Researchers investigating a sample of the hard coating in the transmission electron microscope.
TU Bergakademie Freiberg/D. Müller
Researchers of TU Bergakademie Freiberg cooperate with the Dresden Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) and the Czech branch of the Dormer Pramet Company on the development of novel hard coatings for sustainable use in cutting tools. By working on the nanoscale, they modify the interfaces between individual thin layers in the coatings in order to improve their adhesion. When the drill tips are covered with the innovative coatings, the tool life for drilling, turning or milling can be extended by up to 30 percent.
The examined drill bits consist of stacks of two extremely thin but hard layers that are manufactured in a high-temperature reactor by the industrial partner Dormer Pramet. The top layer consists of aluminum oxide, the underlying layer of titanium carbonitride. When the thin film stacks are used as protective coatings on the inserts in cutting tools, they reduce the wear and enhance the service lives of the insets. Such cutting tools are used in particular for high-speed machining of metals, where high temperatures develop. The main problem of the thin film stacks are the inner interfaces between individual layers. Professor David Rafaja, materials scientist from TU Bergakademie Freiberg, explains: "Due to the different crystal structures of titanium carbonitride and aluminum oxide, the counterpart materials do not fit perfectly onto each other. They do not interlock."
During the drilling process, cracks appear in the aluminum oxide top layer and this peels off. Without the top layer that prevents the oxidation of the titanium carbonitride, the tool parts corrode and the cutting and drilling inserts must be replaced. "If you want to ensure that the tools can stay longer in operation, you must first understand how the interface between the two layers, each being a few micrometers thick, is built-up and why the individual layers are not ideally joined," Professor David Rafaja adds.
Interface studies at the nanoscopic level
The team led by Professor David Rafaja examines the formation of the intermediate layer more closely – at the atomic level. At the interface between the two layers, transition phases and nanoscale structures form, which allow the two materials to interlock like in a zipper, but on the atomic level. The researchers can manipulate the formation of these transition phases through chemical reactions: By adjusting the proportions between the starting materials and the pressure and temperature in the reactor, the team achieves a better compatibility of the crystal structures and a better adhesion of the layers at their interface. In addition, using this approach the researchers can tailor the strain in the crystal lattices. The modified intermediate layers, which are produced under laboratory conditions at the IKTS, serve as a barrier for crack propagation and simultaneously as a diffusion barrier.
