Improvement in heat resistivity of alkaline earth silicate fiber boards by Al4SiC4 coating

Alkaline earth silicate (AES) fiber ceramic board was immersed in a novel coating slurry consisting of Al₄SiC₄ particles and silica-sol, forming a 500-μm coating layer on the AES fiber board. Linear shrinkage of the uncoated AES fiber board was over 3.0% after heating at 1100°C or higher for 8 hours, whereas the linear shrinkage of the AES fiber board with an Al₄SiC₄ coating was below 2.0%. The coating layer of Al₄SiC₄ changed to a hard shell structure consisting of cristobalite, alumina, and mullite after heating. When AES fiber board with an Al₄SiC₄ coating layer was heated at 1200°C for 8 hours, the compressive strength of the board reached 0.45 MPa, 2.5 times greater than that of the original uncoated board.     

Isothermal oxidation behavior of YAl3C3 at 900–1300 °C in air

The isothermal oxidation behavior of YAl₃C₃ in air has been investigated at 900−1300 °C for 20 h. At 900 and 1000 °C, the oxidation kinetic curves of YAl₃C₃ obey parabolic rate law. The oxide scales have a bilayer structure. The outer layer is composed of YAG and amorphous Al₂O₃, and the inner layer is a carbon-rich layer. The oxidation kinetic curves of YAl₃C₃ obey linear law above 1100 °C. The oxide scales have a monolayer structure composed of Al₂O₃ and YAG. The crystallization transformation of amorphous Al₂O₃ causes the oxidation kinetic curve changing from a parabolic to a straight line.     

Improving the High‐Temperature Oxidation Resistance of Nb4AlC3 by Silicon Pack Cementation

Niobium aluminum carbide (Nb₄AlC₃), as a member of the MAX phases, can retain its stiffness and strength up to over 1400°C. However, its applications are limited due to its poor oxidation resistance at high temperatures. In this work, silicon pack cementation has been applied to improve the oxidation resistance of Nb₄AlC₃. After Si pack cementation at 1200°C for 6 h, a dense and uniform silicide coating which was mainly composed of NbSi₂ and SiC and well bonded to the matrix was successfully formed on the surface of Nb₄AlC₃. The Si pack cemented Nb₄AlC₃ shows excellent oxidation resistance up to 1200°C due to the formation of protective Al₂O₃ layer. The oxidation kinetics of the cemented Nb₄AlC₃ obey parabolic law all the way to up to 1200°C, and the parabolic rate constants of cemented Nb₄AlC₃ are in the same order of magnitude as those of Ti₃AlC₂ in the temperature range 1000°C–1200°C. However, the oxidation of the cemented Nb₄AlC₃ was accelerated after oxidation at 1300°C for about 15 h due to the formation of NbAlO₄.     

High-temperature oxidation behaviour of vacuum hot-pressed WC-15 wt% Al2O3 composites

In this study, WC-15 wt% Al₂O₃ composites were prepared using the vacuum hot-pressing sintering method. The high-temperature (600–800 °C) oxidation behaviour of WC-15 wt% Al₂O₃ composites was investigated and compared with that of WC-6wt.%Co cemented carbides. The results showed that the oxidation resistance of WC-15 wt% Al₂O₃ composites was better than that of WC-6wt.%Co cemented carbides at relatively high temperatures (700–800 °C). At 800 °C, an oxide layer was formed on the surface of WC-15 wt% Al₂O₃ composites, which included WO₃ and Al₂O₃. The dispersion of alumina in the composites hindered the further diffusion of oxygen, thus improving the oxidation resistance. The Arrhenius activation energies of WC-15 wt% Al₂O₃ composites and WC-6wt.%Co cemented carbides were 110 ± 1 kJ/mol and 167 ± 2 kJ/mol at 600–800 °C, respectively.     

The effect of Al2O3 on the high‐temperature oxidation resistance of Si‐B‐C ceramic under air atmosphere

Si‐B‐C ceramics were prepared through reaction sintering, and the influence of Al₂O₃ addition on the high‐temperature (1100‐1300°C) oxidation behavior of the material under air atmosphere was studied. The erosion behavior and mechanism are determined from the measurement of weigh changes, microstructure observations, and characterization of the generated oxides on postexposure specimens. Results show that Al₂O₃ is enriched in the oxidized layer, inhibiting the volatilization of B₂O₃ and impeding the crystallization ability of oxide (cristobalite). Narrower erosion layer and less weigh change are observed with Al₂O₃. Low‐frequency Raman results reveals that with the increase in Al₂O₃, the bending vibrations of the BO₄ units and B‐O‐B stretching of the metaborate ring relative intensity are enhanced. Furthermore, high‐frequency Raman results shows that the relative proportion of high‐dimensional vibration modes Q³ and Q⁴ which result in a higher viscosity of melt and a greater resistance of oxygen diffusion are positively correlated with Al₂O₃.     

