Polymer derived oxidation barrier coatings for Mo-Si-B alloys

Thermodynamic analysis was carried out to predict the phase composition of a perhydridopolysilazane-type polymer derived ceramic coating on Mo₅SiB₂ matrix particles after heat treatment. The most probable chemical reactions between these constituents and resulting phases were calculated. The feasibility of PHPS/Mo₅SiB₂ chemical reactions was proved experimentally. An amorphous SiO_xN_y phase and free Si were found by X-ray diffraction analysis and Raman spectroscopy. The presence of these two phases explain the improvement in oxidation resistance of the Mo₅SiB₂ particles, which was found to be as twice as high at 800 °C and 1100 °C in air, compared to the unprotected plain Mo₅SiB₂ phase. The oxidation of the free silicon provided by the PHPS conversion was addressed as an oxygen trap.     

Fabrication of Mo5SiB2-based composites by combustion synthesis involving aluminothermic reduction of MoO3

Fabrication of Mo₅SiB₂–Al₂O₃ composites with a broad range of the Mo₅SiB₂/Al₂O₃ ratio was conducted by thermite-based combustion synthesis in the SHS mode. Thermite reagents of MoO₃ +?2Al were introduced into two Mo?Si–B ternary systems adopting amorphous boron and MoB as their respective boron source. The boron-containing samples were more energetic and were applied to produce composites with Mo₅SiB₂/Al₂O₃ from 1.25 to 2.5, beyond which combustion was extinct. The composites with Mo₅SiB₂/Al₂O₃ from 0.8 to 1.3 were prepared from less exothermic MoB-based samples. Besides causing a decrease in combustion velocity and reaction temperature, the increase of the Mo₅SiB₂/Al₂O₃ proportion led to a transformation in combustion wave propagation from a steady to pulsating mode. For the samples featuring a pulsating combustion wave, the reaction time at peak combustion temperatures was extended and then the evolution of Mo₅SiB₂ from intermediate phases (Mo₃Si and MoB) was significantly improved. The deduced activation energy suggests a lower kinetic barrier for thermite-based combustion synthesis to fabricate Mo₅SiB₂-based composites. The microstructure of synthesized composites indicates that quadrangular Mo₅SiB₂ grains with an average size of 10–15?µm are tightly packed and irregular Al₂O₃ grains are randomly dispersed.     

Polymer derived ceramic materials from Si,B and MoSiB filler-loaded perhydropolysilazane precursor for oxidation protection

Silica-based coating systems were developed using polymer derived ceramics (PDCs) technology. Ceramic composites on the base of a SiO₂ and SiNO matrix and homogeneously distributed Mo₅SiB₂, SiB₆, Si and B fillers were manufactured. The coating systems have low porosity and provide a high oxidation resistance up to 100 h at 800 °C and 1100 °C in air. The influence of temperature and atmosphere of pyrolysis on the polymer precursor, the volume fraction of filler materials on the chemical composition of compacts as well as their high-temperature oxidation protection was investigated.     

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 (2.5 µm), a MoSi₂ layer (18 µm) and a Mo–Mo₅Si₃–Mo₃Si layer (2–4 µm) 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 50 wt%, 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 0.17 wt% after oxidized at 1600 ℃ for 70 h. The Si–MoSi₂ functionally graded coating exhibits better high temperature oxidation resistance than pure MoSi₂ coating.     

Effects of Mo addition on tribological performance of plasma-sprayed Ti–Si–C coatings

Ti–Si–C–Mo composite coatings were fabricated by plasma spraying using Ti, Si, graphite and Mo powders. The effect of Mo on microstructure and tribological performance of the Ti–Si–C coatings were investigated. The results showed that the Ti–Si–C coating consisted of TiC, Ti₃SiC₂, Ti₅Si₃, and residual graphite. The Ti–Si–C–Mo coatings consisted of TiC, Ti₃SiC₂, Ti₅Si₃, residual graphite, Mo and Mo₅Si₃ phases. With increasing Mo contents, the fractions of Mo and Mo₅Si₃ phases increased, and the fractions of Ti₃SiC₂ and Ti₅Si₃ phases decreased. All the coatings existed a typical lamellar structure. The addition of Mo enhanced the hardness and fracture toughness of Ti–Si–C coating by 16% and 52%, respectively. The coating porosity decreased by 57.6%. The wear resistance of the Ti–Si–C coating was also improved and the mass loss decreased by 83%. The wear mechanism of the Ti–Si–C–Mo coatings was the combination of abrasive wear, adhesive wear, and tribo-oxidation wear.     

