Effect of bimodal microstructure on high-temperature properties of nano-reinforced Al2O3–MgO–C refractories

The beneficial effects of adding nanostructured expandable graphite (EG) hybridized yttrium aluminium garnet (EG\YAG) powder as a composite reinforcement in improving the oxidation resistance, hot-strength, and microstructure development in Al₂O₃–MgO–C refractories were studied. The refractory components reinforced with EG\YAG exhibited more than 60% of oxidation resistance enhancement and as high as 200% increase in hot-strength performance over the standard refractories, formulated without EG\YAG. Correlating the damage parameter (D_E) calculations based on ultrasonic measurements with residual strength data (R_c, R_b) showed that there was a progressive increase in R_c and R_b values with consistent reduction in the oxidative damage of EG\YAG reinforced refractories. Analysis indicated that these beneficial features were majorly ascribed to the in-situ development of bimodal microstructure with EG\YAG sintered framework throughout the refractory interior in these new class of reinforced systems. Additionally, the mechanism of toughening and implications of these results to materials design are discussed.     

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.     

Effect of carbon content on the mechanical behavior of MgO–C refractories characterized by Hertzian indentation

The mechanical behavior of magnesia–carbon (MgO–C) refractory materials containing 4, 11, and 19 wt% carbon was investigated by Hertzian indentation tests. No microstructural changes were observed for the various carbon amounts. The slope during loading and the residual displacement after unloading were determined from indentation load–displacement curves. Indentation conditions were varied to study their influence on the mechanical behavior of the MgO–C. Repetitive contact fatigue tests were also conducted. The carbon content and indentation ball size ultimately affected the indentation mechanical behavior. The MgO–C containing 4 wt% carbon exhibited relatively hard and brittle behavior, while the MgO–C containing 19 wt% carbon exhibited a quasiplastic behavior, independent of ball radius indented on the MgO–C. Results indicated that carbon, in suitable amounts, provides the MgO–C with soft and ductile behavior in contact, wear, and impact environments.     

Analysis on microcrystalline graphite and properties of MgO–C refractories with microcrystalline graphite

The composition and microstructure of microcrystalline graphite were studied by X-ray diffraction, differential thermal analysis–thermogravimetry, SEM and energy dispersive analysis in this paper. The chemical composition of ash in microcrystalline graphite was also analysed in the study. The microcrystalline graphite was introduced in MgO–C refractories fabrication to investigate the influence of microcrystalline graphite on the main properties of MgO–C refractories. The oxidation resistance, thermal shock resistance, hot bending strength and expansion rate of MgO–C samples with microcrystalline graphite and flake graphite were investigated in this study. It is indicated that the proper addition amount of microcrystalline graphite in MgO–C refractories should be no more than 4?wt-%.     

Influence of ceramic phase content and its morphology on mechanical properties of MgO–C refractories under high temperature nitriding

To evaluate the influences of ceramic phase content and its morphology on the mechanical properties of MgO–C refractories, pre-prepared Si powder-phenolic resin loaded with Fe₂O₃ was introduced to prepare MgO–C refractories via catalytic nitridation. The effects of nitriding temperature and Fe₂O₃ content on the phase composition, microstructure evolution and properties of MgO–C refractories were studied and compared. The results show that the increase of the nitriding temperature was conducive to the in-situ formation of the ceramic phases, and a new phase of Mg₂SiO₄ was formed at temperatures ≥1450 °C. Both the increase in nitriding temperature and the addition of catalyst could inhibit the growth of α-Si₃N₄ to promote the formation of β-Si₃N₄ and MgSiN₂. In addition, the formation of excessive ceramic phases caused samples after nitriding to expand violently and form more porous, thereby reducing the physical properties of MgO–C refractories.     

Raw materials,microstructure, and properties of MgO–C refractories: Directions for refractory recipe development

A range of steel making vessels and continuous casting components use graphite containing MgO-C refractories that work from ambient to 1600 °C or higher. In the current study, a detailed review on the key importance behind the rightful selection of raw material quality in the development of MgO-C refractories with improved high-temperature microstructure stability is provided. Special cases of carbon\ceramic reinforcements (SiC, nanocarbon, EG, CNT’s, Zircon, Titania) are also included in this review study with the combination of regular raw materials used in refractory formulation such as magnesia, graphite, resin binder, antioxidant additives, and alloys thereof. Additionally, the material design concept based on strength factor (f_s) has been applied to implicate the raw material quality analysis in the development of carbon containing refractory recipe exhibiting satisfactory hot-strength performance with the recyclable MgO-C grog over the commercially available carbon\ceramic reinforcements is discussed.     

