Preparation of infrared high radiation coatings from modified spinel NiFe2O4 and its energy saving applications

The NiFe spinel material itself has good thermal stability and emissivity and can be prepared as an infrared high radiation coating for energy saving applications in industrial high temperature furnace applications. In this study, Cr³⁺ and Cu²⁺ doped spinel NiFe₂O₄ was prepared by solid phase reaction at 1250 °C for 3 h and the microstructure and physicochemical properties of the powder and coating were characterised by XRD, SEM, EDS and IR radiometry. The effect of Cr³⁺ and Cu²⁺ doping on the infrared emissivity of spinel NiFe₂O₄ was investigated and energy saving assessment was carried out in a resistance furnace. The results show that the doping of Cr³⁺ and Cu²⁺ can significantly affect the emissivity of spinel powders in the 2.5–10 μm band, and the coatings prepared from the four powders have an emissivity of up to 0.95 in the 2.5–10 μm band. using this high temperature infrared radiation energy saving coating in a resistance furnace resulted in significant energy savings compared to no coating. The furnace was tested for energy saving by holding the furnace for 2 h and 5 h, and the energy saving efficiency reached 20.7% and 17.0% respectively. The coating was subjected to 10 thermal shock tests from room temperature to 700 °C. The coating bonded well and had good thermal shock resistance. Therefore, the coating has wide application prospects for energy saving applications in the field of industrial high temperature furnaces.     

Infrared radiation and thermal properties of Al-doped SrZrO3 perovskites for potential infrared stealth coating materials in the high-temperature environment

The Al-doped SrZrO₃ perovskite powder with low infrared emissivity at high temperatures was prepared. The infrared radiation performance and thermophysical properties of the perovskites at high temperatures were discussed. As a result, the infrared emissivity of the Al-doped SrZrO₃ perovskite powder is associated with Al³⁺-doping content, phase composition and particle morphology. The flaky particles SrZr_0.85Al_0.15O_2.925 formed by heat treatment at 1000 °C for 6 h have the lowest infrared emissivity of 0.245 in 3–5 μm wavebands at 590 °C. The perovskite powder's infrared emissivity is positively correlated with its electrical resistivity and has no apparent change after heating over 800 °C for long-term. The SrZr_0.85Al_0.15O_2.925 perovskite ceramic formed by pressureless sintering still maintains ideal heat insulation performance with the thermal conductivity from 1.17 to 2.21 W m^?1 K^?1 below 1400 °C. The Al-doped SrZrO₃ perovskite tablet exhibits significant weak radiation intensity due to its characteristics of both low infrared emissivity and thermal conductivity at high temperatures.     

Thermophysical properties and cyclic lifetime of plasma sprayed SrAl12O19 for thermal barrier coating applications

About 6-8 wt% yttria-stabilized zirconia (YSZ) is the industry standard material for thermal barrier coatings (TBC). However, it cannot meet the long-term requirements for advanced engines due to the phase transformation and sintering issues above 1200°C. In this study, we have developed a magnetoplumbite-type SrAl₁₂O₁₉ coating fabricated by atmospheric plasma spray, which shows potential capability to be operated above 1200°C. SrAl₁₂O₁₉ coating exhibits large concentrations of cracks and pores (~26% porosity) after 1000 hours heat treatment at 1300°C, while the total porosity of YSZ coatings progressively decreases from the initial value of ~18% to ~5%. Due to the contribution of porous microstructure, an ultralow thermal conductivity (~1.36 W m⁻¹ K⁻¹) can be maintained for SrAl₁₂O₁₉ coating even after 1000 hours aging at 1300°C, which is far lower than that of the YSZ coating (~1.98 W m⁻¹ K⁻¹). In thermal cyclic fatigue test, the SrAl₁₂O₁₉/YSZ double-ceramic-layer coating undertakes a thermal cycling lifetime of ~512 cycles, which is not only much longer than its single-layer counterpart (~163 cycles), but also superior to that of YSZ coating (~392 cycles). These preliminary results suggest that SrAl₁₂O₁₉ might be a promising alternative TBC material to YSZ for applications above 1200°C.     

