Mold materials with adjustable coefficient of thermal expansion for precision glass molding

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Precision molding is an established technology for the efficient production of aspherical lenses by forming a glass blank with high-precision tools. Tungsten carbide (WC) is mainly used as the forming material. Tungsten carbide is characterized by high temperature resistance, hardness and thermal conductivity. Furthermore, it can be processed by ultra-precision grinding to form tools with mirror-like surfaces. However, tungsten carbide has a decisive deficit: The linear coefficient of thermal expansion (CTE) differs significantly from the CTE of most optical glasses (crown glasses, heavy flint glasses). During the cooling phase of precision molding, the difference in CTE causes so-called "glass shrinkage", which results in a form deviation between the molded lens and the tool and can cause critical stress fields in the glass. Since these phenomena can only be influenced slightly on the process side, the CTE difference between glass and tool limits the application range of precision molding.In this research project MAX-phase composites for glass forming are tested. The CTE of MAX-phase Ti3SiC2 selected for this purpose is 9.1∙10-6 K-1, which is in the range of optical glasses. The addition of silicon or titanium carbide increases the elongation at break and the compressive strength of the Ti3SiC2 composite. The composites are produced by field-assisted sintering (FAST), whereby two routes are followed: (i) using commercial Ti3SiC2 powders and (ii) the in-situ formation of Ti3SiC2. It is systematically investigated how the filling degree of the carbide phase and the sintering parameters influence thermal expansion, microstructure and machining process. In this research project, ultra-precision grinding is transferred to a material that has not been used for glass forming to date. Using a knowledge-based and model-based process design, the influence of kinematics, diamond grain size, bond type, feed rate, cutting speed and cutting depth on the surface integrity of the composites will be investigated. The scientific challenge is to identify a suitable process window for grinding low-crack and defect-free surfaces with low roughness values depending on the material properties.