Modeling the final sintering stage of doped ceramics: mutual interaction between grain growth and densification

Applying the thermodynamic extremal principle, a model for grain growth and densification in the final stage of sintering of doped ceramics was derived, with segregation-dependent interfacial energies and mobilities (or diffusivities). The model demonstrated an interdependence between the driving forces of grain growth and densification during sintering evolution, observed because the surface energy contributes positively to the driving force of grain growth while the GB energy negatively to the driving force of densification. The model was tested in alumina as a host system, and calculations demonstrate that dopants with more negative GB (or surface) segregation enthalpy or which cause lower GB diffusion coefficient can induce higher relative densities at a given grain size. Comparatively studying yttria- and lanthana-doped alumina, the lanthana doping showed significantly enhanced sintering attributed to the larger La³⁺ radius causing a more negative GB segregation energy. This present model is expected to help dopant designing to improve control over sintering.     

Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

The surface- and grain boundary composition of Y, Ce and Ti doped zirconia were studied by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy/Scanning Auger Microscopy. The grain boundaries and free surfaces showed the same enrichment levels. After heat treatment 1000 C all yttria doped samples showed yttrium enrichment. In the ZrO₂-Y₂O₃ system the yttrium enrichment did not depend on the bulk concentration and amounted 30–34 mol% YO_1.5 in all cases. As a consequence the segregation factor increases with decreasing solute concentration in the bulk. The thickness of the segregation layer was about 2–4 nm. In the ternary Y doped systems yttrium is the main segregant. In ceria-doped tetragonal zirconia polycrystals (Ce-TZP) systems significant segregation of cerium starts atT1300C and is mainly attributed to Ce³⁺. In Y,Ti-TZP systems also strong segregation of Ti⁴⁺ occurs. The absolute value of the increase of the surface concentration in fine grained material is smaller than in coarse grained material. This is mainly due to depletion of the bulk.     

The grain size effect on the yttria stabilized zirconia grain boundary conductivity

Conductivity of yttria stabilized zirconia produced from weakly agglomerated nanopowder was investigated by ac impedance spectroscopy. Dense ceramic samples with the grain size from 90 to 800 nm were prepared at the variation of both pressing and sintering conditions. It was found that the bulk conductivity is not affected by grain size, while grain boundary conductivity is dependent on this factor. Observed grain boundary resistance increases with grain size. This relationship is contrary to the previous results obtained for the range from 1 to 18 microm where grain boundary resistance decreased with grain size. Maximum of grain boundary resistance versus ceramics grain size is observed at 450 degrees C for the grain size about 270 nm.     

Some aspects of the anisotropy of grain boundary segregation and wetting

A recently developed model of grain boundary (GB) segregation, in terms of the five macroscopic parameters of GB orientation, has been exercised to explore the anisotropy of GB segregation. The five macroscopic GB orientation parameters are defined by means of the orientations of the two crystallographic planes that terminate the crystals on either side of the GB, and a twist angle. Some important conclusions include the following: (a) the composition of a boundary depends on all five parameters of GB orientation, (b) the segregation profile across a GB depends on the two planes which terminate the adjacent crystals, (c) the composition profile across GB’s terminated by identical crystallographic planes is symmetric, but is asymmetric when GB’s are terminated by different planes, and (d) the strength of the segregation on one side of a GB influences the extent of segregation on the other. Some experimental results on Nb-doped TiO₂ are presented in order to verify above predicted trends. In addition, it is shown that the model predicts the possibility of anisotropic GB wetting transitions as two-phase coexistence is approached.     

Quantifying the effect of fracture surface topography on the scattering of grain boundary segregation measurement by Auger electron spectroscopy

The aim of this paper is to estimate the effect of the geometrical orientation of grain boundary facets on the quantification of grain boundary segregation by Auger electron spectroscopy (AES) on an in-situ fractured surface. Phosphorus grain boundary segregation (PGBS) was studied on 17 different samples (including 3 different steels and 12 different thermal treatments) using a MAC2 analyzer from Riber company. For each sample, 14 to 41 grain boundary facets were analyzed. It is demonstrated that there is proportionality between the PGBS mean value and the PGBS standard deviation obtained on one particular sample: for each sample, the PGBS standard deviation represents approximately 40% of the PGBS mean value.The topography of a typical purely intergranular fracture surface was determined by 3D stereographic observation in a SEM. The influence of surface topography on Auger quantification was then evaluated using the Auger quantification equations. The orientation of grain boundary facets with respect to the primary beam and the Auger analyzer was taken into account. The result is that, considering a homogeneous PGBS on all the grain boundary facets, a large scattering (~ 45%) of the PGBS quantification by AES can be accounted for by a purely geometrical effect (orientation of each facet with respect to the primary beam and to the Auger analyzer). This geometrical effect is nevertheless strongly dependant on the spectrometer geometry (respective position of primary beam, sample and Auger analyzer with respect to each other) and could be reduced by optimizing the relative positions of primary beam, sample and Auger analyzer. The topographical analysis was extended to the case of the well-known CMA (Cylindrical Mirror Analyzer). The scattering of the PGBS measurements with Auger spectrometers fitted with a CMA perpendicular to the fracture surface is expected to be much smaller.     

