Принципы нанометрологии (1027623), страница 30
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Bysimply looking at a surface one can easily tell the difference between a freshlymachined lump of glass and one that has been lapped and fine-polished.Touch was utilized by running a finger or fingernail along a surface to bemeasured and feeling any texture present on the surface. With a few technological modifications, these two methods for measuring surface texture arestill the most widely used today.One of the earliest attempts at controlling surface texture was made in theUSA by a company that mounted samples of textures produced by different117118C H A P T ER 6 : Surface topography measurement instrumentationmethods in cases [15] which were given to the machinist, who was expectedto obtain a texture on his or her workpiece as near to that specified aspossible.
This was a suitable method for controlling the appearance of theworkpiece but did not in any way indicate the magnitude of the surfacetexture.Perhaps the first stylus method was to drag a sapphire needle attached toa pick-up arm across the surface being tested [16]. As with a gramophone, thevibration so produced gave rise to sound in a speaker and variation in theelectrical current reading on a voltmeter. The method was calibrated bycomparing the measured results to those obtained with a sample havinga texture that should have been given to the workpiece.
This method did notgive rise to many benefits over the visual appearance method and it would beexpected that the amplitude of the current reading will bear a greater relationto the pitch of the texture rather than its depth.Few metrologists can doubt the influence on the world of surface texturemeasurement, and indeed the entire field of engineering metrology, played bytwo brothers named Thomas Smithies Taylor and William Taylor, plus theirassociate William S.
Hobson. The three men went into business in Leicester,England, in 1886 manufacturing optical, electrical and scientific instruments[17]. In the 1880s, photography was developing rapidly and Taylor, Taylorand Hobson (TTH) started making photographic lenses. The presentcompany still holds a leading position in the world for cinematograph andtelevision lenses.The first metrology instrument manufactured by TTH was a screwdiameter measuring machine (originally designed by Eden at NPL).
Thisinstrument was used extensively for armaments manufacture during theFirst World War. In 1945 J. Arthur Rank, the British flour miller andmillionaire film magnate, purchased shares in the company. Until 1996,Rank Taylor Hobson was still part of the Rank Organisation.Richard Reason [18], who was employed by TTH, attributed the origin ofsurface stylus measurements to Gustav Schmaltz of Germany in 1929.Schmaltz [19] used a pivoted stylus drawn over the surface with a verylightweight mirror being attached to the stylus.
A beam of light reflected inthe mirror traced a graph on a moving photographic chart, providinga magnified, although distorted, outline of the surface profile. In 1934William Taylor learned of the work of Abbott and Firestone [20] in developingmethods for measuring surface texture. In their 1933 paper Abbott andFirestone discuss the use of a similar instrument to that of Schmaltz andname it a profilograph. Abbott’s instrument was put on the market in 1936.Schmaltz later produced a microscope (known as the light-section microscope) that observed the surface at an angle of incidence of 45 .
This gaveHistorical background of classical surface texture measuring instrumentationadditional magnification (O2) to that of the microscope but was only suitablefor quite coarse surface textures since the optical magnification was necessarily limited.In the mid-1930s the area where accurate surface measurement wasrequired was mainly in finely finished bearing surfaces, such as those used inaircraft engines.
The stylus and mirror arrangement was limited to about4000 magnification but an order of magnitude more was needed. Therefore, Reason rejected optical magnification and used the principles of a stylusdrawn across the surface with a variable inductance pick-up and electronicamplification.Along the lines of Abbott, in 1940 Rolt (at NPL) was pressing for surfacetexture measurement to produce a single number that would define a surfaceand enable comparisons to be made.
The number most readily obtainablefrom a profile graph was the average value, obtained using a planimeter.Eventually, TTH put the Talysurf onto the market. (Note that the nameTalysurf comes from the Latin talea, which roughly translates to ‘measurement’, and not from the name Taylor.) This instrument provided a graph andthe average surface roughness value read directly from a meter. Figure 6.2 isa photograph of the original Talysurf instrument.Another method for measuring surface texture was due to Linnik of theMendelleif Institute in Leningrad (1930) and interferometers for this methodwere made by Hilger and Watts, and by Pitter Valve Engineering in Britain.These interferometric instruments were diffraction limited but paved theFIGURE 6.2 The original Talysurf instrument (courtesy of Taylor Hobson).119120C H A P T ER 6 : Surface topography measurement instrumentationway for a range of non-contacting instruments that is still being increased todate (see section 6.7).In 1947 Reason turned his attention to the measurement of roundnessand in 1949 the first roundness testing machine, the Talyrond, was produced.The Talyrond used a stylus arm and electrical transducer operating on thesame principle as the Talysurf.
