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The ability to manage surface quality is critical in many manufacturing industries because surfaces play a major role in mechanical failures, including catastrophic wear, cracking and deterioration. The desired performance of manufactured parts is very much associated with surface-related factors like friction, cohesion, and adhesion.
Also, determining the mechanical performance of machined parts with various kinds of surfaces can help select the proper kind of surface treatments. For instance, engine parts surfaces call for precise engineering, to a microscopic degree, to ensure they have enough surface roughness to hold onto lubricants – between parts and under pressure – while being smooth enough to avoid metal to metal contact.
Clearly, finding an approach to assess and quantify surface texture is crucial, as texture correlates to various performance functions. Surface Metrology is an approach that encompasses a range of instruments, methods and measurement parameters to measure and analyze surface texture.
Stylus and Optical Profilometry
There are two predominant surface metrology techniques used to measure surface textures: stylus and optical profilometry.
One of the most widely used surface metrology techniques, stylus profilometry involves a stylus tip being moved linearly over a sample. The vertical displacements of the stylus are recorded and used to create a roughness profile. There must be direct contact between the profiler tip and the surface to produce a profile, with the profile based on two-dimensional roughness parameters.
Because stylus profilometry is an extremely long-standing approach, existing test data is mainly in two-dimensional in nature. This method is one of the most cost-friendly approaches. The self-reliance of two-dimensional measuring instruments is also another reason why this technique is commonly used in industry. Stylus profilometry equipment can readily be built into most manufacturing machines for surface quality inspection.
Due to the most recent developments in measurement solutions, there is now a wide variety of optical techniques for characterizing surfaces. Many optical acquisition methodologies can obtain surface topography data from a product, particularly through confocal and electron microscopy. These procedures normally scan a surface with an incident light source and assess the emissive, reflective, or refractive light to get data on a product’s topography. These techniques are often preferred over stylus profiling because of its low invasiveness and superior speed.
Unlike stylus-based measurements, optical techniques mostly supply areal information, making them statically more dependable and more representative.
Primary Form
Primary form describes the overall shape of a surface, which can be assessed quantitatively. For instance, the “primary form” of a lens could be described through an assessment of the lens's curvature as compared to desired specifications. The primary form on a sample can differ based on the region of interest.
Standard primary forms might include curves, planes, and parabolas. Assessing surface shape calls for a field of view large enough to encompass shape geometries.
Roughness
The most popular parameter used to describe roughness is average roughness (Ra), considered to be topography on a scale that may be deemed "texture". Ra is determined by taking the average of the absolute values of profile variations in an evaluation length.
In three-dimensional optical profilometry, roughness is generally expressed as surface area roughness (Sa). Ra can be determined as a line through an area. Interestingly, Sa is also useful to determine average Ra through a surface by averaging multiple profiles.
Fractality
All surfaces have fractal properties, and the term ‘fractality’ in surface metrology is used to the describe the geometric sub-micron variations of a surface. It has been used in the detailed description of surface-to-surface interactions at the microscopic and sub-microscopic levels.
Sources and Further Reading
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