Synthesis and mechanical and thermal properties of (Cr,Mn)2AlC solid solutions

(Cr,Mn)₂AlC solid solutions have attracted much attention due to their magnetic property which will be enhanced with increasing Mn solubility. So far, only 3?at.-% Mn solubility in (Cr,Mn)₂AlC bulk materials has been reported. This work reports on the synthesis of (Cr,Mn)₂AlC solid solutions via a new reaction route with AlCr₂, C and Mn as starting materials. The Mn solubility increases with increasing the starting Mn content. The maximum solubility of Mn in (Cr,Mn)₂AlC was 8.3?at.-%, corresponding to a resultant solid solution (Cr_0.83Mn_0.17)₂AlC. However, increasing the Mn content in the starting mixture caused the formation of impurities in the sintered samples. A dense (Cr_0.95Mn_0.05)₂AlC ceramic has been achieved by hot-pressing the mixture of AlCr₂/C/0.1Mn. The mechanical properties and thermal expansion coefficient of (Cr_0.95Mn_0.05)₂AlC were measured.     

Influence of grain size on high temperature oxidation behavior of Cr2AlC ceramics

The influence of grain size on the oxidation behavior of Cr₂AlC at 1100 °C and 1200 °C for different times was investigated using fine grained (2 μm) and coarse grained (60 μm) samples. The two materials show a good oxidation resistance owing to the formation of a dense and continuous Al₂O₃ layer. The oxidation rate of the fine grained Cr₂AlC is relatively faster than that of the coarse grained Cr₂AlC. The microstructure and phase composition of scale was characterized. After oxidation at 1100 °C and 1200 °C for long times up to 100 h, only a dense and continuous α-Al₂O₃ oxide layer formed on both the fine grained and coarse grained Cr₂AlC. However, after oxidation at 1100 °C for a relatively short 2 h period, a Cr₇C₃ compound was detected beneath the α-Al₂O₃ oxide layer on the coarse grained Cr₂AlC, yet no Cr₇C₃ was found in the fine grained Cr₂AlC. The oxidation mechanism of the fine and the coarse grained Cr₂AlC was discussed.     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

Mechanical properties of low temperature synthesized dense and fine-grained Cr2AlC ceramics

Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr₂AlC ceramic at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3 h, and then subjected to hot pressing at 30 MPa and different temperatures for 1 h in Ar atmosphere. The results show that a dense Cr₂AlC ceramic with a grain size of about 2 μm can be synthesized at a relatively low temperature of 1100 °C. The synthesized fine-grained Cr₂AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr₂AlC (grain size of about 35 μm) as synthesized by hot pressing unmilled Cr, Al and C. The flexural strength, fracture toughness and Vickers hardness of the fine-grained Cr₂AlC were determined and compared with the values for the coarse-grained Cr₂AlC.     

Reactive infiltration processing (RIP) of ultra high temperature ceramics (UHTC) into porous C/C composite tubes

A novel reactive infiltration processing (RIP) technique was employed to infiltrate porous carbon fibre reinforced carbon (C/C) composite hollow tubes with ultra high temperature ceramic (UHTC) particles such as ZrB₂. The C/C composite tubes had initial porosity of ∼60% with a bimodal (10 μm and 100 μm) pore size distribution. A slurry with 40-50% ZrB₂ solid loading particles was used to infiltrate the C/C tubes. Our approach combines in situ ZrB₂ formation with coating of fine ZrB₂ particles on carbon fibre surfaces by a reactive processing method. A Zr and B containing diphasic gel was first prepared using inorganic-organic hybrid precursors of zirconium oxychloride (ZrOCl₂·8H₂O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. Then commercially available ZrB₂ powder was added to this diphasic gel and milled for 6 h. The resultant hybrid slurry was vacuum infiltrated into the porous hollow C/C tubes. The infiltrated tubes were dried and fired for 3 h at 1400 °C in flowing Ar atmosphere to form and coat ZrB₂ on the carbon fibres in situ by carbothermal reaction. Microstructural observation of infiltrated porous C/C composites revealed carbon fibres coating with fine nanosized (∼100 nm) ZrB₂ particles infiltrated to a depth exceeding 2 mm. Ultra high temperature ablation testing for 60 s at 2190 °C suggested formation of ZrO₂ around the inner bore of the downstream surface.     

