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From the operation of the MUPUS thermal probe Spohn et al. (2015) concluded that the material of the nucleus of 67P/Churyumov–Gerasimenko is likely to have a high strength, at least locally at the Philae landing site. In this work we consider the derived strength of the material in order to constrain its granulation. For this purpose we performed numerical simulations of the long-term sintering of ice–dust granular mixtures of different granulation, covered by a dust mantle. The dust mantle has a thickness of 0–16cm, and a (pore size and temperature-dependent) thermal conductivity. According to our simulations a hardened layer at least a meter thick forms beneath the dust only when the grains are tens of microns in radius, or smaller.

Dimensional control is one of the basic problems in ceramic processing, especially for tape cast ceramic sheets which are used to build up multilayer structures for high-integrated components. Uncontrolled anisotropic shrinkage can cause geometrical distortion and circuit failure of multilayer ceramics during sintering. The understanding of the relationship between green tape microstructure and shrinkage anisotropy is of great importance for further miniaturization of multilayer devices. In this study, alumina powders with spherical particle shape were used to cast green tapes. The microstructure as well as the pore orientation and the shrinkage behavior were analyzed. According to the sintering theory, grain growth and pore elimination are the two most important mechanisms to describe sintering shrinkage. In this work, three-dimensional shrinkage behavior of tape cast alumina powders with spherical particle shape was investigated and correlated with pore orientation in the microstructure. Specifically, the reason for the different shrinkage in z-direction compared to the lateral shrinkage is in focus. The study is based on experiments as well as on mathematical visualization.

Sintering response and phase formation during sintering of WC-Co/316L stainless steel composites produced by assembling of powder injection molding (PIM) parts were studied. It is shown that during cosintering a significant mismatch strain (>4 pet) is developed in the temperature range of 1080 degr C to 1350 degr C. This mismatch strain induces biaxial stresses at the interface, leading to interface delamination. Experimental results revealed that sintering at a heating rate of 20 K/min could be used to decrease the mismatch strain to

Micro porous syntactic foams were successfully produced by pressure-assisted infiltration of sintered borosilicate glass bubble structures with AlSi9Cu3 and ZnAl4Cul alloy melts. Specimens with densities as low as 1 g/cm(exp 3) for aluminium and 3 g/cm(exp 3) for zinc and good infiltration qualities could be obtained whereas tests with the magensium alloy MgA15Zn led to extensive reactions between the glass and the magnesium melt. Compression tests with aluminium matrix specimens showed that this material has a very high compression strength and behaves almost like an ideal crash energy absorber. It could be shown that due to its strictly closed and microscopic-scale porosity this kind of foams can be surface treated e.g. by copper-plating and chromatizing or by anodizing processes.

The effect of alumina precursors on microstructure, sintering and creep behavior of sol-gel derived mullite ceramics was studied by scanning and transmission electron microscopy (SEM, TEM and HRTEM), dilatometry and measuring the creep resistance in 4-point bending mode. When aluminum nitrate nonahydrate was used as the precursor, elongated crystals (with longer axis about 5-7 mm) were embedded into a matrix of polyhedral much smaller grains. This morphology was due to overlapping of mullite crystallization and viscous flow sintering. Equiaxial mullite grains with 1.3 μ m mean sizes were obtained using boehmite (γ -AlOOH) as alumina precursor. Transient alumina formed in situ by the decomposition of boehmite shifted the mullite formation above the sintering temperature, and allowed the formation of equiaxial mullite grains. Four-point bending creep behavior of mullite ceramics was studied in the temperature range of 1320-1400°C under the stresses between 40 and 160 MPa. The higher creep rate and the smaller activation energy (Q= 606± ; ; ; ; 33 kJ/mol) exhibited the sample with bimodally distributed grain sizes, whereas a smaller creep rate and the higher activation energy (Q=743± ; ; ; ; 18 kJ/mol) were obtained for mullite ceramic with monomodally distributed grain sizes.

An overview of the achievements of the German priority program “Functionally Graded Materials (FGM)” in the field of processing techniques is given. Established powder processes and techniques involving metal melts are described, and recent developments in the field of graded polymer processing are considered. The importance of modeling of gradient formation, sintering and drying for the production of defect-free parts with predictable gradients in microstructure is discussed, and examples of a successful application of numerical simulations to the processing of functionally graded materials are given.

Micromechanical models for the pressing and sintering of powders are described and macroscopic constitutive equations are derived from them. The effects of cohesion and friction between powder particles on the macroscopic behaviour during compaction are explored. The micromechanical model for sintering is based on the assumption that the surface of the open pore space is in equilibrium and that densification and creep occur by grain boundary diffusion. The thus developed constitutive equations are implemented in finite element codes. A simple example is shown, in which the pressing and sintering of a hard-metal cutting tool with a wedge-type profile is simulated. The simulation correctly predicts large gradients in green density and, as a consequence, large shape distortions during sintering.

Isothermal sintering of SiO 2 aerogels was followed by small angle X-ray scattering using synchrotron radiation. An improved treatment of SAXS data which takes into account the presence of fluctuations explains the discrepancy previously observed with He-pycnometric methods. The matrix density of the gel is shown to increase in several steps, first by a diffusion-controlled mechanism and then by viscous flow.

B6O composites were prepared by hot pressing of B6O powder with alumina, aluminium and carbon additions. Dense materials with hardness higher than 30 GPa and fracture toughness of 3 MPa m1/2 were achieved at 1900 °C. The sintering behaviour and the formed phases were discussed on the bases of thermodynamic calculations. The thermodynamic properties of boron suboxide (B6O) were approximated at high temperatures. The phase equilibria computed based on the obtained thermodynamic properties, agree with the available experimental data. The modelled thermodynamic properties of B6O have been applied to analyse interpretation of the sintering behaviour and the stability of these materials. The thermodynamic calculations and microstructural analysis showed that the densification takes place by liquid phase sintering. Entnommen aus TEMA

The distribution of the sintering additives in the Si3N4 materials with Y2O3 and Al2O3 additives is examined by x-ray defraction and SEM of polished sections of quenched samples. An redistribution of the sintering additives in the temperature range of 1400 to 1650 C was observed.

Al2O3 composite ceramics containing 10 vol% SiC nanoparticles were prepared by pressureless sintering. SiC particles < 150 nm detach from the matrix grain boundaries during grain growth and are predominantly observed in intragranular positions, whereas larger SiC particles retain intergranular positions. Thermal expansion mismatch causes local residual tensile stresses in the matrix grains, giving rise to strain contrasts in TEM imaging; occasionally microcracking around intragranular inclusions ≥ 100 nm is observed in thin TEM foils. It appears that the volume fraction of intragranular SiC particles in the size range 100 < d < 150 nm should not exceed ≈0.5% in high strength/high toughness Al2O3-SiC nanocomposites. The interaction between propagating cracks and internal stress fields around intragranular inclusions forces a transgranular fracture mode. Grain boundary pinning by large intergranular SiC particles, in combination with solute drag inhibits matrix grain growth, while the presence of an amorphous phase at Al2O3 grain boundaries and Al2O3-SiC phase boundaries assists in densifi cation. Excess liquid phase is exuded from the interfaces during consolidation and accumulates in large pores, stabilized by the accidental agglomeration of large SiC particles.