A Multiscale Modelling Approach to Engineering Functional Coatings

Lead Research Organisation: Imperial College London
Department Name: Dept of Chemistry

Abstract

Surface coatings can be used for many important engineering purposes e.g. the protection of substrates from adverse environmental conditions or for specialist optical or electronic applications. Of the various methods available, magnetron sputtering is a process in which the UK is a world leader. The closed-field pulsed magnetron technique is especially useful for producing coatings with excellent film properties and nanoscale thickness control at high deposition rates. For example the industrial process is able to deposit micron thick layers with an RMS roughness of less than 0.5 nm while maintaining high throughput. Due to the precision of the technique, the process has many potential applications in the field of multi-layer optical coatings such as anti-reflection coatings on spectacle lenses, UV and infra-red blockers, lighting filters or conductive oxides for flat screen displays, solar coatings on windows. Besides the more traditional hard coatings for engineering applications. The deposition process is complex. It can involve both physical and chemical processes, spans length scales from a few ?ngstoms up to hundreds of nanometres and time scales from femtoseconds up to minutes. The optical and mechanical properties of the coatings require analysis over similar length scales. As a result, modelling of the processes involved requires a true multi-scale approach both in length and time.The project seeks to understand the deposition and characterisation process of thin film optical materials to determine the important parameters that control the process. There will be an association experimental programme that will use the results of the models to optimise the deposition process and to quantify and standardise the experimental methods used for characterisation of the mechanical properties of the films. To determine the mechanical and optical properties of films requires a knowledge of their bonding arrangements. For example different structures of the oxide TiO2 have different optical properties so it is important to make sure that the deposition process is optimised to give the correct structure. Using state-of-the-art quantum mechanical calculations allows these structures to be related to optical properties. Window glass upon which a solar coating has been deposited has often to be transported over bumpy roads so it is important that the surface does not scratch. Scratching can occur by failure of one of the layers in the coating and a knowledge of the bonding arrangements at the interface can help understand how to stop this failure. These are examples where sub-nanoscale phenomena have an important effect on the practical production of materials used in everyday life and where our modelling can help solve important practical problems. To test the mechanical properties of the films, modern devices of nanoindentation and nanoscratching are used. These examine the mechanical properties of materials over scales of 10's or even 100's on nanometers and full quantum mechanical calculation to understand these processes is too computationally costly. Here we intend to use the quantum calculations to determine interatomic potentials and forces that can be used in a classical particle simulation code which itself will be linked to a finite element calculation, thus bridging length scales of many orders of magnitude. Growth of the thin films occurs at a rate which deposits material at the rate of around 1 monolayer per second. This is a slow process on the atomic scale and so fast techniques will be used computationally to accelerate the growth process and include diffusion within a particle simulation model. With these techniques disparate time scale that vary from picosecond to minutes can be bridged.

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