Good Oxidation Resistance Research Articles

Plasma-assisted particle deposition manufacturing: Multi-functional integrated superhigh temperature thermal protection coating on niobium alloy

Multi-functional integrated thermal protection coating is a promising approach for the high-temperature protection of niobium alloy while facing multiple extremely harsh environments, while hard to avoid the complex/multi-step preparation process. Particularly, a simultaneous demonstration of multi-functional features is still challenging. Herein, a novel HfC-HfO2-MoSi2-Yb2O3 multi-functional layer has been fabricated on the NbSi2 layer surface via plasma-assisted particle deposition manufacturing, endowing the modified silicide-based multilayer composite coating with multiple thermal protective characteristics. The composite coating shows excellent hot corrosion resistance with a corrosion gain of 3.56 mg cm−2 after 200 h, the intact coating structure after three thermal cycles of fast rise and fall from 25 °C–1800 °C, and a high thermal emissivity of above 0.9, as well as the good high-temperature oxidation resistance and ablation resistance demonstrated in our previous study. The superior multiple thermal protective characteristics are attributed to the synergistic effects of multi-functional particles. HfC particle provides the anti-ablation skeleton, MoSi2 particle provides more SiO2 glass phase and seals defects, Yb2O3 particle acts as the stabilizer of glass network, and matching vibration absorption of multiphase/multi-chemical bonds endow the high emissivity of coating. Our work paves the new way and provides an inexpensive and environmentally friendly approach for the development of a new class of multi-functional integrated thermal protection materials.

Oxidation resistance mechanism of copper wire regulated by nano-palladium coating

The microstructure characteristics of palladium coating are the key to the oxidation resistance of palladium-coated copper wire. In this paper, the microstructure characteristics of palladium coating thickness, nanocrystalline size and amorphous band width were controlled by adjusting the process parameters of micro-area coating. The relationship between microstructure characteristics of palladium coating and oxidation resistance was studied. The results show that the wire has good oxidation resistance when the deposition temperature is 400℃ and the copper wire/mold clearances are 2.5 μm. The surface of the copper wire has a palladium coating with a thickness of 74 nm, and the palladium particles are composed of amorphous encapsulated nanocrystals. The relationship between the thickness of palladium coating and the copper wire/mould clearances is y=0.012x+44, and the increase of palladium coating thickness can inhibit the migration of copper ions. The increase in the size of the nanocrystalline and the width of the amorphous band has a certain inhibitory effect on the lattice or grain boundary diffusion of copper to the palladium coating. The multi-stage distorted nanotwins in the amorphous palladium coating can reduce the strain difference at the palladium-copper bonding interface, reduce the diffusion driving force of copper outward, and thus improve the oxidation resistance of palladium-coated copper wires.

AUBERT & DUVAL PER72

Abstract Aubert and Duval PER72 (NiCr18Co15TiMoAI alloy) is a precipitation hardening, nickel-base superalloy. The alloy is solid solution strengthened by additions of chromium, cobalt, molybdenum, and tungsten, while additions of titanium and aluminum provide precipitation strengthening. Aubert & Duval PER72 combines high strength with metallurgical stability as demonstrated by excellent impact strength retention after long exposures at elevated temperatures. It also exhibits good oxidation resistance, as well as excellent resistance to sulfide corrosion. This datasheet provides information on composition, physical properties, elasticity, and tensile properties. It also includes information on low temperature performance, high temperature performance, and corrosion resistance as well as forming and heat treating. Filing Code: Ni-787. Producer or source: Aubert & Duval S.A. (a member of the Eramet Group).

Preparation and properties of Ta fiber reinforced high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-SiC composite ceramics

