Patterning

There are an increasing number of metal surfaces, which are deliberately structured using some regular array of surface height features, which can be referred to as geometrical patterns. The periodic character of those features implies that they are deliberately designed in order to achieve a required performance gain. Different applications will require different geometrical features of patterns. These can be groves, pyramidal recesses, circular or elliptical dimples, squares, etc. In any case they are usually small enough to use the term micro-patterns. Macro patterns and stochastic
geometrical surface structuring are not considered in this synthesis. It should be noted that the terminology used here is not established yet and different terms may refer to the same thing.

There are various methods, which can be used to produce micro-patterns. They encompass mechanical processes (e.g. diamond machining, indentation), lithographic/etching processes (e.g. LIGA), energy beam processes (e.g. laser beam) and coating processes (e.g. PVD). Since, except indentation, all these processes are covered by other partners in task 7.3, this synthesis will address only a mechanical process of indentation.

Technology description

Micro-indentation is a mechanical method based on indentation of a micro-indenter into a metallic surface. The method could be derived from a microhardness testing with, for example, a Vickers diamond pyramid indenter (Fig. 1). Also other indenter shapes can be used for micro-indentation for patterning purposes. A common shape is a cone with a spherical tip. The materials used for those indenters are diamond and sintered carbides chosen for their hardness and strength.

Indentation is a metal forming process, which involves material displacement by a hard tool forced into the material. Thus pile-up or sink-in of the material surrounding the indentation would be expected. Also, elastic recovery and sometimes secondary yielding occur on indenter retraction. All these phenomena influence the indentation profile error. However, in most applications this is not a major concern. Since the process is characterised by high hydrostatic pressure, it can be applied to both ductile and not so ductile metals.

For patterning purposes, where thousands of indentations are required, the process has to be fast to yield reasonable productivity. This changes the character of the process from static one, as in hardness measurements, to a dynamic one. Another requirement is a controllable position of each indent and its depth. This can be achieved by employing accurate multi-axis positioning stages associated with either the surface treated or the indenter. The depth control is related to the way the indenter is driven into the material. In this context, mechanical and piezoelectric drives can be
considered.

Indentation can be applied to flat, cylindrical and curved irregular surfaces. It can produce discrete
impressions or continuous profiles by incremental indentation. The current
laboratory state of the art enables producing 1-15 µm deep spherical dimples in a 60HRC steel at a
rate of 75 Hz.

Technology limits

Since the method is still under development, its limits are not clear yet. However, there are several
issues, which may pose limitations. They can be grouped into surface, tool and process related
factors.

Surface type and quality

While flat and cylindrical surfaces require only 2 axes for position control of the indent, irregular curved surfaces require a 5-axis positioning system. The intricacy of these surfaces is limited by the access of the indenter. Shallow impressions require a fine surface roughness. Surface waviness can also pose problems with depth control. The tool system developed in Strathclyde University under EU project MACHMINI overcomes such constrains.

Indenter design and manufacture

When designing indenters, there is a limit on their slenderness due to the value and direction of the indentation force. The value of the force depends on the type of processed material and the indent’s depth while its direction depends on the tangential component of indenter’s velocity during indentation. Manufacturing of micro-indenters made of various materials to match the processed material and having different geometries as required by applications is a technological challenge. The geometrical accuracy of indenters is an important factor in minimising profile errors. Their surface
finish will affect friction and tool life.

Process capability and control

There is an inherent material displacement, which, for higher indentation depths, leads to noticeable geometrical changes such as material pile-up and sink-in around impressions. To a certain extent these can be dealt with by lapping. Nevertheless, these effects limit the indentation depth. The control of this depth, especially for uneven surfaces, is a technical challenge. Another issue is the accuracy of positioning, which has to be adequate for a given indentation frequency. For continuous positioning, its speed and direction is crucial. The currently achievable indentation frequency is 50Hz. Using an electro-magnetic indenter driving system, this frequency can reach 400 Hz. However, higher frequencies are associated with lower amplitudes and lower forces. Piezoelectric actuators can provide a higher frequency which should be equal to the natural frequency of the system. A combination of lower indentation frequency with, for example, ultrasonic vibration superimposed on indenter is also possible. Another limiting factor will be the speed of positioning, which depends on the pitch of indents and indentation frequency.

Applications

There are numerous potential applications of micro-patterned surfaces.

Tribological applications

It is known that micro-patterned metallic surfaces have a lower wear rate. This is probably due to the entrapment of lubricants in the impressions, which act as lubricating pockets. It is expected that applications such as bearings, piston rings, metal forming and cutting tools will benefit from micro patterning.

Controllable adhesion

Rough surfaces exhibit better adhesion. Controllable roughing by micro patterning can improve
adhesion of seals, increase the grip of miniature grippers and manipulators and provide a better
surface for bonding, coating and painting applications.

Fluid/gas flow improvement

The detailed nature of surfaces in contact with the flowing fluid or gas affects the boundary layer of the flowing medium and hence turbulence and drag. Thus, by deterministic micro patterning, a required flow behaviour can be achieved. Possible applications include marine and aerospace situations as well as hydraulic and pneumatic machines and devices.

Biomedical applications

Medical implants have to be biocompatible and provide good interface with the surrounding tissue. Since roughened implant surfaces provide better integration with the tissue, they are used in many dental and skeletal implants. Shot pinning often used for roughening is not an ideal method though; it lefts a residuum of the blasting agent used. Thus micro patterning based in micro indentation would be a better and cleaner method.

Moulds for replication

Moulds for microinjection moulding and embossing can be micro patterned to replicate the surface pattern on a polymeric component.

Industrial users

Industrial users of micro patterning who use micro indentation in manufacturing are scarce. On the other hand there is a substantial engraving/printing/marking industry, which uses indentation in order to produce larger geometrical features than described in this synthesis.

Equipment suppliers

Following the above comments on industrial users, stylus marking/dot-marking systems may provide valuable clues for designers of micro indentation machines.