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Figure 1: Image courtesy of Cranfield University.
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Figure 2: Image courtesy of Oltmann.
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Figure 3: Image courtesy of A Meier.
Chris Young, Micro PR & Marketing
As the European Society for Precision Engineering and Nanotechnology (euspen) prepares itself to host a Special Interest Group meeting on the subject of Structured and Freeform Surfaces on 9-10 November 2016 at the Technical University of Denmark in Copenhagen, this article looks at this niche of precision engineering, and touches on the key issues and opportunities that exist for manufacturers.
Let’s start with the basics.
When analysing structured and freeform surfaces, what we are looking at in broad terms is the manipulation of surface features and shapes on a micro and nano-scale to add functionality to products and components.
As with any ‘new’ concept in any line of science and technology, there is a huge amount of debate centred around definition, equally as much in fact as is centred around the possibilities, opportunities, and pitfalls that surround technological advancements.
When looking at ‘structured’ surfaces, definition is relatively simple and generally agreed. Regularly structured surfaces are usually characterized by small-scale features, which are repeated with a specific periodic pattern on the surface of interest.
The definition of a freeform surface is a little more complicated.
For some people, a standard aspheric lens surface is seen as a freeform surface. But for others, a freeform surface is one that it is impossible to create by sweeping a 2D curve around an axis or sweeping it along a straight line. The argument goes that such surfaces having been possible through the use of processes such as diamond turning for a long time before the term freeform was even invented.
It seems as if the term ‘freeform’ was introduced to describe surfaces that couldn't be created by such techniques. A freefrom surface therefore is one that is significantly more difficult to design because it has more degrees of freedom than a swept or rotated 2D curve, and is a surface that requires modern manufacturing methods rather than conventional ones in order to fabricate.
Either way, unlike conventional surfaces, freeform surfaces have no axes of rotation, and in the future could almost have any designed shape. Their geometry cannot be described by a single universal equation (as is the case for example with aspheric surfaces), but a myriad of equations.
Freeform in Nature
Much of the more accessible news surrounding freeform surfaces centres around attempts to copy naturally occurring surface features in order to enhance the functionality of products, often referred to as biomimetics. The surface of many plants and the skins of animals have intricate microscale surface features that give rise to properties such as directed water repellency and adhesion, camouflage, etc… Here are a few examples.
Sharkskin-inspired swimsuits were much in the news in 2008 during the summer olympics, since when they have been banned as they give unfair advantage to competitors. Magnified, sharkskin is made up of numerous overlapping scales called “dermal denticles” which have grooves running down their length in line with water flow. These grooves disrupt the formation of eddies, or turbulent swirls of slower water, making the water pass by faster. The rough shape also discourages parasitic growth such as algae and barnacles.
It is now possible to replicate dermal denticles on swimsuits and also for the bottom of boats. When commercial cargo boats can save even a tiny amount in terms of efficiency, less oil is burned, and therefore they become more efficient and less polluting. Researchers have also adapted this technique to create surfaces in hospitals that resist bacteria growth — the bacteria being unable to stick on the rough surface.
Next is the lotus flower which has a micro-rough surface that repels dust and dirt particles, keeping its petals clean. Close up, a host of protuberances can be seen that can fend off specks of dust. When water rolls over a lotus leaf, it collects anything on the surface, leaving it clean. A paint has now been developed with similar properties. The micro-rough surface of the paint repels dust and dirt, diminishing the need to wash the surface that has been painted.
Another example is associated with the Stenocara beetle’s water collection attributes. The beetle lives in a dry desert environment, and is able to survive due to the unique design of its shell. The Stenocara's back is covered in small, smooth bumps that serve as collection points for condensed water. The entire shell is covered in a slick, Teflon-like wax and is channelled so that condensed water from morning fog is funnelled into the beetle's mouth. Scientists have been able to build on a concept inspired by the Stenocara's shell and have crafted a material that collects water from the air more efficiently than existing designs.
Altering Product Functionality.
With the above examples of freeform surfaces inspired by nature, and all other freeform and structured surfaces, the ability to deterministically alter the topographic structure of a surface can have a profound effect on how that surface functions. Whilst much of the work in this area is at the research stage, the number of products that include some form of surface structure control is growing rapidly.
Structured and freeform surfaces have numerous applications ranging from optics to automotive, from aerospace to biomedical, and from micro-fluidics to power generation. The key feature that determines a structured or a freeform surface is that its topography is not just an artifact of the process used to generate the surface, i.e. it has been engineered for a specific function.
Thus, for a structured surface, typical parameters such as Ra do not adequately characterize its properties. A freeform surface can have a topography that significantly departs from a standard geometric element and thus conventional metrology methods tend not to be adequate. For these reasons, such surfaces are a challenge to manufacture and to measure However, their function is by definition profoundly affected by their geometrical characteristics.
The scale of both the challenge and the opportunity provided by structured and freeform surfaces make it a fertile area for discussion and research, and hence the fact that euspen runs a series of Special Interest Groups on this topic.
