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.     

Experimental and first-principles simulation study on oxidation behavior at 1700 °C of Lu2O3–SiC-HfB2 ternary coating for SiC coated carbon/carbon composites

The oxidation behavior and microstructure evolution of Lu₂O₃–SiC-HfB₂ ceramic coating specimen at 1700 °C were investigated systematically by experimental study and first-principles simulation. The prepared ternary coating possesses a compact morphology, which effectively defends C/C substrate against oxidation at 1700 °C for 130 h, showing a good antioxidant property. The formed HfSiO₄, Lu₂Si₂O₇, and HfO₂ with high melting points play an active role in developing the thermal stability of the oxidized scale. Besides, Lu and Hf atoms incline to diffuse into SiO₂, which enhances its structural stability. The improved thermal property of the oxidized scale for the Lu₂O₃–SiC-HfB₂/SiC ceramic coating can delay the effective delivery of oxygen inwardly and thus prolong its oxidation protection time. The quick volatilization of SiO₂ at 1700 °C induces that some glass phase evaporates with being not completely stabilized, which causes the formation of holes and the consumption of the inner coating.     

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.     

A novel combination of precursor pyrolysis assisted sintering and rapid sintering for construction of multi-composition coatings to improve ablation resistance of SiOC ceramic modified carbon fiber needled felt preform composites

A double-layer coating composed of MoSi₂–SiO₂–SiC/ZrB₂–MoSi₂–SiC was designed and successfully constructed by a novel combination of precursor pyrolysis assisted sintering and rapid sintering to improve the ablation resistance of SiOC ceramic modified carbon fiber needled felt preform composites (CSs). The ZrB₂–MoSi₂–SiC inner layer coating was in relatively uniform distribution in the zone of 0–3 mm from the surface of CSs through the slurry/precursor infiltration in vacuum and SiOC precursor pyrolysis assisted sintering, which played a predominant role in improving oxidation and ablation resistance and maintaining the morphology of CSs. The MoSi₂–SiO₂–SiC outer layer coating was prepared by the spray and rapid sintering to further protect CSs from high-temperature oxidation. The ablation resistance of CSs coated with double-layer coating was evaluated by an oxygen-acetylene ablation test under the temperature of 1600–1800 °C with different ablation time of 1000 and 1500 s. The results revealed that the mass recession rates increased with the rise of ablation temperature and extension of ablation time, ranging from 0.47 g/(m²·s) to 0.98 g/(m²·s) at 1600–1800 °C for 1000 s and from 0.72 g/(m²·s) to 0.86 g/(m²·s) for 1000–1500 s at 1700 °C, while the linear recession rates showed negative values at 1700 °C due to the formation of oxides, such as SiO₂ and ZrO₂. The ablation mechanism of the double-layer coating was analyzed and found that a SiO₂–ZrO₂–Mo_4.8Si₃C_0.6 oxidation protection barrier would be formed during the ablation process to prevent the oxygen diffusion into the interior CSs, and this study provided a novel and effective way to fabricate high-temperature oxidation protective and ablation resistant coating.     

Oxidation behaviour and microstructure of a dense MoSi2 ceramic coating on Ta substrate prepared using a novel two-step process

Tantalum (Ta) alloys are important ultra-high-temperature structural materials owing to their excellent high-temperature mechanical properties and processability. However, they exhibit poor high-temperature oxidation resistance. In this study, a dense MoSi₂ ceramic coating was prepared on a Ta substrate using an innovative multi-arc ion plating process and halide activated pack cementation in order to improve its ultra-high-temperature oxidation resistance. This ceramic coating exhibited a low roughness space arithmetic (287.1 ± 26.3 nm) and a dense structure. The relationship between the thickness of the coating and the duration of pack-cementation at 1250 ℃ was parabolic. The coating had a service life of more than 12 h at 1750 ℃, and showed excellent high-temperature oxidation resistance because of the uniform and dense structure of the coating and the rapid formation of a dense SiO₂ layer with low O₂ permeability during high-temperature oxidation.     

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.     

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.     

Oxidation protective MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

A novel kind of dense MoSi₂-SiC-Si coating was prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. Mo-SiC-C precoating was fabricated via slurry dipping method, and then MoSi₂-SiC-Si coating with dense structure consisting of Si, MoSi₂ and SiC was obtained by vapor silicon infiltration process. The isothermal oxidation tests at temperatures from 800 to 1600 °C and TGA test from room temperature to 1500 °C were used to evaluate the oxidation resistance ability of the MoSi₂-SiC-Si coating. The experimental results indicate that the prepared coating has good oxidation protection ability at a wide temperature range from room temperature to 1600 °C. Meanwhile, the oxidation of the coated samples is a weight gain process at temperatures from 800 to 1500 °C due to the formed SiO₂ layer on the surface of coating. After oxidation for 220 h at 1600 °C, the weight loss of the coated sample was only 0.96%, which is considered to be the excessive consumption of the outer coating and the appearance of defects in the coating. Two layers can be observed in the coating after oxidation, namely, SiO₂ layer and MoSi₂-SiC-Si layer.     

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.     

The high temperature oxidation and thermal shock behavior of a dense WSi2–TaSi2 coating on Ta substrate prepared by a novel two-step process

In order to improve the oxidation resistance of the Ta substrate, a novel two-step process including molten salt electrodeposition (Na₂WO₄-WO₃ system) and halide activated pack cementation was adopted to prepare a WSi₂–TaSi₂ coating on tantalum substrate. During the electrodeposition process, dense tungsten coatings were fabricated at current densities of 30 mA/cm², 40 mA/cm² and 50 mA/cm². It was observed that the grain size exhibited a log-normal distribution. When the current density was 40 mA/cm², the grain size and flattest surface of the tungsten coating reached 9.50 ± 0.23 μm and 6.792 μm, respectively. When performing the static oxidation test, the WSi₂–TaSi₂ coating could effectively protect the Ta substrate oxidized at 1600 °C for 30 h. This is attributed to the presence of dense SiO₂ and Ta₂O₅, which acted as a protective layer and suppressed the further penetration of oxygen. Furthermore, due to the matching thermal expansion coefficient between each layer and the sealing ability of semi-molten SiO₂, the four-layer SiO₂–W₅Si₃–WSi₂–Ta₅Si₃ coating could successfully pass 721 thermal shock tests from 1600 °C to room temperature.     

