Influence of Chemical Reactions in Magnesia-Graphite Refractories: I, Effects on Texture and High-Temperature Mechanical Properties

Four formulations of magnesia-graphite-aluminum metal (antioxidant) bricks were prepared from the same raw materials, using the standard commercial practices. Chemical analysis and determination of room-temperature modulus of rupture and Young's modulus, as well as a complete microstructural characterization of the as-received materials, were performed. For high-temperature modulus-of-rupture and Young's modulus data, test samples of the four brick compositions were heated to 1000°, 1200°, and 1450°C in flowing argon (<1000 ppm oxygen at 1000°C) and then loaded mechanically in flexure. Modulus-of-elasticity values ranged from 3.7 to 16.2 GPa and reflected strong effects of aluminum-metal concentration and treatment temperature. Young's modulus evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens. Modulus-of-rupture values ranged from 6 to 21 MPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens at the lower testing temperatures ( T lessthan equal to 1200°C) and by phase assemblages in the surface regions of the specimens-essentially by the presence of the dense MgO zone-at 1450°C.     

Young's Modulus of Elasticity of Carbon‐Bonded Alumina Materials up to 1450°C

Investigating Young's modulus at elevated temperatures supports the understanding of microstructural changes as a function of application temperature. A sintered alumina and three carbon‐bonded alumina materials with carbon contents of 20 and 30 wt% and alumina grain size of 0.6–3 mm were investigated. Young's modulus was measured in a temperature range from 25°C to 1450°C by the impulse excitation technique. The Young's modulus of carbon‐bonded materials increases up to 140% at 1450°C. After one cycle, a decrease of the Young's modulus up to 50% is registered at room temperature. There is a strong hysteresis behavior during one cycle. Thermal expansion measurements show highest expansion for the highest graphite content material. The expansion of alumina grains and graphite flakes, resulting in microcrack generation during cooling and microcrack healing during heating, is reflected in the registered values of the Young's modulus as a function of the temperature. It is assumed, that higher graphite amounts as well as coarse grains lead to lower sintering effects of the microstructure at elevated temperatures and as a result lower values of the Young's modulus have been registered.     

Phase Evolution in Silicon Carbide–Whisker-Reinforced Mullite/Zirconia Composite during Long-Term Oxidation at 1000° to 1350°C

A composite consisting of 30 wt% SiC whiskers and a mullite-based matrix (mullite–32.4 wt% ZrO₂–2.2 wt% MgO) was isothermally exposed in air at 1000°–1350°C, for up to 1000 h. Microstructural evolution in the oxidized samples was investigated using X-ray diffractometry and analytical transmission electron microscopy. Amorphous SiO₂, formed through the oxidation of SiC whiskers, was devitrified into cristobalite at T ≥ 1200°C and into quartz at 1000°C. At T ≥ 1200°C, the reaction between ZrO₂ and SiO₂ resulted in zircon, and prismatic secondary mullite grains were formed via a solution–reprecipitation mechanism in severely oxidized regions. Ternary compounds, such as sapphirine and cordierite, also were found after long-term exposure at T ≥ 1200°C.     

High-Temperature Young's Modulus of Alumina During Sintering

High-temperature Young's modulus of a partially sintered alumina ceramic has been studied dynamically during the sintering process. Comparative, room-temperature Young's modulus data were obtained for a suite of partially sintered alumina compacts with different porosities. The dynamic Young's modulus of a 1200°C partially sintered material was observed to decrease linearly with temperature, but then above 1200°C it increased sharply as sintering and densification of the alumina became dominant. The evolution of the Young's modulus due purely to sintering exhibited an exponential relationship with porosity in excellent agreement with room-temperature measurements of equivalent porous alumina ceramics.     

Crack Healing and Bending Strength of Aluminum Titanate Ceramics at High Temperature

Bending strength and Young's modulus of aluminum titanate ceramics at room temperature to 1300°C were examined. Bending strength increased from 62 MPa at room temperature to 280 MPa at 1100°C. Young's modulus also increased, to 99 GPa at 1100°C. These increments were caused by crack healing. In particular, crack cylinderization occurring at 1000° to 1100°C markedly increased the mechanical strength. The thermal-hysteresis curves also showed healing of grain-boundary cracks.     

