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A quantitative model for liquid phase sintering is developed based on the following ideas. During heating a liquid phase forms, which is easily mobile, wets the solid particles completely, dissolves solid atoms and provides an easy diffusion path for them. The solid density increases by particle rearrangement and by the flattening of particle contacts. Driving (or retarding) forces result from capillary stresses, from applied mechanical stresses, from the pressure of gas entrapped in closed pores and from differences in chemical potential of the dissolved and precipitated matter. At higher densities the driving force may become very small, since the liquid pressure decreases and a negative contribution from the solid-liquid interface energy increases. At this stage grain coarsening plays an important role for the continued filling of larger and larger pores. The model is applied to describe nonisothermal densification curves measured on Si 3 N 4 for various hold temperatures, axial stresses and green densities. Adjusting a moderate number of parameters all having a physical meaning leads to good agreement between theory and experiment. more...

The densification and phase formation of liquid phase sintered silicon carbide (LPSSiC) with 10wt.% additives were investigated. The ratio of the Al(sub 2)O(sub 3)/Y(sub 2)O(sub 3)-additives was changed between 4:1 and 1:2. Densification was carried out by hot pressing and gas pressure sintering. The different densification routes result in different major grain boundary phases-aluminates in gas pressure sintered materials and silicates in hot pressed samples. Thermodynamic calculations were carried out to determine the amount of liquid phase during densification and for the interpretation of the results. more...

Ein wesentlicher Prozessschritt bei der Herstellung von keramischen Mehrlagenschaltungen ist das Sintern. Beim Sintern bildet sich unter dem Einfluss hoher Temperatur aus dem vorgeformten Grünling das feste Bauteil. Die treibende Kraft für das Zusammenwachsen der Pulverteilchen und die Verdichtung des Werkstoffs resultiert aus der großen Energie der Teilchenoberflächen. Zur Verdichtung des Werkstoffs werden verschiedene physikalische Mechanismen genutzt, die man als Festphasen- und Flüssigphasensintern klassifizieren kann. Detail sind z.B. bei German ausführlich beschrieben. more...

A model for liquid phase sintering is examined. The model accounts for solid densification during sintering by particle rearrangement and contact flattening, with large pore fitting and grain coarsening in the final stages of sintering. The liquid phase, which develops during heating, aids the solution and diffusion of solid atoms. The model is used to simulate the sintering of Si3N4. The bulk viscosity is calculated for various sintering histories. The bulk viscosity, subjected to a normalisation, is shown to be approximately a unique function of density. A simplified version of the model is developed for the case where rearrangement is small. The model is applied to WC-Co and is modified to account for solution of WC in Co and rearrangement prior to binder melting. The sintering of a WC-Co turning insert is simulated and conclusions are drawn on the importance if the constitutive model in predicting shape distortions in such cases. more...

The densification curves registered during hot- pressing of silicon nitride are analyzed and put into relation with varying powder properties (granulometric values, oxygen content). Densification within the nonisothermal section can be divided into several stages dominated by various mechanisms, which react differently to processing conditions (temperature-pressure-time) due to altering powder properties. The correlation between powder properties and kinetics of the individual stages can be proven quantifiably. The time- and temperature-dependent densification as well as the densification speed during hot-pressing are very sensitive values for evaluating the influence of powder and raw material properties, a fact not only of sinter-kinetical interest but also of technological importance with respect to process control and quality assurance. more...

The main purpose of the project was to develop a pressureless sintered Silicon Carbide ceramic reinforced with platelets to reach both higher toughness and strength at elevated temperatures. In detail fundamental works in selection of the ceramic matrix were performed, but also forming and pressureless sintering of the platelet composite was carried out. The matrix phase was built up of a Silicon Carbide ceramic using Alumina-Yttria additives. In addition to varying sintering parameterrs the amount and ratio of the additives was changed. It has been shown that alumina content is the main factor that affects sintering activity. By using conventional PM processing a homogeneous and isotropic arrangement of the platelets has been obtained. A 10 to 15 % platelet composite was densified to bigger than 95 %th.D. using the pressureless sintering technique. This result essentially demonstrates the possibility of making dense platelet reinforced composites by pressureless sintering. more...

The structure and the electrical properties of porous and dense liquid-phase sintered silicon carbide ceramics (LPS-SiC), containing yttria and alumina additives, have been studied. The electrical resistance of LPS-SiC-materials varies in a wide range. This paper is focused on the influence of porosity on the electrical resistance of the sintered SiC. The porosity was controlled by the SiC grain size. Porous LPS-SiC materials were found to have substantially lower electrical resistance in comparison to dense materials of the same type. The structure of the materials was investigated by XRD and FESEM. The porous materials consist of large grains due to the coarser starting powders in comparison to the dense materials. This results in a reduction of the dissolved and re-precipitated fraction of the SiC during sintering. Using the in-lens SEM mode, the high conductivity of the formed rims of the SiC grains could be shown. These observations reveal that the rim volume of dense LPS-SiC is much more extended than the rims of porous materials showing the higher conductivity. more...