Infrared radiative performance and anti-ablation behaviour of Sm2O3 modified ZrB2/SiC coatings

To improve the emissivity of ZrB₂/SiC coatings for serving in more serious environment, ZrB₂/SiC coatings with varying contents of high emissivity Sm₂O₃ were fabricated using atmospheric plasma spraying. The microstructure, infrared radiative performance and anti-ablation behaviour of the modified coatings were investigated. The results showed that as the content of Sm₂O₃ increased, the density of the coatings increased because of the low melting point of Sm₂O₃. When the content of Sm₂O₃ was 10 vol%, the coating had the highest emissivity in the 2.5–5 μm band at 1000 °C, up to 0.85, because of the oxygen vacancies promoting additional electronic transitions. Due to the high emissivity, the surface temperature of the coating modified with 10 vol% Sm₂O₃ decreased by 300 °C, which led to little volatilisation of the sealing phase. Further, the mass ablation ratio of the above coating was 3.19 × 10^?4 g/s, decreasing 31% compared to that of a ZrB₂/SiC coating. The formed dense surface structure of the coatings showed considerable oxygen obstructive effects. These findings indicate that the modified coatings show considerable anti-ablation performance, which provides effective anti-ablation protection for the C/C composite substrate.     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Fabrication,thermal shock resistance,and dielectric property of α-Si3N4-based ceramic coating on porous Si3N4 ceramics

A dense α-Si₃N₄-based ceramic protective coating was successfully prepared on porous Si₃N₄ ceramics by a liquid infiltration and filling method. The coating composed of a primary α-Si₃N₄ phase and secondary O'-Sialon, β-Sialon, and Y–Si–Al–O–N glass phase. After thermal shock at ΔT = 1000°C for five times, cracks were produced, but the tip of crack stopped inside the coating; so the coated porous Si₃N₄ ceramics still had a good waterproof ability and its water absorption was only 7%. During thermal shock, toughening mechanisms involving needle-like O'-Sialon particle bridging, crack deflection, and rough fracture, occurred within the cracks, contributing to thermal shock resistance of the coating. The dielectric constant of the coated porous Si₃N₄ ceramics showed a slow increase trend with increasing temperature, and it reached the maximum value of 3.57 at 1100°C at the frequency of 11 GHz. The dielectric loss increased slowly as the temperature increased from room temperature to 900°C, but it started to increase evidently when the temperature was over 900°C.     

Band gap and oxygen vacancy engineering of honeycomb-like CuFe2O4 spinels toward enhanced high infrared emissivity

Recently, copper ferrites have acquired widespread attraction in high infrared radiation fields owing to their remarkable cost efficiency. However, to achieve broader applications under various operating conditions, it is essential to further improve the infrared emissivity, particularly at high temperatures. Herein, the Ni-doped CuFe₂O₄ (NCFO) honeycomb-like frameworks, which are constructed with single-crystal nano-subunits, are successfully fabricated via the scalable sol–gel avenue. The unique porous honeycomb framework endows NCFO with enhanced infrared absorption and relieves the stress between coatings and substrates meanwhile. With both band gap and oxygen vacancy (OV) engineering of CuFe₂O₄ itself via smart Ni doping, a maximum lattice strain, the richest OVs, and the narrowest band gap (∼1.63 eV) are simultaneously achieved for the CuFe₂O₄ with 15% Ni doping (denoted as CNFO-15). Benefiting from the synergy of these external and intrinsic contributions, the CNFO-15 possesses an ultrahigh infrared emissivity (∼0.975) in the wavelength range of 3–5 µm at a test temperature of 800°C. Moreover, the CNFO-15-based coating displays superior infrared radiation performance with outstanding high-temperature resistance. More meaningfully, the constructive design here will provide a distinctive perspective for future large-scale fabrication of advanced high-infrared-emissivity coatings.     