Processing and Mechanical Properties of Ti3SiC2: I, Reaction Path and Microstructure Evolution

In this article, the first part of a two-part study, we report the reaction path and microstructure evolution during the reactive hot isostatic pressing of Ti₃SiC₂, starting with titanium, SiC, and graphite powders. A series of interrupted hot isostatic press runs have been conducted as a function of temperature (1200°–1600°C) and time (0–24 h). Based on X-ray diffractometry and scanning electron microscopy, at 1200°C, the intermediate phases are TiC _x and Ti₅Si₃C _x . Fully dense, essentially single-phase samples are fabricated in the 1450°–1700°C temperature range. The time-temperature processing envelope for fabricating microstructures with small (3–5 μm), large (∼200 μm), and duplex grains, in which large (100–200 μm) Ti₃SiC₂ grains are embedded in a much finer matrix, is delineated. The microstructure evolution is, to a large extent, determined by (i) the presence of unreacted phases, mainly TiC _x , which inhibits grain growth; (ii) a large anisotropy in growth rates along the c and a directions (at 1450°C, growth normal to the basal planes is about an order of magnitude smaller than that parallel to these planes; at 1600°C, the ratio is 4); and (iii) the impingement of grains. Ti₃SiC₂ is thermally stable under vacuum and argon atmosphere at temperatures as high as 1600°C for as long as 24 h. The influence of grain size on the mechanical properties is discussed in the second part of this study.     

Passive and Active Oxidation of Hot–Pressed Silicon Nitride Materials with Two Magnesia Contents

Hot-pressed Si₃N₄ materials containing 1 and 5 wt% MgO were oxidized for 1000 h at 1000°, 1100°, and 1200°C in helium at 0.4 to 0.8 Pa total oxidants. Transition from passive to active oxidation occurred between 1000° and 1100° C, in agreement with published theoretical calculations for pure Si₃N₄. The amounts of both passive and active oxidation were greater for the material containing 5 wt% MgO. Specimen surfaces were porous and oxide–free under active oxidation conditions but contained porous oxide at transition.     

Elasticity and Internal Friction of Polycrystalline Yttrium Oxide

Young's modulus and internal friction of polycrystalline yttrium oxide were determined from room temperature to 1658°C by sonic techniques. A linear relation was found between volume fraction porosity and Young's modulus at room temperature. The effect of porosity on room temperature internal friction is also discussed. Young's modulus decreased linearly with increasing temperature to 1000° to 1100°C, where a slight anomaly was observed accompanied by an internal friction peak. The modulus was again linear from 100° to 13500°C. Above this temperature a rapid decrease in elastic modulus occurred with a rapid increase in internal friction.     

Mechanical and Thermal Properties of Antiperovskite Ti3AlC Prepared by an In Situ Reaction/Hot-Pressing Route

Bulk Ti₃AlC ceramic containing 2.68 wt% TiC was prepared by an in situ reaction/hot-pressing route. The reaction path, microstructure, mechanical and thermal properties were systematically investigated. At room temperature Vickers hardness of Ti₃AlC ceramic is 7.8 GPa. The flexural strength, compressive strength, and fracture toughness are 182, 708 MPa, and 2.6 MPa·m^1/2, respectively. Its apparent Young's modulus, shear modulus, bulk modulus and Possion's ratio are 208.9, 83.4, 140.4 GPa, and 0.25 at room temperature. Apparent Young's modulus decreases slowly with the increasing temperature, and at 1210°C the modulus is 170 GPa. The average coefficient of thermal expansion of Ti₃AlC ceramic is about 10.1 × 10⁻⁶ K⁻¹ in the temperature range of 150°–1200°C. Both the molar heat capacity and thermal conductivity increase with an increase in the temperature. At 300 and 1373 K, the molar heat capacities are 87 and 143·J·(mol·K)⁻¹, while the thermal conductivities are 8.19 and 15.6 W·(m·K)⁻¹, respectively.     

