Continuous SiO2 Research Articles

Effect of Cr on high-temperature oxidation resistance of Cr–Si–Mn alloyed press-hardened steel during press hardening

It is of great interest to overcome the problem of high-temperature oxidation of press-hardened steels (PHS) in press hardening with a cost-effective Cr alloying strategy. Therefore, since Cr is one of the expensive metals, the effect of Cr (Cr ≤ 3 wt%) content on the oxidation resistance of Cr–Si–Mn alloyed steels was investigated. It was found that Cr–Si–Mn alloyed PHS with a Cr content of about 1.5 wt% or less cannot form a continuous Cr2O3 layer during press hardening. However, when the Cr content reaches around 2.5 wt%, the PHS shows excellent oxidation resistance due to the formation of continuous Cr2O3 and amorphous SiO2 layers, as well as an Mn-rich oxide layer. However, further addition of 0.5 wt% could not significantly improve the oxidation resistance of this PHS. Moreover, the increase in Cr content simultaneously enhances the positive role of Si and Mn in high-temperature oxidation resistance. This work provides new insights into the design of the next generation of PHSs with economical and superior performance.

Oxidation behaviors of electrodeposited MoSi2 coating in a wide temperature range from 773 K to 1673 K

In this study, a MoSi2 coating was prepared on the surface of a molybdenum (Mo) substrate using the molten salt electrodeposition method, and the oxidation behavior of the coating over a wide temperature range from 773 K to 1673 K was studied in detail. The phase composition, surface and cross-sectional morphologies of the MoSi2 coatings after oxidation at various temperatures were characterized, and the evolution of the phase and microstructure were analyzed to elucidate the oxidation mechanism. The experimental results showed that the MoSi2 coating exhibited excellent oxidation resistance at lower temperatures (773 K) and did not experience the pesting phenomenon. When exposed to medium and high temperatures, a continuous and dense SiO2 layer was formed on the surface of the MoSi2 coating, which effectively prevented further oxidation inside the coating, thereby protecting the Mo substrate. Meanwhile, at higher temperatures, especially at 1473 K and 1673 K, the solid-phase diffusion of Si towards the substrate gradually transformed the MoSi2 into Mo5Si3 with a lower Si content. Notably, the thickness of the Mo5Si3 layer that was distributed between the original MoSi2 coating and the Mo substrate increased with the oxidation time and temperature. At 1673 K, a Mo3Si layer with a low Si content was formed beneath the Mo5Si3 layer. Thus, the diffusion of Si was an important factor restricting the service life of MoSi2 coating.

Oxidation mechanisms of a Cr[sbnd]Si alloyed coating-free press-hardened steel under simulated press hardening conditions

Press-hardened steel (PHS) undergoes severe high-temperature oxidation after press hardening, which significantly degrades surface quality and increases manufacturing cost. This work reports the mechanism by which a coating-free PHS (CF-PHS) with the simultaneous presence of Cr, Si, and Mn alloying elements achieves excellent high-temperature oxidation resistance. It is found that Si and Mn in the CF-PHS promote the formation of a continuous Cr2O3 layer with good protective effect, thereby improving high-temperature oxidation resistance, during the press hardening. Moreover, the continuous amorphous SiO2 layer formed at the oxide scale/matrix interface and the Mn-rich oxides that improve the compactness of the scale further enhance the oxidation resistance of the CF-PHS.

Polyimide/SiO2 composite aerogels with excellent thermal and sound insulation properties prepared by confined filling method

Aerogels have enormous potential in integrated thermal insulation and noise reduction applications. According to the knowledge of the favorable mechanical and acoustic properties of honeycomb structures. We employed anisotropic “honeycomb-like” polyimide (PI) aerogels as the matrix and utilized a confined filling strategy to directionally filled continuous SiO2 aerogels into the PI aerogel matrix. This resulted in the creation of an exceptional organic–inorganic composite aerogel (PISis) with outstanding thermal and acoustic insulation performance. The radial thermal conductivity of PISis ranged from 22.37 to 27.65 mW·m−1·K−1, and the initial decomposition temperature was approximately 524–557 °C. In the frequency range of 500–6300 Hz, the average sound transmission loss (STL) of the 18 mm thick composite material reached 30.67 ∼ 31.89 dB, and the compressive strength at 10 % strain is 0.416 MPa. The PISis exhibits a density ranging from 0.1012 to 0.1302 g/cm3. These make it an ideal lightweight, high-strength, and efficient material for thermal and acoustic insulation.

