ISO 25178

ISO 25178: Geometric Product Specifications (GPS) – Surface texture: areal is an International Organisation for Standardisation collection of international standards relating to the analysis of 3D areal surface texture.

Structure of the standard

Documents constituting the standard :

  • Part 1: Indication of surface texture
  • Part 2: Terms, definitions and surface texture parameters
  • Part 3: Specification operators
  • Part 6: Classification of methods for measuring surface texture
  • Part 70: Material measures
  • Part 71: Software measurement standards
  • Part 72: XML file format x3p
  • Part 600: Metrological characteristics for areal-topography measuring methods [under development]
  • Part 601: Nominal characteristics of contact (stylus) instruments
  • Part 602: Nominal characteristics of non-contact (confocal chromatic probe) instruments
  • Part 603: Nominal characteristics of non-contact (phase-shifting interferometric microscopy) instruments
  • Part 604: Nominal characteristics of non-contact (coherence scanning interferometry) instruments
  • Part 605: Nominal characteristics of non-contact (point autofocus probe) instruments
  • Part 606: Nominal characteristics of non-contact (focus variation) instruments
  • Part 607: Nominal characteristics of non-contact (confocal microscopy) instruments [under development]
  • Part 700: Calibration of surface texture measuring instruments [under development?]
  • Part 701: Calibration and measurement standards for contact (stylus) instruments

Other documents might be proposed in the future but the structure is now almost defined. Part 600 will replace the common part found in all other parts. When revised, parts 60x will be reduced to only contain descriptions specific to the instrument technology.

New features

It is the first international standard taking into account the specification and measurement of 3D surface texture. In particular, the standard defines 3D surface texture parameters and the associated specification operators. It also describes the applicable measurement technologies, calibration methods, together with the physical calibration standards and calibration software that are required.

A major new feature incorporated into the standard is coverage of non-contact measurement methods, already commonly used by industry, but up until now lacking a standard to support quality audits within the framework of ISO 9000. For the first time, the standard brings 3D surface metrology methods into the official domain, following 2D profilometric methods that have been subject to standards for over 30 years. The same thing applies to measurement technologies that are not restricted to contact measurement (with a diamond point stylus), but can also be optical, such as chromatic confocal gauges and interferometric microscopes.

New definitions

The ISO 25178 standard is considered by TC213 as first and foremost providing a redefinition of the foundations of surface texture, based upon the principle that nature is intrinsically 3D. It is anticipated that future work will extend these new concepts into the domain of 2D profilometric surface texture analysis, requiring a total revision of all current surface texture standards (ISO 4287, ISO 4288, ISO 1302, ISO 11562, ISO 12085, ISO 13565, etc.)

A new vocabulary is imposed:

  • S filter: filter eliminating the smallest scale elements from the surface (or of the shortest wavelength for a linear filter)
  • L filter: filter eliminating the largest scale elements from the surface (or of the longest wavelength for a linear filter)
  • F operator: operator suppressing nominal form.
  • Primary surface: surface obtained after S filtering.
  • S-F surface: surface obtained after applying an F operator to the primary surface.
  • S-L surface: surface obtained after applying an L filter to the S-F surface.
  • Nesting index: index corresponding to the cut-off wavelength of a linear filter, or to the scale of the structuring element of a morphological filter. Under 25178, industry-specific taxonomies such as roughness vs waviness are replaced by the more general concept of "scale limited surface" and "cut-off" by "nesting index".

The new available filters are described in the series of technical specifications included in ISO 16610. These filters include: the Gaussian filter, the spline filter, robust filters, morphological filters, wavelet filters, cascading filters, etc.

Parameters

Generalities

3D areal surface texture parameters are written with the capital letter S (or V) followed by a suffix of one or two small letters. They are calculated on the entire surface and no more by averaging estimations calculated on a number of base lengths, as is the case for 2D parameters. In contrast with 2D naming conventions, the name of a 3D parameter does not reflect the filtering context. For example, Sa always appears regardless of the surface, whereas in 2D there is Pa, Ra or Wa depending on whether the profile is a primary, roughness or waviness profile.

Height parameters

These parameters involve only the statistical distribution of height values along the z axis.