High-entropy boride ceramics are expected to be widely used in aerospace, automotive turbines, and armor protection due to their advantages of high melting point, high hardness, adjustable performance, high-temperature stability, and good oxidation resistance. However, it is urgent to solve the problem of low fracture toughness before application. Therefore, in this paper, a single-phase high-purity (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 powder was prepared by boron/carbon thermal reduction method using a vacuum furnace. The effects of synthesis temperature and C content on the powder were studied. Secondly, HEB ((Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2) powder, SiC powder, and chopped Ta fiber were mixed uniformly, and Ta fiber toughened HEB-SiC composite ceramics were prepared by spark plasma sintering (SPS). The effects of Ta fiber content on the phase composition, microstructure, mechanical properties, and oxidation resistance of the composite ceramics were investigated. The results show that with the increase in synthesis temperature, the HEB powder gradually dissolves, and the solid solution is completely formed at 1700 °C. As the C content increases, the oxygen content and particle size of the powder gradually decrease. Single-phase high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 powders with high purity were prepared at 1700 °C for 1 h with 6 wt% C content. The addition of C will promote the boron/carbon thermal reduction method, reduce the oxygen content, and inhibit grain growth. With the increase of Ta fiber content, the density of HEB-SiC-Taf composite ceramics increased first and then decreased. The hardness gradually decreased, and the fracture toughness gradually increased. When the Ta fiber content was 7 vol%, the fracture toughness was the highest, reaching 5.12 ± 0.39MPa⋅m1/2, which was nearly 45 % higher than that of the composite ceramics without Ta fiber. This is because of the synergistic toughening mechanism of metal toughening and fiber toughening, such as crack deflection, crack bridging, fiber debonding, and fiber pullout, which improves the fracture toughness of the composite ceramics. With the increase in oxidation temperature, B2O3, SiO2, Ta2O5, and various metal oxides appear on the surface of HEB-SiC-Taf composite ceramics. The oxidation depth and weight gain per unit area gradually increase. When the Ta fiber content is 5 vol%, the composite ceramics exhibit the best high temperature stability and oxidation resistance. This is due to the Ta2O5 formed by the oxidation of Ta fibers, which dissolves into the B2O3 glass phase, increasing viscosity and improving high temperature stability while reducing the oxygen diffusion rate.

Enhancing the high-temperature oxidation resistance of C-103 niobium alloy via hybrid treatment combining nickel plating with molten-salt siliconising

ABSTRACT A multi-phase silicide protective coating was prepared by electroplating nickel on the surface of C-103 niobium alloy (89Nb–10Hf–1Ti), followed by molten-salt non-electrolytic siliconising to enhance the high-temperature oxidation resistance of the substrate. The phase composition, microscopic morphology and elemental analysis of the coating were analysed via X-ray diffraction, scanning electron microscope, energy-dispersive spectrometer and hardness distribution. Oxidation characteristics were evaluated by comparing the oxidation inhibition effect of the coating on niobium alloy substrate and that of coating prepared via molten-salt infiltration alone at a constant temperature of 1100°C for 9 h. Results show that a silicide infiltration coating containing about 200-μm-thick compound layer with Ni2Si, NiSi, Nb5Si3, NbSi2, Ni3Si2 and TiNiSi multi-phase composition and ∼10–15-μm-thick diffusion layer were dense with few defects. In the hybrid synthesis process, the upper electroplated nickel coating can promote and modify the growth of subsequent siliconised coating. The duplex prepared silicon infiltration coating can reduce the oxidation rate of niobium alloy substrate by about 1/10, indicating good high-temperature oxidation resistance. This is because the siliconised coating is considerably thick, has a dense blocking structure and generates Ni4Si7Ti4, thereby blocking oxygen.

Novel Fe1.1Co1.1(CrAlx)1.8Gd0.1 high-entropy alloy absorber: Noticeable ferromagnetic transformation and enhanced magnetic loss

High-entropy alloys (HEAs) with excellent soft magnetic properties are gradually emerging as a new generation of electromagnetic wave (EMW) absorption materials. Existing HEAs still cannot efficiently absorb EMW due to inadequate magnetic properties. This paper aims to modulate the ferromagnetic properties of Fe1.1Co1.1(CrAlx)1.8Gd0.1 (x=1, 1.2, 1.4, 1.6) to enhance the permeability and achieve efficient absorption of EMW. The experimental results show that when the ratio of Al to Cr is 1.6:1, the saturation magnetization and permeability reach their highest values. First-principles calculation confirms from an atomic perspective that when the ratio of Al to Cr is 1.4:1, Gd begins to undergo a ferromagnetic transformation. Its magnetic moment increases from −1.2μΒ to 1.4μΒ, and the degree of electron localization also increases. The Fe1.1Co1.1Cr0.75Al1.05Gd0.1 achieves an effective absorption of −51 dB at a thin thickness of 2 mm and an effective absorption bandwidth of 6.2 GHz of 1.4 mm. The cocktail effect also depends on having good corrosion and oxidation resistance. This paper provides theoretical guidance for the design of high-performance soft magnetic EMW absorption materials.