Heat and moisture resistance of divalent metals substituted La-Hexaaluminates prepared by reverse microemulsion

Lanthanum hexaaluminates (LHAs) substituted with divalent metals (Mg, Zn) were successfully prepared by the reverse microemulsion (RM) method and high-temperature treatment. Phase composition, microstructure, porosity characteristics, and their thermal evolution were investigated using X-ray diffractometry, scanning electron microscopy, N₂ isothermal sorption and thermogravimetry-differential scanning calorimetry, respectively. The thermal stability of substituted hexaaluminates was also evaluated in both dry and moisture atmosphere at 1200-1600 °C. Experimental results showed that the RM method promoted the formation of pure and well crystallized magnetoplumbite hexaaluminates with high specific surface areas at temperatures 1100-1200 °C. For both obtained hexaaluminates, their phase composition and nanometer-sized crystallites were retained at temperature as high as 1600 °C in dry air. However, LaMgAl₁₁O₁₉ presented severe deterioration in flowing water vapor at 1300 °C including about 50% weight loss and the remarkable crystalline structure change, whereas the substitution of Zn enabled the LaZnAl₁₁O₁₉ to possess superior moisture resistance. According to the partial charge model of cations in aqueous media, a “hydrolysis-corrosion” mechanism was proposed to describe the degradation of hexaaluminates in high-temperature water vapor. It was revealed that the high-temperature moisture resistance of substituted LHAs was inversely proportional to the hydrolysis tendency of substituted metal cations.     

Oxidation Resistance of AlPO4 Bonded Ceramic Coating Formed on Titanium Alloy for High‐Temperature Applications

AlPO₄ based coatings were prepared on Ti‐6Al‐2Zr‐1Mo‐1V titanium alloy using aluminum phosphate as a binder and Al₂O₃/Cr₂O₃ based mixing particles as the fillers. The microstructure, phase and chemical composition of the coatings were analyzed by SEM, XRD and EDS techniques. The high temperature infrared emissivity values of coated and uncoated titanium samples were tested. The results show that the coating had a higher infrared emissivity value (>0.8) than titanium substrate (0.15–0.3) in the wide wavelength range of 5–20 mm, which is attributed to the uniform dispersion of high emissivity Al₂O₃ and Cr₂O₃ particles in the AlPO₄ binder matrix. The coated titanium samples exhibited excellent oxidation resistance performance with significantly decreased oxidation rates at 600 and 800°C. The mass gain of the coated sample kept at a low and stable constant of 0.15 mg/cm², significantly lower than that of titanium substrate (0.54 mg/cm²) when oxidized at 600°C up to 100 h.     

Microstructure evolution and bonding mechanisms of silica sol bonded coating at elevated temperatures

High emissivity coating plays a critical role in thermal protective system, which can radiate a large amount of aero-convective heat. Silica sol bonded MoSi₂-SiC-Al₂O₃ (S-MSA) coating was proved to be promising for mullite fibrous insulation. However, the bonding mechanisms of the coating at elevated temperatures are not clear. In this work, the S-MSA coatings were heat-treated at temperatures from 600 °C to 1500 °C to reveal the bonding mechanisms at elevated temperatures. The S-MSA coatings go through a relatively stable stage (600 °C–1000 °C), a crystallization stage (1100 °C–1200 °C), and a densification stage (1300 °C–1500 °C) at ever increasing temperatures. Results show that both the contact damage resistance and the bonding strength of the calcined coatings exhibit a decrease followed by an increase at elevated calcination temperatures, with the inflection point at 1200 °C, corresponding to the transition temperature of the bonding mechanisms from 600 °C to 1500 °C.     

