Cost-effective temperature adaptive Cu-based shape memory seals through the synergistic effect of co-microalloying and cooling rate control

Most automotive and satellite microactuators (hydraulic, pneumatic, etc) utilize seals to prevent egress of fluids and ingress of dirt, humidity and other extraneous materials. In these dynamic applications the seal is in contact with the rotating surface of the shaft and has to meet certain requirements of maximum friction, wear and leakage over the working temperature range. This temperature normally varies from about -50 to 150 degrees Celsius in vehicles while in satellites the temperature range is wider (-100 to 200 degrees Celsius) depending on whether the surface is exposed to the sun or in the shadow. The main limitation for using shaft seals at a wide temperature range stems from the differences in dilatation with temperature of the shaft and seal. At low temperature (freezing conditions) the seal-shaft friction is high while at high temperature (above 100 degrees Celsius) the shaft-seal clearance becomes so large that excessive leakage may occur and therefore it is of interest to maintain the pressure constant over the working temperature range. Excessive friction and wear would shorten the service life of seals while leakage would result in lubricant loss (i.e., seizure) or lubricant ingress (i.e., extraneous materials could enter and damage engines and other components). At the same time, seals should wear faster than the shaft on which it is mounted since shafts are more costly and difficult to replace than seals. This suggests the need for novel shaft seals with tuned performance over the working temperature range.
In this research programme I will develop a new strategy for overcoming these limitations consisting of using shape memory alloys with tailored reversibility and wear performance through optimal control of the microstructure and composition. The microstructure, i.e., grain size and distribution, will be tuned upon cooling in a single processing step by controlling the cooling rate and composition.
Novel compositions will be developed through multiple minor element co-addition of relatively low-cost elements such as iron and nickel compared to copper to promote the twinning propensity of austenite. Deformation twinning is a small movement of atoms that occurs in a co-operative process in austenite when the material is subjected to a minimum stress value, resulting in macroscopic deformation. Once the material is twinned, and the force applied released, it keeps its deformed shape unless heated up above a benchmark temperature for which the material recovers to the initial position (i.e., detwinning). Some elements have the ability to decrease the energy required for twining and therefore promotes the twinning propensity (i.e., the ease with which atoms move when strained). This enables to control the temperature, and therefore the shaft-seal contact stress, at which martensite transforms into austenite resulting in a smaller seal diameter and therefore reducing leakage. To decrease the cost of the Cu- based shape memory alloys and make them more appealing for the actuator industry than NiTi alloys, low cost minor elements will be employed. Understanding how to tailor the thermomechanical and tribological performance of these Cu-based shape memory alloys is therefore of utmost importance in fundamental physical metallurgy as well as for industrial applications.
Traditionally, the grain size of rapidly solidified materials has been optimized by tuning the composition, through partial substitution of one element by another or by controlling the cooling rate. However, the synergistic effect resulting from optimizing both parameters is novel and has huge potential for tailoring the properties of shape memory alloys. In this regard, as I have previously observed (S. González et al. Sci. Tech. Adv. Mater. 15, 2014), the addition of some minor elements (such as Fe and Co) at certain concentrations can promote a mechanically-driven martensitic transformation of Cu-containing alloys.

1. Knowledge:The development of novel cost-effective materials with optimized microstructures and compositions has the potential to lead to micro shaft seals with enhanced sealing effect over a wide temperature range and with adequate tribological performance. The approach taken here, based on the synergistic effect for tuning the performance of shape memory alloys, has not been studied before and is therefore novel. This knowledge can be useful to better understand the effect of interaction among microelements and grain size effect on the performance of shape memory alloys, which will make a significant contribution to the knowledge in materials science. By combining both phenomenon, a new technique for tuning the mechanical performance will be developed.
This technique will have an impact on improved sealing effect and therefore will diminish the problems associated with excessive shaft-seal friction at low temperature and excessive fluid leakage and ingress of extraneous materials (dirt, dust, humidity, etc.) into the actuators at high temperature (above 100 degrees Celsius) due to excess shaft-seal clearance. The strategy described here can be of interest beyond the development of shaft seals. For example, it could be useful in enhancing the performance of shape memory thermal switches and thermostats in microactuators.
2. Economy:Micro manufacturing, as an area of high-value manufacturing, is one of the priority areas in the UK Industrial Strategy due to its relevance to the economy. One of my industrial collaborators, EDMzone is a precision engineering company that provides a comprehensive range of engineering services and machining capabilities. EDMzone will be in charge of fabricating seals from the bulk material using EDM (Electron Discharge Machining) and therefore the collaboration will lead to new products and procedures that will help provide better service and solutions to the existing and future customers of EDMzone (see letter of support). The enhanced wear resistance and sealing performance of temperature adaptive shaft seals will provide longer autonomy and durability to actuators were they are coupled to, for example, in Formula One and potential future applications such as the satellite industry.
I will also collaborate with the company Technip Umbilical, which is part of Technip group, a world leader in subsea umbilical solutions with a global international team of nearly 1,000 experienced people located in Newcastle upon Tyne (UK) and other countries. This is a multinational group interested in the oil and gas industry with a number of manufacture units for pipeline, flexible pipe and umbilical. The company has multiple facilities for mechanical tests and they will also provide staff time in the form of meetings for evaluation of the project and discuss the tensile tests performed at the company (see Project Partner Letter of Support).
From the partnership with both companies new sources of funding will be sought to further develop and implement the concepts of this project which would increase the attractiveness of UK as a place to do business and ultimately trigger inward investment. Funding will be requested to Innovate UK (i.e., KTP scheme) and from Europe (Horizon 2020).
3. People and Society:The multidisciplinarity of the project, where materials engineering and micro-manufacturing merge, will lead to a high value product that will be beneficial to the industrial fabric of UK. The postdoctoral researcher will be trained in precision manufacturing and in materials engineering, and in collaboration with me, will disseminate the research outcomes gained in this project through popular science journals such as New Scientist and Physics World. The postdoc will also participate in educational outreach activities such as in ThinkPhysics, a project led by Northumbria University (see Pathways to Impact) to increase participation in STEM subjects by working with children in schools, especially girls.