Effect of different corundum sources on microstructure and properties of Al2O3–Cr2O3 refractories

Al₂O₃–Cr₂O₃ refractories are completely substitutional solid solutions and exhibit better corrosion and abrasion resistance. To enable the comprehensive utilization of it, the microstructure and properties of Al₂O₃–Cr₂O₃ samples with different corundum sources were investigated in this study. The starting sources of corundum sources included sintered tabular corundum, fused white corundum, or brown corundum with minor impurities of β-Al₂O₃ and TiO₂. The results of mechanical test showed that the introduction of white corundum deteriorates the physical structure, while brown corundum acts in an opposite manner. The optimum bonding strength of the Al₂O₃–Cr₂O₃ brick was reached by combining white and brown corundum, whereby rapid neck growth occurred via surface diffusion during solid-phase sintering.     

Mechanism of in-situ formation of spinel and its effect on the mechanical properties of Al2O3–C refractories

This work looked at the in-situ formation mechanism of magnesia alumina spinel in Al₂O₃–C refractories with magnesia addition at different firing temperatures. A comprehensive study on the mechanical properties of Al₂O₃–C refractories was performed in comparison to traditional analogs. The magnesia alumina spinel was in-situ formed at the firing temperature of 1150 °C in Al₂O₃–C refractories. With the increase of the firing temperature, the Al₂O₃ phase was gradually dissolved in spinel phase to form aluminum rich spinel phase, resulting in a decrease in its lattice constant due to the defects formation. The formed spinel phase was homogenously distributed and bonded well with corundum, improving the interfacial bond, load transferring capacity and crack propagation resistance. The formation of spinel phase also enhanced the sintering of the alumina matrix owing to the solid solution of alumina in the spinel. Therefore, the mechanical properties such as cold modulus of rupture and hot modulus of rupture in Al₂O₃–C refractories achieved a substantial enhancement compared with traditional refractories.     

Microstructure and mechanical properties of CaAl12O19 reinforced Al2O3-Cr2O3 composites

Al₂O₃-Cr₂O₃ refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. However, Al₂O₃-Cr₂O₃ refractories have poor mechanical properties and sintering properties. In order to improve the mechanical properties of Al₂O₃-Cr₂O₃ materials, the CaAl₁₂O₁₉ reinforced Al₂O₃-Cr₂O₃ composites were prepared by pressureless sintering process, and the influences of CaO content on the sintering properties, mechanical properties, and microstructure evolution of the composites were studied. The results show that a small amount of CaO can significantly improve the compactness of the composites, which is mainly due to the formed sheet-like CA6 fill the gap between the solid solutions, and reduces the porosity of the composites. In addition, the sheet-like CA6 makes the connection between solid solutions closer, and the intergranular fracture gradually transforms into a mixed mode of intergranular and transgranular fracture. The best mechanical propertie is observed at S4 with the CaO content of 2 wt.%. Compared with sample S0 without CaO, the hardness, compressive strength and flexural strength of the S4 were increased by 35.19 %, 49.69 %, and 68.34 %, respectively. The addition of excessive CaO will deteriorate the mechanical properties of the composites, because the formation of a large number of layered CA6 increases the porosity of the composites. Furthermore, a small amount of CaO addition can significantly improve the thermal shock resistance of the composites. After 10 and 20 thermal shock cycles, the strength loss rates of S4 are only 5.83 % and 8.74 %, respectively.     

Cast refractories of the Al2O3-Cr2O3-CaO system

Conclusions In order to develop stable materials for glassmaking, we studied the physical and technological properties of the refractories belonging to the Al₂O₃-Cr₂O₃-CaO system that contain 5–20% CaO, 15–35% Cr₂O₃ and 45–80% Al₂O₃.The glass resistance of the refractories of the experimental systems (compositions) exceeds that of the BKCh-33 baddeleyite-corundum products by 3–5 times and their thermal shock resistance is superior to that of the well known chromium-containing refractories at comparable levels of mechanical properties.The developed refractories are recommended for the top or the bottom structures of the glassmaking furnaces depending on their glass resistance and thermal shock resistance and for making the refractory components of ferrous metallurgical units that are in contact with highly basic slags.Translated from Ogneupory, No. 3, pp. 23–26, March, 1989.     

