Oxidation behavior of ZrB2-MoSi2-SiC composites in air at 1500 °C

We investigated the oxidation behavior and the effect of the amount of SiC added on oxidation resistance in both hot-pressed ZrB₂-MoSi₂-SiC composites, 55ZrB₂-40MoSi₂-5SiC and 40ZrB₂-40MoSi₂-20SiC (vol.%), exposed to dry air at 1500 °C for up to 10 h. Quantitative electron microprobe analysis characterizations of the chemical compounds of post-oxidized composites were carried out. Parabolic oxidation behavior was observed for both composites. The addition of SiC improved the oxidation resistance of ZrB₂-MoSi₂-SiC composites, and the improvement enhanced with amount of SiC added. The microstructure of the post-oxidized composites consisted of two characteristic regions: oxidized reactive region and unreactive bulk material region. The oxidized reactive region divided into an outermost dense silica-rich scale layer and oxidized reactive mixture layer. The improvement of oxidation resistance with SiC addition is associated with the presence of a thicker dense outermost scale layer which inhibited inward diffusion of oxygen through it.     

Design of an inlaid interface structure to improve the oxidation protective ability of SiC–MoSi2–ZrB2 coating for C/C composites

To improve the oxidation protective ability of SiC–MoSi₂–ZrB₂ coating for carbon/carbon (C/C) composites, pre-oxidation treatment and pack cementation were applied to construct a buffer interface layer between C/C substrate and SiC–MoSi₂–ZrB₂ coating. The tensile strength increased from 2.29 to 3.35 MPa after pre-oxidation treatment, and the mass loss was only 1.91% after oxidation at 1500 °C for 30 h. Compared with the coated C/C composites without pre-oxidation treatment, after 18 thermal cycles from 1500 °C and room temperature, the mass loss was decreased by 30.6%. The improvements of oxidation resistance and mechanical property are primarily attributed to the formation of inlaid interface between the C/C substrate and SiC–MoSi₂–ZrB₂ coating.     

Ablation behavior of C/SiC with YSZ and ZrB2 coatings under high-energy laser irradiation

Carbon fiber-reinforced silicon carbide (C/SiC) composites are important candidates for laser protection materials. In this study, ablation mechanism of C/SiC coated with ZrO₂/Mo and ZrB₂–SiC/ZrO₂/Mo under laser irradiation was studied. ZrB₂–SiC multiphase ceramic and ZrO₂ ceramic were successfully coated on C/SiC composite by atmospheric plasma spraying technology with Mo as transition layer. Phase evolution and morphology of composite were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Moreover, ablation behavior of the composite was investigated by laser confocal microscopy. Results showed that ablation mechanism of C/SiC composite was controlled by phase transformation, thermal reaction, and thermal diffusion, with solid–liquid transition of ZrB₂ and ZrO₂ being dominant factor. Endothermic reaction and good thermal diffusivity of coatings were also important factors affecting ablation performance. Reflectivity effect of ZrO₂ coating was limited under high-energy laser irradiation. Compared with ZrO₂/Mo single-phase-monolayer coating, designed ZrB₂–SiC/ZrO₂/Mo coating showed better ablation performance, and breakdown time of C/SiC increased from 10 to 40 s. The depletion of liquid phase in molten pool was identified as an important factor responsible for rapid failure of C/SiC. The coating failed when the entire liquid phase was consumed within molten pool, followed by rapid damage of C/SiC substrate. Results of this study can provide theoretical guidance and research ideas for design and application of laser protective materials.     

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.     

In situ synthesis of ZrB2–MoSi2 platelet composites: Reactive hot pressing process,microstructure and mechanical properties

Using elemental Zr, B, Mo and Si as raw materials, partially textured ZrB₂–MoSi₂ composites with in situ platelet ZrB₂ grains were fabricated by reactive hot pressing. Synthesized by a combustion reaction, the in situ formed ZrB₂ phase had unique characteristics to grow up to platelet grains, on the surroundings of Mo–Si–B ternary liquid phase at high temperature. Mechanical properties were dependent on grain size and aspect ratio of the ZrB₂ platelets. The rotation and realignment of the platelet ZrB₂ grains under hot pressing led to a partially textured microstructure, which showed anisotropic mechanical properties on different directions of the as-sintered samples. A roadmap of the reaction process, microstructure evolution and texture formation was given to describe the preparation of partially textured ZrB₂–MoSi₂ composites by reactive hot pressing.     