These two, plus other instruments, paved theway for the Talystep instrument, which uses the sensitive electronic transducer technique to measure very small steps or discontinuities in a surfaceand is thus able to measure thin-film steps of near-molecular thickness [21].Further developments in surface texture measurement will be discussed inthe following sections of this chapter.6.4 Surface profile measurementSurface profile measurement is the measurement of a line across the surfacethat can be represented mathematically as a height function with lateraldisplacement, z(x).
With a stylus or optical scanning instrument, profilemeasurement is carried out by traversing the stylus across a line on thesurface. With an areal (see section 6.7.3) optical instrument, a profile isusually extracted in software after an areal measurement has been taken (seesection 6.5). Figure 6.3 shows the result of a profile measurement extractedfrom an areal measurement.When using a stylus instrument, the traversing direction for assessmentpurposes is defined in ISO 4287 [22] as perpendicular to the direction of thelay unless otherwise indicated.
The lay is the direction of the predominantsurface pattern. Lay usually derives from the actual production process usedto manufacture the surface and results in directional striations across theFIGURE 6.3 Example of the result of a profile measurement.Areal surface texture measurementsurface. The appearance of the profile being assessed is affected by thedirection of the view relative to the direction of the lay and it is important totake this into account when interpreting surface texture parameters [1].6.5 Areal surface texture measurementOver the past three decades there has been an increased need to relate surfacetexture to surface function.
Whilst a profile measurement may give somefunctional information about a surface, to really determine functionalinformation, a three-dimensional, or ‘areal’, measurement of the surface isnecessary. Control of the areal nature of a surface allows the manufacturer toalter how a surface interacts with its surroundings. In this way optical,tribological, biological, fluidic and many other properties can be altered[23,24]. For example, control of surface texture is important for:-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 antireflection coatings, waveguides and colour control;-surfaces of microfluidic channels for flow control, mixing, lab-on-achip and biological filtering;-deterministic patterning to control tribological characteristics such asfriction, rheology and wear.There are inherent limitations with 2D surface measurement and characterization.
A fundamental problem is that a 2D profile does not necessarilyindicate functional aspects of the surface. For example, consider the mostcommonly used parameter for 2D surface characterisation, Ra (see section8.2.7.1). Figure 6.4 shows the profiles of two surfaces, both of which returnthe same Ra value when filtered under the same conditions. It can be seenthat the two surfaces have very different features and consequently verydifferent functional properties.With profile measurement and characterization it is often difficult todetermine the exact nature of a topographic feature. Figure 6.5 shows a 2Dprofile and a 3D surface map of the same component covering the samemeasurement area. With the 2D profile alone a discrete pit is measured on121122C H A P T ER 6 : Surface topography measurement instrumentationFIGURE 6.4 Profiles showing the same Ra with differing height distributions.FIGURE 6.5A profile taken froma 3D measurementshows the possibleambiguity of 2Dmeasurement andcharacterization.the surface.
However, when the 3D surface map is examined, it can be seenthat the assumed pit is actually a valley and may have far more bearing on thefunction of the surface than a discrete pit.The measurement of areal surface texture has a number of benefits overprofile measurement. Areal measurements give a more realistic representationof the whole surface and have more statistical significance.
Also, there is lesschance that significant features will be missed by an areal method and themanufacturer gains a better visual record of the overall structure of the surface.6.6 Surface topography measuring instrumentationOver the past one hundred years, and especially in the last thirty years, therehas been an explosion in the number of instruments that are available tomeasure surface texture. The instruments can be divided into three broadSurface topography measuring instrumentationclasses: line profiling, areal topography measuring and area-integratingmethods [25].
Line profiling methods produce a topographic profile, z(x).Areal topography methods produce topographic images, z(x, y). Often, z(x, y)is developed by juxtaposing a set of parallel profiles. Area-integratingmethods measure a representative area of a surface and produce numericalresults that depend on area-integrating properties of the surface.
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