Solidification mechanism and microstructure evolution of Al2O3-ZrO2 ceramic coating prepared by combustion synthesis and thermal explosion spraying

The solidification mechanism and microstructure of the hypoeutectic Al₂O₃-ZrO₂ (Al₂O₃:72 mol%) ultra-fined ceramic coating prepared by combustion synthesis and rapid plate cooling method were analyzed by the heat transfer process and dynamic characteristics. The rapid solidification process inhibited the transformation from the t-ZrO₂ to m-ZrO₂ at low temperatures. The growth rate of the solid-liquid interface to form the amorphous and nano-crystalline Al₂O₃-ZrO₂ was about 65.7 mm/s and 13.7 mm/s, respectively. The mechanism formation of both the amorphous and nano-crystalline areas were analyzed using rapid solidification models. When the growth rate reduced to about 8.23 mm/s, large quantities of nanosized eutectic structures was identified by SEM in the pseudo-eutectic area. The interphase spacing of the eutectic structures was 40–100 nm. In addition, some typical divorced eutectic structures appeared at this area. After that, micron dendrites (0.2–0.6 µm) took the main part when the growth rate decreased to about 3.67 mm/s for such a hypoeutectic Al₂O₃-ZrO₂ binary system. The nano-crystalline area showed the highest nanohardness (22 GPa). This paper may provide new guidance to prepare high performance Al₂O₃-ZrO₂ ceramics both in experiment and theory.     

Effect of an abrupt change in pulling rate on microstructures of directionally solidified Al2O3-Er3Al5O12 eutectic and off-eutectic composite ceramics

Directionally solidified microstructures of Al₂O₃-Er₃Al₅O₁₂ eutectic and off-eutectic in situ composite ceramics were explored under abrupt-change pulling rate conditions. Corresponding temperature distributions and interface locations were studied. In eutectic composition, fluctuation of eutectic spacing occurred when the pulling rate increased abruptly. A gradually increase or abrupt increase in eutectic spacing was observed when the pulling rate decreased abruptly. In hypoeutectic and hypereutectic compositions, formation of the primary phases were suppressed when the pulling rate increased abruptly from 10?µm/s to 100?µm/s, while primary phases precipitated when the pulling rate decreased abruptly from 100?µm/s to 10?µm/s. The interface altitude decreased after the pulling rate increased abruptly, but increased after the pulling rate decreased abruptly. The liquid composition restriction (around the eutectic composition) at the eutectic interface plays an important role in the suppression of the primary dendrite and coupled eutectic oxides can be obtained in off-eutectic compositions even under higher solidification rate conditions.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Synthesis and characterization of Al2O3 - ZrO2-based eutectic ceramic powder material dispersion-hardened with ZrB2 and WB particles prepared by SHS

Method of self-propagating high-temperature synthesis (SHS) was used to obtain ceramic powder material based on Al₂O₃-ZrO₂-WB/ZrB₂ system with size of particles ranging from 10 to 200?µm. The routes of chemical reactions taking place in the process of synthesis depending on the source mixture composition have been determined. It was shown that the formation of zirconium boride strengthening phase depends on the content of source components В and W and combustion temperature of mixture. The analysis of microstructure has shown that powder particles feature complex composite structure consisting of oxide eutectics of Al₂O₃-ZrO₂, individual phases of aluminum oxide and zirconium oxide as well as particles of WB and/or ZrB2. The technological properties of produced powders have been investigated.     

Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-frequency zone melting

Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600?°C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al₂O₃/GdAlO₃ eutectic ceramic was prepared with horizontal high-frequency induction zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic was composed only of the Al₂O₃ phase and GdAlO₃ phase penetrating mutually, and the Al₂O₃ phase was the substrate in which the GdAlO₃ phase was embedded. As the solidification rate increased from 1 to 5?mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5?μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36?GPa and 3.12?MPa?m^1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-frequency induction zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO₃ phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.     