Parameter Description
Sq Root mean square height of the surface
Ssk Skewness of height distribution
Sku Kurtosis of height distribution
Sp Maximum height of peaks
Sv Maximum height of valleys
Sz Maximum height of the surface
Sa Arithmetical mean height of the surface

Spatial parameters

These parameters involve the spatial periodicity of the data, specifically its direction.

Parameter Description
Sal Fastest decay auto-correlation rate
Str Texture aspect ratio of the surface
Std Texture direction of the surface

Hybrid parameters

These parameters relate to the spatial shape of the data.

Parameter Description
Sdq Root mean square gradient of the surface
Sdr Developed area ratio

Functions and related parameters

These parameters are calculated from the material ratio curve (Abbott-Firestone curve).

Parameter Description
Smr Surface bearing area ratio
Sdc Height of surface bearing area ratio
Sxp Peak extreme height
Vm Material volume at a given height
Vv Void volume at a given height
Vmp Material volume of peaks
Vmc Material volume of the core
Vvc Void volume of the core
Vvv Void volume of the valleys

Parameters related to segmentation

These feature parameters are derived from a segmentation of the surface into motifs (dales and hills). Segmentation is carried out using a watershed method.

Parameter Description
Spd Density of peaks
Spc Arithmetic mean peak curvature
S10z 10 point height
S5p 5 point peak height
S5v 5 point valley height
Sda Closed dales area
Sha Closed hills area
Sdv Closed dales volume
Shv Closed hills volume

Software

A consortium of several companies started to work in 2008 on a free implementation of 3D surface texture parameters. The consortium, called OpenGPS [1] later focused its efforts on an XML file format (X3P) that was published under the ISO standard ISO 25178-72. Several commercial packages provide part or all of the parameters defined in ISO 25178, such as MountainsMap from Digital Surf, SPIP from Image Metrology[2] as well as the open source Gwyddion.

Instruments

Part 6 of the standard divides the usable technologies for 3D surface texture measurement into three families:

  1. Topographical instruments: contact and non-contact 3D profilometers, interferometric and confocal microscopes, structured light projectors, stereoscopic microscopes, etc.
  2. Profilometric instruments: contact and non-contact 2D profilometers, line triangulation lasers, etc.
  3. Instruments functioning by integration: pneumatic measurement, capacitive, by optical diffusion, etc.

and defines each of these technologies.

Next, the standard explores a number of these technologies in detail and dedicates two documents to each of them:

  • Part 6xx: nominal characteristics of the instrument
  • Part 7xx: calibration of the instrument

Contact profilometer

Parts 601 and 701 describe the contact profilometer, using a diamond point to measure the surface with the assistance of a lateral scanning device.

Chromatic confocal gauge

Part 602 describes this type of non-contact profilometer, incorporating a single point white light chromatic confocal sensor. The operating principle is based upon the chromatic dispersion of the white light source along the optical axis, via a confocal device, and the detection of the wavelength that is focused on the surface by a spectrometer.

Coherence scanning interferometry

Part 604 describes a class of optical surface measurement methods wherein the localization of interference fringes during a scan of optical path length provides a means to determine surface characteristics such as topography, transparent film structure, and optical properties. The technique encompasses instruments that use spectrally broadband, visible sources (white light) to achieve interference fringe localization). CSI uses either fringe localization alone or in combination with interference fringe phase.

Focus variation

Part 606 describes this type of non-contact areal based method. The operating principle is based on a microscope optics with limited depth of field and a CCD camera. By scanning in vertical direction several images with different focus are gathered. This data is then used to calculate a surface data set for roughness measurement.

See also

References

Digital Surf

Digital Surf is a French software company formed in 1989 mainly known for its Mountains software, that is offered as embedded or optional OEM surface analysis software by the majority of profilometer and microscope manufacturers.

Focus variation

Focus variation is a method to calculate a sharp image and to measure the depth with an optics with limited depth of field.

Geometric dimensioning and tolerancing

Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describe nominal geometry and its allowable variation. It tells the manufacturing staff and machines what degree of accuracy and precision is needed on each controlled feature of the part. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the allowable variation between features.

Dimensioning specifications define the nominal, as-modeled or as-intended geometry. One example is a basic dimension.