Effect of electromagnetic field on slag corrosion resistance of low carbon MgO–C refractories

Using MgO–C refractories containing 6% carbon and the slag with a basicity (CaO/SiO₂) of around 0.8, the melting slag resistance experiments of low carbon MgO–C refractories were carried out in induction furnace and resistance furnace, respectively. The microstructure of low carbon MgO–C refractories corroded by slag under the different conditions was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDAX). The results show that in induction furnace having electromagnetic field (EMF), there are MgFe₂O₄ spinel with a little of Mn ions generated in the interfacial layer. Part of the solid solution is monticellite [CaMgSiO₄] containing a little MnO and FeO. While under the condition of EMF free, there is not MgFe₂O₄ spinel in the interfacial layer and the solid solution is monticellite (CaMgSiO₄). At a high temperature, EMF increases the diffusion coefficient of Fe^2+/3+ ions, which displaces Mg²⁺ and forms MgFe₂O₄ with a little of Mn ions. There are MgAl₂O₄ spinel in the penetration layers under the conditions of both EMF and EMF free. EMF speeds up corrosion of low carbon MgO–C refractories.     

Effect of Mo addition mode on the microstructure and mechanical properties of TiC–high Mn steel cermets

In TiC- and Ti(C,N)-based cermets, the wettability of the ceramic phase with the metallic binder is commonly increased through supplementation with Mo in the form of pure Mo powder or Mo₂C. Herein, TiC–high Mn steel cermets were fabricated by conventional powder metallurgy techniques using Fe–Mo pre-alloyed powders as binders to guarantee uniform Mo distribution, and the cermet preparation process was optimized and investigated in detail. The microstructures of the thus obtained cermets were observed by scanning electron microscopy and compared to those of a Mo-free cermet and a cermet prepared using pure Mo powder. The grain size of Fe–Mo powder cermets exceeded that of the Mo-free cermet but was much smaller and more homogeneous than that of the Mo powder cermet. For Fe–Mo powder cermets, angular and tetragonal TiC particles were observed at Mo contents of <1.2 wt%, while round shapes became dominant at higher Mo contents. The hardness of Fe–Mo powder cermets increased with increasing Mo content, as did transverse rupture strength, which was maximal (2264 MPa) at a Mo content of 2.4 wt%, while impact toughness was maximal (11.2 J/cm²) at a Mo content of 1.2 wt%. The above values exceeded those reported for similar conventional cermets, and the use of Fe–Mo pre-alloyed powder as a metallic binder was therefore concluded to be an attractive strategy of increasing the strength and toughness of TiC–high Mn steel cermets.     

Effects of density and heat treatment of C/C preforms on microstructure and mechanical properties of C/C–SiC composites

Twill multidirectional carbon-fiber-reinforced carbon and silicon carbide composites (i.e., C/C–SiC) were prepared via chemical vapor infiltration combined with reactive melt infiltration process. The effect of heat treatment (HT) on the microstructure and mechanical properties of C/C–SiC composites obtained by C/C preforms with different densities was thoroughly investigated. The results show that as the bulk density of C/C preforms increases, the thickness of the pyrolytic carbon (PyC) layer increases and open pore size distribution narrows, making the bulk density and residual silicon content of C/C–SiC composites decrease. Moreover, the flexural strength and tensile strength of the C/C–SiC composites were improved, which can be attributed to the increased thickness of the PyC layer. The compressive strength reduces due to the decrease of the ceramic phase content. HT improves the graphitization degree of PyC, which reduces the silicon–carbon reaction rate and thereby the content of the SiC phase. HT induces microcracks and porosity but not obviously affects the mechanical properties of C/C–SiC composites. However, the negative impact of HT can be compensated by the increased density of the C/C preforms.     

Atmosphere and carbon effects on microstructure and phase analysis of in situ spinel formation in MgO–C refractories matrix

The effects of carbon, air and reducing atmospheres on microstructure and phase evolution of in situ MgAl₂O₄ spinel (S) formation in the matrix of MgO–C refractories were investigated by X-ray diffraction powder analysis (XRD) and scanning electron microscopy (SEM)/energy-dispersive spectroscopy (EDS) techniques. The formation of spinel started under 1000 °C in both air and reducing atmospheres. The morphology of in situ spinel and its formation mechanism were however different and dependent upon the atmosphere. The solid-state reaction was clarified to be the dominant mechanism of spinel formation in oxide atmosphere, while the gas–solid reaction was found to play a vital role in reducing atmosphere. Reaction of MgO and C in reducing atmosphere led to the formation of Mg_(g) which was found to be partially controlling the in situ spinel formation in the carbon containing samples fired in reducing environment. The results which were necessary are explained with emphasis on MgO–C refractories applications.     

