# Application of Areal Topography Parameters in Surface Characterization
## 2 Areal Topography Parameters from Definition to Application
\(Sq\) is commonly applied in the characterization of optical surfaces [27], metal components produced by additive manufacturing [18], dental implant bonding cements [28] and adhesion of dental restorative composites [29]. \(Sa\) is typically used for assessing machined surfaces [30]. For normally distributed surfaces, \(Ssk\) = 0 and \(Sku\) = 3. The \(Ssk\) parameter is particularly useful in monitoring various types of wear, honing processes, and surface porosity [32]. The \(Sku\) parameter is commonly used to detect surface defects caused by extreme peaks or pits [31], and for identifying irregularities in critical surface components [33]. Since both \(Ssk\) and \(Sku\) involve higher-order statistical moments, a sufficient number of measurements must be conducted to obtain statistically reliable values, and appropriate filtering must be applied to remove spurious peaks or pits. As \(Sp\), \(Sv\), and \(Sz\) are based on individual surface points, their values may lack repeatability. Therefore, when using these parameters, it is essential to apply appropriate cut-off settings to minimize the influence of anomalous peaks or pits. The \(Sz\) parameter is typically used in evaluating coatings and sealing surfaces and in identifying surface anomalies [34]. It is particularly relevant for applications involving fluid retention, such as lubrication or surface coating systems, where the depth profile affects functional performance [35]. The \(Str\) parameter is valuable in identifying directional features of surfaces, especially those produced by multi-step manufacturing processes. It is also used to detect subtle texture orientations on otherwise isotropic surfaces and to monitor machine tool vibrations [36]. \(Sal\), on the other hand, quantifies the distance over which a surface texture remains statistically correlated. It helps determine the appropriate spacing between measurement points to accurately characterize surface texture. Sal finds application in the analysis of tribological behavior (e.g., friction and wear) and in studies of surface interaction with electromagnetic radiation [37; 38]. \(Sdq\) is particularly useful for differentiating surfaces that share the same average roughness (\(Sa\)). It finds application in the analysis of sealing systems, surfaces where cosmetic appearance is important, and in contexts involving fluid interaction with surfaces (e.g., coatings) [39]. This parameter is influenced by both the amplitude and the spacing of the surface texture. For a given \(Sa\) value, a wider spacing between surface features generally results in a lower \(Sdq\), compared to a surface with the same \(Sa\) but smaller spacing (Figure 3). Figure 2: Different surfaces with the same parameter values [31]: \(Sa\) = 16.03 nm and \(Sq\) = 25.4 nm. _Sdr_ can distinguish surfaces that exhibit similar amplitude values (e.g., _Sa_). Typically, as surface complexity increases, _Sdr_ rises regardless of whether _Sa_ changes. This parameter is particularly relevant for applications involving surface coatings, adhesion, and fluid interactions [41]. _Sdr_ is also influenced by texture amplitude and spacing. Thus, a surface with high Sa but widely spaced features may exhibit a lower _Sdr_ compared to a surface with a lower _Sa_ and finely spaced features. In [42], it is demonstrated that the parameters _Sok_ and _Spk_ provide valuable insight into the wear resistance of materials under various operating conditions. _Smr(c)_ is used to assess the amount of residual bearing surface after a certain depth of material has been removed from the surface [43]. A typical application is in specifying engine cylinder bore surfaces before running-in. The bore can be ground to form a plateau-like surface, with fine, pointed structures placed to aid the running-in process of sliding piston rings. The bore can be ground to form a plateau-like surface, with fine, pointed structures placed to aid the running-in process of sliding piston rings [44]. Figure 4 illustrates the material ratio on the observed surface.