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.     

Ultra-high temperature oxidation resistance of a novel (Mo,Hf, W,Ti)Si2 ceramic coating with Nb interlayer on Ta substrate

In order to solve the challenge of recyclability of tantalum substrates in high temperature oxidation environments, a novel MoSi₂-WSi₂-HfSi₂-TiSi₂ composite ceramic coating containing an Nb interlayer was prepared on the surface of tantalum substrate by a three-step method. The mix ceramic silicide coating exhibited superior performance and effective protection for 10.2 h at 1800 °C, possibly due to the formation of an outer SiO₂-HfO₂-HfSiO₄ composite oxide film with low oxygen permeability, moderate viscosity and thermal expansion coefficient, as well as good self-healing ability. Furthermore, the coating successfully passed 537 thermal cycles from room temperature to 1800 °C. The presence of Nb interlayer significantly mitigated the thermal mismatch between the ceramic coating and the tantalum substrate, and the bidirectional diffusion of Nb element during the high temperature oxidation and thermal shock process further reduced the tendency of the coating to crack.     

Synthesis and oxidation behavior of (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramic

(Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ ceramic was synthesized by self-propagating high-temperature synthesis using commercial elemental powders, and then dense monolithic ceramic was prepared via vacuum hot pressing. The oxidation behavior of the bulk ceramic was investigated at 500 °C and 1200 °C. At 500 °C, due to the preferential oxidation of Nb-rich phase and the formation of uniform oxidation layer, the oxidation rate of (Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ is lower than pure MoSi₂. At 1200 °C, (Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ shows better oxidation resistance than MoSi₂, owing to the uniform complex oxide layer with SiO₂, Al₂O₃ and Nb₂O₅ formed on the surface of the prepared ceramic.     

Oxygen barrier resistance of HfB2-MoSi2-TaB2 coatings in a wide temperature region

HfB₂-MoSi₂-based ultra-high temperature ceramic (UHTC) coatings have shown remarkable antioxidant effects owing to the formation of silicate glass layers with low oxygen permeability in high-temperature environments, which shows great potential in the antioxidation of carbon structural materials. To further enhance the oxidation resistance of the HfB₂-MoSi₂-based coating in a wide temperature region, the influence of volume ratio between HfB₂ and TaB₂ on the antioxidant capacity of the HfB₂-MoSi₂-TaB₂ coatings was investigated. The addition of 15 vol% TaB₂ in the 60HfB₂-40MoSi₂ coating delays the initial oxidation temperature of the 60HfB₂-40MoSi₂ sample from 300 °C to 500 °C, which decreased the oxidation loss by 75.85% during dynamic oxidation. In oxidation process at 900 °C and 1700 °C, the weight gains of the 45HfB₂-40MoSi₂–15TaB₂ coating reduced by 78.56% and 63.14%, respectively. Due to the coexistence of 45 vol%HfB₂ and 15 vol%TaB₂, the suitable Ta⁵⁺ promoted the homogenization and dispersion of Hf/Ta-oxides, which forms the coral-like Hf/Ta oxides skeleton in the glass layer, thus preventing the oxygen penetration and decreasing the inert factor of the HfB₂-MoSi₂ coating at 1700 °C by 51.19%. However, excessive TaB₂ weakened the self-healing ability of the Ta-Hf-Si-O glass layer and inhibited the oxygen barrier effect of the HfB₂-MoSi₂-TaB₂ coating.     

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.     

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.     

Prevention of SiC-fiber decomposition via integration of a buffer layer in ZrB2-based ultra-high temperature ceramics

A ZrB₂-based ceramic, containing short Hi-Nicalon SiC fibers, was fabricated with a Mo-impermeable buffer layer sandwiched between bulk and the outermost oxidation resistant ZrB₂–MoSi₂ layer, in order to prevent inward Mo diffusion and associated fiber degradation reactions. This additional layer consisted of ZrB₂ doped with either Si₃N₄ or with the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. Scanning electron microscopy imaging and elemental mapping via energy-dispersive X-ray spectroscopy showed that this tailored sample geometry provides an effective diffusion barrier to prevent the SiC fibers from deterioration due to reactions with Mo or Mo-compounds. In contrast, the structure of the SiC fibers in a reference sample without buffer layer is strongly degraded by MoSi₂ diffusion into the fiber core. The comparison of the three buffer-layer systems showed a moderate alteration of the fiber structure in the case of Si₃N₄ addition, whereas in the PDC-doped samples hardly any structural change within the fibers was observed. A stepwise reaction mechanism is deduced, based on the continuous progression of a reaction zone that propagates toward the ZrB₂–MoSi₂ top layer. The progression of such a reaction zone as a consequence of the different eutectic melts forming in the different layers, that is, first in the SiC-fiber-containing bulk, then in the buffer layer itself, and finally in the top layer at high temperature, allows for an effective separation of the ZrB₂–MoSi₂ top layer from the SiC fibers. Subsequent oxidation at 1500°C and 1650°C for 15 min did not affect the efficiency of all three buffer layers, since no structural changes regarding buffer layer and fibers were observed, as compared to the non-oxidized samples.     

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.     

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.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.