Effect of ZrB2 on the microstructure,mechanical and tribological properties of Mo–Si–B matrix composites

Composites with good mechanical and tribological properties are in high demand for engineering applications. Toward this aim, the Mo–12Si–8.5B alloy with 2.5–10 wt% ZrB₂ ceramic was prepared. The effects of the ZrB₂ content on the microstructure, mechanical properties, and tribological behavior were thoroughly investigated. The composites exhibited reduced density and enhanced hardness and strength owing to the dispersion strengthening of ZrB₂ particles, thus resulting in improved wear resistance. The frictional properties are highly dependent on the ZrB₂ content and counterpart materials. When coupled with GCr15 steel, it shows much slighter abrasive and adhesive wear; therefore, it presents a more preferable anti-wear performance. The wear rate of the composite with 7.5 wt% ZrB₂ showed a minimum value of 2.71 × 10⁻⁷ mm³N⁻¹m⁻¹.     

Effect of Mo and W interlayers on microstructure and mechanical properties of Si3N4–nickel-base superalloy joints

Si₃N₄/nickel-base superalloy (Inconel-625) and Si₃N₄/Si₃N₄ joints with refractory metal (W and Mo) interlayers were vacuum brazed using a Ti-active braze Cu-ABA (92.75Cu–3Si–2Al–2.25Ti) at 1317 K for 30 min with the following interlayered arrangements: Si₃N₄/Mo/W/Inconel and Si₃N₄/Mo/W/Si₃N₄. The joints exhibited Ti segregation at the Si₃N₄/Cu-ABA interface, elemental interdiffusion across the joint interfaces, and sound metallurgical bonding. Knoop microhardness profiles revealed hardness gradients across the joints that mimicked the interlayered arrangement. The compressive shear strength of Si₃N₄/Si₃N₄ joints both with and without W and Mo layers was ∼142 MPa but the strength of Si₃N₄/Inconel joints increased from ∼9 MPa for directly bonded joints without interlayers to 53.5 MPa for joints with Mo and W interlayers.     

Effect of TiB2 particles on microstructure and mechanical properties of B4C–TiB2 ceramics prepared by hot pressing

B₄C-20 wt% TiB₂ ceramics were fabricated by hot pressing B₄C and ball-milled TiB₂ powder mixtures. The effects of the TiB₂ particle size on the microstructure and mechanical properties were investigated. The results showed that the TiB₂ particle size played an important role in the mechanical properties of the B₄C–TiB₂ ceramics. In addition, SiO₂ introduced by ball milling was beneficial for densification but detrimental to the mechanical properties of the B₄C–TiB₂ ceramics. The typical values of relative density, hardness, flexural strength, and fracture toughness of the ceramics were 99.20%, 35.22 GPa, 765 MPa, and 7.69 MPa m^1/2, respectively. The toughening mechanisms of the B₄C–TiB₂ ceramics were explained by crack deflection and crack branching. In this study, the effects of high pressure and temperature caused liquefying SiO₂ to migrate to the surface of B₄C–TiB₂ and react with diffused carbon source in the graphite foil to form a 30 μm thick SiC layered structure, which improved the high-temperature oxidation resistance of the material. The unique SiC layered structure overcame the insufficient oxidation resistance of B₄C and TiB₂, thereby improving the oxidation resistance of the ceramics under high-temperature service conditions.     

Effect of diamond volume fraction on the microstructure and mechanical properties of SPS WC–Co/diamond composites

In this research, the effect of the volume percentage of diamond additive on the sintering behavior, microstructure, and mechanical properties of WC–Co was investigated. WC–Co/diamond composites with different percentages of 0%, 2.5%, 5%, 7.5%, and 10% by volume of diamond were made by spark plasma sintering at 1300°C and 40 MPa for 5 min. A small amount of phase transformation from the diamond phase to the graphite phase was observed. The amount of graphitization was low due to low temperature and short sintering time. The addition of diamond leads to a significant enhancement in both the hardness and fracture toughness of the composites, overcoming the trade-off between hardness and toughness typically observed in WC-based materials. The sample reinforced with 5% by volume of diamond showed simultaneously the highest hardness (22.9 GPa), the highest fracture toughness (22.7 MPa m^1/2), and the highest flexural strength (1896 MPa). The uniform dispersion, good bonding of the superhard diamond phase with the matrix, and the fine microstructure caused the high hardness and toughness of composite. The main effective mechanisms in increasing the fracture toughness of the composite were crack deflection, bridging, and blocking of crack propagation by diamond particles.     