Composition optimization of (Hf, Ta, Zr, Cr)C high‐entropy carbides for good oxidation resistance

AbstractOxidation resistance is crucial to the potential applications of high‐entropy carbides (HECs) at elevated temperatures. Here, we realize the exploration of (Hf, Ta, Zr, Cr)C high‐entropy carbides (HEC‐TM, TM = Hf, Zr, Ta, and Cr) with good oxidation resistance by optimizing their compositions. To be specific, 21 kinds of HEC‐xTM (x = 0–25 mol%) samples are fabricated by a high‐throughput ultrafast high‐temperature sintering technique, followed by oxidation testing at 1673 K for 30 min. Among all the HEC samples, the as‐fabricated HEC‐0Zr samples are proved to possess the best oxidation resistance with an oxidation depth of only 53 µm. Further study on isothermal oxidation kinetics demonstrates that the as‐fabricated HEC‐0Zr samples follow a linear oxidation law. The good oxidation resistance of the as‐fabricated HEC‐0Zr samples is believed to result from the (Ta, Me)2O5 phase with a low melting point, which can promote the densification of the oxide layer. This research opens up a new way for efficiently discovering new HECs for extreme applications.

Printable MXene Ink Hybridized with ZSM-5 for Gas Sensing and CO2 Capturing

In this work we present preparation, characterization and microprinting of Ti3C2/ZSM-5 ink. Printable inks with a viscosity of 5-6 cP were prepared by ultrasonic dispersion of Ti3C2/ZSM-5 nanopowders in DMSO solvent using 1% Polyvinylpyrrolidone (PVP) as dispersant. Brunauer Emmett Teller (BET) results indicate a type IV adsorption isothermal curve for N2 gas with a type H4 retention ring. The data show that the average pore size of the composite material is 23nm, indicating that the material has a micro-mesoporous structure and exhibits excellent adsorption properties for gas sensing applications. However, the adsorption and desorption curves of CO2 shows hysteresis indicating strong interaction between functional groups on the material and CO2 molecules. This material has high CO2 adsorption performance and desorption hysteresis effect, indicating that it can be directly used for CO2 capture. Thermogravimetric (TGA) analysis showed the thermal stability of the composite, with a mass loss of only 5% at 800℃. Ti3C2/ZSM-5 composite ink has excellent anti-oxidation and dispersion properties. When the pure MXene interlayer void is exposed to air, the Ti element in MXene is easily oxidized to TiO2. The introduction of ZSM-5 not only limits the interaction between the functional groups on the surface of MXene, but also fills the layer gap of MXene, effectively reduces the contact with oxygen, and thus shows good oxidation resistance. No characteristic valence signal of TiO2 was observed in XPS results, which confirmed the effectiveness of the method. In addition, imidazole acid molecular sieve frame strengthened MXene adsorption, will have broad prospect of application in gas sensing field. In traditional printing methods, such as drop casting and spin coating, it is difficult to control the uniformity and roughness of coating thickness, which affects the service life and sensing properties of gas sensors. The use of spray atomization technology can maximize the uniform dispersion of ink, and can control the amount of spray, concentration, height and other parameters to control the thickness and roughness of the coating.

Anti-oxidative quartz/carbon-braided fiber reinforced ceramizable composites modified with B4C and AlB2: Mechanical robustness, phase evolution and anti-oxidation mechanism

Carbon fiber reinforced ceramizable phenolic resin matrix composites (Cf/CPR) have been widely used in the field of thermal protection materials. However, the industrial application of the Cf/CPR is restricted by its easy oxidation failure under high temperature condition. In this work, a novel antioxidative ceramizable composites modified with AlB2 and B4C (Cf-Qf/CPR) were prepared by using quartz fiber-coated carbon fiber (Cf-Qf) as reinforcements via co-weaving technique. The thermal stability, mechanical properties, interfacial phase evolution and anti-oxidation mechanism of the composites were investigated. The residue yield and the flexural strength of the composites after treatment at 1200°C for 15 min were 93.4 % and 62.4 MPa, increased by 5.4 % and 67.3 % compared with those of Cf/CPR. Quartz fiber and ceramic protective layer showed synergistic effect to improve the oxidation resistance of carbon fiber and PyC, confirming good high temperature mechanical properties and oxidation resistance of as-received composites.