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.     

Superplasticity of a multiphase fine-grained Mo-Si-B alloy

Mo-9Si-8B-3Hf alloy consisting of a Mo solid solution and intermetallic phases Mo₃Si and Mo₅SiB₂ was fabricated by hot pressing sintering to yield a fine microstructure with all three phases being in the size range of micrometer. The tensile properties of this alloy were evaluated in vacuum at elevated temperatures. This alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400 °C to 1560 °C with strain rate of 3 × 10^− 4 s^− 1. The tensile elongation of 410% was measured at 1560 °C. Grain boundary sliding was the main mechanism of plastic deformation for this alloy.     

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.     

High emissivity MoSi2–ZrO2–borosilicate glass multiphase coating with SiB6 addition for fibrous ZrO2 ceramic

To develop a high emissivity coating on the low thermal conductivity ZrO₂ ceramic insulation for reusable thermal protective system, the MoSi₂–ZrO₂–borosilicate glass multiphase coatings with SiB₆ addition were designed and prepared with slurry dipping and subsequent sintering method. The influence of SiB₆ content on the microstructure, radiative property and thermal shock behavior of the coatings has been investigated. The coating prepared with SiB₆ included the top dense glass layer, the surface porous coating layer and the interfacial transition layer, forming a gradient structure and exhibiting superior compatibility and adherence with the substrate. The emissivity of the coating with 3 wt% SiB₆ addition was up to 0.8 in the range of 0.3–2.5 μm and 0.85 in the range of 0.8–2.5 μm at room temperature, and the “V-shaped grooves” surface roughness morphology had a positive effect on the emissivity. The MZB-3S coating showed excellent thermal shock resistance with only 1.81% weight loss after 10 thermal cycles between 1773 K and room temperature, which was attributed to the synergistic effect of porous gradient structure, self-sealing property of oxidized SiB₆ and the match of thermal expansion coefficient between the coating and substrate. Thus, the high emissivity MoSi₂–ZrO₂–borosilicate glass coating with high temperature resistance presented a promising potential for application in thermal insulation materials.     

Reaction synthesis of spark plasma sintered MoSi2-B4C coatings for oxidation protection of Nb alloy

MoSi₂-B₄C coatings with different B₄C contents were prepared on Nb alloy by spark plasma sintering (SPS) process. Powder mixtures of Mo, Si and B₄C were used as the coating starting materials. Besides MoSi₂ and B₄C phases, small amounts of SiC and MoB are also found in the coatings because of the reactions of Mo, Si and B₄C powders during sintering. Compared with single MoSi₂ coating, the MoSi₂-B₄C coatings show better oxidation resistance at 1450?℃, and dense B₂O₃-SiO₂ oxide scales form after 100?h oxidation. The B₄C or MoB in the MoSi₂-B₄C coatings can serve as the B donor for the formation of B₂O₃. A slight degradation in the microstructure of the MoSi₂-B₄C coatings after oxidation is observed, which can be attributed to the presence of an NbB layer in the inter-diffusion zone of the coatings that retards the inward diffusion of Si from the coating into the substrate alloy. The microstructure development and oxidation behavior of the MoSi₂-B₄C coatings have been discussed.     

