A L Buxton, B G I Dance
Paper presented at ASM International, ISEC Congress. St Paul, Minnesota, USA, 1-3 August 2005
Surfi-Sculpt TM is a new and revolutionary materials processing technology which uses an electron beam to create a customised textured surface. This paper describes the Surfi-Sculpt technology, discusses the benefits it canprovide and provides an overview of the wide range of application areas.
Surfi-Sculpt is a new and revolutionary materials processing technology recently developed and patented by TWI  . An electron beam (EB) is deflected rapidly over a substrate surface to displace material in a controlled manner. The result is a textured surface consisting of an array of protrusions above the original surface and a corresponding array of intrusions or cavities in the substrate. Surfi-Sculpt can be used to produce high aspect ratio spikes up to 2mm in height, blades, channels, swirls and burr-free holes in a range of metals, polymers, ceramics and glasses, in just a matter of seconds per square centimetre. The size, shape and distribution of the features can all be varied to produce a surface tailored for a specific application. Surfi-Sculpt has instigated much industrial interest and is currently being developed for applications as diverse as biomedical implants, wind turbines and jewellery, but the possibilities are endless!
The use of electron beams to weld components is an established process. The intensity and power of the beam, makes electron beam welding particularly successful in situations where a high degree of penetration is required, for example, to join thick section components. The use of electron beams to texture substrates makes use of another characteristic of electron beams - their manoeuvrability.
Electron beam texturing  is a precursor to the Surfi-Sculpt process. When the electron beam comes into contact with the substrate it melts and vaporises the material. Providing that the beam does not penetrate the substrate, the vapour pressure of the molten material causes the material to be expelled from the hole. This material is deposited around the perimeter of the hole. A series of electro-magnetic coils are used to firstly focus the electron beam and then deflect it around the material in a rapid and controlled manner, allowing this process to be carried out at many sites consecutively. The result is an EB textured surface, an example of which is shown in Fig.1. One of the characteristics of an EB textured surface is the re-entrant nature of the features as depicted in Fig.2.
Fig.1. An SEM image of an EB textured surface
Fig.2. A diagram of a cross-section through an EB textured surface showing the re-entrant nature of the displaced material (shaded)
Surfi-Sculpt is an advancement of the texturing process. Having created the molten pool of material, the beam is translated sideways. The combined effects of vapour pressure and surface tension allow the material from the hole to be piled up behind the beam ( Fig.3).
Fig.3. A diagram showing the movement of the electron beam to build up a protrusion
By repeating this process many times at the same site, protrusions of the order of 2mm height and 0.2mm width may be grown, and each will be accompanied by one or more corresponding intrusions or holes. Again, a whole series of protrusions can be built up simultaneously across a substrate ( Fig.4).
Fig.4. An array of Surfi-Sculpt protrusions made simultaneously
With careful control of the electron beam process parameters (beam accelerating potential, beam current, focus, etc), the design of a unique pattern and precisely defined deflection movements, it is possible to create a wide variety of different surfaces.
The Surfi-Sculpt process has been demonstrated on a wide variety of materials and is not limited to metallic substrates. Providing that a material forms a stable liquid phase under the action of the beam, it is possible to process it. Success has even been achieved on some non-conducting materials, which normally charge up under the influence of an electron beam. However, the precise nature of the surface that can be achieved is dependent on the material.
Surfi-Sculpt is a rapid process. It is not uncommon for several hundred features to be built up simultaneously, resulting in a build time per feature in the order of just a few milliseconds. The current development equipment is fitted with a single beam generator. However, it is conceivable that in a production environment, equipment would consist of multiple beams that would be applied simultaneously, resulting in increased processing speeds.
In order to carry out Surfi-Sculpt, an electron beam machine is required, the power of which is dependent upon the substrate material and process type. As with most electron beam processes, work is performed in a vacuum chamber, the size of which is dictated by the size of the components to be treated. To adapt an EB welding machine for Surfi-Sculpting requires the fitting of an intense electron gun and a sophisticated beam deflection system.
The variety of features that it is possible to make is almost limitless. These include; high aspect ratio spikes ( Fig.5), burr-free holes ( Fig.6), blades, channels ( Fig.7) and swirls ( Fig.8).
