Implementing Lubrication in Micro-Electro-Mechanical Systems

Micro-electro-mechanical systems (MEMS) are tiny (sub-millimetre) machines, which have arisen from advances in semiconductor fabrication. Typical MEMS devices include air-bag accelerometers, gyroscopes in smartphones and implanted drug-delivery meters. The MEMS industry is currently worth around 10 billion dollars. Furthermore, their low cost, high tolerances, and ability to combine sensors and actuators with microprocessors, give MEMS the potential to profoundly affect our way of life. Unfortunately however, high friction and wear means that current commercial MEMS designs are confined to non-, or very low sliding devices. This precludes the possibility of rotating or reciprocating MEMS such as micro-engines. Clearly, there are huge possibilities if this can be changed.

Research efforts to tackle the problem of friction in MEMS have suggested lubrication by liquids and vapours as possible solutions since these can continually replenish protective films on rubbing surfaces. Arguably the most promising has been liquid lubrication as my research on silicon micro-contacts has shown. To date, I have demonstrated the effectiveness of liquid lubrication in model, silicon MEMS-type, contacts but no validation has been carried out on a working MEMS device. The proposed project aims to carry out this validation and thus bridge the gap between lubrication research and the production of a commercial sliding MEMS device. To achieve this, I will collaborate with MEMS manufacturers to produce a micro-journal bearing and incorporate this into a MEMS turbine energy harvester. This is a very suitable application since energy harvesters are a rapidly growing area that would significantly benefit if low friction sliding contacts were possible.

The project will break down the bearing production process into a series of studies, each dealing with a different aspect of lubrication and bearing design. These steps include addressing issues such as lubricant containment, evaporation and delivery, optimisation of bearing geometry and adaptation of fabrication techniques. In addition to the goal of producing a MEMS turbine that runs on hydrodynamic micro-bearings, a number of more fundamental avenues of research, involving tribology and silicon MEMS, will be explored. These include a feasibility study into the development of sliding MEMS with compliant surfaces.

Finally, silicon MEMS technology will be used to enhance my tribology research by i) coupling fabricated silicon components with existing infrared microscopy equipment so that the temperature of rough surface contacts can be imaged - this is possible since silicon is transparent to infrared; ii) coating thin-film piezoelectric sensors onto silicon specimens to monitor lubricant film thickness using ultrasound.

The MEMS industry is estimated to be worth approximately 10 billion dollars and this is forecast to rise to 21 billion by 2017 [1]. The proposed research project has potentially far-reaching impacts since its primary goal is to trigger a step change improvement in the design (i.e. the enabling of sliding contacts) of products in this rapidly expanding technology. Furthermore, MEMS devices are present in a range of mass produced components; for instance most modern passenger vehicles contain approximately 50 MEMS sensors and smartphones typically contain five. This means that significant impact and commercial success would result if this research was implemented just a single device.

A specific example of an impact from the research will arise from the energy harvester device that will be developed to provide a compact, high power density, energy source. This should impact on remote sensor technology (e.g. for structural health monitoring of civil infrastructure), where companies require sustainable, high power density, energy sources to replace batteries.

Another important impact may be the emergence of a market in MEMS lubricants: If my research shows that problems of friction and wear in MEMS can be overcome, then devices relying on liquid lubricants will subsequently reach production. This potentially opens up a market in MEMS lubricant manufacture. Furthermore, my research has shown that such lubricants would require a fundamentally different combination of properties - in terms of viscosity, volatility and additive chemistry - than other commercially available lubricants. As described in the Pathways to Impact document, my knowledge of lubrication mechanisms and additives for silicon components, combined with my MEMS lubricant test facilities and network of contacts, make me uniquely able to take advantage of this opportunity.

Additional, albeit more smaller scale, impacts will result from the development of new tools for tribology research. These include silicon fabricated specimens to enable the temperature of rubbing surfaces to be mapped and thin piezoelectric films to monitor film thicknesses in micro-contacts. These techniques can be marketed as add-ons to existing, commercially available, test equipment being manufactured by project partners PCS-Instruments. Commercialisation will proceed under licence to PCS instruments, a company that currently supplies equipment to all the major oil companies [2]. This process should bring revenue and employment to The College, lubricant companies and equipment manufacturers.

As well as its direct effect on the MEMS industry, this project should spark renewed research activity in liquid lubrication of silicon, once the efficiency the prototype turbine energy harvester is demonstrated. Finally, the increased understanding of liquid/additive/silicon interactions should provide valuable information that can be exploited by researchers and companies developing biomedical MEMS such as in-vitro drug delivery devices.

As well as maximising the impact of current research, it is clearly important to encourage the next generation to follow careers in this area, so that they will subsequently work to improve our future state of health, wealth and culture. The support and training in silicon fabrication, tribological testing and numerical modelling that the research associate and PhD student on this project will receive should have such an effect.

References:
[1] Mounier, 2012 Yole Development report, "MEMS Markets & Applications".
[2] pcs-instruments.com