Unidirectional infiltrated PI/SiO2 composite aerogels with a confined reinforcing strategy for integrated thermal and acoustic insulation

It is of great significance to develop integrated thermal insulation and noise reduction materials for energy saving and pollution control. As a nanoporous material, aerogels have excellent thermal and acoustic properties, however, the thermal stability of organic aerogels is insufficient, and the mechanical properties of inorganic aerogels limit their practical application. Herein, we designed a confined reinforcing strategy for organic-inorganic composite aerogels with high strength and excellent acoustic insulation properties. A directional freezing technology was utilized to control the pore structure of a polyimide (PI) aerogel, and then a continuous SiO2 aerogel was filled in the unidirectional pore channels, forming a double-network structure consisting of both PI and SiO2 aerogels. The results showed that the compressive strength of the composites was 1.83 MPa at 10% strain, the initial thermal decomposition temperature was about 530 °C, and the lowest thermal conductivity was 22.37 mW m−1·k−1 at radial direction. Moreover, the average sound transmission loss (STL) of the composites with an 18 mm thickness was 30–33 dB in the range of 500–6300 Hz. This confined reinforcing strategy can safeguard fragile aerogels while maximize their inherent characteristics, which offers a novel approach to the creation of multi-functional aerogel composites.

Advances in Ablation or Oxidation Mechanisms and Behaviors of Carbon Fiber-Reinforced Si-Based Composites.

Composites with excellent thermomechanical and thermochemical properties are urgently needed in the aerospace field, especially for structural applications under high-temperature conditions. Carbon fiber-reinforced Si-based composites are considered the most promising potential high-temperature materials due to their excellent oxidation resistance and ablative behaviors, good structural designability, and excellent mechanical properties. The reinforcement of the relevant composites mainly involves carbon fiber, which possesses good mechanical and temperature resistance abilities. In this paper, the ablation behaviors and mechanisms of related composites are reviewed. For carbon fiber-reinforced pure Si-based composites (C/SiM composites), the anti-ablation mechanism is mainly attributed to the continuous glassy SiO2, which inhibits the damage of the substrate. For C/SiM composite doping with refractory metal compounds, the oxides of Si and refractory metal together protect the main substrate from ablation and oxidation. Moreover, in addition to thermochemical damage, thermophysical and thermomechanical behavior severely destroy the surface coating of the substrate.

Effect of Reaction Temperature on the Molten Salt Electrodeposition of Silicide Coating on Molybdenum Substrate and its Oxidation Behavior

In this study, silicide coatings were successfully prepared on Mo substrates using NaCl-KCl-NaF molten salt as the reaction medium and K2SiF6 as the silicon source. The electrochemical reduction mechanism of Si(IV) was studied by cyclic voltammetry and chronoamperometry technology. The effect of reaction temperature on the phase composition, microstructure, and growth kinetics of the electrodeposited coatings were investigated in detail. The long-term oxidation behaviors of Mo and MoSi2-coated Mo samples at 873 K were also compared. The results revealed that the electrodeposition of silicide coatings was closely related to the reaction temperature. Dense and coherent single-layer MoSi2 coatings could be obtained at 1023 K and 1073 K, while double-layer coatings with inner MoSi2 layer and outer Si layer could be obtained at 1123 K. Under the same electrodeposition time, the overall thickness of the silicide coatings increased with the reaction temperature. Moreover, long-term oxidation tests at 873 K proved that MoSi2 coatings could effectively prevent the Mo substrate from oxidation. The mass gain of the MoSi2-coated Mo sample was only 0.46 wt% after 500 h of oxidation, which could be attributed to the formation of the continuous SiO2 protective layer.

Contribution of Ribbon-Structured SiO2 Films to AlN-Based and AlN/Diamond-Based Lamb Wave Resonators.

New designs based on S0 Lamb modes in AlN thin layer resonating structures coupled with the implementation of structural elements in SiO2, are theoretically analyzed by the Finite Element Method (FEM). This study compares the typical characteristics of different interdigital transducer (IDTs) configurations, involving either a continuous SiO2 cap layer, or structured SiO2 elements, showing their performance in the usual terms of electromechanical coupling coefficient (K2), phase velocity, and temperature coefficient of frequency (TCF), by varying structural parameters and boundary conditions. This paper shows how to reach temperature-compensated, high-performance resonator structures based on ribbon-structured SiO2 capping. The addition of a thin diamond layer can also improve the velocity and electromechanical coupling coefficient, while keeping zero TCF and increasing the solidity of the membranes. Beyond the increase in performance allowed by such resonator configurations, their inherent structure shows additional benefits in terms of passivation, which makes them particularly relevant for sensing applications in stern environments.

