Kurt E. Johanns

Bio: Kurt E. Johanns is an academic researcher from Technische Universität Darmstadt. The author has contributed to research in topics: Indentation & Nanoindentation. The author has an hindex of 16, co-authored 25 publications receiving 774 citations. Previous affiliations of Kurt E. Johanns include Oak Ridge National Laboratory & University of Tennessee.

Measurement of fracture toughness by nanoindentation methods: Recent advances and future challenges

Abstract: In this paper, we describe recent advances and developments for the measurement of fracture toughness at small scales by the use of nanoindentation-based methods including techniques based on micro-cantilever, beam bending and micro-pillar splitting. A critical comparison of the techniques is made by testing a selected group of bulk and thin film materials. For pillar splitting, cohesive zone finite element simulations are used to validate a simple relationship between the critical load at failure, the pillar radius, and the fracture toughness for a range of material properties and coating/substrate combinations. The minimum pillar diameter required for nucleation and growth of a crack during indentation is also estimated. An analysis of pillar splitting for a film on a dissimilar substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties. Experimental results from a selected group of materials show good agreement between single cantilever and pillar splitting methods, while a discrepancy of ∼25% is found between the pillar splitting technique and double-cantilever testing. It is concluded that both the micro-cantilever and pillar splitting techniques are valuable methods for micro-scale assessment of fracture toughness of brittle ceramics, provided the underlying assumptions can be validated. Although the pillar splitting method has some advantages because of the simplicity of sample preparation and testing, it is not applicable to most metals because their higher toughness prevents splitting, and in this case, micro-cantilever bend testing is preferred.

A novel pillar indentation splitting test for measuring fracture toughness of thin ceramic coatings

Abstract: The fracture toughness of thin ceramic films is an important material property that plays a role in determining the in-service mechanical performance and adhesion of this important class of engineering materials. Unfortunately, measurement of thin film fracture toughness is affected by influences from the substrate and the large residual stresses that can exist in the films. In this paper, we explore a promising new technique that potentially overcomes these issues based on nanoindentation testing of micro-pillars produced by focused ion beam milling of the films. By making the pillar diameter approximately equal to its length, the residual stress in the upper portion of the pillar is almost fully relaxed, and when indented with a sharp Berkovich indenter, the pillars fracture by splitting at reproducible loads that are readily quantified by a sudden displacement excursion in the load displacement behaviour. Cohesive finite element simulations are used for analysis and development of a simple relationship...

Effects of indenter angle on micro-scale fracture toughness measurement by pillar splitting

Abstract: We present improvements to a recently developed pillar splitting technique that can be used to characterize the fracture toughness of materials at the micrometer scale. Micro-pillars with different aspect ratios were milled from bulk Si (100) and TiN and CrN thin films, and pillar splitting tests were carried out using four different triangular pyramidal indenters with centerline-to-face angles varying from 35.3° to 65.3°. Cohesive zone finite element modeling (CZ-FEM) was used to evaluate the effect of different material parameters and indenter geometries on the splitting behavior. Pillar splitting experiments revealed a linear relationship between the splitting load and the indenter angle, while CZ-FEM simulations provided the dimensionless coefficients needed to estimate the fracture toughness from the splitting load. The results provide novel insights into the fracture toughness of materials at small-scales using the pillar spitting technique and provide a simple and reliable way to measure fracture toughness over a broad range of material properties.

