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

精密成型是利用高精度工具形成玻璃毛坯，实现非球面透镜高效生产的一种成熟技术。碳化钨（WC）主要用作成形材料。碳化钨具有耐高温、硬度高、导热性好的特点。此外，还可以通过超精密磨削加工，形成镜面状的刀具。然而，碳化钨有一个决定性的缺陷：热膨胀线性系数（CTE）与大多数光学玻璃（冠状玻璃，重燧石玻璃）的CTE有很大不同。在精密成型的冷却阶段，CTE的差异会导致所谓的“玻璃收缩”，导致成型透镜和工具之间的形状偏差，并可能导致玻璃中的临界应力场。由于这些现象只能在工艺方面受到轻微影响，因此玻璃和工具之间的CTE差异限制了精密成型的应用范围。本研究项目对用于玻璃成形的max相复合材料进行了测试。为此选用的max相Ti3SiC2的CTE为9.1∙10-6 K-1，在光学玻璃的范围内。碳化硅或碳化钛的加入提高了Ti3SiC2复合材料的断裂伸长率和抗压强度。复合材料是通过场辅助烧结（FAST）生产的，其中有两种途径：(i)使用商业Ti3SiC2粉末和（ii）原位形成Ti3SiC2。系统地研究了碳化物相填充程度和烧结参数对合金热膨胀、显微组织和加工工艺的影响。在这个研究项目中，超精密磨削被转移到迄今为止尚未用于玻璃成型的材料上。采用基于知识和模型的工艺设计方法，研究了运动学、金刚石晶粒尺寸、结合剂类型、进给速度、切削速度和切削深度对复合材料表面完整性的影响。科学的挑战是确定一个合适的工艺窗口磨削低裂纹和无缺陷的表面粗糙度值取决于材料的性质。

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.