Correlative Chemical Metrology

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The interaction of surfaces is fundamental to how we live our lives. How surfaces behave when they move against each other as part of engineered components in machines depends to a large extend on what they are made from and how they are made. This is important because surfaces made from the wrong material or the wrong surface finish could cause the component to fail before it was designed to do so. This leads to a cost penalty for repair and replacement but there is also the additional energy wastage incurred as the material needs to be recycled and re-made. Thus measuring surface composition and roughness is quite important but to do so accurately involves lots of different scientific techniques which can make it time consuming and expensive. It would be much better from a sustainability perspective if surfaces could be measured quickly and accurately when they were being made at a relatively low cost to give information about both their composition and roughness.This project aims to try and achieve this goal by combining two scientific techniques into one sensor measurement. Roughness is often measured by tracing a diamond tip across a surface to measure differences in height. Diamond is a good material for doing this as it is very hard and is not easily damaged when in contact with surfaces. Diamond is an insulator but this can be changed if it is doped with boron. This makes the diamond conductive and means we could potentially use a technique called electrochemical impedance spectroscopy, or EIS, to measure changes in the contact resistance at different AC frequencies (called the impedance) between the diamond and an engineered surface like a steel. The impedance is likely to change as the probe moves across different parts of the steel structure, for example, it will probably be different for the iron part of the steel compared to a part that has lots of carbon in it. This means we might be able to correlate the high and low points of the roughness to the different materials phases of a surface.It will be quite challenging to achieve this as EIS take several minutes to scan all the AC frequencies, but measuring the topography only takes a few seconds. The influence of vibrations, thermal drift and relative humidity will need to be taken account of when the measurement is performed. The data will be collected from a very sensitive nano-indentation machine that uses capacitance plates to provide very accurate data of surface positions. The amount of water in the air when the measurements are taken will be controlled with a chamber than can be filled with dry nitrogen. This is because water in the air will desorb near the probe when the measurements are made and could allow impedance of ambient surfaces to be measured.If the technique were to work it could be very useful across a number of different sectors. These include manufacturing, where it might be used as a quality control device, checking that manufactured components have been made to the correct surface roughness and that no contamination of the surface is present. When some manufacturing processes go wrong they sometimes 'burn' or oxide the surface and this new sensor might be capable of detecting that before a human notices. Other sectors that could benefit would be the chemical industry, especially the catalysis sector, where the surface area of different catalytic species could be correlated to surface height, allowing optimisation for particular applications. This approach would also be relevant to engineering components that experience sliding or rolling contacts as the technique could determine how surfaces change in response to damage accumulation. This could be from both a surface engineering design optimisation point of view or indeed as a condition monitoring approach, where the surfaces are measuring in-situ within their application environment to warn of potential problems developing during operation.