCrAl alloy. The lowest relative mass change (approx. 1.3%) after 10 oxidation cycles in air at 900?°C (one cycle = 12?h) was measured for the samples with coatings containing hydrophilic nanosilica (Aerosil 380) as filler. The protective efficiency of the coatings in the process of high-temperature oxidation is from 66% to 85%. The thickness of the formed scale and the value of the parabolic rate constant depend on the type of nanopowder in the coating.     

High temperature oxidation behavior of ZrB2-SiC added MoSi2 ceramics

In this paper, MoSi₂, MoSi₂-20?vol% (ZrB₂-20?vol% SiC) and MoSi₂-40?vol% (ZrB₂-20?vol% SiC) ceramics were prepared using pressureless sintering. The oxidation behaviors of these MoSi₂-(ZrB₂-SiC) ceramics were investigated at 1600?°C for different soaking time of 60, 180 and 300?min, respectively. The oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics were studied through weight change test, oxide layer thickness measurement, and microstructure analysis. Further investigation of the oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics was conducted at a higher temperature of 1800?°C for 10?min. The microstructure evolution of the ceramics was also analyzed. It was finally found that the oxidation resistance of MoSi₂ was improved by adding ZrB₂-SiC additives, and the MoSi₂-20?vol% (ZrB₂-20?vol% SiC) ceramic exhibited the optimal oxidation resistance behavior at elevated temperatures. From this study, it is believe that it can give some fundamental understanding and promote the engineering application of MoSi₂-based ceramics at high temperatures.     

A comparative study of MoSi2 coatings manufactured by atmospheric and vacuum plasma spray processes

In this work, MoSi₂ coatings were manufactured by atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS) technologies, respectively. Phase composition and microstructure of the coatings were characterized by X-ray diffraction and scanning electron microscopy. Microhardness, void and oxygen content of the coatings were also determined. Oxidation behavior of the coatings at high temperature was examined. The results showed that the surface of VPS-MoSi₂ coating was dense and homogeneous. However, there were many microcracks formed on the surface of APS-MoSi₂ coating. The VPS-MoSi₂ coating also had lower void and oxygen contents, higher Vickers hardness compared with those of APS-MoSi₂ coating. Besides, oxidation resistance of the VPS-MoSi₂ coating was better than that of APS-MoSi₂ at 1500 °C.     

Oxidation resistance and thermal shock properties of self-healing SiCN/borosilicate glass-B4C-Al2O3 coatings for C/C aircraft brake materials

SiCN/borosilicate glass-B₄C-Al₂O₃ coating was deposited on carbon fiber-reinforced carbon matrix (C/C) brake materials to protect them from oxidation. Microstructural analysis revealed that the coating was dense and uniform. Fabricated coating showed excellent oxidation resistance and significantly low weight losses after oxidation in dry air for 10?h than SiCN/borosilicate glass-B₄C coated samples (ca. 0.12%, 0.51%, and 0.29% at 700, 800, and 900?°C, respectively). B₄C is believed to react with the oxygen diffused into the coating to produce B₂O₃, which could heal cracks of the coating and improve its self-sealing ability and oxidation resistance. The Al₂O₃ present in the outer glass layer is believed to inhibit volatilization of B₂O₃, thereby reducing weight losses in air. Fabricated coating also possessed excellent oxidation resistance under fresh and sea water conditions, with cracks and pores generated during oxidization process being effectively healed. Prepared coating materials showed excellent thermal shock resistances after 50 thermal shock cycles, with weight losses being as low as 0.23%.     

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi₂-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi₂-SiC/SiC coating and continuous ZrSiO₄-SiO₂-ZrO₂ glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6 wt% after 15 times thermal shock tests and 0.12 wt% after isothermal oxidation test at 1500 °C for 20 h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi₂-SiC/SiC coating from further oxidation.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.