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•MoS2 coatings were synthesized by hydrothermal method combined with drop casting.•Annealing treatments were employed to synthesize 2H-MoS2 with various grain sizes.•2H-MoS2 with high ...crystallinity significantly improved lubrication performance.•Enhanced lubricity was attributed to 2H phase and fewer grain boundaries.•2H-MoS2 with high crystallinity had oxidation resistance and less Mo exposure.
Molybdenum disulfide (MoS2), a typical lubricant material in vacuum environments, encounters limitations in broader industrial applications due to its vulnerability to oxidative degradation. Adjusting crystal structure or improving crystallinity can effectively increase the oxidation resistance of MoS2, and then enhance its lubrication performance. Herein, a simple hydrothermal method combined with post-annealing treatments was used to synthesize MoS2 and tailor crystal structure and crystallinity. Structural and morphological analyses indicated that the crystal structure of MoS2 had been transformed from 1T′-MoS2 to 2H-MoS2, and the improved crystallinity with larger grain sizes reduced grain boundaries after annealing treatments. These structural variations were corroborated by in-situ Raman spectroscopy during laser annealing and subsequent tribological evaluation, which demonstrated 2H phase and higher crystallinity were beneficial for improving the oxidation resistance and longevity of MoS2 coating. These results were further validated by density functional theory calculation. The enhanced oxidation resistance could be ascribed to the protective role of the S atomic layer on Mo layer, coupled with high hydrophobicity and reduced grain boundaries by high crystallinity. Furthermore, the oxidation mechanism of MoS2 in the sliding process has been clarified, which can provide a novel pathway to improve its lubrication performance in atmospheric environment.
Ceramic matrix composites are desirable for applications in extreme environments due to the excellent ablation resistance, but few ceramic materials can exhibit a matched load bearing capacity ...currently. The technological potential of functionally graded materials, such as improved ablation resistance and enhanced load-bearing capacity, has opened up the feasibility for designing highly efficient thermal protection materials. In this study, composite insulation tiles, with three-dimensional braided carbon fibers in a ceramic-polymer continuous gradient matrix, are designed and fabricated by using a temperature-controlled polymer-derived ceramic method. The ablation behavior of the specimens exposed to high-temperature high-speed wind tunnel environments has been investigated at a total enthalpy of 15.8 MJ/kg. Temperature response, morphology and composition characteristics are obtained, which allows detecting oxidation resistance and thermal stability of the gradient composites. The results showed that the formation the silicon oxide layer covering the ablated surface and the inner gradient layer composed of amorphous SiCN ceramics display a key role in resisting ablation and reducing heat conduction. Meanwhile, the underlying resin layer can display superior load-bearing capacity at environment temperatures lower than pyrolysis. The present study highlights high ablation behavior of continuous gradient ceramic-polymer composites, offering a possibility in developing thermal protection composites with good trade-off between thermal and mechanical properties.
In this work, high-entropy ceramics (Hf0.25 Zr0.25Ta0.25Nb0.25)C (HZTNC) and HZTNC doped with 20 vol% SiC (HZTNC-SiC) were fabricated by spark plasma sintering. Their oxidation behavior was ...investigated over the temperature range of 1300–1500 °C for up to 60 min. Both HZTNC and HZTNC-SiC exhibited good oxidation resistance, and their weight change as a function of oxidation time obeyed a parabolic law. Through XRD, microstructure observation, and elemental mapping analysis of the oxide layers, it was found that the formation of Nb2Zr6O17, Hf6Ta2O17, and (Ta, Nb)2O5 mixed-oxide layers effectively protected the matrix from further oxidation. Microcracks began to appear on the oxide layer of HZTNC at high temperatures after 60 min of oxidation. However, the addition of SiC in HZTNC suppressed these microcracks at high temperatures due to the active oxidation of SiC. Compared with the oxides formed on HZTNC, the additional formation of Hf(Zr)SiO4 on HZTNC-SiC could further improve its oxidation resistance over HZTNC ceramics.
Alloying with Si or Ta is an effective method to upgrade the properties of TiAlN coatings. In the present work, we prepared Ti0.54Al0.46N, Ti0.46Al0.45Si0.09N as well as Ti0.42Al0.43Si0.09Ta0.06N ...coatings, and then investigated the effect of Si and Ta mixed doping on the structure, mechanical and thermal properties of TiAlN coatings. The hardness of cubic Ti0.54Al0.46N coating is 28.6 ± 0.9 GPa, whereas the mixed cubic and wurtzite Ti0.46Al0.45Si0.09N coating presents a higher hardness of 29.1 ± 1.1 GPa. The Si and Ta mixed doping leads to a maximum hardness of 34.1 ± 0.8 GPa for cubic Ti0.42Al0.43Si0.09Ta0.06N coating. The thermal stability of TiAlN coatings is more beneficial from the Si and Ta mixed doping. After annealing at 1100 °C, the Ti0.42Al0.43Si0.09Ta0.06N coating reaches a peak hardness of 37.9 ± 1.1 GPa due to the age-hardening effect from the spinodal decomposition, whereas the hardness of Ti0.54Al0.46N and Ti0.46Al0.45Si0.09N coatings decrease to 30.4 ± 1.3 and 32.9 ± 0.9 GPa, respectively. Furthermore, the Ti0.42Al0.43Si0.09Ta0.06N coating shows the best oxidation resistance. After oxidation at 1000 °C for 15 h, the oxide scale thickness of Ti0.42Al0.43Si0.09Ta0.06N is only ∼0.56 μm, whereas ∼0.91 μm thick oxide scale is formed on the Ti0.46Al0.45Si0.09N coating.