Anti-ablation performance of plasma sprayed ZrB2/SiC coatings on C/C substrates and the influence of a chemical vapor deposited SiC interlayer

To address the industrial usage of carbon/carbon (C/C) substrates with plasma-sprayed ZrB2/SiC coatings, a continuous and dense chemical vapor deposition (CVD)-SiC interlayer of 150 µm was deposited on a C/C substrate. The integration of CVD-SiC with the C/C substrate and plasma sprayed (PS) ZrB2/SiC coatings results in a tight interface. The SiC interlayer significantly reducing thermal stress in the optimized composites up to 1828 Mpa, a decrease of approximately 60% compared to standard composites without CVD-SiC. Ablation tests conducted showed that the optimized composite displayed remarkable mass ablation rate of − 11.7% after ablation for 600 s, with a notable increase of − 3.5% even after ablation for 1800 s. After 600 s of ablation, the exposed SiO2 layer is approximately 500 µm in size. After 900 s of ablation, the exposed SiO2 layer is approximately 1500 µm. After 1800 s of ablation, the SiO2 is completely evaporated, exposing the C/C matrix, at which point the coating has failed. The SiC interlayer improve the ablation resistance due to the continuous SiO2 film formed by the self-sealing and O anti-diffusive properties.

Influence of Silicon and Chromium on the Na2SO4-Induced Hot Corrosion Behavior of Titanium Alloys

Titanium alloys are widely used as construction materials in the aerospace and automotive industries. They have many advantages but also have limitations related to their susceptibility to high-temperature oxidation and hot corrosion. Many efforts to increase the lifetime of components made of titanium alloys have been reported in the literature; the most promising ones involve the deposition of coatings. The present paper is focused on the development of coatings containing chromium and silicon, and their further evaluation in hot corrosion tests. It was proved that the Cr-Si coatings were more effective than Si coatings alone in protecting the titanium alloys against Na2SO4-induced hot corrosion at 800 °C. The enhanced corrosion resistance was attributed to the preferential formation of a thick and continuous SiO2 layer on the surface and—in the case of titanium aluminide alloy—the growth of an Al2O3-rich inner layer of the scale, promoted by chromium.

Oxidation behavior of Nb-16Si-20Ti-3Al containing nano-ZrC fabricated by laser melting deposition

The effects of nano-ZrC addition on the oxidation resistance of Nb-Si-Ti-based alloy were studied. The results showed that the alloys with different nano-ZrC additions all followed the mixed parabolic law during first 50 h oxidation. However, the oxidation law of the alloys was linear during 50–100 h oxidation. An appropriate amount of nano-ZrC addition improved the oxidation resistance of the alloy, with reduced oxidation weight gain and decreased the oxidation rate constant. The reason was that the nano-ZrC addition refined the microstructure, and promoted the formation of a continuous SiO2 layer, making the oxide layer have better integrity and protection.

Improvement of anti-aging property of UV-curable coatings with silica-coated TiO2

A series of SiO2 coated TiO2 particles (TiO2@SiO2) were prepared by depositing SiO2 layer on the surface of TiO2 particles via an in-situ hydrolysis condensation of tetraethyl silicate, which was used as UV-shielding fillers to prepare UV-curable coatings. The effect of SiO2 layer on the photocatalytic activity, UV-shielding ability and dispersibility of TiO2 particles was systematically explored. The anti-aging property of the UV-cured coating was determined by color change, surface gloss loss, and surface morphology via accelerated UV aging test. It was found that the presence of a continuous and dense SiO2 layer on TiO2 surface could not only improve the dispersibility of TiO2, but also effectively inhibit the photocatalytic activity of TiO2 without sacrificing its UV-shielding ability, thereby significantly enhancing the anti-aging property of UV-curable coatings. Salt spray test shows that the aged UV-cured coating with TiO2@SiO2 exhibited better corrosion protection behavior than the aged pure resin coating as well as the aged TiO2 coating.

