Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

Densification and oxidation behavior of a novel TiB2–MoSi2–CrB2 composite

Titanium diboride (TiB₂) composite with MoSi₂ and CrB₂ has been prepared and tested to possess excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM. Hardness and fracture toughness values were measured. Extensive oxidation studies of the composites were also carried out. Density of ≥ 96% ρ_th was obtained by hot pressing at 1800 °C under a pressure of 35 MPa for 1 h. Hardness and fracture toughness were in the range of 18–24 GPa and 3.5–4 MPa·m^1/2 respectively. Crack branching, deflection and bridging mechanisms were observed in the crack propagation paths. Isothermal and continuous oxidation studies of these composites up to 1000 °C showed improved oxidation resistance with the formation of protective glassy layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Diffusion controlled mechanism of oxidation was observed in the composites.     

The corrosion of a Co–15 wt% Ce alloy in mixed oxidizing-sulfidizing gases at 600–800°C

The corrosion behavior of a Co–Ce alloy containing approximately 15 wt% Ce has been studied at 600–800°C in several H₂–H₂S–CO₂ mixtures, providing sulfur pressures of 10⁻⁸ atm at 600, 700 and 800°C and of 10⁻⁷ atm at 800°C, and oxygen pressures of 10⁻²⁴ atm at 600°C and 10⁻²⁰ atm at 700 and 800°C. At 600°C, the alloy corrodes more slowly than pure cobalt but more rapidly than pure cerium while, at 700°C, it corrodes at about the same rate as pure cerium, but much faster than pure cobalt. At 800°C under 10⁻⁸ atm S₂, i.e. a value below the stability of the cobalt sulfides, the alloy corrodes rather slowly but, under 10⁻⁷ atm S₂, the rate is very high, although slightly lower than that of pure cobalt. The scaling kinetics are generally intermediate between linear and parabolic but are sometimes irregular. The corrosion of this alloy produces multilayered scales, containing an outermost layer of almost pure cobalt sulfide, an intermediate complex layer composed of a mixture of compounds of the two metals and, finally, an innermost region of internal attack of cerium by both oxygen and sulfur. Cerium is not able to diffuse outwards and remains in the alloy consumption region. In the intermediate region cobalt sulfide forms a continuous network which allows the growth of the external CoS_y layer, although at rates that are reduced with respect to those of pure cobalt. Thus, a cerium content of 15 wt% is not sufficient to prevent or even to significantly reduce the sulfidation of the base metal. These results, as well as the details of the microstructure of the scales, are interpreted by taking into account the limited solubility of cerium in the base metal and the presence of an intermetallic compound, rich in cerium, in the alloy.     

Oxidation behavior of ZrC-based composites in static laboratory air up to 1300 °C

Two ZrC-based composites, viz., ZrC + 20 vol.%SiC and ZrC + 20 vol.%SiC + 20 vol.%ZrB₂, were prepared by hot pressing. The oxidation behavior of the composites was studied in the temperature range of 800 °C–1300 °C in air. From the mass gain tests, the surface and cross-sectional SEM observation and EDS analysis of the oxidized samples, it was found that the ZrC + 20 vol.%SiC composite seemed to have a high oxidation resistance below 1000 °C, but the ZrC + 20 vol.%SiC + 20 vol.%ZrB₂ composite appeared a more excellent oxidation resistance up to 1200 °C. It is main reason that the oxidation rate of ZrC is higher than that of SiC below 1300 °C, meaning that SiO₂ from SiC is insufficient to form a dense tight protection film; however, the addition of ZrB₂ particles promotes the formation of more borosilicate above 1000 °C, which exerts more protective effect by self-healing mechanism.     

High temperature oxidation characteristics of Nb–10W–XCr alloys

The effect of Cr content on the microstructure and cyclic oxidation behavior of Nb–10W–XCr alloys with four different compositions has been investigated. Experiments were conducted in air at 900 °C and 1300 °C; the oxidation kinetics have been evaluated in terms of weight change per unit area with respect to exposure time. Alloy's microstructure consists of Nb solid solution phase regions surrounded by a network of NbCr₂ Laves phase. A trend of improvement in oxidation resistance with increase of the intermetallic phase is observed at 1300 °C and oxidation kinetics follow a parabolic behavior. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates than alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄, and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content. Results delineate the influence of microstructure and composition on the oxidation mechanisms of these alloys that represent a promising base for high-temperature intermetallic alloy development.     