Phase stability,thermo-physical properties and thermal cycling behavior of plasma-sprayed CTZ,CTZ/YSZ thermal barrier coatings

ZrO₂ co-stabilized by CeO₂ and TiO₂ with stable, nontransformable tetragonal phase has attracted much attention as a potential material for thermal barrier coatings (TBCs) applied at temperatures >?1200?°C. In this study, ZrO₂ co-stabilized by 15?mol% CeO₂ and 5?mol% TiO₂ (CTZ) and CTZ/YSZ (zirconia stabilized by 7.4?wt% Y₂O₃) double-ceramic-layer TBCs were respectively deposited by atmospheric plasma spraying. The microstructures, phase stability and thermo-physical properties of the CTZ coating were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric-differential scanning calorimeter (TG-DSC), laser pulses and dilatometry. Results showed that the CTZ coating with single tetragonal phase was more stable than the YSZ coating during isothermal heat-treatment at 1300?°C. The CTZ coating had a lower thermal conductivity than that of YSZ coating, decreasing from 0.89?W?m^?1 K^?1 to 0.76?W?m^?1 K^?1 with increasing temperature from room temperature to 1000?°C. The thermal expansion coefficients were in the range of 8.98?×?10^?6 K^?1 – 9.88 ×10^?6 K^?1. Samples were also thermally cycled at 1000?°C and 1100?°C. Failure of the TBCs was mainly a result of the thermal expansion mismatch between CTZ coating and superallloy substrate, the severe coating sintering and the reduction-oxidation of cerium oxide. The thermal durability of the TBCs at 1000?°C can be effectively enhanced by using a YSZ buffer layer, while the thermal cycling life of CTZ/YSZ double-ceramic-layer TBCs at 1100?°C was still unsatisfying. The thermal shock resistance of the CTZ coating should be improved; otherwise the promising properties of CTZ could not be transferred to a well-functioning coating.     

Experiments and transient finite element simulation of γ-Y2Si2O7/B2O3-Al2O3-SiO2 glass coating on porous Si3N4 substrate under thermal shock

A dense γ-Y₂Si₂O₇/B₂O₃-Al₂O₃-SiO₂ glass coating was fabricated by slurry spraying method on porous Si₃N₄ ceramic for water resistance. Thermal shock failure was recognized as one of the key failure modes for porous Si₃N₄ radome materials. In this paper, thermal shock resistance of the coated porous Si₃N₄ ceramics were investigated through rapid quenching thermal shock experiments and transient finite element analysis. Thermal shock resistance of the coating was tested at 700 °C, 800 °C, 900 °C and 1000 °C. Results showed that the cracks initiated within the coating after thermal shock from 800 °C to room temperature, thus leading to the reduction of the water resistance. Based on the finite element simulation results, thermal shock failure tended to occur in the coating layer with increasing temperature gradient, and the critical thermal shock failure temperature was measured as 872.24 °C. The results obtained from finite element analysis agree well with that from the thermal shock tests, indicating accuracy and feasibility of this numerical simulation method. Effects of thermo-physical properties for the coating material on its thermal shock resistance were also discussed. Thermal expansion coefficient of the coating material played a more decisive role in decreasing the tangent tensile stress.     

High-temperature mechanical property and thermal shock resistance of Al2O3f/Al2O3 composites

Continuous alumina fiber–reinforced alumina matrix composites (Al₂O_3f/Al₂O₃ composites) were produced via sol–gel process, then the high-temperature mechanical property and thermal shock resistance of Al₂O_3f/Al₂O₃ composites were investigated. The results showed that the composites exhibited excellent high-temperature properties. The mechanical property of the composites was affected by heat treatment (prepared at 1100°C exhibited the most desirable mechanical property). The tensile strength of the composites abruptly decreased at higher temperatures. Although the mechanical property of the composites deteriorated after the thermal shock test was conducted at high temperatures, they exhibited excellent thermal shock resistance. After 50 thermal shock tests conducted at 1300 and 1500°C, the flexural strength of the composites was found to be 124.34 and 93.04 MPa, thus showing a decrease in strength with the increasing temperature.     