A novel oxide ceramic matrix composite with integrated high-temperature EMW absorption and mechanical performance

Oxide ceramic matrix composites (OCMC) are extensively used as thermal structural materials due to good high-temperature resistance, mechanical properties, and oxidation resistance, but they are rarely regarded as electromagnetic wave absorbing materials due to the high resistivity of oxide ceramics/fibers. However, high-performance Al2O3 fiber can be used to design and fabricate OCMC with integrated high-temperature microwave absorbing and mechanical properties by introducing dielectric material into Si-based ceramic matrix. The N610/Si3N4-SiOC composites have the prior mechanical performance because the uniformly distributed Si3N4 matrix made the fiber bear effectively. After that, the Sc2O3 modified SiOC matrix was filled into inter- and intra-fascicular pores forming interfacial polarization, dipole polarization, multiple reflection and scattering to attenuate the incident electromagnetic wave (EMW). The effective absorption bandwidth (EAB) and the minimum reflection coefficient (RCmin) of N610/Si3N4-SiOC(Sc) composites at the thickness of 2.75 mm reach 3.0 GHz and −42 dB in the X band, respectively. In addition, the EMW absorption performance of the composite is relatively stable with EABmax and RCmin of 3.5 GHz and −41 dB in 4–18 GHz at high temperatures. This work provides a strategy for designing and fabricating OCMC with excellent mechanical properties and EMW absorption properties.

Efficient preparation and oxidation kinetics of Ti3AlC2 powder via coconut shell charcoal as carbon source

Ti3AlC2 powder was prepared via an efficient solid-liquid reaction in flowing argon using coconut shell charcoal as novel carbon source, and phase evolution, microstructure and oxidation behavior of Ti3AlC2 powder were investigated. The results show that coconut shell charcoal is dominated by amorphous carbon after carbonization with some turbostratic graphite-like structure, and large specific surface area of coconut shell charcoal gives its excellent reactivity. Ti3AlC2 powder prepared at 1400 °C for 1.5 h exhibits good crystallinity and typical nanolayered structure. Ti3AlC2 powder remains thermally stable in air below 552 °C, while oxidizing occurs rapidly between 900 and 1100 °C. The isothermal oxidation curves show a parabolic-like, indicating that the oxidation process is primarily controlled by a diffusion mechanism. The good oxidation resistance of Ti3AlC2 powder is attributed to the oxidized products of TiO2 and Al2O3 grains formed a connected and dense protective layer on the particle surface at elevated temperatures.

Low-temperature fabrication of high-entropy rare earth hexaboride powders

Herein, we report a low-temperature method for synthesizing high-entropy rare earth hexaboride powders by reaction among rare earth oxides, B4C, and Al powders. Al was used as a reducing and decarburization agent in this process, which further reduced the Gibbs free energy change for the reaction of RexOy with B4C to form REB6. By investigating the influence of reacting temperature and amount of Al added on preparation of LaB6, the optimum experimental conditions of this method were determined to be 1300 °C and 2 times the stoichiometric ratio of Al. Theoretical supports were proposed to verify the feasibility for formation of high-entropy borides. Then, multicomponent hexaborides La0.5Ce0.5B6, La0.33Ce0.33Nd0.33B6, and La0.25Ce0.25Nd0.25Sm0.25B6 as well as high-entropy hexaboride La0.2Ce0.2Nd0.2Sm0.2Eu0.2B6 powders were successfully prepared by doping with Ce, Nd, Sm, and Eu on LaB6. RE elements with different radius were uniformly distributed in the multicomponent boride powders without obvious segregation. More importantly, the lattice distortion and stress induced by the element substitution contribute to the reduction of the average powder particle size. Finally, the DTA-TG curves show that rare earth hexaborides have good oxidation resistances when the temperature is below about 1000 °C.

Microstructure, high-temperature cyclic oxidation, and hot corrosion behaviors of Inconel 718 alloy produced by laser-induction hybrid cladding

The microstructure evolution, cyclic oxidation, and hot corrosion behavior of the unheated-treated (As-built) and solution + double aging (SDA)-treated Inconel 718 alloys fabricated through laser-induction hybrid cladding (LIHC) were investigated. It was found that the as-built Inconel 718 alloy exhibits a columnar dendritic with intergranular Laves phase. After SDA-treated, γ", γ′, δ and carbide phases precipitated, the Laves phase became fine and dispersed. After cyclic oxidation, the Cr2O3 scales were formed on both as-built and SDA-treated LIHC-produced Inconel 718 alloy, which could provide good cyclic oxidation resistance for the Inconel 718 alloy. However, the as-built and SDA-treated LIHC-produced Inconel 718 alloy suffered severe hot corrosion in 95 wt% Na2SO4 + 5 wt% NaCl mixed salt solution and many corrosion products were generated, including Cr2O3, NiCr2O4, NaNbO3, Na2CrO4, NiFe2O4, Fe3O4, and NiO. Nevertheless, the SDA-treated sample showed better cyclic oxidation and hot corrosion resistance than the as-built sample, owing to the more compact Cr2O3 scale.