Processing of Cr2Nb precursor through powder metallurgy route

Mechanical milling (MM) has been adopted to develop Cr₂Nb intermetallic precursor, required for the processing of Cu-Cr-Nb alloy. Process parameters have been optimized through particle characterisation of powders produced by different milling time and conditions. It has been observed that particle morphology and size change with milling time. A milling time of 15 h, followed by annealing at 1400 °C has been found to be optimum condition to result pure Cr₂Nb powders with desired properties.     

Novel Cr2AlC MAX-phase/SiC fiber composites: Synthesis,processing and tribological response

A new class of ceramic matrix composites based on Cr₂AlC MAX-phase containing 5 and 10 wt.% of SiC fibers was developed in this investigation. The Cr₂AlC/SiC composites were performed through two consecutives steps: i) synthesis of the pure Cr₂AlC phase from its elemental constituents by reactive method under argon atmosphere at 1400 °C and particle size refinement, and ii) processing of Cr₂AlC powder and SiC fibers followed by densification using the field assisted sintering technology/spark plasma sintering. Cr₂AlC/SiC composites presented high density (98.6%) with an excellent dispersion of the fibers within the matrix and a strong matrix/fiber interfase. Tribological behavior of the developed composites was studied under dry conditions to reveal the role played by the SiC fibers. Incorporation of the SiC fibers within the Cr₂AlC matrix reduced the friction coefficient up to 20% for low testing loads, while the wear resistance increased up to 70–80% independently of the applied load.     

Recession behavior of Lu2SiO5 under a high speed steam jet at high temperatures

Study of recession behavior of Lu₂SiO₅ bulk was performed in high speed steam jet with a velocity of ∼50 m/s temperature range between 1300 and 1500 °C for 100 h. X-ray results showed that no phase change was observed for all samples after steam exposure. Detailed scanning electron microscopy examinations showed some cracks formation was observed on the bulk surface for the samples of 1400 and 1500 °C. Also, porous structure was formed on the bulk surface for the samples of 1300 and 1400 °C. As for 1500 °C sample, the porous structure disappeared after exposure test. The high magnification images of 1300 °C sample showed the depletion of grain boundary glassy phase. However, for 1400 °C sample, boundary phase was formed again, and the grain growth can be identified for the sample of 1500 °C. The recession mechanism can be explained by a mass transfer of evaporated species from the bulk surface and the weight loss rate measured can be expressed by Arrhenius plot.     

Suspension stability and sintering influence on yttria-stabilized zirconia fabricated by colloidal processing

The stability of nano-zirconia 3YSZ powder in suspension was extensively studied by the colloidal method, and the optimum sintering temperature of the green sample fabricated through slip casting was determined. Zirconia suspensions with 10 vol% powder loading were prepared with distilled water, and HNO₃ was used to adjust the pH of the suspension to pH 1–6. All of the suspensions were subjected to sedimentation test, and the results showed that the suspensions adjusted to pH 2 had the lowest sediment volume. This finding indicates that a suspension with pH 2 produces higher packing density. Viscosity test was carried out for the suspensions added with dispersant ranging from 0.3 wt% to 0.7 wt% polyethyleneimine (PEI) with and without pH adjustment. The suspension containing 0.5 wt% PEI with pH 2 adjustment produced the lowest viscosity because of interparticle bond breakage in the aggregates, thus forming colloidally stable suspensions. The zirconia suspension containing 0.5 wt% PEI and whose pH was adjusted to pH 2 was chosen to be slip casted into cylindrical shape. Green samples were sintered at various sintering temperatures that ranged from 1100 °C to 1500 °C through a two-step sintering method. The sample sintered at 1500 °C was found to be porosite-free, and its highest relative density was 99.6% of the theoretical density. Morphological studies detected pores in the microstructure of the samples sintered at low sintering temperatures (1100 and 1200 °C). By contrast, the samples sintered at 1400 and 1500 °C were fully densified. However, the grain size of the sample sintered at 1500 °C was 230 nm, which indicated excessive grain growth. The Vickers hardness of the sample sintered at 1400 °C was found to be highest (12.9 GPa) and comparable to results found in the literature.     