Corrosion behavior of a Mo(Si,Al)2 composite at 1700°C in 95% N2 + 5% H2

A Mo(Si, Al)₂ based composite was pre-oxidized to establish an alumina scale on the material surface. Thereafter, the corrosion behavior of the composite was examined at 1700 °C for up to 24 h in 95% N₂ + 5% H₂.The weight change was followed by recording the material weight before and after exposure. The crystalline corrosion products were analyzed with X-ray diffraction (XRD) and the microstructure of the cross sectioned material was characterized using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS).It was shown that AlN and Al₅O₆N layers developed on top of the pre-oxidized alumina layer and alumina threads develop out from the specimen surface. The accompanied aluminum consumption converts the substrate Mo(Si,Al)₂ into Mo₅Si₃ immediately below the alumina scale to the extent that the Mo₅Si₃ becomes porous underneath the alumina scale. Corrosion mechanisms are discussed with the support of thermodynamic calculations.     

A new route to fabricate SiB4 modified C/SiC composites

In order to improve the oxidation and thermal shock resistance of 2D C/SiC composites, dense SiB₄–SiC matrix was in situ formed in 2D C/SiC composites by a joint process of slurry infiltration and liquid silicon infiltration. The synthesis mechanism of SiB₄ was investigated by analyzing the reaction products of B₄C–Si system. Compared with the porous C/SiC composites, the density of C/SiC–SiB₄ composites increased from 1.63 to 2.23 g/cm³ and the flexural strength increased from 135 to 330 MPa. The thermal shock behaviors of C/SiC and C/SiC–SiB₄ composites protected with SiC coating were studied using the method of air quenching. C/SiC–SiB₄ composites displayed good resistance to thermal shock, and retained 95% of the original strength after being quenched in air from 1300 °C to room temperature for 60 cycles, which showed less weight loss than C/SiC composite.     

Enhanced mechanical properties of carbon fibre/lithium aluminosilicate composites modified by SiB6 addition

ABSTRACT

Carbon fibre-reinforced lithium aluminosilicate matrix composites (C_f/LAS) with different SiB₆ contents were prepared by the hot pressing method to assess their mechanical properties and oxidation resistance. Composite that was incorporated with 2 wt-% SiB₆ exhibited the highest flexural strength of 500 ± 22.3 MPa. Weight loss and residual strength of C_f/LAS modified by SiB₆ were analysed. The results indicated that the addition of SiB₆ had a remarkable effect on improving the oxidation resistance for C_f/LAS. To establish a direct relationship among interfacial microstructure, mechanical and oxidation behaviour of the studied composites, their connection was examined and discussed.     

Combustion synthesis of Mo<Subscript>5</Subscript>Si<Subscript>3</Subscript> in chemical furnace

The combustion synthesis of Mo₅Si₃ was carried out in a chemical furnace using 5Ti + 3Si mixtures as a chemical fuel. The synthesized single phase Mo₅Si₃ exhibited a spatial network microstructure and angular texture of crystals. The chemical composition and crystal morphology of intermediate phases suggest that the formation of Mo₅Si₃ occurs via dissolution of Mo in the Si melt followed by reactive diffusion in the supersaturated “liquid Si-Mo melt-solid Mo” system.      

Comparative study of failure mechanisms of MoSi2 coating on Mo1 wire mesh under isothermal oxidation and hot-fire test using a hydroxylammonium nitrate based monopropellant

A MoSi₂ coating was prepared on the Mo1 wire mesh via pack cementation method, and its failure mechanisms under isothermal oxidation and hot-fire test using a hydroxylammonium nitrate based monopropellant were comparatively studied. Under isothermal oxidation at 1300 °C and 1400 °C, degradation of MoSi₂ into Mo₅Si₃ caused failure of the coating, and interdiffusion made a much larger effect relative to oxidation. However, the MoSi₂ coating failed because of the synergy of oxidation, ablation, and interdiffusion under hot-fire test. Besides, dissolution of mullite into SiO₂ and ablation of high velocity flame contributed to the failure of the coating as well.     