Long‐Term High‐Temperature Oxidation of Al10CoCrFeNi30 High‐Entropy Alloy

The combined effects of high‐temperature and long‐term exposure to oxidation environment on an AlCoCrFeNi‐based high entropy alloy (HEA) are investigated. The alloy has a composition of Al10Co20Cr20Fe20Ni30 (at%) and is oxidized in lab air at 1000 °C for 1000 h to emulate working conditions and long operation cycles of gas turbines. The microstructure and oxide formation of the HEA are characterized using scanning electron microscopy, energy‐dispersive spectroscopy, and X‐ray diffraction. The performance of the HEA is compared to that of Inconel 625 and Hastelloy X and shows slower oxidation kinetics than Inconel 625 until 750 h. The HEA has the thinnest oxide layer of the three alloys and exhibits a triplex structure with a continuous Cr2O3 outer scale, a semi‐continuous Al2O3 subscale, and deep AlN precipitates. The matrix shows signs of Cr and Al depletion after 1000 h, leading to its inability to replace the degraded oxide scales that evaporated or spalled, and decreases its long‐term oxidation resistance. Overall, the HEA shows good oxidation resistance until 750 h, but may benefit from a higher Al content on the surface to form a longer‐lasting Al2O3 subscale that would remain protective beyond 750 h.

Functional Nano-Silicon Dioxide Composite Piano Keys to Improve the Performance of Music in Piano Performance Strategy

The texture of piano keys made of traditional materials is too rough, resulting in poor feel and sound quality when playing. In this article, the performance of piano keys is improved by adding functional nano-silica composites. Functional nano-silica composites were prepared by water glass method and gas phase method. It is found that the functional nano-silica composites have good mechanical strength, oxidation resistance, and wear resistance. In this article, the acoustic test of functional nano-silica composites and traditional plastic keys is carried out. It is found that the longer the delay time, the better the performance.

Preparation and high temperature oxidation resistance of AlSiY coating by arc ion plating

AlSiY coating with excellent antioxidant property was deposited using the arc ion plating technique on a nickel-base single crystal superalloy René N5. The microstructures and morphologies of the coatings, the elemental content and composition of the phases were analyzed by scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS), and X-ray diffractometer (XRD). The results showed that the AlSiY coating was primarily comprised with β-NiAl phase with well combined with the substrate after heat treatment. The average oxidation rate of the coating for 300 h oxidation at 1050 ℃ was 0.8570 mg/cm2, superior to that of the substrate with 1.0035 mg/cm2. The coating exhibited good resistance oxidation during 1050 ℃, and the main phase was β-NiAl phase with high Al content after 300 h oxidation, which suggested that the coating could form a dense Al2O3 layer. In the 1100 ℃ oxidation process, although the depleted aluminum zone of γ′-Ni3Al was formed under the oxide layer after 300 h oxidation, the coating still had a high Al content, which could generate a continuous oxide layer on the surface of coating. The average oxidation rate of coating and substrate was 1.4070 mg/cm2, 3.2730 mg/cm2, respectively, which could provide good oxidation resistance for substrate. The investigation could provide some guidance for the preparation and industrial production of aluminide coatings.

Phase formation, structure and properties of quaternary MAX phase thin films in the Cr-V-C-Al system: A combinatorial study

Tailoring of individual atomic layers via alloying to synthesize quaternary MAX phases allows for expanding their chemical diversity and fine-tuning of their properties. In this study, Cr-V/C/Al multilayered precursors were deposited via combinatorial magnetron sputtering, creating nanostructured architectures with periodic stacking of nanolayers and varying Cr:V ratios. Their phase transformation and underlying reaction mechanisms during subsequent thermal annealing in argon towards the prospective quaternary solid solution (CrV)n+1AlCn MAX phases formation was systematically studied. Crystallization of (CrV)2AlC starts at approximately 500°C, while growth of higher-ordered (CrV)4AlC3 is observed from 960°C. Notably, intergrowth of (CrV)2AlC and (CrV)4AlC3 structures with coherent interface suggests that (CrV)2AlC acts as template for nucleation of (CrV)4AlC3. Thermal stability and high-temperature oxidation tests found that (CrV)2AlC exhibits higher thermal stability compared to (CrV)4AlC3 and films with Cr72.7V27.3 displayed good oxidation resistance at 1000°C, forming a protective bilayer oxide scale consisting of (Cr,Al)2O3 and Al2O3.

