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Porous ceramics prepared by the foam replication technique have a high porosity and low mechanical stability. It has been reported that coating such porous ceramics with gelatin allows for an improved compressive strength. Little details regarding the influence of important gelatin parameters such as concentration, temperature and drying conditions as well as bloom number which is a measure of the gel rigidity, on this toughening effect are available. This paper investigates the influence of these parameters on compressive strength of gelatin coated porous hydroxyapatite ceramics. It was found that concentration in the gelatin sols has a marked impact whereas sol temperature, bloom number and differing conditions during subsequent drying have only little influence on the compressive strength of the coated ceramics.
An extensive series of large deformation crushing tests with spruce wood specimens was conducted. Material orientation, lateral constraint and loading rate were varied. Regarding material orientation, a reduction in the softening effect and the general force level was observed with a higher fiber-load angle. A comparison with characteristics gained by application of Hankinson's formula showed discrepancies in compression strength and the beginning of the hardening effect. Lateral constraint of the specimens caused a multiaxial stress state in the specimens, which was quantified with the applied measuring method. Further, a higher force level compared to specimens without lateral constraint and significant hardening effect at large deformations resulted. Thus, the influence of a multiaxial stress state on the force level could be determined. An increase in the loading rate led to higher force levels at any displacement value and material orientation.
Modified Yoldas sols can be used to prepare crack-free well adherent layers on different substrate materials such as sodalime glass, corundum and metal (IN-718). These layers having thicknesses of 0.6 µm are strain tolerant and withstand thermal cycling between 810 °C and room temperature. Introducing alpha-Al2O3 filler particles into modified Yoldas sols allows the preparation of thicker layers of 1.6 µm, which as well can be thermally cycled without noticeable delaminations. Chemical bonding seems to be the predominant layer bonding mechanism.
Alumina coatings with differing phase compositions were deposited on Inconel-718 (IN-718) Ni-base
superalloy substrates using solgel processing. Mass gain measurements served for studying the oxidation
behavior of coated metal surfaces if exposed to 800°C and 900°C for up to 4000 h in static air. Itwas found, that
alumina coatings significantly reduce the oxidation related mass gain of IN-718 even after heating to 900°C
for 4000 h. Transmission electron microscopy (TEM) studies revealed diffusion of elements from the metal
substrate (IN-718) into the alumina coatings and to a much lower degree from the coating into the
substrate. These diffusion processes are greatly influenced by the phase composition of the coatings, especially
by the presence of alpha-alumina.