# Application of Areal Topography Parameters in Surface Characterization
## 2 Areal Topography Parameters from Definition to Application
Different volume parameters are crucial for understanding how much material can be worn away at a given depth of the bearing curve (e.g., _Vmp(p)_) and how much material remains to support the load after upper surface levels are worn down (e.g., _Vmc(p,q)_). These parameters are applied in sealing, where _Vmp(p)_ provides insight into the amount of material available for sealing purposes [45]. _Vv(mr)_, _Vvv(p)_, and _Vvc(p,q)_ quantify the volume of cavities within the surface, with height variations determined by the selected material ratio values. These parameters are useful for determining how much fluid would fill the surface between the selected material ratio heights [46]. Void volume parameters are critical for understanding fluid flow, coating applications, and debris entrapment [47]. Figure 4: The ratio of the material on the observed surface [26]. Figure 3: Different surfaces with the same _Sa_ parameters [40]. _Sa_ = 80 nm, _Sdq_ = 11.0, _Sa_ = 75 mm, _Sdq_ = 0.2. Core void volume (_Vvc_) can be used to assess how much space remains available in the surface core after the surface has undergone a running-in process, leading to reduced peak heights [43]. Meanwhile, _Vvc_(_p_) can be indicative of the residual volume remaining after significant surface wear [48]. The _Std_ parameter measures the angular direction of the dominant surface texture (Figure 5). It is defined relative to the _y_-axis, where a surface aligned along the _y_-axis has an _Std_ value of 0\({}^{\circ}\). Since no dominant surface texture direction is observed in the right image of Figure 5, the _Std_ parameter is undefined. Currently, the _Std_ parameter is not prescribed by ISO 25178-2 [26]. _Std_ is useful for determining the direction of the surface texture relative to the known orientation of the measurement instrument [44]. Its application is particularly relevant in sealing, where subtle changes in the direction of surface texture can lead to adverse conditions [50]. Additionally, _Std_ is used to detect the presence of a preliminary surface treatment process (e.g., turning) that may be removed during subsequent finishing operations (e.g., grinding) [51]. Among surface topography parameters, _Sdq_ stands out as a hybrid descriptor that provides additional insight into the microstructural characteristics of a surface. Unlike spatially oriented parameters such as _Sa_ and _Sz_, _Sdq_ quantifies the average slope of surface features, thereby capturing the uniformity and smoothness of the coating distribution at the nanoscale. Higher _Sdq_ values indicate abrupt height variations and pronounced roughness, whereas lower values reflect a homogeneous and smooth surface. In the study [39], _Sdq_ was employed as an indicator of coating distribution quality under various application and drying methods of epoxy primers. It was found that only the chemical composition of the coating, specifically, the presence of zinc powder, had a statistically significant influence on _Sdq_ values (_p_ < 0.001), while variations in drying techniques and application methods showed no significant effect. These findings suggest that _Sdq_ may serve as a sensitive indicator of microporosity and surface inhomogeneity, particularly in coatings with a high content of metallic pigments. In this context, incorporating _Sdq_ into surface topography analysis represents an important step toward more accurate characterization of functional coatings in engineering applications. In [52], areal topography parameters of cold-forged austenitic stainless steel AISI 304 were analyzed to evaluate their influence on corrosion resistance. Surface characterization was performed using atomic force microscopy (AFM), measuring 3D roughness parameters including _Sa_, _Sz_, _Sv_, _Sp_, _Sku_, _Ssk_, and _Sq_. Statistical analysis (ANOVA) confirmed that surface roughness is significantly affected by the surface preparation method, whereas the degree of plastic deformation had no significant influence.