Decomposition of MgSiN2 in nitrogen atmosphere

Magnesium silicon nitride MgSiN₂ was prepared by direct nitridation of Si/Mg₂Si/Mg/Si₃N₄ powder compact in a temperature range 1350-1420 °C. The thermal stability examination showed that MgSiN₂ is stable up to 1400 °C at 0.1 MPa N₂ pressure. The activation energy of decomposition of MgSiN₂ calculated from the temperature dependence of mass loss in the range of 1400-1650 °C is ΔH = 501 kJ mol⁻¹. The time dependence and nitrogen pressure dependence of MgSiN₂ decomposition was also investigated at constant temperature. MgSiN₂ is stable at 1560 °C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs energy of formation of MgSiN₂ was calculated in a temperature range 25-2200 °C.     

Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

The fabrication of composites formed by alumina grains (95 vol%) in the micrometer size range and aluminium titanate nanoparticles (5 vol%) by reaction sintering of alumina (Al₂O₃) and titania (TiO₂) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400 °C were established from dynamic sintering experiments. Specimens cooled at 5 °C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400 °C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5 °C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.     

Correlation between sintering conditions and water contact angles for Ti–O thick films screen printed on an alumina substrate

TiO₂, TiO_2−x and Ti₃O₅ thick-film structures on corundum Al₂O₃ substrates were prepared using screen-printing technology. The screen-printed deposits were sintered up to 1500 °C in oxidising and reducing atmospheres to vary the Ti⁴⁺/Ti³⁺ ratio and consequently water contact angle. The structure of the thick films was studied with an X-ray powder diffractometer (XRD). The microstructural characteristics and the chemical composition were checked with a scanning electron microscope, equipped with an energy-dispersive spectrometer (EDS). The Ti–O films, up to 55 μm thick, exhibited excellent adhesion to the substrate and had uniform grain- and pore-size distributions. Ti₃O₅ and Al₂O₃ were found to be compatible phases up to 1500 °C in a reducing atmosphere. However, rutile-type TiO₂ and Al₂O₃ are not compatible compounds at temperatures up to 1400 °C, in either oxidising or reducing atmospheres. TiO₂ and TiO_2−x form two types of reaction products with Al₂O₃. These reaction products were found to have various Ti/Al ratios.     

Preparation of SiC nanowires-filled cellular SiCO ceramics from polymeric precursor

SiC nanowires-filled cellular SiCO ceramics were prepared using polyurethane sponge as a porous template infiltrated with silicone resin by pyrolysis at 1400 °C under Ar atmosphere. The pyrolysis temperature was an important parameter affecting the formation of SiC nanowires. The as-prepared sample obtained at 1000 °C was composed of SiCO glasses and turbostratic carbon. The SiCO ceramic was further converted into SiO₂ crystals and amorphous carbon by pyrolysis at 1200 °C. With the increasing pyrolysis temperature, SiC nanocrystals embedded in the non-crystalline SiCO matrix were observed. Furthermore, the SiC nanowires were formed in the pores of the SiCO ceramic. The diameters of the SiC nanowires are in the range 80–150 nm and the lengths are up to several tens of micrometers. The growth mechanism of the nanowires was supported by the vapor-solid mechanism.     

Influence of nano-aluminum filler on the microstructure of SiOC ceramics

Ceramic SiC-mullite-Al₂O₃ based nanocomposites were successfully obtained at temperatures below 1500 °C after pyrolysis and annealing of green compacts prepared by cross-linking and shaping in a warm press step of commercial poly(methylsilsesquioxane) MK polymer mechanically mixed with aluminum filler having nanoparticle size. The heat treatment takes place under the exclusion of oxygen (inert argon atmosphere) and temperatures as low as 800 °C initiate the crystallization of a silicon carbide phase. The influence of the nano-aluminum filler and of the pyrolysis temperature on the crystallization behavior of the materials has been investigated. It was confirmed that an appropriate amount of nano-aluminum filler leads to a cristobalite free bulk SiOC ceramic. In consequence, the received ceramic samples have to be considered as a nano/micro-ceramic composite consisting of crystals of mullite (average dimension in the range of 200 nm), silicon carbide (20 nm) and α-alumina (50 nm).     

Surface modification of 3Y-TZP with cerium oxide

Surface modification with cerium oxide of tetragonal zirconia polycrystals stabilized with 3 mol.% yttria (3Y-TZP) was carried out in order to improve the resistance to low temperature degradation. Specimens were coated with pressed CeO₂ powder and then annealed at 1400 °C and 1500 °C for periods of up to 10 h. Similar treatments were performed in specimens coated with a sub-micron CeO₂ layer by means of magnetron sputtering. Cerium penetration in the surface modified specimens is about 10 μm into the bulk and the grain size increases in the surface layer affected by cerium diffusion. The indentation bulk fracture toughness and Vickers hardness are not affected by the surface modification treatments. Berkovich nanoindentation was performed to observe the contact hardness and elastic modulus at the surface, showing no significant difference after surface modification. Surface modification with ceria induces a large increase in the resistance to hydrothermal ageing without impairing mechanical properties.     