High-temperature hydrogenation behaviour of bulk titanium silicon carbide

Thermal stability of Ti₃SiC₂ was investigated at 1200–1400°C in hydrogen atmosphere for 3 hours. The hydrogenation mechanism was clarified by a combination of X-ray diffraction, scanning electron microscope, Raman spectroscopy and first principles calculation. At 1200°C, a dense and uniform TiSi₂ layer formed on the sample surface, which originated from both the preferable lose of silicon from the Ti₃SiC₂ substrate and the dissociation of Ti₃SiC₂. As temperature increased to 1300°C, TiSi₂ layer began to scale off and presented laminated Ti₃SiC₂ grains beneath this layer, which indicated preferential hydrogenation occurred along the basal planes. This phenomenon was ascribed to the fact that the introduction of H interstitial atom weakened the combination between titanium and silicon interface layer, which was confirmed by first principles calculations. In addition, the formation of TiSi₂ owing to the dissociation of Ti₃SiC₂ caused the volume expansion after hydrogenation, resulting in that majority of TiSi₂ layer spelled off at 1400°C.     

Enhancement of oxidation resistance at 1000–1400 °C of low carbon Al2O3–C refractories with pre-synthesized SiCnw/Al2O3

The oxidation resistance of low carbon Al₂O₃–C refractories with the addition of SiC_nw/Al₂O₃ composite powders and the enhancement mechanisms were investigated. The oxidation resistance was evaluated by oxidation index (O.I.) and oxidation rate constant (k). The enhancement mechanisms of SiC_nw/Al₂O₃ on oxidation resistance were analyzed based on the phases and microstructures. The results showed that the SiC_nw/Al₂O₃ can improve the oxidation resistance of Al₂O₃–C refractories, the O.I. and k of A6 (6 wt% SiC_nw/Al₂O₃ addition) were 26.0% and 34.5% lower than those of reference sample A0, respectively. The oxidation resistance of refractories was improved in a range of 1000–1400 °C due to the introduction of SiC_nw/Al₂O₃. The enhancement mechanisms can be explained that SiC_nw is more susceptible to be oxidized due to its high specific surface area, which expanded the action temperature range of other antioxidants and itself. The mullite and dense protective layer generated during oxidation is also beneficial to impede the diffusion of O₂.     

Oxidation Behavior of SiC Whiskers at 600–1400°C in Air

The oxidation behavior of SiC whiskers (SiC_W) with a diameter size of 50–200 nm has been investigated at 600°C–1400°C in air. Experimental results reveal that SiC_W exhibit a low oxidation rate below 1100°C while a significant larger oxidation rate after that. This can be attributed to the small diameter size of SiC_W, which determines that it is hard to form a protective SiO₂ layer thick enough to hamper the diffusion of oxygen effectively. Both nonisothermal and isothermal oxidation kinetics were studied and the apparent oxidation energy was calculated to further understand the oxidation behavior of the SiC_W.     

Microstructure and High‐temperature Oxidation Behavior of Ti3AlC2/W Composites

Using spark plasma sintering, Ti₃AlC₂/W composites were prepared at 1300°C. They contained “core‐shell” microstructures in which a Ti_xW_1?x “shell” surrounded a W “core”, in a Ti₃AlC₂ matrix. The composite hardness increased with W addition, and the hardening effect is likely achieved by the Ti_xW_1?x interfacial layer providing strong bonding between Ti₃AlC₂ and W, and by the presence of hard W. Microstructural development during high‐temperature oxidation of Ti₃AlC₂/W composites involves α‐Al₂O₃ and rutile (TiO₂) formation ≥1000°C and Al₂TiO₅ formation at ~1400°C while tungsten oxides appear to have volatilized above 800°C. Likely due to exaggerated, secondary grain growth of TiO₂‐doped alumina and the effect of W addition, fine (<1 μm) Al₂O₃ grains formed dense, anisomorphic laths on Ti₃AlC₂/5 wt%W surfaces ≥1200°C and coarsened to large (>5 μm), dense, TiO₂‐doped Al₂O₃ clusters on Ti₃AlC₂/10 wt%W surfaces ≥1400°C. W potentially affects the oxidation behavior of Ti₃AlC₂/W composites beneficially by causing formation of Ti_xW_1?x thus altering the defect structure of Ti₃AlC₂, resulting in Al having a higher activity and by changing the scale morphology by forming dense Al₂O₃ laths in a thinner oxide coating, and detrimentally through release of volatile tungsten oxides generating cavities in the oxide scale. For Ti₃AlC₂/5 wt%W oxidation, the former beneficial effects appear to dominate over the latter detrimental effect.     