Effect of Ta on the high temperature oxidation behavior of Mo-based bulk metallic glasses

Mo-based bulk metallic glasses (BMGs) exhibited very high thermal stability and showed promising applications in high-temperature fields. However, their high-temperature oxidation resistance remains unsatisfactory. In this work, the effect of Ta (0–6 at.%) on the high-temperature oxidation behavior of MoCoB BMG was systematically investigated. The oxidation kinetics of MoCoB BMG and Ta-bearing Mo-based BMGs generally followed a parabolic law at 873–973 K, and the rate constants increased with the increasing temperature. The oxidation products of MoCoB BMG were primarily composed of CoMoO4 and MoO3, and the oxides of Ta-bearing Mo-based BMG were mainly CoMoO4, MoO3, CoTa2O6, and Ta2O5. The addition of Ta can significantly promote adhesion between the substrate and the external oxide layer, forming a dense spinel layer with a diffusion barrier that provides good oxidation resistance to the Mo-based BMG. The thin oxidation layer of Mo44Co34Ta6B16 was only about 13 μm after oxidation at 973 K for 120 h.

Evolution of Microstructure During Stress Relieving Heat Treatment of 1S Aluminium

Aluminium and its alloys are extensively used in nuclear research reactors due to its low neutron absorption coefficient, good heat transfer properties, excellent corrosion and oxidation resistance in air and water environment combined with desirable mechanical properties along with considerable radiation stability. The thin walled tubes are normally manufactured through port hole die extrusion route and used in 'O' tempered condition. The thin tubes in 'O' tempered condition are undergone canning operation which may alter the microstructural features to some extent which may affect mechanical properties. The paper describes the extent to which microstructural and subsequently mechanical properties are altered due to the simulated canning process and further requirements of stress relieving heat treatment to restore the microstructural features and mechanical properties. The following studies are carried out with 1S aluminum tube in 'O' tempered state, simulated canning condition and stress relieved at different temperature, - hardness measurement, X ray line profile analysis for dislocation density measurement, tensile properties measurement using ring tensile testing, electron back scattered diffraction (EBSD) analysis for recrystallization fraction determination and changes in texture components. It is found that the canning operation changes dislocation density, micro-strain, coherent domain size which are well reflected in hardness and ring tensile testing. Subsequent stress relieving at 350°C for 2 hrs. leads to improvement of mechanical properties appreciably.

Investigation of scale formation and degeneration of silicide coating on Nb–Si based alloy in the temperature range from 1250 °C to 1560 °C

Nb silicide coatings modified with active elements have good oxidation resistance, but there is a lack of systematic research on their oxidation behavior at high temperatures above 1250 °C. The aim of this work is to investigate the oxidation behavior of an Al–Y modified silicide coating on Nb–Si based alloy in the temperature range from 1250 °C to 1560 °C. After oxidation at 1250 °C, the scale displays a typical layered structure consisting mainly of a TiO2 outer layer, an amorphous SiO2 matrix layer embedded with TiO2 and ZrSiO4, and an inner layer consisting of SiO2 and Cr2O3. Above 1350 °C, the evaporation reaction of Cr2O3 into CrO3 (g) is significantly accelerated and Cr2O3 disappeared gradually with temperature or time. The TiO2 layers covering on the scales formed at 1250–1500 °C grow along the crystallographic orientation of [100]. The micropores formed on the surface of TiO2 should be attributed to the further oxidation of Cr2O3 into volatile oxide CrO3 (g). The scales exhibit a similar structure consisting mainly of a SiO2 glass matrix embedded with ZrSiO4 and a surface TiO2 layer until 1500 °C. Above the eutectic temperature of SiO2–TiO2 system, the scale is mainly composed of a matrix layer of SiO2–TiO2 eutectic embedded with block TiO2 particles, and a thin SiO2 interface layer at 1560 °C. The degeneration path of the (Nb,X)Si2 coating is (Nb,X)Si2 → (Ti,Nb)5Si4 → γ-(Nb,X)5Si3.