Volume 99, Issue 10

High performance materials are indispensable for economical and environmentally acceptable operation of fusion power and innovative fission reactor systems. In order to provide materials design data for short term needs of the ITER Test Blanket Modules and for a Fusion Demonstration Reactor, the international fusion materials community is concentrating their R&D work mainly on the four reduced-activation structural materials 8 – 9 CrWTa ferritic/martensitic steels, V – Ti – Cr alloys, W alloys and SiC f /SiC composites as well as on functional materials such as CVD diamond windows. Besides the development of the European reduced activation reference steel EUROFER, emphasis will be placed on synergies between fusion and fission materials R&D, namely the development of ductile oxide dispersion strengthened ferritic/martensitic steels with nano-scaled Y 2 O 3 or Y 2 Ti 2 O 7 particles that increase not only high temperature strength but also substantially the microstructural stability against irradiation embrittlement and high temperature ageing. Finally, the corrosion behaviour of zirconium-1.5 wt.% tin alloy, a well established cladding material for nuclear fuel rods, will be shown in different environments.

The inherent brittleness of ceramics led to a probabilistic approach for designing with ceramic materials. For tackling this issue, numerous research projects led to the generation and continuous development of the weakest link postprocessor STAU over the last twenty years. Compared to other similar tools, STAU is unique in its comprehensive capabilities addressing spontaneous fracture, sub-critical crack propagation under transient loading, including a temperature-dependent fracture stress, and crack propagation parameters and analysis of strong stress gradients allowing for solving general thermo-mechanical problems including thermal shock and contact loading. Remarkably, the stress analysis, using commercial finite element codes, does not require a separate meshing with respect to the subsequent probabilistic analysis. Within the framework of a collaborative research effort (SFB 483) ceramics are qualified with respect to friction and wear for highly demanding applications. Here, uncertainty analysis methods for STAU were developed and led to an important extension of STAU for several technological applications. In this paper, the main features of STAU as well as the physical background are revisited and some application cases are shown.

The term “evolution” is used in both biology and materials science. There are however large differences in what is meant by it. It will be shown that one difference is the probability of single or multi step nucleation. Biological structures are too complex to be recreatable by nucleation. They originate by a very rare event. Consequently they are prone to extinction, if they had no chance to multiply and modify in particular environments (populations, generations). Formation of various dispersoid microstructures in Al – Cu alloy is shown as an example of nucleation controlled evolution in materials science.

Machined ceramics and other hard materials exhibit very steep subsurface gradients of residual stresses. X-ray analysis enables the non-destructive assessment of the distributions of these residual stresses versus the distance from the surface via variation of the penetration depth of the X-rays. Conventional evaluation procedures are based on continuous or piecewise defined polynomial functions of stress versus distance, which are fitted to the X-ray data by means of complex solution schemes. These solution schemes are, however, in many cases unstable and difficult to handle. We have developed an entirely novel evaluation procedure which is based on polynomials fitted to the “2θ vs. sin 2 ψ” plots obtained from the X-ray measurements. It is shown that the residual stress profiles can be computed explicitly from these polynomials without any further assumptions. The evaluation procedure is described in detail and demonstrated in a case study on ground alumina ceramics.

The future demands of diesel engines require new options for low-friction and wear-resistant materials in order to increase efficiency and achieve environmentally sound solutions. Efforts are made to improve the performance and reduce the weight of engine blocks by coating the aluminium cylinder bores using thermal-spray processes. Thus, along with other means, today nanocrystalline coatings are considered when looking for the desired combination of structural, productional, and topographical properties. Besides sufficient tribological properties it is important that the composite (base material and coating) allows for prolonged endurance under cyclic mechanical and thermal stresses. Prior to engine tests these coatings are characterised in the laboratory by means of bench tests for adhesive tensile strength, tribological properties and fatigue of the composite. Cavitation tests should reveal the tribological stability of these coatings and their ability to resist high-frequency cyclic impact stresses. The objective in this investigation is to describe the cavitation behaviour and the influence of voids such as pores or oxides.