Effect of Co addition on microstructural and mechanical properties of WC–B4C–SiC composites

In this study, the effect of Co addition on microstructural and mechanical properties of WC-B₄C–SiC composites sintered by spark plasma sintering (SPS) method was investigated. For this purpose, three batches of WC-B₄C–SiC with different contents of Co (10 vol%, 15 vol%, and 20 Vol %) were sintered at 1400 °C. The results of X-ray diffraction (XRD) analysis of the samples indicated the formation of W₂B₅, W₃CoB₃ as well as the remained C phases and unreacted SiC phase. It was observed that by increasing the Co content, the amount of W₂B₅ phase reduces and W₃CoB₃ and C contents increase. Therefore, W₂B₅ peaks were not detected in the sample containing 20vol% Co. Relative density values above 97% were obtained for all the composites. However, a decrease was observed in relative density by increasing the Co content in the composites. The highest flexural strength (510 ± 42 MPa), fracture toughness (10.34 ± 0.82 MPa m^1/2), and hardness (20.63 ± 0.75 GPa) were also obtained for the sample containing 10vol% Co compared to the other samples. In addition, Transgranular fracture of SiC as well as pulling out of W₃CoB₃ and W₂B₅ particles were observed in the fracture surface micrographs of the samples. The presence of micro-cracks in the SiC grains, fracture of W₃CoB₃ grains, and crack deflection was reported as dominant toughening mechanisms.     

Effect of NiO additive on microstructure,mechanical behavior and electrical properties of 0.2PZN–0.8PZT ceramics

A NiO-added Pb((Zn_1/3Nb_2/3)_0.20(Zr_0.50Ti_0.50)_0.80)O₃ system is prepared and investigated. The results reveal that Ni doping induces a phase transformation from the morphotropic phase boundary to the tetragonal phase side. Above the solubility limit of 0.3 wt% in NiO form, excess Ni ions segregate at the grain boundaries and triple junctions, which facilitate the formation of a liquid phase with excess PbO and lead to remarkable grain growth. The mechanical behavior (Vickers hardness (H_v) and fracture toughness (K_IC)) can be tailored by controlling the content of additive; this is accompanied by a transition in the fracture mode changed from transgranular without NiO additive to intergranular with 1.0 wt% NiO additive. Moreover, the NiO addition weakens the dielectric relaxor behavior and improves the piezoelectric properties simultaneously. The 0.2PZN–0.8PZT with 0.5 wt% NiO addition shows good transduction coefficient (d₃₃·g₃₃ = 10,050 × 10^?15 m²/N) and large fracture toughness (K_IC = 1.35 MPa m^1/2).     

Effect of WC particle size on the microstructure and mechanical properties of Ni–Co coarse grain cemented carbide

The effect of different WC grain size additions on the microstructure and grain distribution of Ni–Co coarse crystalline cemented carbide was studied. And then the effect of grain distribution on the mechanical properties of cemented carbide was discussed. The effect of WC grain size on the grain size and coherency of cemented carbide was analyzed by microstructure. And the distribution of grains in the microstructure was investigated by the truncation method. The addition of fine (1.1–1.4 μm), medium (2.3–2.7 μm), and coarse WC (5.6–6.0 μm) particles can increase the nucleation rate of WC grains in the bonded phase. And the higher grain growth driving force can produce the theoretical limitation of nucleation and inhibit the coarsening of WC grains to a certain extent. The WC grain size has an insignificant effect on the frequency of the occurrence of super-coarse grains in coarse crystalline cemented carbide. The average grain size and super coarse grains in microstructure gradually decrease, which promotes the improvement of transverse rupture strength. The increase of the adjacent degree and the decrease of the mean free path reduce which is beneficial to the improvement of the corrosion resistance of the alloy. The best overall performance of the alloy is achieved when fine-grained WC is added.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.     

Influence of nano–micrometre powder blends on microstructure and mechanical properties of silicon nitride

Abstract

A well known route to making tough silicon nitride compositions is to control the grain size and aspect ratio distributions. This is usually done by choosing the appropriate powder characteristics, sintering conditions, as well as sintering additives. The effect of hot pressing a blend of nano and micrometre scale silicon nitride powder is explored here. Microstructures and mechanical properties are determined for these hot pressed ceramics and are compared with a reference silicon nitride. Hardness and fracture toughness are determined at room temperature using hardness indents produced by a macro Vickers hardness indenter. Grain size and aspect ratio distributions and their impact on mechanical properties are presented. Blending of nano and micrometre scale powder is shown to result in a refined microstructure with an increase in the area/volume fraction of finer grains. Rising R curves are established for these ceramics demonstrating toughening behaviour. Crack bridging and crack path deviation are identified as possible toughening mechanisms.     

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.