Interactive grain boundary segregation of nickel and antimony in iron

Abstract

A study of the grain boundary segregation of nickel and antimony in iron is reported in the present paper. It is shown by the results that antimony segregation increases as the bulk nickel and antimony concentrations increase. However, once the solubility limit for antimony in iron is exceeded, the amount of segregation remains essentially constant. Segregation of nickel in iron increases as the bulk concentration of nickel increases and as the bulk concentration of antimony increases. The last effect is observed only when a certain level of antimony is reached, a level that depends on the concentration of nickel. Small additions of antimony, even though they cause an increase in segregation of antimony, do not cause an increase in segregation of nickel and, once the solubility limit for nickel in Fe–Sb alloys is exceeded, the segregation of nickel reaches a plateau. It is only between these two regimes that segregation of nickel is affected by changes in the concentration of antimony. All these results can be explained based on the mutual effects that nickel and antimony have on the solubility of each in iron. The results are not consistent with models based on cosegregation. Finally, other data in the literature are examined; all these data can be explained by an argument based on solubility changes.

MST/795     

Effect of ternary solute interaction on interfacial segregation and grain boundary embrittlement

Effect of ternary solute interaction on interfacial segregation and grain boundary embrittlement in an M–I–J system is modeled on the basis of combined Guttmann and Rice–Wang approaches. It is clearly shown that repulsive I–J interaction strengthens interfacial segregation of the impurity I, suppresses segregation of the solute J, and substantially enhances intergranular embrittlement. Attractive interaction exhibits an opposite effect. Generally, the effect of the ternary interaction is weaker than that of the binary one. Although there are only rare experimental data in this respect, their comparison to model calculations shows a very good agreement.     

First-principles study on the effect and magnetism of iron segregation in Cu grain boundary

The atomic configurations and electronic structures of iron on CuΣ5 symmetrical tilt grain boundary (GB) have been studied based on the density functional theory. Different segregation positions of iron are considered. A weak tendency of iron segregating to GB is arrived due to the segregation energy. In addition, iron segregation shows a cohesion strengthening effect of Cu GB according to Rice–Wang model, which is mainly contributed by the charge redistribution. Finally, an enhancement of the local magnetic moment of iron in Cu GB or bulk or surface is explored due to larger atomic volume than the FCC iron crystal and the Cu atoms surrounding iron are slightly polarized by the doped iron. This study can enrich the understanding of the effects of iron on the cohesion of Cu–Fe alloy and also might supply an indirect guidance to expand the application of Cu–Fe alloy in electronic device manufacture field.     

Comparisons of grain size-density trajectory during spark plasma sintering and hot-pressing of zirconia

Spark plasma sintering (SPS) and hot-pressing (HP) of a granulated stabilized zirconia powder have been investigated for a fixed macroscopic compaction pressure of 100 MPa and a fixed heating rate (25 °C/min for HP, 50 °C/min for SPS). The “relative density/grain size” trajectories have been established for both sintering methods.HP is shown to be similar to SPS for the manufacturing of polycrystalline TZ3Y materials with a final grain size well below the micrometer. Independently of the sintering technology employed, it is interesting to note that three kinds of microstructures are obtained depending on the experimental parameters: porous materials (opened porosity, relative density between 61 and 90%) with a nanometer grain size (around 75-80 nm), dense materials (closed porosity, relative density between 90 and 98%) with a nanometer grain size (around 75-80 nm), fully dense material with a submicron grain size (around 160 nm using SPS and around 105 nm using HP).     

Effect of surface roughness on grain growth and sintering of alumina

The production of ceramic bodies with less surface roughness is industrially important when one considers the aspect of final machining processes. Hence an attempt have been made to study the variation in surface roughness parameters (R _a, R _y, R _z) of alumina having three different kinds of roughness features at different sintering temperatures. Variation in surface roughness properties are also correlated with grain size. R _z shows significant difference between fine and intermediate surfaces, hence predicts small difference in their microstructural features. As a general trend, average grain size increases with increase in sintering temperature, but wide distribution of grains with enhanced non-uniform grain growth is observed when the surface is coarse. Hence, creation of fine surface in the green body is necessary for homogeneously distributed grains with controlled uniform grain growth. The final roughness and grain size of the sintered alumina depend on the initial surface roughness of the green body.