•Ti0.46Al0.45Si0.09N coating is a mixed cubic and wurtzite structure.•Cubic Ti0.42Al0.43Si0.09Ta0.06N coating behaves the maximum hardness.•Ti0.42Al0.43Si0.09Ta0.06N presents the best thermal stability.•Si and Ta mixed doping improves the oxidation resistance of TiAlN.
Copper surfaces that exhibit a wide range of achromatic colors while still metallic have not been studied, despite advancements in antireflection coatings. A series of achromatic copper films grown ...with 111 preferred orientation by depositing 3D porous nanostructures is introduced via coherent/incoherent atomic sputtering epitaxy. The porous copper nanostructures self‐regulate the giant oxidation resistance by constructing a curved surface that generates a series of monoatomic steps, followed by shrinkage of the lattice spacing of one or two surface layers. First‐principles calculations confirm that these structural components cooperatively increase the energy barrier against oxygen penetration. The achromaticity of the single‐crystalline porous copper films is systematically tuned by geometrical parameters such as pore size distribution and 3D linkage. The optimized achromatic copper films with high oxidation resistance show an unusual switching effect between superhydrophilicity and superhydrophobicity. The tailored 3D porous nanostructures can be a candidate material for numerous applications, such as antireflection coatings, microfluidic devices, droplet tweezers, and reversible wettability switches.
A series of achromatic Cu films with semi‐permanent stability and switchable superhydrophobic and superhydrophilic states on rough or curved surfaces is realized by an innovative strategy. Comprehensive atomic‐resolution microscopy reveals that their surfaces are of a series of monoatomic steps and have a counterintuitive lattice contraction, which demonstrates the self‐oxidation resistance of copper.
•Nanostructured YSZ top coat slightly reduced double layered TGO growth rate and weight gain of TBCs.•Y was slightly incorporated in the Al2O3 (TGO) layer of HVOF NiCoCrAlTaY coating as Y containing ...small oxide precipitates.•The segregation of Ta-enriched oxides (TaOx) at the middle of TGO layer led to the formation of some cavities.•TaOx formation in TBCs including MCrAlTaY bond coats can be prevented by presence of Al2O3 film including fine Al-Y oxides.
In this research, the effects of thin Al2O3 diffusion barrier layer (including Y-containing small oxide precipitates) and nanostructured YSZ top coat on the high temperature oxidation behavior of HVOF NiCoCrAlTaY/APS YSZ coatings were examined by using isothermal oxidation tests at 1100 °C. Nanostructured YSZ top coat could slightly reduce the double layered TGO growth rate and weight gain of thermal barrier coating systems. However, Al2O3 diffusion barrier layer (consisting of Y-containing small oxide precipitates) could substantially decline the oxidation rate and Ta movement (Ta-enriched oxides formation) in the NiCoCrAlTaY/Al2O3/nanostructured YSZ coating at 1100 °C.
Oxidation behavior of high-entropy carbide (Hf0.2Ta0.2Zr0.2Ti0.2Nb0.2)C (HTZTNC) was investigated over temperature range of 1400–1600 °C. Results showed improved oxidation resistance of high-entropy ...carbide compared with individual carbide ceramics. In oxide layer, Ta2O5 and Nb2O5 were found to be dominant phases at 1400 °C, whereas ZrTiO4 and HfTiO4 were main phases obtained at 1500 and 1600 °C. Moreover, these complex dense oxide layer structures on the surface of HTZTNC at high temperature led to excellent oxidation resistance. The observation of Ti-depleted layer at 1500 and 1600 °C after 20 min of oxidation indicated that oxidation mechanism involved outward diffusion of titanium oxide, which was further confirmed by reoxidation experiments. In sum, these findings are promising for future development of high-entropy ultrahigh temperature ceramics with good oxidation resistance.
