Thomas Schöberl

Mechanical properties of pyrolysed wood: a nanoindentation study

The present work focuses on changes of mechanical properties in pyrolysed spruce wood as a function of temperature up to 2400°C. Nanoindentation tests are used for the determination of mechanical properties at the scale of single wood cell walls. Hardness, indentation modulus and elasto-plastic/brittle behaviour of the carbonaceous residues are derived as function of pyrolysis temperature. Hardness values increase continuously by more than one order of magnitude to 4.5 GPa at 700°C. The indentation modulus shows complex changes with a minimum of 5 GPa around 400°C and a maximum of 40 GPa around 1000°C. The deformation induced by the indenter is largely visco-plastic in native wood, but it is almost purely elastic in the carbonaceous residue, with particular low values of the indentation ductility index around 700°C. A low density and a strongly cross-linked carbon structure may explain the mechanical behaviour at these intermediate temperatures. A final decrease of the modulus and a slight decrease of ductility for temperatures above 2000°C can be attributed to a continuous structural transition of the material towards graphite-like stacking of carbon sheets and to preferred carbon orientation along the wood cell axis.

Nanoindentation in teeth : influence of experimental conditions on local mechanical properties

The influence of various experimental conditions on the elastic modulus and hardness of human intertubular dentin was studied using instrumented nanoindentation at room temperature. The conditions studied were: dry (chemically dehydrated) versus wet (prepared and nanoindented under Hanks balanced salt solution (HBSS)), the influence of long-term storage under HBSS at ca. 4°C and the influence of deep-freezing under dry and wet conditions. The reversibility of chemical dehydration and the consequences of multiple deep-freezing cycles were also investigated. Two premolars of a 12-year-old human male were chosen for evaluation. The absolute values of the mechanical properties differed by a factor of ∼2 and ∼3 between dry and wet states, whereas trends were conserved. Deep-freezing wet dentin samples weakened their mechanical properties by 20–28%, whereas dehydrated samples suffered no damage. This may be due to mechanical damage via the volume increase in water freezing inside the tubules.

Room temperature deposition of (Ti,Al)N and (Ti,Al)(C,N) coatings by pulsed laser deposition for tribological applications

Abstract Titanium–aluminium based nitride (Ti,Al)N and carbonitride (Ti,Al)(C,N) hard coating systems possess excellent tribological behaviour in metal cutting and polymer forming contacts. In the present work (Ti,Al)N and (Ti,Al)(C,N) coatings were deposited by employing the pulsed laser deposition (PLD) technique. A pulsed Nd:YAG laser with 1064 nm wavelength was used for the vaporization of TiAl targets in low-pressure N2 or N2/C2H2, atmospheres at room temperature. The highly ionized metal vapour was deposited onto polished substrates (molybdenum, AISI D2). The coatings were characterized by light-microscopy, scanning electron microscopy, X-ray diffraction and hardness tests. The variation of the deposition parameters causes a change of the chemical composition, the texture and crystallinity of the coatings and, consequently, the mechanical properties and tribological behaviour. The latter was characterized in pin-on-disc tests at room temperature by using coated discs and uncoated AISI 52100 (DIN 100Cr6) steel and alumina pins as counterparts. The results demonstrate the excellent industrial applicability of these coatings for cold-forming operations: very low-wear rates were found for the (Ti,Al)N coatings. In contrast, the (Ti,Al)(C,N) coatings possess low-friction coefficients of approximately 0.2. As an outstanding advantage of these coatings, which were deposited at the room temperature by the PLD process, their excellent adhesion to the substrate can be pointed out, reaching the highest level (HF 1) in the Rockwell indentation test.

Measurements of mechanical properties in Ni-base superalloys using nanoindentation and atomic force microscopy

Abstract The morphology and the nanomechanical properties of two Ni-base superalloys at length scales of

Influence of the indenter tip geometry and environment on the indentation modulus of enamel

The aim of the investigation was to study the influence of indenter tip geometry on the conventionally obtained indentation modulus of enamel by nanoindentation. Indentation tests on bovine enamel using three different diamond pyramidal indenters with half face angles 65.27°, 45°, and 35.26° were conducted to evaluate the indentation modulus using the Oliver–Pharr method [W.C. Oliver and G.M. Pharr, J. Mater. Res. 7, 1564 (1992)]. In addition, three different dehydration conditions were studied: wet under Hanks balanced salt solution, laboratory dried, and vacuum dehydrated. For the Berkovich indenter (65.27°) and 45° pyramidal indenters, there was only a small difference between indentation modulus values, whereas for the cube-corner indenter (35.26°) a ratio of 2.4 between laboratory dry and wet samples was found. A detailed evaluation, including indentation creep and recovery as well as pileup, resulted in a reduction of this latter ratio to 1.7. This still large difference was rationalized on the basis of the different deformation mechanisms generated by indenters of different face angles.