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.     

Incorporation of BN-coated carbon fibers into ZrB2/SiBCN ceramic composites and their ablation behavior

ZrB₂/SiBCN composites containing carbon fibers coated with BN were prepared using combination of sol-gel and spark plasma sintering (SPS) techniques. Thermal shock and ablation behavior of these composites were thoroughly analyzed. Sintered composites contained ZrB₂, SiC and BN(C) crystalline phases. Thin BN layer passivated carbon fibers and prevented them from reacting with the matrix. With the addition of carbon fiber, diffusion coefficient decreased, and the thermal expansion coefficient increased in comparison to ZrB₂/SiBCN composites without carbon fibers. Ablation tests showed no crack formation after carbon fibers were added to the composites. Ablation center exhibited loose and porous network structure. ZrSiO₄ formed from ZrO₂ and SiO₂ reaction in the central ablation region. Presence of ZrSiO₄ prevented samples from further ablation-related damages.     

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.     

Pressureless sintered in situ toughened ZrB2-SiC platelets ceramics

ZrB₂-SiC composite ceramics were densified by pressureless sintering with addition of Si₃N₄ or MoSi₂ at temperatures that induced SiC anisotropic growth from particles to platelets, within a ZrB₂ matrix with rounded grains. Si₃N₄ addition resulted in the formation of large amounts of liquid phase which enhanced mass transfer mechanisms in terms of matrix grain growth and homogeneous distribution of SiC platelets having an aspect ratio of 3. On the contrary, MoSi₂ helped the densification with local formation of liquid phases leading to a finer matrix with finer SiC platelets, though more agglomerated and with a lower aspect ratio (about 2). These different microstructures had very different fracture properties values, namely a toughness of 3.8 MPa m^1/2 and a strength of 300 MPa for the Si₃N₄-doped composite; toughness of 5 MPa m^1/2 and strength of 410 MPa for the MoSi₂-doped one.     

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.     

Synergistic reinforcement effects of ZrB2 on the ultra-high temperature stability of Cf/SiC composite fabricated by liquid silicon infiltration

A carbon fiber-reinforced silicon carbide (C_f/SiC) composite was fabricated with ZrB₂ via the liquid silicon infiltration (LSI) method. A prepreg was prepared by impregnating the phenolic resin with the ZrB₂ powder. The as-LSIed composites were tested for 5 min with an oxyacetylene torch to evaluate their ablation and oxidation properties under an ultra-high temperature environment. The ZrB₂ powders and SiC matrix between carbon fiber bundles generated a dense ZrO₂-SiO₂ layer, which inhibited further oxygen diffusion into the composite and minimized the ablation and oxidation of the carbon fibers. Weight loss and linear ablation rate were further reduced with the addition of ZrB₂ to the C_f/SiC composite; moreover, the synergistic effect of ZrB₂ and SiC reinforced the ablation properties with increased ZrB₂ content. ZrB₂ also reduced the amount of residual silicon, which was detrimental to the mechanical properties of C_f/SiC composite.     

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.     

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.     

Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

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.     

Structural design and ablation performance of ZrB2/MoSi2 laminated coating for SiC coated carbon/carbon composites

A protective coating alternated with ZrB₂ and MoSi₂ laminated layers was designed and prepared on carbon/carbon (C/C) composites with SiC inner layer by supersonic atmosphere plasma spraying. After ablated at a heat flux of 2.4 MW/m² for 30s, ZrB₂/MoSi₂ laminated coating was in good condition with a linear growth rate and mass gain rate of 1.67 μm/s and 0.44 mg/s, respectively. From the central region to the border region, the calculated residual thermal stress of ZrB₂/MoSi₂ laminated coating decreased at first and then increased rapidly, illustrating the size change of the generated laminated cracks. The alternate design of ZrB₂ layers for erosion and MoSi₂ layers for oxidation resulted in the laminated stress distribution and improved ablation resistance.     

