Surface texture plays a vital role in the functionality of a component. It is estimated that surface effects cause 10% of manufactured parts to fail and can contribute significantly to an advanced nation’s GDP. In the previous century, surface texture was primarily measured by tracing a contacting stylus across the surface and measuring the vertical motion of the stylus as it traverses the surface features.
In most cases only a single line, or surface profile, was measured and this gave rise to enough information to control production, but was limited to looking for process change. However, over the past three decades there has been an increased need to relate surface texture to surface function. Whilst a profile measurement may give some functional information about a surface, to really determine functional information, a three dimensional, or “areal”, measurement of the surface is necessary.
Control of the areal nature of a surface allows the manufacturer to alter the function of the surface with its surroundings or how it interacts with another surface. In this way optical, tribological, biological, fluidic and many other properties can be altered. Examples include:
- surface structuring to encourage the binding of biological molecules, for example proteins, cells or enzymes;
- micro-lens arrays for displays and photo-voltaics;
- prismatic arrays for safety clothing, signage and LED lighting;
- nanostructured surfaces that affect plasmonic interactions for anti-reflection coatings, waveguides and colour control;
- surfaces of microfluidic channels for flow control, mixing, lab-on-a-chip and biological filtering;
- deterministic patterning to control tribological characteristics such as friction, rheology and wear.
The measurement of areal surface texture has a number of benefits over profile measurement. Areal measurements give a more realistic representation of the whole surface and have more statistical significance. Also, there is less chance that significant features will be missed by an areal method and the manufacturer gains a better visual record of the overall structure of the surface.
The need for areal surface texture measurements led to stylus instruments that could measure over an area (a series of usually parallel profiles) and optical techniques. Optical instruments either scan a beam over the surface akin to the stylus instruments, or take an areal measurement by making use of the finite field of view of a microscope objective.
There are currently many commercial instruments that can measure areal surface texture, both stylus and optical. However, instrument comparisons show that stylus and optical techniques only usually agree on surfaces with simple geometry, such as those found on optical flats or step height artefacts. When measuring textured surfaces, instruments rarely agree to within their stated accuracies. These differences can be partially explained by the different interactions of the instrument with the surface being measured, but some differences are down to the instruments being only partially verified, or not verified at all.
It is common to state that an instrument is calibrated simply by measuring a step height artefact. Such an instrument may then be used to measure a range of complex geometries with differing heights and spatial wavelength distributions, and the user may claim that the instrument is still calibrated – a dangerous assumption. For this we cannot put the blame on the user of the instrument – traceable calibration for areal surface texture does not yet exist.
To address this lack of traceability NPL has developed an instrument that can provide the primary traceability for areal surface texture measurement. The instrument uses an air bearing slideway to move the surface being measured along with a contacting stylus. The position of the stylus tip is measured using a multi-axis interferometric arrangement that ensures traceability though the wavelength of the laser source. This instrument can be used to calibrate transfer artefacts that can in turn be used to calibrate industrial stylus and optical instruments. NPL’s current research in this area is to develop the appropriate transfer artefacts along with reference software to calculate areal characterisation parameters. To encourage good practice, NPL will also be developing guides on the calibration and verification of a range of instruments, both stylus and optical.
In 2002, the International Organization for Standardisation (ISO) Technical Committee 213, dealing with Dimensional and Geometrical Product Specifications and Verifications, formed a working group (WG) 16 to address standardisation of areal surface texture measurement methods. WG 16 is developing a number of draft standards encompassing definitions of terms and parameters, calibration methods, file formats and characteristics of instruments. The first standards are expected to be published within a year and drafts are available on the ISO website (www.iso.org and search for 25178 series). But there will be a steep learning curve to change from profile based measurement to areal based measurement and some of the analysis methods are dauntingly complex. Education and awareness, along with good practice guidance, will be needed for industrialists and academics, and NPL is at the forefront of this process.
It really is possible to make a very significant improvement in the energy efficiency and functionality of a product by using three dimensional surface structuring. The companies that will benefit from these improvements will be those that start to consider the manufacturing and metrology solutions now. Even in these difficult financial times, it is investment in technology and new ways of doing things that will allow manufacturers to, first survive, and then prosper. The time is right to start looking at processes that can benefit from surface structuring, start thinking about these issues now!l