High‐temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity

Cr₂AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90‐180 μm, 180‐250 μm, and 250‐400 μm. Sacrificial template technique was used as the processing method, utilizing NH₄HCO₃ as a temporary pore former. Cr₂AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in‐situ formation of an outer thin continuous protective layer of α‐Al₂O₃. The in‐situ α‐Al₂O₃ protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter‐ to transgranular fracture mode. According to the excellent high‐temperature response, heat exchangers and catalyst supports are the potential application of these foams.     

Oxidation and Crack Healing Behavior of a Fine‐Grained Cr2AlC Ceramic

This work reports the oxidation and crack healing behavior of a fine‐grained (~2 μm) Cr₂AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C–1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin α‐Al₂O₃ layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X‐ray tomography. Three‐point bend tests showed the dependence of strength recovery at 1100°C as a function of initial crack length and healing time.     

Mechanical properties and thermal shock resistance of Si2BC3N ceramics with ternary Al4SiC4 additive

Dense Si₂BC₃N ceramics were prepared through SPS sintering the amorphous Si₂BC₃N and Al₄SiC₄ powders obtained from mechanical alloying. The phase compositions, microstructures, and mechanical properties, as well as the thermal shock resistance were investigated. In addition, evaluations of oxidation and the ablation resistance were also preceded. The results show that Al₄SiC₄ phase can be detected at 1200 and 1400?°C under pressureless sintering. However, Al₄SiC₄ can be decomposed to AlN and SiC phases under higher temperatures. As for the bulk Si₂BC₃N ceramics, the Al₄SiC₄ additive induce the development of turbostratic BN(C) plates and improve the relative density consequently. Besides, the Al₄SiC₄ plates are embedded in the matrix of ceramics. Therefore, the mechanical properties and thermal shock resistance are improved apparently with the addition of additive. Meanwhile, the additive containing composites have superior ablation resistance than the pristine Si₂BC₃N ceramics due to their higher relative density.     

Characterization of Ln4Al2O9 (Ln=Y,Sm, Eu,Gd, Tb) rare-earth aluminates as novel high-temperature barrier materials

A family of cuspidine-type rare-earth (RE) aluminates with the general formula Ln₄Al₂O₉ (Ln= Y, Sm, Eu, Gd, Tb) was prepared for potential use as thermal-barrier coating (TBC) materials with appropriate properties. Various trivalent lanthanides were applied to tailor the properties of the oxides for use as ceramic top coat (TC) materials intended for high-temperature applications. Following various heat treatments, the X-ray diffraction (XRD) results obtained demonstrated that Eu₄Al₂O₉ (EuAM) possessed the greatest structural stability of all the samples at 1200 and 1300?°C. Moreover, Y₄Al₂O₉ (YAM) had a long lifetime at 1000?°C, and was stable at 1100?°C. At 1200?°C, Sm₄Al₂O₉ (SmAM) and Gd₄Al₂O₉ (GdAM) were more stable than Tb₄Al₂O₉ (TbAM). However, at 300–1000?°C, the TbAM exhibited the highest thermal expansion coefficient (TEC) of all the samples. At 600?°C, the thermal diffusivity values of the five compositions were favourable, and were lower than that of yttria-stabilised zirconia (YSZ) oxides.     

Thermochemical interactions between CMAS and Ca2Y8(SiO4)6O2 apatite environmental barrier coating material

Thermochemical interactions between Ca₂Y₈(SiO₄)₆O₂ apatite, a potential environmental barrier coating (EBC) material, and a synthetic CMAS having the composition 23.3 CaO - 6.4 MgO - 3.1 Al₂O₃ - 62.5 SiO₂ - 4.1 Na₂O - 0.5 K₂O - 0.04 Fe₂O₃ mole % were investigated. Pellets of apatite + CMAS powder and hot-pressed apatite disc-CMAS couples were annealed at 1200–1500 °C for 1–50 hours in air. Powder X-ray diffraction (XRD) was used to identify the phases present. Polished cross-sections of the heat treated pellets and diffusion couples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). Ca₃Y₂(Si₃O₉)₂ cyclosilicate, apatite, and amorphous phases were present in the samples heat treated at 1200 and 1300 °C, whereas no cyclosilicate was detected in samples annealed at 1400 and 1500 °C. A distinct cyclosilicate layer was observed at the apatite-CMAS interface in the diffusion couples heat treated at 1200 and 1300 °C. However, at 1400 and 1500 °C, due to its much lower viscosity, CMAS quickly infiltrated the apatite substrate through pores and along the grain boundaries and no cyclosilicate was observed; the apatite grains dissolved in molten CMAS followed by re-precipitation of apatite needles within an amorphous phase on cooling.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Increased high temperature strength and oxidation resistance of Al4SiC4 ceramics

