Designing toughening concepts for future hard coatings

Ceramic thin film materials are typically used to protect tools and structural components from harsh environments; they act as diffusion barriers in microelectronics, and are also used in many other applications, like energy storage and conversion. An important issue of this kind of materials is the fatally low resistance to crack extension, which limits the performance in existing applications and barriers new ones. In this project, we will develop advanced concepts for increasing the fracture toughness of hard coating materials. To this end, we will make use of the metastable character of artificial nano-scale materials (e.g., cubic aluminium nitride) and study their potential to act as a toughening agent based on martensitic phase transformation in the vicinity of the crack tip. While phase transformation toughening is effectively used to toughen certain bulk ceramics, metastable nano-materials have been investigated only rarely in this context. The other material system, namely ceramic titanium nitride with metallic aluminium interlayers, is inspired by nature: nacre is built up of stiff layers alternated with thin compliant layers allowing for a high fracture toughness. The synthesis method chosen in the project (physical vapour deposition) enables to mimic such bio-inspired design strategies, which have been optimized over many years. As the dimensions of the film materials are confined to typically 1 to 5 micrometre in film growth direction, state-of-the-art preparation, accurate miniaturized testing, and microscopy techniques are required to quantify the fracture toughness and to understand fundamentally the proposed toughening mechanisms. The complementary expertise of the multi-institutional DACH consortium (TU Wien, FZJ, KIT, MPIE) will serve as a strong basis to break new ground in this field of research.

Ceramic thin-film materials are used to protect tools and components from harsh environmental conditions. They serve as wear-resistant coatings, act as diffusion barriers in microelectronics, and are also applied in areas such as energy conversion and storage. One of the major weaknesses of these materials is their low resistance to crack propagation (low fracture toughness). This limits their performance in existing applications and prevents the exploration of new areas of use. In this project, new concepts were developed to improve the fracture toughness of hard coatings. One focus was on understanding the microstructure. We were able to show that cracks tend to propagate along the boundaries between tiny crystalline regions, known as grain boundaries. However, if the microstructure is designed in such a way that cracks cannot propagate in a straight line but are instead deflected, their advance can be significantly slowed. A second focus was the further development of micromechanical testing methods. Using so-called bridge notches in micro-cantilever specimens, we were able to measure fracture toughness under defined conditions and demonstrate how artifacts caused by preparation with high-energy ion beams can be avoided. These methodological improvements significantly enhance the reliability and comparability of such miniature-scale tests. We also investigated special materials whose internal structure can change under stress - a principle that could theoretically contribute to slowing down crack propagation. Using cubic aluminum nitride as an example, we were not yet able to clearly demonstrate this effect, but we have laid important groundwork for further research. Through close collaboration within an international research network, various areas of expertise were brought together. Our results contribute to the establishment of new design principles for more robust coatings. This can help reduce material usage and extend maintenance intervals in industrial manufacturing and energy technologies.