Mechanical property enhancement in laminates through control of morphology and crystal orientation

This article shows the successful implementation of biological design principles into synthetic laminate materials in order to enhance their mechanical properties. We demonstrate and provide a strategy for laminate thin films, which reveals that the control of local crystal anisotropy across laminates together with the optimized layered arrangement are essential for their mechanical behavior. By the example of a laminate consisting of brittle CrN and ductile Cr layers, enhanced material properties are achieved by taking advantage of the self assembly mechanisms of the heterogeneous material during film growth. The usage of local microstructure analysis by a synchrotron based technique as well as miniature mechanical tests allow to understand the relationship between the apparent local microstructure and the accompanied mechanical properties. A crystallographic orientation relationship between Cr and CrN is elucidated, which leads to decisive mechanical enhancement due to microstructural benefits in terms of texture. This results in enhanced strength and fracture toughness of the laminate compared to its single constituents. The systematic approach gives an insight into the complex coherences of laminate materials, where the used techniques and design principles are universally applicable.

Microstructure-controlled depth gradients of mechanical properties in thin nanocrystalline films: Towards structure-property gradient functionalization

Although the influence of the grain size on the mechanical properties of polycrystalline materials is well understood, the occurrence of depth-gradients of grain size, microstructure, and residual stresses in nanocrystalline thin films and their effect on the functional properties is a phenomenon which has not yet been studied in detail. Hence in this work, single-layered polycrystalline and mosaic epitaxial as well as multilayered CrN thin films were characterized by a combination of averaging as well as depth-resolved experimental techniques, such as cross-sectional X-ray nanodiffraction and small-angle cross-sectional nanoindentation. The results reveal the fundamental relationship between gradients of grain size, microstructure, and stresses, controlled by the film growth conditions, and gradients of hardness and elastic modulus. The effect of the compressive stress and structural defects associated with high particle energy on the mechanical properties of nanocrystalline thin films was found to be do...

Interface fracture and chemistry of a tungsten-based metallization on borophosphosilicate glass

In microelectronic devices, the interface between barrier metal and dielectric is of particular interest for a reliable electronic functionality. However, it is frequently observed that this interface is prone to failure. In this work, the strength of interfaces between an as-deposited borophosphosilicate dielectric glass (BPSG) layer and a W(Ti) metallization with and without Ti interlayer was the centre of interest. Four-point-bending tests were used for the mechanical characterization combined with a topological and chemical analysis of the fracture surfaces. In addition, the interface chemistry was studied locally prior to the testing to search for a possible Ti enrichment at the interface. The fracture results will be discussed taking the chemical and topological information into account.

Microstructural evolution and grain refinement in an intermetallic titanium aluminide alloy with a high molybdenum content: Paper presented at “XV International Conference on Electron Microscopy”, 15–18 September 2014, Cracow, Poland

Abstract As a strong β-stabilizing alloying element, Mo has gained importance for intermetallic β/γ-TiAl alloys. In general, TiAl alloys containing a significant volume fraction of the disordered body-centered cubic β-phase exhibit improved processing characteristics during hot-working. To increase the understanding of the alloying effect of Mo, a model alloy with the chemical composition Ti-44Al-7Mo-0.1B (in at.%) was investigated. In this work, the microstructural evolution after individual heat-treatment steps was studied by means of scanning as well as conventional and in-situ transmission electron microscopy. Additionally, macro-hardness and nanoindentation measurements were conducted to study the change in hardness due to grain refinement and solid-solution hardening. The variation of the observed macro- and nano-hardness corresponds well with the microstructural evolution. The obtained grain refinement effect leads to a significant increase in the macro-hardness, whereas the increase in the average nano-hardness of the individual phases is related to solid-solution hardening.

Nanoindentation applied on a tungsten - copper composite before and after high-pressure torsion

Abstract Nanoindentation experiments were performed before and after severe plastic deformation of W–Cu composites at room temperature, 200, and 400 °C. The strains were induced by high-pressure torsion (HPT). For the highest degrees of deformation, a particle size of 10 – 20 nanometer was achieved. The nanohardness of copper increased remarkably with increasing deformation, the hardness of tungsten was enhanced only slightly. A temperature of 400 °C during HPT significantly lowered the Cu hardness, probably recrystallisation during HPT had taken place, the hardness of tungsten, however, kept unaffected. With decreasing particle size, the influence of the adjacent material on the measured properties increased as well as the scatter of hardness and modulus. Proceeding to verq small particle and grain sizes, the particles became smaller than the size of the indent. Thus, hardness and modulus values reflected some average over that of pure tungstef and copper.