Matthias Lodes

Fabrication and characterisation of a fully auxetic 3D lattice structure via selective electron beam melting

A three-dimensional fully auxetic cellular structure with negative Poissons ratio is presented. Samples are fabricated from Ti6Al4V powder via selective electron beam melting. The influence of the strut thickness and the amplitude of the strut on the mechanical properties and the deformation behaviour of cellular structures is studied.

Self-Supporting Nanocrystalline Diamond Foils – A New Concept for Crystalline Diamond on any Technical Surface

The manufacturing and application of self-supporting nanocrystalline diamond foils is introduced. The high temperature manufacturing of nanocrystalline diamond foils by hot-filament chemical vapour deposition (HFCVD) is separated from the low temperature application, allowing the coating of temperature sensitive materials, which cannot be coated by HFCVD conventionally. By coating appropriate template materials and stripping-off after the CVD-process, self-supporting, flexible nanocrystalline diamond foils with high hardness (> 70 GPa) and very low thermal conductivity (< 1 W/mK) with thicknesses of up to 100 µm can be produced. Lasercutting is an appropriate method for machining any desired geometry. Thus the possibility to use the extreme properties of diamond for protection against friction and wear on new substrate materials, e.g. steels, light metals and polymers, is generated.

Mechanical properties of micro- and nanocrystalline diamond foils

Diamond coating of suitable template materials and subsequent delamination allows for the manufacturing of free-standing diamond foil. The evolution of the microstructure can be influenced by secondary nucleation via control of process conditions in the hot-filament chemical vapour deposition process. Bending tests show extraordinarily high strength (more than 8 GPa), especially for diamond foils with nanocrystalline structure. A detailed fractographic analysis is conducted in order to correlate measured strength values with crack-initiating defects. The size of the failure causing flaw can vary from tens of micrometres to tens of nanometres, depending on the diamond foil microstructure as well as the loading conditions.

The effect of a negative Poisson’s ratio on thermal stresses in cellular metallic structures

To investigate the influence of auxetic behavior on thermally induced stresses, finite element analysis simulations of the displacement and stress fields in locally heated three dimensional auxetic and cubic structures are compared. The simulations were carried out in Comsol for a cubic as well as a chiral array of 30 mm length in each spatial dimension (3 × 3 × 3 unit cells). The center cells of these arrays were heated for 20 s. For two boundary conditions (free and clamped), deformation mechanisms are analyzed. It is found, that the auxetic behavior can effectively reduce thermal stresses by internal node rotation and strut bending, especially for constrained (clamped) boundary conditions. A stress reduction of a factor of 3.3 in comparisons to a simple cubic cell array could be evaluated.