Study of the anticorrosive behavior of samarium as a corrosion inhibitor in multilayer systems for aluminum alloy

This study shows a multilayer system based on samarium compounds as a corrosion inhibitor and a continuous SiO2 layer by atmospheric pressure plasma jet (APPJ) as a protective barrier for aluminim alloy AA3003. One of the main advantages of this new coating is that it does not require vacuum chambers, which makes it easy to incorporate into production lines for automotive and aeronautical components, etc. The deposit of samarium corrosion inhibitor was carried out by two methods for comparison, the immersion method and a novel method to deposit corrosion inhibitor by APPJ. The multilayer system generated was homogeneous, continuous, adherent, and dense. The electrochemical behavior shows that the samarium compound was completely oxidized on coatings by the immersion method and favors corrosion. The APPJ deposition method shows a protective behavior against corrosion by both samarium compounds and silica depositions. XPS analyses show that the amount of Sm(OH)3 increases by the APPJ method compared with the immersion method since the spectrum of O1s is mainly controlled by OH. It was determined that the best processing times for the electrochemical study of the multilayer system were 40 min for the immersion method and 30 s for the APPJ method for the layer of corrosion inhibitor. In the case of the SiO2 barrier layer by APPJ, the best time was 60 s of exposure to the plasma jet and this coating could reduce the corrosion of AA3003 by 31.42%.

Ablation behavior and mechanisms of Cf/(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C–SiC high-entropy ceramic matrix composites

For the first time, air plasma ablation behavior of Cf/(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C–SiC high-entropy ceramic matrix composites was studied systematically under a heat flux of 5 MW/m2, which provided a quasi-real hypersonic service environment at a temperature up to 2430 °C. The Cf/(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C–SiC composites present excellent ablation-resistant performance with a linear recession rate of ∼2.89 μm/s and mass recession rate of ∼2.60 mg/s, which can be attributed to the dense and stable oxides layer formed on the sample surface. (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C is oxidized to high-entropy oxide (TiZrHfNbTa)Ox at the ablation center and highly viscous SiO2 melt with uniformly dispersed (TiZrHfNbTa)Ox microspheres is formed. While at the edge of the ablation center, precipitation occurs during cooling and the oxides layer turns to a plate-like (Hf0.5Zr0.5O2)’ skeleton surrounded by (TiNbTaO7-y)’ nanocrystals and continuous SiO2 melt. In contrast, oxidation of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C to produce (Hf0.5Zr0.5O2)’ and (TiNbTaO7-y)’ is dominant at the ablation transition area and outer area with lower temperature. The multiphase oxides formed during ablation provide a stable and highly self-healing protective layer for the internal materials at ultra-high temperatures.

Improving the high-temperature oxidation resistance of Fe-18Cr-8Ni austenitic stainless steel by cold-rolling treatments

The effect of different degrees of cold rolling on the short-term oxidation behavior of Fe-18Cr-8Ni stainless steel oxidized at 1000 °C in N2-21 vol%O2 for 3 h was investigated. The results show that cold rolling deformation reduces the grain size and coincident site lattice (CSL) grain boundary fraction of the alloy. When the deformation exceeds 20%, the oxidation mechanism changes from internal to external oxidation. And the critical concentration of Cr and Si required for forming the protective oxide scale decreases. The formation of continuous Cr2O3 and SiO2 layers on the alloy prevents the diffusion of oxygen from the surface to the matrix. The cold-rolling treatments improve the oxidation resistance of the Fe-18Cr-8Ni austenitic stainless steel at 1000 °C.

Insights into the oxidation behavior of Ni–25Cr–10Fe–xSi (x = 1, 2, 3, 4 wt%) alloys at 1000°C in ambient air

AbstractThe isothermal oxidation behavior of Ni–25Cr–10Fe–xSi (x = 1, 2, 3, 4 wt%) alloys at 1000°C in ambient air was investigated. The thermodynamics and kinetics of the oxidation behavior were analyzed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, Raman spectroscopy, and the Thermo‐Calc software with the TTNI8 database. The weight gain per unit area of the four alloys showed a parabolic relationship with the oxidation time. The weight gain of the oxides decreased with the addition of larger amounts of Si from 1 to 3 wt%; however, this trend was lost when excessive addition of Si (4 wt% Si) was added. A sufficient amount of discontinuously distributed SiO2 precipitates resulted in better oxidation resistance and adhesion properties than continuous SiO2 layers. The spallation resistance of the oxide scales is believed to be mainly dependent on the distribution of the SiO2 layer rather than the content of SiO2; 3 wt% is a reasonable reference value for Si addition for improving the oxidation resistance of Ni–25Cr–10Fe alloys.

