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Design and Engineering with Hard Materials

The Institute for Materials Applications in Mechanical Applications at RWTH University of Aachen – IWM and the Institute of Applied Powder Metallurgy and Ceramics at RWTH University of Aachen – IAPK had extended an invitation to a colloquium that had been jointly organised by the institutes on 20.09.2022. In machine and plant engineering, hard materials are often associated with the negative characteristic of spontaneous brittle fracture. To enable the use of hard materials despite their low toughness and to utilise their excellent properties, a reliable lifetime prognosis is necessary. What calculation methods and what are the limitations of their application? Under the motto “Design and Engineering with Hard Materials”, at the IWM–IAPK Colloquium, papers on these topics from science and industry were presented. The spectrum ranged from classical structural ceramics and carbides to tool steels.

Papers

Prof. Dr-Ing. C. Broeckmann (IWM–IAPK/ DE), host of the event, was delighted to get Prof. Dr R. Danzer (Montanuniversität Leoben/ AT) on board to present the Keynote on Engineering with Ceramics: Strength and Fracture Statistics. He began by exploring the different failure mechanisms of ceramics (brittle fracture, subcritical, slow crack growth, but also fatigue, creep and corrosion). Linear elastic fracture mechanics can be applied to describe the rapid expansion of cracks in brittle bodies. It should be taken into account, however, that inhomogeneous areas of stress exist. In the field, overload tests are often performed. The factors influencing crack growth are multifaceted (stress intensity, water vapour pressure, temperature, etc.) – it precedes every brittle fracture. The defect with the highest stress intensity triggers it. The cause is often surface defects, but thermal cycling, too. In technical ceramics, over the years, the processes have become cleaner, which has led to more homogeneous and relatively small defects. In the meantime, fracture-statistics-based design methods have been developed and are also applied successfully. Critically, the speaker remarked that there is a lack of “manual” routine to manage the specific challenges (like machining, handling, thermal cycling, edges, surfaces, etc.). Dr-Ing. Atilim Eser (Ivoclar Vivadent AG/LI) spoke on Lifetime Calculation of Dental Ceramics. The Liechtenstein-based dental and medical engineering company Ivoclar registered strong growth in the business year 2021 with sales of over CH 840 million, thanks to new products and further geographic expansion. In product development, “in vitro” and “in vivo” simulation play a key role. That ranges from lifecycle prognosis for restorations on the basis of glass ceramics and zirconium oxide. Linear FEM is used efficiently for ZrO2-based components. With SPT diagrams, it is possible to predict possibly clinical failure rates, and the stress levels to which crowns and dental bridges are exposed during their use. The Calculation and Design of a Ceramic Flat Sliding Valve for Hydraulics was explained by C. Liu (IWM). He came to the following conclusions. Laser-cut ceramic boards adapted to the geometry for flat sliders are not sufficiently fracture-resistant for hydraulic application. Component positions exposed to high stresses should be reworked. Cracks start at highly stressed positions on the Al2O3 slider and the Si3N4 control plate. The calculated fracture probabilities are generally low. Laser structuring increases the bending strength of ceramics and reduces their strength scatter, the “economical” LS68 is a potential method for the future. Dr-Ing. W. Beckert (Fraunhofer IKTS/DE) reported on the Model-Based Design of Ceramic Components and detailed the example of a ceramic spring. Here, precision surface machining has an enormous influence on the quality. The Numerical Design of High- Stressed Ceramic-Metal Composite Components was explained by Dr-Ing. K. Sauerzapfe (Alumina Systems GmbH/DE). The solder geometry is modelled based on empirical values. Material characteristics for soldering metals are simulated as “softer” than datasheet values/measured values for reduction of stress singularities. The metallization layer is not simulated. Fracture probability is used as a design parameter to compare parameter variations. Assessment of design solutions is only possible with comparable cross-linking. The absolute value of the calculated fracture probability cannot be transferred to a real component. Although the simulation results are flawed, it is possible to conduct a comparative assessment of different design variants and obtain an optimised initial geometry for prototype production. Dipl.-Ing. Marc Neumann (IKFVW Freiberg University of Mining and Technology/ DE) spoke on Fracture Behaviour and Subcritical Crack Growth in Flame- Sprayed Al2O3 Materials. The structure of thermally sprayed ceramics exhibits pores and microcracks on account of insufficient crosslinking and internal stresses. Moreover, during cooling, the freezing of metastable phases and amorphous areas can occur. All tested materials showed pronounced Rcurve behaviour and medium-to-high susceptibility to sub-critical crack growth. Dr T. Klünsner (Materials Center Leoben Forschung GmbH/AT) reported on Factors Influencing Deformation and Fatigue of WC–Co Carbides. For productive processes, the criterion “wear before fracture” must be met. Quantitative information on yield and fatigue is important as the yield behaviour influences the fracture behaviour! For carbides, phenomena from the world of both metals and ceramics are relevant! The Influence of Thermally Supported Recompaction on the Strength of Additive- Manufactured WC–Co Carbide was explained by S. Fries (IWM–IAPK). Defects from the PBF-LB process are eliminated in this way. With hot isostatic pressing and sinter-HIPing, the scatter of strength values and internal stresses can be reduced. The Component Design of High- Strength Tools Exposed to Multiaxis Cyclic Loading was discussed by L. Scholl (IWM). Hardly any reliable, quantitative analyses of fatigue and mechanisms in high-carbide tool steels are available. In addition, no validated concepts for multiaxis component and tool design exist. Cold tools are often exposed to disproportionate multi-axis loads. The next IWM–IAPK Colloquium was held in Aachen, Germany, on 23 March 2023 and focused on “Sintering-Based Additive Manufacturing”.


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