Fabrication and characterization of Pr6O11-HfO2 ultra-high temperature infrared radiation coating

In this study, pure HfO₂ and Pr₆O₁₁-HfO₂ coatings were prepared by atmospheric plasma spraying. The chemical compositions, morphologies, infrared radiation performances and thermal resistances of the coatings were characterized. The results showed that doping Pr₆O₁₁ could effectively improve the infrared emittance of the HfO₂ coating. The HfO₂ coating doping with 10 wt. % Pr₆O₁₁ exhibited the highest infrared emittance, which was 0.859 at room temperature and 0.883 at 1600 °C, correspondingly. This was mainly attributed to the oxygen vacancies, which created by the substitution of Hf⁴⁺ by Pr³⁺, could introduce localized energy states within the HfO₂ band gap and increase the lattice distortion, producing lower symmetry vibrations. In addition, the Pr₆O₁₁-HfO₂ infrared radiation coating possessed high tensile adhesive strength and good thermal resistance, which could withstand a high temperature treatment at 1700 °C for at least 50 h without exfoliation, and there was only a slight reduction in emittance.     

High-entropy alloy: An alternative approach for high temperature microwave-infrared compatible stealth

To achieve microwave-infrared compatible stealth in high temperature conditions, high-entropy alloys (HEAs) thin films were deposited on Al₂O₃ matrix by magnetron sputtering technology. Films were annealed to investigate thermal stability at 500 °C, 600 °C and 700 °C, respectively. Results from X-ray diffract meter (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), and Fourier transform infrared spectrometer (FTIR) suggested that high-entropy alloy (HEA) film was seriously oxidized when the annealed temperature reached 700 °C for 6 h, causing a significant decrease of infrared reflectivity. Conversely, HEA films showed low infrared emissivity of 0.09 at 600 °C. Additionally, the films possessed excellent thermal stability at 500 °C for 20 h with low infrared emissivity of 0.11. Finally, a simple metamaterial design utilizing HEA films was proposed for infrared-microwave compatible stealth. With the ability of incorporating excellent thermal stability and durable high temperature stealth performance, the study shows great potential of introducing HEAs in the field of high temperature compatible stealth.     

SiC coating with high crack resistance property for carbon/carbon composites

A novel SiC coating with a relatively high crack resistance property (crack extension force (G_C): 12.0 J·m^?2) and outstanding thermal shock resistance was achieved merely by pack cementation. Compared with the conventional SiC coating with Al₂O₃ addition (AOSC2), SiC coating with Al–B–C additions (ABSC2) possesses refined and denser microstructure owing to different effects in promoting SiC densification under different additions. Therefore, the improvement in microstructures results in superior mechanical capabilities, antioxidation performance (900 °C), and thermal shock resistance (between 1500 °C and room temperature).     

Preparation of a thick NiAl/Al2O3 coating by embedding method and its corrosion resistance under supercritical water environment

One of the critical issues in the application of supercritical water oxidation technology is to improve the corrosion resistance of reactor materials. Use of Al₂O₃ coating is one of the most promising methods to address this issue. In this study, thick NiAl/Al₂O₃ coatings on Inconel 625 substrates were prepared by a consecutive pack embedding and in-situ thermal oxidation process. The effect of aluminizing and oxidation temperature on phase structure and coating thickness is studied. Results show the diffusion of Al from the exterior to the interior of the alloy matrix to form intermetallic compounds between Al and metal elements in the matrix (Ni, Cr, Mo, etc.). Moreover, the coating thickness can reach above 300 μm at the aluminizing temperature of 950 °C. Increasing the aluminizing temperature above 950 °C will not increase the coating thickness further. After high temperature oxidation subsequently, only phases of NiAl and Al₂O₃ were detected. The formation of Al₂O₃ layer can be ascribed to the surface oxidization of Al. And the NiAl between the alloy substrate and Al₂O₃ coating provides an interfacial layer that can alleviate the crack or exfoliation of ceramic coating due to the mismatching of thermal expand coefficient. The thick NiAl/Al₂O₃ coatings prepared by aluminizing 950 °C and oxidizing at 1100 °C exhibit satisfied corrosion resistance after supercritical water test. This work would provide a significant method to develop advanced ceramics coating for the corrosion resistance of alloys.     

