Under the skin of polishing - from nano to macro

This proposal brings together experts in complementary areas of physics, chemistry and engineering, to explore new science with potentially high practical impact.

Processing glass and similar materials to precise, polished surfaces is the "hidden gem" behind many products and services we take for granted - both in precise control of the distribution of light (e.g. anti-glare headlamps), or to focus light in imaging. From medical X-ray cameras to satellite optics, precise, smooth surfaces are required, with surface errors but small fractions of a micron (maybe 1/1000 the width of a human hair), with roughness down to a few atoms. Also, highly localised defects can scatter light, reducing contrast, or lead to component failure in high-power laser applications.

Polishing 'rubs' surfaces to remove damage from prior hard-grinding, and then controls surface-contours to meet design requirements. Historically, these steps were performed by highly-skilled craftspeople, who are in ever-shorter supply as they retire. Modern CNC machines now take much of the drudgery out, but even so, multiple polish/measure cycles are needed to reach refined levels of quality. The basic reason is that, after some 400 years of optical manufacture, the underlying 'rubbing' processes are still far from perfectly understood.

A practical setup typically deploys some kind of rotating tool, fed with a liquid slurry containing a fine abrasive powder. The tool moves over a glass surface, often with complex contours. Details of fluid-flow at the microscopic level between tool and glass are complex, and control local interactions of individual abrasive particles with the glass. Then, at the atomic ('nano') scale, chemical-attack, plastic-flow and brittle-fracture perform a complex 'dance', controlling how material is removed.

Prior work at various institutions has tended to focus on fluid flow OR nano-scale removal, representing distinct disciplines. But, modelling fluid-flow alone (computational fluid dynamics) omits chemistry and fracture-mechanics. Conversely, nano-scale molecular dynamics omits important fluid-flow issues. What nobody has done before, as we propose, is to combine these distinct approaches, supported by real-time process-monitoring data, and high-performance computing. Then CFD can provide molecular dynamics with predicted particle-trajectories, and particles in CFD can be treated as chemically-reactive rather than inert. The models can then by brought together in a unified large-scale and predictive macro model of removal-processes. Often, scientific breakthroughs arise at the INTERFACES between disciplines - precisely where this proposal focusses.

This model will be further developed through polishing trials of complete surfaces, drawing on real-time process-data to predict removal, and post-process measurement of what material has been removed where, plus any defects. This promises to reveal how a surface progresses in real-time, when it is smothered with slurry and invisible to direct inspection. Processes can then be tuned 'on the fly' to keep removal on-target, and improve accuracy of the result. Our aim is then to reduce the number of process cycles required, and give insight into why defects arise and how to control them.

In implementing the above, the mathematical and computer models developed at nano, micro and macro scales will describe fundamental aspects of molecules and fluids. This will be generally applicable, including different materials and abrasives. Another important application arises where the methods could be transformative - processes underlying materials wearing in mechanical systems (bearings, slide-ways, human joint-implants etc). So, what starts out as fundamental research into "intentional wear" in processes such as polishing, promises to have a profoundly significant impact on our understanding and control of "incidental wear" in things that rub - and wear-out - in everyday life!