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.
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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.
Documents constituting the standard :
Other documents are in draft or being drafted and will be published in the next few years. A number of documents that are in draft concern, in particular, other optical measurement technologies (3D confocal microscope, white light interferometric microscope, digital holographic microscope).
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:
The new authorized filters are described in the series of technical specifications included in ISO/TS 16610. These filters include: the Gaussian filter, the spline filter, robust filters, morphological filters, wavelet filters, cascading filters, etc.
3D 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.
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 |
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 |
These parameters relate to the spatial shape of the data.
| Parameter | Description |
|---|---|
| Sdq | Root mean square gradient of the surface |
| Sdr | Developed area ratio |
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 |
These parameters are derived from a segmentation of the surface into motifs (dales and hills). Segmentation is carried out using the watersheds 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 |
At the moment a consortium of several company are working on a free implementation of the above parameters. This is called OpenGPS [1].
Part 6 of the standard divides the usable technologies into three families:
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:
Parts 601 and 701 describe the contact profilometer, using a diamond point to measure the surface with the assistance of a lateral scanning device.
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.
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.