Al₄SiC₄ bulk ceramics were synthesized by reaction hot-pressing using Al, graphite powders and polycarbosilane (PCS) as starting materials. The present work confirmed that this process was an effective method for the preparation of Al₄SiC₄ ceramics having high relative density and well-developed plate-like grains. The mechanical, thermal properties and oxidation behaviors of the Al₄SiC₄ ceramics were also investigated. The flexural strength, fracture toughness (K_IC) and Vickers hardness at room temperature were 297.1 ± 22 MPa, 3.98 ± 0.05 MPa m^1/2, 10.6 ± 1.8 GPa, respectively. The high-temperature bending strength showed an increasing trend with increasing test temperatures, with the value of 449.7 ± 26 MPa at 1300 °C. The thermal expansion coefficient was 6.2 × 10⁻⁶ °C⁻¹ in the temperature range from 200 °C to 1450 °C. The isothermal oxidation of Al₄SiC₄ ceramics at 1200–1600 °C for 10–20 h revealed that it had excellent oxidation resistance.     

Thermochemical degradation of HfSiO4 by molten CMAS

The thermochemical degradation of hafnium silicate (HfSiO₄) was investigated with a molten calcium-magnesium-aluminosilicate (CMAS) glass relevant to gas turbine engine applications. Sintered HfSiO₄ coupons were fabricated, within which wells were drilled and filled with CMAS glass powder at a loading of ～35 mg/cm². Samples were heat treated at 1200°C, 1300°C, 1400°C, and 1500°C for 1 h, 10 h, and 50 h. At 1200°C and 1300°C, slow formation of a Ca₂HfSi₄O₁₂ cyclosilicate phase was observed at the HfSiO₄-CMAS interface. At 1300°C and higher, rapid infiltration of CMAS into the material along the grain boundaries was observed. Initial conjecture into CMAS degradation mechanisms of HfSiO₄ are presented herein.     

Mechanical properties and microstructural evolution of Cansas-III SiC fibers after thermal exposure in different atmospheres

Cansas-III SiC fibers were exposed in argon, air and wet oxygen (12%H₂O+8%O₂+80%Ar) atmospheres for 1 h at 1000–1500 °C. The pristine fiber consisted of β-SiC, free carbon and SiC_xO_y phases. After exposure in air and wet oxygen, an amorphous SiO₂ layer with embedding α-cristobalite crystals formed, while stacking faults were generated in the SiC core to release the residual stress. With the increasing oxidation temperature, lots of pores formed in the oxide layer, accompanied with the thickening, cracking and spallation of oxide layer. The average tensile strength decreased with the exposure temperature increasing and the exposure atmosphere deteriorating (argon→air→wet oxygen). After exposure at 1400 °C in argon and air, the fiber strength retention rates were 84% and 70%, respectively. However, after exposure at 1300 °C in wet oxygen, the strength retention rate was only 51%, indicating the accelerating oxidation and severe strength degradation of fibers.     

Fabrication and characterisation of single-phase Hf2Al4C5 ceramics

Single-phase Hf₂Al₄C₅ ternary carbide was fabricated from Hf/Al/C powder mixtures by pressure assisted sintering techniques such as hot pressing and spark plasma sintering at 1900 °C for 3 h and 10 min, respectively. XRD confirmed that the ternary carbide started to form at temperatures as low as 1500 °C and with total formation of Hf₂Al₄C₅ after reactive sintering for 1 h at 1900 °C. It is evident from HRTEM that two Hf-C layers were sandwiched with 4 Al-C layers (Al₄C₃) in the Hf₂Al₄C₅ ternary carbide. Tight interlocking of grains, faceted grains and stacking faults were occasionally observed. Thermal conductivity of Hf₂Al₄C₅ is measured to be 14 w m^?1k^?1 from room temperature to 1300 °C. The oxidation studies carried out at 1300 °C for 3 h reveal that the oxidation layer thickness is around 220 μm and it contains microcracks closer to sample surface whereas the interface looks seamless without any cracking or spallation of the oxide layer.