Creep Behavior of a Model Refractory System MgO-CaMgSiO4

The effects of the presence of a silicate boundary phase on the high-temperature creep behavior of a model refractory system MgO-CaMgSiO₄ (monticellite, CMS) were studied at 1200° to 1450°C. A change in the dominant mechanism of deformation was determined with increasing temperature and decreasing applied stress. It was concluded that, at 1200°C, deformation is controlled by a dislocation mechanism in the MgO framework, whereas at higher temperatures creep is the result of simultaneous mechanisms but dominated by viscous deformation of the silicate boundary region.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Young's Modulus of Various Refractory Materials as a Function of Temperature

Young's modulus as a function of temperature was determined by a dynamic method for single-crystal sapphire and ruby and for polycrystalline aluminum oxide, magnesium oxide, thorium oxide, mullite, spinel, stabilized zirconium oxide, silicon carbide, and nickel-bonded titanium carbide. For the single crystals, Young's modulus was found to decrease linearly with increasing temperature from 100°C. to the highest temperature of measurement. For all the polycrystalline materials, except silicon carbide, stabilized zirconium oxide, and spinel, Young's modulus was found to decrease approximately linearly with increasing temperature until some temperature range characteristic of the material was reached in which Young's modulus decreased very rapidly and in a nonlinear manner with increasing temperature. This rapid decrease at high temperature is attributed to grain-boundary slip. Stabilized zirconium oxide and spinel were found to have the same rapid decrease in Young's modulus at high temperature, but they also had a decidedly nonlinear temperature dependence at low temperature.     

Phase Equilibria in the System MgO—MgCr2O4

The liquidus, solidus, and subsolidus in the system MgO-MgCrz04 were redetermined. Petro-graphic studies indicated that 47 wt% Cr203 could enter into the periclase lattice at about 235OO C. X-ray and optical data showed that the solid solution decreased to 32 wt% Cr202 at 2165OC, 11 wt% at 1600°C, 5 wt% at 1400°C, 2.5 wt% at 1200°C, and 0 wt% at 1000°C. Petro-graphic studies also suggested that picrochromite could contain about 5 wt% MgO in solid solution. The liquidus was much higher than has been indicated on previously published phase equilibrium diagrams.     

Si diffusion induced adhesion and corrosion resistance in annealed RF sputtered SiC films on graphite substrate

Silicon carbide (SiC) is one of the promising candidates for graphite protection in different applications involving high temperatures and a highly corrosive environment. An ideal Silicon carbide coating should withstand a corrosive environment without compromising its adhesion. Herein, RF magnetron sputtered silicon-rich SiC thin films were deposited on a graphite substrate followed by annealing at 1000 °C, 1200 °C, and 1400 °C in an inert atmosphere. The impact of annealing temperature on microstructure, adhesion and chemical stability of SiC thin films was demonstrated. Different analytical techniques like Scanning electron microscopy (SEM), X-Ray Diffraction (XRD), Fourier's Transform Infrared (FTIR) spectroscopy and nano-indentation were used to study microstructural evaluation and mechanical characteristics. Moreover, the electrochemical analysis (Tafel and Electrochemical impedance spectroscopy) was performed in 3.5% NaCl solution. The microstructural analysis revealed that the amorphous SiC thin film turned into a crystalline and dense film upon annealing. Scanning electron micrographs showed that silicon-rich regions at SiC film surface started to disappear as Si diffuses into graphite matrix at elevated temperatures. Both these factors contributed to improvement in the adhesion of SiC coating with graphite substrate as annealing temperature increased. In addition, the nano-indentation hardness of 5.2 GPa was obtained for as grown sample, which decreased at 1000 °C, and then increased at 1200 °C and 1400 °C. Furthermore, the electrochemical analysis confirmed the enhancement in corrosion resistance upon annealing at a temperature of 1200 °C and 1400 °C due to Si diffusion and film compactness in these samples.     