Investigation on the microstructure of self-reinforced Nd-α-sialon ceramics

α-Sialon ceramics doped with Nd₂O₃ were prepared by hot-pressing sintering at 1900 °C holding for 1 h with a heat preservation for 1 h at 1500 °C. Microscopic observations indicate that elongated α-sialon grains appear and are embedded in the fine equiaxed-grain matrix. A small amount of β-sialon phases and secondary crystallized phases M′ (Nd₂Si_3−xAl_xO_3+xN_4−x) also exist in the Nd-α-sialon ceramic. A core/shell structure can be found in the elongated α-sialon and β-sialon grains with a high aspect ratio, in which some misfit dislocations surround the core. The 21R AlN-polytype was observed by transmission electron microscopy (TEM), which could not be detected by X-ray diffraction (XRD) due to its trace amount. Different nucleation and growth modes of sialon grains were also discussed.     

High-temperature electrical properties of lanthanum beta-alumina solid electrolytes

Impedance spectroscopy measurements were carried out in the 10 Hz to 10 MHz frequency range from 500 to 1200 °C in LaAl₁₁O₁₈ pellets sintered at 1600 °C. The powders were obtained by the polymeric precursor technique. The sintered pellets were nearly single phase LaAl₁₁O₁₈. The bulk electrical resistivity was evaluated from the [−Z″(ω) × Z′(ω)] impedance diagrams. The value of the activation energy for the ionic conduction, 0.89 eV, was determined from the Arrhenius plot of the bulk conductivity. An yttria-stabilized zirconia (YSZ) oxygen pump and an YSZ oxygen sensor were used for providing 10–1500 ppm of partial pressure of oxygen (pO₂) at 1000 °C for determining the electromotive force (emf) in a Pt/LaAl₁₁O₁₈/Cr₂O₃ + Cr electrochemical cell. The results follow the Nernst law. The high signal-to-noise ratio of the emf at low pO₂ values shows that the LaAl₁₁O₁₈ specimens may be used in sensors for detection of oxygen at high temperatures.     

Dynamic corrosion of Al2O3-ZrO2-SiO2 and Cr2O3-containing refractories by molten frits. Part I: Macroscopic analysis

Manufacturing of enamels and frits has undergone dramatic changes since the 1980s. This has required significant efforts in research and development. Typical compositions of frits for ceramic tiles are silica-based with fluxing agents; some of the components are highly corrosive. Improvements in the production of frits would imply the selection of the most adequate refractories as a function of the chemical composition of the considered frit and the fabrication procedure.The refractories currently used in frit furnaces are Al₂O₃-ZrO₂-SiO₂ (AZS) fused cast materials and Cr₂O₃-based materials. In this work, results on dynamic corrosion studies of AZS and Cr₂O₃-based materials by two ZnO-containing frits are described. Experiments have been performed using the “Merry Go Round” test at ≅1500 °C. Macroscopic results are analysed in terms of the remaining volume after the tests, as usually done in the glass industry. The significance and limits of such an approach are discussed.     

Fabrication of polycrystalline (Y0.7Gd0.3)2O3:Eu ceramics: The influence of initial pressure and sintering temperature on its morphology and photoluminescence activity

Nanocrystalline (Y_0.7Gd_0.3)₂O₃ powder, synthetised via polymer complex solution method, was compacted into 25 pellets applying high pressures (173-867 MPa) for 30 s that were subsequently sintered at different temperatures (800-1400 °C) for 18 h. The morphology and optical characteristics of the starting powder and prepared ceramic samples were monitored and discussed in order to identify the changes induced with the variations of initial compacting pressure, which influence is often neglected, and with sintering temperature. The grain size tends to decrease significantly with increasing pressure, even when elevated temperatures are used for annealing, while low compacting pressure resulted in grain coarsening and, in some cases, even in anomalous morphology of ceramic samples. Luminescence emission in ceramic samples decays faster than in nanopowders, that is in complete agreement with the grain formation and gradual transformation to the bulk material. Judd-Ofelt intensity parameters and branching ratios were calculated taking into account the difference in effective refractive index for nanopowder and ceramic samples.