Oxide/oxide composites in the system Cr2O3-Y2O3-Al2O3

Ceramic compacts in the systems Al₂O₃–Y₂O₃, Cr₂O₃–Y₂O₃ and Y₃(Cr_yAl_1-y)₅O₁₂ (Cr-doped YAG) were prepared by solid state reaction in calcined co-precipitated powder mixtures of appropriate compositions. Various solid-solution phases were formed, e.g. Y₃(Al_1-xCr_x)₅O₁₂, YAl_yCr_1-yO₃ and Al_2-xCr_xO₃. Composite materials in the pseudo-binary or ternary systems Al₂O₃–Y₃Al₅O₁₂, Cr₂O₃–Y₂O₃ and Y₃(Al_1–xCr_x)₅O₁₂–YAl_yCr_1–yO₃–(Al_zCr_1−z)₂O₃ were obtained by hot-pressing appropriate powder precursors at 1600–1650°C for 1 h. The microstructure of the prepared materials was studied in a scanning electron microscope with element analysis facilities. X-ray diffraction was used to reveal the phases present and their lattice parameters. The chemical compatibility of these phases was investigated. The results are discussed with a special emphasis on the solubility of Cr in the YAG structure, and on the compatibility relationship between Cr-doped YAG and its neighbouring phases. A gel-coating process for preparing Al₂O₃–YAG composites with tailored microstructures is also described.     

Fabrication of dense nano-laminated tungsten carbide materials doped with Cr3C2/VC through two-step sintering

This work investigates the critical roles of two-step sintering (TSS) and laminated structure on the sintering behavior and mechanical properties of functionally graded WC-TiC-Al₂O₃ nanostructured composite materials doped with Cr₃C₂/VC. Results show that excellent mechanical properties are achieved for tailored TSS conditions with a hardness of 27.91?±?2.3?GPa and a flexural strength of 1423.3?±?23.5?MPa. The desirable mechanical properties are attributed to the suppressed grain growth without densification deterioration. TSS is more effective in facilitating the favorable dispersion of secondary phase toughening nano-particulates in a WC matrix than conventional sintering (CS). Cr₃C₂/VC dopant plays an important role in maximizing and shifting the temperature range of the kinetic window for WC-Al₂O₃ composites. Al₂O₃ crack deflection, transgranular Al₂O₃, microcracking, WC crack bridging and plate-like WC crack deflection are the major toughening mechanisms. Residual surface compressive stress induced by the graded structure is also an appreciated contribution to the improvement of mechanical properties.     

Study of color change and microstructure development of Al2O3–Cr2O3/Cr3C2 nanocomposites prepared by spark plasma sintering

The densification behaviors of Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposites prepared by a Spark Plasma Sintering (SPS) were investigated in this work. The initial powders used for sintering were Al₂O₃–Cr₂O₃, which were prepared by metal organic chemical vapor deposition (MOCVD) in a spout bed. Different colors of the compacts such as green, purple and black were observed after densification process at different SPS temperatures from 1200 °C to 1350 °C. These changes of color were relevant to the existence of secondary phase of green Cr₂O₃, pink solid solution of Cr₂O₃–Al₂O₃ and black Cr₃C₂, which were formed under the different SPS temperature. The secondary phase of Cr₂O₃ retarded the processing of densification for spark plasma sintering at 1200 °C. The Cr₂O₃ reacted with Al₂O₃ to form solid solution of Cr₂O₃–Al₂O₃ and with carbon to form Cr₃C₂ as sintering temperature increased to 1350 °C. The characteristics of high heating rate, shorter sintering time for SPS and the formation of secondary phase of Cr₃C₂ effectively reduced the substrate's grain growth, making Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposites with small grain size.     

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al₂O₃–ZrO₂) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al₂O₃–ZrO₂ composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al₂O₃–ZrO₂ composite ceramic was fabricated via AC flash sintering under 60 mA mm^?2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.     

Laser sintering of Al2O3-based green ceramics prepared by different pre-sintering temperatures

Al₂O₃-based green ceramics are prepared by isostatic cold pressing technology. The prepared green ceramics are pre-sintered at the temperature from room temperature to 1100°C, and then Al₂O₃ ceramics are prepared by laser sintering. The effects of pre-sintering temperatures and laser parameters on mechanical properties and the sintering quality are analyzed. The results show that good crystallinity of Al₂O₃ particles is obtained at a higher pre-sintering temperature. The flexural strength and density of green ceramics increase with the temperature of heat treatment. The flexural strength decreases slightly at ∼200°C due to the paraffin binder disintegration. The pre-sintering temperature and laser processing parameters have a significant influence on the sintering quality. With the increase of laser power and laser frequency, dynamic grain growth occurs, and then grains are refined. The majority of plate-like grains are transformed into long cylindrical-like grains in the severe densification process. However, porous flocculation microstructures are generated on the samples pre-sintered at 1100°C after laser sintering, which is due to the material gasification in atmospheric environment during sintering by infrared laser. More uniform microstructure and better sintering quality of samples pre-sintered at 500°C can be achieved after laser sintering with a relatively narrower grain size distribution.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.     

