TWI Technology Fellow, Alan Taylor and TWI Senior Engineer, Sara Naderizadeh have co-authored an Industrial Member Technical Guide to surface roughness.
The precise and accurate determination of the roughness of a component is a critical element of the engineering process, influencing key functional characteristics such as friction coefficient, wear behaviour, and sealing effectiveness. Even minor deviations can lead to premature failure or reduced efficiency. Surface roughness can impact dimensional tolerance, and the fit of mating surfaces and surface-related metrics are widely used to ensure process control and the quality assurance for a variety of parts across all industry sectors. Surface roughness also influences the strength of joints in parts, particularly if the joint has been created using adhesive or diffusion bonding technologies. Functional and protective coatings are also sensitive to the roughness of a component.
The Importance of Surface Roughness to Industry
Surface roughness measurement is industrially vital because it directly influences product performance, durability and compliance. It underpins quality control, process optimisation and functional assurance across all sectors.
In manufacturing, roughness affects friction, wear, sealing and fatigue life. Accurate measurements are essential to ensure parts fit, function, and last as intended. Tight tolerances are frequently specified, but often these relate to a small number of parameters that are well recognised, but which do not give a complete understanding of a complex topography.
For example, parts being prepared for joining are often specified to have a maximum roughness as defined by the average roughness value, Ra. Such a measure however, would not specifically identify a surface that was dominated by ‘hills’ or one dominated by ‘dales’, these are topography differences that may result in variances in behaviours or properties. Similarly, surface preparation for coating or adhesive bonding often involves increasing the roughness of a part to provide mechanical interlocking, however the guidance provided is often based on a simple “surface profile” value.
The use of a wider range of parameters and particularly comparative parameters help to provide a more comprehensive description of the surface topography of a workpiece. For example, the skewness parameter, Rsk helps to identify whether a surface is peak or dale dominated, which is critical for tribological functions such as friction, wear and lubrication. The Kurtosis parameter, Rku, informs whether a surface has sharp asperities or broad plateaus, which, respectively, influence coating and adhesive success or improved load bearing. These parameters compliment other height parameters such as Ra by describing the shape of the height distribution and not just its magnitude.
Fundamental Concepts
Surfaces typically contain multiple features over a range of scales. A common maxim is that the Earth is smoother than a table tennis ball when viewed from space. However, at a human scale, the Earth’s surface is clearly not smooth; there are mountains, valleys, hills, abysses, crevices, slopes, plains and plateaus. These features are typically unique - with varying heights, depths, gradients, separations and sizes. Engineering surfaces have many of the same features, albeit on a much smaller scale, however the relative size, number, and sharpness of these features all impact the roughness of the component.
Roughness, waviness and form all describe surface texture, but at different scales:
- Roughness relates to fine irregularities often seen in engineering workpieces as a consequence of manufacture or machining
- Waviness describes medium-scale undulations, such as those generated by vibration or deflection
- Form refers to the macro-shape of a surface.
Together they define to total surface topography, which influences dimensional accuracy and functional properties such as wear.
Categorisation of Roughness
There are a great many terms used to describe surface topographies, which fall under two broad categories:
- Geometrical parameters: These are single value metrics for specific aspects of surface texture. They can be derived from profile or area based measurements, and are widely used to specify surface finish. The main parameters are categorised as relating to height, spatial separation, hybrid parameters, material ratio functions and parameters, stratified surfaces, and volume.
- Geometrical features: These terms are used to describe surfaces as continuous fields and are often represented as a height map. These terms can capture local variations, periodicity and can be used in simulations and finite element analysis
Standards
There are a range of overarching, broadly geographically directed, roughness standards, which differ between Europe, North America, Japan, and China. However, global trade is driving a harmonisation of roughness standards, with the ISO taxonomy being increasingly accepted with regional standards moving into alignment.
The transition towards a harmonised approach, being spearheaded by ISO, has resulted in a consolidation of key standards and the development of two interconnected models, the Geometrical Product Specifications (GPS) matrix model and the Chain Link Model.
The purpose of the GPS matrix model is to organise international standards for GPS across the full specification-verification-calibration chain. It is a taxonomy of standards not a process model with seven columns that each represent a category of standards.
Meanwhile, the Chain Link Model shows the process flow and specifically the flow of traceability and uncertainty in a measurement system. Its aim is to highlight risks and to support quality assurance and to ensure each link is robust and traceable.
These two models are used in tandem to determine factors such as the benchmarking of supplier systems or laboratory protocols, the identification of applicable standards, the steps in a traceable measurement system, areas where may uncertainty arise or traceability break down, and compliance with the ISO GPS itself.
Roughness Measurement Techniques
Conventionally, roughness has been measured using contact methods, such as stylus profilometry. These methods remain the benchmark and provide roughness average, Ra, maximum roughness height, Rz and related parameters with a high degree of accuracy.
As with a vinyl record player, the profilometer is equipped with a stylus tip, which traces the surface of the test piece, with the vertical motion of the stylus being detected electrically. These signals go through an amplification and digital conversion procedure prior to being recorded.