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.     

Erosion mechanism of ternary-phase SiC/ZrB2-MoSi2-SiC ultra-high temperature multilayer coating under supersonic flame at 90° angle with speed of 1400 m/s (Mach 4)

A ternary-phase SiC/ZrB₂-MoSi₂-SiC multilayer coating was prepared on graphite by two-step reactive melt infiltration (RMI) method. The formation mechanism of the coating was studied by HSC chemistry software 6.0. The erosion resistance of the coating was investigated by supersonic flame erosion test at 90° angle, temperature of 2173 K and speed of 1400 m/s (Mach 4) for 120 s. Erosion test results revealed that the SiC/ZrB₂-MoSi₂-SiC multilayer coating had very good erosion resistance. Weight change percentage, mass erosion rate and linear erosion rate of the coating were −0.18 %, −0.027 × 10⁻³g cm⁻² s⁻¹ and 0.33 μm s⁻¹, respectively. Microstructural characterization demonstrated that interesting structures such as rod-like, flake-like, spherical, worm-like and fibrous structures were formed during erosion test. The erosion mechanism of ZrB₂-MoSi₂-SiC coatings is controlled both chemically and mechanically. The reduction of chemical degradation can be attributed to the presence of MoSi₂ particles and the reduction of mechanical degradation can be related to the presence of ZrB₂ particles.     

Influence of chromium diboride on the oxidation resistance of ZrB2-MoSi2 and ZrB2-SiC ceramics

The effect of chromium diboride addition on the densification process and oxidation behavior of two ZrB₂-MoSi₂ and ZrB₂-SiC baseline systems was studied. CrB₂ was beneficial in lowering the sintering temperature owing to the tendency of its oxide to react with MoSi₂ and SiC forming low-melting phases that helped the powder consolidation. Oxidation at 1500 °C induced the formation of further boron oxide as first consequence. In one case, when CrB₂ was combined with MoSi₂, an improved oxidation resistance was observed due to the stabilization of Cr-borides in the subscales saturated with B₂O₃. In the other case, when it was combined with SiC, the excessive low viscosity of the borosilicate glass facilitated the consumption of a thicker portion of materials as compared to the ZrB₂-SiC reference.     

Oxidation inhibition behaviors of HfB2-MoSi2-SiC oxygen blocking coating prepared by spark plasma sintering

The HfB₂-MoSi₂-SiC oxygen blocking coatings were prepared by the spark plasma sintering (SPS) technique, whose oxidation inhibition ability was further strengthened by the pre-oxidation treatment. The effect of MoSi₂ content and pre-oxidation treatment process on the oxygen blocking ability of the HfB₂-MoSi₂-SiC coating at 1973 K were conducted. After SPS, for the HfB₂-MoSi₂-SiC coatings with 20 wt%, 40 wt%, and 60 wt% MoSi₂, the relative density of the coatings are 92.6%, 93.9%, and 85.6%, respectively. Owing to the enhanced compactness of the coatings, increasing MoSi₂ content can significantly improve the protection efficiency of the coatings during the activation oxidation stage. However, due to the increased formation of gaseous by-products during the inerting oxidation stage, excessive MoSi₂ weaken the oxidation inhibition ability of the coatings. The sufficient dispersion of Hf-oxides nanocrystals in the glass layer conduces to enhance the oxygen blocking ability of the glass layer, making the 40HfB₂-40MoSi₂-20SiC coating present the best oxidation protective ability. The pre-oxidation treatment at 1773 K conduces to form the steady glass layer with fewer defects at the cost of a lower oxidation consumption of the coating, which enhanced the protection efficiency of the coating from 96.9% to 99.8% and reduced the oxygen permeability from 0.13% to 0.028%.     

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.     

Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

The high temperature compressive strength behavior of zirconium diboride (ZrB₂)-silicon carbide (SiC) particulate composites containing either carbon powder or SCS-9a silicon carbide fibers was evaluated in air. Constant strain rate compression tests have been performed on these materials at room temperature, 1400, and 1550 °C. The degradation of the mechanical properties as a result of atmospheric air exposure at high temperatures has also been studied as a function of exposure time. The ZrB₂-SiC material shows excellent strength of 3.1 ± 0.2 GPa at room temperature and 0.9 ± 0.1 GPa at 1400 °C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB₂-SiC-C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB₂-SiC composites contain significant matrix microcracking due to residual thermal stresses, and show poor mechanical properties and oxidation resistance. After exposure to air at high temperatures an external SiO₂ layer is formed, beneath which ZrB₂ oxidizes to ZrO₂. A significant reduction in room temperature strength occurs after 16-24 h of exposure to air at 1400 °C for the ZrB₂-SiC material, while for the ZrB₂-SiC-C composition this reduction is observed after less than 16 h. The thickness of the oxide layer was measured as a function of exposure time and temperatures and the details of oxidation process has been discussed.     

Fabrication and characterization of ZrB2–SiC ceramic electrodes coated with a proton conducting, SiO2-rich glass layer

3.5 μm thin layers of dense, crack-free, proton conducting, SiO₂-rich glass have been developed on ZrB₂–SiC ceramic composites, by thermal oxidation at 1400 °C for 30 min in air. A conductivity of 2 mS cm⁻¹ at 25 °C was found, as measured by AC impedance and steady-state voltammetry, and was estimated at ca. 2 × 10⁻² S cm⁻¹ at 80 °C. A striking behaviour of the oxidized ZrB₂–SiC composites is also pointed out: underneath the glass layer, there is a porous layer rich in electronic conductive ZrB₂, without well-defined interface between them, i.e., exhibiting a composition gradient in oxygen. In other words, protonic half-fuel cells could be fabricated under such conditions, for future use in hydrogen or direct alcohol fuel cells.     

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.     

Processing and properties of ZrB2-SiC composites obtained by aqueous tape casting and hot pressing

ZrB₂-SiC composites with different SiC content were prepared through aqueous tape casting and hot pressing. The influences of dispersant, SiC content and binder content on the rheological properties of slurries were investigated and the conditions for preparing stable ZrB₂-SiC suspensions were optimized. After tape casting and drying, the green ZrB₂-SiC tapes showed good flexibility, lubricious surface and homogeneous microstructure. The ZrB₂ ceramics could be densified to 97.2% after hot-pressing, while the ZrB₂ containing 20 and 30 vol% SiC ceramics were nearly fully densified (>99%). The sintered ZrB₂-20 vol% SiC ceramic had improved mechanical properties compared with ZrB₂ ceramic. Further increase in SiC content resulted in lower flexural strength and fracture toughness. SEM and TEM showed a fine microstructure with a clear grain boundary. The fracture mode changed from intragranular type for ZrB₂ to both intragranular and intergranular type for ZrB₂-SiC composites.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

Densification of ZrB2-based composites and their mechanical and physical properties: A review

This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB₂ ceramics and ZrB₂-based composites. Hot-pressing is the most commonly used densification method for the ZrB₂-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB₂-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB₂ ceramics and ZrB₂-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB₂-based composites is in the range of 2–6 MPa m^1/2. Fine-grained ZrB₂ ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi₂. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB₂-based ceramics. In addition, the ZrB₂-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB₂-based composites attractive candidates for high-temperature thermomechanical structural applications.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Oxidation of ZrB2–SiC‐Graphite Composites Under Low Oxygen Partial Pressures of 500 and 1500 Pa at 1800°C

The oxidation behavior of ZrB₂–SiC‐graphite composites under low oxygen partial pressures of 500 and 1500 Pa at 1800°C was investigated. The phase composition and microstructure of the oxidized scale were characterized using TEM, SEM, XRD, XPS, EDS. The analytical results indicated that a low oxygen partial pressure had a remarkable effect on the oxidation mechanism of ZrB₂–SiC‐graphite composites. When oxidized at 1500 Pa, the oxidation kinetics was controlled by the rate of oxygen diffusion into the composite. When the composite was oxidized at 500 Pa, control of the oxidation kinetics changed from the rate of oxygen diffusion to the rate of the oxidation reaction. The rate of oxidation decreased with decreasing oxygen partial pressure. Higher partial pressures of oxygen resulted in less oxidation resistance by the ZrB₂–SiC‐graphite composites.     