MoSi2和(Mo,W)Si2涂层的宽温域氧化过程

采用包渗法在Mo及Mo?W基体上分别制备MoSi2及(Mo,W)Si2涂层,研究了W掺杂对MoSi2涂层抗氧化性能的影响规律和作用机理。结果表明,W元素固溶到MoSi2涂层中,形成(Mo,W)Si2固溶体,涂层微观结构更加致密化。在1600℃高温下静态氧化,(Mo,W)Si2涂层抗氧化失效时间长达70 h,1200℃下氧化1000 h仍具有良好的防护性能,抗氧化性能大幅提升。加入W元素阻碍了Si元素与基体间的扩散反应,降低了涂层中Si元素的消耗速率,显著增强了(Mo,W)Si2涂层抗高温氧化性能。在500℃低温下静态氧化50 h,与MoSi2涂层相比,(Mo,W)Si2涂层氧化产生明显的“Pest”现象,涂层严重粉化失效。加入W元素降低了涂层中Si元素的扩散速率,导致低温下涂层表面无法形成致密氧化层,加剧涂层的快速氧化。     

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.     

Microstructure and oxidation resistant of Si–NbSi2 coating on Nb substrate at 800°C and 1000°C

The Si–NbSi₂ composite coating with a smooth surface was successfully prepared on Nb substrate by hot dip silicon-plating (HDS) technology. The composite coating is composed of Si outer layer, NbSi₂ interlayer and Nb₅Si₃ interfacial layer. And the average surface roughness (RSa) and specific surface area growth rate (Sdr) are only 0.275 μm and 2.85%, respectively. The cyclic oxidation test shows that the Si–NbSi₂ composite coating has a very excellent oxidative resistance after oxidation at 800 °C for different times. After oxidation for 40 h, the Δm/S and oxide layer thickness of the coating are only 3.72 mg/cm² and 8 μm, respectively. After oxidation at 1000 °C for 20 h, the coating surface is almost completely covered by a dense SiO₂ layer, the Δm/S and oxide layer thickness of the coating are 7.28 mg/cm² and 15 μm, respectively. The Si–NbSi₂ composite coating presents good self-healing ability and excellent oxidation resistance, which can significantly prolong the service life of bare Nb in oxidation environment.     

TaSi2–SiC high-emissivity coating modified by SiB6 on flexible alumina fibre fabric with enhanced oxidation resistance and high interfacial bonding strength

To improve the oxidation inhibition of TaSi₂-based high-emissivity coatings at high temperatures, TaSi₂–SiC coating modified by SiB₆ was prepared on the surface of alumina fibre fabrics. The effects of the SiB₆ content on the surface appearance and emissivity of the coating were investigated, and the mechanical properties of the coated fabrics were compared. When the SiB₆ content in the coating was 2.5%, the borosilicate glass liquid phase generated by SiB₆ oxidation effectively prevented the oxidation of TaSi₂. The bond strength between the coatings and fibre fabric was 207 kPa after calcination at 1200 °C, which was 39% higher than that of the coated fabric without SiB₆. The emissivity of the TaSi₂–SiC coating, modified by a SiB₆ content of 2.5%, reached above 0.92 after calcination at 1200 °C for 5 h. Therefore, the TaSi₂–SiC high-emissivity coating modified by SiB₆ has good application prospects in the field of thermal protection.     

Oxidation and cracking/spallation resistance of ZrB2–SiC–TaSi2–Si coating on siliconized graphite at 1500 °C in air

A ZrB₂–SiC–TaSi₂–Si coating on siliconized graphite substrate was prepared by a combination process of slurry brushing and vapor silicon infiltration. The high-temperature oxidation behavior and cracking/spallation resistance of the as-prepared coating were investigated in detail. It was revealed that the oxidation kinetics at 1500 °C in static air followed a parabolic law with a relatively low oxidation rate constant down to 0.27 mg/(cm²·h^0.5). The crack area ratio of the as-prepared coating was determined as 3.8 × 10⁻³ after severe thermal cycling from 1500 °C to room temperature for 20 times. Apart from the formation of ZrO₂ as skeleton phase with SiO₂ as infilling species, the good oxidation and cracking/spallation resistance of the coating also could be attributed to its unique duplex-layered structure, i.e., a dense ZrB₂–SiC–TaSi₂ major layer filled with Si and an outermost Si cladding top layer. Meanwhile, the strong adhesion strength of the SiC transition layer with the graphite substrate and the outer ZrB₂–SiC–TaSi₂–Si layer was a vital factor as well.