Microstructural evolution and enhanced mechanical properties of 95Al2O3–Yb/Mg/Si ceramics by optimizing sintering temperature

Designed component of 0.95Al₂O₃–0.015Yb₂O₃–0.01MgO–0.025SiO₂ (95Al₂O₃–Yb/Mg/Si) ceramics were prepared by the traditional mixed-oxide method in the sintering temperature range of 1450–1700 °C. The influence of sintering temperature on the crystal structure, densification, microstructure, mechanical, friction and wear properties of 95Al₂O₃-YbMgSi ceramics were systematically investigated. XRD and SEM analysis results revealed that the increase in sintering temperature was very beneficial to eliminate the pores, increase the density and grain size, which obeys the common grain growth law. Both the flexural strength and hardness of obtained samples were increased almost linearly when the sintering temperature increased from 1450 °C to 1700 °C. The ceramics sintered at 1650 °C showed the optimum properties: H_v = 1812.3, σ = 151.3 MPa, μ = 0.41, ρ = 3.72 g/cm³ and K_c = 8.06e^?5 mm³/N·m, respectively. Furthermore, the results of friction and wear experiments suggested that the 95Al₂O₃–Yb/Mg/Si ceramic prepared at the optimizing sintering process exhibited more stable friction state and lower wear degree under non-lubricated conditions. The enhanced mechanical properties could be attributed to their structure densification, pore elimination and gain growth with the increase of sintering temperature.     

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al₂O₃ and Y₂O₃ powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al₂O₃–Y₂O₃ mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al₂O₃–Y₂O₃ mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.     

Densification behaviors and high-temperature characteristics of Si3N4 sintered bodies using Al2O3–Yb2O3 additives

The densification behaviors (include α–β transformation) and high-temperature characteristics (especially oxidation resistance and high-temperature strength properties) of Si₃N₄ sintered bodies using Al₂O₃–Yb₂O₃ based sintering additive are investigated.Densification and α–β transformation behaviors were investigated by varying the compositions of Al₂O₃–Yb₂O₃ additives. In terms of the influence of the Y₂O₃/Al₂O₃ ratio on densification behavior, a greater Yb₂O₃/Al₂O₃ ratio tends to inhibit densification. The α–β transformation tended to be delayed in sintered bodies with a small additive amount of 3.4 mass%. Compared with the transformation behaviors of the sintered bodies using Al₂O₃–Y₂O₃ additives, those using Al₂O₃–Yb₂O₃ additives exhibited a narrower temperature zone for α–β transformation, which attributed to the finer structure for the sintered body using Al₂O₃–Yb₂O₃ additives. This is affected by the difference in solubility of Si₃N₄ in the two kinds of glass phase.High room temperature strength of 900–1000 MPa was obtained for sintered bodies with a 10.0 mass% addition of additives, and this is considered to be due to the finer micro-structure. Precipitation of a Yb₄Si₂N₂O₇ phase at the grain boundary glass phase, as induced by crystallization processing, enables the improvement of 1300 °C strength to about 650–720 MPa. Crystallization processing resulted in a 30% reduction in the amount of weight change during oxidation (from 3.42 to 2.46 mg/cm²), demonstrating the effectiveness in improving oxidation resistance.     

Synthesis and characterization of Al2O3-TiC-ZrO2 ceramics with intragranular nanostructure by spark plasma sintering

In this paper, Al₂O₃-TiC ceramic composites with intragranular nano-ZrO₂ were prepared in vacuum by spark plasma sintering (SPS). The effect of ZrO₂ particles with different nano-sizes on the microstructure and mechanical properties of ceramics was studied. The results show that SPS can achieve relative densification of materials without generating new impurity phases. At the same time, the sintering densification temperature of ceramic materials can be reduced by adding ZrO₂ (20 nm) particles. Under the action of SPS strong electric field, the nano-ZrO₂ adsorbed on the surface of the matrix particles can enter the interior of matrix grains, and form intragranular nanostructures when the grain boundaries move and the particles merge. The microstructure and mechanical properties of ceramic materials can be improved through the intragranular structure formed by nanoparticles. The main reasons for the increased strength and toughness of ceramic materials are crack deflection, crack bridging and transgranular fracture.     

MgO−Al2O3−Cr2O3 refractories of increased corrosion resistance for nonferrous metallurgical furnaces

The paper sums up a study that had as its objective to develop MgO−Al₂O₃−Cr₂O₃ refractories of increased corrosion resistance for harsh service conditions. The refractories thus developed, designated PShKhM-1 and PShKhM-2, have better high-temperature strength and abrasion resistance than PKhS refractories manufactured commercially. The experimental refractories will enhance the durability of furnace and converter linings, extend their campaigns, step up their productivity, and reduce consumption of refractories and repair costs. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 1, pp. 33–37, January, 1998.     

