Jens Emmerlich

A proposal for an unusually stiff and moderately ductile hard coating material: Mo2BC

The elastic properties of Mo2BC were studied using ab initio calculations. The calculated bulk modulus of 324 GPa is 45% larger than that of Ti0.25Al0.75N and 14% smaller than that of c-BN, indicating a highly stiff material. The bulk modulus (B) to shear modulus (G) ratio is 1.72 at the transition from brittle to ductile behaviour. This, in combination with a positive Cauchy pressure (c12 − c44), suggests moderate ductility. When compared with a typical hard protective coating such as Ti0.25Al0.75N (B = 178 GPa; B/G = 1.44; negative Cauchy pressure), Mo2BC displays considerable potential as protective coating for metal cutting applications. In order to test this proposal, Mo2BC thin films were synthesized using dc magnetron sputtering from three plasma sources on Al2O3(0 0 0 1) at a substrate temperature of ~900 °C. The calculated lattice parameters are in good agreement with values determined from x-ray diffraction. The measured Youngs modulus values of ~460 ± 21 GPa are in excellent agreement with the 470 GPa value obtained by calculations. Scanning probe microscopy imaging of the residual indent revealed no evidence for crack formation as well as significant pile-up, which is consistent with the moderate plasticity predicted. The apparent contradiction between moderate ductility on the one hand and indentation hardness values of 29 GPa can be understood by considering the electronic structure particularly the extreme anisotropy. The presence of stiff Mo–C and Mo–B layers with metallic interlayer bonding enables this intriguing and unexpected property combination.

Phase stability and elastic properties of Tan+1AlCn (n = 1–3) at high pressure and elevated temperature

We have studied the electronic structure of Tan+1AlCn (space group P63/mmc,n = 1?3) under uniform compression from 0 to 60?GPa and at temperatures from 0 to 1500?K using ab initio calculations. These phases can be characterized by alternating layers of high and low electron density and are referred to as nanolaminates. At 0?K we observe similar compressibilities in both the a and c directions for all phases investigated. This is unusual for nanolaminates. Based on the density of states analysis, we propose that these similar compressibilities may be caused by an increase in Ta?Al and Ta?Ta bonding strength as well as a stronger long-range interaction between TaC?TaC layers. No evidence of a phase transition is observed as the pressure is increased to 60?GPa. However, as the temperature is increased to approximately 1000?K without applying pressure, a first-order phase transition occurs in Ta3AlC2. These results are relevant for applications of Tan+1AlCn at elevated temperature and pressure.

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.

Surface chemistry of TiAlN and TiAlNO coatings deposited by means of high power pulsed magnetron sputtering

It is shown that the investigated TiAlN and TiAlNO protective coatings show an enrichment of oxygen in the surface near region with concurrently higher oxidation states of titanium. We suggest that surface oxidation is caused by reaction with atmospheric oxygen at room temperature and/or with residual gas immediately after deposition during cooling in the chamber. As this coating system is used during cutting and forming operations, the here-established difference between the coatings composition and the composition of the surface near region of the coating is of great relevance for understanding the interactions thereof with the materials to be cut or formed.

Systematic study on the electronic structure and mechanical properties of X2BC (X?=?Mo, Ti, V, Zr, Nb, Hf, Ta and W)

In this work the electronic structure and mechanical properties of the phases X(2)BC with X =Ti, V, Zr, Nb, Mo, Hf, Ta, W (Mo(2)BC-prototype) were studied using ab initio calculations. As the valence electron concentration (VEC) per atom is increased by substitution of the transition metal X, the six very strong bonds between the transition metal and the carbon shift to lower energies relative to the Fermi level, thereby increasing the bulk modulus to values of up to 350 GPa, which corresponds to 93% of the value reported for c-BN. Systems with higher VEC appear to be ductile as inferred from both the more positive Cauchy pressure and the larger value of the bulk to shear modulus ratio (B/G). The more ductile behavior is a result of the more delocalized interatomic interactions due to larger orbital overlap in smaller unit cells. The calculated phase stabilities show an increasing trend as the VEC is decreased. This rather unusual combination of high stiffness and moderate ductility renders X(2)BC compounds with X = Ta, Mo and W as promising candidates for protection of cutting and forming tools.