Understanding the oxidation kinetics of (Ti0.8Nb0.2)C and (Ti0.8Nb0.2)C-SiC composite in high-temperature water vapor

(Ti0.8Nb0.2)C exhibits high oxidation resistance in water vapor up to 1300 ºC, with the addition of 10% wt. SiC, even higher oxidation resistance could maintain up to 1500 ºC. The oxidation scales of both (Ti0.8Nb0.2)C and (Ti0.8Nb0.2)C-SiC composite show preferential oxidation and steep, opposite chemical gradients of Ti and Nb, implying that the predominant controlling factor is the outward diffusion of the metal elements by the interstitialcy mechanism. Dense and oriented (Ti1−xNbx)O2 oxidation scale accounts for the high oxidation resistance. A continuous and dense SiO2 layer further improved the oxidation resistance in (Ti0.8Nb0.2)C-SiC composite.

Formation and oxidation behavior of refractory high-entropy silicide (NbMoTaW)Si2 coating

A single high-entropy silicide (NbMoTaW)Si2 coating was prepared on NbMoTaW refractory alloy by a halide-activated pack cementation process. The formation mechanism and growth kinetics of the (NbMoTaW)Si2 coatings were analysed. The oxidation behavior of the (NbMoTaW)Si2 coatings was investigated at 1300 ℃ for 24 h in air. The results showed that the high entropy effect and sluggish effect of the high-entropy NbMoTaW alloy makes it form a single-phase silicide (NbMoTaW)Si2 during the Si diffusion growth process. The lattice distortion effect of substrate provides a channel for the diffusion of Si to accelerate its diffusion rate. The (NbMoTaW)Si2 coating effectively protects the NbMoTaW alloy from oxidation at 1300 ℃. The oxidation resistance of the coating is attributed to the preferential oxidation of Si to produce a dense continuous SiO2 scale and the precipitation of WSi2 to suppress the interdiffusion of the silicide layer and the alloy substrate.

Revealing the long-term oxidation and carburization mechanism of 310S SS and Alloy 800H exposed to supercritical carbon dioxide

The oxidation/carburization behavior and failure mechanism of 310S stainless steel (SS) and Alloy 800H exposed to supercritical carbon dioxide at 650 °C/20 MPa were investigated. The results show that the oxidation kinetics of 310S SS and Alloy 800H both approximately follow the parabolic oxidation law. The weight gain of Alloy 800H is higher than 310S SS for the first 2000 h exposure. While with the increase of exposure time to 3000 h, the weight gain of Alloy 800H decreases, and becomes lower than 310S SS. Cr2O3 is formed on the surface of 310S SS, while a multilayer structure mainly composed of outer MnCr2O4-Cr2O3 layer and inner MnCr2O4-SiO2 layer is formed on Alloy 800H. Carburization happens in the oxidation process of 310S SS and Alloy 800H. However, the carburization performances of the two kinds of materials are different. For Alloy 800H, a C layer is deposited nearby the O/M interface, which is because that the inner MnCr2O4 and SiO2 layer in the oxide scale acts as a barrier for the inward diffusion of C-containing substances. For 310S SS, a carburized region with more and bigger carbides is formed in the matrix alloy beneath the Cr2O3 layer. The oxide spallation occurs on Alloy 800H after 2000 h exposure, which explains the reason that the weight gain of Alloy 800H decreases with the further increment of testing time to 3000 h. The spallation of oxide film on Alloy 800H is likely to be attributed to the presence of the inner MnCr2O4 layer, continuous SiO2 layer and the deposited amorphous C nearby the O/M interface.

Porosity tailoring of electrophoretically derived zirconia coatings using acidic and alkaline-based sol-gel post-treatment to enhance anti-corrosion performance

In this work, the post-treatment sol-gel methods based on acidic and alkaline catalyzed tetraethylorthosilicate (TEOS) solutions were applied to refine the porosity network of electrophoretically deposited ZrO2 coating on anodically oxidized 316 L substrate to improve the corrosion resistance. The ZrO2 coating dipped in alkaline TEOS solution manifested the best barrier performance, arising from the formation of a dense continuous SiO2 interlayer at the coating/substrate interface as well as narrowing the interconnected pores in the coating's body. This was related to the easily inward diffusion of the alkaline hydrolyzed TEOS molecules with an inert surface charge into the negatively charged ZrO2 coating through the open pores toward the interface. However, the acidic-catalyzed TEOS molecules mostly preferred to form a relatively thin SiO2 layer on the surface of ZrO2 coating, which provided a poor barrier performance. This corresponded to the established electrostatic repulsion forces between the positively charged ZrO2 particles and acidic-hydrolyzed TEOS molecules with positive surface charge, which restricted the unlimited diffusion of TEOS molecules. Based on the microscopic observations and electrochemical impedance results, a possible mechanism for refinement of the porous network of ZrO2 coating and formation of SiO2 layers in the acidic and alkaline TEOS solutions was suggested.