Thermal Degradation of Fiber Coatings in Mullite-Fiber-Reinforced Mullite Composites

The thermal degradation behavior of single-layer BN and of double-layer BN/SiC chemically vapor-deposited fiber coatings in mullite-fiber-reinforced mullite composites was investigated by means of transmission electron microscopy after processing and heat treatment of the composites at 1000°, 1200°, and 1300°C for 6 h in air. The single-layer BN coatings were ˜0.7 mu m thick and consisted of turbostratic BN with (0001) basal planes lying parallel to the surfaces of the fibers plus nanosized areas that had no preferential orientation. This microstructure remained unchanged up to 1000°C; however, distinct coarsening of the randomly oriented BN crystallites occurred in the temperature range of 1000°-1200°C. The single-layer BN coatings were stable against oxidation, up to 1200°C. At higher temperatures, degradation of the coatings via oxidation occurred. Double-layer BN/SiC coating systems consisted of BN that was 0.08 mu m thick and SiC layers that were 0.16 mu m thick and deposited onto the mullite fibers. The turbostratic BN was highly anisotropic and did not undergo any microstructural change, up to 1300°C. The outer SiC layer of the double-layer coating system improved the oxidation resistance of BN in the 1200°-1300°C temperature range, despite a partial oxidation of SiC to SiO₂.     

Grain Growth in Microwave-Annealed Alumina

Normal grain growth in dense, fine-grained, aluminum oxide-0.1 wt% MgO was studied under both conventional furnace and 28-GHz microwave furnace annealing conditions. The microstructural changes that occurred were the same for both sets of samples; soap bubble microstructures were observed and the aspect ratios and shape factors did not change during the anneals. The kinetics of grain growth were greatly increased by the 28-GHz microwave anneals; e.g., the grain growth rate at 1500°C in the microwave furnace was the same as the rate at 1700°C in the conventional furnace. Also, the activation energy for grain growth was reduced by the microwave anneal from 590 kJ/mol (conventional) to 480 kJ/mol (microwave). Finally, these results demonstrate that a "microwave effect" can exist in a dense ceramic body and that no free pore-solid interfaces are necessary.     

Low-Temperature Fine-Grained Alumina–Aluminum Titanate Composites Produced from a Titania-Coated Alumina Precursor

A method for the preparation of alumina–aluminum titanate (AT) composites, which can be sintered to high density with a fine-grained microstructure at <1450°C, is reported. The composite precursor is alumina particles coated by sol–gel-derived titania, which reacts during sintering to form AT in situ at temperature as low as 1300°C. The composite can be sintered at 1350°C to 98% density with 1.5–2.0 μm grain size. Other composites containing 5–50 wt% AT are also investigated.     

High-Temperature Strength and Fracture Toughness of Y2O3-Partially-Stabilized ZrO2/Al2O3 Composites

The temperature dependence of bending strength, fracture toughness, and Young's modulus of composite materials fabricated in the ZrO₂ (Y₂O₃)-Al₂O₃ system were examined. The addition of A1203 enhanced the high-temperature strength. Isostatically hot-pressed, 60 wt% ZrO₂ (2 mol% Y₂O₃)/40 wt% Al₂O₃ exhibited an extremely high strength, 1000 MPa, at 1000°C.     

Hot Isostatic Press Sintering and Properties of Silicon Nitride without Additives

Fully densified silicon nitride without additives was fabricated by means of hot isostatic pressing. The sintering process of highly pure powder was investigated with special interest in the evolution of α–β phase transformation, densification, and microstructure development. It was observed that the transformation occurred without a liquid phase below 1730°C, which corresponds to the melting point of SiO₂. Above 1730°C, the densification and β-grain elongation accelerated concurrently because of the appearance of liquid SiO₂. However, full densification was attained at 1950°C together with marked grain growth. Flexural strength, microhardness, fracture toughness, and Young's modulus of sintered bodies were measured as a function of temperature. In the sintered body started from highly pure powder, excellent MOR behavior was found up to 1400°C. Impurity content of a few hundred ppm was found to be sufficient to make densification easy and to degrade high-temperature strength.