Tools for hot wire rolling are exposed to high thermal, mechanical and tribological loadings during service. Ceramic materials such as silicon nitride possess good mechanical and tribological properties up to temperatures where most of today' roller materials do not exhibit acceptable reliability. In the present paper basic mechanical properties under thermal and mechanical loading as well as the damage behaviour of Si 3 N 4 have been investigated in high cycle thermal fatigue and dynamic as well as cyclic four point bending tests. In fretting fatigue tests the behaviour of Si 3 N 4 under near-service conditions, as they appear during application in the hot wire rolling process, has been analysed. It is shown that in the temperature range investigated pure thermally induced loading leads to the same damage mechanism as pure mechanical loading and therefore thermally induced stresses can be directly added to any mechanical loading. The isothermal four point bending tests confirm the existence of a cyclic fatigue effect. With the fretting fatigue experiments the limits of high-performance ceramics such as Si 3 N 4 are shown. Under the complex loading, surface flaws are produced leading to a severe lifetime reduction compared to plain mechanical and thermal loading.

Hydrogen diffusion coefficients D H were determined in various commercial β titanium alloys and binary titanium alloys of the Ti – Mo and Ti – V systems, since molybdenum and vanadium are the most important β-stabilizing alloying elements. For this purpose, hydrogen concentration step profiles were established in long thin titanium rod samples via electrochemical hydrogenation. Subsequent diffusion annealing at various temperatures led to hydrogen diffusion inside the samples. The hydrogen concentration profiles established were evaluated either numerically by means of a finite-difference application or analytically by applying the Matano technique and the hydrogen diffusion coefficients were calculated. The results show that the D H -values increase with increasing β stability, obey an Arrhenius-type temperature dependence and are not affected by hydrogen concentration.

Lifetime tests under static load are often carried out in order to determine the relation v(K) of subcritical crack growth. Depending on the scatter behaviour of the most serious cracks in a specimen a number of immediate failures can occur during load application and some specimens will not fracture within a certain maximum lifetime limit. In order to get additional information on subcritical crack growth behaviour, the authors recommend performing lifetime tests at an increased load level. Appropriate procedures for the evaluation of such tests will be outlined. First, the procedure is explained for the simple case of the power-law crack growth relation. Then, a modified lifetime evaluation is proposed that needs no special crack growth law. The methods described are applied to static lifetime measurements on SiAlON in distilled water at 20 °C as the environment.

Measurements of the stress – strain response of hairlike gecko setae are presented. These setae have diameters in the micron range and are part of the attachment organs that enable lizards to cling to surfaces. Three test methods were employed: (a) applying tension with a piezoresistive cantilever inside a focused ion beam microscope under vacuum conditions, (b) three-point bending in air using an atomic force microscope, and (c) nanoindentation tests on the circumference of the seta. Elastic moduli were found to depend strongly on the method of testing: Tensile testing in vacuum yielded an elastic modulus of 7.3 ± 1.0 GPa which was roughly five to six times higher than in bending and in nanoindentation tests at ambient conditions. The results are discussed in terms of the anisotropic nanostructure of the setae and humidity effects. They constitute valuable input parameters for optimizing the performance of artificial attachment devices.

The influence of Fe acceptor and Fe – F acceptor – donor co-doping on the dielectric response of screen-printed Ba 0.6 Sr 0.4 TiO 3 thick-films on alumina substrates has been investigated. The Ba 0.6 Sr 0.4 TiO 3 powders were synthesized by freeze-drying of sols. Permittivity, dielectric loss, and tunability were investigated at kHz frequencies with a planar metal-insulator-metal capacitor structure, and at GHz frequencies up to 40 GHz using coplanar waveguide structures. Pure acceptor doping by Fe was found to have a distinct influence on permittivity and dielectric loss-factor at kHz-frequencies and at GHz frequencies due to an internal bias field and pairs of associated defects. Co-doping with F significantly suppresses the influence of the internal bias field and defect-associates at low and microwave frequencies. The commutation quality factor at 10 GHz and E eff = 5.8 V μm -1 was increased by co-doping the thick-films with Fe and F. Such doped Ba 0.6 Sr 0.4 TiO 3 films show a high potential for tunable microwave applications.

Degradation evolution and failure mechanisms of air plasma-sprayed thermal barrier coatings during thermal cycling were investigated using microstructural and acoustic emission analysis. The microcrack evolution observed suggests that the life-time is governed by the kinetics of crack formation, growth and linking of individual cracks. The damage in the thermal barrier coatings mainly occurs during cooling due to thermal-expansion mismatch stresses at the metal – ceramic interface. The effect of the minimum cycling temperature on the lifetime was found to be much more pronounced than that of a variation in cooling rate. Experimental results were supported by finite element modeling of the stress distribution at the metal – ceramic interface during cooling at different rates.