Microstructures and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic by laser floating zone melting with high temperature gradient

Directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic rods with high densities and low solidification defects are prepared by laser floating zone melting at solidification rate from 2 to 200 μm/s. The microstructure evolution, eutectic growth behavior and mechanical properties are investigated. At low solidification rates (<30 μm/s), the eutectic rods present a homogeneous irregular eutectic microstructure, whereas cellular microstructure containing regular lamella/rod structure is developed at higher solidification rates. The relationship is established between the eutectic interphase spacing and solidification rate, which follows the Magnin-Kurz eutectic model. The Vickers hardness (15.9–17.3 GPa) increases slightly with decreasing interphase spacing, but the fracture toughness (4.08 MPa m^1/2) shows little dependence with the solidification rate. Different crack propagation mechanisms are revealed among the indentation cracks. The flexural strength at ambient temperature reaches up to 1.14 GPa for the eutectic grown at 100 μm/s. The fracture surface analysis indicates that the surface defects are the main crack source.     

In situ fabrication and microstructural evolution of Al2O3-ZrO2 nanoeutectic ceramics by a novel pulse discharge plasma assisted melting method

The present work concentrated on a novel one-step pulse discharge plasma melting method for in situ fabricating Al₂O₃-ZrO₂ binary eutectic in a short processing time. Al₂O₃-ZrO₂ eutectic exhibited well-aligned fibrous and colonial structure with a minimum lamellar spacing of ～250?nm at the growth rate of ～176.0?μm/s. The lamellar spacing was well consistent with the inverse-square-root dependence on the growth rate as $\lambda =2.90\times v^{?1/2}$, and the constant of proportionality of 2.90 was close to 3.32 predicted by the JH eutectic theory. The formation mechanism of Al₂O₃-ZrO₂ eutectic on primary faceted-alumina crystals was also proposed. The Al₂O₃-ZrO₂ eutectic originated from the faceted Al₂O₃ primary crystals, which played the leading role in the coupled growth process. The interfacial fracture behavior across the coarse intercolony regions was explored, and the fracture toughness of Al₂O₃-ZrO₂ eutectic ceramics was comparable to those achieved by traditional directional solidification.     

Investigation of the solidification behavior of Al2O3/YAG/YSZ ceramic in situ composite with off-eutectic composition

Directionally solidified Al₂O₃/YAG/YSZ ceramic in situ composite is an interesting candidate for the manufacture of turbine blade because of its excellent mechanical property. In the present study, two directionally solidified hypoeutectic and hypereutectic Al₂O₃/YAG/YSZ ceramic in situ composites are prepared by laser zone remelting, aiming to investigate the solidification behavior of the ternary composite with off-eutectic composition under high-temperature gradient. The results show that the composition and laser scanning rate significantly influence the solidification microstructure. The ternary in situ composite presents ultra-fine microstructure, and the eutectic interspacing is refined with the increase of the scanning rate. The Al₂O₃/YAG/YSZ hypoeutectic ceramic displays an irregular hypoeutectic network structure consisting of a primary Al₂O₃/YAG binary eutectic and fine Al₂O₃/YAG/YSZ ternary eutectic. Only at low scanning rate, homogeneous ternary eutectic-like microstructures are obtained in the hypoeutectic composition. Meanwhile, the Al₂O₃/YAG/YSZ hypereutectic ceramic shows homogeneous eutectic-like microstructure in most cases and the eutectic interspacing is finer than the ternary eutectic. Furthermore, the formation and evolution mechanism of the off-eutectic microstructure of the ternary composite are discussed.     

Microstructure control,competitive growth and precipitation rule in faceted Al2O3/Er3Al5O12 eutectic in situ composite ceramics prepared by laser floating zone melting

Microstructure control and competitive growth of Al₂O₃/Er₃Al₅O₁₂ eutectic/off-eutectics are explored over wide ranges of solidification rates and compositions. Gradual transformation phenomenon of microstructure morphology from complete eutectic to eutectic + coarse Er₃Al₅O₁₂ phase and to eutectic + Er₃Al₅O₁₂ dendrite is observed and the corresponding influence factors are evaluated. Competitive growth between single-phase Al₂O₃ (or Er₃Al₅O₁₂) dendrite and eutectic is analyzed and coupled growth zone is mapped through comparing interface temperatures of different patterns of microstructures. The complete eutectic microstructure could be obtained at Al₂O₃/Er₃Al₅O₁₂ hypoeutectic (Al₂O₃-17.5 mol% Er₂O₃) under fast solidification rate and the onset growth rate (?0.94 × 10⁴ μm/s) estimated from the measured eutectic spacing (?150 nm) fits well with the result calculated on the basis of competitive growth (?1.27 × 10⁴ μm/s). Transformation of microstructure from irregular eutectic to regular eutectic and probable adjustment mechanism of eutectic spacings are discussed when the eutectic spacings refined from micron-scale (<10 μm) to nano-scale (?20 nm).     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.     