Supersonic atmospheric plasma sprayed ZrO2 and ZrB2-SiC/ZrO2 coatings on Cf/Mg composites for anticorrosion

To improve the corrosion resistance of the carbon fiber reinforced magnesium matrix composites (C_f/Mg composites), ZrO₂ and ZrB₂-SiC/ZrO₂ composite coatings were prepared by supersonic atmospheric plasma spraying (SAPS) on C_f/Mg composites. The microstructure and phase composition of the coatings before and after the corrosion test were investigated. Open circuit potential and potentiodynamic polarization tests were measured at room temperature. Results revealed that the corrosion current density ($i_{\mathit{corr}}$) of the ZrO₂ coated C_f/Mg composites decreased by one order while the ZrB_2-SiC/ZrO₂ coated C_f/Mg composites reduced by two orders. Compared with C_f/Mg composites, the corrosion potential ($E_{\mathit{corr}}$) of the ZrO₂ and ZrB₂-SiC/ZrO₂ coated C_f/Mg composites increased by 220.5?mV and 1021.8?mV respectively, indicating that the ZrB₂-SiC/ZrO₂ composite coatings greatly improve the corrosion resistance of C_f/Mg composites. The uniform distribution of the SiC particles with small grain size in ZrB₂ is responsible for the densification of the coating. The ZrB₂-SiC/ZrO₂ composite coatings provide a barrier for the substrate to impede the entry of Cl^- in the corrosion solution, thus exhibiting a better corrosion resistance than the ZrO₂ coating.     

Low temperature synthesis of ZrB2-SiC powders by molten salt magnesiothermic reduction and their oxidation resistance

Herein, we prepare phase-pure ZrB₂-SiC composite powders by molten-salt-mediated reduction of ZrSiO₄/B₂O₃/activated carbon mixtures with Mg, showing that the phase composition and morphology of the above composites is influenced by firing temperature, B:Zr and C:Si molar ratios, and the amount of excess Mg. Notably, phase-pure ZrB₂-SiC powder with a ZrB₂:SiC weight ratio of ~75:25 could be obtained by 3-h firing at 1200?°C, i.e., at a temperature lower than that used for conventional carbothermal reduction by at least 200?°C. As-prepared ZrB₂-SiC composites exhibited grain sizes of several microns and comprised SiC nanoparticles well distributed in the ZrB₂ matrix. Finally, the oxidation activation energies of the prepared ZrB₂ and ZrB₂-SiC powders were determined as 326 and 381?kJ/mol, respectively, which demonstrated that the introduction of SiC improved the oxidation resistance of monolithic ZrB₂.     

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.     

Effect of the surface microstructure of SiC inner coating on the bonding strength and ablation resistance of ZrB2-SiC coating for C/C composites

The present study has been conducted in order to investigate the effect of the surface morphology of SiC inner coating on the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating for C/C composites. The microstructure of SiC inner coatings prepared by chemical vapor deposition and pack cementation at different temperatures were analyzed by X-ray diffraction, scanning electron microscopy, and 3D Confocal Laser Scanning Microscope. Tensile bonding strength and oxyacetylene ablation testing were used to characterize the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating, respectively. Results show that SiC inner coating prepared by chemical vapor deposition has a smooth surface, which is not beneficial to improve the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating. SiC inner coating prepared by pack cementation at 2000 °C has a rugged surface with the roughness of 72.15 µm, and the sprayed ZrB₂-SiC coating with it as inner layer exhibits good bonding strength and ablation resistance.     

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.     

Investigation of MoSi2 melt infiltrated RSiC and its oxidation behavior

Melt infiltration process was employed to fill molten MoSi₂ into the pores of recrystallized silicon carbide (RSiC) to improve the oxidation resistance of RSiC, and an almost fully dense RSiC-MoSi₂ composite with the microstructure of two-phase interpenetrated network was obtained. The phase compositions of the composites are mainly SiC and MoSi₂, including a small amount of Mo_4.8Si₃C_0.6 and Mo₅Si₃. The flexural strength of the composites at room temperature was 11-32% higher than that of as-received RSiC. The cyclic oxidation tests at 1500 °C indicated that both the RSiC-MoSi₂ composites and RSiC exhibited a parabolic oxidation behavior, and the corresponding parabolic rate constants of the composites were almost an order of magnitude lower than those of RSiC, resulting in a great improvement of oxidation resistance of RSiC-MoSi₂.     

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.