Amorphous hydrocarbon (a-C: H) films have enormous potential as low friction, wear resistant coatings. Here, we present a plasma assisted chemical vapour deposition process for a-C: H that exhibits growth rates of 100 nm min – 1 and higher. The tribological performance of the resulting a-C: H films has been studied experimentally by reciprocating sliding of an a-C: H-coated Si 3 N 4 ball on an a-C: H-coated 100Cr6 steel substrate and by subsequent micro Raman spectroscopy of the wear track. Running-in of the coatings is observed and characterised by a rapid decrease in the friction coefficient accompanied by a significant increase in sp 2 hybridisation in the wear track. In order to gain a deeper understanding of the underlying running-in mechanisms, the sliding of two a-C: H films under a load of 5 GPa has been studied by classical molecular dynamics employing a range-corrected Brenner bond-order potential. The simulations reproduce the experimental trends and explain the running-in by a combination of smoothing and chemical passivation of both tribosurfaces. Consequently, both mechanisms should be controlled in order to produce tribological coatings for applications with optimum energy-efficiency.

This paper describes the use of thin film high-throughput experimentation methods for the efficient development of multifunctional materials, using Ni – Ti – X and ferromagnetic shape memory alloys as examples. The thin films were fabricated in the form of binary, ternary, and quaternary materials libraries by special magnetron sputter deposition processes. These materials libraries were subsequently processed and characterized by high-throughput experimentation methods in order to relate compositional information with structural and functional properties. For this, appropriate visualization of the data is necessary. Results show that the martensitically transforming regions in ternary thin films are generally larger than was known from literature. Within these regions, the variation of the functional properties can be mapped with respect to the composition and microstructure, and thus the most suitable materials for applications can be effectively selected.

The in-situ transmission electron microscopy technique allows direct observations of formation and growth of stress-induced martensite in pseudoelastic NiTi shape memory alloys. The present paper reports on the development of a miniature test procedure for in-situ straining experiments with specimens taken from small components. The deformation of an ultra-fine grained NiTi specimen is characterised by transmission electron microscopy (at early loading stages) and by optical microscopy (at larger strains). A complementary finite element analysis of the complex strain state in the specimen rationalises why the stress-induced martensitic transformation first occurs in the thin foil region of the specimen before spreading towards the outer rim of the specimen.

Effective material application and miniaturization, both indispensable to modern product development and production, demand enhanced manufacturing processes suitable for both micro devices and economic series production. For micro parts made of polymeric materials, micro injection molding represents such a method and has already reached an industrially viable status. For manufacturing of ceramic products micro powder injection molding is a promising option because it combines the possibility of large-scale series production with a wide range of materials, thus possessing a considerable economic potential. An enhanced variant, micro two-component injection molding enables, for example, the fabrication of micro components consisting of two ceramic materials with different physical properties and, furthermore, significantly minimizes mounting expenditure.

The aim of the presented study is to investigate the influence of the different σ-phase morphologies on the mechanical, wear and corrosive behaviour of the duplex stainless steel 1.4462 (X2CrNiMoN 22 5 3). Different amounts of σ-phase were precipitated in tensile, impact strength, wear and corrosion samples by annealing at 750 °C, 850 °C and 938 °C. σ-phase shows a detrimental influence on the impact strength and the corrosion behaviour. To a very limited extent it can be used to slightly raise the strength under quasi-static load and to improve the wear resistance. Finally, it can be concluded that σ-phase has more unfavourable influences on the material' behaviour than advantageous ones. Therefore, it should be avoided in duplex stainless steels.

Cathode material for Li-ion batteries can be synthesised by r.f. magnetron sputtering of LiCoO 2 targets in a pure Ar plasma. This technique is suitable for large-scale implementation in foil coating set-ups. By choosing the process parameters and by employing post heat treatment nanocrystalline, stoichiometrical LiCoO 2 films can be fabricated which exhibit the desired high temperature phase. The determination of the elementary composition is possible by optical emission spectroscopy including plasma stimulation and carrier gas temperature extraction. The proof of crystal structure is carried out by X-ray diffraction and Raman spectroscopy. Heat treatment can be conventionally realised in a furnace or by laser impact. With regard to increasing the power density, the surface of the cathode material can be enhanced six-fold by laser-assisted surface patterning.