Microstructure refinement approaches of melt-grown Al2O3/YAG/ZrO2 eutectic bulk

Microstructure developments of melt-grown Al₂O₃/YAG/ZrO₂ ceramic bulks were investigated by controlling composition, cooling rate, heterogeneous nucleation sites and melt superheating treatment. The solidification microstructure of sample with hypereutectic composition (ZrO₂ 20 mol%) is finer than that with hypoeutectic or eutectic ones. With increasing the cooling rate, microstructure of melt-grown samples develops from colony to dendrite and finally to cell. The microscopy and the components of samples vary with the melt superheating temperature and the type of heterogeneous nucleation sites. The microstructure evolutions of melt-grown Al₂O₃/YAG/ZrO₂ eutectic relate to the melt undercooling level and the solid–liquid interfaces stability.     

Insight into the complex coupled growth behavior of Al2O3/YAG eutectic ceramic based on the evolutions of microstructure and crystallographic texture

Clarifying the dependence of microstructure and crystallographic texture on growth kinetics is critical concerns for mechanical properties prediction in complex faceted eutectic growth. In this study, we present the comprehensive investigations of microstructure and crystallographic texture characteristic in directionally solidified Al₂O₃/YAG eutectic that rely on its coupled growth mechanism. The growth mechanism of eutectic spacing is also well depicted by the Magnin-Kurz (MK) model associated with morphological criteria for branching in faceted phase. As increasing the solidification rate (10 µm/s∼200 µm/s), the regularization tendency for irregular three-dimensional interpenetrated (TDI) structure in geometric category is stronger. Whereas, the enhanced kinetic undercooling increases the misorientation and interfacial energy, which is attributed to the fact that the growth kinetics is more dominant than the minimum interface energy in this work. Additionally, the fluctuation tendency of eutectic spacing and crystallographic orientation with the solidification rate is diametrically opposite. The colony structure consisting of coupled irregular eutectic is formed by the planar interface instability, and its formation mechanism is revealed by the misorientation distribution. This comprehensive research contributes to the design of high-performance eutectic ceramics considering the influence of microstructural features and crystallographic growth modes.     

Formation mechanism of a wrinkled and textured Al2O3-ZrO2 nanoeutectic rapidly solidified from oxy-acetylene flame remelting

A highly wrinkled and textured Al₂O₃-ZrO₂ nanoeutectic induced by oxy-acetylene flame remelting was elaborated for the first time from the perspective of morphological evolution and formation mechanism. The rapidly solidified Al₂O₃-ZrO₂ ceramic exhibits a wrinkled cellular nanoeutectic structure with a minimum interphase spacing of 0.12 μm. The Jackson-Hunt (JH) eutectic theory is more appropriate to predict the growth of Al₂O₃-ZrO₂ nanoeutectic induced by oxy-acetylene flame remelting than the Trivedi-Magnin-Kurz (TMK) model. The bulk growth rate of 7.8 μm³/s is close to the theoretically predicted value of 11.0 μm³/s by the JH model. The eutectic growth rates of Al₂O₃-ZrO₂ with average interphase spacings of 0.12 and 0.3 μm are estimated to be 763.8 and 122.2 μm/s, respectively. Especially, a mesoscopic insight into formation mechanism of the wrinkled and textured eutectic was proposed from a three-dimensional perspective based on both the crystal growth orientation and the airflow effect.     

Formation mechanism and process optimization of nano Al2O3-ZrO2 eutectic ceramic via laser engineered net shaping (LENS)

Periodic banded structure has great influence on the microstructure and fracture toughness of eutectic ceramic produced by LENS system. Formation Mechanism of periodic banded structure and influence regularity from laser power, scanning speed, feeding rate and feeding proportion were studied. X-ray diffractometer analysis and scanning electron microscopy observation were carried out for sample phase composition and microstructure respectively. Microhardness was measured by vivtorinox hardness tester, and surface fracture toughness was calculated by indentation crack length. The results show that periodic banded structure is divided into organizational coarsening zone and divorced eutectic zone. Due to the influence of upper cladding layer heat, the eutectic structure in the interlayer bonding zone is obviously coarsening in organization coarsening zone. When laser power is lower, Al₂O₃ phase takes the lead in nucleation and grows up quickly, forming a discrete block structure of Al₂O₃ particles in divorced eutectic area. When laser power is higher, ZrO₂ phase takes the lead in nucleation and forms a discrete block structure of ZrO₂ particles. Al₂O₃-ZrO₂ ceramics with the shape of thin-walled and cylinder were manufactured under the optimized process conditions, whose independent nucleation disappears and the thickness of periodic banded structure reduces to 10 µm. The eutectic spacing of prototype structure reaches 120–130 nm, has uniform microstructure and good surface morphology. The average micro-hardness is 18.59 GPa. Surface fracture toughness increases to 6.52 MPa m^1/2.     