Electrical and thermophysical properties of ZrB2 and HfB2 based composites

Electrical resistivities, thermal conductivities and thermal expansion coefficients of hot-pressed ZrB₂–SiC, ZrB₂–SiC–Si₃N₄, ZrB₂–ZrC–SiC–Si₃N₄ and HfB₂–SiC composites have been evaluated. Effects of Si₃N₄ and ZrC additions on electrical and thermophysical properties of ZrB₂–SiC composite have been investigated. Further, properties of ZrB₂–SiC and HfB₂–SiC composites have been compared. Electrical resistivities (at 25 °C), thermal conductivities (between 25 and 1300 °C) and thermal expansion coefficients (over 25–1000 °C) have been determined by four-probe method, laser flash method and thermo-mechanical analyzer, respectively. Experimental results have shown reasonable agreement with theoretical predictions. Electrical resistivities of ZrB₂-based composites are lower than that of HfB₂–SiC composite. Thermal conductivity of ZrB₂ increases with addition of SiC, while it decreases on ZrC addition, which is explained considering relative contributions of electrons and phonons to thermal transport. As expected, thermal expansion coefficient of each composite is reduced by SiC additions in 25–200 °C range, while it exceeds theoretical values at higher temperatures.     

Effects of Si3N4 and WC on the oxidation resistance of ZrB2/SiC ceramic tool materials

ZrB₂/SiC, ZrB₂/SiC/Si₃N₄ and ZrB₂/SiC/WC ceramic tool materials were prepared by spark plasma sintering technology, and their oxidation resistance was tested at different oxidation temperatures. When the oxidation temperature is 1300 °C, the oxide layer thickness, oxidation weight gain and flexural strength of ZrB₂/SiC/Si₃N₄ ceramic tool material after oxidation are 8.476 μm, 1.436 mg cm^?2 and 891.0 MPa, respectively. Compared with ZrB₂/SiC ceramic tool materials, the oxide layer thickness and oxidation weight gain are reduced by 8.2% and 11.8%, respectively, and the flexural strength after oxidation is increased by 116.1%. However, the addition of WC significantly reduces the oxidation resistance of the ceramic tool material. A dense oxide film is formed on the surface of ZrB₂/SiC/Si₃N₄ ceramic tool material during oxidation, which effectively prevents oxygen from entering the inside of the material, thereby improving the oxidation resistance of the ceramic tool material.     

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.     

Effect of tantalum addition on microstructure and oxidation of spark plasma sintered ZrB2-20vol% SiC composites

Almost full density (>99% theoretical density (ρ_th)) was achieved for ZrB₂-20vol% SiC-Xwt.% Ta (X = 2,5, 5 and 10) composites after Spark Plasma Sintering (SPS) (Temperature: 1900 °C, Pressure: 50 MPa; Time: 3 min). The microstructure of ZrB₂-based composites exhibited core-rim structure and it consists of major crystalline phases (ZrB₂ core, (Zr, Ta)B₂ rim, SiC), minor amounts of ZrO₂ and (Zr, Ta)C solid solution phases. Both the specific weight (from 22.91 to 18.77 mg/cm²) and oxide layer thickness (401–195 μm) of ZrB₂-20vol% SiC composites decreased with increasing addition of Ta after the isothermal oxidation at 1500 °C for 10 h in air. The cross-sectional microstructure of oxidized samples displayed presence of a stack of three distinctive layers, which includes thick dense SiO₂ top layer, SiC depleted intermediate layer and unreacted bulk. The present work clearly demonstrated the advantage of tantalum addition in improving the oxidation resistance of ZrB₂-20vol% SiC.