Surface finish plays a significant role in the functions and reliabilities of materials and devices. Understanding surface wear and its underlying causes can be critical to the manufacture and maintenance of automotive and aerospace parts, such as bearings, seals, drive trains, shafts and brake components.
Improving the adhesion energy between coatings and substrates can make parts more reliable. For example, by controlling the surface roughness of engine parts, lubrication can be improved as lubricant trapped on the surface is tailored by surface texture optimisation.
Additionally, by controlling the properties of the surface texture, visual effects can be changed significantly, such as making car paint look premium.
Limitations Of Stylus Profiling
Quantitative measurements of surface finish can be traced back to the 1930s. A tiny stylus was scanned across the sample surface and the vertical movement of the stylus was recorded against the lateral position, forming a line profile.
From the line profile, more than 100 parameters have been defined to describe the surface texture, including commonly used average roughness (Ra), root mean square roughness (Rq), peak counts (RPc) and more.
However, the stylus profiling method has a few limitations. First, stylus profiling is a contact-based technique; there is a possibility of damaging or contaminating the sample.
In addition, the size of stylus limits the spatial resolution of this method. Lower spatial resolution may result in measured results that are not relevant to the application. For example, roughness at a resolution of ten micrometres may not affect surface adhesion energy.
The third limitation of stylus profiling is its limited sampling size, where only a line is measured and important characteristics of the surface might be missed.
To circumvent this problem, most commercial stylus profilers now have 3D mapping, which is performed by scanning multiple lines to form a 3D surface. However, the time taken for one measurement can take hours to perform. This makes it prohibitive to use 3D mapping in routine surface measurements.
Measurements Through Optical Interference
Surface measurement of a turbine blade
It is highly desirable to have a non-contact, fast, high-resolution, and 3D surface measurement technique for development and quality control. The answer is 3D optical microscopes: these devices measure surface finish through optical interference technology.
A resolution of sub-nanometre in Z and sub-micrometre in XY has been demonstrated on 3D optical microscopes. The typical time used for one measurement ranges from a few seconds to a few minutes depending on the surface roughness.
The 3D optical profiling data gathered would be the equivalent to taking hundreds of parallel line scans with the stylus profilers, which could easily take many hours to complete.
One unique merit of 3D optical microscopes such as Bruker’s NPFLEX 3D surface metrology system is that the sub-nanometre resolution in Z is independent of the measurement range in XYZ. For some samples, the height variation in one field of view can be up to several millimetres.
The device can also measure sub-nanometre resolution within the 10 mm Z range. In terms of XY dimensions, one measurement can cover an area from tens micrometres to a few millimetres by using different objectives.
If an even larger measurement area is required, the 3D microscope can do a stitching scan, where a series of single measurements will be stitched together to form a large area up to eight inches in XY. In routine measurements for quality control processes, all the measurements can also be automated.
After each measurement, the required surface parameters can be calculated automatically and checked against the preset criteria to report a fail or pass. If robotics is integrated, the 3D optical microscope can also be used as a sorting tool based on part quality.
Data Analysis Provides A Better Understanding
The rich information in the 3D data provides a more comprehensive understanding of the surface.
For example, shape and volume of each corrosion pit can be analysed automatically through one measurement. Spectral distribution and angular distribution of surface finish can be calculated automatically, which is important to understand the root cause of such surface texture and quality control for some products, such as sealing components.
To meet the requirements of different applications, all the surface parameters in ISO standard have been implemented in the analysis software, including commonly used roughness parameters for 2D profile and 3D surface, spectrum for periodicity and directionality analysis of surface texture, geometric parameter extraction, such as height, depth, width, area and volume. To support production environment and eliminate human error, data analysis and data logging can be automated.
In summary, 3D optical profiling provides a versatile, rapid, non-contact characterisation of surface texture for both research laboratories and production floors. 3D optical microscopes are a vital metrology platform for precision engineering, engineering materials, microelectronics, manufacturing, automation and quality control.
APMEN Metrology & Design, Mar 2017