Hot corrosion properties of composite coatings in the presence of NaCl at 700 and 900 °C

Cyclic hot corrosion tests have been carried out on three coatings (one NiCoCrAlY and two composite coatings) at 700 and 900 °C. The kinetic curves and evolution of microstructure show that the composite coating with a Cr-base interlayer performs best. The Cr₂O₃ scale is more effective to protect the coating at 700 °C than that at 900 °C. The corrosion process is accelerated by NaCl via forming volatile MCl_x and inducing the formation of molten voids in the coating or extra oxidation at the interface of fusant/oxide scale, determined by the temperature and the compositions of the coating.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

Mechanosynthesis of rhenium carbide at ambient pressure and temperature

We report the synthesis of Re₂C at ambient pressure and temperature. The formation of rhenium carbide (Re₂C) from the elements was obtained by mechanochemical treatment after 640 min of milling. The microscopy analysis shows polyhedral particle agglomerates with diameters of less than 600 nm. Thermogravimetric analysis results revealed the stability of material at up to 800 °C as well as chemical and surface analysis of polyhedral particles which show oxidative properties and a surface area of 2.0 m² g^− 1, respectively.     

Behaviour and mechanisms of alkali-sulphate-induced hot corrosion on composite coatings at 900 °C

Three coating systems (one single MCrAlY, two composite coatings with/without the Cr-rich interlayer) have been prepared by the arc ion plating (AIP) and electroplating methods. Hot corrosion of the coatings at 900 °C by alkali sulphates shows that a composite coating with a chromium-rich interlayer exhibits the best corrosion resistance. The MCrAlY coating was severely deteriorated in 100 h since its non-productive (Ni,Co)Al₂O₄ and Cr₂O₃ scales induce internal sulfidation and oxidation. However, the microstructural involutions seldom occur within composite coatings, especially for the one with the Cr-rich interlayer. Mechanisms of hot corrosion of the three coatings are discussed.     

Oxidation kinetics of the pack siliconized TiAl-based alloy and microstructure evolution of the coating

The cyclic oxidation resistance of a two-phase TiAl-based alloy was remarkably improved with the formation of composite coating by siliconization with mixed powder of 15 wt%Si + 85 wt%Al₂O₃. The composite coating consists of a Ti₅Si₃-based inner layer and an Al₂O₃-based outer layer. The cyclic oxidation test at 900 °C showed that increasing the siliconization temperature benefits the oxidation resistance. The higher the siliconization temperature, the stabler the Ti₅Si₃-based layer and the more Al₂O₃ concentrated in the outer layer. Usually, the oxidation curves consist of three regions, the parabolic, the linear and the quadratic. For the specimen that was siliconized at 1250 °C for 2 h, however, only the parabolic region appeared during the whole cyclic oxidation test at 900 °C up to 1000 h. The weight gain is less than 0.3 mg cm⁻² after cyclic oxidation at 900 °C for 1000 h, corresponding to a parabolic oxidation rate constant, K_p ≈ 6.03 × 10⁻⁵ mg²/(cm⁴ h). Such a low oxidation rate is attributed to the composite layer of the specimen with a stable Ti₅Si₃-based layer and a dense Al₂O₃-based layer.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Oxidation behaviour of the Mo–Si–B and Mo–Si–B–Al alloys in the temperature range of 700–1300 °C

The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo₃Si, Mo₅SiB₂ and SiO₂ or α-Al₂O₃ phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO₃ as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO₂ and MoO₃, while those formed at 900 °C or above contain Mo, MoO₂ and SiO₂. In addition, α-Al₂O₃ or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B₂O₃–SiO₂.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.     

