David E. Hajas

The Sandfish＇s Skin： Morphology, Chemistry and Reconstruction

The sandfish is a lizard having the remarkable ability to move in desert sand in a swimming-like fashion. The most outstanding adaptations to this mode of life are the low friction behaviour and the extensive abrasion resistance of the sandfish skin against sand, outperforming even steel. We investigated the topography, the composition and the mechanical properties of sandfish scales. These consist of glycosylated keratins with high amount of sulphur but no hard inorganic material, such as silicates or lime. Remarkably, atomic force microscopy shows an almost complete absence of attractive forces between the scale surface and a silicon tip, suggesting that this is responsible for the unusual tribological properties. The unusual glycosylation of the keratins was found to be absolutely necessary for the described phenomenon. The scales were dissolved and reconstituted on a polymer surface resulting in properties similar to the original scale. Thus, we provide a pathway towards exploitation of the reconstituted scale material for future engineering applications.

Thermal and chemical stability of Cr2AlC in contact with α-Al2O3 and NiAl

Abstract The thermal and chemical stability of Cr2AlC, Cr2AlC–NiAl and Cr2AlC–α-Al2O3 have been studied. No enthalpic effects indicating phase transformations or chemical reactions were observed for the Cr2AlC–α-Al2O3 powder mixtures at T < 1476°C, while a melting onset temperature of 1310°C was measured for the Cr2AlC–NiAl powder mixture by differential thermal analysis. However, the formation of chromium carbide is also predicted to occur below this temperature based on our thermodynamic calculations, and was observed by X-ray diffraction of powders annealed at 1000 and 1300°C. The carbide formation is caused by the reaction of Cr2AlC with NiAl and can be understood by considering the large solid solution range of Al in NiAl. These results indicate that the application potential of Cr2AlC–NiAl composites is limited by the Al solubility in the NiAl.

Microstructure and mechanical properties of continuous Al2O3 fibre reinforced Ni45Al45Cr7.5Ta2.5 alloy (IP75) matrix composites

Single crystalline Al2O3 fibres (sapphire), coated with the NiAl alloy IP75 by physical vapour deposition (PVD), were assembled to fabricate composites by means of diffusion bonding. The microstructure and chemistry of both as-coated fibre and as-diffusion bonded composites were investigated by electron microscopy and microanalysis. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature, and the composite tensile strength was measured at 900°C and 1100°C. An amorphous layer with a thickness of about 400 nm formed between the fibre and the matrix during the PVD process and was maintained during diffusion bonding. A Laves phase precipitated along NiAl grain boundaries in the IP75 matrix. This caused a lower tensile strength of the IP75/Al2O3 composite at high temperatures compared to as-cast monolithic IP75 and rendered the composite useless for structural applications.