Rachman Chaim

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Abstract Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth.

Transparent nanocrystalline MgO by rapid and low-temperature spark plasma sintering

We investigated superfast densification of nanocrystalline MgO powders by spark plasma sintering (SPS) between 700 °C and 825 °C under applied pressures of 100and 150 MPa. Fully-dense transparent nanocrystalline MgO with a 52-nm average grain size was fabricated at 800 °C and 150 MPa for 5 min. In-line transmissionsof 40% and 60% were measured compared to MgO single crystal, for the yellowand red wavelengths, respectively. Densification occurs by particles sliding over each other; the nanometric grain size and pores lead to the optical transparency. The light brownish color of the nanocrystalline MgO is due to the oxygen vacancy color centers, originating from the reducing atmosphere of the SPS process.

Electrochemical ZrO2 and Al2O3 coatings on SiC substrates

SiC was electrochemically coated with ZrO2 and with Al2O3 from 0.1 m aqueous solutions of metal-nitrate-hydrates with ethanol added. Amorphous zirconia and alumina coatings were formed with current densities from 10 to 70 mA cm−2, and deposition durations of 1–60 min. The as-deposited coatings contained microcracks caused by drying shrinkage. Sintering of zirconia at 900 °C for 1 h and of alumina at 1200 °C for 2 h in air was accompanied by crystallization to a mixture of tetragonal and monoclinic phases in the former and to α-alumina in the latter. The absence of intermediate phases between the coatings and the substrates and the good adherence of the sintered coatings indicate the high-temperature stability of these coatings.

Elastic moduli of grain boundaries in nanocrystalline MgO ceramics

Dense MgO ceramics with nanometer to submicrometer grain size were fabricated byhigh-temperature hot-isostatic pressing, low-temperature hot-pressing, and spark plasmasintering. The elastic properties were determined by sound wave velocitymeasurements. Young’s and shear moduli of nanocrystalline MgO were lower by 13%than those with submicrometer grain size. Softening of the elastic properties wasanalyzed and related to the lower density and lower elastic moduli of the grainboundaries compared to the crystal interior. Young’s and shear moduli of the grainboundaries were evaluated as 90 and 34 GPa, respectively. This leads to a more than3-fold decrease in the effective elastic moduli with the decrease of grain size into thenanometer range.

Joining of yttria-tetragonal stabilized zirconia polycrystals using nanocrystals

Superplastic flow has been successfully used to join fully dense 3 mol% Y{sub 2}O{sub 3}-tetragonal ZrO{sub 2} polycrystals (Y-TZP) at temperatures as low as 1,350 C, a temperature at which direct diffusional bonding would be unlikely to produce a strong, pore-free joint. The objective of the present work was to determine whether bonding temperatures could be further reduced. To achieve lower bonding temperatures, the authors have investigated the bonding of conventional Y-TZP in which a nanocrystalline Y-TZP with a 20-nm grain size is used as the interlayer between two pieces of 0.3 {micro}m grain sizes Y-TZP. Little is known about the deformation of fully dense nanocrystalline Y-TZP, but recent work indicates that above a threshold stress, the principal deformation mechanism would be grain boundary sliding that results in superplastic flow. Questions on the deformation mechanism in the nanocrystalline Y-TZP are being addressed as part of a larger investigation of high-temperature compressive creep behavior, currently in progress.

Mechanical alloying of the immiscible system W-Cu

Abstract The formation of nanocrystalline and amorphous phases in the immiscible binary system W-Cu by mechanical alloying was studied. By HRTEM it was found that the amorphous matrix contains distorted tungsten crystallites with 1 to 1.5 nm size. In addition, the amorphous phase was found at the surface of the original particles. By X-ray analysis, the lattice parameter of the BCC-W, the relative amount of the amorphous phase and its atomic environment at different mechanical alloying times are studied. Beyond a certain time, MA is found to cause amorphization rather than increasing the solid solubility.

Creep of nanocrystalline Y-SZP ceramics

Abstract Fully dense nanocrystalline Yttria-Partially Stabilized Zirconia with a grain size of 40 nm has been crept in compression at temperatures ranged between 1100°C and 1200°C and strain rates between 5 × 10 −7 s −1 and 10 −4 s −1 . The creep parameters n = 1.4 and Q = 660 kJ/mol were obtained and the microstructure was observed before and after deformation. The microscopic features and macroscopic parameters showed that these nanocrystalline ceramics probably cannot be deformed superplastically.

Electrochemical Al2O3–ZrO2 composite coatings on non-oxide ceramic substrates

Aqueous solutions of xAl(NO3)3+(1−x)ZrO(NO3)2 were used for electrodeposition of ceramic Al2O3–ZrO2 composite on TiC, TiB2 and SiC sunstrates. The weight of the deposit was studied versus the duration of deposition, the current density and the temperature of the bath for Al-rich (x=0.9), Zr-rich (x=0.4) and eutectic (x=0.75) electrolyte compositions. Optimal current densities and durations of deposition were determined to obtain maxima weights of deposits. Amorphous deposits with thicknesses up to 10 μm were formed. The microstructure and microchemical composition of the as-deposited and sintered deposits were characterized. Increase in the temperature of the bath inhibited microcracking due to shrinkage during drying. Coated TiC substrates exhibited enhanced oxidation resistance in air at 1100°C.

Densification and preservation of ceramic nanocrystalline character by spark plasma sintering

Abstract Spark plasma sintering is a hot pressing technique where rapid heating by dc electric pulses is used simultaneously with applied pressure. Thus, spark plasma sintering is highly suitable for rapid densification of ceramic nanoparticles and preservation of the final nanostructure. A considerable portion of the shrinkage during densification of the green compact of nanoparticles in the first and intermediate stages of sintering occurs during heating by particle rearrangement by sliding and rotation. Further densification to the final stage of sintering takes place by either plastic yield or diffusional processes. Full densification in the final stage of sintering is associated with diffusional processes only. Nanoparticle sliding and rotation during heating may also lead to grain coalescence, with much faster kinetics than normal grain growth at higher temperatures. Based on existing models for particle rearrangement and sliding, the contributions of these processes in conjunction with nanoparticle properties and process parameters were highlighted.

Microstructure evolution and ordering in commercial Mg-PSZ

Sintered commercial ZrO2-9 mol % MgO (PSZ) alloy was heat-treated at different temperatures in the range 900 to 1400° C. The microstructure was studied using transmission electron microscopy (TEM). The as-sintered material was characterized by either fine tetragonal precipitation in cubic matrix grains, or coarser precipitates which had transformed martensitically to the monoclinic symmetry. The diffuse scattering intensity (DSI) was observed to originate from the cubic lattice, and was correlated with the short-range ordering of the oxygen vacancies present in the cubic matrix. However, by annealing at temperatures below 1100° C for relatively short times, long-range ordering occurred in the cubic matrix. The ordered phase wasβ-Mg2Zr5O12 with a rhombohedral symmetry, which belongs to the homologous series of MnO2n-2(M7O12) defect structures derived from the CaF2-type structure. The ordering process is characteristic only for the cubic regions between the fine-tetragonal precipitates. This microstructure is considered to be a pseudo-equilibrium state and is related to the limited extent of diffusion.