Dual-phase nanostructuring as a route to high-strength magnesium alloys

Abstract: Combining the benefits of nanocrystals with those of amorphous metallic glasses leads to a dual-phase material—comprising sub-10-nanometre-sized nanocrystalline grains embedded in amorphous glassy shells—that exhibits a strength approaching the ideal theoretical limit. Nanostructuring of crystalline metal alloys can yield high-strength materials, but these tend to soften as the strain is increased. Ge Wu et al. describe a strategy that combines the benefits of nanocrystallinity with those of single-phase amorphous metallic glasses to yield a dual-phase material—nanocrystalline grains each enclosed in an amorphous glassy shell—that exhibits strength approaching the ideal theoretical limit. They demonstrate this approach with a magnesium alloy and prepare the strongest thin films yet achieved for any magnesium alloy. The authors suggest that this material could be a promising coating for wear-resistant surfaces. It is not easy to fabricate materials that exhibit their theoretical ‘ideal’ strength. Most methods of producing stronger materials are based on controlling defects to impede the motion of dislocations, but such methods have their limitations. For example, industrial single-phase nanocrystalline alloys1,2 and single-phase metallic glasses3 can be very strong, but they typically soften at relatively low strains (less than two per cent) because of, respectively, the reverse Hall–Petch effect4 and shear-band formation. Here we describe an approach that combines the strengthening benefits of nanocrystallinity with those of amorphization to produce a dual-phase material that exhibits near-ideal strength at room temperature and without sample size effects. Our magnesium-alloy system consists of nanocrystalline cores embedded in amorphous glassy shells, and the strength of the resulting dual-phase material is a near-ideal 3.3 gigapascals—making this the strongest magnesium-alloy thin film yet achieved. We propose a mechanism, supported by constitutive modelling, in which the crystalline phase (consisting of almost-dislocation-free grains of around six nanometres in diameter) blocks the propagation of localized shear bands when under strain; moreover, within any shear bands that do appear, embedded crystalline grains divide and rotate, contributing to hardening and countering the softening effect of the shear band.

Elastic Properties of the Solid Electrolyte Li7La3Zr2O12 (LLZO)

Abstract: The oxide known as LLZO, with nominal composition Li7La3Zr2O12, is a promising solid electrolyte for Li-based batteries due to its high Li-ion conductivity and chemical stability with respect to lithium. Solid electrolytes may also enable the use of metallic Li anodes by serving as a physical barrier that suppresses dendrite initiation and propagation during cycling. Prior linear elasticity models of the Li electrode/solid electrolyte interface suggest that the stability of this interface is highly dependent on the elastic properties of the solid separator. For example, dendritic suppression is predicted to be enhanced as the electrolyte’s shear modulus increases. In the present study a combination of first-principles calculations, acoustic impulse excitation measurements, and nanoindentation experiments are used to determine the elastic constants and moduli for high-conductivity LLZO compositions based on Al and Ta doping. The calculated and measured isotropic shear moduli are in good agreement and fall ...

Unravelling the physics of size-dependent dislocation-mediated plasticity

Abstract: Size-affected dislocation-mediated plasticity is important in a wide range of materials and technologies. Here we develop a generalized size-dependent dislocation-based model that predicts strength as a function of crystal/grain size and the dislocation density. Threedimensional (3D) discrete dislocation dynamics (DDD) simulations reveal the existence of a well-defined relationship between strength and dislocation microstructure at all length scales for both single crystals and polycrystalline materials. The results predict a transition from dislocation-source strengthening to forest-dominated strengthening at a size-dependent critical dislocation density. It is also shown that the Hall–Petch relationship can be physically interpreted by coupling with an appropriate kinetic equation of the evolution of the dislocation density in polycrystals. The model is shown to be in remarkable agreement with experiments. This work presents a micro-mechanistic framework to predict and interpret strength sizescale effects, and provides an avenue towards performing multiscale simulations without ad hoc assumptions.

Source Truncation and Exhaustion: Insights from Quantitative in situ TEM Tensile Testing

Abstract: A unique method for quantitative in situ nano- tensile testing in a transmission electron microscope employing focused ion beam fabricated specimens was developed. Experi- ments were performed on copper samples with minimum dimensions in the 100200 nm regime oriented for either single slip or multiple slip, respectively. We observe that both frequently discussed mechanisms, truncation of spiral disloca- tion sources and exhaustion of defects available within the specimen, contribute to high strengths and related size-effects insmallvolumes.Thissuggeststhatinthesubmicrometerrange these mechanisms should be considered simultaneously rather than exclusively.