Thermal shock resistance of Al2O3/SiO2 composites by sol-gel

In this paper, the SiO₂ ceramic matrix composites were reinforced by the two-dimensional (2D) braided Al₂O₃ fibers by sol-gel. To develop the high performance aeroengine with excellent resistance to thermal shock for advanced aerospace application, two different thermal shock temperatures (1100?°C and 1300?°C) and three different thermal shock cycles (10, 20 and 30 cycles) were tested and compared in this paper; besides, the thermal shock resistance of Al₂O₃/SiO₂ composites was investigated in air. Our results suggested that, the flexural strength of the untreated composites was 78.157?MPa, while the residual strength of Al₂O₃/SiO₂ composites under diverse thermal shock cycles and temperatures had accounted for about 95% and 50% of the untreated composites, respectively. Meanwhile, the density and porosity of the composites were gradually increased with the increase in test temperature. Moreover, the changes in fracture morphology and micro-structural evolution of the composites were also observed. Our observations indicated that, the fracture morphology of the composites mainly exhibited ductile fracture at the thermal shock temperature of 1100?°C, whereas brittle fracture at the thermal shock temperature of 1300?°C. Additionally, Al₂O₃/SiO₂ composites belonged to the Oxide/Oxide CMCs, so no new phase was formed after thermal shock tests. Above all, findings of this paper showed that Al₂O₃/SiO₂ composites had displayed outstanding thermal shock resistance.     

Preparation and structure control of a scalelike MoSi2-borosilicate glass coating with improved contact damage and thermal shock resistance

The scalelike coatings with MoSi₂ as emittance agent, flake fused quartz as coarse fillers, silica sol as dispersive medium of coating slurry, were prepared on rigid mullite fibrous ceramics to avoid fatal radial cracks and improve thermal shock resistance via combining slurry method with sol-gel method. And the particle size distribution of fused silica was changed to get different crack structure in scalelike coatings. Microstructure and phase composition of the coatings with different crack structure were investigated comprehensively. Contact damage resistance, thermal shock resistance and infrared radiating property were also studied. The results showed that only the coatings with mixing large and small particles of fused silica as coarse fillers could form crack network. The scalelike coatings with crack network went through 25 thermal cycles between 1500 °C and room temperature without peeling and spalling, and the bonding strength did not decrease after test. The scalelike structure with crack network also avoided radial cracks and exhibited some softness and less stiffness in Hertzian indentation test. The emissivity of coatings with different crack structure were all higher than 0.85; and the coatings with lower proportion of crack areas had higher emissivity.     

Ca-Mn co-doping LaCrO3 coating with high emissivity and good mechanical property for enhancing high-temperature radiant heat dissipation

Two techniques, including spray drying and electrostatic spray, were applied to produce feedstocks for preparing Ca-Mn co-doping LaCrO₃ ceramic coatings with two different structures on Ni-based alloy by the atmospheric plasma spraying method. The results show that coating from feedstocks produced by spray drying exhibits lower roughness and porosity than the coating from feedstocks produced by electrostatic spray due to the full melting of smaller feedstocks. Higher proportion of melting zones is beneficial to enhance the ratio of hardness to modulus to improve wear resistance. The emissivity of the coatings with roughness from 0.65 µm to 4.6 µm is all above 0.9 in the waveband of 1–14 µm at room temperature. What’s more, structure-dependent emissivity is affected by surface roughness and pore size due to the infrared scattering. The temperature-dependent thermal infrared emissivity at 1–14 µm decreases with the increasing temperature, and is still above 0.67 at 1200 °C.