3D surface parameters (ISO 25178-2): Actual meaning of Spk and its relationship to Vmp

• The paper examines the standard (ISO 13565-2/ISO 25178-2) instructions on how to calculate surface parameters Spk and Svk.
• It is demonstrated that the instructions do not lead to values that might be considered as “average heights” as described in the standard.
• Results are shown for surface measurements on a ground part that agree with the mathematical deduction of the paper and corresponding calculation for a theoretical Gaussian height distribution.
• The relationship between the functional parameters of the “k” series and the new volume parameters is examined. As a result it is proposed to retain the Sk parameter for core height definition and to drop Spk and Svk.

3D parameters are important in tribological studies and many of them show strong correlation with surface operational performance. Characterization of cylinder liner surfaces is a good example of the use of 3D roughness parameters. Standard ISO 25178-2:2012 defines new parameters retaining some of the old ones, even though presented as their equivalent 3D successors. That is the case for instance of the “k” series: Spk, Svk and Sk. This paper covers an analysis of the relationship between Spk and the new volume parameter Vmp where it is shown that it is not correct to designate Spk as the average height of the protruding peaks above the core surface as stated in the standard. It is also suggested that Sk should be maintained but that the new volume parameters, through their relationship to the “k series”, make it possible to discard Spk and Svk.