Effects of fine grains and sintering additives on stereolithography additive manufactured Al2O3 ceramic

Al₂O₃ ceramics are fabricated by stereolithography based additive manufacturing in present reports. To improve the densification and performance of Al₂O₃ ceramic, the introduction of fine grains or sintering additives has been studied by traditional fabrication techniques. However, no research has focused on the effects of adding fine grains and sintering additives on the stereolithography additive manufactured Al₂O₃ ceramic. In this study, both fine grains and sintering additives were added firstly, and then the effects of fine grains and sintering additives on the relative density, microstructure, mechanical properties, and physical properties of the stereolithography additive manufactured Al₂O₃ ceramics were investigated. Finally, defect-free Al₂O₃ ceramic lattice structures with high precise and high compressive strength were manufactured.     

Effects of different particle size of spinel and Y2O3 additive of the periclase-spinel refractory on the sintering densification and corrosion resistance to copper smelting slag

In order to analyze the sintering densification and copper smelting slag corrosion resistance of periclase-spinel refractories, the periclase-spinel refractories were prepared with fused magnesia, magnesia-rich spinel, industrial alumina, and yttrium oxide as the main raw materials. The different particle sizes of spinel in material and with or without Y₂O₃ additive were studied. The study demonstrated that: (1) The different particle sizes of spinel in periclase-spinel refractories can result in different effects. Adding particle spinel to the refractory can improve the strength and corrosion resistance of the periclase-spinel refractories. The addition of spinel and magnesia powders in the matrix resulted in cracks due to the great difference of coefficient of thermal expansion between magnesia and spinel. The reduction in bulk density and strength of the material decreased slag penetration resistance because of its poor sintering properties. While adding the alumina in the matrix can further fill the crack and prevent slag penetration by the volume expansion of in-situ reaction to form spinel. (2) The periclase-spinel refractories can be reacted with Cu slag to form a Mg₂FeO₄ insulating layer as the iron ion becomes oxidized. Adding Y₂O₃ in periclase-spinel refractories can result in grain boundary phase reconstruction, which can promote sintering densification, improve the slag physical infiltration resistance, and improve the chemical corrosion resistance of materials.     

Flash sintering of 3YSZ/Al2O3-platelet composites

Preparation of 3YSZ/Al₂O₃-platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al₂O₃-platelet composites are prepared at low temperature of 492°C-645°C in 30 seconds by flash sintering under the electric field of 300-800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash-sintered 3YSZ/Al₂O₃-platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size.     

Microwave hybrid and conventional sintering of Al2O3 and Al2O3/ZrO2 multilayers fabricated by aqueous tape casting

In this study, microwave hybrid sintering and conventional sintering of Al₂O₃- and Al₂O₃/ZrO₂-laminated structures fabricated via aqueous tape casting were investigated. A combination of process temperature control rings and thermocouples was used to measure the sample surface temperatures more accurately. Microwave hybrid sintering caused higher densification and resulted in higher hardness in Al₂O₃ and Al₂O₃/ZrO₂ than in their conventionally sintered counterparts. The flexural strength of microwave-hybrid-sintered Al₂O₃/ZrO₂ was 70.9% higher than that of the conventionally sintered composite, despite a lower sintering temperature. The fracture toughness of the microwave-hybrid-sintered Al₂O₃ increased remarkably by 107.8% despite a decrease in the relative density when only 3 wt.% t-ZrO₂ was added. The fracture toughness of the microwave-hybrid-sintered Al₂O₃/ZrO₂ was significantly higher (247.7%) than that of the conventionally sintered composite. A higher particle coordination and voids elimination due to the tape casting and the lamination processes, the microwave effect, the stress-induced martensitic phase transformation, and the grain refinement phenomenon are regarded as the main reasons for the mentioned outcomes.     

B6O materials with Al2O3/Y2O3 additives densified by FAST/SPS and HIP

Fully densified B₆O materials with Al₂O₃/Y₂O₃ sintering additives amounts systematically varied between 0 and 15 vol.% and Al₂O₃/(Al₂O₃ + Y₂O₃) molar ratios of 0.05–1 were prepared by FAST/SPS and HIP at sintering temperatures between 1725 °C and 1900 °C. Their densification and microstructure were correlated with measured mechanical properties. The addition of low additive amounts in the range of 2–3 vol.% was found to increase the fracture toughness and strength from 2.0 MPa m^1/2 (SEVNB) and 420 MPa for pure B₆O to about 3.0 MPa m^1/2 and 540 MPa, but it had no effect on the hardness, which remained at a high level of 30–36 GPa (HV_0.4). Higher additive contents did not yield a further improvement in the toughness but resulted in a reduction in hardness and strength.