Tolerancing specifications define the allowable variation for the form and possibly the size of individual features, and the allowable variation in orientation and location between features. Two examples are linear dimensions and feature control frames using a datum reference (both shown above).There are several standards available worldwide that describe the symbols and define the rules used in GD&T. One such standard is American Society of Mechanical Engineers (ASME) Y14.5-2009. This article is based on that standard, but other standards, such as those from the International Organization for Standardization (ISO), may vary slightly. The Y14.5 standard has the advantage of providing a fairly complete set of standards for GD&T in one document. The ISO standards, in comparison, typically only address a single topic at a time. There are separate standards that provide the details for each of the major symbols and topics below (e.g. position, flatness, profile, etc.).

ISO 16610

ISO 16610: Geometrical product specifications (GPS) – Filtration is a standard series on filters for surface texture, and provides guidance on the use of these filters in various applications.

Filters are used in surface texture in order reduce the bandwidth of analysis in order to obtain functional correlation with physical phenomena such as friction, wear, adhesion, etc. For example, filters are used to separate roughness and waviness from the primary profile, or to create a multiscale decomposition in order to identify the scale at which a phenomenon occurs.

Historically, the first roughness measuring instruments - stylus profilometer - used to have electronic filters made of capacitors and resistors that filtered out low frequencies in order to retain frequencies that represent roughness. Later, digital filters replaced analog filters and international standards such as ISO 11562 for the Gaussian filter were published.

List of International Organization for Standardization standards, 24000-25999

This is a list of published International Organization for Standardization (ISO) standards and other deliverables. For a complete and up-to-date list of all the ISO standards, see the ISO catalogue.The standards are protected by copyright and most of them must be purchased. However, about 300 of the standards produced by ISO and IEC's Joint Technical Committee 1 (JTC1) have been made freely and publicly available.

Micro pitting

Micro pitting is a fatigue failure of the surface of a material commonly seen in rolling bearings and gears.

It is also known as grey staining, micro spalling or frosting.

MountainsMap

Mountains is an image analysis and surface metrology software platform published by the company Digital Surf. Its core is micro-topography, the science of studying surface texture and form in 3D at the microscopic scale. The software is dedicated to profilometers, 3D light microscopes ("MountainsMap"), scanning electron microscopes ("MountainsSEM") and scanning probe microscopes ("MountainsSPIP").

Surface metrology

Surface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface fractality and surface roughness are the parameters most commonly associated with the field. It is important to many disciplines and is mostly known for the machining of precision parts and assemblies which contain mating surfaces or which must operate with high internal pressures.

Surface finish may be measured in two ways: contact and non-contact methods. Contact methods involve dragging a measurement stylus across the surface; these instruments are called profilometers. Non-contact methods include: interferometry, digital holography, confocal microscopy, focus variation, structured light, electrical capacitance, electron microscopy, and photogrammetry.

Surface roughness

Surface roughness often shortened to roughness, is a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. In surface metrology, roughness is typically considered to be the high-frequency, short-wavelength component of a measured surface. However, in practice it is often necessary to know both the amplitude and frequency to ensure that a surface is fit for a purpose.

Roughness plays an important role in determining how a real object will interact with its environment. In tribology, rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces. Roughness is often a good predictor of the performance of a mechanical component, since irregularities on the surface may form nucleation sites for cracks or corrosion. On the other hand, roughness may promote adhesion. Generally speaking, rather than scale specific descriptors, cross-scale descriptors such as surface fractality provide more meaningful predictions of mechanical interactions at surfaces including contact stiffness and static friction.Although a high roughness value is often undesirable, it can be difficult and expensive to control in manufacturing. For example, it is difficult and expensive to control surface roughness of fused deposition modelling (FDM) manufactured parts.

Decreasing the roughness of a surface usually increases its manufacturing cost. This often results in a trade-off between the manufacturing cost of a component and its performance in application.

Roughness can be measured by manual comparison against a "surface roughness comparator" (a sample of known surface roughness), but more generally a surface profile measurement is made with a profilometer. These can be of the contact variety (typically a diamond stylus) or optical (e.g.: a white light interferometer or laser scanning confocal microscope).

However, controlled roughness can often be desirable. For example, a gloss surface can be too shiny to the eye and too slippery to the finger (a touchpad is a good example) so a controlled roughness is required. This is a case where both amplitude and frequency are very important.

ISO standards by standard number
1–9999
10000–19999
20000+

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