Protective coatings in the field of high-performance components require high resistance against mechanical impacts. These impacts are typically divided in two big groups – mechanical impact in the range of the ultimate yield strength (or above) and cyclic loads way below any critical stress state (important for fatigue mechanisms). Within this project, we investigate in more detail the first loading scenario, meaning mechanical impacts with a high stress level, especially focusing on erosion (Domestic Object Damage). For erosion resistant coating materials, different coating parameters are relevant such as hardness, Young’s Modulus, or fracture toughness (just to mention the most important ones), which need to be balanced. In addition, the influence of increasing temperatures is also an important point (change of H, E, KIC - creep resistance, oxidation resistance). Especially, for the development of new coating materials for such applications a basic understanding between the bonding nature, mechanical parameters, and the application is highly important.
Figure 1: Typical hexagonal α-TMB2 supercell used as input for theoretical DFT and Lobster calculations.
First of all, the design of the coating material starts at the atomic scale range using atomistic modelling methods to gain a deeper insight on the bonding nature of novel coating materials. Density Functional Theory (DFT) using the Vienna Ab-Initio Simulation Package (VASP) is a common tool for the screening of various materials system. Moreover, to get a deeper insight on the bonding nature we use the Local Orbital Basis Suite Towards Electronic-Structure Reconstruction (LOBSTER) which is a rather new technique for calculating crystal orbital overlap populations (COOP) quantifying atomic bondings.
Figure 2: DC magnetron sputtered WB2 and CrB2 coatings used for solid particle erosion tests.
In a second step, the most promising material systems are synthesized by DC magnetron sputtering but also arc evaporation in a lab-scale environment and characterized in detail to verify the theoretical predictions. Techniques like nanoindentation, X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), white light interferometry (profilometry), energy dispersive x-ray spectroscopy (EDX) and atom probe tomography (APT) will be conducted to extend the understanding of the investigated material systems.
Figure 3: Air jet erosion tester used for lab-scaled solid particle erosion (SPE) tests.
Furthermore, on a lab scale range, these coating materials are loaded with application related environments. By means of solid particle erosion tests of selected material systems the (fracture and erosion) resistance against impacting particles (SPE) is investigated. The facilities make it possible to generate realistic testing conditions by adjusting temperatures up to 1000 °C. The gained results will give further insights into erosion behavior and connected mechanical properties and the possibility for improvements.
Thermomechanical fatigue phenomena strongly limit the life time of modern high-performance components and therefore demand innovative coating materials extending the life-time through predictable crack propagation. Especially, an in-depth knowledge on the decisive failure criteria of ceramic thin film materials – generally associated with an intrinsic lack in ductility – under long-term mechanical loading is paramount to enhance limited bulk material properties. Literature reports on fatigue resistance, especially of hard coatings but also thin films in general, are relatively rare . Thus, an extensive analysis of different coating classes and architectural designs with respect to fatigue phenomena (e.g. LCF, HCF, or extrusion formation) is of great interest.
This work focusses on novel methodologies for the investigation of the fatigue properties of PVD deposited thin film materials. Using different approaches on several length scales – from the nanometre to the millimetre scale – the key aspect sits on a more general understanding of the failure criteria of ceramic- and metal-based thin films under cyclic mechanical and thermal loads. Using FIB prepared micro-geometries, fatigue testing in combination with in-situ synchrotron X-ray diffraction experiments will be employed to characterise the fatigue properties of coatings from the aspect of changing bond states – i.e. altered ratio of ionic, covalent, or metallic bonds – and altered growth morphologies. These results are also correlated with dynamic-mechanical analysis to obtain further insights on possible size-effects and the behaviour of the system coating-substrate in general. This comprehensive approach should identify the most critical aspects with respect to fatigue life of protective coating materials.
Figure 1: Evolution of mechanical fatigue testing of PVD deposited thin film materials through several length scales: from FIB prepared micro-geometries, to dynamic-mechanical analysis of the coating-substrate system, to application related testing. Furthermore, the extension towards thermo-mechanical fatigue testing through coupled high-temperature fatigue testing is presented.
 X. Luo, B. Zhang, G. Zhang, Fatigue of metals at nanoscale: Metal thin films and conductive interconnects for flexible device application, Nano Materials Science. 1 (2019) 198–207.
On the strive for higher efficiencies and environmental sustainability materials which sustain highly demanding environments including high temperatures are needed. If such components are operated at the desired high temperatures in air, the destruction through oxidation becomes very crucial. Therefore, the goal of this project is to develop coatings which protect the component against oxidation at high temperatures above state of the art, 1100 °C and higher.
Figure 1: Schematic of a substrate-coating-oxide scale stack with indicated diffusion processes.
For the coating synthesis we are using Physical vapor deposition (PVD) techniques as there the design capabilities are nearly unlimited. Afterwards the coatings are oxidized in a combined DTA/TG system or a conventional furnace. Applying high resolution techniques, we then yield for a profound insight into the scale formation and diffusion processes in order to be able to further slow down the oxide scale growth and enable long time protection at high temperatures. The experimental work is further assisted by DFT calculations applying MD-VASP to study the initial stage of the oxide scale formation.
Figure 2: MD-VASP simulation cell
Diffusion driven mechanisms govern crucial processes within physical vapor deposited (PVD) thin film materials and have an influence on every aspect related to thin films synthesis, properties and performance. Compared to bulk solids, diffusion in PVD thin films differs due to the higher density of defects as a consequence of the low temperature growth from the vapor phase. These structural defects are expected to provide fast routes for diffusion of species within the films, as they are in close proximity to interfaces or directly connected to surfaces (e.g. columnar boundaries). As a result, the morphology and microstructure of the synthesized film will highly influence the performance of the coating materials in various applications where diffusion driven processes are dominant. Prominent examples for such processes are oxidation and corrosion in high temperature environments where the diffusion of attacking species lead to film degradation. Additionally, nowadays we have increasing demand on applications involving hydrogen storage and production where new barrier coating materials are need to be developed to inhibit hydrogen diffusion [1, 2].
Figure 1: Schematic figure for diffusion mechanisms in PVD thin films
Understanding the correlation between active diffusion mechanisms and the morphology of the films is crucial for tailoring and developing new coating materials with desired properties for energy related applications. By combining advanced high-resolution techniques such as atom probe tomography (APT) which can provide 3D chemical mapping at the atomic level with transmission electron microscopy we could gain in-depth knowledge of the diffusion pathways in the film microstructures. Additionally, we employ atomistic modelling for studying diffusion in thin films by performing ab initio theoretical calculations based on density functional theory (DFT) to predict and investigate the incorporation and diffusion characteristics of non-metal species (e.g. O, H) along grain boundaries for different material systems.
- Zhou, T., et al., Microstructure and hydrogen impermeability of titanium nitride thin films deposited by direct current reactive magnetron sputtering. Journal of Alloys and Compounds, 2016. 688: p. 44-50.
- Tamura, M. and T. Eguchi, Nanostructured thin films for hydrogen-permeation barrier. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2015. 33(4).