Synthesis of monodispersed La2Ce2O7 nanocrystals via hydrothermal method: A study of crystal growth and sintering behavior

Monodispersed and uniformly distributed La₂Ce₂O₇ nanocrystals were synthesized via the hydrothermal method using polyethyleneglycol (PEG) as surfactant. X-ray diffraction (XRD), Thermogravimetric analysis/Differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and High resolution transmission electron microscopy (HRTEM) were utilized to characterize the thermal decomposition, phase structure and morphology of the products. Qualitative analysis indicates that the products are comprised of well-dispersed square particles with cubic fluorite structure. The specific surface area and the average crystallite size of the as-prepared products are 195.59 m² g^− 1 and 10–15 nm, respectively. The low effective activation energy (15.27 ± 0.03 kJ mol^− 1) for crystal growth was obtained in the calcination temperature range of 700–1300 °C. The sintering behavior of the compacted body was also investigated, revealing that La₂Ce₂O₇ has a low relative density and open channel morphology.     

High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate

The two-layer MoSi₂/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200 °C for 100 h and at 1300 °C for 80 h were 0.270 and 0.499 mg/cm², respectively. Compared with the monolithic MoSi₂ coatings, the transformation of MoSi₂ phase in the MoSi₂/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo₅Si₃ layer in the composite coatings was two orders of magnitude lower than that of the Mo₅Si₃ layer in the monolithic coatings at 1300 °C. The microstructural degradation of MoSi₂ in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi₂ coatings at high temperatures.     

Dislocation climb in Mo5SiB2 during high-temperature deformation

During high-temperature compression tests on intermetallic Mo₅SiB₂, the dislocation microstructures vary with increasing temperature and strain rate. At 1400 °C, an increasing tendency exists for slip planes to be of an unexpected type (e.g., {143) and {523)) as a function of the decreasing strain rate and increasing strain that originates from a dislocation climb. As the temperature increases to 1600 °C, the internal strain rate of 6.07 × 10^− 3 s^− 1 from the dislocation climb at 4% strain exceeds the applied value of 1.67 × 10^− 3 s^− 1, and thus, the climb mainly controls the plastic strain, as evidenced by a strength that is lower than that at 1200 °C under the same conditions.     

Electrical properties,sintering and thermal expansion behavior of composite ceramic interconnecting materials,La0.7Ca0.3CrO3−δ/Y0.2Ce0.8O1.9 for SOFCs

This paper presents a high-performance interconnect ceramic for solid oxide fuel cells (SOFCs), based on a modification of La_0.7Ca_0.3CrO_3−δ (LCC). It was found that addition of a small amount of YDC (Y_0.2Ce_0.8O_1.9) into LCC dramatically increased the electrical conductivity. For the best system, LCC + 3 wt.% YDC, the electrical conductivity reached 104.8 S cm⁻¹ at 800 °C in air. The electrical conductivity of the specimen with 2 wt.% YDC in H₂ at 800 °C was 5.9 S cm⁻¹. With the increase of YDC content, the relative density increased, reaching 97.6% when the YDC content was 10 wt.%. The average coefficient of thermal expansion (CTE) at 30–1000 °C in air increased with YDC content, ranging from 11.12 × 10⁻⁶ K⁻¹ to 15.34 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating that it is still an electronically conducting ceramic. Therefore, it is a very promising interconnecting ceramic for SOFCs.     

Novel thermal protection coating based on Zr0.75Ce0.25O2/phosphate duplex system for polyimide matrix composites fabricated via a combined sol–gel/sealing treatment process

Thermal protection coating based on Zr_0.75Ce_0.25O₂/phosphate system was fabricated on polymer–matrix composites via a combined sol–gel/sealing treatment process. Phosphates sealed the cracks and enhanced the adhesion property via chemical bonding and binding. The Zr_0.75Ce_0.25O₂/phosphate duplex coating exhibited good thermal shock resistance and improved thermal oxidation resistance of the substrate. Due to the protection of the duplex coating, the weight loss of the specimen reduced from (4.83 ± 0.12)% to (0.98 ± 0.08)% and the mass ablation rate decreased from 0.088 ± 0.002 mg cm⁻² s⁻¹ to 0.018 ± 0.002 mg cm⁻² s⁻¹ when testing at 810 °C. Coating failure was attributed to the formation of cracks and delamination.