MAX tools for glass forming

Molding materials with adjustable coefficient of thermal expansion for glass forming

Motivation

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 predominantly used as the molding material. Tungsten carbide is characterized by high temperature resistance, hardness and thermal conductivity. Furthermore, it can be processed into forming tools with mirror-like surfaces by ultra-precision grinding. However, tungsten carbide has a crucial shortcoming: the coefficient of linear thermal expansion (CTE) difference of about 4∙10-6 K-1 is significantly lower than the CTE of most optical glasses. During the cooling phase of precision molding, so-called "glass shrinkage" occurs due to the CTE difference, resulting in a shape deviation between the molded lens and the mold, as well as critical stress fields in the glass. Since these phenomena can only be influenced slightly on the process side, the CTE difference between glass and mold limits the application range of precision molding.

Objective

In this research project MAX-phase composites for glass forming are tested. The CTE of the MAX phase Ti3SiC2 selected for this purpose is 9.1∙10-6 K-1, which is in the range of optical glasses. The introduction of silicon or titanium carbide increases the elongation at break, hardness and compressive strength compared with the pure MAX phase, but at the same time reduces thermal expansion. In particular, the thermal expansion coefficient of the composite is to be adjusted via the filling ratio.

Project contents

The composites are prepared by field-assisted sintering (FAST) in two different routes:

(i) use of commercial Ti3SiC2 powders. (ii) the in-situ formation of Ti3SiC2.

It is systematically investigated how the initial particle size, the carbide phase filling degree and the sintering parameters influence the thermal expansion, the thermal conductivity, the microstructure and the machining process. At the project partner, ultra-precision grinding is being applied to a material that has not been used for glass forming to date. Through knowledge-based and model-based process design, the influence of kinematics, diamond grain size, bond type, feed rate, cutting speed and depth of cut on the surface integrity of the composites will be investigated.

Partner

Machine Tool Laboratory WZL of RWTH Aachen University

Funding

Funded by German Research Society