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al₂O₃–Y₂O₃–ZrO₂ ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al₂O₃ tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.     

Optical absorption and selective thermal emission in directionally solidified Al2O3-Er3Al5O12 and Al2O3-Er3Al5O12-ZrO2 eutectics

Al₂O₃-Er₃Al₅O₁₂ and Al₂O₃-Er₃Al₅O₁₂-ZrO₂ eutectic ceramic rods were directionally solidified using the laser floating zone technique at several growth rates. Binary eutectic microstructure consisted in a three-dimensional interpenetrated network of the eutectic phases whereas the ternary eutectic showed a geometrical microstructure at low growth rates and a nanofibrillar pattern at high rates. The microstructure size was strongly dependent on the growth rate, decreasing when the processing rate increased. The optical absorption was measured in the samples at room temperature and Judd–Ofelt analysis was used to model the optical absorption of the Er³⁺ ions. Thermal emission of the eutectic rods was studied at temperatures up to 1600 °C. An intense narrow emission band at 1.55 μm matching with the sensitive region of the GaSb photoconverter was obtained. The intensity of the selective emission band is larger for the binary eutectic than for the ternary compound and increases as the microstructural size decreases.     

Microstructure, growth mechanism and mechanical property of Al2O3-based eutectic ceramic in situ composites

Directionally solidified Al₂O₃-based eutectic ceramic in situ composites with inherently high melting point, low density, excellent microstructure stability, outstanding resistance to creep, corrosion and oxidation at elevated temperature, have attracted significant interest as promising candidate for high-temperature application. This paper reviews the recent research progress on Al₂O₃-based eutectic ceramic in situ composites in State Key Laboratory of Solidification Processing. Al₂O₃/YAG binary eutectic and Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics are prepared by laser zone melting, electron beam floating zone melting and laser direct forming, respectively. The processing control, solidification characteristic, microstructure evolution, eutectic growth mechanism, phase interface structure, mechanical property and toughening mechanism are investigated. The high thermal gradient and cooling rate during solidification lead to the refined microstructure with minimum eutectic spacing of 100 nm. Besides the typical faceted/faceted eutectic growth manner, the faceted to non-faceted growth transition is found. The room-temperature hardness H_V and fracture toughness K_IC are measured with micro-indentation method. For Al₂O₃/YAG/ZrO₂, K_IC = 8.0 ± 2.0 MPa m^1/2 while for Al₂O₃/YAG, K_IC = 3.6 ± 0.4 MPa m^1/2. It is expectable that directionally solidified Al₂O₃-based eutectic ceramics are approaching practical application with the advancement of processing theory, technique and apparatus.     

Hydrogen production by steam reforming of LNG over Ni/Al2O3-ZrO2 catalysts: Effect of ZrO2 and preparation method of Al2O3-ZrO2

An Al₂O₃-ZrO₂ support was prepared by grafting a zirconium precursor onto the surface of commercial γ-Al₂O₃. A physical mixture of Al₂O₃-ZrO₂ was also prepared for the purpose of comparison. Ni/Al₂O₃-ZrO₂ catalysts were then prepared by an impregnation method, and were applied to the hydrogen production by steam reforming of liquefied natural gas (LNG). The effect ZrO₂ and preparation method of Al₂O₃-ZrO₂ on the performance of supported nickel catalysts in the steam reforming of LNG was investigated. The Al₂O₃-ZrO₂ prepared by a grafting method was more efficient as a support for nickel catalyst than the physical mixture of Al₂O₃-ZrO₂ in the hydrogen production by steam reforming of LNG. The well-developed tetragonal phase of ZrO₂ and the high dispersion of ZrO₂ on the surface of γ-Al₂O₃ were responsible for the enhanced catalytic performance of Ni/Al₂O₃-ZrO₂ prepared by way of a grafting method.