Metrology Concerns
In the world of precision, micro, and nano manufacturing, there is one homily that underlies everything that designers, researchers, and manufacturers do, — “if you can’t measure it, you can’t make it!”
Making micro parts and components — or larger parts with micro and nano features, — the pressure is constantly on sophisticated metrology technologies to keep up. Nowhere is this more the case than in the area of structured and freeform surfaces.
Over recent years, there has been a paradigm shift in the discipline of surface metrology, driven in large part by the growth in nanotechnology and the dawn of the next generation of ultra-precision surfaces.
Advances in nanotechnology have brought into clear focus the issues of the ability to measure and qualify commercial micro and nanometer scale manufactured components. As the dimensions of a product become smaller, the importance of the surface and its properties become the dominant factor in the successful functionality of that product. Measurement of surface texture is becoming increasingly critical because of its direct link to part functionality. Manufactured items such as micro- and nanometer scale transistors, micro electro mechanical systems (MEMS), and nano-electro mechanical systems, optics, and structured surface products are clear evidence of products where the surface plays the dominant role.
The next generation of ultra-precision surfaces will not only be very smooth, but will also have the specification of surface form at levels approaching atomic magnitude. These surfaces will relate to a wide range of devices and components, including micro-electronic devices (where semi-conductor surfaces have to be extremely flat in order to pack more and more transistors and other microscopic components into the same area), optics in ground- and space-based telescopes, in defence and satellite-based imaging systems, and in large laser facilities, where smooth surfaces with complex optical shapes are required with exceptional precision. Similar accuracy is also required in implantable medical devices such as hip and knee joints, where micrometre form and nanometer roughness requirements are specified in order to reduce the generation of wear debris.
When it comes to freeform surfaces made by one or a combination of ultra precision multi-axis freeform machining a very real difficulty is how to measure such surfaces with the required sub-micrometre form accuracy and sub-nanometre surface texture.
As a consequence of the geometrical complexities of freeform surfaces, the ability to achieve a good freeform surface depends to a great extent on the prowess of machine operators and trial and error. However, there are measurement and characterization techniques for freeform surface constantly under development, although there is still no mature traceable measurement methodology for the complete range of complex geometric surfaces. Instead, traditional measurement techniques ranging from contact to non-contact and from points to areal measurement are partially used for freeform surface measurement,
Fabricating Processes
Various replication processes exist that are used in the manufacture of structured and freeform surfaces such a imprinting and embossing, and an area of very productive research is also centred around the use of additive manufacturing techniques.
Imprinting processes have attracted increasing attention for their potential as manufacturing techniques for micro- to nanometre-scale features especially in polymers. Such processes include hot micro-embossing, nano-imprint lithography, UV-embossing, and others. Broadly, these processes involve the use of a tool to replicate micro- to nanometer-scale features in a substrate.
Replication fidelity is often high, so the geometry, surface texture, and other characteristics of the finished part are largely determined by the tool. Tooling is a critical factor for the overall success of imprinting processes, not only in terms of the quality of individual parts, but also for realizing the potential of imprinting as a mass-manufacturing technique.
Each tooling material and production technique has unique fundamental limits in terms of minimum feature size, aspect ratio, surface finish, and other attributes. These limits place additional constraints on production quality and tool durability. In many cases, however, these constraints are not yet known.
Imprinting techniques have demonstrated potential for manufacturing of polymer-based devices with features at the micro/nano scale. In order to achieve this potential on a mass scale, fundamental issues such as production rate and quality, must be considered. Imprint tooling strongly influences the characteristics of the final product, and the production and quality of tooling is a vital concern for micro- and nano-manufacturing.
In the area of additive manufacturing, selective laser melting is a technique that potentially enables new, complex-shaped geometries. It allows for the building of prototypes of freeform products from polymer-based materials with functional surfaces to which coatings can then be applied. Alternatively, SLM can create aluminium-based prototypes that do not require coating. Both reduce the manufacturing time and the carbon footprint of complex-shaped components.
SLM has been found to be an efficient method for the fabrication of optical components, with a carbon footprint that compares favourably with that of conventional fabrication techniques. Although SLM makes it relatively easy to generate a rigid structure that serves as a mounting base for additional parts, the requirement for a high reflection grade is a serious challenge.
In the area of microfluidics — as another example — the search for economically viable micro-fabrication technologies to replace the more commonly used glass etching or injection moulding processes are moving on apace. Roll-to-roll production as a high volume process has become the emerging fabrication technology for integrated, complex high technology products recently. Differently functionalized polymer films enable researchers to create a new generation of lab-on-a-chip devices by combining electronic, microfluidic and optical functions in multilayer architecture. For replication of microfluidic and optical functions via roll-to-roll production process competitive approaches are available. One of them is to imprint fluidic channels and optical structures of micro- or nanometer scale from embossing rollers into ultraviolet (UV) curable lacquers on polymer substrates.
Structured and Freeform Surfaces — SIG
euspen’s Special Interest Group focused on structured and freeform surfaces covers all the key areas of research and development in the area of metrology and replication processes associated with complex surfaces. The next event will be held 9-11 November 2016 in Copenhagen, Denmark.