For diesel oxidation catalyst (DOC), the sintering and oxidation of active Pt0 nanoparticles leads to severe deactivation during long-term operating condition. Here, we have synthesized SiO2-modified ...Pt/SiO2-Al2O3 (Pt-SiAl@Si) catalysts, and this silica coating layer not only can inhibit the migration of precious metals through physical isolation, but also restrain Pt0 oxidation by interfacial electron interaction between Pt and acid sites (Si-OH-Al). Before and after hydrothermal aging, the maximum NO conversion of Pt-SiAl@Si decreased slightly by 5%, showing excellent durability. TEM and CO chemisorption unveiled that no obvious differences in Pt size between fresh and aged Pt-SiAl@Si. 1H NMR and NH3-DRIFTS confirmed that Pt-SiAl@Si possessed more Brönsted acid sites (Si-OH-Al). And the Pt−acid sites interface caused more active Pt0 by the electronic effect. This bifunctional overcoating allows a wide application in other catalytic materials for various reactions.
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•The SiO2 overlayer of Pt-SiAl@Si is bifunctional.•SiO2 shell as a physical barrier has an ability to retard sintering of Pt nanoparticles.•Pt-acid sites interfaces arising from interaction between SiO2 shell and support enhanced the oxidation-resistance of Pt0.•The increased acidity of catalyst promoted the fast desorption and decomposition of nitrate species, facilitating NO oxidation.
•Multi-particles/multilayer synergistic design strategy for enhancing the high-temperature oxidation resistance.•In-situ Nb2O5-SiO2 layer provides a good interface matching between the NbSi2 layer ...and particle deposition layer.•The multi-particles exhibit an excellent coupling effect for retarding the oxidation rate of silicide coating.•“Hf-Yb” skeleton significantly improves the thermal stability and stress damage tolerance of the oxide layer.
Multilayer composite design for thermal protective coating has proven to be promising for improving the high-temperature oxidation resistance of aerospace components. However, a series of problems such as the interface mismatch, dependable production method, and difficulty in forming a stable oxide scale, etc., make the scalable production of the multilayer composite coating with high efficiency, and low cost, along with achieving superior high-temperature oxidation resistance remains a long-time endeavor. Herein, a brand-new NbSi2/Nb2O5-SiO2/HfC-HfO2-MoSi2-Yb2O3 multilayer protective ceramic coating is synthesized by the multi-particles/multilayer synergistic design strategy. The multi-particles outer layer acts as the first oxygen erosion barrier, while the in-situ Nb2O5-SiO2 layer and the NbSi2 bottom layer are the second and third oxygen shielding layer for niobium alloys separately. Moreover, the in-situ Nb2O5-SiO2 layer also provides a good interface matching, and the co-existent multilayer makes a powerful thermal protection system. The incorporation of the high content of multi-particles in the outer layer is not only beneficial to reduce the inward oxygen flux through the oxygen diffusion barrier layer, but also improves the stress damage tolerance and thermal stability of the oxide scale by forming HfSiO4-Yb2O3-SiO2 skeleton layer. Thereby, the multilayer-coated sample reduces the oxide scale thickness to 128 μm after being exposed to 1200 ℃ for 120 h, which is only 67 % compared to the NbSi2-coated sample (100 h, 192 μm), and simultaneously holding the intact structure after a short-time oxygen acetylene ablation above 1800 ℃. This work paves a new way to design next-generation thermal protective coatings with great potential for applications.
•Low-temperature hydrolysis of carbonyl sulfur in complex gas fractions of blast furnace gas.•Needle-like are obtained due toMoO3 nanoparticle insert in the of potassium oxide deposited on alumina ...Nano sheet structures, enhance the catalytic properties of the prepared composites.•The Mo atoms transfer electrons and excite the adsorbed H2O to produce –OH, while enhancing the basic sites that can stabilize the hydrolysis of COS. The synergistic effect of Mo species and K2O over Al2O3 promoted the removal of COS and inhibited the production of sulfate.•The introduction of Mo and K increases the number of weakly basic sites in the catalyst while decreasing the amount of lattice oxygen in the catalyst, and the NH3 adsorption IR shows a decrease in acidic sites.•COS is first adsorbed on the catalyst surface, bicarbonate and thiocarbonate are the main intermediates of the catalyst, and H2O is involved in the reaction by reacting with transition species to eventually generate sulfate to deactivate the catalyst.
Water vapor, O2 and various sulfides in blast furnace gases affect the carbonyl sulfide (COS) hydrolysis reaction. In this study, we investigated the hydrolysis and deactivation mechanisms of a series of catalysts that were synthesized by depositing K2O and MoO3 on industrial alumina-based catalysts. Alumina nanosheets were deposited with potassium oxide, in which MoO3 nanoparticles were incorporated resulting in needle-like structures. The modified catalysts had substantially enhanced COS hydrolysis activity at low temperatures. The introduction of K2O and MoO3 significantly increased the number of weakly basic sites on the catalysts and reduced the lattice oxygen content. The catalysts exhibited high oxidation resistance and hydrolysis properties. According to the poisoning mechanism, the deposition of sulfate species tended to reduce weakly basic sites. High-valent Mo easily excited the adsorbed H2O on the catalysts, producing –OH groups that facilitated the COS hydrolysis reaction. This modification facilitates the further investigation of catalysts capable of promoting COS hydrolysis and their application in the manufacturing industry.