Fig.5. High aspect ratio spikes created using Surfi-Sculpt
Fig.6. Holes created using Surfi-Sculpt, showing the absence of burrs
Fig.7. Channels created using Surfi-Sculpt
Fig.8. Swirls created using Surfi-Sculpt
Material from a single slot may be used to produce a corresponding protrusion, or material from a number of slots may be piled up at one central location to create a single large multi-axial protrusion as shown in Fig.9. It has even proved possible to treat curved surfaces using a continuous travelling pattern as shown in Fig.10.
Fig.9. A Surfi-Sculpt protrusion created by moving material from six slots
Fig.10. Surfi-Sculpting of curved surfaces
Within any pattern, the size, shape, angle of incidence ( Fig.11) and distribution of the features can all be varied to produce customised surfaces. At the present time, protrusions ranging in height from 200µm to 2mm have been successfully made.
Fig.11. A Surfi-Sculpt protrusion grown at an angle to the substrate
Surfi-Sculpt offers benefits in a wide range of industrial applications.
It is proving successful as a preparation for surfaces prior to joining. The re-entrant features provide improvements in mechanical interlocking with adjoining parts and the protrusions help to distribute the stresses more evenly across a joint interface. The flexibility of the process may also be exploited to tailor-make a surface. For example, protrusions may be aligned in the direction of maximum stress or the density of features may be altered to distribute the stress uniformly throughout a component. The fact that the process is carried out under vacuum, and consequently produces a 'clean' surface, offers obvious advantages for any bonding application. Promise has been shown in the use of Surfi-Sculpt to promote adhesion between a substrate and a coating. Fig.12 shows a titanium surface coated by a thermally sprayed alumina coating.
Fig.12. Alumina coated titanium
In this case, the coating follows the profile of the surface to some extent. However, there was evidence that the coating was preferentially filling the cavities and it is expected that a finer scale surface profile would allow a smooth surface to be created.
Following the increasing use of composite materials in applications where weight saving is critical, the joining of composites to existing metal components has provided a huge challenge to the automotive, aerospace and power sectors. Surfi-Sculpt is proving to be an attractive solution for the manufacture of composite to metal (Comeld TM ) joints ( Fig.13). The protrusions penetrate the fibres in the composite, preventing sudden bond-line failure from occurring. Tests revealed that Comeld joints absorbed twice as much energy before failure than control joints and displayed a more progressive and consequently detectable failure mode.
Fig.13. A Comeld joint of approximately 6mm thickness
The series of protrusions and slots/holes achievable using Surfi-Sculpt has the potential to control gas and liquid flow. This aspect makes it suitable for applications involving the mixing of gases/liquids (eg. within engines) and where aerodynamic, hydrodynamic or thermally enhanced surfaces are required. Coupled with the accompanying significant increase in surface area, the surfaces are highly suitable for situations involving heat dissipation or chemical reactions such as catalysis. Surfi-Sculpt could also prove to be invaluable in the manufacture of biologically compatible surfaces.
There are many other possibilities, which would allow the creation of functional surfaces, including:
- the processing of pre-coated materials to create local variations in surface properties
- the combination of Surfi-Sculpting and alloying within a single process to engineer the mechanical, thermal, chemical, electrical or magnetic properties of a surface
- the Surfi-Sculpting of shape-memory alloys.
It is anticipated that in the future finer scale features will be possible using even smaller diameter electron beams.
Advancements in electron beam technology have resulted in the development of a new surface processing technique - Surfi-Sculpt. It enables a wide variety of surface features to be produced rapidly and has shown promise in improving joints between composites and metals, in creating hydrodynamic and aerodynamic surfaces and enhancing the surface properties through coatings. TWI is already developing the process for use in several application areas using it's unique development capability in Cambridge, UK and would welcome collaboration with potential industrial users of the technology in any area of application.
The authors would like to express their thanks to the many colleagues at TWI who have assisted in this work.
- International Patent Publication Number WO 2004/028731 A1. 'Workpiece Structure Modification.' Applicant: The Welding Institute. Inventors: Bruce Guy Irvine Dance and Ewen James Crawford Kellar.
- International Patent Publication Number WO 2002/094497 A3. 'Modulated Surface Modification.' Applicant: The Welding Institute. Inventors: Bruce Guy Irvine Dance.